Resistive switching near electrode interfaces: Estimations by a current model

NASA Astrophysics Data System (ADS)

Schroeder, Herbert; Zurhelle, Alexander; Stemmer, Stefanie; Marchewka, Astrid; Waser, Rainer

2013-02-01

The growing resistive switching database is accompanied by many detailed mechanisms which often are pure hypotheses. Some of these suggested models can be verified by checking their predictions with the benchmarks of future memory cells. The valence change memory model assumes that the different resistances in ON and OFF states are made by changing the defect density profiles in a sheet near one working electrode during switching. The resulting different READ current densities in ON and OFF states were calculated by using an appropriate simulation model with variation of several important defect and material parameters of the metal/insulator (oxide)/metal thin film stack such as defect density and its profile change in density and thickness, height of the interface barrier, dielectric permittivity, applied voltage. The results were compared to the benchmarks and some memory windows of the varied parameters can be defined: The required ON state READ current density of 105 A/cm2 can only be achieved for barriers smaller than 0.7 eV and defect densities larger than 3 × 1020 cm-3. The required current ratio between ON and OFF states of at least 10 requests defect density reduction of approximately an order of magnitude in a sheet of several nanometers near the working electrode.

Lattice structures and electronic properties of CIGS/CdS interface: First-principles calculations

NASA Astrophysics Data System (ADS)

Tang, Fu-Ling; Liu, Ran; Xue, Hong-Tao; Lu, Wen-Jiang; Feng, Yu-Dong; Rui, Zhi-Yuan; Huang, Min

2014-07-01

Using first-principles calculations within density functional theory, we study the atomic structures and electronic properties of the perfect and defective (2VCu+InCu) CuInGaSe2/CdS interfaces theoretically, especially the interface states. We find that the local lattice structure of (2VCu+InCu) interface is somewhat disorganized. By analyzing the local density of states projected on several atomic layers of the two interfaces models, we find that for the (2VCu+InCu) interface the interface states near the Fermi level in CuInGaSe2 and CdS band gap regions are mainly composed of interfacial Se-4p, Cu-3d and S-3p orbitals, while for the perfect interface there are no clear interface states in the CuInGaSe2 region but only some interface states which are mainly composed of S-3p orbitals in the valance band of CdS region.

Non-contact, non-destructive, quantitative probing of interfacial trap sites for charge carrier transport at semiconductor-insulator boundary

DOE Office of Scientific and Technical Information (OSTI.GOV)

Choi, Wookjin; Miyakai, Tomoyo; Sakurai, Tsuneaki

The density of traps at semiconductor–insulator interfaces was successfully estimated using microwave dielectric loss spectroscopy with model thin-film organic field-effect transistors. The non-contact, non-destructive analysis technique is referred to as field-induced time-resolved microwave conductivity (FI-TRMC) at interfaces. Kinetic traces of FI-TRMC transients clearly distinguished the mobile charge carriers at the interfaces from the immobile charges trapped at defects, allowing both the mobility of charge carriers and the number density of trap sites to be determined at the semiconductor-insulator interfaces. The number density of defects at the interface between evaporated pentacene on a poly(methylmethacrylate) insulating layer was determined to be 10{supmore » 12 }cm{sup −2}, and the hole mobility was up to 6.5 cm{sup 2} V{sup −1} s{sup −1} after filling the defects with trapped carriers. The FI-TRMC at interfaces technique has the potential to provide rapid screening for the assessment of interfacial electronic states in a variety of semiconductor devices.« less

Interface Energy Alignment of Atomic-Layer-Deposited VOx on Pentacene: an in Situ Photoelectron Spectroscopy Investigation.

PubMed

Zhao, Ran; Gao, Yuanhong; Guo, Zheng; Su, Yantao; Wang, Xinwei

2017-01-18

Ultrathin atomic-layer-deposited (ALD) vanadium oxide (VO x ) interlayer has recently been demonstrated for remarkably reducing the contact resistance in organic electronic devices (Adv. Funct. Mater. 2016, 26, 4456). Herein, we present an in situ photoelectron spectroscopy investigation (including X-ray and ultraviolet photoelectron spectroscopies) of ALD VO x grown on pentacene to understand the role of the ALD VO x interlayer for the improved contact resistance. The in situ photoelectron spectroscopy characterizations allow us to monitor the ALD growth process of VO x and trace the evolutions of the work function, pentacene HOMO level, and VO x defect states during the growth. The initial VO x growth is found to be partially delayed on pentacene in the first ∼20 ALD cycles. The underneath pentacene layer is largely intact after ALD. The ALD VO x is found to contain a high density of defect states starting from 0.67 eV below the Fermi level, and the energy level of these defect states is in excellent alignment with the HOMO level of pentacene, which therefore allows these VO x defect states to provide an efficient hole-injection pathway at the contact interface.

Total photoelectron yield spectroscopy of energy distribution of electronic states density at GaN surface and SiO2/GaN interface

NASA Astrophysics Data System (ADS)

Ohta, Akio; Truyen, Nguyen Xuan; Fujimura, Nobuyuki; Ikeda, Mitsuhisa; Makihara, Katsunori; Miyazaki, Seiichi

2018-06-01

The energy distribution of the electronic state density of wet-cleaned epitaxial GaN surfaces and SiO2/GaN structures has been studied by total photoelectron yield spectroscopy (PYS). By X-ray photoelectron spectroscopy (XPS) analysis, the energy band diagram for a wet-cleaned epitaxial GaN surface such as the energy level of the valence band top and electron affinity has been determined to obtain a better understanding of the measured PYS signals. The electronic state density of GaN surface with different carrier concentrations in the energy region corresponding to the GaN bandgap has been evaluated. Also, the interface defect state density of SiO2/GaN structures was also estimated by not only PYS analysis but also capacitance–voltage (C–V) characteristics. We have demonstrated that PYS analysis enables the evaluation of defect state density filled with electrons at the SiO2/GaN interface in the energy region corresponding to the GaN midgap, which is difficult to estimate by C–V measurement of MOS capacitors.

Capacitance Techniques | Photovoltaic Research | NREL

Science.gov Websites

transient spectroscopy generated graph showing six defect levels; DLTS signal (Y-axis) versus Temperature (X -axis). DLTS characterizes defect levels to assist in identification of impurities and potential levels of interface states (or both) that often exist between the surfaces of dissimilar materials. Deep

Water promotes the sealing of nanoscale packing defects in folding proteins.

PubMed

Fernández, Ariel

2014-05-21

A net dipole moment is shown to arise from a non-Debye component of water polarization created by nanoscale packing defects on the protein surface. Accordingly, the protein electrostatic field exerts a torque on the induced dipole, locally impeding the nucleation of ice at the protein-water interface. We evaluate the solvent orientation steering (SOS) as the reversible work needed to align the induced dipoles with the Debye electrostatic field and computed the SOS for the variable interface of a folding protein. The minimization of the SOS is shown to drive protein folding as evidenced by the entrainment of the total free energy by the SOS energy along trajectories that approach a Debye limit state where no torque arises. This result suggests that the minimization of anomalous water polarization at the interface promotes the sealing of packing defects, thereby maintaining structural integrity and committing the protein chain to fold.

Structural and electronic properties of the transition layer at the SiO{sub 2}/4H-SiC interface

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, Wenbo; Wang, Dejun, E-mail: dwang121@dlut.edu.cn; Zhao, Jijun

Using first-principles methods, we generate an amorphous SiO{sub 2}/4H-SiC interface with a transition layer. Based this interface model, we investigate the structural and electronic properties of the interfacial transition layer. The calculated Si 2p core-level shifts for this interface are comparable to the experimental data, indicating that various SiC{sub x}O{sub y} species should be present in this interface transition layer. The analysis of the electronic structures reveals that the tetrahedral SiC{sub x}O{sub y} structures cannot introduce any of the defect states at the interface. Interestingly, our transition layer also includes a C-C=C trimer and SiO{sub 5} configurations, which lead tomore » the generation of interface states. The accurate positions of Kohn-Sham energy levels associated with these defects are further calculated within the hybrid functional scheme. The Kohn-Sham energy levels of the carbon trimer and SiO{sub 5} configurations are located near the conduction and valence band of bulk 4H-SiC, respectively. The result indicates that the carbon trimer occurred in the transition layer may be a possible origin of near interface traps. These findings provide novel insight into the structural and electronic properties of the realistic SiO{sub 2}/SiC interface.« less

Colloidal Particle Adsorption at Water-Water Interfaces with Ultralow Interfacial Tension

NASA Astrophysics Data System (ADS)

Keal, Louis; Colosqui, Carlos E.; Tromp, R. Hans; Monteux, Cécile

2018-05-01

Using fluorescence confocal microscopy we study the adsorption of single latex microparticles at a water-water interface between demixing aqueous solutions of polymers, generally known as a water-in-water emulsion. Similar microparticles at the interface between molecular liquids have exhibited an extremely slow relaxation preventing the observation of expected equilibrium states. This phenomenon has been attributed to "long-lived" metastable states caused by significant energy barriers Δ F ˜γ Ad≫kBT induced by high interfacial tension (γ ˜10-2 N /m ) and nanoscale surface defects with characteristic areas Ad≃10 - 30 nm2 . For the studied water-water interface with ultralow surface tension (γ ˜10-4 N /m ) we are able to characterize the entire adsorption process and observe equilibrium states prescribed by a single equilibrium contact angle independent of the particle size. Notably, we observe crossovers from fast initial dynamics to slower kinetic regimes analytically predicted for large surface defects (Ad≃500 nm2). Moreover, particle trajectories reveal a position-independent damping coefficient that is unexpected given the large viscosity contrast between phases. These observations are attributed to the remarkably diffuse nature of the water-water interface and the adsorption and entanglement of polymer chains in the semidilute solutions. This work offers some first insights on the adsorption dynamics or kinetics of microparticles at water-water interfaces in biocolloidal systems.

Topologically protected bound states in photonic parity-time-symmetric crystals.

PubMed

Weimann, S; Kremer, M; Plotnik, Y; Lumer, Y; Nolte, S; Makris, K G; Segev, M; Rechtsman, M C; Szameit, A

2017-04-01

Parity-time (PT)-symmetric crystals are a class of non-Hermitian systems that allow, for example, the existence of modes with real propagation constants, for self-orthogonality of propagating modes, and for uni-directional invisibility at defects. Photonic PT-symmetric systems that also support topological states could be useful for shaping and routing light waves. However, it is currently debated whether topological interface states can exist at all in PT-symmetric systems. Here, we show theoretically and demonstrate experimentally the existence of such states: states that are localized at the interface between two topologically distinct PT-symmetric photonic lattices. We find analytical closed form solutions of topological PT-symmetric interface states, and observe them through fluorescence microscopy in a passive PT-symmetric dimerized photonic lattice. Our results are relevant towards approaches to localize light on the interface between non-Hermitian crystals.

Defect Characterization, Imaging, and Control in Wide-Bandgap Semiconductors and Devices

NASA Astrophysics Data System (ADS)

Brillson, L. J.; Foster, G. M.; Cox, J.; Ruane, W. T.; Jarjour, A. B.; Gao, H.; von Wenckstern, H.; Grundmann, M.; Wang, B.; Look, D. C.; Hyland, A.; Allen, M. W.

2018-03-01

Wide-bandgap semiconductors are now leading the way to new physical phenomena and device applications at nanoscale dimensions. The impact of defects on the electronic properties of these materials increases as their size decreases, motivating new techniques to characterize and begin to control these electronic states. Leading these advances have been the semiconductors ZnO, GaN, and related materials. This paper highlights the importance of native point defects in these semiconductors and describes how a complement of spatially localized surface science and spectroscopy techniques in three dimensions can characterize, image, and begin to control these electronic states at the nanoscale. A combination of characterization techniques including depth-resolved cathodoluminescence spectroscopy, surface photovoltage spectroscopy, and hyperspectral imaging can describe the nature and distribution of defects at interfaces at both bulk and nanoscale surfaces, their metal interfaces, and inside nanostructures themselves. These features as well as temperature and mechanical strain inside wide-bandgap device structures at the nanoscale can be measured even while these devices are operating. These advanced capabilities enable several new directions for describing defects at the nanoscale, showing how they contribute to device degradation, and guiding growth processes to control them.

First-principles study on C=C defects near SiC/SiO2 interface: Defect passivation by double-bond saturation

NASA Astrophysics Data System (ADS)

Tajima, Nobuo; Kaneko, Tomoaki; Yamasaki, Takahiro; Nara, Jun; Schimizu, Tatsuo; Kato, Koichi; Ohno, Takahisa

2018-04-01

Thermally produced SiC/SiO2 stacking in SiC MOSFETs creates defect-related interfacial states in and around the band gap of SiC. These interfacial states can cause serious reliability problems such as threshold voltage shift, as well as efficiency problems such as channel mobility degradation. Carbon species having C=C double bonds have been suggested as one of the origins of these interfacial states. We have theoretically shown that this type of defect produces interfacial states in and around the band gap of SiC, and that they can be removed by saturating the C=C double bond by reactions with H2 and F2. The single-bond products of these reactions are found to be stable at regular device operation temperatures.

Research on c-HfO2 (0 0 1)/α -Al2O3 (1 -1 0 2) interface in CTM devices based on first principle theory

NASA Astrophysics Data System (ADS)

Lu, Wenjuan; Dai, Yuehua; Wang, Feifei; Yang, Fei; Ma, Chengzhi; Zhang, Xu; Jiang, Xianwei

2017-12-01

With the growing application of high-k dielectrics, the interface between HfO2 and Al2O3 play a crucial role in CTM devices. To clearly understand the interaction of the HfO-AlO interface at the atomic and electronic scale, the bonding feature, electronic properties and charge localized character of c- HfO2 (0 0 1)/α-Al2O3 (1 -1 0 2) interface has been investigated by first principle calculations. The c- HfO2 (0 0 1)/α-Al2O3 (1 -1 0 2) interface has adhesive energy about -1.754 J/m2, suggesting that this interface can exist stably. Through analysis of Bader charge and charge density difference, the intrinsic interfacial gap states are mainly originated from the OII and OIII types oxygen atoms at the interface, and only OIII type oxygen atoms can localized electrons effectively and are provided with good reliability during P/E cycles, which theoretically validate the experimental results that HfO2/Al2O3 multi-layered charge trapping layer can generate more effective traps in memory device. Furthermore, the influence of interfacial gap states during P/E cycles in the defective interface system have also been studied, and the results imply that defective system displays the degradation on the reliability during P/E cycles, while, the charge localized ability of interfacial states is stronger than intrinsic oxygen vacancy in the trapping layer. Besides, these charge localized characters are further explained by the analysis of the density of states correspondingly. In sum, our results compare well with similar experimental observations in other literatures, and the study of the interfacial gap states in this work would facilitate further development of interface passivation.

Coarse-grained molecular dynamics modeling of the kinetics of lamellar BCP defect annealing

NASA Astrophysics Data System (ADS)

Peters, Andrew J.; Lawson, Richard A.; Nation, Benjamin D.; Ludovice, Peter J.; Henderson, Clifford L.

2015-03-01

Directed self-assembly of block copolymers (BCPs) is a process that has received great interest in the field of nanomanufacturing in the past decade, and great strides towards forming high quality aligned patterns have been made. But state of the art methods still yield defectivities orders of magnitude higher than is necessary in semi-conductor fabrication even though free energy calculations suggest that equilibrium defectivities are much lower than is necessary for economic semi-conductor fabrication. This disparity suggests that the main problem may lie in the kinetics of defect removal. This work uses a coarse-grained model to study the rates, pathways, and dependencies of healing a common defect to give insight into the fundamental processes that control defect healing and give guidance on optimal process conditions for BCP-DSA. It is found that infinitely thick films yield an exponential drop in defect heal rate above χN ~ 30. Below χN ~ 30, the rate of transport was similar to the rate at which the transition state was reached so that the overall rate changed only slightly. The energy barrier in periodic simulations increased with 0.31 χN on average. Thin film simulations show no change in rate associated with the energy barrier below χN ~ 50, and then show an increase in energy barrier scaling with 0.16χN. Thin film simulations always begin to heal at either the free interface or the BCP-underlayer interface where the increased A-B contact area associated with the transition state will be minimized, while the infinitely thick films must start healing in the bulk where the A-B contact area is increased. It is also found that cooperative chain movement is required for the defect to start healing.

Observation of interface defects in thermally oxidized SiC using positron annihilation

NASA Astrophysics Data System (ADS)

Dekker, James; Saarinen, Kimmo; Ólafsson, Halldór; Sveinbjörnsson, Einar Ö.

2003-03-01

Positron annihilation has been applied to study thermally oxidized 4H- and 6H-SiC. The SiC/SiO2 interface is found to contain a high density of open-volume defects. The positron trapping at the interface defects correlates with the charge of the interface determined by capacitance-voltage experiments. For oxides grown on n-SiC substrates, the positron annihilation characteristics at these defects are nearly indistinguishable from those of a silicon/oxide interface, with no discernable contribution from C-related bonds or carbon clusters. These results indicate that those defects at the SiC/oxide interface, which are visible to positrons, are similar to those at the Si/oxide interface. The positron annihilation characteristics suggest that these defects are vacancies surrounded by oxygen atoms.

DLTS analysis of amphoteric interface defects in high-TiO{sub 2} MOS structures prepared by sol-gel spin-coating

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kumar, Arvind; Mondal, Sandip; Rao, K. S. R. Koteswara, E-mail: ksrkrao@physics.iisc.ernet.in, E-mail: raoksrk@gmail.com

2015-11-15

High-κ TiO{sub 2} thin films have been fabricated from a facile, combined sol – gel spin – coating technique on p and n type silicon substrate. XRD and Raman studies headed the existence of anatase phase of TiO{sub 2} with a small grain size of 18 nm. The refractive index ‘n’ quantified from ellipsometry is 2.41. AFM studies suggest a high quality, pore free films with a fairly small surface roughness of 6 Å. The presence of Ti in its tetravalent state is confirmed by XPS analysis. The defect parameters observed at the interface of Si/TiO{sub 2} were studied bymore » capacitance – voltage (C – V) and deep level transient spectroscopy (DLTS). The flat – band voltage (V{sub FB}) and the density of slow interface states estimated are – 0.9, – 0.44 V and 5.24×10{sup 10}, 1.03×10{sup 11} cm{sup −2}; for the NMOS and PMOS capacitors, respectively. The activation energies, interface state densities and capture cross – sections measured by DLTS are E{sub V} + 0.30, E{sub C} – 0.21 eV; 8.73×10{sup 11}, 6.41×10{sup 11} eV{sup −1} cm{sup −2} and 5.8×10{sup −23}, 8.11×10{sup −23} cm{sup 2} for the NMOS and PMOS structures, respectively. A low value of interface state density in both P- and N-MOS structures makes it a suitable alternate dielectric layer for CMOS applications. And also very low value of capture cross section for both the carriers due to the amphoteric nature of defect indicates that the traps are not aggressive recombination centers and possibly can not contribute to the device operation to a large extent.« less

Hydrogen Peroxide Formation and pH Changes at Rock-Water Interface during Stressing

NASA Astrophysics Data System (ADS)

Xie, S.; Kulahci, I.; Cyr, G.; Tregloan-Reed, J.; Balk, M.; Rothschild, L. J.; Freund, F. T.

2008-12-01

Common igneous and high-grade metamorphic rocks contain dormant defects, which become activated when stressed. They release electronic charge carriers, in particular defect electrons associated with O- states in a matrix of O2-. Known as 'positive holes' or pholes for short, the O- states can spread out of the stressed rock volume, travel along stress gradients over distances on the order of meters in the lab and probably over kilometers in the field. They carry a current, which can flow through meters of rock in the laboratory, probably tens of kilometers in the field. At rock-water interfaces the O- states turn into O radicals, which subtract H from H2O, forming OH- in the rock surface and PH radicals in the water. Two OH combine to H2O2. In the process the pH becomes more acidic. The discovery of H2O2 formation at rock-water interfaces as part of stress- activated currents on the tectonically active Earth may help us better understand the oxidation of the early Earth and the evolution of early Life.

Classification of defects in honeycomb composite structure of helicopter rotor blades

NASA Astrophysics Data System (ADS)

Balaskó, M.; Sváb, E.; Molnár, Gy.; Veres, I.

2005-04-01

The use of non-destructive testing methods to qualify the state of rotor blades with respect to their expected flight hours, with the aim to extend their lifetime without any risk of breakdown, is an important financial demand. In order to detect the possible defects in the composite structure of Mi-8 and Mi-24 type helicopter rotor blades used by the Hungarian Army, we have performed combined neutron- and X-ray radiography measurements at the Budapest Research Reactor. Several types of defects were detected, analysed and typified. Among the most frequent and important defects observed were cavities, holes and/or cracks in the sealing elements on the interface of the honeycomb structure and the section boarders. Inhomogeneities of the resin materials (resin-rich or starved areas) at the core-honeycomb surfaces proved to be an other important point. Defects were detected at the adhesive filling, and water percolation was visualized at the sealing interfaces of the honeycomb sections. Corrosion effects, and metal inclusions have also been detected.

Surface acceptor states in MBE-grown CdTe layers

NASA Astrophysics Data System (ADS)

Wichrowska, Karolina; Wosinski, Tadeusz; Tkaczyk, Zbigniew; Kolkovsky, Valery; Karczewski, Grzegorz

2018-04-01

A deep-level hole trap associated with surface defect states has been revealed with deep-level transient spectroscopy investigations of metal-semiconductor junctions fabricated on nitrogen doped p-type CdTe layers grown by the molecular-beam epitaxy technique. The trap displayed the hole-emission activation energy of 0.33 eV and the logarithmic capture kinetics indicating its relation to extended defect states at the metal-semiconductor interface. Strong electric-field-induced enhancement of the thermal emission rate of holes from the trap has been attributed to the phonon-assisted tunneling effect from defect states involving very large lattice relaxation around the defect and metastability of its occupied state. Passivation with ammonium sulfide of the CdTe surface, prior to metallization, results in a significant decrease in the trap density. It also results in a distinct reduction in the width of the surface-acceptor-state-induced hysteresis loops in the capacitance vs. voltage characteristics of the metal-semiconductor junctions.

Interface effects on calculated defect levels for oxide defects

NASA Astrophysics Data System (ADS)

Edwards, Arthur; Barnaby, Hugh; Schultz, Peter; Pineda, Andrew

2014-03-01

Density functional theory (DFT) has had impressive recent success predicting defect levels in insulators and semiconductors [Schultz and von Lillienfeld, 2009]. Such success requires care in accounting for long-range electrostatic effects. Recently, Komsa and Pasquarello have started to address this problem in systems with interfaces. We report a multiscale technique for calculating electrostatic energies for charged defects in oxide of the metal-oxide-silicon (MOS) system, but where account is taken of substrate doping density, oxide thickness, and gate bias. We use device modeling to calculate electric fields for a point charge a fixed distance from the interface, and used the field to numerically calculate the long-range electrostatic interactions. We find, for example, that defect levels in the oxide do depend on both the magnitude and the polarity the substrate doping density. Furthermore, below 20 Å, oxide thickness also has significant effects. So, transferring results directly from bulk calculations leads to inaccuracies up to 0.5 eV- half of the silicon band gap. We will present trends in defect levels as a function of device parameters. We show that these results explain previous experimental results, and we comment on their potential impact on models for NBTI. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the United States Department of Energy's National Nuclear Security Administration under co.

Defect States in InP/InGaAs/InP Heterostructures by Current-Voltage Characteristics and Deep Level Transient Spectroscopy.

PubMed

Vu, Thi Kim Oanh; Lee, Kyoung Su; Lee, Sang Jun; Kim, Eun Kyu

2018-09-01

We studied defect states in In0.53Ga0.47As/InP heterojunctions with interface control by group V atoms during metalorganic chemical vapor (MOCVD) deposition. From deep level transient spectroscopy (DLTS) measurements, two defects with activation energies of 0.28 eV (E1) and 0.15 eV (E2) below the conduction band edge, were observed. The defect density of E1 for In0.53Ga0.47As/InP heterojunctions with an addition of As and P atoms was about 1.5 times higher than that of the heterojunction added P atom only. From the temperature dependence of current- voltage characteristics, the thermal activation energies of In0.53Ga0.47As/InP of heterojunctions were estimated to be 0.27 and 0.25 eV, respectively. It appeared that the reverse light current for In0.53Ga0.47As/InP heterojunction added P atom increased only by illumination of a 940 nm-LED light source. These results imply that only the P addition at the interface can enhance the quality of InGaAs/InP heterojunction.

Electrochemical impedance spectroscopy for quantitative interface state characterization of planar and nanostructured semiconductor-dielectric interfaces

NASA Astrophysics Data System (ADS)

Meng, Andrew C.; Tang, Kechao; Braun, Michael R.; Zhang, Liangliang; McIntyre, Paul C.

2017-10-01

The performance of nanostructured semiconductors is frequently limited by interface defects that trap electronic carriers. In particular, high aspect ratio geometries dramatically increase the difficulty of using typical solid-state electrical measurements (multifrequency capacitance- and conductance-voltage testing) to quantify interface trap densities (D it). We report on electrochemical impedance spectroscopy (EIS) to characterize the energy distribution of interface traps at metal oxide/semiconductor interfaces. This method takes advantage of liquid electrolytes, which provide conformal electrical contacts. Planar Al2O3/p-Si and Al2O3/p-Si0.55Ge0.45 interfaces are used to benchmark the EIS data against results obtained from standard electrical testing methods. We find that the solid state and EIS data agree very well, leading to the extraction of consistent D it energy distributions. Measurements carried out on pyramid-nanostructured p-Si obtained by KOH etching followed by deposition of a 10 nm ALD-Al2O3 demonstrate the application of EIS to trap characterization of a nanostructured dielectric/semiconductor interface. These results show the promise of this methodology to measure interface state densities for a broad range of semiconductor nanostructures such as nanowires, nanofins, and porous structures.

Study of spin-dependent transitions and spin coherence at the (111) oriented phosphorous doped crystalline silicon to silicon dioxide interface using pulsed electrically detected magnetic resonance

NASA Astrophysics Data System (ADS)

Paik, Seoyoung

A study of spin-dependent electronic transitions at the (111) oriented phosphorous doped crystalline silicon (c-Si) to silicon dioxide (SiO 2) interface is presented for [31P] = 1015 cm-3 and [31P] = 1016 cm -3 and a temperature range between T ≈ 5K and T ≈ 15K. Using pulsed electrically detected magnetic resonance (pEDMR), spin-dependent transitions involving 31P donor states and two different interface states are observed, namely (i) Pb centers which can be identified by their characteristic anisotropy and (ii) the E' center which is attributed to defects of the near interface SiO 2 bulk. Correlation measurements of the dynamics of spin-dependent recombination confirm that previously proposed transitions between 31P and the interface defects take place. The influence of these near interface transitions on the 31P donor spin coherence time T 2 as well as the donor spin-lattice relaxation time T 1 is then investigated by comparison of spin Hahn echo decay measurements obtained from conventional bulk sensitive pulsed electron paramagnetic resonance and surface sensitive pEDMR measurements, as well as surface sensitive electrically detected inversion recovery experiments. The measurements reveal that the T2 times of both interface states and 31P donor electrons spins in proximity of them are consistently shorter than the T1 times, and both T2 and T1 times of the near interface donors are reduced by several orders of magnitude from those in the bulk, at T ≤ 13 K. The T 2 times of the 31P donor electrons are in agreement with the prediction by De Sousa that they are limited by interface defect-induced field noise. To further investigate the dynamic properties of spin-dependent near interface processes, electrical detection of spin beat oscillation between resonantly induced spin-Rabi nutation is conducted at the phosphorous doped (1016cm-3) Si(111)/SiO2 interface. Predictions of Rabi beat oscillations based on several different spin-pair models are compared with measured Rabi beat nutation data. Due to the g-factor anisotropy of the Pb center (a silicon surface dangling bond), one can tune intra-pair Larmor frequency differences (Larmor separations) by orientation of the crystal with regard to an external magnetic field. Since Larmor separation governs the number of beating spin-pairs, crystal orientation can control the beat current. This is used to identify spin states that are paired by mutual electronic transitions. Based on the agreement between hypothesis and data, the experiments confirm the presence of the previously observed 31P-P b transition and the previously hypothesized P b to near interface SiO2 bulk state (E' center) transition.

Crystal orientation induced spin Rabi beat oscillations of point defects at the c-Si(111)/ SiO 2 interface

NASA Astrophysics Data System (ADS)

Paik, Seoyoung; Lee, Sang-Yun; Boehme, Christoph

2011-03-01

Spin-dependent electronic transitions such as certain charge carrier recombination and transport processes in semiconductors are usually governed by the Pauli blockade within pairs of two paramagnetic centers. One implication of this is that the manipulation of spin states, e.g. by magnetic resonant excitation, can produce changes to electric currents of the given semiconductor material. If both spins are changed at the same time, quantum beat effects such as beat oscillation between resonantly induced spin Rabi nutation becomes detectable through current measurements. Here, we report on electrically detected spin Rabi beat oscillation caused by pairs of 31 P donor states and Pb interface defects at the phosphorous doped Si(111)/ Si O2 interface. Due to the g-factor anisotropy of the Pb center we can tune the intra pair Larmor frequency difference (so called Larmor separation) through orientation of the sample with regard to the external magnetic field. As the Larmor separation governs the spin Rabi beat oscillation, we show experimentally how the crystal orientation can influence the beat effect.

Molecular dynamics modeling of helium bubbles in austenitic steels

NASA Astrophysics Data System (ADS)

Jelea, A.

2018-06-01

The austenitic steel devices from pressurized water reactors are continuously subjected to neutron irradiation that produces crystalline point defects and helium atoms in the steel matrix. These species evolve into large defects such as dislocation loops and helium filled bubbles. This paper analyzes, through molecular dynamics simulations with recently developed interatomic potentials, the impact of the helium/steel interface on the helium behavior in nanosize bubbles trapped in an austenitic steel matrix. It is shown that the repulsive helium-steel interactions induce higher pressures in the bubble compared to bulk helium at the same temperature and average density. A new equation of state for helium is proposed in order to take into account these interface effects.

Asymmetric interaction of point defects and heterophase interfaces in ZrN/TaN multilayered nanofilms.

PubMed

Lao, Yuanxia; Hu, Shuanglin; Shi, Yunlong; Deng, Yu; Wang, Fei; Du, Hao; Zhang, Haibing; Wang, Yuan

2017-01-05

Materials with a high density of heterophase interfaces, which are capable of absorbing and annihilating radiation-induced point defects, can exhibit a superior radiation tolerance. In this paper, we investigated the interaction behaviors of point defects and heterophase interfaces by implanting helium atoms into the ZrN/TaN multilayered nanofilms. It was found that the point defect-interface interaction on the two sides of the ZrN/TaN interface was asymmetric, likely due to the difference in the vacancy formation energies of ZrN and TaN. The helium bubbles could migrate from the ZrN layers into the TaN layers through the heterophase interfaces, resulting in a better crystallinity of the ZrN layers and a complete amorphization of the TaN layers. The findings provided some clues to the fundamental behaviors of point defects near the heterophase interfaces, which make us re-examine the design rules of advanced radiation-tolerant materials.

Asymmetric interaction of point defects and heterophase interfaces in ZrN/TaN multilayered nanofilms

NASA Astrophysics Data System (ADS)

Lao, Yuanxia; Hu, Shuanglin; Shi, Yunlong; Deng, Yu; Wang, Fei; Du, Hao; Zhang, Haibing; Wang, Yuan

2017-01-01

Materials with a high density of heterophase interfaces, which are capable of absorbing and annihilating radiation-induced point defects, can exhibit a superior radiation tolerance. In this paper, we investigated the interaction behaviors of point defects and heterophase interfaces by implanting helium atoms into the ZrN/TaN multilayered nanofilms. It was found that the point defect-interface interaction on the two sides of the ZrN/TaN interface was asymmetric, likely due to the difference in the vacancy formation energies of ZrN and TaN. The helium bubbles could migrate from the ZrN layers into the TaN layers through the heterophase interfaces, resulting in a better crystallinity of the ZrN layers and a complete amorphization of the TaN layers. The findings provided some clues to the fundamental behaviors of point defects near the heterophase interfaces, which make us re-examine the design rules of advanced radiation-tolerant materials.

Asymmetric interaction of point defects and heterophase interfaces in ZrN/TaN multilayered nanofilms

PubMed Central

Lao, Yuanxia; Hu, Shuanglin; Shi, Yunlong; Deng, Yu; Wang, Fei; Du, Hao; Zhang, Haibing; Wang, Yuan

2017-01-01

Materials with a high density of heterophase interfaces, which are capable of absorbing and annihilating radiation-induced point defects, can exhibit a superior radiation tolerance. In this paper, we investigated the interaction behaviors of point defects and heterophase interfaces by implanting helium atoms into the ZrN/TaN multilayered nanofilms. It was found that the point defect-interface interaction on the two sides of the ZrN/TaN interface was asymmetric, likely due to the difference in the vacancy formation energies of ZrN and TaN. The helium bubbles could migrate from the ZrN layers into the TaN layers through the heterophase interfaces, resulting in a better crystallinity of the ZrN layers and a complete amorphization of the TaN layers. The findings provided some clues to the fundamental behaviors of point defects near the heterophase interfaces, which make us re-examine the design rules of advanced radiation-tolerant materials. PMID:28053307

Fabrication of CuInS2-sensitized solar cells via an improved SILAR process and its interface electron recombination.

PubMed

Xu, Xueqing; Wan, Qingcui; Luan, Chunyan; Mei, Fengjiao; Zhao, Qian; An, Ping; Liang, Zhurong; Xu, Gang; Zapien, Juan Antonio

2013-11-13

Tetragonal CuInS2 (CIS) has been successfully deposited onto mesoporous TiO2 films by in-sequence growth of InxS and CuyS via a successive ionic layer absorption and reaction (SILAR) process and postdeposition annealing in sulfur ambiance. X-ray diffraction and Raman measurements showed that the obtained tetragonal CIS consisted of a chalcopyrite phase and Cu-Au ordering, which related with the antisite defect states. For a fixed Cu-S deposition cycle, an interface layer of β-In2S3 formed at the TiO2/CIS interface with suitable excess deposition of In-S. In the meantime, the content of the Cu-Au ordering phase decreased to a reasonable level. These facts resulted in the retardance of electron recombination in the cells, which is proposed to be dominated by electron transfer from the conduction band of TiO2 to the unoccupied defect states in CIS via exponentially distributed surface states. As a result, a relatively high efficiency of ~0.92% (V(oc) = 0.35 V, J(sc) = 8.49 mA cm(-2), and FF = 0.31) has been obtained. Last, but not least, with an overloading of the sensitizers, a decrease in the interface area between the sensitized TiO2 and electrolytes resulted in deceleration of hole extraction from CIS to the electrolytes, leading to a decrease in the fill factor of the solar cells. It is indicated that the unoccupied states in CIS with energy levels below EF0 of the TiO2 films play an important role in the interface electron recombination at low potentials and has a great influence on the fill factor of the solar cells.

Investigation of 3C-SiC/SiO2 interfacial point defects from ab initio g-tensor calculations and electron paramagnetic resonance measurements

NASA Astrophysics Data System (ADS)

Nugraha, T. A.; Rohrmueller, M.; Gerstmann, U.; Greulich-Weber, S.; Stellhorn, A.; Cantin, J. L.; von Bardeleben, J.; Schmidt, W. G.; Wippermann, S.

SiC is widely used in high-power, high-frequency electronic devices. Recently, it has also been employed as a building block in nanocomposites used as light absorbers in solar energy conversion devices. Analogous to Si, SiC features SiO2 as native oxide that can be used for passivation and insulating layers. However, a significant number of defect states are reported to form at SiC/SiO2 interfaces, limiting mobility and increasing recombination of free charge carriers. We investigated the growth of oxide on different 3C-SiC surfaces from first principles. Carbon antisite Csi defects are found to be strongly stabilized in particular at the interface, because carbon changes its hybridization from sp3 in the SiC-bulk to sp2 at the interface, creating a dangling bond inside a porous region of the SiO2 passivating layer. Combining ab initio g-tensor calculations and electron paramagnetic resonance (EPR) measurements, we show that Csi defects explain the measured EPR signatures, while the hyperfine structure allows to obtain local structural information of the oxide layer. Financial support from BMBF NanoMatFutur Grant 13N12972 and DFG priority program SPP-1601 is gratefully acknowledged.

The relationship between grain boundary structure, defect mobility, and grain boundary sink efficiency

PubMed Central

Uberuaga, Blas Pedro; Vernon, Louis J.; Martinez, Enrique; Voter, Arthur F.

2015-01-01

Nanocrystalline materials have received great attention due to their potential for improved functionality and have been proposed for extreme environments where the interfaces are expected to promote radiation tolerance. However, the precise role of the interfaces in modifying defect behavior is unclear. Using long-time simulations methods, we determine the mobility of defects and defect clusters at grain boundaries in Cu. We find that mobilities vary significantly with boundary structure and cluster size, with larger clusters exhibiting reduced mobility, and that interface sink efficiency depends on the kinetics of defects within the interface via the in-boundary annihilation rate of defects. Thus, sink efficiency is a strong function of defect mobility, which depends on boundary structure, a property that evolves with time. Further, defect mobility at boundaries can be slower than in the bulk, which has general implications for the properties of polycrystalline materials. Finally, we correlate defect energetics with the volumes of atomic sites at the boundary. PMID:25766999

The relationship between grain boundary structure, defect mobility, and grain boundary sink efficiency

DOE PAGES

Uberuaga, Blas Pedro; Vernon, Louis J.; Martinez, Enrique; ...

2015-03-13

Nanocrystalline materials have received great attention due to their potential for improved functionality and have been proposed for extreme environments where the interfaces are expected to promote radiation tolerance. However, the precise role of the interfaces in modifying defect behavior is unclear. Using long-time simulations methods, we determine the mobility of defects and defect clusters at grain boundaries in Cu. We find that mobilities vary significantly with boundary structure and cluster size, with larger clusters exhibiting reduced mobility, and that interface sink efficiency depends on the kinetics of defects within the interface via the in-boundary annihilation rate of defects. Thus,more » sink efficiency is a strong function of defect mobility, which depends on boundary structure, a property that evolves with time. Further, defect mobility at boundaries can be slower than in the bulk, which has general implications for the properties of polycrystalline materials. Finally, we correlate defect energetics with the volumes of atomic sites at the boundary.« less

Nature of electron trap states under inversion at In0.53Ga0.47As/Al2O3 interfaces

NASA Astrophysics Data System (ADS)

Colleoni, Davide; Pourtois, Geoffrey; Pasquarello, Alfredo

2017-03-01

In and Ga impurities substitutional to Al in the oxide layer resulting from diffusion out of the substrate are identified as candidates for electron traps under inversion at In0.53Ga0.47As/Al2O3 interfaces. Through density-functional calculations, these defects are found to be thermodynamically stable in amorphous Al2O3 and to be able to capture two electrons in a dangling bond upon breaking bonds with neighboring O atoms. Through a band alignment based on hybrid functional calculations, it is inferred that the corresponding defect levels lie at ˜1 eV above the conduction band minimum of In0.53Ga0.47As, in agreement with measured defect densities. These results support the technological importance of avoiding cation diffusion into the oxide layer.

PyCDT: A Python toolkit for modeling point defects in semiconductors and insulators

DOE PAGES

Broberg, Danny; Medasani, Bharat; Zimmermann, Nils E. R.; ...

2018-02-13

Point defects have a strong impact on the performance of semiconductor and insulator materials used in technological applications, spanning microelectronics to energy conversion and storage. The nature of the dominant defect types, how they vary with processing conditions, and their impact on materials properties are central aspects that determine the performance of a material in a certain application. This information is, however, difficult to access directly from experimental measurements. Consequently, computational methods, based on electronic density functional theory (DFT), have found widespread use in the calculation of point-defect properties. Here we have developed the Python Charged Defect Toolkit (PyCDT) tomore » expedite the setup and post-processing of defect calculations with widely used DFT software. PyCDT has a user-friendly command-line interface and provides a direct interface with the Materials Project database. This allows for setting up many charged defect calculations for any material of interest, as well as post-processing and applying state-of-the-art electrostatic correction terms. Our paper serves as a documentation for PyCDT, and demonstrates its use in an application to the well-studied GaAs compound semiconductor. As a result, we anticipate that the PyCDT code will be useful as a framework for undertaking readily reproducible calculations of charged point-defect properties, and that it will provide a foundation for automated, high-throughput calculations.« less

PyCDT: A Python toolkit for modeling point defects in semiconductors and insulators

NASA Astrophysics Data System (ADS)

Broberg, Danny; Medasani, Bharat; Zimmermann, Nils E. R.; Yu, Guodong; Canning, Andrew; Haranczyk, Maciej; Asta, Mark; Hautier, Geoffroy

2018-05-01

Point defects have a strong impact on the performance of semiconductor and insulator materials used in technological applications, spanning microelectronics to energy conversion and storage. The nature of the dominant defect types, how they vary with processing conditions, and their impact on materials properties are central aspects that determine the performance of a material in a certain application. This information is, however, difficult to access directly from experimental measurements. Consequently, computational methods, based on electronic density functional theory (DFT), have found widespread use in the calculation of point-defect properties. Here we have developed the Python Charged Defect Toolkit (PyCDT) to expedite the setup and post-processing of defect calculations with widely used DFT software. PyCDT has a user-friendly command-line interface and provides a direct interface with the Materials Project database. This allows for setting up many charged defect calculations for any material of interest, as well as post-processing and applying state-of-the-art electrostatic correction terms. Our paper serves as a documentation for PyCDT, and demonstrates its use in an application to the well-studied GaAs compound semiconductor. We anticipate that the PyCDT code will be useful as a framework for undertaking readily reproducible calculations of charged point-defect properties, and that it will provide a foundation for automated, high-throughput calculations.

PyCDT: A Python toolkit for modeling point defects in semiconductors and insulators

DOE Office of Scientific and Technical Information (OSTI.GOV)

Broberg, Danny; Medasani, Bharat; Zimmermann, Nils E. R.

Point defects have a strong impact on the performance of semiconductor and insulator materials used in technological applications, spanning microelectronics to energy conversion and storage. The nature of the dominant defect types, how they vary with processing conditions, and their impact on materials properties are central aspects that determine the performance of a material in a certain application. This information is, however, difficult to access directly from experimental measurements. Consequently, computational methods, based on electronic density functional theory DFT), have found widespread use in the calculation of point defect properties. Here we have developed the Python Charged Defect Toolkit (PyCDT)more » to expedite the setup and post-processing of defect calculations with widely used DFT software. PyCDT has a user-friendly command-line interface and provides a direct interface with the Materials Project database. This allows for setting up many charged defect calculations for any material of interest, as well as post-processing and applying state-of-the-art electrostatic correction terms. Our paper serves as a documentation for PyCDT, and demonstrates its use in an application to the well-studied GaAs compound semiconductor. We anticipate that the PyCDT code will be useful as a framework for undertaking readily reproducible calculations of charged point-defect properties, and that it will provide a foundation for automated, high-throughput calculations.« less

PyCDT: A Python toolkit for modeling point defects in semiconductors and insulators

DOE Office of Scientific and Technical Information (OSTI.GOV)

Broberg, Danny; Medasani, Bharat; Zimmermann, Nils E. R.

Point defects have a strong impact on the performance of semiconductor and insulator materials used in technological applications, spanning microelectronics to energy conversion and storage. The nature of the dominant defect types, how they vary with processing conditions, and their impact on materials properties are central aspects that determine the performance of a material in a certain application. This information is, however, difficult to access directly from experimental measurements. Consequently, computational methods, based on electronic density functional theory (DFT), have found widespread use in the calculation of point-defect properties. Here we have developed the Python Charged Defect Toolkit (PyCDT) tomore » expedite the setup and post-processing of defect calculations with widely used DFT software. PyCDT has a user-friendly command-line interface and provides a direct interface with the Materials Project database. This allows for setting up many charged defect calculations for any material of interest, as well as post-processing and applying state-of-the-art electrostatic correction terms. Our paper serves as a documentation for PyCDT, and demonstrates its use in an application to the well-studied GaAs compound semiconductor. As a result, we anticipate that the PyCDT code will be useful as a framework for undertaking readily reproducible calculations of charged point-defect properties, and that it will provide a foundation for automated, high-throughput calculations.« less

Air-annealing of Cu(In, Ga)Se2/CdS and performances of CIGS solar cells

NASA Astrophysics Data System (ADS)

Niu, X.; Zhu, H.; Liang, X.; Guo, Y.; Li, Z.; Mai, Y.

2017-12-01

In this study, the annealing treatment on Cu(In, Ga)Se2 (CIGS)/CdS interface in air is systematically investigated under different annealing temperatures from room temperature to 150 °C and different durations. It is found that when CIGS/CdS interface is annealed for a proper duration the corresponding CIGS thin film solar cells show enhanced open circuit voltage (Voc) and fill factor (FF) as well as corresponding conversion efficiency. The capacitance-voltage (C-V) and time-resolved photoluminescence (TR-PL) measurement results indicate that the CIGS thin film solar cells exhibit an increase in net defect density (NCV) and long lifetime for the carriers, respectively, after the annealing treatment of CIGS/CdS at a mediate annealing temperature here. Moreover, the net defect density of annealed solar cells at higher annealing temperatures for a long duration is reduced. All the variations in the solar cell performances, NCV and carrier lifetime would be related to the passivation of Se vacancies and InCu defects, surface (interface) states as well as positive interface discharges and Cu migration etc. A high efficiency CIGS solar cell of 14.4% is achieved. The optimized solar cell of 17.2% with a MgF2 anti-reflective layer has been obtained.

Massive Interfacial Reconstruction at Misfit Dislocations in Metal/Oxide Interfaces

DOE Office of Scientific and Technical Information (OSTI.GOV)

Choudhury, Samrat; Morgan, Dane; Uberuaga, Blas P.

Electronic structure calculations were performed to study the role of misfit dislocations on the structure and chemistry of a metal/oxide interface. We found that a chemical imbalance exists at the misfit dislocation which leads to dramatic changes in the point defect content at the interface – stabilizing the structure requires removing as much as 50% of the metal atoms and insertion of a large number of oxygen interstitials. The exact defect composition that stabilizes the interface is sensitive to the external oxygen partial pressure. We relate the preferred defect structure at the interface to a competition between chemical and strainmore » energies as defects are introduced.« less

Massive Interfacial Reconstruction at Misfit Dislocations in Metal/Oxide Interfaces

DOE PAGES

Choudhury, Samrat; Morgan, Dane; Uberuaga, Blas P.

2014-10-17

Electronic structure calculations were performed to study the role of misfit dislocations on the structure and chemistry of a metal/oxide interface. We found that a chemical imbalance exists at the misfit dislocation which leads to dramatic changes in the point defect content at the interface – stabilizing the structure requires removing as much as 50% of the metal atoms and insertion of a large number of oxygen interstitials. The exact defect composition that stabilizes the interface is sensitive to the external oxygen partial pressure. We relate the preferred defect structure at the interface to a competition between chemical and strainmore » energies as defects are introduced.« less

a Positron 2D-ACAR Study of the Silicon-Dioxide Interface and the Point Defects in the Semi-Insulating Gallium Arsenide

NASA Astrophysics Data System (ADS)

Peng, Jianping

The SiO_2-Si system has been the subject of extensive study for several decades. Particular interest has been paid to the interface between Si single crystal and the amorphous SiO_2 which determines the properties and performances of devices. This is significant because of the importance of Si technology in the semiconductor industry. The development of the high-intensity slow positron beam at Brookhaven National Laboratory make it possible to study this system for the first time using the positron two-dimensional angular correlation of annihilation radiation (2D-ACAR) technique. 2D-ACAR is a well established and is a non-destructive microscopic probe for studying the electronic structure of materials, and for doing the depth-resolved measurements. Some unique information was obtained from the measurements performed on the SiO_2-Si system: Positronium (Ps) atoms formation and trapping in microvoids in both oxide and interface regions; and positron annihilation at vacancy-like defects in the interface region which can be attributed to the famous Pb centers. The discovery of the microvoids in the interface region may have some impact on the fabrication of the next generation electronic devices. Using the conventional 2D-ACAR setup with a ^{22}Na as positron source, we also studied the native arsenic (As) vacancy in the semi -insulating gallium-arsenide (SI-GaAs), coupled with in situ infrared light illumination. The defect spectrum was obtained by comparing the spectrum taken without photo -illumination to the spectrum taken with photo-illumination. The photo-illumination excited electrons from valence band to the defect level so that positrons can become localized in the defects. The two experiments may represent a new direction of the application of positron 2D-ACAR technique on the solid state physics and materials sciences.

Defect assistant band alignment transition from staggered to broken gap in mixed As/Sb tunnel field effect transistor heterostructure

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhu, Y.; Jain, N.; Vijayaraghavan, S.

2012-11-01

The compositional dependence of effective tunneling barrier height (E{sub beff}) and defect assisted band alignment transition from staggered gap to broken gap in GaAsSb/InGaAs n-channel tunnel field effect transistor (TFET) structures were demonstrated by x-ray photoelectron spectroscopy (XPS). High-resolution x-ray diffraction measurements revealed that the active layers are internally lattice matched. The evolution of defect properties was evaluated using cross-sectional transmission electron microscopy. The defect density at the source/channel heterointerface was controlled by changing the interface properties during growth. By increasing indium (In) and antimony (Sb) alloy compositions from 65% to 70% in In{sub x}Ga{sub 1-x}As and 60% to 65%more » in GaAs{sub 1-y}Sb{sub y} layers, the E{sub beff} was reduced from 0.30 eV to 0.21 eV, respectively, with the low defect density at the source/channel heterointerface. The transfer characteristics of the fabricated TFET device with an E{sub beff} of 0.21 eV show 2 Multiplication-Sign improvement in ON-state current compared to the device with E{sub beff} of 0.30 eV. On contrary, the value of E{sub beff} was decreased from 0.21 eV to -0.03 eV due to the presence of high defect density at the GaAs{sub 0.35}Sb{sub 0.65}/In{sub 0.7}Ga{sub 0.3}As heterointerface. As a result, the band alignment was converted from staggered gap to broken gap, which leads to 4 orders of magnitude increase in OFF-state leakage current. Therefore, a high quality source/channel interface with a properly selected E{sub beff} and well maintained low defect density is necessary to obtain both high ON-state current and low OFF-state leakage in a mixed As/Sb TFET structure for high-performance and lower-power logic applications.« less

Directional charge transfer mediated by mid-gap states: A transient absorption spectroscopy study of CdSe quantum dot/β-Pb 0.33V 2O 5 heterostructures

DOE PAGES

Milleville, Christopher C.; Pelcher, Kate E.; Sfeir, Matthew Y.; ...

2016-02-15

For solar energy conversion, not only must a semiconductor absorb incident solar radiation efficiently but also its photoexcited electron—hole pairs must further be separated and transported across interfaces. Charge transfer across interfaces requires consideration of both thermodynamic driving forces as well as the competing kinetics of multiple possible transfer, cooling, and recombination pathways. In this work, we demonstrate a novel strategy for extracting holes from photoexcited CdSe quantum dots (QDs) based on interfacing with β-Pb 0.33V 2O 5 nanowires that have strategically positioned midgap states derived from the intercalating Pb 2+ ions. Unlike midgap states derived from defects or dopants,more » the states utilized here are derived from the intrinsic crystal structure and are thus homogeneously distributed across the material. CdSe/β-Pb 0.33V 2O 5 heterostructures were assembled using two distinct methods: successive ionic layer adsorption and reaction (SILAR) and linker-assisted assembly (LAA). Transient absorption spectroscopy measurements indicate that, for both types of heterostructures, photoexcitation of CdSe QDs was followed by the transfer of electrons to the conduction band of β-Pb 0.33V 2O 5 nanowires and holes to the midgap states of β-Pb 0.33V 2O 5 nanowires. Holes were transferred on time scales less than 1 ps, whereas electrons were transferred more slowly on time scales of ~2 ps. In contrast, for analogous heterostructures consisting of CdSe QDs interfaced with V 2O 5 nanowires (wherein midgap states are absent), only electron transfer was observed. Interestingly, electron transfer was readily achieved for CdSe QDs interfaced with V 2O 5 nanowires by the SILAR method; however, for interfaces incorporating molecular linkers, electron transfer was observed only upon excitation at energies substantially greater than the bandgap absorption threshold of CdSe. Furthermore, transient absorbance decay traces reveal longer excited-state lifetimes (1–3 μs) for CdSe/β-Pb 0.33V 2O 5 heterostructures relative to bare β-Pb 0.33V 2O 5 nanowires (0.2 to 0.6 μs); the difference is attributed to surface passivation of intrinsic surface defects in β-Pb 0.33V 2O 5 upon interfacing with CdSe.« less

First-principles thermodynamics and defect kinetics guidelines for engineering a tailored RRAM device

DOE Office of Scientific and Technical Information (OSTI.GOV)

Clima, Sergiu, E-mail: clima@imec.be; Chen, Yang Yin; Goux, Ludovic

Resistive Random Access Memories are among the most promising candidates for the next generation of non-volatile memory. Transition metal oxides such as HfOx and TaOx attracted a lot of attention due to their CMOS compatibility. Furthermore, these materials do not require the inclusion of extrinsic conducting defects since their operation is based on intrinsic ones (oxygen vacancies). Using Density Functional Theory, we evaluated the thermodynamics of the defects formation and the kinetics of diffusion of the conducting species active in transition metal oxide RRAM materials. The gained insights based on the thermodynamics in the Top Electrode, Insulating Matrix and Bottommore » Electrode and at the interfaces are used to design a proper defect reservoir, which is needed for a low-energy reliable switching device. The defect reservoir has also a direct impact on the retention of the Low Resistance State due to the resulting thermodynamic driving forces. The kinetics of the diffusing conducting defects in the Insulating Matrix determine the switching dynamics and resistance retention. The interface at the Bottom Electrode has a significant impact on the low-current operation and long endurance of the memory cell. Our first-principles findings are confirmed by experimental measurements on fabricated RRAM devices.« less

Defect states at organic-inorganic interfaces: Insight from first principles calculations for pentaerythritol tetranitrate on MgO surface

NASA Astrophysics Data System (ADS)

Tsyshevsky, Roman V.; Rashkeev, Sergey N.; Kuklja, Maija M.

2015-07-01

Light-responsive organic-inorganic interfaces offer experimental opportunities that are otherwise difficult to achieve. Since laser light can be manipulated very precisely, it becomes possible to engineer selective, predictive, and highly controlled interface properties. Photochemistry of organic-inorganic energetic interfaces is a rapidly emerging research field in which energy absorption and interface stability mechanisms have yet to be established. To explore the interaction of the laser irradiation with molecular materials, we performed first principle calculations of a prototype organic-inorganic interface between a nitroester (pentaerythritol tetranitrate, PETN, C5H8N4O12) and a magnesium oxide (MgO) surface. We found that the light absorption is defined by the band alignment between interface components and interfacial charge transfer coupled with electronic states in the band gap, generated by oxide surface defects. Hence the choice of an oxide substrate and its morphology makes the optical absorption tunable and governs both the energy accumulation and energy release at the interface. The obtained results offer a possible consistent interpretation of experiments on selective laser initiation of energetic materials, which reported that the presence of metal oxide additives triggered the photoinitiation by excitation energy much lower than the band gap. We suggest that PETN photodecomposition is catalyzed by oxygen vacancies (F0 centers) at the MgO surface. Our conclusions predict ways for a complete separation of thermo- and photo-stimulated interface chemistry of molecular materials, which is imperative for highly controllable fast decomposition and was not attainable before. The methodology described here can be applied to any type of molecular material/wide band gap dielectric interfaces. It provides a solid basis for novel design and targeted improvements of organic-inorganic interfaces with desired properties that promise to enable vastly new concepts of energy storage and conversion, photocatalysis, and molecular electronics.

Growth experiment of narrow band-gap semiconductor PbSnTe single crystals in space (M-1)

NASA Technical Reports Server (NTRS)

Yamada, Tomoaki

1993-01-01

An experiment on crystal growth of Pb(1-x)Sn(x)Te in microgravity is planned. This material is an alloy of the compound semiconductors PbTe and SnTe. It is a promising material for infrared diode lasers and detectors in the wavelength region between 6 and 30 micron. Since the electrical properties of Pb(1-x)Sn(x)Te depend greatly on the Pb/Sn ratio and crystalline defects as well as impurity concentration, homogeneous, defect-free, high-quality crystals are anticipated. Although many growth methods, such as the pulling method, the Bridgman method, the vapor growth method, etc., have been applied to the growth of Pb(1-x)Sn(x)Te, large, homogeneous, low-defect-density crystals have not yet been grown on Earth. The unsuccessful results were caused by buoyancy-driven convection in the fluids induced by the specific gravity difference between heated and cooled fluids on Earth. A crystal is grown by cooling the melt from one end of the ampoule. In crystal growth from the melt, about 30 percent of the SnTe in the melt is rejected at the solid-liquid interface during solidification. On Earth, the rejected SnTe is completely mixed with the remaining melt by convection in the melt. Therefore, SnTe concentration in the melt, and accordingly in the crystal, increases as the crystal grows. In the microgravity environment, buoyancy-driven convection is suppressed because the specific gravity difference is negligible. In that case, the rejected SnTe remains at the solid-liquid interface and its concentration increases only at the interface. If the growth rate is higher than the PbTe-SnTe interdiffusion rate, the amount of SnTe which diffuses from the interface into the melt increases as SnTe piles up at the interface, and finally it balances the amount of rejected SnTe during solidification, resulting in steady-state SnTe transportation at the interface. By using this principle, compositionally homogeneous crystals can be grown. Furthermore, low-defect-density crystals will be grown in microgravity, because convection causes crystalline defects by mising hot and cold fluids and generating temperature fluctuations in them.

Non-random walk diffusion enhances the sink strength of semicoherent interfaces

DOE PAGES

Vattré, A.; Jourdan, T.; Ding, H.; ...

2016-01-29

Clean, safe and economical nuclear energy requires new materials capable of withstanding severe radiation damage. One strategy of imparting radiation resistance to solids is to incorporate into them a high density of solid-phase interfaces capable of absorbing and annihilating radiation-induced defects. Here we show that elastic interactions between point defects and semicoherent interfaces lead to a marked enhancement in interface sink strength. Our conclusions stem from simulations that integrate first principles, object kinetic Monte Carlo and anisotropic elasticity calculations. Surprisingly, the enhancement in sink strength is not due primarily to increased thermodynamic driving forces, but rather to reduced defect migrationmore » barriers, which induce a preferential drift of defects towards interfaces. The sink strength enhancement is highly sensitive to the detailed character of interfacial stresses, suggesting that ‘super-sink’ interfaces may be designed by optimizing interface stress fields. Lastly, such interfaces may be used to create materials with unprecedented resistance to radiation-induced damage.« less

Temperature dependent recombination dynamics in InP/ZnS colloidal nanocrystals

NASA Astrophysics Data System (ADS)

Shirazi, R.; Kopylov, O.; Kovacs, A.; Kardynał, B. E.

2012-08-01

In this letter, we investigate exciton recombination in InP/ZnS core-shell colloidal nanocrystals over a wide temperature range. Over the entire range between room temperature and liquid helium temperature, multi-exponential exciton decay curves are observed and well explained by the presence of bright and dark exciton states, as well as defect states. Two different types of defect are present: one located at the core-shell interface and the other on the surface of the nanocrystal. Based on the temperature dependent contributions of all four states to the total photoluminescence signal, we estimate that the four states are distributed within a 20 meV energy band in nanocrystals that emit at 1.82 eV.

Vacancy-type defects in Al2O3/GaN structure probed by monoenergetic positron beams

NASA Astrophysics Data System (ADS)

Uedono, Akira; Nabatame, Toshihide; Egger, Werner; Koschine, Tönjes; Hugenschmidt, Christoph; Dickmann, Marcel; Sumiya, Masatomo; Ishibashi, Shoji

2018-04-01

Defects in the Al2O3(25 nm)/GaN structure were probed by using monoenergetic positron beams. Al2O3 films were deposited on GaN by atomic layer deposition at 300 °C. Temperature treatment above 800 °C leads to the introduction of vacancy-type defects in GaN due to outdiffusion of atoms from GaN into Al2O3. The width of the damaged region was determined to be 40-50 nm from the Al2O3/GaN interface, and some of the vacancies were identified to act as electron trapping centers. In the Al2O3 film before and after annealing treatment at 300-900 °C, open spaces with three different sizes were found to coexist. The density of medium-sized open spaces started to decrease above 800 °C, which was associated with the interaction between GaN and Al2O3. Effects of the electron trapping/detrapping processes of interface states on the flat band voltage and the defects in GaN were also discussed.

Transport Physics Mechanisms in Thin-Film Oxides.

NASA Astrophysics Data System (ADS)

Tierney, Brian D.; Hjalmarson, Harold P.; Jacobs-Gedrim, Robin B.; James, Conrad D.; Marinella, Matthew M.

A physics-based model of electron transport mechanisms in metal-insulating oxide-metal (M-I-M) systems is presented focusing on transport through the metal-oxide interfaces and in the bulk of the oxide. Interface tunneling, such as electron tunneling between the metal and the conduction band, or to oxide defect states, is accounted for via a WKB model. The effects of thermionic emission are also included. In the bulk of the oxide, defect-site hopping is dominant. Corresponding continuum calculations are performed for Ta2O5 M-I-M systems utilizing two different metal electrodes, e.g., platinum and tantalum. Such an asymmetrical M-I-M structure, applicable to resistive memory applications or oxide-based capacitors, reveals that the current can be either bulk or interface limited depending on the bias polarity and concentration of oxygen vacancy defects. Also, the dominance of some transport mechanisms over others is shown to be due to a complex interdependence between the vacancy concentration and bias polarity. Sandia National Laboratories is a multi-mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Designing an in-situ ultrasonic nondestructive evaluation system for ultrasonic additive manufacturing

NASA Astrophysics Data System (ADS)

Nadimpalli, Venkata K.; Nagy, Peter B.

2018-04-01

Ultrasonic Additive Manufacturing (UAM) is a solid-state layer by layer manufacturing process that utilizes vibration induced plastic deformation to form a metallurgical bond between a thin layer and an existing base structure. Due to the vibration based bonding mechanism, the quality of components at each layer depends on the geometry of the structure. In-situ monitoring during and between UAM manufacturing steps offers the potential for closed-loop control to optimize process parameters and to repair existing defects. One interface that is most prone to delamination is the base/build interface and often UAM component height and quality are limited by failure at the base/build interface. Low manufacturing temperatures and favorable orientation of typical interface defects in UAM make ultrasonic NDE an attractive candidate for online monitoring. Two approaches for in-situ NDE are discussed and the design of the monitoring system optimized so that the quality of UAM components is not affected by the addition of the NDE setup. Preliminary results from in-situ ultrasonic NDE indicate the potential to be utilized for online qualification, closed-loop control and offline certification of UAM components.

Non destructive examination of interface of molecular assembly

NASA Astrophysics Data System (ADS)

Perez, Guy; Richard, Isaline; Lecomte, Jean-Claude

2017-11-01

Molecular assembly interfaces can be characterised by mechanical testing and/or the interaction between waves and the interface. The disadvantage of the mechanical approach is that new defects may be produced at the interface, or existing defects may be destroyed. Using the interaction between waves and the interface is a non-destructive approach. But what kind of waves should be used? Electromagnetic waves in the visible range depend on wave attenuation in the material, infrared waves also depend on the thickness and X-ray waves have a too short a wave length to detect interface defects. In this article, the use of acoustic waves is proposed for non-destructive examination of molecular assembly interfaces. Acoustic wave propagation is very sensitive to variations in interface characteristics depending on whether the waves are reflected or transmitted. To improve the sensitivity and resolution of this technique, small wave lengths have been used with a scanning acoustic microscope (S.A.M.) with a band width from 1MHz to 400 MHz. After a short description of the principle of the method, results are given for different types of components. Different applications of acoustic microscopy are proposed for non-destructive examination of interfaces and defect detection in materials.

Thermodynamics of surface defects at the aspirin/water interface

NASA Astrophysics Data System (ADS)

Schneider, Julian; Zheng, Chen; Reuter, Karsten

2014-09-01

We present a simulation scheme to calculate defect formation free energies at a molecular crystal/water interface based on force-field molecular dynamics simulations. To this end, we adopt and modify existing approaches to calculate binding free energies of biological ligand/receptor complexes to be applicable to common surface defects, such as step edges and kink sites. We obtain statistically accurate and reliable free energy values for the aspirin/water interface, which can be applied to estimate the distribution of defects using well-established thermodynamic relations. As a show case we calculate the free energy upon dissolving molecules from kink sites at the interface. This free energy can be related to the solubility concentration and we obtain solubility values in excellent agreement with experimental results.

The Interface Influence in TiN/SiN x Multilayer Nanocomposite Under Irradiation

NASA Astrophysics Data System (ADS)

Uglov, V. V.; Safronov, I. V.; Kvasov, N. T.; Remnev, G. E.; Shimanski, V. I.

2018-01-01

The paper focuses on studying the kinetics of radiation-induced point defects formed in TiN/SiN x multilayer nanocomposites with account of their generation, diffusion recombination, and the influence of sinks functioning as interfaces. In order to describe the kinetics in nanocrystalline TiN and amorphous SiN x phases, a finite-difference method is used to solve the system of balance kinetic equations for absolute defect concentrations depending on the spatiotemporal variables. A model of the disclination-dislocation interface structure is used to study the absorption of radiation-induced point defects on the boundaries in created stress fields. It is shown that the interface effectively absorbs point defects in these phases of TiN/SiN x multilayer nanocomposite, thereby reducing their amount within the space between phases. This behavior of point defects partially explains a mechanism of the radiation resistance in this type of nanocomposites.

Recombination in Perovskite Solar Cells: Significance of Grain Boundaries, Interface Traps, and Defect Ions.

PubMed

Sherkar, Tejas S; Momblona, Cristina; Gil-Escrig, Lidón; Ávila, Jorge; Sessolo, Michele; Bolink, Henk J; Koster, L Jan Anton

2017-05-12

Trap-assisted recombination, despite being lower as compared with traditional inorganic solar cells, is still the dominant recombination mechanism in perovskite solar cells (PSCs) and limits their efficiency. We investigate the attributes of the primary trap-assisted recombination channels (grain boundaries and interfaces) and their correlation to defect ions in PSCs. We achieve this by using a validated device model to fit the simulations to the experimental data of efficient vacuum-deposited p-i-n and n-i-p CH 3 NH 3 PbI 3 solar cells, including the light intensity dependence of the open-circuit voltage and fill factor. We find that, despite the presence of traps at interfaces and grain boundaries (GBs), their neutral (when filled with photogenerated charges) disposition along with the long-lived nature of holes leads to the high performance of PSCs. The sign of the traps (when filled) is of little importance in efficient solar cells with compact morphologies (fused GBs, low trap density). On the other hand, solar cells with noncompact morphologies (open GBs, high trap density) are sensitive to the sign of the traps and hence to the cell preparation methods. Even in the presence of traps at GBs, trap-assisted recombination at interfaces (between the transport layers and the perovskite) is the dominant loss mechanism. We find a direct correlation between the density of traps, the density of mobile ionic defects, and the degree of hysteresis observed in the current-voltage ( J - V ) characteristics. The presence of defect states or mobile ions not only limits the device performance but also plays a role in the J - V hysteresis.

Inspecting the microstructure of electrically active defects at the Ge/GeOx interface

NASA Astrophysics Data System (ADS)

Fanciulli, Marco; Baldovino, Silvia; Molle, Alessandro

2012-02-01

High mobility substrates are important key elements in the development of advanced devices targeting a vast range of functionalities. Among them, Ge showed promising properties promoting it as valid candidate to replace Si in CMOS technology. However, the electrical quality of the Ge/oxide interface is still a problematic issue, in particular for the observed inversion of the n-type Ge surface, attributed to the presence of dangling bonds inducing a severe band bending [1]. In this scenario, the identification of electrically active defects present at the Ge/oxide interface and the capability to passivate or anneal them becomes a mandatory issue aiming at an electrically optimized interface. We report on the application of highly sensitive electrically detected magnetic resonance (EDMR) techniques in the investigation of defects at the interface between Ge and GeO2 (or GeOx), including Ge dangling bonds and defects in the oxide [2]. In particular we will investigate how different surface orientations, e.g. the (001) against the (111) Ge surface, impacts the microstructure of the interface defects. [1] P. Tsipas and A. Dimoulas, Appl. Phys. Lett. 94, 012114 (2009) [2] S. Baldovino, A. Molle, and M. Fanciulli, Appl. Phys. Lett. 96, 222110 (2010)

Positron annihilation studies of silicon-based materials

NASA Astrophysics Data System (ADS)

Petkov, Mihail Petkov

Positron Annihilation Spectroscopy (PAS) is used as a defect-profiling tool in the characterization of Si-based materials. PAS, in conjunction with variable energy positron beams, is a non-destructive depth-profiling probe, ideally suited for studying thin films, multi-layered structures, and buried interfaces. Its sensitivity to open-volume defects covers a wide range of defect sizes and concentrations, and surpasses that of most other techniques. This dissertation presents PAS investigations of electrical, chemical and mechanical properties of a number of advanced materials for future use by the semiconductor industry. Among the subjects of this work are: hydrogenated amorphous silicon (a-Si:H) for use in solar cells and flat-panel displays; low dielectric constant materials (low-k) for interlayer dielectrics; and thin-gate transistors, focusing on the defects at the Si/SiO 2 interface, which limit the device reliability. Results from extensive research on various possibilities to enhance the PAS capability by increasing its efficiency are presented in the appendices. The recognition of different dangling bond defects for low defect densities is achieved in these first PAS studies of void-free a-Si:H. Direct evidence of the existence of dopant-defect complexes is obtained for the first time. This research lays the foundation for future studies of the role of the impurities in light- and thermal degradation of a-Si:H PAS was applied to the characterization of porous low-k dielectrics. The annihilation observables are correlated with the dielectric properties of the material and their preparation conditions. PAS is the only non-destructive local k-probe, and the only tool for measuring void densities and sizes. The method is also sensitive to the chemical environment of the voids, seen during oxidation, water absorption, and forming gas anneal. Industrial research, partially based on these results, is currently in progress at IBM. A decade-old controversy, involving different models of defect states at Si/SiO2 interfaces, has been resolved. The two-defect model was confirmed and previous results were reevaluated. Research in this area will promote the use of PAS as an on-line diagnostic tool in the manufacturing of integrated circuits.

Linear complexions: Confined chemical and structural states at dislocations

NASA Astrophysics Data System (ADS)

Kuzmina, M.; Herbig, M.; Ponge, D.; Sandlöbes, S.; Raabe, D.

2015-09-01

For 5000 years, metals have been mankind’s most essential materials owing to their ductility and strength. Linear defects called dislocations carry atomic shear steps, enabling their formability. We report chemical and structural states confined at dislocations. In a body-centered cubic Fe-9 atomic percent Mn alloy, we found Mn segregation at dislocation cores during heating, followed by formation of face-centered cubic regions but no further growth. The regions are in equilibrium with the matrix and remain confined to the dislocation cores with coherent interfaces. The phenomenon resembles interface-stabilized structural states called complexions. A cubic meter of strained alloy contains up to a light year of dislocation length, suggesting that linear complexions could provide opportunities to nanostructure alloys via segregation and confined structural states.

Spins and photons: connecting quantum registers in diamond

NASA Astrophysics Data System (ADS)

Childress, Lily

2012-06-01

Long-lived electronic and nuclear spin states have made the nitrogen-vacancy (NV) defect in diamond a leading candidate for quantum information processing in the solid state. Multi-qubit quantum registers formed by single defects and nearby nuclear spins can currently be controlled and detected with high fidelity. Nevertheless, development of coherent connections between distant NVs remains an outstanding challenge. One advantage to working with solid-state defects is the opportunity to integrate them with microfabricated mechanical, electronic, or optical devices; in principle, such devices could mediate interactions between registers, turning them into nodes within a larger quantum network. In the last few months, several experiments have made key steps toward realizing a coherent quantum interface between individual NV centers using a mechanical quantum bus [1] or optical channels [2,3]. This talk will explore the current state of the art, and report on recent observation of two photon quantum interference between different gate-tunable defect centers [2]. These results pave the way towards measurement-based entanglement between remote NV centers and the realization of quantum networks with solid-state spins.[4pt] [1] Kolkowitz et al., Science 335, 1603 (2012)[2] Bernien et al., Phys. Rev. Lett. 108, 043604 (2012)[3] Sipahigil et al., http://lanl.arxiv.org/abs/1112.3975

The frictional response of patterned soft polymer surfaces

NASA Astrophysics Data System (ADS)

Rand, Charles J.

2008-10-01

Friction plays an intricate role in our everyday lives, it is therefore critical to understand the underlying features of friction to better help control and manipulate the response anywhere two surfaces in contact move past each other by a sliding motion. Here we present results targeting a thorough understanding of soft material friction and how it can be manipulated with patterns. We found that the naturally occurring length scale or periodicity (lambda) of frictionally induced patterns, Schallamach waves, could be described using two materials properties (critical energy release rate Gc and complex modulus (E*), i.e. lambdainfinity Gc /E*). Following this, we evaluated the effect of a single defect at a sliding interface. Sliding over a defect can be used to model the sliding from one feature to another in a patterned surface. Defects decreased the sliding frictional force by as much as 80% sliding and this decrease was attributed to changes in tangential stiffness of the sliding interface. The frictional response of surface wrinkles, where multiple edges or defects are acting in concert, was also evaluated. Wrinkles were shown to decrease friction (F) and changes in contact area (A) could not describe this decrease. A tangential stiffness correction factor (fx) and changes in the critical energy release rate were used to describe this deviation (F infinity Gc *A*fx/ℓ, where ℓ is a materials defined length scale of dissipation). This scaling can be used to describe the friction of any topographically patterned surface including the Gecko's foot, where the feature size is smaller than ℓ and thus replaces ℓ, increasing the friction compared to a flat surface. Also, mechanically-induced surface defects were used to align osmotically driven surface wrinkles by creating stress discontinuities that convert the global biaxial stress state to local uniaxial stresses. Defect spacing was used to control the alignment process at the surface of the wrinkled rigid film/soft elastomer interface. These aligned wrinkled surfaces can be used to tune the adhesion and friction of an interface. The work presented here gives insight into tuning the friction of a soft polymeric surface as well as understanding the friction of complex hierarchical structures.

Point defect stability in a semicoherent metallic interface

NASA Astrophysics Data System (ADS)

González, C.; Iglesias, R.; Demkowicz, M. J.

2015-02-01

We present a comprehensive density functional theory (DFT) -based study of different aspects of one vacancy and He impurity atom behavior at semicoherent interfaces between the low-solubility transition metals Cu and Nb. Such interfaces have not been previously modeled using DFT. A thorough analysis of the stability and mobility of the two types of defects at the interfaces and neighboring internal layers has been performed and the results have been compared to the equivalent cases in the pure metallic matrices. The different behavior of fcc and bcc metals on both sides of the interface has been specifically assessed. The modeling effort undertaken is the first attempt to study the stability and defect energetics of noncoherent Cu/Nb interfaces from first principles, in order to assess their potential use in radiation-resistant materials.

Structural manipulation of the graphene/metal interface with Ar+ irradiation

NASA Astrophysics Data System (ADS)

Åhlgren, E. H.; Hämäläinen, S. K.; Lehtinen, O.; Liljeroth, P.; Kotakoski, J.

2013-10-01

Controlled defect creation is a prerequisite for the detailed study of disorder effects in materials. Here, we irradiate a graphene/Ir(111) interface with low-energy Ar+ to study the induced structural changes. Combining computer simulations and scanning-probe microscopy, we show that the resulting disorder manifests mainly in the forms of intercalated metal adatoms and vacancy-type defects in graphene. One prominent feature at higher irradiation energies (from 1 keV up) is the formation of linelike depressions, which consist of sequential graphene defects created by the ion channeling within the interface, much like a stone skipping on water. Lower energies result in simpler defects, down to 100 eV, where more than one defect in every three is a graphene single vacancy.

Study of Silicon/silicon, Silicon/silicon Dioxide, and Metal-Oxide

NASA Astrophysics Data System (ADS)

Leung, To Chi

A variable-energy positron beam is used to study Si/Si, Si/SiO_2, and metal-oxide -semiconductor (MOS) structures. The capability of depth resolution and the remarkable sensitivity to defects have made the positron annihilation technique a unique tool in detecting open-volume defects in the newly innovated low temperature (300^circC) molecular-beam-epitaxy (MBE) Si/Si. These two features of the positron beam have further shown its potential role in the study of the Si/SiO_2. Distinct annihilation characteristics has been observed at the interface and has been studied as a function of the sample growth conditions, annealing (in vacuum), and hydrogen exposure. The MOS structure provides an effective way to study the electrical properties of the Si/SiO_2 interface as a function of applied bias voltage. The annihilation characteristics show a large change as the device condition is changed from accumulation to inversion. The effect of forming gas (FG) anneal is studied using positron annihilation and the result is compared with capacitance-voltage (C -V) measurements. The reduction in the number of interface states is found correlated with the changes in the positron spectra. The present study shows the importance of the positron annihilation technique as a non-contact, non-destructive, and depth-sensitive characterization tool to study the Si-related systems, in particular, the Si/SiO_2 interface which is of crucial importance in semiconductor technology, and fundamental understanding of the defects responsible for degradation of the electrical properties.

Role of the dielectric for the charging dynamics of the dielectric/barrier interface in AlGaN/GaN based metal-insulator-semiconductor structures under forward gate bias stress

NASA Astrophysics Data System (ADS)

Lagger, P.; Steinschifter, P.; Reiner, M.; Stadtmüller, M.; Denifl, G.; Naumann, A.; Müller, J.; Wilde, L.; Sundqvist, J.; Pogany, D.; Ostermaier, C.

2014-07-01

The high density of defect states at the dielectric/III-N interface in GaN based metal-insulator-semiconductor structures causes tremendous threshold voltage drifts, ΔVth, under forward gate bias conditions. A comprehensive study on different dielectric materials, as well as varying dielectric thickness tD and barrier thickness tB, is performed using capacitance-voltage analysis. It is revealed that the density of trapped electrons, ΔNit, scales with the dielectric capacitance under spill-over conditions, i.e., the accumulation of a second electron channel at the dielectric/AlGaN barrier interface. Hence, the density of trapped electrons is defined by the charging of the dielectric capacitance. The scaling behavior of ΔNit is explained universally by the density of accumulated electrons at the dielectric/III-N interface under spill-over conditions. We conclude that the overall density of interface defects is higher than what can be electrically measured, due to limits set by dielectric breakdown. These findings have a significant impact on the correct interpretation of threshold voltage drift data and are of relevance for the development of normally off and normally on III-N/GaN high electron mobility transistors with gate insulation.

Defect interactions with stepped CeO₂/SrTiO₃ interfaces: implications for radiation damage evolution and fast ion conduction.

PubMed

Dholabhai, Pratik P; Aguiar, Jeffery A; Misra, Amit; Uberuaga, Blas P

2014-05-21

Due to reduced dimensions and increased interfacial content, nanocomposite oxides offer improved functionalities in a wide variety of advanced technological applications, including their potential use as radiation tolerant materials. To better understand the role of interface structures in influencing the radiation damage tolerance of oxides, we have conducted atomistic calculations to elucidate the behavior of radiation-induced point defects (vacancies and interstitials) at interface steps in a model CeO2/SrTiO3 system. We find that atomic-scale steps at the interface have substantial influence on the defect behavior, which ultimately dictate the material performance in hostile irradiation environments. Distinctive steps react dissimilarly to cation and anion defects, effectively becoming biased sinks for different types of defects. Steps also attract cation interstitials, leaving behind an excess of immobile vacancies. Further, defects introduce significant structural and chemical distortions primarily at the steps. These two factors are plausible origins for the enhanced amorphization at steps seen in our recent experiments. The present work indicates that comprehensive examination of the interaction of radiation-induced point defects with the atomic-scale topology and defect structure of heterointerfaces is essential to evaluate the radiation tolerance of nanocomposites. Finally, our results have implications for other applications, such as fast ion conduction.

Quantum Control and Entanglement of Spins in Silicon Carbide

NASA Astrophysics Data System (ADS)

Klimov, Paul

Over the past several decades silicon carbide (SiC) has matured into a versatile material platform for high-power electronics and optoelectronic and micromechanical devices. Recent advances have also established SiC as a promising host for quantum technologies based on the spin of intrinsic defects, with the potential of leveraging existing device fabrication protocols alongside solid-state quantum control. Among these defects are the divacancies and related color centers, which have ground-state electron-spin triplets with coherence times as long as one millisecond and built-in optical interfaces operating near the telecommunication wavelengths. This rapidly developing field has prompted research into the SiC material host to understand how defect-bound electron spins interact with their surrounding nuclear spin bath. Although nuclear spins are a major source of decoherence in color-center spin systems, they are also a valuable resource since they can have coherence times that are orders of magnitude longer than electron spins. In this talk I will discuss our recent efforts to interface defect-bound electron spins in SiC with the nuclear spins of naturally occurring 29Si and 13C isotopic defects. I will discuss how the hyperfine interaction can be used to strongly initialize them, to coherently control them, to read them out, and to produce genuine electron-nuclear ensemble entanglement, all at ambient conditions. These demonstrations motivate further research into spins in SiC for prospective quantum technologies. In collaboration with A. Falk, D. Christle, K. Miao, H. Seo, V. Ivady, A. Gali, G. Galli, and D. D. Awschalom. This research was supported by the AFOSR, the NSF DMR-1306300, and the NSF Materials Research Science and Engineering Center.

Ultrafast photocarrier dynamics related to defect states of Si1-xGex nanowires measured by optical pump-THz probe spectroscopy.

PubMed

Bae, Jung Min; Lee, Woo-Jung; Jung, Seonghoon; Ma, Jin Won; Jeong, Kwang-Sik; Oh, Seung Hoon; Kim, Seongsin M; Suh, Dongchan; Song, Woobin; Kim, Sunjung; Park, Jaehun; Cho, Mann-Ho

2017-06-14

Slightly tapered Si 1-x Ge x nanowires (NWs) (x = 0.29-0.84) were synthesized via a vapor-liquid-solid procedure using Au as a catalyst. We measured the optically excited carrier dynamics of Si 1-x Ge x NWs as a function of Ge content using optical pump-THz probe spectroscopy. The measured -ΔT/T 0 signals of Si 1-x Ge x NWs were converted into conductivity in the THz region. We developed a fitting formula to apply to indirect semiconductors such as Si 1-x Ge x , which explains the temporal population of photo-excited carriers in the band structure and the relationship between the trapping time and the defect states on an ultrafast time scale. From the fitting results, we extracted intra- and inter-valley transition times and trapping times of electrons and holes of Si 1-x Ge x NWs as a function of Ge content. On the basis of theoretical reports, we suggest a physical model to interpret the trapping times related to the species of interface defect states located at the oxide/NW: substoichiometric oxide states of Si(Ge) 0+,1+,2+ , but not Si(Ge) 3+ , could function as defect states capturing photo-excited electrons or holes and could determine the different trapping times of electrons and holes depending on negatively or neutrally charged states.

Identifying Read/Write Speeds for Field-Induced Interfacial Resistive Switching.

NASA Astrophysics Data System (ADS)

Tsui, Stephen; Das, Nilanjan; Wang, Yaqi; Xue, Yuyi; Chu, C. W.

2007-03-01

Efforts continue to explore new phenomena that may allow for next generation nonvolatile memory technology. Much attention has been drawn to the field-induced resistive switch occurring at the interface between a metal electrode and perovskite oxide. The switch between high (off) and low (on) resistance states is controlled by the polarity of applied voltage pulsing. Characterization of Ag-Pr0.7Ca0.3MnO3 interfaces via impedance spectroscopy shows that the resistances above 10^6 Hz are the same at the on and off states, which limits the reading speed to far slower than the applied switching pulses, or device write speed at the order of 10^7 Hz. We deduce that the switching interface is percolative in nature and that small local rearrangement of defect structures may play a major role.

Carrier collection losses in interface passivated amorphous silicon thin-film solar cells

DOE Office of Scientific and Technical Information (OSTI.GOV)

Neumüller, A., E-mail: alex.neumueller@next-energy.de; Sergeev, O.; Vehse, M.

In silicon thin-film solar cells the interface between the i- and p-layer is the most critical. In the case of back diffusion of photogenerated minority carriers to the i/p-interface, recombination occurs mainly on the defect states at the interface. To suppress this effect and to reduce recombination losses, hydrogen plasma treatment (HPT) is usually applied. As an alternative to using state of the art HPT we apply an argon plasma treatment (APT) before the p-layer deposition in n-i-p solar cells. To study the effect of APT, several investigations were applied to compare the results with HPT and no plasma treatmentmore » at the interface. Carrier collection losses in resulting solar cells were examined with spectral response measurements with and without bias voltage. To investigate single layers, surface photovoltage and X-ray photoelectron spectroscopy (XPS) measurements were conducted. The results with APT at the i/p-interface show a beneficial contribution to the carrier collection compared with HPT and no plasma treatment. Therefore, it can be concluded that APT reduces the recombination centers at the interface. Further, we demonstrate that carrier collection losses of thin-film solar cells are significantly lower with APT.« less

Tuning Interfacial Thermal Conductance of Graphene Embedded in Soft Materials by Vacancy Defects

DOE PAGES

Liu, Ying; Hu, Chongze; Huang, Jingsong; ...

2015-06-23

Nanocomposites based on graphene dispersed in matrices of soft materials are promising thermal management materials. Their effective thermal conductivity depends on both the thermal conductivity of graphene and the conductance of the thermal transport across graphene-matrix interfaces. Here we report on molecular dynamics simulations of the thermal transport across the interfaces between defected graphene and soft materials in two different modes: in the across mode, heat enters graphene from one side of its basal plane and leaves through the other side; in the non-across mode, heat enters or leaves a graphene simultaneously from both sides of its basal plane. Wemore » show that, as the density of vacancy defects in graphene increases from 0 to 8%, the conductance of the interfacial thermal transport in the across mode increases from 160.4 16 to 207.8 11 MW/m2K, while that in the non-across mode increases from 7.2 0.1 to 17.8 0.6 MW/m2K. The molecular mechanisms for these variations of thermal conductance are clarified by using the phonon density of states and structural characteristics of defected graphenes. On the basis of these results and effective medium theory, we show that it is possible to enhance the effective thermal conductivity of thermal nanocomposites by tuning the density of vacancy defects in graphene despite the fact that graphene s thermal conductivity always decreases as vacancy defects are introduced.« less

Radiation Tolerant Interfaces: Influence of Local Stoichiometry at the Misfit Dislocation on Radiation Damage Resistance of Metal/Oxide Interfaces

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shutthanandan, Vaithiyalingam; Choudhury, Samrat; Manandhar, Sandeep

To understand how variations in interface properties such as misfit-dislocation density and local chemistry affect radiation-induced defect absorption and recombination, we have explored a model system of CrxV1-x alloy epitaxial films deposited on MgO single crystals. By controlling film composition, the lattice mismatch with MgO was adjusted so that the misfit-dislocation density varies at the interface. These interfaces were exposed to irradiation and in situ results show that the film with a semi-coherent interface (Cr) withstands irradiation while V film, which has similar semi-coherent interface like Cr, showed the largest damage. Theoretical calculations indicate that, unlike at metal/metal interfaces, themore » misfit dislocation density does not dominate radiation damage tolerance at metal/oxide interfaces. Rather, the stoichiometry, and the precise location of the misfit-dislocation density relative to the interface, drives defect behavior. Together, these results demonstrate the sensitivity of defect recombination to interfacial chemistry and provide new avenues for engineering radiation-tolerant nanomaterials.« less

Steady distribution structure of point defects near crystal-melt interface under pulling stop of CZ Si crystal

NASA Astrophysics Data System (ADS)

Abe, T.; Takahashi, T.; Shirai, K.

2017-02-01

In order to reveal a steady distribution structure of point defects of no growing Si on the solid-liquid interface, the crystals were grown at a high pulling rate, which Vs becomes predominant, and the pulling was suddenly stopped. After restoring the variations of the crystal by the pulling-stop, the crystals were then left in prolonged contact with the melt. Finally, the crystals were detached and rapidly cooled to freeze point defects and then a distribution of the point defects of the as-grown crystals was observed. As a result, a dislocation loop (DL) region, which is formed by the aggregation of interstitials (Is), was formed over the solid-liquid interface and was surrounded with a Vs-and-Is-free recombination region (Rc-region), although the entire crystals had been Vs rich in the beginning. It was also revealed that the crystal on the solid-liquid interface after the prolonged contact with the melt can partially have a Rc-region to be directly in contact with the melt, unlike a defect distribution of a solid-liquid interface that has been growing. This experimental result contradicts a hypothesis of Voronkov's diffusion model, which always assumes the equilibrium concentrations of Vs and Is as the boundary condition for distribution of point defects on the growth interface. The results were disscussed from a qualitative point of view of temperature distribution and thermal stress by the pulling-stop.

Hot-electron-induced hydrogen redistribution and defect generation in metal-oxide-semiconductor capacitors

NASA Astrophysics Data System (ADS)

Buchanan, D. A.; Marwick, A. D.; Dimaria, D. J.; Dori, L.

1994-09-01

Redistribution of hydrogen caused by hot-electron injection has been studied by hydrogen depth profiling with N-15 nuclear reaction analysis and electrical methods. Internal photoemission and Fowler-Nordheim injection were used for electron injection into large Al-gate and polysilicon-gate capacitors, respectively. A hydrogen-rich layer (about 10(exp 15) atoms/sq cm) observed at the Al/SiO2 interface was found to serve as the source of hydrogen during the hot-electron stress. A small fraction of the hydrogen released from this layer was found to be retrapped near the Si/SiO2 interface for large electron fluences in the Al-gate samples. Within the limit of detectability, about 10(exp 14)/sq cm, no hydrogen was measured using nuclear reaction analysis in the polysilicon-gate samples. The buildup of hydrogen at the Si/SiO2 interface exhibits a threshold at about 1 MV/cm, consistent with the threshold for electron heating in SiO2. In the 'wet' SiO2 films with purposely introduced excess hydrogen, the rate of hydrogen buildup at the Si/SiO2 interface is found to be significantly greater than that found in the 'dry' films. During electron injection, hydrogen redistribution was also confirmed via the deactivation of boron dopant in the silicon substrate. The generation rates of interface states, neutral electron traps, and anomalous positive charge are found to increase with increasing hydrogen buildup in the substrate and the initial hydrogen concentration in the film. It is concluded that the generation of defects is preceded by the hot-electron-induced release and transport of atomic hydrogen and it is the chemical reaction of this species within the metal-oxide-semiconductor structure that generates the electrically active defects.

Role of point defects and HfO2/TiN interface stoichiometry on effective work function modulation in ultra-scaled complementary metal-oxide-semiconductor devices

NASA Astrophysics Data System (ADS)

Pandey, R. K.; Sathiyanarayanan, Rajesh; Kwon, Unoh; Narayanan, Vijay; Murali, K. V. R. M.

2013-07-01

We investigate the physical properties of a portion of the gate stack of an ultra-scaled complementary metal-oxide-semiconductor (CMOS) device. The effects of point defects, such as oxygen vacancy, oxygen, and aluminum interstitials at the HfO2/TiN interface, on the effective work function of TiN are explored using density functional theory. We compute the diffusion barriers of such point defects in the bulk TiN and across the HfO2/TiN interface. Diffusion of these point defects across the HfO2/TiN interface occurs during the device integration process. This results in variation of the effective work function and hence in the threshold voltage variation in the devices. Further, we simulate the effects of varying the HfO2/TiN interface stoichiometry on the effective work function modulation in these extremely-scaled CMOS devices. Our results show that the interface rich in nitrogen gives higher effective work function, whereas the interface rich in titanium gives lower effective work function, compared to a stoichiometric HfO2/TiN interface. This theoretical prediction is confirmed by the experiment, demonstrating over 700 meV modulation in the effective work function.

Interface investigation of solution processed high- κ ZrO2/Si MOS structure by DLTS

NASA Astrophysics Data System (ADS)

Kumar, Arvind; Mondal, Sandip; Rao, Ksr Koteswara

The interfacial region is dominating due to the continuous downscaling and integration of high- k oxides in CMOS applications. The accurate characterization of high- k oxides/semiconductor interface has the significant importance towards its usage in memory and thin film devices. The interface traps at the high - k /semiconductor interface can be quantified by deep level transient spectroscopy (DLTS) with better accuracy in contrast to capacitance-voltage (CV) and conductance technique. We report the fabrication of high- k ZrO2 films on p-Si substrate by a simple and inexpensive sol-gel spin-coating technique. Further, the ZrO2/Si interface is characterized through DLTS. The flat-band voltage (VFB) and the density of slow interface states (oxide trapped charges) extracted from CV characteristics are 0.37 V and 2x10- 11 C/cm2, respectively. The activation energy, interface state density and capture cross-section quantified by DLTS are EV + 0.42 eV, 3.4x1011 eV- 1 cm- 2 and 5.8x10- 18 cm2, respectively. The high quality ZrO2 films own high dielectric constant 15 with low leakage current density might be an appropriate insulating layer in future electronic application. The low value of interface state density and capture cross-section are the indication of high quality interface and the defect present at the interface may not affect the device performance to a great extent. The DLTS study provides a broad understanding about the traps present at the interface of spin-coated ZrO2/Si.

Correlation of film morphology and defect content with the charge-carrier transport in thin-film transistors based on ZnO nanoparticles

DOE Office of Scientific and Technical Information (OSTI.GOV)

Polster, S.; Jank, M. P. M.; Frey, L.

2016-01-14

The correlation of defect content and film morphology with the charge-carrier transport in field-effect devices based on zinc oxide nanoparticles was investigated. Changes in the defect content and the morphology were realized by annealing and sintering of the nanoparticle thin films. Temperature-dependent electrical measurements reveal that the carrier transport is thermally activated for both the unsintered and sintered thin films. Reduced energetic barrier heights between the particles have been determined after sintering. Additionally, the energetic barrier heights between the particles can be reduced by increasing the drain-to-source voltage and the gate-to-source voltage. The changes in the barrier height are discussedmore » with respect to information obtained by scanning electron microscopy and photoluminescence measurements. It is found that a reduction of surface states and a lower roughness at the interface between the particle layer and the gate dielectric lead to lower barrier heights. Both surface termination and layer morphology at the interface affect the barrier height and thus are the main criteria for mobility improvement and device optimization.« less

In situ study of heavy ion irradiation response of immiscible Cu/Fe multilayers

DOE PAGES

Chen, Youxing; Li, Nan; Bufford, Daniel Charles; ...

2016-04-09

By providing active defect sinks that capture and annihilate radiation induced defect clusters immiscible metallic multilayers with incoherent interfaces can effectively reduce defect density in ion irradiated metals. Although it is anticipated that defect density within the layers should vary as a function of distance to the layer interface, there is, to date, little in situ TEM evidence to validate this hypothesis. In our study monolithic Cu films and Cu/Fe multilayers with individual layer thickness, h, of 100 and 5 nm were subjected to in situ Cu ion irradiation at room temperature to nominally 1 displacement-per-atom inside a transmission electronmore » microscope. Rapid formation and propagation of defect clusters were observed in monolithic Cu, whereas fewer defects with smaller dimensions were generated in Cu/Fe multilayers with smaller h. Moreover, in situ video shows that the cumulative defect density in Cu/Fe 100 nm multilayers indeed varies, as a function of distance to the layer interfaces, supporting a long postulated hypothesis.« less

Improving Defect-Based Quantum Emitters in Silicon Carbide via Inorganic Passivation.

PubMed

Polking, Mark J; Dibos, Alan M; de Leon, Nathalie P; Park, Hongkun

2018-01-01

Defect-based color centers in wide-bandgap crystalline solids are actively being explored for quantum information science, sensing, and imaging. Unfortunately, the luminescent properties of these emitters are frequently degraded by blinking and photobleaching that arise from poorly passivated host crystal surfaces. Here, a new method for stabilizing the photoluminescence and charge state of color centers based on epitaxial growth of an inorganic passivation layer is presented. Specifically, carbon antisite-vacancy pairs (CAV centers) in 4H-SiC, which serve as single-photon emitters at visible wavelengths, are used as a model system to demonstrate the power of this inorganic passivation scheme. Analysis of CAV centers with scanning confocal microscopy indicates a dramatic improvement in photostability and an enhancement in emission after growth of an epitaxial AlN passivation layer. Permanent, spatially selective control of the defect charge state can also be achieved by exploiting the mismatch in spontaneous polarization at the AlN/SiC interface. These results demonstrate that epitaxial inorganic passivation of defect-based quantum emitters provides a new method for enhancing photostability, emission, and charge state stability of these color centers. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

A new photoluminescence emission peak of ZnO SiO2 nanocomposites and its energy transfer to Eu3+ ions

NASA Astrophysics Data System (ADS)

Hong, Jian-He; Cong, Chang-Jie; Zhang, Zhi-Guo; Zhang, Ke-Li

2007-07-01

This work reports a new photoluminescence (PL) emission peak at about 402 nm from amorphous ZnO nanoparticles in a silica matrix, and the energy transfer from it to Eu3+ ions. The amorphous ZnO SiO2 nanocomposites were prepared by the sol gel method, which is verified by X-ray diffraction (XRD) profiles and FT IR spectra. The luminescence emission spectra are fitted by four Gauss profiles, two of which at longer wavelength are due to the defects of the material and the others to amorphous ZnO nanoparticles and the Zn O Si interface state. With the reduction of Zn/Si ratio and diethanolamine, the relative intensities of visible emission decrease. The weak visible emission is due to the reduction of defects after calcined at high temperature. The new energy state at the Zn O Si interface results in strong emission at about 402 nm. When Eu3+ ions are co-doped, weak energy transfer from ZnO SiO2 nanocomposites to Eu3+ emission are observed in the excitation spectra.

Linear complexions: Confined chemical and structural states at dislocations.

PubMed

Kuzmina, M; Herbig, M; Ponge, D; Sandlöbes, S; Raabe, D

2015-09-04

For 5000 years, metals have been mankind's most essential materials owing to their ductility and strength. Linear defects called dislocations carry atomic shear steps, enabling their formability. We report chemical and structural states confined at dislocations. In a body-centered cubic Fe-9 atomic percent Mn alloy, we found Mn segregation at dislocation cores during heating, followed by formation of face-centered cubic regions but no further growth. The regions are in equilibrium with the matrix and remain confined to the dislocation cores with coherent interfaces. The phenomenon resembles interface-stabilized structural states called complexions. A cubic meter of strained alloy contains up to a light year of dislocation length, suggesting that linear complexions could provide opportunities to nanostructure alloys via segregation and confined structural states. Copyright © 2015, American Association for the Advancement of Science.

Radiation Tolerant Interfaces: Influence of Local Stoichiometry at the Misfit Dislocation on Radiation Damage Resistance of Metal/Oxide Interfaces

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shutthanandan, Vaithiyalingam; Choudhury, Samrat; Manandhar, Sandeep

The interaction of radiation with materials controls the performance, reliability, and safety of many structures in nuclear power systems. Revolutionary improvements in radiation damage resistance may be attainable if methods can be found to manipulate interface properties to give optimal interface stability and point defect recombination capability. To understand how variations in interface properties such as misfit dislocation density and local chemistry affect radiation-induced defect absorption and recombination, a model system of metallic Cr xV 1-x (0 ≤ x ≤ 1) epitaxial films deposited on MgO(001) single crystal substrates has been explored in this paper. By controlling film composition, themore » lattice mismatch between the film and MgO is adjusted to vary the misfit dislocation density at the metal/oxide interface. The stability of these interfaces under various irradiation conditions is studied experimentally and theoretically. The results indicate that, unlike at metal/metal interfaces, the misfit dislocation density does not dominate radiation damage tolerance at metal/oxide interfaces. Rather, the stoichiometry and the location of the misfit dislocation extra half-plane (in the metal or the oxide) drive radiation-induced defect behavior. Finally, together, these results demonstrate the sensitivity of defect recombination to interfacial chemistry and provide new avenues for engineering radiation-tolerant nanomaterials for next-generation nuclear power plants.« less

Radiation Tolerant Interfaces: Influence of Local Stoichiometry at the Misfit Dislocation on Radiation Damage Resistance of Metal/Oxide Interfaces

DOE PAGES

Shutthanandan, Vaithiyalingam; Choudhury, Samrat; Manandhar, Sandeep; ...

2017-04-24

The interaction of radiation with materials controls the performance, reliability, and safety of many structures in nuclear power systems. Revolutionary improvements in radiation damage resistance may be attainable if methods can be found to manipulate interface properties to give optimal interface stability and point defect recombination capability. To understand how variations in interface properties such as misfit dislocation density and local chemistry affect radiation-induced defect absorption and recombination, a model system of metallic Cr xV 1-x (0 ≤ x ≤ 1) epitaxial films deposited on MgO(001) single crystal substrates has been explored in this paper. By controlling film composition, themore » lattice mismatch between the film and MgO is adjusted to vary the misfit dislocation density at the metal/oxide interface. The stability of these interfaces under various irradiation conditions is studied experimentally and theoretically. The results indicate that, unlike at metal/metal interfaces, the misfit dislocation density does not dominate radiation damage tolerance at metal/oxide interfaces. Rather, the stoichiometry and the location of the misfit dislocation extra half-plane (in the metal or the oxide) drive radiation-induced defect behavior. Finally, together, these results demonstrate the sensitivity of defect recombination to interfacial chemistry and provide new avenues for engineering radiation-tolerant nanomaterials for next-generation nuclear power plants.« less

Atomic and electronic structure of Lomer dislocations at CdTe bicrystal interface

PubMed Central

Sun, Ce; Paulauskas, Tadas; Sen, Fatih G.; Lian, Guoda; Wang, Jinguo; Buurma, Christopher; Chan, Maria K. Y.; Klie, Robert F.; Kim, Moon J.

2016-01-01

Extended defects are of considerable importance in determining the electronic properties of semiconductors, especially in photovoltaics (PVs), due to their effects on electron-hole recombination. We employ model systems to study the effects of dislocations in CdTe by constructing grain boundaries using wafer bonding. Atomic-resolution scanning transmission electron microscopy (STEM) of a [1–10]/(110) 4.8° tilt grain boundary reveals that the interface is composed of three distinct types of Lomer dislocations. Geometrical phase analysis is used to map strain fields, while STEM and density functional theory (DFT) modeling determine the atomic structure at the interface. The electronic structure of the dislocation cores calculated using DFT shows significant mid-gap states and different charge-channeling tendencies. Cl-doping is shown to reduce the midgap states, while maintaining the charge separation effects. This report offers novel avenues for exploring grain boundary effects in CdTe-based solar cells by fabricating controlled bicrystal interfaces and systematic atomic-scale analysis. PMID:27255415

Atomic and electronic structure of Lomer dislocations at CdTe bicrystal interface

DOE PAGES

Sun, Ce; Paulauskas, Tadas; Sen, Fatih G.; ...

2016-06-03

Extended defects are of considerable importance in determining the electronic properties of semiconductors, especially in photovoltaics (PVs), due to their effects on electron-hole recombination. We employ model systems to study the effects of dislocations in CdTe by constructing grain boundaries using wafer bonding. Atomic-resolution scanning transmission electron microscopy (STEM) of a [1–10]/ (110) 4.8° tilt grain boundary reveals that the interface is composed of three distinct types of Lomer dislocations. Geometrical phase analysis is used to map strain fields, while STEM and density functional theory (DFT) modeling determine the atomic structure at the interface. The electronic structure of the dislocationmore » cores calculated using DFT shows significant mid-gap states and different charge-channeling tendencies. Cl-doping is shown to reduce the midgap states, while maintaining the charge separation effects. In conclusion, this report offers novel avenues for exploring grain boundary effects in CdTe-based solar cells by fabricating controlled bicrystal interfaces and systematic atomic-scale analysis.« less

The study of mechanical behavior on the interface between calcar-defect femur and restorations by means of finite element analysis

NASA Astrophysics Data System (ADS)

Shi, X. H.; Jiang, W.; Chen, H. Z.; Zou, W.; Wang, W. D.; Guo, Z.; Luo, J. M.; Gu, Z. W.; Zhang, X. D.

2008-11-01

The mechanical behaviors of calcar-defected femur and restorations under physiological load are the key factors that will greatly influence the success of femur calcar defect repairing, especially the stress distribution on the bone-restoration interface. 3D finite elements analysis (FEA) was used to analyze the mechanical characters on the interfaces between femoral calcar defects and bone cement or HA restorations. Under the load of two times of a human weight (1436.03 N) and with the increase of the defect dimension from 6 mm to 12 mm, the maximal stresses on the surface of restorations are from 7.06 MPa to 11.89 MPa for bone cement and 2.97-9 MPa for HA separately. In this condition, HA restoration will probably be broken on the bone-restoration interface when the defect diameter is beyond 8 mm. Furthermore, under the load of 1.5 times of a human weight, HA restoration would not be safe unless the defect dimension is smaller than 10 mm, because the maximal stress (4.62 MPa) on the restoration is only a little lower than compressive strength of HA, otherwise the bone fixation device should be applied to ensure the safety. It is relatively safe for all restorations under all the tested defect sizes when the load is just the weight of a human body.

Interfacial Charge-Carrier Trapping in CH3NH3PbI3-Based Heterolayered Structures Revealed by Time-Resolved Photoluminescence Spectroscopy.

PubMed

Yamada, Yasuhiro; Yamada, Takumi; Shimazaki, Ai; Wakamiya, Atsushi; Kanemitsu, Yoshihiko

2016-06-02

The fast-decaying component of photoluminescence (PL) under very weak pulse photoexcitation is dominated by the rapid relaxation of the photoexcited carriers into a small number of carrier-trapping defect states. Here, we report the subnanosecond decay of the PL under excitation weaker than 1 nJ/cm(2) both in CH3NH3PbI3-based heterostructures and bare thin films. The trap-site density at the interface was evaluated on the basis of the fluence-dependent PL decay profiles. It was found that high-density defects determining the PL decay dynamics are formed near the interface between CH3NH3PbI3 and the hole-transporting Spiro-OMeTAD but not at the CH3NH3PbI3/TiO2 interface and the interior regions of CH3NH3PbI3 films. This finding can aid the fabrication of high-quality heterointerfaces, which are required improving the photoconversion efficiency of perovskite-based solar cells.

TEM-EELS Investigation of Boron and Phosphorus Passivated 4H-SiC/SiO2 Interface Structures

NASA Astrophysics Data System (ADS)

Klingshirn, Christopher; Taillon, Joshua; Liu, Gang; Dhar, Sarit; Feldman, Leonard; Zheleva, Tsvetanka; Lelis, Aivars; Salamanca-Riba, Lourdes

A high density of electronic defects at the SiC/SiO2 interface adversely affects SiC-based metal oxide semiconductor devices. Various treatments are known to improve device performance. Annealing in a nitric oxide (NO) environment, for example, passivates electronic defects at the interface and raises the carrier mobility in the active region to 35-40 cm2/Vs, but the effect saturates after about 60 minutes of annealing. Passivation with phosphorus or boron improves upon NO by a factor of 2, increasing the mobility to over 90 cm2/Vs.2 We investigate the chemical and structural effects of these treatments on the SiC/SiO2 transition layer using high-resolution transmission electron microscopy (HRTEM) and high angle annular dark field (HAADF). Electron energy loss spectroscopy Spectrum Imaging (EELS SI) collected across the transition region allow identification of the width, composition and types of bonding at the transition layer. Advanced machine learning techniques applied to the EELS data reveal intermediate bonding states within this region. Supported by ARL under Grant No. W911NF1420110.

Energy shadowing correction of ultrasonic pulse-echo records by digital signal processing

NASA Technical Reports Server (NTRS)

Kishonio, D.; Heyman, J. S.

1985-01-01

A numerical algorithm is described that enables the correction of energy shadowing during the ultrasonic testing of bulk materials. In the conventional method, an ultrasonic transducer transmits sound waves into a material that is immersed in water so that discontinuities such as defects can be revealed when the waves are reflected and then detected and displayed graphically. Since a defect that lies behind another defect is shadowed in that it receives less energy, the conventional method has a major drawback. The algorithm normalizes the energy of the incoming wave by measuring the energy of the waves reflected off the water/air interface. The algorithm is fast and simple enough to be adopted for real time applications in industry. Images of material defects with the shadowing corrections permit more quantitative interpretation of the material state.

Kinetic Monte Carlo simulation of dopant-defect systems under submicrosecond laser thermal processes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fisicaro, G.; Pelaz, Lourdes; Lopez, P.

2012-11-06

An innovative Kinetic Monte Carlo (KMC) code has been developed, which rules the post-implant kinetics of the defects system in the extremely far-from-the equilibrium conditions caused by the laser irradiation close to the liquid-solid interface. It considers defect diffusion, annihilation and clustering. The code properly implements, consistently to the stochastic formalism, the fast varying local event rates related to the thermal field T(r,t) evolution. This feature of our numerical method represents an important advancement with respect to current state of the art KMC codes. The reduction of the implantation damage and its reorganization in defect aggregates are studied as amore » function of the process conditions. Phosphorus activation efficiency, experimentally determined in similar conditions, has been related to the emerging damage scenario.« less

A defect in holographic interpretations of tensor networks

NASA Astrophysics Data System (ADS)

Czech, Bartlomiej; Nguyen, Phuc H.; Swaminathan, Sivaramakrishnan

2017-03-01

We initiate the study of how tensor networks reproduce properties of static holographic space-times, which are not locally pure anti-de Sitter. We consider geometries that are holographically dual to ground states of defect, interface and boundary CFTs and compare them to the structure of the requisite MERA networks predicted by the theory of minimal updates. When the CFT is deformed, certain tensors require updating. On the other hand, even identical tensors can contribute differently to estimates of entanglement entropies. We interpret these facts holographically by associating tensor updates to turning on non-normalizable modes in the bulk. In passing, we also clarify and complement existing arguments in support of the theory of minimal updates, propose a novel ansatz called rayed MERA that applies to a class of generalized interface CFTs, and analyze the kinematic spaces of the thin wall and AdS3-Janus geometries.

Record surface state mobility and quantum Hall effect in topological insulator thin films via interface engineering

DOE PAGES

Koirala, Nikesh; Han, Myung -Geun; Brahlek, Matthew; ...

2015-11-19

Material defects remain as the main bottleneck to the progress of topological insulators (TIs). In particular, efforts to achieve thin TI samples with dominant surface transport have always led to increased defects and degraded mobilities, thus making it difficult to probe the quantum regime of the topological surface states. Here, by utilizing a novel buffer layer scheme composed of an In 2Se 3/(Bi 0.5In 0.5) 2Se 3 heterostructure, we introduce a quantum generation of Bi 2Se 3 films with an order of magnitude enhanced mobilities than before. Furthermore, this scheme has led to the first observation of the quantum Hallmore » effect in Bi 2Se 3.« less

Topological Defects in Topological Insulators and Bound States at Topological Superconductor Vortices

PubMed Central

Parente, Vincenzo; Campagnano, Gabriele; Giuliano, Domenico; Tagliacozzo, Arturo; Guinea, Francisco

2014-01-01

The scattering of Dirac electrons by topological defects could be one of the most relevant sources of resistance in graphene and at the boundary surfaces of a three-dimensional topological insulator (3D TI). In the long wavelength, continuous limit of the Dirac equation, the topological defect can be described as a distortion of the metric in curved space, which can be accounted for by a rotation of the Gamma matrices and by a spin connection inherited with the curvature. These features modify the scattering properties of the carriers. We discuss the self-energy of defect formation with this approach and the electron cross-section for intra-valley scattering at an edge dislocation in graphene, including corrections coming from the local stress. The cross-section contribution to the resistivity, ρ, is derived within the Boltzmann theory of transport. On the same lines, we discuss the scattering of a screw dislocation in a two-band 3D TI, like Bi1−xSbx, and we present the analytical simplified form of the wavefunction for gapless helical states bound at the defect. When a 3D TI is sandwiched between two even-parity superconductors, Dirac boundary states acquire superconductive correlations by proximity. In the presence of a magnetic vortex piercing the heterostructure, two Majorana states are localized at the two interfaces and bound to the vortex core. They have a half integer total angular momentum each, to match with the unitary orbital angular momentum of the vortex charge. PMID:28788537

Topological Defects in Topological Insulators and Bound States at Topological Superconductor Vortices.

PubMed

Parente, Vincenzo; Campagnano, Gabriele; Giuliano, Domenico; Tagliacozzo, Arturo; Guinea, Francisco

2014-03-04

The scattering of Dirac electrons by topological defects could be one of the most relevant sources of resistance in graphene and at the boundary surfaces of a three-dimensional topological insulator (3D TI). In the long wavelength, continuous limit of the Dirac equation, the topological defect can be described as a distortion of the metric in curved space, which can be accounted for by a rotation of the Gamma matrices and by a spin connection inherited with the curvature. These features modify the scattering properties of the carriers. We discuss the self-energy of defect formation with this approach and the electron cross-section for intra-valley scattering at an edge dislocation in graphene, including corrections coming from the local stress. The cross-section contribution to the resistivity, ρ, is derived within the Boltzmann theory of transport. On the same lines, we discuss the scattering of a screw dislocation in a two-band 3D TI, like Bi 1-x Sb x , and we present the analytical simplified form of the wavefunction for gapless helical states bound at the defect. When a 3D TI is sandwiched between two even-parity superconductors, Dirac boundary states acquire superconductive correlations by proximity. In the presence of a magnetic vortex piercing the heterostructure, two Majorana states are localized at the two interfaces and bound to the vortex core. They have a half integer total angular momentum each, to match with the unitary orbital angular momentum of the vortex charge.

Ab Initio Study of the Atomic Level Structure of the Rutile TiO2(110)-Titanium Nitride (TiN) Interface.

PubMed

Gutiérrez Moreno, José Julio; Nolan, Michael

2017-11-01

Titanium nitride (TiN) is widely used in industry as a protective coating due to its hardness and resistance to corrosion and can spontaneously form a thin oxide layer when it is exposed to air, which could modify the properties of the coating. With limited understanding of the TiO 2 -TiN interfacial system at present, this work aims to describe the structural and electronic properties of oxidized TiN based on a density functional theory (DFT) study of the rutile TiO 2 (110)-TiN(100) interface model system, also including Hubbard +U correction on Ti 3d states. The small lattice mismatch gives a good stability to the TiO 2 -TiN interface after depositing the oxide onto TiN through the formation of interfacial Ti-O bonds. Our DFT+U study shows the presence of Ti 3+ cations in the TiO 2 region, which are preferentially located next to the interface region as well as the rotation of the rutile TiO 2 octahedra in the interface structure. The DFT+U TiO 2 electronic density of states (EDOS) shows localized Ti 3+ defect states forming in the midgap between the top edge of the valence and the bottom of the conduction band. We increase the complexity of our models by the introduction of nonstoichiometric compositions. Although the vacancy formation energies for Ti in TiN (E vac (Ti) ≥ 4.03 eV) or O in the oxide (E vac (O) ≥ 3.40 eV) are quite high relative to perfect TiO 2 -TiN, defects are known to form during the oxide growth and can therefore be present after TiO 2 formation. Our results show that a structure with exchanged O and N can lie 0.82 eV higher in energy than the perfect system, suggesting the stability of structures with interdiffused O and N anions at ambient conditions. The presence of N in TiO 2 introduces N 2p states localized between the top edge of the O 2p valence states and the midgap Ti 3+ 3d states, thus reducing the band gap in the TiO 2 region for the exchanged O/N interface EDOS. The outcomes of these simulations give us a most comprehensive insight on the atomic level structure and the electronic properties of oxidized TiN surfaces.

Evaluation of border traps and interface traps in HfO2/MoS2 gate stacks by capacitance–voltage analysis

NASA Astrophysics Data System (ADS)

Zhao, Peng; Khosravi, Ava; Azcatl, Angelica; Bolshakov, Pavel; Mirabelli, Gioele; Caruso, Enrico; Hinkle, Christopher L.; Hurley, Paul K.; Wallace, Robert M.; Young, Chadwin D.

2018-07-01

Border traps and interface traps in HfO2/few-layer MoS2 top-gate stacks are investigated by C–V characterization. Frequency dependent C–V data shows dispersion in both the depletion and accumulation regions for the MoS2 devices. The border trap density is extracted with a distributed model, and interface traps are analyzed using the high-low frequency and multi-frequency methods. The physical origins of interface traps appear to be caused by impurities/defects in the MoS2 layers, performing as band tail states, while the border traps are associated with the dielectric, likely a consequence of the low-temperature deposition. This work provides a method of using multiple C–V measurements and analysis techniques to analyze the behavior of high-k/TMD gate stacks and deconvolute border traps from interface traps.

An open-architecture approach to defect analysis software for mask inspection systems

NASA Astrophysics Data System (ADS)

Pereira, Mark; Pai, Ravi R.; Reddy, Murali Mohan; Krishna, Ravi M.

2009-04-01

Industry data suggests that Mask Inspection represents the second biggest component of Mask Cost and Mask Turn Around Time (TAT). Ever decreasing defect size targets lead to more sensitive mask inspection across the chip, thus generating too many defects. Hence, more operator time is being spent in analyzing and disposition of defects. Also, the fact that multiple Mask Inspection Systems and Defect Analysis strategies would typically be in use in a Mask Shop or a Wafer Foundry further complicates the situation. In this scenario, there is a need for a versatile, user friendly and extensible Defect Analysis software that reduces operator analysis time and enables correct classification and disposition of mask defects by providing intuitive visual and analysis aids. We propose a new vendor-neutral defect analysis software, NxDAT, based on an open architecture. The open architecture of NxDAT makes it easily extensible to support defect analysis for mask inspection systems from different vendors. The capability to load results from mask inspection systems from different vendors either directly or through a common interface enables the functionality of establishing correlation between inspections carried out by mask inspection systems from different vendors. This capability of NxDAT enhances the effectiveness of defect analysis as it directly addresses the real-life scenario where multiple types of mask inspection systems from different vendors co-exist in mask shops or wafer foundries. The open architecture also potentially enables loading wafer inspection results as well as loading data from other related tools such as Review Tools, Repair Tools, CD-SEM tools etc, and correlating them with the corresponding mask inspection results. A unique concept of Plug-In interface to NxDAT further enhances the openness of the architecture of NxDAT by enabling end-users to add their own proprietary defect analysis and image processing algorithms. The plug-in interface makes it possible for the end-users to make use of their collected knowledge through the years of experience in mask inspection process by encapsulating the knowledge into software utilities and plugging them into NxDAT. The plug-in interface is designed with the intent of enabling the pro-active mask defect analysis teams to build competitive differentiation into their defect analysis process while protecting their knowledge internally within their company. By providing interface with all major standard layout and mask data formats, NxDAT enables correlation of defect data on reticles with design and mask databases, further extending the effectiveness of defect analysis for D2DB inspection. NxDAT also includes many other advanced features for easy and fast navigation, visual display of defects, defect selection, multi-tier classification, defect clustering and gridding, sophisticated CD and contact measurement analysis, repeatability analysis such as adder analysis, defect trend, capture rate etc.

Measuring Te inclusion uniformity over large areas for CdTe/CZT imaging and spectrometry sensors

NASA Astrophysics Data System (ADS)

Bolke, Joe; O'Brien, Kathryn; Wall, Peter; Spicer, Mike; Gélinas, Guillaume; Beaudry, Jean-Nicolas; Alexander, W. Brock

2017-09-01

CdTe and CZT materials are technologies for gamma and x-ray imaging for applications in industry, homeland security, defense, space, medical, and astrophysics. There remain challenges in uniformity over large detector areas (50 75 mm) due to a combination of material purity, handling, growth process, grown in defects, doping/compensation, and metal contacts/surface states. The influence of these various factors has yet to be explored at the large substrate level required for devices with higher resolution both spatially and spectroscopically. In this study, we looked at how the crystal growth processes affect the size and density distributions of microscopic Te inclusion defects. We were able to grow single crystals as large as 75 mm in diameter and spatially characterize three-dimensional defects and map the uniformity using IR microscopy. We report on the pattern of observed defects within wafers and its relation to instabilities at the crystal growth interface.

Effect of antimony on the deep-level traps in GaInNAsSb thin films

DOE Office of Scientific and Technical Information (OSTI.GOV)

Islam, Muhammad Monirul, E-mail: islam.monir.ke@u.tsukuba.ac.jp; Miyashita, Naoya; Ahsan, Nazmul

2014-09-15

Admittance spectroscopy has been performed to investigate the effect of antimony (Sb) on GaInNAs material in relation to the deep-level defects in this material. Two electron traps, E1 and E2 at an energy level 0.12 and 0.41 eV below the conduction band (E{sub C}), respectively, were found in undoped GaInNAs. Bias-voltage dependent admittance confirmed that E1 is an interface-type defect being spatially localized at the GaInNAs/GaAs interface, while E2 is a bulk-type defect located around mid-gap of GaInNAs layer. Introduction of Sb improved the material quality which was evident from the reduction of both the interface and bulk-type defects.

Density-functional study on the dopant-segregation mechanism: Chemical potential dependence of dopant-defect complex at Si/SiO2 interface

NASA Astrophysics Data System (ADS)

Kawai, Hiroki; Nakasaki, Yasushi; Kanemura, Takahisa; Ishihara, Takamitsu

2018-04-01

Dopant segregation at Si/SiO2 interface has been a serious problem in silicon device technology. This paper reports a comprehensive density-functional study on the segregation mechanisms of boron, phosphorous, and arsenic at the Si/SiO2 interface. We found that three kinds of interfacial defects, namely, interstitial oxygen, oxygen vacancy, and silicon vacancy with two oxygen atoms, are stable in the possible chemical potential range. Thus, we consider these defects as trap sites for the dopants. For these defects, the dopant segregation energies, the electrical activities of the trapped dopants, and the kinetic energy barriers of the trapping/detrapping processes are calculated. As a result, trapping at the interstitial oxygen site is indicated to be the most plausible mechanism of the dopant segregation. The interstitial oxygen works as a major trap site since it has a high areal density at the Si/SiO2 interface due to the low formation energy.

Topological interface physics in spinor Bose-Einstein condensates

NASA Astrophysics Data System (ADS)

Borgh, Magnus; Ruostekoski, Janne

2013-05-01

We present an experimentally viable scheme whereby the physics of coherent interfaces between topologically distinct regions can be studied in an atomic quantum gas. The interface engineering is achieved using the internal spin structures of atoms together with local control over interaction strengths. We consider a coherent interface between polar and ferromagnetic regions of a spin-1 Bose-Einstein condensate and show that defects representing different topologies can connect continuously across the boundary. We show that energy minimization leads to nontrivial interface-crossing defect structures, demonstrating how the method can be used to study stability properties of field-theoretical solitons. We demonstrate, e.g., the formation of a half-quantum vortex arch, an Alice arch, on the interface, exhibiting the topological charge of a point defect. We also demonstrate an energetically stable connection of a coreless vortex to two half-quantum vortices. Our method can be extended to study interface physics in spin-2 and spin-3 BECs with richer phenomenology, or in strongly correlated optical-lattice systems. We acknowledge financial support from the Leverhulme Trust.

Instabilities and patterns in an active nematic film

NASA Astrophysics Data System (ADS)

Srivastava, Pragya; Marchetti, Cristina

2015-03-01

Experiments on microtubule bundles confined to an oil-water interface have motivated extensive theoretical studies of two-dimensional active nematics. Theoretical models taking into account the interplay between activity, flow and order have remarkably reproduced several experimentally observed features of the defect-dynamics in these ``living'' nematics. Here, we derive minimal description of a two-dimensional active nematic film confined between walls. At high friction, we eliminate the flow to obtain closed equations for the nematic order parameter, with renormalized Frank elastic constants. Active processes can render the ``Frank'' constants negative, resulting in the instability of the uniformly ordered nematic state. The minimal model yields emergent patterns of growing complexity with increasing activity, including bands and turbulent dynamics with a steady density of topological defects, as obtained with the full hydrodynamic equations. We report on the scaling of the length scales of these patterns and of the steady state number of defects with activity and system size. National Science Foundation grant DMR-1305184 and Syracuse Soft Matter Program.

Interface-defect-mediated photocatalysis of mesocrystalline ZnO assembly synthesized in-situ via a template-free hydrothermal approach

NASA Astrophysics Data System (ADS)

Wang, Hui; Wang, Cuicui; Chen, Qifeng; Ren, Baosheng; Guan, Ruifang; Cao, Xiaofeng; Yang, Xiaopeng; Duan, Ran

2017-08-01

Both architecture construction and defects engineering of photocatalysts are highly vital in the photocatalytic activity. We report herein that the interface-defect-mediated photocatalytic activity of pompon-like ZnO (P-ZnO) mesocrystal photocatalyst synthesized via an aqueous approach, in the presence of sodium citrate without any other organic templates. The microstructure and defects of the diverse ZnO photocatalysts were examined with various techniques. The results indicated that the P-ZnO assemblies were composed of mesocrystal nanosheets exposed high energy (002) facet with high crystallinity. More importantly, the defects located at the interfaces among the nanocrystals in ZnO mesocrystals played an important role in the photocatalytic activity than that of interstitial zinc vacancies in bulk, which was confirmed by photocatalytic degradation of organic pollutants, such as methylene blue (MB) and 2,4,6-trichlorophenol (2,4,6-TCP). The results showed that the P-ZnO exhibited higher photocatalytic activity than that of the nanosized ZnO (N-ZnO), which could be attributed to not only the unique mesocrystal structure and high energy (002) facet exposed, but also the defects located at interfaces among nanocrystals in ZnO mesocrystals. In addition, the formation mechanism of the P-ZnO was investigated via a time-dependent method. It was found that the formation of P-ZnO hierarchical architecture assembled with ZnO mesocrystals involved a nonclassical crystallization growth and Ostwald Ripening process. This study provides a perspective on the improvement in photocatalytic activity via adjusting the bulk and interface defects and construction of hierarchical architectures of semiconductors.

Voltage and partial pressure dependent defect chemistry in (La,Sr)FeO3–δ thin films investigated by chemical capacitance measurements

PubMed Central

Rupp, Ghislain M.; Fleig, Jürgen

2018-01-01

La0.6Sr0.4FeO3–δ (LSF) thin films of different thickness were prepared by pulsed laser deposition on yttria stabilized zirconia (YSZ) and characterized by using three electrode impedance spectroscopy. Electrochemical film capacitance was analyzed in relation to oxygen partial pressure (0.25 mbar to 1 bar), DC polarization (0 m to –600 m) and temperature (500 to 650 °C). For most measurement parameters, the chemical bulk capacitance dominates the overall capacitive properties and the corresponding defect chemical state depends solely on the oxygen chemical potential inside the film, independent of atmospheric oxygen pressure and DC polarization. Thus, defect chemical properties (defect concentrations and defect formation enthalpies) could be deduced from such measurements. Comparison with LSF defect chemical bulk data from the literature showed good agreement for vacancy formation energies but suggested larger electronic defect concentrations in the films. From thickness-dependent measurements at lower oxygen chemical potentials, an additional capacitive contribution could be identified and attributed to the LSF|YSZ interface. Deviations from simple chemical capacitance models at high pressures are most probably due to defect interactions. PMID:29671421

Voltage and partial pressure dependent defect chemistry in (La,Sr)FeO3-δ thin films investigated by chemical capacitance measurements.

PubMed

Schmid, Alexander; Rupp, Ghislain M; Fleig, Jürgen

2018-05-03

La0.6Sr0.4FeO3-δ (LSF) thin films of different thickness were prepared by pulsed laser deposition on yttria stabilized zirconia (YSZ) and characterized by using three electrode impedance spectroscopy. Electrochemical film capacitance was analyzed in relation to oxygen partial pressure (0.25 mbar to 1 bar), DC polarization (0 m to -600 m) and temperature (500 to 650 °C). For most measurement parameters, the chemical bulk capacitance dominates the overall capacitive properties and the corresponding defect chemical state depends solely on the oxygen chemical potential inside the film, independent of atmospheric oxygen pressure and DC polarization. Thus, defect chemical properties (defect concentrations and defect formation enthalpies) could be deduced from such measurements. Comparison with LSF defect chemical bulk data from the literature showed good agreement for vacancy formation energies but suggested larger electronic defect concentrations in the films. From thickness-dependent measurements at lower oxygen chemical potentials, an additional capacitive contribution could be identified and attributed to the LSF|YSZ interface. Deviations from simple chemical capacitance models at high pressures are most probably due to defect interactions.

Heterogeneous nucleation on rough surfaces: Generalized Gibbs' approach.

PubMed

Abyzov, Alexander S; Schmelzer, Jürn W P; Davydov, Leonid N

2017-12-07

Heterogeneous nucleation (condensation) of liquid droplets from vapor (gas) on a defective solid surface is considered. The vapor is described by the van der Waals equation of state. The dependence of nucleating droplet parameters on droplet size is accounted for within the generalized Gibbs approach. As a surface defect, a conic void is taken. This choice allows us to simplify the analysis and at the same time to follow the main aspects of the influence of the surface roughness on the nucleation process. Similar to condensation on ideal planar surfaces, the contact angle and catalytic factor for heterogeneous nucleation on a rough surface depend on the degree of vapor overcooling. In the case of droplet formation on a hydrophilic surface of a conic void, the nucleation rate considerably increases in comparison with the condensation on a planar interface. In fact, the presence of a defect on the hydrophilic surface leads to a considerable shift of the spinodal towards lower supersaturation in comparison with heterogeneous nucleation on a planar interface. With the decrease in the void cone angle, the heterogeneous spinodal approaches the binodal, and the region of metastability is diminished at the expense of the instability region.

HgCdTe Surface and Defect Study Program.

DTIC Science & Technology

1984-07-01

double layer heterojunction (DLHJ) devices. There are however many complications on this once we consider implanted junctions, LWIR devices or even the...It is not possible from this measurement to discriminate between real interface states and charge nonuniformities . Admittance spectroscopy (discussed...earlier) and deep level transient spectroscopy (DLTS) are not usually affected by these nonuniformities due to their observation of a speci- fic

Charge Trapping Properties of Ge Nanocrystals Grown via Solid-State Dewetting

DOE Office of Scientific and Technical Information (OSTI.GOV)

Johnston, Steven; Jadli, I.; Aouassa, M.

2018-05-04

In the present work, we report on the charge trapping properties of Germanium Nanocrystals (Ge NCs) self assembled on SiO2 thin layer for promising applications in next-generation non volatile memory by the means of Deep Level Transient Spectroscopy (DLTS) and high frequency C-V method. The Ge NCs were grown via dewetting phenomenon at solid state by Ultra-High Vacuum (UHV) annealing and passivated with silicon before SiO2 capping. The role of the surface passivation is to reduce the electrical defect density at the Ge NCs-SiO2 interface. The presence of the Ge NCs in the oxide of the MOS capacitors strongly affectsmore » the C-V characteristics and increases the accumulation capacitance, causes a negative flat band voltage (VFB) shift. The DLTS has been used to study the individual Ge NCs as a single point deep level defect in the oxide. DLTS reveals two main features: the first electron traps around 255 K could correspond to dangling bonds at the Si/SiO2 interface and the second, at high-temperature (>300 K) response, could be originated from minority carrier generation in Ge NCs.« less

Coherently Coupled ZnO and VO2 Interface studied by Photoluminescence and electrical transport across a phase transition

NASA Astrophysics Data System (ADS)

Srivastava, Amar; Saha, S.; Annadi, A.; Zhao, Y. L.; Gopinadhan, K.; Wang, X.; Naomi, N.; Liu, Z. Q.; Dhar, S.; Herng, T. S.; Nina, Bao; Ariando, -; Ding, Jun; Venkatesan, T.

2012-02-01

In this work we report a study of a coherently coupled interface consisting of a ZnO layer grown on top of an oriented VO2 layer on sapphire by photoluminescence and electrical transport measurements across the VO2 metal insulator phase transition (MIT). The photoluminescence of the ZnO layer showed a broad hysteresis induced by the phase transition of VO2 while the width of the electrical hysteresis was narrow and unaffected by the over layer. The enhanced width of the PL hysteresis was due to the formation of defects during the MIT as evidenced by a broad hysteresis in the opposite direction to that of the band edge PL in the defect luminescense. Unlike VO2 the defects in ZnO did not fully recover across the phase transition. From the defect luminescence data, oxygen interstitials were found to be the predominant defects in ZnO mediated by the strain from the VO2 phase transition. Such coherently coupled interfaces could be of use in characterizing the stability of a variety of interfaces and also for novel device application.

Quantum Properties of Dichroic Silicon Vacancies in Silicon Carbide

NASA Astrophysics Data System (ADS)

Nagy, Roland; Widmann, Matthias; Niethammer, Matthias; Dasari, Durga B. R.; Gerhardt, Ilja; Soykal, Öney O.; Radulaski, Marina; Ohshima, Takeshi; Vučković, Jelena; Son, Nguyen Tien; Ivanov, Ivan G.; Economou, Sophia E.; Bonato, Cristian; Lee, Sang-Yun; Wrachtrup, Jörg

2018-03-01

Although various defect centers have displayed promise as either quantum sensors, single photon emitters, or light-matter interfaces, the search for an ideal defect with multifunctional ability remains open. In this spirit, we study the dichroic silicon vacancies in silicon carbide that feature two well-distinguishable zero-phonon lines and analyze the quantum properties in their optical emission and spin control. We demonstrate that this center combines 40% optical emission into the zero-phonon lines showing the contrasting difference in optical properties with varying temperature and polarization, and a 100% increase in the fluorescence intensity upon the spin resonance, and long spin coherence time of their spin-3 /2 ground states up to 0.6 ms. These results single out this defect center as a promising system for spin-based quantum technologies.

Time domain optical coherence tomography investigation of bone matrix interface in rat femurs

NASA Astrophysics Data System (ADS)

Rusu, Laura-Cristina; Negruá¹±iu, Meda-Lavinia; Sinescu, Cosmin; Hoinoiu, Bogdan; Topala, Florin-Ionel; Duma, Virgil-Florin; Rominu, Mihai; Podoleanu, Adrian G.

2013-08-01

The materials used to fabricate scaffolds for tissue engineering are derived from synthetic polymers, mainly from the polyester family, or from natural materials (e.g., collagen and chitosan). The mechanical properties and the structural properties of these materials can be tailored by adjusting the molecular weight, the crystalline state, and the ratio of monomers in the copolymers. Quality control and adjustment of the scaffold manufacturing process are essential to achieve high standard scaffolds. Most scaffolds are made from highly crystalline polymers, which inevitably result in their opaque appearance. Their 3-D opaque structure prevents the observation of internal uneven surface structures of the scaffolds under normal optical instruments, such as the traditional light microscope. The inability to easily monitor the inner structure of scaffolds as well as the interface with the old bone poses a major challenge for tissue engineering: it impedes the precise control and adjustment of the parameters that affect the cell growth in response to various mimicked culture conditions. The aim of this paper is to investigate the interface between the femur rat bone and the new bone that is obtained using a method of tissue engineering that is based on different artificial matrixes inserted in previously artificially induced defects. For this study, 15 rats were used in conformity with ethical procedures. In all the femurs a round defect was induced by drilling with a 1 mm spherical Co-Cr surgical drill. The matrixes used were Bioss and 4bone. These materials were inserted into the induced defects. The femurs were investigated at 1 week, 1 month, 2 month and three month after the surgical procedures. The interfaces were examined using Time Domain (TD) Optical Coherence Tomography (OCT) combined with Confocal Microscopy (CM). The optical configuration uses two single mode directional couplers with a superluminiscent diode as the source centered at 1300 nm. The scanning procedure is similar to that used in any CM, where the fast scanning is en-face (line rate) and the scanning in depth is much slower (at the frame rate). The results showed open interfaces due to the insufficient healing process, as well as closed interfaces due to a new bone formation inside the defect. The conclusion of this study is that TD-OCT can act as a valuable tool in the investigation of the interface between the old bone and the one that has been newly induced due to the osteoinductive process.

Topological phononic insulator with robust pseudospin-dependent transport

NASA Astrophysics Data System (ADS)

Xia, Bai-Zhan; Liu, Ting-Ting; Huang, Guo-Liang; Dai, Hong-Qing; Jiao, Jun-Rui; Zang, Xian-Guo; Yu, De-Jie; Zheng, Sheng-Jie; Liu, Jian

2017-09-01

Topological phononic states, which facilitate unique acoustic transport around defects and disorders, have significantly revolutionized our scientific cognition of acoustic systems. Here, by introducing a zone folding mechanism, we realize the topological phase transition in a double Dirac cone of the rotatable triangular phononic crystal with C3 v symmetry. We then investigate the distinct topological edge states on two types of interfaces of our phononic insulators. The first one is a zigzag interface which simultaneously possesses a symmetric mode and an antisymmetric mode. Hybridization of the two modes leads to a robust pseudospin-dependent one-way propagation. The second one is a linear interface with a symmetric mode or an antisymmetric mode. The type of mode is dependent on the topological phase transition of the phononic insulators. Based on the rotatability of triangular phononic crystals, we consider several complicated contours defined by the topological zigzag interfaces. Along these contours, the acoustic waves can unimpededly transmit without backscattering. Our research develops a route for the exploration of the topological phenomena in experiments and provides an excellent framework for freely steering the acoustic backscattering-immune propagation within topological phononic structures.

Effect of intermixing at CdS/CdTe interface on defect properties

NASA Astrophysics Data System (ADS)

Park, Ji-Sang; Yang, Ji-Hui; Barnes, Teresa; Wei, Su-Huai

2016-07-01

We investigated the stability and electronic properties of defects in CdTe1-xSx that can be formed at the CdS/CdTe interface. As the anions mix at the interface, the defect properties are significantly affected, especially those defects centered at cation sites like Cd vacancy, VCd, and Te on Cd antisite, TeCd, because the environment surrounding the defect sites can have different configurations. We show that at a given composition, the transition energy levels of VCd and TeCd become close to the valence band maximum when the defect has more S atoms in their local environment, thus improving the device performance. Such beneficial role is also found at the grain boundaries when the Te atom is replaced by S in the Te-Te wrong bonds, reducing the energy of the grain boundary level. On the other hand, the transition levels with respect to the valence band edge of CdTe1-xSx increases with the S concentration as the valence band edge decreases with the S concentration, resulting in the reduced p-type doping efficiency.

Gamma ray irradiated silicon nanowires: An effective model to investigate defects at the interface of Si/SiOx

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yin, Kui; Zhao, Yi; Liu, Liangbin

2014-01-20

The effect of gamma ray irradiation on silicon nanowires was investigated. Here, an additional defect emerged in the gamma-ray-irradiated silicon nanowires and was confirmed with electron spin resonance spectra. {sup 29}Si nuclear magnetic resonance spectroscopy showed that irradiation doses had influence on the Q{sup 4} unit structure. This phenomenon indicated that the unique core/shell structure of silicon nanowires might contribute to induce metastable defects under gamma ray irradiation, which served as a satisfactory model to investigate defects at the interface of Si/SiOx.

Interface Energetics and Chemical Doping of Organic Electronic Materials

NASA Astrophysics Data System (ADS)

Kahn, Antoine

2014-03-01

The energetics of organic semiconductors and their interfaces are central to the performance of organic thin film devices. The relative positions of charge transport states across the many interfaces of multi-layer OLEDs, OPV cells and OFETs determine in great part the efficiency and lifetime of these devices. New experiments are presented here, that look in detail at the position of these transport states and associated gap states and electronic traps that tail into the energy gap of organic molecular (e.g. pentacene) or polymer (P3HT, PBDTTT-C) semiconductors, and which directly affect carrier mobility in these materials. Disorder, sometime caused by simple exposure to an inert gas, impurities and defects are at the origin of these electronic gap states. Recent efforts in chemical doping in organic semiconductors aimed at mitigating the impact of electronic gap states are described. An overview of the reducing or oxidizing power of several n- and p-type dopants for vacuum- or solution-processed films, and their effect on the electronic structure and conductivity of both vacuum- and solution-processed organic semiconductor films is given. Finally, the filling (compensation) of active gap states via doping is investigated on the electron-transport materials C60 and P(NDI2OD-T2) , and the hole-transport polymer PBDTTT-C.

Model for thickness dependence of radiation charging in MOS structures

NASA Technical Reports Server (NTRS)

Viswanathan, C. R.; Maserjian, J.

1976-01-01

The model considers charge buildup in MOS structures due to hole trapping in the oxide and the creation of sheet charge at the silicon interface. The contribution of hole trapping causes the flatband voltage to increase with thickness in a manner in which square and cube dependences are limiting cases. Experimental measurements on samples covering a 200 - 1000 A range of oxide thickness are consistent with the model, using independently obtained values of hole-trapping parameters. An important finding of our experimental results is that a negative interface charge contribution due to surface states created during irradiation compensates most of the positive charge in the oxide at flatband. The tendency of the surface states to 'track' the positive charge buildup in the oxide, for all thicknesses, applies both in creation during irradiation and in annihilation during annealing. An explanation is proposed based on the common defect origin of hole traps and potential surface states.

Organic Solar Cells: Degradation Processes and Approaches to Enhance Performance

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fungura, Fadzai

2016-12-17

Intrinsic photodegradation of organic solar cells, theoretically attributed to C-H bond rearrangement/breaking, remains a key commercialization barrier. This work presents, via dark electron paramagnetic resonance (EPR), the first experimental evidence for metastable C dangling bonds (DBs) (g=2.0029±0.0004) formed by blue/UV irradiation of polymer:fullerene blend films in nitrogen. The DB density increased with irradiation and decreased ~4 fold after 2 weeks in the dark. The dark EPR also showed increased densities of other spin-active sites in photodegraded polymer, fullerene, and polymer:fullerene blend films, consistent with broad electronic measurements of fundamental properties, including defect/gap state densities. The EPR enabled identification of defectmore » states, whether in the polymer, fullerene, or at the donor/acceptor (D/A) interface. Importantly, the EPR results indicate that the DBs are at the D/A interface, as they were present only in the blend films. The role of polarons in interface DB formation is also discussed.« less

Controlling the energy of defects and interfaces in the amplitude expansion of the phase-field crystal model

NASA Astrophysics Data System (ADS)

Salvalaglio, Marco; Backofen, Rainer; Voigt, Axel; Elder, Ken R.

2017-08-01

One of the major difficulties in employing phase-field crystal (PFC) modeling and the associated amplitude (APFC) formulation is the ability to tune model parameters to match experimental quantities. In this work, we address the problem of tuning the defect core and interface energies in the APFC formulation. We show that the addition of a single term to the free-energy functional can be used to increase the solid-liquid interface and defect energies in a well-controlled fashion, without any major change to other features. The influence of the newly added term is explored in two-dimensional triangular and honeycomb structures as well as bcc and fcc lattices in three dimensions. In addition, a finite-element method (FEM) is developed for the model that incorporates a mesh refinement scheme. The combination of the FEM and mesh refinement to simulate amplitude expansion with a new energy term provides a method of controlling microscopic features such as defect and interface energies while simultaneously delivering a coarse-grained examination of the system.

Analysis of Recombination in CdTe Heterostructures With Time-Resolved Two-Photon Excitation Microscopy

DOE PAGES

Kuciauskas, Darius; Wernsing, Keith; Jensen, Soren Alkaersig; ...

2016-11-01

Here, we used time-resolved photoluminescence microscopy to analyze charge carrier transport and recombination in CdTe double heterostructures fabricated by molecular beam epitaxy (MBE). This allowed us to determine the charge carrier mobility in this system, which was found to be 500-625 cm 2/(V s). Charge carrier lifetimes in the 15-100 ns range are limited by the interface recombination, and the data indicate higher interface recombination velocity near extended defects. This study describes a new method to analyze the spatial distribution of the interface recombination velocity and the interface defects in semiconductor heterostructures.

Analysis of Recombination in CdTe Heterostructures With Time-Resolved Two-Photon Excitation Microscopy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kuciauskas, Darius; Wernsing, Keith; Jensen, Soren Alkaersig

Here, we used time-resolved photoluminescence microscopy to analyze charge carrier transport and recombination in CdTe double heterostructures fabricated by molecular beam epitaxy (MBE). This allowed us to determine the charge carrier mobility in this system, which was found to be 500-625 cm 2/(V s). Charge carrier lifetimes in the 15-100 ns range are limited by the interface recombination, and the data indicate higher interface recombination velocity near extended defects. This study describes a new method to analyze the spatial distribution of the interface recombination velocity and the interface defects in semiconductor heterostructures.

A correlation between the defect states and yellow luminescence in AlGaN/GaN heterostructures

NASA Astrophysics Data System (ADS)

Jana, Dipankar; Sharma, T. K.

2017-07-01

AlGaN/GaN heterostructures are investigated by performing complementary spectroscopic measurements under novel experimental configurations. Distinct features related to the band edge of AlGaN and GaN layers are clearly observed in surface photovoltage spectroscopy (SPS) spectra. A few more SPS features, which are associated with defects in GaN, are also identified by performing the pump-probe SPS measurements. SPS results are strongly corroborated by the complementary photoluminescence and photoluminescence excitation (PLE) measurements. A correlation between the defect assisted SPS features and yellow luminescence (YL) peak is established by performing pump-probe SPS and PLE measurements. It is found that CN-ON donor complex is responsible for the generation of YL peak in our sample. Further, the deep trap states are found to be present throughout the entire GaN epilayer. It is also noticed that the deep trap states lying at the GaN/Fe-GaN interface make a strong contribution to the YL feature. A phenomenological model is proposed to explain the intensity dependence of the YL feature and the corresponding SPS features in a pump-probe configuration, where a reasonable agreement between the numerical simulations and experimental results is achieved.

Dynamics of Defects and Dopants in Complex Systems: Si and Oxide Surfaces and Interfaces

NASA Astrophysics Data System (ADS)

Kirichenko, Taras; Yu, Decai; Banarjee, Sanjay; Hwang, Gyeong

2004-10-01

Fabrication of forthcoming nanometer scale electronic devices faces many difficulties including formation of extremely shallow and highly doped junctions. At present, ultra-low-energy ion implantation followed by high-temperature thermal annealing is most widely used to fabricate such ultra-shallow junctions. In the process, a great challenge lies in achieving precise control of redistribution and electrical activation of dopant impurities. Native defects (such as vacancies and interstitials) generated during implantation are known to be mainly responsible for the TED and also influence significantly the electrical activation/deactivation. Defect-dopant dynamics is rather well understood in crystalline Si and SiO2. However, little is known about their diffusion and annihilation (or precipitation) at the surfaces and interfaces, despite its growing importance in determining junction profiles as device dimensions get smaller. In this talk, we will present our density functional theory calculation results on the atomic and electronic structure and dynamical behavior of native defects and dopant-defect complexes in disordered/strained Si and oxide systems, such as i) clean and absorbent-modified Si(100) surface and subsurface layers, ii) amorphous-crystalline Si interfaces and iii) amorphous SiO2/Si interfaces. The fundamental understanding and data is essential in developing a comprehensive kinetic model for junction formation, which would contribute greatly in improving current process technologies.

Temperature Dependence Discontinuity in the Stability of Manganese doped Ceria Nanocrystals

DOE PAGES

Wu, Longjia; Dholabhai, Pratik; Uberuaga, Blas P.; ...

2017-01-05

CeO 2 has strong potential for chemical-looping water splitting. It has been shown that manganese doping decreases interface energies of CeO 2, allowing increased stability of high surface areas in this oxygen carrier oxide. The phenomenon is related to the segregation of Mn3+ at interfaces, which causes a measurable decrease in excess energy. Here in the present work, it is shown that, despite the stability of nanocrystals of manganese-doped CeO 2 with relation to undoped CeO 2, the effect is strongly dependent on the oxidation state of manganese, i.e., on the temperature. At temperatures below 800 °C, Mn is inmore » the 3+ valence state, and coarsening is hindered by the reduced interface energetics, showing smaller crystal sizes with increasing Mn content. At temperatures above 800 °C, Mn is reduced to its 2+ valence state, and coarsening is enhanced with increasing Mn content. Atomistic simulations show the segregation of Mn to grain boundaries is relatively insensitive to the charge state of the dopant. However, point defect modeling finds that the reduced state causes a decrease in cation vacancy concentration and an increase in cation interstitials, reducing drag forces for grain boundary mobility and increasing growth rates.« less

Modeling electronic trap state distributions in nanocrystalline anatase

NASA Astrophysics Data System (ADS)

Le, Nam; Schweigert, Igor

The charge transport properties of nanocrystalline TiO2 films, and thus the catalytic performance of devices that incorporate them, are affected strongly by the spatial and energetic distribution of localized electronic trap states. Such traps may arise from a variety of defects: Ti interstitials, O vacancies, step edges at surfaces, and grain boundaries. We have developed a procedure for applying density functional theory (DFT) and density functional tight binding (DFTB) calculations to characterize distributions of localized states arising from multiple types of defects. We have applied the procedure to investigate how the morphologies of interfaces between pairs of attached anatase nanoparticles determine the energies of trap states therein. Our results complement recent experimental findings that subtle changes in the morphology of highly porous TiO2 aerogel networks can have a dramatic effect on catalytic performance, which was attributed to changes in the distribution of trap states. This work was supported by the U.S. Naval Research Laboratory via the National Research Council and by the Office of Naval Research through the U.S. Naval Research Laboratory.

Entanglement across extended random defects in the XX spin chain

NASA Astrophysics Data System (ADS)

Juhász, Róbert

2017-08-01

We study the half-chain entanglement entropy in the ground state of the spin-1/2 XX chain across an extended random defect, where the strength of disorder decays with the distance from the interface algebraically as Δ_l∼ l-κ . In the whole regime κ≥slant 0 , the average entanglement entropy is found to increase logarithmically with the system size L as S_L≃\\frac{c_eff(κ)}{6}\\ln L+const , where the effective central charge c_eff(κ) depends on κ. In the regime κ<1/2 , where the extended defect is a relevant perturbation, the strong-disorder renormalization group method gives c_eff(κ)=(1-2κ)\\ln2 , while, in the regime κ≥slant 1/2 , where the extended defect is irrelevant in the bulk, numerical results indicate a non-zero effective central charge, which increases with κ. The variation of c_eff(κ) is thus found to be non-monotonic and discontinuous at κ=1/2 .

The influence of interfacial defects on fast charge trapping in nanocrystalline oxide-semiconductor thin film transistors

NASA Astrophysics Data System (ADS)

Kim, Taeho; Hur, Jihyun; Jeon, Sanghun

2016-05-01

Defects in oxide semiconductors not only influence the initial device performance but also affect device reliability. The front channel is the major carrier transport region during the transistor turn-on stage, therefore an understanding of defects located in the vicinity of the interface is very important. In this study, we investigated the dynamics of charge transport in a nanocrystalline hafnium-indium-zinc-oxide thin-film transistor (TFT) by short pulse I-V, transient current and 1/f noise measurement methods. We found that the fast charging behavior of the tested device stems from defects located in both the front channel and the interface, following a multi-trapping mechanism. We found that a silicon-nitride stacked hafnium-indium-zinc-oxide TFT is vulnerable to interfacial charge trapping compared with silicon-oxide counterpart, causing significant mobility degradation and threshold voltage instability. The 1/f noise measurement data indicate that the carrier transport in a silicon-nitride stacked TFT device is governed by trapping/de-trapping processes via defects in the interface, while the silicon-oxide device follows the mobility fluctuation model.

Dominance of debonding defect of CFST on PZT sensor response considering the meso-scale structure of concrete with multi-scale simulation

NASA Astrophysics Data System (ADS)

Xu, Bin; Chen, Hongbing; Mo, Y.-L.; Zhou, Tianmin

2018-07-01

Piezoelectric-lead-zirconate-titanate(PZT)-based interface debonding defects detection for concrete filled steel tubulars (CFSTs) has been proposed and validated through experiments, and numerical study on its mechanism has been carried out recently by assuming that concrete material is homogenous. However, concrete is composed of coarse and fine aggregates, mortar and interface transition zones (ITZs) and even initial defects and is a typical nonhomogeneous material and its mesoscale structure might affect the wave propagation in the concrete core of CFST members. Therefore, it is significantly important to further investigate the influence of mesoscale structure of concrete on the stress wave propagation and the response of embedded PZT sensor for the interface debonding detection. In this study, multi-physical numerical simulation on the wave propagation and embedded PZT sensor response of rectangular CFST members with numerical concrete core considering the randomness in circular aggregate distribution, and coupled with surface-mounted PZT actuator and embedded PZT sensor is carried out. The effect of randomness in the circular aggregates distribution and the existence of ITZs are discussed. Both a local stress wave propagation behavior including transmission, reflection, and diffraction at the interface between concrete core and steel tube under a pulse signal excitation and a global wave field in the cross-section of the rectangular CFST models without and with interface debonding defects under sweep frequency excitation are simulated. The sensitivity of an evaluation index based on wavelet packet analysis on the embedded PZT sensor response on the variation of mesoscale parameters of concrete core without and with different interface debonding defects under sweep frequency voltage signal is investigated in details. The results show that the effect of the interface debondings on the embedded PZT measurement is dominant when compared to the meso-scale structures of concrete core. This study verified the feasibility of the PZT based debonding detection for rectangular CFST members even the meso-scale structure of concrete core is considered.

Expanding the Bethe/Gauge dictionary

NASA Astrophysics Data System (ADS)

Bullimore, Mathew; Kim, Hee-Cheol; Lukowski, Tomasz

2017-11-01

We expand the Bethe/Gauge dictionary between the XXX Heisenberg spin chain and 2d N = (2, 2) supersymmetric gauge theories to include aspects of the algebraic Bethe ansatz. We construct the wave functions of off-shell Bethe states as orbifold defects in the A-twisted supersymmetric gauge theory and study their correlation functions. We also present an alternative description of off-shell Bethe states as boundary conditions in an effective N = 4 supersymmetric quantum mechanics. Finally, we interpret spin chain R-matrices as correlation functions of Janus interfaces for mass parameters in the supersymmetric quantum mechanics.

Comparison and characterization of different tunnel layers, suitable for passivated contact formation

NASA Astrophysics Data System (ADS)

Ling, Zhi Peng; Xin, Zheng; Ke, Cangming; Jammaal Buatis, Kitz; Duttagupta, Shubham; Lee, Jae Sung; Lai, Archon; Hsu, Adam; Rostan, Johannes; Stangl, Rolf

2017-08-01

Passivated contacts for solar cells can be realized using a variety of differently formed ultra-thin tunnel oxide layers. Assessing their interface properties is important for optimization purposes. In this work, we demonstrate the ability to measure the interface defect density distribution D it(E) and the fixed interface charge density Q f for ultra-thin passivation layers operating within the tunnel regime (<2 nm). Various promising tunnel layer candidates [i.e., wet chemically formed SiO x , UV photo-oxidized SiO x , and atomic layer deposited (ALD) AlO x ] are investigated for their potential application forming electron or hole selective tunnel layer passivated contacts. In particular, ALD AlO x is identified as a promising tunnel layer candidate for hole-extracting passivated contact formation, stemming from its high (negative) fixed interface charge density in the order of -6 × 1012 cm-2. This is an order of magnitude higher compared to wet chemically or UV photo-oxidized formed silicon oxide tunnel layers, while keeping the density of interface defect states D it at a similar level (in the order of ˜2 × 1012 cm-2 eV-1). This leads to additional field effect passivation and therefore to significantly higher measured effective carrier lifetimes (˜2 orders of magnitude). A surface recombination velocity of ˜40 cm/s has been achieved for a 1.5 nm thin ALD AlO x tunnel layer prior to capping by an additional hole transport material, like p-doped poly-Si or PEDOT:PSS.

A sharp interface model for void growth in irradiated materials

NASA Astrophysics Data System (ADS)

Hochrainer, Thomas; El-Azab, Anter

2015-03-01

A thermodynamic formalism for the interaction of point defects with free surfaces in single-component solids has been developed and applied to the problem of void growth by absorption of point defects in irradiated metals. This formalism consists of two parts, a detailed description of the dynamics of defects within the non-equilibrium thermodynamic frame, and the application of the second law of thermodynamics to provide closure relations for all kinetic equations. Enforcing the principle of non-negative entropy production showed that the description of the problem of void evolution under irradiation must include a relationship between the normal fluxes of defects into the void surface and the driving thermodynamic forces for the void surface motion; these thermodynamic forces are identified for both vacancies and interstitials and the relationships between these forces and the normal point defect fluxes are established using the concepts of transition state theory. The latter theory implies that the defect accommodation into the surface is a thermally activated process. Numerical examples are given to illustrate void growth dynamics in this new formalism and to investigate the effect of the surface energy barriers on void growth. Consequences for phase field models of void growth are discussed.

Freedom and the State: Kant on Revolution and International Interface

DTIC Science & Technology

1999-11-11

physical and psychological nature and many of them make us selfish and disagreeable beings. In fact, Kant contends that human beings are naturally...circumstances include a "defective education, bad company,... the viciousness of a natural disposition insensitive to shame,... levity and thoughtlessness...possibility of ethical action. Unless we are capable of choosing duty over inclinations, we cannot be held responsible for our actions (GMM 49). This

Advanced energy materials (Preface)

NASA Astrophysics Data System (ADS)

Titus, Elby; Ventura, João; Araújo, João Pedro; Campos Gil, João

2017-12-01

Advances in material science make it possible to fabricate the building blocks of an entirely new generation of hierarchical energy materials. Recent developments were focused on functionality and areas connecting macroscopic to atomic and nanoscale properties, where surfaces, defects, interfaces and metastable state of the materials played crucial roles. The idea is to combine both, the top-down and bottom-up approach as well as shape future materials with a blend of both the paradigms.

Transport Physics in Thin-Film Oxides: From Capacitors to Memristors1

NASA Astrophysics Data System (ADS)

Tierney, Brian; Hjalmarson, Harold; McLain, Michael; Hughart, David; Marinella, Matthew; Mamaluy, Denis; Gao, Xujiao

A physics-based model of transport mechanisms in metal-insulator-metal (M-I-M) systems is developed to explain transport through the metal-oxide interfaces and in the bulk of the insulating oxide. Interface tunneling, such as that between the metal to the conduction band or bound defect states, is accounted for by a WKB model. Our model also incorporates the evolution of the associated oxide defect chemistry. Continuum calculations are performed for both Ta2O5 M-I-M capacitors and TaOx-Based M-I-M memristors, as both devices are structurally similar and can be characterized by a common set of transport mechanisms. However, due to the electroforming process for which memristors are subjected, different transport mechanisms dominate for each type of device. Also, the effects of pulsed ionizing radiation from an external source are included in the model. It is shown that such radiation can be used to probe whether the M-I-M system is in a capacitive or memristive state. 1Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Improved Si0.5Ge0.5/Si interface quality achieved by the process of low energy hydrogen plasma cleaning and investigation of interface quality with positron annihilation spectroscopy

NASA Astrophysics Data System (ADS)

Liao, M.-H.; Chen, C.-H.

2013-04-01

The Positron Annihilation Spectra (PAS), Raman, and Photoluminescence spectroscopy reveal that Si0.5Ge0.5/Si interface quality can be significantly improved by the low energy plasma cleaning process using hydrogen. In the PAS, the particularly small value of lifetime and intensity near the Si0.5Ge0.5/Si interface in the sample with the treatment indicate that the defect concentration is successfully reduced 2.25 times, respectively. Fewer defects existed in the Si0.5Ge0.5/Si interface result in the high compressive strain about 0.36% in the top epi-Si0.5Ge0.5 layer, which can be observed in Raman spectra and stronger radiative recombination rate about 1.39 times for the infrared emission, which can be observed in the photoluminescence spectra. With better Si0.5Ge0.5/Si interface quality, the SiGe-based devices can have better optical and electrical characteristics for more applications in the industry. The PAS is also demonstrated that it is the useful methodology tool to quantify the defect information in the SiGe-based material.

Interface studies of N2 plasma-treated ZnSnO nanowire transistors using low-frequency noise measurements.

PubMed

Kim, Seongmin; Kim, Hwansoo; Janes, David B; Ju, Sanghyun

2013-08-02

Due to the large surface-to-volume ratio of nanowires, the quality of nanowire-insulator interfaces as well as the nanowire surface characteristics significantly influence the electrical characteristics of nanowire transistors (NWTs). To improve the electrical characteristics by doping or post-processing, it is important to evaluate the interface characteristics and stability of NWTs. In this study, we have synthesized ZnSnO (ZTO) nanowires using the chemical vapor deposition method, characterized the composition of ZTO nanowires using x-ray photoelectron spectroscopy, and fabricated ZTO NWTs. We have characterized the current-voltage characteristics and low-frequency noise of ZTO NWTs in order to investigate the effects of interface states on subthreshold slope (SS) and the noise before and after N2 plasma treatments. The as-fabricated device exhibited a SS of 0.29 V/dec and Hooge parameter of ~1.20 × 10(-2). Upon N2 plasma treatment with N2 gas flow rate of 40 sccm (20 sccm), the SS improved to 0.12 V/dec (0.21 V/dec) and the Hooge parameter decreased to ~4.99 × 10(-3) (8.14 × 10(-3)). The interface trap densities inferred from both SS and low-frequency noise decrease upon plasma treatment, with the highest flow rate yielding the smallest trap density. These results demonstrate that the N2 plasma treatment decreases the interface trap states and defects on ZTO nanowires, thereby enabling the fabrication of high-quality nanowire interfaces.

Topologically protected bound states in one-dimensional Floquet acoustic waveguide systems

NASA Astrophysics Data System (ADS)

Peng, Yu-Gui; Geng, Zhi-Guo; Zhu, Xue-Feng

2018-03-01

Topological manipulation of sound has recently been a hot spot in acoustics due to the fascinating property of defect immune transport. To the best of our knowledge, the studies on one-dimensional (1D) topological acoustic systems hitherto mainly focus on the case of the Su-Schrieffer-Heeger model. Here, we show that topologically protected bound states may also exist in 1D periodically modulated acoustic waveguide systems, viz., 1D Floquet topological insulators. The results show that tuning the coupling strength in a waveguide lattice could trigger topological phase transition, which gives rise to topologically protected interface states as we put together two waveguide lattices featured with different topological phases or winding numbers. However, for the combined lattice, input at the waveguides other than the interfacial ones will excite bulk states. We have further verified the robustness of interface bound states against the variation of coupling strengths between the two distinct waveguide lattices. This work extends the scope of topological acoustics and may promote potential applications for acoustic devices with topological functionalities.

Investigation on Explosive Welding of Zr53Cu35Al12 Bulk Metallic Glass with Crystalline Copper

NASA Astrophysics Data System (ADS)

Feng, Jianrui; Chen, Pengwan; Zhou, Qiang

2018-05-01

A Zr53Cu35Al12 bulk metallic glass (BMG) was welded to a crystalline Cu using explosive welding technique. The morphology and the composition of the composite were characterized using optical microscopy, scanning electron microscopy, energy-dispersive x-ray spectroscopy and transmission electron microscopy. The investigation indicated that the BMG and Cu were tightly joined together without visible defects, and a thin diffusion layer appeared at the interface. The captured jet at the end of the welding region mostly comes from the Cu side. Amorphous and partially crystallized structures have been observed within the diffusion layer, but the BMG in close proximity to the interface still retains its amorphous state. Nanoindentation tests reveal that the interface exhibits an increment in hardness compared with the matrix on both sides.

Evaluation of Surface Cleaning of Si(211) for Molecular-Beam Epitaxy Deposition of Infrared Detectors

NASA Astrophysics Data System (ADS)

Jaime-Vasquez, M.; Jacobs, R. N.; Benson, J. D.; Stoltz, A. J.; Almeida, L. A.; Bubulac, L. O.; Chen, Y.; Brill, G.

2010-07-01

We report an assessment of the reproducibility of the HF cleaning process and As passivation prior to the nucleation of ZnTe on the Si(211) surface using temperature desorption spectroscopy, ion scattering spectroscopy, and electron spectroscopy. Observations suggest full H coverage of the Si(211) surface with mostly monohydride and small amounts of dihydride states, and that F is uniformly distributed across the top layer as a physisorbed species. Variations in major contaminants are observed across the Si surface and at the CdTe-ZnTe/Si interface. Defects act as getters for impurities present on the Si surface, and some are buried under the CdTe/ZnTe heterostructure. Overall, the data show evidence of localized concentration of major impurities around defects, supporting the hypothesis of a physical model explaining the electrical activation of defects in long-wave infrared (LWIR) HgCdTe/CdTe/Si devices.

Modification of electron states in CdTe absorber due to a buffer layer in CdTe/CdS solar cells

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fedorenko, Y. G., E-mail: y.fedorenko@liverpool.ac.uk; Major, J. D.; Pressman, A.

2015-10-28

By application of the ac admittance spectroscopy method, the defect state energy distributions were determined in CdTe incorporated in thin film solar cell structures concluded on ZnO, ZnSe, and ZnS buffer layers. Together with the Mott-Schottky analysis, the results revealed a strong modification of the defect density of states and the concentration of the uncompensated acceptors as influenced by the choice of the buffer layer. In the solar cells formed on ZnSe and ZnS, the Fermi level and the energy position of the dominant deep trap levels were observed to shift closer to the midgap of CdTe, suggesting the mid-gapmore » states may act as recombination centers and impact the open-circuit voltage and the fill factor of the solar cells. For the deeper states, the broadening parameter was observed to increase, indicating fluctuations of the charge on a microscopic scale. Such changes can be attributed to the grain-boundary strain and the modification of the charge trapped at the grain-boundary interface states in polycrystalline CdTe.« less

Atomistic study on the FCC/BCC interface structure with {112}KS orientation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kang, Keonwook; Beyerlein, Irene; Han, Weizhong

2011-09-23

In this study, atomistic simulation is used to explore the atomic interface structure, the intrinsic defect network, and mechanism of twin formation from the {112}KS Cu-Nb interface. The interface structure of different material systems AI-Fe and AI-Nb are also compared with Cu-Nb interface.

Coherently coupled ZnO and VO2 interface studied by photoluminescence and electrical transport across a phase transition

NASA Astrophysics Data System (ADS)

Srivastava, Amar; Herng, T. S.; Saha, Surajit; Nina, Bao; Annadi, A.; Naomi, N.; Liu, Z. Q.; Dhar, S.; Ariando; Ding, J.; Venkatesan, T.

2012-06-01

We have investigated the photoluminescence and electrical properties of a coherently coupled interface consisting of a ZnO layer grown on top of an oriented VO2 layer on sapphire across the phase transition of VO2. The band edge and defect luminescence of the ZnO overlayer exhibit hysteresis in opposite directions induced by the phase transition of VO2. Concomitantly the phase transition of VO2 was seen to induce defects in the ZnO layer. Such coherently coupled interfaces could be of use in characterizing the stability of a variety of interfaces in situ and also for novel device application.

Structural plasticity: how intermetallics deform themselves in response to chemical pressure, and the complex structures that result.

PubMed

Berns, Veronica M; Fredrickson, Daniel C

2014-10-06

Interfaces between periodic domains play a crucial role in the properties of metallic materials, as is vividly illustrated by the way in which the familiar malleability of many metals arises from the formation and migration of dislocations. In complex intermetallics, such interfaces can occur as an integral part of the ground-state crystal structure, rather than as defects, resulting in such marvels as the NaCd2 structure (whose giant cubic unit cell contains more than 1000 atoms). However, the sources of the periodic interfaces in intermetallics remain mysterious, unlike the dislocations in simple metals, which can be associated with the exertion of physical stresses. In this Article, we propose and explore the concept of structural plasticity, the hypothesis that interfaces in complex intermetallic structures similarly result from stresses, but ones that are inherent in a defect-free parent structure, rather than being externally applied. Using DFT-chemical pressure analysis, we show how the complex structures of Ca2Ag7 (Yb2Ag7 type), Ca14Cd51 (Gd14Ag51 type), and the 1/1 Tsai-type quasicrystal approximant CaCd6 (YCd6 type) can all be traced to large negative pressures around the Ca atoms of a common progenitor structure, the CaCu5 type with its simple hexagonal 6-atom unit cell. Two structural paths are found by which the compounds provide relief to the Ca atoms' negative pressures: a Ca-rich pathway, where lower coordination numbers are achieved through defects eliminating transition metal (TM) atoms from the structure; and a TM-rich path, along which the addition of spacer Cd atoms provides the Ca coordination environments greater independence from each other as they contract. The common origins of these structures in the presence of stresses within a single parent structure highlights the diverse paths by which intermetallics can cope with competing interactions, and the role that structural plasticity may play in navigating this diversity.

On correction of model of stabilization of distribution of concentration of radiation defects in a multilayer structure with account experiment data

NASA Astrophysics Data System (ADS)

Pankratov, E. L.

2018-05-01

We introduce a model of redistribution of point radiation defects, their interaction between themselves and redistribution of their simplest complexes (divacancies and diinterstitials) in a multilayer structure. The model gives a possibility to describe qualitatively nonmonotonicity of distributions of concentrations of radiation defects on interfaces between layers of the multilayer structure. The nonmonotonicity was recently found experimentally. To take into account the nonmonotonicity we modify recently used in literature model for analysis of distribution of concentration of radiation defects. To analyze the model we used an approach of solution of boundary problems, which could be used without crosslinking of solutions on interfaces between layers of the considered multilayer structures.

Origin of the energy level alignment at organic/organic interfaces: The role of structural defects

NASA Astrophysics Data System (ADS)

Bussolotti, Fabio; Yang, Jinpeng; Hinderhofer, Alexander; Huang, Yuli; Chen, Wei; Kera, Satoshi; Wee, Andrew T. S.; Ueno, Nobuo

2014-03-01

In this paper, the electronic properties of as-deposited and N2-exposedCuPc/F16CuPc interface, a prototype system for organic photovoltaic applications, are investigated by using ultralow background, high-sensitivity photoemission spectroscopy. It is found that (i) N2 exposure significantly modifies the energy level alignment (ELA) at the interface between CuPc and F16CuPc layer and (ii) the direction of the N2-induced energy level shift of the CuPc depends on the position of the Fermi level (EF) in the CuPc highest occupied molecular orbital-lowest unoccupied molecular orbital gap of the as-deposited film. These observations are related to the changes in the density of gap states (DOGS) produced by structural imperfections in the molecular packing geometry, as introduced by the N2 penetration into the CuPc layer. This result demonstrates the key role of structure-induced DOGS in controlling the ELA at organic/organic interfaces.

Low-temperature preparation of GaN-SiO2 interfaces with low defect density. II. Remote plasma-assisted oxidation of GaN and nitrogen incorporation

NASA Astrophysics Data System (ADS)

Bae, Choelhwyi; Lucovsky, Gerald

2004-11-01

Low-temperature remote plasma-assisted oxidation and nitridation processes for interface formation and passivation have been extended from Si and SiC to GaN. The initial oxidation kinetics and chemical composition of thin interfacial oxide were determined from analysis of on-line Auger electron spectroscopy features associated with Ga, N, and O. The plasma-assisted oxidation process is self-limiting with power-law kinetics similar to those for the plasma-assisted oxidation of Si and SiC. Oxidation using O2/He plasma forms nearly pure GaOx, and oxidation using 1% N2O in N2 forms GaOxNy with small nitrogen content, ~4-7 at. %. The interface and dielectric layer quality was investigated using fabricated GaN metal-oxide-semiconductor capacitors. The lowest density of interface states was achieved with a two-step plasma-assisted oxidation and nitridation process before SiO2 deposition.

Interface Defect States and Charge Transport Properties in Low-Cost Photovoltaic Devices made from Scalable Deposition Methods

NASA Astrophysics Data System (ADS)

Marin, Andrew; Munoz-Rojas, David; Iza, Diana; Gershon, Talia; MacManus-Driscoll, Judith

2011-03-01

In-plane (parallel to the substrate) polymer diffusion at and near interfaces has significant implications for polymeric surfactants used in tertiary oil recovery, exfoliation of clay sheets in polymer nano-composites, and several other high technology applications. Here, we report a study on the in-plane diffusion of whole polymer chains confined between interfaces using fluorescence recovery after photobleaching. Adapted from quantitative biology, FRAP provides a platform to independently study the effect of temperature, molecular weight, and film thickness on in-plane diffusion of polymers confined between interfaces. Fluorescently labeled polymers were synthesized, spin coated onto quartz substrates and the self-diffusion coefficient was measured by irreversibly photobleaching fluorophores in a pre-defined pattern and monitoring recovery of fluorescence over time. Preliminary results indicate that for thick films the diffusion coefficient is consistent with bulk values. The authors would like to thank the Gates-Cambridge Trust and the International Copper Association.

Apparatus and method for characterizing thin film and interfaces using an optical heat generator and detector

DOEpatents

Maris, H.J.; Stoner, R.J.

1998-05-05

An optical heat generation and detection system generates a first non-destructive pulsed beam of electromagnetic radiation that is directed upon a sample containing at least one interface between similar or dissimilar materials. The first pulsed beam of electromagnetic radiation, a pump beam, produces a non-uniform temperature change within the sample. A second non-destructive pulsed beam of electromagnetic radiation, a probe beam, is also directed upon the sample. Physical and chemical properties of the materials, and of the interface, are measured by observing changes in a transient optical response of the sample to the probe beam, as revealed by a time dependence of changes in, by example, beam intensity, direction, or state of polarization. The system has increased sensitivity to interfacial properties including defects, contaminants, chemical reactions and delaminations, as compared to conventional non-destructive, non-contact techniques. One feature of this invention is a determination of a Kapitza resistance at the interface, and the correlation of the determined Kapitza resistance with a characteristic of the interface, such as roughness, delamination, the presence of contaminants, etc. 31 figs.

Apparatus and method for characterizing thin film and interfaces using an optical heat generator and detector

DOEpatents

Maris, Humphrey J; Stoner, Robert J

1998-01-01

An optical heat generation and detection system generates a first non-destructive pulsed beam of electromagnetic radiation that is directed upon a sample containing at least one interface between similar or dissimilar materials. The first pulsed beam of electromagnetic radiation, a pump beam (21a), produces a non-uniform temperature change within the sample. A second non-destructive pulsed beam of electromagnetic radiation, a probe beam (21b), is also directed upon the sample. Physical and chemical properties of the materials, and of the interface, are measured by observing changes in a transient optical response of the sample to the probe beam, as revealed by a time dependence of changes in, by example, beam intensity, direction, or state of polarization. The system has increased sensitivity to interfacial properties including defects, contaminants, chemical reactions and delaminations, as compared to conventional non-destructive, non-contact techniques. One feature of this invention is a determination of a Kapitza resistance at the interface, and the correlation of the determined Kapitza resistance with a characteristic of the interface, such as roughness, delamination, the presence of contaminants, etc.

Microstructure and conductance-slope of InAs/GaSb tunnel diodes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Iutzi, Ryan M., E-mail: iutzi@mit.edu; Fitzgerald, Eugene A.

2014-06-21

InAs/GaSb and similar materials systems have generated great interest as a heterojunction for tunnel field effect transistors (TFETs) due to favorable band alignment. However, little is currently understood about how such TFETs are affected by materials defects and nonidealities. We present measurements of the conductance slope for various InAs/GaSb heterojunctions via two-terminal electrical measurements, which removes three-terminal parasitics and enables direct study on the effect of microstructure on tunnelling. Using this, we can predict how subthreshold swings in TFETs can depend on microstructure. We also demonstrate growth and electrical characterization for structures grown by metalorganic chemical vapor deposition (MOCVD)—a generallymore » more scalable process compared with molecular beam epitaxy (MBE). We determine that misfit dislocations and point defects near the interface can lead to energy states in the band-gap and local band bending that result in trap-assisted leakage routes and nonuniform band alignment across the junction area that lower the steepness of the conductance slope. Despite the small lattice mismatch, misfit dislocations still form in InAs on GaSb due to relaxation as a result of large strain from intermixed compositions. This can be circumvented by growing GaSb on InAs, straining the GaSb underlayer, or lowering the InAs growth temperature in the region of the interface. The conductance slope can also be improved by annealing the samples at higher temperatures, which we believe acts to annihilate point defects and average out major fluctuations in band alignment across the interface. Using a combination of these techniques, we can greatly improve the steepness of the conductance slope which could result in steeper subthreshold swings in TFETs in the future.« less

Defects and Interfaces on PtPb Nanoplates Boost Fuel Cell Electrocatalysis.

PubMed

Sun, Yingjun; Liang, Yanxia; Luo, Mingchuan; Lv, Fan; Qin, Yingnan; Wang, Lei; Xu, Chuan; Fu, Engang; Guo, Shaojun

2018-01-01

Nanostructured Pt is the most efficient single-metal catalyst for fuel cell technology. Great efforts have been devoted to optimizing the Pt-based alloy nanocrystals with desired structure, composition, and shape for boosting the electrocatalytic activity. However, these well-known controls still show the limited ability in maximizing the Pt utilization efficiency for achieving more efficient fuel cell catalysis. Herein, a new strategy for maximizing the fuel cell catalysis by controlling/tuning the defects and interfaces of PtPb nanoplates using ion irradiation technique is reported. The defects and interfaces on PtPb nanoplates, controlled by the fluence of incident C + ions, make them exhibit the volcano-like electrocatalytic activity for methanol oxidation reaction (MOR), ethanol oxidation reaction (EOR), and oxygen reduction reaction (ORR) as a function of ion irradiation fluence. The optimized PtPb nanoplates with the mixed structure of dislocations, subgrain boundaries, and small amorphous domains are the most active for MOR, EOR, and ORR. They can also maintain high catalytic stability in acid solution. This work highlights the impact and significance of inducing/controlling the defects and interfaces on Pt-based nanocrystals toward maximizing the catalytic performance by advanced ion irradiation strategy. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Final Technical Report for DE-SC0001878 [Theory and Simulation of Defects in Oxide Materials

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chelikowsky, James R.

2014-04-14

We explored a wide variety of oxide materials and related problems, including materials at the nanoscale and generic problems associated with oxide materials such as the development of more efficient computational tools to examine these materials. We developed and implemented methods to understand the optical and structural properties of oxides. For ground state properties, our work is predominantly based on pseudopotentials and density functional theory (DFT), including new functionals and going beyond the local density approximation (LDA): LDA+U. To study excited state properties (quasiparticle and optical excitations), we use time dependent density functional theory, the GW approach, and GW plusmore » Bethe-Salpeter equation (GW-BSE) methods based on a many-body Green function approaches. Our work focused on the structural, electronic, optical and magnetic properties of defects (such as oxygen vacancies) in hafnium oxide, titanium oxide (both bulk and clusters) and related materials. We calculated the quasiparticle defect states and charge transition levels of oxygen vacancies in monoclinic hafnia. we presented a milestone G0W0 study of two of the crystalline phases of dye-sensitized TiO{sub 2} clusters. We employed hybrid density functional theory to examine the electronic structure of sexithiophene/ZnO interfaces. To identify the possible effect of epitaxial strain on stabilization of the ferromagnetic state of LaCoO{sub 3} (LCO), we compare the total energy of the magnetic and nonmagnetic states of the strained theoretical bulk structure.« less

Characterization of Defects in Scaled Mis Dielectrics with Variable Frequency Charge Pumping

NASA Astrophysics Data System (ADS)

Paulsen, Ronald Eugene

1995-01-01

Historically, the interface trap has been extensively investigated to determine the effects on device performance. Recently, much attention has been paid to trapping in near-interface oxide traps. Performance of high precision analog circuitry is affected by charge trapping in near-interface oxide traps which produces hysteresis, charge redistribution errors, and dielectric relaxation effects. In addition, the performance of low power digital circuitry, with reduced noise margins, may be drastically affected by the threshold voltage shifts associated with charge trapping in near -interface oxide traps. Since near-interface oxide traps may substantially alter the performance of devices, complete characterization of these defects is necessary. In this dissertation a new characterization technique, variable frequency charge pumping, is introduced which allows charge trapped at the interface to be distinguished from the charge trapped within the oxide. The new experimental technique is an extension of the charge pumping technique to low frequencies such that tunneling may occur from interface traps to near-interface oxide traps. A generalized charge pumping model, based on Shockley-Read-Hall statistics and trap-to-trap tunneling theory, has been developed which allows a more complete characterization of near-interface oxide traps. A pair of coupled differential equations governing the rate of change of occupied interface and near-interface oxide traps have been developed. Due to the experimental conditions in the charge pumping technique the equations may be decoupled, leading to an equation governing the rate of change of occupied interface traps and an equation governing the rate of change of occcupied near-interface oxide traps. Solving the interface trap equation and applying non-steady state charge dynamics leads to an interface trap component of the charge pumping current. In addition, solution to the near-interface oxide trap equation leads to an additional oxide trap component to the charge pumping current. Numerical simulations have been performed to support the analytical development of the generalized charge pumping model. By varying the frequency of the applied charge pumping waveform and monitoring the charge recombined per cycle, the contributions from interface traps may be separated from the contributions of the near-interface oxide traps. The generalized charge pumping model allows characterization of the density and spatial distribution of near-interface oxide traps from this variable frequency charge pumping technique. Characterization of interface and near-interface oxide trap generation has been performed on devices exposed to ionizing radiation, hot electron injection, and high -field/Fowler-Nordheim stressing. Finally, using SONOS nonvolatile memory devices, a framework has been established for experimentally determining not only the spatial distribution of near-interface oxide traps, but also the energetic distribution. An experimental approach, based on tri-level charge pumping, is discussed which allows the energetic distribution of near-interface oxide traps to be determined.

Atomic Resolution Imaging of Nanoscale Chemical Expansion in PrxCe1-xO2-δ during In Situ Heating.

PubMed

Swallow, Jessica G; Lee, Ja Kyung; Defferriere, Thomas; Hughes, Gareth M; Raja, Shilpa N; Tuller, Harry L; Warner, Jamie H; Van Vliet, Krystyn J

2018-02-27

Thin film nonstoichiometric oxides enable many high-temperature applications including solid oxide fuel cells, actuators, and catalysis. Large concentrations of point defects (particularly, oxygen vacancies) enable fast ionic conductivity or gas exchange kinetics in these materials but also manifest as coupling between lattice volume and chemical composition. This chemical expansion may be either detrimental or useful, especially in thin film devices that may exhibit enhanced performance through strain engineering or decreased operating temperatures. However, thin film nonstoichiometric oxides can differ from bulk counterparts in terms of operando defect concentrations, transport properties, and mechanical properties. Here, we present an in situ investigation of atomic-scale chemical expansion in Pr x Ce 1-x O 2-δ (PCO), a mixed ionic-electronic conducting oxide relevant to electrochemical energy conversion and high-temperature actuation. Through a combination of electron energy loss spectroscopy and transmission electron microscopy with in situ heating, we characterized chemical strains and changes in oxidation state in cross sections of PCO films grown on yttria-stabilized zirconia (YSZ) at temperatures reaching 650 °C. We quantified, both statically and dynamically, the nanoscale chemical expansion induced by changes in PCO redox state as a function of position and direction relative to the film-substrate interface. Additionally, we observed dislocations at the film-substrate interface, as well as reduced cation localization to threading defects within PCO films. These results illustrate several key aspects of atomic-scale structure and mechanical deformation in nonstoichiometric oxide films that clarify distinctions between films and bulk counterparts and that hold several implications for operando chemical expansion or "breathing" of such oxide films.

Focal cartilage defect compromises fluid-pressure dependent load support in the knee joint.

PubMed

Dabiri, Yaghoub; Li, LePing

2015-06-01

A focal cartilage defect involves tissue loss or rupture. Altered mechanics in the affected joint may play an essential role in the onset and progression of osteoarthritis. The objective of the present study was to determine the compromised load support in the human knee joint during defect progression from the cartilage surface to the cartilage-bone interface. Ten normal and defect cases were simulated with a previously tested 3D finite element model of the knee. The focal defects were considered in both condyles within high load-bearing regions. Fluid pressurization, anisotropic fibril-reinforcement, and depth-dependent mechanical properties were considered for the articular cartilages and menisci. The results showed that a small cartilage defect could cause 25% reduction in the load support of the knee joint due to a reduced capacity of fluid pressurization in the defect cartilage. A partial-thickness defect could cause a fluid pressure decrease or increase in the remaining underlying cartilage depending on the defect depth. A cartilage defect also increased the shear strain at the cartilage-bone interface, which was more significant with a full-thickness defect. The effect of cartilage defect on the fluid pressurization also depended on the defect sites and contact conditions. In conclusion, a focal cartilage defect causes a fluid-pressure dependent load reallocation and a compromised load support in the joint, which depend on the defect depth, site, and contact condition. Copyright © 2015 John Wiley & Sons, Ltd.

Synergy of elastic and inelastic energy loss on ion track formation in SrTiO 3

DOE Office of Scientific and Technical Information (OSTI.GOV)

Weber, William J.; Zarkadoula, Eva; Pakarinen, Olli H.

2015-01-12

While the interaction of energetic ions with solids is well known to result in inelastic energy loss to electrons and elastic energy loss to atomic nuclei in the solid, the coupled effects of these energy losses on defect production, nanostructure evolution and phase transformations in ionic and covalently bonded materials are complex and not well understood due to dependencies on electron-electron scattering processes, electron-phonon coupling, localized electronic excitations, diffusivity of charged defects, and solid-state radiolysis. Here we show that a colossal synergy occurs between inelastic energy loss and pre-existing atomic defects created by elastic energy loss in single crystal strontiummore » titanate (SrTiO 3), resulting in the formation of nanometer-sized amorphous tracks, but only in the narrow region with pre-existing defects. These defects locally decrease the electronic and atomic thermal conductivities and increase electron-phonon coupling, which locally increase the intensity of the thermal spike for each ion. This work identifies a major gap in understanding on the role of defects in electronic energy dissipation and electron-phonon coupling; it also provides insights for creating novel interfaces and nanostructures to functionalize thin film structures, including tunable electronic, ionic, magnetic and optical properties.« less

Oxygen-induced defects at the lead halide perovskite/graphene oxide interfaces

DOE PAGES

Acik, Muge; Park, In Kee; Koritala, Rachel E.; ...

2017-12-21

Here, graphene oxide or its reduced derivative (GO/RGO) replace metal oxides in perovskite photovoltaics to achieve energy band alignment for minimization of the energy barriers at the film interfaces allowing efficient charge transport, and eliminate stability issues. However, the power conversion efficiencies fall in a wide range (~0.6–18%). Therefore, the perovskite growth and nucleation on GO/RGO require fundamental understanding to improve device function for controlled fabrication, which remain a major challenge. We analyze the surface morphology and crystallization of the lead halide perovskites (MAPbX 3) at 20–300 °C on GO using X-ray diffraction and photoelectron spectroscopy. To determine defect mechanismsmore » and their composition, we perform in situ transmission infrared and micro Raman spectroscopy, and the cross-sectional scanning microscopy that captures interfacial imperfections with the oxygen defects. We demonstrate the oxygen-induced defects at the MAPbX 3/GO interfaces that initiate at room temperature, and occur through the nucleophilic substitution reactions. Unexpectedly, structural defects nucleate in GO forming chemically reduced GO, and modify the surface morphology that yield a poor perovskite growth. Our theoretical studies also reveal that energetically favorable, exothermic reactions between the halides of the perovskite precursors and the oxygen groups of GO generate acidic reaction by-products ( i.e. HX), that confirm the formation of oxygen-induced defects.« less

Oxygen-induced defects at the lead halide perovskite/graphene oxide interfaces

DOE Office of Scientific and Technical Information (OSTI.GOV)

Acik, Muge; Park, In Kee; Koritala, Rachel E.

Here, graphene oxide or its reduced derivative (GO/RGO) replace metal oxides in perovskite photovoltaics to achieve energy band alignment for minimization of the energy barriers at the film interfaces allowing efficient charge transport, and eliminate stability issues. However, the power conversion efficiencies fall in a wide range (~0.6–18%). Therefore, the perovskite growth and nucleation on GO/RGO require fundamental understanding to improve device function for controlled fabrication, which remain a major challenge. We analyze the surface morphology and crystallization of the lead halide perovskites (MAPbX 3) at 20–300 °C on GO using X-ray diffraction and photoelectron spectroscopy. To determine defect mechanismsmore » and their composition, we perform in situ transmission infrared and micro Raman spectroscopy, and the cross-sectional scanning microscopy that captures interfacial imperfections with the oxygen defects. We demonstrate the oxygen-induced defects at the MAPbX 3/GO interfaces that initiate at room temperature, and occur through the nucleophilic substitution reactions. Unexpectedly, structural defects nucleate in GO forming chemically reduced GO, and modify the surface morphology that yield a poor perovskite growth. Our theoretical studies also reveal that energetically favorable, exothermic reactions between the halides of the perovskite precursors and the oxygen groups of GO generate acidic reaction by-products ( i.e. HX), that confirm the formation of oxygen-induced defects.« less

Memristive behavior of the SnO2/TiO2 interface deposited by sol-gel

NASA Astrophysics Data System (ADS)

Boratto, Miguel H.; Ramos, Roberto A.; Congiu, Mirko; Graeff, Carlos F. O.; Scalvi, Luis V. A.

2017-07-01

A novel and cheap Resistive Random Access Memory (RRAM) device is proposed within this work, based on the interface between antimony doped Tin Oxide (4%at Sb:SnO2) and Titanium Oxide (TiO2) thin films, entirely prepared through a low-temperature sol-gel process. The device was fabricated on glass slides using evaporated aluminum electrodes. Typical bipolar memristive behavior under cyclic voltage sweeping and square wave voltages, with well-defined high and low resistance states (HRS and LRS), and set and reset voltages are shown in our samples. The switching mechanism, explained by charges trapping/de-trapping by defects in the SnO2/TiO2 interface, is mainly driven by the external electric field. The calculated on/off ratio was about 8 × 102 in best conditions with good reproducibility over repeated measurement cycles under cyclic voltammetry and about 102 under applied square wave voltage.

Post deposition annealing effect on the properties of Al2O3/InP interface

NASA Astrophysics Data System (ADS)

Kim, Hogyoung; Kim, Dong Ha; Choi, Byung Joon

2018-02-01

Post deposition in-situ annealing effect on the interfacial and electrical properties of Au/Al2O3/n-InP junctions were investigated. With increasing the annealing time, both the barrier height and ideality factor changed slightly but the series resistance decreased significantly. Photoluminescence (PL) measurements showed that the intensities of both the near band edge (NBE) emission from InP and defect-related bands (DBs) from Al2O3 decreased with 30 min annealing. With increasing the annealing time, the diffusion of oxygen (indium) atoms into Al2O3/InP interface (into Al2O3 layer) occurred more significantly, giving rise to the increase of the interface state density. Therefore, the out-diffusion of oxygen atoms from Al2O3 during the annealing process should be controlled carefully to optimize the Al2O3/InP based devices.

Low-temperature diffusion assisted by femtosecond laser-induced modifications at Ni/SiC interface

NASA Astrophysics Data System (ADS)

Okada, Tatsuya; Tomita, Takuro; Ueki, Tomoyuki; Hashimoto, Takuya; Kawakami, Hiroki; Fuchikami, Yuki; Hisazawa, Hiromu; Tanaka, Yasuhiro

2018-01-01

We investigated low-temperature diffusion at the Ni/SiC interface with the assistance of femtosecond laser-induced modifications. Cross sections of the laser-irradiated lines of two different pulse energies — 0.84 and 0.60 J/cm2 in laser fluence — were compared before and after annealing at 673 K. At the laser fluence of 0.60 J/cm2, a single flat Ni-based particle was formed at the interface after annealing. The SiC crystal under the particle was defect-free. The present results suggest the potential application of femtosecond laser-induced modifications to the low-temperature fabrication of contacts at the interface without introducing crystal defects, e.g., dislocations and stacking faults, in SiC.

Different matrix evaluation for the bone regeneration of rats' femours using time domain optical coherence tomography

NASA Astrophysics Data System (ADS)

Rusu, Laura-Cristina; Negrutiu, Meda Lavinia; Sinescu, Cosmin; Hoinoiu, Bogdan; Zaharia, Cristian; Ardelean, Lavinia; Duma, Virgil-Florin; Podoleanu, Adrian G.

2014-01-01

The osteoconductive materials are important in bone regeneration procedures. Three dimensional (3D) reconstructions were obtained from the analysis. The aim of this study is to investigate the interface between the femur rat bone and the new bone that is obtained using a method of tissue engineering that is based on two artificial matrixes inserted in previously artificially induced defects. For this study, under strict supervision 20 rats were used in conformity with ethical procedures. In all the femurs a round defect was induced by drilling with a 1 mm spherical Co-Cr surgical drill. The matrixes used were IngeniOss (for ten samples) and 4Bone(for the other ten samples). These materials were inserted into the induced defects. The femurs were investigated at 1 month, after the surgical procedures. The interfaces were examined using Time Domain (TD) Optical Coherence Tomography (OCT) combined with Confocal Microscopy (CM). The scanning procedure is similar to that used in any CM, where the fast scanning is en-face (line rate) and the scanning in depth is much slower (at the frame rate). The optical configuration uses two single mode directional couplers with a superluminiscent diode as the source centered at 1300 nm. The results showed open interfaces due to the insufficient healing process, as well as closed interfaces due to a new bone formation inside the defect. The conclusion of this study is that TD-OCT can act as a valuable tool in the investigation of the interface between the old bone and the one that has been newly created due to the osteoinductive process. The TD-OCT has proven a valuable tool for the non-invasive evaluation of the matrix bone interfaces.

Vitreoretinal Interface Characteristics in Eyes with Idiopathic Macular Holes: Qualitative and Quantitative Analysis.

PubMed

Seyhan Karatepe, Arzu; Menteş, Jale; Erakgün, E Tansu; Afrashi, Filiz; Nalçacı, Serhad; Akkın, Cezmi; Ateş, Yeşim

2018-04-01

To determine the qualitative and quantitative vitreoretinal interface characteristics with spectral domain optical coherence tomography (SD-OCT) in eyes with macular hole (MH) and investigate their relation with best corrected visual acuity (BCVA) and MH duration. Sixty-one eyes of 46 consecutive patients diagnosed with idiopathic MH were included in the study. The mean age of the patients was 66.7±7.5 (51-79) years. Complete ophthalmologic examination and SD-OCT examination were performed in all eyes and MH stages were determined according to SD-OCT findings. Qualitative characteristics of the vitreoretinal interface were investigated, including vitreomacular traction, vitreopapillary traction, maculopapillary traction, vitreoschisis, intraretinal cyst, presence of epiretinal membrane, and the integrity of the photoreceptor inner segment-outer segment junction (IS/OS) and external limiting membrane (ELM). In addition, MH diameter, MH base diameter (MHBD), ELM defect diameter, IS/OS defect diameter, and MH height were quantitatively measured and the MH index was calculated. Out of 61 eyes, 9.8% were classified as stage 1a, 19.7% as stage 1b, 18% as stage 2, 23% as stage 3, and 29.5% as stage 4. Mean BCVA was 0.28±0.24 (1 mps-1.0) Snellen and MH duration was 10.08±18.6 (1-108) months. The most common interface characteristics associated with MH were determined as intraretinal cyst (91.8%), IS/OS defect (78.7%) and ELM defect (63.9%). Duration and stage of MH were inversely proportional to BCVA but directly proportional to the presence and diameter of IS/OS and ELM defects. BCVA was significantly lower in eyes with IS/OS and ELM defects (p<0.0001; p<0.0001 Mann-Whitney U test). We determined that the most important factors affecting BCVA in cases with idiopathic MH were MH stage, MH duration, MHBD, and the presence and diameter of IS/OS and ELM defects, which suggests that these parameters should be considered while making decisions about prognosis and treatment.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Takabe, Ryota; Du, Weijie; Takeuchi, Hiroki

Undoped n-type BaSi{sub 2} films were grown on Si(111) by molecular beam epitaxy, and the valence band (VB) offset at the interface between the BaSi{sub 2} and its native oxide was measured by hard x-ray photoelectron spectroscopy (HAXPES) at room temperature. HAXPES enabled us to investigate the electronic states of the buried BaSi{sub 2} layer non-destructively thanks to its large analysis depth. We performed the depth-analysis by varying the take-off angle (TOA) of photoelectrons as 15°, 30°, and 90° with respect to the sample surface and succeeded to obtain the VB spectra of the BaSi{sub 2} and the native oxidemore » separately. The VB maximum was located at −1.0 eV from the Fermi energy for the BaSi{sub 2} and −4.9 eV for the native oxide. We found that the band bending did not occur near the native oxide/BaSi{sub 2} interface. This result was clarified by the fact that the core-level emission peaks did not shift regardless of TOA (i.e., analysis depth). Thus, the barrier height of the native oxide for the minority-carriers in the undoped n-BaSi{sub 2} (holes) was determined to be 3.9 eV. No band bending in the BaSi{sub 2} close to the interface also suggests that the large minority-carrier lifetime in undoped n-BaSi{sub 2} films capped with native oxide is attributed not to the band bending in the BaSi{sub 2}, which pushes away photogenerated minority carriers from the defective surface region, but to the decrease of defective states by the native oxide.« less

Two Stages of Surface-Defect Formation in a MOS Structure under Low-Dose Rate Gamma Irradiation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Popov, V. D., E-mail: wdpopov@mail.ru

2016-03-15

The results of an experimental study of how surface defects are formed at the Si–SiO{sub 2} interface at γ-radiation dose rates of P = 0.1 and 1.0 rad/s are reported. It is found that the surface defects are formed in two stages. The defect-formation mechanisms are analyzed.

Effect of interfacial composition on Ag-based Ohmic contact of GaN-based vertical light emitting diodes

NASA Astrophysics Data System (ADS)

Wu, Ning; Xiong, Zhihua; Qin, Zhenzhen

2018-02-01

By investigating the effect of a defective interface structure on Ag-based Ohmic contact of GaN-based vertical light-emitting diodes, we found a direct relationship between the interfacial composition and the Schottky barrier height of the Ag(111)/GaN(0001) interface. It was demonstrated that the Schottky barrier height of a defect-free Ag(111)/GaN(0001) interface was 2.221 eV, and it would be dramatically decreased to 0.375 eV with the introduction of one Ni atom and one Ga vacancy at the interface structure. It was found that the tunability of the Schottky barrier height can be attributed to charge accumulations around the interfacial defective regions and an unpinning of the Fermi level, which explains the experimental phenomenon of Ni-assisted annealing improving the p-type Ohmic contact characteristic. Lastly, we propose a new method of using Cu as an assisted metal to realize a novel Ag-based Ohmic contact. These results provide a guideline for the fabrication of high-quality Ag-based Ohmic contact of GaN-based vertical light-emitting diodes.

Tracking ion irradiation effects using buried interface devices

NASA Astrophysics Data System (ADS)

Cutshall, D. B.; Kulkarni, D. D.; Miller, A. J.; Harriss, J. E.; Harrell, W. R.; Sosolik, C. E.

2018-05-01

We discuss how a buried interface device, specifically a metal-oxide-semiconductor (MOS) capacitor, can be utilized to track effects of ion irradiation on insulators. We show that the exposure of oxides within unfinished capacitor devices to ions can lead to significant changes in the capacitance of the finished devices. For multicharged ions, these capacitive effects can be traced to defect production within the oxide and ultimately point to a role for charge-dependent energy loss. In particular, we attribute the stretchout of the capacitance-voltage curves of MOS devices that include an irradiated oxide to the ion irradiation. The stretchout shows a power law dependence on the multicharged ion charge state (Q) that is similar to that observed for multicharged ion energy loss in other systems.

Size and location of defects at the coupling interface affect lithotripter performance.

PubMed

Li, Guangyan; Williams, James C; Pishchalnikov, Yuri A; Liu, Ziyue; McAteer, James A

2012-12-01

Study Type--Therapy (case series) Level of Evidence 4. What's known on the subject? and What does the study add? In shock wave lithotripsy air pockets tend to get caught between the therapy head of the lithotripter and the skin of the patient. Defects at the coupling interface hinder the transmission of shock wave energy into the body, reducing the effectiveness of treatment. This in vitro study shows that ineffective coupling not only blocks the transmission of acoustic pulses but also alters the properties of shock waves involved in the mechanisms of stone breakage, with the effect dependent on the size and location of defects at the coupling interface. • To determine how the size and location of coupling defects caught between the therapy head of a lithotripter and the skin of a surrogate patient (i.e. the acoustic window of a test chamber) affect the features of shock waves responsible for stone breakage. • Model defects were placed in the coupling gel between the therapy head of a Dornier Compact-S electromagnetic lithotripter (Dornier MedTech, Kennesaw, GA, USA) and the Mylar (biaxially oriented polyethylene terephthalate) (DuPont Teijin Films, Chester, VA, USA) window of a water-filled coupling test system. • A fibre-optic probe hydrophone was used to measure acoustic pressures and map the lateral dimensions of the focal zone of the lithotripter. • The effect of coupling conditions on stone breakage was assessed using gypsum model stones. • Stone breakage decreased in proportion to the area of the coupling defect; a centrally located defect blocking only 18% of the transmission area reduced stone breakage by an average of almost 30%. • The effect on stone breakage was greater for defects located on-axis and decreased as the defect was moved laterally; an 18% defect located near the periphery of the coupling window (2.0 cm off-axis) reduced stone breakage by only ~15% compared to when coupling was completely unobstructed. • Defects centred within the coupling window acted to narrow the focal width of the lithotripter; an 8.2% defect reduced the focal width ~30% compared to no obstruction (4.4 mm vs 6.5 mm). • Coupling defects located slightly off centre disrupted the symmetry of the acoustic field; an 18% defect positioned 1.0 cm off-axis shifted the focus of maximum positive pressure ~1.0 mm laterally. • Defects on and off-axis imposed a significant reduction in the energy density of shock waves across the focal zone. • In addition to blocking the transmission of shock-wave energy, coupling defects also disrupt the properties of shock waves that play a role in stone breakage, including the focal width of the lithotripter and the symmetry of the acoustic field • The effect is dependent on the size and location of defects, with defects near the centre of the coupling window having the greatest effect. • These data emphasize the importance of eliminating air pockets from the coupling interface, particularly defects located near the centre of the coupling window. © 2012 BJU INTERNATIONAL.

Defects in paramagnetic Co-doped ZnO films studied by transmission electron microscopy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kovacs, Andras; Ney, A.; Duchamp, Martial

2013-12-23

We have studied planar defects in epitaxial Co:ZnO dilute magnetic semiconductor thin films deposited on c-plane sapphire (Al2O3) and the Co:ZnO/Al2O3 interface structure at atomic resolution using aberration-corrected transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS). Comparing Co:ZnO samples deposited by pulsed laser deposition and reactive magnetron sputtering, both exhibit extrinsic stacking faults, incoherent interface structures, and compositional variations within the first 3-4 Co:ZnO layers at the interface.. In addition, we have measured the local strain which reveals the lattice distortion around the stacking faults.

Mechanical evaluation of a tissue-engineered zone of calcification in a bone–hydrogel osteochondral construct

PubMed Central

Hollenstein, Jérôme; Terrier, Alexandre; Cory, Esther; Chen, Albert C.; Sah, Robert L.; Pioletti, Dominique P.

2016-01-01

The objective of this study was to test the hypothesis that mechanical properties of artificial osteochondral constructs can be improved by a tissue-engineered zone of calcification (teZCC) at the bone–hydrogel interface. Experimental push-off tests were performed on osteochondral constructs with or without a teZCC. In parallel, a numerical model of the osteochondral defect treatment was developed and validated against experimental results. Experimental results showed that the shear strength at the bone–hydrogel interface increased by 100% with the teZCC. Numerical predictions of the osteochondral defect treatment showed that the shear stress at the bone–hydrogel interface was reduced with the teZCC. We conclude that a teZCC in osteochondral constructs can provide two improvements. First, it increases the strength of the bone–hydrogel interface and second, it reduces the stress at this interface. PMID:23706035

Radiation response of oxide-dispersion-strengthened alloy MA956 after self-ion irradiation

NASA Astrophysics Data System (ADS)

Chen, Tianyi; Kim, Hyosim; Gigax, Jonathan G.; Chen, Di; Wei, Chao-Chen; Garner, F. A.; Shao, Lin

2017-10-01

We studied the radiation-induced microstructural evolution of an oxide-dispersion-strengthened (ODS) ferritic alloy, MA956, to 180 dpa using 3.5 MeV Fe2+ ions. Post-irradiation examination showed that voids formed rather early and almost exclusively at the particle-matrix interfaces. Surprisingly, voids formed even in the injected interstitial zone. Comparisons with studies on other ODS alloys with smaller and largely coherent dispersoids irradiated at similar conditions revealed that the larger and not completely coherent oxide particles in MA956 serve as defect collectors which promote nucleation of voids at their interface. The interface configuration, which is related to particle type, crystal structure and size, is one of the important factors determining the defect-sink properties of particle-matrix interfaces.

Real-space mapping of electronic orbitals.

PubMed

Löffler, Stefan; Bugnet, Matthieu; Gauquelin, Nicolas; Lazar, Sorin; Assmann, Elias; Held, Karsten; Botton, Gianluigi A; Schattschneider, Peter

2017-06-01

Electronic states are responsible for most material properties, including chemical bonds, electrical and thermal conductivity, as well as optical and magnetic properties. Experimentally, however, they remain mostly elusive. Here, we report the real-space mapping of selected transitions between p and d states on the Ångström scale in bulk rutile (TiO 2 ) using electron energy-loss spectrometry (EELS), revealing information on individual bonds between atoms. On the one hand, this enables the experimental verification of theoretical predictions about electronic states. On the other hand, it paves the way for directly investigating electronic states under conditions that are at the limit of the current capabilities of numerical simulations such as, e.g., the electronic states at defects, interfaces, and quantum dots. Copyright © 2017 Elsevier B.V. All rights reserved.

Abnormal bias dependence of magnetoresistance in CoFeB/MgO/Si spin-injection tunnel contacts

DOE Office of Scientific and Technical Information (OSTI.GOV)

Park, June-Young; Park, Byong-Guk, E-mail: bgpark@kaist.ac.kr; Baek, Seung-heon Chris

We report a strong bias voltage dependence of magnetoresistance (MR) in CoFeB/MgO/Si spin-injection tunnel contacts using the three-terminal Hanle geometry. When a bias voltage is relatively large, the MR is composed of two characteristic signals: a conventional Hanle signal observed at a low magnetic field, which is due to the precession of injected spins, and another signal originating from the rotation of the magnetization at a larger magnetic field. In contrast, for a small bias voltage, additional signals appear at a wide range of magnetic fields, which occasionally overwhelms the conventional Hanle signals. Because the additional signals are pronounced atmore » a low bias and are significantly reduced by annealing at moderate temperatures, they can be attributed to multi-step tunneling via defect states at the interfaces or tunnel barrier. Our results demonstrate that the spin injection signal caused by the defect states can be evaluated by its bias voltage dependence.« less

Slow-muon study of quaternary solar-cell materials: Single layers and p -n junctions

NASA Astrophysics Data System (ADS)

Alberto, H. V.; Vilão, R. C.; Vieira, R. B. L.; Gil, J. M.; Weidinger, A.; Sousa, M. G.; Teixeira, J. P.; da Cunha, A. F.; Leitão, J. P.; Salomé, P. M. P.; Fernandes, P. A.; Törndahl, T.; Prokscha, T.; Suter, A.; Salman, Z.

2018-02-01

Thin films and p -n junctions for solar cells based on the absorber materials Cu (In ,G a ) Se2 and Cu2ZnSnS4 were investigated as a function of depth using implanted low energy muons. The most significant result is a clear decrease of the formation probability of the Mu+ state at the heterojunction interface as well as at the surface of the Cu (In ,G a ) Se2 film. This reduction is attributed to a reduced bonding reaction of the muon in the absorber defect layer at its surface. In addition, the activation energies for the conversion from a muon in an atomiclike configuration to a anion-bound position are determined from temperature-dependence measurements. It is concluded that the muon probe provides a measurement of the effective surface defect layer width, both at the heterojunctions and at the films. The CIGS surface defect layer is crucial for solar-cell electrical performance and additional information can be used for further optimizations of the surface.

Vibrational characteristics of FRP-bonded concrete interfacial defects in a low frequency regime

NASA Astrophysics Data System (ADS)

Cheng, Tin Kei; Lau, Denvid

2014-04-01

As externally bonded fiber-reinforced polymer (FRP) is a critical load-bearing component of strengthened or retrofitted civil infrastructures, the betterment of structural health monitoring (SHM) methodology for such composites is imperative. Henceforth the vibrational characteristics of near surface interfacial defects involving delamination and trapped air pockets at the FRP-concrete interface are investigated in this study using a finite element approach. Intuitively, due to its lower interfacial stiffness compared with an intact interface, a damaged region is expected to have a set of resonance frequencies different from an intact region when excited by acoustic waves. It has been observed that, when excited acoustically, both the vibrational amplitudes and frequency peaks in the response spectrum of the defects demonstrate a significant deviation from an intact FRP-bonded region. For a thin sheet of FRP bonded to concrete with sizable interfacial defects, the fundamental mode under free vibration is shown to be relatively low, in the order of kHz. Due to the low resonance frequencies of the defects, the use of low-cost equipment for interfacial defect detection via response spectrum analysis is highly feasible.

Spatially Resolved Nano-Scale Characterization of Electronic States in SrTiO3(001) Surfaces by STM/STS

NASA Astrophysics Data System (ADS)

Iwaya, Katsuya; Ohsawa, Takeo; Shimizu, Ryota; Hashizume, Tomihiro; Hitosugi, Taro

2012-02-01

We have performed low temperature scanning tunneling microscopy/spectroscopy (STM/STS) measurements on TiO2-terminated SrTiO3(001) thin film surfaces. The conductance map exhibited electronic modulations that were completely different from the surface structure. We also found that the electronic modulations were strongly dependent on temperature and the density of atomic defects associated with oxygen vacancies. These results suggest the existence of strongly correlated two-dimensional electronic states near the SrTiO3 surface, implying the importance of electron correlation at the interfaces of SrTiO3-related heterostructures.

Portable Handheld Optical Window Inspection Device

NASA Technical Reports Server (NTRS)

Ihlefeld, Curtis; Dokos, Adam; Burns, Bradley

2010-01-01

The Portable Handheld Optical Window Inspection Device (PHOWID) is a measurement system for imaging small defects (scratches, pits, micrometeor impacts, and the like) in the field. Designed primarily for window inspection, PHOWID attaches to a smooth surface with suction cups, and raster scans a small area with an optical pen in order to provide a three-dimensional image of the defect. PHOWID consists of a graphical user interface, motor control subsystem, scanning head, and interface electronics, as well as an integrated camera and user display that allows a user to locate minute defects before scanning. Noise levels are on the order of 60 in. (1.5 m). PHOWID allows field measurement of defects that are usually done in the lab. It is small, light, and attaches directly to the test article in any orientation up to vertical. An operator can scan a defect and get useful engineering data in a matter of minutes. There is no need to make a mold impression for later lab analysis.

Defect Characterization in SiGe/SOI Epitaxial Semiconductors by Positron Annihilation

PubMed Central

2010-01-01

The potential of positron annihilation spectroscopy (PAS) for defect characterization at the atomic scale in semiconductors has been demonstrated in thin multilayer structures of SiGe (50 nm) grown on UTB (ultra-thin body) SOI (silicon-on-insulator). A slow positron beam was used to probe the defect profile. The SiO2/Si interface in the UTB-SOI was well characterized, and a good estimation of its depth has been obtained. The chemical analysis indicates that the interface does not contain defects, but only strongly localized charged centers. In order to promote the relaxation, the samples have been submitted to a post-growth annealing treatment in vacuum. After this treatment, it was possible to observe the modifications of the defect structure of the relaxed film. Chemical analysis of the SiGe layers suggests a prevalent trapping site surrounded by germanium atoms, presumably Si vacancies associated with misfit dislocations and threading dislocations in the SiGe films. PMID:21170391

Characterization of charge trapping phenomena at III-N/dielectric interfaces

NASA Astrophysics Data System (ADS)

Stradiotto, Roberta; Pobegen, Gregor; Ostermaier, Clemens; Grasser, Tibor

2016-11-01

Charge trapping related phenomena are among the most serious reliability issues in GaN/AlGaN MIS-HEMTs technology. Today, many research efforts are undertaken to investigate and identify the defects responsible for device degradation. This work focuses on the trap sites located close to the interface with the dielectric, which are responsible for large voltage drifts in on-state conditions. We study the response of GaN/AlGaN/SiN systems to small and large signal excitation. Measurements performed with a lock-in amplifier enable us to deeply understand the dynamic behavior because of the improved time resolution and the versatility of the instrument. We investigate the frequency dispersion and the hysteresis of these devices and conclude that direct analysis of impedance characteristics is not sufficient to extract information about the interface trap response. We propose a methodology to study trapping phenomena based on transient measurement analysis, describing the approximations made and their effect on the accuracy of the result. Results on MIS test structures confirm the existence of a broad distribution of trap states. Capture time constants are found to be uniformly distributed in the experimental time window between 50 μs and 100 s.

First-principles study of point defects at a semicoherent interface

DOE PAGES

Metsanurk, E.; Tamm, A.; Caro, A.; ...

2014-12-19

Most of the atomistic modeling of semicoherent metal-metal interfaces has so far been based on the use of semiempirical interatomic potentials. Here, we show that key conclusions drawn from previous studies are in contradiction with more precise ab-initio calculations. In particular we find that single point defects do not delocalize, but remain compact near the interfacial plane in Cu-Nb multilayers. Lastly, we give a simple qualitative explanation for this difference on the basis of the well known limited transferability of empirical potentials.

Design Strategies for High-Efficiency CdTe Solar Cells

NASA Astrophysics Data System (ADS)

Song, Tao

With continuous technology advances over the past years, CdTe solar cells have surged to be a leading contributor in thin-film photovoltaic (PV) field. While empirical material and device optimization has led to considerable progress, further device optimization requires accurate device models that are able to provide an in-depth understanding of CdTe device physics. Consequently, this thesis is intended to develop a comprehensive model system for high-efficiency CdTe devices through applying basic design principles of solar cells with numerical modeling and comparing results with experimental CdTe devices. The CdTe absorber is central to cell performance. Numerical simulation has shown the feasibility of high energy-conversion efficiency, which requires both high carrier density and long minority carrier lifetime. As the minority carrier lifetime increases, the carrier recombination at the back surface becomes a limitation for cell performance with absorber thickness < 3 microm. Hence, either a thicker absorber or an appropriate back-surface-field layer is a requisite for reducing the back-surface recombination. When integrating layers into devices, more careful design of interfaces is needed. One consideration is the emitter/absorber interface. It is shown that a positive conduction-band offset DeltaEC ("spike") at the interface is beneficial to cell performance, since it can induce a large valence-band bending which suppresses the hole injection near the interface for the electron-hole recombination, but too large a spike is detrimental to photocurrent transport. In a heterojunction device with many defects at the emitter/absorber interface (high SIF), a thin and highly-doped emitter can induce strong absorber inversion and hence help maintain good cell performance. Performance losses from acceptor-type interface defects can be significant when interface defect states are located near mid-gap energies. In terms of specific emitter materials, the calculations suggest that the (Mg,Zn)O alloy with 20% Mg, or a similar type-I heterojunction partner with moderate DeltaE C (e.g., Cd(S,O) or (Cd,Mg)Te with appropriate oxygen or magnesium ratios) should yield higher voltages and would therefore be better candidates for the CdTe-cell emitter. The CdTe/substrate interface is also of great importance, particularly in the growth of epitaxial monocrystalline CdTe cells. Several substrate materials have been discussed and all have challenges. These have generally been addressed through the addition of intermediate layers between the substrate and CdTe absorber. InSb is an attractive substrate choice for CdTe devices, because it has a close lattice match with CdTe, it has low resistivity, and it is easy to contact. However, the valence-band alignment between InSb and p-type CdTe, which can both impede hole current and enhance forward electron current, is not favorable. In addition, the CdTe/back contact interface plays a significant role in carrier transport for conventional polycrystalline thin-film CdTe devices. A significant back-contact barrier φb caused by metallic contact with low work function can block hole transport and enhance the forward current and thus result in a reduced VOC, particularly with fully-depleted CdTe devices. A buffer contact layer between CdTe absorber and metallic contact is strongly needed to mitigate this detrimental impact. The simulation has shown that a thin tellurium (Te) buffer as well as a highly doped p-type CdTe layer can assume such a role by reducing the downward valence-band bending caused by large φb and hence enhancing the extraction of the charge carriers. Finally, experimental CdTe cells are discussed in parallel with the simulation results to identify limiting mechanisms and give guidance for future efficiency improvement. For the monocrystalline CdTe cells made at NREL, it is found that the sputter damage causing large numbers of defect states near the Cd(S,O)/CdTe interface plays an important role in limiting cell performance, particularly for cells with low oxygen Cd(

Novel wavelet threshold denoising method in axle press-fit zone ultrasonic detection

NASA Astrophysics Data System (ADS)

Peng, Chaoyong; Gao, Xiaorong; Peng, Jianping; Wang, Ai

2017-02-01

Axles are important part of railway locomotives and vehicles. Periodic ultrasonic inspection of axles can effectively detect and monitor axle fatigue cracks. However, in the axle press-fit zone, the complex interface contact condition reduces the signal-noise ratio (SNR). Therefore, the probability of false positives and false negatives increases. In this work, a novel wavelet threshold function is created to remove noise and suppress press-fit interface echoes in axle ultrasonic defect detection. The novel wavelet threshold function with two variables is designed to ensure the precision of optimum searching process. Based on the positive correlation between the correlation coefficient and SNR and with the experiment phenomenon that the defect and the press-fit interface echo have different axle-circumferential correlation characteristics, a discrete optimum searching process for two undetermined variables in novel wavelet threshold function is conducted. The performance of the proposed method is assessed by comparing it with traditional threshold methods using real data. The statistic results of the amplitude and the peak SNR of defect echoes show that the proposed wavelet threshold denoising method not only maintains the amplitude of defect echoes but also has a higher peak SNR.

Early stages of plasma induced nitridation of Si (111) surface and study of interfacial band alignment

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shetty, Satish; Shivaprasad, S. M., E-mail: smsprasad@jncasr.ac.in

2016-02-07

We report here a systematic study of the nitridation of the Si (111) surface by nitrogen plasma exposure. The surface and interface chemical composition and surface morphology are investigated by using RHEED, X-ray photoelectron spectroscopy, and atomic force microscopy (AFM). At the initial stage of nitridation two superstructures—“8 × 8” and “8/3 × 8/3”—form, and further nitridation leads to 1 × 1 stoichiometric silicon nitride. The interface is seen to have the Si{sup 1+} and Si{sup 3+} states of silicon bonding with nitrogen, which suggests an atomically abrupt and defect-free interface. The initial single crystalline silicon nitride layers are seen to become amorphous at higher thicknesses.more » The AFM image shows that the nitride nucleates at interfacial dislocations that are connected by sub-stoichiometric 2D-nitride layers, which agglomerate to form thick overlayers. The electrical properties of the interface yield a valence band offset that saturates at 1.9 eV and conduction band offset at 2.3 eV due to the evolution of the sub-stoichiometric interface and band bending.« less

Effects of crystallization interfaces on irradiated ferroelectric thin films

NASA Astrophysics Data System (ADS)

Brewer, S. J.; Williams, S. C.; Cress, C. D.; Bassiri-Gharb, N.

2017-11-01

This work investigates the role of crystallization interfaces and chemical heterogeneity in the radiation tolerance of chemical solution-deposited lead zirconate titanate (PZT) thin films. Two sets of PZT thin films were fabricated with crystallization performed at (i) every deposited layer or (ii) every three layers. The films were exposed to a range of 60Co gamma radiation doses, between 0.2 and 20 Mrad, and their functional response was compared before and after irradiation. The observed trends indicate enhancements of dielectric, ferroelectric, and piezoelectric responses at low radiation doses and degradation of the same at higher doses. Response enhancements are expected to result from low-dose (≤2 Mrad), ionizing radiation-induced charging of internal interfaces—an effect that results in neutralization of pre-existing internal bias in the samples. At higher radiation doses (>2 Mrad), accumulation and self-ordering of radiation-modified, mobile, oxygen vacancy-related defects contribute to degradation of dielectric, ferroelectric, and piezoelectric properties, exacerbated in the samples with more crystallization layers, potentially due to increased defect accumulation at these internal interfaces. These results suggest that the interaction between radiation and crystallization interfaces is multifaceted—the effects of ionization, domain wall motion, point defect mobility, and microstructure are considered.

Interfacial micropore defect formation in PEDOT:PSS-Si hybrid solar cells probed by TOF-SIMS 3D chemical imaging.

PubMed

Thomas, Joseph P; Zhao, Liyan; Abd-Ellah, Marwa; Heinig, Nina F; Leung, K T

2013-07-16

Conducting p-type polymer layers on n-type Si have been widely studied for the fabrication of cost-effective hybrid solar cells. In this work, time-of-flight secondary ion mass spectrometry (TOF-SIMS) is used to provide three-dimensional chemical imaging of the interface between poly(3,4-ethylene-dioxythiophene):polystyrenesulfonate (PEDOT:PSS) and SiOx/Si in a hybrid solar cell. To minimize structural damage to the polymer layer, an Ar cluster sputtering source is used for depth profiling. The present result shows the formation of micropore defects in the interface region of the PEDOT:PSS layer on the SiOx/Si substrate. This interfacial micropore defect formation becomes more prominent with increasing thickness of the native oxide layer, which is a key device parameter that greatly affects the hybrid solar cell performance. Three-dimensional chemical imaging coupled with Ar cluster ion sputtering has therefore been demonstrated as an emerging technique for probing the interface of this and other polymer-inorganic systems.

Unraveling the Semiconducting/Metallic Discrepancy in Ni 3(HITP) 2

DOE PAGES

Foster, Michael E.; Sohlberg, Karl; Allendorf, Mark D.; ...

2018-01-10

Here, Ni 3(2,3,6,7,10,11-hexaiminotriphenylene) 2 is a π-stacked layered metal–organic framework material with extended π-conjugation that is analogous to graphene. Published experimental results indicate that the material is semiconducting, but all theoretical studies to date predict the bulk material to be metallic. Given that previous experimental work was carried out on specimens containing complex nanocrystalline microstructures and the tendency for internal interfaces to introduce transport barriers, we apply DFT to investigate the influence of internal interface defects on the electronic structure of Ni 3(HITP) 2. The results show that interface defects can introduce a transport barrier by breaking the π-conjugation and/ormore » decreasing the dispersion of the electronic bands near the Fermi level. We demonstrate that the presence of defects can open a small gap, in the range of 15–200 meV, which is consistent with the experimentally inferred hopping barrier.« less

The study on the electrical resistivity of Cu/V multilayer films subjected to helium (He) ion irradiation

NASA Astrophysics Data System (ADS)

Wang, P. P.; Xu, C.; Fu, E. G.; Du, J. L.; Gao, Y.; Wang, X. J.; Qiu, Y. H.

2018-05-01

Sputtering-deposited Cu/V multilayer films with the individual layer thickness varying from 2.5 nm to 100 nm were irradiated by 1 MeV helium (He) ion at the fluence of 6 ×1016 ions ·cm-2 at room temperature. The resistivity of Cu/V multilayer films after ion irradiation was evaluated as a function of individual layer thickness at 300 K and compared with their resistivity before ion irradiation. The results show that the resistivity change before and after ion irradiation is largely determined by the interface structure, grain boundary and radiation induced defects. A model amended based on the model used in describing the resistivity of as-deposited Cu/V multilayer films was proposed to describe the resistivity of ion irradiated Cu/V multilayer films by considering the point defects induced by ion irradiation, the effect of interface absorption on defects and the effect of interface microstructure in the multilayer films.

Electrical Characterization of Defects Created by γ-Radiation in HfO2-Based MIS Structures for RRAM Applications

NASA Astrophysics Data System (ADS)

García, H.; González, M. B.; Mallol, M. M.; Castán, H.; Dueñas, S.; Campabadal, F.; Acero, M. C.; Sambuco Salomone, L.; Faigón, A.

2018-04-01

The γ-radiation effects on the electrical characteristics of metal-insulator-semiconductor capacitors based on HfO2, and on the resistive switching characteristics of the structures have been studied. The HfO2 was grown directly on silicon substrates by atomic layer deposition. Some of the capacitors were submitted to a γ ray irradiation using three different doses (16 kGy, 96 kGy and 386 kGy). We studied the electrical characteristics in the pristine state of the capacitors. The radiation increased the interfacial state densities at the insulator/semiconductor interface, and the slow traps inside the insulator near the interface. However, the leakage current is not increased by the irradiation, and the conduction mechanism is Poole-Frenkel for all the samples. The switching characteristics were also studied, and no significant differences were obtained in the performance of the devices after having been irradiated, indicating that the fabricated capacitors present good radiation hardness for its use as a RS element.

Theoretical study of optical properties of anti phase domains in GaP

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tea, E., E-mail: etea.contact@gmail.com; FOTON INSA-Rennes; Vidal, J.

III-V/Si heterostructures are currently investigated for silicon photonics and solar energy conversion. In particular, dilute nitride alloy GaAsPN grown on a GaP/Si platform exhibits lattice match with Si and an optimal band gap configuration for tandem solar cell devices. However, monolithic “coherent” growth of the GaP thin layer on Si suffers from the nucleation of extended structural defects, which can hamper device operation as well as the GaP/Si interface level and through their propagation inside the overall heterostructure. However, the effect of such structural defects on optical and transport properties is actually not well understood in details. In this letter,more » we investigate the anti phase domains defect (also called inversion domains) by means of ab initio calculations giving insights into the alteration of optical and transport properties of GaP due to the defective GaP/Si interface.« less

Comparison of acellular dermal matrix and synthetic mesh for lateral chest wall reconstruction in a rabbit model.

PubMed

Holton, Luther H; Chung, Thomas; Silverman, Ronald P; Haerian, Hafez; Goldberg, Nelson H; Burrows, Whitney M; Gobin, Andrea; Butler, Charles E

2007-04-01

Synthetic mesh is used for chest wall reconstruction, but infection or exposure can occur and necessitate removal. Human acellular dermal matrix (AlloDerm) has been used to reconstruct musculofascial defects in the trunk with low infection and herniation rates. AlloDerm may have advantages over synthetic mesh for chest wall reconstruction. This study compared outcomes and repair strengths of AlloDerm to expanded polytetrafluoroethylene mesh used for repair of rib cage defects. A 3 x 3-cm, full-thickness, lateral rib cage defect was created in each rabbit and repaired with expanded polytetrafluoroethylene (n = 8) or acellular dermal matrix (n = 9). At 4 weeks, the animals were euthanized and evaluated for lung herniation/dehiscence, strength of adhesions between the implant and intrapleural structures, and breaking strength of the implant materials and the implant-fascia interface. Tissue sections were analyzed with histologic and immunohistochemical staining to evaluate cellular infiltration and vascularization. No herniation or dehiscence occurred with either material. The incidence and strength of adhesions was similar between materials. The mean breaking strength of the AlloDerm-fascia interface (14.5 +/- 8.9 N) was greater than the expanded polytetrafluoroethylene-fascia interface (8.7 +/- 4.4 N; p = 0.027) and similar to the rib-intercostal-rib interface of the contralateral native chest wall (14.0 +/- 5.6 N). The AlloDerm grafts became infiltrated with cells and vascularized after implantation. AlloDerm used for chest wall reconstruction results in greater implant-defect interface strength than expanded polytetrafluoroethylene. The ability of AlloDerm to become vascularized and remodeled by autologous cells and to resist infection may be advantageous for chest wall reconstruction.

Mechanism of electromigration failure in Damascene processed copper interconnects

NASA Astrophysics Data System (ADS)

Michael, Nancy Lyn

2002-11-01

A major unresolved issue in Cu interconnect reliability is the interface role in the failure mechanism of real structures. The present study investigates failure in single-level damascene Cu interconnects with variations in interface condition, passivation and barrier, and linewidth. In the first phase, accelerated electromigration testing of 0.25mum Cu interconnects capped with SiN or SiCN, shows that lifetime and failure mode vary with capping layer. The first mode, seen primarily in SiN samples, is characterized by gradual resistance increase and extensive interface damage, believed to result from failure led by interface electromigration. The competing failure mode, found in SiCN capped samples, is characterized by abrupt resistance increase and localized voiding. The second phase fixes SiCN as the capping material and varies barrier material and line width. The three barrier materials, Ta, TaN, and Ta/TaN, produce similar lifetime statistics and failure is abrupt. Line width, however, does have a strong influence on failure time. The line width/grain size ratio ranged from 0.53 to 2.2 but does not correlate with mean time to failure (MTF). The strong dependence on interface fraction, combined with the conclusion from phase one that interface electromigration is not rate controlling, suggests another mechanism related to the interface is a controlling factor. The possibility that contamination and defects at the interface are key to this failure mode was investigated using electro-thermal fatigue (ETF). In ETF, where lines are simultaneously subjected to thermal cycling and constant current, damage caused by thermal stress is accelerated. Tests reveal that in 80 nm lines, transient failure occurs at times far below MTF in electromigration tests at higher temperatures. Failure found in ETF is clearly a result of damage growth due to thermal/mechanical stress rather than electromigration. At the stress levels created by the moderate ETF test conditions, the only place voids are likely to nucleate and grow is at pre-existing defects and impurities. In narrower lines, where smaller voids can cause catastrophic damage, defects have a greater effect on MTF. Results from this investigation suggest that impurities and defects in the Cu and at the interface, must be carefully controlled to make reliable narrow Cu interconnects.

Relaxation, Structure and Properties of Semi-coherent Interfaces

DOE PAGES

Shao, Shuai; Wang, Jian

2015-11-05

Materials containing high density of interfaces are promising candidates for future energy technologies, because interfaces acting as sources, sinks, and barriers for defects can improve mechanical and irradiation properties of materials. Semi-coherent interface widely occurring in various materials is composed of a network of misfit dislocations and coherent regions separated by misfit dislocations. Lastly, in this article, we review relaxation mechanisms, structure and properties of (111) semi-coherent interfaces in face centered cubic structures.

Metal-induced gap states in ferroelectric capacitors and its relationship with complex band structures

NASA Astrophysics Data System (ADS)

Junquera, Javier; Aguado-Puente, Pablo

2013-03-01

At metal-isulator interfaces, the metallic wave functions with an energy eigenvalue within the band gap decay exponentially inside the dielectric (metal-induced gap states, MIGS). These MIGS can be actually regarded as Bloch functions with an associated complex wave vector. Usually only real values of the wave vectors are discussed in text books, since infinite periodicity is assumed and, in that situation, wave functions growing exponentially in any direction would not be physically valid. However, localized wave functions with an exponential decay are indeed perfectly valid solution of the Schrodinger equation in the presence of defects, surfaces or interfaces. For this reason, properties of MIGS have been typically discussed in terms of the complex band structure of bulk materials. The probable dependence on the interface particulars has been rarely taken into account explicitly due to the difficulties to include them into the model or simulations. We aim to characterize from first-principles simulations the MIGS in realistic ferroelectric capacitors and their connection with the complex band structure of the ferroelectric material. We emphasize the influence of the real interface beyond the complex band structure of bulk materials. Financial support provided by MICINN Grant FIS2009-12721-C04-02, and by the European Union Grant No. CP-FP 228989-2 ``OxIDes''. Computer resources provided by the RES.

Physical criteria for the interface passivation layer in hydrogenated amorphous/crystalline silicon heterojunction solar cell

NASA Astrophysics Data System (ADS)

Zhao, Lei; Wang, Guanghong; Diao, Hongwei; Wang, Wenjing

2018-01-01

AFORS-HET (automat for simulation of heterostructures) simulation was utilized to explore the physical criteria for the passivation layer in hydrogenated amorphous/crystalline silicon heterojunction (SHJ) solar cells, by systematically investigating the solar cell current density-voltage (J-V) performance as a function of the interface defect density (D it) at the passivation layer/c-Si hetero-interface, the thickness (t) of the passivation layer, the bandgap (E g) of the passivation layer, and the density of dangling bond states (D db)/band tail states (D bt) in the band gap of the passivation layer. The corresponding impact regulations were presented clearly. Except for D it, the impacts of D db, D bt and E g are strongly dependent on the passivation layer thickness t. While t is smaller than 4-5 nm, the solar cell performance is less sensitive to the variation of D db, D bt and E g. Low D it at the a-Si:H/c-Si interface and small thickness t are the critical criteria for the passivation layer in such a case. However, if t has to be relatively larger, the microstructure, i.e. the material quality, including D db, D bt and E g, of the passivation layer should be controlled carefully. The mechanisms involved were analyzed and some applicable methods to prepare the passivation layer were proposed.

Towards quantum networks of single spins: analysis of a quantum memory with an optical interface in diamond.

PubMed

Blok, M S; Kalb, N; Reiserer, A; Taminiau, T H; Hanson, R

2015-01-01

Single defect centers in diamond have emerged as a powerful platform for quantum optics experiments and quantum information processing tasks. Connecting spatially separated nodes via optical photons into a quantum network will enable distributed quantum computing and long-range quantum communication. Initial experiments on trapped atoms and ions as well as defects in diamond have demonstrated entanglement between two nodes over several meters. To realize multi-node networks, additional quantum bit systems that store quantum states while new entanglement links are established are highly desirable. Such memories allow for entanglement distillation, purification and quantum repeater protocols that extend the size, speed and distance of the network. However, to be effective, the memory must be robust against the entanglement generation protocol, which typically must be repeated many times. Here we evaluate the prospects of using carbon nuclear spins in diamond as quantum memories that are compatible with quantum networks based on single nitrogen vacancy (NV) defects in diamond. We present a theoretical framework to describe the dephasing of the nuclear spins under repeated generation of NV spin-photon entanglement and show that quantum states can be stored during hundreds of repetitions using typical experimental coupling parameters. This result demonstrates that nuclear spins with weak hyperfine couplings are promising quantum memories for quantum networks.

Charge transport model in solid-state avalanche amorphous selenium and defect suppression design

NASA Astrophysics Data System (ADS)

Scheuermann, James R.; Miranda, Yesenia; Liu, Hongyu; Zhao, Wei

2016-01-01

Avalanche amorphous selenium (a-Se) in a layer of High Gain Avalanche Rushing Photoconductor (HARP) is being investigated for its use in large area medical imagers. Avalanche multiplication of photogenerated charge requires electric fields greater than 70 V μm-1. For a-Se to withstand this high electric field, blocking layers are used to prevent the injection of charge carriers from the electrodes. Blocking layers must have a high injection barrier and deep trapping states to reduce the electric field at the interface. In the presence of a defect in the blocking layer, a distributed resistive layer (DRL) must be included into the structure to build up space charge and reduce the electric field in a-Se and the defect. A numerical charge transport model has been developed to optimize the properties of blocking layers used in various HARP structures. The model shows the incorporation of a DRL functionality into the p-layer can reduce dark current at a point defect by two orders of magnitude by reducing the field in a-Se to the avalanche threshold. Hole mobility in a DRL of ˜10-8 cm2 V-1 s-1 at 100 V μm-1 as demonstrated by the model can be achieved experimentally by varying the hole mobility of p-type organic or inorganic semiconductors through doping, e.g., using Poly(9-vinylcarbozole) doped with 1%-3% (by weight) of poly(3-hexylthiopene).

Interface roughness induced asymmetric magnetic property in sputter-deposited Co/CoO/Co exchange coupled trilayers

NASA Astrophysics Data System (ADS)

Wang, J.; Sannomiya, T.; Shi, J.; Nakamura, Y.

2012-04-01

The effect of interface roughness on magnetic properties of exchange coupled polycrystalline Co/CoO(tAF)/Co trilayers has been investigated by varying antiferromagnetic layer (CoO) thickness. It has been found that the upper CoO/Co interface becomes rougher with increasing CoO layer thickness, resulting in stronger exchange bias of the upper interface than the lower one. The interfacial exchange coupling is strengthened by the increase of defect-generated uncompensated antiferromagnetic spins; such spins form coupling with spins in the Co layer at the interface. As a result, the CoO layer thickness dependence of exchange bias is much enhanced for the upper Co layer. The transition from anisotropic magnetoresistance to isotropic magnetoresistance for the top Co layer has also been found. This could be attributed to the defects, probably partial thin oxide layers, between Co grains in the top Co layer that leads a switch from spin-orbit scattering related magnetoresistance to spin-dependent electron scattering dominated magnetoresistance.

Interface traps and quantum size effects on the retention time in nanoscale memory devices

PubMed Central

2013-01-01

Based on the analysis of Poisson equation, an analytical surface potential model including interface charge density for nanocrystalline (NC) germanium (Ge) memory devices with p-type silicon substrate has been proposed. Thus, the effects of Pb defects at Si(110)/SiO2, Si(111)/SiO2, and Si(100)/SiO2 interfaces on the retention time have been calculated after quantum size effects have been considered. The results show that the interface trap density has a large effect on the electric field across the tunneling oxide layer and leakage current. This letter demonstrates that the retention time firstly increases with the decrease in diameter of NC Ge and then rapidly decreases with the diameter when it is a few nanometers. This implies that the interface defects, its energy distribution, and the NC size should be seriously considered in the aim to improve the retention time from different technological processes. The experimental data reported in the literature support the theoretical expectation. PMID:23984827

Interface-Driven Phenomena in Solids: Thermodynamics, Kinetics and Chemistry

DOE PAGES

Abdeljawad, Fadi; Foiles, Stephen M.

2016-05-04

The study of materials interfaces dates back over a century. In solid systems and from an engineering perspective, free surfaces and internal (grain and/or phase) boundaries influence a wide range of properties, such as thermal, electrical and optical transport, and mechanical ones. The properties and the role of interfaces has been discussed extensively in various reviews such as by Sutton and Balluffi. As the characteristic feature size of a materials system (i.e., grain size) is decreased to the nanometer scale, interface-driven physics is expected to dominate due to the increased density of such planar defects. Moreover, interfacial attributes, thermodynamics, andmore » mobility play a key role in phase transformations, such as solidification dynamics and structural transitions in solids, and in homogenization and microstructural evolution processes, such as grain growth, coarsening, and recrystallization. In summary, the set of articles published in this special topic titled: “Interface-Driven Phenomena in Solids: Thermodynamics, Kinetics and Chemistry” covers topics related to microstructure evolution, segregation/adsorption phenomena and interface interactions with other materials defects.« less

Interface-Driven Phenomena in Solids: Thermodynamics, Kinetics and Chemistry

DOE Office of Scientific and Technical Information (OSTI.GOV)

Abdeljawad, Fadi; Foiles, Stephen M.

The study of materials interfaces dates back over a century. In solid systems and from an engineering perspective, free surfaces and internal (grain and/or phase) boundaries influence a wide range of properties, such as thermal, electrical and optical transport, and mechanical ones. The properties and the role of interfaces has been discussed extensively in various reviews such as by Sutton and Balluffi. As the characteristic feature size of a materials system (i.e., grain size) is decreased to the nanometer scale, interface-driven physics is expected to dominate due to the increased density of such planar defects. Moreover, interfacial attributes, thermodynamics, andmore » mobility play a key role in phase transformations, such as solidification dynamics and structural transitions in solids, and in homogenization and microstructural evolution processes, such as grain growth, coarsening, and recrystallization. In summary, the set of articles published in this special topic titled: “Interface-Driven Phenomena in Solids: Thermodynamics, Kinetics and Chemistry” covers topics related to microstructure evolution, segregation/adsorption phenomena and interface interactions with other materials defects.« less

Screening mechanisms at polar oxide heterointerfaces

DOE PAGES

Hong, Seungbum; Nakhmanson, Serge M.; Fong, Dillon D.

2016-06-16

The interfaces of polar oxide heterostructures can display electronic properties unique from the oxides they border, as they require screening from either internal or external sources of charge. The screening mechanism depends on a variety of factors, including the band structure at the interface, the presence of point defects or adsorbates, whether or not the oxide is ferroelectric, and whether or not an external field is applied. In this review, we discuss both theoretical and experimental aspects of different screening mechanisms, giving special emphasis to ways in which the mechanism can be altered to provide novel or tunable functionalities. Wemore » begin with a theoretical introduction to the problem and highlight recent progress in understanding the impact of point defects on polar interfaces. Different case studies are then discussed, for both the high thickness regime, where interfaces must be screened and each interface can be considered separately, and the low thickness regime, where the degree and nature of screening can be manipulated and the interfaces are close enough to interact. As a result, we end with a brief outlook toward new developments in this rapidly progressing field.« less

Method for reducing or eliminating interface defects in mismatched semiconductor epilayers

DOEpatents

Fitzgerald, Jr., Eugene A.; Ast, Dieter G.

1992-01-01

The present invention and process relates to crystal lattice mismatched semiconductor composite having a first semiconductor layer and a second semiconductor growth layer deposited thereon to form an interface wherein the growth layer can be deposited at thicknesses in excess of the critical thickness, even up to about 10.times. critical thickness. Such composite has an interface which is substantially free of interface defects. For example, the size of the growth areas in a mismatched In.sub.0.05 Ga.sub.0.95 As/(001)GaAs interface was controlled by fabricating 2-.mu.m high pillars of various lateral geometries and lateral dimensions before the epitaxial deposition of 3500.ANG. of In.sub.0.05 Ga.sub.0.95 As. The linear dislocation density at the interface was reduced from >5000 dislocations/cm to about zero for 25-.mu.m lateral dimensions and to less than 800 dislocations/cm for lateral dimensions as large as 100 .mu.m. The fabricated pillars control the lateral dimensions of the growth layer and block the glide of misfit dislocations with the resultant decrease in dislocation density.

Method for reducing or eliminating interface defects in mismatched semiconductor eiplayers

DOEpatents

Fitzgerald, Jr., Eugene A.; Ast, Dieter G.

1991-01-01

The present invention and process relates to crystal lattice mismatched semiconductor composite having a first semiconductor layer and a second semiconductor growth layer deposited thereon to form an interface wherein the growth layer can be deposited at thicknesses in excess of the critical thickness, even up to about 10x critical thickness. Such composite has an interface which is substantially free of interface defects. For example, the size of the growth areas in a mismatched In.sub.0.05 Ga.sub.0.95 As/(001)GaAs interface was controlled by fabricating 2-.mu.m high pillars of various lateral geometries and lateral dimensions before the epitaxial deposition of 3500.ANG. of In.sub.0.05 Ga.sub.0.95 As. The linear dislocation density at the interface was reduced from >5000 dislocations/cm to about zero for 25-.mu.m lateral dimensions and to less than 800 dislocations/cm for lateral dimensions as large as 100 .mu.m. The fabricated pillars control the lateral dimensions of the growth layer and block the glide of misfit dislocations with the resultant decrease in dislocation density.

Method for reducing or eliminating interface defects in mismatched semiconductor epilayers

DOEpatents

Fitzgerald, E.A. Jr.; Ast, D.G.

1992-10-20

The present invention and process relates to crystal lattice mismatched semiconductor composite having a first semiconductor layer and a second semiconductor growth layer deposited thereon to form an interface wherein the growth layer can be deposited at thicknesses in excess of the critical thickness, even up to about 10[times] critical thickness. Such composite has an interface which is substantially free of interface defects. For example, the size of the growth areas in a mismatched In[sub 0.05]Ga[sub 0.95]As/(001)GaAs interface was controlled by fabricating 2-[mu]m high pillars of various lateral geometries and lateral dimensions before the epitaxial deposition of 3500 [angstrom] of In[sub 0.05]Ga[sub 0.95]As. The linear dislocation density at the interface was reduced from >5000 dislocations/cm to about zero for 25-[mu]m lateral dimensions and to less than 800 dislocations/cm for lateral dimensions as large as 100 [mu]m. The fabricated pillars control the lateral dimensions of the growth layer and block the glide of misfit dislocations with the resultant decrease in dislocation density. 7 figs.

Phonon interference control of atomic-scale metamirrors, meta-absorbers, and heat transfer through crystal interfaces

NASA Astrophysics Data System (ADS)

Kosevich, Yu. A.; Potyomina, L. G.; Darinskii, A. N.; Strelnikov, I. A.

2018-03-01

The paper theoretically studies the possibility of using the effects of phonon interference between paths through different interatomic bonds for the control of phonon heat transfer through internal crystal interfaces and for the design of phonon metamirrors and meta-absorbers. These metamirrors and meta-absorbers are considered to be defect nanolayers of atomic-scale thicknesses embedded in a crystal. Several analytically solvable three-dimensional lattice-dynamics models of the phonon metamirrors and meta-absorbers at the internal crystal planes are described. It is shown that due to destructive interference in the two or more phonon paths, the internal crystal planes, fully or partially filled with weakly bound or heavy-isotope defect atoms, can completely reflect or completely absorb phonons at the transmission antiresonances, whose wavelengths are larger than the effective thickness of the metamirror or meta-absorber. Due to cooperative superradiant effect, the spectral widths of the two-path interference antiresonances for the plane waves are given by the square of partial filling fraction in the defect crystal plane. Our analysis reveals that the presence of two or more phonon paths plays the dominant role in the emergence of the transmission antiresonances in phonon scattering at the defect crystal planes and in reduction of the thermal interface conductance in comparison with the Fano-resonance concept. We study analytically phonon transmission through internal crystal plane in a model cubic lattice of Si-like atoms, partially filled with Ge-like defect atoms. Such a plane can serve as interference phonon metamirror with the transmission antiresonances in the vicinities of eigenmode frequencies of Ge-like defect atoms in the terahertz frequency range. We predict the extraordinary phonon transmission induced by the two-path constructive interference of the lattice waves in resonance with the vibrations of rare host atoms, periodically distributed in the crystal plane almost completely filled with heavy-isotope defects. We show that the phonon-interference-induced transparency can be produced by the defect nanolayer with the non-nearest-neighbor interactions, filled with two types of isotopes with relatively small difference in masses or binding force constants. In this case, relatively broad transmission antiresonance is accompanied by the narrow transmission peak close to the antiresonance frequency. We describe the softening of the flexural surface acoustic wave, localized at the embedded defect nanolayer, caused by negative surface stress in the layer. The surface wave softening results in spatially periodic static bending deformation of the embedded nanolayer with the definite wave number. The latter effect is estimated for graphene monolayer embedded in a strained matrix of polyethylene. We analyze the effect of nonlinearity in the dynamics of defect atoms on the one- and two-path phonon interference and show that the interference transmission resonances and antiresonances are shifted in frequencies but not completely suppressed by rather strong anharmonicity of interatomic bonds. The reduction of the Kapitza thermal interface conductance caused by the destructive phonon interference in a defect monolayer is described. We show that the additional relatively weak non-nearest-neighbor interactions through the defect crystal plane filled with heavy isotopes substantially reduces the interface thermal conductance, and this effect is stronger in the three-dimensional system than in the quasi-one-dimensional systems studied previously.

Strength and Microstructure of Ceramics

DTIC Science & Technology

1989-11-01

processing defects (pores or inclusions), etc. Theoretically, flaws have been represented as scaled-down versions of large cracks, so that the...no spurious reflections. confirming that the defects were not microtwins, From the TEM evidence. alhing with corresponding observations of fault...Lawn Vol. 71. No. I Interfaces. can be viewred as high-energy planar defects . AS Such. V. Conclusions they represent favored sites for microcrack

Nanoscale interfacial defect shedding in a growing nematic droplet.

PubMed

Gurevich, Sebastian; Provatas, Nikolas; Rey, Alejandro

2017-08-01

Interfacial defect shedding is the most recent known mechanism for defect formation in a thermally driven isotropic-to-nematic phase transition. It manifests in nematic-isotropic interfaces going through an anchoring switch. Numerical computations in planar geometry established that a growing nematic droplet can undergo interfacial defect shedding, nucleating interfacial defect structures that shed into the bulk as +1/2 point defects. By extending the study of interfacial defect shedding in a growing nematic droplet to larger length and time scales, and to three dimensions, we unveil an oscillatory growth mode involving shape and anchoring transitions that results in a controllable regular distributions of point defects in planar geometry, and complex structures of disclination lines in three dimensions.

Role of Defects and Adsorbed Water Film in Influencing the Electrical, Optical and Catalytic Properties of Transition Metal Oxides

NASA Astrophysics Data System (ADS)

Wang, Qi

Transition metal oxides (TMOs) constitute a large group of materials that exhibit a wide range of optical, electrical, electrochemical, dielectric and catalytic properties, and thus making them highly regarded as promising materials for a variety of applications in next generation electronic, optoelectronic, catalytic, photonic, energy storage and energy conversion devices. Some of the unique properties of TMOs are their strong electron-electron correlations that exists between the valence electrons of narrow d- or f-shells and their ability to exist in variety of oxidation states. This gives TMOs an enormous range of fascinating electronic and other physical properties. Many of these remarkable properties of TMOs arises from the complex surface charge transfer processes at the oxide surface/electrochemical redox species interface and non-stoichiometry due to the presence of lattice vacancies that may cause significant perturbation to the electronic structure of the material. Stoichiometry, oxidation state of the metal center and lattice vacancy defects all play important roles in affecting the physical properties, electronic structures, device behavior and other functional properties of TMOs. However, the underlying relationships between them is not clearly known. For instance, the exchange of electrons between adsorbates and defects can lead to the passivation of existing defect states or formation of new defects, both of which affect defect equilibria, and consequently, functional properties. In depth understanding of the role of lattice defects on the electrical, catalytic and optical properties of TMOs is central to further expansion of the technological applications of TMO based devices. The focus of this work is to elucidate the interactions of vacancy defects with various electrochemical adsorbates in TMOs. The ability to directly probe the interactions of vacancy defects with gas and liquid phase species under in-operando conditions is highly desirable to obtain a mechanistic understanding of the charge transfer process. We have developed a spectroscopic technique for studying vacancy defects in TMOs using near-infrared photoluminescence (NIR-PL) spectroscopy and showed that this technique is uniquely suited for studying defect-adsorbate interactions. In this work, a series of studies were carried out to elucidate the underlying structure-defect-property correlations of TMOs and their role in catalyzing electrical and electrochemical properties. In the first study, we report a new type of electrical phase transition in p-type, non-stoichiometric nickel oxide involving a semiconductor-to-insulator-to-metal transition along with the complete change of conductivity from p- to n-type at room temperature induced by electrochemical Li+ intercalation. Direct observation of vacancy-ion interactions using in-situ NIR-PL show that the transition is a result of passivation of native nickel (cationic) vacancy defects and subsequent formation of oxygen (anionic) vacancy defects driven by Li+ insertion into the lattice. X-ray photoemission spectroscopy studies performed to examine the changes in the oxidation states of nickel due to defect interactions support the above conclusions. In the second study, main effects of oxygen vacancy defects on the electronic and optical properties of V2O5 nanowires were studied using in-situ Raman, photoluminescence, absorption, and photoemission spectroscopy. We show that both thermal reduction and electrochemical reduction via Li+ insertion results in the creation of oxygen vacancy defects in the crystal that leads to band filling and an increase in the optical band gap of V2O5 from 1.95 eV to 2.45 eV, an effect known as the Burstein-Moss effect. In the third study, we report a new type of semiconductor-adsorbed water interaction in metal oxides known as "electrochemical surface transfer doping," a phenomenon that has been previously been observed on hydrogen-terminated diamond, carbon nanotube, gallium nitride and zinc oxide. Most TMOs at room temperature are known to be strongly hydrated. We show that an adsorbed water film present on the surface of TMOs facilitates the dissolution of gaseous species and promotes charge transfers at the adsorbed-water/oxide interfaces. Further, we show the role of vacancy defects in enhancing catalytic processes by directly monitoring the charge transfer process between gaseous species and vacancy defects in non-stoichiometric p-type nickel oxide and n-type tungsten oxide using in-situ NIR-PL, electrical resistance, and X-ray photoelectron spectroscopy. We find the importance of adsorbed water and vacancy defects in affecting catalytic, electronic, electrical, and optical changes such as insulator-to-metal transitions and radiative emissions during electrochemical reactions. In addition, we demonstrate that electrochemical surface transfer doping exists in another system, specifically, in gallium nitride, and the presence of this adsorbed water film present on the surface of GaN induces electron transfer from GaN that leads to the formation of an electron depletion region on the surface.

Electrical and structural characterizations of crystallized Al{sub 2}O{sub 3}/GaN interfaces formed by in situ metalorganic chemical vapor deposition

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liu, X., E-mail: xliu@ece.ucsb.edu; Yeluri, R.; Kim, J.

2016-01-07

Al{sub 2}O{sub 3} films were grown in situ by metalorganic chemical vapor deposition at 900 °C on GaN of both Ga- and N-face polarities. High-resolution transmission electron microscopy revealed that the Al{sub 2}O{sub 3} films were crystalline and primarily γ-phase. The Al{sub 2}O{sub 3}/Ga-GaN and Al{sub 2}O{sub 3}/N-GaN interfaces were both atomically sharp, and the latter further exhibited a biatomic step feature. The corresponding current-voltage (J-V) characteristics were measured on a metal-Al{sub 2}O{sub 3}-semiconductor capacitor (MOSCAP) structure. The leakage current was very high when the Al{sub 2}O{sub 3} thickness was comparable with the size of the crystalline defects, but was suppressedmore » to the order of 1 × 10{sup −8} A/cm{sup 2} with larger Al{sub 2}O{sub 3} thicknesses. The interface states densities (D{sub it}) were measured on the same MOSCAPs by using combined ultraviolet (UV)-assisted capacitance-voltage (C-V), constant capacitance deep level transient spectroscopy (CC-DLTS), and constant capacitance deep level optical spectroscopy (CC-DLOS) techniques. The average D{sub it} measured by CC-DLTS and CC-DLOS were 6.6 × 10{sup 12} and 8.8 × 10{sup 12} cm{sup −2} eV{sup −1} for Al{sub 2}O{sub 3}/Ga-GaN and 8.6 × 10{sup 12} and 8.6 × 10{sup 12 }cm{sup −2} eV{sup −1} for Al{sub 2}O{sub 3}/N-GaN, respectively. The possible origins of the positive (negative) polarization compensation charges in Al{sub 2}O{sub 3}/Ga-GaN (Al{sub 2}O{sub 3}/N-GaN), including the filling of interface states and the existence of structure defects and impurities in the Al{sub 2}O{sub 3} layer, were discussed in accordance with the experimental results and relevant studies in the literature.« less

Defect-driven interfacial electronic structures at an organic/metal-oxide semiconductor heterojunction.

PubMed

Winget, Paul; Schirra, Laura K; Cornil, David; Li, Hong; Coropceanu, Veaceslav; Ndione, Paul F; Sigdel, Ajaya K; Ginley, David S; Berry, Joseph J; Shim, Jaewon; Kim, Hyungchui; Kippelen, Bernard; Brédas, Jean-Luc; Monti, Oliver L A

2014-07-16

The electronic structure of the hybrid interface between ZnO and the prototypical organic semiconductor PTCDI is investigated via a combination of ultraviolet and X-ray photoelectron spectroscopy (UPS/XPS) and density functional theory (DFT) calculations. The interfacial electronic interactions lead to a large interface dipole due to substantial charge transfer from ZnO to 3,4,9,10-perylenetetracarboxylicdiimide (PTCDI), which can be properly described only when accounting for surface defects that confer ZnO its n-type properties. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Magnetorheological finishing for removing surface and subsurface defects of fused silica optics

NASA Astrophysics Data System (ADS)

Catrin, Rodolphe; Neauport, Jerome; Taroux, Daniel; Cormont, Philippe; Maunier, Cedric; Lambert, Sebastien

2014-09-01

We investigate the capacity of magnetorheological finishing (MRF) process to remove surface and subsurface defects of fused silica optics. Polished samples with engineered surface and subsurface defects were manufactured and characterized. Uniform material removals were performed with a QED Q22-XE machine using different MRF process parameters in order to remove these defects. We provide evidence that whatever the MRF process parameters are, MRF is able to remove surface and subsurface defects. Moreover, we show that MRF induces a pollution of the glass interface similar to conventional polishing processes.

Impact of extended defects on recombination in CdTe heterostructures grown by molecular beam epitaxy

NASA Astrophysics Data System (ADS)

Zaunbrecher, Katherine N.; Kuciauskas, Darius; Swartz, Craig H.; Dippo, Pat; Edirisooriya, Madhavie; Ogedengbe, Olanrewaju S.; Sohal, Sandeep; Hancock, Bobby L.; LeBlanc, Elizabeth G.; Jayathilaka, Pathiraja A. R. D.; Barnes, Teresa M.; Myers, Thomas H.

2016-08-01

Heterostructures with CdTe and CdTe1-xSex (x ˜ 0.01) absorbers between two wider-band-gap Cd1-xMgxTe barriers (x ˜ 0.25-0.3) were grown by molecular beam epitaxy to study carrier generation and recombination in bulk materials with passivated interfaces. Using a combination of confocal photoluminescence (PL), time-resolved PL, and low-temperature PL emission spectroscopy, two extended defect types were identified and the impact of these defects on charge-carrier recombination was analyzed. The dominant defects identified by confocal PL were dislocations in samples grown on (211)B CdTe substrates and crystallographic twinning-related defects in samples on (100)-oriented InSb substrates. Low-temperature PL shows that twin-related defects have a zero-phonon energy of 1.460 eV and a Huang-Rhys factor of 1.50, while dislocation-dominated samples have a 1.473-eV zero-phonon energy and a Huang-Rhys factor of 1.22. The charge carrier diffusion length near both types of defects is ˜6 μm, suggesting that recombination is limited by diffusion dynamics. For heterostructures with a low concentration of extended defects, the bulk lifetime was determined to be 2.2 μs with an interface recombination velocity of 160 cm/s and an estimated radiative lifetime of 91 μs.

Impact of extended defects on recombination in CdTe heterostructures grown by molecular beam epitaxy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zaunbrecher, Katherine N.; Kuciauskas, Darius; Swartz, Craig H.

Heterostructures with CdTe and CdTe 1-xSex (x ~ 0.01) absorbers between two wider-band-gap Cd1-xMgxTe barriers (x ~ 0.25-0.3) were grown by molecular beam epitaxy to study carrier generation and recombination in bulk materials with passivated interfaces. Using a combination of confocal photoluminescence (PL), time-resolved PL, and low-temperature PL emission spectroscopy, two extended defect types were identified and the impact of these defects on charge-carrier recombination was analyzed. The dominant defects identified by confocal PL were dislocations in samples grown on (211)B CdTe substrates and crystallographic twinning-related defects in samples on (100)-oriented InSb substrates. Low-temperature PL shows that twin-related defects havemore » a zero-phonon energy of 1.460 eV and a Huang-Rhys factor of 1.50, while dislocation-dominated samples have a 1.473-eV zero-phonon energy and a Huang-Rhys factor of 1.22. The charge carrier diffusion length near both types of defects is ~6 um, suggesting that recombination is limited by diffusion dynamics. For heterostructures with a low concentration of extended defects, the bulk lifetime was determined to be 2.2 us with an interface recombination velocity of 160 cm/s and an estimated radiative lifetime of 91 us.« less

Sensitivity of indentation testing to step-off edges and interface integrity in cartilage repair.

PubMed

Bae, Won C; Law, Amanda W; Amiel, David; Sah, Robert L

2004-03-01

Step-off edges and tissue interfaces are prevalent in cartilage injury such as after intra-articular fracture and reduction, and in focal defects and surgical repair procedures such as osteochondral graft implantation. It would be useful to assess the function of injured or donor tissues near such step-off edges and the extent of integration at material interfaces. The objective of this study was to determine if indentation testing is sensitive to the presence of step-off edges and the integrity of material interfaces, in both in vitro simulated repair samples of bovine cartilage defect filled with fibrin matrix, and in vivo biological repair samples from a goat animal model. Indentation stiffness decreased at locations approaching a step-off edge, a lacerated interface, or an integrated interface in which the distal tissue was relatively soft. The indentation stiffness increased or remained constant when the site of indentation approached an integrated interface in which the distal tissue was relatively stiff or similar in stiffness to the tissue being tested. These results indicate that indentation testing is sensitive to step-off edges and interface integrity, and may be useful for assessing cartilage injury and for following the progression of tissue integration after surgical treatments.

Impurity distribution and microstructure of Ga-doped ZnO films grown by molecular beam epitaxy

NASA Astrophysics Data System (ADS)

Kvit, A. V.; Yankovich, A. B.; Avrutin, V.; Liu, H.; Izyumskaya, N.; Özgür, Ü.; Morkoç, H.; Voyles, P. M.

2012-12-01

We report microstructural characterization of heavily Ga-doped ZnO (GZO) thin films on GaN and sapphire by aberration-corrected scanning transmission electron microscopy. Growth under oxygen-rich and metal-rich growth conditions leads to changes in the GZO polarity and different extended defects. For GZO layers on sapphire, the primary extended defects are voids, inversion domain boundaries, and low-angle grain boundaries. Ga doping of ZnO grown under metal-rich conditions causes a switch from pure oxygen polarity to mixed oxygen and zinc polarity in small domains. Electron energy loss spectroscopy and energy dispersive spectroscopy spectrum imaging show that Ga is homogeneous, but other residual impurities tend to accumulate at the GZO surface and at extended defects. GZO grown on GaN on c-plane sapphire has Zn polarity and no voids. There are misfit dislocations at the interfaces between GZO and an undoped ZnO buffer layer and at the buffer/GaN interface. Low-angle grain boundaries are the only threading microstructural defects. The potential effects of different extended defects and impurity distributions on free carrier scattering are discussed.

Lifetime degradation of n-type Czochralski silicon after hydrogenation

NASA Astrophysics Data System (ADS)

Vaqueiro-Contreras, M.; Markevich, V. P.; Mullins, J.; Halsall, M. P.; Murin, L. I.; Falster, R.; Binns, J.; Coutinho, J.; Peaker, A. R.

2018-04-01

Hydrogen plays an important role in the passivation of interface states in silicon-based metal-oxide semiconductor technologies and passivation of surface and interface states in solar silicon. We have shown recently [Vaqueiro-Contreras et al., Phys. Status Solidi RRL 11, 1700133 (2017)] that hydrogenation of n-type silicon slices containing relatively large concentrations of carbon and oxygen impurity atoms {[Cs] ≥ 1 × 1016 cm-3 and [Oi] ≥ 1017 cm-3} can produce a family of C-O-H defects, which act as powerful recombination centres reducing the minority carrier lifetime. In this work, evidence of the silicon's lifetime deterioration after hydrogen injection from SiNx coating, which is widely used in solar cell manufacturing, has been obtained from microwave photoconductance decay measurements. We have characterised the hydrogenation induced deep level defects in n-type Czochralski-grown Si samples through a series of deep level transient spectroscopy (DLTS), minority carrier transient spectroscopy (MCTS), and high-resolution Laplace DLTS/MCTS measurements. It has been found that along with the hydrogen-related hole traps, H1 and H2, in the lower half of the gap reported by us previously, hydrogenation gives rise to two electron traps, E1 and E2, in the upper half of the gap. The activation energies for electron emission from the E1 and E2 trap levels have been determined as 0.12, and 0.14 eV, respectively. We argue that the E1/H1 and E2/H2 pairs of electron/hole traps are related to two energy levels of two complexes, each incorporating carbon, oxygen, and hydrogen atoms. Our results show that the detrimental effect of the C-O-H defects on the minority carrier lifetime in n-type Si:O + C materials can be very significant, and the carbon concentration in Czochralski-grown silicon is a key parameter in the formation of the recombination centers.

Defect control of conventional and anomalous electron transport at complex oxide interfaces

DOE PAGES

Gunkel, F.; Bell, Chris; Inoue, Hisashi; ...

2016-08-30

Using low-temperature electrical measurements, the interrelation between electron transport, magnetic properties, and ionic defect structure in complex oxide interface systems is investigated, focusing on NdGaO 3/SrTiO 3 (100) interfaces. Field-dependent Hall characteristics (2–300 K) are obtained for samples grown at various growth pressures. In addition to multiple electron transport, interfacial magnetism is tracked exploiting the anomalous Hall effect (AHE). These two properties both contribute to a nonlinearity in the field dependence of the Hall resistance, with multiple carrier conduction evident below 30 K and AHE at temperatures ≲10 K. Considering these two sources of nonlinearity, we suggest a phenomenological modelmore » capturing the complex field dependence of the Hall characteristics in the low-temperature regime. Our model allows the extraction of the conventional transport parameters and a qualitative analysis of the magnetization. The electron mobility is found to decrease systematically with increasing growth pressure. This suggests dominant electron scattering by acceptor-type strontium vacancies incorporated during growth. The AHE scales with growth pressure. In conclusion, the most pronounced AHE is found at increased growth pressure and, thus, in the most defective, low-mobility samples, indicating a correlation between transport, magnetism, and cation defect concentration.« less

Interface Trap Density Reduction for Al2O3/GaN (0001) Interfaces by Oxidizing Surface Preparation prior to Atomic Layer Deposition.

PubMed

Zhernokletov, Dmitry M; Negara, Muhammad A; Long, Rathnait D; Aloni, Shaul; Nordlund, Dennis; McIntyre, Paul C

2015-06-17

We correlate interfacial defect state densities with the chemical composition of the Al2O3/GaN interface in metal-oxide-semiconductor (MOS) structures using synchrotron photoelectron emission spectroscopy (PES), cathodoluminescence and high-temperature capacitance-voltage measurements. The influence of the wet chemical pretreatments involving (1) HCl+HF etching or (2) NH4OH(aq) exposure prior to atomic layer deposition (ALD) of Al2O3 were investigated on n-type GaN (0001) substrates. Prior to ALD, PES analysis of the NH4OH(aq) treated surface shows a greater Ga2O3 component compared to either HCl+HF treated or as-received surfaces. The lowest surface concentration of oxygen species is detected on the acid etched surface, whereas the NH4OH treated sample reveals the lowest carbon surface concentration. Both surface pretreatments improve electrical characteristics of MOS capacitors compared to untreated samples by reducing the Al2O3/GaN interface state density. The lowest interfacial trap density at energies in the upper band gap is detected for samples pretreated with NH4OH. These results are consistent with cathodoluminescence data indicating that the NH4OH treated samples show the strongest band edge emission compared to as-received and acid etched samples. PES results indicate that the combination of reduced carbon contamination while maintaining a Ga2O3 interfacial layer by NH4OH(aq) exposure prior to ALD results in fewer interface traps after Al2O3 deposition on the GaN substrate.

Modeling of the interfacial separation work in relation to impurity concentration in adjoining materials

NASA Astrophysics Data System (ADS)

Alekseev, Ilia M.; Makhviladze, Tariel M.; Minushev, Airat Kh.; Sarychev, Mikhail E.

2009-10-01

On the basis of the general thermodynamic approach developed in a model describing the influence of point defects on the separation work at an interface of solid materials is developed. The kinetic equations describing the defect exchange between the interface and the material bulks are formulated. The model have been applied to the case when joined materials contain such point defects as impurity atoms (interstitial and substitutional), concretized the main characteristic parameters required for a numerical modeling as well as clarified their domains of variability. The results of the numerical modeling concerning the dependences on impurity concentrations and the temperature dependences are obtained and analyzed. Particularly, the effects of interfacial strengthening and adhesion incompatibility predicted analytically for the case of impurity atoms are verified and analyzed.

Modeling of the interfacial separation work in relation to impurity concentration in adjoining materials

NASA Astrophysics Data System (ADS)

Alekseev, Ilia M.; Makhviladze, Tariel M.; Minushev, Airat Kh.; Sarychev, Mikhail E.

2010-02-01

On the basis of the general thermodynamic approach developed in a model describing the influence of point defects on the separation work at an interface of solid materials is developed. The kinetic equations describing the defect exchange between the interface and the material bulks are formulated. The model have been applied to the case when joined materials contain such point defects as impurity atoms (interstitial and substitutional), concretized the main characteristic parameters required for a numerical modeling as well as clarified their domains of variability. The results of the numerical modeling concerning the dependences on impurity concentrations and the temperature dependences are obtained and analyzed. Particularly, the effects of interfacial strengthening and adhesion incompatibility predicted analytically for the case of impurity atoms are verified and analyzed.

Morphological characterization of dental prostheses interfaces using optical coherence tomography

NASA Astrophysics Data System (ADS)

Sinescu, Cosmin; Negrutiu, Meda L.; Ionita, Ciprian; Marsavina, Liviu; Negru, Radu; Caplescu, Cristiana; Bradu, Adrian; Topala, Florin; Rominu, Roxana O.; Petrescu, Emanuela; Leretter, Marius; Rominu, Mihai; Podoleanu, Adrian G.

2010-03-01

Fixed partial prostheses as integral ceramic, polymers, metal-ceramic or metal-polymers bridges are mainly used in the frontal part of the dental arch (especially the integral bridges). They have to satisfy high stress as well as esthetic requirements. The masticatory stress may induce fractures of the bridges. These may be triggered by initial materials defects or by alterations of the technological process. The fractures of these bridges lead to functional, esthetic and phonetic disturbances which finally render the prosthetic treatment inefficient. Dental interfaces represent one of the most significant aspects in the strength of the dental prostheses under the masticatory load. The purpose of this study is to evaluate the capability of optical coherence tomography (OCT) to characterize the dental prostheses interfaces. The materials used were several fixed partial prostheses integral ceramic, polymers, metal-ceramic and metal-polymers bridges. It is important to produce both C-scans and B-scans of the defects in order to differentiate morphological aspects of the bridge infrastructures. The material defects observed with OCT were investigated with micro-CT in order to prove their existence and positions. In conclusion, it is important to have a non invasive method to investigate dental prostheses interfaces before the insertion of prostheses in the oral cavity.

Spatially-resolved studies on the role of defects and boundaries in electronic behavior of 2D materials

NASA Astrophysics Data System (ADS)

Hus, Saban M.; Li, An-Ping

2017-08-01

Two-dimensional (2D) materials are intrinsically heterogeneous. Both localized defects, such as vacancies and dopants, and mesoscopic boundaries, such as surfaces and interfaces, give rise to compositional or structural heterogeneities. The presence of defects and boundaries can break lattice symmetry, modify the energy landscape, and create quantum confinement, leading to fascinating electronic properties different from the ;ideal; 2D sheets. This review summarizes recent progress in understanding the roles of defects and boundaries in electronic, magnetic, thermoelectric, and transport properties of 2D layered materials. The focus is on the understanding of correlation of atomic-scale structural information with electronic functions by interrogating heterogeneities individually. The materials concerned are graphene, transition metal dichalcogenides (TMDs), hexagonal boron nitride (hBN), and topological insulators (TIs). The experimental investigations benefit from new methodologies and techniques in scanning tunneling microscopy (STM), including spin-polarized STM, scanning tunneling potentiometry (STP), scanning tunneling thermopower microscopy, and multi-probe STM. The experimental effort is complemented by the computational and theoretical approaches, capable of discriminating between closely competing states and achieving the length scales necessary to bridge across features such as local defects and complex heterostructures. The goal is to provide a general view of current understanding and challenges in studying the heterogeneities in 2D materials and to evaluate the potential of controlling and exploiting these heterogeneities for novel functionalities and electron devices.

Water on Graphene-Coated TiO2: Role of Atomic Vacancies

PubMed Central

2018-01-01

Beyond two-dimensional (2D) materials, interfaces between 2D materials and underlying supports or 2D-coated metal or metal oxide nanoparticles exhibit excellent properties and promising applications. The hybrid interface between graphene and anatase TiO2 shows great importance in photocatalytic, catalytic, and nanomedical applications due to the excellent and complementary properties of the two materials. Water, as a ubiquitous and essential element in practical conditions and in the human body, plays a significant role in the applications of graphene/TiO2 composites for both electronic devices and nanomedicine. Carbon vacancies, as common defects in chemically prepared graphene, also need to be considered for the application of graphene-based materials. Therefore, the behavior of water on top and at the interface of defective graphene on anatase TiO2 surface was systematically investigated by dispersion-corrected hybrid density functional calculations. The presence of the substrate only slightly enhances the on-top adsorption and reduces the on-top dissociation of water on defective graphene. However, at the interface, dissociated water is largely preferred compared with undissociated water on bare TiO2 surface, showing a prominent cover effect. Reduced TiO2 may further induce oxygen diffusion into the bulk. Our results are helpful to understand how the presence of water in the surrounding environment affects structural and electronic properties of the graphene/TiO2 interface and thus its application in photocatalysis, electronic devices, and nanomedicine. PMID:29368503

FeS2 /CoS2 Interface Nanosheets as Efficient Bifunctional Electrocatalyst for Overall Water Splitting.

PubMed

Li, Yuxuan; Yin, Jie; An, Li; Lu, Min; Sun, Ke; Zhao, Yong-Qin; Gao, Daqiang; Cheng, Fangyi; Xi, Pinxian

2018-05-28

Electrochemical water splitting to produce hydrogen and oxygen, as an important reaction for renewable energy storage, needs highly efficient and stable catalysts. Herein, FeS 2 /CoS 2 interface nanosheets (NSs) as efficient bifunctional electrocatalysts for overall water splitting are reported. The thickness and interface disordered structure with rich defects of FeS 2 /CoS 2 NSs are confirmed by atomic force microscopy and high-resolution transmission electron microscopy. Furthermore, extended X-ray absorption fine structure spectroscopy clarifies that FeS 2 /CoS 2 NSs with sulfur vacancies, which can further increase electrocatalytic performance. Benefiting from the interface nanosheets' structure with abundant defects, the FeS 2 /CoS 2 NSs show remarkable hydrogen evolution reaction (HER) performance with a low overpotential of 78.2 mV at 10 mA cm -2 and a superior stability for 80 h in 1.0 m KOH, and an overpotential of 302 mV at 100 mA cm -2 for the oxygen evolution reaction (OER). More importantly, the FeS 2 /CoS 2 NSs display excellent performance for overall water splitting with a voltage of 1.47 V to achieve current density of 10 mA cm -2 and maintain the activity for at least 21 h. The present work highlights the importance of engineering interface nanosheets with rich defects based on transition metal dichalcogenides for boosting the HER and OER performance. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Isolated Spin Qubits in SiC with a High-Fidelity Infrared Spin-to-Photon Interface

NASA Astrophysics Data System (ADS)

Christle, David J.; Klimov, Paul V.; de las Casas, Charles F.; Szász, Krisztián; Ivády, Viktor; Jokubavicius, Valdas; Ul Hassan, Jawad; Syväjärvi, Mikael; Koehl, William F.; Ohshima, Takeshi; Son, Nguyen T.; Janzén, Erik; Gali, Ádám; Awschalom, David D.

2017-04-01

The divacancies in SiC are a family of paramagnetic defects that show promise for quantum communication technologies due to their long-lived electron spin coherence and their optical addressability at near-telecom wavelengths. Nonetheless, a high-fidelity spin-photon interface, which is a crucial prerequisite for such technologies, has not yet been demonstrated. Here, we demonstrate that such an interface exists in isolated divacancies in epitaxial films of 3C-SiC and 4H-SiC. Our data show that divacancies in 4H-SiC have minimal undesirable spin mixing, and that the optical linewidths in our current sample are already similar to those of recent remote entanglement demonstrations in other systems. Moreover, we find that 3C-SiC divacancies have a millisecond Hahn-echo spin coherence time, which is among the longest measured in a naturally isotopic solid. The presence of defects with these properties in a commercial semiconductor that can be heteroepitaxially grown as a thin film on Si shows promise for future quantum networks based on SiC defects.

Atomic and electronic structure of a copper/graphene interface as prepared and 1.5 years after

NASA Astrophysics Data System (ADS)

Boukhvalov, D. W.; Bazylewski, P. F.; Kukharenko, A. I.; Zhidkov, I. S.; Ponosov, Yu. S.; Kurmaev, E. Z.; Cholakh, S. O.; Lee, Y. H.; Chang, G. S.

2017-12-01

We report the results of X-ray spectroscopy and Raman measurements of as-prepared graphene on a high quality copper surface and the same materials after 1.5 years under different conditions (ambient and low humidity). The obtained results were compared with density functional theory calculations of the formation energies and electronic structures of various structural defects in graphene/Cu interfaces. For evaluation of the stability of the carbon cover, we propose a two-step model. The first step is oxidation of the graphene, and the second is perforation of graphene with the removal of carbon atoms as part of the carbon dioxide molecule. Results of the modeling and experimental measurements provide evidence that graphene grown on high-quality copper substrate becomes robust and stable in time (1.5 years). However, the stability of this interface depends on the quality of the graphene and the number of native defects in the graphene and substrate. The effect of the presence of a metallic substrate with defects on the stability and electronic structure of graphene is also discussed

A deep-level transient spectroscopy study of gamma-ray irradiation on the passivation properties of silicon nitride layer on silicon

NASA Astrophysics Data System (ADS)

Dong, Peng; Yu, Xuegong; Ma, Yao; Xie, Meng; Li, Yun; Huang, Chunlai; Li, Mo; Dai, Gang; Zhang, Jian

2017-08-01

Plasma-enhanced chemical vapor deposited silicon nitride (SiNx) films are extensively used as passivation material in the solar cell industry. Such SiNx passivation layers are the most sensitive part to gamma-ray irradiation in solar cells. In this work, deep-level transient spectroscopy has been applied to analyse the influence of gamma-ray irradiation on the passivation properties of SiNx layer on silicon. It is shown that the effective carrier lifetime decreases with the irradiation dose. At the same time, the interface state density is significantly increased after irradiation, and its energy distribution is broadened and shifts deeper with respect to the conduction band edge, which makes the interface states becoming more efficient recombination centers for carriers. Besides, C-V characteristics show a progressive negative shift with increasing dose, indicating the generation of effective positive charges in SiNx films. Such positive charges are beneficial for shielding holes from the n-type silicon substrates, i. e. the field-effect passivation. However, based on the reduced carrier lifetime after irradiation, it can be inferred that the irradiation induced interface defects play a dominant role over the trapped positive charges, and therefore lead to the degradation of passivation properties of SiNx on silicon.

A Theoretical Simulation of the Radiation Responses of Si, Ge, and Si/Ge Superlattice to Low-Energy Irradiation.

PubMed

Jiang, Ming; Xiao, Haiyan; Peng, Shuming; Yang, Guixia; Liu, Zijiang; Qiao, Liang; Zu, Xiaotao

2018-05-02

In this study, the low-energy radiation responses of Si, Ge, and Si/Ge superlattice are investigated by an ab initio molecular dynamics method and the origins of their different radiation behaviors are explored. It is found that the radiation resistance of the Ge atoms that are around the interface of Si/Ge superlattice is comparable to bulk Ge, whereas the Si atoms around the interface are more difficult to be displaced than the bulk Si, showing enhanced radiation tolerance as compared with the bulk Si. The mechanisms for defect generation in the bulk and superlattice structures show somewhat different character, and the associated defects in the superlattice are more complex. Defect formation and migration calculations show that in the superlattice structure, the point defects are more difficult to form and the vacancies are less mobile. The enhanced radiation tolerance of the Si/Ge superlattice will benefit for its applications as electronic and optoelectronic devices under radiation environment.

A Theoretical Simulation of the Radiation Responses of Si, Ge, and Si/Ge Superlattice to Low-Energy Irradiation

NASA Astrophysics Data System (ADS)

Jiang, Ming; Xiao, Haiyan; Peng, Shuming; Yang, Guixia; Liu, Zijiang; Qiao, Liang; Zu, Xiaotao

2018-05-01

In this study, the low-energy radiation responses of Si, Ge, and Si/Ge superlattice are investigated by an ab initio molecular dynamics method and the origins of their different radiation behaviors are explored. It is found that the radiation resistance of the Ge atoms that are around the interface of Si/Ge superlattice is comparable to bulk Ge, whereas the Si atoms around the interface are more difficult to be displaced than the bulk Si, showing enhanced radiation tolerance as compared with the bulk Si. The mechanisms for defect generation in the bulk and superlattice structures show somewhat different character, and the associated defects in the superlattice are more complex. Defect formation and migration calculations show that in the superlattice structure, the point defects are more difficult to form and the vacancies are less mobile. The enhanced radiation tolerance of the Si/Ge superlattice will benefit for its applications as electronic and optoelectronic devices under radiation environment.

Carbon, oxygen and their interaction with intrinsic point defects in solar silicon ribbon material: A speculative approach

NASA Technical Reports Server (NTRS)

Goesele, U.; Ast, D. G.

1983-01-01

Some background information on intrinsic point defects is provided and on carbon and oxygen in silicon in so far as it may be relevant for the efficiency of solar cells fabricated from EFG ribbon material. The co-precipitation of carbon and oxygen and especially of carbon and silicon self interstitials are discussed. A simple model for the electrical activity of carbon-self-interstitial agglomerates is presented. The self-interstitial content of these agglomerates is assumed to determine their electrical activity and that both compressive stresses (high self-interstitial content) and tensile stresses (low self-interstitial content) give rise to electrical activity of the agglomerates. The self-interstitial content of these carbon-related agglomerates may be reduced by an appropriate high temperature treatment and enhanced by a supersaturation of self-interstitials generated during formation of the p-n junction of solar cells. Oxygen present in supersaturation in carbon-rich silicon may be induced to form SiO, precipitates by self-interstitials generated during phosphorus diffusion. It is proposed that the SiO2-Si interface of the precipates gives rise to a continuum of donor stables and that these interface states are responsible for at least part of the light inhancement effects observed in oxygen containing EFG silicon after phosphorus diffusion.

Polarization Coupling in Ferroelectric Multilayers as a Function of Interface Charge Concentration

NASA Astrophysics Data System (ADS)

Okatan, Mahmut; Mantese, Joseph; Alpay, Pamir

2009-03-01

Intriguing properties of multilayered and graded ferroelectrics follow from the electrostatic and electromechanical interactions. The strength of the interlayer coupling depends on the concentration of interfacial defects with short-range local electrostatic fields. Defects may locally relax polarization differences and thus reduce the commensurate bound charge concentration at the interlayer interfaces. In this talk, we develop a theoretical analysis based on non-linear thermodynamics coupled with basic electrostatic relations to understand the role of charge compensation at the interlayer interfaces. The results show multilayered ferroelectrics with systematic variations in the composition may display a colossal dielectric response depending upon the interlayer electrostatic interactions. It is expected that other properties such as the pyroelectric and piezoelectric response will yield concomitant increases through the dielectric permittivity.

Relaxation mechanisms, structure and properties of semi-coherent interfaces

DOE PAGES

Shao, Shuai; Wang, Jian

2015-10-15

In this work, using the Cu–Ni (111) semi-coherent interface as a model system, we combine atomistic simulations and defect theory to reveal the relaxation mechanisms, structure, and properties of semi-coherent interfaces. By calculating the generalized stacking fault energy (GSFE) profile of the interface, two stable structures and a high-energy structure are located. During the relaxation, the regions that possess the stable structures expand and develop into coherent regions; the regions with high-energy structure shrink into the intersection of misfit dislocations (nodes). This process reduces the interface excess potential energy but increases the core energy of the misfit dislocations and nodes.more » The core width is dependent on the GSFE of the interface. The high-energy structure relaxes by relative rotation and dilatation between the crystals. The relative rotation is responsible for the spiral pattern at nodes. The relative dilatation is responsible for the creation of free volume at nodes, which facilitates the nodes’ structural transformation. Several node structures have been observed and analyzed. In conclusion, the various structures have significant impact on the plastic deformation in terms of lattice dislocation nucleation, as well as the point defect formation energies.« less

(001) 3C SiC/Ni contact interface: In situ XPS observation of annealing induced Ni2Si formation and the resulting barrier height changes

NASA Astrophysics Data System (ADS)

Tengeler, Sven; Kaiser, Bernhard; Chaussende, Didier; Jaegermann, Wolfram

2017-04-01

The electronic states of the (001) 3C SiC/Ni interface prior and post annealing are investigated via an in situ XPS interface experiment, allowing direct observation of the induced band bending and the transformation from Schottky to ohmic behaviour for the first time. A single domain (001) 3C SiC sample was prepared via wet chemical etching. Nickel was deposited on the sample in multiple in situ deposition steps via RF sputtering, allowing observation of the 3C SiC/Ni interface formation. Over the course of the experiments, an upward band bending of 0.35 eV was observed, along with defect induced Fermi level pinning. This indicates a Schottky type contact behaviour with a barrier height of 0.41 eV. The subsequent annealing at 850 °C for 5 min resulted in the formation of a Ni2Si layer and a reversal of the band bending to 0.06 eV downward. Thus explaining the ohmic contact behaviour frequently reported for annealed n-type 3C SiC/Ni contacts.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shao, Shuai; Wang, Jian

In this work, using the Cu–Ni (111) semi-coherent interface as a model system, we combine atomistic simulations and defect theory to reveal the relaxation mechanisms, structure, and properties of semi-coherent interfaces. By calculating the generalized stacking fault energy (GSFE) profile of the interface, two stable structures and a high-energy structure are located. During the relaxation, the regions that possess the stable structures expand and develop into coherent regions; the regions with high-energy structure shrink into the intersection of misfit dislocations (nodes). This process reduces the interface excess potential energy but increases the core energy of the misfit dislocations and nodes.more » The core width is dependent on the GSFE of the interface. The high-energy structure relaxes by relative rotation and dilatation between the crystals. The relative rotation is responsible for the spiral pattern at nodes. The relative dilatation is responsible for the creation of free volume at nodes, which facilitates the nodes’ structural transformation. Several node structures have been observed and analyzed. In conclusion, the various structures have significant impact on the plastic deformation in terms of lattice dislocation nucleation, as well as the point defect formation energies.« less

Temperature dependence of trapping effects in metal gates/Al2O3/InGaAs stacks

NASA Astrophysics Data System (ADS)

Palumbo, F.; Pazos, S.; Aguirre, F.; Winter, R.; Krylov, I.; Eizenberg, M.

2017-06-01

The influence of the temperature on Metal Gate/Al2O3/n-InGaAs stacks has been studied by means of capacitance-voltage (C-V) hysteresis and flat band voltage as function of both negative and positive stress fields. It was found that the de-trapping effect decreases at low-temperature, indicating that the de-trapping of trapped electrons from oxide traps may be performed via Al2O3/InGaAs interface defects. The dependence of the C-V hysteresis on the stress field at different temperatures in our InGaAs stacks can be explained in terms of the defect spatial distribution. An oxide defect distribution can be found very close to the metal gate/Al2O3 interface. On the other side, the Al2O3/InGaAs interface presents defects distributed from the interface into the bulk of the oxide, showing the influence of InGaAs on Al2O3 in terms of the spatial defect distribution. At the present, he is a research staff of the National Council of Science and Technology (CONICET), working in the National Commission of Atomic Energy (CNEA) in Buenos Aires, Argentina, well embedded within international research collaboration. Since 2008, he is Professor at the National Technological University (UTN) in Buenos Aires, Argentina. Dr. Palumbo has received research fellowships from: Marie Curie Fellowship within the 7th European Community Framework Programme, Abdus Salam International Centre for Theoretical Physics (ICTP) Italy, National Council of Science and Technology (CONICET) Argentina, and Consiglio Nazionale delle Ricerche (CNR) Italy. He is also a frequent scientific visitor of academic institutions as IMM-CNR-Italy, Minatec Grenoble-France, the Autonomous University of Barcelona-Spain, and the Israel Institute of Technology-Technion. He has authored and co-authored more than 50 papers in international conferences and journals.

Impact of extended defects on recombination in CdTe heterostructures grown by molecular beam epitaxy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zaunbrecher, Katherine N.; National Renewable Energy Laboratory, Golden, Colorado 80401; Kuciauskas, Darius

Heterostructures with CdTe and CdTe{sub 1-x}Se{sub x} (x ∼ 0.01) absorbers between two wider-band-gap Cd{sub 1-x}Mg{sub x}Te barriers (x ∼ 0.25–0.3) were grown by molecular beam epitaxy to study carrier generation and recombination in bulk materials with passivated interfaces. Using a combination of confocal photoluminescence (PL), time-resolved PL, and low-temperature PL emission spectroscopy, two extended defect types were identified and the impact of these defects on charge-carrier recombination was analyzed. The dominant defects identified by confocal PL were dislocations in samples grown on (211)B CdTe substrates and crystallographic twinning-related defects in samples on (100)-oriented InSb substrates. Low-temperature PL shows that twin-related defects have amore » zero-phonon energy of 1.460 eV and a Huang-Rhys factor of 1.50, while dislocation-dominated samples have a 1.473-eV zero-phonon energy and a Huang-Rhys factor of 1.22. The charge carrier diffusion length near both types of defects is ∼6 μm, suggesting that recombination is limited by diffusion dynamics. For heterostructures with a low concentration of extended defects, the bulk lifetime was determined to be 2.2 μs with an interface recombination velocity of 160 cm/s and an estimated radiative lifetime of 91 μs.« less

Topological defects in electric double layers of ionic liquids at carbon interfaces

DOE PAGES

Black, Jennifer M.; Okatan, Mahmut Baris; Feng, Guang; ...

2015-06-07

The structure and properties of the electrical double layer in ionic liquids is of interest in a wide range of areas including energy storage, catalysis, lubrication, and many more. Theories describing the electrical double layer for ionic liquids have been proposed, however a full molecular level description of the double layer is lacking. To date, studies have been predominantly focused on ion distributions normal to the surface, however the 3D nature of the electrical double layer in ionic liquids requires a full picture of the double layer structure not only normal to the surface, but also in plane. Here wemore » utilize 3D force mapping to probe the in plane structure of an ionic liquid at a graphite interface and report the direct observation of the structure and properties of topological defects. The observation of ion layering at structural defects such as step-edges, reinforced by molecular dynamics simulations, defines the spatial resolution of the method. Observation of defects allows for the establishment of the universality of ionic liquid behavior vs. separation from the carbon surface and to map internal defect structure. In conclusion, these studies offer a universal pathway for probing the internal structure of topological defects in soft condensed matter on the nanometer level in three dimensions.« less

Electronic properties of MoS2 / MoOx interfaces: Implications in Tunnel Field Effect Transistors and Hole Contacts

DOE PAGES

Santosh, K. C.; Longo, Roberto; Addou, Rafik; ...

2016-09-26

In an electronic device based on two dimensional (2D) transitional metal dichalcogenides (TMDs), finding a low resistance metal contact is critical in order to achieve the desired performance. However, due to the unusual Fermi level pinning in metal/2D TMD interface, the performance is limited. Here, we investigate the electronic properties of TMDs and transition metal oxide (TMO) interfaces (MoS 2/MoO 3) using density functional theory (DFT). Our results demonstrate that, due to the large work function of MoO 3 and the relative band alignment with MoS 2, together with small energy gap, the MoS 2/MoO 3 interface is a goodmore » candidate for a tunnel field effect (TFET)-type device. Moreover, if the interface is not stoichiometric because of the presence of oxygen vacancies in MoO 3, the heterostructure is more suitable for p-type (hole) contacts, exhibiting an Ohmic electrical behavior as experimentally demonstrated for different TMO/TMD interfaces. Our results reveal that the defect state induced by an oxygen vacancy in the MoO3 aligns with the valance band of MoS 2, showing an insignificant impact on the band gap of the TMD. This result highlights the role of oxygen vacancies in oxides on facilitating appropriate contacts at the MoS 2 and MoO x (x < 3) interface, which consistently explains the available experimental observations.« less

Electronic properties of MoS2/MoOx interfaces: Implications in Tunnel Field Effect Transistors and Hole Contacts

PubMed Central

K. C., Santosh; Longo, Roberto C.; Addou, Rafik; Wallace, Robert M.; Cho, Kyeongjae

2016-01-01

In an electronic device based on two dimensional (2D) transitional metal dichalcogenides (TMDs), finding a low resistance metal contact is critical in order to achieve the desired performance. However, due to the unusual Fermi level pinning in metal/2D TMD interface, the performance is limited. Here, we investigate the electronic properties of TMDs and transition metal oxide (TMO) interfaces (MoS2/MoO3) using density functional theory (DFT). Our results demonstrate that, due to the large work function of MoO3 and the relative band alignment with MoS2, together with small energy gap, the MoS2/MoO3 interface is a good candidate for a tunnel field effect (TFET)-type device. Moreover, if the interface is not stoichiometric because of the presence of oxygen vacancies in MoO3, the heterostructure is more suitable for p-type (hole) contacts, exhibiting an Ohmic electrical behavior as experimentally demonstrated for different TMO/TMD interfaces. Our results reveal that the defect state induced by an oxygen vacancy in the MoO3 aligns with the valance band of MoS2, showing an insignificant impact on the band gap of the TMD. This result highlights the role of oxygen vacancies in oxides on facilitating appropriate contacts at the MoS2 and MoOx (x < 3) interface, which consistently explains the available experimental observations. PMID:27666523

Interactions of Organic Solvents at Graphene/α-Al 2 O 3 and Graphene Oxide/α-Al 2 O 3 Interfaces Studied by Sum Frequency Generation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Achtyl, Jennifer L.; Vlassiouk, Ivan V.; Dai, Sheng

2014-07-17

The adsorption of 1-hexanol from cyclohexane-d12 at single-layer graphene/α-Al2O3 interfaces was probed at mole percent values as low as 0.05 in the C–H stretching region using vibrational sum frequency generation (SFG). The SFG spectra are indiscernible from those obtained in the absence of graphene, and from those obtained in the presence of graphene oxide films prepared via oxygen plasma treatment of pristine single-layer graphene. A Langmuir adsorption model yields observed free adsorption energies of -19.9(5) to -20.9(3) kJ/mol for the three interfaces. The results indicate that the molecular structure of the hexanol alkyl chain is subject to the same orientationmore » distribution when graphene, oxidized or not, is present or absent at the α-Al2O3/cyclohexane-d12 interface. Moreover, it appears that the adsorption of 1-hexanol in this binary mixture is driven by hexanol interactions with the underlying oxide support, and that a single layer of graphene does not influence the extent of this interaction, even when defects are introduced to it. Finally, our structural and quantitative thermodynamic data provide important benchmarks for theoretical calculations and atomistic simulations of liquid/graphene interfaces. We hypothesize that defects emerging in graphene during operation of any device application that relies on layered solvent/graphene/oxide interfaces have little impact on the interfacial structure or thermodynamics, at least for the binary mixture and over the range of defect densities probed in our studies.« less

Asymmetry of radiation damage properties in Al-Ti nanolayers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Setyawan, Wahyu; Gerboth, Matthew D.; Yao, Bo

2014-02-01

Molecular dynamics (MD) simulations were employed with empirical potentials to study the effects of multilayer interfaces and interface spacing in Al-Ti nanolayers. Several model interfaces derived from stacking of close-packed layers or face-centered cubic \\{100\\} layers were investigated. The simulations reveal significant and important asymmetries in defect production withmore » $$\\sim$$60\\% of vacancies created in Al layers compared to Ti layers within the Al-Ti multilayer system. The asymmetry in the creation of interstitials is even more pronounced. The asymmetries cause an imbalance in the ratio of vacancies and interstitials in films of dissimilar materials leading to $>$$90\\% of the surviving interstitials located in the Al layers. While in the close-packed nanolayers the interstitials migrate to the atomic layers adjacent to the interface of the Al layers, in the \\{100\\} nanolayers the interstitials migrate to the center of the Al layers and away from the interfaces. The degree of asymmetry and defect ratio imbalance increases as the layer spacing decreases in the multilayer films. Underlying physical processes are discussed including the interfacial strain fields and the individual elemental layer stopping power in nanolayered systems. In addition, experimental work was performed on low-dose (10$$^{16}$ atoms/cm$^2$) helium (He) irradiation on Al/Ti nanolayers (5 nm per film), resulting in He bubble formation $$\\sim$$1 nm in diameter in the Ti film near the interface. The correlation between the preferential flux of displaced atoms from Ti films to Al films during the defect production that is revealed in the simulations and the morphology and location of He bubbles from the experiments is discussed.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Marrakchi, G.; Barbier, D.; Guillot, G.

Electrical and deep level transient spectroscopy measurements on Schottky barriers were performed in order to characterize electrically active defects in n-type GaAs (Bridgman substrates or liquid-phase epitaxial layers) after pulsed electron beam annealing. Both surface damage and bulk defects were observed in the Bridgman substrates depending on the pulse energy density. No electron traps were detected in the liquid-phase epitaxial layers before and after annealing for an energy density of 0.4 J/cm/sup 2/. The existence of an interfacial insulating layer at the metal-semiconductor interface, associated with As out-diffusion during the pulsed electron irradiation, was revealed by the abnormally high valuesmore » of the Schottky barrier diffusion potential. Moreover, two new electron traps with activation energy of 0.35 and 0.43 eV, called EP1 and EP2, were introduced in the Bridgman substrates after pulsed electron beam annealing. The presence of these traps, related to the As evaporation, was tentatively attributed to the decrease of the EL2 electron trap signal after 0.4-J/cm/sup 2/ annealing. It is proposed that these new defects states are due to the decomposition of the As/sub Ga/-As/sub i/ complex recently considered as the most probable defect configuration for the dominant EL2 electron trap usually detected in as-grown GaAs substrates.« less

Magnetization due to localized states on graphene grain boundary

PubMed Central

Dutta, Sudipta; Wakabayashi, Katsunori

2015-01-01

Magnetism in graphene has been found to originate from various defects, e.g., vacancy, edge formation, add-atoms etc. Here, we discuss about an alternate route of achieving magnetism in graphene via grain boundary. During chemical vapor deposition of graphene, several graphene nucleation centers grow independently and face themselves with unusual bonding environment, giving rise to the formation of grain boundaries. We investigate the origin of magnetism in such grain boundaries within first-principles calculations, by letting two nucleation centers interact with each other at their interface. We observe formation of unprecedented point defect, consisting of fused three-membered and larger carbon rings, which induces net magnetization to graphene quantum dots. In case of periodic lattices, the appearance of array of point defects leads to the formation of magnetic grain boundaries. The net magnetization on these defects arises due to the deviation from bipartite characteristics of pristine graphene. We observe magnetic grain boundary induced dispersion less flat bands near Fermi energy, showing higher localization of electrons. These flat bands can be accessed via small doping, leading to enhanced magnetism. Moreover, the grain boundaries can induce asymmetric spin conduction behavior along the cross boundary direction. These properties can be exploited for sensor and spin-filtering applications. PMID:26145161

Influence of ZnO seed layer precursor molar ratio on the density of interface defects in low temperature aqueous chemically synthesized ZnO nanorods/GaN light-emitting diodes

NASA Astrophysics Data System (ADS)

Alnoor, Hatim; Pozina, Galia; Khranovskyy, Volodymyr; Liu, Xianjie; Iandolo, Donata; Willander, Magnus; Nur, Omer

2016-04-01

Low temperature aqueous chemical synthesis (LT-ACS) of zinc oxide (ZnO) nanorods (NRs) has been attracting considerable research interest due to its great potential in the development of light-emitting diodes (LEDs). The influence of the molar ratio of the zinc acetate (ZnAc): KOH as a ZnO seed layer precursor on the density of interface defects and hence the presence of non-radiative recombination centers in LT-ACS of ZnO NRs/GaN LEDs has been systematically investigated. The material quality of the as-prepared seed layer as quantitatively deduced by the X-ray photoelectron spectroscopy is found to be influenced by the molar ratio. It is revealed by spatially resolved cathodoluminescence that the seed layer molar ratio plays a significant role in the formation and the density of defects at the n-ZnO NRs/p-GaN heterostructure interface. Consequently, LED devices processed using ZnO NRs synthesized with molar ratio of 1:5 M exhibit stronger yellow emission (˜575 nm) compared to those based on 1:1 and 1:3 M ratios as measured by the electroluminescence. Furthermore, seed layer molar ratio shows a quantitative dependence of the non-radiative defect densities as deduced from light-output current characteristics analysis. These results have implications on the development of high-efficiency ZnO-based LEDs and may also be helpful in understanding the effects of the ZnO seed layer on defect-related non-radiative recombination.

Techniques and software tools for estimating ultrasonic signal-to-noise ratios

NASA Astrophysics Data System (ADS)

Chiou, Chien-Ping; Margetan, Frank J.; McKillip, Matthew; Engle, Brady J.; Roberts, Ronald A.

2016-02-01

At Iowa State University's Center for Nondestructive Evaluation (ISU CNDE), the use of models to simulate ultrasonic inspections has played a key role in R&D efforts for over 30 years. To this end a series of wave propagation models, flaw response models, and microstructural backscatter models have been developed to address inspection problems of interest. One use of the combined models is the estimation of signal-to-noise ratios (S/N) in circumstances where backscatter from the microstructure (grain noise) acts to mask sonic echoes from internal defects. Such S/N models have been used in the past to address questions of inspection optimization and reliability. Under the sponsorship of the National Science Foundation's Industry/University Cooperative Research Center at ISU, an effort was recently initiated to improve existing research-grade software by adding graphical user interface (GUI) to become user friendly tools for the rapid estimation of S/N for ultrasonic inspections of metals. The software combines: (1) a Python-based GUI for specifying an inspection scenario and displaying results; and (2) a Fortran-based engine for computing defect signal and backscattered grain noise characteristics. The latter makes use of several models including: the Multi-Gaussian Beam Model for computing sonic fields radiated by commercial transducers; the Thompson-Gray Model for the response from an internal defect; the Independent Scatterer Model for backscattered grain noise; and the Stanke-Kino Unified Model for attenuation. The initial emphasis was on reformulating the research-grade code into a suitable modular form, adding the graphical user interface and performing computations rapidly and robustly. Thus the initial inspection problem being addressed is relatively simple. A normal-incidence pulse/echo immersion inspection is simulated for a curved metal component having a non-uniform microstructure, specifically an equiaxed, untextured microstructure in which the average grain size may vary with depth. The defect may be a flat-bottomed-hole reference reflector, a spherical void or a spherical inclusion. In future generations of the software, microstructures and defect types will be generalized and oblique incidence inspections will be treated as well. This paper provides an overview of the modeling approach and presents illustrative results output by the first-generation software.

Influence of processing factors on the stability of model mayonnaise with whole egg during long-term storage.

PubMed

Ariizumi, Masahiro; Kubo, Megumi; Handa, Akihiro; Hayakawa, Takashi; Matsumiya, Kentaro; Matsumura, Yasuki

2017-04-01

Mayonnaise-like oil-in-water emulsions with different stabilities-evaluated from the degree of macroscopic defects, e.g., syneresis-were prepared by different formulations and processing conditions (egg yolk weight, homogenizer speed, and vegetable oil temperature). Emulsions prepared with lower egg yolk content were destabilized for shorter periods. The long-term stability of emulsions was weakly related to initial properties, e.g., oil droplet distribution and protein coverage at the interface. Protein aggregation between oil droplets was observed and would be responsible for the instability of emulsions exhibited by the appearance defects. SDS-PAGE results for adsorbed and unadsorbed proteins at the O/W interface suggested that predominant constituents adsorbed onto the interface were egg white proteins as compared with egg yolk components when the amount of added egg yolk was low. In present condition, egg white proteins adsorbed at the O/W interface could be a bridge of neighboring oil droplets thereby causing flocculation in emulsions.

PREFACE: The International Workshop on Positron Studies of Defects 2014

NASA Astrophysics Data System (ADS)

Sugita, Kazuki; Shirai, Yasuharu

2016-01-01

The International Workshop on Positron Studies of Defects 2014 (PSD-14) was held in Kyoto, Japan from 14-19 September, 2014. The PSD Workshop brought together positron scientists interested in studying defects to an international platform for presenting and discussing recent results and achievements, including new experimental and theoretical methods in the field. The workshop topics can be characterized as follows: • Positron studies of defects in semiconductors and oxides • Positron studies of defects in metals • New experimental methods and equipment • Theoretical calculations and simulations of momentum distributions, positron lifetimes and other characteristics for defects • Positron studies of defects in combination with complementary methods • Positron beam studies of defects at surfaces, interfaces, in sub-surface regions and thin films • Nanostructures and amorphous materials

Thermal stability of Ag, Al, Sn, Pb, and Hg films reinforced by 2D (C, Si) crystals and the formation of interfacial fluid states in them upon heating. MD experiment

NASA Astrophysics Data System (ADS)

Polukhin, V. A.; Kurbanova, E. D.

2016-02-01

Molecular dynamics simulation is used to study the thermal stability of the interfacial states of metallic Al, Ag, Sn, Pb, and Hg films (i.e., the structural elements of superconductor composites and conducting electrodes) reinforced by 2D graphene and silicene crystals upon heating up to disordering and to analyze the formation of nonautonomous fluid pseudophases in interfaces. The effect of perforation defects in reinforcing 2D-C and 2D-Si planes with passivated edge covalent bonds on the atomic dynamics is investigated. As compared to Al and Ag, the diffusion coefficients in Pd and Hg films increase monotonically with temperature during thermally activated disordering processes, the interatomic distances decrease, the sizes decrease, drops form, and their density profile grows along the normal. The coagulation of Pb and Hg drops is accompanied by a decrease in the contact angle, the reduction of the interface contact with graphene, and the enhancement of its corrugation (waviness).

Interfacial and electrical properties of InGaAs metal-oxide-semiconductor capacitor with TiON/TaON multilayer composite gate dielectric

NASA Astrophysics Data System (ADS)

Wang, L. S.; Xu, J. P.; Liu, L.; Lu, H. H.; Lai, P. T.; Tang, W. M.

2015-03-01

InGaAs metal-oxide-semiconductor (MOS) capacitors with composite gate dielectric consisting of Ti-based oxynitride (TiON)/Ta-based oxynitride (TaON) multilayer are fabricated by RF sputtering. The interfacial and electrical properties of the TiON/TaON/InGaAs and TaON/TiON/InGaAs MOS structures are investigated and compared. Experimental results show that the former exhibits lower interface-state density (1.0 × 1012 cm-2 eV-1 at midgap), smaller gate leakage current (9.5 × 10-5 A/cm2 at a gate voltage of 2 V), larger equivalent dielectric constant (19.8), and higher reliability under electrical stress than the latter. The involved mechanism lies in the fact that the ultrathin TaON interlayer deposited on the sulfur-passivated InGaAs surface can effectively reduce the defective states and thus unpin the Femi level at the TaON/InGaAs interface, improving the electrical properties of the device.

Electric polarization control of magnetoresistance in complex oxide heterojunctions

NASA Astrophysics Data System (ADS)

Swartz, Adrian G.; Inoue, Hisashi; Hwang, Harold Y.

2016-10-01

Lorentzian magnetoresistance (L-MR) has been widely observed in three-terminal ferromagnet-nonmagnet (FM-NM) tunnel junctions. One possible explanation for this behavior is ensemble dephasing (Hanle effect) of a spin accumulation, potentially offering a powerful approach for characterizing the spin lifetime of emerging spintronics materials. However, discrepancies between the extracted spin parameters with known materials properties has cast doubt on this interpretation for most implementations. Here, we have developed a method to control band alignments in perovskite oxide heterostructures through the use of epitaxial interface dipoles, providing a highly effective method for manipulating the Schottky barrier height and contact resistance. Using these atomically engineered heterojunctions, we are able to tune key parameters relevant to various spin accumulation models, providing an experimental platform which can test their applicability. We find that the observed L-MR is inconsistent with an interpretation of spin accumulation in either the NM material or in interface states. Rather, we consider a mechanism analogous to Coulomb blockade in quantum dots, where spin-dependent tunneling through an ensemble of interfacial defect states is controlled by local and external magnetic fields.

Positron beam study of indium tin oxide films on GaN

NASA Astrophysics Data System (ADS)

Cheung, C. K.; Wang, R. X.; Beling, C. D.; Djurisic, A. B.; Fung, S.

2007-02-01

Variable energy Doppler broadening spectroscopy has been used to study open-volume defects formed during the fabrication of indium tin oxide (ITO) thin films grown by electron-beam evaporation on n-GaN. The films were prepared at room temperature, 200 and 300 °C without oxygen and at 200 °C under different oxygen partial pressures. The results show that at elevated growth temperatures the ITO has fewer open volume sites and grows with a more crystalline structure. High temperature growth, however, is not sufficient in itself to remove open volume defects at the ITO/GaN interface. Growth under elevated temperature and under partial pressure of oxygen is found to further reduce the vacancy type defects associated with the ITO film, thus improving the quality of the film. Oxygen partial pressures of 6 × 10-3 mbar and above are found to remove open volume defects associated with the ITO/GaN interface. The study suggests that, irrespective of growth temperature and oxygen partial pressure, there is only one type of defect in the ITO responsible for trapping positrons, which we tentatively attribute to the oxygen vacancy.

Diffraction inspired unidirectional and bidirectional beam splitting in defect-containing photonic structures without interface corrugations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Colak, Evrim; Serebryannikov, Andriy E., E-mail: andser@amu.edu.pl; Usik, P. V.

2016-05-21

It is shown that strong diffractions and related dual-beam splitting can be obtained at transmission through the nonsymmetric structures that represent two slabs of photonic crystal (PhC) separated by a single coupled-cavity type defect layer, while there are no grating-like corrugations at the interfaces. The basic operation regimes include unidirectional and bidirectional splitting that occur due to the dominant contribution of the first positive and first negative diffraction orders to the transmission, which is typically connected with different manifestations of the asymmetric transmission phenomenon. Being the main component of the resulting transmission mechanism, diffractions appear owing to the effect exertedmore » by the defect layer that works like an embedded diffractive element. Two mechanisms can co-exist in one structure, which differ, among others, in that whether dispersion allows coupling of zero order to a wave propagating in the regular, i.e., defect-free PhC segments or not. The possibility of strong diffractions and efficient splitting related to it strongly depend on the dispersion properties of the Floquet-Bloch modes of the PhC. Existence of one of the studied transmission scenarios is not affected by location of the defect layer.« less

Dimensional control of defect dynamics in perovskite oxide superlattices

NASA Astrophysics Data System (ADS)

Bredeson, Isaac; Zhang, Lipeng; Kent, P. R. C.; Cooper, Valentino R.; Xu, Haixuan

2018-03-01

Point defects play a critical role in the structural, physical, and interfacial properties of perovskite oxide superlattices. However, understanding of the fundamental properties of point defects in superlattices, especially their transport properties, is rather limited. Here, we report predictions of the stability and dynamics of oxygen vacancies in SrTi O3/PbTi O3 oxide superlattices using first-principles calculations in combination with the kinetic Monte Carlo method. By varying the stacking period, i.e., changing of n in n STO /n PTO , we discover a crossover from three-dimensional diffusion to primarily two-dimensional planar diffusion. Such planar diffusion may lead to novel designs of ionic conductors. We show that the dominant vacancy position may vary in the superlattices, depending on the superlattice structure and stacking period, contradicting the common assumption that point defects reside at interfaces. Moreover, we predict a significant increase in room-temperature ionic conductivity for 3STO/3PTO relative to the bulk phases. Considering the variety of cations that can be accommodated in perovskite superlattices and the potential mismatch of spin, charge, and orbitals at the interfaces, this paper identifies a pathway to control defect dynamics for technological applications.

Phase behavior of charged hydrophobic colloids on flat and spherical surfaces

NASA Astrophysics Data System (ADS)

Kelleher, Colm P.

For a broad class of two-dimensional (2D) materials, the transition from isotropic fluid to crystalline solid is described by the theory of melting due to Kosterlitz, Thouless, Halperin, Nelson and Young (KTHNY). According to this theory, long-range order is achieved via elimination of the topological defects which proliferate in the fluid phase. However, many natural and man-made 2D systems posses spatial curvature and/or non-trivial topology, which require the presence of topological defects, even at T=0. In principle, the presence of these defects could profoundly affect the phase behavior of such a system. In this thesis, we develop and characterize an experimental system of charged colloidal particles that bind electrostatically to the interface between an oil and an aqueous phase. Depending on how we prepare the sample, this fluid interface may be flat, spherical, or have a more complicated geometry. Focusing on the cases where the interface is flat or spherical, we measure the interactions between the particles, and probe various aspects of their phase behavior. On flat interfaces, this phase behavior is well-described by KTHNY theory. In spherical geometries, however, we observe spatial structures and inhomogeneous dynamics that cannot be captured by the measures traditionally used to describe flat-space phase behavior. We show that, in the spherical system, ordering is achieved by a novel mechanism: sequestration of topological defects into freely-terminating grain boundaries ("scars"), and simultaneous spatial organization of the scars themselves on the vertices of an icosahedron. The emergence of icosahedral order coincides with the localization of mobility into isolated "lakes" of fluid or glassy particles, situated at the icosahedron vertices. These lakes are embedded in a rigid, connected "continent" of locally crystalline particles.

Gradient nano-engineered in situ forming composite hydrogel for osteochondral regeneration.

PubMed

Radhakrishnan, Janani; Manigandan, Amrutha; Chinnaswamy, Prabu; Subramanian, Anuradha; Sethuraman, Swaminathan

2018-04-01

Fabrication of anisotropic osteochondral-mimetic scaffold with mineralized subchondral zone and gradient interface remains challenging. We have developed an injectable semi-interpenetrating network hydrogel construct with chondroitin sulfate nanoparticles (ChS-NPs) and nanohydroxyapatite (nHA) (∼30-90 nm) in chondral and subchondral hydrogel zones respectively. Mineralized subchondral hydrogel exhibited significantly higher osteoblast proliferation and alkaline phosphatase activity (p < 0.05). Osteochondral hydrogel exhibited interconnected porous structure and spatial variation with gradient interface of nHA and ChS-NPs. Microcomputed tomography (μCT) demonstrated nHA gradation while rheology showed predominant elastic modulus (∼930 Pa) at the interface. Co-culture of osteoblasts and chondrocytes in gradient hydrogels showed layer-specific retention of cells and cell-cell interaction at the interface. In vivo osteochondral regeneration by biphasic (nHA or ChS) and gradient (nHA + ChS) hydrogels was compared with control using rabbit osteochondral defect after 3 and 8 weeks. Complete closure of defect was observed in gradient (8 weeks) while defect remained in other groups. Histology demonstrated collagen and glycosaminoglycan deposition in neo-matrix and presence of hyaline cartilage-characteristic matrix, chondrocytes and osteoblasts. μCT showed mineralized neo-tissue formation, which was confined within the defect with higher bone mineral density in gradient (chondral: 0.42 ± 0.07 g/cc, osteal: 0.64 ± 0.08 g/cc) group. Further, biomechanical push-out studies showed significantly higher load for gradient group (378 ± 56 N) compared to others. Thus, the developed nano-engineered gradient hydrogel enhanced hyaline cartilage regeneration with subchondral bone formation and lateral host-tissue integration. Copyright © 2018 Elsevier Ltd. All rights reserved.

N-SCAN: new vibromodulation system for detection and monitoring of cracks and other contact-type defects

NASA Astrophysics Data System (ADS)

Donskoy, Dmitri; Ekimov, Alexander; Luzzato, Emile; Lottiaux, Jean-Louis; Stoupin, Stanislav; Zagrai, Andrei

2003-08-01

In recent years, innovative vibro-modulation technique has been introduced for detection of contact-type interfaces such as cracks, debondings, and delaminations. The technique utilizes the effect of nonlinear interaction of ultrasound and vibrations at the interface of the defect. Vibration varies on the contact area of the interface modulating passing through ultrasonic wave. The modulation manifests itself as additional side-band spectral components with the combination frequencies in the spectrum of the received signal. The presence of these components allows for detection and differentiation of the contact-type defects from other structural and material inhomogeneities. Vibro-modulation technique has been implemented in N-SCAN damage detection system. The system consists of a digital synthesizer, high and low frequency amplifiers, a magnetostrictive shaker, ultrasonic transducers and a PC-based data acquisition/processing station with N-SCAN software. The ability of the system to detect contact-type defects was experimentally verified using specimens of simple and complex geometries made of steel, aluminum, composites and other structural materials. N-SCAN proved to be very effective for nondestructive testing of full-scale structures ranging from 24 foot-long gun barrels to stainless steel pipes used in nuclear power plants. Among advantages of the system are applicability for the wide range of structural materials and for structures with complex geometries, real time data processing, convenient interface for system operation, simplicity of interpretation of results, no need for sensor scanning along structure, onsite inspection of large structures at a fraction of time as compared with conventional techniques. This paper describes the basic principles of nonlinear vibro-modulation NDE technique, some theoretical background for nonlinear interaction and justification of signal processing algorithm. It is also presents examples of practical implementation and application of the technique.

Hydrogen-bond Specific Materials Modification in Group IV Semiconductors

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tolk, Norman H.; Feldman, L. C.; Luepke, G.

Executive summary Semiconductor dielectric crystals consist of two fundamental components: lattice atoms and electrons. The former component provides a crystalline structure that can be disrupted by various defects or the presence of an interface, or by transient oscillations known as phonons. The latter component produces an energetic structure that is responsible for the optical and electronic properties of the material, and can be perturbed by lattice defects or by photo-excitation. Over the period of this project, August 15, 1999 to March 31, 2015, a persistent theme has been the elucidation of the fundamental role of defects arising from the presencemore » of radiation damage, impurities (in particular, hydrogen), localized strain or some combination of all three. As our research effort developed and evolved, we have experienced a few title changes, which reflected this evolution. Throughout the project, ultrafast lasers usually in a pump-probe configuration provided the ideal means to perturb and study semiconductor crystals by both forms of excitation, vibrational (phonon) and electronic (photon). Moreover, we have found in the course of this research that there are many interesting and relevant scientific questions that may be explored when phonon and photon excitations are controlled separately. Our early goals were to explore the dynamics of bond-selective vibrational excitation of hydrogen from point defects and impurities in crystalline and amorphous solids, initiating an investigation into the behavior of hydrogen isotopes utilizing a variety of ultrafast characterization techniques, principally transient bleaching spectroscopy to experimentally obtain vibrational lifetimes. The initiative could be divided into three related areas: (a) investigation of the change in electronic structure of solids due to the presence of hydrogen defect centers, (b) dynamical studies of hydrogen in materials and (c) characterization and stability of metastable hydrogen impurity states under transient compression. This research focused on the characterization of photon and ion stimulated hydrogen related defect and impurity reactions and migration in solid state matter, which requires a detailed understanding of the rates and pathways of vibrational energy flow, of the transfer channels and of the coupling mechanisms between local vibrational modes (LVMs) and phonon bath as well as the electronic system of the host material. It should be stressed that researchers at Vanderbilt and William and Mary represented a unique group with a research focus and capabilities for low temperature creation and investigation of such material systems. Later in the program, we carried out a vigorous research effort addressing the roles of defects, interfaces, and dopants on the optical and electronic characteristics of semiconductor crystals, using phonon generation by means of ultrafast coherent acoustic phonon (CAP) spectroscopy, nonlinear characterization using second harmonic generation (SHG), and ultrafast pump-and-probe reflectivity and absorption measurements. This program featured research efforts from hydrogen defects in silicon alone to other forms of defects such as interfaces and dopant layers, as well as other important semiconducting systems. Even so, the emphasis remains on phenomena and processes far from equilibrium, such as hot electron effects and travelling localized phonon waves. This program relates directly to the mission of the Department of Energy. Knowledge of the rates and pathways of vibrational energy flow in condensed matter is critical for understanding dynamical processes in solids including electronically, optically and thermally stimulated defect and impurity reactions and migration. The ability to directly probe these pathways and rates allows tests of theory and scaling laws at new levels of precision. Hydrogen embedded in model crystalline semiconductors and metal oxides is of particular interest, since the associated local mode can be excited cleanly, and is usually well-separated in energy from the phonon bath. These basic dynamical studies have provided new insights for example into the fundamental mechanisms that control proton diffusion in these oxides. This area of materials science has largely fulfilled its promise to identify degradation mechanisms in electronic and optoelectronic devices, and to advance solid oxide proton conductors for fuel cells, gas sensors and proton-exchange membrane applications. It also provides the basis for innovations in materials synthesis involving atomic-selective diffusion and desorption.« less

Enhancement of the Stability of Fluorine Atoms on Defective Graphene and at Graphene/Fluorographene Interface.

PubMed

Ao, Zhimin; Jiang, Quanguo; Li, Shuang; Liu, Hao; Peeters, Francois M; Li, Sean; Wang, Guoxiu

2015-09-09

Fluorinated graphene is one of the most important derivatives of graphene and has been found to have great potential in optoelectronic and photonic nanodevices. However, the stability of F atoms on fluorinated graphene under different conditions, which is essential to maintain the desired properties of fluorinated graphene, is still unclear. In this work, we investigate the diffusion of F atoms on pristine graphene, graphene with defects, and at graphene/fluorographene interfaces by using density functional theory calculations. We find that an isolated F atom diffuses easily on graphene, but those F atoms can be localized by inducing vacancies or absorbates in graphene and by creating graphene/fluorographene interfaces, which would strengthen the binding energy of F atoms on graphene and increase the diffusion energy barrier of F atoms remarkably.

Controlling the defects and transition layer in SiO2 films grown on 4H-SiC via direct plasma-assisted oxidation

PubMed Central

Kim, Dae-Kyoung; Jeong, Kwang-Sik; Kang, Yu-Seon; Kang, Hang-Kyu; Cho, Sang W.; Kim, Sang-Ok; Suh, Dongchan; Kim, Sunjung; Cho, Mann-Ho

2016-01-01

The structural stability and electrical performance of SiO2 grown on SiC via direct plasma-assisted oxidation were investigated. To investigate the changes in the electronic structure and electrical characteristics caused by the interfacial reaction between the SiO2 film (thickness ~5 nm) and SiC, X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), density functional theory (DFT) calculations, and electrical measurements were performed. The SiO2 films grown via direct plasma-assisted oxidation at room temperature for 300s exhibited significantly decreased concentrations of silicon oxycarbides (SiOxCy) in the transition layer compared to that of conventionally grown (i.e., thermally grown) SiO2 films. Moreover, the plasma-assisted SiO2 films exhibited enhanced electrical characteristics, such as reduced frequency dispersion, hysteresis, and interface trap density (Dit ≈ 1011 cm−2 · eV−1). In particular, stress induced leakage current (SILC) characteristics showed that the generation of defect states can be dramatically suppressed in metal oxide semiconductor (MOS) structures with plasma-assisted oxide layer due to the formation of stable Si-O bonds and the reduced concentrations of SiOxCy species defect states in the transition layer. That is, energetically stable interfacial states of high quality SiO2 on SiC can be obtained by the controlling the formation of SiOxCy through the highly reactive direct plasma-assisted oxidation process. PMID:27721493

Emission efficiency limit of Si nanocrystals

PubMed Central

Limpens, Rens; Luxembourg, Stefan L.; Weeber, Arthur W.; Gregorkiewicz, Tom

2016-01-01

One of the important obstacles on the way to application of Si nanocrystals for development of practical devices is their typically low emissivity. In this study we explore the limits of external quantum yield of photoluminescence of solid-state dispersions of Si nanocrystals in SiO2. By making use of a low-temperature hydrogen passivation treatment we demonstrate a maximum emission quantum efficiency of approximately 35%. This is the highest value ever reported for this type of material. By cross-correlating PL lifetime with EQE values, we obtain a comprehensive understanding of the efficiency limiting processes induced by Pb-defects. We establish that the observed record efficiency corresponds to an interface density of Pb-centers of 1.3 × 1012 cm12, which is 2 orders of magnitude higher than for the best Si/SiO2 interface. This result implies that Si nanocrystals with up to 100% emission efficiency are feasible. PMID:26786062

Molecular Dynamics Simulations of Shock Wave Propagation across the Nitromethane Crystal-Melt Interface

NASA Astrophysics Data System (ADS)

Jiang, Shan; Sewell, Thomas D.; Thompson, Donald L.

2015-06-01

We are interested in understanding the fundamental processes that occur during propagation of shock waves across the crystal-melt interface in molecular substances. We have carried out molecular dynamics simulations of shock passage from the nitromethane (100)-oriented crystal into the melt and vice versa using the fully flexible, non-reactive Sorescu, Rice, and Thompson force field. A stable interface was established for a temperature near the melting point by using a combination of isobaric-isothermal (NPT) and isochoric-isothermal (NVT) simulations. The equilibrium bulk and interfacial regions were characterized using spatial-temporal distributions of molecular number density, kinetic and potential energy, and C-N bond orientations. Those same properties were calculated as functions of time during shock propagation. As expected, the local temperatures (intermolecular, intramolecular, and total) and stress states differed significantly between the liquid and crystal regions and depending on the direction of shock propagation. Substantial differences in the spatial distribution of shock-induced defect structures in the crystalline region were observed depending on the direction of shock propagation. Research supported by the U.S. Army Research Office.

Catalytically enhanced thermal decomposition of chemically grown silicon oxide layers on Si(001)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Leroy, F., E-mail: leroy@cinam.univ-mrs.fr; Passanante, T.; Cheynis, F.

2016-03-14

The thermal decomposition of Si dioxide layers formed by wet chemical treatment on Si(001) has been studied by low-energy electron microscopy. Independent nucleations of voids occur into the Si oxide layers that open by reaction at the void periphery. Depending on the voids, the reaction rates exhibit large differences via the occurrence of a nonlinear growth of the void radius. This non-steady state regime is attributed to the accumulation of defects and silicon hydroxyl species at the SiO{sub 2}/Si interface that enhances the silicon oxide decomposition at the void periphery.

Abnormal hump in capacitance-voltage measurements induced by ultraviolet light in a-IGZO thin-film transistors

NASA Astrophysics Data System (ADS)

Tsao, Yu-Ching; Chang, Ting-Chang; Chen, Hua-Mao; Chen, Bo-Wei; Chiang, Hsiao-Cheng; Chen, Guan-Fu; Chien, Yu-Chieh; Tai, Ya-Hsiang; Hung, Yu-Ju; Huang, Shin-Ping; Yang, Chung-Yi; Chou, Wu-Ching

2017-01-01

This work demonstrates the generation of abnormal capacitance for amorphous indium-gallium-zinc oxide (a-InGaZnO4) thin-film transistors after being subjected to negative bias stress under ultraviolet light illumination stress (NBIS). At various operation frequencies, there are two-step tendencies in their capacitance-voltage curves. When gate bias is smaller than threshold voltage, the measured capacitance is dominated by interface defects. Conversely, the measured capacitance is dominated by oxygen vacancies when gate bias is larger than threshold voltage. The impact of these interface defects and oxygen vacancies on capacitance-voltage curves is verified by TCAD simulation software.

Deformation in amorphous–crystalline nanolaminates—an effective-temperature theory and interaction between defects

DOE PAGES

Lieou, Charles K. C.; Mayeur, Jason R.; Beyerlein, Irene J.

2017-02-24

Experiments and atomic-scale simulations suggest that the transmission of plasticity carriers in deforming amorphous–crystalline nanolaminates is mediated by the biphase interface between the amorphous and crystalline layers. In this study, we present a micromechanics model for these biphase nanolaminates that describes defect interactions through the amorphous–crystalline interface (ACI). The model is based on an effective-temperature framework to achieve a unified description of the slow, configurational atomic rearrangements in both phases when driven out of equilibrium. We show how the second law of thermodynamics constrains the density of defects and the rate of configurational rearrangements, and apply this framework to dislocationsmore » in crystalline solids and shear transformation zones (STZs) in amorphous materials. The effective-temperature formulation enables us to interpret the observed movement of dislocations to the ACI and the production of STZs at the interface as a 'diffusion' of configurational disorder across the material. Finally, we demonstrate favorable agreement with experimental findings reported in (Kim et al 2011 Adv. Funct. Mater. 21 4550–4), and demonstrate how the ACI acts as a sink of dislocations and a source of STZs.« less

Defect generation in amorphous-indium-gallium-zinc-oxide thin-film transistors by positive bias stress at elevated temperature

DOE Office of Scientific and Technical Information (OSTI.GOV)

Um, Jae Gwang; Mativenga, Mallory; Jang, Jin, E-mail: jjang@khu.ac.kr

2014-04-07

We report on the generation and characterization of a hump in the transfer characteristics of amorphous indium gallium zinc-oxide thin-film transistors by positive bias temperature stress. The hump depends strongly on the gate bias stress at 100 °C. Due to the hump, the positive shift of the transfer characteristic in deep depletion is always smaller that in accumulation. Since, the latter shift is twice the former, with very good correlation, we conclude that the effect is due to creation of a double acceptor, likely to be a cation vacancy. Our results indicate that these defects are located near the gate insulator/activemore » layer interface, rather than in the bulk. Migration of donor defects from the interface towards the bulk may also occur under PBST at 100 °C.« less

Segregation of liquid crystal mixtures in topological defects

DOE PAGES

Rahimi, Mohammad; Ramezani-Dakhel, Hadi; Zhang, Rui; ...

2017-04-28

The structure and physical properties of liquid crystal (LC) mixtures are a function of composition, and small changes can have pronounced effects on observables, such as phase-transition temperatures. Traditionally, LC mixtures have been assumed to be compositionally homogenous. The results of chemically detailed simulations presented here show that this is not the case; pronounced deviations of the local order from that observed in the bulk at defects and interfaces lead to significant compositional segregation effects. More specifically, two disclination lines are stabilized in this work by introducing into a nematic liquid crystal mixture a cylindrical body that exhibits perpendicular anchoring.more » Here, it is found that the local composition deviates considerably from that of the bulk at the interface with the cylinder and in the defects, thereby suggesting new assembly and synthetic strategies that may capitalize on the unusual molecular environment provided by liquid crystal mixtures.« less

Inexpensive Eddy-Current Standard

NASA Technical Reports Server (NTRS)

Berry, Robert F., Jr.

1985-01-01

Radial crack replicas serve as evaluation standards. Technique entails intimately joining two pieces of appropriate aluminum alloy stock and centering drilled hole through and along interface. Bore surface of hole presents two vertical stock interface lines 180 degrees apart. These lines serve as radial crack defect replicas during eddy-current technique setup and verification.

Li + Defects in a Solid-State Li Ion Battery: Theoretical Insights with a Li 3 OCl Electrolyte

DOE Office of Scientific and Technical Information (OSTI.GOV)

Stegmaier, Saskia; Voss, Johannes; Reuter, Karsten

In a solid-state Li ion battery, the solid-state electrolyte exits principally in regions of high externally applied potentials, and this varies rapidly at the interfaces with electrodes due to the formation of electrochemical double layers. Here, we investigate the implications of these for a model solid-state Li ion battery Li|Li 3OCl|C, where C is simply a metallic intercalation cathode. We use DFT to calculate the potential dependence of the formation energies of the Li + charge carriers in superionic Li 3OCl. We find that Li+ vacancies are the dominant species at the cathode while Li+ interstitials dominate at the anode.more » With typical Mg aliovalent doping of Li 3OCl, Li + vacancies dominate the bulk of the electrolyte as well, with freely mobile vacancies only ~ 10 -4 of the Mg doping density at room temperature. We study the repulsive interaction between Li+ vacancies and find that this is extremely short range, typically only one lattice constant due to local structural relaxation around the vacancy and this is significantly shorter than pure electrostatic screening. We model a Li 3OCl- cathode interface by treating the cathode as a nearly ideal metal using a polarizable continuum model with an ε r = 1000. There is a large interface segregation free energy of ~ - 1 eV per Li + vacancy. Combined with the short range for repulsive interactions of the vacancies, this means that very large vacancy concentrations will build up in a single layer of Li 3OCl at the cathode interface to form a compact double layer. The calculated potential drop across the interface is ~ 3 V for a nearly full concentration of vacancies at the surface. This suggests that nearly all the cathode potential drop in Li 3OCl occurs at the Helmholtz plane rather than in a diffuse space-charge region. We suggest that the conclusions found here will be general to other superionic conductors as well.« less

Li + Defects in a Solid-State Li Ion Battery: Theoretical Insights with a Li 3 OCl Electrolyte

DOE PAGES

Stegmaier, Saskia; Voss, Johannes; Reuter, Karsten; ...

2017-04-26

In a solid-state Li ion battery, the solid-state electrolyte exits principally in regions of high externally applied potentials, and this varies rapidly at the interfaces with electrodes due to the formation of electrochemical double layers. Here, we investigate the implications of these for a model solid-state Li ion battery Li|Li 3OCl|C, where C is simply a metallic intercalation cathode. We use DFT to calculate the potential dependence of the formation energies of the Li + charge carriers in superionic Li 3OCl. We find that Li+ vacancies are the dominant species at the cathode while Li+ interstitials dominate at the anode.more » With typical Mg aliovalent doping of Li 3OCl, Li + vacancies dominate the bulk of the electrolyte as well, with freely mobile vacancies only ~ 10 -4 of the Mg doping density at room temperature. We study the repulsive interaction between Li+ vacancies and find that this is extremely short range, typically only one lattice constant due to local structural relaxation around the vacancy and this is significantly shorter than pure electrostatic screening. We model a Li 3OCl- cathode interface by treating the cathode as a nearly ideal metal using a polarizable continuum model with an ε r = 1000. There is a large interface segregation free energy of ~ - 1 eV per Li + vacancy. Combined with the short range for repulsive interactions of the vacancies, this means that very large vacancy concentrations will build up in a single layer of Li 3OCl at the cathode interface to form a compact double layer. The calculated potential drop across the interface is ~ 3 V for a nearly full concentration of vacancies at the surface. This suggests that nearly all the cathode potential drop in Li 3OCl occurs at the Helmholtz plane rather than in a diffuse space-charge region. We suggest that the conclusions found here will be general to other superionic conductors as well.« less

Dislocation and Structural Studies at Metal-Metallic Glass Interface at Low Temperature

NASA Astrophysics Data System (ADS)

Gupta, Pradeep; Yedla, Natraj

2017-12-01

In this paper, molecular dynamics (MD) simulation deformation studies on the Al (metal)-Cu50Zr50 (metallic glass) model interface is carried out based on cohesive zone model. The interface is subjected to mode-I loading at a strain rate of 109 s-1 and temperature of 100 K. The dislocations reactions and evolution of dislocation densities during the deformation have been investigated. Atomic interactions between Al, Cu and Zr atoms are modeled using EAM (embedded atom method) potential, and a timestep of 0.002 ps is used for performing the MD simulations. A circular crack and rectangular notch are introduced at the interface to investigate the effect on the deformation behavior and fracture. Further, scale size effect is also investigated. The structural changes and evolution of dislocation density are also examined. It is found that the dominant deformation mechanism is by Shockley partial dislocation nucleation. Amorphization is observed in the Al regions close to the interface and occurs at a lower strain in the presence of a crack. The total dislocation density is found to be maximum after the first yield in both the perfect and defect interface models and is highest in the case of perfect interface with a density of 6.31 × 1017 m-2. In the perfect and circular crack defect interface models, it is observed that the fraction of Shockley partial dislocation density decreases, whereas that of strain rod dislocations increases with increase in strain.

Effect of PECVD SiNx/SiOyNx-Si interface property on surface passivation of silicon wafer

NASA Astrophysics Data System (ADS)

Jia, Xiao-Jie; Zhou, Chun-Lan; Zhu, Jun-Jie; Zhou, Su; Wang, Wen-Jing

2016-12-01

It is studied in this paper that the electrical characteristics of the interface between SiOyNx/SiNx stack and silicon wafer affect silicon surface passivation. The effects of precursor flow ratio and deposition temperature of the SiOyNx layer on interface parameters, such as interface state density Dit and fixed charge Qf, and the surface passivation quality of silicon are observed. Capacitance-voltage measurements reveal that inserting a thin SiOyNx layer between the SiNx and the silicon wafer can suppress Qf in the film and Dit at the interface. The positive Qf and Dit and a high surface recombination velocity in stacks are observed to increase with the introduced oxygen and minimal hydrogen in the SiOyNx film increasing. Prepared by deposition at a low temperature and a low ratio of N2O/SiH4 flow rate, the SiOyNx/SiNx stacks result in a low effective surface recombination velocity (Seff) of 6 cm/s on a p-type 1 Ω·cm-5 Ω·cm FZ silicon wafer. The positive relationship between Seff and Dit suggests that the saturation of the interface defect is the main passivation mechanism although the field-effect passivation provided by the fixed charges also make a contribution to it. Project supported by the National High Technology Research and Development Program of China (Grant No. 2015AA050302) and the National Natural Science Foundation of China (Grant No. 61306076).

Influence of ZnO seed layer precursor molar ratio on the density of interface defects in low temperature aqueous chemically synthesized ZnO nanorods/GaN light-emitting diodes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Alnoor, Hatim, E-mail: hatim.alnoor@liu.se; Iandolo, Donata; Willander, Magnus

Low temperature aqueous chemical synthesis (LT-ACS) of zinc oxide (ZnO) nanorods (NRs) has been attracting considerable research interest due to its great potential in the development of light-emitting diodes (LEDs). The influence of the molar ratio of the zinc acetate (ZnAc): KOH as a ZnO seed layer precursor on the density of interface defects and hence the presence of non-radiative recombination centers in LT-ACS of ZnO NRs/GaN LEDs has been systematically investigated. The material quality of the as-prepared seed layer as quantitatively deduced by the X-ray photoelectron spectroscopy is found to be influenced by the molar ratio. It is revealedmore » by spatially resolved cathodoluminescence that the seed layer molar ratio plays a significant role in the formation and the density of defects at the n-ZnO NRs/p-GaN heterostructure interface. Consequently, LED devices processed using ZnO NRs synthesized with molar ratio of 1:5 M exhibit stronger yellow emission (∼575 nm) compared to those based on 1:1 and 1:3 M ratios as measured by the electroluminescence. Furthermore, seed layer molar ratio shows a quantitative dependence of the non-radiative defect densities as deduced from light-output current characteristics analysis. These results have implications on the development of high-efficiency ZnO-based LEDs and may also be helpful in understanding the effects of the ZnO seed layer on defect-related non-radiative recombination.« less

Surface roughness effects on contact line motion with small capillary number

NASA Astrophysics Data System (ADS)

Yang, Feng-Chao; Chen, Xiao-Peng; Yue, Pengtao

2018-01-01

In this work, we investigate how surface roughness influences contact line dynamics by simulating forced wetting in a capillary tube. The tube wall is decorated with microgrooves and is intrinsically hydrophilic. A phase-field method is used to capture the fluid interface and the moving contact line. According to the numerical results, a criterion is proposed to judge whether the grooves are entirely wetted or not at vanishing capillary numbers. When the contact line moves over a train of grooves, the apparent contact angle exhibits a periodic nature, no matter whether the Cassie-Baxter or the Wenzel state is achieved. The oscillation amplitude of apparent contact angle is analyzed and found to be inversely proportional to the interface area. The contact line motion can be characterized as stick-jump-slip in the Cassie-Baxter state and stick-slip in the Wenzel state. By comparing to the contact line dynamics on smooth surfaces, equivalent microscopic contact angles and slip lengths are obtained. The equivalent slip length in the Cassie-Baxter state agrees well with the theoretical model in the literature. The equivalent contact angles are, however, much greater than the predictions of the Cassie-Baxter model and the Wenzel model for equilibrium stable states. Our results reveal that the pinning of the contact line at surface defects effectively enhances the hydrophobicity of rough surfaces, even when the surface material is intrinsically hydrophilic and the flow is under the Wenzel state.

Bioactive Scaffolds for Regeneration of Cartilage and Subchondral Bone Interface

PubMed Central

Deng, Cuijun; Zhu, Huiying; Li, Jiayi; Feng, Chun; Yao, Qingqiang; Wang, Liming; Chang, Jiang; Wu, Chengtie

2018-01-01

The cartilage lesion resulting from osteoarthritis (OA) always extends into subchondral bone. It is of great importance for simultaneous regeneration of two tissues of cartilage and subchondral bone. 3D-printed Sr5(PO4)2SiO4 (SPS) bioactive ceramic scaffolds may achieve the aim of regenerating both of cartilage and subchondral bone. We hypothesized that strontium (Sr) and silicon (Si) ions released from SPS scaffolds play a crucial role in osteochondral defect reconstruction. Methods: SPS bioactive ceramic scaffolds were fabricated by a 3D-printing method. The SEM and ICPAES were used to investigate the physicochemical properties of SPS scaffolds. The proliferation and maturation of rabbit chondrocytes stimulated by SPS bioactive ceramics were measured in vitro. The stimulatory effect of SPS scaffolds for cartilage and subchondral bone regeneration was investigated in vivo. Results: SPS scaffolds significantly stimulated chondrocyte proliferation, and SPS extracts distinctly enhanced the maturation of chondrocytes and preserved chondrocytes from OA. SPS scaffolds markedly promoted the regeneration of osteochondral defects. The complex interface microstructure between cartilage and subchondral bone was obviously reconstructed. The underlying mechanism may be related to Sr and Si ions stimulating cartilage regeneration by activating HIF pathway and promoting subchondral bone reconstruction through activating Wnt pathway, as well as preserving chondrocytes from OA via inducing autophagy and inhibiting hedgehog pathway. Conclusion: Our findings suggest that SPS scaffolds can help osteochondral defect reconstruction and well reconstruct the complex interface between cartilage and subchondral bone, which represents a promising strategy for osteochondral defect regeneration. PMID:29556366

Trap-assisted tunneling in Si-InAs nanowire heterojunction tunnel diodes.

PubMed

Bessire, Cedric D; Björk, Mikael T; Schmid, Heinz; Schenk, Andreas; Reuter, Kathleen B; Riel, Heike

2011-10-12

We report on the electrical characterization of one-sided p(+)-si/n-InAs nanowire heterojunction tunnel diodes to provide insight into the tunnel process occurring in this highly lattice mismatched material system. The lattice mismatch gives rise to dislocations at the interface as confirmed by electron microscopy. Despite this, a negative differential resistance with peak-to-valley current ratios of up to 2.4 at room temperature and with large current densities is observed, attesting to the very abrupt and high-quality interface. The presence of dislocations and other defects that increase the excess current is evident in the first and second derivative of the I-V characteristics as distinct peaks arising from trap-and phonon-assisted tunneling via the corresponding defect levels. We observe this assisted tunneling mainly in the forward direction and at low reverse bias but not at higher reverse biases because the band-to-band generation rates are peaked in the InAs, which is also confirmed by modeling. This indicates that most of the peaks are due to dislocations and defects in the immediate vicinity of the interface. Finally, we also demonstrate that these devices are very sensitive to electrical stress, in particular at room temperature, because of the extremely high electrical fields obtained at the abrupt junction even at low bias. The electrical stress induces additional defect levels in the band gap, which reduce the peak-to-valley current ratios.

Temperature dependent current transport of Pd/ZnO nanowire Schottky diodes

NASA Astrophysics Data System (ADS)

Gayen, R. N.; Bhattacharyya, S. R.; Jana, P.

2014-09-01

Zinc oxide (ZnO) nanowire based Schottky barrier diodes are fabricated by depositing Pd metal contact on top of vertically well-aligned ZnO nanowire arrays. A vertical array of ZnO nanowires on indium tin oxide (ITO) coated glass substrates is synthesized by hybrid wet chemical route. Scanning electron microscopy (SEM), x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) measurement confirm the formation of stoichiometric well-aligned hexagonal (h-ZnO) nanowire arrays with wurtzite structure. Temperature dependent current-voltage (I-V) measurements on palladium-ZnO (Pd/ZnO) nanowire Schottky junctions in the temperature range 303-383 K exhibit excellent rectifying character. From these nonlinear I-V plots, different electrical parameters of diode-like reverse saturation current, barrier height and ideality factor are determined as a function of temperature assuming pure thermionic emission model. The ideality factor is found to decrease while the barrier height increases with the increase in temperature. The series resistance values calculated from Cheung’s functions also show temperature dependency. Such behavior can be attributed to the presence of defects that traps carriers, and barrier height inhomogeneity at the interface of the barrier junction. After barrier height inhomogeneity correction, considering a Gaussian distributed barrier height fluctuation across the Pd/ZnO interface, the estimated values of mean barrier height and modified Richardson constant are more closely matched to the theoretically predicted value for Pd/ZnO Schottky barrier diodes. The variation of density of interface states as a function of interface state energy is also calculated.

ZnO Schottky barriers and Ohmic contacts

NASA Astrophysics Data System (ADS)

Brillson, Leonard J.; Lu, Yicheng

2011-06-01

ZnO has emerged as a promising candidate for optoelectronic and microelectronic applications, whose development requires greater understanding and control of their electronic contacts. The rapid pace of ZnO research over the past decade has yielded considerable new information on the nature of ZnO interfaces with metals. Work on ZnO contacts over the past decade has now been carried out on high quality material, nearly free from complicating factors such as impurities, morphological and native point defects. Based on the high quality bulk and thin film crystals now available, ZnO exhibits a range of systematic interface electronic structure that can be understood at the atomic scale. Here we provide a comprehensive review of Schottky barrier and ohmic contacts including work extending over the past half century. For Schottky barriers, these results span the nature of ZnO surface charge transfer, the roles of surface cleaning, crystal quality, chemical interactions, and defect formation. For ohmic contacts, these studies encompass the nature of metal-specific interactions, the role of annealing, multilayered contacts, alloyed contacts, metallization schemes for state-of-the-art contacts, and their application to n-type versus p-type ZnO. Both ZnO Schottky barriers and ohmic contacts show a wide range of phenomena and electronic behavior, which can all be directly tied to chemical and structural changes on an atomic scale.

Comparative analysis of the H 2 passivation of interface defects at the {(100) Si}/{SiO 2} interface using electron spin resonance

NASA Astrophysics Data System (ADS)

Stesmans, A.

1996-01-01

The passivation with molecular hydrogen in the range 213-234°C of the interfacial Pb0 and Pb1 defects in {(100) Si}/{SiO 2}, thermally grown at low temperature (<750°C), has been analyzed by K-band electron spin resonance. The passivation kinetics are found to be well described by the same defect-H 2 reaction limited model applying to the interfacial Pb defect (∘SiSi 3) in {(111) Si}/{SiO 2} grown at 850°C. However, unlike Pb, that was typified by a single-valued activation energy for passivation Ea = 1.66 eV, both Pb0 and Pb1 are found to exhibit a Gaussian spread σEa ˜ 0.15 eV around their respective meanEa values, deduced as 1.51 and 1.57 ± 0.3 eV. The similar passivation kinetics are in line with assigning the Pb0 and Pb1 defects, like Pb, to an interfacial unpaired sp3 Si hybrid. However, as there is no fundamental difference between Pb0 and Pb1 regarding passivation in H 2, more specfic identification of Pb with either Pb0 or Pb1 , if any, cannot be concluded.

Interplay of point defects, biaxial strain, and thermal conductivity in homoepitaxial SrTiO3 thin films

NASA Astrophysics Data System (ADS)

Wiedigen, S.; Kramer, T.; Feuchter, M.; Knorr, I.; Nee, N.; Hoffmann, J.; Kamlah, M.; Volkert, C. A.; Jooss, Ch.

2012-02-01

Separating out effects of point defects and lattice strain on thermal conductivity is essential for improvement of thermoelectric properties of SrTiO3. We study relations between defects generated during deposition, induced lattice strain, and their impact on thermal conductivity κ in homoepitaxial SrTiO3 films prepared by ion-beam sputtering. Lowering the deposition temperature gives rise to lattice expansion by enhancement of point defect density which increases the hardness of the films. Due to a fully coherent substrate-film interface, the lattice misfit induces a large biaxial strain. However, we can show that the temperature dependence of κ is mainly sensitive on the defect concentration.

Giant Tunnel Magnetoresistance with a Single Magnetic Phase-Transition Electrode

NASA Astrophysics Data System (ADS)

Zhang, Jia; Chen, X. Z.; Song, C.; Feng, J. F.; Wei, H. X.; Lü, Jing-Tao

2018-04-01

The magnetic phase-transition tunnel-magnetoresistance (MPT-TMR) effect with a single magnetic electrode is investigated by first-principles calculations. The calculations show that the MPT-TMR of an α'-FeRh /MgO /Cu tunnel junction can be as high as hundreds of percent when the magnetic structure of α'-FeRh changes from G -type antiferromagnetic (G -AFM ) to ferromagnetic order. This type of MPT-TMR may be superior to the tunnel anisotropic magnetoresistance because of its huge magnetoresistance effect and similar structural simplicity. The main mechanism for the giant MPT-TMR can be attributed to the formation of interface resonant states at the G -AFM FeRh /MgO interface. A direct FeRh /MgO interface is found to be necessary for achieving a high MPT-TMR experimentally. Moreover, we find the α'-FeRh /MgO interface with FeRh in the ferromagnetic phase has nearly full spin polarization due to the negligible majority transmission and significantly different Fermi surface of two spin channels. Thus, it may act as a highly efficient and tunable spin injector. In addition, the electric-field-driven MPT of FeRh-based heteromagnetic nanostructures can be utilized to design various energy-efficient tunnel-junction structures and the corresponding lower-power-consumption devices. We also discuss the consequence of various junction defects on MPT-TMR. The interface oxygen layer is found to be detrimental to MPT-TMR. The sign of MPT-TMR is reversed with Rh termination due to the lack of contribution from the interface resonance states. However, the MPT-TMR may be robust against the oxygen vacancy inside of MgO and the shift of the Fermi energy. Our results will stimulate further experimental investigations of MPT-TMR and other fascinating phenomenon of FeRh-based tunnel junctions that may be promising in antiferromagnetic spintronics.

Structural and mechanical implications of PMMA implant shape and interface geometry in cranioplasty--A finite element study.

PubMed

Ridwan-Pramana, Angela; Marcián, Petr; Borák, Libor; Narra, Nathaniel; Forouzanfar, Tim; Wolff, Jan

2016-01-01

This computational study investigates the effect of shape (defect contour curvature) and bone-implant interface (osteotomy angle) on the stress distribution within PMMA skull implants. Using finite element methodology, 15 configurations--combinations of simplified synthetic geometric shapes (circular, square, triangular, irregular) and interface angulations--were simulated under 50N static loads. Furthermore, the implant fixation devices were modelled and analysed in detail. Negative osteotomy configurations demonstrated the largest stresses in the implant (275 MPa), fixation devices (1258 MPa) and bone strains (0.04). The circular implant with zero and positive osteotomy performed well with maximum observed magnitudes of--implant stress (1.2 MPa and 1.2 MPa), fixation device stress (11.2 MPa and 2.2 MPa), bone strain (0.218e-3 and 0.750e-4). The results suggest that the preparation of defect sites is a critical procedure. Of the greatest importance is the angle at which the edges of the defect are sawed. If under an external load, the implant has no support from the interface and the stresses are transferred to the fixation devices. This can endanger their material integrity and lead to unphysiological strains in the adjacent bone, potentially compromising the bone morphology required for anchoring. These factors can ultimately weaken the stability of the entire implant assembly. Copyright © 2015 European Association for Cranio-Maxillo-Facial Surgery. Published by Elsevier Ltd. All rights reserved.

Defects in zinc oxide grown by pulsed laser deposition

NASA Astrophysics Data System (ADS)

Ling, Francis C. C.; Wang, Zilan; Ping Ho, Lok; Younas, M.; Anwand, W.; Wagner, A.; Su, S. C.; Shan, C. X.

2016-01-01

ZnO films are grown on c-plane sapphire using the pulsed laser deposition method. Systematic studies on the effects of annealing are performed to understand the thermal evolutions of the defects in the films. Particular attention is paid to the discussions of the ZnO/sapphire interface thermal stability, the Zn-vacancy related defects having different microstructures, the origins of the green luminescence (∼2.4-2.5 eV) and the near band edge (NBE) emission at 3.23 eV.

Charged Defects-Induced Resistive Switching in Sb2Te3 Memristor

NASA Astrophysics Data System (ADS)

Zhang, J. J.; Liu, N.; Sun, H. J.; Yan, P.; Li, Y.; Zhong, S. J.; Xie, S.; Li, R. J.; Miao, X. S.

2016-02-01

Resistive switching (RS) characteristics of Ta/Sb2Te3/Ta and Ag/Sb2Te3/Ta memory devices have been investigated. The I- V curves show the bipolar RS at room temperature. We have demonstrated that the redistribution and migration of charged defects are responsible for the memristive switching. By using Ag electrode instead of Ta, more defects can be created near the Ag/Sb2Te3 interface, which is a feasible method to eliminate the electroforming process.

Size and Location of Defects at the Coupling Interface Affect Lithotripter Performance

PubMed Central

Li, Guangyan; Williams, James C.; Pishchalnikov, Yuri A.; Liu, Ziyue; McAteer, James A.

2012-01-01

OBJECTIVE To determine how the size and location of coupling defects caught between the therapy head of a lithotripter and the skin of a surrogate patient (acoustic window of a test chamber) affect the features of shock waves responsible for stone breakage. METHODS Model defects were placed in the coupling gel between the therapy head of a Dornier Compact-S electromagnetic lithotripter and the Mylar window of a water-filled coupling test system. A fiber-optic hydrophone was used to measure acoustic pressures and map the lateral dimensions of the focal zone of the lithotripter. The effect of coupling conditions on stone breakage was assessed using Gypsum model stones. RESULTS Stone breakage decreased in proportion to the area of the coupling defect; a centrally located defect blocking only 18% of the transmission area reduced stone breakage by an average of almost 30%. The effect on stone breakage was greater for defects located on-axis and decreased as the defect was moved laterally; an 18% defect located near the periphery of the coupling window (2.0 cm off-axis) reduced stone breakage by only ~15% compared to when coupling was completely unobstructed. Defects centered within the coupling window acted to narrow the focal width of the lithotripter; an 8.2% defect reduced the focal width ~30% compared to no obstruction (4.4 mm versus 6.5 mm). Coupling defects located slightly off center disrupted the symmetry of the acoustic field; an 18% defect positioned 1.0 cm off-axis shifted the focus of maximum positive pressure ~1.0 mm laterally. Defects on and off-axis imposed a significant reduction in the energy density of shock waves across the focal zone. CONCLUSIONS In addition to blocking the transmission of shock wave energy, coupling defects also disrupt the properties of shock waves that play a role in stone breakage, including the focal width of the lithotripter and the symmetry of the acoustic field; the effect is dependent on the size and location of defects, with defects near the center of the coupling window having the greatest effect. These data emphasize the importance of eliminating air pockets from the coupling interface, particularly defects located near the center of the coupling window. PMID:22938566

Interfacial Cation-Defect Charge Dipoles in Stacked TiO2/Al2O3 Gate Dielectrics.

PubMed

Zhang, Liangliang; Janotti, Anderson; Meng, Andrew C; Tang, Kechao; Van de Walle, Chris G; McIntyre, Paul C

2018-02-14

Layered atomic-layer-deposited and forming-gas-annealed TiO 2 /Al 2 O 3 dielectric stacks, with the Al 2 O 3 layer interposed between the TiO 2 and a p-type germanium substrate, are found to exhibit a significant interface charge dipole that causes a ∼-0.2 V shift of the flat-band voltage and suppresses the leakage current density for gate injection of electrons. These effects can be eliminated by the formation of a trilayer dielectric stack, consistent with the cancellation of one TiO 2 /Al 2 O 3 interface dipole by the addition of another dipole of opposite sign. Density functional theory calculations indicate that the observed interface-dependent properties of TiO 2 /Al 2 O 3 dielectric stacks are consistent in sign and magnitude with the predicted behavior of Al Ti and Ti Al point-defect dipoles produced by local intermixing of the Al 2 O 3 /TiO 2 layers across the interface. Evidence for such intermixing is found in both electrical and physical characterization of the gate stacks.

Strain relaxation of CdTe on Ge studied by medium energy ion scattering

NASA Astrophysics Data System (ADS)

Pillet, J. C.; Pierre, F.; Jalabert, D.

2016-10-01

We have used the medium energy ion scattering (MEIS) technique to assess the strain relaxation in molecular-beam epitaxial (MBE) grown CdTe (2 1 1)/Ge (2 1 1) system. A previous X-ray diffraction study, on 10 samples of the same heterostructure having thicknesses ranging from 25 nm to 10 μm has allowed the measurement of the strain relaxation on a large scale. However, the X-ray diffraction measurements cannot achieve a stress measurement in close proximity to the CdTe/Ge interface at the nanometer scale. Due to the huge lattice misfit between the CdTe and Ge, a high degree of disorder is expected at the interface. The MEIS in channeling mode is a good alternative in order to profile defects with a high depth resolution. For a 21 nm thick CdTe layer, we observed, at the interface, a high density of Cd and/or Te atoms moved from their expected crystallographic positions followed by a rapid recombination of defects. Strain relaxation mechanisms in the vicinity of the interface are discussed

Structures, Properties and Defects of SrTiO3/GaAs Hetero-interfaces

NASA Astrophysics Data System (ADS)

Hong, Liang; Bhatnagar, Kunal; Droopad, Ravi; Öğüt, Serdar; Klie, Robert

SrTiO3 thin film can be epitaxially grown on GaAs substrate and used as a platform for growing other oxides to create functional metal-oxide-semiconductor devices, where a high-quality SrTiO3/GaAs interface is essential. We studied the structural and electronic properties of SrTiO3/GaAs hetero-interfaces at atomic level using scanning transmission electron microscopy and first-principles calculations. Our results suggest the preferred termination of GaAs (001) is significantly dependent on the oxygen concentration in the first oxide layer. The favorable interface structure is characterized as oxygen-deficient SrO in contact with arsenic and is observed in both experiment and simulation. The electronic properties are calculated and found to be tunable by interfacial defects such as oxygen, gallium and arsenic vacancies. This work was supported by the National Science Foundation (Grant No. DMR-1408427). This work made use of instruments in the Electron Microscopy Service and the High Performance Computing Clusters at University of Illinois at Chicago.

The interface quality of Ge nanoparticles grown in thick silica matrix

NASA Astrophysics Data System (ADS)

Dasović, J.; Dubček, P.; Pucić, I.; Bernstorff, S.; Radić, N.; Pivac, B.

2017-08-01

Germanium nanoparticles, or Ge quantum dots (QDs), embedded in different transparent dielectric matrix exhibit properties significantly different from the same bulk semiconductor and therefore exhibit a considerable potential for applications in advanced electronic and optoelectronic devices. It is expected that the quantum confinement effect will tune the optical bandgap simply by varying the QDs size. Nevertheless, the question remains whether and how the defects often present in the matrix or at interfaces affect their properties. A thick (SiO2 + Ge) layer was deposited by magnetron sputtering and after suitable thermal treatment spherical Ge QDs were formed in SiO2 matrix with rather narrow size distribution, as confirmed by GIWAXS and GISAXS analysis. It is shown that the formed surface/interface of the QDs with the matrix was rough with fractal nature. Annealing in N2 atmosphere produced photoluminescence (PL) in the visible part of the spectrum which consists of three contributions. All are attributed to structural defects at or close to the Ge/SiOx interface. Time-resolved PL results support the assumption that the three components are dominant in the observed luminescence.

Oxidation of GaSb(100) and its control studied by scanning tunneling microscopy and spectroscopy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mäkelä, J., E-mail: jaakko.m.makela@utu.fi, E-mail: pekka.laukkanen@utu.fi, E-mail: rmwallace@utdallas.edu; Tuominen, M.; Yasir, M.

2015-08-10

Atomic-scale knowledge and control of oxidation of GaSb(100), which is a potential interface for energy-efficient transistors, are still incomplete, largely due to an amorphous structure of GaSb(100) oxides. We elucidate these issues with scanning-tunneling microscopy and spectroscopy. The unveiled oxidation-induced building blocks cause defect states above Fermi level around the conduction-band edge. By interconnecting the results to previous photoemission findings, we suggest that the oxidation starts with substituting second-layer Sb sites by oxygen. Adding small amount of indium on GaSb(100), resulting in a (4 × 2)-In reconstruction, before oxidation produces a previously unreported, crystalline oxidized layer of (1 × 3)-O free of gap states.

Interface for the rapid analysis of liquid samples by accelerator mass spectrometry

DOEpatents

Turteltaub, Kenneth; Ognibene, Ted; Thomas, Avi; Daley, Paul F; Salazar Quintero, Gary A; Bench, Graham

2014-02-04

An interface for the analysis of liquid sample having carbon content by an accelerator mass spectrometer including a wire, defects on the wire, a system for moving the wire, a droplet maker for producing droplets of the liquid sample and placing the droplets of the liquid sample on the wire in the defects, a system that converts the carbon content of the droplets of the liquid sample to carbon dioxide gas in a helium stream, and a gas-accepting ion source connected to the accelerator mass spectrometer that receives the carbon dioxide gas of the sample in a helium stream and introduces the carbon dioxide gas of the sample into the accelerator mass spectrometer.

Improvement of charge-pumping electrically detected magnetic resonance and its application to silicon metal-oxide-semiconductor field-effect transistor

NASA Astrophysics Data System (ADS)

Hori, Masahiro; Tsuchiya, Toshiaki; Ono, Yukinori

2017-01-01

Charge-pumping electrically detected magnetic resonance (CP EDMR), or EDMR in the CP mode, is improved and applied to a silicon metal-oxide-semiconductor field-effect transistor (MOSFET). Real-time monitoring of the CP process reveals that high-frequency transient currents are an obstacle to signal amplification for EDMR. Therefore, we introduce cutoff circuitry, leading to a detection limit for the number of spins as low as 103 for Si MOS interface defects. With this improved method, we demonstrate that CP EDMR inherits one of the most important features of the CP method: the gate control of the energy window of the detectable interface defects for spectroscopy.

Computational design of surfaces, nanostructures and optoelectronic materials

NASA Astrophysics Data System (ADS)

Choudhary, Kamal

Properties of engineering materials are generally influenced by defects such as point defects (vacancies, interstitials, substitutional defects), line defects (dislocations), planar defects (grain boundaries, free surfaces/nanostructures, interfaces, stacking faults) and volume defects (voids). Classical physics based molecular dynamics and quantum physics based density functional theory can be useful in designing materials with controlled defect properties. In this thesis, empirical potential based molecular dynamics was used to study the surface modification of polymers due to energetic polyatomic ion, thermodynamics and mechanics of metal-ceramic interfaces and nanostructures, while density functional theory was used to screen substituents in optoelectronic materials. Firstly, polyatomic ion-beams were deposited on polymer surfaces and the resulting chemical modifications of the surface were examined. In particular, S, SC and SH were deposited on amorphous polystyrene (PS), and C2H, CH3, and C3H5 were deposited on amorphous poly (methyl methacrylate) (PMMA) using molecular dynamics simulations with classical reactive empirical many-body (REBO) potentials. The objective of this work was to elucidate the mechanisms by which the polymer surface modification took place. The results of the work could be used in tailoring the incident energy and/or constituents of ion beam for obtaining a particular chemistry inside the polymer surface. Secondly, a new Al-O-N empirical potential was developed within the charge optimized many body (COMB) formalism. This potential was then used to examine the thermodynamic stability of interfaces and mechanical properties of nanostructures composed of aluminum, its oxide and its nitride. The potentials were tested for these materials based on surface energies, defect energies, bulk phase stability, the mechanical properties of the most stable bulk phase, its phonon properties as well as with a genetic algorithm based evolution theory of the materials to ensure that no spurious phases had a lower cohesive energy. Thirdly, lanthanide doped and co-doped Y3Al5O 12 were examined using density functional theory (DFT) with semi-local and local functional. Theoretical results were compared and validated with experimental data and new co-doped materials with high efficiency were predicted. Finally, Transition element doped CH3NH3PbI3 were studied with DFT for validation of the model with experimental data and replacement materials for toxic Pb were predicted.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yang, Yongsoo; Chen, Chien-Chun; Scott, M. C.

Perfect crystals are rare in nature. Real materials often contain crystal defects and chemical order/disorder such as grain boundaries, dislocations, interfaces, surface reconstructions and point defects. Such disruption in periodicity strongly affects material properties and functionality. Despite rapid development of quantitative material characterization methods, correlating three-dimensional (3D) atomic arrangements of chemical order/disorder and crystal defects with material properties remains a challenge. On a parallel front, quantum mechanics calculations such as density functional theory (DFT) have progressed from the modelling of ideal bulk systems to modelling ‘real’ materials with dopants, dislocations, grain boundaries and interfaces; but these calculations rely heavily onmore » average atomic models extracted from crystallography. To improve the predictive power of first-principles calculations, there is a pressing need to use atomic coordinates of real systems beyond average crystallographic measurements. Here we determine the 3D coordinates of 6,569 iron and 16,627 platinum atoms in an iron-platinum nanoparticle, and correlate chemical order/disorder and crystal defects with material properties at the single-atom level. We identify rich structural variety with unprecedented 3D detail including atomic composition, grain boundaries, anti-phase boundaries, anti-site point defects and swap defects. We show that the experimentally measured coordinates and chemical species with 22 picometre precision can be used as direct input for DFT calculations of material properties such as atomic spin and orbital magnetic moments and local magnetocrystalline anisotropy. The work presented here combines 3D atomic structure determination of crystal defects with DFT calculations, which is expected to advance our understanding of structure–property relationships at the fundamental level.« less

Deciphering chemical order/disorder and material properties at the single-atom level.

PubMed

Yang, Yongsoo; Chen, Chien-Chun; Scott, M C; Ophus, Colin; Xu, Rui; Pryor, Alan; Wu, Li; Sun, Fan; Theis, Wolfgang; Zhou, Jihan; Eisenbach, Markus; Kent, Paul R C; Sabirianov, Renat F; Zeng, Hao; Ercius, Peter; Miao, Jianwei

2017-02-01

Perfect crystals are rare in nature. Real materials often contain crystal defects and chemical order/disorder such as grain boundaries, dislocations, interfaces, surface reconstructions and point defects. Such disruption in periodicity strongly affects material properties and functionality. Despite rapid development of quantitative material characterization methods, correlating three-dimensional (3D) atomic arrangements of chemical order/disorder and crystal defects with material properties remains a challenge. On a parallel front, quantum mechanics calculations such as density functional theory (DFT) have progressed from the modelling of ideal bulk systems to modelling 'real' materials with dopants, dislocations, grain boundaries and interfaces; but these calculations rely heavily on average atomic models extracted from crystallography. To improve the predictive power of first-principles calculations, there is a pressing need to use atomic coordinates of real systems beyond average crystallographic measurements. Here we determine the 3D coordinates of 6,569 iron and 16,627 platinum atoms in an iron-platinum nanoparticle, and correlate chemical order/disorder and crystal defects with material properties at the single-atom level. We identify rich structural variety with unprecedented 3D detail including atomic composition, grain boundaries, anti-phase boundaries, anti-site point defects and swap defects. We show that the experimentally measured coordinates and chemical species with 22 picometre precision can be used as direct input for DFT calculations of material properties such as atomic spin and orbital magnetic moments and local magnetocrystalline anisotropy. This work combines 3D atomic structure determination of crystal defects with DFT calculations, which is expected to advance our understanding of structure-property relationships at the fundamental level.

Identification of Defect Candidates and their Effects on Carrier Lifetimes and Dark Currents in InAs/InAsSb Strained-Layer Superlattices for Infrared Detectors

NASA Astrophysics Data System (ADS)

Kioussis, Nicholas

The InAs/GaSb and InAs/InAsSb type-II strain-layer superlattices (T2SLS) are of great importance and show great promise for mid-wave and long-wave infrared (IR) detectors for a variety of civil and military applications. The T2SLS offer several advantages over present day detection technologies including suppressed Auger recombination relative to the bulk MCT material, high quantum efficiencies, and commercial availability of low defect density substrates. While the T2SLS detectors are approaching the empirical Rule-07 benchmark of MCT's performance level, the dark-current density is still significantly higher than that of bulk MCT detectors. One of the major origins of dark current is associated with the Shockley-Read- Hall (SRH) process in the depletion region of the detector. I will present results of ab initio electronic structure calculations of the stability of a wide range of point defects [As and In vacancies, In, As and Sb antisites, In interstitials, As interstitials, and Sb interstitials] in various charged states in bulk InAs, InSb, and InAsSb systems and T2SLS. I will also present results of the transition energy levels. The calculations reveal that compared to defects in bulk materials, the formation and defect properties in InAs/InAsSb T2SLS can be affected by various structural features, such as strain, interface, and local chemical environment. I will present examples where the effect of strain or local chemical environment shifts the transition energy levels of certain point defects either above or below the conduction band minimum, thus suppressing their contribution to the SRH recombination.

Quasiparticle and hybrid density functional methods in defect studies: An application to the nitrogen vacancy in GaN

NASA Astrophysics Data System (ADS)

Lewis, D. K.; Matsubara, M.; Bellotti, E.; Sharifzadeh, S.

2017-12-01

Defects in semiconductors can play a vital role in the performance of electronic devices, with native defects often dominating the electronic properties of the semiconductor. Understanding the relationship between structural defects and electronic function will be central to the design of new high-performance materials. In particular, it is necessary to quantitatively understand the energy and lifetime of electronic states associated with the defect. Here, we apply first-principles density functional theory (DFT) and many-body perturbation theory within the GW approximation to understand the nature and energy of the defect states associated with a charged nitrogen vacancy on the electronic properties of gallium nitride (GaN), as a model of a well-studied and important wide gap semiconductor grown with defects. We systematically investigate the sources of error associated with the GW approximation and the role of the underlying atomic structure on the predicted defect state energies. Additionally, analysis of the computed electronic density of states (DOS) reveals that there is one occupied defect state 0.2 eV below the valence band maximum and three unoccupied defect states at energy of 0.2-0.4 eV above the conduction band minimum, suggesting that this defect in the +1 charge state will not behave as a carrier trap. Furthermore, we compare the character and energy of the defect state obtained from GW and DFT using the HSE approximate density functional and find excellent agreement. This systematic study provides a more complete understanding of how to obtain quantitative defect energy states in bulk semiconductors.

Evolution of Radiation Induced Defects in SiC: A Multiscale Simulation Approach

NASA Astrophysics Data System (ADS)

Jiang, Hao

Because of various excellent properties, SiC has been proposed for many applications in nuclear reactors including cladding layers in fuel rod, fission products container in TRISO fuel, and first wall/blanket in magnetic controlled fusion reactors. Upon exposure to high energy radiation environments, point defects and defect clusters are generated in materials in amounts significantly exceeding their equilibrium concentrations. The accumulation of defects can lead to undesired consequences such as crystalline-to-amorphous transformation1, swelling, and embrittlement, and these phenomena can adversely affect the lifetime of SiC based components in nuclear reactors. It is of great importance to understand the accumulation process of these defects in order to estimate change in properties of this material and to design components with superior ability to withstand radiation damages. Defect clusters are widely in SiC irradiated at the operation temperatures of various reactors. These clusters are believed to cause more than half of the overall swelling of irradiated SiC and can potentially lead to lowered thermal conductivity and mechanical strength. It is critical to understand the formation and growth of these clusters. Diffusion of these clusters is one importance piece to determine the growth rate of clusters; however it is unclear so far due to the challenges in simulating rare events. Using a combination of kinetic Activation Relaxation Technique with empirical potential and ab initio based climbing image nudged elastic band method, I performed an extensive search of the migration paths of the most stable carbon tri-interstitial cluster in SiC. This research reveals paths with the lowest energy barriers to migration, rotation, and dissociation of the most stable cluster. Based on these energy barriers, I concluded defect clusters are thermally immobile at temperatures lower than 1500 K and can dissociate into smaller clusters and single interstitials at temperatures beyond that. Even though clusters cannot diffuse by thermal vibrations, we found they can migrate at room temperature under the influence of electron radiation. This is the first direct observation of radiation-induced diffusion of defect clusters in bulk materials. We show that the underlying mechanism of this athermal diffusion is elastic collision between incoming electrons and cluster atoms. Our findings suggest that defect clusters may be mobile under certain irradiation conditions, changing current understanding of cluster annealing process in irradiated SiC. With the knowledge of cluster diffusion in SiC demonstrated in this thesis, we now become able to predict cluster evolution in SiC with good agreement with experimental measurements. This ability can enable us to estimate changes in many properties of irradiated SiC relevant for its applications in reactors. Internal interfaces such as grain boundaries can behave as sinks to radiation induced defects. The ability of GBs to absorb, transport, and annihilate radiation-induced defects (sink strength) is important to understand radiation response of polycrystalline materials and to better design interfaces for improved resistance to radiation damage. Nowadays, it is established GBs' sink strength is not a static property but rather evolves with many factors, including radiation environments, grain size, and GB microstructure. In this thesis, I investigated the response of small-angle tilt and twist GBs to point defects fluxes in SiC. First of all, I found the pipe diffusion of interstitials in tilt GBs is slower than bulk diffusion. This is because the increased interatomic distance at dislocation cores raises the migration barrier of interstitial dumbbells. Furthermore, I show that both the annihilation of interstitials at jogs and jog nucleation from clusters are diffusion-controlled and can occur under off-stoichiometric interstitial fluxes. Finally, a dislocation line model is developed to predict the role of tilt GBs in annihilating radiation damage. The model predicts the role of tilt GBs in annihilating defects depends on the rate of defects segregation to and diffusion along tilt GBs. Tilt GBs mainly serve as diffusion channel for defects to reach other sinks when defect diffusivity is high at boundaries. When defect diffusivity is low, most of the defects segregated to tilt GBs are annihilated by dislocation climb. Up-to-date, the response of twist GBs under irradiation has been rarely reported in literature and is still unclear. It is important to develop atom scale insight on this question in order to predict twist GBs' sink strength for a better understanding of radiation response of polycrystalline materials. By using a combination of molecular dynamics and grand canonical Monte Carlo, here I demonstrate the defect kinetics in {001} and {111} twist GBs and the microstructural evolution of these GBs under defect fluxes in SiC. I found due to the deep potential well for interstitials at dislocation intersections within the interface, the mobility of defects on dislocation grid is retard and this leads to defect accumulation at GBs for many cases. Furthermore, I conclude both types of twist GBs have to form mixed dislocations with edge component in order to absorb accumulated interstitials at the interface. The formation of mixed dislocation is either by interstitial loop nucleation or by dislocation reactions at the interface. The continuous formation and climb of these mixed dislocations make twist GBs unsaturatable sinks to radiation induced defects.

Isolated oxygen defects in 3C- and 4H-SiC: A theoretical study

NASA Astrophysics Data System (ADS)

Gali, A.; Heringer, D.; Deák, P.; Hajnal, Z.; Frauenheim, Th.; Devaty, R. P.; Choyke, W. J.

2002-09-01

Ab initio calculations in the local-density approximation have been carried out in SiC to determine the possible configurations of the isolated oxygen impurity. Equilibrium geometry and occupation levels were calculated. Substitutional oxygen in 3C-SiC is a relatively shallow effective mass like double donor on the carbon site (OC) and a hyperdeep double donor on the Si site (OSi). In 4H-SiC OC is still a double donor but with a more localized electron state. In 3C-SiC OC is substantially more stable under any condition than OSi or interstitial oxygen (Oi). In 4H-SiC OC is also the most stable one except for heavy n-type doping. We propose that OC is at the core of the electrically active oxygen-related defect family found by deep level transient spectroscopy in 4H-SiC. The consequences of the site preference of oxygen on the SiC/SiO2 interface are discussed.

Physics through the 1990s: Condensed-matter physics

NASA Technical Reports Server (NTRS)

1986-01-01

The volume presents the current status of condensed-matter physics from developments since the 1970s to opportunities in the 1990s. Topics include electronic structure, vibrational properties, critical phenomena and phase transitions, magnetism, semiconductors, defects and diffusion, surfaces and interfaces, low-temperature physics, liquid-state physics, polymers, nonlinear dynamics, instabilities, and chaos. Appendices cover the connections between condensed-matter physics and applications of national interest, new experimental techniques and materials, laser spectroscopy, and national facilities for condensed-matter physics research. The needs of the research community regarding support for individual researchers and for national facilities are presented, as are recommendations for improved government-academic-industrial relations.

Enhanced Efficiency of Polymer Light-Emitting Diodes by Dispersing Dehydrated Nanotube Titanic Acid in the Hole-buffer Layer

NASA Astrophysics Data System (ADS)

Qian, L.; Xu, Z.; Teng, F.; Duan, X.-X.; Jin, Z.-S.; Du, Z.-L.; Li, F.-S.; Zheng, M.-J.; Wang, Y.-S.

2007-06-01

Efficiency of polymer light-emitting diodes (PLEDs) with poly(2-methoxy-5-(2-ethyl hexyloxy)- p-phenylene vinylene) (MEH-PPV) as an emitting layer was improved if a dehydrated nanotubed titanic acid (DNTA) doped hole-buffer layer polyethylene dioxythiophene (PEDOT) was used. Photoluminescence (PL) and Raman spectra indicated a stronger interaction between DNTA and sulfur atom in thiophene of PEDOT, which suppresses the chemical interaction between vinylene of MEH-PPV and thiophene of PEDOT. The interaction decreases the defect states in an interface region to result in enhancement in device efficiency, even though the hole transporting ability of PEDOT was decreased.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nippert, Felix, E-mail: felix@physik.tu-berlin.de; Callsen, Gordon; Westerkamp, Steffen

We investigate industrial-grade InGaN/GaN quantum wells (QWs) emitting in the green spectral region under high, resonant pumping conditions. Consequently, an ubiquitous high energy luminescence is observed that we assign to a polarization field Confined Hole Continuum (CHC). Our finding is supported by a unique combination of experimental techniques, including transmission electron microscopy, (time-resolved) photoluminescence under various excitation conditions, and electroluminescence, which confirm an extended out-of-plane localization of the CHC-states. The larger width of this localization volume surpasses the QW thickness, yielding enhanced non-radiative losses due to point defects and interfaces, whereas the energetic proximity to the bulk valence band statesmore » promotes carrier leakage.« less

Vortex properties of two-dimensional superconducting Pb films.

PubMed

Ning, Y X; Song, C L; Wang, Y L; Chen, Xi; Jia, J F; Xue, Q K; Ma, X C

2010-02-17

Using low temperature scanning tunnelling microscopy/spectroscopy (STM/STS) we have investigated the vortex behaviours of two-dimensional superconducting Pb films at different thicknesses. STS at the vortex core shows an evolution of electronic states with film thickness. Transition from the clean limit to the dirty limit of superconductivity is identified, which can be ascribed to the decreased electronic mean free path induced by stronger scattering from the disordered interface at smaller thicknesses. A magnetic field dependent vortex core size is observed even for such a low- κ superconductor. The weak pinning induced by surface defects leads to the formation of a distorted hexagonal vortex lattice.

Effective defect diffusion lengths in Ar-ion bombarded 3C-SiC

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bayu Aji, L. B.; Wallace, J. B.; Shao, L.

Above room temperature, SiC exhibits pronounced processes of diffusion and interaction of radiation-generated point defects. Here, we use the recently developed pulsed ion beam method to measure effective defect diffusion lengths in 3C-SiC bombarded in the temperature range of 25–200 °C with 500 keV Ar ions. Results reveal a diffusion length of ~10 nm, which exhibits a weak temperature dependence, changing from 9 to 13 nm with increasing temperature. Lastly, these results have important implications for understanding and predicting radiation damage in SiC and for the development of radiation-resistant materials via interface-mediated defect reactions.

Effective defect diffusion lengths in Ar-ion bombarded 3C-SiC

DOE PAGES

Bayu Aji, L. B.; Wallace, J. B.; Shao, L.; ...

2016-04-14

Above room temperature, SiC exhibits pronounced processes of diffusion and interaction of radiation-generated point defects. Here, we use the recently developed pulsed ion beam method to measure effective defect diffusion lengths in 3C-SiC bombarded in the temperature range of 25–200 °C with 500 keV Ar ions. Results reveal a diffusion length of ~10 nm, which exhibits a weak temperature dependence, changing from 9 to 13 nm with increasing temperature. Lastly, these results have important implications for understanding and predicting radiation damage in SiC and for the development of radiation-resistant materials via interface-mediated defect reactions.

Characterization of high-quality kerfless epitaxial silicon for solar cells: Defect sources and impact on minority-carrier lifetime

DOE PAGES

Kivambe, Maulid M.; Powell, Douglas M.; Castellanos, Sergio; ...

2017-11-14

We investigate the types and origins of structural defects in thin (<100 μm) kerfless epitaxial single crystal silicon grown on top of reorganized porous silicon layers. Although the structural defect density is low (has average defect density < 10 4 cm -2), localized areas with a defect density > 10 5 cm -2 are observed. Cross-sectional and systematic plan-view defect etching and microscopy reveals that the majority of stacking faults and dislocations originate at the interface between the porous silicon layer and the epitaxial wafer. Localised dislocation clusters are observed in regions of collapsed/deformed porous silicon and at decorated stackingmore » faults. In localized regions of high extended defect density, increased minority-carrier recombination activity is observed. Evidence for impurity segregation to the extended defects (internal gettering), which is known to exacerbate carrier recombination is demonstrated. In conclusion, the impact of the defects on material performance and substrate re-use is also discussed.« less

Characterization of high-quality kerfless epitaxial silicon for solar cells: Defect sources and impact on minority-carrier lifetime

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kivambe, Maulid M.; Powell, Douglas M.; Castellanos, Sergio

We investigate the types and origins of structural defects in thin (<100 μm) kerfless epitaxial single crystal silicon grown on top of reorganized porous silicon layers. Although the structural defect density is low (has average defect density < 10 4 cm -2), localized areas with a defect density > 10 5 cm -2 are observed. Cross-sectional and systematic plan-view defect etching and microscopy reveals that the majority of stacking faults and dislocations originate at the interface between the porous silicon layer and the epitaxial wafer. Localised dislocation clusters are observed in regions of collapsed/deformed porous silicon and at decorated stackingmore » faults. In localized regions of high extended defect density, increased minority-carrier recombination activity is observed. Evidence for impurity segregation to the extended defects (internal gettering), which is known to exacerbate carrier recombination is demonstrated. In conclusion, the impact of the defects on material performance and substrate re-use is also discussed.« less

Characterization of high-quality kerfless epitaxial silicon for solar cells: Defect sources and impact on minority-carrier lifetime

NASA Astrophysics Data System (ADS)

Kivambe, Maulid M.; Powell, Douglas M.; Castellanos, Sergio; Jensen, Mallory Ann; Morishige, Ashley E.; Lai, Barry; Hao, Ruiying; Ravi, T. S.; Buonassisi, Tonio

2018-02-01

We investigate the types and origins of structural defects in thin (<100 μm) kerfless epitaxial single crystal silicon grown on top of reorganized porous silicon layers. Although the structural defect density is low (has average defect density < 104 cm-2), localized areas with a defect density > 105 cm-2 are observed. Cross-sectional and systematic plan-view defect etching and microscopy reveals that the majority of stacking faults and dislocations originate at the interface between the porous silicon layer and the epitaxial wafer. Localised dislocation clusters are observed in regions of collapsed/deformed porous silicon and at decorated stacking faults. In localized regions of high extended defect density, increased minority-carrier recombination activity is observed. Evidence for impurity segregation to the extended defects (internal gettering), which is known to exacerbate carrier recombination is demonstrated. The impact of the defects on material performance and substrate re-use is also discussed.

Using high pressure to study thermal transport and phonon scattering mechanisms

NASA Astrophysics Data System (ADS)

Hohensee, Gregory Thomas

The aerospace industry studies nanocomposites for heat dissipation and moderation of thermal expansion, and the semiconductor industry faces a Joule heating barrier in devices with high power density. My primary experimental tools are the diamond anvil cell (DAC) coupled with time-domain thermoreflectance (TDTR). TDTR is a precise optical method well-suited to measuring thermal conductivities and conductances at the nanoscale and across interfaces. The DAC-TDTR method yields thermal property data as a function of pressure, rather than temperature. This relatively unexplored independent variable can separate the components of thermal conductance and serve as an independent test for phonon-defect scattering models. I studied the effect of non-equilibrium thermal transport at the aluminum-coated surface of an exotic cuprate material Ca9La5Cu 24O41, which boasts a tenfold enhanced thermal conductivity along one crystalline axis where two-leg copper-oxygen spin-ladder structures carry heat in the form of thermalized magnetic excitations. Highly anisotropic materials are of interest for controlled thermal management applications, and the spin-ladder magnetic heat carriers ("magnons") are not well understood. I found that below room temperature, the apparent thermal conductivity of Ca9La5Cu24O41 depends on the frequency of the applied surface heating in TDTR. This occurs because the thermal penetration depth in the TDTR experiment is comparable to the length-scale for the equilibration of the magnons that are the dominant channel for heat conduction and the phonons that dominate the heat capacity. I applied a two-temperature model to analyze the TDTR data and extracted an effective volumetric magnon-phonon coupling parameter g for Ca9La5Cu24O 41 at temperatures from 75 K to 300 K; g varies by approximately two orders of magnitude over this range of temperature and has the value g = 1015 W m-3 K-1 near the peak of the thermal conductivity at T ≈ 180 K. To examine intrinsic phonon-mediated interface conductance between dissimilar materials, I applied DAC-TDTR to measure the thermal conductance of a series of metal-diamond interfaces as a function of pressure up to 50 GPa. The thermal conductance of interfaces between metals and diamond, which has a comparatively high Debye temperature, is often greater than can be accounted for by two phonon-processes, and the nature of heat transport between such dissimilar materials is central to the thermal design of composite materials. The high pressures achievable in a diamond anvil cell can significantly extend the metal phonon density of states to higher frequencies, and can also suppress extrinsic effects by greatly stiffening interface bonding. I measured the interface thermal conductances of Pb, Au0.95Pd0.05, Pt, and Al films deposited on Type 1A natural [100] and Type 2A synthetic [110] diamond anvils, from ambient pressure to 50 GPa. In all cases, the thermal conductances increase weakly or saturate to similar values at high pressure. My results suggest that anharmonic conductance at metal-diamond interfaces is controlled by partial transmission processes, where a diamond phonon that inelastically scatters at the interface absorbs or emits a metal phonon. The thermal conductivity and absolute electrical resistivity of metallic silicon have not been measured previously. I performed regular and beam-offset TDTR to establish the thermal conductivities of Si and Si0.991Ge 0.009 across the semiconductor-metal phase transition and up to 45 GPa. The thermal conductivities of metallic Si and Si(Ge) are comparable to aluminum and indicative of predominantly electronic heat carriers. Metallic Si and Si(Ge) have a transport anisotropy of approximately 1.4, similar to that of beryllium, due to the primitive hexagonal crystal structure. I used the Wiedemann-Franz law to derive the associated electrical resistivity, and found it consistent with the Bloch-Gruneisen model. Not all crystalline point defects are alike in how they scatter phonons and reduce the thermal conductivity of mixed crystals. Heat-carrying phonons in iron (Fe) doped MgO, or [Mg,Fe]O ferropericlase, are known to be resonantly scattered by interaction with a 3.3 THz electronic transition in the high-spin state of the Fe impurities. At sufficiently high pressures, the Fe atoms transition from a high-spin to a low-spin state, which eliminates the resonant interaction and reduces the Fe atoms to simpler point defect phonon scatterers. To study the behavior of phonon-defect scattering with and without this resonant scattering process, I measured the thermal conductivity of Mg0.92Fe0.08 O ferropericlase up to and above the 40--60 GPa spin transition. Fe-doped MgO (ferropericlase) is also a model system relevant to geophysical modeling of the Earth's core-mantle boundary, so data on its thermal transport under pressure is valuable in itself. (Abstract shortened by UMI.).

Irradiation Induced Microstructure Evolution in Nanostructured Materials: A Review

PubMed Central

Liu, Wenbo; Ji, Yanzhou; Tan, Pengkang; Zang, Hang; He, Chaohui; Yun, Di; Zhang, Chi; Yang, Zhigang

2016-01-01

Nanostructured (NS) materials may have different irradiation resistance from their coarse-grained (CG) counterparts. In this review, we focus on the effect of grain boundaries (GBs)/interfaces on irradiation induced microstructure evolution and the irradiation tolerance of NS materials under irradiation. The features of void denuded zones (VDZs) and the unusual behavior of void formation near GBs/interfaces in metals due to the interactions between GBs/interfaces and irradiation-produced point defects are systematically reviewed. Some experimental results and calculation results show that NS materials have enhanced irradiation resistance, due to their extremely small grain sizes and large volume fractions of GBs/interfaces, which could absorb and annihilate the mobile defects produced during irradiation. However, there is also literature reporting reduced irradiation resistance or even amorphization of NS materials at a lower irradiation dose compared with their bulk counterparts, since the GBs are also characterized by excess energy (compared to that of single crystal materials) which could provide a shift in the total free energy that will lead to the amorphization process. The competition of these two effects leads to the different irradiation tolerance of NS materials. The irradiation-induced grain growth is dominated by irradiation temperature, dose, ion flux, character of GBs/interface and nanoprecipitates, although the decrease of grain sizes under irradiation is also observed in some experiments. PMID:28787902

Role of Grain Boundaries under Long-Time Radiation

NASA Astrophysics Data System (ADS)

Zhu, Yichao; Luo, Jing; Guo, Xu; Xiang, Yang; Chapman, Stephen Jonathan

2018-06-01

Materials containing a high proportion of grain boundaries offer significant potential for the development of radiation-resistant structural materials. However, a proper understanding of the connection between the radiation-induced microstructural behavior of a grain boundary and its impact at long natural time scales is still missing. In this Letter, point defect absorption at interfaces is summarized by a jump Robin-type condition at a coarse-grained level, wherein the role of interface microstructure is effectively taken into account. Then a concise formula linking the sink strength of a polycrystalline aggregate with its grain size is introduced and is well compared with experimental observation. Based on the derived model, a coarse-grained formulation incorporating the coupled evolution of grain boundaries and point defects is proposed, so as to underpin the study of long-time morphological evolution of grains induced by irradiation. Our simulation results suggest that the presence of point defect sources within a grain further accelerates its shrinking process, and radiation tends to trigger the extension of twin boundary sections.

Optical coherence tomography and confocal microscopy investigations of dental prostheses

NASA Astrophysics Data System (ADS)

Negrutiu, Meda L.; Sinescu, Cosmin; Hughes, Michael; Bradu, Adrian; Rominu, Mihai; Todea, Carmen; Dobre, George; Podoleanu, Adrian

2008-09-01

Dental prostheses are very complex systems, heterogenous in structure, made up from various materials, with different physical properties. An essential question mark is on the physical, chemical and mechanical compatibility between these materials. They have to satisfy high stress requirements as well as esthetic challenges. The masticatory stress may induce fractures of the prostheses, which may be triggered by initial materials defects or by alterations of the technological process. The failures of dental prostheses lead to functional, esthetic and phonetic disturbances which finally render the prosthetic treatment inefficient. The purpose of this study is to evaluate the capability of en-face optical coherence tomography as a possible non-invasive high resolution method in supplying the necessary information on the material defects of dental prostheses and microleakage at prosthetic interfaces. C-scan and B-scan OCT images as well as confocal images are acquired from a large range of samples. Gaps between the dental interfaces and material defects are clearly exposed. We conclude that OCT can successfully be used as a noninvasive analysis method.

Magnetic states of linear defects in graphene monolayers: Effects of strain and interaction

NASA Astrophysics Data System (ADS)

Alexandre, Simone S.; Nunes, R. W.

2017-08-01

The combined effects of defect-defect interaction and strains of up to 10% on the onset of magnetic states in the quasi-one-dimensional electronic states generated by the so-called 558 linear defect in graphene monolayers are investigated by means of ab initio calculations. Results are analyzed on the basis of the heuristics of the Stoner criterion. We find that conditions for the emergence of magnetic states on the 558 defect can be tuned by uniaxial tensile parallel strains (along the defect direction) as well as by uniaxial compressive perpendicular strains, at both limits of isolated and interacting 558 defects. Parallel tensile strains and perpendicular compressive strains are shown to give rise to two cooperative effects that favor the emergence of itinerant magnetism on the 558 defect in graphene: enhancement of the density of states (DOS) of the resonant defect states in the region of the Fermi level and tuning of the Fermi level to the maximum of the related DOS peak. On the other hand, parallel compressive strains and perpendicular tensile strains are shown to be detrimental to the development of magnetic states in the 558 defect, because in these cases the Fermi level is found to shift away from the maximum of the DOS of the defect states. Effects of isotropic and unisotropic biaxial strains are also analyzed in terms of the conditions encoded in the Stoner criterion.

Defect assisted coupling of a MoS2/TiO2 interface and tuning of its electronic structure.

PubMed

Chen, Guifeng; Song, Xiaolin; Guan, Lixiu; Chai, Jianwei; Zhang, Hui; Wang, Shijie; Pan, Jisheng; Tao, Junguang

2016-09-02

Although MoS2 based heterostructures have drawn increased attention, the van der Waals forces within MoS2 layers make it difficult for the layers to form strong chemical coupled interfaces with other materials. In this paper, we demonstrate the successful strong chemical attachment of MoS2 on TiO2 nanobelts after appropriate surface modifications. The etch-created dangling bonds on TiO2 surfaces facilitate the formation of a steady chemically bonded MoS2/TiO2 interface. With the aid of high resolution transmission electron microscope measurements, the in-plane structure registry of MoS2/TiO2 is unveiled at the atomic scale, which shows that MoS2[1-10] grows along the direction of TiO2[001] and MoS2[110] parallel to TiO2[100] with every six units of MoS2 superimposed on five units of TiO2. Electronically, type II band alignments are realized for all surface treatments. Moreover, the band offsets are delicately correlated to the surface states, which plays a significant role in their photocatalytic performance.

Effect of electrode manufacturing defects on electrochemical performance of lithium-ion batteries: Cognizance of the battery failure sources

DOE PAGES

Mohanty, D.; Hockaday, E.; Li, J.; ...

2016-02-21

During LIB electrode manufacturing, it is difficult to avoid the certain defects that diminish LIB performance and shorten the life span of the batteries. This study provides a systematic investigation correlating the different plausible defects (agglomeration/blisters, pinholes/divots, metal particle contamination, and non-uniform coating) in a LiNi 0.5Mn 0.3Co 0.2O 2 positive electrode with its electrochemical performance. Additionally, an infrared thermography technique was demonstrated as a nondestructive tool to detect these defects. The findings show that cathode agglomerates aggravated cycle efficiency, and resulted in faster capacity fading at high current density. Electrode pinholes showed substantially lower discharge capacities at higher currentmore » densities than baseline NMC 532 electrodes. Metal particle contaminants have an extremely negative effect on performance, at higher C-rates. The electrodes with more coated and uncoated interfaces (non-uniform coatings) showed poor cycle life compared with electrodes with fewer coated and uncoated interfaces. Further, microstructural investigation provided evidence of presence of carbon-rich region in the agglomerated region and uneven electrode coating thickness in the coated and uncoated interfacial regions that may lead to the inferior electrochemical performance. In conclusion, this study provides the importance of monitoring and early detection of the electrode defects during LIB manufacturing processes to minimize the cell rejection rate after fabrication and testing.« less

Low voltage-driven oxide phototransistors with fast recovery, high signal-to-noise ratio, and high responsivity fabricated via a simple defect-generating process

PubMed Central

Yun, Myeong Gu; Kim, Ye Kyun; Ahn, Cheol Hyoun; Cho, Sung Woon; Kang, Won Jun; Cho, Hyung Koun; Kim, Yong-Hoon

2016-01-01

We have demonstrated that photo-thin film transistors (photo-TFTs) fabricated via a simple defect-generating process could achieve fast recovery, a high signal to noise (S/N) ratio, and high sensitivity. The photo-TFTs are inverted-staggered bottom-gate type indium-gallium-zinc-oxide (IGZO) TFTs fabricated using atomic layer deposition (ALD)-derived Al2O3 gate insulators. The surfaces of the Al2O3 gate insulators are damaged by ion bombardment during the deposition of the IGZO channel layers by sputtering and the damage results in the hysteresis behavior of the photo-TFTs. The hysteresis loops broaden as the deposition power density increases. This implies that we can easily control the amount of the interface trap sites and/or trap sites in the gate insulator near the interface. The photo-TFTs with large hysteresis-related defects have high S/N ratio and fast recovery in spite of the low operation voltages including a drain voltage of 1 V, positive gate bias pulse voltage of 3 V, and gate voltage pulse width of 3 V (0 to 3 V). In addition, through the hysteresis-related defect-generating process, we have achieved a high responsivity since the bulk defects that can be photo-excited and eject electrons also increase with increasing deposition power density. PMID:27553518

Positron annihilation spectroscopy: Applications to Si, ZnO, and multilayer semiconductor structures

NASA Astrophysics Data System (ADS)

Schaffer, J. P.; Rohatgi, A.; Dewald, A. B.; Frost, R. L.; Pang, S. K.

1989-11-01

The potential of positron annihilation spectroscopy (PAS) for defect characterization at the atomic scale in semiconductors is demonstrated for Si, ZnO, and multilayer structures, such as an AlGaAs/GaAs solar cell. The types of defects discussed include: i) vacancy complexes, oxygen impurities and dopants, ii) the influence of cooling rates on spatial non-uniformities in defects, and iii) characterization of buried interfaces. In sev-eral instances, the results of the PAS investigations are correlated with data from other established semiconductor characterization techniques.

Electron emission from deep level defects EL2 and EL6 in semi-insulating GaAs observed by positron drift velocity transient measurements

NASA Astrophysics Data System (ADS)

Tsia, J. M.; Ling, C. C.; Beling, C. D.; Fung, S.

2002-09-01

A plus-or-minus100 V square wave applied to a Au/semi-insulating SI-GaAs interface was used to bring about electron emission from and capture into deep level defects in the region adjacent to the interface. The electric field transient resulting from deep level emission was studied by monitoring the positron drift velocity in the region. A deep level transient spectrum was obtained by computing the trap emission rate as a function of temperature and two peaks corresponding to EL2 (Ea=0.81plus-or-minus0.15 eV) and EL6 (Ea=0.30plus-or-minus0.12 eV) have been identified.

Electron and hole relaxation pathways in semiconductor quantum dots

DOE Office of Scientific and Technical Information (OSTI.GOV)

Klimov, V.I.; McBranch, D.W.; Leatherdale, C.A.

1999-11-01

Femtosecond (fs) broad-band transient absorption (TA) is used to study the intraband relaxation and depopulation dynamics of electron and hole quantized states in CdSe nanocrystals (NC{close_quote}s) with a range of surface properties. Instead of the drastic reduction in the energy relaxation rate expected due to a {open_quotes}phonon bottleneck,{close_quotes} we observe a fast subpicosecond 1P-to-1S electron relaxation, with the rate exceeding that due to phonon emission in bulk semiconductors. The energy relaxation is enhanced with reducing the NC{close_quote}s radius, and does not show any dependence on the NC surface properties (quality of the surface passivation). These data indicate that electron energymore » relaxation occurs by neither multiphonon emission nor by coupling to surface defects, but is likely meditated by Auger-type electron-hole energy transfer. We use fs infrared TA to probe electron and hole intraband transitions, which allows us to distinguish between electron and hole relaxation pathways leading to the depopulation of NC quantized states. In contrast to the electron relaxation, which is controlled by NC surface passivation, the depopulation of hole quantized states is extremely fast (sub-ps-to-ps time scales) in all types of samples, independent of NC surface treatment (including NC{close_quote}s overcoated with a ZnS layer). Our results indicate that ultrafast hole dynamics are not due to trapping at localized surface defects such as a vacancy, but rather arise from relaxation into intrinsic NC states or intrinsically unpassivated interface states. {copyright} {ital 1999} {ital The American Physical Society}« less

Entanglement between random and clean quantum spin chains

NASA Astrophysics Data System (ADS)

Juhász, Róbert; Kovács, István A.; Roósz, Gergő; Iglói, Ferenc

2017-08-01

The entanglement entropy in clean, as well as in random quantum spin chains has a logarithmic size-dependence at the critical point. Here, we study the entanglement of composite systems that consist of a clean subsystem and a random subsystem, both being critical. In the composite, antiferromagnetic XX-chain with a sharp interface, the entropy is found to grow in a double-logarithmic fashion {{ S}}∼ \\ln\\ln(L) , where L is the length of the chain. We have also considered an extended defect at the interface, where the disorder penetrates into the homogeneous region in such a way that the strength of disorder decays with the distance l from the contact point as  ∼l-κ . For κ<1/2 , the entropy scales as {{ S}}(κ) ≃ \\frac{\\ln 2 (1-2κ)}{6}{\\ln L} , while for κ ≥slant 1/2 , when the extended interface defect is an irrelevant perturbation, we recover the double-logarithmic scaling. These results are explained through strong-disorder RG arguments.

Emitter Choice for Epitaxial CdTe Solar Cells

DOE Office of Scientific and Technical Information (OSTI.GOV)

Song, Tao; Kanevce, Ana; Sites, James R.

2016-11-21

High-quality epitaxial CdTe layers with low defect density and high carrier concentration have been demonstrated by several research groups. Nevertheless, one primary challenge for high-performance epitaxial CdTe solar cells is how to choose a suitable emitter partner for the junction formation. The numerical simulations show that a type I heterojunction with small conduction band offset (0.1 eV = ..delta..Ec = 0.3 eV) is necessary to maintain a good cell efficiency even with large interface recombination. Otherwise, a small 'cliff' can assist interface recombination causing smaller Voc, and a large 'spike' (..delta..Ec = 0.4 eV) can impede the photo current andmore » lead to a reduction of JSC and FF. Among the three possible emitters, CdS, CdMgTe, and MgZnO, CdMgTe (with ~30% Mg) and MgZnO (with ~ 20% Mg) are likely to be a better choice since their type-I junction can tolerate a larger density of interface defects.« less

A novel ultrasonic NDE for shrink fit welded structures using interface waves.

PubMed

Lee, Jaesun; Park, Junpil; Cho, Younho

2016-05-01

Reactor vessel inspection is a critical part of safety maintenance in a nuclear power plant. The inspection of shrink fit welded structures in a reactor nozzle can be a challenging task due to the complicated geometry. Nozzle inspection using pseudo interface waves allows us to inspect the nozzle from outside of the nuclear reactor. In this study, layered concentric pipes were manufactured with perfect shrink fit conditions using stainless steel 316. The displacement distributions were calculated with boundary conditions for a shrink fit welded structure. A multi-transducer guided wave phased array system was employed to monitor the welding quality of the nozzle end at a distance from a fixed position. The complicated geometry of a shrink fit welded structure can be overcome by using the pseudo interface waves in identifying the location and size of defects. The experimental results demonstrate the feasibility of detecting weld delamination and defects. Copyright © 2016 Elsevier B.V. All rights reserved.

Deciphering chemical order/disorder and material properties at the single-atom level

DOE PAGES

Yang, Yongsoo; Chen, Chien-Chun; Scott, M. C.; ...

2017-02-01

Perfect crystals are rare in nature. Real materials often contain crystal defects and chemical order/disorder such as grain boundaries, dislocations, interfaces, surface reconstructions and point defects. Such disruption in periodicity strongly affects material properties and functionality. Despite rapid development of quantitative material characterization methods, correlating three-dimensional (3D) atomic arrangements of chemical order/disorder and crystal defects with material properties remains a challenge. On a parallel front, quantum mechanics calculations such as density functional theory (DFT) have progressed from the modelling of ideal bulk systems to modelling ‘real’ materials with dopants, dislocations, grain boundaries and interfaces; but these calculations rely heavily onmore » average atomic models extracted from crystallography. To improve the predictive power of first-principles calculations, there is a pressing need to use atomic coordinates of real systems beyond average crystallographic measurements. Here we determine the 3D coordinates of 6,569 iron and 16,627 platinum atoms in an iron-platinum nanoparticle, and correlate chemical order/disorder and crystal defects with material properties at the single-atom level. We identify rich structural variety with unprecedented 3D detail including atomic composition, grain boundaries, anti-phase boundaries, anti-site point defects and swap defects. We show that the experimentally measured coordinates and chemical species with 22 picometre precision can be used as direct input for DFT calculations of material properties such as atomic spin and orbital magnetic moments and local magnetocrystalline anisotropy. The work presented here combines 3D atomic structure determination of crystal defects with DFT calculations, which is expected to advance our understanding of structure–property relationships at the fundamental level.« less

Storing quantum information for 30 seconds in a nanoelectronic device.

PubMed

Muhonen, Juha T; Dehollain, Juan P; Laucht, Arne; Hudson, Fay E; Kalra, Rachpon; Sekiguchi, Takeharu; Itoh, Kohei M; Jamieson, David N; McCallum, Jeffrey C; Dzurak, Andrew S; Morello, Andrea

2014-12-01

The spin of an electron or a nucleus in a semiconductor naturally implements the unit of quantum information--the qubit. In addition, because semiconductors are currently used in the electronics industry, developing qubits in semiconductors would be a promising route to realize scalable quantum information devices. The solid-state environment, however, may provide deleterious interactions between the qubit and the nuclear spins of surrounding atoms, or charge and spin fluctuations arising from defects in oxides and interfaces. For materials such as silicon, enrichment of the spin-zero (28)Si isotope drastically reduces spin-bath decoherence. Experiments on bulk spin ensembles in (28)Si crystals have indeed demonstrated extraordinary coherence times. However, it remained unclear whether these would persist at the single-spin level, in gated nanostructures near amorphous interfaces. Here, we present the coherent operation of individual (31)P electron and nuclear spin qubits in a top-gated nanostructure, fabricated on an isotopically engineered (28)Si substrate. The (31)P nuclear spin sets the new benchmark coherence time (>30 s with Carr-Purcell-Meiboom-Gill (CPMG) sequence) of any single qubit in the solid state and reaches >99.99% control fidelity. The electron spin CPMG coherence time exceeds 0.5 s, and detailed noise spectroscopy indicates that--contrary to widespread belief--it is not limited by the proximity to an interface. Instead, decoherence is probably dominated by thermal and magnetic noise external to the device, and is thus amenable to further improvement.

Some fundamental and applicative properties of [polymer/nano-SiC] hybrid nanocomposites

NASA Astrophysics Data System (ADS)

Kassiba, A.; Bouclé, J.; Makowska-Janusik, M.; Errien, N.

2007-08-01

Hybrid nanocomposites which combine polymer as host matrix and nanocrystals as active elements are promising functional materials for electronics, optics or photonics. In these systems, the physical response is governed by the nanocrystal features (size, surface and defect states), the polymer properties and the polymer-nanocrystal interface. This work reviews some selective nanostructured architectures based on active elements such as silicon carbide (SiC) nanocrystals and polymer host matrices. Beyond an overview of some key properties of the nanocrystals, a main part will be devoted to the electro-optical (EO) properties of SiC based hybrid systems where SiC nanocrystals are embedded in polymer matrices of different chemical nature such as poly-(methylmethacrylate) (PMMA), poly-vinylcarbazole (PVK) or polycarbonate. Using this approach, the organic-inorganic interface effects are emphasised with regard to the dielectric or hole transporting behaviour of PMMA and PVK respectively. These effects are illustrated through different EO responses associated with hybrid composites based on PMMA or PVK.

Real-space imaging of a topologically protected edge state with ultracold atoms in an amplitude-chirped optical lattice

PubMed Central

Leder, Martin; Grossert, Christopher; Sitta, Lukas; Genske, Maximilian; Rosch, Achim; Weitz, Martin

2016-01-01

To describe a mobile defect in polyacetylene chains, Su, Schrieffer and Heeger formulated a model assuming two degenerate energy configurations that are characterized by two different topological phases. An immediate consequence was the emergence of a soliton-type edge state located at the boundary between two regions of different configurations. Besides giving first insights in the electrical properties of polyacetylene materials, interest in this effect also stems from its close connection to states with fractional charge from relativistic field theory. Here, using a one-dimensional optical lattice for cold rubidium atoms with a spatially chirped amplitude, we experimentally realize an interface between two spatial regions of different topological order in an atomic physics system. We directly observe atoms confined in the edge state at the intersection by optical real-space imaging and characterize the state as well as the size of the associated energy gap. Our findings hold prospects for the spectroscopy of surface states in topological matter and for the quantum simulation of interacting Dirac systems. PMID:27767054

Effect of Interaction of the Temperature Field and Supersaturation on the Morphology of the Solid-Vapor Interface in Crystal Growth by Physical Vapor Transport

NASA Technical Reports Server (NTRS)

Grasza, K.; Palosz, W.; Curreri, Peter A. (Technical Monitor)

2002-01-01

An in-situ study of the morphology of the solid-vapor interface during iodine crystal growth was done. The conditions for terrace growth, flat faces formation and retraction, competition between sources of steps, formation of protrusions, surface roughening, and defect overgrowth are demonstrated and discussed.

Reactive transport of CO2-rich fluids in simulated wellbore interfaces: Experiments and models exploring behaviour on length scales of 1 to 6 m

NASA Astrophysics Data System (ADS)

Wolterbeek, T. K. T.; Raoof, A.; Peach, C. J.; Spiers, C. J.

2016-12-01

Defects present at casing-cement interfaces in wellbores constitute potential pathways for CO2 to migrate from geological storage systems. It is essential to understand how the transport properties of such pathways evolve when penetrated by CO2-rich fluids. While numerous studies have explored this problem at the decimetre length-scale, the 1-10-100 m scales relevant for real wellbores have received little attention. The present work addresses the effects of long-range reactive transport on a length scale of 1-6 m. This is done by means of a combined experimental and modelling study. The experimental work consisted of flow-through tests, performed on cement-filled steel tubes, 1-6 m in length, containing artificially debonded cement-interfaces. Four tests were performed, at 60-80 °C, imposing flow-through of CO2-rich fluid at mean pressures of 10-15 MPa, controlling the pressure difference at 0.12-4.8 MPa, while measuring flow-rate. In the modelling work, we developed a numerical model to explore reactive transport in CO2-exposed defects on a similar length scale. The formulation adopted incorporates fluid flow, advective and diffusive solute transport, and CO2-cement chemical reactions. Our results show that long-range reactive transport strongly affects the permeability evolution of CO2-exposed defects. In the experiments, sample permeability decreased by 2-4 orders, which microstructural observations revealed was associated with downstream precipitation of carbonates, possibly aided by migration of fines. The model simulations show precipitation in initially open defects produces a sharp decrease in flow rate, causing a transition from advection to diffusion-dominated reactive transport. While the modelling results broadly reproduce the experimental observations, it is further demonstrated that non-uniformity in initial defect aperture has a profound impact on self-sealing behaviour and system permeability evolution on the metre scale. The implication is that future reactive transport models and wellbore scale analyses must include defects with variable aperture in order to obtain reliable upscaling relations.

Molecular dynamics studies of simple membrane-water interfaces: Structure and functions in the beginnings of cellular life

NASA Technical Reports Server (NTRS)

Pohorille, Andrew; Wilson, Michael A.

1995-01-01

Molecular dynamics computer simulations of the structure and functions of a simple membrane are performed in order to examine whether membranes provide an environment capable of promoting protobiological evolution. Our model membrane is composed of glycerol 1-monooleate. It is found that the bilayer surface fluctuates in time and space, occasionally creating thinning defects in the membrane. These defects are essential for passive transport of simple ions across membranes because they reduce the Born barrier to this process by approximately 40%. Negative ions are transferred across the bilayer more readily than positive ions due to favorable interactions with the electric field at the membrane-water interface. Passive transport of neutral molecules is, in general, more complex than predicted by the solubility-diffusion model. In particular, molecules which exhibit sufficient hydrophilicity and lipophilicity concentrate near membrane surfaces and experience 'interfacial resistance' to transport. The membrane-water interface forms an environment suitable for heterogeneous catalysis. Several possible mechanisms leading to an increase of reaction rates at the interface are discussed. We conclude that vesicles have many properties that make them very good candidates for earliest protocells. Some potentially fruitful directions of experimental and theoretical research on this subject are proposed.

Effect of Interface Shape and Magnetic Field on the Microstructure of Bulk Ge:Ga

NASA Technical Reports Server (NTRS)

Cobb, S. D.; Szofran, F. R.; Volz, M. P.

1999-01-01

Thermal and compositional gradients induced during the growth process contribute significantly to the development of defects in the solidified boule. Thermal gradients and the solid-liquid interface shape can be greatly effected by ampoule material. Compositional gradients are strongly influenced by interface curvature and convective flow in the liquid. Results of this investigation illustrate the combined influences of interface shape and convective fluid flow. An applied magnetic field was used to reduce the effects of convective fluid flow in the electrically conductive melt during directional solidification. Several 8 mm diameter boules of Ga-doped Ge were grown at different field strengths, up to 5 Tesla, in four different ampoule materials. Compositional profiles indicate mass transfer conditions ranged from completely mixed to diffusion controlled. The influence of convection in the melt on the developing crystal microstructure and defect density was investigated as a function of field strength and ampoule material. Chemical etching and electron backscattered electron diffraction were used to map the crystal structure of each boule along the center plane. Dislocation etch pit densities were measured for each boule. Results show the influence of magnetic field strength and ampoule material on overall crystal quality.

Modeling the interface of platinum and α-quartz(001): Implications for sintering

DOE PAGES

Plessow, Philipp N.; Sánchez-Carrera, Roel S.; Li, Lin; ...

2016-05-04

We present a first-principles study which aims to understand the metal–support interaction of platinum nanoparticles on α-quartz(001) and, more generally, silica. The thermodynamic stability of the α-quartz(001) surface and its interface with Pt(111) are investigated as a function of temperature and partial pressure of H 2O and O 2. Potential defects in the α-quartz(001) surface as well as the adsorption energies of the Pt atom are also studied. This allows us to draw conclusions concerning nanoparticle shape and the resistance toward particle migration based on the interface free energies. We find that, as for the clean α-quartz(001) surface, a dry,more » reconstructed interface is expected at temperatures that are high but within experimentally relevant ranges. On an ideal, dry, reconstructed surface, particle migration is predicted to be a fast sintering mechanism. On real surfaces, defects may locally prevent reconstruction and act as anchoring points. Finally, the energetics of the adsorption of platinum atoms on α-quartz(001) do not support surface-mediated single-atom migration as a viable path for sintering on the investigated surfaces.« less

Determination of the Dynamics of Healing at the Tissue-Implant Interface by Means of Microcomputed Tomography and Functional Apparent Moduli

PubMed Central

Chang, Po-Chun; Seol, Yang-Jo; Goldstein, Steven A.; Giannobile, William V.

2014-01-01

Purpose It is currently a challenge to determine the biomechanical properties of the hard tissue–dental implant interface. Recent advances in intraoral imaging and tomographic methods, such as microcomputed tomography (micro-CT), provide three-dimensional details, offering significant potential to evaluate the bone-implant interface, but yield limited information regarding osseointegration because of physical scattering effects emanating from metallic implant surfaces. In the present study, it was hypothesized that functional apparent moduli (FAM), generated from functional incorporation of the peri-implant structure, would eliminate the radiographic artifact–affected layer and serve as a feasible means to evaluate the biomechanical dynamics of tissue-implant integration in vivo. Materials and Methods Cylindric titanium mini-implants were placed in osteotomies and osteotomies with defects in rodent maxillae. The layers affected by radiographic artifacts were identified, and the pattern of tissue-implant integration was evaluated from histology and micro-CT images over a 21-day observation period. Analyses of structural information, FAM, and the relationship between FAM and interfacial stiffness (IS) were done before and after eliminating artifacts. Results Physical artifacts were present within a zone of about 100 to 150 μm around the implant in both experimental defect situations (osteotomy alone and osteotomy + defect). All correlations were evaluated before and after eliminating the artifact-affected layers, most notably during the maturation period of osseointegration. A strong correlation existed between functional bone apparent modulus and IS within 300 μm at the osteotomy defects (r > 0.9) and functional composite tissue apparent modulus in the osteotomy defects (r > 0.75). Conclusion Micro-CT imaging and FAM were of value in measuring the temporal process of tissue-implant integration in vivo. This approach will be useful to complement imaging technologies for longitudinal monitoring of osseointegration. PMID:23377049

XPS studies of MgO based magnetic tunnel junction structures

NASA Astrophysics Data System (ADS)

Read, John; Mather, Phil; Tan, Eileen; Buhrman, Robert

2006-03-01

The very high tunneling magnetoresistance (TMR) obtained in MgO magnetic tunnel junctions (MTJ)^(1,2) motivates the investigation of the electronic properties of the MgO barrier layer and the study of the ferromagnetic metal - MgO interface chemistry. Such large TMR values are predicted by theory due to the high degree of order apparent in the barrier and electrode materials. However, as grown ultra-thin MgO films generally contain defects that can influence electron transport properties through the creation of low energy states within the bulk MgO band-gap. We will report the results of x-ray photoelectron spectroscopy (XPS) studies of (001) textured ultra-thin MgO layers that are prepared by RF magnetron sputtering and electron beam evaporation on ordered ferromagnetic electrodes and in ordered MTJ structures with and without post growth vacuum annealing. XPS spectra for both MgO deposition techniques clearly indicate a surface oxygen species that is likely bound by defects in the oxide^(3) in half-formed junctions and improvements in MgO quality after counter electrode deposition. We will discuss our results regarding the chemical properties of the oxide and its interfaces directed towards possibly providing guidance to engineer improved MgO MTJ devices. [1] S.S.P. Parkin et. al., Nature Materials, 3, 862 (2004). [2] S. Yuasa et. al., Nature Materials, 3, 868 (2004). [3] E. Tan et. al. , Phys. Rev. B. , 71, 161401 (2005).

Mobile interfaces: Liquids as a perfect structural material for multifunctional, antifouling surfaces

DOE PAGES

Grinthal, Alison; Aizenberg, Joanna

2013-10-14

Life creates some of its most robust, extreme surface materials not from solids but from liquids: a purely liquid interface, stabilized by underlying nanotexture, makes carnivorous plant leaves ultraslippery, the eye optically perfect and dirt-resistant, our knees lubricated and pressure-tolerant, and insect feet reversibly adhesive and shape-adaptive. Novel liquid surfaces based on this idea have recently been shown to display unprecedented omniphobic, self-healing, anti-ice, antifouling, optical, and adaptive properties. In this Perspective, we present a framework and a path forward for developing and designing such liquid surfaces into sophisticated, versatile multifunctional materials. Drawing on concepts from solid materials design andmore » fluid dynamics, we outline how the continuous dynamics, responsiveness, and multiscale patternability of a liquid surface layer can be harnessed to create a wide range of unique, active interfacial functions-able to operate in harsh, changing environments-not achievable with static solids. We discuss how, in partnership with the underlying substrate, the liquid surface can be programmed to adaptively and reversibly reconfigure from a defect-free, molecularly smooth, transparent interface through a range of finely tuned liquid topographies in response to environmental stimuli. In conclusion, with nearly unlimited design possibilities and unmatched interfacial properties, liquid materials-as long-term stable interfaces yet in their fully liquid state-may potentially transform surface design everywhere from medicine to architecture to energy infrastructure.« less

La interstitial defect-induced insulator-metal transition in the oxide heterostructures LaAl O3 /SrTi O3

NASA Astrophysics Data System (ADS)

Zhou, Jun; Yang, Ming; Feng, Yuan Ping; Rusydi, Andrivo

2017-11-01

Perovskite oxide interfaces have attracted tremendous research interest for their fundamental physics and promising all-oxide electronic applications. Here, based on first-principles calculations, we propose a surface La interstitial promoted interface insulator-metal transition in LaAl O3 /SrTi O3 (110). Compared with surface oxygen vacancies, which play a determining role on the insulator-metal transition of LaAl O3 /SrTi O3 (001) interfaces, we find that surface La interstitials can be more experimentally realistic and accessible for manipulation and more stable in an ambient atmospheric environment. Interestingly, these surface La interstitials also induce significant spin-splitting states with a Ti dy z/dx z character at a conducting LaAl O3 /SrTi O3 (110) interface. On the other hand, for insulating LaAl O3 /SrTi O3 (110) (<4 unit cells LaAl O3 thickness), a distortion between La (Al) and O atoms is found at the LaAl O3 side, partially compensating the polarization divergence. Our results reveal the origin of the metal-insulator transition in LaAl O3 /SrTi O3 (110) heterostructures, and also shed light on the manipulation of the superior properties of LaAl O3 /SrTi O3 (110) for different possibilities in electronic and magnetic applications.

Mitigation of substrate defects in reticles using multilayer buffer layers

DOEpatents

Mirkarimi, Paul B.; Bajt, Sasa; Stearns, Daniel G.

2001-01-01

A multilayer film is used as a buffer layer to minimize the size of defects on a reticle substrate prior to deposition of a reflective coating on the substrate. The multilayer buffer layer deposited intermediate the reticle substrate and the reflective coating produces a smoothing of small particles and other defects on the reticle substrate. The reduction in defect size is controlled by surface relaxation during the buffer layer growth process and by the degree of intermixing and volume contraction of the materials at the multilayer interfaces. The buffer layers are deposited at near-normal incidence via a low particulate ion beam sputtering process. The growth surface of the buffer layer may also be heated by a secondary ion source to increase the degree of intermixing and improve the mitigation of defects.

The effect of crystallographic misorientation and interfacial separation on jump-to-contact behavior and defect generation in aluminum

NASA Astrophysics Data System (ADS)

Khajehvand, Milad; Sepehrband, Panthea

2018-07-01

The jump-to-contact (JC) phenomenon for (111)-oriented surfaces in aluminum at room temperature is studied via molecular dynamics simulations. The effect of crystallographic misorientation and interfacial distance on the JC behavior and distribution of the resultant defects at the interface is investigated. The effect of misorientation on the critical distance for JC is found to be negligible. However, when JC occurs, different distribution of defects is observed for various misorientation angles. The density of defects is shown to be a function of interfacial distance for low misorientation angles, but independent of it for misorientation angles of ∼30 ± 10°.

Enhancement of tunneling current in phosphorene tunnel field effect transistors by surface defects.

PubMed

Lu, Juan; Fan, Zhi-Qiang; Gong, Jian; Chen, Jie-Zhi; ManduLa, Huhe; Zhang, Yan-Yang; Yang, Shen-Yuan; Jiang, Xiang-Wei

2018-02-21

The effects of the staggered double vacancies, hydrogen (H), 3d transition metals, for example cobalt, and semiconductor covalent atoms, for example, germanium, nitrogen, phosphorus (P) and silicon adsorption on the transport properties of monolayer phosphorene were studied using density functional theory and non-equilibrium Green's function formalism. It was observed that the performance of the phosphorene tunnel field effect transistors (TFETs) with an 8.8 nm scaling channel length could be improved most effectively, if the adatoms or vacancies were introduced at the source channel interface. For H and P doped devices, the upper limit of on-state currents of phosphorene TFETs were able to be quickly increased to 2465 μA μm -1 and 1652 μA μm -1 , respectively, which not only outperformed the pristine sample, but also met the requirements for high performance logic applications for the next decade in the International Technology Roadmap for Semiconductors (ITRS). It was proved that the defect-induced band gap states make the effective tunneling path between the conduction band (CB) and valence band (VB) much shorter, so that the carriers can be injected easily from the left electrode, then transfer to the channel. In this regard, the tunneling properties of phosphorene TFETs can be manipulated using surface defects. In addition, the effects of spin polarization on the transport properties of doped phosphorene TFETs were also rigorously considered, H and P doped TFETs could achieve a high ON current of 1795 μA μm -1 and 1368 μA μm -1 , respectively, which is closer to realistic nanodevices.

Adhesion of voids to bimetal interfaces with non-uniform energies

DOE PAGES

Zheng, Shijian; Shao, Shuai; Zhang, Jian; ...

2015-10-21

Interface engineering has become an important strategy for designing radiation-resistant materials. Critical to its success is fundamental understanding of the interactions between interfaces and radiation-induced defects, such as voids. Using transmission electron microscopy, here we report an interesting phenomenon in their interaction, wherein voids adhere to only one side of the bimetal interfaces rather than overlapping them. We show that this asymmetrical void-interface interaction is a consequence of differing surface energies of the two metals and non-uniformity in their interface formation energy. Specifically, voids grow within the phase of lower surface energy and wet only the high-interface energy regions. Furthermore,more » because this outcome cannot be accounted for by wetting of interfaces with uniform internal energy, our report provides experimental evidence that bimetal interfaces contain non-uniform internal energy distributions. Ultimately, this work also indicates that to design irradiation-resistant materials, we can avoid void-interface overlap via tuning the configurations of interfaces.« less

Robustness of Topological Superconductivity in Solid State Hybrid Structures

NASA Astrophysics Data System (ADS)

Sitthison, Piyapong

The non-Abelian statistics of Majorana fermions (MFs) makes them an ideal platform for implementing topological quantum computation. In addition to the fascinating fundamental physics underlying the emergence of MFs, this potential for applications makes the study of these quasiparticles an extremely popular subject in condensed matter physics. The commonly called `Majorana fermions' are zero-energy bound states that emerge near boundaries and defects in topological superconducting phases, which can be engineered, for example, by proximity coupling strong spin-orbit coupling semiconductor nanowires and ordinary s-wave superconductors. The stability of these bound states is determined by the stability of the underlying topological superconducting phase. Hence, understanding their stability (which is critical for quantum computation), involves studying the robustness of the engineered topological superconductors. This work addresses this important problem in the context of two types of hybrid structures that have been proposed for realizing topological superconductivity: topological insulator - superconductor (TI-SC) and semiconductor - superconductor (SM-SC) nanostructures. In both structures, electrostatic effects due to applied external potentials and interface-induced potentials are significant. This work focuses on developing a theoretical framework for understanding these effects, to facilitate the optimization of the nanostructures studied in the laboratory. The approach presented in this thesis is based on describing the low-energy physics of the hybrid structure using effective tight-binding models that explicitly incorporate the proximity effects emerging at interfaces. Generically, as a result of the proximity coupling to the superconductor, an induced gap emerges in the semiconductor (topological insulator) sub-system. The strength of the proximity-induced gap is determined by the transparency of the interface and by the amplitude of the low- energy SM (TI) states at the interface. In turn, this amplitude is strongly impacted by electrostatic effects. In addition, these effects control the value of the chemical potential in the nanowire (nanoribbon), as well as the strength of the Rashba-type spin-orbit coupling - two key parameters that determine the stability of the topological superconducting phase. To account for these critical effects, a numerically efficient Poisson-Schrodinger scheme is developed.

Simulation approach for optimization of ZnO/c-WSe{}_{2} heterojunction solar cells

NASA Astrophysics Data System (ADS)

Huang, Shihua; Li, Qiannan; Chi, Dan; Meng, Xiuqing; He, Lü

2017-04-01

Taking into account defect density in WSe{}2, interface recombination between ZnO and WSe{}2, we presented a simulation study of ZnO/crystalline WSe{}2 heterojunction (HJ) solar cell using wxAMPS simulation software. The optimal conversion efficiency 39.07% for n-ZnO/p-c-WSe{}2 HJ solar cell can be realized without considering the impact of defects. High defect density (> 1.0× {10}11 cm{}-2) in c-WSe{}2 and large trap cross-section (> 1.0 × 10{}-10 cm{}2) have serious impact on solar cell efficiency. A thin p-WSe{}2 layer is intentionally inserted between ZnO layer and c-WSe{}2 to investigate the effect of the interface recombination. The interface properties are very crucial to the performance of ZnO/c-WSe{}2HJ solar cell. The affinity of ZnO value range between 3.7-4.5 eV gives the best conversion efficiency. Project supported by the Natural Science Foundation of Zhejiang Province (No. LY17F040001), the Open Project Program of Surface Physics Laboratory (National Key Laboratory) of Fudan University (No. KF2015_02), the Open Project Program of National Laboratory for Infrared Physics, Chinese Academy of Sciences (No. M201503), the Zhejiang Provincial Science and Technology Key Innovation Team (No. 2011R50012), and the Zhejiang Provincial Key Laboratory (No. 2013E10022).

Tendon Reattachment to Bone in an Ovine Tendon Defect Model of Retraction Using Allogenic and Xenogenic Demineralised Bone Matrix Incorporated with Mesenchymal Stem Cells.

PubMed

Thangarajah, Tanujan; Shahbazi, Shirin; Pendegrass, Catherine J; Lambert, Simon; Alexander, Susan; Blunn, Gordon W

2016-01-01

Tendon-bone healing following rotator cuff repairs is mainly impaired by poor tissue quality. Demineralised bone matrix promotes healing of the tendon-bone interface but its role in the treatment of tendon tears with retraction has not been investigated. We hypothesized that cortical demineralised bone matrix used with minimally manipulated mesenchymal stem cells will result in improved function and restoration of the tendon-bone interface with no difference between xenogenic and allogenic scaffolds. In an ovine model, the patellar tendon was detached from the tibial tuberosity and a complete distal tendon transverse defect measuring 1 cm was created. Suture anchors were used to reattach the tendon and xenogenic demineralised bone matrix + minimally manipulated mesenchymal stem cells (n = 5), or allogenic demineralised bone matrix + minimally manipulated mesenchymal stem cells (n = 5) were used to bridge the defect. Graft incorporation into the tendon and its effect on regeneration of the enthesis was assessed using histomorphometry. Force plate analysis was used to assess functional recovery. Compared to the xenograft, the allograft was associated with significantly higher functional weight bearing at 6 (P = 0.047), 9 (P = 0.028), and 12 weeks (P = 0.009). In the allogenic group this was accompanied by greater remodeling of the demineralised bone matrix into tendon-like tissue in the region of the defect (p = 0.015), and a more direct type of enthesis characterized by significantly more fibrocartilage (p = 0.039). No failures of tendon-bone healing were noted in either group. Demineralised bone matrix used with minimally manipulated mesenchymal stem cells promotes healing of the tendon-bone interface in an ovine model of acute tendon retraction, with superior mechanical and histological results associated with use of an allograft.

Effect of defective collagen synthesis on epithelial implant interface: lathyritic model in dogs. An experimental preliminary study.

PubMed

Cengiz, Murat Inanç; Kirtiloğlu, Tuğrul; Acikgoz, Gökhan; Trisi, Paolo; Wang, Hom-Lay

2012-04-01

Peri-implant mucosa is composed of 2 compartments: a marginal junctional epithelium and a zone of connective tissue attachment. Both structures consist mainly of collagen. Lathyrism is characterized by defective collagen synthesis due to inhibition of lysyl oxidase, an enzyme that is essential for interfibrillar collagen cross-linking. The lathyritic agent beta-aminoproprionitrile (β-APN) is considered a suitable agent to disrupt the connective tissue metabolism. Therefore, the purpose of this study was to assess the effect of defective connective tissue metabolism on epithelial implant interface by using β-APN created chronic lathyrism in the canine model. Two 1-year-old male dogs were included in this study. A β-APN dosage of 5 mg/0.4 mL/volume 100 g/body weight was given to the test dog for 10 months, until lathyritic symptoms developed. After this, the mandibular premolar teeth (p2, p3, p4) of both dogs were atraumatically extracted, and the investigators waited 3 months before implants were placed. In the test dog, 3 implants were placed in the left mandible, and 2 implants were placed in the right mandible. In the control dog, 2 implants were placed in the left mandibular premolar site. The dogs were sacrificed 10 months after healing. Peri-implant tissues obtained from the dogs were examined histomorphologically and histopathologically. Bone to implant contact (BIC) values and bone volumes (BV) were lower in the lathyritic group compared to the control group; however, no statistical significance was found. Significant histologic and histomorphometric changes were observed in peri-implant bone, connective tissue, and peri-implant mucosal width between test and control implants. Defective collagen metabolism such as lathyrism may negatively influence the interface between implant and surrounding soft tissue attachment.

Characterization of HgCdTe and Related Materials For Third Generation Infrared Detectors

NASA Astrophysics Data System (ADS)

Vaghayenegar, Majid

Hg1-xCdxTe (MCT) has historically been the primary material used for infrared detectors. Recently, alternative substrates for MCT growth such as Si, as well as alternative infrared materials such as Hg1-xCdxSe, have been explored. This dissertation involves characterization of Hg-based infrared materials for third generation infrared detectors using a wide range of transmission electron microscopy (TEM) techniques. A microstructural study on HgCdTe/CdTe heterostructures grown by MBE on Si (211) substrates showed a thin ZnTe layer grown between CdTe and Si to mediate the large lattice mismatch of 19.5%. Observations showed large dislocation densities at the CdTe/ZnTe/Si (211) interfaces, which dropped off rapidly away from the interface. Growth of a thin HgTe buffer layer between HgCdTe and CdTe layers seemed to improve the HgCdTe layer quality by blocking some defects. A second study investigated the correlation of etch pits and dislocations in as-grown and thermal-cycle-annealed (TCA) HgCdTe (211) films. For as-grown samples, pits with triangular and fish-eye shapes were associated with Frank partial and perfect dislocations, respectively. Skew pits were determined to have a more complex nature. TCA reduced the etch-pit density by 72%. Although TCA processing eliminated the fish-eye pits, dislocations reappeared in shorter segments in the TCA samples. Large pits were observed in both as-grown and TCA samples, but the nature of any defects associated with these pits in the as-grown samples is unclear. Microstructural studies of HgCdSe revealed large dislocation density at ZnTe/Si(211) interfaces, which dropped off markedly with ZnTe thickness. Atomic-resolution STEM images showed that the large lattice mismatch at the ZnTe/Si interface was accommodated through {111}-type stacking faults. A detailed analysis showed that the stacking faults were inclined at angles of 19.5 and 90 degrees at both ZnTe/Si and HgCdSe/ZnTe interfaces. These stacking faults were associated with Shockley and Frank partial dislocations, respectively. Initial attempts to delineate individual dislocations by chemical etching revealed that while the etchants successfully attacked defective areas, many defects in close proximity to the pits were unaffected.

An Exploration of the Phases and Structure Formation in Active Nematic Materials Using an Overdamped Continuum Theory

NASA Astrophysics Data System (ADS)

Putzig, Elias

Active nematics are a class of nonequilibrium systems which have received much attention in the form of continuum models in recent years. For the dense, highly ordered case which is of particular interest, these models focus almost exclusively on suspensions of active particles in which the flow of the medium plays a key role in the dynamical equations. Many active nematics, however, reside at an interface or on a surface where friction excludes the effects of long-range flow. In the following pages we shall construct a general model which describes these systems with overdamped dynamical equations. Through numerical and analytical investigation we detail how many of the striking nonequilibrium behaviors of active nematics arise in such systems. We shall first discuss how the activity in these systems gives rise to an instability in the nematic ordered state. This instability leads to phase-separation in which bands of ordered active nematic are interspersed with bands of the disordered phase. We expose the factors which control the density contrast and the stability of these bands through numerical investigation. We then turn to the highly ordered phase of active nematic materials, in which striking nonequilibrium behaviors such as the spontaneous formation, self-propulsion, and ordering of charge-half defects occurs. We extend the overdamped model of an active nematic to describe these behaviors by including the advection of the director by the active forces in the dynamical equations. We find a new instability in the ordered state which gives rise to defect formation, as well as an analog of the instability which is seen in models of active nematic suspensions. Through numerical investigations we expose a rich phenomenology in the neighborhood of this new instability. The phenomenology includes a state in which the orientations of motile, transient defects form long-range order. This is the first continuum model to contain such a state, and we compare the behavior seen here with similar states seen in the experiments and simulations of Stephen DeCamp and Gabriel Redner et. al. [1]. Finally, we propose the measurement of defect shape as a mechanism for the comparison between continuum theories of active nematics and the experimental and simulated realiza- tions of these systems. We present a method for making these measurements which allows for averaging and statistical analysis, and use this method to determine how the shapes of defects depend on the parameters of our continuum theory. We then compare these with the shapes of defects which we measure in the experiments and simulations mentioned above in order to place these systems in the parameter space of our model. It is our hope that this mechanism for comparison between models and realizations of active nematics will provide a key to pairing the two more closely.

Automated identification and indexing of dislocations in crystal interfaces

DOE PAGES

Stukowski, Alexander; Bulatov, Vasily V.; Arsenlis, Athanasios

2012-10-31

Here, we present a computational method for identifying partial and interfacial dislocations in atomistic models of crystals with defects. Our automated algorithm is based on a discrete Burgers circuit integral over the elastic displacement field and is not limited to specific lattices or dislocation types. Dislocations in grain boundaries and other interfaces are identified by mapping atomic bonds from the dislocated interface to an ideal template configuration of the coherent interface to reveal incompatible displacements induced by dislocations and to determine their Burgers vectors. Additionally, the algorithm generates a continuous line representation of each dislocation segment in the crystal andmore » also identifies dislocation junctions.« less

Freckle Defect Formation near the Casting Interfaces of Directionally Solidified Superalloys

PubMed Central

Hong, Jianping; Ma, Dexin; Wang, Jun; Wang, Fu; Sun, Baode; Dong, Anping; Li, Fei; Bührig-Polaczek, Andreas

2016-01-01

Freckle defects usually appear on the surface of castings and industrial ingots during the directional solidification process and most of them are located near the interface between the shell mold and superalloys. Ceramic cores create more interfaces in the directionally solidified (DS) and single crystal (SX) hollow turbine blades. In order to investigate the location of freckle occurrence in superalloys, superalloy CM247 LC was directionally solidified in an industrial-sized Bridgman furnace. Instead of ceramic cores, Alumina tubes were used inside of the casting specimens. It was found that freckles occur not only on the casting external surfaces, but also appear near the internal interfaces between the ceramic core and superalloys. Meanwhile, the size, initial position, and area of freckle were investigated in various diameters of the specimens. The initial position of the freckle chain reduces when the diameter of the rods increase. Freckle area follows a linear relationship in various diameters and the average freckle fraction is 1.1% of cross sectional area of casting specimens. The flow of liquid metal near the interfaces was stronger than that in the interdendritic region in the mushy zone, and explained why freckle tends to occur on the outer or inner surfaces of castings. This new phenomenon suggests that freckles are more likely to occur on the outer or inner surfaces of the hollow turbine blades. PMID:28774050

Freckle Defect Formation near the Casting Interfaces of Directionally Solidified Superalloys.

PubMed

Hong, Jianping; Ma, Dexin; Wang, Jun; Wang, Fu; Sun, Baode; Dong, Anping; Li, Fei; Bührig-Polaczek, Andreas

2016-11-16

Freckle defects usually appear on the surface of castings and industrial ingots during the directional solidification process and most of them are located near the interface between the shell mold and superalloys. Ceramic cores create more interfaces in the directionally solidified (DS) and single crystal (SX) hollow turbine blades. In order to investigate the location of freckle occurrence in superalloys, superalloy CM247 LC was directionally solidified in an industrial-sized Bridgman furnace. Instead of ceramic cores, Alumina tubes were used inside of the casting specimens. It was found that freckles occur not only on the casting external surfaces, but also appear near the internal interfaces between the ceramic core and superalloys. Meanwhile, the size, initial position, and area of freckle were investigated in various diameters of the specimens. The initial position of the freckle chain reduces when the diameter of the rods increase. Freckle area follows a linear relationship in various diameters and the average freckle fraction is 1.1% of cross sectional area of casting specimens. The flow of liquid metal near the interfaces was stronger than that in the interdendritic region in the mushy zone, and explained why freckle tends to occur on the outer or inner surfaces of castings. This new phenomenon suggests that freckles are more likely to occur on the outer or inner surfaces of the hollow turbine blades.

Persistence time of charge carriers in defect states of molecular semiconductors.

PubMed

McMahon, David P; Troisi, Alessandro

2011-06-07

Charge carriers in organic crystals are often trapped in point defects. The persistence time of the charge in these defect states is evaluated by computing the escape rate from this state using non-adiabatic rate theory. Two cases are considered (i) the hopping between separate identical defect states and (ii) the hopping between a defect state and the bulk (delocalized) states. We show that only the second process is likely to happen with realistic defect concentrations and highlight that the inclusion of an effective quantum mode of vibration is essential for accurate computation of the rate. The computed persistence time as a function of the trap energy indicates that trap states shallower than ∼0.3 eV cannot be effectively investigated with some slow spectroscopic techniques such as THz spectroscopy or EPR commonly used to study the nature of excess charge in semiconductors.

NREL Collaboration Breaks 1-Volt Barrier in CdTe Solar Technology

DOE Office of Scientific and Technical Information (OSTI.GOV)

2016-05-01

NREL scientists have worked with Washington State University and the University of Tennessee to improve the maximum voltage available from CdTe solar cells. Changes in dopants, stoichiometry, interface design, and defect chemistry improved the CdTe conductivity and carrier lifetime by orders of magnitude, thus enabling CdTe solar cells with open-circuit voltages exceeding 1 volt for the first time. Values of current density and fill factor for CdTe solar cells are already at high levels, but sub-par voltages has been a barrier to improved efficiencies. With voltages pushed beyond 1 volt, CdTe cells have a path to produce electricity at costsmore » less than fossil fuels.« less

Recent Progress in Nanoelectrical Characterizations of CdTe and Cu(In,Ga)Se2

DOE Office of Scientific and Technical Information (OSTI.GOV)

Jiang, Chun-Sheng; To, Bobby; Glynn, Stephen

2016-11-21

We report two recent nanoelectrical characterizations of CdTe and Cu(In, Ga)Se2 (CIGS) thin-film solar cells by developing atomic force microscopy-based nanoelectrical probes. Charges trapped at defects at the CdS/CdTe interface were probed by Kelvin probe force microscopy (KPFM) potential mapping and by ion-milling the CdTe superstrate device in a bevel glancing angle of ~0.5 degrees. The results show randomly distributed donor-like defects at the interface. The effect of K post-deposition treatment on the near-surface region of the CIGS film was studied by KPFM potential and scanning spreading resistance microscopy (SSRM) resistivity mapping, which shows passivation of grain-boundary potential and improvementmore » of resistivity uniformity by the K treatment.« less

Photoresponse in graphene induced by defect engineering

NASA Astrophysics Data System (ADS)

Du, Ruxia; Wang, Wenhui; Du, Jianxin; Guo, Xitao; Liu, Er; Bing, Dan; Bai, Jing

2016-11-01

We present a photoresponse study on a lateral defect/pristine graphene junction device fabricated by a simple plasma irradiation method. The junction between pristine graphene and plasma-modified graphene was created by controlling the location of Ar+ plasma treatment. We found that a distinct photocurrent was generated at the junction by photocurrent line scanning measurements, and further analysis reveals that the photo-thermoelectric (PTE) effect, instead of the photovoltaic (PV) effect, dominates the photocurrent generation at the interface. Additionally, the obtained results suggest that tuning the defect density could be effective in modulating the optoelectronic performance of junctions in our device.

Defect engineering of the electronic transport through cuprous oxide interlayers

NASA Astrophysics Data System (ADS)

Fadlallah, Mohamed M.; Eckern, Ulrich; Schwingenschlögl, Udo

2016-06-01

The electronic transport through Au-(Cu2O)n-Au junctions is investigated using first-principles calculations and the nonequilibrium Green’s function method. The effect of varying the thickness (i.e., n) is studied as well as that of point defects and anion substitution. For all Cu2O thicknesses the conductance is more enhanced by bulk-like (in contrast to near-interface) defects, with the exception of O vacancies and Cl substitutional defects. A similar transmission behavior results from Cu deficiency and N substitution, as well as from Cl substitution and N interstitials for thick Cu2O junctions. In agreement with recent experimental observations, it is found that N and Cl doping enhances the conductance. A Frenkel defect, i.e., a superposition of an O interstitial and O substitutional defect, leads to a remarkably high conductance. From the analysis of the defect formation energies, Cu vacancies are found to be particularly stable, in agreement with earlier experimental and theoretical work.

Tunneling Spectroscopy Studies of Epitaxial Graphene on Silicon Carbide(0001) and Its Interfaces

NASA Astrophysics Data System (ADS)

Sandin, Andreas Axel Tomas

A two dimensional network of sp2 bonded carbon atoms is defined as graphene. This novel material possesses remarkable electronic properties due to its unique band structure at the vicinity of the Fermi energy. The toughest challenge to bring use of graphene electronic properties in device geometries is that graphene is exceptionally sensitive to its electrical environment for integration into macroscopic system of electrical contacts and substrates. One of the most promising substrates for graphene is the polar surfaces of SiC for the reason it can be grown epitaxially by sublimating Si from the top-most SiC atomic layers. In this work, the interfaces of graphene grown on the Si-terminated polar surface SiC(0001) is studied in UHV using scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), low energy electron diffraction (LEED) and auger electron Spectroscopy (AES). STM is used image the graphene surface and interfaces with the capability of atomic resolution. LEED is used to study surface atomic reciprocal ordering and AES is used to determine surface atomic composition during the graphene formation. Interfacial layer (Buffer layer), Single layer graphene and bilayer graphene are identified electronically by means of probing the first member of the image potential derived state. This state is found by dZ/dV spectroscopy in the high energy unoccupied states and is exceptionally sensitive to electrostatic changes to the surface which is detected by energy shifts of image potential states (IPS). This sensitivity is utilized to probe the graphene screening of external electric fields by varying the electric field in the tunneling junction and addresses the fact that charged impurity scattering is likely to be crucial for epitaxial graphene on SiC(0001) when it comes to transport parameters. Shifts of IPS energy position has also been used verify work function changes for identification of several Sodium Intercalation structures of epitaxial graphene. STS, STM along with DFT calculations are used to determine the interface location of Sodium, SiC-bufferlayer or bufferlayer-graphene intercalation. In this thesis, STM, and STS are used to study the interactions of paramagnetic FePc molecules with epitaxial graphene. The molecules, FePc, is found to interact with the graphene substrate where STM images show substrate induced orientation of FePc densely packed square lattice structure. At sub-monolayer coverages, FePc form a molecular gas at room temperature suggesting a low diffusion barrier on the graphene lattice. The substrate interaction is probed by STS and show an abnormally low LUMO energy that suggest strong electronic coupling between graphene and FePc. DFT calculations support the experimental observations and predict a spin-dependent molecule-graphene hybridization close to the Fermi energy in unoccupied states. For majority spins, DFT demonstrates the Dirac cone splits and a delocalized hybrid state is found in the band gap. For minority spin the Dirac cone is intact with energy of Dirac point empty. In addition, a novel method of improving UHV graphene growth on SiC(0001) is presented. During growth the SiC surface is exposed to atomic hydrogen which allows selective etching of Si over Carbon. This result in more uniform non-thermal formation of the buffer layer with many fewer defects and thus leads to nearly pit-free and defect-free thermal graphene layers.

Pseudo-time-reversal symmetry and topological edge states in two-dimensional acoustic crystals

PubMed Central

Mei, Jun; Chen, Zeguo; Wu, Ying

2016-01-01

We propose a simple two-dimensional acoustic crystal to realize topologically protected edge states for acoustic waves. The acoustic crystal is composed of a triangular array of core-shell cylinders embedded in a water host. By utilizing the point group symmetry of two doubly degenerate eigenstates at the Γ point, we can construct pseudo-time-reversal symmetry as well as pseudo-spin states in this classical system. We develop an effective Hamiltonian for the associated dispersion bands around the Brillouin zone center, and find the inherent link between the band inversion and the topological phase transition. With numerical simulations, we unambiguously demonstrate the unidirectional propagation of acoustic edge states along the interface between a topologically nontrivial acoustic crystal and a trivial one, and the robustness of the edge states against defects with sharp bends. Our work provides a new design paradigm for manipulating and transporting acoustic waves in a topologically protected manner. Technological applications and devices based on our design are expected in various frequency ranges of interest, spanning from infrasound to ultrasound. PMID:27587311

Probing the Origin of Interfacial Carriers in SrTiO 3$-$LaCrO 3 Superlattices

DOE PAGES

Comes, Ryan B.; Spurgeon, Steven R.; Kepaptsoglou, Despoina M.; ...

2017-01-13

Emergent phenomena at complex oxide interfaces could provide the basis for a wide variety of next-generation devices, including photovoltaics and spintronics. To date, detailed characterization and computational modeling of interfacial defects, cation intermixing, and film stoichiometry have helped to explain many of the novel behaviors observed at a single heterojunction. Unfortunately, many of the techniques employed to characterize a single heterojunction are less effective for a superlattice made up of a repeating series of interfaces that induce collective interfacial phenomena throughout a film. These repeating interfaces present an untapped opportunity to introduce an additional degree of complexity, such as confinedmore » electric fields, that cannot be realized in a single heterojunction. In this work, we explore the properties of SrTiO 3–LaCrO 3 superlattices to understand the role of defects, including variations in cation stoichiometry of individual layers of the superlattice, intermixing across interfaces, and interfacial oxygen vacancies. Using X-ray photoelectron spectroscopy (XPS) and scanning transmission electron microscopy electron energy-loss spectroscopy (STEM-EELS), we quantify the stoichiometry of individual layers of the superlattice and determine the degree of intermixing in these materials. By comparing these results to both density functional theory (DFT) models and STEM-EELS measurements of the Ti and Cr valence in each layer of the superlattice, we correlate different types of defects with the associated materials properties of the superlattice. In conclusion, we show that a combination of ab initio modeling and complementary structural characterization methods can offer unique insight into structure–property relationships in many oxide superlattice systems.« less

Effects of interface morphology and TGO thickness on residual stress of EB-PVD thermal barrier coatings

NASA Astrophysics Data System (ADS)

Chen, Jianwei; Zhao, Yang; Ma, Jian

2015-04-01

The residual stress of electron beam-physical vapor deposition (EB-PVD) thermal barrier coatings (TBC) is complex and difficult to be obtained. In this paper, the interface morphology of TBCs subjected to cyclic heating and cooling was observed by SEM. Based on the thermal elastic-plastic finite method, corresponding interface model of TBCs was established. The residual stress of EB-PVD TBCs with different interface morphologies and TGO thicknesses was calculated using the FE method without regard to the presence of cracks and defects. The result shows that the distribution of residual stress is significantly affected by the interface morphology, and the growth of TGO also has influence on the residual stress of TC and TGO.

Probing defect states in polycrystalline GaN grown on Si(111) by sub-bandgap laser-excited scanning tunneling spectroscopy

NASA Astrophysics Data System (ADS)

Hsiao, F.-M.; Schnedler, M.; Portz, V.; Huang, Y.-C.; Huang, B.-C.; Shih, M.-C.; Chang, C.-W.; Tu, L.-W.; Eisele, H.; Dunin-Borkowski, R. E.; Ebert, Ph.; Chiu, Y.-P.

2017-01-01

We demonstrate the potential of sub-bandgap laser-excited cross-sectional scanning tunneling microscopy and spectroscopy to investigate the presence of defect states in semiconductors. The characterization method is illustrated on GaN layers grown on Si(111) substrates without intentional buffer layers. According to high-resolution transmission electron microscopy and cathodoluminescence spectroscopy, the GaN layers consist of nanoscale wurtzite and zincblende crystallites with varying crystal orientations and hence contain high defect state densities. In order to discriminate between band-to-band excitation and defect state excitations, we use sub-bandgap laser excitation. We probe a clear increase in the tunnel current at positive sample voltages during sub-bandgap laser illumination for the GaN layer with high defect density, but no effect is found for high quality GaN epitaxial layers. This demonstrates the excitation of free charge carriers at defect states. Thus, sub-bandgap laser-excited scanning tunneling spectroscopy is a powerful complimentary characterization tool for defect states.

Ordered defect compounds in CuInSe{sub 2} for photovoltaic solar cell application

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sato, K.; Katayama-Yoshida, H.

2014-02-21

Due to the complete compensation, defect complex (2V{sub Cu}+In{sub Cu}), namely two Cu vacancies and In located at Cu site, is stable in CuInSe{sub 2} (CIS). It is known that the series of ordered defect compounds (ODC) are constracted by ordering the defect complex. Based on the total energy calcalation by using the Korringa-Kohn-Rostoker coherent potential approxiamtion (KKR-CPA) method, we discuss phase separation of the CIS with the defect complexes into ODC and CIS. Since the band alignment between ODC and CIS is calculated to be type 2, effective electron-hole separation at the interface between ODC and CIS can bemore » expected. This causes the enhancement of conversion efficiency of CIS-based solar cell materials.« less

Electrically detected magnetic resonance of carbon dangling bonds at the Si-face 4H-SiC/SiO2 interface

NASA Astrophysics Data System (ADS)

Gruber, G.; Cottom, J.; Meszaros, R.; Koch, M.; Pobegen, G.; Aichinger, T.; Peters, D.; Hadley, P.

2018-04-01

SiC based metal-oxide-semiconductor field-effect transistors (MOSFETs) have gained a significant importance in power electronics applications. However, electrically active defects at the SiC/SiO2 interface degrade the ideal behavior of the devices. The relevant microscopic defects can be identified by electron paramagnetic resonance (EPR) or electrically detected magnetic resonance (EDMR). This helps to decide which changes to the fabrication process will likely lead to further increases of device performance and reliability. EDMR measurements have shown very similar dominant hyperfine (HF) spectra in differently processed MOSFETs although some discrepancies were observed in the measured g-factors. Here, the HF spectra measured of different SiC MOSFETs are compared, and it is argued that the same dominant defect is present in all devices. A comparison of the data with simulated spectra of the C dangling bond (PbC) center and the silicon vacancy (VSi) demonstrates that the PbC center is a more suitable candidate to explain the observed HF spectra.

Rapid Detection Methods for Asphalt Pavement Thicknesses and Defects by a Vehicle-Mounted Ground Penetrating Radar (GPR) System

PubMed Central

Dong, Zehua; Ye, Shengbo; Gao, Yunze; Fang, Guangyou; Zhang, Xiaojuan; Xue, Zhongjun; Zhang, Tao

2016-01-01

The thickness estimation of the top surface layer and surface layer, as well as the detection of road defects, are of great importance to the quality conditions of asphalt pavement. Although ground penetrating radar (GPR) methods have been widely used in non-destructive detection of pavements, the thickness estimation of the thin top surface layer is still a difficult problem due to the limitations of GPR resolution and the similar permittivity of asphalt sub-layers. Besides, the detection of some road defects, including inadequate compaction and delamination at interfaces, require further practical study. In this paper, a newly-developed vehicle-mounted GPR detection system is introduced. We used a horizontal high-pass filter and a modified layer localization method to extract the underground layers. Besides, according to lab experiments and simulation analysis, we proposed theoretical methods for detecting the degree of compaction and delamination at the interface, respectively. Moreover, a field test was carried out and the estimated results showed a satisfactory accuracy of the system and methods. PMID:27929409

MCM ring hexamerization is a prerequisite for DNA-binding

DOE PAGES

Froelich, Clifford A.; Nourse, Amanda; Enemark, Eric J.

2015-09-13

The hexameric Minichromosome Maintenance (MCM) protein complex forms a ring that unwinds DNA at the replication fork in eukaryotes and archaea. Our recent crystal structure of an archaeal MCM N-terminal domain bound to single-stranded DNA (ssDNA) revealed ssDNA associating across tight subunit interfaces but not at the loose interfaces, indicating that DNA-binding is governed not only by the DNA-binding residues of the subunits (MCM ssDNA-binding motif, MSSB) but also by the relative orientation of the subunits. We now extend these findings to show that DNA-binding by the MCM N-terminal domain of the archaeal organism Pyrococcus furiosus occurs specifically in themore » hexameric oligomeric form. We show that mutants defective for hexamerization are defective in binding ssDNA despite retaining all the residues observed to interact with ssDNA in the crystal structure. One mutation that exhibits severely defective hexamerization and ssDNA-binding is at a conserved phenylalanine that aligns with the mouse Mcm4(Chaos3) mutation associated with chromosomal instability, cancer, and decreased intersubunit association.« less

Rapid Detection Methods for Asphalt Pavement Thicknesses and Defects by a Vehicle-Mounted Ground Penetrating Radar (GPR) System.

PubMed

Dong, Zehua; Ye, Shengbo; Gao, Yunze; Fang, Guangyou; Zhang, Xiaojuan; Xue, Zhongjun; Zhang, Tao

2016-12-06

The thickness estimation of the top surface layer and surface layer, as well as the detection of road defects, are of great importance to the quality conditions of asphalt pavement. Although ground penetrating radar (GPR) methods have been widely used in non-destructive detection of pavements, the thickness estimation of the thin top surface layer is still a difficult problem due to the limitations of GPR resolution and the similar permittivity of asphalt sub-layers. Besides, the detection of some road defects, including inadequate compaction and delamination at interfaces, require further practical study. In this paper, a newly-developed vehicle-mounted GPR detection system is introduced. We used a horizontal high-pass filter and a modified layer localization method to extract the underground layers. Besides, according to lab experiments and simulation analysis, we proposed theoretical methods for detecting the degree of compaction and delamination at the interface, respectively. Moreover, a field test was carried out and the estimated results showed a satisfactory accuracy of the system and methods.

Public Health Practice of Population-Based Birth Defects Surveillance Programs in the United States.

PubMed

Mai, Cara T; Kirby, Russell S; Correa, Adolfo; Rosenberg, Deborah; Petros, Michael; Fagen, Michael C

2016-01-01

Birth defects remain a leading cause of infant mortality in the United States and contribute substantially to health care costs and lifelong disabilities. State population-based surveillance systems have been established to monitor birth defects, yet no recent systematic examination of their efforts in the United States has been conducted. To understand the current population-based birth defects surveillance practices in the United States. The National Birth Defects Prevention Network conducted a survey of US population-based birth defects activities that included questions about operational status, case ascertainment methodology, program infrastructure, data collection and utilization, as well as priorities and challenges for surveillance programs. Birth defects contacts in the United States, including District of Columbia and Puerto Rico, received the survey via e-mail; follow-up reminders via e-mails and telephone were used to ensure a 100% response rate. Forty-three states perform population-based surveillance for birth defects, covering approximately 80% of the live births in the United States. Seventeen primarily use an active case-finding approach and 26 use a passive case-finding approach. These programs all monitor major structural malformations; however, passive case-finding programs more often monitor a broader list of conditions, including developmental conditions and newborn screening conditions. Active case-finding programs more often use clinical reviewers, cover broader pregnancy outcomes, and collect more extensive information, such as family history. More than half of the programs (24 of 43) reported an ability to conduct follow-up studies of children with birth defects. The breadth and depth of information collected at a population level by birth defects surveillance programs in the United States serve as an important data source to guide public health action. Collaborative efforts at the state and national levels can help harmonize data collection and increase utility of birth defects programs.

Mapping Ionic Currents and Reactivity on the Nanoscale: Electrochemical Strain Microscopy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kalinin, S.V.

2010-10-19

Solid-state electrochemical processes in oxides underpin a broad spectrum of energy and information storage devices, ranging from Li-ion and Li-air batteries, to solid oxide fuel cells (SOFC) to electroresistive and memristive systems. These functionalities are controlled by the bias-driven diffusive and electromigration transport of mobile ionic species, as well as intricate a set of electrochemical and defect-controlled reactions at interfaces and in bulk. Despite the wealth of device-level and atomistic studies, little is known on the mesoscopic mechanisms of ion diffusion and electronic transport on the level of grain clusters, individual grains, and extended defects. The development of the capabilitymore » for probing ion transport on the nanometer scale is a key to deciphering complex interplay between structure, functionality, and performance in these systems. Here we introduce Electrochemical Strain Microscopy, a scanning probe microscopy technique based on strong strain-bias coupling in the systems in which local ion concentrations are changed by electrical fields. The imaging capability, as well as time- and voltage spectroscopies analogous to traditional current based electrochemical characterization methods are developed. The reversible intercalation of Li and mapping electrochemical activity in LiCoO2 is demonstrated, illustrating higher Li diffusivity at non-basal planes and grain boundaries. In Si-anode device structure, the direct mapping of Li diffusion at extended defects and evolution of Li-activity with charge state is explored. The electrical field-dependence of Li mobility is studied to determine the critical bias required for the onset of electrochemical transformation, allowing reaction and diffusion processes in the battery system to be separated at each location. Finally, the applicability of ESM for probing oxygen vacancy diffusion and oxygen reduction/evolution reactions is illustrated, and the high resolution ESM maps are correlated with aberration corrected scanning transmission electron microscopy imaging. The future potential for deciphering mechanisms of electrochemical transformations on an atomically-defined single-defect level is discussed.« less

Positronic probe of vacancy defects on surfaces of Au nanoparticles embedded in MgO

NASA Astrophysics Data System (ADS)

Xu, Jun; Moxom, J.; Somieski, B.; White, C. W.; Mills, A. P., Jr.; Suzuki, R.; Ishibashi, S.

2001-09-01

Clusters of four atomic vacancies were found in Au nanoparticle-embedded MgO by positron lifetime spectroscopy [Phys. Rev. Lett. 83, 4586 (1999)]. These clusters were also suggested to locate at the surface of Au nanoparticles by one-detector measurements of Doppler broadening of annihilation radiation. In this work we provide evidence, using two-detector coincidence experiments of Doppler broadening (2D-DBAR), to clarify that these vacancy clusters reside on the surfaces of Au nanoparticles. This work also demonstrates a method for identifying defects at nanomaterials interfaces: a combination of both positron lifetime spectroscopy, which tells the type of the defects, and 2D-DBAR measurements, which reveals chemical environment of the defects.

Organic molecules as tools to control the growth, surface structure, and redox activity of colloidal quantum dots.

PubMed

Weiss, Emily A

2013-11-19

In order to achieve efficient and reliable technology that can harness solar energy, the behavior of electrons and energy at interfaces between different types or phases of materials must be understood. Conversion of light to chemical or electrical potential in condensed phase systems requires gradients in free energy that allow the movement of energy or charge carriers and facilitate redox reactions and dissociation of photoexcited states (excitons) into free charge carriers. Such free energy gradients are present at interfaces between solid and liquid phases or between inorganic and organic materials. Nanostructured materials have a higher density of these interfaces than bulk materials. Nanostructured materials, however, have a structural and chemical complexity that does not exist in bulk materials, which presents a difficult challenge: to lower or eliminate energy barriers to electron and energy flux that inevitably result from forcing different materials to meet in a spatial region of atomic dimensions. Chemical functionalization of nanostructured materials is perhaps the most versatile and powerful strategy for controlling the potential energy landscape of their interfaces and for minimizing losses in energy conversion efficiency due to interfacial structural and electronic defects. Colloidal quantum dots are semiconductor nanocrystals synthesized with wet-chemical methods and coated in organic molecules. Chemists can use these model systems to study the effects of chemical functionalization of nanoscale organic/inorganic interfaces on the optical and electronic properties of a nanostructured material, and the behavior of electrons and energy at interfaces. The optical and electronic properties of colloidal quantum dots have an intense sensitivity to their surface chemistry, and their organic adlayers make them dispersible in solvent. This allows researchers to use high signal-to-noise solution-phase spectroscopy to study processes at interfaces. In this Account, I describe the varied roles of organic molecules in controlling the structure and properties of colloidal quantum dots. Molecules serve as surfactant that determines the mechanism and rate of nucleation and growth and the final size and surface structure of a quantum dot. Anionic surfactant in the reaction mixture allows precise control over the size of the quantum dot core but also drives cation enrichment and structural disordering of the quantum dot surface. Molecules serve as chemisorbed ligands that dictate the energetic distribution of surface states. These states can then serve as thermodynamic traps for excitonic charge carriers or couple to delocalized states of the quantum dot core to change the confinement energy of excitonic carriers. Ligands, therefore, in some cases, dramatically shift the ground state absorption and photoluminescence spectra of quantum dots. Molecules also act as protective layers that determine the probability of redox processes between quantum dots and other molecules. How much the ligand shell insulates the quantum dot from electron exchange with a molecular redox partner depends less on the length or degree of conjugation of the native ligand and more on the density and packing structure of the adlayer and the size and adsorption mode of the molecular redox partner. Control of quantum dot properties in these examples demonstrates that nanoscale interfaces, while complex, can be rationally designed to enhance or specify the functionality of a nanostructured system.

Optical Fluorescence Microscopy for Spatially Characterizing Electron Transfer across a Solid-Liquid Interface on Heterogeneous Electrodes.

PubMed

Choudhary, Eric; Velmurugan, Jeyavel; Marr, James M; Liddle, James A; Szalai, Veronika

2016-01-01

Heterogeneous catalytic materials and electrodes are used for (electro)chemical transformations, including those important for energy storage and utilization. 1, 2 Due to the heterogeneous nature of these materials, activity measurements with sufficient spatial resolution are needed to obtain structure/activity correlations across the different surface features (exposed facets, step edges, lattice defects, grain boundaries, etc.). These measurements will help lead to an understanding of the underlying reaction mechanisms and enable engineering of more active materials. Because (electro)catalytic surfaces restructure with changing environments, 1 it is important to perform measurements in operando . Sub-diffraction fluorescence microscopy is well suited for these requirements because it can operate in solution with resolution down to a few nm. We have applied sub-diffraction fluorescence microscopy to a thin cell containing an electrocatalyst and a solution containing the redox sensitive dye p-aminophenyl fluorescein to characterize reaction at the solid-liquid interface. Our chosen dye switches between a nonfluorescent reduced state and a one-electron oxidized bright state, a process that occurs at the electrode surface. This scheme is used to investigate the activity differences on the surface of polycrystalline Pt, in particular to differentiate reactivity at grain faces and grain boundaries. Ultimately, this method will be extended to study other dye systems and electrode materials.

Localized states and their stability in an anharmonic medium with a nonlinear defect

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gerasimchuk, I. V., E-mail: igor.gera@gmail.com

2015-10-15

A comprehensive analysis of soliton states localized near a plane defect (a defect layer) possessing nonlinear properties is carried out within a quasiclassical approach for different signs of nonlinearity of the medium and different characters of interaction of elementary excitations of the medium with the defect. A quantum interpretation is given to these nonlinear localized modes as a bound state of a large number of elementary excitations. The domains of existence of such states are determined, and their properties are analyzed as a function of the character of interaction of elementary excitations between each other and with the defect. Amore » full analysis of the stability of all the localized states with respect to small perturbations of amplitude and phase is carried out analytically, and the frequency of small oscillations of the state localized on the defect is determined.« less

Simulation of the zero-temperature behavior of a three-dimensional elastic medium

NASA Astrophysics Data System (ADS)

McNamara, David; Middleton, A. Alan; Zeng, Chen

1999-10-01

We have performed numerical simulation of a three-dimensional elastic medium, with scalar displacements, subject to quenched disorder. In the absence of topological defects this system is equivalent to a (3+1)-dimensional interface subject to a periodic pinning potential. We have applied an efficient combinatorial optimization algorithm to generate exact ground states for this interface representation. Our results indicate that this Bragg glass is characterized by power law divergences in the structure factor S(k)~Ak-3. We have found numerically consistent values of the coefficient A for two lattice discretizations of the medium, supporting universality for A in the isotropic systems considered here. We also examine the response of the ground state to the change in boundary conditions that corresponds to introducing a single dislocation loop encircling the system. The rearrangement of the ground state caused by this change is equivalent to the domain wall of elastic deformations which span the dislocation loop. Our results indicate that these domain walls are highly convoluted, with a fractal dimension df=2.60(5). We also discuss the implications of the domain wall energetics for the stability of the Bragg glass phase. Elastic excitations similar to these domain walls arise when the pinning potential is slightly perturbed. As in other disordered systems, perturbations of relative strength δ introduce a new length scale L*~δ-1/ζ beyond which the perturbed ground state becomes uncorrelated with the reference (unperturbed) ground state. We have performed a scaling analysis of the response of the ground state to the perturbations and obtain ζ=0.385(40). This value is consistent with the scaling relation ζ=df/2-θ, where θ characterizes the scaling of the energy fluctuations of low energy excitations.

Identification and Control of Gravity Related Defect Formation During Melt Growth of Electro-Optic Single Crystals Bismuth Silicate(Bi12SiO20)

NASA Technical Reports Server (NTRS)

Becia, Piotr; Wiegel, Michaela E. K.

2004-01-01

A research carried out under Award Number NAG8-1487 was aimed at to the design, conduct and analysis of experiments directed at the identification and control of gravitational effects on crystal growth, segregation and defect formation in the Sillenite system: bismuth silicate (Bi(12)SiO(20)). Correlation analyses was conducted in order to establish the influence of gravity related defects introduced during crystal growth on critical, application specific properties. Achievement of the states objective was conducted during the period from Feb. 01, 1998 to Dec. 31, 2003 with the following anticipated milestones: 1. Establishment of capabilities for (a) reproducible Czochralski and Bridgman-type growth of BSO single crystals and (b) for comprehensive analysis of crystalline and chemical defects as well as for selective property characterization of grown crystals (year 1). 2. Design and execution of critical space growth experiment(s) based on analyses of prefatory space results (experiments aimed at establishing the viability of planned approaches and procedures) and on unresolved issues related to growth, segregation and defect formation associated with conventional growth in Bridgman geometries. Comparative analysis of growth under conventional and under mu-g conditions; identification of gravity related defect formation during conventional Bridgman growth and formulation of approaches for their control (years 2 and 3). Development of charge confinement system which permits growth interface demarcation (in a mu-g environment) as well as minimization of confinement related stress and contamination during growth; design of complementary mu-g growth experiments aimed at quantitative mu-g growth and segregation analyses (year 4). 3. Conduct of quantitative mu-g growth experiments directed at: (a) identification and control of gravity related crystalline and chemical defect formation during single crystal growth of Bi(12)SiO(20) and at (b) defect engineering -the development of approaches to the controlled generation during crystal growth of specified point defects in homogeneous distribution (year 5). The proposed research places focus on a class of materials which have outstanding electrical and optical properties but have so far failed to reach their potential, primarily because of our inability to control adequately their stoichiometry and crystal defect formation as well as confinement related contamination and lattice stress.

Surface Structure and Surface Electronic States Related to Plasma Cleaning of Silicon and Germanium

NASA Astrophysics Data System (ADS)

Cho, Jaewon

This thesis discusses the surface structure and the surface electronic states of Si and Ge(100) surfaces as well as the effects of oxidation process on the silicon oxide/Si(100) interface structure. The H-plasma exposure was performed in situ at low temperatures. The active species, produced in the H-plasma by the rf-excitation of H_2 gas, not only remove microcontaminants such as oxygen and carbon from the surface, but also passivate the surface with atomic hydrogen by satisfying the dangling bonds of the surface atoms. The surfaces were characterized by Angle Resolved UV-Photoemission Spectroscopy (ARUPS) and Low Energy Electron Diffraction (LEED). In the case of Si(100), H-plasma exposure produced ordered H-terminated crystallographic structures with either a 2 x 1 or 1 x 1 LEED pattern. The hydride phases, found on the surfaces of the cleaned Si(100), were shown to depend on the temperature of the surface during H-plasma cleaning. The electronic states for the monohydride and dihydride phases were identified by ARUPS. When the plasma cleaned surface was annealed, the phase transition from the dihydride to monohydride was observed. The monohydride Si-H surface bond was stable up to 460^circC, and the dangling bond surface states were identified after annealing at 500^circC which was accompanied by the spectral shift. The H-terminated surface were characterized to have a flat band structure. For the Ge(100) surface, an ordered 2 x 1 monohydride phase was obtained from the surface cleaned at 180 ^circC. After plasma exposure at <=170^circC a 1 x 1 surface was observed, but the ARUPS indicated that the surface was predominantly composed of disordered monohydride structures. After annealing above the H-dissociation temperatures, the shift in the spectrum was shown to occur with the dangling bond surface states. The H-terminated surfaces were identified to be unpinned. The interface structure of silicon oxide/Si(100) was studied using ARUPS. Spectral shifts were observed, which were dependent on the processes of surface preparation and oxidation. The shift was characterized in association with the band bending. The origins of the spectral shifts were discussed, including defects at interface and H-passivation in Si. The interface structure is considered to be dependent on the surface preparation and oxidation process.

Investigation of (Ti-Zr-Hf-V-Nb)N Multicomponent Nanostructured Coatings before and after Thermal Annealing by Nuclear Physics Methods of Analysis

NASA Astrophysics Data System (ADS)

Pogrebnjak, A. D.; Beresnev, V. M.; Bondar', A. V.; Kaverin, M. V.; Ponomarev, A. G.

2013-10-01

(Ti-Zr-Hf-V-Nb)N multicomponent nanostructured coatings with thickness of 1.0-1.4 μm synthesized by the method of cathode arc-vapor deposition at temperatures of 250-300°С are investigated by various mutually complementary methods of elemental structural analysis using slow positron beams (SPB), proton microbeam based particle-induced x-ray emission (μ-PIXE), energy-dispersive x-ray spectroscopy (EDS) and scanning electron microscopy (SEM) analyses based on electron micro- and nanobeams, x-ray diffraction (XRD) method of phase structural analysis, and the "a-sin2φ" method of measuring a stressed-strained state (x-ray tensometry). The elemental composition, microstructure, residual stress in nanograins, profiles of defect and atom distributions with depth and over the coating surface in 3D-representation are studied for these coatings, and their phase composition, severely strained state, and composition of coatings before and after annealing at Tann = 600°С for annealing time τ = 30 min are investigated. It is demonstrated that the oxidation resistance of the examined coatings can be significantly increased by high-temperature annealing that leads to the formation of elastic severely strained compression state of the coating. Redistribution of elements and defects, their segregation near the interface boundaries and around grains and subgrains in the process of thermostimulated diffusion, and termination of spinodal segregation without considerable change of the average nanograin size are revealed.

Fuel swelling and interaction layer formation in the SELENIUM Si and ZrN coated U(Mo) dispersion fuel plates irradiated at high power in BR2

NASA Astrophysics Data System (ADS)

Leenaers, A.; Van den Berghe, S.; Koonen, E.; Kuzminov, V.; Detavernier, C.

2015-03-01

In the framework of the SELENIUM project two full size flat fuel plates were produced with respectively Si and ZrN coated U(Mo) particles and irradiated in the BR2 reactor at SCK•CEN. Non-destructive analysis of the plates showed that the fuel swelling profiles of both SELENIUM plates were very similar to each other and none of the plates showed signs of pillowing or excessive swelling at the end of irradiation at the highest power position (local maximum 70% 235U). The microstructural analysis showed that the Si coated fuel has less interaction phase formation at low burn-up but at the highest burn-ups, defects start to develop on the IL-matrix interface. The ZrN coated fuel, shows a virtual absence of reaction between the U(Mo) and the Al, up to high fission densities after which the interaction layer formation starts and defects develop in the matrix near the U(Mo) particles. It was found and is confirmed by the SELENIUM (Surface Engineering of Low ENrIched Uranium-Molybdenum) experiment that there are two phenomena at play that need to be controlled: the formation of an interaction layer and swelling of the fuel. As the interaction layer formation occurs at the U(Mo)-matrix interface, applying a diffusion barrier (coating) at that interface should prevent the interaction between U(Mo) and the matrix. The U(Mo) swelling, observed to proceed at an accelerating rate with respect to fission density accumulation, is governed by linear solid state swelling and fission gas bubble swelling due to recrystallization of the fuel. The examination of the SELENIUM fuel plates clearly show that for the U(Mo) dispersion fuel to be qualified, the swelling rate at high burn-up needs to be reduced.

In situ transmission electron microscopy He + implantation and thermal aging of nanocrystalline iron

DOE PAGES

Muntifering, Brittany R.; Fang, Youwu; Leff, Asher C.; ...

2016-10-04

Due to their high density of interfaces, nanostructured material are hypothesized to show a higher tolerance to radiation damage compared to conventional coarse-grained materials and are on interest for use in future nuclear reactors. In order to investigate the roles of vacancies, self-interstitials, and helium during defect accumulation, and the thermal evolution of such defects, a complex set of in situ TEM experiments were performed in nanocrystalline iron.

Semi-empirical device model for Cu2ZnSn(S,Se)4 solar cells

NASA Astrophysics Data System (ADS)

Gokmen, Tayfun; Gunawan, Oki; Mitzi, David B.

2014-07-01

We present a device model for the hydrazine processed kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cell with a world record efficiency of ˜12.6%. Detailed comparison of the simulation results, performed using wxAMPS software, to the measured device parameters shows that our model captures the vast majority of experimental observations, including VOC, JSC, FF, and efficiency under normal operating conditions, and temperature vs. VOC, sun intensity vs. VOC, and quantum efficiency. Moreover, our model is consistent with material properties derived from various techniques. Interestingly, this model does not have any interface defects/states, suggesting that all the experimentally observed features can be accounted for by the bulk properties of CZTSSe. An electrical (mobility) gap that is smaller than the optical gap is critical to fit the VOC data. These findings point to the importance of tail states in CZTSSe solar cells.

Applications of surface analysis and surface theory in tribology

NASA Technical Reports Server (NTRS)

Ferrante, John

1988-01-01

Tribology, the study of adhesion, friction and wear of materials is a complex field which requires a knowledge of solid state physics, surface physics, chemistry, material science and mechanical engineering. It has been dominated, however, by the more practical need to make equipment work. With the advent of surface analysis and advances in surface and solid state theory, a new dimension has been added to the analysis of interactions at tribological interfaces. In this paper the applications of tribological studies and their limitations are presented. Examples from research at the NASA Lewis Research Center are given. Emphasis is on fundamental studies involving the effects of monolayer coverage and thick films on friction and wear. A summary of the current status of theoretical calculations of defect energetics is presented. In addition, some new theoretical techniques which enable simplified quantitative calculations of adhesion, fracture and friction are discussed.

Applications of surface analysis and surface theory in tribology

NASA Technical Reports Server (NTRS)

Ferrante, John

1989-01-01

Tribology, the study of adhesion, friction and wear of materials, is a complex field which requires a knowledge of solid state physics, surface physics, chemistry, material science, and mechanical engineering. It has been dominated, however, by the more practical need to make equipment work. With the advent of surface analysis and advances in surface and solid-state theory, a new dimension has been added to the analysis of interactions at tribological interfaces. In this paper the applications of tribological studies and their limitations are presented. Examples from research at the NASA Lewis Research Center are given. Emphasis is on fundamental studies involving the effects of monolayer coverage and thick films on friction and wear. A summary of the current status of theoretical calculations of defect energetics is presented. In addition, some new theoretical techniques which enable simplified quantitative calculations of adhesion, fracture, and friction are discussed.

III-V heterostructure tunnel field-effect transistor.

PubMed

Convertino, C; Zota, C B; Schmid, H; Ionescu, A M; Moselund, K E

2018-07-04

The tunnel field-effect transistor (TFET) is regarded as one of the most promising solid-state switches to overcome the power dissipation challenge in ultra-low power integrated circuits. TFETs take advantage of quantum mechanical tunneling hence exploit a different current control mechanism compared to standard MOSFETs. In this review, we describe state-of-the-art development of TFET both in terms of performances and of materials integration and we identify the main remaining technological challenges such as heterojunction defects and oxide/channel interface traps causing trap-assisted-tunneling (TAT). Mesa-structures, planar as well as vertical geometries are examined. Conductance slope analysis on InAs/GaSb nanowire tunnel diodes are reported, these two-terminal measurements can be relevant to investigate the tunneling behavior. A special focus is dedicated to III-V heterostructure TFET, as different groups have recently shown encouraging results achieving the predicted sub-thermionic low-voltage operation.

III–V heterostructure tunnel field-effect transistor

NASA Astrophysics Data System (ADS)

Convertino, C.; Zota, C. B.; Schmid, H.; Ionescu, A. M.; Moselund, K. E.

2018-07-01

The tunnel field-effect transistor (TFET) is regarded as one of the most promising solid-state switches to overcome the power dissipation challenge in ultra-low power integrated circuits. TFETs take advantage of quantum mechanical tunneling hence exploit a different current control mechanism compared to standard MOSFETs. In this review, we describe state-of-the-art development of TFET both in terms of performances and of materials integration and we identify the main remaining technological challenges such as heterojunction defects and oxide/channel interface traps causing trap-assisted-tunneling (TAT). Mesa-structures, planar as well as vertical geometries are examined. Conductance slope analysis on InAs/GaSb nanowire tunnel diodes are reported, these two-terminal measurements can be relevant to investigate the tunneling behavior. A special focus is dedicated to III–V heterostructure TFET, as different groups have recently shown encouraging results achieving the predicted sub-thermionic low-voltage operation.

Particle protection capability of SEMI-compliant EUV-pod carriers

NASA Astrophysics Data System (ADS)

Huang, George; He, Long; Lystad, John; Kielbaso, Tom; Montgomery, Cecilia; Goodwin, Frank

2010-04-01

With the projected rollout of pre-production extreme ultraviolet lithography (EUVL) scanners in 2010, EUVL pilot line production will become a reality in wafer fabrication companies. Among EUVL infrastructure items that must be ready, EUV mask carriers remain critical. To keep non-pellicle EUV masks free from particle contamination, an EUV pod concept has been extensively studied. Early prototypes demonstrated nearly particle-free results at a 53 nm PSL equivalent inspection sensitivity during EUVL mask robotic handling, shipment, vacuum pump-purge, and storage. After the passage of SEMI E152, which specifies the EUV pod mechanical interfaces, standards-compliant EUV pod prototypes, including a production version inner pod and prototype outer pod, were built and tested. Their particle protection capability results are reported in this paper. A state-of-the-art blank defect inspection tool was used to quantify their defect protection capability during mask robotic handling, shipment, and storage tests. To ensure the availability of an EUV pod for 2010 pilot production, the progress and preliminary test results of pre-production EUV outer pods are reported as well.

Effect of thermal annealing on the structure and magnetism of Fe-doped ZnO nanocrystals synthesized by solid state reaction

NASA Astrophysics Data System (ADS)

Wang, Dong; Chen, Z. Q.; Wang, D. D.; Gong, J.; Cao, C. Y.; Tang, Z.; Huang, L. R.

2010-11-01

High purity Fe 2O 3/ZnO nanocomposites were annealed in air at different temperatures between 100 and 1200 °C to get Fe-doped ZnO nanocrystals. The structure and grain size of the Fe 2O 3/ZnO nanocomposites were investigated by X-ray diffraction 2θ scans. Annealing induces an increase of the grain size from 25 to 195 nm and appearance of franklinite phase of ZnFe 2O 4. Positron annihilation measurements reveal large number of vacancy defects in the interface region of the Fe 2O 3/ZnO nanocomposites, and they are gradually recovered with increasing annealing temperature. After annealing at temperatures higher than 1000 °C, the number of vacancies decreases to the lower detection limit of positrons. Room temperature ferromagnetism can be observed in Fe-doped ZnO nanocrystals using physical properties measurement system. The ferromagnetism remains after annealing up to 1000 °C, suggesting that it is not related with the interfacial defects.

3D Printing of Materials with Tunable Failure via Bioinspired Mechanical Gradients.

PubMed

Kokkinis, Dimitri; Bouville, Florian; Studart, André R

2018-05-01

Mechanical gradients are useful to reduce strain mismatches in heterogeneous materials and thus prevent premature failure of devices in a wide range of applications. While complex graded designs are a hallmark of biological materials, gradients in manmade materials are often limited to 1D profiles due to the lack of adequate fabrication tools. Here, a multimaterial 3D-printing platform is developed to fabricate elastomer gradients spanning three orders of magnitude in elastic modulus and used to investigate the role of various bioinspired gradient designs on the local and global mechanical behavior of synthetic materials. The digital image correlation data and finite element modeling indicate that gradients can be effectively used to manipulate the stress state and thus circumvent the weakening effect of defect-rich interfaces or program the failure behavior of heterogeneous materials. Implementing this concept in materials with bioinspired designs can potentially lead to defect-tolerant structures and to materials whose tunable failure facilitates repair of biomedical implants, stretchable electronics, or soft robotics. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Depth-Dependent Defect Studies Using Coherent Acoustic Phonons

DTIC Science & Technology

2014-09-29

using CAP waves as an active moving interface to induce local changes in electric, acoustic , and optical properties. This is able to generate ultrafast...the elastic strain component [6]. b) Modification of the crystal lattice due to transient strain caused by the coherent acoustic phonon wave . The...opto-electronic properties of materials. We are also using CAP waves as an active moving interface to induce local changes in electric, acoustic , and

Direct Probing of the Dielectric Scavenging-Layer Interface in Oxide Filamentary-Based Valence Change Memory.

PubMed

Celano, Umberto; Op de Beeck, Jonathan; Clima, Sergiu; Luebben, Michael; Koenraad, Paul M; Goux, Ludovic; Valov, Ilia; Vandervorst, Wilfried

2017-03-29

A great improvement in valence change memory performance has been recently achieved by adding another metallic layer to the simple metal-insulator-metal (MIM) structure. This metal layer is often referred to as oxygen exchange layer (OEL) and is introduced between one of the electrodes and the oxide. The OEL is believed to induce a distributed reservoir of defects at the metal-insulator interface thus providing an unlimited availability of building blocks for the conductive filament (CF). However, its role remains elusive and controversial owing to the difficulties to probe the interface between the OEL and the CF. Here, using Scalpel SPM we probe multiple functions of the OEL which have not yet been directly measured, for two popular VCMs material systems: Hf/HfO 2 and Ta/Ta 2 O 5 . We locate and characterize in three-dimensions the volume containing the oxygen exchange layer and the CF with nanometer lateral resolution. We demonstrate that the OEL induces a thermodynamic barrier for the CF and estimate the minimum thickness of the OEL/oxide interface to guarantee the proper switching operations is ca. 3 nm. Our experimental observations are combined to first-principles thermodynamics and defect kinetics to elucidate the role of the OEL for device optimization.

Dynamic fracture and hot-spot modeling in energetic composites

NASA Astrophysics Data System (ADS)

Grilli, Nicolò; Duarte, Camilo A.; Koslowski, Marisol

2018-02-01

Defects such as cracks, pores, and particle-matrix interface debonding affect the sensitivity of energetic materials by reducing the time-to-ignition and the threshold pressure to initiate an explosion. Frictional sliding of preexisting cracks is considered to be one of the most important causes of localized heating. Therefore, understanding the dynamic fracture of crystalline energetic materials is of extreme importance to assess the reliability and safety of polymer-bonded explosives. Phase field damage model simulations, based on the regularization of the crack surface as a diffuse delta function, are used to describe crack propagation in cyclotetramethylene-tetranitramine crystals embedded in a Sylgard matrix. A thermal transport model that includes heat generation by friction at crack interfaces is coupled to the solution of crack propagation. 2D and 3D dynamic compression simulations are performed with different boundary velocities and initial distributions of cracks and interface defects to understand their effect on crack propagation and heat generation. It is found that, at an impact velocity of 400 m/s, localized damage at the particle-binder interface is of key importance and that the sample reaches temperatures high enough to create a hot-spot that will lead to ignition. At an impact velocity of 10 m/s, preexisting cracks advanced inside the particle, but the increase of temperature will not cause ignition.

Multilayer scaffolds in orthopaedic tissue engineering.

PubMed

Atesok, Kivanc; Doral, M Nedim; Karlsson, Jon; Egol, Kenneth A; Jazrawi, Laith M; Coelho, Paulo G; Martinez, Amaury; Matsumoto, Tomoyuki; Owens, Brett D; Ochi, Mitsuo; Hurwitz, Shepard R; Atala, Anthony; Fu, Freddie H; Lu, Helen H; Rodeo, Scott A

2016-07-01

The purpose of this study was to summarize the recent developments in the field of tissue engineering as they relate to multilayer scaffold designs in musculoskeletal regeneration. Clinical and basic research studies that highlight the current knowledge and potential future applications of the multilayer scaffolds in orthopaedic tissue engineering were evaluated and the best evidence collected. Studies were divided into three main categories based on tissue types and interfaces for which multilayer scaffolds were used to regenerate: bone, osteochondral junction and tendon-to-bone interfaces. In vitro and in vivo studies indicate that the use of stratified scaffolds composed of multiple layers with distinct compositions for regeneration of distinct tissue types within the same scaffold and anatomic location is feasible. This emerging tissue engineering approach has potential applications in regeneration of bone defects, osteochondral lesions and tendon-to-bone interfaces with successful basic research findings that encourage clinical applications. Present data supporting the advantages of the use of multilayer scaffolds as an emerging strategy in musculoskeletal tissue engineering are promising, however, still limited. Positive impacts of the use of next generation scaffolds in orthopaedic tissue engineering can be expected in terms of decreasing the invasiveness of current grafting techniques used for reconstruction of bone and osteochondral defects, and tendon-to-bone interfaces in near future.

Defect-induced instability mechanisms of sputtered amorphous indium tin zinc oxide thin-film transistors

NASA Astrophysics Data System (ADS)

Park, Jinhee; Rim, You Seung; Li, Chao; Wu, Jiechen; Goorsky, Mark; Streit, Dwight

2018-04-01

We report the device performance and stability of sputtered amorphous indium-tin-zinc-oxide (ITZO) thin-film transistors as a function of oxygen ratio [O2/(Ar + O2)] during growth. Increasing the oxygen ratio enhanced the incorporation of oxygen during ITZO film growth and reduced the concentration of deep-level defects associated with oxygen vacancies. Under illumination with no bias stress, device stability and persistent photocurrent were improved with increased oxygen ratio. Bias stress tests of the devices were also performed with and without illumination. While high oxygen ratio growth conditions resulted in decreased deep-level oxygen vacancies in the ITZO material, the same conditions resulted in degradation of the interfacial layer between the ITZO channel and dielectric due to the migration of energetic oxygen ions to the interface. Therefore, when bias stress was applied, increased carrier trap density at the interface led to a decrease in device stability that offsets any improvement in the material itself. In order to take advantage of the improved ITZO material growth at a high oxygen ratio, the interface-related problems must be solved.

Optical transitions in two-dimensional topological insulators with point defects

NASA Astrophysics Data System (ADS)

Sablikov, Vladimir A.; Sukhanov, Aleksei A.

2016-12-01

Nontrivial properties of electronic states in topological insulators are inherent not only to the surface and boundary states, but to bound states localized at structure defects as well. We clarify how the unusual properties of the defect-induced bound states are manifested in optical absorption spectra in two-dimensional topological insulators. The calculations are carried out for defects with short-range potential. We find that the defects give rise to the appearance of specific features in the absorption spectrum, which are an inherent property of topological insulators. They have the form of two or three absorption peaks that are due to intracenter transitions between electron-like and hole-like bound states.

Interpretation and Regulation of Electronic Defects in IGZO TFTs Through Materials & Processes

NASA Astrophysics Data System (ADS)

Mudgal, Tarun

The recent rise in the market for consumer electronics has fueled extensive research in the field of display. Thin-Film Transistors (TFTs) are used as active matrix switching devices for flat panel displays such as LCD and OLED. The following investigation involves an amorphous metal-oxide semiconductor that has the potential for improved performance over current technology, while maintaining high manufacturability. Indium-Gallium-Zinc-Oxide (IGZO) is a semiconductor material which is at the onset of commercialization. The low-temperature large-area deposition compatibility of IGZO makes it an attractive technology from a manufacturing standpoint, with an electron mobility that is 10 times higher than current amorphous silicon technology. The stability of IGZO TFTs continues to be a challenge due to the presence of defect states and problems associated with interface passivation. The goal of this dissertation is to further the understanding of the role of defect states in IGZO, and investigate materials and processes needed to regulate defects to the level at which the associated influence on device operation is controlled. The relationships between processes associated with IGZO TFT operation including IGZO sputter deposition, annealing conditions and back-channel passivation are established through process experimentation, materials analysis, electrical characterization, and modeling of electronic properties and transistor behavior. Each of these components has been essential in formulating and testing several hypotheses on the mechanisms involved, and directing efforts towards achieving the goal. Key accomplishments and quantified results are summarized as follows: • XPS analysis identified differences in oxygen vacancies in samples before and after oxidizing ambient annealing at 400 °C, showing a drop in relative integrated area of the O-1s peak from 32% to 19%, which experimentally translates to over a thousand fold decrease in the channel free electron concentration. • Transport behavior at cryogenic temperatures identified variable range hopping as the electron transport mechanism at temperature below 130 K, whereas at temperature greater than 130 K, the current vs temperature response followed an Arrhenius relationship consistent with extended state transport. • Refinement of an IGZO material model for TCAD simulation, which consists of oxygen vacancy donors providing an integrated space charge concentration NVO = +5e15 cm-3, and acceptor-like band-tail states with a total integrated ionized concentration of NTA = -2e18 cm-3. An intrinsic electron mobility was established to be micron = 12.7 cm2/V·s. • A SPICE-compatible 2D on-state operation model for IGZO TFTs has been developed which includes the integration of drain-impressed deionization of band-tail states and results in a 2D modification of free channel charge. The model provides an exceptional match to measured data and TCAD simulation, with model parameters for channel mobility (microch = 12 cm2/V·s) and threshold voltage (V T = 0.14 V) having a close match to TCAD analogs. • TCAD material and device models for bottom-gate and double-gate TFT configurations have been developed which depict the role of defect states on device operation, as well as provide insight and support of a presented hypothesis on DIBL like device behavior associated with back-channel interface trap inhomogeneity. This phenomenon has been named Trap Associated Barrier Lowering (TABL). • A process integration scheme has been developed that includes IGZO back-channel passivation with PECVD SiO2, furnace annealing in O2 at 400 °C, and a thin capping layer of alumina deposited via atomic layer deposition. This process supports device stability when subjected to negative and positive bias stress conditions, and thermal stability up to 140 °C. It also enables TFT operation at short channel lengths (Leff 3 microm) with steep subthreshold characteristics (SS 120 mV/dec). The details of these contributions in the interpretation and regulation of electronic defect states in IGZO TFTs is presented, along with the support of device characteristics that are among the best reported in the literature. Additional material on a complementary technology which utilizes flash-lamp annealing of amorphous silicon will also be described. Flash-Lamp Annealed Polycrystalline Silicon (FLAPS) has realized n-channel and p-channel TFTs with promising results, and may provide an option for future applications with the highest performance demands. IGZO is rapidly emerging as the candidate to replace a-Si:H and address the performance needs of display products produced by large panel manufacturing.

Self-regulation of charged defect compensation and formation energy pinning in semiconductors

PubMed Central

Yang, Ji-Hui; Yin, Wan-Jian; Park, Ji-Sang; Wei, Su-Huai

2015-01-01

Current theoretical analyses of defect properties without solving the detailed balance equations often estimate Fermi-level pinning position by omitting free carriers and assume defect concentrations can be always tuned by atomic chemical potentials. This could be misleading in some circumstance. Here we clarify that: (1) Because the Fermi-level pinning is determined not only by defect states but also by free carriers from band-edge states, band-edge states should be treated explicitly in the same footing as the defect states in practice; (2) defect formation energy, thus defect density, could be pinned and independent on atomic chemical potentials due to the entanglement of atomic chemical potentials and Fermi energy, in contrast to the usual expectation that defect formation energy can always be tuned by varying the atomic chemical potentials; and (3) the charged defect compensation behavior, i.e., most of donors are compensated by acceptors or vice versa, is self-regulated when defect formation energies are pinned. The last two phenomena are more dominant in wide-gap semiconductors or when the defect formation energies are small. Using NaCl and CH3NH3PbI3 as examples, we illustrate these unexpected behaviors. Our analysis thus provides new insights that enrich the understanding of the defect physics in semiconductors and insulators. PMID:26584670

Magnesium acceptor in gallium nitride. II. Koopmans-tuned Heyd-Scuseria-Ernzerhof hybrid functional calculations of its dual nature and optical properties

NASA Astrophysics Data System (ADS)

Demchenko, D. O.; Diallo, I. C.; Reshchikov, M. A.

2018-05-01

The problem of magnesium acceptor in gallium nitride is that experimental photoluminescence measurements clearly reveal a shallow defect state, while most theoretical predictions favor a localized polaronic defect state. To resolve this contradiction, we calculate properties of magnesium acceptor using the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional, tuned to fulfill the generalized Koopmans condition. We test Koopmans tuning of HSE for defect calculations in GaN using two contrasting test cases: a deep state of gallium vacancy and a shallow state of magnesium acceptor. The obtained parametrization of HSE allows calculations of optical properties of acceptors using neutral defect-state eigenvalues, without relying on corrections due to charged defects in periodic supercells. Optical transitions and vibrational properties of M gGa defect are analyzed to bring the dual (shallow and deep) nature of this defect into accord with experimental photoluminescence measurements of the ultraviolet band in Mg-doped GaN samples.

Engineering Human Immunodeficiency Virus 1 Protease Heterodimers as Macromolecular Inhibitors of Viral Maturation

NASA Astrophysics Data System (ADS)

McPhee, Fiona; Good, Andrew C.; Kuntz, Irwin D.; Craik, Charles S.

1996-10-01

Dimerization of human immunodeficiency virus type 1 protease (HIV-1 PR) monomers is an essential prerequisite for viral proteolytic activity and the subsequent generation of infectious virus particles. Disruption of the dimer interface inhibits this activity as does formation of heterodimers between wild-type and defective monomers. A structure-based approach was used to identify amino acid substitutions at the dimer interface of HIV-1 PR that facilitate preferential association of heterodimers and inhibit self-association of the defective monomers. Expression of the designed PR monomers inhibits activity of wild-type HIV-1 PR and viral infectivity when assayed in an ex vivo model system. These results show that it is possible to design PR monomers as macromolecular inhibitors that may provide an alternative to small molecule inhibitors for the treatment of HIV infection.

Phases, phase equilibria, and phase rules in low-dimensional systems

DOE Office of Scientific and Technical Information (OSTI.GOV)

Frolov, T., E-mail: timfrol@berkeley.edu; Mishin, Y., E-mail: ymishin@gmu.edu

2015-07-28

We present a unified approach to thermodynamic description of one, two, and three dimensional phases and phase transformations among them. The approach is based on a rigorous definition of a phase applicable to thermodynamic systems of any dimensionality. Within this approach, the same thermodynamic formalism can be applied for the description of phase transformations in bulk systems, interfaces, and line defects separating interface phases. For both lines and interfaces, we rigorously derive an adsorption equation, the phase coexistence equations, and other thermodynamic relations expressed in terms of generalized line and interface excess quantities. As a generalization of the Gibbs phasemore » rule for bulk phases, we derive phase rules for lines and interfaces and predict the maximum number of phases than may coexist in systems of the respective dimensionality.« less

Anomalously deep polarization in SrTiO3 (001) interfaced with an epitaxial ultrathin manganite film

DOE PAGES

Wang, Zhen; Tao, Jing; Yu, Liping; ...

2016-10-17

Using atomically-resolved imaging and spectroscopy, we reveal a remarkably deep polarization in non-ferroelectric SrTiO 3 near its interface with an ultrathin nonmetallic film of La 2/3Sr 1/3MnO 3. Electron holography shows an electric field near the interface in SrTiO 3, yielding a surprising spontaneous polarization density of ~ 21 μC/cm 2. Combining the experimental results with first principles calculations, we propose that the observed deep polarization is induced by the electric field originating from oxygen vacancies that extend beyond a dozen unit-cells from the interface, thus providing important evidence of the role of defects in the emergent interface properties ofmore » transition metal oxides.« less

Cryogenic Quenching Process for Electronic Part Screening

NASA Technical Reports Server (NTRS)

Sheldon, Douglas J.; Cressler, John

2011-01-01

The use of electronic parts at cryogenic temperatures (less than 100 C) for extreme environments is not well controlled or developed from a product quality and reliability point of view. This is in contrast to the very rigorous and well-documented procedures to qualify electronic parts for mission use in the 55 to 125 C temperature range. A similarly rigorous methodology for screening and evaluating electronic parts needs to be developed so that mission planners can expect the same level of high reliability performance for parts operated at cryogenic temperatures. A formal methodology for screening and qualifying electronic parts at cryogenic temperatures has been proposed. The methodology focuses on the base physics of failure of the devices at cryogenic temperatures. All electronic part reliability is based on the bathtub curve, high amounts of initial failures (infant mortals), a long period of normal use (random failures), and then an increasing number of failures (end of life). Unique to this is the development of custom screening procedures to eliminate early failures at cold temperatures. The ability to screen out defects will specifically impact reliability at cold temperatures. Cryogenic reliability is limited by electron trap creation in the oxide and defect sites at conductor interfaces. Non-uniform conduction processes due to process marginalities will be magnified at cryogenic temperatures. Carrier mobilities change by orders of magnitude at cryogenic temperatures, significantly enhancing the effects of electric field. Marginal contacts, impurities in oxides, and defects in conductor/conductor interfaces can all be magnified at low temperatures. The novelty is the use of an ultra-low temperature, short-duration quenching process for defect screening. The quenching process is designed to identify those defects that will precisely (and negatively) affect long-term, cryogenic part operation. This quenching process occurs at a temperature that is at least 25 C colder than the coldest expected operating temperature. This quenching process is the opposite of the standard burn-in procedure. Normal burn-in raises the temperature (and voltage) to activate quickly any possible manufacturing defects remaining in the device that were not already rejected at a functional test step. The proposed inverse burn-in or quenching process is custom-tailored to the electronic device being used. The doping profiles, materials, minimum dimensions, interfaces, and thermal expansion coefficients are all taken into account in determining the ramp rate, dwell time, and temperature.

Thermal evolution of defects in undoped zinc oxide grown by pulsed laser deposition

NASA Astrophysics Data System (ADS)

Wang, Zilan; Su, Shichen; Ling, Francis Chi-Chung; Anwand, W.; Wagner, A.

2014-07-01

Undoped ZnO films are grown by pulsed laser deposition on c-plane sapphire with different oxygen pressures. Thermal evolutions of defects in the ZnO films are studied by secondary ion mass spectroscopy (SIMS), Raman spectroscopy, and positron annihilation spectroscopy (PAS), and with the electrical properties characterized by the room temperature Hall measurement. Oxygen deficient defect related Raman lines 560 cm-1 and 584 cm-1 are identified and their origins are discussed. Thermal annealing induces extensive Zn out-diffusion at the ZnO/sapphire interface and leaves out Zn-vacancy in the ZnO film. Two types of Zn-vacancy related defects with different microstructures are identified in the films. One of them dominates in the samples grown without oxygen. Annealing the sample grown without oxygen or growing the samples in oxygen would favor the Zn-vacancy with another microstructure, and this Zn-vacancy defect persists after 1100 °C annealing.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Buttler, D J

The Java Metadata Facility is introduced by Java Specification Request (JSR) 175 [1], and incorporated into the Java language specification [2] in version 1.5 of the language. The specification allows annotations on Java program elements: classes, interfaces, methods, and fields. Annotations give programmers a uniform way to add metadata to program elements that can be used by code checkers, code generators, or other compile-time or runtime components. Annotations are defined by annotation types. These are defined the same way as interfaces, but with the symbol {at} preceding the interface keyword. There are additional restrictions on defining annotation types: (1) Theymore » cannot be generic; (2) They cannot extend other annotation types or interfaces; (3) Methods cannot have any parameters; (4) Methods cannot have type parameters; (5) Methods cannot throw exceptions; and (6) The return type of methods of an annotation type must be a primitive, a String, a Class, an annotation type, or an array, where the type of the array is restricted to one of the four allowed types. See [2] for additional restrictions and syntax. The methods of an annotation type define the elements that may be used to parameterize the annotation in code. Annotation types may have default values for any of its elements. For example, an annotation that specifies a defect report could initialize an element defining the defect outcome submitted. Annotations may also have zero elements. This could be used to indicate serializability for a class (as opposed to the current Serializability interface).« less

Modelling of thermal field and point defect dynamics during silicon single crystal growth using CZ technique

NASA Astrophysics Data System (ADS)

Sabanskis, A.; Virbulis, J.

2018-05-01

Mathematical modelling is employed to numerically analyse the dynamics of the Czochralski (CZ) silicon single crystal growth. The model is axisymmetric, its thermal part describes heat transfer by conduction and thermal radiation, and allows to predict the time-dependent shape of the crystal-melt interface. Besides the thermal field, the point defect dynamics is modelled using the finite element method. The considered process consists of cone growth and cylindrical phases, including a short period of a reduced crystal pull rate, and a power jump to avoid large diameter changes. The influence of the thermal stresses on the point defects is also investigated.

OCT for early quality evaluation of tooth-composite bond in clinical trials.

PubMed

Haak, Rainer; Schmidt, Patrick; Park, Kyung-Jin; Häfer, Matthias; Krause, Felix; Ziebolz, Dirk; Schneider, Hartmut

2018-06-19

To evaluate early quality of composite restorations with a universal adhesive in different application modes clinically and with optical coherence tomography (OCT). 22 patients with four non-carious cervical lesions each received composite restorations (Filtek Supreme TM XTE, 3 M). The universal adhesive Scotchbond Universal TM (SBU, 3 M) was applied with three etching protocols: self-etch (SE), selective-enamel-etch (SEE) and etch-and-rinse (ER). The etch-and-rinse adhesive OptiBond TM FL (OFL, Kerr) served as a control. Restorations were imaged by OCT (Thorlabs) directly after application (t 0 ). After 14 days (t 1 ) and 6 month (t 2 ) OCT imaging (interfacial adhesive defects) was repeated combined with clinical assessment (FDI criteria). Groups were compared by Friedman-/Wilcoxon- and McNemar-Test. No differences were seen clinically between groups (p i ≥ 0.500). OCT assessment revealed more adhesive defects at the enamel interface with SBU/SE at t 0- t 2 compared to all groups (p i ≤ 0.016). OFL showed more defects than SBU/ER (t 1 : p = 0.01; t 2 : p = 0.083). At dentin/cementum interface OFL exhibited more adhesive defects than SBU with all conditioning modes (t 0 , t 1 , p i ≤ 0.003) and at t 2 to SBU/SE and SBU/ER (p < 0.001). Since t 1 defects with SBU were detected more frequently in the SE and SEE modes compared to ER (p i ≤ 0.037). In contrast to SBU defects increased with OFL up to t 2 (p i ≤ 0.007). In contrast to clinical evaluation, OCT revealed subtle adhesive defects directly after application that might interfere with clinical success. It was demonstrated that ER doesn't decrease initial adhesion of SBU to dentin. Copyright © 2018. Published by Elsevier Ltd.

Lithium diffusion at Si-C interfaces in silicon-graphene composites

DOE Office of Scientific and Technical Information (OSTI.GOV)

Odbadrakh, Khorgolkhuu; McNutt, N. W.; Nicholson, D. M.

2014-08-04

Models of intercalated Li and its diffusion in Si-Graphene interfaces are investigated using density functional theory. Results suggest that the presence of interfaces alters the energetics of Li binding and diffusion significantly compared to bare Si or Graphene surfaces. Our results show that cavities along reconstructed Si surface provide diffusion paths for Li. Diffusion barriers calculated along these cavities are significantly lower than penetration barriers to bulk Si. Interaction with Si surface results in graphene defects, creating Li diffusion paths that are confined along the cavities but have still lower barrier than in bulk Si.

Stitching h-BN by atomic layer deposition of LiF as a stable interface for lithium metal anode

PubMed Central

Xie, Jin; Liao, Lei; Gong, Yongji; Li, Yanbin; Shi, Feifei; Pei, Allen; Sun, Jie; Zhang, Rufan; Kong, Biao; Subbaraman, Ram; Christensen, Jake; Cui, Yi

2017-01-01

Defects are important features in two-dimensional (2D) materials that have a strong influence on their chemical and physical properties. Through the enhanced chemical reactivity at defect sites (point defects, line defects, etc.), one can selectively functionalize 2D materials via chemical reactions and thereby tune their physical properties. We demonstrate the selective atomic layer deposition of LiF on defect sites of h-BN prepared by chemical vapor deposition. The LiF deposits primarily on the line and point defects of h-BN, thereby creating seams that hold the h-BN crystallites together. The chemically and mechanically stable hybrid LiF/h-BN film successfully suppresses lithium dendrite formation during both the initial electrochemical deposition onto a copper foil and the subsequent cycling. The protected lithium electrodes exhibit good cycling behavior with more than 300 cycles at relatively high coulombic efficiency (>95%) in an additive-free carbonate electrolyte. PMID:29202031

Stitching h-BN by atomic layer deposition of LiF as a stable interface for lithium metal anode

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xie, Jin; Liao, Lei; Gong, Yongji

Defects are important features in two-dimensional (2D) materials that have a strong influence on their chemical and physical properties. Through the enhanced chemical reactivity at defect sites (point defects, line defects, etc.), one can selectively functionalize 2D materials via chemical reactions and thereby tune their physical properties. We demonstrate the selective atomic layer deposition of LiF on defect sites of h-BN prepared by chemical vapor deposition. The LiF deposits primarily on the line and point defects of h-BN, thereby creating seams that hold the h-BN crystallites together. The chemically and mechanically stable hybrid LiF/h-BN film successfully suppresses lithium dendrite formationmore » during both the initial electrochemical deposition onto a copper foil and the subsequent cycling. In conclusion, the protected lithium electrodes exhibit good cycling behavior with more than 300 cycles at relatively high coulombic efficiency (>95%) in an additive-free carbonate electrolyte.« less

Vacancy clustering and its dissociation process in electroless deposited copper films studied by monoenergetic positron beams

NASA Astrophysics Data System (ADS)

Uedono, A.; Yamashita, Y.; Tsutsui, T.; Dordi, Y.; Li, S.; Oshima, N.; Suzuki, R.

2012-05-01

Positron annihilation was used to probe vacancy-type defects in electroless deposited copper films. For as-deposited films, two different types of vacancy-type defects were found to coexist; these were identified as vacancy aggregates (V3-V4) and larger vacancy clusters (˜V10). After annealing at about 200 °C, the defects started to diffuse toward the surface and aggregate. The same tendency has been observed for sulfur only, suggesting the formation of complexes between sulfur and vacancies. The defect concentration near the Cu/barrier-metal interface was high even after annealing above 600 °C, and this was attributed to an accumulation of vacancy-impurity complexes. The observed defect reactions were attributed to suppression of the vacancy diffusion to sinks through the formation of impurity-vacancy complexes. It was shown that electroless plating has a high potential to suppress the formation of voids/hillocks caused by defect migration.

Stitching h-BN by atomic layer deposition of LiF as a stable interface for lithium metal anode

DOE PAGES

Xie, Jin; Liao, Lei; Gong, Yongji; ...

2017-11-29

Defects are important features in two-dimensional (2D) materials that have a strong influence on their chemical and physical properties. Through the enhanced chemical reactivity at defect sites (point defects, line defects, etc.), one can selectively functionalize 2D materials via chemical reactions and thereby tune their physical properties. We demonstrate the selective atomic layer deposition of LiF on defect sites of h-BN prepared by chemical vapor deposition. The LiF deposits primarily on the line and point defects of h-BN, thereby creating seams that hold the h-BN crystallites together. The chemically and mechanically stable hybrid LiF/h-BN film successfully suppresses lithium dendrite formationmore » during both the initial electrochemical deposition onto a copper foil and the subsequent cycling. In conclusion, the protected lithium electrodes exhibit good cycling behavior with more than 300 cycles at relatively high coulombic efficiency (>95%) in an additive-free carbonate electrolyte.« less

Degradation of polyfluorene-type polymers: interface and bulk-related defects

NASA Astrophysics Data System (ADS)

Gamerith, Stefan; Gadermaier, Christoph; Nothofer, Heinz G.; Scherf, Ullrich; List, Emil J.

2004-09-01

The origin of a broad low-energy photo-luminescence (PL) and electro-luminescence (EL) band emerging upon oxidative degradation of hihgly emissive polyfluorenes (PFs) has recently been identified as the emission from on-chain keto defects acting as exciton and/or charge traps. In this work we compare several polyfluorenes with respect to their stability upon thermal degradation, and their stability upon fabrication and operation of PF-based polymer light emitting devices (PLEDs). We show that in addition to the keto emission a second type of defect emission, which is related to the deposition of the metal electrode, can also affect the color purity of PF-PLEDs. Investigated materials are a poly(9,9 dialkylfluorene) with hexahydrofarnesyl sidechains (PF111/12) a poly(9,9 dialkylfluorene) with ethyl-hexyl sidechains (PF 2/6) and two different slightly branched spiro-PFs with and without triphenylamine endcappers, respetively. We find significant differences in the spectral stability of the polymers which may on the one hand be explained by a difference of the chemical stability of the polymers but to some extent must be explained withiin the picture of excited energy migration. Regarding a comparison of the polymers, the end-capped spiro-type PF shows an overall improved performance compared to the other investigated polymers provided that the evaporation process of the metal cathode of an PLED is well controlled to avoid the formation of emissive defects at the interface.

Point Defects in Oxides: Tailoring Materials Through Defect Engineering

NASA Astrophysics Data System (ADS)

Tuller, Harry L.; Bishop, Sean R.

2011-08-01

Optimization of electrical, optical, mechanical, and other properties of many advanced, functional materials today relies on precise control of point defects. This article illustrates the progress that has been made in elucidating the often complex equilibria exhibited by many materials by examining two recently well-characterized model systems, TlBr for radiation detection and PrxCe1-xO2-δ, of potential interest in solid-oxide fuel cells. The interplay between material composition, electrical conductivity, and mechanical properties (electrochemomechanics) is discussed, and implications in these relations, for example, enhancing electrical properties through large mechanical strains, are described. The impact of space charge and strain fields at interfaces, particularly important in nanostructure materials, is also emphasized. Key experimental techniques useful in characterizing bulk and surface defects are summarized and reviewed.

Response function of a moving contact line

NASA Astrophysics Data System (ADS)

Perrin, H.; Belardinelli, D.; Sbragaglia, M.; Andreotti, B.

2018-04-01

The hydrodynamics of a liquid-vapor interface in contact with a heterogeneous surface is largely impacted by the presence of defects at the smaller scales. Such defects introduce morphological disturbances on the contact line and ultimately determine the force exerted on the wedge of liquid in contact with the surface. From the mathematical point of view, defects introduce perturbation modes, whose space-time evolution is governed by the interfacial hydrodynamic equations of the contact line. In this paper we derive the response function of the contact line to such generic perturbations. The contact line response may be used to design simplified one-dimensional time-dependent models accounting for the complexity of interfacial flows coupled to nanoscale defects, yet offering a more tractable mathematical framework to explore contact line motion through a disordered energy landscape.

Study on clinical application of nano-hydroxyapatite bone in bone defect repair.

PubMed

Zhu, Weimin; Wang, Daping; Xiong, Jianyi; Liu, Jianquan; You, Wei; Huang, Jianghong; Duan, Li; Chen, Jielin; Zeng, Yanjun

2015-01-01

To study the clinical effect of bone defect treated with nano-hydroxyapatite(Nano-HA) artificial bone. From September 2009 to June 2012, 27 cases of bone defect were analyzed retrospectively. The position of bone defect included humerus, radius, ulna, femur, tibia and calcaneus. The range of bone defect was from 0.3 × 1.0 cm to 3 × 6.5 cm. Among them, there were 22 cases with fractures and 5 cases with tumors. All patients were treated with Nano-HA artificial bone. The ability of bone defect repair was evaluated by X-ray exams performed preoperatively and postoperatively. HSS scores were adopted for final evaluation at the latest follow-up. The patients were followed up from 11 to 26 months (average of 18.5 months). No general side effects occurred. X-ray photo showed an integrity interface between Nano-HA and bone. Primary healing was obtained in all cases without any complication. The Nano-HA artificial bone had a good biocompatibility and could be an ideal artificial bone in the reconstruction of bone defect.

Tailoring Heterovalent Interface Formation with Light

DOE PAGES

Park, Kwangwook; Alberi, Kirstin

2017-08-17

Integrating different semiconductor materials into an epitaxial device structure offers additional degrees of freedom to select for optimal material properties in each layer. However, interface between materials with different valences (i.e. III-V, II-VI and IV semiconductors) can be difficult to form with high quality. Using ZnSe/GaAs as a model system, we explore the use of UV illumination during heterovalent interface growth by molecular beam epitaxy as a way to modify the interface properties. We find that UV illumination alters the mixture of chemical bonds at the interface, permitting the formation of Ga-Se bonds that help to passivate the underlying GaAsmore » layer. Illumination also helps to reduce defects in the ZnSe epilayer. Furthermore, these results suggest that moderate UV illumination during growth may be used as a way to improve the optical properties of both the GaAs and ZnSe layers on either side of the interface.« less

Interaction of bimetallic PtCo layers with bare and graphene-covered ZnO(0001) supports

NASA Astrophysics Data System (ADS)

Luo, Wen; Mélart, Christophe; Rach, Alain; Sutter, Christophe; Zafeiratos, Spyridon

2018-03-01

PtCo bimetallic overlayers supported on bare and graphene covered ZnO(0001) substrates have been successfully prepared and used to investigate the effect of graphene interlayer on the arrangement and the redox behaviour of PtCo. We found that Co is readily oxidized at the PtCo/ZnO interface during annealing in ultra-high vacuum (UHV) and low pressure O2 atmosphere, while after inserting a layer of graphene in-between, the oxidation of Co is restricted. In addition, the reduction of Co oxides by H2 is more pronounced when PtCo is supported on graphene covered ZnO. Apart from the cobalt oxidation state, graphene insertion at the interface also influences the PtCo arrangement by favouring their intermixing. Raman spectra show that low intensity defects are introduced into graphene layer after the deposition of PtCo and are enhanced by high temperature annealing. This study highlights the prospect of using graphene to tune the interaction between alloys and oxide supports which finds potential applications in catalysis.

Oxide Structure Dependence of SiO2/SiOx/3C-SiC/n-Type Si Nonvolatile Resistive Memory on Memory Operation Characteristics

NASA Astrophysics Data System (ADS)

Yamaguchi, Yuichiro; Shouji, Masatsugu; Suda, Yoshiyuki

2012-11-01

We have investigated the dependence of the oxide layer structure of our previously proposed metal/SiO2/SiOx/3C-SiC/n-Si/metal metal-insulator-semiconductor (MIS) resistive memory device on the memory operation characteristics. The current-voltage (I-V) measurement and X-ray photoemission spectroscopy results suggest that SiOx defect states mainly caused by the oxidation of 3C-SiC at temperatures below 1000 °C are related to the hysteresis memory behavior in the I-V curve. By restricting the SiOx interface region, the number of switching cycles and the on/off current ratio are more enhanced. Compared with a memory device formed by one-step or two-step oxidation of 3C-SiC, a memory device formed by one-step oxidation of Si/3C-SiC exhibits a more restrictive SiOx interface with a more definitive SiO2 layer and higher memory performances for both the endurance switching cycle and on/off current ratio.

Capacitance-voltage characteristics of sub-nanometric Al2O3 / TiO2 laminates: dielectric and interface charge densities.

PubMed

Kahouli, Abdelkader; Elbahri, Marwa Ben; Lebedev, Oleg; Lüders, Ulrike

2017-07-12

Advanced amorphous sub-nanometric laminates based on TiO 2 and Al 2 O 3 were deposited by atomic layer deposition at low temperature. Low densities of 'slow' and 'fast' interface states are achieved with values of 3.96 · 10 10 cm -2 and 4.85 · 10 -9 eV -1 cm -2 , respectively, by using a 40 nm laminate constituted of 0.7 nm TiO 2 and 0.8 nm Al 2 O 3 . The sub-nanometric laminate shows a low hysteresis width of 20 mV due to the low oxide charge density of about 3.72 · 10 11 cm -2 . Interestingly, such properties are required for stable and reliable performance of MOS capacitors and transistor operation. Thus, decreasing the individual layer thickness to the sub-nanometric range and combining two dielectric materials with oppositely charged defects may play a major role in the electrical response, highly promising for the application in future micro and nano-electronics applications.

High-Performance Ultrathin Organic-Inorganic Hybrid Silicon Solar Cells via Solution-Processed Interface Modification.

PubMed

Zhang, Jie; Zhang, Yinan; Song, Tao; Shen, Xinlei; Yu, Xuegong; Lee, Shuit-Tong; Sun, Baoquan; Jia, Baohua

2017-07-05

Organic-inorganic hybrid solar cells based on n-type crystalline silicon and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) exhibited promising efficiency along with a low-cost fabrication process. In this work, ultrathin flexible silicon substrates, with a thickness as low as tens of micrometers, were employed to fabricate hybrid solar cells to reduce the use of silicon materials. To improve the light-trapping ability, nanostructures were built on the thin silicon substrates by a metal-assisted chemical etching method (MACE). However, nanostructured silicon resulted in a large amount of surface-defect states, causing detrimental charge recombination. Here, the surface was smoothed by solution-processed chemical treatment to reduce the surface/volume ratio of nanostructured silicon. Surface-charge recombination was dramatically suppressed after surface modification with a chemical, associated with improved minority charge-carrier lifetime. As a result, a power conversion efficiency of 9.1% was achieved in the flexible hybrid silicon solar cells, with a substrate thickness as low as ∼14 μm, indicating that interface engineering was essential to improve the hybrid junction quality and photovoltaic characteristics of the hybrid devices.

Arresting relaxation in Pickering Emulsions

NASA Astrophysics Data System (ADS)

Atherton, Tim; Burke, Chris

2015-03-01

Pickering emulsions consist of droplets of one fluid dispersed in a host fluid and stabilized by colloidal particles absorbed at the fluid-fluid interface. Everyday materials such as crude oil and food products like salad dressing are examples of these materials. Particles can stabilize non spherical droplet shapes in these emulsions through the following sequence: first, an isolated droplet is deformed, e.g. by an electric field, increasing the surface area above the equilibrium value; additional particles are then adsorbed to the interface reducing the surface tension. The droplet is then allowed to relax toward a sphere. If more particles were adsorbed than can be accommodated by the surface area of the spherical ground state, relaxation of the droplet is arrested at some non-spherical shape. Because the energetic cost of removing adsorbed colloids exceeds the interfacial driving force, these configurations can remain stable over long timescales. In this presentation, we present a computational study of the ordering present in anisotropic droplets produced through the mechanism of arrested relaxation and discuss the interplay between the geometry of the droplet, the dynamical process that produced it, and the structure of the defects observed.

Enhanced red photoluminescence of quartz by silicon nanocrystals thin film deposition

NASA Astrophysics Data System (ADS)

Momeni, A.; Pourgolestani, M.; Taheri, M.; Mansour, N.

2018-03-01

The room-temperature photoluminescence properties of silicon nanocrystals (SiNCs) thin film on a quartz substrate were investigated, which presents the red emission enhancement of quartz. We show that the photoluminescence intensity of quartz, in the wavelength range of 640-700 nm, can be enhanced as much as 15-fold in the presence of the SiNCs thin film. Our results reveal that the defect states at the SiNCs/SiO2 interface can be excited more efficiently by indirect excitation via the SiNCs, leading to the prominent red photoluminescence enhancement under the photo-excitation in the range of 440-470 nm. This work suggests a simple pathway to improve silicon-based light emitting devices for photonic applications.

Computational Study on Atomic Structures, Electronic Properties, and Chemical Reactions at Surfaces and Interfaces and in Biomaterials

NASA Astrophysics Data System (ADS)

Takano, Yu; Kobayashi, Nobuhiko; Morikawa, Yoshitada

2018-06-01

Through computer simulations using atomistic models, it is becoming possible to calculate the atomic structures of localized defects or dopants in semiconductors, chemically active sites in heterogeneous catalysts, nanoscale structures, and active sites in biological systems precisely. Furthermore, it is also possible to clarify physical and chemical properties possessed by these nanoscale structures such as electronic states, electronic and atomic transport properties, optical properties, and chemical reactivity. It is sometimes quite difficult to clarify these nanoscale structure-function relations experimentally and, therefore, accurate computational studies are indispensable in materials science. In this paper, we review recent studies on the relation between local structures and functions for inorganic, organic, and biological systems by using atomistic computer simulations.

Epitaxial Zn quantum dots coherently grown on Si(1 1 1): growth mechanism, nonlinear optical and chemical states analyses

NASA Astrophysics Data System (ADS)

Huang, Bo-Jia; Kao, Li-Chi; Brahma, Sanjaya; Jeng, Yu-En; Chiu, Shang-Jui; Ku, Ching-Shun; Lo, Kuang-Yao

2017-05-01

Oxide- and defect-free metal/semiconductor interface is important to improve Ohmic contact for the suppression of electron scattering and the avoidance of an extrinsic surface state in estimating the barrier of the Schottky contact at the nanodevice interface. This study reports the growth mechanism of Zn quantum dots coherently grown on Si(1 1 1) and the physical phenomena of the crystalline, nonlinear optics, and the chemical states of Zn quantum dots. Epitaxial Zn quantum dots were coherently formed on a non-oxide Si(1 1 1) surface through the liquid- to solid-phase transformation as a result of pattern matching between the Zn(0 0 2) and Si(1 1 1) surfaces. The growth mechanism of constrained Zn quantum dots grown through strategic magnetron radio frequency sputtering is complex. Some factors, such as substrate temperature, hydrogen gas flow, and negative DC bias, influence the configuration of epitaxial Zn quantum dots. In particular, hydrogen gas plays an important role in reducing the ZnO+ and native oxide that is bombarded by accelerated ions, thereby enhancing the Zn ion surface diffusion. The reduction reaction can be inspected by distinguishing the chemical states of ZnO/Zn quantum dots from natural oxidation or the states of Zn 3d through the analysis of x-ray absorption near the edge structure spectrum. The complex growth mechanism can be systematically understood by analyzing a noncancelled anisotropic 3 m dipole from reflective second harmonic generation and inspecting the evolution between the Zn(0 0 2) and Zn(1 1 1) peaks of the collective ZnO/Zn quantum dots in synchrotron XRD.

Self-regulation of charged defect compensation and formation energy pinning in semiconductors

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yang, Ji -Hui; Yin, Wan -Jian; Park, Ji -Sang

2015-11-20

Current theoretical analyses of defect properties without solving the detailed balance equations often estimate Fermi-level pinning position by omitting free carriers and assume defect concentrations can be always tuned by atomic chemical potentials. This could be misleading in some circumstance. Here we clarify that: (1) Because the Fermi-level pinning is determined not only by defect states but also by free carriers from band-edge states, band-edge states should be treated explicitly in the same footing as the defect states in practice; (2) defect formation energy, thus defect density, could be pinned and independent on atomic chemical potentials due to the entanglementmore » of atomic chemical potentials and Fermi energy, in contrast to the usual expectation that defect formation energy can always be tuned by varying the atomic chemical potentials; and (3) the charged defect compensation behavior, i.e., most of donors are compensated by acceptors or vice versa, is self-regulated when defect formation energies are pinned. The last two phenomena are more dominant in wide-gap semiconductors or when the defect formation energies are small. Using NaCl and CH 3NH 3PbI 3 as examples, we illustrate these unexpected behaviors. Furthermore, our analysis thus provides new insights that enrich the understanding of the defect physics in semiconductors and insulators.« less

Effects of fO2, fH2O and aoxide on formation and density of extended planar defects in olivine

NASA Astrophysics Data System (ADS)

Burgess, K.; Cooper, R. F.

2011-12-01

Melt inclusions are used in geochemistry to inform our understanding of many physiochemical processes taking place in the mantle, such as melting, melt-rock interactions and magma mixing. Fundamental to this interpretation of melt inclusions is the assumption that they act as closed systems, i.e., they are chemically isolated after trapping and preserve primitive magma compositions. However, recent work indicates that volatiles (e.g., H and F) can be rapidly reset [Portnyagin et al., 2008], and the diffusion mechanisms and rates in tracer diffusion experiments, specifically of REEs, are a matter of some debate [Spandler and O'Neill, 2010; Cherniak, 2010]. The compendium of observations and experiments suggests a role of planar extended defects in effecting and affecting diffusion kinetics in olivine. Planar extended defects are the exothermic condensation of charged point defects into two-dimensional structures, their third dimension insufficient (i.e., sub-unit cell) to describe them as a unique phase. These planar defects, in a manner similar to mechanisms of "pipe" diffusion along dislocations and of grain boundary diffusion, can lead to measured diffusivities far greater than the lattice diffusivity, and their overall effect on flux is proportional to their spatial density [cf. Hart, 1957]. High-resolution TEM and AEM investigation of experimental olivine-basalt samples show the presence of planar defects near the olivine-melt interface, with the area fraction of the high-contrast defects in the images being greatest at high fO2 and/or fH2O while temperature has an effect on the defect dimensions but not total areal density. EDS analysis of the interface regions indicate high Ti/Ca and Ti/Al ratios compared to the glass; the stability of intercalated humite-type defects in olivine, a planar defect type found in some natural olivines [e.g., Risold et al., 2001; Hermann et al., 2007], is increased to higher temperature by the incorporation of Ti. Activities of oxides clearly affect the presence and density of the defects. Olivine-ilmenite experiments were also carried out in varying fO2 and fH2O conditions. Thermodynamic calculations for concentrations of point defects, defect association(s) and defect condensation in olivine can relate experimental data for measured diffusivities to discerning natural conditions where condensed-defect, fast-path diffusion in olivine could be significant. Planar extended defects can potentially play a role in the kinetics of deformation of olivine in the mantle, particularly as the condensation reaction lowers the activity of mobile point defects. Cherniak, Am. Mineral. 95 (2010) 362-368. Hart, Acta Met. 5 (1957) 597. Hermann et al., Contrib. Mineral. Petrol. 153 (2007) 417-428. Portnyagin et al., Earth Planet. Sci.Lett. 272 (2008) 541-552. Risold et al., Contrib. Mineral. Petrol. 142 (2001) 619-628. Spandler and O'Neill, Contrib. Mineral. Petrol. 159 (2010) 791-818.

Photoluminescence Dynamics of Aryl sp 3 Defect States in Single-Walled Carbon Nanotubes

DOE PAGES

Hartmann, Nicolai F.; Velizhanin, Kirill A.; Haroz, Erik H.; ...

2016-08-16

Photoluminescent defect states introduced by sp 3 functionalization of semiconducting carbon nanotubes are rapidly emerging as important routes for boosting emission quantum yields and introducing new functionality. Knowledge of the relaxation dynamics of these states is required for understanding how functionalizing agents (molecular dopants) may be designed to access specific behaviors. We measure photoluminescence (PL) decay dynamics of sp 3 defect states introduced by aryl functionalization of the carbon nanotube surface. Results are given for five different nanotube chiralities, each doped with a range of aryl functionality. We find the PL decays of these sp 3 defect states are biexponential,more » with both components relaxing on timescales of ~ 100 ps. Exciton trapping at defects is found to increases PL lifetimes by a factor of 5-10, in comparison to those for the free exciton. A significant chirality dependence is observed in the decay times, ranging from 77 ps for (7,5) nanotubes to > 600 ps for (5,4) structures. The strong correlation of time constants with emission energy indicates relaxation occurs via multiphonon decay processes, with close agreement to theoretical expectations. Variation of the aryl dopant further modulates decay times by 10-15%. The aryl defects also affect PL lifetimes of the free E 11 exciton. Shortening of the E 11 bright state lifetime as defect density increases provides further confirmation that defects act as exciton traps. A similar shortening of the E11 dark exciton lifetime is found as defect density increases, providing strong experimental evidence that dark excitons are also trapped at such defect sites.« less

Photoluminescence Dynamics of Aryl sp 3 Defect States in Single-Walled Carbon Nanotubes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hartmann, Nicolai F.; Velizhanin, Kirill A.; Haroz, Erik H.

Photoluminescent defect states introduced by sp 3 functionalization of semiconducting carbon nanotubes are rapidly emerging as important routes for boosting emission quantum yields and introducing new functionality. Knowledge of the relaxation dynamics of these states is required for understanding how functionalizing agents (molecular dopants) may be designed to access specific behaviors. We measure photoluminescence (PL) decay dynamics of sp 3 defect states introduced by aryl functionalization of the carbon nanotube surface. Results are given for five different nanotube chiralities, each doped with a range of aryl functionality. We find the PL decays of these sp 3 defect states are biexponential,more » with both components relaxing on timescales of ~ 100 ps. Exciton trapping at defects is found to increases PL lifetimes by a factor of 5-10, in comparison to those for the free exciton. A significant chirality dependence is observed in the decay times, ranging from 77 ps for (7,5) nanotubes to > 600 ps for (5,4) structures. The strong correlation of time constants with emission energy indicates relaxation occurs via multiphonon decay processes, with close agreement to theoretical expectations. Variation of the aryl dopant further modulates decay times by 10-15%. The aryl defects also affect PL lifetimes of the free E 11 exciton. Shortening of the E 11 bright state lifetime as defect density increases provides further confirmation that defects act as exciton traps. A similar shortening of the E11 dark exciton lifetime is found as defect density increases, providing strong experimental evidence that dark excitons are also trapped at such defect sites.« less

Understanding and Calibrating Density-Functional-Theory Calculations Describing the Energy and Spectroscopy of Defect Sites in Hexagonal Boron Nitride.

PubMed

Reimers, Jeffrey R; Sajid, A; Kobayashi, Rika; Ford, Michael J

2018-03-13

Defect states in 2-D materials present many possible uses but both experimental and computational characterization of their spectroscopic properties is difficult. We provide and compare results from 13 DFT and ab initio computational methods for up to 25 excited states of a paradigm system, the V N C B defect in hexagonal boron nitride (h-BN). Studied include: (i) potentially catastrophic effects for computational methods arising from the multireference nature of the closed-shell and open-shell states of the defect, which intrinsically involves broken chemical bonds, (ii) differing results from DFT and time-dependent DFT (TDDFT) calculations, (iii) comparison of cluster models to periodic-slab models of the defect, (iv) the starkly differing effects of nuclear relaxation on the various electronic states that control the widths of photoabsorption and photoemission spectra as broken bonds try to heal, (v) the effect of zero-point energy and entropy on free-energy differences, (vi) defect-localized and conduction/valence-band transition natures, and (vii) strategies needed to ensure that the lowest-energy state of a defect can be computationally identified. Averaged state-energy differences of 0.3 eV are found between CCSD(T) and MRCI energies, with thermal effects on free energies sometimes also being of this order. However, DFT-based methods can perform very poorly. Simple generalized-gradient functionals like PBE fail at the most basic level and should never be applied to defect states. Hybrid functionals like HSE06 work very well for excitations within the triplet manifold of the defect, with an accuracy equivalent to or perhaps exceeding the accuracy of the ab initio methods used. However, HSE06 underestimates triplet-state energies by on average of 0.7 eV compared to closed-shell singlet states, while open-shell singlet states are predicted to be too low in energy by 1.0 eV. This leads to misassignment of the ground state of the V N C B defect. Long-range corrected functionals like CAM-B3LYP are shown to work much better and to represent the current entry level for DFT calculations on defects. As significant differences between cluster and periodic-slab models are also found, the widespread implementation of such functionals in periodic codes is in urgent need.

Enhanced direct-gap light emission from Si-capped n+-Ge epitaxial layers on Si after post-growth rapid cyclic annealing: impact of non-radiative interface recombination toward Ge/Si double heterostructure lasers.

PubMed

Higashitarumizu, Naoki; Ishikawa, Yasuhiko

2017-09-04

Enhanced direct-gap light emission is reported for Si-capped n + -Ge layers on Si after post-growth rapid cyclic annealing (RCA), and impact of non-radiative recombination (NRR) at the Ge/Si interface is discussed toward Ge/Si double heterostructure (DH) lasers. P-doped n + -Ge layer (1 × 10 19 cm -3 , 400 nm) is grown on Si by ultra-high vacuum chemical vapor deposition, followed by a growth of Si capping layer (5 nm) to form a Si/Ge/Si DH structure. Post-growth RCA to eliminate defects in Ge is performed in N 2 at temperatures between 900°C and 780°C, where the annealing time is minimized to be 5 s in each RCA cycle to prevent an out-diffusion of P dopants from the Ge surface. Direct-gap photoluminescence (PL) intensity at 1.6 µm increases with the RCA cycles up to 40, although the threading dislocation density in Ge is not reduced after 3 cycles in the present condition. The PL enhancement is ascribed to the suppression of NRR at the Ge/Si interface, where an intermixed SiGe alloy is formed. For Ge/Si DH lasers, NRR at the Ge/Si interface is found to have a significant impact on the threshold current density Jth. In order to achieve Jth on the order of 1 kA/cm 2 , similar to III-V lasers, the interface recombination velocity S is required below 10 3 cm/s in spite of S as large as 10 5 cm/s at the ordinary defect-rich Ge/Si interface.

Role of Sink Density in Nonequilibrium Chemical Redistribution in Alloys

DOE PAGES

Martinez, Enrique Saez; Senninger, Oriane; Caro, Alfredo; ...

2018-03-08

Nonequilibrium chemical redistribution in open systems submitted to external forces, such as particle irradiation, leads to changes in the structural properties of the material, potentially driving the system to failure. Such redistribution is controlled by the complex interplay between the production of point defects, atomic transport rates, and the sink character of the microstructure. In this work, we analyze this interplay by means of a kinetic Monte Carlo (KMC) framework with an underlying atomistic model for the Fe-Cr model alloy to study the effect of ideal defect sinks on Cr concentration profiles, with a particular focus on the role ofmore » interface density. We observe that the amount of segregation decreases linearly with decreasing interface spacing. Within the framework of the thermodynamics of irreversible processes, a general analytical model is derived and assessed against the KMC simulations to elucidate the structure-property relationship of this system. Interestingly, in the kinetic regime where elimination of point defects at sinks is dominant over bulk recombination, the solute segregation does not directly depend on the dose rate but only on the density of sinks. Furthermore, this model provides new insight into the design of microstructures that mitigate chemical redistribution and improve radiation tolerance.« less

Role of Sink Density in Nonequilibrium Chemical Redistribution in Alloys

DOE Office of Scientific and Technical Information (OSTI.GOV)

Martinez, Enrique Saez; Senninger, Oriane; Caro, Alfredo

Nonequilibrium chemical redistribution in open systems submitted to external forces, such as particle irradiation, leads to changes in the structural properties of the material, potentially driving the system to failure. Such redistribution is controlled by the complex interplay between the production of point defects, atomic transport rates, and the sink character of the microstructure. In this work, we analyze this interplay by means of a kinetic Monte Carlo (KMC) framework with an underlying atomistic model for the Fe-Cr model alloy to study the effect of ideal defect sinks on Cr concentration profiles, with a particular focus on the role ofmore » interface density. We observe that the amount of segregation decreases linearly with decreasing interface spacing. Within the framework of the thermodynamics of irreversible processes, a general analytical model is derived and assessed against the KMC simulations to elucidate the structure-property relationship of this system. Interestingly, in the kinetic regime where elimination of point defects at sinks is dominant over bulk recombination, the solute segregation does not directly depend on the dose rate but only on the density of sinks. Furthermore, this model provides new insight into the design of microstructures that mitigate chemical redistribution and improve radiation tolerance.« less

Probing and Manipulating the Interfacial Defects of InGaAs Dual-Layer Metal Oxides at the Atomic Scale.

PubMed

Wu, Xing; Luo, Chen; Hao, Peng; Sun, Tao; Wang, Runsheng; Wang, Chaolun; Hu, Zhigao; Li, Yawei; Zhang, Jian; Bersuker, Gennadi; Sun, Litao; Pey, Kinleong

2018-01-01

The interface between III-V and metal-oxide-semiconductor materials plays a central role in the operation of high-speed electronic devices, such as transistors and light-emitting diodes. The high-speed property gives the light-emitting diodes a high response speed and low dark current, and they are widely used in communications, infrared remote sensing, optical detection, and other fields. The rational design of high-performance devices requires a detailed understanding of the electronic structure at this interface; however, this understanding remains a challenge, given the complex nature of surface interactions and the dynamic relationship between the morphology evolution and electronic structures. Herein, in situ transmission electron microscopy is used to probe and manipulate the structural and electrical properties of ZrO 2 films on Al 2 O 3 and InGaAs substrate at the atomic scale. Interfacial defects resulting from the spillover of the oxygen-atom conduction-band wavefunctions are resolved. This study unearths the fundamental defect-driven interfacial electric structure of III-V semiconductor materials and paves the way to future high-speed and high-reliability devices. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Role of Sink Density in Nonequilibrium Chemical Redistribution in Alloys

NASA Astrophysics Data System (ADS)

Martínez, Enrique; Senninger, Oriane; Caro, Alfredo; Soisson, Frédéric; Nastar, Maylise; Uberuaga, Blas P.

2018-03-01

Nonequilibrium chemical redistribution in open systems submitted to external forces, such as particle irradiation, leads to changes in the structural properties of the material, potentially driving the system to failure. Such redistribution is controlled by the complex interplay between the production of point defects, atomic transport rates, and the sink character of the microstructure. In this work, we analyze this interplay by means of a kinetic Monte Carlo (KMC) framework with an underlying atomistic model for the Fe-Cr model alloy to study the effect of ideal defect sinks on Cr concentration profiles, with a particular focus on the role of interface density. We observe that the amount of segregation decreases linearly with decreasing interface spacing. Within the framework of the thermodynamics of irreversible processes, a general analytical model is derived and assessed against the KMC simulations to elucidate the structure-property relationship of this system. Interestingly, in the kinetic regime where elimination of point defects at sinks is dominant over bulk recombination, the solute segregation does not directly depend on the dose rate but only on the density of sinks. This model provides new insight into the design of microstructures that mitigate chemical redistribution and improve radiation tolerance.

Novel highly ordered core–shell nanoparticles

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dey, Sonal; Hossain, Mohammad D.; Mayanovic, Robert A.

2016-10-26

Core–shell nanoparticles have potential for a wide range of applications due to the tunability of their magnetic, catalytic, electronic, optical, and other physicochemical properties. A frequent drawback in the design of core–shell nanoparticles and nanocrystals is the lack of control over an extensive, disordered, and compositionally distinct interface that occurs due to the dissimilarity of structural and compositional phases of the core and shell. In this work, we demonstrate a new hydrothermal nanophase epitaxy (HNE) technique to synthesize highly structurally ordered α-Cr 2O 3@α-Co 0.38Cr 1.62O 2.92 inverted core–shell nanoparticles (CSNs) with evidence for the nanoscale growth of corundum structuremore » beginning from the core and extending completely into the shell of the CSNs with minimal defects at the interface. The high-resolution TEM results show a sharp interface exhibiting epitaxial atomic registry of shell atoms over highly ordered core atoms. The XPS and Co K-edge XANES analyses indicate the +2 oxidation state of cobalt is incorporated in the shell of the CSNs. Our XPS and EXAFS results are consistent with oxygen vacancy formation in order to maintain charge neutrality upon substitution of the Co 2+ ion for the Cr 3+ ion in the α-Co 0.38Cr 1.62O 2.92 shell. Furthermore, the CSNs exhibit the magnetic exchange bias effect, which is attributed to the exchange anisotropy at the interface made possible by the nanophase epitaxial growth of the α-Co 0.38Cr 1.62O 2.92 shell on the α-Cr 2O 3 core of the nanoparticles. The combination of a well-structured, sharp interface and novel nanophase characteristics is highly desirable for nanostructures having enhanced magnetic properties.« less

Investigation of the interface characteristics of Y2O3/GaAs under biaxial strain, triaxial strain, and non-strain conditions

NASA Astrophysics Data System (ADS)

Shi, Li-Bin; Liu, Xu-Yang; Dong, Hai-Kuan

2016-09-01

We investigate the interface behaviors of Y2O3/GaAs under biaxial strain, triaxial strain, and non-strain conditions. This study is performed by first principles calculations based on density functional theory (DFT). First of all, the biaxial strain is realized by changing the lattice constants in ab plane. Averaged electrostatic potential (AEP) is aligned by establishing Y2O3 and GaAs (110) surfaces. The band offsets of Y2O3/GaAs interface under biaxial strain are investigated by generalized gradient approximation and Heyd-Scuseria-Ernzerhof (HSE) functionals. The interface under biaxial strain is suitable for the design of metal oxide semiconductor (MOS) devices because the valence band offsets (VBO) and conduction band offsets (CBO) are larger than 1 eV. Second, the triaxial strain is applied to Y2O3/GaAs interface by synchronously changing the lattice constants in a, b, and c axis. The band gaps of Y2O3 and GaAs under triaxial strain are investigated by HSE functional. We compare the VBO and CBO under triaxial strain with those under biaxial strain. Third, in the absence of lattice strain, the formation energies, charge state switching levels, and migration barriers of native defects in Y2O3 are assessed. We investigate how they will affect the MOS device performance. It is found that VO+2 and Oi-2 play a very dangerous role in MOS devices. Finally, a direct tunneling leakage current model is established. The model is used to analyze current and voltage characteristics of the metal/Y2O3/GaAs.

Agrichemicals in surface water and birth defects in the United States

PubMed Central

Winchester, Paul D; Huskins, Jordan; Ying, Jun

2009-01-01

Objectives: To investigate if live births conceived in months when surface water agrichemicals are highest are at greater risk for birth defects. Methods: Monthly concentrations during 1996–2002 of nitrates, atrazine and other pesticides were calculated using United States Geological Survey's National Water Quality Assessment data. Monthly United States birth defect rates were calculated for live births from 1996 to 2002 using United States Centers for Disease Control and Prevention natality data sets. Birth defect rates by month of last menstrual period (LMP) were then compared to pesticide/nitrate means using logistical regression models. Results: Mean concentrations of agrichemicals were highest in April–July. Total birth defects, and eleven of 22 birth defect subcategories, were more likely to occur in live births with LMPs between April and July. A significant association was found between the season of elevated agrichemicals and birth defects. Conclusion: Elevated concentrations of agrichemicals in surface water in April–July coincided with higher risk of birth defects in live births with LMPs April–July. While a causal link between agrichemicals and birth defects cannot be proven from this study an association might provide clues to common factors shared by both variables. PMID:19183116

Annihilating nanoscale defects | Argonne National Laboratory

Science.gov Websites

molecules must follow to find defect-free states and designed a process that delivers industry-standard predict the path molecules must follow to find defect-free states and designed a process that delivers deeper than others. The system prefers defect-free stability, which can be characterized by the deepest

Hybrid deposition of thin film solid oxide fuel cells and electrolyzers

DOEpatents

Jankowski, A.F.; Makowiecki, D.M.; Rambach, G.D.; Randich, E.

1998-05-19

The use of vapor deposition techniques enables synthesis of the basic components of a solid oxide fuel cell (SOFC); namely, the electrolyte layer, the two electrodes, and the electrolyte-electrode interfaces. Such vapor deposition techniques provide solutions to each of the three critical steps of material synthesis to produce a thin film solid oxide fuel cell (TFSOFC). The electrolyte is formed by reactive deposition of essentially any ion conducting oxide, such as defect free, yttria stabilized zirconia (YSZ) by planar magnetron sputtering. The electrodes are formed from ceramic powders sputter coated with an appropriate metal and sintered to a porous compact. The electrolyte-electrode interface is formed by chemical vapor deposition of zirconia compounds onto the porous electrodes to provide a dense, smooth surface on which to continue the growth of the defect-free electrolyte, whereby a single fuel cell or multiple cells may be fabricated. 8 figs.

Atomic Origins of the Self-Healing Function in Cement–Polymer Composites

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nguyen, Manh-Thuong; Wang, Zheming; Rod, Kenton A.

Motivated by recent advances in self-healing cement and epoxy polymer composites, we present a combined ab initio molecular dynamics and sum frequency generation (SFG) spectroscopy study of a calcium-silicate-hydrate/polymer interface. On stable, low-defect surfaces, the polymer only weakly adheres through coordination and hydrogen bonding interactions and can be easily mobilized towards defected surfaces. Conversely, on fractured surfaces, the polymer strongly anchors through ionic Ca-O bonds resulting from the deprotonation of polymer hydroxyl groups. In addition, polymer S-S groups are turned away from the cement/polymer interface, allowing for the self-healing function within the polymer. The overall elasticity and healing properties ofmore » these composites stem from a flexible hydrogen bonding network that can readily adapt to surface morphology. The theoretical vibrational signals associated with the proposed cement-polymer interfacial chemistry were confirmed experimentally by SFG spectroscopy.« less

Engineering human immunodeficiency virus 1 protease heterodimers as macromolecular inhibitors of viral maturation.

PubMed Central

McPhee, F; Good, A C; Kuntz, I D; Craik, C S

1996-01-01

Dimerization of human immunodeficiency virus type 1 protease (HIV-1 PR) monomers is an essential prerequisite for viral proteolytic activity and the subsequent generation of infectious virus particles. Disruption of the dimer interface inhibits this activity as does formation of heterodimers between wild-type and defective monomers. A structure-based approach was used to identify amino acid substitutions at the dimer interface of HIV-1 PR that facilitate preferential association of heterodimers and inhibit self-association of the defective monomers. Expression of the designed PR monomers inhibits activity of wild-type HIV-1 PR and viral infectivity when assayed in an ex vivo model system. These results show that it is possible to design PR monomers as macromolecular inhibitors that may provide an alternative to small molecule inhibitors for the treatment of HIV infection. Images Fig. 1 Fig. 2 Fig. 3 Fig. 4 PMID:8876160

Direct observation of electron emission and recombination processes by time domain measurements of charge pumping current

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hori, Masahiro, E-mail: hori@eng.u-toyama.ac.jp; Watanabe, Tokinobu; Ono, Yukinori

2015-01-26

To analyze the charge pumping (CP) sequence in detail, the source/drain electron current and the substrate hole current under the CP mode of transistors are simultaneously monitored in the time domain. Peaks are observed in both the electron and hole currents, which are, respectively, attributed to the electron emission from the interface defects and to the recombination with holes. The peak caused by the electron emission is found to consist of two components, strongly suggesting that the present time-domain measurement can enable us to resolve different kinds of interface defects. Investigating the correlation between the number of emitted and recombinedmore » electrons reveals that only one of the two components contributes to the CP current for the gate-pulse fall time from 6.25 × 10{sup −4} to 1.25 × 10{sup −2} s.« less

Atomic Origins of the Self-Healing Function in Cement-Polymer Composites.

PubMed

Nguyen, Manh-Thuong; Wang, Zheming; Rod, Kenton A; Childers, M Ian; Fernandez, Carlos; Koech, Phillip K; Bennett, Wendy D; Rousseau, Roger; Glezakou, Vassiliki-Alexandra

2018-01-24

Motivated by recent advances in self-healing cement and epoxy polymer composites, we present a combined ab initio molecular dynamics and sum frequency generation (SFG) vibrational spectroscopy study of a calcium-silicate-hydrate/polymer interface. On stable, low-defect surfaces, the polymer only weakly adheres through coordination and hydrogen bonding interactions and can be easily mobilized toward defected surfaces. Conversely, on fractured surfaces, the polymer strongly anchors through ionic Ca-O bonds resulting from the deprotonation of polymer hydroxyl groups. In addition, polymer S-S groups are turned away from the cement-polymer interface, allowing for the self-healing function within the polymer. The overall elasticity and healing properties of these composites stem from a flexible hydrogen bonding network that can readily adapt to surface morphology. The theoretical vibrational signals associated with the proposed cement-polymer interfacial chemistry were confirmed experimentally by SFG vibrational spectroscopy.

Hybrid deposition of thin film solid oxide fuel cells and electrolyzers

DOEpatents

Jankowski, Alan F.; Makowiecki, Daniel M.; Rambach, Glenn D.; Randich, Erik

1999-01-01

The use of vapor deposition techniques enables synthesis of the basic components of a solid oxide fuel cell (SOFC); namely, the electrolyte layer, the two electrodes, and the electrolyte-electrode interfaces. Such vapor deposition techniques provide solutions to each of the three critical steps of material synthesis to produce a thin film solid oxide fuel cell (TFSOFC). The electrolyte is formed by reactive deposition of essentially any ion conducting oxide, such as defect free, yttria stabilized zirconia (YSZ) by planar magnetron sputtering. The electrodes are formed from ceramic powders sputter coated with an appropriate metal and sintered to a porous compact. The electrolyte-electrode interface is formed by chemical vapor deposition of zirconia compounds onto the porous electrodes to provide a dense, smooth surface on which to continue the growth of the defect-free electrolyte, whereby a single fuel cell or multiple cells may be fabricated.

Hybrid deposition of thin film solid oxide fuel cells and electrolyzers

DOEpatents

Jankowski, Alan F.; Makowiecki, Daniel M.; Rambach, Glenn D.; Randich, Erik

1998-01-01

The use of vapor deposition techniques enables synthesis of the basic components of a solid oxide fuel cell (SOFC); namely, the electrolyte layer, the two electrodes, and the electrolyte-electrode interfaces. Such vapor deposition techniques provide solutions to each of the three critical steps of material synthesis to produce a thin film solid oxide fuel cell (TFSOFC). The electrolyte is formed by reactive deposition of essentially any ion conducting oxide, such as defect free, yttria stabilized zirconia (YSZ) by planar magnetron sputtering. The electrodes are formed from ceramic powders sputter coated with an appropriate metal and sintered to a porous compact. The electrolyte-electrode interface is formed by chemical vapor deposition of zirconia compounds onto the porous electrodes to provide a dense, smooth surface on which to continue the growth of the defect-free electrolyte, whereby a single fuel cell or multiple cells may be fabricated.

Stability of boron-doped graphene/copper interface: DFT, XPS and OSEE studies

NASA Astrophysics Data System (ADS)

Boukhvalov, D. W.; Zhidkov, I. S.; Kukharenko, A. I.; Slesarev, A. I.; Zatsepin, A. F.; Cholakh, S. O.; Kurmaev, E. Z.

2018-05-01

Two different types of boron-doped graphene/copper interfaces synthesized using two different flow rates of Ar through the bubbler containing the boron source were studied. X-ray photoelectron spectra (XPS) and optically stimulated electron emission (OSEE) measurements have demonstrated that boron-doped graphene coating provides a high corrosion resistivity of Cu-substrate with the light traces of the oxidation of carbon cover. The density functional theory calculations suggest that for the case of substitutional (graphitic) boron-defect only the oxidation near boron impurity is energetically favorable and creation of the vacancies that can induce the oxidation of copper substrate is energetically unfavorable. In the case of non-graphitic boron defects oxidation of the area, a nearby impurity is metastable that not only prevent oxidation but makes boron-doped graphene. Modeling of oxygen reduction reaction demonstrates high catalytic performance of these materials.

Geometric Defects in Quantum Hall States

NASA Astrophysics Data System (ADS)

Gromov, Andrey

I will describe a geometric analogue of Laughlin quasiholes in fractional quantum Hall (FQH) states. These ``quasiholes'' are generated by an insertion of quantized fluxes of curvature - which can be modeled by branch points of a certain Riemann surface - and, consequently, are related to genons. Unlike quasiholes, the genons are not excitations, but extrinsic defects. Fusion of genons describes the response of an FQH state to a process that changes (effective) topology of the physical space. These defects are abelian for IQH states and non-abelian for FQH states. I will explain how to calculate an electric charge, geometric spin and adiabatic mutual statistics of the these defects. Leo Kadanoff Fellowship.

Breakdown of the Debye polarization ansatz at protein-water interfaces

NASA Astrophysics Data System (ADS)

Fernández Stigliano, Ariel

2013-06-01

The topographical and physico-chemical complexity of protein-water interfaces scales down to the sub-nanoscale range. At this level of confinement, we demonstrate that the dielectric structure of interfacial water entails a breakdown of the Debye ansatz that postulates the alignment of polarization with the protein electrostatic field. The tendencies to promote anomalous polarization are determined for each residue type and a particular kind of structural defect is shown to provide the predominant causal context.

Defect and interface analyses of non-stoichiometric n-type GaSb thin films grown on Ge(100) substrates by rapid thermal annealing

NASA Astrophysics Data System (ADS)

Nishimoto, Naoki; Fujihara, Junko; Yoshino, Katsumi

2018-05-01

In this study, Ga0.6Sb0.4 thin films were grown on quartz and Ge(100) 1° off-axis substrates by RF magnetron sputtering at 500 °C. Ga0.6Sb0.4/Ge(100) shows n-type conductivity at room temperature (RT) and p-type conductivity at low temperatures, whereas undoped GaSb thin films exhibit p-type conductivity, irrespective of their growth methods and conditions. Their electrical properties were determined by rapid thermal annealing, which revealed that Ga0.6Sb0.4/Ge(100) contains two types of acceptors and two types of donors. The acceptors are considered to be GaSb and electrically active sites on dislocations originating at the Ga0.6Sb0.4/Ge(100) interface, while donors are believed to be Gai and electrically active sites originating at the Ga0.6Sb0.4/Ge(100) interface. In these acceptors and donors, the shallow donor concentration is higher than the shallow acceptor concentration, and the shallow donor level is deeper than the shallow acceptor level. Thus, we concluded that Ga0.6Sb0.4/Ge(100) shows n-type conductivity at RT due to electrically active sites originating at the Ga0.6Sb0.4/Ge(100) interface and native defects originating from excess Ga.

Role of surface defects on the formation of the 2-dimensional electron gas at polar interfaces

NASA Astrophysics Data System (ADS)

Artacho, Emilio; Aguado-Puente, Pablo

2014-03-01

The discovery of a 2-dimensional electron gas (2DEG) at the interface between two insulators, LaAlO3 and SrTiO3, has fuelled a great research activity on this and similar systems in the last years. The electronic reconstruction model, typically invoked to explain the formation of the 2DEG, while being intuitive and successful on predicting fundamental aspects of this phenomenon like the critical thickness of LaAlO3, fails to explain many other experimental observations. Oxygen vacancies, on the other hand, are known to dramatically affect the physical behaviour of this system, but their role at the atomic level is far from well understood. Here we perform ab initio simulations in order to assess whether the formation of oxygen vacancies at the surface of the polar material can account for various recent experimental results that defy the current theoretical understanding of these interfaces. We simulate SrTiO3/LaAlO3 slabs with various concentrations of surface oxygen vacancies and analyze the role of the defects on the formation of the metallic interface, their electrostatic coupling with the 2DEG and the interplay with the different instabilities of the materials involved. Financial support from Spanish MINECO under grant FIS2012-37549-C05-01. Computational resources provided by the Red Espñola de Supercomputación and DIPC.

Design of defect spins in piezoelectric aluminum nitride for solid-state hybrid quantum technologies

DOE PAGES

Seo, Hosung; Govoni, Marco; Galli, Giulia

2016-02-15

Spin defects in wide-band gap semiconductors are promising systems for the realization of quantum bits, or qubits, in solid-state environments. To date, defect qubits have only been realized in materials with strong covalent bonds. Here, we introduce a strain-driven scheme to rationally design defect spins in functional ionic crystals, which may operate as potential qubits. In particular, using a combination of state-of-the-art ab-initio calculations based on hybrid density functional and many-body perturbation theory, we predicted that the negatively charged nitrogen vacancy center in piezoelectric aluminum nitride exhibits spin-triplet ground states under realistic uni- and bi-axial strain conditions; such states maymore » be harnessed for the realization of qubits. As a result, the strain-driven strategy adopted here can be readily extended to a wide range of point defects in other wide-band gap semiconductors, paving the way to controlling the spin properties of defects in ionic systems for potential spintronic technologies.« less

State memory in solution gated epitaxial graphene

NASA Astrophysics Data System (ADS)

Butko, A. V.; Butko, V. Y.; Lebedev, S. P.; Lebedev, A. A.; Davydov, V. Y.; Smirnov, A. N.; Eliseyev, I. A.; Dunaevskiy, M. S.; Kumzerov, Y. A.

2018-06-01

We studied electrical transport in transistors fabricated on a surface of high quality epitaxial graphene with density of defects as low as 5·1010 cm-2 and observed quasistatic hysteresis with a time constant in a scale of hours. This constant is in a few orders of magnitude greater than the constant previously reported in CVD graphene. The hysteresis observed here can be described as a shift of ∼+2V of the Dirac point measured during a gate voltage increase from the position of the Dirac point measured during a gate voltage decrease. This hysteresis can be characterized as a nonvolatile quasistatic state memory effect in which the state of the gated graphene is determined by its initial state prior to entering the hysteretic region. Due to this effect the difference in resistance of the gated graphene measured in the hysteretic region at the same applied voltages can be as high as 70%. The observed effect can be explained by assuming that charge carriers in graphene and oppositely charged molecular ions from the solution form quasistable interfacial complexes at the graphene interface. These complexes likely preserve the initial state by preventing charge carriers in graphene from discharging in the hysteretic region.

Defects, Entropy, and the Stabilization of Alternative Phase Boundary Orientations in Battery Electrode Particles

DOE Office of Scientific and Technical Information (OSTI.GOV)

Heo, Tae Wook; Tang, Ming; Chen, Long-Qing

Using a novel statistical approach that efficiently explores the space of possible defect configurations, our present study investigates the chemomechanical coupling between interfacial structural defects and phase boundary alignments within phase-separating electrode particles. Applied to the battery cathode material Li XFePO 4 as an example, the theoretical analysis reveals that small, defect-induced deviations from an ideal interface can lead to dramatic shifts in the orientations of phase boundaries between Li-rich and Li-lean phases, stabilizing otherwise unfavorable orientations. Significantly, this stabilization arises predominantly from configurational entropic factors associated with the presence of the interfacial defects rather than from absolute energetic considerations.more » The specific entropic factors pertain to the diversity of defect configurations and their contributions to rotational/orientational rigidity of phase boundaries. Comparison of the predictions with experimental observations indicates that the additional entropy contributions indeed play a dominant role under actual cycling conditions, leading to the conclusion that interfacial defects must be considered when analyzing the stability and evolution kinetics of the internal phase microstructure of strongly phase-separating systems. Possible implications for tuning the kinetics of (de)lithiation based on selective defect incorporation are discussed. Ultimately, this understanding can be generalized to the chemomechanics of other defective solid phase boundaries.« less

Defects, Entropy, and the Stabilization of Alternative Phase Boundary Orientations in Battery Electrode Particles

DOE PAGES

Heo, Tae Wook; Tang, Ming; Chen, Long-Qing; ...

2016-01-04

Using a novel statistical approach that efficiently explores the space of possible defect configurations, our present study investigates the chemomechanical coupling between interfacial structural defects and phase boundary alignments within phase-separating electrode particles. Applied to the battery cathode material Li XFePO 4 as an example, the theoretical analysis reveals that small, defect-induced deviations from an ideal interface can lead to dramatic shifts in the orientations of phase boundaries between Li-rich and Li-lean phases, stabilizing otherwise unfavorable orientations. Significantly, this stabilization arises predominantly from configurational entropic factors associated with the presence of the interfacial defects rather than from absolute energetic considerations.more » The specific entropic factors pertain to the diversity of defect configurations and their contributions to rotational/orientational rigidity of phase boundaries. Comparison of the predictions with experimental observations indicates that the additional entropy contributions indeed play a dominant role under actual cycling conditions, leading to the conclusion that interfacial defects must be considered when analyzing the stability and evolution kinetics of the internal phase microstructure of strongly phase-separating systems. Possible implications for tuning the kinetics of (de)lithiation based on selective defect incorporation are discussed. Ultimately, this understanding can be generalized to the chemomechanics of other defective solid phase boundaries.« less

[Biomaterials in bone repair].

PubMed

Puska, Mervi; Aho, Allan J; Vallittu, Pekka K

2013-01-01

In orthopedics, traumatology, and craniofacial surgery, biomaterials should meet the clinical demands of bone that include shape, size and anatomical location of the defect, as well as the physiological load-bearing stresses. Biomaterials are metals, ceramics, plastics or materials of biological origin. In the treatment of large defects, metallic endoprostheses or bone grafts are employed, whereas ceramics in the case of small defects. Plastics are employed on the artificial joint surfaces, in the treatment of vertebral compression fractures, and as biodegradable screws and plates. Porosity, bioactivity, and identical biomechanics to bone are fundamental for achieving a durable, well-bonded, interface between biomaterial and bone. In the case of severe bone treatments, biomaterials should also imply an option to add biologically active substances.

Understanding individual defects in CdTe thin-film solar cells via STEM: From atomic structure to electrical activity

DOE PAGES

Li, Chen; Poplawsky, Jonathan; Yan, Yanfa; ...

2017-07-01

Here in this paper we review a systematic study of the structure-property correlations of a series of defects in CdTe solar cells. A variety of experimental methods, including aberration-corrected scanning transmission electron microscopy, electron energy loss spectroscopy, energy dispersive X-ray spectroscopy, and electron-beam-induced current have been combined with density-functional theory. The research traces the connections between the structures and electrical activities of individual defects including intra-grain partial dislocations, grain boundaries and the CdTe/CdS interface. The interpretations of the physical origin underlying the structure-property correlation provide insights that should further the development of future CdTe solar cells.

Understanding individual defects in CdTe thin-film solar cells via STEM: From atomic structure to electrical activity

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, Chen; Poplawsky, Jonathan; Yan, Yanfa

Here in this paper we review a systematic study of the structure-property correlations of a series of defects in CdTe solar cells. A variety of experimental methods, including aberration-corrected scanning transmission electron microscopy, electron energy loss spectroscopy, energy dispersive X-ray spectroscopy, and electron-beam-induced current have been combined with density-functional theory. The research traces the connections between the structures and electrical activities of individual defects including intra-grain partial dislocations, grain boundaries and the CdTe/CdS interface. The interpretations of the physical origin underlying the structure-property correlation provide insights that should further the development of future CdTe solar cells.

Influence of superconductor film composition on adhesion strength of coated conductors

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kesgin, Ibrahim; Khatri, Narayan; Liu, Yuhao

The effect of high temperature superconductor (HTS) film composition on the adhesion strength of rare- earth barium copper oxide coated conductors (CCs) has been studied. It has been found that the mechanical integrity of the superconductor layer is very susceptible to the defects especially those along the ab plane, probably due to the weak interfaces between the defects and the matrix. Gd and Y in the standard composition were substituted with Sm and the number of in-plane defects was drastically reduced. Consequently, a four-fold increase in adhesion or peeling strength in Sm-based CCs was achieved compared to the standard GdYBCOmore » samples.« less

Study of defects in an electroresistive Au/La2/3Sr1/3MnO3/SrTiO3(001) heterostructure by positron annihilation

NASA Astrophysics Data System (ADS)

Ferragut, R.; Dupaquier, A.; Brivio, S.; Bertacco, R.; Egger, W.

2011-09-01

Defects in an ultrathin Au/La2/3Sr1/3MnO3/SrTiO3 (Au/LSMO/STO) heterostructure displaying electroresistive behavior were studied using variable energy positron annihilation spectroscopy. Vacancy-like defects were found to be the dominant positron traps in the LSMO and STO thin perovskite oxides with a number density >1017 cm-3 and 2 × 1017 cm-3 in the STO substrate. High defect density was revealed by strong positron trapping at the Au/LSMO interface. Oxygen deficiency in LSMO would be the main source of these traps. Besides, a low density of sub-nano voids of ˜6 Å was found in the substrate and in the thin LSMO/STO films.

Carrier quenching in InGaP/GaAs double heterostructures

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wells, Nathan P., E-mail: nathan.p.wells@aero.org; Driskell, Travis U.; Hudson, Andrew I.

2015-08-14

Photoluminescence measurements on a series of GaAs double heterostructures demonstrate a rapid quenching of carriers in the GaAs layer at irradiance levels below 0.1 W/cm{sup 2} in samples with a GaAs-on-InGaP interface. These results indicate the existence of non-radiative defect centers at or near the GaAs-on-InGaP interface, consistent with previous reports showing the intermixing of In and P when free As impinges on the InGaP surface during growth. At low irradiance, these defect centers can lead to sub-ns carrier lifetimes. The defect centers involved in the rapid carrier quenching can be saturated at higher irradiance levels and allow carrier lifetimes tomore » reach hundreds of nanoseconds. To our knowledge, this is the first report of a nearly three orders of magnitude decrease in carrier lifetime at low irradiance in a simple double heterostructure. Carrier quenching occurs at irradiance levels near the integrated Air Mass Zero (AM0) and Air Mass 1.5 (AM1.5) solar irradiance. Additionally, a lower energy photoluminescence band is observed both at room and cryogenic temperatures. The temperature and time dependence of the lower energy luminescence is consistent with the presence of an unintentional InGaAs or InGaAsP quantum well that forms due to compositional mixing at the GaAs-on-InGaP interface. Our results are of general interest to the photovoltaic community as InGaP is commonly used as a window layer in GaAs based solar cells.« less

Photocharge accumulation and recombination in perovskite solar cells regarding device performance and stability

NASA Astrophysics Data System (ADS)

Li, Yusheng; Li, Yiming; Shi, Jiangjian; Li, Hongshi; Zhang, Huiyin; Wu, Jionghua; Li, Dongmei; Luo, Yanhong; Wu, Huijue; Meng, Qingbo

2018-01-01

Photocharge accumulation and recombination in perovskite solar cells have been systematically investigated in this paper by electrochemical spectroscopy and transient photocurrent/photovoltage methods. It is found that the non-equilibrium photocharges stored in the selective charge transport layers follow a backward recombination mechanism. That is, the photocharges are first captured by the interface defects corresponding to the fast photovoltage decay, while the bulk charge recombination instead of the diffusion process dominates the slow photovoltage decay process. Further investigation reveals that the device degradation preferentially takes place at the interface under working conditions, which thus can confirm the importance of interface engineering to enhance the device stability.

General theories and features of interfacial thermal transport

NASA Astrophysics Data System (ADS)

Zhou, Hangbo; Zhang, Gang

2018-03-01

A clear understanding and proper control of interfacial thermal transport is important in nanoscale device. In this review, we first discuss the theoretical methods to handle the interfacial thermal transport problem, such as the macroscopic model, molecular dynamics, lattice dynamics and modern quantum transport theories. Then we discuss various effects that can significantly affect the interfacial thermal transport, such as the formation of chemical bonds at interface, defects and interface roughness, strain and substrates, atomic species and mass ratios, structural orientations. Then importantly, we analyze the role of inelastic scatterings at the interface, and discuss its application in thermal rectifications. Finally, the challenges and promising directions are discussed.

Assembling non-ferromagnetic materials to ferromagnetic architectures using metal-semiconductor interfaces

PubMed Central

Ma, Ji; Liu, Chunting; Chen, Kezheng

2016-01-01

In this work, a facile and versatile solution route was used to fabricate room-temperature ferromagnetic fish bone-like, pteridophyte-like, poplar flower-like, cotton-like Cu@Cu2O architectures and golfball-like Cu@ZnO architecture. The ferromagnetic origins in these architectures were found to be around metal-semiconductor interfaces and defects, and the root cause for their ferromagnetism lay in charge transfer processes from metal Cu to semiconductors Cu2O and ZnO. Owing to different metallization at their interfaces, these architectures exhibited different ferromagnetic behaviors, including coercivity, saturation magnetization as well as magnetic interactions. PMID:27680286

films on silicon at different annealing temperatures

NASA Astrophysics Data System (ADS)

Zhao, Yan; Zhou, Chunlan; Zhang, Xiang; Zhang, Peng; Dou, Yanan; Wang, Wenjing; Cao, Xingzhong; Wang, Baoyi; Tang, Yehua; Zhou, Su

2013-03-01

Thermal atomic layer-deposited (ALD) aluminum oxide (Al2O3) acquires high negative fixed charge density ( Q f) and sufficiently low interface trap density after annealing, which enables excellent surface passivation for crystalline silicon. Q f can be controlled by varying the annealing temperatures. In this study, the effect of the annealing temperature of thermal ALD Al2O3 films on p-type Czochralski silicon wafers was investigated. Corona charging measurements revealed that the Q f obtained at 300°C did not significantly affect passivation. The interface-trapping density markedly increased at high annealing temperature (>600°C) and degraded the surface passivation even at a high Q f. Negatively charged or neutral vacancies were found in the samples annealed at 300°C, 500°C, and 750°C using positron annihilation techniques. The Al defect density in the bulk film and the vacancy density near the SiO x /Si interface region decreased with increased temperature. Measurement results of Q f proved that the Al vacancy of the bulk film may not be related to Q f. The defect density in the SiO x region affected the chemical passivation, but other factors may dominantly influence chemical passivation at 750°C.

A low thermal impact annealing process for SiO2-embedded Si nanocrystals with optimized interface quality

NASA Astrophysics Data System (ADS)

Hiller, Daniel; Gutsch, Sebastian; Hartel, Andreas M.; Löper, Philipp; Gebel, Thoralf; Zacharias, Margit

2014-04-01

Silicon nanocrystals (Si NCs) for 3rd generation photovoltaics or optoelectronic applications can be produced by several industrially compatible physical or chemical vapor deposition technologies. A major obstacle for the integration into a fabrication process is the typical annealing to form and crystallize these Si quantum dots (QDs) which involves temperatures ≥1100 °C for 1 h. This standard annealing procedure allows for interface qualities that correspond to more than 95% dangling bond defect free Si NCs. We study the possibilities to use rapid thermal annealing (RTA) and flash lamp annealing to crystallize the Si QDs within seconds or milliseconds at high temperatures. The Si NC interface of such samples exhibits huge dangling bond defect densities which makes them inapplicable for photovoltaics or optoelectronics. However, if the RTA high temperature annealing is combined with a medium temperature inert gas post-annealing and a H2 passivation, luminescent Si NC fractions of up to 90% can be achieved with a significantly reduced thermal load. A new figure or merit, the relative dopant diffusion length, is introduced as a measure for the impact of a Si NC annealing procedure on doping profiles of device structures.

Mechanism-based corrector combination restores ΔF508-CFTR folding and function

PubMed Central

Okiyoneda, Tsukasa; Veit, Guido; Dekkers, Johanna F.; Bagdany, Miklos; Soya, Naoto; Xu, Haijin; Roldan, Ariel; Verkman, Alan S.; Kurth, Mark; Simon, Agnes; Hegedus, Tamas; Beekman, Jeffrey M.; Lukacs, Gergely L.

2013-01-01

The most common cystic fibrosis (CF) mutation, ΔF508 in the nucleotide binding domain-1 (NBD1), impairs CFTR coupled-domain folding, plasma membrane (PM) expression, function and stability. VX-809, a promising investigational corrector of ΔF508-CFTR misprocessing, has limited clinical benefit and incompletely understood mechanism, hampering drug development. Based on the effect of second site suppressor mutations, robust ΔF508-CFTR correction likely requires stabilization of NBD1 and the membrane spanning domains (MSDs)-NBD1 interface, both established primary conformational defects. Here, we elucidated the molecular targets of available correctors; class-I stabilizes the NBD1-MSD1/2 interface, class-II targets NBD2, and only chemical chaperones, surrogates of class-III correctors, stabilize the human ΔF508-NBD1. While VX-809 can correct missense mutations primarily destabilizing the NBD1-MSD1/2 interface, functional PM expression of ΔF508-CFTR also requires compounds that counteract the NBD1 and NBD2 stability defects in CF bronchial epithelial cells and intestinal organoids. Thus, structure-guided corrector combination represents an effective approach for CF therapy. PMID:23666117

Electronic properties of defects in silicon and related materials

NASA Astrophysics Data System (ADS)

Mitromara, Niki

Efforts in the current semiconductor industry are focused on the production of smaller, more efficient and inexpensive devices of higher packing density. As silicon is the dominant semiconductor implemented for the fabrication of the majority of semiconductor devices, perpetual research has focused on the improvement of its properties and the realisation of the most efficient structures. This thesis presents the electrical characterisation of two different diode structures that are important for the present and future generations of electronic devices.The first part of the thesis is focused on the electrical characterisation of Ultra-Shallow Junction (USJs) Si diodes. Both p+n and n+p USJ structures that contained different implants were examined. These were very highly doped and intended to simulate the situation where a doping well is formed after heavy doping in Si for the fabrication of transistors currently used in Complementary-Metal-Oxide-Semiconductor (CMOS) technology. The implanted USJ diodes were provided by NXP, Belgium and contact deposition was performed before their electrical characterisation as part of this project. Subsequently the p+n and n+p USJ diodes were characterised by the use of Capacitance-Voltage (CV), Current-Voltage (IV), Deep Level Transient Spectroscopy (DLTS) and high resolution Laplace DLTS (LDLTS). DLTS and LDLTS are very powerful spectroscopic techniques for the profiling of defects in the bandgap of a semiconductor as well as for the identification of the electrical signatures of these defects. Transient-Enhanced Diffusion (TED) related defects were detected in these diodes as the presence of mainly carbon-related interstitial complexes was observed. In addition, certain vacancy or vacancy-dopant related levels were also discerned.The second part of this thesis presents the electrical characterisation from Schottky p-diamond/p-Si and p-diamond/n-Si p-n diodes. These diodes were readily provided, grown by the Chemical Vapour Deposition (CVD) technique, for the electrical characterisation that was performed as part of this project. The purpose of characterising both Schottky and p-n diamond on Si diodes was to detect defects near the surface of the films and near the interface with Si and hence provide a comparison between defects present at the beginning and end of growth. More defects were found near the interface with Si and the majority of observed defects were related to extended defects while the presence of grain boundaries in polycrystalline diamond was discussed.

Non-radiative recombination in Ge{sub 1−y}Sn{sub y} light emitting diodes: The role of strain relaxation in tuned heterostructure designs

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gallagher, J. D.; Xu, C.; Smith, D. J.

This paper describes the properties of Ge{sub 1−y}Sn{sub y} light emitting diodes with a broad range of Sn concentrations (y = 0.0–0.11). The devices are grown upon Si(100) platforms using ultra-low temperature deposition of highly reactive Ge and Sn hydrides. The device fabrication adopts two new photodiode designs which lead to optimized performance and enables a systematic study of the effects of strain relaxation on emission efficiency. In contrast with n-Ge/i-Ge{sub 1−y}Sn{sub y}/p-Ge analogs, which in most cases contain two defected interfaces, our designs include a p-layer with composition Ge{sub 1−z}Sn{sub z} chosen to be z < y to facilitate light extraction, but withmore » z close enough to y to guarantee no strain relaxation at the i/p interface. In addition, a Ge{sub 1−x}Sn{sub x} alloy is also used for the n layer, with compositions in the 0 ≤ x ≤ y range, so that defected and non-defected n/i interfaces can be studied. The electroluminescence spectra vs the Sn content y in the intrinsic layer of the diodes exhibit a monotonic shift in the emission wavelength from 1550 nm to 2500 nm. On the other hand, the emission intensities show a complex dependence that cannot be explained solely on the basis of Sn concentrations. Detailed theoretical modeling of these intensities makes it possible to extract recombination lifetimes that are found to be more than three times longer in samples in which strain relaxation has not occurred at the n-i interface, demonstrating the existence of a large non-radiative contribution from the relaxation defects. This finding is particularly significant for direct gap diodes with y > 0.09, for which it is practically impossible to avoid strain relaxation in n-Ge/i-Ge{sub 1−y}Sn{sub y}/p-Ge analogs. The new designs introduced here open the door to the fabrication of highly efficient electrically pumped systems for applications in future generations of integrated photonics.« less

Tuning of Schottky barrier height of Al/n-Si by electron beam irradiation

NASA Astrophysics Data System (ADS)

Vali, Indudhar Panduranga; Shetty, Pramoda Kumara; Mahesha, M. G.; Petwal, V. C.; Dwivedi, Jishnu; Choudhary, R. J.

2017-06-01

The effect of electron beam irradiation (EBI) on Al/n-Si Schottky diode has been studied by I-V characterization at room temperature. The behavior of the metal-semiconductor (MS) interface is analyzed by means of variations in the MS contact parameters such as, Schottky barrier height (ΦB), ideality factor (n) and series resistance (Rs). These parameters were found to depend on the EBI dose having a fixed incident beam of energy 7.5 MeV. At different doses (500, 1000, 1500 kGy) of EBI, the Schottky contacts were prepared and extracted their contact parameters by applying thermionic emission and Cheung models. Remarkably, the tuning of ΦB was observed as a function of EBI dose. The improved n with increased ΦB is seen for all the EBI doses. As a consequence of which the thermionic emission is more favored. However, the competing transport mechanisms such as space charge limited emission, tunneling and tunneling through the trap states were ascribed due to n > 1. The analysis of XPS spectra have shown the presence of native oxide and increased radiation induced defect states. The thickness variation in the MS interface contributing to Schottky contact behavior is discussed. This study explains a new technique to tune Schottky contact parameters by metal deposition on the electron beam irradiated n-Si wafers.

Fabrication of Cu-Ni mixed phase layer using DC electroplating and suppression of Kirkendall voids in Sn-Ag-Cu solder joints

NASA Astrophysics Data System (ADS)

Chee, Sang-Soo; Lee, Jong-Hyun

2014-05-01

A solderable layer concurrently containing Cu-rich and Ni-rich phases (mixed-phase layer, MPL) was fabricated by direct current electroplating under varying process conditions. Current density was considered as the main parameter to adjust the microstructure and composition of MPL during the electroplating process, and deposit thickness were evaluated as functions of plating time. As a result, it was observed that the coral-like structure that consisted of Cu-rich and Ni-rich phases grew in the thickness direction. The most desirable microstructure was obtained at a relatively low current density of 0.4 mA/cm2. In other words, the surface was the smoothest and defect-free at this current density. The electroplating rate was slightly enhanced with an increase in current density. Investigations of its solid-state reaction properties, including the formation of Kirkendall voids, were also carried out after reflow soldering with Sn-3.0 Ag-0.5 Cu solder balls. In the solid-state aging experiment at 125°C, Kirkendall voids at the normal Sn-3.0 Ag-0.5 Cu solder/Cu interface were easily formed after just 240 h. Meanwhile, the presence of an intermetallic compound (IMC) layer created in the solder/MPL interface indicated a slightly lower growth rate, and no Kirkendall voids were observed in the IMC layer even after 720 h.

Effects of nitrogen incorporation in HfO(2) grown on InP by atomic layer deposition: an evolution in structural, chemical, and electrical characteristics.

PubMed

Kang, Yu-Seon; Kim, Dae-Kyoung; Kang, Hang-Kyu; Jeong, Kwang-Sik; Cho, Mann-Ho; Ko, Dae-Hong; Kim, Hyoungsub; Seo, Jung-Hye; Kim, Dong-Chan

2014-03-26

We investigated the effects of postnitridation on the structural characteristics and interfacial reactions of HfO2 thin films grown on InP by atomic layer deposition (ALD) as a function of film thickness. By postdeposition annealing under NH3 vapor (PDN) at 600 °C, an InN layer formed at the HfO2/InP interface, and ionized NHx was incorporated in the HfO2 film. We demonstrate that structural changes resulting from nitridation of HfO2/InP depend on the film thickness (i.e., a single-crystal interfacial layer of h-InN formed at thin (2 nm) HfO2/InP interfaces, whereas an amorphous InN layer formed at thick (>6 nm) HfO2/InP interfaces). Consequently, the tetragonal structure of HfO2 transformed into a mixture structure of tetragonal and monoclinic because the interfacial InN layer relieved interfacial strain between HfO2 and InP. During postdeposition annealing (PDA) in HfO2/InP at 600 °C, large numbers of oxidation states were generated as a result of interfacial reactions between interdiffused oxygen impurities and out-diffused InP substrate elements. However, in the case of the PDN of HfO2/InP structures at 600 °C, nitrogen incorporation in the HfO2 film effectively blocked the out-diffusion of atomic In and P, thus suppressing the formation of oxidation states. Accordingly, the number of interfacial defect states (Dit) within the band gap of InP was significantly reduced, which was also supported by DFT calculations. Interfacial InN in HfO2/InP increased the electron-barrier height to ∼0.6 eV, which led to low-leakage-current density in the gate voltage region over 2 V.

Nanoscale patterning controls inorganic-membrane interface structure

NASA Astrophysics Data System (ADS)

Almquist, Benjamin D.; Verma, Piyush; Cai, Wei; Melosh, Nicholas A.

2011-02-01

The ability to non-destructively integrate inorganic structures into or through biological membranes is essential to realizing full bio-inorganic integration, including arrayed on-chip patch-clamps, drug delivery, and biosensors. Here we explore the role of nanoscale patterning on the strength of biomembrane-inorganic interfaces. AFM measurements show that inorganic probes functionalized with hydrophobic bands with thicknesses complimentary to the hydrophobic lipid bilayer core exhibit strong attachment in the bilayer. As hydrophobic band thickness increases to 2-3 times the bilayer core the interfacial strength decreases, comparable to homogeneously hydrophobic probes. Analytical calculations and molecular dynamics simulations predict a transition between a `fused' interface and a `T-junction' that matches the experimental results, showing lipid disorder and defect formation for thicker bands. These results show that matching biological length scales leads to more intimate bio-inorganic junctions, enabling rational design of non-destructive membrane interfaces.The ability to non-destructively integrate inorganic structures into or through biological membranes is essential to realizing full bio-inorganic integration, including arrayed on-chip patch-clamps, drug delivery, and biosensors. Here we explore the role of nanoscale patterning on the strength of biomembrane-inorganic interfaces. AFM measurements show that inorganic probes functionalized with hydrophobic bands with thicknesses complimentary to the hydrophobic lipid bilayer core exhibit strong attachment in the bilayer. As hydrophobic band thickness increases to 2-3 times the bilayer core the interfacial strength decreases, comparable to homogeneously hydrophobic probes. Analytical calculations and molecular dynamics simulations predict a transition between a `fused' interface and a `T-junction' that matches the experimental results, showing lipid disorder and defect formation for thicker bands. These results show that matching biological length scales leads to more intimate bio-inorganic junctions, enabling rational design of non-destructive membrane interfaces. Electronic supplementary information (ESI) available: Breakthrough rate as a function of force plots for 5 nm, 10 nm and ∞-probes.. See DOI: 10.1039/c0nr00486c

Surface smoothing and template partitioning for cranial implant CAD

NASA Astrophysics Data System (ADS)

Min, Kyoung-june; Dean, David

2005-04-01

Employing patient-specific prefabricated implants can be an effective treatment for large cranial defects (i.e., > 25 cm2). We have previously demonstrated the use of Computer Aided Design (CAD) software that starts with the patient"s 3D head CT-scan. A template is accurately matched to the pre-detected skull defect margin. For unilateral cranial defects the template is derived from a left-to-right mirrored skull image. However, two problems arise: (1) slice edge artifacts generated during isosurface polygonalization are inherited by the final implant; and (2) partitioning (i.e., cookie-cutting) the implant surface from the mirrored skull image usually results in curvature discontinuities across the interface between the patient"s defect and the implant. To solve these problems, we introduce a novel space curve-to-surface partitioning algorithm following a ray-casting surface re-sampling and smoothing procedure. Specifically, the ray-cast re-sampling is followed by bilinear interpolation and low-pass filtering. The resulting surface has a highly regular grid-like topological structure of quadrilaterally arranged triangles. Then, we replace the regions to be partitioned with predefined sets of triangular elements thereby cutting the template surface to accurately fit the defect margin at high resolution and without surface curvature discontinuities. Comparisons of the CAD implants for five patients against the manually generated implant that the patient actually received show an average implant-patient gap of 0.45mm for the former and 2.96mm for the latter. Also, average maximum normalized curvature of interfacing surfaces was found to be smoother, 0.043, for the former than the latter, 0.097. This indicates that the CAD implants would provide a significantly better fit.

Electronic and structural properties of vacancies and hydrogen adsorbates on trilayer graphene

NASA Astrophysics Data System (ADS)

Menezes, Marcos G.; Capaz, Rodrigo B.

2015-08-01

Using ab initio calculations, we study the electronic and structural properties of vacancies and hydrogen adsorbates on trilayer graphene. Those defects are found to share similar low-energy electronic features, since they both remove a pz electron from the honeycomb lattice and induce a defect level near the Fermi energy. However, a vacancy also leaves unpaired σ electrons on the lattice, which lead to important structural differences and also contribute to magnetism. We explore both ABA and ABC stackings and compare properties such as formation energies, magnetic moments, spin density and the local density of states (LDOS) of the defect levels. These properties show a strong sensitivity to the layer in which the defect is placed and smaller sensitivities to sublattice placing and stacking type. Finally, for the ABC trilayer, we also study how these states behave in the presence of an external field, which opens a tunable gap in the band structure of the non-defective system. The pz defect states show a strong hybridization with band states as the field increases, with reduction and eventually loss of magnetization, and a non-magnetic, midgap-like state is found when the defect is at the middle layer.

Electronic and Structural Properties of Vacancies and Hydrogen Adsorbates on Trilayer Graphene

NASA Astrophysics Data System (ADS)

Menezes, Marcos; Capaz, Rodrigo

2015-03-01

Using ab initio calculations, we study the electronic and structural properties of vacancies and hydrogen adsorbates on trilayer graphene. Those defects are found to share similar low-energy electronic features, since they both remove a pz electron from the honeycomb lattice and induce a defect level near the Fermi energy. However, a vacancy also leaves unpaired σ electrons on the lattice, which lead to important structural differences and also contribute to magnetism. We explore both ABA and ABC stackings and compare properties such as formation energies, magnetic moments, spin density and the local density of states (LDOS) of the defect levels. These properties show a strong sensitivity to the layer in which the defect is placed and smaller sensitivities to sublattice placing and stacking type. Finally, for the ABC trilayer, we also study how these states behave in the presence of an external electrical field, which opens a tunable gap in the band structure of the non-defective system. The pz defect states show a strong hybridization with band states as the field increases, with reduction and eventually loss of magnetization, and a non-magnetic, midgap-like state is found when the defect is at the middle layer.

Interface Mediated Nucleation and Growth of Dislocations in fcc-bcc nanocomposite

NASA Astrophysics Data System (ADS)

Zhang, Ruifeng; Wang, Jian; Beyerlein, Irene J.; Germann, Timothy C.

2011-03-01

Heterophase interfaces play a crucial role in determining material strength for nanostructured materials because they can block, store, nucleate, and remove dislocations, the essential defects that enable plastic deformation. Much recent theoretical and experimental effort has been conducted on nanostructured Cu-Nb multilayer composites that exhibited extraordinarily high strength, ductility, and resistance to radiation and mechanical loading. In decreasing layer thicknesses to the order of a few tens of nanometers or less, the deformation behavior of such composites is mainly controlled by the Cu/Nb interface. In this work, we focus on the cooperative mechanisms of dislocation nucleation and growth from Cu/Nb interfaces, and their interaction with interface. Two types of experimentally observed Cu/Nb incoherent interfaces are comparatively studied. We found that the preferred dislocation nucleation sites are closely related to atomic interface structure, which in turn, depend on the orientation relationship. The activation stress and energies for an isolated Shockley dislocation loop of different sizes from specific interface sites depend strongly on dislocation size, atomic interface pattern, and loading conditions. Such findings provide important insight into the mechanical response of a wide range of fcc/bcc metallic nanocomposites via atomic interface design.

Defect mediated ferromagnetism in Ni-doped ZnO nanocrystals evidenced by positron annihilation spectroscopy

NASA Astrophysics Data System (ADS)

Chen, Zhi-Yuan; Chen, Z. Q.; Zou, B.; Zhao, X. G.; Tang, Z.; Wang, S. J.

2012-10-01

NiO/ZnO nanocomposites with NiO content of 4 at. % and 20 at. % were annealed up to 1200 °C to get Ni doped ZnO nanocrystals. Raman scattering spectra illustrate a broad and strong band at 500-600cm-1 in all nanocomposites after annealing above 700 °C, which suggests incorporation of Ni in the ZnO lattice. However, x-ray diffraction measurements show that NiO phase can be still observed in all nanocomposites after annealing, which indicates that Ni is partially doped into the ZnO structure. Positron annihilation measurements reveal large number of vacancy defects in the interface region of all nanocomposites, and they are gradually recovered with increasing annealing temperature up to 1000 °C. Room temperature ferromagnetism can be observed in the NiO/ZnO nanocomposites, which is stronger in the 20 at. % NiO/ZnO nanocomposites, and the magnetization decreases continuously with increasing annealing temperature. This indicates that the ferromagnetism at low annealing temperatures originates from the NiO nanograins, and they become antiferromanetic after subsequent higher temperature annealing which leads to the weakening of ferromagnetism. After annealing up to 1000 °C, the ferromagnetism in both the two samples becomes nearly invisible. The disappearance of ferromagnetism shows good coincidence with the recovery of vacancy defects in NiO/ZnO nanocomposites. It can be inferred that the ferromagnetism is mediated by vacancy defects which are distributed in the interface region.

The microstructure of laterally seeded silicon-on-oxide

NASA Astrophysics Data System (ADS)

Pinizzotto, R. F.; Lam, H. W.; Vaandrager, B. L.

1982-03-01

The production of large scale integrated circuits in thin silicon films on insulating substrates is currently of much interest in the electronics industry. One of the most promising techniques of forming this composite structure is by lateral seeding. We have used optical microscopy and transmission electron microscopy to characterize the microstructure of silicon-on-oxide formed by scanning CW laser induced lateral epitaxy. The primary defects are dislocations. Dislocation rearrangement leads to the formation of both small angle boundaries (stable, regular dislocation arrays) and grain boundaries. The grains were found to be misoriented to the <100> direction perpendicular to the film plane by ≤ 4° and to the <100> directions in the plane of the film by ≤ 2°. Internal reflection twins are a common defect. Microtwinning was found to occur at the vertical step caused by the substrate-oxide interface if the substrate to oxide step height was > 120 nm. The microstructure is continuous across successive scan lines. Microstructural defects are found to initiate at the same topographical location in different oxide pads. We propose that this is due to the meeting of two crystallization growth fronts. The liquid silicon between the fronts causes large stresses in this area because of the 9% volume increase during solidification. The defects observed in the bulk may form by a similar mechanism or by dislocation generation at substrate-oxide interface irregularities. The models predict that slower growth leads to improved material quality. This has been observed experimentally.

The detection and quantification of the defects in adhesive bonded joints of the piezoelectric sensors by infrared thermographic nondestructive testing

NASA Astrophysics Data System (ADS)

Vinod, P. N.; Joseph, Sherin; John, Reji

2017-04-01

In this paper, efficacy of pulsed thermography technique has been explored for the first time for the detection and quantification of the subsurface defects present in the rubber-encapsulated piezoelectric sensors. Initial experiments were performed on adhesively bonded joints of the rubber/Al or rubber/PZT control samples to find out an optimum acquisition time for the 3-mm rubber encapsulants. Thermographic measurements were performed in the reflection mode and acquired thermal images were analysed and processed images were described in terms of the phase images. The defective regions are identified as delamination of the adhesive joints at the interface of rubber and PZT stacks, and presence of porosity in the encapsulation in the inspected hydrophone. The defect depths of the observed anomalies were calculated empirically from the plots of the peak time of thermal contrast (tmax) maximum and thermal contrast maximum (Cmax) for a particular defect. The estimated defect depths of the prominent porosity observed in the PZT hydrophone are found nearly 1 mm from the surface.

Diameter Dependence of Planar Defects in InP Nanowires

PubMed Central

Wang, Fengyun; Wang, Chao; Wang, Yiqian; Zhang, Minghuan; Han, Zhenlian; Yip, SenPo; Shen, Lifan; Han, Ning; Pun, Edwin Y. B.; Ho, Johnny C.

2016-01-01

In this work, extensive characterization and complementary theoretical analysis have been carried out on Au-catalyzed InP nanowires in order to understand the planar defect formation as a function of nanowire diameter. From the detailed transmission electron microscopic measurements, the density of stacking faults and twin defects are found to monotonically decrease as the nanowire diameter is decreased to 10 nm, and the chemical analysis clearly indicates the drastic impact of In catalytic supersaturation in Au nanoparticles on the minimized planar defect formation in miniaturized nanowires. Specifically, during the chemical vapor deposition of InP nanowires, a significant amount of planar defects is created when the catalyst seed sizes are increased with the lower degree of In supersaturation as dictated by the Gibbs-Thomson effect, and an insufficient In diffusion (or Au-rich enhancement) would lead to a reduced and non-uniform In precipitation at the NW growing interface. The results presented here provide an insight into the fabrication of “bottom-up” InP NWs with minimized defect concentration which are suitable for various device applications. PMID:27616584

Diameter Dependence of Planar Defects in InP Nanowires.

PubMed

Wang, Fengyun; Wang, Chao; Wang, Yiqian; Zhang, Minghuan; Han, Zhenlian; Yip, SenPo; Shen, Lifan; Han, Ning; Pun, Edwin Y B; Ho, Johnny C

2016-09-12

In this work, extensive characterization and complementary theoretical analysis have been carried out on Au-catalyzed InP nanowires in order to understand the planar defect formation as a function of nanowire diameter. From the detailed transmission electron microscopic measurements, the density of stacking faults and twin defects are found to monotonically decrease as the nanowire diameter is decreased to 10 nm, and the chemical analysis clearly indicates the drastic impact of In catalytic supersaturation in Au nanoparticles on the minimized planar defect formation in miniaturized nanowires. Specifically, during the chemical vapor deposition of InP nanowires, a significant amount of planar defects is created when the catalyst seed sizes are increased with the lower degree of In supersaturation as dictated by the Gibbs-Thomson effect, and an insufficient In diffusion (or Au-rich enhancement) would lead to a reduced and non-uniform In precipitation at the NW growing interface. The results presented here provide an insight into the fabrication of "bottom-up" InP NWs with minimized defect concentration which are suitable for various device applications.

Hidden Order as a Source of Interface Superconductivity

NASA Astrophysics Data System (ADS)

Moor, Andreas; Volkov, Anatoly; Efetov, Konstantin

2015-03-01

We propose a new mechanism of the interfacial superconductivity observed in many heterostructures composed of different materials including high-temperature superconductors. Our proposal is based on the use of the Ginzburg-Landau equations applicable to a wide class of systems. The system under consideration is assumed to have, alongside the superconducting order parameter, also another competing order that might be a charge- or spin-density wave. At certain temperatures or doping level the superconducting state is not realized (thus, ``hidden''), while the amplitude of another order parameter corresponds to a minimum of the free energy. We also assume that at an interface or at a defect, the non-superconducting order parameter is suppressed (strongly or weakly), e.g., due to an enhanced impurity scattering. The local superconductivity is shown to emerge at the interface, and the spatial dependence of the corresponding order parameter is described by the Gross-Pitaevskii equation. The quantized values of the temperature and doping levels, at which Δ (x) arises, are determined by the ``energy'' levels of the linearized Gross-Pitaevskii equation, i.e., of the Schrodinger equation. Interestingly, the local superconductivity arises even at a small suppression of the rival order. We appreciate the support from DFG via the Projekt EF 11/8-1; K. B. E. gratefully acknowledges the financial support of the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST ``MISiS.''

Reaction mechanism of a PbS-on-ZnO heterostructure and enhanced photovoltaic diode performance with an interface-modulated heterojunction energy band structure.

PubMed

Li, Haili; Jiao, Shujie; Ren, Jinxian; Li, Hongtao; Gao, Shiyong; Wang, Jinzhong; Wang, Dongbo; Yu, Qingjiang; Zhang, Yong; Li, Lin

2016-02-07

A room temperature successive ionic layer adsorption and reaction (SILAR) method is introduced for fabricating quantum dots-on-wide bandgap semiconductors. Detailed exploration of how SILAR begins and proceeds is performed by analyzing changes in the electronic structure of related elements at interfaces by X-ray photoelectric spectroscopy, together with characterization of optical properties and X-ray diffraction. The distribution of PbS QDs on ZnO, which is critical for optoelectrical applications of PbS with a large dielectric constant, shows a close relationship with the dipping order. A successively deposited PbS QDs layer is obtained when the sample is first immersed in Na2S solution. This is reasonable because the initial formation of different chemical bonds on ZnO nanorods is closely related to dangling bonds and defect states on surfaces. Most importantly, dipping order also affects their optoelectrical characteristics greatly, which can be explained by the heterojunction energy band structure related to the interface. The formation mechanism for PbS QDs on ZnO is confirmed by the fact that the photovoltaic diode device performance is closely related to the dipping order. Our atomic-scale understanding emphasises the fundamental role of surface chemistry in the structure and tuning of optoelectrical properties, and consequently in devices.

Defect-Mediated Molecular Interaction and Charge Transfer in Graphene Mesh-Glucose Sensors.

PubMed

Kwon, Sun Sang; Shin, Jae Hyeok; Choi, Jonghyun; Nam, SungWoo; Park, Won Il

2017-04-26

We report the role of defects in enzymatic graphene field-effect transistor sensors by introducing engineered defects in graphene channels. Compared with conventional graphene sensors (Gr sensors), graphene mesh sensors (GM sensors), with an array of circular holes, initially exhibited a higher irreversible response to glucose, involving strong chemisorption to edge defects. However, after immobilization of glucose oxidase, the irreversibility of the responses was substantially diminished, without any reduction in the sensitivity of the GM sensors (i.e., -0.53 mV/mM for the GM sensor vs -0.37 mV/mM for Gr sensor). Furthermore, multiple cycle operation led to rapid sensing and improved the reversibility of GM sensors. In addition, control tests with sensors containing a linker showed that sensitivity was increased in Gr sensors but decreased in GM sensors. Our findings indicate that edge defects can be used to replace linkers for immobilization of glucose oxidase and improve charge transfer across glucose oxidase-graphene interfaces.

Atomic-Scale Mechanisms of Defect-Induced Retention Failure in Ferroelectrics.

PubMed

Li, Linze; Zhang, Yi; Xie, Lin; Jokisaari, Jacob R; Beekman, Christianne; Yang, Jan-Chi; Chu, Ying-Hao; Christen, Hans M; Pan, Xiaoqing

2017-06-14

The ability to switch the ferroelectric polarization using an electric field makes ferroelectrics attractive for application in nanodevices such as high-density memories. One of the major challenges impeding this application, however, has been known as "retention failure", which is a spontaneous process of polarization back-switching that can lead to data loss. This process is generally thought to be caused by the domain instability arising from interface boundary conditions and countered by defects, which can pin the domain wall and impede the back-switching. Here, using in situ transmission electron microscopy and atomic-scale scanning transmission electron microscopy, we show that the polarization retention failure can be induced by commonly observed nanoscale impurity defects in BiFeO 3 thin films. The interaction between polarization and the defects can also lead to the stabilization of novel functional nanodomains with mixed-phase structures and head-to-head polarization configurations. Thus, defect engineering provides a new route for tuning properties of ferroelectric nanosystems.

Characterization of structural and electrostatic complexity in pentacene thin films by scanning probe microscopy

NASA Astrophysics Data System (ADS)

Puntambekar, Kanan Prakash

The advancement of organic electronics for applications in solar energy conversion, printed circuitry, displays, and solid-state lighting depends upon optimization of structure and properties for a variety of organic semiconductor interfaces. Organic semiconductor/insulator (O/I) and organic-metal (O/M) interfaces, in particular, are critical to the operation of organic thin film transistors (OTFTs) currently being developed for printed flexible electronics. Scanning probe microscopy (SPM) is a powerful tool to isolate and characterize the bottlenecks to charge transport at these interfaces. This thesis establishes a direct correlation between the structural disorder and electrical complexity at these interfaces, using various SPM based methods and discusses the implications of such complexity on device performance. To examine the O/M interfaces, surface potentials of operating pentacene TFTs with two different contact geometries (bottom or top) were mapped by Kelvin probe force microscopy (KFM). The surface potential distribution was used to isolate the potential drops at the source and drain contacts. Simultaneously obtained topography and surface potential maps elucidated the correlation between the morphology and contact resistance at the O/M interface; the bottom contact TFTs were observed to be contact limited at large gate voltages, while the top contact TFTs were not contact limited. A direct correlation between structural defects and electric potential variations at the pentacene and silicon dioxide, a common insulator, is demonstrated. Lateral force microscopy (LFM) generates striking images of the polycrystalline microstructure of a monolayer thick pentacene film, allowing clear visualization of the grain boundary network. Further more, surface potential wells localized at the grain boundaries were observed by KFM, suggesting that the grain boundaries may serve as charge carrier (hole) traps. Line dislocations were also revealed in the second monolayer by chemical etching and SPM and produce strong variations in the surface potential that must affect the interfacial charge conductance. Structural disorder at the O/I and O/M interfaces degrades both injection and transport of charge, and therefore needs to be minimized. Thus both visualization and correlation of structural and electrical complexity at these interfaces have important implications for understanding electrical transport in OTFTs and for defining strategies to improve device performance.

Enhancement of Radiation Tolerance by Interfaces in Nanostructured Metallic Materials

DTIC Science & Technology

2013-06-05

MeV Ar+ ions, respectively [14],[15]. Chimi et al. measured the electrical resistivity of irradiated nc gold and found that enlarged defect...2011.10.052 07/28/2011 1.00 Nan Li, M. S. Martin, O. Anderoglu, A. Misra, L. Shao, H. Wang, X. Zhang. He ion irradiation damage in Al? Nb ...climb at interfaces in nanolayered Al/ Nb composites, Scripta Materialia, (08 2010): 363. doi: 10.1016/j.scriptamat.2010.04.005 07/28/2011 5.00 E.G. Fu

Crystal-Phase Quantum Wires: One-Dimensional Heterostructures with Atomically Flat Interfaces.

PubMed

Corfdir, Pierre; Li, Hong; Marquardt, Oliver; Gao, Guanhui; Molas, Maciej R; Zettler, Johannes K; van Treeck, David; Flissikowski, Timur; Potemski, Marek; Draxl, Claudia; Trampert, Achim; Fernández-Garrido, Sergio; Grahn, Holger T; Brandt, Oliver

2018-01-10

In semiconductor quantum-wire heterostructures, interface roughness leads to exciton localization and to a radiative decay rate much smaller than that expected for structures with flat interfaces. Here, we uncover the electronic and optical properties of the one-dimensional extended defects that form at the intersection between stacking faults and inversion domain boundaries in GaN nanowires. We show that they act as crystal-phase quantum wires, a novel one-dimensional quantum system with atomically flat interfaces. These quantum wires efficiently capture excitons whose radiative decay gives rise to an optical doublet at 3.36 eV at 4.2 K. The binding energy of excitons confined in crystal-phase quantum wires is measured to be more than twice larger than that of the bulk. As a result of their unprecedented interface quality, these crystal-phase quantum wires constitute a model system for the study of one-dimensional excitons.

Interaction of polymer-coated silicon nanocrystals with lipid bilayers and surfactant interfaces

NASA Astrophysics Data System (ADS)

Elbaradei, Ahmed; Brown, Samuel L.; Miller, Joseph B.; May, Sylvio; Hobbie, Erik K.

2016-10-01

We use photoluminescence (PL) microscopy to measure the interaction between polyethylene-glycol-coated (PEGylated) silicon nanocrystals (SiNCs) and two model surfaces: lipid bilayers and surfactant interfaces. By characterizing the photostability, transport, and size-dependent emission of the PEGylated nanocrystal clusters, we demonstrate the retention of red PL suitable for detection and tracking with minimal blueshift after a year in an aqueous environment. The predominant interaction measured for both interfaces is short-range repulsion, consistent with the ideal behavior anticipated for PEGylated phospholipid coatings. However, we also observe unanticipated attractive behavior in a small number of scenarios for both interfaces. We attribute this anomaly to defective PEG coverage on a subset of the clusters, suggesting a possible strategy for enhancing cellular uptake by controlling the homogeneity of the PEG corona. In both scenarios, the shape of the apparent potential is modeled through the free or bound diffusion of the clusters near the confining interface.