Science.gov

Sample records for bulk semiconductors surfaces

  1. Interaction of graphene quantum dots with bulk semiconductor surfaces

    NASA Astrophysics Data System (ADS)

    Mohapatra, P. K.; Kushavah, Dushyant; Mohapatra, J.; Singh, B. P.

    2015-05-01

    Highly luminescent graphene quantum dots (GQDs) are synthesized through thermolysis of glucose. The average lateral size of the synthesized GQDs is found to be ˜5 nm. The occurrence of D and G band at 1345 and 1580 cm-1 in Raman spectrum confirms the presence of graphene layers. GQDs are mostly consisting of 3 to 4 graphene layers as confirmed from the AFM measurements. Photoluminescence (PL) measurement shows a distinct broadening of the spectrum when GQDs are on the semiconducting bulk surface compared to GQDs in water. The time resolved PL measurement shows a significant shortening in PL lifetime due to the substrate interaction on GQDs compared to the GQDs in solution phase.

  2. Interaction of graphene quantum dots with bulk semiconductor surfaces

    SciTech Connect

    Mohapatra, P. K.; Singh, B. P.; Kushavah, Dushyant; Mohapatra, J.

    2015-05-15

    Highly luminescent graphene quantum dots (GQDs) are synthesized through thermolysis of glucose. The average lateral size of the synthesized GQDs is found to be ∼5 nm. The occurrence of D and G band at 1345 and 1580 cm{sup −1} in Raman spectrum confirms the presence of graphene layers. GQDs are mostly consisting of 3 to 4 graphene layers as confirmed from the AFM measurements. Photoluminescence (PL) measurement shows a distinct broadening of the spectrum when GQDs are on the semiconducting bulk surface compared to GQDs in water. The time resolved PL measurement shows a significant shortening in PL lifetime due to the substrate interaction on GQDs compared to the GQDs in solution phase.

  3. Quasiparticle energy studies of bulk semiconductors, surfaces and nanotubes

    SciTech Connect

    Blase, Xavier Francois

    1994-12-01

    Effects of many-body effects on electronic excitation energies (quasiparticle band structure) of these materials are explored. GW approximation, including local field effects, for self-energy operator is used to calculate quasi-particle energies. The newly discovered carbon nanotubes are studied; structural stability and band structures are calculated. BN nanotubes are also studied, and their stability is predicted. Unexpected electronic features are predicted for both systems. Filling of carbon nanotubes with metal atoms and the doping of BN nanotubes by carbon and other impurites is also studied. The occupied surface states at H/Si(111)-(1x1) surface are studied; it is shown that the electronic structure requires a full quasiparticle calculation even for this simple chemisorption system. The core level shift of the Si 2p levels for atoms near the H/Si(111)-(1x1) surface is calculated; a simple first order perturbation theory using pseudopotential and the local density approximation gives good results for the photoemission spectra of the core electrons. The quasiparticle energies of bulk hexagonal BN and those of an isolated BN sheet are studied; this provides an understanding of the quasiparticle band structure of BN nanotubes. A nearly free electron state with a wavefunction in the interlayer or vacuum region composes the bottom of the conduction bands. A mixed-space formalism is presented for calculating the dynamical screening effects and electron self-energy operator in solids; this provides an efficient algorithm to calculate quasiparticle energies for large systems.

  4. Two-dimensional bulk bands and surface resonances originated from (100) surfaces of III-V semiconductor compounds

    NASA Astrophysics Data System (ADS)

    Olguín, D.; de Coss, R.; Baquero, R.

    1996-07-01

    We have calculated the electronic band structure of the (100) surface of the III-V zinc blende semiconductor compounds, using the standard tight binding method and the surface Green's function matching method. We have found that the creation of the surface gives place to new states in the electronic structure: surface resonances and two dimensional bulk states. The two dimensional bulk states are of the same character of those reported recently in CdTe(100) [Phys. Rev. 50, 1980 (1994)]. We analyze the states in the valence band region and compare with photoemission spectroscopy data.

  5. Bulk and surface properties of ZnTe-ZnS system semiconductors

    NASA Astrophysics Data System (ADS)

    Kirovskaya, I. A.; Mironova, E. V.; Kosarev, B. A.; Nor, P. E.; Bukashkina, T. L.

    2016-10-01

    Physicochemical studies of a new ZnTe-ZnS semiconductor system are conducted. It is found that at certain ratios of binary components, substitutional solid solutions with a cubic sphalerite structure are formed in this system. Interrelated laws governing changes in the bulk (crystal chemical, structural) and surface (acid-base) properties with varying system composition are identified. It is assumed they can be attributed to the nature of the active (acid-base) sites. The presented data, observed patterns, and an interpretation of them are used not only to confirm earlier proposed mechanisms of atomic-molecular interaction on diamond-like semiconductors, but to search for promising materials for use in highly sensitive selective sensors for environmental and medical purposes as well.

  6. (001)-surface-induced bulk states and surface resonances in II-VI zinc-blende semiconductors

    NASA Astrophysics Data System (ADS)

    Olguín, D.; Baquero, R.

    1995-06-01

    In a previous paper [Phys. Rev. 50, 1980 (1994)] we gave an account of the nondispersive band found experimentally at -4.4 for CdTe(001) by Niles and Höchst. We have characterized this band as a surface-induced bulk state. In a second paper we showed that a similar state does exist in II-VI and III-V zinc-blende semiconductor compounds. In this paper we show that there are more such states within the valence-band energy interval. We use tight-binding Hamiltonians and the surface-Green's-function matching method to calculate the surface resonances and surface-induced bulk states in the zinc-blende semiconductors CdTe, CdSe, ZnTe, and ZnSe. We find one distinctive surface resonance for the cation- and two for the anion-terminated (001) surface and three (001)-surface-induced bulk states with energies that correspond to the value of the heavy-hole, light-hole, and spin-orbit bands at X.

  7. A Theoretical Study of Bulk and Surface Diffusion Processes for Semiconductor Materials Using First Principles Calculations

    NASA Astrophysics Data System (ADS)

    Roehl, Jason L.

    discovered for a Ga adatom relaxing from heights of 3 and 0.5 A from the surface. These two sets show significant differences in the interaction of the Ga adatom with surface As dimers and an electronic signature of the differences in this interaction was identified. The energetic barriers to diffusion were computed between various adsorption sites. Diffusion profiles for native Cd and S, adatom and vacancy, and non-native interstitial adatoms of Te, Cu and Cl were investigated in bulk wurtzite CdS. The interstitial diffusion paths considered in this work were chosen parallel to c-axis as it represents the path encountered by defects diffusing from the CdTe layer. Because of the lattice mismatch between zinc-blende CdTe and hexagonal wurtzite CdS, the c-axis in CdS is normal to the CdTe interface. The global minimum and maximum energy positions in the bulk unit cell vary for different diffusing species. This results in a significant variation, in the bonding configurations and associated strain energies of different extrema positions along the diffusion paths for various defects. The diffusion barriers range from a low of 0.42 eV for an S interstitial to a high of 2.18 eV for a S vacancy. The computed 0.66 eV barrier for a Cu interstitial is in good agreement with experimental values in the range of 0.58 - 0.96 eV reported in the literature. There exists an electronic signature in the local density of states for the s- and d-states of the Cu interstitial at the global maximum and global minimum energy position. The work presented in this thesis is an investigation into diffusion processes for semiconductor bulk and surfaces. The work provides information about these processes at a level of control unavailable experimentally giving an elaborate description into physical and electronic properties associated with diffusion at its most basic level. Not only does this work provide information about GaAs, CdTe and CdS, it is intended to contribute to a foundation of knowledge that

  8. Importance of bulk states for the electronic structure of semiconductor surfaces: implications for finite slabs.

    PubMed

    Sagisaka, Keisuke; Nara, Jun; Bowler, David

    2017-04-12

    We investigate the influence of slab thickness on the electronic structure of the Si(1 0 0)- p([Formula: see text]) surface in density functional theory (DFT) calculations, considering both density of states and band structure. Our calculations, with slab thicknesses of up to 78 atomic layers, reveal that the slab thickness profoundly affects the surface band structure, particularly the dangling bond states of the silicon dimers near the Fermi level. We find that, to precisely reproduce the surface bands, the slab thickness needs to be large enough to completely converge the bulk bands in the slab. In the case of the Si(1 0 0) surface, the dispersion features of the surface bands, such as the band shape and width, converge when the slab thickness is larger than 30 layers. Complete convergence of both the surface and bulk bands in the slab is only achieved when the slab thickness is greater than 60 layers.

  9. Importance of bulk states for the electronic structure of semiconductor surfaces: implications for finite slabs

    NASA Astrophysics Data System (ADS)

    Sagisaka, Keisuke; Nara, Jun; Bowler, David

    2017-04-01

    We investigate the influence of slab thickness on the electronic structure of the Si(1 0 0)- p(2× 2 ) surface in density functional theory (DFT) calculations, considering both density of states and band structure. Our calculations, with slab thicknesses of up to 78 atomic layers, reveal that the slab thickness profoundly affects the surface band structure, particularly the dangling bond states of the silicon dimers near the Fermi level. We find that, to precisely reproduce the surface bands, the slab thickness needs to be large enough to completely converge the bulk bands in the slab. In the case of the Si(1 0 0) surface, the dispersion features of the surface bands, such as the band shape and width, converge when the slab thickness is larger than 30 layers. Complete convergence of both the surface and bulk bands in the slab is only achieved when the slab thickness is greater than 60 layers.

  10. Semiconductor surface roughness: Dependence on sign and magnitude of bulk strain

    SciTech Connect

    Xie, Y.H.; Gilmer, G.H.; Roland, C.; Silverman, P.J.; Buratto, S.K.; Cheng, J.Y.; Fitzgerald, E.A.; Kortan, A.R.; Schuppler, S.; Marcus, M.A.; Citrin, P.H. Department of Physics, North Carolina State University, Raleigh, North Carolina 27695 )

    1994-11-28

    Changes in surface roughness have been studied as a function of bulk compressive and tensile strains (biaxial in the plane of the sample surface) in thin films of compositionally uniform and dislocation-free Ge[sub 0.5]Si[sub 0.5]. A pronounced surface roughness is observed only for films under compressive strains exceeding 1.4%. Molecular dynamics simulations show that this striking result has its origin in the strain-induced lowering of surface step free energies.

  11. Coexistence of bulk and surface polaritons in a magnetic-semiconductor superlattice influenced by a transverse magnetic field

    NASA Astrophysics Data System (ADS)

    Tuz, Vladimir R.; Fesenko, Volodymyr I.; Fedorin, Illia V.; Sun, Hong-Bo; Han, Wei

    2017-03-01

    It is demonstrated that the effect of coexistence of bulk and surface polaritons within the same frequency band and the wavevector space can be achieved in a magnetic-semiconductor superlattice, providing a conscious choice of characteristic resonant frequencies and material fractions of the structure's underlying components as well as geometry of the external static magnetic field. The study is based on the effective medium theory, which is involved to calculate dispersion characteristics of the long-wavelength electromagnetic modes of ordinary and extraordinary bulk polaritons, and hybrid surface polaritons with dominant longitudinal component of either magnetic (HE) or electric (EH) field derived via averaged expressions with respect to the effective constitutive parameters of the superlattice.

  12. Transport noise arising from diffusion and bulk or surface generation recombination in semiconductors

    NASA Astrophysics Data System (ADS)

    Mehta, H.

    The type of noise spectra in solids or solid state devices, which can arise from noise due to a combination of causes, such as diffusion of carriers, generation and recombination (g-r) processes of carriers in the bulk and at the surface was studied using techniques to be described apply equally well to the problem of heat diffusion, with heat transfer or reflection at the boundaries of the system, we have, however, mainly the carrier noise problem in mind. The physical noise sources, the diffusion source, g-r source and surface source, Clarke and Voss' P source are discussed as well as transport noise in symmetrical embedded bodies, in nonsymmetrical embedded bodies, in symmetrical nonembedded bodies and in nonsymmetrical nonembedded bodies. Results indicate that volume g-r and diffusion processes give rather smooth spectra, but, without a l/f range. The high frequency asymptote is always w-3/2 (embedded case) or omega to the minus 2 power (nonembedded case).

  13. Artificial atoms on semiconductor surfaces

    PubMed Central

    Tisdale, W. A.; Zhu, X.-Y.

    2011-01-01

    Semiconductor nanocrystals are called artificial atoms because of their atom-like discrete electronic structure resulting from quantum confinement. Artificial atoms can also be assembled into artificial molecules or solids, thus, extending the toolbox for material design. We address the interaction of artificial atoms with bulk semiconductor surfaces. These interfaces are model systems for understanding the coupling between localized and delocalized electronic structures. In many perceived applications, such as nanoelectronics, optoelectronics, and solar energy conversion, interfacing semiconductor nanocrystals to bulk materials is a key ingredient. Here, we apply the well established theories of chemisorption and interfacial electron transfer as conceptual frameworks for understanding the adsorption of semiconductor nanocrystals on surfaces, paying particular attention to instances when the nonadiabatic Marcus picture breaks down. We illustrate these issues using recent examples from our laboratory. PMID:21097704

  14. Study of surface and bulk electronic structure of II-VI semiconductor nanocrystals using Cu as a nanosensor.

    PubMed

    Grandhi, G Krishnamurthy; Tomar, Renu; Viswanatha, Ranjani

    2012-11-27

    Efficiency of the quantum dots based solar cells relies on charge transfer at the interface and hence on the relative alignment of the energy levels between materials. Despite a high demand to obtain size specific band offsets, very few studies exist where meticulous methods like photoelectron spectroscopy are used. However, semiconductor charging during measurements could result in indirect and possibly inaccurate measurements due to shift in valence and conduction band position. Here, in this report, we devise a novel method to study the band offsets by associating an atomic like state with the conduction band and hence obtaining an internal standard. This is achieved by doping copper in semiconductor nanocrystals, leading to the development of a characteristic intragap Cu-related emission feature assigned to the transition from the conduction band to the atomic-like Cu d state. Using this transition we determine the relative band alignment of II-VI semiconductor nanocrystals as a function of size in the below 10 nm size regime. The results are in excellent agreement with the available photoelectron spectroscopy data as well as the theoretical data. We further use this technique to study the excitonic band edge variation as a function of temperature in CdSe nanocrystals. Additionally, surface electronic structure of CdSe nanocrystals have been studied using quantitative measurements of absolute quantum yield and PL decay studies of the Cu related emission and the excitonic emission. The role of TOP and oleic acid as surface passivating ligand molecules has been studied for the first time.

  15. Bulk rectification effect in a polar semiconductor

    NASA Astrophysics Data System (ADS)

    Ideue, T.; Hamamoto, K.; Koshikawa, S.; Ezawa, M.; Shimizu, S.; Kaneko, Y.; Tokura, Y.; Nagaosa, N.; Iwasa, Y.

    2017-06-01

    Noncentrosymmetric conductors are an interesting material platform, with rich spintronic functionalities and exotic superconducting properties typically produced in polar systems with Rashba-type spin-orbit interactions. Polar conductors should also exhibit inherent nonreciprocal transport, in which the rightward and leftward currents differ from each other. But such a rectification is difficult to achieve in bulk materials because, unlike the translationally asymmetric p-n junctions, bulk materials are translationally symmetric, making this phenomenon highly nontrivial. Here we report a bulk rectification effect in a three-dimensional Rashba-type polar semiconductor BiTeBr. Experimentally observed nonreciprocal electric signals are quantitatively explained by theoretical calculations based on the Boltzmann equation considering the giant Rashba spin-orbit coupling. The present result offers a microscopic understanding of the bulk rectification effect intrinsic to polar conductors as well as a simple electrical means to estimate the spin-orbit parameter in a variety of noncentrosymmetric systems.

  16. Bulk Rashba Semiconductors and Related Quantum Phenomena.

    PubMed

    Bahramy, Mohammad Saeed; Ogawa, Naoki

    2017-03-29

    Bithmuth tellurohalides BiTeX (X = Cl, Br and I) are model examples of bulk Rashba semiconductors, exhibiting a giant Rashba-type spin splitting among their both valence and conduction bands. Extensive spectroscopic and transport experiments combined with the state-of-the-art first-principles calculations have revealed many unique quantum phenomena emerging from the bulk Rashba effect in these systems. The novel features such as the exotic inter- and intra-band optical transitions, enhanced magneto-optical response, divergent orbital dia-/para-magnetic susceptibility and helical spin textures with a nontrivial Berry's phase in the momentum space are among the salient discoveries, all arising from this effect. Also, it is theoretically proposed and indications have been experimentally reported that bulk Rashba semiconductors such as BiTeI have the capability of becoming a topological insulator under the application of a hydrostatic pressure. Here, we overview these studies and show that BiTeX are an ideal platform to explore the next aspects of quantum matter, which could ultimately be utilized to create spintronic devices with novel functionalities.

  17. Discrete energy bands in bulk semiconductors

    NASA Astrophysics Data System (ADS)

    Du, Maohua; Shi, Hongliang

    2015-03-01

    Bulk semiconductors typically have continuous valence and conduction bands. Discrete energy levels and bands have been sought after for various applications. For instance, discrete energy levels existing in semiconductor nanocrystals, or quantum does (QDs) have been proposed as a mechanism to suppress hot carrier thermalization and to enhance carrier multiplication in QD solar cells. Impurity bands in the band gap have been introduced for intermediate-band solar cells and for efficient visible light absorption and photocatalysis. In this talk, we show by first principles calculations that, in a multinary compound, a combination of large electronegativity difference between different cations (anions) and large nearest-neighbor distances in cation (anion) sublattices can lead to the splitting of the conduction (valence) band, resulting in several discrete and narrow energy bands separated by large energy gaps. We also discuss applications that may benefit from such electronic structure.

  18. Circular photon drag effect in bulk semiconductors

    NASA Astrophysics Data System (ADS)

    Shalygin, V. A.; Moldavskaya, M. D.; Danilov, S. N.; Farbshtein, I. I.; Golub, L. E.

    2017-06-01

    We report on the observation of the circular photon drag effect in a bulk semiconductor. The photocurrent caused by a transfer of both linear and angular momenta of photons to charge carriers is detected in tellurium. Dependencies of the photocurrent on the light polarization and on the incidence angle agree with the symmetry analysis of the circular photon drag effect. Experimental spectral data on the photocurrent in mid-infrared range qualitatively agree with a microscopic model of the circular photon drag effect considering intersubband optical transitions of holes in tellurium.

  19. Modeling direct interband tunneling. I. Bulk semiconductors

    SciTech Connect

    Pan, Andrew; Chui, Chi On

    2014-08-07

    Interband tunneling is frequently studied using the semiclassical Kane model, despite uncertainty about its validity. Revisiting the physical basis of this formula, we find that it neglects coupling to other bands and underestimates transverse tunneling. As a result, significant errors can arise at low and high fields for small and large gap materials, respectively. We derive a simple multiband tunneling model to correct these defects analytically without arbitrary parameters. Through extensive comparison with band structure and quantum transport calculations for bulk InGaAs, InAs, and InSb, we probe the accuracy of the Kane and multiband formulas and establish the superiority of the latter. We also show that the nonlocal average electric field should be used when applying either of these models to nonuniform potentials. Our findings are important for efficient analysis and simulation of bulk semiconductor devices involving tunneling.

  20. Short- and long-range-order features in the electronic structure of bulk and surface vacancies in diamond-structure semiconductors

    NASA Astrophysics Data System (ADS)

    Louis, E.; Menéndez, C.; Vergés, J. A.

    1981-09-01

    Some questions concerning the nature of localized electron states at vacancies in covalent semiconductors are examined. In order to make the calculations easier, a honeycomb lattice, instead of a diamond lattice, is first considered. Two simple Hamiltonians are used: a one-state (s-like) Hamiltonian and an sp2 Hamiltonian of the Weaire-Thorpe-Leman-Friedel type. Short- and long-range-order effects are studied by solving the Hamiltonians within different degrees of approximation: the Bethe lattice, cluster-Bethe lattice, and exact solutions are presented. Vacancies at bulk and (111) surface are discussed. The analysis of the results is facilitated by comparing them with the results obtained by exactly solving a full sp3 Hamiltonian for a diamond-structure semiconductor (Si). It is shown that whereas the gap states (dangling-bond-like) are mainly determined by short-range order, the states deep in the valence band, of A1 symmetry, are associated with the rings of atoms characteristic of the diamond structure.

  1. Semiconductor surface protection material

    NASA Technical Reports Server (NTRS)

    Packard, R. D. (Inventor)

    1973-01-01

    A method and a product for protecting semiconductor surfaces is disclosed. The protective coating material is prepared by heating a suitable protective resin with an organic solvent which is solid at room temperature and converting the resulting solution into sheets by a conventional casting operation. Pieces of such sheets of suitable shape and thickness are placed on the semiconductor areas to be coated and heat and vacuum are then applied to melt the sheet and to drive off the solvent and cure the resin. A uniform adherent coating, free of bubbles and other defects, is thus obtained exactly where it is desired.

  2. Detection of Berry's phase in a Bulk Rashba semiconductor.

    PubMed

    Murakawa, H; Bahramy, M S; Tokunaga, M; Kohama, Y; Bell, C; Kaneko, Y; Nagaosa, N; Hwang, H Y; Tokura, Y

    2013-12-20

    The motion of electrons in a solid has a profound effect on its topological properties and may result in a nonzero Berry's phase, a geometric quantum phase encoded in the system's electronic wave function. Despite its ubiquity, there are few experimental observations of Berry's phase of bulk states. Here, we report detection of a nontrivial π Berry's phase in the bulk Rashba semiconductor BiTeI via analysis of the Shubnikov-de Haas (SdH) effect. The extremely large Rashba splitting in this material enables the separation of SdH oscillations, stemming from the spin-split inner and outer Fermi surfaces. For both Fermi surfaces, we observe a systematic π-phase shift in SdH oscillations, consistent with the theoretically predicted nontrivial π Berry's phase in Rashba systems.

  3. Photocontrol of Dirac electrons in a bulk Rashba semiconductor

    NASA Astrophysics Data System (ADS)

    Ogawa, N.; Bahramy, M. S.; Kaneko, Y.; Tokura, Y.

    2014-09-01

    We demonstrate the generation of circularly polarized light induced current of bulk Dirac electrons at room temperature by exploiting a giant Rashba effect in a bulk semiconductor. The photocurrent is spin polarized due to the spin-momentum locking of the electronic states, which is manifested by a sign reversal upon flipping either the photon helicity or the sign of the Rashba parameter, without any stray current. The action spectra revealed the photon energy range, where the photocurrent is carried by the Dirac electrons at the inner Fermi surface. This photogalvanic control is enabled by the sizable spin splittings both at the valence and conduction bands with the same helicity, and also by a number of optical transition pathways compared to those in the two-dimensional Rashba systems. An efficient coupling between the photon field and large spin-orbit interaction is accordingly proposed to allow the universal control of Dirac electrons.

  4. Defects at semiconductor surfaces

    NASA Astrophysics Data System (ADS)

    Henzler, Martin

    1985-04-01

    Low Energy Electron Diffraction (LEED) is widely used for detection of periodicity at the surface and of atom arrangement within the unit cell. Experiments and results, however, are increasing, which use the spot profile analysis (SPA-LEED) for the study of nonperiodic surfaces. Here the kinematical approximation provides a wider range of validity than expected. For semiconductors defects are especially important, since the surface states in the gap are determined or strongly influenced by almost any kind of defects at the surface. Atomic steps at the interface {Si}/{SiO2} have been shown to be correlated with many electronic properties of MOS devices like mobility, interface states and fixed charge. The epitaxy on Si and GaAs has been studied with LEED and RHEED, showing the density of the nuclei during formation of a layer and the layer-by-layer growth. The formation of metal suicides in the monolayer range is accompanied by many different superstructures and other rearrangements. It is demonstrated, that the new high resolution instruments provide additional qualitative and quantitative informations on any kind of surface defects.

  5. Method of passivating semiconductor surfaces

    DOEpatents

    Wanlass, Mark W.

    1990-01-01

    A method of passivating Group III-V or II-VI semiconductor compound surfaces. The method includes selecting a passivating material having a lattice constant substantially mismatched to the lattice constant of the semiconductor compound. The passivating material is then grown as an ultrathin layer of passivating material on the surface of the Group III-V or II-VI semiconductor compound. The passivating material is grown to a thickness sufficient to maintain a coherent interface between the ultrathin passivating material and the semiconductor compound. In addition, a device formed from such method is also disclosed.

  6. Method of passivating semiconductor surfaces

    DOEpatents

    Wanlass, M.W.

    1990-06-19

    A method is described for passivating Group III-V or II-VI semiconductor compound surfaces. The method includes selecting a passivating material having a lattice constant substantially mismatched to the lattice constant of the semiconductor compound. The passivating material is then grown as an ultrathin layer of passivating material on the surface of the Group III-V or II-VI semiconductor compound. The passivating material is grown to a thickness sufficient to maintain a coherent interface between the ultrathin passivating material and the semiconductor compound. In addition, a device formed from such method is also disclosed.

  7. Spin Splitting and Spin Current in Strained Bulk Semiconductors

    SciTech Connect

    Bernevig, B.Andrei; Zhang, Shou-Cheng; /Stanford U., Phys. Dept.

    2010-01-15

    We present a theory for two recent experiments in bulk strained semiconductors and show that a new, previously overlooked, strain spin-orbit coupling term may play a fundamental role. We propose simple experiments that could clarify the origin of strain-induced spin-orbit coupling terms in inversion asymmetric semiconductors. We predict that a uniform magnetization parallel to the electric field will be induced in the samples studied in for specific directions of the applied electric field. We also propose special geometries to detect spin currents in strained semiconductors.

  8. Crystal growth of semiconductor bulk crystals

    SciTech Connect

    Kakimoto, Koichi

    2010-07-22

    This course is aimed at showing how to grow bulk crystals by using several methods. The course involves the following points. The growth methods of Bridgman and Czochralski will be introduced. The course also focuses on the mechanism of some processes with consideration of the basic phenomenon. Experimental and numerical examples of the methods will also be introduced.

  9. Surface recombination in semiconductors

    SciTech Connect

    Langer, J.M.; Walukiewicz, W.

    1995-07-01

    We propose two general criteria for a surface defect state to act as an efficient, nonradiative recombination center. The first is that the thermal ionization energy should not deviate from the mid-gap energy by more than the relaxation energy of the defect, In this case the activation energy for the recombination is given by the barrier for the capture of the first carrier, whereas the second carrier is captured athermally. The second citerion is related to the position of the average dangling bond energy relative to the band edges. If, as in the cases of InP or InAs, it is located close to a band edge, a low surface recombination velocity is expected. However a much faster recombination is predicated and experimentally observed in the materials with the average dangling bond energy located close to the mid-gap. The relevance of these criteria for the novel wide-gap optoelectronic materials is discussed.

  10. Modeling and simulation of bulk gallium nitride power semiconductor devices

    NASA Astrophysics Data System (ADS)

    Sabui, G.; Parbrook, P. J.; Arredondo-Arechavala, M.; Shen, Z. J.

    2016-05-01

    Bulk gallium nitride (GaN) power semiconductor devices are gaining significant interest in recent years, creating the need for technology computer aided design (TCAD) simulation to accurately model and optimize these devices. This paper comprehensively reviews and compares different GaN physical models and model parameters in the literature, and discusses the appropriate selection of these models and parameters for TCAD simulation. 2-D drift-diffusion semi-classical simulation is carried out for 2.6 kV and 3.7 kV bulk GaN vertical PN diodes. The simulated forward current-voltage and reverse breakdown characteristics are in good agreement with the measurement data even over a wide temperature range.

  11. Surface Flashover of Semiconductors: A Fundamental Study

    DTIC Science & Technology

    1993-06-16

    mechanisms responsible for inducing surface flashover in semiconductors , par- ticularly silicon. In order to achieve this...of semiconductors . 2.9.2 Our Modelfor the Surface Flashover Mechanism We have developed a model in which the flashover initiation process occurs on...is described in detail in the following pages. 3-1 4. CUMULATIVE LIST OF PUBLICATIONS 1. "A Mechanism for Surface Flashover of Semiconductors ,"

  12. Compound semiconductor surfaces and interfaces

    NASA Astrophysics Data System (ADS)

    Wilmsen, C. W.

    All semiconductor devices and integrated circuits require dielectric layers for interconnect isolation, gate oxides, passivation, etc. Thermally grown SiO2 is the primary dielectric film for silicon technology. This ONR contract investigated the chemistry and electrical properties of the interface between III-V compound semiconductors and electrical properties of grown and deposited oxides. When this study began, very little was known about these oxides or their interfaces. The results of the research supported by this contract has added substantially to the fundamental understanding of the oxide growth, properties and electronic structure. Much of the experimental work was obtained with surface analytical techniques and significant contributions were also made in this field. The original ONR contract began in October 1975 and initially concentrated on the grown oxides of InP and GaAs. This work evolved over the years to include the study of deposited insulator/InP interfaces and trapping at the interface. The ONR contract has supported the publication of 31 journal articles and book chapters and numerous conference presentations.

  13. Stable surface passivation process for compound semiconductors

    DOEpatents

    Ashby, Carol I. H.

    2001-01-01

    A passivation process for a previously sulfided, selenided or tellurated III-V compound semiconductor surface. The concentration of undesired mid-gap surface states on a compound semiconductor surface is reduced by the formation of a near-monolayer of metal-(sulfur and/or selenium and/or tellurium)-semiconductor that is effective for long term passivation of the underlying semiconductor surface. Starting with the III-V compound semiconductor surface, any oxidation present thereon is substantially removed and the surface is then treated with sulfur, selenium or tellurium to form a near-monolayer of chalcogen-semiconductor of the surface in an oxygen-free atmosphere. This chalcogenated surface is then contacted with a solution of a metal that will form a low solubility chalcogenide to form a near-monolayer of metal-chalcogen-semiconductor. The resulting passivating layer provides long term protection for the underlying surface at or above the level achieved by a freshly chalcogenated compound semiconductor surface in an oxygen free atmosphere.

  14. Chemical modification of semiconductor surfaces

    NASA Technical Reports Server (NTRS)

    Finklea, H. O.

    1981-01-01

    Results of research on the chemical modification of TiO2 powders in the gas phase and the examination of the modified powders by infrared absorption spectroscopy are comprehensively summarized. The range of information obtainable by IR spectroscopy of chemically modified semiconductors, and a definition of the optimum reaction conditions for synthesizing a monolayer of methylsilanes using vapor phase reaction conditions were considered.

  15. Hamiltonian decomposition for bulk and surface states.

    PubMed

    Sasaki, Ken-Ichi; Shimomura, Yuji; Takane, Yositake; Wakabayashi, Katsunori

    2009-04-10

    We demonstrate that a tight-binding Hamiltonian with nearest- and next-nearest-neighbor hopping integrals can be decomposed into bulk and boundary parts for honeycomb lattice systems. The Hamiltonian decomposition reveals that next-nearest-neighbor hopping causes sizable changes in the energy spectrum of surface states even if the correction to the energy spectrum of bulk states is negligible. By applying the Hamiltonian decomposition to edge states in graphene systems, we show that the next-nearest-neighbor hopping stabilizes the edge states. The application of Hamiltonian decomposition to a general lattice system is discussed.

  16. Molecular engineering of semiconductor surfaces and devices.

    PubMed

    Ashkenasy, Gonen; Cahen, David; Cohen, Rami; Shanzer, Abraham; Vilan, Ayelet

    2002-02-01

    Grafting organic molecules onto solid surfaces can transfer molecular properties to the solid. We describe how modifications of semiconductor or metal surfaces by molecules with systematically varying properties can lead to corresponding trends in the (electronic) properties of the resulting hybrid (molecule + solid) materials and devices made with them. Examples include molecule-controlled diodes and sensors, where the electrons need not to go through the molecules (action at a distance), suggesting a new approach to molecule-based electronics.

  17. Surface passivation process of compound semiconductor material using UV photosulfidation

    DOEpatents

    Ashby, Carol I. H.

    1995-01-01

    A method for passivating compound semiconductor surfaces by photolytically disrupting molecular sulfur vapor with ultraviolet radiation to form reactive sulfur which then reacts with and passivates the surface of compound semiconductors.

  18. Surface Conduction in III-V Semiconductor Infrared Detector Materials

    NASA Astrophysics Data System (ADS)

    Sidor, Daniel Evan

    III-V semiconductors are increasingly used to produce high performance infrared photodetectors; however a significant challenge inherent to working with these materials is presented by unintended electrical conduction pathways that form along their surfaces. Resulting leakage currents contribute to system noise and are ineffectively mitigated by device cooling, and therefore limit ultimate performance. When the mechanism of surface conduction is understood, the unipolar barrier device architecture offers a potential solution. III-V bulk unipolar barrier detectors that effectively suppress surface leakage have approached the performance of the best II-VI pn-based structures. This thesis begins with a review of empirically determined Schottky barrier heights and uses this information to present a simple model of semiconductor surface conductivity. The model is validated through measurements of degenerate n-type surface conductivity on InAs pn junctions, and non-degenerate surface conductivity on GaSb pn junctions. It is then extended, along with design principles inspired by the InAs-based nBn detector, to create a flat-band pn-based unipolar barrier detector possessing a conductive surface but free of detrimental surface leakage current. Consideration is then given to the relative success of these and related bulk detectors in suppressing surface leakage when compared to analogous superlattice-based designs, and general limitations of unipolar barriers in suppressing surface leakage are proposed. Finally, refinements to the molecular beam epitaxy crystal growth techniques used to produce InAs-based unipolar barrier heterostructure devices are discussed. Improvements leading to III-V device performance well within an order of magnitude of the state-of-the-art are demonstrated.

  19. Surface-enhanced Raman spectroscopy of semiconductor nanostructures

    NASA Astrophysics Data System (ADS)

    Milekhin, A. G.; Sveshnikova, L. L.; Duda, T. A.; Yeryukov, N. A.; Rodyakina, E. E.; Gutakovskii, A. K.; Batsanov, S. A.; Latyshev, A. V.; Zahn, D. R. T.

    2016-01-01

    We review our recent results concerning surface-enhanced Raman scattering (SERS) by confined optical and surface optical phonons in semiconductor nanostructures including CdS, CuS, GaN, and ZnO nanocrystals, GaN and ZnO nanorods, and AlN nanowires. Enhancement of Raman scattering by confined optical phonons as well as appearance of new Raman modes with the frequencies different from those in ZnO bulk attributed to surface optical modes is observed in a series of nanostructures having different morphology located in the vicinity of metal nanoclusters (Ag, Au, and Pt). Assignment of surface optical modes is based on calculations performed in the frame of the dielectric continuum model. It is established that SERS by phonons has a resonant character. A maximal enhancement by optical phonons as high as 730 is achieved for CdS nanocrystals in double resonant conditions at the coincidence of laser energy with that of electronic transitions in semiconductor nanocrystals and localized surface plasmon resonance in metal nanoclusters. Even a higher enhancement is observed for SERS by surface optical modes in ZnO nanocrystals (above 104). Surface enhanced Raman scattering is used for studying phonon spectrum in nanocrystal ensembles with an ultra-low areal density on metal plasmonic nanostructures.

  20. Surface Bonding Effects in Compound Semiconductor Nanoparticles: II

    SciTech Connect

    Helen H. Farrell

    2008-07-01

    Small nanoparticles have a large proportion of their atoms either at or near the surface, and those in clusters are essentially all on the surface. As a consequence, the details of the surface structure are of paramount importance in governing the overall stability of the particle. Just as with bulk materials, factors that determine this stability include “bulk” structure, surface reconstruction, charge balance and hybridization, ionicity, strain, stoichiometry, and the presence of adsorbates. Needless to say, many of these factors, such as charge balance, hybridization and strain, are interdependent. These factors all contribute to the overall binding energy of clusters and small nanoparticles and play a role in determining the deviations from an inverse size dependence that we have previously reported for compound semiconductor materials. Using first-principles density functional theory calculations, we have explored how these factors influence particle stability under a variety of conditions.

  1. Electrons, Phonons and Excitons at Semiconductor Surfaces

    NASA Astrophysics Data System (ADS)

    Pollmann, Johannes; Krueger, Peter; Mazur, Albert; Rohlfing, Michael

    We briefly address 'state-of-the-art' ab-initio calculations of basic properties of semiconductor surfaces such as their atomic configuration, electronic structure and surface vibrations, as well as, their optical properties and compare exemplary results with experimental data. The surface structure and the electronic ground state are described within the local density approximation (LDA) of density functional theory (DFT). The description of excited electronic states requires to take dynamical correlations in the many electron system into account, which is achieved to a considerable extent within the GW approximation (GWA) leading to the concept of the quasiparticle bandstructure. Surface phonons are treated from first principles within density functional perturbation theory (DFPT). The theory of optical surface properties and surface excitons, in particular, requires to account for the electron-hole Coulomb correlation which is done in the framework of the Bethe-Salpeter equation (BSE). These methods yield results which are in good agreement with experiment and can significantly contribute to an interpretation of experimental data from high-resolution surface microscopy and spectroscopy thus enhancing our current understanding of semiconductor surfaces.

  2. Bulk water freezing dynamics on superhydrophobic surfaces

    NASA Astrophysics Data System (ADS)

    Chavan, S.; Carpenter, J.; Nallapaneni, M.; Chen, J. Y.; Miljkovic, N.

    2017-01-01

    In this study, we elucidate the mechanisms governing the heat-transfer mediated, non-thermodynamic limited, freezing delay on non-wetting surfaces for a variety of characteristic length scales, Lc (volume/surface area, 3 mm < Lc < 6 mm) using carefully designed freezing experiments in a temperature-controlled, zero-humidity environment on thin water slabs. To probe the effect of surface wettability, we investigated the total time for room temperature water to completely freeze into ice on superhydrophilic ( θaapp→ 0°), hydrophilic (0° < θa < 90°), hydrophobic (90° < θa < 125°), and superhydrophobic ( θaapp→ 180°) surfaces. Our results show that at macroscopic length scales, heat conduction through the bulk water/ice layer dominates the freezing process when compared to heat conduction through the functional coatings or nanoscale gaps at the superhydrophobic substrate-water/ice interface. In order to verify our findings, and to determine when the surface structure thermal resistance approaches the water/ice resistance, we fabricated and tested the additional substrates coated with commercial superhydrophobic spray coatings, showing a monotonic increase in freezing time with coating thickness. The added thermal resistance of thicker coatings was much larger than that of the nanoscale superhydrophobic features, which reduced the droplet heat transfer and increased the total freezing time. Transient finite element method heat transfer simulations of the water slab freezing process were performed to calculate the overall heat transfer coefficient at the substrate-water/ice interface during freezing, and shown to be in the range of 1-2.5 kW/m2K for these experiments. The results shown here suggest that in order to exploit the heat-transfer mediated freezing delay, thicker superhydrophobic coatings must be deposited on the surface, where the coating resistance is comparable to the bulk water/ice conduction resistance.

  3. Separation of the surface and bulk recombination in silicon by means of transient photoluminescence

    NASA Astrophysics Data System (ADS)

    Heinz, Friedemann D.; Warta, Wilhelm; Schubert, Martin C.

    2017-01-01

    The bulk and surface recombination determine the electrical performance of many semiconductor devices. Yet, the experimental determination and separation of both surface and bulk recombination rate remains challenging. This paper presents the measurement and separation of the bulk and surface recombination in silicon by means of time resolved photoluminescence spectroscopy. The high temporal resolution of the applied time correlated single photon counting technique is exploited to access the photoluminescence response of a silicon sample upon pulsed excitation in the nanosecond to millisecond regime on a sub-cm2 area. A rigorous data fitting algorithm based on two dimensional numeric simulations of the induced charge carrier dynamics is applied to extract all information on bulk and surface recombination properties from the recorded photoluminescence transients. Using different samples with symmetric as well as asymmetric surface recombination properties, we demonstrate the capabilities of the proposed contactless and nondestructive technique, which may be applicable to silicon based mono- or multi-junction devices.

  4. "Dip-Pen" nanolithography on semiconductor surfaces.

    PubMed

    Ivanisevic, A; Mirkin, C A

    2001-08-15

    Dip-Pen Nanolithography (DPN) uses an AFM tip to deposit organic molecules through a meniscus onto an underlying substrate under ambient conditions. Thus far, the methodology has been developed exclusively for gold using alkyl or aryl thiols as inks. This study describes the first application of DPN to write organic patterns with sub-100 nm dimensions directly onto two different semiconductor surfaces: silicon and gallium arsenide. Using hexamethyldisilazane (HMDS) as the ink in the DPN procedure, we were able to utilize lateral force microscopy (LFM) images to differentiate between oxidized semiconductor surfaces and patterned areas with deposited monolayers of HMDS. The choice of the silazane ink is a critical component of the process since adsorbates such as trichlorosilanes are incompatible with the water meniscus and polymerize during ink deposition. This work provides insight into additional factors, such as temperature and adsorbate reactivity, that control the rate of the DPN process and paves the way for researchers to interface organic and biological structures generated via DPN with electronically important semiconductor substrates.

  5. Multiscale approach to the electronic structure of doped semiconductor surfaces

    NASA Astrophysics Data System (ADS)

    Sinai, Ofer; Hofmann, Oliver T.; Rinke, Patrick; Scheffler, Matthias; Heimel, Georg; Kronik, Leeor

    2015-02-01

    The inclusion of the global effects of semiconductor doping poses a unique challenge for first-principles simulations, because the typically low concentration of dopants renders an explicit treatment intractable. Furthermore, the width of the space-charge region (SCR) at charged surfaces often exceeds realistic supercell dimensions. Here, we present a multiscale technique that fully addresses these difficulties. It is based on the introduction of a charged sheet, mimicking the SCR-related field, along with free charge which mimics the bulk charge reservoir, such that the system is neutral overall. These augment a slab comprising "pseudoatoms" possessing a fractional nuclear charge matching the bulk doping concentration. Self-consistency is reached by imposing charge conservation and Fermi level equilibration between the bulk, treated semiclassically, and the electronic states of the slab, which are treated quantum-mechanically. The method, called CREST—the charge-reservoir electrostatic sheet technique—can be used with standard electronic structure codes. We validate CREST using a simple tight-binding model, which allows for comparison of its results with calculations encompassing the full SCR explicitly. Specifically, we show that CREST successfully predicts scenarios spanning the range from no to full Fermi level pinning. We then employ it with density functional theory, obtaining insight into the doping dependence of the electronic structures of the metallic "clean-cleaved" Si(111) surface and its semiconducting (2 ×1 ) reconstructions.

  6. Gated Hall effect of nanoplate devices reveals surface-state-induced surface inversion in iron pyrite semiconductor.

    PubMed

    Liang, Dong; Cabán-Acevedo, Miguel; Kaiser, Nicholas S; Jin, Song

    2014-12-10

    Understanding semiconductor surface states is critical for their applications, but fully characterizing surface electrical properties is challenging. Such a challenge is especially crippling for semiconducting iron pyrite (FeS2), whose potential for solar energy conversion has been suggested to be held back by rich surface states. Here, by taking advantage of the high surface-to-bulk ratio in nanostructures and effective electrolyte gating, we develop a general method to fully characterize both the surface inversion and bulk electrical transport properties for the first time through electrolyte-gated Hall measurements of pyrite nanoplate devices. Our study shows that pyrite is n-type in the bulk and p-type near the surface due to strong inversion and yields the concentrations and mobilities of both bulk electrons and surface holes. Further, solutions of the Poisson equation reveal a high-density of surface holes accumulated in a 1.3 nm thick strong inversion layer and an upward band bending of 0.9-1.0 eV. This work presents a general methodology for using transport measurements of nanostructures to study both bulk and surface transport properties of semiconductors. It also suggests that high-density of surface states are present on surface of pyrite, which partially explains the universal p-type conductivity and lack of photovoltage in polycrystalline pyrite.

  7. Theory of electron g-tensor in bulk and quantum-well semiconductors

    NASA Astrophysics Data System (ADS)

    Lau, Wayne H.; Flatte', Michael E.

    2004-03-01

    We present quantitative calculations for the electron g-tensors in bulk and quantum-well semiconductors based on a generalized P.p envelope function theory solved in a fourteen-band restricted basis set. The dependences of g-tensor on structure, magnetic field, carrier density, temperature, and spin polarization have been explored and will be described. It is found that at temperatures of a few Kelvin and fields of a few Tesla, the g-tensors for bulk semiconductors develop quasi-steplike dependences on carrier density or magnetic field due to magnetic quantization, and this effect is even more pronounced in quantum-well semiconductors due to the additional electric quantization along the growth direction. The influence of quantum confinement on the electron g-tensors in QWs is studied by examining the dependence of electron g-tensors on well width. Excellent agreement between these calculated electron g-tensors and measurements [1-2] is found for GaAs/AlGaAs QWs. This work was supported by DARPA/ARO. [1] A. Malinowski and R. T. Harley, Phys. Rev. B 62, 2051 (2000);[2] Le Jeune et al., Semicond. Sci. Technol. 12, 380 (1997).

  8. Scattering of carriers by coupled plasmon-phonon modes in bulk polar semiconductors and polar semiconductor heterostructures

    NASA Astrophysics Data System (ADS)

    Hauber, Anna; Fahy, Stephen

    2017-01-01

    We present a general treatment of carrier scattering by coupled phonon-plasmon collective modes in polar semiconductors, taking anharmonic phonon decay into account and self-consistently calculating carrier momentum relaxation rates and carrier mobility in a parabolic band model. We iteratively solve the weak-field Boltzmann equations for carriers and collective modes and obtain their nonequilibrium distribution functions. Both the scattering rates and the anharmonic decay of the coupled modes are expressed through the total dielectric function of the semiconductor, consisting of a damped lattice dielectric function, and a temperature dependent random phase approximation dielectric function for the carrier plasma. We show that the decay of the coupled modes has a significant effect on the contribution to the mobility limited by carrier-coupled mode scattering. We also propose a scalar quantity, the phonon dissipation weight factor, with which this effect can be estimated from an analytic expression. We apply this treatment to dynamically screened electron-longitudinal optical phonon scattering in bulk polar semiconductors, and to dynamically screened remote phonon scattering in polar heterostructures where monolayers of MoS2 are sandwiched between various polar dielectrics. We find that a dynamic treatment of the remote phonon scattering yields mobilities up to 75% higher than a static screening approximation does for structures which consist of a monolayer of MoS2 between hafnia and silica. Moreover, we show that accounting for the nonzero thickness of the MoS2 interface layer has an important effect on the calculated mobility in the same structure.

  9. Challenges and opportunities in chemical functionalization of semiconductor surfaces

    NASA Astrophysics Data System (ADS)

    Gao, Fei; Teplyakov, Andrew V.

    2017-03-01

    The field of chemical functionalization of semiconductor surfaces has developed tremendously over the last several decades. Since silicon occupied the main portion of the industrial applications of semiconductors, understanding its surface chemistry at a molecular level is very advanced. This particular field has also benefited from combining the work on well-defined clean silicon surfaces in vacuum, well-characterized chemically-passivated surfaces in ambient, and current and potential applications of the resulting interfaces. This article will use this work to build a generalized evaluation of the developments in surface chemical functionalization, specifically addressing organic functionalization of semiconductors, and consider the challenges and opportunities for further evolution of the field.

  10. Bulk-mediated surface transport in the presence of bias

    NASA Astrophysics Data System (ADS)

    Berezhkovskii, Alexander M.; Dagdug, Leonardo; Bezrukov, Sergey M.

    2017-07-01

    Surface transport, when the particle is allowed to leave the surface, travel in the bulk for some time, and then return to the surface, is referred to as bulk-mediated surface transport. Recently, we proposed a formalism that significantly simplifies analysis of bulk-mediated surface diffusion [A. M. Berezhkovskii, L. Dagdug, and S. M. Bezrukov, J. Chem. Phys. 143, 084103 (2015)]. Here this formalism is extended to bulk-mediated surface transport in the presence of bias, i.e., when the particle has arbitrary drift velocities on the surface and in the bulk. A key advantage of our approach is that the transport problem reduces to that of a two-state problem of the particle transitions between the surface and the bulk. The latter can be solved with relative ease. The formalism is used to find the Laplace transforms of the first two moments of the particle displacement over the surface in time t at arbitrary values of the particle drift velocities and diffusivities on the surface and in the bulk. This allows us to analyze in detail the time dependence of the effective drift velocity of the particle on the surface, which can be highly nontrivial.

  11. Non-equilibrium optical phonon dynamics in bulk and low-dimensional semiconductors

    NASA Astrophysics Data System (ADS)

    Srivastava, G. P.

    2007-02-01

    We present theoretical investigations of the intrinsic dynamics of long-wavelength non-equilibrium optical phonons in bulk and low-dimensional semiconductors. The theory is based on the application of Fermi's golden rule formula, with phonon dispersion relations as well as crystal anharmonicity considered in the framework of isotropic continuum model. Contributions to the decay rates of the phonon modes are discussed in terms of four possible channels: Klemens channel (into two acoustic daughter modes), generalised Ridley channel (into one acoustic and one optical mode), generalised Vallee-Bogani channel (into a lower mode of the same branch and an acoustic mode), and Barman-Srivastava channel (into two lower-branch optical modes). The role of crystal structure and cation/anion mass ratio in determining the lifetime of such modes in bulk semiconductors is highlighted. Estimates of lifetimes of such modes in silicon nanowires and carbon nanotubes will also be presented. The results support and explain available experimental data, and make predictions in some cases.

  12. Quantum kinetic theory of optically injected electrical and spin currents in bulk semiconductors

    NASA Astrophysics Data System (ADS)

    Prepelita, Oleg; Sipe, J. E.

    2002-03-01

    We consider the relaxation of coherently controlled currents and spin currents in bulk semiconductors. The currents are injected by simultaneous irradiation with two laser beams; the magnitude of both currents is controlled by the relative phases of these two fields [1,2]. At low injected carrier densities the relaxation of the currents occurs because of the interaction of carriers with the equilibrium optical phonons. Using a quantum master equation approach, Boltzmann type microscopic equations for the averages of the electron-hole subsystem were obtained. From these microscopic equations a simple system of linear differential equations for the evolution of the macroscopic electrical current and spin current in semiconductors can be obtained and easily solved, thus vastly simplifying the calculation of current and spin current relaxation. The developed theory was applied to bulk GaAs. 1.A. Hache, Y. Kostoulas, J. L. P. Hughes, J. E. Sipe, and H. M. van Driel, Phys. Rev. Lett. 78, 306 (1997). 2.R. D. R. Bhat and J. E. Sipe, Phys. Rev. Lett. 85, 5432 (2000).

  13. Surface roughness scattering of electrons in bulk mosfets

    SciTech Connect

    Zuverink, Amanda Renee

    2015-11-01

    Surface-roughness scattering of electrons at the Si-SiO2 interface is a very important consideration when analyzing Si metal-oxide-semiconductor field-effect transistors (MOSFETs). Scattering reduces the mobility of the electrons and degrades the device performance. 250-nm and 50-nm bulk MOSFETs were simulated with varying device parameters and mesh sizes in order to compare the effects of surface-roughness scattering in multiple devices. The simulation framework includes the ensemble Monte Carlo method used to solve the Boltzmann transport equation coupled with a successive over-relaxation method used to solve the two-dimensional Poisson's equation. Four methods for simulating the surface-roughness scattering of electrons were implemented on both devices and compared: the constant specularity parameter, the momentum-dependent specularity parameter, and the real-space-roughness method with both uniform and varying electric fields. The specularity parameter is the probability of an electron scattering speculariy from a rough surface. It can be chosen as a constant, characterizing partially diffuse scattering of all electrons from the surface the same way, or it can be momentum dependent, where the size of rms roughness and the normal component of the electron wave number determine the probability of electron-momentum randomization. The real-space rough surface method uses the rms roughness height and correlation length of an actual MOSFET to simulate a rough interface. Due to their charge, electrons scatter from the electric field and not directly from the surface. If the electric field is kept uniform, the electrons do not perceive the roughness and scatter as if from a at surface. However, if the field is allowed to vary, the electrons scatter from the varying electric field as they would in a MOSFET. These methods were implemented for both the 50-nm and 250-nm MOSFETs, and using the rms roughness heights and correlation lengths for real devices. The

  14. Photoreactions on (110) compound semiconductor surfaces

    NASA Astrophysics Data System (ADS)

    Lasky, Peter John

    Photo-initiated and thermal reactions of alkyl halides and dimethyl metals on (110) GaAs and CdTe surfaces have been studied under Ultra High Vacuum (UHV) conditions. These specific systems have been chosen because they allow investigation of mechanisms underlying reactions relevant to semiconductor processing. Thermal reactivity studies, binding energy measurements, coverage calibrations, and analyses of photoreaction products have been made using Temperature Programmed Desorption (TPD). The dynamics of photoreactions, induced by irradiation from excimer and excimer-pumped dye lasers, have been investigated using Time of Flight (TOF) methods with a quadrupole mass spectrometer. On these (110) semiconductor surfaces, methyl halides, CH3X (X = Cl, Br, I), exhibit molecular adsorption and desorption at [<]240 K. Modeling of TPD spectra with a repulsive interadsorbate force gives some indication of the formation of ordered overlayers with a tilted orientation. Upon irradiation, methyl fragments are ejected into the vacuum and angular-resolved detection reflects the tilt of the C-X bond axis. For the GaAs case, the orientation has been confirmed by Near- edge X-ray Adsorption Fine Structure (NEXAFS) measurements and corroborated by ab initio calculations done in collaboration with Professor Friesner's group. TOF studies indicate that, in the first monolayer, dissociation proceeds via creation of hot electrons in the substrate and transfer to adsorbate molecules resulting in a process analogous to Dissociative Electron Attachment (DEA) seen in the gas phase. The initial excitation energy necessary to induce this hot electron chemistry with CH3Br on GaAs(110) has been accurately determined and is below that needed for photoemission. With multilayer coverages, direct photolysis is observed; however, it is quenched in the first monolayer. Similar effects are observed in other alkyl halides. (CH3)2M, M = Cd, Zn adsorb molecularly on GaAs(110) at 80 K, but upon heating

  15. Depletion-mode polysilicon optical modulators in a bulk complementary metal-oxide semiconductor process.

    PubMed

    Shainline, Jeffrey M; Orcutt, Jason S; Wade, Mark T; Nammari, Kareem; Tehar-Zahav, Ofer; Sternberg, Zvi; Meade, Roy; Ram, Rajeev J; Stojanović, Vladimir; Popović, Miloš A

    2013-08-01

    We demonstrate depletion-mode carrier-plasma optical modulators fabricated in a bulk complementary metal-oxide semiconductor (CMOS), DRAM-emulation process. To the best of our knowledge, these are the first depletion-mode modulators demonstrated in polycrystalline silicon and in bulk CMOS. The modulators are based on novel optical microcavities that utilize periodic spatial interference of two guided modes to create field nulls along waveguide sidewalls. At these nulls, electrical contacts can be placed while preserving a high optical Q. These cavities enable active devices in a process with no partial silicon etch and with lateral p-n junctions. We demonstrate two device variants at 5 Gbps data modulation rate near 1610 nm wavelength. One design shows 3.1 dB modulation depth with 1.5 dB insertion loss and an estimated 160 fJ/bit energy consumption, while a more compact device achieves 4.2 dB modulation depth with 4.0 dB insertion loss and 60 fJ/bit energy consumption. These modulators represent a significant breakthrough in enabling active photonics in bulk silicon CMOS--the platform of the majority of microelectronic logic and DRAM processes--and lay the groundwork for monolithically integrated CMOS-to-DRAM photonic links.

  16. Shubnikov-de Haas Oscillations in the Bulk Rashba Semiconductor BiTeI

    SciTech Connect

    Bell, C.; Bahramy, M.S.; Murakawa, H.; Checkelsky, J.G.; Arita, R.; Kaneko, Y.; Onose, Y.; Nagaosa, N.; Tokura, Y.; Hwang, H.Y.

    2012-07-11

    Bulk magnetoresistance quantum oscillations are observed in high quality single crystal samples of BiTeI. This compound shows an extremely large internal spin-orbit coupling, associated with the polarity of the alternating Bi, Te, and I layers perpendicular to the c-axis. The corresponding areas of the inner and outer Fermi surfaces around the A-point show good agreement with theoretical calculations, demonstrating that the intrinsic bulk Rashba-type splitting is nearly 360 meV, comparable to the largest spin-orbit coupling generated in heterostructures and at surfaces.

  17. Continuity of the nucleation of bulk and surface phases.

    PubMed

    Sear, Richard P

    2008-10-28

    Nucleation of a new bulk phase is almost always heterogeneous: It occurs at a surface. Nucleation of a new surface phase also occurs at a surface of course. We study nucleation in an Ising model with both bulk and surface (prewetting) transitions. We find no clear distinction between nucleation of the bulk and surface phases. As the bulk transition is crossed, the phase that is nucleating changes from being a new surface phase to being a new bulk phase. However, neither the nucleation rate nor the nucleus itself changes discontinuously. It appears that because the nucleus is only approximately two spins high, it is oblivious to the fact that on one side of the bulk transition it will grow to form a thick wetting layer, while on the other side it will grow to form a bulk phase of limitless thickness. The phase diagram of hard spheres at a hard wall is analogous to that of our model. Therefore, we suggest that the nucleation of the crystal phase of hard spheres at a hard wall may also be indistinguishable from that associated with a prefreezing surface transition.

  18. - and Mesoscopic Patterns on Semiconductor Surfaces

    NASA Astrophysics Data System (ADS)

    Samuilov, Vladimir; Seo, Young-Soo; Ksenevich, Vitaly; Galibert, Jean; Sokolov, Jonathan; Rafailovich, Miriam

    2002-03-01

    A novel and simple approach of self-organized fabrication of two dimensional mesoscopic networks with the feature size down to 50 nm has been developed. The technique is based on the self-organized patterning in a thin layer of complex liquid (polymer solution) in the presence of humid atmosphere. Two dimensional mesoscopic honeycomb-shaped carbon structures were produced by high temperature annealing of nitrocellulose precursors [1]. The polymer network was also utilized as a mask for further reactive ion etching of surfaces with epi-layer of GaAs [2,3] and GaAs/AlGaAs d -doped heterostructures [4]. These structures can be considered as regular arrays of mesoscopic cells or 2-D photonic band gap crystals. Also, we have used diblock-copolymer system, self-assembled with L-B technique, to produce nano-patterns. These structures were utilized as templates for introducing metal nanopatterns on semiconductor surfaces by reactive ion beam etching for magnetic storage systems [5] and DNA separation on a flat surface [6] devices. 1. V.A. Samuilov, J. Galibert, V.K. Ksenevich, V.J. Goldman, M. Rafailovich, J. Sokolov, I.A. Bashmakov, V.A. Dorosinets, Magnetotransport in mesoscopic carbon networks, Physica B, 294-295, p.p.319-323, 2001. 2. V.A. Samuilov, I.B. Butylina, L.V. Govor, V.K. Ksenevich, I.A. Bashmakov, I.M. Grigorieva, L.V. Solovjova, Fabrication of regular mesoscopic networks of GaAs wires, Superlattices and Microstructures, 25: (1-2) p.p.127-130, 1999. 3. V.A. Samuilov, I.B. Butylina, V.K. Ksenevich, G. Kiss, G. Remenyi, Observation of transport in mesoscopic honeycomb-shaped networks, Superlattices and Microstructures, 25: (1-2) p.p.197-202, 1999 4. J. Galibert, V.A. Samuilov, V.K. Ksenevich, M. Rafailovich, J. Sokolov, Magnetoresistance of low dimensional mesoscopic honeycomb-shape GaAs networks, Physica B, 294-295, p.p.314-318, 2001. 5. S. Zhu, R.J. Gambino, M.H. Rafailovich, J. Sokolov, S.A. Schwarz, and R.D. Gomez, "Microscopic Magnetic Characterization of

  19. Determine electric field directions at semiconductor surfaces by femtosecond frequency domain interferometric second harmonic (FDISH) generation

    NASA Astrophysics Data System (ADS)

    Nelson, C. A.; Zhu, X.-Y.

    2016-10-01

    Optical excitations at semiconductor surfaces or interfaces are accompanied by transient interfacial electric fields due to charge redistribution or transfer. While such transient fields may be probed by time-resolved second harmonic generation (TR-SHG), it is difficult to determine the field direction, which is invaluable to unveiling the underlying physics. Here we apply a time-resolved frequency domain interferometric second harmonic (TR-FDISH) generation technique to determine the phase relationship between the SH field emitted from bulk GaAs(1 0 0) and the transient SH field from the space charge region. The interference between these two SH fields allow us to unambiguously determine the directions of transient electric fields. Since SH fields from a static bulk contribution and a changing electric field contribution are present at most semiconductor surfaces or interfaces under optical excitation, the TR-FDISH technique is of general significance to probing the dynamics of interfacial charge transfer/redistribution.

  20. Growth of Bulk Wide Bandgap Semiconductor Crystals and Their Potential Applications

    NASA Technical Reports Server (NTRS)

    Chen, Kuo-Tong; Shi, Detang; Morgan, S. H.; Collins, W. Eugene; Burger, Arnold

    1997-01-01

    Developments in bulk crystal growth research for electro-optical devices in the Center for Photonic Materials and Devices since its establishment have been reviewed. Purification processes and single crystal growth systems employing physical vapor transport and Bridgman methods were assembled and used to produce high purity and superior quality wide bandgap materials such as heavy metal halides and II-VI compound semiconductors. Comprehensive material characterization techniques have been employed to reveal the optical, electrical and thermodynamic properties of crystals, and the results were used to establish improved material processing procedures. Postgrowth treatments such as passivation, oxidation, chemical etching and metal contacting during the X-ray and gamma-ray device fabrication process have also been investigated and low noise threshold with improved energy resolution has been achieved.

  1. Phonon-Electron Interactions in Piezoelectric Semiconductor Bulk Acoustic Wave Resonators

    PubMed Central

    Gokhale, Vikrant J.; Rais-Zadeh, Mina

    2014-01-01

    This work presents the first comprehensive investigation of phonon-electron interactions in bulk acoustic standing wave (BAW) resonators made from piezoelectric semiconductor (PS) materials. We show that these interactions constitute a significant energy loss mechanism and can set practical loss limits lower than anharmonic phonon scattering limits or thermoelastic damping limits. Secondly, we theoretically and experimentally demonstrate that phonon-electron interactions, under appropriate conditions, can result in a significant acoustic gain manifested as an improved quality factor (Q). Measurements on GaN resonators are consistent with the presented interaction model and demonstrate up to 35% dynamic improvement in Q. The strong dependencies of electron-mediated acoustic loss/gain on resonance frequency and material properties are investigated. Piezoelectric semiconductors are an extremely important class of electromechanical materials, and this work provides crucial insights for material choice, material properties, and device design to achieve low-loss PS-BAW resonators along with the unprecedented ability to dynamically tune resonator Q. PMID:25001100

  2. Electrical and optical conductivities of hole gas in p-doped bulk III-V semiconductors

    NASA Astrophysics Data System (ADS)

    Mawrie, Alestin; Halder, Pushpajit; Ghosh, Barun; Ghosh, Tarun Kanti

    2016-09-01

    We study electrical and optical conductivities of hole gas in p-doped bulk III-V semiconductors described by the Luttinger Hamiltonian. We provide exact analytical expressions of the Drude conductivity, inverse relaxation time for various impurity potentials, Drude weight, and optical conductivity in terms of the Luttinger parameters γ1 and γ2. The back scattering is completely suppressed as a result of the helicity conservation of the heavy and light hole states. The energy dependence of the relaxation time for the hole states is different from the Brooks-Herring formula for electron gas in n-doped semiconductors. We find that the inverse relaxation time of heavy holes is much less than that of the light holes for Coulomb-type and Gaussian-type impurity potentials and vice-versa for a short-range impurity potential. The Drude conductivity increases non-linearly with the increase in the hole density. The exponent of the density dependence of the conductivity is obtained in the Thomas-Fermi limit. The Drude weight varies linearly with the density even in the presence of the spin-orbit coupling. The finite-frequency optical conductivity goes as √{ ω} , and its amplitude strongly depends on the Luttinger parameters. The Luttinger parameters can be extracted from the optical conductivity measurement.

  3. Large enhancements of thermopower and carrier mobility in quantum dot engineered bulk semiconductors.

    PubMed

    Liu, Yuanfeng; Sahoo, Pranati; Makongo, Julien P A; Zhou, Xiaoyuan; Kim, Sung-Joo; Chi, Hang; Uher, Ctirad; Pan, Xiaoqing; Poudeu, Pierre F P

    2013-05-22

    The thermopower (S) and electrical conductivity (σ) in conventional semiconductors are coupled adversely through the carriers' density (n) making it difficult to achieve meaningful simultaneous improvements in both electronic properties through doping and/or substitutional chemistry. Here, we demonstrate the effectiveness of coherently embedded full-Heusler (FH) quantum dots (QDs) in tailoring the density, mobility, and effective mass of charge carriers in the n-type Ti(0.1)Zr(0.9)NiSn half-Heusler matrix. We propose that the embedded FH QD forms a potential barrier at the interface with the matrix due to the offset of their conduction band minima. This potential barrier discriminates existing charge carriers from the conduction band of the matrix with respect to their relative energy leading to simultaneous large enhancements of the thermopower (up to 200%) and carrier mobility (up to 43%) of the resulting Ti(0.1)Zr(0.9)Ni(1+x)Sn nanocomposites. The improvement in S with increasing mole fraction of the FH-QDs arises from a drastic reduction (up to 250%) in the effective carrier density coupled with an increase in the carrier's effective mass (m*), whereas the surprising enhancement in the mobility (μ) is attributed to an increase in the carrier's relaxation time (τ). This strategy to manipulate the transport behavior of existing ensembles of charge carriers within a bulk semiconductor using QDs is very promising and could pave the way to a new generation of high figure of merit thermoelectric materials.

  4. Collective bulk carrier delocalization driven by electrostatic surface charge accumulation.

    PubMed

    Nakano, M; Shibuya, K; Okuyama, D; Hatano, T; Ono, S; Kawasaki, M; Iwasa, Y; Tokura, Y

    2012-07-25

    In the classic transistor, the number of electric charge carriers--and thus the electrical conductivity--is precisely controlled by external voltage, providing electrical switching capability. This simple but powerful feature is essential for information processing technology, and also provides a platform for fundamental physics research. As the number of charges essentially determines the electronic phase of a condensed-matter system, transistor operation enables reversible and isothermal changes in the system's state, as successfully demonstrated in electric-field-induced ferromagnetism and superconductivity. However, this effect of the electric field is limited to a channel thickness of nanometres or less, owing to the presence of Thomas-Fermi screening. Here we show that this conventional picture does not apply to a class of materials characterized by inherent collective interactions between electrons and the crystal lattice. We prepared metal-insulator-semiconductor field-effect transistors based on vanadium dioxide--a strongly correlated material with a thermally driven, first-order metal-insulator transition well above room temperature--and found that electrostatic charging at a surface drives all the previously localized charge carriers in the bulk material into motion, leading to the emergence of a three-dimensional metallic ground state. This non-local switching of the electronic state is achieved by applying a voltage of only about one volt. In a voltage-sweep measurement, the first-order nature of the metal-insulator transition provides a non-volatile memory effect, which is operable at room temperature. Our results demonstrate a conceptually new field-effect device, extending the concept of electric-field control to macroscopic phase control.

  5. Effect of bulk aging on surface diffusion of glasses

    NASA Astrophysics Data System (ADS)

    Brian, Caleb W.; Zhu, Lei; Yu, Lian

    2014-02-01

    The effect of physical aging on surface diffusion has been determined for two organic glasses, Indomethacin and Nifedipine. The two systems exhibit similar aging kinetics typical of organic glasses. Surface diffusivity remains unchanged despite significant bulk aging that nearly equilibrates the systems and increases the bulk relaxation time by orders of magnitude. The finding is relevant for understanding the stability of amorphous materials and the formation of low-energy glasses by vapor deposition.

  6. Thermal and Electric Cloaking Effect in Concentric Composite Made of Homogeneous Bulk and Porous Semiconductors

    NASA Astrophysics Data System (ADS)

    Tarkhanyan, Roland H.; Niarchos, Dimitris G.

    2015-12-01

    A new material platform is presented to manipulate heat and charge transportation in steady-state conditions. More precisely, we investigate the conceptual realization of a concentric composite made of the same isotropic and homogeneous semiconductor material with layers of different porosity, and show the possibility of a simultaneous cloaking performance in such a device for both heat flux and electric current without disturbing external fields. The background medium in the composite is a porous material with a periodical 3D cubic lattice of spherical hollow pores while the cylindrical shell is made from the same bulk material with zero porosity. A sound analytical expression is found for the volume fraction of the pores at which bi-functional cloaking effect can be realized. To validate our theoretical results, we also demonstrate the temperature and heat flux profiles as well as the voltage and current profiles in numerical simulations for a composite consisting of bulk (cylindrical shell) and porous (background) n-type silicon layers.

  7. Surface and bulk photochemistry of solids

    NASA Astrophysics Data System (ADS)

    Franchy, René

    1998-06-01

    This article reviews aspects of photochemistry on solid surfaces. In order to understand the photo-induced processes a brief introduction is given to the interaction between light and the solid-gas interfaces. The adsorption of molecules on solid surfaces, and the negative ion resonances (NIR) by inelastic electron scattering are briefly discussed. There are three photoinduced processes which occur on surfaces: photoinduced desorption (PID), photoinduced dissociation and photoinduced reactions. The mechanisms of the photoinduced processes are discussed and related to the experimentally determined cross sections. Photoinduced processes are driven: (i) by direct electronic excitation of the adsorbate, (ii) by substrate excitation, (iii) by both adsorbate and substrate excitation, or (iv) by charge transfer dissociation. The usual experimental methods and the light sources are presented. Most of the experimental examples deal with photo-stimulated experiments in the IR-visible and UV region. In some examples photon stimulated desorption of ions of physisorbed molecules is studied by using VUV synchrotron radiation in the energy range between 13-40 eV. In addition to desorption and dissociation, photo-excited molecules can react with each other and form new chemical bonds leading to new species on the surface. Photoinduced polymerizations of formaldehyde on Ag(111) occur after irradiation with light at 0034-4885/61/6/003/img1.

  8. Surface-active state of semiconductor materials based on CdTe-AIIS systems

    NASA Astrophysics Data System (ADS)

    Kirovskaya, I. A.; Nor, P. E.; Ushakov, O. V.; Pogodin, S. N.

    2017-08-01

    Surface properties (surface chemical and acid-base state) of binary and multicomponent semiconductors, i.e. CdTe-CdS and CdTe-ZnS systems components, were studied. The influence of mutual binary component CdTe on the surface acid-base state is demonstrated. Similar regularities in acid-base properties changes due to composition were determined, as well as their correlations with the regularities in bulk properties changes in the row of initial binary compounds. The conclusion was made that semiconductors - the components of the systems with acid surfaces (pHiso<7) - can be used as the materials for sensors on basic gases micro-impurities. The possibility of applying the stated regularities and correlations to facilitate the search for such materials is also noted.

  9. Mapping vibrational surface and bulk modes in a single nanocube

    NASA Astrophysics Data System (ADS)

    Lagos, Maureen J.; Trügler, Andreas; Hohenester, Ulrich; Batson, Philip E.

    2017-03-01

    Imaging of vibrational excitations in and near nanostructures is essential for developing low-loss infrared nanophotonics, controlling heat transport in thermal nanodevices, inventing new thermoelectric materials and understanding nanoscale energy transport. Spatially resolved electron energy loss spectroscopy has previously been used to image plasmonic behaviour in nanostructures in an electron microscope, but hitherto it has not been possible to map vibrational modes directly in a single nanostructure, limiting our understanding of phonon coupling with photons and plasmons. Here we present spatial mapping of optical and acoustic, bulk and surface vibrational modes in magnesium oxide nanocubes using an atom-wide electron beam. We find that the energy and the symmetry of the surface polariton phonon modes depend on the size of the nanocubes, and that they are localized to the surfaces of the nanocube. We also observe a limiting of bulk phonon scattering in the presence of surface phonon modes. Most phonon spectroscopies are selectively sensitive to either surface or bulk excitations; therefore, by demonstrating the excitation of both bulk and surface vibrational modes using a single probe, our work represents advances in the detection and visualization of spatially confined surface and bulk phonons in nanostructures.

  10. Mapping vibrational surface and bulk modes in a single nanocube.

    PubMed

    Lagos, Maureen J; Trügler, Andreas; Hohenester, Ulrich; Batson, Philip E

    2017-03-22

    Imaging of vibrational excitations in and near nanostructures is essential for developing low-loss infrared nanophotonics, controlling heat transport in thermal nanodevices, inventing new thermoelectric materials and understanding nanoscale energy transport. Spatially resolved electron energy loss spectroscopy has previously been used to image plasmonic behaviour in nanostructures in an electron microscope, but hitherto it has not been possible to map vibrational modes directly in a single nanostructure, limiting our understanding of phonon coupling with photons and plasmons. Here we present spatial mapping of optical and acoustic, bulk and surface vibrational modes in magnesium oxide nanocubes using an atom-wide electron beam. We find that the energy and the symmetry of the surface polariton phonon modes depend on the size of the nanocubes, and that they are localized to the surfaces of the nanocube. We also observe a limiting of bulk phonon scattering in the presence of surface phonon modes. Most phonon spectroscopies are selectively sensitive to either surface or bulk excitations; therefore, by demonstrating the excitation of both bulk and surface vibrational modes using a single probe, our work represents advances in the detection and visualization of spatially confined surface and bulk phonons in nanostructures.

  11. Dielectric response of a semi-infinite layered electron gas and Raman scattering from its bulk and surface plasmons

    NASA Astrophysics Data System (ADS)

    Jain, Jainendra K.; Allen, Philip B.

    1985-07-01

    An exact solution of the random-phase-approximation equations is worked out for the density-density correlation function of a semi-infinite system of two-dimensional electron-gas layers, with different dielectrics outside and inside the layered system. From this solution, analytic formulas are derived for the dispersion relations of the bulk and surface plasmons and for the intensity of the light scattered inelastically from such a system. The intensity is written as a sum of the bulk and the surface terms. The theory is applied to semiconductor multilayers. The line shape of the bulk-plasmon peak, obtained after cancellation of van Hove singularities in the bulk piece by the surface piece, is compared with experiment. Conditions for observation of the Giuliani-Quinn surface plasmon are outlined.

  12. Photovoltage inversion effect and its application to semiconductor surface studies: CdS.

    NASA Technical Reports Server (NTRS)

    Lagowski, J.; Balestra, C. L.; Gatos, H. C.

    1971-01-01

    Experimental results on the surface states of the basal plane of CdS are presented and discussed in the light of a model of the photovoltage inversion effect. It is found that the surface states associated with this effect are in poor communication with the bulk and under normal conditions may require long periods of time to reach equilibrium. A combination of photovoltage inversion, photovoltage quenching, and photovoltage spectroscopy is believed to constitute a very effective tool for the study of surfaces of wide energy gap semiconductors.

  13. Teflon, Kapton surface and bulk effects

    NASA Technical Reports Server (NTRS)

    Robinson, J. C.; Mccargo, M.

    1985-01-01

    Two samples of 2 mil aluminized/Kapton thermal blanket and two samples of Teflon were received for evaluation. Their location on the solar maximum is given. The Kapton top layer of the thermal blanket had been exposed to the LEO atomic oxygen environment and shows the surface degradation due to atomic oxygen attack resulting in a diffuse character over most of the surface. The backside Kapton layer which was unexposed appeared to the eye to be in virgin condition. Another sample exhibited similar properties, but was, in large part, covered with Kapton adhesive tape and it was not possible to obtain usable specimens for analysis. One of the Teflon samples which was exposed shows signs of heavy degradation including attack on the Ag/Inconel backside by ultraviolet and atomic oxygen. The other Teflon sample which was unexposed was only slightly fogged on the Teflon side and the Ag/Inconel appeared untouched. These samples were subjected to several chemical and physical analyses, the results of which are discussed.

  14. The Positron as a Probe for Studying Bulk and Defect Properties in Semiconductors

    NASA Astrophysics Data System (ADS)

    Dlubek, G.; Brümmer, O.

    Positron lifetime measurements are used to study various doped and undoped III-V compound semiconductors like GaAs, GaP, InAs and InP. In some as-grown crystals native vacancies (V, V) or their complexes with dopants (TeAsV, VPZnInV) are detected with maximum concentrations of a few 1018 cm-3. In undoped GaAs neutral As vacancies V are identified, the concentration of which varies locally. The vacancies have their origin in deviations from the stoichiometrie composition of the compound. They disappear at 500°C. At this temperature defects created by irradiation with fast neutrons anneal also out. The relation between the bulk positron lifetime and the density and polarization of valence electrons is analyzed. Further, the potential of the positron annihilation method in identifying and characterizing vacancy-type defects in semiconductors is discussed.Translated AbstractDas Positron als Sonde zur Untersuchung der Volumen- und Fehlstelleneigenschaften in HalbleiternPositronenlebensdauermessungen werden angewandt, um verschiedene dotierte und undotierte AIIIBV-Verbindungshalbleiter, wie GaAs, GaP, InAs und InP, zu untersuchen. In einigen Kristallen werden Leerstellen (V, V) oder ihre Komplexe mit Dotierungen (TeAsV, VPZnInV) mit maximalen Konzentrationen von einigen 1018 cm-3 nachgewiesen. In undotierten GaAs werden neutrale As-Leerstellen V identifiziert, deren Konzentration lokal variiert. Die Leerstellen haben ihren Ursprung in Abweichungen von der stöchiometrischen Zusammensetzung der Verbindung. Sie verschwinden bei 500°C. Bei dieser Temperatur heilen auch Defekte aus, welche durch Bestrahlung mit schnellen Neutronen erzeugt wurden. Die Beziehung zwischen der Positronenlebensdauer im Kristall und der Dichte und Polarisierung der Valenzelektronen wird analysiert. Weiterhin wird das Potential der Methode der Positronenannihilation zur Identifizierung und Charakterisierung leerstellenartiger Defekte in Halbleitern diskutiert.

  15. Zinc uptake by brain cells: `surface' versus `bulk'

    NASA Astrophysics Data System (ADS)

    DeStasio, Gelsomina; Pochon, S.; Lorusso, G. F.; Tonner, B. P.; Mercanti, Delio; Ciotti, M. Teresa; Oddo, Nino; Galli, Paolo; Perfetti, P.; Margaritondo, G.

    1996-08-01

    The uptake of zinc by cerebellar rat cultures upon exposure to 0022-3727/29/8/023/img12 solutions was comparatively investigated using two well known condensed matter physics techniques: synchrotron photoelectron spectromicroscopy and inductively coupled plasma atomic emission spectroscopy. The objective was to apply a strategy - well known in surface physics - to distinguish between `surface' and `bulk' phenomena. The results clearly demonstrate that exposure significantly enhances the bulk (cell cytoplasm) Zn concentration with respect to the physiological level, whereas the effect on the surface (cell membrane) is negligible.

  16. Surface resistance measurement of modified QMG superconducting bulks

    NASA Astrophysics Data System (ADS)

    Saito, A.; Sekiya, N.; Teshima, H.; Obara, H.; Noguchi, Y.; Hirano, H.; Hirano, S.; Ohshima, S.

    2006-10-01

    We investigated the microwave properties of high-temperature superconductor (HTS) bulks for high-power microwave applications. The HTS bulk samples of Dy-Ba-Cu-O and Gd-Ba-Cu-O, were prepared using the modified quench and melt growth (QMG) process. The relationship among the bulk surface mirror polishing process, oxygen annealing process, and surface resistance (Rs) was also investigated. The Rs of the HTS bulks was measured using the dielectric resonator method at 21.8 GHz for the TE011 mode. The measurement temperature was varied from 20 to 95 K. We found that the annealing after surface mirror polishing was more effective to obtain low Rs than polishing after annealing. Although the Rs value of the optimized Dy-Ba-Cu-O bulk sample at 20 K was twofold larger than that of the high-quality YBa2Cu3O7-δ thin film fabricated by THEVA GmbH, the Rs value was even low enough to be used for microwave devices. These results indicate that Dy-Ba-Cu-O bulk superconductors are feasible for applications to high-power transmit filters for mobile communication systems.

  17. Intersubband collective excitations at the surface of a semiconductor superlattice

    NASA Astrophysics Data System (ADS)

    Hawrylak, Pawel; Wu, Ji-Wei; Quinn, J. J.

    1985-06-01

    The intersubband and intrasubband collective excitations of a semi-infinite semiconductor superlattice are investigated. Surface intersubband excitations, similar to the intrasubband surface modes predicted by Giuliani and Quinn, are found for wavelengths shorter than a critical value λ*, which depends on the material parameters.

  18. The DIET from semiconductor surfaces by excitation of valence electrons

    NASA Astrophysics Data System (ADS)

    Kanasaki, Jun'ichi; Okano, Akiko; Ishikawa, Ken'ichi; Nakai, Yasuo; Itoh, Noriaki

    1995-06-01

    We discuss the desorption induced by electronic transitions (DIET) of constituent atoms from several types of non-metallic solids, particularly the DIET from semiconductors by valence electron excitations. We first classify the non-metallic solids into type A, in which no self-trapping of excitons occurs, and type B, in which self-trapping occurs. We argue that in type B solids the localization of electron-hole pairs or excitons through the self-trapping on the surfaces induces the Menzel-Gomer-Redhead-type anti-bonding state resulting in DIET. For the DIET from type A non-metals, typically semiconductors, in which the self-trapping is not induced, we derive two important characteristics: (1) the emission is related to defects on the surfaces and (2) single electronic excitation cannot induce the emissions. The recent experimental observations of laser-induced emissions satisfying these characteristics for the DIET from semiconductors are surveyed. Furthermore we present experimental evidence demonstrating that the observed emissions are of the electronic origin: the emission occurs dominantly when the excitation is localized on the surface. Finally, surface phenomena, such as laser ablation and dry etching, related to the DIET from semiconductors, and the applications of the DIET from semiconductors are discussed.

  19. Surface plasmon based engineering of semiconductor nanowire optics

    NASA Astrophysics Data System (ADS)

    Aspetti, Carlos Octavio

    Semiconductor nanowires combine the material properties of semiconductors, which are ubiquitous in modern technology, with nanoscale dimensions and as such, are firmly poised at the forefront of nanotechnology research. The rich physics of semiconductor nanowire optics, in particular, arises from the increased interaction between light and matter that occurs when light is confined to dimensions below the size of its wavelength, in other words, when the nanowire serves as a light trapping optical cavity, which itself is also a source of light. Light confinement is taken to new extremes by coupling to the surface plasmon modes of metallic nanostructures, where light acquires mixed photonic and electronic character, and which may focus light to deep-subwavelength regions amenable to the dimensions of the electron wave. This thesis examines how the integration of "plasmonic optical cavities" and semiconductor nanowires leads to substantial modification (and enhancement) of the optical properties of the same, resulting in orders-of-magnitude faster and more efficient light emission with colors that may be tuned as a function of optical cavity geometry. Furthermore, this method is applied to nanowires composed of both direct and indirect bandgap semiconductor materials resulting in applications such as light emission from high-energy states in light emitting materials, highly enhanced broadband light emission from nominally non-light emitting (dark) materials, and broadband (and anomalous) enhancement of light absorption in various materials, all the while maintaining the unifying theme of employing integrated plasmonic-semiconductor optical cavities to achieve tailored optical properties. We begin with a review of the electromagnetic properties of optical cavities, surface plasmon-enhanced light emission in semiconductors, and the key physical properties of semiconductor nanowires. It goes without saying that this thesis work resides at the interface between optical

  20. Polycrystalline silicon ring resonator photodiodes in a bulk complementary metal-oxide-semiconductor process.

    PubMed

    Mehta, Karan K; Orcutt, Jason S; Shainline, Jeffrey M; Tehar-Zahav, Ofer; Sternberg, Zvi; Meade, Roy; Popović, Miloš A; Ram, Rajeev J

    2014-02-15

    We present measurements on resonant photodetectors utilizing sub-bandgap absorption in polycrystalline silicon ring resonators, in which light is localized in the intrinsic region of a p+/p/i/n/n+ diode. The devices, operating both at λ=1280 and λ=1550  nm and fabricated in a complementary metal-oxide-semiconductor (CMOS) dynamic random-access memory emulation process, exhibit detection quantum efficiencies around 20% and few-gigahertz response bandwidths. We observe this performance at low reverse biases in the range of a few volts and in devices with dark currents below 50 pA at 10 V. These results demonstrate that such photodetector behavior, previously reported by Preston et al. [Opt. Lett. 36, 52 (2011)], is achievable in bulk CMOS processes, with significant improvements with respect to the previous work in quantum efficiency, dark current, linearity, bandwidth, and operating bias due to additional midlevel doping implants and different material deposition. The present work thus offers a robust realization of a fully CMOS-fabricated all-silicon photodetector functional across a wide wavelength range.

  1. Dynamical thermoelectric coefficients of bulk semiconductor crystals: Towards high thermoelectric efficiency at high frequencies

    SciTech Connect

    Ezzahri, Younès Joulain, Karl

    2014-06-14

    We investigate in this work the fundamental behavior of the dynamical thermoelectric coefficients of a bulk cubic semiconductor (SC) crystal. The treatment is based on solving Boltzmann electron transport equation in the frequency domain after simultaneous excitations by dynamical temperature and electric potential gradients, within the framework of the single relaxation time approximation. The SC crystal is assumed to be a linear, elastic homogenous, and isotropic medium having a parabolic energy band structure. We further assume to deal with one type of carriers (electrons or holes) that reside in a single energy band, and we neglect any phonon drag effect. Our approach allows us to obtain very compact expressions for the different dynamical thermoelectric coefficients that nicely capture the essential features of the dynamics of electron transport. We emphasize our study about the dynamical behavior of the thermoelectric figure of merit ZT(Ω) of the SC crystal by considering the coupled electron-phonon transport. Our study revealed a very interesting and compelling result in which ZT increases in the high frequency regime with respect to its steady-state value. The fundamental reason of this enhancement is due to the intrinsic uncoupling in the dynamics of electrons and phonons in the high frequency regime.

  2. Bulk Growth of Wide Band Gap II-VI Compound Semiconductors by Physical Vapor Transport

    NASA Technical Reports Server (NTRS)

    Su, Ching-Hua

    1997-01-01

    The mechanism of physical vapor transport of II-VI semiconducting compounds was studied both theoretically, using a one-dimensional diffusion model, as well as experimentally. It was found that the vapor phase stoichiometry is critical in determining the vapor transport rate. The experimental heat treatment methods to control the vapor composition over the starting materials were investigated and the effectiveness of the heat treatments was confirmed by partial pressure measurements using an optical absorption technique. The effect of residual (foreign) gas on the transport rate was also studies theoretically by the diffusion model and confirmed experimentally by the measurements of total pressure and compositions of the residual gas. An in-situ dynamic technique for the transport rate measurements and a further extension of the technique that simultaneously measured the partial pressures and transport rates were performed and, for the first time, the experimentally determined mass fluxes were compared with those calculated, without any adjustable parameters, from the diffusion model. Using the information obtained from the experimental transport rate measurements as guideline high quality bulk crystal of wide band gap II-VI semiconductor were grown from the source materials which undergone the same heat treatment methods. The grown crystals were then extensively characterized with emphasis on the analysis of the crystalline structural defects.

  3. System suitable for observing surface plasmon in a semi-infinite semiconductor superlattice

    NASA Astrophysics Data System (ADS)

    Zhu, Yun; Cai, Shengshan; Zhou, Shixun

    1987-08-01

    A model of semi-infinite semiconductor superlattice topped with a metal-insulator (M-I-SL) is suggested. A modified Giuliani-Quinn surface plasmon is found. It is interesting to note that the frequency and critical wavelength can be arbitrarily chosen by varying thickness of the insulator. In particular, a new type of surface plasmon with null critical wave vector exists only below the bulk plasmon continuum, and the frequency is directly related to the ratio of thickness of the insulator d to the superlattice spacing a.

  4. Observation of core-level binding energy shifts between (100) surface and bulk atoms of epitaxial CuInSe{sub 2}

    SciTech Connect

    Nelson, A.J.; Berry, G.; Rockett, A.

    1997-04-01

    Core-level and valence band photoemission from semiconductors has been shown to exhibit binding energy differences between surface atoms and bulk atoms, thus allowing one to unambiguously distinguish between the two atomic positions. Quite clearly, surface atoms experience a potential different from the bulk due to the lower coordination number - a characteristic feature of any surface is the incomplete atomic coordination. Theoretical accounts of this phenomena are well documented in the literature for III-V and II-VI semiconductors. However, surface state energies corresponding to the equilibrium geometry of (100) and (111) surfaces of Cu-based ternary chalcopyrite semiconductors have not been calculated or experimental determined. These compounds are generating great interest for optoelectronic and photovoltaic applications, and are an isoelectronic analog of the II-VI binary compound semiconductors. Surface core-level binding energy shifts depend on the surface cohesive energies, and surface cohesive energies are related to surface structure. For ternary compound semiconductor surfaces, such as CuInSe{sub 2}, one has the possibility of variations in surface stoichiometry. Applying standard thermodynamical calculations which consider the number of individual surface atoms and their respective chemical potentials should allow one to qualitatively determine the magnitude of surface core-level shifts and, consequently, surface state energies.

  5. Exceptional surface and bulk electronic structures in a topological insulator, Bi2Se3

    PubMed Central

    Biswas, Deepnarayan; Thakur, Sangeeta; Balakrishnan, Geetha; Maiti, Kalobaran

    2015-01-01

    The outstanding problem in topological insulators is the bulk metallicity underneath topologically ordered surface states and the appearance of Dirac point far away from the Fermi energy. Enormous efforts are being devoted to get the Dirac point at the Fermi level via exposure to foreign materials so that these materials can be used in technology and realize novel fundamental physics. Ironically, the conclusion of bulk metallicity in the electronic structure is essentially based on the angle resolved photoemission spectroscopy, a highly surface sensitive technique. Here, we employed state-of-the-art hard x-ray photoemission spectroscopy with judiciously chosen experiment geometry to delineate the bulk electronic structure of a topological insulator and a potential thermoelectric material, Bi2Se3. The results exhibit signature of insulating bulk electronic structure with tiny intensities at akin to defect/vacancy induced doped states in the semiconductors. The core level spectra exhibit intense plasmon peak associated to core level excitations manifesting the signature of coupling of electrons to the collective excitations, a possible case of plasmon-phonon coupling. In addition, a new loss feature appear in the core level spectra indicating presence of additional collective excitations in the system. PMID:26644075

  6. Immobilization of biomolecules on semiconductor surfaces

    NASA Astrophysics Data System (ADS)

    Joensson, U.; Malmqvist, M.; Nilsson, H.; Olofsson, G.; Roennberg, I.

    1983-09-01

    A reproducible, stable and functional introduction of reactive groups on oxide covered silicon surfaces used in chemically sensitive field effect transistors and optical methods based on light reflection is described. Biomolecules, such as antibodies, antigens and enzymes, were covalently attached to the surface modified silicon via a thiol disulfide exchange reaction. The immobilization technique eliminates the risk of crosslinking and homopolymerization, giving monolayer coverage in close contact with the surface. The technique was used for immobilized protein A and interaction of such surfaces with immunoglobulins. The result was evaluated by in situ ellipsometry, which gives the amount of immobilized and interacting material on the surfaces.

  7. Impurities, Defects and Diffusion in Semiconductors: Bulk and Layered Structures. Materials Research Society Symposium Proceedings. Volume 163

    DTIC Science & Technology

    1990-11-21

    Selective Area Deposition and Etching (161) Properties of II-VI Semiconductors: Bulk Crystals, Expitaxial Films, Quantum Well Structures, and Dilute...Pfeiffer, H. Skudlik, and D. Steiner STRAIN INDUCED INTRINSIC QUANTUM WELLS AS THE ORIGIN OF BROAD BAND PHOTOLUMINESCENCE IN SILICON CONTAINING EXTENDED...FREE TO BOUND TRANSITIONS IN Ga xAlX As/GaAs QUANTUM WELLS 313 Donald C. Reynolds and K.K. Bajaj DECAY MEASUREMENTS OF FREE AND BOUND EXCITON RECOM

  8. Concentration polarization, surface currents, and bulk advection in a microchannel

    NASA Astrophysics Data System (ADS)

    Nielsen, Christoffer P.; Bruus, Henrik

    2014-10-01

    We present a comprehensive analysis of salt transport and overlimiting currents in a microchannel during concentration polarization. We have carried out full numerical simulations of the coupled Poisson-Nernst-Planck-Stokes problem governing the transport and rationalized the behavior of the system. A remarkable outcome of the investigations is the discovery of strong couplings between bulk advection and the surface current; without a surface current, bulk advection is strongly suppressed. The numerical simulations are supplemented by analytical models valid in the long channel limit as well as in the limit of negligible surface charge. By including the effects of diffusion and advection in the diffuse part of the electric double layers, we extend a recently published analytical model of overlimiting current due to surface conduction.

  9. Surface and bulk contribution to Cu(111) quantum efficiency

    SciTech Connect

    Pedersoli, Emanuele; Greaves, Corin Michael Ricardo; Wan, Weishi; Coleman-Smith, Christopher; Padmore, Howard A.; Pagliara, Stefania; Cartella, Andrea; Lamarca, Fabrizio; Ferrini, Gabriele; Galimberti, Gianluca; Montagnese, Matteo; dal Conte, Stefano; Parmigiani, Fulvio

    2008-11-04

    The quantum efficiency (QE) of Cu(111) is measured for different impinging light angles with photon energies just above the work function. We observe that the vectorial photoelectric effect, an enhancement of the QE due to illumination with light with an electric vector perpendicular to the sample surface, is stronger in the more surface sensitive regime. This can be explained by a contribution to photoemission due to the variation in the electromagnetic potential at the surface. The contributions of bulk and surface electrons can then be determined.

  10. Investigation on properties of ultrafast switching in a bulk gallium arsenide avalanche semiconductor switch

    SciTech Connect

    Hu, Long; Su, Jiancang; Ding, Zhenjie; Hao, Qingsong; Yuan, Xuelin

    2014-03-07

    Properties of ultrafast switching in a bulk gallium arsenide (GaAs) avalanche semiconductor switch based on semi-insulating wafer, triggered by an optical pulse, were analyzed using physics-based numerical simulations. It has been demonstrated that when a voltage with amplitude of 5.2 kV is applied, after an exciting optical pulse with energy of 1 μJ arrival, the structure with thickness of 650 μm reaches a high conductivity state within 110 ps. Carriers are created due to photons absorption, and electrons and holes drift to anode and cathode terminals, respectively. Static ionizing domains appear both at anode and cathode terminals, and create impact-generated carriers which contribute to the formation of electron-hole plasma along entire channel. When the electric field in plasma region increases above the critical value (∼4 kV/cm) at which the electrons drift velocity peaks, a domain comes into being. An increase in carrier concentration due to avalanche multiplication in the domains reduces the domain width and results in the formation of an additional domain as soon as the field outside the domains increases above ∼4 kV/cm. The formation and evolution of multiple powerfully avalanching domains observed in the simulations are the physical reasons of ultrafast switching. The switch exhibits delayed breakdown with the characteristics affected by biased electric field, current density, and optical pulse energy. The dependence of threshold energy of the exciting optical pulse on the biased electric field is discussed.

  11. Metal-Induced Reconstructions on Semiconductor Surfaces

    NASA Astrophysics Data System (ADS)

    Li, Lian

    1995-01-01

    Scanning tunneling microscopy (STM) has been used to study the surface morphology induced by In, Ga, and Sn overlayers on Si(100) and Si(311) surfaces. STM and low-energy electron microscopy (LEEM) were used to study the surface morphology of Pb overlayer grown on Si(100) -(2 x 1). Reconstructions on clean alpha -SiC(0001) and alpha-SiC(0001) surfaces and those induced by deposition of Ti were also studied. We found that deposition of In produced step bunching and faceting on vicinal Si(100) surface. At coverages >=1 ML, the original step direction of (01 1) changed to a zig-zag along low-index (010) and (001) directions, and the bunched steps became {130} facets. At annealing temperatures >=550^ circC, the {130} facets disappeared and the bunched steps were restored with meandering step directions. For the case of Pb adsorption on Si(100)-(2 x 1) surface, surface reconstructions of (2 x 2), c(4 x 8), (2 x 1), and c(4 x 4) were observed as Pb coverages increased from 0.5 to 1.5 ML. After the appearance of the c(4 x 4) phase, Pb islands with threefold-symmetric (111) orientation were observed on the twofold or fourfold-symmetric Si(100) surface. Both Sn and Ga adsorption on Si(100)-(2 x 1) and Si(311)-(3 x 2) surfaces induced complex surface morphologies when the coverage exceeded 1 ML. Rectangular and square depressions shaped like inverted pyramids were formed at Ga coverage above 1 ML. The walls of the inverted pyramids were (311). Deposition of Sn on Si(100) at coverages exceeding 2 ML also produced (311) facets, but they formed the faces of long parallel and orthogonal prisms, giving rise to a large scale maze-like structure on the surface. Deposition of Ga on the Si(311)-(3 x 2) surface produced ordered (211) and (611) facets at coverage above 1 ML. On the other hand, deposition of Sn on Si(311)-(3 x 2) did not produce any facet. Instead, layer-by-layer growth of (1 x 1) pseudomorphic Sn was observed on the Si(311) surface up to 6 ML. Ge growth on Si(100

  12. Nonadiabatic bulk-surface oscillations in driven topological insulators

    NASA Astrophysics Data System (ADS)

    Kolodrubetz, Michael; Fregoso, Benjamin M.; Moore, Joel E.

    2016-11-01

    Recent theoretical and experimental work has suggested the tantalizing possibility of opening a topological gap upon driving the surface states of a three-dimensional strong topological insulator (TI) with circularly polarized light. With this motivation, we study the response of TIs to a driving field that couples to states near the surface. We unexpectedly find coherent oscillations between the surface and the bulk and trace their appearance to unavoidable resonances caused by photon absorption from the drive. We show how these resonant oscillations may be captured by the Demkov-Osherov model of multilevel Landau-Zener physics, leading to nontrivial consequences such as the loss of adiabaticity upon slow ramping of the amplitude. We numerically demonstrate that these oscillations are observable in the time-dependent Wigner distribution, which is directly measurable in time-resolved angle-resolved photoemission spectroscopy (ARPES) experiments. Our results apply to any system with surface states in the presence of a gapped bulk, and thus suggest experimental signatures of a generic surface-bulk coupling mechanism that is fundamental for proposals to engineer nontrivial states by periodic driving.

  13. Surface Brillouin scattering in semiconductor Fibonacci multilayers

    NASA Astrophysics Data System (ADS)

    Xia, Hua; Zhang, X. K.; Chen, K. J.; Cheng, G. X.; Feng, D.; Socino, G.; Palmieri, L.; Carlotti, G.; Fioretto, D.; Nizzoli, F.

    1990-12-01

    Brillouin spectra from surface (Rayleigh) acoustic waves in a-Si:H/a-SiNx:H (x~=0.94) Fibonacci multilayers have been measured with a tandem multipass Fabry-Pérot interferometer. To make a detailed analysis for the observed surface (Rayleigh) waves, following Merlin et al. [Phys. Rev. Lett. 55, 1678 (1985)] and Dharma-wardana et al. [Phys. Rev. Lett. 58, 1761 (1987)], we have formulated the propagation of surface acoustic waves parallel to the surface of the quasiperiodic multilayers using the Fourier-transform method in the long-wavelength regime. We found that the obtained Brillouin spectra can be accounted for by this new derivation of the Fourier transform of the surface acoustic waves, but a rather fascinating feature for the phonon frequency is that it is slightly modified by quasiperiodic indices (n,m) or p. Such a modulation is attributed to the existence of a small but nonvanishing acoustic impedance. Our theoretical approach gives the correct order of amplitude modulation for surface (Rayleigh) waves within an error of <1%. We thereby obtain an overall understanding of the elastic properties of the Fibonacci multilayers.

  14. Point defect reactions at surfaces and in bulk metals

    NASA Astrophysics Data System (ADS)

    Flynn, C. P.

    2005-02-01

    This paper describes the time evolution of reacting defect assemblies both in bulk metals and on their surfaces. Three areas are treated. The first describes the linear response of reacting assemblies to perturbing fields such as irradiation or temperature change. Alternative long wavelength limits identified here concern: (i) independent diffusion of vacancy- and interstitial-type defects to sinks; and (ii) joint diffusion of defects down a chemical potential gradient, with a separate branch of solutions associated with recombination. The second topic concerns definitions of the chemical potential μ* and temperature T* associated with the defect system itself, as distinct from the properties of the embedding lattice. The utility of these quantities is illustrated by examples including those pertaining to rapid temperature change. μ* and T differ from the lattice values μ,T , to an extent that determines possible energy and particle transfer in such processes as nucleation of new sinks and precipitation from the defect assembly. The role of these quantities in relaxation modes is clarified. Finally, an appendix discusses an approximate model of defect behavior in the bulk, and a speculative discussion of defect behavior on surfaces, both positing homologous properties of the defect systems in metals, when scaled to the melting temperature Tm . These characteristics of a standard metal and a standard close-packed metal surface are employed in the text to identify and contrast typical behaviors of the bulk and surface defect systems of metals. Universal properties that follow from these models are discussed in a second appendix.

  15. Experimental demonstration of surface and bulk plasmon polaritons in hypergratings

    PubMed Central

    Sreekanth, Kandammathe Valiyaveedu; De Luca, Antonio; Strangi, Giuseppe

    2013-01-01

    Hyperbolic metamaterials (HMMs) represent a novel class of fascinating anisotropic plasmonic materials, supporting highly confined bulk plasmon polaritons in addition to the surface plasmon polaritons. However, it is very challenging to tailor and excite those modes at optical frequencies using prism coupling technique because of the intrinsic difficulties to engineer non-traditional optical properties using artificial nanostructures and the unavailability of high refractive index prisms for matching the momentum between the incident light and the guided modes. Here, we experimentally demonstrate the excitation of both surface and bulk plasmon polaritons in a HMM through a grating coupling technique of surface plasmon excitation that makes use a hypergrating, which is a combined structure of metallic diffraction grating and HMM. Initially, we propose an optical hyperbolic metamaterial based on Au/TiO2 multilayers and confirm the hyperbolic dispersion, and the presence of high-k modes in the fabricated HMM. Reflection measurements as a function of incident angle and excitation wavelength show the existence of both surface and bulk plasmon polaritons inside the hypergrating. The proposed configuration is expected to find potential applications in bio-chemical sensors, integrated optics and optical sub-wavelength imaging. PMID:24256947

  16. Properties of II-VI Semiconductors: Bulk Crystals, Epitaxial Films, Quantum Well Structures, and Dilute Magnetic Systems. Materials Research Society Symposium Proceedings. Volume 161

    DTIC Science & Technology

    1990-11-21

    quantum well (MQW) structures, which can confine electrons and holes in a two-dimensional well , fabricated by MBE [2] and MOCVD [3]. Despite the...N Pie MA’ FERIA -LS - RESEAR(--’H -)CIFFY VOLUME 161 Properties of 11-VI Semiconductors: Bulk Crystals, Epitaxial Films, Quantum Well Structures...Semiconductors: Bulk Crystals, Epitaxial Films, Quantum Well Structures, and Dilute Magnet;-- Systems :1ity CodeS JLECTE0 Nov 15 1990 SDISTRI:7UTICN SAT EM~

  17. Surface characterization of semiconductor photocathode structures

    NASA Astrophysics Data System (ADS)

    Liu, Zhi

    The need for a high performance photocathode in the electron beam lithography and microscopy is well established. Previous research demonstrated high brightness (1 x 108 A/cm2-sr at 3 KeV), and an energy spread as low as 50meV at room temperature for a GaAs based negative electron affinity (NEA) cathode in a sealed-off tube. However the GaAs cathodes suffer rapid decay in an open vacuum system. Achieving a clean, stoichiometric and repeatable GaAs(100) surface was the first step in this study. Based on the knowledge obtained from synchrotron radiation photoelectron spectroscopy, we successfully developed and optimized a reliable surface cleaning technique for our GaAs photocathodes. The fully activated photocathode and its decay under different vacuum conditions were investigated. The NEA activation layer is about 1 nm thick and was very vulnerable to oxygen in the system. A revised double dipole structural model was proposed to explain how the Cs/O co-deposition could produce a NEA surface. We found the chemical changes of oxygen species in the activation layer caused the initial quantum yield (QY) decay of the cathode. Further exposure to oxygen oxidized the substrate and permanently reduced the QY to zero. Energy distribution curve measurements of GaAs(100) and GaN(0001) NEA surfaces were performed under laser illumination. We found that the main contribution to the total emitted current of NEA GaAs and GaN surfaces was due to the electrons that were lost an average 140meV and 310 meV respectively in the near surface region prior to emission into vacuum. This energy loss is due not to the scattering through Cs or Cs/O layer; In GaN, it is probably due to a Gunn-like effect involving inter-valley phonon scattering within the band-bending region. We observed a highly directional emission profile from GaAs cathodes (electrons emitted within a semi-angle of 15° relative to the surface normal). In practice, it is expected that the highly directional photoemission

  18. Surface-Dominated Transport on a Bulk Topological Insulator

    NASA Astrophysics Data System (ADS)

    Hofmann, Philip; Barreto, Lucas; Kühnemund, Lisa; Edler, Frederik; Tegenkamp, Christoph; Mi, Jianli; Bremholm, Martin; Brummerstedt Iversen, Bo; Frydendahl, Christian; Bianchi, Marco

    2014-03-01

    Topological insulators are guaranteed to support metallic surface states on an insulating bulk, and one should thus expect that the electronic transport in these materials is dominated by the surfaces states. Alas, due to the high remaining bulk conductivity, surface contributions to transport have so-far only been singled out indirectly via quantum oscillation, or for devices based on gated and doped topological insulator thin films, a situation in which the surface carrier mobility could be limited by defect and interface scattering. Here we present a direct measurement of surface-dominated conduction on an atomically clean surface of Bi2Te2Se. Using nano-scale four point setups with variable contact distance, we show that the transport at 30 K is two-dimensional rather than three-dimensional and by combining these measurements with angle-resolved photoemission results from the same crystals, we find a surface state mobility of 390(30) cm2V-1s-1 at 30 K at a carrier concentration of 8.71(7) ×1012 cm-2.

  19. MEED studies of thin metal film covered semiconductor surfaces

    NASA Astrophysics Data System (ADS)

    Shimaoka, G.

    1991-06-01

    Results of observations of thin metal films deposited on clean surfaces of semiconductors, such as Si, GaAs and GaP, in the early stage of deposition in UHV, are reported with particular emphasis on in-situ MEED observations at 1-5 kV using a grazing angle of incidence. Various atomic rearrangements and reconstructions due to thin metal deposits were observed, for example: 2 × 1 and 4 × 5 reconstruction of Ni on a clean Si(110)16 × 2 surface, and formation of a one-dimensional lattice of Ag on a coplanar plane of GaAs (001) and GaP (001) and (011). These results are also discussed in terms of surface structures with special emphasis on the early stages of metal deposition and on the metal/semiconductor interface.

  20. Electroless silver plating of the surface of organic semiconductors.

    PubMed

    Campione, Marcello; Parravicini, Matteo; Moret, Massimo; Papagni, Antonio; Schröter, Bernd; Fritz, Torsten

    2011-10-04

    The integration of nanoscale processes and devices demands fabrication routes involving rapid, cost-effective steps, preferably carried out under ambient conditions. The realization of the metal/organic semiconductor interface is one of the most demanding steps of device fabrication, since it requires mechanical and/or thermal treatments which increment costs and are often harmful in respect to the active layer. Here, we provide a microscopic analysis of a room temperature, electroless process aimed at the deposition of a nanostructured metallic silver layer with controlled coverage atop the surface of single crystals and thin films of organic semiconductors. This process relies on the reaction of aqueous AgF solutions with the nonwettable crystalline surface of donor-type organic semiconductors. It is observed that the formation of a uniform layer of silver nanoparticles can be accomplished within 20 min contact time. The electrical characterization of two-terminal devices performed before and after the aforementioned treatment shows that the metal deposition process is associated with a redox reaction causing the p-doping of the semiconductor. © 2011 American Chemical Society

  1. Bulk Surface Momentum Parameters for Satellite-Derived Vegetation Fields

    NASA Technical Reports Server (NTRS)

    Jasinski, Michael F.; Borak, Jordan; Crago, Richard

    2005-01-01

    The bulk aerodynamic parameters associated with the absorption of surface momentum by vegetated landscapes are theoretically estimated within the context of Raupach's roughness sublayer formulation. The parameters include the bulk plant drag coefficient, maximum u*/U(sub h), sheltering coefficient, and canopy area density at onset of sheltering. Parameters are estimated for the four principal IGBP land cover classes within the U.S. Southern Great Plains: evergreen needleleaf forests, grasslands, croplands, and open shrublands. The estimation approach applies the Method of Moments to roughness data from several international field experiments and other published sources. The results provide the necessary land surface parameters for satellite-based estimation of momentum aerodynamic roughness length and zero-plane displacement height for seasonally variable vegetation fields employed in most terrestrial and atmospheric simulation models used today. Construction of sample displacement and roughness maps over the Southern United States using MODIS land products demonstrates the potential of this approach for regional to global applications.

  2. Apparent Charge Transfer at Semiconductor Surfaces

    SciTech Connect

    Carpinelli, Joseph M.; Stumpf, Roland R.; Weitering, Hanno H.

    1999-05-11

    We investigate the apparent charge transfer between adatoms in the GeXPb[l.XjGe(lll) interface both experimentally and theoretically. Scanning tunneling microscopy and surface core level measurements suggest significant charge transfer from the Ge adatoms to the Pb adatoms. However, first-principles calculations unambiguously find that the total electronic displacement is negligibly small, and that the results of published experiments can be explained as a result of bond rearrangement.

  3. Bulk charge carrier transport in push-pull type organic semiconductor.

    PubMed

    Karak, Supravat; Liu, Feng; Russell, Thomas P; Duzhko, Volodimyr V

    2014-12-10

    Operation of organic electronic and optoelectronic devices relies on charge transport properties of active layer materials. The magnitude of charge carrier mobility, a key efficiency metrics of charge transport properties, is determined by the chemical structure of molecular units and their crystallographic packing motifs, as well as strongly depends on the film fabrication approaches that produce films with different degrees of anisotropy and structural order. Probed by the time-of-flight and grazing incidence X-ray diffraction techniques, bulk charge carrier transport, molecular packing, and film morphology in different structural phases of push-pull type organic semiconductor, 7,7'-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b']dithiophene-2,6-diyl)bis(6-fluoro-4-(5'-hexyl-[2,2'-bithiophen]-5yl)benzo[c][1,2,5] thiadiazole), one of the most efficient small-molecule photovoltaic materials to-date, are described herein. In the isotropic phase, the material is ambipolar with high mobilities for a fluid state. The electron and hole mobilities at the phase onset at 210.78 °C are 1.0 × 10(-3) cm(2)/(V s) and 6.5 × 10(-4) cm(2)/(V s), respectively. Analysis of the temperature and electric field dependences of the mobilities in the framework of Gaussian disorder formalism suggests larger energetic and positional disorder for electron transport sites. Below 210 °C, crystallization into a polycrystalline film with a triclinic unit cell symmetry and high degree of anisotropy leads to a 10-fold increase of hole mobility. The mobility is limited by the charge transfer along the direction of branched alkyl side chains. Below 90 °C, faster cooling rates produce even higher hole mobilities up to 2 × 10(-2) cm(2)/(V s) at 25 °C because of the more isotropic orientations of crystalline domains. These properties facilitate in understanding efficient material performance in photovoltaic devices and will guide further development of materials and devices.

  4. Surface processing for bulk niobium superconducting radio frequency cavities

    NASA Astrophysics Data System (ADS)

    Kelly, M. P.; Reid, T.

    2017-04-01

    The majority of niobium cavities for superconducting particle accelerators continue to be fabricated from thin-walled (2-4 mm) polycrystalline niobium sheet and, as a final step, require material removal from the radio frequency (RF) surface in order to achieve performance needed for use as practical accelerator devices. More recently bulk niobium in the form of, single- or large-grain slices cut from an ingot has become a viable alternative for some cavity types. In both cases the so-called damaged layer must be chemically etched or electrochemically polished away. The methods for doing this date back at least four decades, however, vigorous empirical studies on real cavities and more fundamental studies on niobium samples at laboratories worldwide have led to seemingly modest improvements that, when taken together, constitute a substantial advance in the reproducibility for surface processing techniques and overall cavity performance. This article reviews the development of niobium cavity surface processing, and summarizes results of recent studies. We place some emphasis on practical details for real cavity processing systems which are difficult to find in the literature but are, nonetheless, crucial for achieving the good and reproducible cavity performance. New approaches for bulk niobium surface treatment which aim to reduce cost or increase performance, including alternate chemical recipes, barrel polishing and ‘nitrogen doping’ of the RF surface, continue to be pursued and are closely linked to the requirements for surface processing.

  5. Surface processing for bulk niobium superconducting radio frequency cavities

    DOE PAGES

    Kelly, M. P.; Reid, T.

    2017-02-21

    The majority of niobium cavities for superconducting particle accelerators continue to be fabricated from thin-walled (2-4mm) polycrystalline niobium sheet and, as a final step, require material removal from the radio frequency (RF) surface in order to achieve performance needed for use as practical accelerator devices. More recently bulk niobium in the form of, single-or large-grain slices cut from an ingot has become a viable alternative for some cavity types. In both cases the so-called damaged layer must be chemically etched or electrochemically polished away. The methods for doing this date back at least four decades, however, vigorous empirical studies onmore » real cavities and more fundamental studies on niobium samples at laboratories worldwide have led to seemingly modest improvements that, when taken together, constitute a substantial advance in the reproducibility for surface processing techniques and overall cavity performance. This article reviews the development of niobium cavity surface processing, and summarizes results of recent studies. We place some emphasis on practical details for real cavity processing systems which are difficult to find in the literature but are, nonetheless, crucial for achieving the good and reproducible cavity performance. New approaches for bulk niobium surface treatment which aim to reduce cost or increase performance, including alternate chemical recipes, barrel polishing and 'nitrogen doping' of the RF surface, continue to be pursued and are closely linked to the requirements for surface processing.« less

  6. Ultraviolet-Ozone Cleaning of Semiconductor Surfaces

    DTIC Science & Technology

    1992-10-01

    cleaning; (b) after UV/ozone cleaning. 4 3. Absorption spectrum of oxygen . 7 4. Absorption spectrum of ozone. 7 5. Schematic drawing of a UV/ozone...A clean glass surface was obtained after 15 hours of exposure to the UV radiation in air. In a vacuum system at 104 torr of oxygen , clean gold... oxygen . It took 60 minutes in 20 torr of oxygen , and no cleaning effect was observed in 1 torr after 60 minutes of cleaning (47). (It should be noted

  7. Ultraviolet-Ozone Cleaning of Semiconductor Surfaces

    DTIC Science & Technology

    1992-01-01

    to the UV radiation in air. In a vacuum system at 10-4 tort of oxygen , clean gold surfaces were produced after about two hours of UV exposure. During...torr of oxygen , and no cleaning effect was observed in 1 torr after 60 minutes of cleaning (46). (It should be noted, however, that the cleaning...source can be used as an "ozone killer." For example, in one cryopumped vacuum system, UV/ozone cleaning was performed in up to 20 torr of oxygen . After

  8. Surface and Interface Engineering of Organometallic and Two Dimensional Semiconductor

    NASA Astrophysics Data System (ADS)

    Park, Jun Hong

    For over half a century, inorganic Si and III-V materials have led the modern semiconductor industry, expanding to logic transistor and optoelectronic applications. However, these inorganic materials have faced two different fundamental limitations, flexibility for wearable applications and scaling limitation as logic transistors. As a result, the organic and two dimensional have been studied intentionally for various fields. In the present dissertation, three different studies will be presented with followed order; (1) the chemical response of organic semiconductor in NO2 exposure. (2) The surface and stability of WSe2 in ambient air. (3) Deposition of dielectric on two dimensional materials using organometallic seeding layer. The organic molecules rely on the van der Waals interaction during growth of thin films, contrast to covalent bond inorganic semiconductors. Therefore, the morphology and electronic property at surface of organic semiconductor in micro scale is more sensitive to change in gaseous conditions. In addition, metal phthalocyanine, which is one of organic semiconductor materials, change their electronic property as reaction with gaseous analytes, suggesting as potential chemical sensing platforms. In the present part, the growth behavior of metal phthalocyanine and surface response to gaseous condition will be elucidated using scanning tunneling microscopy (STM). In second part, the surface of layered transition metal dichalcogenides and their chemical response to exposure ambient air will be investigated, using STM. Layered transition metal dichalcogenides (TMDs) have attracted widespread attention in the scientific community for electronic device applications because improved electrostatic gate control and suppression of short channel leakage resulted from their atomic thin body. To fabricate the transistor based on TMDs, TMDs should be exposed to ambient conditions, while the effect of air exposure has not been understood fully. In this part

  9. Two-stage model of nanocone formation on a surface of elementary semiconductors by laser radiation

    PubMed Central

    2012-01-01

    In this work, we study the mechanism of nanocone formation on a surface of elementary semiconductors by Nd:YAG laser radiation. Our previous investigations of SiGe and CdZnTe solid solutions have shown that nanocone formation mechanism is characterized by two stages. The first stage is characterized by formation of heterostructure, for example, Ge/Si heterostructure from SiGe solid solutions, and the second stage is characterized by formation of nanocones by mechanical plastic deformation of the compressed Ge layer on Si due to mismatch of Si and Ge crystalline lattices. The mechanism of nanocone formation for elementary semiconductors is not clear until now. Therefore, the main goal of our investigations is to study the stages of nanocone formation in elementary semiconductors. A new mechanism of p-n junction formation by laser radiation in the elementary semiconductor as a first stage of nanocone formation is proposed. We explain this effect by the following way: p-n junction is formed by generation and redistribution of intrinsic point defects in temperature gradient field – the thermogradient effect, which is caused by strongly absorbed laser radiation. According to the thermogradient effect, interstitial atoms drift towards the irradiated surface, but vacancies drift to the opposite direction – in the bulk of semiconductor. Since interstitials in Ge crystal are of n-type and vacancies are known to be of p-type, a n-p junction is formed. The mechanism is confirmed by the appearance of diode-like current–voltage characteristics after i-Ge irradiation crystal by laser radiation. The mechanism in Si is confirmed by conductivity type inversion and increased microhardness of Si crystal. The second stage of nanocone formation is laser heating up of top layer enriched by interstitial atoms with its further plastic deformation due to compressive stress caused by interstitials in the top layer and vacancies in the buried layer. PMID:22849869

  10. Surface and bulk contributions to nematic order reconstruction.

    PubMed

    Amoddeo, A; Barberi, R; Lombardo, G

    2012-06-01

    Nematic molecules confined in an asymmetric π cell and subjected to strong electric fields exhibit textural distortions involving nematic order variations, described by the Landau-de Gennes Q-tensor theory. We investigated the evolution of order variations as function of the applied electric pulse amplitude and of the nematic surface pretilt anchoring angles by implementing a Q-tensor model with a moving mesh finite element method. The proposed technique is able to clearly distinguish the bulk and the surface order reconstruction which occur in the cell.

  11. Visible light surface emitting semiconductor laser

    DOEpatents

    Olbright, Gregory R.; Jewell, Jack L.

    1993-01-01

    A vertical-cavity surface-emitting laser is disclosed comprising a laser cavity sandwiched between two distributed Bragg reflectors. The laser cavity comprises a pair of spacer layers surrounding one or more active, optically emitting quantum-well layers having a bandgap in the visible which serve as the active optically emitting material of the device. The thickness of the laser cavity is m .lambda./2n.sub.eff where m is an integer, .lambda. is the free-space wavelength of the laser radiation and n.sub.eff is the effective index of refraction of the cavity. Electrical pumping of the laser is achieved by heavily doping the bottom mirror and substrate to one conductivity-type and heavily doping regions of the upper mirror with the opposite conductivity type to form a diode structure and applying a suitable voltage to the diode structure. Specific embodiments of the invention for generating red, green, and blue radiation are described.

  12. Probing surface recombination velocities in semiconductors using two-photon microscopy

    PubMed Central

    Gaury, Benoit; Haney, Paul M.

    2016-01-01

    The determination of minority-carrier lifetimes and surface recombination velocities is essential for the development of semiconductor technologies such as solar cells. The recent development of two-photon time-resolved microscopy allows for better measurements of bulk and subsurface interfaces properties. Here we analyze the diffusion problem related to this optical technique. Our three-dimensional treatment enables us to separate lifetime (recombination) from transport effects (diffusion) in the photoluminescence intensity. It also allows us to consider surface recombination occurring at a variety of geometries: a single plane (representing an isolated exposed or buried interface), two parallel planes (representing two inequivalent interfaces), and a spherical surface (representing the enclosing surface of a grain boundary). We provide fully analytical results and scalings directly amenable to data fitting, and apply those to experimental data collected on heteroepitaxial CdTe/ZnTe/Si. PMID:27182082

  13. Probing surface recombination velocities in semiconductors using two-photon microscopy

    SciTech Connect

    Gaury, Benoit; Haney, Paul M.

    2016-03-28

    The determination of minority-carrier lifetimes and surface recombination velocities is essential for the development of semiconductor technologies such as solar cells. The recent development of two-photon time-resolved microscopy allows for better measurements of bulk and subsurface interfaces properties. Here, we analyze the diffusion problem related to this optical technique. Our three-dimensional treatment enables us to separate lifetime (recombination) from transport effects (diffusion) in the photoluminescence intensity. It also allows us to consider surface recombination occurring at a variety of geometries: a single plane (representing an isolated exposed or buried interface), a two parallel planes (representing two inequivalent interfaces), and a spherical surface (representing the enclosing surface of a grain boundary). We provide fully analytical results and scalings directly amenable to data fitting and apply those to experimental data collected on heteroepitaxial CdTe/ZnTe/Si.

  14. Relaxation Models of the (110) Zinc-Blende III-V Semiconductor Surfaces: Density Functional Study

    SciTech Connect

    Ye, H.; Chen, G.; Wu, Y.; Zhu, Y.; Wei, S. H.

    2008-11-01

    Clean III-V zinc-blende (110) surfaces are the most extensively studied semiconductor surface. For conventional III-V compounds such as GaAs and InP, the surface relaxation follows a bond rotation relaxation model. However, for III-nitrides recent study indicates that they follow a bond-constricting relaxation model. First-principles atom relaxation calculations are performed to explore the origin of the difference between the two groups of materials. By analyzing the individual shift trends and ionic properties of the top layer anions and cations, we attribute the difference between the conventional and nitride III-V compounds to the strong electronegativity of N, which leads to the s{sup 2}p{sup 3} pyramid bond angle to be larger than the ideal one in bulk (109.5{sup o}). The general trends of the atomic relaxation at the III-nitrides (110) surfaces are explained.

  15. Tapping of Love waves in an isotropic surface waveguide by surface-to-bulk wave transduction.

    NASA Technical Reports Server (NTRS)

    Tuan, H.-S.; Chang, C.-P.

    1972-01-01

    A theoretical study of tapping a Love wave in an isotropic microacoustic surface waveguide is given. The surface Love wave is tapped by partial transduction into a bulk wave at a discontinuity. It is shown that, by careful design of the discontinuity, the converted bulk wave power and the radiation pattern may be controlled. General formulas are derived for the calculation of these important characteristics from a relatively general surface contour deformation.

  16. Surface melting of clusters and implications for bulk matter

    NASA Astrophysics Data System (ADS)

    Cheng, Hai-Ping; Berry, R. Stephen

    1992-06-01

    Surface melting on clusters is investigated by a combination of analytic modeling and computer simulation. Homogeneous argonlike clusters bound by Lennard-Jones forces and Cu-like clusters bound by ``embedded-atom'' potentials are the systems considered. Molecular-dynamics calculations have been carried out for clusters with 40-147 atoms. Well below the bulk melting temperature, the surfaces become very soft, exhibiting well-defined diffusion constants even while the cores remain nearly rigid and solidlike. The simulations, particularly animations, of atomic motion reveal that the surface melting is associated not with amorphous, random surface structures in constant, irregular motion, but rather with large-amplitude, organized, collective motion of most of the surface atoms accompanied by a few detached atoms (``floaters'') and holes. At any time, a few of the surface atoms are out of the surface layer, leaving vacancies; these promoted particles wander diffusively, the holes also but less so; the floaters occasionally exchange with atoms in the surface layer. This result is the basis for an analytic, statistical model. The caloric curves, particularly the latent heats, together with the results from an analytical model, show that surface melting of clusters is a ``phase change'' different from the homogeneous melting of clusters.

  17. Pump-probe surface photovoltage spectroscopy measurements on semiconductor epitaxial layers

    SciTech Connect

    Jana, Dipankar Porwal, S.; Sharma, T. K. Oak, S. M.; Kumar, Shailendra

    2014-04-15

    Pump-probe Surface Photovoltage Spectroscopy (SPS) measurements are performed on semiconductor epitaxial layers. Here, an additional sub-bandgap cw pump laser beam is used in a conventional chopped light geometry SPS setup under the pump-probe configuration. The main role of pump laser beam is to saturate the sub-bandgap localized states whose contribution otherwise swamp the information related to the bandgap of material. It also affects the magnitude of Dember voltage in case of semi-insulating (SI) semiconductor substrates. Pump-probe SPS technique enables an accurate determination of the bandgap of semiconductor epitaxial layers even under the strong influence of localized sub-bandgap states. The pump beam is found to be very effective in suppressing the effect of surface/interface and bulk trap states. The overall magnitude of SPV signal is decided by the dependence of charge separation mechanisms on the intensity of the pump beam. On the contrary, an above bandgap cw pump laser can be used to distinguish the signatures of sub-bandgap states by suppressing the band edge related feature. Usefulness of the pump-probe SPS technique is established by unambiguously determining the bandgap of p-GaAs epitaxial layers grown on SI-GaAs substrates, SI-InP wafers, and p-GaN epilayers grown on Sapphire substrates.

  18. Bulk and surface half-metallicity: Metastable zinc-blende TiSb

    NASA Astrophysics Data System (ADS)

    Gao, G. Y.; Yao, K. L.

    2012-07-01

    Motivated by the recent experimental fabrication of half-metallic MnSb and CrSb thin films with metastable zinc-blende structure [Aldous et al. Phys. Rev. B 85, 060403(R) (2012); Deng et al. J. Appl. Phys. 99, 093902 (2006)], we investigate the structural, electronic, and magnetic properties of TiSb in both ground-state NiAs and metastable zinc-blende phases by using the first-principles calculations. It is shown that the ground-state NiAs phase is metallic and nonmagnetic, but the metastable zinc-blende phase exhibits half-metallic ferromagnetism with a magnetic moment of 1.00 μB per formula unit. We also reveal that the half-metallicity in bulk ZB TiSb is lost at the Sb-terminated (001) surface due to the appearance of surface states within the gap of the minority-spin channel, but the Ti-terminated (001) surface retains the bulk half-metallicity, which makes ZB TiSb a promising candidate for the possible epitaxial growth of half-metallic thin films or multilayers on semiconductor substrates for spintronic applications.

  19. Analysis of tuning methods in semiconductor frequency-selective surfaces

    NASA Astrophysics Data System (ADS)

    Shemelya, Corey; Palm, Dominic; Fip, Tassilo; Rahm, Marco

    2017-02-01

    Advanced technology, such as sensing and communication equipment, has recently begun to combine optically sensitive nano-scale structures with customizable semiconductor material systems. Included within this broad field of study is the aptly named frequency-selective surface; which is unique in that it can be artificially designed to produce a specific electromagnetic or optical response. With the inherent utility of a frequency-selective surface, there has been an increased interest in the area of dynamic frequency-selective surfaces, which can be altered through optical or electrical tuning. This area has had exciting break throughs as tuning methods have evolved; however, these methods are typically energy intensive (optical tuning) or have met with limited success (electrical tuning). As such, this work investigates multiple structures and processes which implement semiconductor electrical biasing and/or optical tuning. Within this study are surfaces ranging from transmission meta-structures to metamaterial surface-waves and the associated coupling schemes. This work shows the utility of each design, while highlighting potential methods for optimizing dynamic meta-surfaces. As an added constraint, the structures were also designed to operate in unison with a state-of-the-art Ti:Sapphire Spitfire Ace and Spitfire Ace PA dual system (12 Watt) with pulse front matching THz generation and an EOS detection system. Additionally, the Ti:Sapphire laser system would provide the means for optical tunablity, while electrical tuning can be obtained through external power supplies.

  20. Enhanced infrared magneto-optical response of the nonmagnetic semiconductor BiTeI driven by bulk Rashba splitting.

    PubMed

    Demkó, L; Schober, G A H; Kocsis, V; Bahramy, M S; Murakawa, H; Lee, J S; Kézsmárki, I; Arita, R; Nagaosa, N; Tokura, Y

    2012-10-19

    We study the magneto-optical (MO) response of the polar semiconductor BiTeI with giant bulk Rashba spin splitting at various carrier densities. Despite being nonmagnetic, the material is found to yield a huge MO activity in the infrared region under moderate magnetic fields (up to 3 T). Our first-principles calculations show that the enhanced MO response of BiTeI comes mainly from the intraband transitions between the Rashba-split bulk conduction bands. These transitions connecting electronic states with opposite spin directions become active due to the presence of strong spin-orbit interaction and give rise to distinct features in the MO spectra with a systematic doping dependence. We predict an even more pronounced enhancement in the low-energy MO response and dc Hall effect near the crossing (Dirac) point of the conduction bands.

  1. Study on the nonlinear polarization rotation law in a bulk semiconductor optical amplifier in a pump-probe scheme

    NASA Astrophysics Data System (ADS)

    Feng, Xianghua; Ji, Jiarong; Dou, Wenhua; Zhang, Guomin

    2012-10-01

    The physical mechanisms for the polarization rotation of the light in a bulk semiconductor optical amplifier (SOA) originate from the significant nonuniform distributions of carrier density across the active region. Due to this carrier density's nonuniformity, the effective refractive indexes experienced by transverse-electric (TE) and transverse-magnetic (TM) modes of the probe are different. This results in a phase shift between TE and TM modes of the light upon leaving the SOA. The bulk SOA polarization rotation's law can be analyzed theoretically and experimentally based on the method of measuring output power in a pump-probe scheme. The experiment employs polarizer driving by walking electromotor and power meter, the light power of every orientation is measured. The transformation law of output polarization is find for obvious polarization rotation in other perpendicular axes based on connection of ellipse in difference axes.

  2. Hot electron dynamics at semiconductor surfaces: Implications for quantum dot photovoltaics

    NASA Astrophysics Data System (ADS)

    Tisdale, William A., III

    Finding a viable supply of clean, renewable energy is one of the most daunting challenges facing the world today. Solar cells have had limited impact in meeting this challenge because of their high cost and low power conversion efficiencies. Semiconductor nanocrystals, or quantum dots, are promising materials for use in novel solar cells because they can be processed with potentially inexpensive solution-based techniques and because they are predicted to have novel optoelectronic properties that could enable the realization of ultra-efficient solar power converters. However, there is a lack of fundamental understanding regarding the behavior of highly-excited, or "hot," charge carriers near quantum-dot and semiconductor interfaces, which is of paramount importance to the rational design of high-efficiency devices. The elucidation of these ultrafast hot electron dynamics is the central aim of this Dissertation. I present a theoretical framework for treating the electronic interactions between quantum dots and bulk semiconductor surfaces and propose a novel experimental technique, time-resolved surface second harmonic generation (TR-SHG), for probing these interactions. I then describe a series of experimental investigations into hot electron dynamics in specific quantum-dot/semiconductor systems. A two-photon photoelectron spectroscopy (2PPE) study of the technologically-relevant ZnO(1010) surface reveals ultrafast (sub-30fs) cooling of hot electrons in the bulk conduction band, which is due to strong electron-phonon coupling in this highly polar material. The presence of a continuum of defect states near the conduction band edge results in Fermi-level pinning and upward (n-type) band-bending at the (1010) surface and provides an alternate route for electronic relaxation. In monolayer films of colloidal PbSe quantum dots, chemical treatment with either hydrazine or 1,2-ethanedithiol results in strong and tunable electronic coupling between neighboring quantum dots

  3. Separation of surface and bulk minority-carrier lifetimes in silicon

    NASA Technical Reports Server (NTRS)

    Carroll, K. A.; Casper, K. J.

    1974-01-01

    A photoexcitation technique is suggested which is direct and convenient, requiring little special equipment. No change in the dimensions of the crystal sample is necessary. Although related to the technique of interposing a filter between the light source and the sample, it differs in that the filter is part of the crystal itself. Basically, one of the electrodes is split into two electrodes, one collecting charge from carriers excited in the bulk. This phenomenon in which the carriers follow very closely the electric field lines has been used in position-sensitive semiconductor radiation detectors (Norbeck and Carlson, 1963) but has not been applied to lifetime measurements. In a sense, the split electrode creates a sample without a surface.

  4. Investigation of Surface Breakdown on Semiconductor Devices Using Optical Probing Techniques.

    DTIC Science & Technology

    1990-01-01

    18] L. Bovino , T. Burke, R. Youmans, M. Weiner, and J. Car, r, "Recent Advances in Optically C’ntrolled Bulk Semiconductor Switches," Digest of...Comp. Simul. 5 (3), 175 (1988). [321 M. Weiner, L. Bovino , R. Youmans, and T. Burke, "Modeling of the Optically Conrolled Semiconductor Switch," J

  5. Bulk and surface magnetoinductive breathers in binary metamaterials.

    PubMed

    Molina, M I; Lazarides, N; Tsironis, G P

    2009-10-01

    We investigate theoretically the existence of bulk and surface discrete breathers in a one-dimensional magnetic metamaterial comprised of a periodic binary array of split-ring resonators; the two types of resonators used have different resonant frequencies caused by unequal slit sizes. We use the rotating-wave approximation and construct several types of breather excitations both for the energy-conserving as well as dissipative-driven case; we corroborate these approximate results trough numerically exact computations. We demonstrate that discrete breathers can appear spontaneously in the dissipative-driven system as a result of a fundamental instability.

  6. Bulk and surface sensitivity of a resonant waveguide grating imager

    NASA Astrophysics Data System (ADS)

    Orgovan, Norbert; Kovacs, Boglarka; Farkas, Eniko; Szabó, Bálint; Zaytseva, Natalya; Fang, Ye; Horvath, Robert

    2014-02-01

    We report the assessment of the sensitivity of a microplate-compatible resonant waveguide grating imager. The sensitivity to bulk refractive index changes was determined using a serial dilution of glycerol solution with the help of a refractometer. The surface sensitivity was examined using layer-by-layer polyelectrolyte films in conjunction with optical waveguide lightmode spectroscopy and characterized by the binding of acetazolamide to immobilized carbonic anhydrase under microfluidics. The results suggest that the imager has a limit of detection down to 2.2 × 10-6 for refractive index change and 0.078 ng/cm2 for the adsorbed mass.

  7. Time domain terahertz spectroscopy of semiconductor bulk and multiple quantum wells structures

    NASA Astrophysics Data System (ADS)

    Chen, Yue

    A time-domain terahertz spectroscopic system with high source power (average power > 10 nW) and high signal-to- noise ratio (>104) was developed and used to study ultrafast electronic processes in semiconductor structures. The physics of the spectroscopy, the theoretical basis of the interferometry, the model of the electron-electromagnetic field interaction, and the principle of experimental data processing are presented. The first direct measurement of the intervalley scattering time in In 0.53Ga0.47As was performed. The intervalley scattering time constants obtained were τLΓ = 35 fs and τLΓ = 450 fs. The spectroscopic data showed that at low carrier density the carrier- carrier scattering is unimportant. The intervalley deformation potential was obtained from the measured intervalley scattering time constant τ LΓ. The transient conductivity was obtained using time-domain terahertz spectroscopy. The frequency dependent terahertz spectroscopy enabled us to uniquely determine the transient mobility and density. The transient electron mobility is ~5200 cm2/Vs, which is less than the Hall mobility. For large photocarrier densities, this discrepancy is attributed to the additional momentum relaxation associated with electron-hole scattering. Using pump pulses with wavelength of 810 run, the electron trapping time in low-temperature-grown GaAs was accurately determined. The measured trapping time is slightly larger than that observed from a band-edge pump- probe measurements. We argue that the terahertz technique provides the most reliable measure of carrier lifetime due to the unique interaction. The carrier dynamics of low-temperature-grown InGaAs bulk and InGaAs/InAlAs multiple quantum wells were investigated. We were able to differentiate the two dominant mechanisms in the electron decay process, trapping and recombination. A trapping time as fast as 1.3-2.6 ps was observed for photo-excited electrons. The effects of Be-doping and growth temperature on the

  8. Chemisorption and Reactions of Molecules on Semiconductor Surfaces.

    DTIC Science & Technology

    1987-05-18

    relevant to Surface Enhanced Raman and Sur- face Chemistry were also investigated. Later work on Si(l1l) surfaces included studies of oxidation, water ...a 2K cal/mole weakly bound, Inclined, nitrogen lone-pair bonded phase (Ref. 21). d. Benine, cyclohexane, methanol, water , n-hexane ethylene have also...silicon monoxide diatomic molecule, and a bulk-like monoxide phase are observed to form (Ref. 42). 38. Formation of embedded metallic microclusters on

  9. Transport Imaging: Developing an Optical Technique to Characterize Bulk Semiconductor Materials for Next Generation Radiation Detectors

    DTIC Science & Technology

    2009-06-01

    OF PAGES 79 14. SUBJECT TERMS Cathodoluminescence, Diffusion, Drift, Mobility, Lifetime, Bismuth Ferrite , BiFeO3 , Semiconductor, Transport...34 VI. CATHODOLUMINESCENCE OF BISMUTH FERRITE ................................ 51...developed. This technique is then illustrated experimentally on gallium arsenide. Bismuth ferrite (BFO) is a new promising high Z material that is

  10. Adhesion: role of bulk viscoelasticity and surface roughness

    NASA Astrophysics Data System (ADS)

    Lorenz, B.; Krick, B. A.; Mulakaluri, N.; Smolyakova, M.; Dieluweit, S.; Sawyer, W. G.; Persson, B. N. J.

    2013-06-01

    We study the adhesion between smooth polydimethylsiloxane (PDMS) rubber balls and smooth and rough poly(methyl methacrylate) (PMMA) surfaces, and between smooth silicon nitride balls and smooth PDMS surfaces. From the measured viscoelastic modulus of the PDMS rubber we calculate the viscoelastic contribution to the crack-opening propagation energy γeff(v,T) for a wide range of crack tip velocities v and for several temperatures T. The Johnson-Kendall-Roberts (JKR) contact mechanics theory is used to analyze the ball pull-off force data, and γeff(v,T) is obtained for smooth and rough surfaces. We conclude that γeff(v,T) has contributions of similar magnitude from both the bulk viscoelastic energy dissipation close to the crack tip, and from the bond-breaking process at the crack tip. The pull-off force on the rough surfaces is strongly reduced compared to that of the flat surface, which we attribute mainly to the decrease in the area of contact on the rough surfaces.

  11. Adhesion: role of bulk viscoelasticity and surface roughness.

    PubMed

    Lorenz, B; Krick, B A; Mulakaluri, N; Smolyakova, M; Dieluweit, S; Sawyer, W G; Persson, B N J

    2013-06-05

    We study the adhesion between smooth polydimethylsiloxane (PDMS) rubber balls and smooth and rough poly(methyl methacrylate) (PMMA) surfaces, and between smooth silicon nitride balls and smooth PDMS surfaces. From the measured viscoelastic modulus of the PDMS rubber we calculate the viscoelastic contribution to the crack-opening propagation energy γeff(v,T) for a wide range of crack tip velocities v and for several temperatures T. The Johnson-Kendall-Roberts (JKR) contact mechanics theory is used to analyze the ball pull-off force data, and γeff(v,T) is obtained for smooth and rough surfaces. We conclude that γeff(v,T) has contributions of similar magnitude from both the bulk viscoelastic energy dissipation close to the crack tip, and from the bond-breaking process at the crack tip. The pull-off force on the rough surfaces is strongly reduced compared to that of the flat surface, which we attribute mainly to the decrease in the area of contact on the rough surfaces.

  12. Surface and Bulk Carbide Transformations in High-Speed Steel

    NASA Astrophysics Data System (ADS)

    Godec, M.; Večko Pirtovšek, T.; Šetina Batič, B.; McGuiness, P.; Burja, J.; Podgornik, B.

    2015-11-01

    We have studied the transformation of carbides in AISI M42 high-speed steels in the temperature window used for forging. The annealing was found to result in the partial transformation of the large, metastable M2C carbides into small, more stable grains of M6C, with an associated change in the crystal orientation. In addition, MC carbides form during the transformation of M2C to M6C. From the high-speed-steel production point of view, it is beneficial to have large, metastable carbides in the cast structure, which later during annealing, before the forging, transform into a structure of polycrystalline carbides. Such carbides can be easily decomposed into several small carbides, which are then randomly distributed in the microstructure. The results also show an interesting difference in the carbide-transformation reactions on the surface versus the bulk of the alloy, which has implications for in-situ studies of bulk phenomena that are based on surface observations.

  13. Surface and Bulk Carbide Transformations in High-Speed Steel.

    PubMed

    Godec, M; Večko Pirtovšek, T; Šetina Batič, B; McGuiness, P; Burja, J; Podgornik, B

    2015-11-05

    We have studied the transformation of carbides in AISI M42 high-speed steels in the temperature window used for forging. The annealing was found to result in the partial transformation of the large, metastable M2C carbides into small, more stable grains of M6C, with an associated change in the crystal orientation. In addition, MC carbides form during the transformation of M2C to M6C. From the high-speed-steel production point of view, it is beneficial to have large, metastable carbides in the cast structure, which later during annealing, before the forging, transform into a structure of polycrystalline carbides. Such carbides can be easily decomposed into several small carbides, which are then randomly distributed in the microstructure. The results also show an interesting difference in the carbide-transformation reactions on the surface versus the bulk of the alloy, which has implications for in-situ studies of bulk phenomena that are based on surface observations.

  14. Surface and Bulk Carbide Transformations in High-Speed Steel

    PubMed Central

    Godec, M.; Večko Pirtovšek, T.; Šetina Batič, B.; McGuiness, P.; Burja, J.; Podgornik, B.

    2015-01-01

    We have studied the transformation of carbides in AISI M42 high-speed steels in the temperature window used for forging. The annealing was found to result in the partial transformation of the large, metastable M2C carbides into small, more stable grains of M6C, with an associated change in the crystal orientation. In addition, MC carbides form during the transformation of M2C to M6C. From the high-speed-steel production point of view, it is beneficial to have large, metastable carbides in the cast structure, which later during annealing, before the forging, transform into a structure of polycrystalline carbides. Such carbides can be easily decomposed into several small carbides, which are then randomly distributed in the microstructure. The results also show an interesting difference in the carbide-transformation reactions on the surface versus the bulk of the alloy, which has implications for in-situ studies of bulk phenomena that are based on surface observations. PMID:26537780

  15. Bulk and surface calorimetric measurements at CO wavelengths.

    PubMed

    Allen, S D; Rudisill, J E

    1977-11-01

    Laser calorimetry was used to measure the optical absorption of several candidate window materials for application at CO laser wavelengths. These materials include KCl, CaF(2), SrF(2), and ZnSe. The long, thin bar technique was used to separate bulk and surface absorption contributions by means of their time dependencies. One sample of KCl, known to be low absorbing at 10.6 microm, exhibited no measurable absorption within the sensitivity of the calorimeter (beta(t) < 2 x 10(-6) cm(-1)). The absorption coefficients measured for two samples of CaF(2) at 5.41 microm were higher than those measured by other investigators at 5.25 microm, but, when fitted to an exponential dependence on wavelength, compared favorably with currently measured values. To compare results from different laboratories, precise definition of the spectral power distribution is essential. The surface absorption was, in all cases, small relative to similar measurements at 10.6 microm; in several cases it was not separable from bulk absorption by the technique used.

  16. Applications of surface acoustic and shallow bulk acoustic wave devices

    NASA Astrophysics Data System (ADS)

    Campbell, Colin K.

    1989-10-01

    Surface acoustic wave (SAW) device coverage includes delay lines and filters operating at selected frequencies in the range from about 10 MHz to 11 GHz; modeling with single-crystal piezoelectrics and layered structures; resonators and low-loss filters; comb filters and multiplexers; antenna duplexers; harmonic devices; chirp filters for pulse compression; coding with fixed and programmable transversal filters; Barker and quadraphase coding; adaptive filters; acoustic and acoustoelectric convolvers and correlators for radar, spread spectrum, and packet radio; acoustooptic processors for Bragg modulation and spectrum analysis; real-time Fourier-transform and cepstrum processors for radar and sonar; compressive receivers; Nyquist filters for microwave digital radio; clock-recovery filters for fiber communications; fixed-, tunable-, and multimode oscillators and frequency synthesizers; acoustic charge transport; and other SAW devices for signal processing on gallium arsenide. Shallow bulk acoustic wave device applications include gigahertz delay lines, surface-transverse-wave resonators employing energy-trapping gratings, and oscillators with enhanced performance and capability.

  17. Tailoring GaN semiconductor surfaces with biomolecules.

    PubMed

    Estephan, Elias; Larroque, Christian; Cuisinier, Frédéric J G; Bálint, Zoltán; Gergely, Csilla

    2008-07-24

    Functionalization of semiconductors constitutes a crucial step in using these materials for various electronic, photonic, biomedical, and sensing applications. Within the various possible approaches, selection of material-binding biomolecules from a random biological library, based on the natural recognition of proteins or peptides toward specific material, offers many advantages, most notably biocompatibility. Here we report on the selective functionalization of GaN, an important semiconductor that has found broad uses in the past decade due to its efficient electroluminescence and pronounced chemical stability. A 12-mer peptide ("GaN_probe") with specific recognition for GaN has evolved. The subtle interplay of mostly nonpolar hydrophobic and some polar amino acidic residues defines the high affinity adhesion properties of the peptide. The interaction forces between the peptide and GaN are quantified, and the hydrophobic domain of the GaN_probe is identified as primordial for the binding specificity. These nanosized binding blocks are further used for controlled placement of biotin-streptavidin complexes on the GaN surface. Thus, the controlled grow of a new, patterned inorganic-organic hybrid material is achieved. Tailoring of GaN by biological molecules can lead to a new class of nanostructured semiconductor-based devices.

  18. Surface and bulk hot electron dynamics in silicon

    NASA Astrophysics Data System (ADS)

    Jeong, Seongtae; Bokor, Jeffrey

    1997-03-01

    The direct time domain study of hot electron dynamics on the silicon surface has been an active area of research. Dynamics in Si(100) surface states was observed(M.W. Rowe, H. Liu, G. P. Williams, Jr., and R. T. Williams, Phys. Rev. B 47, 2048 (1993)) as well as cooling of a hot but thermal distribution of carriers in bulk silicon(J. R. Goldman, and J. A. Prybyla, Phys. Rev. Lett. 72, 1364 (1994)). In this work, a time-resolved photoemission study on the Si(100)2x1 surface with 1.55 eV pump and 4.66 eV probe with 0.2 psec time resolution is reported. It is observed that two-photon absorption is responsible for high kinetic energy electrons above the conduction band minimum (CBM) but direct single-photon excitation into surface states and conduction band states followed by the surface recombination dominates the dynamics. Also observed are an early nonthermal electronic distribution in silicon and its transition into a thermal one followed by a rapid cooling.

  19. Electron microscopy techniques for evaluating epitaxial and bulk III-V compound semiconductors

    SciTech Connect

    Frigeri, C.

    1996-12-01

    Electron microscopy is an important technique to study interfaces and microdefects in advanced III-V compound semiconductors. The paper briefly reviews some of the TEM methods used to this purpose and shows examples of their application to the characterization of epitaxial structures such as InGaAs/GaAs and GaAs/Ge as well as processed substrates like implanted InP.

  20. Theory of surfactant-mediated growth on semiconductor surfaces

    NASA Astrophysics Data System (ADS)

    Kaxiras, Efthimios; Kandel, Daniel

    1996-08-01

    The surfactant effect, first demonstrated by Copel et al. [Phys. Rev. Lett. 63 (1989) 632] by using As to promote epitaxial growth of Ge on Si(100), has now been studied in a wide variety of systems, thus making systematic studies possible. We present theoretical models that account for the observed behavior of various surfactants on semiconductor surfaces, including homo-epitaxial and hetero-epitaxial growth. The theoretical models include first-principles calculations of the relative energy of different structures associated with surfactant layers and the activation energies for diffusion and exchange mechanisms, as well as solid-on-solid Monte Carlo simulations.

  1. Bulk and surface characterization of novel photoresponsive polymeric systems

    NASA Astrophysics Data System (ADS)

    Venkataramani, Shivshankar

    This dissertation presents a detailed characterization of two important classes of photoresponsive polymers-polydiacetylenes (PDAs) and azopolymers. Bulk and surface characterization techniques were used to evaluate the structure-property relationships of the PDAs and surface characterization, in particular-atomic force microscopy (AFM) was used to characterize the azopolymers. PDAs from bis-alkylurethanes of 5,7 dodecadiyn 1,12-diol (viz.,) ETCD, IPUDO and PUDO are of particular interest in view of reports of reversible thermochromic and photochromic phase transitions in these materials. Thermochromism in the above PDAs is associated with a first order phase transition involving expansion of the crystallographic unit cell, the preservation of the urethane hydrogen bonding and possibly some relief of mechanical strain upon heating. Insights into thermochromism obtained from studies of nonthermochromic forms of PDA-ETCD are discussed. Some of the bulk characterization experiments reported In the literature are repeated. The motivation to investigate the surface morphology of the PDA single crystals using AFM was derived from Raman spectroscopy studies of various PDAs in which dispersion of the Raman spectrum indicating surface heterogeneity was observed. Micron scale as well as molecularly resolved images were obtained The micron scale images indicated a variable surface of the crystals. The molecularly resolved images showed a well defined 2-D lattice and are interpreted in terms of known crystallographic data. The surface parameters obtained from AFM measurements are similar to those determined from X-ray diffraction. During an attempt of AFM imaging of IPUDO crystals exposed to 254 nm ultraviolet light, it was observed that these crystals undergo a "macroscopic shattering". In the interest of rigorously defining conditions for photochromism, this research has undertaken a combined study of the surface morphology of the above mentioned PDA crystals by AFM and the

  2. Nonequilibrium optical properties in semiconductors from first principles: A combined theoretical and experimental study of bulk silicon

    NASA Astrophysics Data System (ADS)

    Sangalli, Davide; Dal Conte, Stefano; Manzoni, Cristian; Cerullo, Giulio; Marini, Andrea

    2016-05-01

    The calculation of the equilibrium optical properties of bulk silicon by using the Bethe-Salpeter equation solved in the Kohn-Sham basis represents a cornerstone in the development of an ab-initio approach to the optical and electronic properties of materials. Nevertheless, calculations of the transient optical spectrum using the same efficient and successful scheme are scarce. We report, here, a joint theoretical and experimental study of the transient reflectivity spectrum of bulk silicon. Femtosecond transient reflectivity is compared to a parameter-free calculation based on the nonequilibrium Bethe-Salpeter equation. By providing an accurate description of the experimental results we disclose the different phenomena that determine the transient optical response of a semiconductor. We give a parameter-free interpretation of concepts such as bleaching, photoinduced absorption, and stimulated emission, beyond the Fermi golden rule. We also introduce the concept of optical gap renormalization, as a generalization of the known mechanism of band gap renormalization. The present scheme successfully describes the case of bulk silicon, showing its universality and accuracy.

  3. The role of strain in the surface structures of III-V alloyed semiconductor films

    NASA Astrophysics Data System (ADS)

    Bickel, Jessica E.

    As length scales continue to decrease, it is vital to understand the fundamental physical parameters governing surfaces and surface interactions. In semiconductors particularly, surface reconstructions are known to impact film growth, bulk atomic ordering and the development of interfacial structure, all of which can drastically impact device growth. While the parameters that determine surface reconstructions in homoepitaxially grown films are well known and understood, those that impact alloy film growth are less studied. This work examines the impact of strain on alloy surface reconstructions, using the III-V semiconductors as a model system for any covalently bonded crystal structure. The presence of surface reconstruction coexistence in both mixed cation and mixed anion systems suggests that localized strain fields on alloy surfaces stabilize elastic relaxation at boundaries, resulting in more complex surface structures than those seen on binary, unstrained films. Atomic size mismatch strain is shown to induce an ordering in alloyed surface reconstructions that is not seen in the non-alloyed constituent surfaces. Lattice mismatch strain is shown to both stabilize new reconstructions not common to the homoepitaxial system and to induce surface reconstruction coexistence on alloy surfaces. The supplied flux of material is shown to affect the kinetics of transformation between the two coexisting surface reconstructions and an incorporation model for material on the alloy surface is developed. The effects of strained surface reconstructions on subsequent film growth is explored and it is shown that identical films grown on two different surfaces have very different strain relaxation profiles, surface topographies and defect structures. The strain fields of surface reconstructions and defects are also shown to interact which may have an impact on the insertion of dislocations in these films. Combined together, this deep understanding of the role that alloy induced

  4. Coupling of Surface Plasmons and Semiconductor Nanocrystals for Nanophotonics Applications

    NASA Astrophysics Data System (ADS)

    Jayanti, Sriharsha V.

    The goal of this thesis is to engineer the interaction between surface plasmons and semiconductor nanocrystals for nanophotonic applications. Plasmonic metals support surface plasmon polaritons, hybrid photon and electron waves that propagate along a metal-dielectric interface. Unlike photons, surface plasmons can be confined in sub-diffraction geometries. This has two important consequences: 1) optical devices can be designed at the nanoscale, and 2) the high density of electromagnetic fields allows study of enhanced light-matter interactions. Surface plasmons have been exploited to demonstrate components of optoelectronic circuits, optical antennas, surface enhanced spectroscopy, enhanced fluorescence from fluorophores, and nanolasers. Despite the advances, surface plasmon losses limit their propagation lengths to tens of micrometers in the visible wavelengths, hindering many applications. Recently, the template-stripping approach was shown to fabricate metal films that exhibit larger grains and smoother surface, reducing the grain boundary and roughness scattering. To further improve the plasmonic properties, we investigate the importance of deposition conditions in the template-stripping approach. We provide insight and recipes to enhance the plasmonic performance of the most commonly used metals in the ultraviolet, visible, and near-infrared. We also explore the potential of low temperatures to improve the performance of metal films, where the electron-electron and electron-phonon scattering should be reduced. This sets a limit on the minimum loss metals can exhibit. Using this knowledge, we study the optical properties of quantum-confined semiconductor nanocrystals near metal structures. Semiconductor nanocrystals have many attractive characteristics that make them suitable for solid-state lighting and solar cells among others. Specifically, CdSe nanocrystals have been heavily studied for their large absorption and emission cross-sections, size dependent

  5. Light scattering from human corneal grafts: Bulk and surface contribution

    NASA Astrophysics Data System (ADS)

    Latour, Gaël; Georges, Gaëlle; Lamoine, Laure Siozade; Deumié, Carole; Conrath, John; Hoffart, Louis

    2010-09-01

    The cornea is the only transparent tissue in the body. The transparency is the main characteristic of the corneal tissue, and depends not only on the transmission coefficient but also on the losses by scattering and absorption. The scattering properties of the cornea tissues become one of the most important parameters in the case of the corneal graft. These scattering properties are studied in this paper in the reflected half area, similar to the diagnosis configuration. We quantify the influence of the cornea thickness and of the epithelial layer on scattering level. The technique of ellipsometry on scattered field is also used to analyze the polarization properties in order to determine the origin of scattering (surface and/or bulk).

  6. Novel Bulk Silicon Lateral Double-Diffused Metal-Oxide-Semiconductor Field-Effect Transistors Using Step Thickness Technology in Drift Region

    NASA Astrophysics Data System (ADS)

    Huang, Shi; Guo, Yufeng; Yao, Jiafei; Hua, Tingting; Zhang, Jun; Zhang, Changchun; Ji, Xincun

    2013-12-01

    In this paper, a novel bulk silicon lateral double-diffused metal-oxide-semiconductor field-effect transistors (LDMOS) using step thickness technology in drift region is proposed. The drift region is divided into several zones with different thicknesses increasing from source to drain. Owing to modulation effect of the step thickness drift region, new additional electric field peaks are introduced in the drift region, thus leading to the reduction of the surface electric fields and the increase of the breakdown voltage. The influences of device parameters on breakdown voltage and specific on-resistance are investigated using semiconductor device simulator, MEDICI. The simulation results indicate that an 18.4% increase in the breakdown voltage and a 42.5% increase in the figure of merit (FOM) are obtained in the novel device in comparison with the conventional LDMOS. Furthermore, single step can lead to approximately ideal FOM in comparison with the multiple steps, so that can obtain a suitable trade-off between fabrication costs and performance.

  7. Semiconductor-to-metal transition in the bulk of WSe2 upon potassium intercalation

    NASA Astrophysics Data System (ADS)

    Ahmad, Mushtaq; Müller, Eric; Habenicht, Carsten; Schuster, Roman; Knupfer, Martin; Büchner, Bernd

    2017-04-01

    We present electron energy-loss spectroscopic measurements of potassium (K) intercalated tungsten diselenide (WSe2). After exposure of pristine WSe2 films to potassium, we observe a charge carrier plasmon excitation at about 0.97 eV, which indicates a semiconductor-to-metal transition. Our data reveal the formation of one particular doped K-WSe2 phase. A Kramers-Kronig analysis allows the determination of the dielectric function and the estimation of the composition of K0.6WSe2. Momentum dependent measurements reveal a substantial plasmon dispersion to higher energies.

  8. Semiconductor-to-metal transition in the bulk of WSe2 upon potassium intercalation.

    PubMed

    Ahmad, Mushtaq; Müller, Eric; Habenicht, Carsten; Schuster, Roman; Knupfer, Martin; Büchner, Bernd

    2017-04-26

    We present electron energy-loss spectroscopic measurements of potassium (K) intercalated tungsten diselenide (WSe2). After exposure of pristine WSe2 films to potassium, we observe a charge carrier plasmon excitation at about 0.97 eV, which indicates a semiconductor-to-metal transition. Our data reveal the formation of one particular doped K-WSe2 phase. A Kramers-Kronig analysis allows the determination of the dielectric function and the estimation of the composition of K0.6WSe2. Momentum dependent measurements reveal a substantial plasmon dispersion to higher energies.

  9. Bulk and surface polymer composites prepared in supercritical carbon dioxide

    NASA Astrophysics Data System (ADS)

    Kung, Edward

    This dissertation describes the use of supercritical carbon dioxide (SC CO2) as an aid in fabricating polymer/polymer composites. Monomers and initiators were infused into solid polymer substrates using SC CO 2. The monomers were subsequently polymerized within the substrates to form composites. CO2 swells the polymer substrate and increases the diffusively of reactants within the substrate. The solvent strength of SC CO2 is tunable allowing control over the degree of swelling and over the partitioning behavior of the reactants. CO2 can be easily removed from the final products. First, polystyrene/polyethylene bulk composites were investigated. Styrene and a radical initiator were infused into and reacted throughout the bulk of polyethylene substrates. The composite composition was controlled by controlling infusion time, reaction time and partitioning conditions. Characterization of the composites showed that the crystalline domains of the polyethylene were unaffected. Styrene infused into and polymerized within only the amorphous domains of polyethylene. Polyethylene and polystyrene are immiscible; the semicrystalline nature of polyethylene frustrated gross phase separation of the polystyrene. The resulting "kinetically trapped" phase morphology gave the composites interesting mechanical properties. The phase morphology was characterized, and the polystyrene was found to reside within the interlamellar regions and the centers of the polyethylene spherulites. The polystyrene formed a continuous "scaffold" that reinforced the polyethylene. The reinforcement provided efficient and dramatic improvement in the composite modulus and strength. However, the composites fracture toughness decreased with increasing polystyrene content. The fracture behavior was correlated to the microstructural damage mechanisms in the composites. Second, surface composites were investigated. Using a two-stage process, ethyl 2-cyanoacrylate (ECA) monomer was anionically polymerized in the

  10. New materials based on CdTe-AIIBVI systems with cationic and anionic substitution. Bulk and surface properties

    NASA Astrophysics Data System (ADS)

    Kirovskaya, I. A.; Bukashkina, T. L.; Ekkert, R. V.; Murashova, A. O.

    2017-08-01

    The bulk (crystallochemical, structural) and surface properties (acid-base, optical) of the multicomponent and binary semiconductors of CdTe-CdSe, CdTe-ZnSe systems have been examined. The relationships of the changes between both correlating compositions have been investigated. The possibility of using the detected correlations for a less expensive way to find new materials-primary sensor transducers - has been shown. In this aspect, recommendations how to use the received materials in the sensors in CO micro-impurities have been made.

  11. Modeling direct band-to-band tunneling: From bulk to quantum-confined semiconductor devices

    SciTech Connect

    Carrillo-Nuñez, H.; Ziegler, A.; Luisier, M.; Schenk, A.

    2015-06-21

    A rigorous framework to study direct band-to-band tunneling (BTBT) in homo- and hetero-junction semiconductor nanodevices is introduced. An interaction Hamiltonian coupling conduction and valence bands (CVBs) is derived using a multiband envelope method. A general form of the BTBT probability is then obtained from the linear response to the “CVBs interaction” that drives the system out of equilibrium. Simple expressions in terms of the one-electron spectral function are developed to compute the BTBT current in two- and three-dimensional semiconductor structures. Additionally, a two-band envelope equation based on the Flietner model of imaginary dispersion is proposed for the same purpose. In order to characterize their accuracy and differences, both approaches are compared with full-band, atomistic quantum transport simulations of Ge, InAs, and InAs-Si Esaki diodes. As another numerical application, the BTBT current in InAs-Si nanowire tunnel field-effect transistors is computed. It is found that both approaches agree with high accuracy. The first one is considerably easier to conceive and could be implemented straightforwardly in existing quantum transport tools based on the effective mass approximation to account for BTBT in nanodevices.

  12. Towards p × n transverse thermoelectrics: extreme anisotropic conduction in bulk doped semiconductor thin films via proton implantation

    NASA Astrophysics Data System (ADS)

    Tang, Yang; Koblmüller, G.; Riedl, H.; Grayson, M.

    2016-03-01

    Transverse thermoelectrics promise entirely new strategies for integrated cooling elements for optoelectronics. The recently introduced p × n-type transverse thermoelectric paradigm indicates that the most important step to engineering artificial transverse thermoelectrics is to create alternate p- and n-doped layers with orthogonally oriented anisotropic conductivity. This paper studies an approach to creating extreme anisotropic conductivity in bulk-doped semiconductor thin films via ion implantation. This approach defines an array of parallel conduction channels with photolithographic patterning of an SiO2 mask layer, followed by proton implantation. With a 10 μm channel width and 20 μm pitch, both n-type and p-type Al0.42 Ga0.58As thin films demonstrate a conductivity anisotropy ratio σ /σ⊥ > 104 at room temperature, while the longitudinal resistivity along the channel direction after implantation only increased by a factor of 3.3 ˜ 3.6. This approach can be readily adapted to other semiconductor materials for artificial p × n-type transverse thermoelectrics as other applications.

  13. Photoinduced "stick-slip" on superhydrophilic semiconductor surfaces.

    PubMed

    Denison, Kieth R; Boxall, Colin

    2007-04-10

    Transparent mesoporous TiO2 (M-TiO2) thin films were prepared on quartz via a reverse micelle, sol-gel, spin-coating technique. Films were characterized by atomic force microscopy (AFM) and Raman and UV-vis spectroscopies and were found to be mostly anatase with low surface roughness (Rt approximately 5 nm). The time dependence of film photoinduced superhydrophilicity (PISH) was measured by observation of the spreading of a sessile water drop using a new, continuous measurement technique wherein the drop was first applied to the semiconductor surface and then was filmed while it and the underlying substrate were illuminated by 315 nm ultraband gap light. Results obtained at 100% relative humidity (RH) at 293 K showed that drops on M-TiO2 surfaces exhibited a photoinduced "stick-slip" behavior, the first time such an effect has been observed. The thermodynamic driving force for this photoinduced stick-slip was the departure of the system from capillary equilibrium as, with increasing illumination time, the concentration of surface Ti-OH groups increased and the equilibrium contact angle of the drop, theta0, decreased. A simple theoretical description of photoinduced stick-slip is derived and is used to calculate a value of the potential energy barrier associated with surface inhomogeneities that oppose onset of movement of the triple line, U = 6.63 x 10(-6) J m(-1). This is the first time that U has been quantified for a surface with photoinduced superhydrophilicity. Triple line retreat measurements on an evaporating drop on M-TiO2 in the dark, RH = 60%, T = 293 K, gave a value of U = 9.4 x 10(-6) J m(-1), indicating that U decreases upon UV illumination and that U in the light is primarily associated with inhomogeneities that are unaffected by an increase in the surface Ti-OH population, such as the physical roughness of the surface. In the dark evaporation experiment, the drop was found to retreat with an areal velocity of 1.48 x 10(-8) m2 s(-1). However, under UV

  14. Quantifying bulk and surface recombination processes in nanostructured water splitting photocatalysts via in situ ultrafast spectroscopy.

    PubMed

    Appavoo, Kannatassen; Liu, Mingzhao; Black, Charles T; Sfeir, Matthew Y

    2015-02-11

    A quantitative description of recombination processes in nanostructured semiconductor photocatalysts-one that distinguishes between bulk (charge transport) and surface (chemical reaction) losses-is critical for advancing solar-to-fuel technologies. Here we present an in situ experimental framework that determines the bias-dependent quantum yield for ultrafast carrier transport to the reactive interface. This is achieved by simultaneously measuring the electrical characteristics and the subpicosecond charge dynamics of a heterostructured photoanode in a working photoelectrochemical cell. Together with direct measurements of the overall incident-photon-to-current efficiency, we illustrate how subtle structural modifications that are not perceivable by conventional X-ray diffraction can drastically affect the overall photocatalytic quantum yield. We reveal how charge carrier recombination losses occurring on ultrafast time scales can limit the overall efficiency even in nanostructures with dimensions smaller than the minority carrier diffusion length. This is particularly true for materials with high carrier concentration, where losses as high as 37% are observed. Our methodology provides a means of evaluating the efficacy of multifunctional designs where high overall efficiency is achieved by maximizing surface transport yield to near unity and utilizing surface layers with enhanced activity.

  15. Free Surface Properties of III-V Compound Semiconductor Surfaces.

    DTIC Science & Technology

    1980-06-01

    Review of Modern Physics Vol. 42 #3 July 1980, p. 317. 2. A. Kahn, G. Cisneros, M. Bonn, P. Mark and C.B. Duke, Surface Science 71, 387 (1978). 3. A...Kahn, E. So, P. Mark , C.B. Duke, J. Vac. Sci. Technol. 15, 580 (1978). 4. R.J. Meyer, C.B. Duke, A. Paton, A. Kahn, E. So, P. Mark , Phys. Rev. B19 (1979...Yeh, J. Tsang, A. Kahn, P. Mark , Phys. Rev. (to be published). 7. E. So, Ph.D. Dissertation, Princeton University, Department of Electrical Engineering

  16. Synthetic Strategies for Semiconductor Nanocrystals Expressing Localized Surface Plasmon Resonance.

    PubMed

    Niezgoda, J Scott; Rosenthal, Sandra J

    2016-03-03

    The field of semiconductor plasmonics has grown rapidly since its outset, only roughly six years ago, and now includes many crystalline substances ranging from GeTe to wide-bandgap transition-metal oxides. One byproduct of this proliferation is the sea of differing synthetic methods to realize localized surface plasmon resonances (LSPRs) based on the studied material. Strategies vary widely from material to material, but all have the common goal of introducing extremely high carrier densities to the semiconductor system. This doping results in tunable, size-quantized, and on/off-switchable LSPR modes, which are a complete departure from traditional metal-nanoparticle-based plasmon resonances. This Minireview will provide an overview of the current state of nanocrystal and quantum-dot plasmonics and the physical basis thereof, however its main purpose is to summarize the methods for realizing LSPRs in the various syntheses and systems that have been reported to date. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  17. Effect of surface modification on semiconductor nanocrystal fluorescence lifetime.

    PubMed

    Ruedas-Rama, Maria J; Orte, Angel; Hall, Elizabeth A H; Alvarez-Pez, Jose M; Talavera, Eva M

    2011-04-04

    Semiconductor nanocrystals, namely, quantum dots (QDs), present a set of unique photoluminescence properties, which has led to increased interest in using them as advantageous alternatives to conventional organic dyes. Many applications of QDs involve surface modification to enhance the solubility or biocompatibility of the QDs. One of the least exploited properties of QDs is the very long photoluminescence lifetime that usually has complex kinetics owing to the effect of quantum confinement. Herein, we describe the effect of different surface modifications on the photoluminescence decay kinetics of QDs. The different surface modifications were carefully chosen to provide lipophilic or water-soluble QDs with either positive or negative surface net charges. We also survey the effect on the QD lifetime of several ligands that interact with the QD surface, such as organic chromophores or fluorescent proteins. The results obtained demonstrate that time-resolved fluorescence is a useful tool for QD-based sensing to set the basis for the development of time-resolved-based nanosensors.

  18. The use of bulk states to accelerate the band edge statecalculation of a semiconductor quantum dot

    SciTech Connect

    Vomel, Christof; Tomov, Stanimire Z.; Wang, Lin-Wang; Marques,Osni A.; Dongarra, Jack J.

    2006-05-10

    We present a new technique to accelerate the convergence of the folded spectrum method in empirical pseudopotential band edge state calculations for colloidal quantum dots. We use bulk band states of the materials constituent of the quantum dot to construct initial vectors and a preconditioner. We apply these to accelerate the convergence of the folded spectrum method for the interior states at the top of the valence and the bottom of the conduction band. For large CdSe quantum dots, the number of iteration steps until convergence decreases by about a factor of 4 compared to previous calculations.

  19. Frontier induced semi-infinite-medium (FISIM) states at semiconductor surfaces and interfaces

    NASA Astrophysics Data System (ADS)

    Olguın, D.; Rodrıguez, J. A.; Baquero, R.

    2003-02-01

    In a previous work we have discussed the valence band electronic structure of a (001) oriented surface (semi-infinite medium) of some II-VI wide band gap zinc-blende semiconductor compounds. For these systems, we have found three characteristic surface resonances, besides the known bulk bands (hh, lh and spin-orbit bands). Two of these resonances correspond to the anion terminated surface and the third one to the cation terminated one. We have shown, specifically, that three non dispersive (001)-surface-induced bulk states, in the Γ - X direction of the 2D Brillouin zone, do exist and are characteristic of these systems. The existence of these states has been confirmed, independently, by two experimental groups and further evidence of our predictions has been more recently found. In order to continue with the description of these states, in this work, we briefly review the main characteristics of the electronic structure of the (001)-surfaces to up-date their analysis and we present new results concerning the existence of the same kind of states in Cu-based calcopyrites and at interfaces. This shows that, in general, the non-dispersive states occur in several, if not all, crystal surfaces, and, on general grounds, as the consequence of introducing to an infinite medium a frontier of any kind (not only with the vacuum). For that reason we propose here, to name them, more appropriately as Frontier Induced Semi-Infinite Medium (FISIM) states. We present in this paper two new interesting cases where the non-dispersive states appear. First, the (112)-oriented CuInSe2 calcopyrite surface and, secondly, the interface CdTe/CdSexTe1-x (x = 0.15) which, essentially, does not introduce the additional effects due to lattice mismatch so that the FISIM states are clearly seen. We have calculated them for a broader range of x and for other II-VI and III-V semiconductor compounds to check that the result is general. The surface and the interface that we present here, allow us to

  20. Atomistic simulations of bulk, surface and interfacial polymer properties

    NASA Astrophysics Data System (ADS)

    Natarajan, Upendra

    In chapter I, quasi-static molecular mechanics based simulations are used to estimate the activation energy of phenoxy rings flips in the amorphous region of a semicrystalline polyimide. Intra and intermolecular contributions to the flip activation energy, the torsional cooperativity accompanying the flip, and the effect of the flip on the motion in the glassy bulk state, are looked at. Also, comparison of the weighted mean activation energy is made with experimental data from solid state NMR measurements; the simulated value being 17.5 kcal/mol., while the experimental value was observed to be 10.5 kcal/mol. Chapter II deals with construction of random copolymer thin films of styrene-butadiene (SB) and styrene-butadiene-acrylonitrile (SBA). The structure and properties of the free surfaces presented by these thin films are analysed by, the atom mass density profiles, backbone bond orientation function, and the spatial distribution of acrylonitrile groups and styrene rings. The surface energies of SB and SBA are calculated using an atomistic equation and are compared with experimental data in the literature. In chapter III, simulations of polymer-polymer interfaces between like and unlike polymers, specifically cis-polybutadiene (PBD) and atatic polypropylene (PP), are presented. The structure of an incompatible polymer-polymer interface, and the estimation of the thermodynamic work of adhesion and interfacial energy between different incompatible polymers, form the focus here. The work of adhesion is calculated using an atomistic equation and is further used in a macroscopic equation to estimate the interfacial energy. The interfacial energy is compared with typical values for other immiscible systems in the literature. The interfacial energy compared very well with interfacial energy values for a few other immiscible hydrocarbon pairs. In chapter IV, the study proceeds to look at the interactions between nonpolar and polar small molecules with SB and SBA thin

  1. Semiconductor meta-surface based perfect light absorber.

    PubMed

    Liu, Guiqiang; Nie, Yiyou; Fu, Guolan; Liu, Xiaoshan; Liu, Yi; Tang, Li; Liu, Zhengqi

    2017-04-21

    We numerically proposed and demonstrated a semiconductor meta-surface light absorber, which consists of a silicon patches array on a silicon thin-film and an opaque silver substrate. The Mie resonances of the silicon patches and the fundamental cavity mode of the ultra-thin silicon film couple strongly to the incident optical field, leading to a multi-band perfect absorption. The maximal absorption is above 99.5% and the absorption is polarization-independent. Moreover, the absorption behavior is scalable in the frequency region via tuning the structural parameters. These features hold the absorber platform with wide applications in optoelectronics such as hot-electron excitation and photo-detection.

  2. Note: All solid-state high repetitive sub-nanosecond risetime pulse generator based on bulk gallium arsenide avalanche semiconductor switches.

    PubMed

    Hu, Long; Su, Jiancang; Ding, Zhenjie; Hao, Qingsong; Fan, Yajun; Liu, Chunliang

    2016-08-01

    An all solid-state high repetitive sub-nanosecond risetime pulse generator featuring low-energy-triggered bulk gallium arsenide (GaAs) avalanche semiconductor switches and a step-type transmission line is presented. The step-type transmission line with two stages is charged to a potential of 5.0 kV also biasing at the switches. The bulk GaAs avalanche semiconductor switch closes within sub-nanosecond range when illuminated with approximately 87 nJ of laser energy at 905 nm in a single pulse. An asymmetric dipolar pulse with peak-to-peak amplitude of 9.6 kV and risetime of 0.65 ns is produced on a resistive load of 50 Ω. A technique that allows for repetition-rate multiplication of pulse trains experimentally demonstrated that the parallel-connected bulk GaAs avalanche semiconductor switches are triggered in sequence. The highest repetition rate is decided by recovery time of the bulk GaAs avalanche semiconductor switch, and the operating result of 100 kHz of the generator is discussed.

  3. Bulk gallium nitride based electronic devices: Schottky diodes, Schottky-type ultraviolet photodetectors and metal-oxide-semiconductor capacitors

    NASA Astrophysics Data System (ADS)

    Zhou, Yi

    Gallium Nitride (GaN) is one of most promising semiconductor materials for high power, high temperature and high frequency applications. Due to the lack of native substrates for homoepitaxial growth, GaN electronic devices have been conventionally fabricated on epitaxial GaN layers grown on foreign substrates, mostly sapphire. This scheme complicates the fabrication process and compromises the device performance due to the large amount of native defects within the heteroepitaxial layer. In order to fabricate devices with improved performance and simplified fabrication processes, it is desirable to utilize high quality bulk GaN substrates. Recent developments in Hydride Vapor Phase Epitaxy (HVPE) technology have enabled the successful growth of free-standing GaN wafers with very low dislocation densities. This dissertation reports some developments in the device fabrication, performance and simulation based on bulk GaN substrates. We have fabricated vertical geometry Schottky diodes with a full backside ohmic contact using a bulk GaN substrate. The absence of the sapphire substrate, improved ohmic contact scheme and the vertical transport mode greatly enhance the forward current conduction of the bulk GaN Schottky diode. The device also displays a high reverse breakdown voltage and ultrafast reverse recovery characteristics. The low dislocation density of the substrate allows the fabrication of Schottky-type ultraviolet photodetectors with ultralow dark currents. The large band gap of GaN provides the intrinsic "visible blindness" of the UV photodetector. The device displays a reasonably high responsivity and a good linearity of photocurrent with UV irradiance. We have also fabricated MOS capacitors using a thermally oxidized bulk GaN substrate. The thermal gallium oxide is characterized and its oxidation mechanism and etching process are explored. The thermal grown Ga2O 3/GaN interface displays a relatively lower interface density of state as compared to the

  4. Selection and mass spectrometry characterization of peptides targeting semiconductor surfaces.

    PubMed

    Estephan, Elias; Larroque, Christian; Bec, Nicole; Martineau, Pierre; Cuisinier, Frédéric J G; Cloitre, Thierry; Gergely, Csilla

    2009-12-15

    We report on elaboration of 12-mer peptides that reveal specific recognition for the following semiconductor (SC) surfaces: GaAs(100), InAs(100), GaN(0001), ZnSe(100), ZnTe(100), GaAs(111)A, GaSb(100), CdSe(100). A M13 bacteriophage library was used to screen 10(9) different 12-mer peptides against these substrates to finally isolate, in maximum six amplification cycles, peptides that bind to the target surfaces. The specific peptides for the InAs and ZnSe surfaces were obtained. Contrary, for the other SC surfaces several peptides with high affinities have been isolated. Aiming for a better specificity, when the phage display has been conducted through six cycles, the screening procedure got dominated by a phage present in the M13 bacteriophage library and the SVSVGMKPSPRP peptide has been selected for different SCs. The high amplification potential of this phage has been observed previously with different targets. Thus, precaution should be undertaken in defining adhesion peptides with the phage display technique and real affinity of the obtained biolinkers should be studied with other methods. We employed mass spectrometry (MALDI-TOF/TOF) to demonstrate the preferential attachment (or not) of the SVSVGMKPSPRP peptide to the different SC surfaces. This allows us to define a realistic selection of the expressed peptides presenting affinity for the studied eight SC surfaces. We demonstrate that with increasing the dielectric constants of the employed solvents, adhesion of the SVSVGMKPSPRP peptide onto GaN(0001) is hindered. 2009 Wiley Periodicals, Inc.

  5. Anomalous acoustoelectric effect in semiconductor layered structures using separated medium configuration

    NASA Astrophysics Data System (ADS)

    Abedin, M. N.; Strashilov, V. L.; Das, P.

    1990-01-01

    An anomalous acoustoelectric effect is observed in semiconductor layered structures and bulk semiconductors due to semiconductor surface conditions. We report preliminary results of this effect in semiconductors using the nondestructive surface acoustic wave (SAW) technique. The magnitude and polarity of the acoustoelectric voltages in GaAs/AlAs superlattices exhibit strong SAW frequency dependencies, a phenomenon that is not observed in bulk semiconductors. The anomalous acoustoelectric voltage (AAV) is detected in high electron mobility transistor (HEMT) and also bulk semiconductors as a function of bias voltage.

  6. Surface Hydrogen and Subsurface Hydrogen: Their Roles in Bulk Absorption and Surface Reaction

    NASA Astrophysics Data System (ADS)

    Fukutani, Katsuyuki

    Hydrogen adsorbed on metal surfaces possibly penetrates into “subsurface” sites, which might further diffuse into bulk. When temperature is raised, on the other hand, such absorbed hydrogen diffuses back to the surface via the subsurface site eventually desorbing from the surface. The kinetics of these absorption and desorption are ideally expressed by the potential energy surfaces of hydrogen near the surfaces. This article describes how the potential of hydrogen is described, and how the surface and subsurface sites influence the kinetics of absorption and desorption for Pd and Ni as examples. As well as these phenomena, the subsurface sites could serve to promote particular hydrogenation reactions occurring at surfaces. The mechanism of subsurface chemistry is discussed.

  7. Temperature and doping dependent changes in surface recombination during UV illumination of (Al)GaN bulk layers

    SciTech Connect

    Netzel, Carsten; Jeschke, Jörg; Brunner, Frank; Knauer, Arne; Weyers, Markus

    2016-09-07

    We have studied the effect of continuous illumination with above band gap energy on the emission intensity of polar (Al)GaN bulk layers during the photoluminescence experiments. A temporal change in emission intensity on time scales from seconds to hours is based on the modification of the semiconductor surface states and the surface recombination by the incident light. The temporal behavior of the photoluminescence intensity varies with the parameters such as ambient atmosphere, pretreatment of the surface, doping density, threading dislocation density, excitation power density, and sample temperature. By means of temperature-dependent photoluminescence measurements, we observed that at least two different processes at the semiconductor surface affect the non-radiative surface recombination during illumination. The first process leads to an irreversible decrease in photoluminescence intensity and is dominant around room temperature, and the second process leads to a delayed increase in intensity and becomes dominant around T = 150–200 K. Both processes become slower when the sample temperature decreases from room temperature. They cease for T < 150 K. Stable photoluminescence intensity at arbitrary sample temperature was obtained by passivating the analyzed layer with an epitaxially grown AlN cap layer.

  8. Localized photovoltaic investigations on organic semiconductors and bulk heterojunction solar cells

    PubMed Central

    Kollender, Jan Philipp; Gasiorowski, Jacek; Sariciftci, Niyazi Serdar; Mardare, Andrei Ionut; Hassel, Achim Walter

    2014-01-01

    Newly synthesized organic electronics materials are often available in submicrogram amounts only. Photoelectrochemical scanning droplet cell microscopy is a powerful method that allows a comprehensive characterisation of such small amounts including oxidation, reduction potentials, doping, determination of charge carriers, band gap, charge capacity, over-oxidation sensitivity and many more. Localized photoelectrochemical characterization of the poly[4,8-bis-substituted-benzo[1,2-b:4,5-b0]dithiophene-2,6-diyl-alt-4-substituted-thieno [3,4-b] thiophene-2,6-diyl] (PBDTTT-c) and PBDTTT-c:PCBM bulk heterojunction was performed using photoelectrochemical scanning droplet cell microscopy (PE-SDCM). The optical properties and the real and imaginary part of the dielectric function, of the polymer were determined using spectroscopic ellipsometry. The photoelectrochemical characterizations were performed in a three and two electrode configuration of PE-SDCM under laser and white light illumination. The effect of illumination was characterized using dark/illumination sequences. The stability of the photocurrent was studied using longer term (600 s) illumination. Finally the effect of cell configuration and illumination conditions on the photovoltage was studied. PMID:27877711

  9. Crystallisation-enhanced bulk hole mobility in phenothiazine-based organic semiconductors

    NASA Astrophysics Data System (ADS)

    Shinde, D. B.; Salunke, Jagadish K.; Candeias, Nuno R.; Tinti, Francesca; Gazzano, Massimo; Wadgaonkar, P. P.; Priimagi, Arri; Camaioni, Nadia; Vivo, Paola

    2017-04-01

    A series of three novel donor-acceptor systems based on C(3)-malononitrile-substituted phenothiazines was synthesised in good overall yields and their thermal, spectroscopic, and electrochemical properties were characterised. The compounds were prepared through a sequence of Ullmann-coupling, Vilsmeier-Haack formylation and Knoevenagel-condensation, followed by Suzuki-coupling reactions for introduction of aryl substitutents at C(7) position of the phenothiazine. The introduction of a donor unit at the C(7) position exhibited a weak impact on the optical and electrochemical characteristics of the compounds and led to amorphous films with bulk hole mobilities in the typical range reported for phenothiazines, despite the higher charge delocalisation as attested by computational studies. In contrast, highly ordered films were formed when using the C(7)-unsubstituted 3-malononitrile phenothiazine, exhibiting an outstanding mobility of 1 × 10-3 cm2 V-1 s-1, the highest reported for this class of compounds. Computational conformational analysis of the new phenothizanes suggested that free rotation of the substitutents at the C(7) position suppresses the ordering of the system, thereby hampering suitable packing of the new materials needed for high charge carrier mobility.

  10. Influence of the side chain and substrate on polythiophene thin film surface, bulk, and buried interfacial structures.

    PubMed

    Xiao, Minyu; Jasensky, Joshua; Zhang, Xiaoxian; Li, Yaoxin; Pichan, Cayla; Lu, Xiaolin; Chen, Zhan

    2016-08-10

    The molecular structures of organic semiconducting thin films mediate the performance of various devices composed of such materials. To fully understand how the structures of organic semiconductors alter on substrates due to different polymer side chains and different interfacial interactions, thin films of two kinds of polythiophene derivatives with different side-chains, poly(3-hexylthiophene) (P3HT) and poly(3-potassium-6-hexanoate thiophene) (P3KHT), were deposited and compared on various surfaces. A combination of analytical tools was applied in this research: contact angle goniometry and X-ray photoelectron spectroscopy (XPS) were used to characterize substrate dielectric surfaces with varied hydrophobicity for polymer film deposition; X-ray diffraction and UV-vis spectroscopy were used to examine the polythiophene film bulk structure; sum frequency generation (SFG) vibrational spectroscopy was utilized to probe the molecular structures of polymer film surfaces in air and buried solid/solid interfaces. Both side-chain hydrophobicity and substrate hydrophobicity were found to mediate the crystallinity of the polythiophene film, as well as the orientation of the thiophene ring within the polymer backbone at the buried polymer/substrate interface and the polymer thin film surface in air. For the same type of polythiophene film deposited on different substrates, a more hydrophobic substrate surface induced thiophene ring alignment with the surface normal at both the buried interface and on the surface in air. For different films (P3HT vs. P3KHT) deposited on the same dielectric substrate, a more hydrophobic polythiophene side chain caused the thiophene ring to align more towards the surface at the buried polymer/substrate interface and on the surface in air. We believe that the polythiophene surface, bulk, and buried interfacial molecular structures all influence the hole mobility within the polythiophene film. Successful characterization of an organic conducting

  11. Diffusion length measurements in bulk and epitaxially grown 3-5 semiconductors using charge collection microscopy

    NASA Technical Reports Server (NTRS)

    Leon, R. P.

    1987-01-01

    Diffusion lengths and surface recombination velocities were measured in GaAs diodes and InP finished solar cells. The basic techniques used was charge collection microscopy also known as electron beam induced current (EBIC). The normalized currents and distances from the pn junction were read directly from the calibrated curves obtained while using the line scan mode in an SEM. These values were then equated to integral and infinite series expressions resulting from the solution of the diffusion equation with both extended generation and point generation functions. This expands previous work by examining both thin and thick samples. The surface recombination velocity was either treated as an unknown in a system of two equations, or measured directly using low e(-) beam accelerating voltages. These techniques give accurate results by accounting for the effects of surface recombination and the finite size of the generation volume.

  12. Diffusion length measurement in bulk and epitaxially grown III-V semiconductors using charge collection microscopy

    NASA Technical Reports Server (NTRS)

    Leon, R. P.

    1987-01-01

    Diffusion lengths and surface recombination velocities were measured in GaAs diodes and InP finished solar cells. The basic technique used was charge collection microscopy, also known as electron beam induced current (EBIC). The normalized currents and distances from the pn junction were read directly from the calibrated curves obtained while using the line-scan mode in an SEM. These values were then equated to integral and infinite series expressions resulting from the solution of the diffusion equation with both extended-generation and point-generation functions. This expands previous work by examining both thin and thick samples. The surface recombination velocity was either treated as an unknown in a system of two equations or measured directly using low e(-) beam accelerating voltages. These techniques give accurate results by accounting for the effects of surface recombination and the finite size of the generation volume.

  13. Toward understanding the electrical properties of metal/semiconductor Schottky contacts: The effects of barrier inhomogeneities and geometry in bulk and nanoscale structures

    NASA Astrophysics Data System (ADS)

    Sarpatwari, Karthik

    The work presented in this thesis comprises of two parts. Part I deals with Schottky contacts to the wide bandgap (WBG) semiconductors SiC, GaN and ZnO. These semiconductors offer great promise for a wide variety of electronic and optoelectronic applications. Schottky barriers to WBG semiconductors are attractive in particular for high temperature/high power diodes, photodetectors, and gas sensors. However, the Schottky barriers exhibit non-ideal behavior, due in part to inhomogeneities originating from immature crystal growth and device processing technologies. Apart from being a versatile electronic component, the Schottky diode is a valuable test structure. The Schottky contact is routinely used to probe substrate and epilayer quality by different electrical characterization techniques. It is well established that the current-voltage-temperature ( I-V-T) characteristics of Schottky contacts are routinely affected by the presence of barrier height inhomogeneities (BHI). Consequently, Schottky diode parameters such as the Schottky barrier height and the Richardson constant extracted using the I-V-T measurements can deviate from their actual values. The effects of BHI on the extracted Schottky barrier height have been studied in the literature. However, the effects of BHI on the Richardson constant have not been thoroughly explored and are the focus of the first part of this thesis. Based on the inhomogeneous Schottky barrier model provided by Tung, a new method for the extraction of the Richardson constant is developed. The new method is applied to the Richardson constant determination of n-type ZnO and GaN. Excellent agreement with the theoretical value is obtained in both cases. The advent of the nanoelectronics era has resulted in the Schottky contact evolving from the relatively simple, planar structure into a more complex structure. Compared to bulk Schottky contacts, the Schottky barrier properties are expected to be widely different at the nanoscale. For

  14. First-principles study of semiconductor and metal surfaces

    NASA Astrophysics Data System (ADS)

    Kim, Sungho

    In this dissertation, we study the electronic and geometric structure of semiconductors and metal surfaces based on quantum mechanical first-principles calculations. We determine the geometry of vacancy defects of hydrogen adsorbed on a Pd(111) surface by treating the motion of a hydrogen atom, in addition to electrons, quantum mechanically. The calculated ground state wave function has high probability density in the hcp site located at the center of the vacancy instead of the fcc sites where the potential is minimum and hydrogen atoms on a Pd(111) surface normally adsorb. The geometry of quantum mechanically determined divacancy provides a simple and clear explanation for the scanning tunneling microscopy (STM) images of these defects that appear as three-lobed objects as observed in recent experiments [Mitsui, et al, Nature 422, 705 (2003)]. We employ the same principle to successfully elucidate the STM images of larger size vacancy defects. Our model also provides a compelling argument to explain the unusual recent experimental result that aggregates of three or more hydrogen vacancies are much more active in adsorption of hydrogen molecules while two-vacancy defects are never inactive. The InAs (110) surfaces appear lower than GaSb in STM images. This height difference is caused primarily by differences in the electronic structure of the two materials according to our calculations in a good agreement with measurements. In contrast, local variations in the apparent height of (110) surface atoms at InSb- or GaAs-like interfaces arise primarily from geometric distortions associated with local differences in bond length. The arsenic atoms adsorb preferably at the bridge sites between the dimerized Sb atoms on Sb-terminating (001) surfaces. Indium atoms, on the other hand, have somewhat equal probabilities at a few different sites on Ga-terminating (001) surfaces. Our calculated energies for atomic intermixing indicate that anion exchanges are exothermic for As

  15. Stability analysis and simulations of coupled bulk-surface reaction–diffusion systems

    PubMed Central

    Madzvamuse, Anotida; Chung, Andy H. W.; Venkataraman, Chandrasekhar

    2015-01-01

    In this article, we formulate new models for coupled systems of bulk-surface reaction–diffusion equations on stationary volumes. The bulk reaction–diffusion equations are coupled to the surface reaction–diffusion equations through linear Robin-type boundary conditions. We then state and prove the necessary conditions for diffusion-driven instability for the coupled system. Owing to the nature of the coupling between bulk and surface dynamics, we are able to decouple the stability analysis of the bulk and surface dynamics. Under a suitable choice of model parameter values, the bulk reaction–diffusion system can induce patterning on the surface independent of whether the surface reaction–diffusion system produces or not, patterning. On the other hand, the surface reaction–diffusion system cannot generate patterns everywhere in the bulk in the absence of patterning from the bulk reaction–diffusion system. For this case, patterns can be induced only in regions close to the surface membrane. Various numerical experiments are presented to support our theoretical findings. Our most revealing numerical result is that, Robin-type boundary conditions seem to introduce a boundary layer coupling the bulk and surface dynamics. PMID:25792948

  16. Low-dimensional transport and large thermoelectric power factors in bulk semiconductors by band engineering of highly directional electronic states.

    PubMed

    Bilc, Daniel I; Hautier, Geoffroy; Waroquiers, David; Rignanese, Gian-Marco; Ghosez, Philippe

    2015-04-03

    Thermoelectrics are promising for addressing energy issues but their exploitation is still hampered by low efficiencies. So far, much improvement has been achieved by reducing the thermal conductivity but less by maximizing the power factor. The latter imposes apparently conflicting requirements on the band structure: a narrow energy distribution and a low effective mass. Quantum confinement in nanostructures and the introduction of resonant states were suggested as possible solutions to this paradox, but with limited success. Here, we propose an original approach to fulfill both requirements in bulk semiconductors. It exploits the highly directional character of some orbitals to engineer the band structure and produce a type of low-dimensional transport similar to that targeted in nanostructures, while retaining isotropic properties. Using first-principle calculations, the theoretical concept is demonstrated in Fe2YZ Heusler compounds, yielding power factors 4 to 5 times larger than in classical thermoelectrics at room temperature. Our findings are totally generic and rationalize the search of alternative compounds with similar behavior. Beyond thermoelectricity, these might be relevant also in the context of electronic, superconducting, or photovoltaic applications.

  17. Terahertz surface plasmon polaritons on a semiconductor surface structured with periodic V-grooves.

    PubMed

    Li, Shanshan; Jadidi, Mohammad M; Murphy, Thomas E; Kumar, Gagan

    2013-03-25

    We demonstrate propagation of terahertz waves confined to a semiconductor surface that is periodically corrugated with V-shaped grooves. A one-dimensional array of V-grooves is fabricated on a highly-doped silicon surface, using anisotropic wet-etching of crystalline silicon, thereby forming a plasmonic waveguide. Terahertz time domain spectroscopy is used to characterize the propagation of waves near the corrugated surface. We observe that the grating structure creates resonant modes that are confined near the surface. The degree of confinement and frequency of the resonant mode is found to be related to the pitch and depth of the V-grooves. The surface modes are confirmed through both numerical simulations and experimental measurements. Not only does the V-groove geometry represent a new and largely unexplored structure for supporting surface waves, but it also enables the practical fabrication of terahertz waveguides directly on semiconductor surfaces, without relying on reactive-ion etching or electroplating of sub-millimeter metallic surfaces.

  18. Surface-segregated monolayers: a new type of ordered monolayer for surface modification of organic semiconductors.

    PubMed

    Wei, Qingshuo; Tajima, Keisuke; Tong, Yujin; Ye, Shen; Hashimoto, Kazuhito

    2009-12-09

    We report a new type of ordered monolayer for the surface modification of organic semiconductors. Fullerene derivatives with fluorocarbon chains ([6,6]-phenyl-C(61)-buryric acid 1H,1H-perfluoro-1-alkyl ester or FC(n)) spontaneously segregated as a monolayer on the surface of a [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) film during a spin-coating process from the mixture solutions, as confirmed by X-ray photoelectron spectroscopy (XPS). Ultraviolet photoelectron spectroscopy (UPS) showed the shift of ionization potentials (IPs) depending on the fluorocarbon chain length, indicating the formation of surface dipole moments. Surface-sensitive vibrational spectroscopy, sum frequency generation (SFG) revealed the ordered molecular orientations of the C(60) moiety in the surface FC(n) layers. The intensity of the SFG signals from FC(n) on the surface showed a clear odd-even effect when the length of the fluorocarbon chain was changed. This new concept of the surface-segregated monolayer provides a facile and versatile approach to modifying the surface of organic semiconductors and is applicable to various organic optoelectronic devices.

  19. Manipulating Surface Energy to form Compound Semiconductor Nanostructures

    NASA Astrophysics Data System (ADS)

    DeJarld, Matthew T.

    Nanostructures have been lauded for their quantum confinement capabilities and potential applications in future devices. Compound semiconductor nanostructures are being integrated into the next generation of photovoltaic and light emitting devices to take advantage of their unique optical characteristics. Despite their promise, adoption of nanostructure based devices has been slow. This is due in large part to difficulties in effective fabrication and processing steps. By manipulating the surface energy of various components during growth, we can control the final structure and corresponding optoelectronic characteristics. Specifically I will present on GaSb quantum dots embedded in GaAs and GaAs nanowires using novel substrate and catalyst materials. GaSb quantum dots embedded in a GaAs matrix are ideal for devices that require capture of minority carriers as they exhibit a type II band offset with carrier concentration in the valence band. However, during GaAs capping, there is a strong driving force for the dot to demolish into a distribution of intact dots, rings, and GaSb material clusters. We demonstrate the ability to mitigate this effect using both chemical and kinetic means: we alter the surface chemistry via the addition of aluminum, and use droplet epitaxy as an alternative quantum dot formation method. Secondly, the growth of high quality GaAs on silicon has always been restricted due to material incompatibilities. With the emergence of increasingly smaller low power electronics, there is a demand to integrate optoelectronic devices directly on the surface of CMOS sensor stacks. Utilizing the vapor-liquid-solid growth mechanism we are able to demonstrate the growth of high quality GaAs nanowires on polycrystalline substrates at low temperatures. This allows for the growth of III-V nanowire based devices directly on the metal pads of pre-packaged CMOS chips. We also investigate the potential use of bismuth as an alternative to gold for catalyzing

  20. Atomically Flat Surfaces Developed for Improved Semiconductor Devices

    NASA Technical Reports Server (NTRS)

    Powell, J. Anthony

    2001-01-01

    New wide bandgap semiconductor materials are being developed to meet the diverse high temperature, -power, and -frequency demands of the aerospace industry. Two of the most promising emerging materials are silicon carbide (SiC) for high-temperature and high power applications and gallium nitride (GaN) for high-frequency and optical (blue-light-emitting diodes and lasers) applications. This past year Glenn scientists implemented a NASA-patented crystal growth process for producing arrays of device-size mesas whose tops are atomically flat (i.e., step-free). It is expected that these mesas can be used for fabricating SiC and GaN devices with major improvements in performance and lifetime. The promising new SiC and GaN devices are fabricated in thin-crystal films (known as epi films) that are grown on commercial single-crystal SiC wafers. At this time, no commercial GaN wafers exist. Crystal defects, known as screw defects and micropipes, that are present in the commercial SiC wafers propagate into the epi films and degrade the performance and lifetime of subsequently fabricated devices. The new technology isolates the screw defects in a small percentage of small device-size mesas on the surface of commercial SiC wafers. This enables atomically flat surfaces to be grown on the remaining defect-free mesas. We believe that the atomically flat mesas can also be used to grow GaN epi films with a much lower defect density than in the GaN epi films currently being grown. Much improved devices are expected from these improved low-defect epi films. Surface-sensitive SiC devices such as Schottky diodes and field effect transistors should benefit from atomically flat substrates. Also, we believe that the atomically flat SiC surface will be an ideal surface on which to fabricate nanoscale sensors and devices. The process for achieving atomically flat surfaces is illustrated. The surface steps present on the "as-received" commercial SiC wafer is also illustrated. because of the

  1. Surface chemistry on semiconductors studied by molecular-beam reactive scattering

    NASA Astrophysics Data System (ADS)

    Yu, Ming L.; DeLouise, Lisa A.

    1994-01-01

    This Report reviews the use of molecular-beam reactive scattering to study the surface reactions of gas molecules on semiconductors which have relevance to microelectronic technologies. Modern semiconductor fabrication techniques rely heavily on dry processes where gas-surface reactions are the basic premise. This article focuses on the use of supersonic molecular-beam-surface scattering to study the dynamics and kinetics of surface reactions connected with the growth and etching processes on semiconductor surfaces. The discussion on growth processes covers the oxidation of silicon and germanium, the tungsten-hexafluoride-based tungsten deposition, and the organometallic chemical vapor deposition of gallium arsenide. The discussion on etching processes covers the halogen-based etching of gallium arsenide and silicon. An overview of the experimental technique and the underlying principles in surface-reaction dynamics and kinetics is included for readers in the technology area. The potential use of the molecular beams for actual semiconductor materials processing is also discussed.

  2. Chemical, electronic, and magnetic structure of LaFeCoSi alloy: Surface and bulk properties

    SciTech Connect

    Lollobrigida, V.; Basso, V.; Kuepferling, M.; Coïsson, M.; Olivetti, E. S.; Celegato, F.; Borgatti, F.; Torelli, P.; Panaccione, G.; Tortora, L.; Stefani, G.; Offi, F.

    2014-05-28

    We investigate the chemical, electronic, and magnetic structure of the magnetocaloric LaFeCoSi compound with bulk and surface sensitive techniques. We put in evidence that the surface retains a soft ferromagnetic behavior at temperatures higher than the Curie temperature of the bulk due to the presence of Fe clusters at the surface only. This peculiar magnetic surface effect is attributed to the exchange interaction between the ferromagnetic Fe clusters located at the surface and the bulk magnetocaloric alloy, and it is used here to monitor the magnetic properties of the alloy itself.

  3. Sub-nanometer resolution of an organic semiconductor crystal surface using friction force microscopy in water.

    PubMed

    Pimentel, Carlos; Varghese, Shinto; Yoon, Seong-Jun; Park, Soo Young; Gierschner, Johannes; Gnecco, Enrico; Pina, Carlos M

    2016-04-06

    Organic semiconductors (OSC) are attracting much interest for (opto)electronic applications, such as photovoltaics, LEDs, sensors or solid state lasers. In particular, crystals formed by small π-conjugated molecules have shown to be suitable for constructing OSC devices. However, the (opto)electronic properties are complex since they depend strongly on both the mutual orientation of molecules as well as the perfection of bulk crystal surfaces. Hence, there is an urgent need to control nano-topographic OSC features in real space. Here we show that friction force microscopy in water is a very suitable technique to image the free surface morphology of an OSC single crystal (TDDCS) with sub-nanometer resolution. We demonstrate the power of the method by direct correlation to the structural information extracted from combined single crystal (SC-) and specular (s-) XRD studies, which allows us to identify the pinning centers encountered in the stick-slip motion of the probing tip with the topmost methyl groups on the TDDCS surface.

  4. A multi-scale approach to the electronic structure of doped semiconductor surfaces

    NASA Astrophysics Data System (ADS)

    Sinai, Ofer; Hofmann, Oliver T.; Rinke, Patrick; Scheffler, Matthias; Heimel, Georg; Kronik, Leeor

    2015-03-01

    The inclusion of the global effects of semiconductor doping poses a unique challenge for first-principles simulations, because the typically low concentration of dopants renders an explicit treatment intractable. Furthermore, the width of the space-charge region (SCR) at charged surfaces often exceeds realistic supercell dimensions. We present a multi-scale technique that addresses these difficulties. It is based on the introduction of excess charge, mimicking free charge carriers from the SCR, along with a fixed sheet of counter-charge mimicking the SCR-related field. Self-consistency is obtained by imposing charge conservation and Fermi level equilibration between the bulk, treated semi-classically, and the electronic states of the slab/surface, which are treated quantum-mechanically. The method, called CREST - the Charge-Reservoir Electrostatic Sheet Technique - can be used with standard electronic structure codes. We validate CREST using a simple tight-binding model, which allows for comparison of its results with calculations encompassing the full SCR explicitly. We then employ it with density functional theory, obtaining insight into the doping dependence of the electronic structures of the metallic clean-cleaved Si(111) surface and its semiconducting (2x1) reconstructions.

  5. Bulk and surface controlled diffusion of fission gas atoms

    SciTech Connect

    Andersson, Anders D.

    2012-08-09

    Fission gas retention and release impact nuclear fuel performance by, e.g., causing fuel swelling leading to mechanical interaction with the clad, increasing the plenum pressure and reducing the gap thermal conductivity. All of these processes are important to understand in order to optimize operating conditions of nuclear reactors and to simulate accident scenarios. Most fission gases have low solubility in the fuel matrix, which is especially pronounced for large fission gas atoms such as Xe and Kr, and as a result there is a significant driving force for segregation of gas atoms to extended defects such as grain boundaries or dislocations and subsequently for nucleation of gas bubbles at these sinks. Several empirical or semi-empirical models have been developed for fission gas release in nuclear fuels, e.g. [1-6]. One of the most commonly used models in fuel performance codes was published by Massih and Forsberg [3,4,6]. This model is similar to the early Booth model [1] in that it applies an equivalent sphere to separate bulk UO{sub 2} from grain boundaries represented by the sphere circumference. Compared to the Booth model, it also captures trapping at grain boundaries, fission gas resolution and it describes release from the boundary by applying timedependent boundary conditions to the circumference. In this work we focus on the step where fission gas atoms diffuse from the grain interior to the grain boundaries. The original Massih-Forsberg model describes this process by applying an effective diffusivity divided into three temperature regimes. In this report we present results from density functional theory calculations (DFT) that are relevant for the high (D{sub 3}) and intermediate (D{sub 2}) temperature diffusivities of fission gases. The results are validated by making a quantitative comparison to Turnbull's [8-10] and Matzke's data [12]. For the intrinsic or high temperature regime we report activation energies for both Xe and Kr diffusion in UO

  6. Determination of bulk diffusion lengths for angle-lapped semiconductor material via the scanning electron microscope: A theoretical analysis

    NASA Technical Reports Server (NTRS)

    Vonroos, O.

    1978-01-01

    A standard procedure for the determination of the minority carrier diffusion length by means of a scanning electron microscope (SEM) consists in scanning across an angle-lapped surface of a P-N junction and measuring the resultant short circuit current I sub sc as a function of beam position. A detailed analysis of the I sub sc originating from this configuration is presented. It is found that, for a point source excitation, the I sub sc depends very simply on x, the variable distance between the surface and the junction edge. The expression for the I sub sc of a planar junction device is well known. If d, the constant distance between the plane of the surface of the semiconductor and the junction edge in the expression for the I of a planar junction is merely replaced by x, the variable distance of the corresponding angle-lapped junction, an expression results which is correct to within a small fraction of a percent as long as the angle between the surfaces, 2 theta sub 1, is smaller than 10 deg.

  7. Investigation of Semiconductor Surface Structure by Transmission Ion Channeling.

    NASA Astrophysics Data System (ADS)

    Lyman, Paul Francis

    The primary thrust of this dissertation is the investigation of the composition and structure of two important surface systems on Si, and the study of how this structure evolves under the influence of ion bombardment or film growth. I have studied the initial stages of oxidation of Si immediately following removal of a surface oxide by an HF etch. I have also studied the structure of Ge deposited on clean Si(100) at low temperatures. These systems are of considerable technological interest, but were chosen because they naturally pose fundamental questions regarding physical and chemical processes at surfaces. In the study of the oxidation of Si, I have focused on the influence of the bombarding ion beam in altering the structure and composition of the surface layer. Thus, the system then provides a natural vehicle to study ion-induced chemistry. In the study of low-temperature growth of Ge, I have focused on the structure of the Ge layer and the evolution of that structure upon further deposition or upon heating. This simple system is a model one for observing strained semiconductor heteroepitaxial growth. The primary probe for these studies was transmission channeling of MeV ions. The sensitivity of this technique to correlations between the substrate and an overlayer allowed us to make the following observations. The O, Si and H bound in the thin oxide formed after an HF etch and H_2O rinse occupy preferred positions with respect to the Si matrix. Upon ion bombardment, the O further reacts with the Si (the reaction proceeds linearly with the ion fluence) and the portion of the H that is uncorrelated to the substrate is preferentially desorbed. For the case of Ge growth on Si(100)-(2 x 1) at room temperature, a substantial fraction of the Ge films is strained to occupy sites having the lattice constant of the Si substrate (pseudomorphic growth). A model for film growth is proposed in which pseudomorphic domains constitute roughly half of the Ge films up to a

  8. Tunable Surface Plasmon and Phonon Polariton Interactions for Moderately Doped Semiconductor Surfaces

    PubMed Central

    Janipour, Mohsen; Misirlioglu, Ibrahim Burc; Sendur, Kursat

    2016-01-01

    Spatial charge distribution for biased semiconductors fundamentally differs from metals since they can allow inhomogeneous charge distributions due to penetration of the electric field, as observed in the classical Schottky junctions. Similarly, the electrostatics of the dielectric/semiconductor interface can lead to a carrier depletion or accumulation in the semiconductor side when under applied bias. In this study, we demonstrate that the inhomogeneous carrier accumulation in a moderately p-doped GaAs–dielectric interface can be tailored for tunable plasmonics by an external voltage. Solving Maxwell’s equations in the doped GaAs-dielectric stack, we investigate the tunability of the surface plasmon and phonon polaritons’ interaction via an external bias. The plasmonic mode analysis of such an interface reveals interesting dispersion curves for surface plasmon and phonon polariton interactions that are not possible in metals. We show that the plasmon dispersion curve can be engineered through an external bias using the inherent properties of the p-doped GaAs– dielectric interface. PMID:27698393

  9. Tunable Surface Plasmon and Phonon Polariton Interactions for Moderately Doped Semiconductor Surfaces

    NASA Astrophysics Data System (ADS)

    Janipour, Mohsen; Misirlioglu, Ibrahim Burc; Sendur, Kursat

    2016-10-01

    Spatial charge distribution for biased semiconductors fundamentally differs from metals since they can allow inhomogeneous charge distributions due to penetration of the electric field, as observed in the classical Schottky junctions. Similarly, the electrostatics of the dielectric/semiconductor interface can lead to a carrier depletion or accumulation in the semiconductor side when under applied bias. In this study, we demonstrate that the inhomogeneous carrier accumulation in a moderately p-doped GaAs–dielectric interface can be tailored for tunable plasmonics by an external voltage. Solving Maxwell’s equations in the doped GaAs-dielectric stack, we investigate the tunability of the surface plasmon and phonon polaritons’ interaction via an external bias. The plasmonic mode analysis of such an interface reveals interesting dispersion curves for surface plasmon and phonon polariton interactions that are not possible in metals. We show that the plasmon dispersion curve can be engineered through an external bias using the inherent properties of the p-doped GaAs– dielectric interface.

  10. Observing the interplay between surface and bulk optical nonlinearities in thin van der Waals crystals

    PubMed Central

    Deckoff-Jones, Skylar; Zhang, Jingjing; Petoukhoff, Christopher E.; Man, Michael K.L.; Lei, Sidong; Vajtai, Robert; Ajayan, Pulickel M.; Talbayev, Diyar; Madéo, Julien; Dani, Keshav M.

    2016-01-01

    Van der Waals materials, existing in a range of thicknesses from monolayer to bulk, allow for interplay between surface and bulk nonlinearities, which otherwise dominate only at atomically-thin or bulk extremes, respectively. Here, we observe an unexpected peak in intensity of the generated second harmonic signal versus the thickness of Indium Selenide crystals, in contrast to the quadratic increase expected from thin crystals. We explain this by interference effects between surface and bulk nonlinearities, which offer a new handle on engineering the nonlinear optical response of 2D materials and their heterostructures. PMID:26936437

  11. Observing the interplay between surface and bulk optical nonlinearities in thin van der Waals crystals

    NASA Astrophysics Data System (ADS)

    Deckoff-Jones, Skylar; Zhang, Jingjing; Petoukhoff, Christopher E.; Man, Michael K. L.; Lei, Sidong; Vajtai, Robert; Ajayan, Pulickel M.; Talbayev, Diyar; Madéo, Julien; Dani, Keshav M.

    2016-03-01

    Van der Waals materials, existing in a range of thicknesses from monolayer to bulk, allow for interplay between surface and bulk nonlinearities, which otherwise dominate only at atomically-thin or bulk extremes, respectively. Here, we observe an unexpected peak in intensity of the generated second harmonic signal versus the thickness of Indium Selenide crystals, in contrast to the quadratic increase expected from thin crystals. We explain this by interference effects between surface and bulk nonlinearities, which offer a new handle on engineering the nonlinear optical response of 2D materials and their heterostructures.

  12. Application of PECVD for bulk and surface passivation of high efficiency silicon solar cells

    SciTech Connect

    Krygowski, T.; Doshi, P.; Cai, L.; Doolittle, A.; Rohatgi, A.

    1995-08-01

    Plasma enhanced chemical vapor deposition (PECVD) passivation of bulk and surface defects has been shown to be an important technique to improve the performance of multicrystalline silicon (mc-Si) and single crystalline silicon solar cells. In this paper, we report the status of our on-going investigation into the bulk and surface passivation properties of PECVD insulators for photovoltaic applications. The objective of this paper is to demonstrate the ability of PECVD films to passivate the front (emitter) surface, bulk, and back surface by proper tailoring of deposition and post-PECVD annealing conditions.

  13. Surface analysis of chalcogenide semiconductors used in photovoltaics (Conference Presentation)

    NASA Astrophysics Data System (ADS)

    Rockett, Angus A.

    2017-04-01

    Both CdTe and Cu(In,Ga)Se2 have produced highly efficient thin film solar cells, exceeding 22% in champion devices. Both are also manufactured in large scales and show promise as future energy technologies. However, understanding the current collection mechanisms and mechanisms of instability in the devices remain a concern. To address these questions, we have used scanning probe and photoemission spectroscopies to study the response of chalcogenide materials to light and how charge is collected. Results of scanning microwave impedance microscopy and conductive atomic force microscopy show dramatic differences in the behavior of CdTe and Cu(In,Ga)Se2 (CIGS). The results include characterization of the effect of CdCl2 treatment on the properties CdTe grains and grain boundaries. This treatment dramatically increases the current collection in the grain boundaries. Thus we show that CdTe solar cells operate apparently by generation of electron hole pairs in the CdTe grains and collection of electrons to the grain boundaries. By contrast, CIGS grains show little or no contrast between the grains and grain boundaries and no obvious conduction pathway through the grain boundaries appears to exist. Our surface analysis results are supplemented with other measurements of both surface and bulk microchemistry and microstructure.

  14. Photoprecursor Approach Enables Preparation of Well-Performing Bulk-Heterojunction Layers Comprising a Highly Aggregating Molecular Semiconductor.

    PubMed

    Suzuki, Mitsuharu; Yamaguchi, Yuji; Takahashi, Kohei; Takahira, Katsuya; Koganezawa, Tomoyuki; Masuo, Sadahiro; Nakayama, Ken-ichi; Yamada, Hiroko

    2016-04-06

    Active-layer morphology critically affects the performance of organic photovoltaic cells, and thus its optimization is a key toward the achievement of high-efficiency devices. However, the optimization of active-layer morphology is sometimes challenging because of the intrinsic properties of materials such as strong self-aggregating nature or low miscibility. This study postulates that the "photoprecursor approach" can serve as an effective means to prepare well-performing bulk-heterojunction (BHJ) layers containing highly aggregating molecular semiconductors. In the photoprecursor approach, a photoreactive precursor compound is solution-deposited and then converted in situ to a semiconducting material. This study employs 2,6-di(2-thienyl)anthracene (DTA) and [6,6]-phenyl-C71-butyric acid methyl ester as p- and n-type materials, respectively, in which DTA is generated by the photoprecursor approach from the corresponding α-diketone-type derivative DTADK. When only chloroform is used as a cast solvent, the photovoltaic performance of the resulting BHJ films is severely limited because of unfavorable film morphology. The addition of a high-boiling-point cosolvent, o-dichlorobenzene (o-DCB), to the cast solution leads to significant improvement such that the resulting active layers afford up to approximately 5 times higher power conversion efficiencies. The film structure is investigated by two-dimensional grazing-incident wide-angle X-ray diffraction, atomic force microscopy, and fluorescence microspectroscopy to demonstrate that the use of o-DCB leads to improvement in film crystallinity and increase in charge-carrier generation efficiency. The change in film structure is assumed to originate from dynamic molecular motion enabled by the existence of solvent during the in situ photoreaction. The unique features of the photoprecursor approach will be beneficial in extending the material and processing scopes for the development of organic thin-film devices.

  15. Bulk crystal growth, and high-resolution x-ray diffraction results of LiZnP semiconductor material

    NASA Astrophysics Data System (ADS)

    Montag, Benjamin W.; Reichenberger, Michael A.; Sunder, Madhana; Ugorowski, Philip B.; Nelson, Kyle A.; McGregor, Douglas S.

    2015-06-01

    Nowotny-Juza compounds continue to be explored as a candidate for solid-state neutron detectors. Such a device would have greater efficiency, in a compact form, than present day gas-filled 3He and 10BF3 detectors. The 6Li(n,t)4He reaction yields a total Q-value of 4.78 MeV, larger than 10B, an energy easily identified above background radiations. Hence, devices fabricated from semiconducting compounds containing either natural Li (nominally 7.5% 6Li) or enriched 6Li (usually 95% 6Li) may provide a semiconductor material for compact high efficiency neutron detectors. Starting material was synthesized by preparing equimolar portions of Li, Zn, and P sealed under vacuum (10-6 Torr) in quartz ampoules lined with boron nitride and subsequently reacted in a compounding furnace [1]. The synthesized material showed signs of high impurity levels from material and electrical property characterizations. A static vacuum sublimation in quartz was performed to help purify the synthesized material [2]. Bulk crystalline samples were grown from the purified material. An ingot 9.6 mm in diameter and 4.0 mm in length was harvested. Individual samples were characterized for crystallinity on a Bruker AXS Inc. D2 CRYSO, energy dispersive x-ray diffractometer, and a Bruker AXS D8 DISCOVER, high-resolution x-ray diffractometer with a 0.004° beam divergence. The (220) orientation was characterized as the main orientation with the D2 CRYSO, and confirmed with the D8 DISCOVER. An out-of-plane high-resolution rocking curve yielded a 0.417° full width at half maximum (FWHM) for the (220) LiZnP. In-plane ordering was confirmed by observation of the (311) orientation, where a rocking curve was collected with a FWHM of 0.294°.

  16. Interaction of hydrogen with ZnO: surface adsorption versus bulk diffusion.

    PubMed

    Doh, W H; Roy, P C; Kim, C M

    2010-11-02

    The interaction of hydrogen (H) and a ZnO(0001)-O surface has been investigated using a temperature programmed desorption (TPD) technique. When the surface is exposed to atomic hydrogen below 400 K, hydrogen is adsorbed on the surface. As the hydrogen exposure increases, bulk diffusion of hydrogen takes place. The existence of surface and bulk hydrogen has been confirmed using X-ray photoelectron spectroscopy (XPS). When the ZnO surface dosed with hydrogen is heated, surface hydrogen is desorbed at 432 K and bulk hydrogen is evolved at ∼539 K. Diffusion of hydrogen into the ZnO bulk is an activated process, and the activation energy is estimated to be 0.19 eV.

  17. Spin mapping of surface and bulk Rashba states in ferroelectric α -GeTe(111) films

    NASA Astrophysics Data System (ADS)

    Elmers, H. J.; Wallauer, R.; Liebmann, M.; Kellner, J.; Morgenstern, M.; Wang, R. N.; Boschker, J. E.; Calarco, R.; Sánchez-Barriga, J.; Rader, O.; Kutnyakhov, D.; Chernov, S. V.; Medjanik, K.; Tusche, C.; Ellguth, M.; Volfova, H.; Borek, St.; Braun, J.; Minár, J.; Ebert, H.; Schönhense, G.

    2016-11-01

    The breaking of bulk inversion symmetry in ferroelectric semiconductors causes a Rashba-type spin splitting of electronic bulk bands. This is shown by a comprehensive mapping of the spin polarization of the electronic bands in ferroelectric α -GeTe(111) films using a time-of-flight momentum microscope equipped with an imaging spin filter that enables a simultaneous measurement of more than 10 000 data points. The experiment reveals an opposite spin helicity of the inner and outer Rashba bands with a different spin polarization in agreement with theoretical predictions, confirming a complex spin texture of bulk Rashba states. The outer band has about twice larger spin polarization than the inner one, giving evidence of a spin-orbit effect being related to the orbital composition of the band states. The switchable inner electric field of GeTe implies new functionalities for spintronic devices.

  18. Generation of terahertz radiation by a surface ballistic photocurrent in semiconductors under subpicosecond laser excitation

    SciTech Connect

    Ziaziulia, P. A.; Malevich, V. L.; Manak, I. S.; Krotkus, A.

    2012-02-15

    An analytical model describing the onset of a surface ballistic photocurrent in cubic semiconductors under femtosecond laser excitation is proposed. It is shown that the contribution of the photocurrent component parallel to the surface to the generation of terahertz pulses may be comparable to the contribution of the perpendicular component. Consideration of the cubic symmetry of a semiconductor leads to the azimuthal anisotropy of terahertz generation.

  19. Wavelength-resonant surface-emitting semiconductor laser

    DOEpatents

    Brueck, Steven R. J.; Schaus, Christian F.; Osinski, Marek A.; McInerney, John G.; Raja, M. Yasin A.; Brennan, Thomas M.; Hammons, Burrell E.

    1989-01-01

    A wavelength resonant semiconductor gain medium is disclosed. The essential feature of this medium is a multiplicity of quantum-well gain regions separated by semiconductor spacer regions of higher bandgap. Each period of this medium consisting of one quantum-well region and the adjacent spacer region is chosen such that the total width is equal to an integral multiple of 1/2 the wavelength in the medium of the radiation with which the medium is interacting. Optical, electron-beam and electrical injection pumping of the medium is disclosed. This medium may be used as a laser medium for single devices or arrays either with or without reflectors, which may be either semiconductor or external.

  20. Surface to bulk Fermi arcs via Weyl nodes as topological defects

    PubMed Central

    Kim, Kun Woo; Lee, Woo-Ram; Kim, Yong Baek; Park, Kwon

    2016-01-01

    A hallmark of Weyl semimetal is the existence of surface Fermi arcs. An intriguing question is what determines the connectivity of surface Fermi arcs, when multiple pairs of Weyl nodes are present. To answer this question, we show that the locations of surface Fermi arcs are predominantly determined by the condition that the Zak phase integrated along the normal-to-surface direction is . The Zak phase can reveal the peculiar topological structure of Weyl semimetal directly in the bulk. Here, we show that the winding of the Zak phase around each projected Weyl node manifests itself as a topological defect of the Wannier–Stark ladder, energy eigenstates under an electric field. Remarkably, this leads to bulk Fermi arcs, open-line segments in the bulk spectra. Bulk Fermi arcs should exist in conjunction with surface counterparts to conserve the Weyl fermion number under an electric field, which is supported by explicit numerical evidence. PMID:27845342

  1. Surface to bulk Fermi arcs via Weyl nodes as topological defects

    NASA Astrophysics Data System (ADS)

    Kim, Kun Woo; Lee, Woo-Ram; Kim, Yong Baek; Park, Kwon

    2016-11-01

    A hallmark of Weyl semimetal is the existence of surface Fermi arcs. An intriguing question is what determines the connectivity of surface Fermi arcs, when multiple pairs of Weyl nodes are present. To answer this question, we show that the locations of surface Fermi arcs are predominantly determined by the condition that the Zak phase integrated along the normal-to-surface direction is . The Zak phase can reveal the peculiar topological structure of Weyl semimetal directly in the bulk. Here, we show that the winding of the Zak phase around each projected Weyl node manifests itself as a topological defect of the Wannier-Stark ladder, energy eigenstates under an electric field. Remarkably, this leads to bulk Fermi arcs, open-line segments in the bulk spectra. Bulk Fermi arcs should exist in conjunction with surface counterparts to conserve the Weyl fermion number under an electric field, which is supported by explicit numerical evidence.

  2. 40 CFR 761.265 - Sampling bulk PCB remediation waste and porous surfaces.

    Code of Federal Regulations, 2012 CFR

    2012-07-01

    ... 40 Protection of Environment 32 2012-07-01 2012-07-01 false Sampling bulk PCB remediation waste..., DISTRIBUTION IN COMMERCE, AND USE PROHIBITIONS Cleanup Site Characterization Sampling for PCB Remediation Waste in Accordance with § 761.61(a)(2) § 761.265 Sampling bulk PCB remediation waste and porous surfaces...

  3. 40 CFR 761.265 - Sampling bulk PCB remediation waste and porous surfaces.

    Code of Federal Regulations, 2014 CFR

    2014-07-01

    ... 40 Protection of Environment 31 2014-07-01 2014-07-01 false Sampling bulk PCB remediation waste..., DISTRIBUTION IN COMMERCE, AND USE PROHIBITIONS Cleanup Site Characterization Sampling for PCB Remediation Waste in Accordance with § 761.61(a)(2) § 761.265 Sampling bulk PCB remediation waste and porous surfaces...

  4. 40 CFR 761.265 - Sampling bulk PCB remediation waste and porous surfaces.

    Code of Federal Regulations, 2010 CFR

    2010-07-01

    ..., DISTRIBUTION IN COMMERCE, AND USE PROHIBITIONS Cleanup Site Characterization Sampling for PCB Remediation Waste in Accordance with § 761.61(a)(2) § 761.265 Sampling bulk PCB remediation waste and porous surfaces... 40 Protection of Environment 30 2010-07-01 2010-07-01 false Sampling bulk PCB remediation...

  5. Semiconductor with protective surface coating and method of manufacture thereof. [Patent application

    DOEpatents

    Hansen, W.L.; Haller, E.E.

    1980-09-19

    Passivation of predominantly crystalline semiconductor devices is provided for by a surface coating of sputtered hydrogenated amorphous semiconductor material. Passivation of a radiation detector germanium diode, for example, is realized by sputtering a coating of amorphous germanium onto the etched and quenched diode surface in a low pressure atmosphere of hydrogen and argon. Unlike prior germanium diode semiconductor devices, which must be maintained in vacuum at cryogenic temperatures to avoid deterioration, a diode processed in the described manner may be stored in air at room temperature or otherwise exposed to a variety of environmental conditions. The coating compensates for pre-existing undesirable surface states as well as protecting the semiconductor device against future impregnation with impurities.

  6. Organic functionalization of group IV semiconductor surfaces: principles, examples, applications, and prospects

    NASA Astrophysics Data System (ADS)

    Bent, Stacey F.

    2002-03-01

    Organic functionalization is emerging as an important area in the development of new semiconductor-based materials and devices. Direct, covalent attachment of organic layers to a semiconductor interface provides for the incorporation of many new properties, including lubrication, optical response, chemical sensing, or biocompatibility. Methods by which to incorporate organic functionality to the surfaces of semiconductors have seen immense progress in recent years, and in this article several of these approaches are reviewed. Examples are included from both dry and wet processing environments. The focus of the article is on attachment strategies that demonstrate the molecular nature of the semiconductor surface. In many cases, the surfaces mimic the reactivity of their molecular carbon or organosilane counterparts, and examples of functionalization reactions are described in which direct analogies to textbook organic and inorganic chemistry can be applied. This article addresses the expected impact of these functionalization strategies on emerging technologies in nanotechnology, sensing, and bioengineering.

  7. - and Mesoscopic Soft Condensed Matter Architectures on Semiconductor Surfaces

    NASA Astrophysics Data System (ADS)

    Samuilov, Vladimir; Seo, Young-Soo; Ksenevich, Vitaly; Galibert, Jean; Sokolov, John; Rafailovich, Miriam

    2003-03-01

    A novel and simple approach of self-organized fabrication of two dimensional mesoscopic networks with the feature size down to 50 nm has been developed. The technique is based on the self-organized patterning in a thin layer of complex liquid (polymer solution) in the presence of humid atmosphere. Two dimensional mesoscopic honeycomb-shaped carbon structures were produced by high temperature annealing of nitrocellulose precursors [1]. The polymer network was also utilized as a mask for further reactive ion etching of surfaces with epi-layer of GaAs [2,3] and GaAs/AlGaAs ? -doped heterostructures [4]. The electrical transport in the obtained structures was studied in a temperature range from 1.9 to 300 K and in pulsed magnetic fields up to 35 T. A crossover from the Mott variable range hopping to the Colomb-gap Efros-Shklovskii variable range hopping has been observed experimentally in mesoscopic carbon structures [1]. At low fields, ln(R/R0) is proportional to B2. In the intermediate range, the magnetoresistance is linear on B. At high temperatures, if the hopping distance is comparable to the localization length, the observed small negative magnetoresistance in our samples is consistent to the weak-localization picture. Magnetoresistance of patterned GaAs/AlGaAs ?-doped structures is negative [4], which is relater to quantum interference in hopping regime. At B=0, the resistance showed typical behavior of a two-dimensional hopping. Below about 20 K, the data followed the Mott variable-range-hopping mechanism for two dimensions. Also, we have used diblock-copolymer system, self-assembled with L-B technique, to produce nano-patterns. These structures were utilized as templates for introducing metal nanopatterns on semiconductor surfaces by reactive ion beam etching for magnetic storage systems [5] and DNA separation on a flat surface [6] devices. 1. V.A. Samuilov, J. Galibert, V.K. Ksenevich, V.J. Goldman, M. Rafailovich, J. Sokolov, I.A. Bashmakov, V.A. Dorosinets

  8. STM study of surface structures formed by metal adsorption on semiconductor surfaces

    NASA Astrophysics Data System (ADS)

    Yoon, Myonggeun

    By utilizing a variety of the metal-induced superstructures and growth of metal adsorbates on semiconductor surfaces, we have studied how both the arrangements of atoms in topmost surfaces and the bonding mechanism actually affect both the atomic structures and electronic properties of the surface. This work describes the results of experimental studies of the metal adsorbates on two semiconductor surfaces, Si(111) and Ge(111), using scanning tunneling microscopy (STM), large modulation-amplitude local-barrier-height (LM-LBH) imaging as primary tools for structural analysis, and scanning tunneling spectroscopy (STS) as a tool for electronic analysis. Firstly, we have obtained new real-space images of the filled dangling-bond states of the alkali-metal induced 3x1 reconstruction of the Si(111) surface associated with a recently proposed Si=Si double-bond stabilized surface structure. Our new experimental evidence reveals significant subtle differences between this local bonding on Na/Si(111)-(3x1) and Na/Ge(111)-(3x1) which argues a strong case for a previously proposed "Honeycomb-chain-channel model" but with a relaxation of a strict double-bond requirement, particularly on the Ge surface. Secondly, the adsorption mechanism and the origin of the In, Al & Sn-induced surface electronic states on the Si(111)-(7x7) surface have been studied using bias-dependent scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Our experimental evidence suggests that all these metal atoms are covalently bonding with Si topmost atoms on Si(111)-(7x7) surface while In and Al atoms possibly substitute for Si adatoms in the 7x7 unit cell during room temperature adsorption. Both mechanisms remove intrinsic metallic surface states caused by partially occupied Si adatom dangling bonds, opening a bandgap at the surface and producing a metal-insulator transition. Finally, we report on the self-assembly of a superlattice of nanodots of different elements (Sn, In) on a

  9. Reliability of AlGaN/GaN high electron mobility transistors on low dislocation density bulk GaN substrate: Implications of surface step edges

    SciTech Connect

    Killat, N. E-mail: Martin.Kuball@bristol.ac.uk; Montes Bajo, M.; Kuball, M. E-mail: Martin.Kuball@bristol.ac.uk; Paskova, T.; Evans, K. R.; Leach, J.; Li, X.; Özgür, Ü.; Morkoç, H.; Chabak, K. D.; Crespo, A.; Gillespie, J. K.; Fitch, R.; Kossler, M.; Walker, D. E.; Trejo, M.; Via, G. D.; Blevins, J. D.

    2013-11-04

    To enable gaining insight into degradation mechanisms of AlGaN/GaN high electron mobility transistors, devices grown on a low-dislocation-density bulk-GaN substrate were studied. Gate leakage current and electroluminescence (EL) monitoring revealed a progressive appearance of EL spots during off-state stress which signify the generation of gate current leakage paths. Atomic force microscopy evidenced the formation of semiconductor surface pits at the failure location, which corresponds to the interaction region of the gate contact edge and the edges of surface steps.

  10. Reliability of AlGaN/GaN high electron mobility transistors on low dislocation density bulk GaN substrate: Implications of surface step edges

    NASA Astrophysics Data System (ADS)

    Killat, N.; Montes Bajo, M.; Paskova, T.; Evans, K. R.; Leach, J.; Li, X.; Özgür, Ü.; Morkoç, H.; Chabak, K. D.; Crespo, A.; Gillespie, J. K.; Fitch, R.; Kossler, M.; Walker, D. E.; Trejo, M.; Via, G. D.; Blevins, J. D.; Kuball, M.

    2013-11-01

    To enable gaining insight into degradation mechanisms of AlGaN/GaN high electron mobility transistors, devices grown on a low-dislocation-density bulk-GaN substrate were studied. Gate leakage current and electroluminescence (EL) monitoring revealed a progressive appearance of EL spots during off-state stress which signify the generation of gate current leakage paths. Atomic force microscopy evidenced the formation of semiconductor surface pits at the failure location, which corresponds to the interaction region of the gate contact edge and the edges of surface steps.

  11. Laboratory Experiment in Semiconductor Surface-Field Effects

    ERIC Educational Resources Information Center

    Goodman, F. R.; And Others

    1974-01-01

    A laboratory instructional program involving metal-insulator-semiconductor (MIS) devices is described. In the first of a two-part experiment, students become familiar with the important parameters of a simple MIS device and learn measurement techniques; in the second part, device fabrication procedures are learned. (DT)

  12. Laboratory Experiment in Semiconductor Surface-Field Effects

    ERIC Educational Resources Information Center

    Goodman, F. R.; And Others

    1974-01-01

    A laboratory instructional program involving metal-insulator-semiconductor (MIS) devices is described. In the first of a two-part experiment, students become familiar with the important parameters of a simple MIS device and learn measurement techniques; in the second part, device fabrication procedures are learned. (DT)

  13. Ultrashort pulse generation from vertical cavity surface emitting semiconductor lasers

    NASA Astrophysics Data System (ADS)

    Jasim, Khalil E.

    This work presents the first demonstration of a passively modelocked extended vertical cavity surface emitting laser (VECSEL) diode. Three cavity configurations were used to sustain stable passive modelocking operation: the Z-shaped, V-shaped and linear cavities. A semiconductor saturable absorber mirror (SESAM) used to triggered passive modelocking of the VECSEL diode. The SESAM device was used as a nonlinear high reflector in the Z-shaped and V-shaped cavity configurations, while it served as an output coupler (SESAMOC) in the linear cavity after the substrate was being angle polished and antireflection coated to eliminate any etalon effects. Many examples of VECSEL diode passive modelocking results will be presented. The standard non-collinear second-harmonic autocorrelation technique has been used to measure the generated pulse width, which was as small as 23 psec. The VECSEL-SESAM configuration has generated stable pulse trains at repetition rates ranging from 1 GHz to approximately 6 GHz, depending on the resonator configuration. Modelocking operation was stable and robust as amplitude noise measurements revealed a noise level ˜0.8%. Moreover, harmonic passive modelocking operation has been observed for the first time during the investigation of modelocking dynamics and stability in the regime of strong self-feedback coupling >10%. A reverse biased p-i-n QW device has enabled the generation of a stable pulse train at 15 GHz with pulse duration close to 15 psec and amplitude noise level ˜0.3%. However, due to design limitations of both the active and passive VECSELs, driving the system to produce repetition rates close to 20 GHz resulted in pulse amplitude variation and an unavoidable DC background. These initial results suggest the possibility of design and fabrication of an integrated or monolithic structure, which may lead to operation of the device at repetition rates beyond 50 GHz with sub-ps pulse durations. Although our VECSEL diode emits 980 nm

  14. Charge-transfer excitons at organic semiconductor surfaces and interfaces.

    PubMed

    Zhu, X-Y; Yang, Q; Muntwiler, M

    2009-11-17

    When a material of low dielectric constant is excited electronically from the absorption of a photon, the Coulomb attraction between the excited electron and the hole gives rise to an atomic H-like quasi-particle called an exciton. The bound electron-hole pair also forms across a material interface, such as the donor/acceptor interface in an organic heterojunction solar cell; the result is a charge-transfer (CT) exciton. On the basis of typical dielectric constants of organic semiconductors and the sizes of conjugated molecules, one can estimate that the binding energy of a CT exciton across a donor/acceptor interface is 1 order of magnitude greater than k(B)T at room temperature (k(B) is the Boltzmann constant and T is the temperature). How can the electron-hole pair escape this Coulomb trap in a successful photovoltaic device? To answer this question, we use a crystalline pentacene thin film as a model system and the ubiquitous image band on the surface as the electron acceptor. We observe, in time-resolved two-photon photoemission, a series of CT excitons with binding energies < or = 0.5 eV below the image band minimum. These CT excitons are essential solutions to the atomic H-like Schrodinger equation with cylindrical symmetry. They are characterized by principal and angular momentum quantum numbers. The binding energy of the lowest lying CT exciton with 1s character is more than 1 order of magnitude higher than k(B)T at room temperature. The CT(1s) exciton is essentially the so-called exciplex and has a very low probability of dissociation. We conclude that hot CT exciton states must be involved in charge separation in organic heterojunction solar cells because (1) in comparison to CT(1s), hot CT excitons are more weakly bound by the Coulomb potential and more easily dissociated, (2) density-of-states of these hot excitons increase with energy in the Coulomb potential, and (3) electronic coupling from a donor exciton to a hot CT exciton across the D

  15. Surface dynamics dominated by bulk thermal defects -- the case of NiAl (110).

    SciTech Connect

    Nobel, J. A.; McCarty, Kevin F.; Bartelt, Norman Charles

    2004-09-01

    We find that small temperature changes cause steps on the NiAl(110) surface to move. We show that this step motion occurs because mass is transferred between the bulk and the surface as the concentration of bulk thermal defects (i.e., vacancies) changes with temperature. Since the change in an island's area with a temperature change is found to scale strictly with the island's step length, the thermally generated defects are created (annihilated) very near the surface steps. To quantify the bulk/surface exchange, we oscillate the sample temperature and measure the amplitude and phase lag of the system response, i.e., the change in an island's area normalized to its perimeter. Using a one-dimensional model of defect diffusion through the bulk in a direction perpendicular to the surface, we determine the migration and formation energies of the bulk thermal defects. During surface smoothing, we show that there is no flow of material between islands on the same terrace and that all islands in a stack shrink at the same rate. We conclude that smoothing occurs by mass transport through the bulk of the crystal rather than via surface diffusion. Based on the measured relative sizes of the activation energies for island decay, defect migration, and defect formation, we show that attachment/detachment at the steps is the rate-limiting step in smoothing.

  16. A cochlear implant fabricated using a bulk silicon-surface micromachining process

    NASA Astrophysics Data System (ADS)

    Bell, Tracy Elizabeth

    1999-11-01

    This dissertation presents the design and fabrication of two generations of a silicon microelectrode array for use in a cochlear implant. A cochlear implant is a device that is inserted into the inner ear and uses electrical stimulation to provide sound sensations to the profoundly deaf. The first-generation silicon cochlear implant is a passive device fabricated using silicon microprobe technology developed at the University of Michigan. It contains twenty-two iridium oxide (IrO) stimulating sites that are 250 mum in diameter and spaced at 750 mum intervals. In-vivo recordings were made in guinea pig auditory cortex in response to electrical stimulation with this device, verifying its ability to electrically evoke an auditory response. Auditory thresholds as low as 78 muA were recorded. The second-generation implant is a thirty-two site, four-channel device with on-chip CMOS site-selection circuitry and integrated position sensing. It was fabricated using a novel bulk silicon surface micromachining process which was developed as a part of this dissertation work. While the use of semiconductor technology offers many advantages in fabricating cochlear implants over the methods currently used, it was felt that even further advantages could be gained by developing a new micromachining process which would allow circuitry to be distributed along the full length of the cochlear implant substrate. The new process uses electropolishing of an n+ bulk silicon sacrificial layer to undercut and release n- epitaxial silicon structures from the wafer. An extremely abrupt etch-stop between the n+ and n- silicon is obtained, with no electropolishing taking place in the n-type silicon that is doped lower than 1 x 1017 cm-3 in concentration. Lateral electropolishing rates of up to 50 mum/min were measured using this technique, allowing one millimeter-wide structures to be fully undercut in as little as 10 minutes. The new micromachining process was integrated with a standard p

  17. Reverse Non-Equilibrium Molecular Dynamics Demonstrate That Surface Passivation Controls Thermal Transport at Semiconductor-Solvent Interfaces.

    PubMed

    Hannah, Daniel C; Gezelter, J Daniel; Schaller, Richard D; Schatz, George C

    2015-06-23

    We examine the role played by surface structure and passivation in thermal transport at semiconductor/organic interfaces. Such interfaces dominate thermal transport in semiconductor nanomaterials owing to material dimensions much smaller than the bulk phonon mean free path. Utilizing reverse nonequilibrium molecular dynamics simulations, we calculate the interfacial thermal conductance (G) between a hexane solvent and chemically passivated wurtzite CdSe surfaces. In particular, we examine the dependence of G on the CdSe slab thickness, the particular exposed crystal facet, and the extent of surface passivation. Our results indicate a nonmonotonic dependence of G on ligand-grafting density, with interfaces generally exhibiting higher thermal conductance for increasing surface coverage up to ∼0.08 ligands/Å(2) (75-100% of a monolayer, depending on the particular exposed facet) and decreasing for still higher coverages. By analyzing orientational ordering and solvent penetration into the ligand layer, we show that a balance of competing effects is responsible for this nonmonotonic dependence. Although the various unpassivated CdSe surfaces exhibit similar G values, the crystal structure of an exposed facet nevertheless plays an important role in determining the interfacial thermal conductance of passivated surfaces, as the density of binding sites on a surface determines the ligand-grafting densities that may ultimately be achieved. We demonstrate that surface passivation can increase G relative to a bare surface by roughly 1 order of magnitude and that, for a given extent of passivation, thermal conductance can vary by up to a factor of ∼2 between different surfaces, suggesting that appropriately tailored nanostructures may direct heat flow in an anisotropic fashion for interface-limited thermal transport.

  18. Improving the Bulk Formula for Sea-Surface Fluxes

    DTIC Science & Technology

    2011-03-14

    weak SST heterogeneity. J. Geophys. Res, 115, D11103,doi:10.1029/2009JD013161. Vickers D and L. Mahrt 2010: Sea-surface roughness lengths in the midlatitude coastal zone. Quart. J. Roy. Meterol. Soc. 136, 1089 -1093.

  19. Simultaneous surface and bulk sensitive XAS measurements of magnetic particle clusters

    NASA Astrophysics Data System (ADS)

    Swaraj, S.; Dietrich, P. M.; Unger, W. E. S.

    2017-06-01

    Magnetic iron oxide nanoparticle clusters (mnpc) coated with organic stabilizers were investigated using scanning transmission x-ray microscopy (STXM). Simultaneous surface and bulk sensitive Fe L3 edge absorption spectra, obtained using a photomultiplier tube and a channeltron, were used to detect subtle changes in the oxidation state in the surface and bulk of Iron Oxide mnpc. The effectiveness of this mode of STXM operation is demonstrated for these nanoparticle clusters.

  20. Contactless nondestructive measurement of bulk and surface recombination using frequency-modulated free carrier absorption

    NASA Astrophysics Data System (ADS)

    Sanii, F.; Giles, F. P.; Schwartz, R. J.; Gray, J. L.

    1992-03-01

    A measurement procedure is described which allows the contactless measurement of bulk lifetime and surface recombination. The procedure uses the the free-carrier absorption of a long-wavelength laser beam by a modulated free-carrier wave to measure and separate the bulk recombination from the surface recombination. The dependence of the absorption on the modulation frequency is used to accomplish the separation. Limitations of the technique are also discussed.

  1. Room-temperature 2D semiconductor activated vertical-cavity surface-emitting lasers.

    PubMed

    Shang, Jingzhi; Cong, Chunxiao; Wang, Zilong; Peimyoo, Namphung; Wu, Lishu; Zou, Chenji; Chen, Yu; Chin, Xin Yu; Wang, Jianpu; Soci, Cesare; Huang, Wei; Yu, Ting

    2017-09-14

    Two-dimensional (2D) semiconductors are opening a new platform for revitalizing widely spread optoelectronic applications. The realisation of room-temperature vertical 2D lasing from monolayer semiconductors is fundamentally interesting and highly desired for appealing on-chip laser applications such as optical interconnects and supercomputing. Here, we present room-temperature low-threshold lasing from 2D semiconductor activated vertical-cavity surface-emitting lasers (VCSELs) under continuous-wave pumping. 2D lasing is achieved from a 2D semiconductor. Structurally, dielectric oxides were used to construct the half-wavelength-thick cavity and distributed Bragg reflectors, in favour of single-mode operation and ultralow optical loss; in the cavity centre, the direct-bandgap monolayer WS2 was embedded as the gain medium, compatible with the planar VCSEL configuration and the monolithic integration technology. This work demonstrates 2D semiconductor activated VCSELs with desirable emission characteristics, which represents a major step towards practical optoelectronic applications of 2D semiconductor lasers.Two-dimensional materials have recently emerged as interesting materials for optoelectronic applications. Here, Shang et al. demonstrate two-dimensional semiconductor activated vertical-cavity surface-emitting lasers where both the gain material and the lasing characteristics are two-dimensional.

  2. The influence of the semiconductor and dielectric properties on surface flashover in silicon-dielectric systems

    SciTech Connect

    Gradinaru, G.; Madangarli, V.P.; Sudarshan, T.S. . Dept. of Electrical and Computer Engineering)

    1994-07-01

    New experimental results on surface flashover are reported for high field silicon-dielectric systems. Different conditions of the lateral surface, contacts and ambient dielectrics have been studied. The strong influence of the semiconductor quality, and that of the dielectric properties, on the prebreakdown and breakdown response of the system, is demonstrated. All experimental results strongly support the conclusion that surface flashover in silicon systems is a physical process totally different from semiconductor surface breakdown. This conclusion has important practical application in the improvement of the performance of photoconductive power switches, severely limited by premature breakdown effects.

  3. Bulk growth and surface characterization of epitaxy ready cadmium zinc telluride substrates for use in IR imaging applications

    NASA Astrophysics Data System (ADS)

    Flint, J. P.; Martinez, B.; Betz, T. E. M.; MacKenzie, J.; Kumar, F. J.; Bindley, G.

    2016-05-01

    Cadmium Zinc Telluride (CZT) is an important compound semiconductor material upon which Mercury Cadmium Telluride (MCT) layers are deposited epitaxially to form structures that are used in high performance detectors covering a wide infrared (IR) spectral band. The epitaxial growth of high quality MCT layers presents many technical challenges and a critical determinant of material performance is the quality of the underlying bulk CZT substrate. CZT itself is a difficult material to manufacture where traditional methods of bulk growth are complex and low yielding, which constrains the supply of commercially available substrates. In this work we report on the epitaxy-ready finishing of Travelling Heather Method (THM) grown Cd0.96Zn0.04Te substrates. The THM method is well established for the growth of high quality CZT crystals used in nuclear, X-ray and spectroscopic imaging applications and in this work we demonstrate the application of this technique to the growth of IR specification CZT substrates with areas of up to 5 cm x 5 cm square. We will discuss the advantages of the THM method over alternative methods of bulk CZT growth where the high yield and material uniformity advantages of this technique will be demonstrated. Chemo-mechanical polishing (CMP) of 4 cm x 4 cm CZT substrates reveals that III-V (InSb/GaSb) like levels of epitaxy-ready surface finishing may be obtained with modified process chemistries. Surface quality assessments will be made by various surface analytical and microscopy techniques from which the suitability of the material for subsequent assessment of quality by epitaxial growth will be ascertained.

  4. Electrospray deposition of organic molecules on bulk insulator surfaces.

    PubMed

    Hinaut, Antoine; Pawlak, Rémy; Meyer, Ernst; Glatzel, Thilo

    2015-01-01

    Large organic molecules are of important interest for organic-based devices such as hybrid photovoltaics or molecular electronics. Knowing their adsorption geometries and electronic structures allows to design and predict macroscopic device properties. Fundamental investigations in ultra-high vacuum (UHV) are thus mandatory to analyze and engineer processes in this prospects. With increasing size, complexity or chemical reactivity, depositing molecules by thermal evaporation becomes challenging. A recent way to deposit molecules in clean conditions is Electrospray Ionization (ESI). ESI keeps the possibility to work with large molecules, to introduce them in vacuum, and to deposit them on a large variety of surfaces. Here, ESI has been successfully applied to deposit triply fused porphyrin molecules on an insulating KBr(001) surface in UHV environment. Different deposition coverages have been obtained and characterization of the surface by in-situ atomic force microscopy working in the non-contact mode shows details of the molecular structures adsorbed on the surface. We show that UHV-ESI, can be performed on insulating surfaces in the sub-monolayer regime and to single molecules which opens the possibility to study a variety of complex molecules.

  5. Effect of intense terahertz laser and magnetic fields on the binding energy and the transition energy of shallow impurity in a bulk semiconductor

    NASA Astrophysics Data System (ADS)

    Wang, Weiyang; Xu, Lei; Wu, Bo; Zhang, Sha; Wei, Xiangfei

    2017-09-01

    The influences of intense terahertz laser and magnetic fields on shallow-donor states in GaAs bulk semiconductors in the Faraday geometry are studied theoretically in the framework of the effective-mass approximation. The interaction between the laser field and the semiconductor is treated nonperturbatively by solving analytically the time-dependent Schrödinger equation in which the two external fields are included exactly. In the nonresonant region, we have found that the binding and transition energies decrease with increasing laser-field intensity or decreasing laser-field frequency, and the binding energy increases with magnetic field. For relatively low radiation levels, the transition energy first slowly decreases with increasing magnetic field, but after a critical value, it rapidly increases with increasing magnetic field. However, it slowly decreases with magnetic field when the laser-field intensity is strong enough. Furthermore, in the vicinity of the resonant regime, the oscillatory behaviours of the binding and transition energies with laser-field frequency and magnetic field are observed. These results obtained indicate the possibility of manipulating the shallow impurity states in semiconductor by changing the intense laser-field frequency and intensity and the magnetic field, which gives a new degree of freedom in semiconductor device application.

  6. Li(Zn,Co,Mn)As: A bulk form diluted magnetic semiconductor with Co and Mn co-doping at Zn sites

    NASA Astrophysics Data System (ADS)

    Chen, Bijuan; Deng, Zheng; Li, Wenmin; Gao, Moran; Zhao, Jianfa; Zhao, Guoqiang; Yu, Shuang; Wang, Xiancheng; Liu, Qingqing; Jin, Changqing

    2016-11-01

    We report the synthesis and characterization of a series of bulk forms of diluted magnetic semiconductors Li(Zn1-x-yCoxMny)As with a crystal structure close to that of III-V diluted magnetic semiconductor (Ga,Mn)As. No ferromagnetic order occurs with single (Zn,Co) or (Zn, Mn) substitution in the parent compound LiZnAs. Only with co-doped Co and Mn ferromagnetic ordering can occur at the Curie temperature ˜40 K. The maximum saturation moment of the this system reached to 2.17 μB /Mn , which is comparable to that of Li (Zn,Mn)As. It is the first time that a diluted magnetic semiconductor with co-doping Co and Mn into Zn sites is achieved in "111" LiZnAs system, which could be utilized to investigate the basic science of ferromagnetism in diluted magnetic semiconductors. In addition, ferromagnetic Li(Zn,Co,Mn)As, antiferromagnetic LiMnAs, and superconducting LiFeAs share square lattice at As layers, which may enable the development of novel heterojunction devices in the future.

  7. Determination of Surface Recombination Velocities at Contacts in Organic Semiconductor Devices Using Injected Carrier Reservoirs

    NASA Astrophysics Data System (ADS)

    Sandberg, Oskar J.; Sandén, Simon; Sundqvist, Anton; Smâtt, Jan-Henrik; Österbacka, Ronald

    2017-02-01

    A method to determine surface recombination velocities at collecting contacts in interface-limited organic semiconductor devices, based on the extraction of injected carrier reservoirs in a single-carrier sandwich-type structure, is presented. The analytical framework is derived and verified with drift-diffusion simulations. The method is demonstrated on solution-processed organic semiconductor devices with hole-blocking TiO2/organic and SiO2/organic interfaces, relevant for solar cell and transistor applications, respectively.

  8. The coexistence of surface magnetoplasmons (SMPs) and bulk magnetoplasmons (BMPs) in SIS waveguide with the Voigt configuration magnetization

    NASA Astrophysics Data System (ADS)

    Zhu, Qiong-gan; Wang, Zhi-guo

    2016-05-01

    The coexistence behavior of surface magnetoplasmons (SMPs) and bulk magnetoplasmons (BMPs) is discussed on a platform constructed by the Semicondutor-Insulator-Semiconductor (SIS) waveguide with the Voigt configuration magnetization. It is found that the coexistence of SMPs and BMPs stems from the nonzero off-diagonal terms of permittivity tensors of the top and the bottom semiconductor materials (SM) claddings which are induced by the external magnetic field. In this case, the impendence of SM for SMPs contains two contributions associated with both the transversal and the longitudinal wave vectors of SMPs. When the impendence matching condition of SMPs exciting in SIS waveguide is satisfied in the propagating band of BMPs, the coexistence of these two modes thus appears. The results show that the forward-propagating SMPs only coexists with the lower BMPs mode, however, the backward-propagating SMPs coexists with the higher BMPs mode when the top and the bottom SM claddings are magnetized by equal amplitude magnetic field but with opposite direction. In addition, the influences of external-magnetic-field intensity, insulator permittivity and waveguide width on the coexisting frequency widths are also presented.

  9. Optical Real Time Signal Processors and Semiconductor Surface and Semiconductor-Electrolyte Interface Study Using Acoustic Surface Wave.

    DTIC Science & Technology

    1981-08-01

    Resistivity GaAs", J. Appl. Phys., Vol. 50, pp. 4942-4950, 1979. Copies of the papers are available upon request. 2 5. J . Scott Moore and P. Das, "The...Papers presented with support from the contract 1. J . Scott Moore and P. Das, "Hot Electron Effects in Quasi-Two Dimensional Semiconductors

  10. In situ Magnetotransport Measurements in Ultrathin Bi Films: Evidence for Surface-Bulk Coherent Transport

    NASA Astrophysics Data System (ADS)

    Aitani, Masaki; Hirahara, Toru; Ichinokura, Satoru; Hanaduka, Masahiro; Shin, Dongyoon; Hasegawa, Shuji

    2014-11-01

    We performed in situ magnetotransport measurements on ultrathin Bi(111) films [4-30 bilayers (BLs), 16-120 Å thick] to elucidate the role of bulk or surface states in the transport phenomena. We found that the temperature dependence of the film conductivity shows no thickness dependence for the 6-16 BL films and is affected by the electron-electron scattering, suggesting surface-state dominant contribution. In contrast, the weak antilocalization effect observed by applying a magnetic field shows clear thickness dependence, indicating bulk transport. This apparent inconsistency is explained by a coherent bulk-surface coupling that produces a single channel transport. For the films thicker than 20 BLs, the behavior changes drastically which can likely be interpreted as a bulk dominant conduction.

  11. Relationship between surface concentration of L-leucine and bulk powder properties in spray dried formulations.

    PubMed

    Mangal, Sharad; Meiser, Felix; Tan, Geoffrey; Gengenbach, Thomas; Denman, John; Rowles, Matthew R; Larson, Ian; Morton, David A V

    2015-08-01

    The amino acid L-leucine has been demonstrated to act as a lubricant and improve the dispersibility of otherwise cohesive fine particles. It was hypothesized that optimum surface L-leucine concentration is necessary to achieve optimal surface and bulk powder properties. Polyvinylpyrrolidone was spray dried with different concentration of L-leucine and the change in surface composition of the formulations was determined using X-ray photoelectron spectroscopy (XPS) and time of flight-secondary ion mass spectrometry (ToF-SIMS). The formulations were also subjected to powder X-ray diffraction analysis in order to understand the relationship between surface concentration and solid-state properties of L-leucine. In addition, the morphology, surface energy and bulk cohesion of spray dried formulations were also assessed to understand the relation between surface L-leucine concentration and surface and bulk properties. The surface concentration of L-leucine increased with higher feed concentrations and plateaued at about 10% L-leucine. Higher surface L-leucine concentration also resulted in the formation of larger L-leucine crystals and not much change in crystal size was noted above 10% L-leucine. A change in surface morphology of particles from spherical to increasingly corrugated was also observed with increasing surface l-leucine concentration. Specific collapsed/folded over particles were only seen in formulations with 10% or higher l-leucine feed concentration suggesting a change in particle surface formation process. In addition, bulk cohesion also reduced and approached a minimum with 10% L-leucine concentration. Thus, the surface concentration of L-leucine governs particle formation and optimum surface L-leucine concentration results in optimum surface and bulk powder properties.

  12. Data analysis and calibration for a bulk-refractive-index-compensated surface plasmon resonance affinity sensor

    NASA Astrophysics Data System (ADS)

    Chinowsky, Timothy M.; Yee, Sinclair S.

    2002-02-01

    Surface plasmon resonance (SPR) affinity sensing, the problem of bulk refractive index (RI) interference in SPR sensing, and a sensor developed to overcome this problem are briefly reviewed. The sensor uses a design based on Texas Instruments' Spreeta SPR sensor to simultaneously measure both bulk and surface RI. The bulk RI measurement is then used to compensate the surface measurement and remove the effects of bulk RI interference. To achieve accurate compensation, robust data analysis and calibration techniques are necessary. Simple linear data analysis techniques derived from measurements of the sensor response were found to provide a versatile, low noise method for extracting measurements of bulk and surface refractive index from the raw sensor data. Automatic calibration using RI gradients was used to correct the linear estimates, enabling the sensor to produce accurate data even when the sensor has a complicated nonlinear response which varies with time. The calibration procedure is described, and the factors influencing calibration accuracy are discussed. Data analysis and calibration principles are illustrated with an experiment in which sucrose and detergent solutions are used to produce changes in bulk and surface RI, respectively.

  13. Analysis of Surface and Bulk Behavior in Ni-Pd Alloys

    NASA Technical Reports Server (NTRS)

    Bozzolo, Guillermo; Noebe, Rondald D.

    2003-01-01

    The most salient features of the surface structure and bulk behavior of Ni-Pd alloys have been studied using the BFS method for alloys. Large-scale atomistic simulations were performed to investigate surface segregation profiles as a function of temperature, crystal face, and composition. Pd enrichment of the first layer was observed in (111) and (100) surfaces, and enrichment of the top two layers occurred for (110) surfaces. In all cases, the segregation profile shows alternate planes enriched and depleted in Pd. In addition, the phase structure of bulk Ni-Pd alloys as a function of temperature and composition was studied. A weak ordering tendency was observed at low temperatures, which helps explain the compositional oscillations in the segregation profiles. Finally, based on atom-by-atom static energy calculations, a comprehensive explanation for the observed surface and bulk features will be presented in terms of competing chemical and strain energy effects.

  14. Two-band superconductivity of bulk and surface states in Ag thin films on Nb

    NASA Astrophysics Data System (ADS)

    Tomanic, Tihomir; Schackert, Michael; Wulfhekel, Wulf; Sürgers, Christoph; Löhneysen, Hilbert v.

    2016-12-01

    We use epitaxial strain to spatially tune the bottom of the surface-state band ESS of Ag(111) islands on Nb(110). Bulk and surface-state contributions to the Ag(111) local density of states (LDOS) can be separated with scanning tunneling spectroscopy. For thick islands (≈20 nm), the Ag surface states are decoupled from the Ag bulk states and the superconductive gap induced by proximity to Nb is due to bulk states only. However, for thin islands (3-4 nm), surface-state electrons develop superconducting correlations as identified by a complete energy gap in the LDOS when ESS is smaller than but close to the Fermi level. The induced superconductivity in this case is of a two-band nature and appears to occur when the surface-state wave function reaches down to the Ag/Nb interface.

  15. Collective excitations of spherical semiconductor nanoparticles

    NASA Astrophysics Data System (ADS)

    Moradi, Afshin

    2016-10-01

    In this article, we study the dispersion properties of bulk and surface electrostatic oscillations of a spherical quantum electron-hole semiconductor plasma as a simple model of a semiconductor nanoparticle. We derive general dispersion relation for both bulk and surface modes, using quantum hydrodynamic theory (including the electrons and holes quantum recoil effects, quantum statistical pressures of the plasma species, as well as exchange and correlation effects) in conjunction with Poisson’s equation and appropriate boundary conditions. We show that for the arbitrary value of angular quantum number {\\ell }≥slant 1 there are only two surface plasmon modes, but two infinite series of bulk modes for {\\ell }≥slant 0 that owe their existence to the curvature of the interface. We use the typical values of GaAs semiconductor to compute the bulk and surface mode frequencies for different value of {\\ell }.

  16. Bulk and surface effects in segmented high purity germanium detectors

    NASA Astrophysics Data System (ADS)

    Abt, I.; Caldwell, A.; Dönmez, B.; Irlbeck, S.; Majorovits, B.; Volynets, O.

    2013-08-01

    Segmented high-purity germanium detectors have been developed for a variety of experiments. The segmentation is used to augment the excellent energy resolution of such a device with spatial information to disentangle event topologies. Several performance aspects of true-coaxial segmented detectors are presented, especially the effects due to the crystallographic axes and the problem of events close to the surfaces of the detector. A test stand and Monte Carlo tools developed to study such effects are introduced. The simulation tools can also be used to design novel detectors, such as segmented point-contact detectors. A particular design is presented and discussed.

  17. Latex Surface and Bulk Coagulation Induced by Solvent Vapors.

    PubMed

    Braga; da Silva Md; Cardoso; Galembeck

    2000-08-01

    Latex exposure to solvent vapors leads to highly specific changes in latex stability as well as on the morphologies of the particle association products, depending on the latex and solvent used. Examples of solvent vapor-induced aggregation are given: surface films are obtained on two PS latexes; in one case, the film surface is mirror-reflective and very flat, as evidenced by AFM. Another PS latex coagulates under exposure to acetone vapors, and the morphologies of the coagula are highly sensitive to the exposure conditions. This latex yields a highly porous foam-like structure, in which particles are strongly coalesced but form percolating patches around the pores. The same latex but under other conditions produces a coagulum of large numbers of aggregated particles with a raspberry-like morphology. Density centrifugation experiments show that the effect of solvents on different latex fractions is not uniform, and some fractions show larger density changes than others, thus evidencing a variability in their swelling ability. Copyright 2000 Academic Press.

  18. A New Commercializable Route for the Preparation of Single-Source Precursors for Bulk, Thin-Film, and Nanocrystallite I-III-IV Semiconductors

    NASA Technical Reports Server (NTRS)

    Banger, Kulbinder K.; Jin, Michael H. C.; Harris, Jerry D.; Fanwick, Philip E.; Hepp, Aloysius F.

    2004-01-01

    We report a new simplified synthetic procedure for commercial manufacture of ternary single source precursors (SSP). This new synthetic process has been successfully implemented to fabricate known SSPs on bulk scale and the first liquid SSPs to the semiconductors CuInSe2 and AgIn(x)S(y). Single crystal X-ray determination reveals the first unsolvated ternary AgInS SSP. SSPs prepared via this new route have successfully been used in a spray assisted chemical vapor deposition (CVD) process to deposit polycrystalline thin films, and for preparing ternary nanocrystallites.

  19. Magnetism on the surface of the bulk paramagnetic intermetallic compound YCo2.

    PubMed

    Khmelevskyi, S; Mohn, P; Redinger, J; Weinert, M

    2005-04-15

    Using full-potential electronic structure calculations, we predict that the (111) surface of the cubic Laves phase Pauli paramagnet YCo2 is ferromagnetic. The magnetism of the (111) surface is independent of the termination of the surface, does not extend beyond two Co layers, and is related to the field-induced metamagnetism of the bulk. YCo2 appears to be a prominent candidate to demonstrate the phenomenon of surface-induced itinerant magnetism localized in two dimensions.

  20. Disentanglement of surface and bulk Rashba spin splittings in noncentrosymmetric BiTeI.

    PubMed

    Landolt, Gabriel; Eremeev, Sergey V; Koroteev, Yury M; Slomski, Bartosz; Muff, Stefan; Neupert, Titus; Kobayashi, Masaki; Strocov, Vladimir N; Schmitt, Thorsten; Aliev, Ziya S; Babanly, Mahammad B; Amiraslanov, Imamaddin R; Chulkov, Evgueni V; Osterwalder, Jürg; Dil, J Hugo

    2012-09-14

    BiTeI has a layered and noncentrosymmetric structure where strong spin-orbit interaction leads to a giant Rashba spin splitting in the bulk bands. We present direct measurements of the bulk band structure obtained with soft x-ray angle-resolved photoemission (ARPES), revealing the three-dimensional Fermi surface. The observed spindle torus shape bears the potential for a topological transition in the bulk by hole doping. Moreover, the bulk electronic structure is clearly disentangled from the two-dimensional surface electronic structure by means of high-resolution and spin-resolved ARPES measurements in the ultraviolet regime. All findings are supported by ab initio calculations.

  1. Surface plasmon polariton amplification in semiconductor film / graphene / dielectric structure by direct electric current

    NASA Astrophysics Data System (ADS)

    Moiseev, Sergey G.; Dadoenkova, Yuliya S.; Zolotovskii, Igor O.; Abramov, Aleksei S.; Pavlov, Dmitrii A.; Anzulevich, Anton P.

    2017-09-01

    An amplification of surface plasmon polaritons due to the transfer of electromagnetic energy from a drift current wave into a far-infrared surface wave propagating along a semiconductor-dielectric boundary in waveguide geometry is studied. It is shown that the amplification coefficient of slow surface plasmon polaritons can reach values substantially exceeding the ohmic loss coefficient when phase matching condition is satisfied, i. e., when the phase velocity of the surface wave approaches the drift velocity of charge carriers.

  2. Disentangling the surface and bulk electronic structures of LaOFeAs

    DOE PAGES

    Zhang, P.; Ma, J.; Qian, T.; ...

    2016-09-20

    We performed a comprehensive angle-resolved photoemission spectroscopy study of the electronic band structure of LaOFeAs single crystals. We found that samples cleaved at low temperature show an unstable and very complicated band structure, whereas samples cleaved at high temperature exhibit a stable and clearer electronic structure. Using in situ surface doping with K and supported by first-principles calculations, we identify both surface and bulk bands. Our assignments are confirmed by the difference in the temperature dependence of the bulk and surface states.

  3. Near-surface and bulk behavior of Ag in SiC

    SciTech Connect

    Xiao, Haiyan; Zhang, Yanwen; Snead, Lance Lewis; Shutthanandan, Vaithiyalingam; Xue, Haizhou; Weber, William J

    2012-01-01

    The diffusive release of fission products, such as Ag, from TRISO particles at high temperatures has raised concerns regarding safe and economic operation of advanced nuclear reactors. Understanding the mechanisms of Ag diffusion is thus of crucial importance for effective retention of fission products. Two mechanisms, i.e., grain boundary diffusion and vapor or surface diffusion through macroscopic structures such as nano-pores or nano-cracks, remain in debate. In the present work, an integrated computational and experimental study of the near-surface and bulk behavior of Ag in silicon carbide (SiC) has been carried out. The ab initio calculations show that Ag prefers to adsorb on the SiC surface rather than in the bulk, and the mobility of Ag on the surface is high. The energy barrier for Ag desorption from the surface is calculated to be 0.85-1.68 eV, and Ag migration into bulk SiC through equilibrium diffusion process is not favorable. Experimentally, Ag ions are implanted into SiC to produce Ag profiles buried in the bulk and peaked at the surface. High-temperature annealing leads to Ag release from the surface region instead of diffusion into the interior of SiC. It is suggested that surface diffusion through mechanical structural imperfection, such as vapor transport through cracks in SiC coatings, may be a dominating mechanism accounting for Ag release from the SiC in the nuclear reactor.

  4. Guided evolution of bulk metallic glass nanostructures: A platform for designing three-dimensional electrocatalytic surfaces

    DOE PAGES

    Doubek, Gustavo; Sekol, Ryan C.; Li, Jinyang; ...

    2015-12-22

    Precise control over catalyst surface composition and structure is necessary to improve the function of electrochemical systems. To that end, bulk metallic glass (BMG) alloys with atomically dispersed elements provide a highly processable, nanoscale platform for electrocatalysis and surface modification. Here we report on nanostructures of Pt-based BMGs that are modified with various subtractive and additive processes to improve their electrochemical performance.

  5. Surface flashover threshold and switched fields of photoconductive semiconductor switches

    SciTech Connect

    Loubriel, G.M.; O'Malley, M.W.; Zutavern, F.J.; McKenzie, B.B.; Conley, W.R.

    1988-01-01

    We have shown that Si Photoconductive Semiconductor Switches (PCSS) can be used to switch high voltages (up to 123 kV), high fields (up to 82 kV/cm) and high currents (2.8 kA). The ability of the samples to withstand this type of high voltage, high current switching depends on the way in which the current penetrates the semiconductor. The appropriate use of water or contacts greatly improves the switching capability. We have also shown that the wafers can support large currents (4.0 kA for GaAs and 2.8 kA for Si) and large linear current densities (3.2 kA/cm for GaAs and 1.4 kA/cm for Si). For GaAs this linear current density corresponds to about 1 Ma/cm/sup 2/ given a penetration depth of about 10/sup /minus/3/ cm. 4 refs., 4 figs., 2 tabs.

  6. Disentangling surface and bulk transport in topological-insulator p -n junctions

    NASA Astrophysics Data System (ADS)

    Backes, Dirk; Huang, Danhong; Mansell, Rhodri; Lanius, Martin; Kampmeier, Jörn; Ritchie, David; Mussler, Gregor; Gumbs, Godfrey; Grützmacher, Detlev; Narayan, Vijay

    2017-09-01

    By combining n -type Bi2Te3 and p -type Sb2Te3 topological insulators, vertically stacked p -n junctions can be formed, allowing to position the Fermi level into the bulk band gap and also tune between n - and p -type surface carriers. Here, we use low-temperature magnetotransport measurements to probe the surface and bulk transport modes in a range of vertical Bi2Te3/Sb2Te3 heterostructures with varying relative thicknesses of the top and bottom layers. With increasing thickness of the Sb2Te3 layer we observe a change from n - to p -type behavior via a specific thickness where the Hall signal is immeasurable. Assuming that the the bulk and surface states contribute in parallel, we can calculate and reproduce the dependence of the Hall and longitudinal components of resistivity on the film thickness. This highlights the role played by the bulk conduction channels which, importantly, cannot be probed using surface-sensitive spectroscopic techniques. Our calculations are then buttressed by a semiclassical Boltzmann transport theory which rigorously shows the vanishing of the Hall signal. Our results provide crucial experimental and theoretical insights into the relative roles of the surface and bulk in the vertical topological p -n junctions.

  7. Nonlinear scaling of surface water diffusion with bulk water viscosity of crowded solutions.

    PubMed

    Franck, John M; Scott, John A; Han, Songi

    2013-03-20

    The translational hydration dynamics within 0.5-1.5 nm of the surface of a DPPC liposome, a model biomacromolecular surface, is analyzed by the recently developed Overhauser dynamic nuclear polarization (ODNP) technique. We find that dramatic changes to the bulk solvent cause only weak changes in the surface hydration dynamics. Specifically, both a >10-fold increase in bulk viscosity and the restriction of diffusion by confinement on a multiple nm length-scale change the local translational diffusion coefficient of the surface water surrounding the lipid bilayer by <2.5-fold. By contrast, previous ODNP studies have shown that changes to the biomacromolecular surface induced by folding, binding, or aggregation can cause local hydration dynamics to vary by factors of up to 30. We suggest that the surface topology and chemistry at the ≤1.5 nm scale, rather than the characteristics of the solvent, nearly exclusively determine the macromolecule's surface hydration dynamics.

  8. Edge Dislocations Triggered Surface Instability in Tensile Epitaxial Hexagonal Nitride Semiconductor.

    PubMed

    Cheng, Jianpeng; Yang, Xuelin; Zhang, Jie; Hu, Anqi; Ji, Panfeng; Feng, Yuxia; Guo, Lei; He, Chenguang; Zhang, Lisheng; Xu, Fujun; Tang, Ning; Wang, Xinqiang; Shen, Bo

    2016-12-14

    Understanding the semiconductor surface and its properties including surface stability, atomic morphologies, and even electronic states is of great importance not only for understanding surface growth kinetics but also for evaluating the degree to which they affect the devices' performance. Here, we report studies on the nanoscale fissures related surface instability in AlGaN/GaN heterostructures. Experimental results reveal that edge dislocations are actually the root cause of the surface instability. The nanoscale fissures are initially triggered by the edge dislocations, and the subsequent evolution is associated with tensile lattice-mismatch stress and hydrogen etching. Our findings resolve a long-standing problem on the surface instability in AlGaN/GaN heterostructures and will also lead to new understandings of surface growth kinetics in other hexagonal semiconductor systems.

  9. Electron beam dose dependence of surface recombination velocity and surface space charge in semiconductor nanowires

    NASA Astrophysics Data System (ADS)

    Donatini, Fabrice; Sartel, Corinne; Sallet, Vincent; Pernot, Julien

    2017-06-01

    The characterization of nanowires (NWs) often requires the use of scanning electron beam techniques because of their high spatial resolution. However, the impact of the high energetic electron beam on the physical parameters under investigation is rarely taken into account. In this work, a combination of optical and electrical techniques is involved for the measurement of the electron beam dose (EBD) dependence of cathodoluminescence intensity, exciton diffusion length and electrical resistance in ZnO NWs. Large EBD dependences of these key parameters are observed and their reversibility is investigated. The results are discussed in terms of bulk and surface reversible modifications. In particular, the behaviors of surface recombination velocity and surface space charge under electron beam exposure are determined and simulated. This study points out that caution must be taken and experimental protocols must be well defined when measuring physical parameters of NWs using electron beam techniques.

  10. In-crystal and surface charge transport of electric-field-induced carriers in organic single-crystal semiconductors.

    PubMed

    Takeya, J; Kato, J; Hara, K; Yamagishi, M; Hirahara, R; Yamada, K; Nakazawa, Y; Ikehata, S; Tsukagoshi, K; Aoyagi, Y; Takenobu, T; Iwasa, Y

    2007-05-11

    Gate-voltage dependence of carrier mobility is measured in high-performance field-effect transistors of rubrene single crystals by simultaneous detection of the longitudinal conductivity sigma(square) and Hall coefficient R(H). The Hall mobility mu(H) (identical with sigma(square)R(H)) reaches nearly 10 cm(2)/V s when relatively low-density carriers (<10(11) cm(-2)) distribute into the crystal. mu(H) rapidly decreases with higher-density carriers as they are essentially confined to the surface and are subjected to randomness of the amorphous gate insulators. The mechanism to realize high carrier mobility in the organic transistor devices involves intrinsic-semiconductor character of the high-purity organic crystals and diffusive bandlike carrier transport in the bulk.

  11. Structural Studies of Clean Semiconductor Surfaces and Metal-Semiconductor Interfaces by Photoemission Extended X-Ray Absorption Fine Structure Spectroscopy.

    NASA Astrophysics Data System (ADS)

    Mangat, Pawitterjit Singh

    We determined the atomic geometries for clean InP(110)-(1 x 1) and Si(111)-(2 x 1) surfaces and Al/InP(110), Ag/InP(110), Bi/InP(110), Na/InP(110) and Al/Si(111) interfaces by photoemission extended x-ray absorption fine structure (PEXAFS) spectroscopy to understand the correlation between electrical Schottky barrier heights and interfacial structure. P 2p PEXAFS for the InP(110) surface and Si 2p PEXAFS for the Si(111) surface were acquired which yielded information on the short range order of substrate atoms on the surface or at the interface. For Al/Si(111) interfaces, we also obtained Al 2p PEXAFS. The data analyzed by Fourier analysis and curve-fitting procedures. The theoretical backscattering phase function of McKale et al. (J. Am. Chem. Soc. 110, 3763 (1988)) and absorber phase function of Teo and Lee (J. Am. Chem. Soc. 101, 2815 (1979)) were used for phase analysis to determine the interatomic bond lengths. For the clean InP(110) surface, we observed surface relaxation. For the Si(111)-(2 x 1) surface, we found 10% contraction in the second near neighborhood Si-Si distance which is not reported in any model. For low coverage reactive metal (Al, Na)/InP(110) interfaces, we observed metal induced surface structural changes which involve removal of relaxation and change in the basis of the surface unit mesh of the substrate. For Ag/InP(110) interfaces, the noble metal atoms were found to remove the relaxation of the first P-In bond length at the interface. These changes in the substrate might bring in interface states within the semiconductor band gap and, consequently, influencing Fermi-level pinning during the Schottky barrier formation. For the Bi/InP(110) interfaces, the relaxation of the clean InP(110) surface is not removed by the deposited Bi atoms. Hence, the Bi/InP(110) interface might not have Fermi-level pinning by interface states due to the interfacial structure of InP. For Al/Si(111) interfaces, the Al atoms do not induce drastic surface

  12. Electrostatic Surface Waves on Semi-Bounded Quantum Electron-Hole Semiconductor Plasmas

    NASA Astrophysics Data System (ADS)

    Moradi, Afshin

    2017-03-01

    The electrostatic surface waves on semi-bounded quantum electron-hole semiconductor plasmas are studied within the framework of the quantum hydrodynamic model, including the electrons and holes quantum recoil effects, quantum statistical pressures of the plasma species, as well as exchange and correlation effects. The dispersion characteristics of surface electrostatic oscillations are investigated by using the typical values of GaAs, GaSb and GaN semiconductors. Numerical results show the existence of one low-frequency branch due to the mass difference between the electrons and holes in addition to one high-frequency branch due to charge-separation effects.

  13. Near-surface and bulk behavior of Ag in SiC

    SciTech Connect

    Xiao, Haiyan Y.; Zhang, Yanwen; Snead, Lance L.; Shutthanandan, V.; Xue, Haizhou; Weber, William J.

    2012-01-01

    The diffusive release of fission products, such as Ag, from TRISO particles at high temperatures has raised concerns regarding safe and economic operation of advanced nuclear reactors. Understanding the mechanisms of Ag diffusion is thus of crucial importance for effective retention of fission products. Two mechanisms, i.e., grain boundary diffusion and vapor or surface diffusion through macroscopic structures such as nano-pores or nano-cracks, remain in debate. In the present work, an integrated computational and experimental study of the nearsurface and bulk behavior of Ag in silicon carbide (SiC) has been carried out. The ab initio calculations show that Ag prefers to adsorb on the SiC surface rather than in the bulk, and the mobility of Ag on the surface is high. The energy barrier for Ag desorption from the surface is calculated to be 0.85~1.68 eV, and Ag migration into bulk SiC through equilibrium diffusion process is not favorable. Experimentally, Ag ions are implanted into SiC to produce Ag profiles buried in the bulk and peaked at the surface. High-temperature annealing leads to Ag release from the surface region instead of diffusion into the interior of SiC. It is suggested that surface diffusion through mechanical structural imperfection, such as vapor transport through cracks in 2 SiC coatings, may be a dominating mechanism accounting for Ag release from the SiC in the nuclear reactor.

  14. Surfaces of Homopolar Amorphous Semiconductors: Definition, Characterization, and Density of Surface States

    NASA Astrophysics Data System (ADS)

    Richmond, Daniel Lee

    To rigorously investigate the contribution of surfaces to the density of electronic states of a-Si (Ge), and the effect of the topology on the density of surface states, a surface for amorphous homopolar tetrahedral solids is defined. Continuous random network models are statistically analyzed for homogeneity. Various possible surfaces generated from these models are examined with the result that a spherical surface is found to be most representative of a surface from a homogeneous infinite amorphous solid. The homopolar amorphous surface is characterized by a wealth of steric and dangling bond configurations. Surface atoms can have one, two, or three dangling bonds, and can have from zero to three nearest neighbor surface atoms. The density of dangling bonds is 0.106 bonds/(ANGSTROM)('2). Reconstruction enables a 96% reduction in the density of dangling bonds. The ring structure of the surface atoms is significantly different from the ring structure of the bulk atoms. The topological effects on the density of surface states is exhaustively treated using a s-band Hamiltonian. Rings of different sizes uniquely contribute to the density of states. Other topological properties, such as multiple dangling bonds per surface atom and near neighbor surface atoms are treated. The effects on the density of states by the surface in the valence band and energy gap of a tetrahedral solid is investigated using a two parameter Hamiltonian. The local and configuration averaged density of states are computed for the dangling bond and four back bond hybrids. The ring structure affects the density of surface states in the valence band, but not the more localized energy gap states. The antibonding spectral feature in the energy gap deriving from surface atoms with two or three dangling bonds is independent of all topological effects, while the bonding spectral feature from these same surface atoms is not. The spectral feature due to surface atoms with only one dangling bond is also

  15. Visualizing weakly bound surface Fermi arcs and their correspondence to bulk Weyl fermions.

    PubMed

    Batabyal, Rajib; Morali, Noam; Avraham, Nurit; Sun, Yan; Schmidt, Marcus; Felser, Claudia; Stern, Ady; Yan, Binghai; Beidenkopf, Haim

    2016-08-01

    Fermi arcs are the surface manifestation of the topological nature of Weyl semimetals, enforced by the bulk-boundary correspondence with the bulk Weyl nodes. The surface of tantalum arsenide, similar to that of other members of the Weyl semimetal class, hosts nontopological bands that obscure the exploration of this correspondence. We use the spatial structure of the Fermi arc wave function, probed by scanning tunneling microscopy, as a spectroscopic tool to distinguish and characterize the surface Fermi arc bands. We find that, as opposed to nontopological states, the Fermi arc wave function is weakly affected by the surface potential: it spreads rather uniformly within the unit cell and penetrates deeper into the bulk. Fermi arcs reside predominantly on tantalum sites, from which the topological bulk bands are derived. Furthermore, we identify a correspondence between the Fermi arc dispersion and the energy and momentum of the bulk Weyl nodes that classify this material as topological. We obtain these results by introducing an analysis based on the role the Bloch wave function has in shaping quantum electronic interference patterns. It thus carries broader applicability to the study of other electronic systems and other physical processes.

  16. Visualizing weakly bound surface Fermi arcs and their correspondence to bulk Weyl fermions

    PubMed Central

    Batabyal, Rajib; Morali, Noam; Avraham, Nurit; Sun, Yan; Schmidt, Marcus; Felser, Claudia; Stern, Ady; Yan, Binghai; Beidenkopf, Haim

    2016-01-01

    Fermi arcs are the surface manifestation of the topological nature of Weyl semimetals, enforced by the bulk-boundary correspondence with the bulk Weyl nodes. The surface of tantalum arsenide, similar to that of other members of the Weyl semimetal class, hosts nontopological bands that obscure the exploration of this correspondence. We use the spatial structure of the Fermi arc wave function, probed by scanning tunneling microscopy, as a spectroscopic tool to distinguish and characterize the surface Fermi arc bands. We find that, as opposed to nontopological states, the Fermi arc wave function is weakly affected by the surface potential: it spreads rather uniformly within the unit cell and penetrates deeper into the bulk. Fermi arcs reside predominantly on tantalum sites, from which the topological bulk bands are derived. Furthermore, we identify a correspondence between the Fermi arc dispersion and the energy and momentum of the bulk Weyl nodes that classify this material as topological. We obtain these results by introducing an analysis based on the role the Bloch wave function has in shaping quantum electronic interference patterns. It thus carries broader applicability to the study of other electronic systems and other physical processes. PMID:27551687

  17. Tuning apparent friction coefficient by controlled patterning bulk metallic glasses surfaces

    PubMed Central

    Li, Ning; Xu, Erjiang; Liu, Ze; Wang, Xinyun; Liu, Lin

    2016-01-01

    Micro-honeycomb structures with various pitches between adjacent cells were hot-embossed on Zr35Ti30Cu8.25Be26.75 bulk metallic glass surface. The effect of pitch geometry on the frictional behavior of metallic glass surface was systematically investigated. The results revealed that all textured metallic glass surfaces show a reduction in friction coefficient compared to smooth surface. More intriguingly, the friction coefficient first decreased and then increased gradually with increasing pitches. Such unique behavior can be understood fundamentally from the perspective of competing effects between contact area and local stress level with increasing pitches. This finding not only enhance the in-depth understanding of the mechanism of the significant role of surface topography on the frictional behavior of metallic glass surface, but also opens a new route towards other functional applications for bulk metallic glasses. PMID:27991571

  18. Tuning apparent friction coefficient by controlled patterning bulk metallic glasses surfaces

    NASA Astrophysics Data System (ADS)

    Li, Ning; Xu, Erjiang; Liu, Ze; Wang, Xinyun; Liu, Lin

    2016-12-01

    Micro-honeycomb structures with various pitches between adjacent cells were hot-embossed on Zr35Ti30Cu8.25Be26.75 bulk metallic glass surface. The effect of pitch geometry on the frictional behavior of metallic glass surface was systematically investigated. The results revealed that all textured metallic glass surfaces show a reduction in friction coefficient compared to smooth surface. More intriguingly, the friction coefficient first decreased and then increased gradually with increasing pitches. Such unique behavior can be understood fundamentally from the perspective of competing effects between contact area and local stress level with increasing pitches. This finding not only enhance the in-depth understanding of the mechanism of the significant role of surface topography on the frictional behavior of metallic glass surface, but also opens a new route towards other functional applications for bulk metallic glasses.

  19. Tuning apparent friction coefficient by controlled patterning bulk metallic glasses surfaces.

    PubMed

    Li, Ning; Xu, Erjiang; Liu, Ze; Wang, Xinyun; Liu, Lin

    2016-12-19

    Micro-honeycomb structures with various pitches between adjacent cells were hot-embossed on Zr35Ti30Cu8.25Be26.75 bulk metallic glass surface. The effect of pitch geometry on the frictional behavior of metallic glass surface was systematically investigated. The results revealed that all textured metallic glass surfaces show a reduction in friction coefficient compared to smooth surface. More intriguingly, the friction coefficient first decreased and then increased gradually with increasing pitches. Such unique behavior can be understood fundamentally from the perspective of competing effects between contact area and local stress level with increasing pitches. This finding not only enhance the in-depth understanding of the mechanism of the significant role of surface topography on the frictional behavior of metallic glass surface, but also opens a new route towards other functional applications for bulk metallic glasses.

  20. A new approach to the problem of bulk-mediated surface diffusion

    PubMed Central

    Berezhkovskii, Alexander M.; Dagdug, Leonardo; Bezrukov, Sergey M.

    2015-01-01

    This paper is devoted to bulk-mediated surface diffusion of a particle which can diffuse both on a flat surface and in the bulk layer above the surface. It is assumed that the particle is on the surface initially (at t = 0) and at time t, while in between it may escape from the surface and come back any number of times. We propose a new approach to the problem, which reduces its solution to that of a two-state problem of the particle transitions between the surface and the bulk layer, focusing on the cumulative residence times spent by the particle in the two states. These times are random variables, the sum of which is equal to the total observation time t. The advantage of the proposed approach is that it allows for a simple exact analytical solution for the double Laplace transform of the conditional probability density of the cumulative residence time spent on the surface by the particle observed for time t. This solution is used to find the Laplace transform of the particle mean square displacement and to analyze the peculiarities of its time behavior over the entire range of time. We also establish a relation between the double Laplace transform of the conditional probability density and the Fourier-Laplace transform of the particle propagator over the surface. The proposed approach treats the cases of both finite and infinite bulk layer thicknesses (where bulk-mediated surface diffusion is normal and anomalous at asymptotically long times, respectively) on equal footing. PMID:26328814

  1. Bulk and Surface Molecular Orientation Distribution in Injection-molded Liquid Crystalline Polymers: Experiment and Simulation

    SciTech Connect

    Fang, J.; Burghardt, W; Bubeck, R; Burgard, S; Fischer, D

    2010-01-01

    Bulk and surface distributions of molecular orientation in injection-molded plaques of thermotropic liquid crystalline polymers (TLCPs) have been studied using a combination of techniques, coordinated with process simulations using the Larson-Doi 'polydomain' model. Wide-angle X-ray scattering was used to map out the bulk orientation distribution. Fourier Transform Infrared Attenuated Total Reflectance (FTIR-ATR) and Near-Edge X-ray Absorption Fine Structure (NEXAFS) were utilized to probe the molecular orientation states to within about {approx}5 {micro}m and {approx}2 nm, respectively, of the sample surface. These noninvasive, surface-sensitive techniques yield reasonable self-consistency, providing complementary validation of the robustness of these methods. An analogy between Larson-Doi and fiber orientation models has allowed the first simulations of TLCP injection molding. The simulations capture many fine details in the bulk orientation distribution across the sample plaque. Direct simulation of surface orientation at the level probed by FTIR-ATR and NEXAFS was not possible due to the limited spatial resolution of the simulations. However, simulation results extracted from the shear-dominant skin region are found to provide a qualitatively accurate indicator of surface orientation. Finally, simulations capture the relation between bulk and surface orientation states across the different regions of the sample plaque.

  2. Surface plasmon-polariton resonance at diffraction of THz radiation on semiconductor gratings

    NASA Astrophysics Data System (ADS)

    Spevak, I. S.; Kuzmenko, A. A.; Tymchenko, M.; Gavrikov, V. K.; Shulga, V. M.; Feng, J.; Sun, H. B.; Kamenev, Yu. E.; Kats, A. V.

    2016-08-01

    Resonance diffraction of THz hidrogen cyanide laser radiation on a semiconductor (InSb) grating is studied both experimentally and theoretically. The specular reflectivity suppression due to the resonance excitation of the THz surface plasmon-polariton is observed on a pure semiconductor grating and on semiconductor gratings covered with a thin dielectric layer. The dielectric coating of the grating results in the resonance shift and widening depending both on the layer thickness and dielectric properties. A simple analytical theory of the resonance diffraction on rather shallow gratings covered with a dielectric layer is presented, and the results are in a good accordance with the experimental data. Analytical expressions for the resonance shift and broadening are essential for the resonance properties understanding and useful for sensing data interpretation of the agents deposited on the grating surface.

  3. The role of relative rate constants in determining surface state phenomena at semiconductor-liquid interfaces.

    PubMed

    Iqbal, Asif; Hossain, Md Sazzad; Bevan, Kirk H

    2016-10-26

    In this work, we present a theoretical study of surface state occupation statistics at semiconductor-liquid interfaces, as it pertains to the evolution of H2 and O2 through water splitting. Our approach combines semiclassical charge transport and electrostatics at the semiconductor-liquid junction, with a master rate equation describing surface state mediated electron/hole transfer. As a model system we have studied the TiO2-water junction in the absence of illumination, where it is shown that surface states might not always equilibrate with the semiconductor. Non-trivial electrostatics, for example including a shifting of the Mott-Schottky plateau in capacitive measurements, are explored when deep-level surface states partially equilibrate with the liquid. We also endeavor to explain observations of non-linearity present in Mott-Schottky plots, as they pertain to surface state occupation statistics. In general, it is intended that the results of this work will serve to further the use and development of quantitative device modeling techniques in the description of H2 evolution at semiconductor-liquid junctions.

  4. Dynamics of Epitaxy on the Nano-sized Semiconductor Surfaces

    DTIC Science & Technology

    2006-09-05

    on the Si(100) substrates by means of lithography and anisotropic wet chemical etch. When the patterned surface consists only of low-index surfaces...structure with the flat (111) sidewalls on the Si(100) substrates by means of lithography and anisotropic wet chemical etch. When the patterned surface...photolithography and e-beam lithography followed by anisotropic wet chemical etch. The evolution of the surface structure on both the (100) top terraces and

  5. Niobium pentoxide: a promising surface-enhanced Raman scattering active semiconductor substrate

    NASA Astrophysics Data System (ADS)

    Shan, Yufeng; Zheng, Zhihui; Liu, Jianjun; Yang, Yong; Li, Zhiyuan; Huang, Zhengren; Jiang, Dongliang

    2017-03-01

    Surface-enhanced Raman scattering technique, as a powerful tool to identify the molecular species, has been severely restricted to the noble metals. The surface-enhanced Raman scattering substrates based on semiconductors would overcome the shortcomings of metal substrates and promote development of surface-enhanced Raman scattering technique in surface science, spectroscopy, and biomedicine studies. However, the detection sensitivity and enhancement effects of semiconductor substrates are suffering from their weak activities. In this work, a semiconductor based on Nb2O5 is reported as a new candidate for highly sensitive surface-enhanced Raman scattering detection of dye molecules. The largest enhancement factor value greater than 107 was observed with the laser excitation at 633 and 780 nm for methylene blue detection. As far as literature review shows, this is in the rank of the highest sensitivity among semiconductor materials; even comparable to the metal nanostructure substrates with "hot spots". The impressive surface-enhanced Raman scattering activities can be attributed to the chemical enhancement dominated by the photo-induced charge transfer, as well as the electromagnetic enhancement, which have been supported by the density-functional-theory and finite element method calculation results. The chemisorption of dye on Nb2O5 creates a new highest occupied molecular orbital and lowest unoccupied molecular orbital contributed by both fragments in the molecule-Nb2O5 system, which makes the charge transfer more feasible with longer excitation wavelength. In addition, the electromagnetic enhancement mechanism also accounts for two orders of magnitude enhancement in the overall enhancement factor value. This work has revealed Nb2O5 nanoparticles as a new semiconductor surface-enhanced Raman scattering substrate that is able to replace noble metals and shows great potentials applied in the fields of biology related.

  6. Surface Stability and Growth Kinetics of Compound Semiconductors: An Ab Initio-Based Approach

    PubMed Central

    Kangawa, Yoshihiro; Akiyama, Toru; Ito, Tomonori; Shiraishi, Kenji; Nakayama, Takashi

    2013-01-01

    We review the surface stability and growth kinetics of III-V and III-nitride semiconductors. The theoretical approach used in these studies is based on ab initio calculations and includes gas-phase free energy. With this method, we can investigate the influence of growth conditions, such as partial pressure and temperature, on the surface stability and growth kinetics. First, we examine the feasibility of this approach by comparing calculated surface phase diagrams of GaAs(001) with experimental results. In addition, the Ga diffusion length on GaAs(001) during molecular beam epitaxy is discussed. Next, this approach is systematically applied to the reconstruction, adsorption and incorporation on various nitride semiconductor surfaces. The calculated results for nitride semiconductor surface reconstructions with polar, nonpolar, and semipolar orientations suggest that adlayer reconstructions generally appear on the polar and the semipolar surfaces. However, the stable ideal surface without adsorption is found on the nonpolar surfaces because the ideal surface satisfies the electron counting rule. Finally, the stability of hydrogen and the incorporation mechanisms of Mg and C during metalorganic vapor phase epitaxy are discussed. PMID:28811438

  7. Nanochemistry at the atomic scale revealed in hydrogen-induced semiconductor surface metallization

    NASA Astrophysics Data System (ADS)

    Derycke, Vincent; Soukiassian, Patrick G.; Amy, Fabrice; Chabal, Yves J.; D'Angelo, Marie D.; Enriquez, Hanna B.; Silly, Mathieu G.

    2003-04-01

    Passivation of semiconductor surfaces against chemical attack can be achieved by terminating the surface-dangling bonds with a monovalent atom such as hydrogen. Such passivation invariably leads to the removal of all surface states in the bandgap, and thus to the termination of non-metallic surfaces. Here we report the first observation of semiconductor surface metallization induced by atomic hydrogen. This result, established by using photo-electron and photo-absorption spectroscopies and scanning tunnelling techniques, is achieved on a Si-terminated cubic silicon carbide (SiC) surface. It results from competition between hydrogen termination of surface-dangling bonds and hydrogen-generated steric hindrance below the surface. Understanding the ingredient for hydrogen-stabilized metallization directly impacts the ability to eliminate electronic defects at semiconductor interfaces critical for microelectronics, provides a means to develop electrical contacts on high-bandgap chemically passive materials, particularly for interfacing with biological systems, and gives control of surfaces for lubrication, for example of nanomechanical devices.

  8. Nanochemistry at the atomic scale revealed in hydrogen-induced semiconductor surface metallization.

    PubMed

    Derycke, Vincent; Soukiassian, Patrick G; Amy, Fabrice; Chabal, Yves J; D'angelo, Marie D; Enriquez, Hanna B; Silly, Mathieu G

    2003-04-01

    Passivation of semiconductor surfaces against chemical attack can be achieved by terminating the surface-dangling bonds with a monovalent atom such as hydrogen. Such passivation invariably leads to the removal of all surface states in the bandgap, and thus to the termination of non-metallic surfaces. Here we report the first observation of semiconductor surface metallization induced by atomic hydrogen. This result, established by using photo-electron and photo-absorption spectroscopies and scanning tunnelling techniques, is achieved on a Si-terminated cubic silicon carbide (SiC) surface. It results from competition between hydrogen termination of surface-dangling bonds and hydrogen-generated steric hindrance below the surface. Understanding the ingredient for hydrogen-stabilized metallization directly impacts the ability to eliminate electronic defects at semiconductor interfaces critical for microelectronics, provides a means to develop electrical contacts on high-bandgap chemically passive materials, particularly for interfacing with biological systems, and gives control of surfaces for lubrication, for example of nanomechanical devices.

  9. Josephson Supercurrent through the Topological Surface States of Strained Bulk HgTe

    NASA Astrophysics Data System (ADS)

    Oostinga, Jeroen B.; Maier, Luis; Schüffelgen, Peter; Knott, Daniel; Ames, Christopher; Brüne, Christoph; Tkachov, Grigory; Buhmann, Hartmut; Molenkamp, Laurens W.

    2013-04-01

    Strained bulk HgTe is a three-dimensional topological insulator, whose surface electrons have a high mobility (˜30000cm2/Vs), while its bulk is effectively free of mobile charge carriers. These properties enable a study of transport through its unconventional surface states without being hindered by a parallel bulk conductance. Here, we show transport experiments on HgTe-based Josephson junctions to investigate the appearance of the predicted Majorana states at the interface between a topological insulator and a superconductor. Interestingly, we observe a dissipationless supercurrent flow through the topological surface states of HgTe. The current-voltage characteristics are hysteretic at temperatures below 1 K, with critical supercurrents of several microamperes. Moreover, we observe a magnetic-field-induced Fraunhofer pattern of the critical supercurrent, indicating a dominant 2π-periodic Josephson effect in the unconventional surface states. Our results show that strained bulk HgTe is a promising material system to get a better understanding of the Josephson effect in topological surface states, and to search for the manifestation of zero-energy Majorana states in transport experiments.

  10. Revealing Surface States in In-Doped SnTe Nanoplates with Low Bulk Mobility.

    PubMed

    Shen, Jie; Xie, Yujun; Cha, Judy J

    2015-06-10

    Indium (In) doping in topological crystalline insulator SnTe induces superconductivity, making In-doped SnTe a candidate for a topological superconductor. SnTe nanostructures offer well-defined nanoscale morphology and high surface-to-volume ratios to enhance surface effects. Here, we study In-doped SnTe nanoplates, In(x)Sn(1-x)Te, with x ranging from 0 to 0.1 and show they superconduct. More importantly, we show that In doping reduces the bulk mobility of In(x)Sn(1-x)Te such that the surface states are revealed in magnetotransport despite the high bulk carrier density. This is manifested by two-dimensional linear magnetoresistance in high magnetic fields, which is independent of temperature up to 10 K. Aging experiments show that the linear magnetoresistance is sensitive to ambient conditions, further confirming its surface origin. We also show that the weak antilocalization observed in In(x)Sn(1-x)Te nanoplates is a bulk effect. Thus, we show that nanostructures and reducing the bulk mobility are effective strategies to reveal the surface states and test for topological superconductors.

  11. Bulk-Induced 1/f Noise at the Surface of Three-Dimensional Topological Insulators.

    PubMed

    Bhattacharyya, Semonti; Banerjee, Mitali; Nhalil, Hariharan; Islam, Saurav; Dasgupta, Chandan; Elizabeth, Suja; Ghosh, Arindam

    2015-12-22

    Slow intrinsic fluctuations of resistance, also known as the flicker noise or 1/f-noise, in the surface transport of strong topological insulators (TIs) is a poorly understood phenomenon. Here, we have systematically explored the 1/f-noise in field-effect transistors (FET) of mechanically exfoliated Bi1.6Sb0.4Te2Se TI films when transport occurs predominantly via the surface states. We find that the slow kinetics of the charge disorder within the bulk of the TI induces mobility fluctuations at the surface, providing a new source of intrinsic 1/f-noise that is unique to bulk TI systems. At small channel thickness, the noise magnitude can be extremely small, corresponding to the phenomenological Hooge parameter γH as low as ≈10(-4), but it increases rapidly when channel thickness exceeds ∼1 μm. From the temperature (T)-dependence of noise, which displayed sharp peaks at characteristic values of T, we identified generation-recombination processes from interband transitions within the TI bulk as the dominant source of the mobility fluctuations in surface transport. Our experiment not only establishes an intrinsic microscopic origin of noise in TI surface channels, but also reveals a unique spectroscopic information on the impurity bands that can be useful in bulk TI systems in general.

  12. Ultrafast photo-induced electron-transfer from coumarin dyes adsorbed on semiconductor nanoclusters surfaces

    SciTech Connect

    Castner, E.W. Jr.; Murakoshi, Kei; Yanagida, Shozo

    1997-12-31

    Photosensitizers adsorbed directly on semiconductor surfaces provide the basis for solar photoelectrochemical devices. Semiconductor nanoclusters in solution allow for simple preparation of the photosensitized surface, with very large surface area. To optimize the efficiency of the photoelectrochemical cycle, one must know the rates of forward and reverse electron-transfer from the excited state dye to the semiconductor conduction band. We present measurements of the ultrafast forward electron-transfer rates for three coumarin dyes (Coumarins 343, D-1421, and D-126) bound by the carboxylic acid group to the metal cationic sites of TiO{sub 2} and ZnO semiconductor nanocluster surfaces. These rates are obtained directly from the ultrafast fluorescence dynamics, measured using the femotosecond upconversion technique. Coumarins 343 and D-1421 on TiO{sub 2} nanoclusters in aqueous solution display very rapid forward electron-transfer with rate constants exceeding 10{sup 13} s{sup -1}. Substantially slower rates are observed for the same coumarin TiO{sub 2} systems in methanol solution.

  13. Contribution of the comonomers to the bulk and surface properties of methacrylate copolymers.

    PubMed

    Hermitte, L; Thomas, F; Bougaran, R; Martelet, C

    2004-04-01

    Relationships between formulation, bulk properties, and surface properties are investigated on series of copolymers prepared with hydroxyethylmethacrylate (HEMA), methylmethacrylate (MMA), and ethylmethacrylate (EMA) monomers, and on the homopolymers PMMA and PHEMA. The bulk water content, swelling ratio, and static (sessile drop and captive bubble) and dynamic (Wilhelmy plate technique) contact angles and the electrokinetic potential (streaming potential) are measured. The bulk water content and swelling ratio of HEMA copolymers are proportional to the amount of HEMA and are linearly correlated to the contact angle hysteresis. Periodic instabilities in the wetting cycles, similar to Haines jumps, are observed with HEMA copolymers and support a bidirectional relaxation of the hydrophilic groups respectively towards external water and capillary water. The origin of the electrokinetic potential of these nonionizable polymers is attributed to specific adsorption of [Formula: see text] ions. Its dependence on surface hydrophobicity and statistical length of the side-chains is interpreted in terms of the properties of water molecules near the interface.

  14. Predictions of point defect, surface, and interface properties in semiconductors using first-principles calculations

    SciTech Connect

    Oba, Fumiyasu

    2016-08-26

    The energetics, structures, and properties of lattice defects in semiconductors are discussed on the basis of predictions using first-principles calculations. New insights into the atomistic and electronic structure of point defects are obtained, including significant off-centering of Ti antisite defects and local octahedral rotation around O vacancies in SrTiO{sub 3}, both of which are accompanied by electron localization. Another example is the formation of a luminous dopant-vacancy complex in cubic BN. Band alignment at semiconductor surfaces and heterointerfaces is also discussed, with a focus on approximation dependence.

  15. Decoupling Bulk and Surface Contributions in Water- Splitting Photocatalysts by In Situ Ultrafast Spectroscopy

    SciTech Connect

    Appavoo, Kannatassen; Mingzhao, Liu; Black, Charles T.; Sfeir, Matthew Y.

    2015-05-10

    By performing ultrafast emission spectroscopy in an operating, bias-controlled photoelectrochemical cell, we distinguish between bulk (charge transport) and surface (chemical reaction) recombination processes in a nanostructured photocatalyst and correlate its electronic properties directly with its incident-photon-to-current efficiency.

  16. Weak localization and antilocalization in topological insulator thin films with coherent bulk-surface coupling

    NASA Astrophysics Data System (ADS)

    Garate, Ion

    2014-03-01

    This talk will review the theory and experiments concerning quantum corrections to conductivity in thin films of three dimensional topological insulators. In particular, I will discuss how the magnitude and sign of the low-field magnetoresistance are influenced by bulk-surface coupling, gate voltage, size quantization effects and magnetic order. The speaker is financially supported by Canada's NSERC.

  17. Surface and bulk-loss reduction research by low-energy hydrogen doping

    NASA Technical Reports Server (NTRS)

    Fonash, S.

    1985-01-01

    Surface and bulk loss reduction by low energy hydrogen doping of silicon solar cells was examined. Hydrogen ions provided a suppression of space charge recombination currents. Implantation of hydrogen followed by the anneal cycle caused more redistribution of boron than the anneal which could complicate processing. It was demonstrated that passivation leads to space charge current reduction.

  18. Optical find of hypersonic surface acoustic waves in bulk transparent materials

    NASA Astrophysics Data System (ADS)

    Jiménez Riobóo, Rafael J.; Sánchez-Sánchez, Alberto; Prieto, Carlos

    2016-07-01

    It is shown that direct information from surface acoustic waves (SAWs) of bulk transparent materials can be obtained by using Brillouin light scattering (BLS). The study of surface phonons by means of an optical spectroscopy such as BLS has been historically constrained to nontransparent and highly reflecting bulk and film samples or even to very thin films deposited on reflecting substrates. Probably due to its low signal and to the narrow window in experimental conditions, it was assumed for years that bulk transparent samples were not suited for Brillouin spectroscopy in order to get information on SAWs, negating this optical technique in the search for SAW properties. The reported experiments on transparent glasses and single crystals (cubic MgO and trigonal sapphire) prove that there is no intrinsic physical reason not to collect SAW propagation velocity data from transparent bulk samples and opens a challenge to apply the Brillouin spectroscopy in a wider scenario to obtain direct information, in a nondestructive and contactless way, about SAWs in bulk materials.

  19. Tailoring the Spectroscopic Properties of Semiconductor Nanowires via Surface-Plasmon-Based Optical Engineering

    PubMed Central

    2014-01-01

    Semiconductor nanowires, due to their unique electronic, optical, and chemical properties, are firmly placed at the forefront of nanotechnology research. The rich physics of semiconductor nanowire optics arises due to the enhanced light–matter interactions at the nanoscale and coupling of optical modes to electronic resonances. Furthermore, confinement of light can be taken to new extremes via coupling to the surface plasmon modes of metal nanostructures integrated with nanowires, leading to interesting physical phenomena. This Perspective will examine how the optical properties of semiconductor nanowires can be altered via their integration with highly confined plasmonic nanocavities that have resulted in properties such as orders of magnitude faster and more efficient light emission and lasing. The use of plasmonic nanocavities for tailored optical absorption will also be discussed in order to understand and engineer fundamental optical properties of these hybrid systems along with their potential for novel applications, which may not be possible with purely dielectric cavities. PMID:25396030

  20. Three-dimensional surface inspection for semiconductor components with fringe projection profilometry

    NASA Astrophysics Data System (ADS)

    Deng, Fuqin; Ding, Yi; Peng, Kai; Xi, Jiangtao; Yin, Yongkai; Zhu, Ziqi

    2016-11-01

    With the increasing integration level of components in modern electronic devices, three-dimensional automated optical inspection has been widely used in the manufacturing process of electronic and communication industries to improve the product quality. In this paper, we develop a three-dimensional inspection and metrology system for semiconductor components with fringe projection profilometry, which is composed of industry camera, telecentric lens and projection module. This system is used to measure the height, flatness, volume, shape, coplanarity for quality checking. To detect the discontinuous parts in the internal surface of semiconductor components, we employ the fringes with multiple spatial frequencies to avoid the measurement ambiguity. The complete three-dimensional information of semiconductor component is obtained by fusing the absolute phase maps from different views. The practical inspection results show that the depth resolution of our system reaches 10 μm . This system can be further embedded for the online inspection of various electronic and communication products.

  1. Surface Preparation and Deposited Gate Oxides for Gallium Nitride Based Metal Oxide Semiconductor Devices

    PubMed Central

    Long, Rathnait D.; McIntyre, Paul C.

    2012-01-01

    The literature on polar Gallium Nitride (GaN) surfaces, surface treatments and gate dielectrics relevant to metal oxide semiconductor devices is reviewed. The significance of the GaN growth technique and growth parameters on the properties of GaN epilayers, the ability to modify GaN surface properties using in situ and ex situ processes and progress on the understanding and performance of GaN metal oxide semiconductor (MOS) devices are presented and discussed. Although a reasonably consistent picture is emerging from focused studies on issues covered in each of these topics, future research can achieve a better understanding of the critical oxide-semiconductor interface by probing the connections between these topics. The challenges in analyzing defect concentrations and energies in GaN MOS gate stacks are discussed. Promising gate dielectric deposition techniques such as atomic layer deposition, which is already accepted by the semiconductor industry for silicon CMOS device fabrication, coupled with more advanced physical and electrical characterization methods will likely accelerate the pace of learning required to develop future GaN-based MOS technology.

  2. Surface Passivation for 3-5 Semiconductor Processing: Stable Gallium Sulphide Films by MOCVD

    NASA Technical Reports Server (NTRS)

    Macinnes, Andrew N.; Jenkins, Phillip P.; Power, Michael B.; Kang, Soon; Barron, Andrew R.; Hepp, Aloysius F.; Tabib-Azar, Massood

    1994-01-01

    Gallium sulphide (GaS) has been deposited on GaAs to form stable, insulating, passivating layers. Spectrally resolved photoluminescence and surface recombination velocity measurements indicate that the GaS itself can contribute a significant fraction of the photoluminescence in GaS/GaAs structures. Determination of surface recombination velocity by photoluminescence is therefore difficult. By using C-V analysis of metal-insulator-semiconductor structures, passivation of the GaAs with GaS films is quantified.

  3. Surface plasmon polariton amplification in metal-semiconductor structures.

    PubMed

    Fedyanin, Dmitry Yu; Arsenin, Aleksey V

    2011-06-20

    We propose a novel scheme of surface plasmon polariton (SPP) amplification that is based on a minority carrier injection in a Schottky diode. This scheme uses compact electrical pumping instead of bulky optical pumping. Compact size and a planar structure of the proposed amplifier allow one to utilize it in integrated plasmonic circuits and couple it easily to passive plasmonic devices. Moreover, this technique can be used to obtain surface plasmon lasing.

  4. Curved grating fabrication techniques for concentric-circle grating, surface-emitting semiconductor lasers

    NASA Technical Reports Server (NTRS)

    Jordan, Rebecca H.; King, Oliver; Wicks, Gary W.; Hall, Dennis G.; Anderson, Erik H.; Rooks, Michael J.

    1993-01-01

    We describe the fabrication and operational characteristics of a novel, surface-emitting semiconductor laser that makes use of a concentric-circle grating to both define its resonant cavity and to provide surface emission. A properly fabricated circular grating causes the laser to operate in radially inward- and outward-going circular waves in the waveguide, thus, introducing the circular symmetry needed for the laser to emit a beam with a circular cross-section. The basic circular-grating-resonator concept can be implemented in any materials system; an AlGaAs/GaAs graded-index, separate confinement heterostructure (GRINSCH), single-quantum-well (SQW) semiconductor laser, grown by molecular beam epitaxy (MBE), was used for the experiments discussed here. Each concentric-circle grating was fabricated on the surface of the AlGaAs/GaAs semiconductor laser. The circular pattern was first defined by electron-beam (e-beam) lithography in a layer of polymethylmethacrylate (PMMA) and subsequently etched into the semiconductor surface using chemically-assisted (chlorine) ion-beam etching (CAIBE). We consider issues that affect the fabrication and quality of the gratings. These issues include grating design requirements, data representation of the grating pattern, and e-beam scan method. We provide examples of how these techniques can be implemented and their impact on the resulting laser performance. A comparison is made of the results obtained using two fundamentally different electron-beam writing systems. Circular gratings with period lambda = 0.25 microns and overall diameters ranging from 80 microns to 500 microns were fabricated. We also report our successful demonstration of an optically pumped, concentric-circle grating, semiconductor laser that emits a beam with a far-field divergence angle that is less than one degree. The emission spectrum is quite narrow (less than 0.1 nm) and is centered at wavelength lambda = 0.8175 microns.

  5. Concurrence of bulk and surface order reconstruction to the relaxation of frustrated nematics

    NASA Astrophysics Data System (ADS)

    Amoddeo, Antonino

    2016-08-01

    Applying appropriate electric pulses to a nematic liquid crystal confined between plates, the bulk order reconstruction can occur, a mechanism allowing the switching between topologically different nematic textures without any director rotation. Using a moving mesh finite element method we describe the order tensor dynamics for a nematic inside an asymmetric n-cell, putting in evidence as textural distortions induced by strong asymmetries can be relaxed via both bulk and surface order reconstruction, occurring close to a confining plate with different time duration.

  6. Nanoscale surface structuring during ion bombardment of elemental semiconductors

    NASA Astrophysics Data System (ADS)

    Anzenberg, Eitan

    2013-01-01

    Nano-patterning of surfaces with uniform ion bombardment yields a rich phase-space of topographic patterns. Particle irradiation can cause surface ultra-smoothing or self-organized nanoscale pattern formation in surface topography. Topographic pattern formation has previously been attributed to the effects of the removal of target atoms by sputter erosion. In this thesis, the surface morphology evolution of Si(100) and Ge(100) during low energy ion bombardment of Ar+ and Kr+ ions, respectively, is studied. Our facilities for studies of surface processes at the National Synchrotron Light Source (NSLS) allow in-situ characterization of surface morphology evolution during ion bombardment using grazing incidence small angle x-ray scattering (GISAXS). This technique is used to measure in reciprocal space the kinetics of formation or decay of correlated nanostructures on the surface, effectively measuring the height-height correlations. A linear model is used to characterize the early time kinetic behavior during ion bombardment as a function of ion beam incidence angle. The curvature coefficients predicted by the widely used erosive model of Bradley and Harper are quantitatively negligible and of the wrong sign when compared to the observed effect in both Si and Ge. A mass-redistribution model explains the observed ultra-smoothing at low angles, exhibits an instability at higher angles, and predicts the observed 45° critical angle separating these two regimes in Si. The Ge surface evolution during Kr+ irradiation is qualitatively similar to that observed for Ar+ irradiation of Si at the same ion energy. However, the critical angle for Ge cannot be quantitatively reproduced by the simple mass redistribution model. Crater function theory, as developed by Norris et al., incorporates both mass redistributive and erosive effects, and predicts constraining relationships between curvature coefficients. These constraints are compared to experimental data of both Si and Ge

  7. First-principles investigations of III-nitride bulk and surface properties

    NASA Astrophysics Data System (ADS)

    Dreyer, Cyrus Eduard

    The III-nitride semiconductors, including AlN, InN, GaN, and BN have been demonstrated as technologically exciting materials for a wide range of device applications. With band gaps that span the visible range, GaN, InN, and InGaN alloys are used for high efficiency light emitting diodes for general lighting, as well as laser diodes for optical storage. The wide gaps, large band offsets, and polarization fields in AlN, GaN, and AlGaN alloys are promising for high-frequency, high-power transistors with applications in power conversion and radio frequency amplifiers. Despite the plethora of attractive material parameters of the III-nitride materials there are several issues that significantly limit the efficiency of devices and range of possible applications. In this study, we use first-principles electronic structure calculations to explore several of these properties relevant to understanding growth, processing, and device design. Arguably the most detrimental issue in this material system is the lack of widely available, cost-effective substrates for the growth of films and devices. Heteroepitaxy, as well as the lattice mismatch between the layers of different III-nitride alloys in heterostructures, results in residual stresses in films and devices. Such stress will alter the electronic structure of the materials, so it is necessary for device design to be able to quantify these effects. We explore the influence of strain on the effective mass of carriers in GaN and AlN, a parameter that is tied to the conductivity. In addition, films under tensile strain can crack if the strain energy is sufficient. We explore the propensity for AlN, GaN, and AlGaN to crack on different crystallographic planes. There has been significant work done to overcome the issue of residual strains in III-nitride films, both through the growth of bulk crystals for substrates, and the growth of structures such as nanowires that avoid many of the thickness and alloy-content limitations of

  8. Carbon dioxide capture by an amine functionalized ionic liquid: fundamental differences of surface and bulk behavior.

    PubMed

    Niedermaier, Inga; Bahlmann, Matthias; Papp, Christian; Kolbeck, Claudia; Wei, Wei; Krick Calderón, Sandra; Grabau, Mathias; Schulz, Peter S; Wasserscheid, Peter; Steinrück, Hans-Peter; Maier, Florian

    2014-01-08

    Carbon dioxide (CO2) absorption by the amine-functionalized ionic liquid (IL) dihydroxyethyldimethylammonium taurinate at 310 K was studied using surface- and bulk-sensitive experimental techniques. From near-ambient pressure X-ray photoelectron spectroscopy at 0.9 mbar CO2, the amount of captured CO2 per mole of IL in the near-surface region is quantified to ~0.58 mol, with ~0.15 mol in form of carbamate dianions and ~0.43 mol in form of carbamic acid. From isothermal uptake experiments combined with infrared spectroscopy, CO2 is found to be bound in the bulk as carbamate (with nominally 0.5 mol of CO2 bound per 1 mol of IL) up to ~2.5 bar CO2, and as carbamic acid (with nominally 1 mol CO2 bound per 1 mol IL) at higher pressures. We attribute the fact that at low pressures carbamic acid is the dominating species in the near-surface region, while only carbamate is formed in the bulk, to differences in solvation in the outermost IL layers as compared to the bulk situation.

  9. Low temperature dynamics of surface and bulk electronic structure of quantum dots

    NASA Astrophysics Data System (ADS)

    Krishnamurthy Grandhi, G.; Tomar, Renu; Viswanatha, Ranjani

    2017-09-01

    Absolute energies of band edges have proven to be very important for various applications like hydrogen generation, solar water splitting and solar cell optimization. Energy differences as small as 50–100 meV have been shown to largely affect device efficiencies. Device operational temperature can vary largely and temperature dependence of band gap is well known in bulk semiconductor literature. However, there are only a few studies on variation of band gap in quantum dots and none of them characterize the relative energy variation of band edges in spite of their importance in various applications. This is mainly due to the absence of an internal standard that can be used to study the variation of band edges. Here, in this paper, we introduce a technique wherein we utilize Cu dopant emission as an internal probe. Using this technique, we report the variation of band gap, conduction band and valence band edges of CdS and CdSe quantum dots as a function of temperature and size. We found that band gap variation is similar to that of bulk but with a higher average phonon energy. The band edge variation is characterized by a dominant conduction band shift for larger sizes with decreasing temperature while the smaller size QDs show the variation in both conduction band and valence band. Further, we have also utilized this method to study the binding energy of the trap states as a function of temperature using Cu photoluminescence quantum yield and average lifetime of Cu photoluminescence.

  10. Near-Surface Electronic Contribution to Semiconductor Elasticity

    NASA Astrophysics Data System (ADS)

    Lin, J. T.; Shuvra, P. D.; McNamara, S.; Gong, H.; Liao, W.; Davidson, J. L.; Walsh, K. M.; Alles, M. L.; Alphenaar, B. W.

    2017-09-01

    The influence of the carrier concentration on the elasticity is measured for a microscale silicon resonator. UV radiation is used to generate a surface charge that gates the underlying carrier concentration, as indicated by the device resistance. Correlated with the carrier concentration change is a drop in the resonant frequency that persists for 60 h following exposure. Model calculations show that the change in resonant frequency is due to the modification of the elastic modulus in the near-surface region. This effect becomes increasingly important as device dimensions are reduced to the nanometer scale, and contributes an important source of instability for microscale and nanoscale electromechanical devices operating in radiation environments.

  11. Chemical surface deposition of ultra-thin semiconductors

    DOEpatents

    McCandless, Brian E.; Shafarman, William N.

    2003-03-25

    A chemical surface deposition process for forming an ultra-thin semiconducting film of Group IIB-VIA compounds onto a substrate. This process eliminates particulates formed by homogeneous reactions in bath, dramatically increases the utilization of Group IIB species, and results in the formation of a dense, adherent film for thin film solar cells. The process involves applying a pre-mixed liquid coating composition containing Group IIB and Group VIA ionic species onto a preheated substrate. Heat from the substrate causes a heterogeneous reaction between the Group IIB and VIA ionic species of the liquid coating composition, thus forming a solid reaction product film on the substrate surface.

  12. Bulk transport measurements in ZnO: The effect of surface electron layers

    NASA Astrophysics Data System (ADS)

    Allen, M. W.; Swartz, C. H.; Myers, T. H.; Veal, T. D.; McConville, C. F.; Durbin, S. M.

    2010-02-01

    Magnetotransport measurements and x-ray photoemission spectroscopy were used to investigate the surface conductivity of ZnO. Near-surface downward band bending, consistent with electron accumulation, was found on the polar and nonpolar faces of bulk ZnO single crystals. A significant polarity effect was observed in that the downward band bending was consistently stronger on the Zn-polar face and weaker on the O-polar face. The surface electron accumulation layer was found to significantly influence the electrical properties of high resistivity, hydrothermally grown bulk ZnO crystals at temperatures below 200 K, and is largely responsible for the anomalously low electron mobility reported for this material.

  13. Study of the ink-paper interaction by image analysis: surface and bulk inspection

    NASA Astrophysics Data System (ADS)

    Fiadeiro, Paulo T.; de O. Mendes, António; M. Ramos, Ana M.; L. de Sousa, Sónia C.

    2013-11-01

    In this work, two optical systems previously designed and implemented by our research team, were used to enable the surface and bulk inspection of the ink-paper interaction by image analysis. Basically, the first system works by ejecting micro-liter ink drops onto the papers surface while monitoring the event under three different views over time. The second system is used for sectioning the paper samples through their thickness and to simultaneously acquire images of the ink penetration of each section cut. In the performed experiments, three black inks of different brands and a common copy paper were chosen, used, and tested with the two developed optical systems. Both qualitative and quantitative analyses were carried out at the surface level and in the bulk of the paper. In terms of conclusions, it was shown that the three tested ink-paper combinations revealed very distinct characteristics.

  14. PLASMA TREATMENT OF BULK Nb SURFACE IN THE Ar/Cl2 DISCHARGE

    SciTech Connect

    Marija Raskovic; H. Phillips; Anne-Marie Valente

    2008-02-12

    The preparation of the cavity walls has been one of the major challenges in the superconducting radio-frequency (SRF) accelerator technology. Therefore, constant research and development effort is devoted to develop surface preparation processes that will improve roughness and lower the level of impurities, like hydrogen or oxygen, embedded in bulk Nb, having in the same time reasonable etching rates. Plasma based surface modification provides an excellent opportunity to achieve these goals. We present Ar/Cl2 discharge treatment of bulk Nb where we achieved etching rates comparable to the rates obtained with the electropolishing method without introducing impurities in Nb. The current experiments were performed on disk shaped Nb samples, exposed to plasma produced in a microwave discharge system. Surface composition and topology measurements were carried out before and after plasma treatment. Upon determining optimal experimental conditions on disk shaped samples, we will apply the same procedure on the single cell cavities, pursuing improvement of their RF performance.

  15. Vapor-Liquid-Solid Etch of Semiconductor Surface Channels by Running Gold Nanodroplets

    PubMed Central

    Nikoobakht, Babak; Herzing, Andrew; Muramoto, Shin; Tersoff, Jerry

    2016-01-01

    We show that Au nanoparticles spontaneously move across the (001) surface of InP, InAs, and GaP when heated in the presence of water vapor. As they move, the particles etch crystallographically aligned grooves into the surface. We show that this process is a negative analog of the vapor-liquid-solid (VLS) growth of semiconductor nanowires: semiconductor dissolves into the catalyst, and reacts with water vapor at the catalyst surface to create volatile oxides, depleting the dissolved cations and anions and so sustaining the dissolution process. This VLS etching process provides a new tool for directed assembly of structures with sub-lithographic dimensions, as small as a few nanometers in diameter. Au particles above 100 nm in size do not exhibit this process but remain stationary, with oxide accumulating around the particles. PMID:26599639

  16. Surface plasmon polariton assisted red shift in excitonic emission of semiconductor microflowers

    NASA Astrophysics Data System (ADS)

    Parameswaran, Chithra; Warrier, Anita R.; Bingi, Jayachandra; Vijayan, C.

    2014-10-01

    We report on the study of metal nanoparticle-semiconductor hybrid system composed of β-indium sulfide (β-In2S3) and gold (Au) nanoparticles. β-In2S3 micron sized flower like structures (˜1 μm) and Au nanoparticles (˜10 nm) were synthesized by chemical route. These Au nanoparticles have surface plasmon resonance at ˜ 520 nm. We study the influence of Au surface plasmon polaritons on the radiative properties of the β-In2S3 microflowers. As a result of the coupling between the surface plasmon polaritons and the excitons there is a red shift ˜ 50 nm in emission spectrum of hybrid β-In2S3-Au system. Such hybrid systems provide scope for a control on the optical properties of semiconductor microstructures, thus rendering them suitable for specific device applications in optoelectronics and photovoltaics.

  17. Surface plasmon polariton assisted red shift in excitonic emission of semiconductor microflowers

    SciTech Connect

    Parameswaran, Chithra; Warrier, Anita R. Bingi, Jayachandra Vijayan, C.

    2014-10-15

    We report on the study of metal nanoparticle-semiconductor hybrid system composed of β-indium sulfide (β-In{sub 2}S{sub 3}) and gold (Au) nanoparticles. β-In{sub 2}S{sub 3} micron sized flower like structures (∼1 μm) and Au nanoparticles (∼10 nm) were synthesized by chemical route. These Au nanoparticles have surface plasmon resonance at ∼ 520 nm. We study the influence of Au surface plasmon polaritons on the radiative properties of the β-In{sub 2}S{sub 3} microflowers. As a result of the coupling between the surface plasmon polaritons and the excitons there is a red shift ∼ 50 nm in emission spectrum of hybrid β-In{sub 2}S{sub 3}-Au system. Such hybrid systems provide scope for a control on the optical properties of semiconductor microstructures, thus rendering them suitable for specific device applications in optoelectronics and photovoltaics.

  18. Ice-nucleating particles in Canadian Arctic sea-surface microlayer and bulk seawater

    NASA Astrophysics Data System (ADS)

    Irish, Victoria E.; Elizondo, Pablo; Chen, Jessie; Chou, Cédric; Charette, Joannie; Lizotte, Martine; Ladino, Luis A.; Wilson, Theodore W.; Gosselin, Michel; Murray, Benjamin J.; Polishchuk, Elena; Abbatt, Jonathan P. D.; Miller, Lisa A.; Bertram, Allan K.

    2017-09-01

    The sea-surface microlayer and bulk seawater can contain ice-nucleating particles (INPs) and these INPs can be emitted into the atmosphere. Our current understanding of the properties, concentrations, and spatial and temporal distributions of INPs in the microlayer and bulk seawater is limited. In this study we investigate the concentrations and properties of INPs in microlayer and bulk seawater samples collected in the Canadian Arctic during the summer of 2014. INPs were ubiquitous in the microlayer and bulk seawater with freezing temperatures in the immersion mode as high as -14 °C. A strong negative correlation (R = -0. 7, p = 0. 02) was observed between salinity and freezing temperatures (after correction for freezing depression by the salts). One possible explanation is that INPs were associated with melting sea ice. Heat and filtration treatments of the samples show that the INPs were likely heat-labile biological materials with sizes between 0.02 and 0.2 µm in diameter, consistent with previous measurements off the coast of North America and near Greenland in the Arctic. The concentrations of INPs in the microlayer and bulk seawater were consistent with previous measurements at several other locations off the coast of North America. However, our average microlayer concentration was lower than previous observations made near Greenland in the Arctic. This difference could not be explained by chlorophyll a concentrations derived from satellite measurements. In addition, previous studies found significant INP enrichment in the microlayer, relative to bulk seawater, which we did not observe in this study. While further studies are needed to understand these differences, we confirm that there is a source of INP in the microlayer and bulk seawater in the Canadian Arctic that may be important for atmospheric INP concentrations.

  19. Surface recombination velocity at n-CdSe semiconductor-electrolyte interface

    NASA Astrophysics Data System (ADS)

    Bessler-Podorowski, Pnina

    1989-06-01

    The influence of the concentration of divers electrolyte solutions on the surface recombination velocity of n-CdSe immersed in them was investigated. The semiconductor was irradiated with a 25-psec, 532-nanometer laser pulse and photoluminescence decays from the crystal surface were measured. Surface recombination velocities were evaluated by fitting the experimental decay curves to simulated decay curves. A crystal which had been chemically etched showed a significantly longer photoluminescence decay time than a mechanically polished one, when immersed in distilled water, and a surface recombination velocity of less than 1,000 cm/sec. The surface recombination velocity increased when the crystal was immersed in aqueous solutions of CuSO4, NaI, NaI/I2, AgNO3 and HgCl2, and methanol solutions of bromide. The velocity increased with solution concentration, to a maximum of 100,000-1,000,000 cm/sec. Immersion in aqueous solutions of LiCl and Cr(NO3)3 induced recombination velocity increases, but only at high concentrations; these changes were attributed to impurities. In aqueous ZnSO4 solutions, there was no velocity change until the concentration reached 0.01M, when the velocity decreased. The surface recombination velocity increases are caused by surface states which result from intrinsic states of the crystal, unsaturated bonds on the surface, defects, or adsorption of ions from the solution. Only molecules having a negative free energy of absorption can be adsorbed, and the solubility product of the adsorbed cation with Se, the semiconductor anion, must be lower than the solubility product of the semiconductor, CdSe. When an anion is adsorbed, the solubility product of the adsorbed anion with Cd(2+), the semiconductor cation, must be lower than the solubility product of CdSe. Thus, cations which form selenide salts or anions which form cadmium salts in which the solubility products are higher than the solubility product of CdSe would not be adsorbed on the Cd

  20. Atomic-level imaging, processing and characterization of semiconductor surfaces

    DOEpatents

    Kazmerski, L.L.

    1995-08-22

    A method for selecting and removing single specific atoms from a solid material surface uses photon biasing to break down bonds that hold the selected atom in the lattice and to reduce barrier effects that hold the atom from transferring to a probe. The photon bias is preferably light or other electromagnetic radiation with a wavelength and frequency that approximately matches the wave function of the target atom species to be removed to induce high energy, selective thermionic-like vibration. An electric field potential is then applied between the probe and the surface of the solid material to pull the atom out of the lattice and to transfer the atom to the probe. Different extrinsic atoms can be installed in the lattice sites that are vacated by the removed atoms by using a photon bias that resonates the extrinsic atom species, reversing polarity of the electric field, and blowing gas comprising the extrinsic atoms through a hollow catheter probe. 8 figs.

  1. Atomic-level imaging, processing and characterization of semiconductor surfaces

    DOEpatents

    Kazmerski, Lawrence L.

    1995-01-01

    A method for selecting and removing single specific atoms from a solid material surface uses photon biasing to break down bonds that hold the selected atom in the lattice and to reduce barrier effects that hold the atom from transferring to a probe. The photon bias is preferably light or other electromagnetic radiation with a wavelength and frequency that approximately matches the wave function of the target atom species to be removed to induce high energy, selective thermionic-like vibration. An electric field potential is then applied between the probe and the surface of the solid material to pull the atom out of the lattice and to transfer the atom to the probe. Different extrinsic atoms can be installed in the lattice sites that are vacated by the removed atoms by using a photon bias that resonates the extrinsic atom species, reversing polarity of the electric field, and blowing gas comprising the extrinsic atoms through a hollow catheter probe.

  2. Picosecond Laser Pulse Interactions with Metallic and Semiconductor Surfaces.

    DTIC Science & Technology

    1984-11-01

    between the hot, dense carrier plasma and the lattice vibrations of less than two picoseconds. A careful observation and analysis of the photoelectric...photoelectric effect, an emission proportional to the square of the intensity. This is due to photoemis- Ssion from the hot, dense carrier plasma created...by the same pulse, or by a preceding picosecond pulse. The lifetime of this plasma surface layer is lim- ited by diffusion of the hot carriers out of

  3. Bulk-surface relationship of an electronic structure for high-throughput screening of metal oxide catalysts

    NASA Astrophysics Data System (ADS)

    Kweun, Joshua Minwoo; Li, Chenzhe; Zheng, Yongping; Cho, Maenghyo; Kim, Yoon Young; Cho, Kyeongjae

    2016-05-01

    Designing metal-oxides consisting of earth-abundant elements has been a crucial issue to replace precious metal catalysts. To achieve efficient screening of metal-oxide catalysts via bulk descriptors rather than surface descriptors, we investigated the relationship between the electronic structure of bulk and that of the surface for lanthanum-based perovskite oxides, LaMO3 (M = Ti, V, Cr, Mn, Fe, Co, Ni, Cu). Through density functional theory calculations, we examined the d-band occupancy of the bulk and surface transition-metal atoms (nBulk and nSurf) and the adsorption energy of an oxygen atom (Eads) on (001), (110), and (111) surfaces. For the (001) surface, we observed strong correlation between the nBulk and nSurf with an R-squared value over 94%, and the result was interpreted in terms of ligand field splitting and antibonding/bonding level splitting. Moreover, the Eads on the surfaces was highly correlated with the nBulk with an R-squared value of more than 94%, and different surface relaxations could be explained by the bulk electronic structure (e.g., LaMnO3 vs. LaTiO3). These results suggest that a bulk-derived descriptor such as nBulk can be used to screen metal-oxide catalysts.

  4. Coupled instabilities of surface crease and bulk bending during fast free swelling of hydrogels.

    PubMed

    Takahashi, Riku; Ikura, Yumihiko; King, Daniel R; Nonoyama, Takayuki; Nakajima, Tasuku; Kurokawa, Takayuki; Kuroda, Hirotoshi; Tonegawa, Yoshihiro; Gong, Jian Ping

    2016-06-21

    Most studies on hydrogel swelling instability have been focused on a constrained boundary condition. In this paper, we studied the mechanical instability of a piece of disc-shaped hydrogel during free swelling. The fast swelling of the gel induces two swelling mismatches; a surface-inner layer mismatch and an annulus-disc mismatch, which lead to the formation of a surface crease pattern and a saddle-like bulk bending, respectively. For the first time, a stripe-like surface crease that is at a right angle on the two surfaces of the gel was observed. This stripe pattern is related to the mechanical coupling of surface instability and bulk bending, which is justified by investigating the swelling-induced surface pattern on thin hydrogel sheets fixed onto a saddle-shaped substrate prior to swelling. A theoretical mechanism based on an energy model was developed to show an anisotropic stripe-like surface crease pattern on a saddle-shaped surface. These results might be helpful to develop novel strategies for controlling crease patterns on soft and wet materials by changing their three-dimensional shape.

  5. Tuning the reactivity of semiconductor surfaces by functionalization with amines of different basicity

    PubMed Central

    Bent, Stacey F.; Kachian, Jessica S.; Rodríguez-Reyes, Juan Carlos F.; Teplyakov, Andrew V.

    2011-01-01

    Surface functionalization of semiconductors has been the backbone of the newest developments in microelectronics, energy conversion, sensing device design, and many other fields of science and technology. Over a decade ago, the notion of viewing the surface itself as a chemical reagent in surface reactions was introduced, and adding a variety of new functionalities to the semiconductor surface has become a target of research for many groups. The electronic effects on the substrate have been considered as an important consequence of chemical modification. In this work, we shift the focus to the electronic properties of the functional groups attached to the surface and their role on subsequent reactivity. We investigate surface functionalization of clean Si(100)-2 × 1 and Ge(100)-2 × 1 surfaces with amines as a way to modify their reactivity and to fine tune this reactivity by considering the basicity of the attached functionality. The reactivity of silicon and germanium surfaces modified with ethylamine (CH3CH2NH2) and aniline (C6H5NH2) is predicted using density functional theory calculations of proton attachment to the nitrogen of the adsorbed amine to differ with respect to a nucleophilic attack of the surface species. These predictions are then tested using a model metalorganic reagent, tetrakis(dimethylamido)titanium (((CH3)2N)4Ti, TDMAT), which undergoes a transamination reaction with sufficiently nucleophilic amines, and the reactivity tests confirm trends consistent with predicted basicities. The identity of the underlying semiconductor surface has a profound effect on the outcome of this reaction, and results comparing silicon and germanium are discussed. PMID:21068370

  6. Bulk electrical properties of single-walled carbon nanotubes immobilized by dielectrophoresis: evidence of metallic or semiconductor behavior.

    PubMed

    Mureau, Natacha; Watts, Paul C P; Tison, Yann; Silva, S Ravi P

    2008-06-01

    We report the electrical characterization of single-walled carbon nanotubes (SWCNTs) trapped between two electrodes by dielectrophoresis (DEP). At high frequency, SWCNTs collected by DEP are expected to be of metallic type. Indeed current-voltage (I-V) measurements for devices made at 10 MHz show high values of conductivity and exhibit metallic behavior with linear and symmetric electrical features attributed to ohmic conduction. At low frequency, SWCNTs attracted by DEP are expected to be of semiconducting nature. Devices made at 10 kHz behave as semiconductors and demonstrate nonlinear and rectifying electrical characteristics with conductivities many orders of magnitude below the sample resulting from high-frequency immobilization of SWCNTs. Conducting atomic force microscopy (C-AFM) and current density calculation results are presented to reinforce results obtained by I-V measurements which clearly show type separation of SWCNTs after DEP experiments.

  7. Improving the Performance of Semiconductor Sensor Devices Using Surface Functionalization

    NASA Astrophysics Data System (ADS)

    Rohrbaugh, Nathaniel W.

    As production and understanding of III-nitride growth has progressed, this class of material has been used for its semiconducting properties in the fields of computer processing, microelectronics, and LEDs. As understanding of materials properties has advanced, devices were fabricated to be sensitive to environmental surroundings such as pH, gas, or ionic concentration. Simultaneously the world of pharmaceuticals and environmental science has come to the age where the use of wearable devices and active environmental sensing can not only help us learn more about our surroundings, but help save lives. At the crossroads of these two fields work has been done in marrying the high stability and electrical properties of the III-nitrides with the needs of a growing sensor field for various environments and stimuli. Device architecture can only get one so far, and thus the need for well understood surface functionalization techniques has arisen in the field of III-nitride environmental sensing. Many existing schemes for functionalization involve chemistries that may be unfriendly to a biological environment, unstable in solution, or expensive to produce. One possible solution to these issues is the work presented here, which highlights a surface modification scheme utilizing phosphonic acid based chemistry and biomolecular attachment. This dissertation presents a set of studies and experiments quantifying and analyzing the response behaviors of AlGaN/GaN field effect transistor (FET) devices via their interfacial electronic properties. Additional investigation was done on the modification of these surfaces, effects of stressful environmental conditions, and the utility of the phosphonic acid surface treatments. Signals of AlGaN/GaN FETs were measured as IDrain values and in the earliest study an average signal increase of 96.43% was observed when surfaces were incubated in a solution of a known recognition peptide sequence (SVSVGMKPSPRP). This work showed that even without

  8. Novel Surface Preparation and Contacts for CdZnTe Nuclear Radiation Detectors Using Patterned Films of Semiconductors and Insulators

    NASA Astrophysics Data System (ADS)

    Burger, Arnold; Groza, Michael; Conway, Adam; Payne, Steve

    2013-04-01

    The semiconductor Cadmium Zinc Telluride (CZT) has emerged as the material of choice for room temperature detection of X-rays and gamma-rays. The detectors will cover the energy range from 30 keV to several MeV, and will achieve excellent 662 keV energy resolution. The development of high resolution gamma ray detectors based on CZT is dependent on low electronic noise levels. One common source of noise is the surface leakage current, which limits the performance of advanced readout schemes such as the coplanar grid and pixelated architectures with steering grids. Excessive bulk leakage current can result from one of several surface effects: leaky native oxides, unsatisfied bonds, and surface damage. We propose to fabricate and test oriented [111] CZT crystals with thicknesses up to 1.5 cm with an innovative detection technique based on co-planar or other electron only transport designs using plasma processing, thin film sputtering, chemical passivation and wet etching techniques. Compared to conventional pixel detectors, the proposed contact configuration needs lower power consumption and a lower cost. The detector design can be used for building very low-cost handheld radiation detection devices.

  9. Direct laser fabrication of nanowires on semiconductor surfaces

    NASA Astrophysics Data System (ADS)

    Haghizadeh, Anahita; Yang, Haeyeon

    2016-03-01

    Periodic nanowires are observed from (001) orientation of Si and GaAs when the surfaces are irradiated interferentially by high power laser pulses. These nanowires are self-assembled and can be strain-free while their period is consistent with interference period. The nanowire morphologies are studied by atomic force microscopy. The observed period between nanowires depends on the wavelengths used and interference angle. The nanowire width increases with laser intensity. The narrowest nanowires observed have the width smaller than 20 nm, which is more than 10 times smaller than the interference period.

  10. Combining Bulk/Surface Engineering of Hematite To Synergistically Improve Its Photoelectrochemical Water Splitting Performance.

    PubMed

    Yuan, Yufei; Gu, Jiuwang; Ye, Kai-Hang; Chai, Zhisheng; Yu, Xiang; Chen, Xiaobo; Zhao, Chuanxi; Zhang, Yuanming; Mai, Wenjie

    2016-06-29

    One of the most promising candidates for photoelectrochemical (PEC) water splitting photoanode is hematite (α-Fe2O3) due to its narrow bandgap and chemical stability. However, the poor bulk/surface kinetics of hematite limits its PEC performance. Herein, a facile two-step approach is reported to synergistically improve the PEC performance of Fe2O3. First, through bulk engineering of Ti doping, the photocurrent density of Ti-Fe2O3 photoanode (1.68 mA cm(-2) at 1.23 VRHE) shows a 3-fold increase compared with that of pure Fe2O3 photoanode (0.50 mA cm(-2) at 1.23 VRHE). Second, the photocurrent density of Ti-Fe2O3 photoanode could be further enhanced to 2.31 mA cm(-2) by surface engineering of FeOOH. The enhanced PEC water splitting performance is proposed to be the synergistic effect of bulk and surface engineering, which can be mainly attributed to the great increase of charge separation efficiency and surface transfer efficiency.

  11. Laser Measurement of SAM Bulk and Surface Wave Amplitudes for Material Microstructure Analysis

    SciTech Connect

    Ken L. Telschow; Chiaki Miyasaka; David L. Cottle

    2005-07-01

    Scanning Acoustic Microscopy (SAM) at ultra high frequencies has proven to be a useful tool for investigating materials on the scale of individual grains. This technique is normally performed in a reflection mode from one side of a sample surface. Information about the generation and transmission of bulk acoustic waves into the material is inferred from the reflection signal amplitude. We present an adaptation to the SAM method whereby the acoustic bulk waves are directly visualized through laser acoustic detection. Ultrasonic waves were emitted from a nominal 200 MHz point focus acoustic lens into a thin silicon plate (thickness 75ìm) coupled with distilled water. A scanned laser beam detected the bulk and surface acoustic waves at the opposite surface of the thin silicon plate. Distinct amplitude patterns exhibiting the expected symmetry for Silicon were observed that alter in predictable ways as the acoustic focal point was moved throughout the plate. Predictions of the acoustic wave fields generated by the acoustic lens within and at the surface of the Silicon are being investigated through the angular spectrum of plane waves approach. Results shall be presented for plates with (100) and (111) orientations followed by discussion of applications of the technique for material microstructure analysis.

  12. Electronic structure of reconstructed InAs(001) surfaces - identification of bulk and surface bands based on their symmetries

    NASA Astrophysics Data System (ADS)

    Olszowska, Natalia; Kolodziej, Jacek J.

    2016-02-01

    Using angle-resolved photoelectron spectroscopy (ARPES) band structures of indium- and arsenic-terminated InAs(001) surfaces are investigated. These surfaces are highly reconstructed, elementary cells of their lattices contain many atoms in different chemical configurations, and moreover, they are composed of domains having related but different reconstructions. These domain-type surface reconstructions result in the reciprocal spaces containing regions with well-defined k→∥-vector and regions with not-well-defined one. In the ARPES spectra most of the surface related features appear as straight lines in the indeterminate k→∥-vector space. It is shown that, thanks to differences in crystal and surface symmetries, the single photon energy ARPES may be successfully used for classification of surface and bulk bands of electronic states on complex, highly reconstructed surfaces instead of the most often used variable photon energy studies.

  13. Semiconductor surface emitting lasers for photon pairs generation

    NASA Astrophysics Data System (ADS)

    Vanbever, Luc R.; Karpov, Evgueni; Panajotov, Krassimir

    2017-10-01

    We study the feasibility of generating photon pairs in a resonant Vertical-Cavity Surface-Emitting Laser (VCSEL) as a result of a third-order non-linear, four wave mixing interaction. We focus on degenerate four wave mixing in the spontaneous regime where two pump photons are annihilated to create a pair of signal and idler photons. Using the methods of quantum optics, we calculate the two-photon production rate, the spectrum of the generated photons, and the signal-idler cross-correlations. We highlight how the dispersion of the medium in the VCSEL cavity (a regular GaAs configuration) significantly diminishes the two-photon production rate. Based on our results, we enumerate the characteristics of a VCSEL that would be suitable for photon pair generation.

  14. Surface-state spin textures in strained bulk HgTe: Strain-induced topological phase transitions

    NASA Astrophysics Data System (ADS)

    Kirtschig, Frank; van den Brink, Jeroen; Ortix, Carmine

    2016-12-01

    The opening of a band gap due to compressive uniaxial strain renders bulk HgTe a strong three-dimensional topological insulator with protected gapless surface states at any surface. By employing a six-band k .p model, we determine the spin textures of the topological surface states of bulk HgTe uniaxially strained along the (100 ) direction. We show that at the (010 ) and (001 ) surfaces, an increase in the strain magnitude triggers a topological phase transition where the winding number of the surface-state spin texture is flipped while the four topological invariants characterizing the bulk band structure of the material are unchanged.

  15. Bulk anisotropic excitons in type-II semiconductors built with 1D and 2D low-dimensional structures

    NASA Astrophysics Data System (ADS)

    Coyotecatl, H. A.; Del Castillo-Mussot, M.; Reyes, J. A.; Vazquez, G. J.; Montemayor-Aldrete, J. A.; Reyes-Esqueda, J. A.; Cocoletzi, G. H.

    2005-08-01

    We used a simple variational approach to account for the difference in the electron and hole effective masses in Wannier-Mott excitons in type-II semiconducting heterostructures in which the electron is constrained in an one-dimensional quantum wire (1DQW) and the hole is in a two-dimensional quantum layer (2DQL) perpendicular to the wire or viceversa. The resulting Schrodinger equation is similar to that of a 3D bulk exciton because the number of free (nonconfined) variables is three; two coming from the 2DQL and one from the 1DQW. In this system the effective electron-hole interaction depends on the confinement potentials.

  16. Metal-oxide-semiconductor field effect transistor humidity sensor using surface conductance

    NASA Astrophysics Data System (ADS)

    Song, Seok-Ho; Yang, Hyun-Ho; Han, Chang-Hoon; Ko, Seung-Deok; Lee, Seok-Hee; Yoon, Jun-Bo

    2012-03-01

    This letter presents a metal-oxide-semiconductor field effect transistor based humidity sensor which does not use any specific materials to sense the relative humidity. We simply make use of the low pressure chemical vapor deposited (LPCVD) silicon dioxide's surface conductance change. When the gate is biased and then floated, the electrical charge in the gate is dissipated through the LPCVD silicon dioxide's surface to the surrounding ground with a time constant depending on the surface conductance which, in turn, varies with humidity. With this method, extremely high sensitivity was achieved—the charge dissipation speed increased thousand times as the relative humidity increased.

  17. Surface and bulk modified high capacity layered oxide cathodes with low irreversible capacity loss

    DOEpatents

    Manthiram, Arumugam; Wu, Yan

    2010-03-16

    The present invention includes compositions, surface and bulk modifications, and methods of making of (1-x)Li[Li.sub.1/3Mn.sub.2/3]O.sub.2.xLi[Mn.sub.0.5-yNi.sub.0.5-yCo.sub.2- y]O.sub.2 cathode materials having an O3 crystal structure with a x value between 0 and 1 and y value between 0 and 0.5, reducing the irreversible capacity loss in the first cycle by surface modification with oxides and bulk modification with cationic and anionic substitutions, and increasing the reversible capacity to close to the theoretical value of insertion/extraction of one lithium per transition metal ion (250-300 mAh/g).

  18. Glass formation, chemical properties and surface analysis of Cu-based bulk metallic glasses.

    PubMed

    Qin, Chunling; Zhao, Weimin; Inoue, Akihisa

    2011-01-01

    This paper reviews the influence of alloying elements Mo, Nb, Ta and Ni on glass formation and corrosion resistance of Cu-based bulk metallic glasses (BMGs). In order to obtain basic knowledge for application to the industry, corrosion resistance of the Cu-Hf-Ti-(Mo, Nb, Ta, Ni) and Cu-Zr-Ag-Al-(Nb) bulk glassy alloy systems in various solutions are reported in this work. Moreover, X-ray photoelectron spectroscopy (XPS) analysis is performed to clarify the surface-related chemical characteristics of the alloy before and after immersion in the solutions; this has lead to a better understanding of the correlation between the surface composition and the corrosion resistance.

  19. Imaging of surface spin textures on bulk crystals by scanning electron microscopy

    PubMed Central

    Akamine, Hiroshi; Okumura, So; Farjami, Sahar; Murakami, Yasukazu; Nishida, Minoru

    2016-01-01

    Direct observation of magnetic microstructures is vital for advancing spintronics and other technologies. Here we report a method for imaging surface domain structures on bulk samples by scanning electron microscopy (SEM). Complex magnetic domains, referred to as the maze state in CoPt/FePt alloys, were observed at a spatial resolution of less than 100 nm by using an in-lens annular detector. The method allows for imaging almost all the domain walls in the mazy structure, whereas the visualisation of the domain walls with the classical SEM method was limited. Our method provides a simple way to analyse surface domain structures in the bulk state that can be used in combination with SEM functions such as orientation or composition analysis. Thus, the method extends applications of SEM-based magnetic imaging, and is promising for resolving various problems at the forefront of fields including physics, magnetics, materials science, engineering, and chemistry. PMID:27872493

  20. Imaging of surface spin textures on bulk crystals by scanning electron microscopy.

    PubMed

    Akamine, Hiroshi; Okumura, So; Farjami, Sahar; Murakami, Yasukazu; Nishida, Minoru

    2016-11-22

    Direct observation of magnetic microstructures is vital for advancing spintronics and other technologies. Here we report a method for imaging surface domain structures on bulk samples by scanning electron microscopy (SEM). Complex magnetic domains, referred to as the maze state in CoPt/FePt alloys, were observed at a spatial resolution of less than 100 nm by using an in-lens annular detector. The method allows for imaging almost all the domain walls in the mazy structure, whereas the visualisation of the domain walls with the classical SEM method was limited. Our method provides a simple way to analyse surface domain structures in the bulk state that can be used in combination with SEM functions such as orientation or composition analysis. Thus, the method extends applications of SEM-based magnetic imaging, and is promising for resolving various problems at the forefront of fields including physics, magnetics, materials science, engineering, and chemistry.

  1. Glass Formation, Chemical Properties and Surface Analysis of Cu-Based Bulk Metallic Glasses

    PubMed Central

    Qin, Chunling; Zhao, Weimin; Inoue, Akihisa

    2011-01-01

    This paper reviews the influence of alloying elements Mo, Nb, Ta and Ni on glass formation and corrosion resistance of Cu-based bulk metallic glasses (BMGs). In order to obtain basic knowledge for application to the industry, corrosion resistance of the Cu–Hf–Ti–(Mo, Nb, Ta, Ni) and Cu–Zr–Ag–Al–(Nb) bulk glassy alloy systems in various solutions are reported in this work. Moreover, X-ray photoelectron spectroscopy (XPS) analysis is performed to clarify the surface-related chemical characteristics of the alloy before and after immersion in the solutions; this has lead to a better understanding of the correlation between the surface composition and the corrosion resistance. PMID:21731441

  2. Imaging of surface spin textures on bulk crystals by scanning electron microscopy

    NASA Astrophysics Data System (ADS)

    Akamine, Hiroshi; Okumura, So; Farjami, Sahar; Murakami, Yasukazu; Nishida, Minoru

    2016-11-01

    Direct observation of magnetic microstructures is vital for advancing spintronics and other technologies. Here we report a method for imaging surface domain structures on bulk samples by scanning electron microscopy (SEM). Complex magnetic domains, referred to as the maze state in CoPt/FePt alloys, were observed at a spatial resolution of less than 100 nm by using an in-lens annular detector. The method allows for imaging almost all the domain walls in the mazy structure, whereas the visualisation of the domain walls with the classical SEM method was limited. Our method provides a simple way to analyse surface domain structures in the bulk state that can be used in combination with SEM functions such as orientation or composition analysis. Thus, the method extends applications of SEM-based magnetic imaging, and is promising for resolving various problems at the forefront of fields including physics, magnetics, materials science, engineering, and chemistry.

  3. Surface and bulk modified high capacity layered oxide cathodes with low irreversible capacity loss

    NASA Technical Reports Server (NTRS)

    Manthiram, Arumugam (Inventor); Wu, Yan (Inventor)

    2010-01-01

    The present invention includes compositions, surface and bulk modifications, and methods of making of (1-x)Li[Li.sub.1/3Mn.sub.2/3]O.sub.2.xLi[Mn.sub.0.5-yNi.sub.0.5-yCo.sub.2- y]O.sub.2 cathode materials having an O3 crystal structure with a x value between 0 and 1 and y value between 0 and 0.5, reducing the irreversible capacity loss in the first cycle by surface modification with oxides and bulk modification with cationic and anionic substitutions, and increasing the reversible capacity to close to the theoretical value of insertion/extraction of one lithium per transition metal ion (250-300 mAh/g).

  4. Distinct surface and bulk charge density waves in ultrathin 1 T -Ta S2

    NASA Astrophysics Data System (ADS)

    He, Rui; Okamoto, Junichi; Ye, Zhipeng; Ye, Gaihua; Anderson, Heidi; Dai, Xia; Wu, Xianxin; Hu, Jiangping; Liu, Yu; Lu, Wenjian; Sun, Yuping; Pasupathy, Abhay N.; Tsen, Adam W.

    2016-11-01

    We employ low-frequency Raman spectroscopy to study the nearly commensurate (NC) to commensurate (C) charge density wave (CDW) transition in 1 T -Ta S2 ultrathin flakes protected from oxidation. We identify additional modes originating from C-phase CDW phonons that are distinct from those seen in bulk 1 T -Ta S2 . We attribute these to CDW modes from the surface layers. By monitoring individual modes with temperature, we find that surfaces undergo a separate, low-hysteresis NC-C phase transition that is decoupled from the transition in the bulk layers. This indicates the activation of a secondary phase nucleation process in the limit of weak interlayer interaction, which can be understood from energy considerations.

  5. Mapping polarization induced surface band bending on the Rashba semiconductor BiTeI

    PubMed Central

    Butler, Christopher John; Yang, Hung-Hsiang; Hong, Jhen-Yong; Hsu, Shih-Hao; Sankar, Raman; Lu, Chun-I; Lu, Hsin-Yu; Yang, Kui-Hon Ou; Shiu, Hung-Wei; Chen, Chia-Hao; Kaun, Chao-Cheng; Shu, Guo-Jiun; Chou, Fang-Cheng; Lin, Minn-Tsong

    2014-01-01

    Surfaces of semiconductors with strong spin-orbit coupling are of great interest for use in spintronic devices exploiting the Rashba effect. BiTeI features large Rashba-type spin splitting in both valence and conduction bands. Either can be shifted towards the Fermi level by surface band bending induced by the two possible polar terminations, making Rashba spin-split electron or hole bands electronically accessible. Here we demonstrate the first real-space microscopic identification of each termination with a multi-technique experimental approach. Using spatially resolved tunnelling spectroscopy across the lateral boundary between the two terminations, a previously speculated on p-n junction-like discontinuity in electronic structure at the lateral boundary is confirmed experimentally. These findings realize an important step towards the exploitation of the unique behaviour of the Rashba semiconductor BiTeI for new device concepts in spintronics. PMID:24898943

  6. Mapping polarization induced surface band bending on the Rashba semiconductor BiTeI

    NASA Astrophysics Data System (ADS)

    Butler, Christopher John; Yang, Hung-Hsiang; Hong, Jhen-Yong; Hsu, Shih-Hao; Sankar, Raman; Lu, Chun-I.; Lu, Hsin-Yu; Yang, Kui-Hon Ou; Shiu, Hung-Wei; Chen, Chia-Hao; Kaun, Chao-Cheng; Shu, Guo-Jiun; Chou, Fang-Cheng; Lin, Minn-Tsong

    2014-06-01

    Surfaces of semiconductors with strong spin-orbit coupling are of great interest for use in spintronic devices exploiting the Rashba effect. BiTeI features large Rashba-type spin splitting in both valence and conduction bands. Either can be shifted towards the Fermi level by surface band bending induced by the two possible polar terminations, making Rashba spin-split electron or hole bands electronically accessible. Here we demonstrate the first real-space microscopic identification of each termination with a multi-technique experimental approach. Using spatially resolved tunnelling spectroscopy across the lateral boundary between the two terminations, a previously speculated on p-n junction-like discontinuity in electronic structure at the lateral boundary is confirmed experimentally. These findings realize an important step towards the exploitation of the unique behaviour of the Rashba semiconductor BiTeI for new device concepts in spintronics.

  7. Mapping polarization induced surface band bending on the Rashba semiconductor BiTeI.

    PubMed

    Butler, Christopher John; Yang, Hung-Hsiang; Hong, Jhen-Yong; Hsu, Shih-Hao; Sankar, Raman; Lu, Chun-I; Lu, Hsin-Yu; Yang, Kui-Hon Ou; Shiu, Hung-Wei; Chen, Chia-Hao; Kaun, Chao-Cheng; Shu, Guo-Jiun; Chou, Fang-Cheng; Lin, Minn-Tsong

    2014-06-05

    Surfaces of semiconductors with strong spin-orbit coupling are of great interest for use in spintronic devices exploiting the Rashba effect. BiTeI features large Rashba-type spin splitting in both valence and conduction bands. Either can be shifted towards the Fermi level by surface band bending induced by the two possible polar terminations, making Rashba spin-split electron or hole bands electronically accessible. Here we demonstrate the first real-space microscopic identification of each termination with a multi-technique experimental approach. Using spatially resolved tunnelling spectroscopy across the lateral boundary between the two terminations, a previously speculated on p-n junction-like discontinuity in electronic structure at the lateral boundary is confirmed experimentally. These findings realize an important step towards the exploitation of the unique behaviour of the Rashba semiconductor BiTeI for new device concepts in spintronics.

  8. Compositional correlation and anticorrelation in quaternary alloys: competition between bulk thermodynamics and surface kinetics.

    PubMed

    Albrecht, M; Abu-Farsakh, H; Remmele, T; Geelhaar, L; Riechert, H; Neugebauer, J

    2007-11-16

    We analyze the atomistic mechanisms driving the compositional correlation of In and N in the quaternary Inx Ga1-xAs1-yNy alloys combining atomic scale chemical analysis in transmission electron microscopy and density-functional theory calculations. Our results show that for typical growth conditions surface kinetics prevail over bulk thermodynamics resulting in a hitherto unexpected compositional anticorrelation between In and N.

  9. Spatial distribution of the intensity noise of a vertical-cavity surface-emitting semiconductor laser.

    PubMed

    Bramati, A; Hermier, J P; Khoury, A Z; Giacobino, E; Schnitzer, P; Michalzik, R; Ebeling, K J; Poizat, J P; Grangier, P

    1999-07-01

    We studied anticorrelated quantum fluctuations between the TEM(00) and the TEM(01) transverse modes of a vertical-cavity surface-emitting semiconductor laser by measuring the transverse spatial distribution of the laser beam intensity noise. Our experimental results are found to be in good agreement with the predictions of a phenomenological model that accounts for quantum correlations between transverse modes in a light beam.

  10. Superconducting topological surface states in the noncentrosymmetric bulk superconductor PbTaSe2

    PubMed Central

    Guan, Syu-You; Chen, Peng-Jen; Chu, Ming-Wen; Sankar, Raman; Chou, Fangcheng; Jeng, Horng-Tay; Chang, Chia-Seng; Chuang, Tien-Ming

    2016-01-01

    The search for topological superconductors (TSCs) is one of the most urgent contemporary problems in condensed matter systems. TSCs are characterized by a full superconducting gap in the bulk and topologically protected gapless surface (or edge) states. Within each vortex core of TSCs, there exists the zero-energy Majorana bound states, which are predicted to exhibit non-Abelian statistics and to form the basis of the fault-tolerant quantum computation. To date, no stoichiometric bulk material exhibits the required topological surface states (TSSs) at the Fermi level (EF) combined with fully gapped bulk superconductivity. We report atomic-scale visualization of the TSSs of the noncentrosymmetric fully gapped superconductor PbTaSe2. Using quasi-particle scattering interference imaging, we find two TSSs with a Dirac point at E ≅ 1.0 eV, of which the inner TSS and the partial outer TSS cross EF, on the Pb-terminated surface of this fully gapped superconductor. This discovery reveals PbTaSe2 as a promising candidate for TSC. PMID:28138520

  11. Surface studies of gallium nitride quantum dots grown using droplet epitaxy on bulk, native substrates

    NASA Astrophysics Data System (ADS)

    Jones, Christina; Jeon, Sunyeol; Goldman, Rachel; Yacoby, Yizhak; Clarke, Roy

    Gallium nitride (GaN) and its applications in light-emitting diodes play an integral part in efficient, solid-state lighting, as evidenced by its recognition in the 2014 Nobel prize in physics. In order to push this technology towards higher efficiency and reliability and lower cost, we must understand device growth on bulk GaN substrates, which have lower defect densities and strain than template GaN substrates grown on sapphire. In this work, we present our findings on the surface properties of GaN quantum dots (QDs) grown on commercial bulk GaN. QDs are grown using the droplet epitaxy method and analyzed using a surface X-ray diffraction technique called Coherent Bragg Rod Analysis (COBRA), which uses phase retrieval to reconstruct atomic positions near the substrate surface. While several QD growth conditions in our study produce dense QDs, COBRA reveals that only low nitridation temperatures result in GaN QDs that are coherent with the bulk GaN substrate. Results are supported with atomic force microscopy and high-resolution transmission electron microscopy.

  12. Superconducting topological surface states in the noncentrosymmetric bulk superconductor PbTaSe2.

    PubMed

    Guan, Syu-You; Chen, Peng-Jen; Chu, Ming-Wen; Sankar, Raman; Chou, Fangcheng; Jeng, Horng-Tay; Chang, Chia-Seng; Chuang, Tien-Ming

    2016-11-01

    The search for topological superconductors (TSCs) is one of the most urgent contemporary problems in condensed matter systems. TSCs are characterized by a full superconducting gap in the bulk and topologically protected gapless surface (or edge) states. Within each vortex core of TSCs, there exists the zero-energy Majorana bound states, which are predicted to exhibit non-Abelian statistics and to form the basis of the fault-tolerant quantum computation. To date, no stoichiometric bulk material exhibits the required topological surface states (TSSs) at the Fermi level (EF) combined with fully gapped bulk superconductivity. We report atomic-scale visualization of the TSSs of the noncentrosymmetric fully gapped superconductor PbTaSe2. Using quasi-particle scattering interference imaging, we find two TSSs with a Dirac point at E ≅ 1.0 eV, of which the inner TSS and the partial outer TSS cross EF, on the Pb-terminated surface of this fully gapped superconductor. This discovery reveals PbTaSe2 as a promising candidate for TSC.

  13. Analysis of bulk and surface contributions in the neutron skin of nuclei

    SciTech Connect

    Warda, M.; Vinas, X.; Roca-Maza, X.; Centelles, M.

    2010-05-15

    The neutron skin thickness of nuclei is a sensitive probe of the nuclear symmetry energy and has multiple implications for nuclear and astrophysical studies. However, precision measurements of this observable are difficult to obtain. The analysis of the experimental data may imply some assumptions about the bulk or surface nature of the formation of the neutron skin. Here we study the bulk or surface character of neutron skins of nuclei following from calculations with Gogny, Skyrme, and covariant nuclear mean-field interactions. These interactions are successful in describing nuclear charge radii and binding energies but predict different values for neutron skins. We perform the study by fitting two-parameter Fermi distributions to the calculated self-consistent neutron and proton densities. We note that the equivalent sharp radius is a more suitable reference quantity than the half-density radius parameter of the Fermi distributions to discern between the bulk and surface contributions in neutron skins. We present calculations for nuclei in the stability valley and for the isotopic chains of Sn and Pb.

  14. Differentiation of surface and bulk conductivities in topological insulator via four-probe spectroscopy

    NASA Astrophysics Data System (ADS)

    Li, An-Ping; Durand, Corentin; Hus, Saban; Zhang, Xiaoguang; McGuire, Michael; Chen, Yong

    The direct measurement of the topological surface states (TSS) conductivity is often hard to achieve due to the pronounced contribution from the bulk conduction channel. Here, we show a new method to differentiate conductivities from the surface states and the coexisting bulk states in topological insulators (TI) using a four-probe transport spectroscopy in a multi-probe scanning tunneling microscopy system. In contrast to conventional models that assume two resistors in parallel to count for both the TSS and bulk conductance channels, we derive a scaling relation of measured resistance with respect to varying inter-probe spacing for two interconnected conduction channels, which allows quantitative determination of conductivities from both channels. Using this method, we demonstrate the separation of 2D and 3D conduction in TI by comparing the conductance scaling of Bi2Se3, Bi2Te2Se, and Sb-doped Bi2Se3 with that of a pure 2D conductance of graphene on SiC substrate. We also quantitatively show the effect of surface doping carriers on the 2D conductance enhancement in TI. The method offers an approach to understanding not just the topological insulators but also the 2D to 3D crossover of conductance in other complex systems. This research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.

  15. Bulk and surface phase transitions in the three-dimensional O(4) spin model

    NASA Astrophysics Data System (ADS)

    Deng, Youjin

    2006-05-01

    We investigate the O(4) spin model on the simple-cubic lattice by means of the Wolff cluster algorithm. Using the toroidal boundary condition, we locate the bulk critical point at coupling Kc=0.935856(2) , and determine the bulk thermal magnetic renormalization exponents as yt=1.3375(15) and yh=2.4820(2) , respectively. The universal ratio Q=⟨m2⟩2/⟨m4⟩ is also determined as 0.9142(1). The precision of these estimates significantly improves over that of the existing results. Then, we simulate the critical O(4) model with two open surfaces on which the coupling strength K1 can be varied. At the ordinary transitions, the surface magnetic exponent is determined as yh1(o)=1.0202(12) . Further, we find a so-called special surface transition at κ=K1/K-1=1.258(20) . At this point, the surface thermal exponent yt1(s) is rather close to zero, and we cannot exclude that the corresponding surface transition is Kosterlitz-Thouless-like. The surface magnetic exponent is yh1(s)=1.816(2) .

  16. Bulk and surface phase transitions in the three-dimensional O4 spin model.

    PubMed

    Deng, Youjin

    2006-05-01

    We investigate the O(4) spin model on the simple-cubic lattice by means of the Wolff cluster algorithm. Using the toroidal boundary condition, we locate the bulk critical point at coupling K(c) = 0.935 856(2), and determine the bulk thermal magnetic renormalization exponents as y(t) = 1.337 5(15) and y(h) = 2.482 0(2), respectively. The universal ratio Q=m(2)(2)/m(4) is also determined as 0.9142(1). The precision of these estimates significantly improves over that of the existing results. Then, we simulate the critical O(4) model with two open surfaces on which the coupling strength K(1) can be varied. At the ordinary transitions, the surface magnetic exponent is determined as y((o))(h1) = 1.020 2(12). Further, we find a so-called special surface transition at (k) = K(1)/K-1 = 1.258(20). At this point, the surface thermal exponent y(s)(t1) is rather close to zero, and we cannot exclude that the corresponding surface transition is Kosterlitz-Thouless-like. The surface magnetic exponent is y((s))/h1 = 1.816(2).

  17. Chalcogenopyrylium Dyes with Anchors to Nanoparticle and Semiconductor Surfaces

    NASA Astrophysics Data System (ADS)

    Bedics, Matthew Allen

    Surface enhanced Raman scattering (SERS) has gained widespread attention as a biomedical imaging technique due to its multiplexing capabilities and the low limits of detection (LODs) of SERS-nanoprobes. The library of available reporter molecules, which are used to generate unique SERS spectra, was previously limited to commercially available dyes or a small group of cyanine reporters. Herein, the design and synthesis of a novel group of chalcogenopyrylium SERS reporters is described. These dyes have a high affinity for Au and absorption maxima that range into the NIR region. These reporter molecules enabled the use of the 1280 nm laser, which was previously incompatible with SERS imaging. Also, nanoprobe LODs using these dyes as reporters are lower than any previously documented systems, with a 100 aM LOD using a 785 nm excitation and multiple examples of fM to pM LODs using a 1064 nm or 1280 nm excitation source. Nanoprobes functionalized with these compounds have also been successfully utilized in vivo, and produce more intense SERS spectra as compared to a commonly used cyanine reporter. Dye sensitized solar cells (DSSCs) have produced considerable interest as an alternative to conventionally used Si-based solar cells. Specifically, DSSCs that use metal-free organic dyes as sensitizers are important due to the lower cost and the use of earth abundant materials as starting materials. Herein, a group of chalcogenopyrylium dyes were appended with an anchoring group to TiO2, which enables the use of these dyes as sensitizers. Structural modifications were used to extend absorption maxima into the near-infrared region of the light spectrum and to evaluate the effect that dye aggregation has on device performance. The monomethine dyes successfully produced a photocurrent, with incident photon to current efficiency values as high as 20%. Aggregation was found to benefit these systems due to the spectral broadening of aggregated dyes, and consequent increased range of

  18. Relative influence of surface states and bulk impurities on the electrical properties of Ge nanowires.

    PubMed

    Zhang, Shixiong; Hemesath, Eric R; Perea, Daniel E; Wijaya, Edy; Lensch-Falk, Jessica L; Lauhon, Lincoln J

    2009-09-01

    We quantitatively examine the relative influence of bulk impurities and surface states on the electrical properties of Ge nanowires with and without phosphorus (P) doping. The unintentional impurity concentration in nominally undoped Ge nanowires is less than 2 x 10(17) cm(-3) as determined by atom probe tomography. Surprisingly, P doping of approximately 10(18) cm(-3) reduces the nanowire conductivity by 2 orders of magnitude. By modeling the contributions of dopants, impurities, and surface states, we confirm that the conductivity of nominally undoped Ge nanowires is mainly due to surface state induced hole accumulation rather than impurities introduced by catalyst. In P-doped nanowires, the surface states accept the electrons generated by the P dopants, reducing the conductivity and leading to ambipolar behavior. In contrast, intentional surface-doping results in a high conductivity and recovery of n-type characteristics.

  19. Improvement of the Sintered Surface and Bulk of the Product Via Differentiating Laser Sintering (Melting) Modes

    NASA Astrophysics Data System (ADS)

    Saprykina, N. A.; Saprykin, A. A.; Arkhipova, D. A.; Borovikov, I. F.

    2016-08-01

    Selective laser sintering (melting) enables using metal powdered materials to manufacture products of any geometrical complexity, requiring no preliminary costs to prepare processing equipment. However, quality of the sintered surface is often inadequate as against the product manufactured traditionally. Manufacturing a high quality product requires solution of such vital task as prediction of the sintered surface roughness. The authors address to the effect of laser sintering modes on roughness of the surface, sintered of copper powdered material PMS-l (IIMC-1). The dependence of roughness of the surface layer sintered of copper powder material PMS-l upon sintering process conditions is expressed mathematically. The authors suggest differentiating sintering modes to improve the sintered surface and the bulk of the product and dividing them into rough, semi-finishing, and finishing ones.

  20. High-coherent-power, two-dimensional grating surface-emitting (GSE) semiconductor lasers

    NASA Astrophysics Data System (ADS)

    Li, Shuang

    High-power semiconductor lasers, with coherent radiation, are attractive sources for many applications. However, achieving stable, coherent radiation to watt-range power from monolithic semiconductor lasers has been a challenge. This work covers the study and development of high power coherent semiconductor lasers employing novel-types of both surface-emitting and edge-emitting structures. Surface-emitting (SE) semiconductor lasers are preferred over edge-emitting lasers due to their inherent reliability, scalability, and packaging advantages. Horizontal-cavity, grating SE semiconductor lasers are promising candidates for high-power coherent sources. Here we present the design and analysis of a two-dimensional (2D) horizontal-cavity GSE laser (so called ROW-SEDFB laser), for which 2nd-order, distributed feedback/distributed Bragg reflector (DFB/DBR) gratings with central pi phaseshift are preferentially placed in the element regions of a resonant-optical-waveguide (ROW) structure. We find that beside their usual functions (feedback and outcoupling), the gratings act as an effective array-mode selector. The in-phase mode is strongly favored to lase around its resonance due both to better field overlap with the active-grating (i.e., DFB) and to lower interelement loss than the other array modes. For 20-element arrays with 700/600mum-long DFB/DBR gratings, and of 100mum-wide lateral dimension, high intermodal discrimination is obtained. The primary mechanisms behind this discrimination are found to be: absorption losses for the interelement field to the metal contact and to a semiconductor/metal grating layer, and the longitudinal guided-field overlap with the DFB region. The discrimination can be further enhanced by introducing free-carrier absorption in the interelement regions. The device has relatively uniform guided-field profiles in both lateral and longitudinal directions and a strong built-in index profile in the lateral direction. These features make the ROW

  1. Bulk and surface properties of magnesium peroxide MgO2

    NASA Astrophysics Data System (ADS)

    Esch, Tobit R.; Bredow, Thomas

    2016-12-01

    Magnesium peroxide has been identified in Mg/air batteries as an intermediate in the oxygen reduction reaction (ORR) [1]. It is assumed that MgO2 is involved in the solid-electrolyte interphase on the cathode surface. Therefore its structure and stability play a crucial role in the performance of Mg/air batteries. In this work we present a theoretical study of the bulk and low-index surface properties of MgO2. All methods give a good account of the experimental lattice parameters for MgO2 and MgO bulk. The reaction energies, enthalpies and free energies for MgO2 formation from MgO are compared among the different DFT methods and with the local MP2 method. A pronounced dependence from the applied functional is found. At variance with a previous theoretical study but in agreement with recent experiments we find that the MgO2 formation reaction is endothermic (HSE06-D3BJ: ΔH = 51.9 kJ/mol). The stability of low-index surfaces MgO2 (001) (Es = 0.96 J/m2) and (011) (Es = 1.98 J/m2) is calculated and compared to the surface energy of MgO (001). The formation energy of neutral oxygen vacancies in the topmost layer of the MgO2 (001) surface is calculated and compared with defect formation energies for MgO (001).

  2. Ultra-thin silver-coated tilted fiber grating for surface and bulk refractive index measurement

    NASA Astrophysics Data System (ADS)

    Qiu, Xuhui; Guo, Tuan; Liu, Fu; Guan, Bai-Ou; Tam, Hwa-Yaw; Albert, Jacques

    2015-09-01

    An ultra-thin silver-coated tilted fiber Bragg grating (TFBG) sensor with clear surface plasmon resonance (SPR) together with strong evanescent wave in transmission for "surface" and "bulk" surrounding refractive index (SRI) measurement is proposed and experimentally demonstrated. The thickness of the silver coating over the fiber surface is precisely controlled at 12~16 nm (much thinner than 40~50 nm for traditional SPR excitation). The transmission spectrum of the sensor provides a fine comb of narrowband resonances that overlap with the broader absorption of the surface plasmon and thus provide a unique tool to measure small shifts of the plasmon and identify the "surface" SRI changes with high accuracy. Meanwhile, the ultra-thin nanometric-coating permits part of high-order cladding modes to become leaky modes which have a large sensitivity to variations in the background solution for "bulk" SRI measurement. Experimental results show that above two resonances have an inverse amplitude responses to the SRI changing. Biological solutions (urine of rats with different concentration of Aquaporin) with different RI ranging from 1.3400 to 1.3408 were clearly discriminated in-situ by using the differential amplitude monitoring between "cut-off" cladding resonance and plasmonic resonance, with an amplitude variation sensitivity of ~8100 dB/RIU and a limit of detection of ~10-5 RIU.

  3. Topological Polymer Dispersed Liquid Crystals with Bulk Nematic Defect Lines Pinned to Handlebody Surfaces

    NASA Astrophysics Data System (ADS)

    Campbell, Michael G.; Tasinkevych, Mykola; Smalyukh, Ivan I.

    2014-05-01

    Polymer dispersed liquid crystals are a useful model system for studying the relationship between surface topology and defect structures. They are comprised of a polymer matrix with suspended spherical nematic drops and are topologically constrained to host defects of an elementary hedgehog charge per droplet, such as bulk or surface point defects or closed disclination loops. We control the genus of the closed surfaces confining such micrometer-sized nematic drops with tangential boundary conditions for molecular alignment imposed by the polymer matrix, allowing us to avoid defects or, on the contrary, to generate them in a controlled way. We show, both experimentally and through numerical modeling, that topological constraints in nematic microdrops can be satisfied by hosting topologically stable half-integer bulk defect lines anchored to opposite sides of handlebody surfaces. This enriches the interplay of topologies of closed surfaces and fields with nonpolar symmetry, yielding new unexpected configurations that cannot be realized in vector fields, having potential implications for topologically similar defects in cosmology and other fields.

  4. Nickel Alloy Primary Water Bulk Surface and SCC Corrosion Film Analytical Characterization and SCC Mechanistic Implications

    SciTech Connect

    Morton, D.; Lewis, N.; Hanson, M.; Rice, S.; Sanders, P.

    2007-04-18

    Alloy 600 corrosion coupon tests were performed: (1) to quantify the temperature dependency of general corrosion and (2) to characterize the composition and structure of bulk surface corrosion films for comparison with ongoing primary water SCC (PWSCC) crack tip corrosion film analyses. Results suggest that the thermal activation energy of Alloy 600 corrosion is consistent with the thermal activation energy of nickel alloy PWSCC. Analytical investigations of the structure and composition of Alloy 600 bulk surface corrosion oxides revealed a duplex (inner and outer) oxide layer structure. The outer layer is discontinuous and comprised of relatively large (1 to 3 {micro}m) nickel ferrite crystals and smaller ({approx}0.1 {micro}m) chromium containing nickel ferrite crystals. The inner layer consists of a relatively continuous chromite spinel (major phase) and chromia (Cr{sub 2}O{sub 3} minor phase) which formed through non-selective oxidation. Chromia and dealloyed Alloy 600 (highly Ni enriched metal) were only observed at 337 C (640 F) and only along the boundaries of deformation induced fine grains and subcells. Specimens having deformation free surfaces exhibited continuous uniform inner chromite spinel oxide layers. Specimens with machining induced surface deformation produced non-uniform inner layer oxides (chromite spinel, Cr{sub 2}O{sub 3} and unoxidized material). PWSCC crack tip oxides, in contrast, were fine grain (no duplex structure) and consisted of both chromium rich spinels and ''NiO'' structure oxides. Generally, nickel rich oxides were more abundant under more oxidized conditions (reduced coolant hydrogen) and spinel rich crack tip oxides were favored under more reducing conditions (increased coolant hydrogen). Bulk surface corrosion film thickness did not correlate with observed SCC growth rates. These results suggest that corrosion is not the rate controlling step of PWSCC but rather that PWSCC and corrosion have a common rate controlling sub

  5. Exploiting imperfections in the bulk to direct assembly of surface colloids

    PubMed Central

    Cavallaro, Marcello; Gharbi, Mohamed A.; Beller, Daniel A.; Čopar, Simon; Shi, Zheng; Baumgart, Tobias; Yang, Shu; Kamien, Randall D.; Stebe, Kathleen J.

    2013-01-01

    We exploit the long-ranged elastic fields inherent to confined nematic liquid crystals (LCs) to assemble colloidal particles trapped at the LC interface into reconfigurable structures with complex symmetries and packings. Spherical colloids with homeotropic anchoring trapped at the interface between air and the nematic LC 4-cyano-4′-pentylbiphenyl create quadrupolar distortions in the director field causing particles to repel and consequently form close-packed assemblies with a triangular habit. Here, we report on complex open structures organized via interactions with defects in the bulk. Specifically, by confining the nematic LC in an array of microposts with homeotropic anchoring conditions, we cause defect rings to form at well-defined locations in the bulk of the sample. These defects source elastic deformations that direct the assembly of the interfacially trapped colloids into ring-like assemblies, which recapitulate the defect geometry even when the microposts are completely immersed in the nematic. When the surface density of the colloids is high, they form a ring near the defect and a hexagonal lattice far from it. Because topographically complex substrates are easily fabricated and LC defects are readily reconfigured, this work lays the foundation for a versatile, robust mechanism to direct assembly dynamically over large areas by controlling surface anchoring and associated bulk defect structure. PMID:24191037

  6. Surface and bulk crystallization of amorphous solid water films: Confirmation of “top-down” crystallization

    SciTech Connect

    Yuan, Chunqing; Smith, R. Scott; Kay, Bruce D.

    2016-01-11

    Here, the crystallization kinetics of nanoscale amorphous solid water (ASW) films are investigated using temperature-programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS). TPD measurements are used to probe surface crystallization and RAIRS measurements are used to probe bulk crystallization. Isothermal TPD results show that surface crystallization is independent of the film thickness (from 100 to 1000 ML). Conversely, the RAIRS measurements show that the bulk crystallization time increases linearly with increasing film thickness. These results suggest that nucleation and crystallization begin at the ASW/vacuum interface and then the crystallization growth front propagates linearly into the bulk. This mechanism was confirmed by selective placement of an isotopic layer (5% D2O in H2O) at various positions in an ASW (H2O) film. In this case, the closer the isotopic layer was to the vacuum interface, the earlier the isotopic layer crystallized. These experiments provide direct evidence to confirm that ASW crystallization in vacuum proceeds by a “top-down” crystallization mechanism.

  7. Surface and bulk crystallization of amorphous solid water films: Confirmation of “top-down” crystallization

    SciTech Connect

    Yuan, Chunqing; Smith, R. Scott; Kay, Bruce D.

    2016-10-01

    The crystallization kinetics of nanoscale amorphous solid water (ASW) films are investigated using temperature-programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS). TPD measurements are used to probe surface crystallization and RAIRS measurements are used to probe bulk crystallization. Isothermal TPD results show that surface crystallization is independent of the film thickness (from 100 to 1000 ML). Conversely, the RAIRS measurements show that the bulk crystallization time increases linearly with increasing film thickness. These results suggest that nucleation and crystallization begin at the ASW/vacuum interface and then the crystallization growth front propagates linearly into the bulk. This mechanism was confirmed by selective placement of an isotopic layer (5% D2O in H2O) at various positions in an ASW (H2O) film. In this case, the closer the isotopic layer was to the vacuum interface, the earlier the isotopic layer crystallized. These experiments provide direct evidence to confirm that ASW crystallization in vacuum proceeds by a “top-down” crystallization mechanism.

  8. Surface and bulk crystallization of amorphous solid water films: Confirmation of “top-down” crystallization

    DOE PAGES

    Yuan, Chunqing; Smith, R. Scott; Kay, Bruce D.

    2016-01-11

    Here, the crystallization kinetics of nanoscale amorphous solid water (ASW) films are investigated using temperature-programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS). TPD measurements are used to probe surface crystallization and RAIRS measurements are used to probe bulk crystallization. Isothermal TPD results show that surface crystallization is independent of the film thickness (from 100 to 1000 ML). Conversely, the RAIRS measurements show that the bulk crystallization time increases linearly with increasing film thickness. These results suggest that nucleation and crystallization begin at the ASW/vacuum interface and then the crystallization growth front propagates linearly into the bulk. This mechanism wasmore » confirmed by selective placement of an isotopic layer (5% D2O in H2O) at various positions in an ASW (H2O) film. In this case, the closer the isotopic layer was to the vacuum interface, the earlier the isotopic layer crystallized. These experiments provide direct evidence to confirm that ASW crystallization in vacuum proceeds by a “top-down” crystallization mechanism.« less

  9. Effect of Oxygen Adsorbates on Terahertz Emission Properties of Various Semiconductor Surfaces Covered with Graphene

    NASA Astrophysics Data System (ADS)

    Bagsican, Filchito Renee; Zhang, Xiang; Ma, Lulu; Wang, Minjie; Murakami, Hironaru; Vajtai, Robert; Ajayan, Pulickel M.; Kono, Junichiro; Tonouchi, Masayoshi; Kawayama, Iwao

    2016-11-01

    We have studied coherent terahertz (THz) emission from graphene-coated surfaces of three different semiconductors—InP, GaAs, and InAs—to provide insight into the influence of O2 adsorption on charge states and dynamics at the graphene/semiconductor interface. The amplitude of emitted THz radiation from graphene-coated InP was found to change significantly upon desorption of O2 molecules by thermal annealing, while THz emission from bare InP was nearly uninfluenced by O2 desorption. In contrast, the amount of change in the amplitude of emitted THz radiation due to O2 desorption was essentially the same for graphene-coated GaAs and bare GaAs. However, in InAs, neither graphene coating nor O2 adsorption/desorption affected the properties of its THz emission. These results can be explained in terms of the effects of adsorbed O2 molecules on the different THz generation mechanisms in these semiconductors. Furthermore, these observations suggest that THz emission from graphene-coated semiconductors can be used for probing surface chemical reactions (e.g., oxidation) as well as for developing O2 gas sensor devices.

  10. Generalized Electron Counting in Determination of Metal-Induced Reconstruction of Compound Semiconductor Surfaces

    SciTech Connect

    Zhang, Lixin; Wang, E. G.; Xue, Qi-Kun; Zhang, S. B.; Zhang, Zhenyu

    2006-01-01

    Based on theoretical analysis, first-principles calculations, and experimental observations, we establish a generic guiding principle, embodied in generalized electron counting (GEC), that governs the surface reconstruction of compound semiconductors induced by different metal adsorbates. Within the GEC model, the adsorbates serve as an electron bath, donating or accepting the right number of electrons as the host surface chooses a specific reconstruction that obeys the classic electron-counting model. The predictive power of the GEC model is illustrated for a wide range of metal adsorbates.

  11. Bulk Crystal Growth, and High-Resolution X-ray Diffraction Results of LiZnAs Semiconductor Material

    NASA Astrophysics Data System (ADS)

    Montag, Benjamin W.; Reichenberger, Michael A.; Sunder, Madhana; Ugorowski, Philip B.; Nelson, Kyle A.; Henson, Luke C.; McGregor, Douglas S.

    2017-08-01

    LiZnAs is being explored as a candidate for solid-state neutron detectors. The compact form, solid-state device would have greater efficiency than present day gas-filled 3He and 10BF3 detectors. Devices fabricated from LiZnAs having either natural Li (nominally 7.5% 6Li) or enriched 6Li (usually 95% 6Li) as constituent atoms may provide a material for compact high efficiency neutron detectors. The 6Li( n, t)4He reaction yields a total Q-value of 4.78 MeV, an energy larger than that of the 10B reaction, which can easily be identified above background radiations. LiZnAs material was synthesized by preparing equimolar portions of Li, Zn, and As sealed under vacuum (10-6 Torr) in quartz ampoules lined with boron nitride and subsequently reacted in a compounding furnace (Montag et al. in J Cryst Growth 412:103, 2015). The raw synthesized LiZnAs was purified by a static vacuum sublimation in quartz (Montag et al. in J Cryst Growth 438:99, 2016). Bulk crystalline LiZnAs ingots were grown from the purified material with a high-temperature Bridgman-style growth process described here. One of the largest LiZnAs ingots harvested was 9.6 mm in diameter and 4.2 mm in length. Samples were harvested from the ingot and were characterized for crystallinity using a Bruker AXS Inc. D8 AXS Inc. D2 CRYSO, energy dispersive x-ray diffractometer, and a Bruker AXS Inc. D8 DISCOVER, high-resolution x-ray diffractometer equipped with molybdenum radiation, Gobel mirror, four bounce germanium monochromator and a scintillation detector. The primary beam divergence was determined to be 0.004°, using a single crystal Si standard. The x-ray based characterization revealed that the samples nucleated in the (110) direction and a high-resolution open detector rocking curve recorded on the (220) LiZnAs yielded a full width at half maximum (FWHM) of 0.235°. Sectional pole figures using off-axis reflections of the (211) LiZnAs confirmed in-plane ordering, and also indicated the presence of multiple

  12. Roughening of an interface in a system with surface or bulk disorder

    NASA Astrophysics Data System (ADS)

    Corberi, Federico; Lippiello, Eugenio; Zannetti, Marco

    2016-05-01

    We study numerically the roughening properties of an interface in a two-dimensional Ising model with either random bonds or random fields, which are representative of universality classes where disorder acts only on the interface or also away from it, in the bulk. The dynamical structure factor shows a rich crossover pattern from the form of a pure system at large wavevectors k, to a different behavior, typical of the kind of disorder, at smaller k's. For the random field model a second crossover is observed from the typical behavior of a system where disorder is only effective on the surface, as the random bond model, to the truly large scale behavior, where bulk-disorder is important, that is observed at the smallest wavevectors.

  13. Nonreciprocal propagation of surface plasmon mode guided through graphene layer on magnetized semiconductor

    NASA Astrophysics Data System (ADS)

    Bhagyaraj, C.; Mathew, Vincent

    2017-01-01

    This paper discusses the nonreciprocal effect induced by magnetized semiconductor substrate on surface plasmon mode guided through monolayer graphene. Dispersion relation for the fundamental antisymmetric mode is derived analytically. Nonreciprocal propagation characteristics of fundamental mode is studied as a function of wavelength, graphene layer chemical potential and biasing magnetic field. Fundamental mode exhibits appreciable nonreciprocal dispersion for transversal magnetization of semiconductor substrate in midinfrared and terahertz frequencies. Cutoff wavelength for backward propagating mode is observed above 2.5 T of external biasing field. Cutoff wavelength is found to be decreasing with increase in the biasing magnetic field and cladding index, also identified to be independent of graphene layer chemical potential. Proposed waveguide structure suggests the possibility of realizing one way propagating plasmonic waveguides with widely tunable guiding characteristics and related functional devices such as isolators, modulators, phase shifters and switches for integrated photonic circuits.

  14. Simulation Study on Semiconductor Coupled Surface Acoustic Wave Convolver through a Multi-Strip Electrodes

    NASA Astrophysics Data System (ADS)

    Hohkawa, Kohji; Suda, Takaya; Aoki, Yusuke; Kaneshiro, Chinami; Koh, Keishin

    2001-05-01

    This paper presents results of simulation study on a semiconductor coupled surface acoustic wave (SAW) convolver, in which the propagating SAW on a highly coupling coefficient piezoelectric substrate, couples with a bonded semiconductor diodes through multi-strip electrodes. We focus our study on a relatively wide band device which is the main feature of a highly efficiency device. By using a simple analysis and circuit simulator, based on the simulation program with integrated circuit emphasis (SPICE), we clarified the effect of device parameters, such as the shape of multi-strip tapping electrodes, characteristics of diode, impedance matching condition, kinds of transmission code and electro-mechanical coupling coefficient of SAW, on the device performances. We discussed the phenomenon, which cause the degradation, focusing on the frequency domain. We also clarified the essential problems of second order effect on the wide bandwidth device, which should be solved.

  15. Semiconductor nanocrystals covalently bound to solid inorganic surfaces using self-assembled monolayers

    DOEpatents

    Alivisatos, A. Paul; Colvin, Vicki L.

    1998-01-01

    Methods are described for attaching semiconductor nanocrystals to solid inorganic surfaces, using self-assembled bifunctional organic monolayers as bridge compounds. Two different techniques are presented. One relies on the formation of self-assembled monolayers on these surfaces. When exposed to solutions of nanocrystals, these bridge compounds bind the crystals and anchor them to the surface. The second technique attaches nanocrystals already coated with bridge compounds to the surfaces. Analyses indicate the presence of quantum confined clusters on the surfaces at the nanolayer level. These materials allow electron spectroscopies to be completed on condensed phase clusters, and represent a first step towards synthesis of an organized assembly of clusters. These new products are also disclosed.

  16. Semiconductor nanocrystals covalently bound to solid inorganic surfaces using self-assembled monolayers

    DOEpatents

    Alivisatos, A.P.; Colvin, V.L.

    1998-05-12

    Methods are described for attaching semiconductor nanocrystals to solid inorganic surfaces, using self-assembled bifunctional organic monolayers as bridge compounds. Two different techniques are presented. One relies on the formation of self-assembled monolayers on these surfaces. When exposed to solutions of nanocrystals, these bridge compounds bind the crystals and anchor them to the surface. The second technique attaches nanocrystals already coated with bridge compounds to the surfaces. Analyses indicate the presence of quantum confined clusters on the surfaces at the nanolayer level. These materials allow electron spectroscopies to be completed on condensed phase clusters, and represent a first step towards synthesis of an organized assembly of clusters. These new products are also disclosed. 10 figs.

  17. ANALYSES AND COMPARISON OF BULK AND COIL SURFACE SAMPLES FROM THE DWPF SLURRY MIX EVAPORATOR

    SciTech Connect

    Hay, M.; Nash, C.; Stone, M.

    2012-02-17

    Sludge samples from the DWPF Slurry Mix Evaporator (SME) heating coil frame and coil surface were characterized to identify differences that might help identify heat transfer fouling materials. The SME steam coils have seen increased fouling leading to lower boil-up rates. Samples of the sludge were taken from the coil frame somewhat distant from the coil (bulk tank material) and from the coil surface (coil surface sample). The results of the analysis indicate the composition of the two SME samples are very similar with the exception that the coil surface sample shows {approx}5-10X higher mercury concentration than the bulk tank sample. Elemental analyses and x-ray diffraction results did not indicate notable differences between the two samples. The ICP-MS and Cs-137 data indicate no significant differences in the radionuclide composition of the two SME samples. Semi-volatile organic analysis revealed numerous organic molecules, these likely result from antifoaming additives. The compositions of the two SME samples also match well with the analyzed composition of the SME batch with the exception of significantly higher silicon, lithium, and boron content in the batch sample indicating the coil samples are deficient in frit relative to the SME batch composition.

  18. Photocatalysis with chromium-doped TiO2: bulk and surface doping.

    PubMed

    Ould-Chikh, Samy; Proux, Olivier; Afanasiev, Pavel; Khrouz, Lhoussain; Hedhili, Mohamed N; Anjum, Dalaver H; Harb, Moussab; Geantet, Christophe; Basset, Jean-Marie; Puzenat, Eric

    2014-05-01

    The photocatalytic properties of TiO2 modified by chromium are usually found to depend strongly on the preparation method. To clarify this problem, two series of chromium-doped titania with a chromium content of up to 1.56 wt % have been prepared under hydrothermal conditions: the first series (Cr:TiO2) is intended to dope the bulk of TiO2, whereas the second series (Cr/TiO2) is intended to load the surface of TiO2 with Cr. The catalytic properties have been compared in the photocatalytic oxidation of formic acid. Characterization data provides evidence that in the Cr/TiO2 catalysts chromium is located on the surface of TiO2 as amorphous CrOOH clusters. In contrast, in the Cr:TiO2 series, chromium is mostly dissolved in the titania lattice, although a minor part is still present on the surface. Photocatalytic tests show that both series of chromium-doped titania demonstrate visible-light-driven photo-oxidation activity. Surface-doped Cr/TiO2 solids appear to be more efficient photocatalysts than the bulk-doped Cr:TiO2 counterparts.

  19. Differentiation of surface and bulk conductivities in topological insulator via four-probe spectroscopy

    SciTech Connect

    Zhang, Xiaoguang; McGuire, Michael A.; Chen, Yong P.; Li, An -Ping; Durand, Corentin; Hus, Saban M.; Ma, Chuanxu; Hu, Yang; Cao, Helin; Miotkowski, Ireneusz

    2016-03-08

    Topological insulators, with characteristic topological surface states, have emerged as a new state of matter with rich potentials for both fundamental physics and device applications. However, the experimental detection of the surface transport has been hampered by the unavoidable extrinsic conductivity associated with the bulk crystals. Here we show that a four-probe transport spectroscopy in a multi-probe scanning tunneling microscopy system can be used to differentiate conductivities from the surface states and the coexisting bulk states in topological insulators. We derive a scaling relation of measured resistance with respect to varying inter-probe spacing for two interconnected conduction channels, which allows quantitative determination of conductivities from both channels. Using this method, we demonstrate the separation of 2D and 3D conduction in topological insulators by comparing the conductance scaling of Bi2Se3, Bi2Te2Se, and Sb-doped Bi2Se3 with that of a pure 2D conductance of graphene on SiC substrate. We also report the 2D conductance enhancement due to the surface doping effect in topological insulators. This technique can be applied to reveal 2D to 3D crossover of conductance in other complex systems.

  20. Skyrmions in quasi-2D chiral magnets with broken bulk and surface inversion symmetry (Presentation Recording)

    NASA Astrophysics Data System (ADS)

    Randeria, Mohit; Banerjee, Sumilan; Rowland, James

    2015-09-01

    Most theoretical studies of chiral magnetism, and the resulting spin textures, have focused on 3D systems with broken bulk inversion symmetry, where skyrmions are stabilized by easy-axis anisotropy. In this talk I will describe our results on 2D and quasi-2D systems with broken surface inversion, where we find [1] that skyrmion crystals are much more stable than in 3D, especially for the case of easy-plane anisotropy. These results are of particular interest for thin films, surfaces, and oxide interfaces [2], where broken surface-inversion symmetry and Rashba spin-orbit coupling naturally lead to both the chiral Dzyaloshinskii-Moriya (DM) interaction and to easy-plane compass anisotropy. I will then turn to systems that break both bulk and surface inversion, resulting in two distinct DM terms arising from Dresselhaus and Rashba spin-orbit coupling. I will describe [3] the evolution of the skyrmion structure and of the phase diagram as a function of the ratio of Dresselhaus and Rashba terms, which can be tuned by varying film thickness and strain. [1] S. Banerjee, J. Rowland, O. Erten, and M. Randeria, PRX 4, 031045 (2014). [2] S. Banerjee, O. Erten, and M. Randeria, Nature Phys. 9, 626 (2013). [3] J. Rowland, S. Banerjee and M. Randeria, (unpublished).

  1. Differentiation of surface and bulk conductivities in topological insulator via four-probe spectroscopy

    DOE PAGES

    Zhang, Xiaoguang; McGuire, Michael A.; Chen, Yong P.; ...

    2016-03-08

    Topological insulators, with characteristic topological surface states, have emerged as a new state of matter with rich potentials for both fundamental physics and device applications. However, the experimental detection of the surface transport has been hampered by the unavoidable extrinsic conductivity associated with the bulk crystals. Here we show that a four-probe transport spectroscopy in a multi-probe scanning tunneling microscopy system can be used to differentiate conductivities from the surface states and the coexisting bulk states in topological insulators. We derive a scaling relation of measured resistance with respect to varying inter-probe spacing for two interconnected conduction channels, which allowsmore » quantitative determination of conductivities from both channels. Using this method, we demonstrate the separation of 2D and 3D conduction in topological insulators by comparing the conductance scaling of Bi2Se3, Bi2Te2Se, and Sb-doped Bi2Se3 with that of a pure 2D conductance of graphene on SiC substrate. We also report the 2D conductance enhancement due to the surface doping effect in topological insulators. This technique can be applied to reveal 2D to 3D crossover of conductance in other complex systems.« less

  2. Comparing near-surface and bulk densities of asteroids using radar scattering properties

    NASA Astrophysics Data System (ADS)

    Zambrano Marin, Luisa Fernanda; Nolan, Michael C.; Taylor, Patrick A.; Virkki, Anne

    2016-10-01

    Dual-polarization radar measurements of asteroids provide a joint constraint on the near-surface density and porosity, which can give insights on asteroid composition and evolution. Magri et al. (2001) used (433) Eros radar and spacecraft data as calibration for estimating the near-surface densities and porosities of 45 other radar-detected asteroids (36 main-belt and 9 near-Earth). At that time, only (433) Eros had both radar observations and a measured bulk density. Now that there have been spacecraft observations of several other asteroids and radar measurements of the densities of several binary near-Earth asteroids with various compositions, we can expand the calibration to include those objects. We begin by applying the method of Magri et al. to Ceres, Vesta, Itokawa, 1994 CC, 2001 SN263, 1998 QE2, and 2000 DP107 to explore the differences between the bulk density and the near-surface density measured with radar. We expect significant differences between Ceres and Vesta and the small near-Earth asteroids as the porosities of these objects are expected to be quite different. However, we expect that small binary objects likely have similar internal structures, so that any differences should depend on composition and perhaps surface weathering.Reference: Magri et al., "Radar constraints on asteroid Properties using 433 Eros as ground truth". Meteoritics & Planetary Science 36, 1697-1709, 2001.

  3. The surface as molecular reagent: organic chemistry at the semiconductor interface

    NASA Astrophysics Data System (ADS)

    Filler, Michael A.; Bent, Stacey F.

    2003-09-01

    Methods for the incorporation of organic functionality onto semiconductor surfaces have seen immense progress in recent years. Of the multiple methods developed, the direct, covalent attachment of organic moieties is valuable because it allows for excellent control of the interfacial properties. This review article will focus on a number of synthetic strategies that have been developed to exploit the unique reactivity of group-IV surfaces under vacuum. A picture of the semiconductor surface and its reactions will be developed within the standard framework of organic chemistry with emphasis on the importance of combined experimental and theoretical approaches. Three broad areas of organic chemistry will be highlighted, including nucleophilic/electrophilic, pericyclic, and aromatic reactions. The concept of nucleophilicity and electrophilicity will be discussed within the context of dative bonding and proton transfer of amines and alcohols. Pericyclic reactions cover the [4 + 2] or Diels-Alder cycloaddition, [2 + 2] cycloaddition, dipolar, and ene reactions. Examples include the reactions of alkenes, dienes, ketones, nitriles, and related multifunctional molecules at the interface. Aromaticity and the use of directing groups to influence the distribution of surface products will be illustrated with benzene, xylene, and heteroaromatic compounds. Finally, multifunctional molecules are used to describe the competition and selectively observed among different surface reactions.

  4. Chemical and thermodynamic control of the surface of semiconductor nanocrystals for designer white light emitters.

    PubMed

    Krause, Michael M; Mooney, Jonathan; Kambhampati, Patanjali

    2013-07-23

    Small CdSe semiconductor nanocrystals with diameters below 2 nm are thought to emit white light due to random surface defects which result in a broad distribution of midgap emitting states, thereby preventing rational design of small nanocrystal white light emitters. We perform temperature dependent photoluminescence experiments before and after ligand exchange and electron transfer simulations to reveal a very simple microscopic picture of the origin of the white light. These experiments and simulations reveal that these small nanocrystals can be physically modeled in precisely the same way as normal-sized semiconductor nanocrystals; differences in their emission spectra arise from their surface thermodynamics. The white light emission is thus a consequence of the thermodynamic relationship between a core excitonic state and an optically bright surface state with good quantum yield. By virtue of this understanding of the surface and the manner in which it is coupled to the core excitonic states of these nanocrystals, we show both chemical and thermodynamic control of the photoluminescence spectra. We find that using both temperature and appropriate choice in ligands, one can rationally control the spectra so as to engineer the surface to target color rendering coordinates for displays and white light emitters.

  5. Schottky-barrier-free contacts with two-dimensional semiconductors by surface-engineered MXenes

    DOE PAGES

    Liu, Yuanyue; Xiao, Hai; Goddard, III, William A.

    2016-11-22

    Two-dimensional (2D) metal carbides and nitrides, called MXenes, have attracted great interest for applications such as energy storage. Here we demonstrate their potential as Schottky-barrier-free metal contacts to 2D semiconductors, providing a solution to the contact-resistance problem in 2D electronics. Based on first principles calculations, we find that the surface chemistry strongly affects the Fermi level of MXenes: O termination always increases the work function with respect to that of bare surface, OH always decreases it, while F exhibits either trend depending on the specific material. This phenomenon originates from the effect of surface dipoles, which together with the weakmore » Fermi level pinning, enable Schottky-barrier-free hole (or electron) injection into 2D semiconductors through van der Waals junctions with some of the O-terminated (or all the OH-terminated) MXenes. Furthermore, we suggest synthetic routes to control the surface terminations based on the calculated formation energies. Finally, this study enhances the understanding of the correlation between surface chemistry and electronic/transport properties of 2D materials, and also gives practical predictions for improving 2D electronics.« less

  6. Schottky-barrier-free contacts with two-dimensional semiconductors by surface-engineered MXenes

    SciTech Connect

    Liu, Yuanyue; Xiao, Hai; Goddard, III, William A.

    2016-11-22

    Two-dimensional (2D) metal carbides and nitrides, called MXenes, have attracted great interest for applications such as energy storage. Here we demonstrate their potential as Schottky-barrier-free metal contacts to 2D semiconductors, providing a solution to the contact-resistance problem in 2D electronics. Based on first principles calculations, we find that the surface chemistry strongly affects the Fermi level of MXenes: O termination always increases the work function with respect to that of bare surface, OH always decreases it, while F exhibits either trend depending on the specific material. This phenomenon originates from the effect of surface dipoles, which together with the weak Fermi level pinning, enable Schottky-barrier-free hole (or electron) injection into 2D semiconductors through van der Waals junctions with some of the O-terminated (or all the OH-terminated) MXenes. Furthermore, we suggest synthetic routes to control the surface terminations based on the calculated formation energies. Finally, this study enhances the understanding of the correlation between surface chemistry and electronic/transport properties of 2D materials, and also gives practical predictions for improving 2D electronics.

  7. Modelling the chemistry of Mn-doped MgO for bulk and (100) surfaces.

    PubMed

    Logsdail, Andrew J; Downing, Christopher A; Keal, Thomas W; Sherwood, Paul; Sokol, Alexey A; Catlow, C Richard A

    2016-10-19

    We have investigated the energetic properties of Mn-doped MgO bulk and (100) surfaces using a QM/MM embedding computational method, calculating the formation energy for doped systems, as well as for surface defects, and the subsequent effect on chemical reactivity. Low-concentration Mn doping is endothermic for isovalent species in the bulk but exothermic for higher oxidation states under p-type conditions, and compensated by electrons going to the Fermi level rather than cation vacancies. The highest occupied dopant Mn 3d states are positioned in the MgO band gap, about 4.2 eV below the vacuum level. Surface Mn-doping is more favourable than subsurface doping, and marginally exothermic on a (100) surface at high O2 pressures. For both types of isovalent Mn-doped (100) surfaces, the formation energy for catalytically important oxygen defects is less than for pristine MgO, with F(0) and F(2+)-centres favoured in n- and p-type conditions, respectively. In addition, F(+)-centres are stabilised by favourable exchange coupling between the Mn 3d states and the vacancy-localised electrons, as verified through calculation of the vertical ionisation potential. The adsorption of CO2 on to the pristine and defective (100) surface is used as a probe of chemical reactivity, with isovalent subsurface Mn dopants mildly affecting reactivity, whereas isovalent surface-positioned Mn strongly alters the chemical interactions between the substrate and adsorbate. The differing chemical reactivity, when compared to pristine MgO, justifies further detailed investigations for more varied oxidation states and dopant species.

  8. Kinetics of proton diffusion in the regimes of fast and slow exchange between the membrane surface and the bulk solution.

    PubMed

    Medvedev, E S; Stuchebrukhov, A A

    2006-02-01

    The phenomenological model developed in our recent publications [9,10] is used to investigate the kinetics of proton diffusion from a source to a detector on the membrane surface. In most cases the observed kinetics shows a single diffusional maximum with the exponential ascending front and the power-law descending tail. The kinetics depends on the distance between the source and the detector. If the detector is located inside the proton collecting antenna, the kinetics corresponds to the surface diffusion at the times near the maximum and shortly thereafter, and it turns into the bulk diffusion kinetics at longer times, after the equilibrium is established between the membrane surface and the bulk solution. If the detector is located outside the antenna, the kinetics corresponds to the bulk diffusion at all times where the signal is nonvanishing. What is seen at locations near the antenna radius depends on the exchange regime. In the regime of fast exchange between the surface and the bulk as compared to the bulk diffusion, the kinetics shows a single peak whose location is intermediate between the peaks for the surface and bulk diffusion. In the regime of slow exchange there are two maxima corresponding to the surface and bulk diffusion. In buffered solutions the antenna radius decreases with increasing buffer concentration, which changes the kinetics from the surface to bulk diffusion. The theory is applied to interprete recent experiments on a phospholipid membrane [25]. It is found that (i) the fast exchange regime is operating since only a single maximum is observed; (ii) the shift of the maximum toward longer times with increasing buffer concentration is a manifestation of the transition from the surface to bulk diffusion kinetics.

  9. Electronic properties of surfaces, subsurface layers and interfaces for semiconductor and semimetal solid solutions

    NASA Astrophysics Data System (ADS)

    Romanov, O. V.; Belygh, A. M.; Krasnov, E. L.; Kalenik, V. I.

    1992-05-01

    The electronic and physical-chemical properties of restorded (dDX≤0.5-1.0 nm) surfaces at the depth of the space-charge region and of semiconductor(semimetal)-native anodic oxide (dDX=1.0-100 nm) interfaces have been investigated by the combined field effect in electrolytes (CFEE) method at T = 273-305 K and in metal-oxide-semiconductor (MOS) systems at T = 77-305 K for monocrystalline solid solutions CdxHg1-xTe (0≤x≤0.4) and MnxHg1-xTe (0≤x≤0.24). The interpretation of experimental capacitance-potential and current-potential characteristics has been effectuated allowing for the degeneracy of the electron-hole gas and conduction band non-parabolicity (considered in the Kane approximation). The electronic properties of the surface and subsurface regions have been determined: the forbidden zone (Eg), the thermodynamic equilibrium (ni) and non-equilibrium (nix) carrier concentrations for intrinsic material or the doping level (NA,D) for extrinsic material, the density-of- states effective masses (mcx, mhx), the equilibrium semiconductor(semimetal) surface potential (Vs0) and total electric charge captured at the interface defects (Qtot0), the fast surface states (FSS) density (NSS). The FSS density and the total electric charge at the interface (Qtotox) increase drastically (0.5-2 orders) as a function of the anodic oxide thickness at dOX < 8-12 nm.

  10. Excitation and propagation of shear-horizontal-type surface and bulk acoustic waves.

    PubMed

    Hashimoto, K Y; Yamaguchi, M

    2001-09-01

    This paper reviews the basic properties of shear-horizontal (SH)-type surface acoustic waves (SAWs) and bulk acoustic waves (BAWs). As one of the simplest cases, the structure supporting Bleustein-Gulyaev-Shimizu waves is considered, and their excitation and propagation are discussed from various view points. First, the formalism based on the complex integral theory is presented, where the surface is assumed to be covered with an infinitesimally thin metallic film, and it is shown how the excitation and propagation of SH-type waves are affected by the surface perturbation. Then, the analysis is extended to a periodic grating structure, and the behavior of SH-type SAWs under the grating structure is discussed. Finally, the origin of the leaky nature is explained.

  11. Bulk and surface electronic structure of actinide, rare earth, and transition metal elements and compounds

    SciTech Connect

    Wills, J.W.; Eriksson, O.

    1996-07-01

    This is the final report for a three-year, Laboratory Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). This project sought to study of unusual magnetic and structural properties of rare earth, actinide, and transition metals through high-precision electronic structure calculations. Magnetic moment anisotropies in bulk and surface systems were studied, with emphasis on novel surfaces with unusual magnetic properties with possible applicability in magnetic recording. The structural stability, bonding properties, and elastic response of the actinides, as well as transition and rare earth elements and compounds, were also studied. The project sought to understand the unusual crystallographic and cohesive properties of the actinides and the importance of correlation to structural stability and the nature of the delocalization transition in these elements. Theoretical photoemission spectra, including surface effects, were calculated for rare earths and actinides.

  12. Surface properties and photocatalytic activity of KTaO3, CdS, MoS2 semiconductors and their binary and ternary semiconductor composites.

    PubMed

    Bajorowicz, Beata; Cybula, Anna; Winiarski, Michał J; Klimczuk, Tomasz; Zaleska, Adriana

    2014-09-24

    Single semiconductors such as KTaO3, CdS MoS2 or their precursor solutions were combined to form novel binary and ternary semiconductor nanocomposites by the calcination or by the hydro/solvothermal mixed solutions methods, respectively. The aim of this work was to study the influence of preparation method as well as type and amount of the composite components on the surface properties and photocatalytic activity of the new semiconducting photoactive materials. We presented different binary and ternary combinations of the above semiconductors for phenol and toluene photocatalytic degradation and characterized by X-ray powder diffraction (XRD), UV-Vis diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) specific surface area and porosity. The results showed that loading MoS2 onto CdS as well as loading CdS onto KTaO3 significantly enhanced absorption properties as compared with single semiconductors. The highest photocatalytic activity in phenol degradation reaction under both UV-Vis and visible light irradiation and very good stability in toluene removal was observed for ternary hybrid obtained by calcination of KTaO3, CdS, MoS2 powders at the 10:5:1 molar ratio. Enhanced photoactivity could be related to the two-photon excitation in KTaO3-CdS-MoS2 composite under UV-Vis and/or to additional presence of CdMoO4 working as co-catalyst.

  13. Surface Majorana fermions and bulk collective modes in superfluid 3He-B

    NASA Astrophysics Data System (ADS)

    Park, YeJe; Chung, Suk Bum; Maciejko, Joseph

    2015-02-01

    The theoretical study of topological superfluids and superconductors has so far been carried out largely as a translation of the theory of noninteracting topological insulators into the superfluid language, whereby one replaces electrons by Bogoliubov quasiparticles and single-particle band Hamiltonians by Bogoliubov-de Gennes Hamiltonians. Band insulators and superfluids are, however, fundamentally different: While the former exist in the absence of interparticle interactions, the latter are broken symmetry states that owe their very existence to such interactions. In particular, unlike the static energy gap of a band insulator, the gap in a superfluid is due to a dynamical order parameter that is subject to both thermal and quantum fluctuations. In this work, we explore the consequences of bulk quantum fluctuations of the order parameter in the B phase of superfluid 3He on the topologically protected Majorana surface states. Neglecting the high-energy amplitude modes, we find that one of the three spin-orbit Goldstone modes in 3He-B couples to the surface Majorana fermions. This coupling in turn induces an effective short-range two-body interaction between the Majorana fermions, with coupling constant inversely proportional to the strength of the nuclear dipole-dipole interaction in bulk 3He. A mean-field theory suggests that the surface Majorana fermions in 3He-B may be in the vicinity of a metastable gapped time-reversal-symmetry-breaking phase.

  14. Computer simulation of bulk mechanical properties and surface hydration of biomaterials.

    PubMed

    Raffaini, Giuseppina; Elli, Stefano; Ganazzoli, Fabio

    2006-06-01

    Some intrinsic properties of biomaterials are calculated with atomistic computer simulations through energy minimizations and molecular dynamics methods. The mechanical properties of bulk polymers such as poly(vinyl alcohol) and poly(ethylene terephthalate) are obtained in terms of the Young's modulus, the bulk and shear moduli, and the Poisson ratio below the glass transition temperature. The calculated values apply to an ideal, defect-free sample, and therefore, they correspond to the theoretical upper limit for the mechanical behavior of these materials. The surface hydration of the same polymers and of graphite is analyzed in terms of the statistical distribution of the water molecules near the surfaces of these materials that range from hydrophilic to strongly hydrophobic. Consistent with recent spectroscopic evidence, it is found that water forms relatively ordered hydration shells driven by hydrogen bonds above the hydrophilic surface, but is highly disordered over the hydrophobic one. Therefore, it is suggested that computer simulations provide a new useful tool to investigate various aspects of biomaterials.

  15. Optical properties of thin semiconductor device structures with reflective back-surface layers

    SciTech Connect

    Clevenger, M.B.; Murray, C.S.; Ringel, S.A.; Sachs, R.N.; Qin, L.; Charache, G.W.; Depoy, D.M.

    1998-11-01

    Ultrathin semiconductor device structures incorporating reflective internal or back surface layers have been investigated recently as a means of improving photon recuperation, eliminating losses associated with free carrier absorption in conductive substrates and increasing the above bandgap optical thickness of thermophotovoltaic device structures. However, optical losses in the form of resonance absorptions in these ultrathin devices have been observed. This behavior in cells incorporating epitaxially grown FeAl layers and in devices that lack a substrate but have a back-surface reflector (BSR) at the rear of the active layers has been studied experimentally and modeled effectively. For thermophotovoltaic devices, these resonances represent a significant loss mechanism since the wavelengths at which they occur are defined by the active TPV cell thickness of {approximately} 2--5 microns and are in a spectral range of significant energy content for thermal radiators. This study demonstrates that ultrathin semiconductor structures that are clad by such highly reflective layers or by films with largely different indices of refraction display resonance absorptions that can only be overcome through the implementation of some external spectral control strategy. Effective broadband, below-bandgap spectral control using a back-surface reflector is only achievable using a large separation between the TPV active layers and the back-surface reflector.

  16. Coupling Emission from Single Localized Defects in Two-Dimensional Semiconductor to Surface Plasmon Polaritons.

    PubMed

    Cai, Tao; Dutta, Subhojit; Aghaeimeibodi, Shahriar; Yang, Zhili; Nah, Sanghee; Fourkas, John T; Waks, Edo

    2017-10-10

    Coupling of an atom-like emitter to surface plasmons provides a path toward significant optical nonlinearity, which is essential in quantum information processing and quantum networks. A large coupling strength requires nanometer-scale positioning accuracy of the emitter near the surface of the plasmonic structure, which is challenging. We demonstrate the coupling of single localized defects in a tungsten diselenide (WSe2) monolayer self-aligned to the surface plasmon mode of a silver nanowire. The silver nanowire induces a strain gradient on the monolayer at the overlapping area, leading to the formation of localized defect emission sites that are intrinsically close to the surface plasmon. We measured an average coupling efficiency with a lower bound of 26% ± 11% from the emitter into the plasmonic mode of the silver nanowire. This technique offers a way to achieve efficient coupling between plasmonic structures and localized defects of two-dimensional semiconductors.

  17. Photochemical etching during ultraviolet photolytic deposition of metal films on semiconductor surfaces

    NASA Astrophysics Data System (ADS)

    Kowalczyk, Steven P.; Miller, D. L.

    1986-01-01

    UV photochemical deposition of Sn films on GaAs (001) surfaces from a variety of tin-containing compounds (tetramethyltin, tetrabutyltin, dibutyltin dibromide, stannic chloride, hexamethylditin, dibutyltin sulfide, and iodotrimethyltin) was studied. X-ray photoelectron spectroscopy showed that during the initial stages of deposition from the halogenated compounds, the GaAs surface was photochemically etched, most likely by a halogen radical species. The photochemical etching resulted in an arsenic deficient surface which was particularly dramatic for the case of SnCl4. These results have important implications for the choice of sources for photochemical deposition when the metal-semiconductor interface is important and for photochemical etching if stoichiometric surfaces are required.

  18. Super-roughening: A new phase transition on the surfaces of crystals with quenched bulk disorder

    NASA Astrophysics Data System (ADS)

    Toner, John; Divincenzo, D. P.

    1990-01-01

    We present and study a model for surface fluctuations and equilibrium crystal shapes in solids with quenched bulk translational disorder but infinitely long-ranged orientational order. Strictly speaking, such surfaces have no sharp surface phase transition. However, for reasonable values of the bulk correlation length ξB (ξB>~30 Å should be sufficient), an experimentally sharp ``super-roughening'' transition occurs at a temperature TSR. This transition separates a high-temperature ``rough'' phase of the surface from a low-temperature ``super-rough'' phase that, counterintuitively, is even rougher. Specifically, the root-mean-square equilibrium vertical fluctuation in the position of the interface

    ¯ 1/2 diverge like √lnL as the length L of the surface -->∞ for T>TSR (just as in ordered solids for T greater than the roughening temperature TR), while

    ¯ 1/2lnL for T measured in surface-sensitive scattering experiments (e.g., anti-Bragg x-ray scattering) to go from algebraic decay C(qzx)~||x||-η(qz) in the rough phase to short-ranged order C(qzx)~||x-h¯(qz)ln(||x||) in the super-rough phase. The functional dependence of η(qz) on qz differs from that for fluctuating surfaces of both bulk ordered solids (above TR) and liquids. We identify an experimentally measurable correlation length ξSR that diverges as T-->TSR- as exp[ATSR2/(TSR-T)2], where A is a constant of order ln-4||ξB/a|| and a is a lattice constant. The equilibrium crystal shapes do not have facets in either the rough or the super-rough phase. At low temperatures in the super-rough phase, however, nearly flat regions appear, with a radius of curvature scaling like (ξB)-1.

  19. Nanostructured organic semiconductor films for molecular detection with surface-enhanced Raman spectroscopy

    NASA Astrophysics Data System (ADS)

    Yilmaz, Mehmet; Babur, Esra; Ozdemir, Mehmet; Gieseking, Rebecca L.; Dede, Yavuz; Tamer, Ugur; Schatz, George C.; Facchetti, Antonio; Usta, Hakan; Demirel, Gokhan

    2017-09-01

    π-Conjugated organic semiconductors have been explored in several optoelectronic devices, yet their use in molecular detection as surface-enhanced Raman spectroscopy (SERS)-active platforms is unknown. Herein, we demonstrate that SERS-active, superhydrophobic and ivy-like nanostructured films of a molecular semiconductor, α,ω-diperfluorohexylquaterthiophene (DFH-4T), can be easily fabricated by vapour deposition. DFH-4T films without any additional plasmonic layer exhibit unprecedented Raman signal enhancements up to 3.4 × 103 for the probe molecule methylene blue. The combination of quantum mechanical computations, comparative experiments with a fluorocarbon-free α,ω-dihexylquaterthiophene (DH-4T), and thin-film microstructural analysis demonstrates the fundamental roles of the π-conjugated core fluorocarbon substitution and the unique DFH-4T film morphology governing the SERS response. Furthermore, Raman signal enhancements up to ~1010 and sub-zeptomole (<10-21 mole) analyte detection were accomplished by coating the DFH-4T films with a thin gold layer. Our results offer important guidance for the molecular design of SERS-active organic semiconductors and easily fabricable SERS platforms for ultrasensitive trace analysis.

  20. Platinum nanoparticles on gallium nitride surfaces: effect of semiconductor doping on nanoparticle reactivity.

    PubMed

    Schäfer, Susanne; Wyrzgol, Sonja A; Caterino, Roberta; Jentys, Andreas; Schoell, Sebastian J; Hävecker, Michael; Knop-Gericke, Axel; Lercher, Johannes A; Sharp, Ian D; Stutzmann, Martin

    2012-08-01

    Platinum nanoparticles supported on n- and p-type gallium nitride (GaN) are investigated as novel hybrid systems for the electronic control of catalytic activity via electronic interactions with the semiconductor support. In situ oxidation and reduction were studied with high pressure photoemission spectroscopy. The experiments revealed that the underlying wide-band-gap semiconductor has a large influence on the chemical composition and oxygen affinity of supported nanoparticles under X-ray irradiation. For as-deposited Pt cuboctahedra supported on n-type GaN, a higher fraction of oxidized surface atoms was observed compared to cuboctahedral particles supported on p-type GaN. Under an oxygen atmosphere, immediate oxidation was recorded for nanoparticles on n-type GaN, whereas little oxidation was observed for nanoparticles on p-type GaN. Together, these results indicate that changes in the Pt chemical state under X-ray irradiation depend on the type of GaN doping. The strong interaction between the nanoparticles and the support is consistent with charge transfer of X-ray photogenerated free carriers at the semiconductor-nanoparticle interface and suggests that GaN is a promising wide-band-gap support material for photocatalysis and electronic control of catalysis.

  1. Electrically detected and conventional magnetic resonance investigation of surface and bulk states in polyaniline thin films

    NASA Astrophysics Data System (ADS)

    Castro, Fernando A.; Graeff, Carlos F. O.

    2007-04-01

    Electrically detected magnetic resonance (EDMR) and electron paramagnetic resonance (EPR) were used to investigate emeraldine base polyaniline films. The magnetic susceptibility presented a Curie (localized spins)—Pauli (delocalized spins) transition at 240 K, when we also observed a transition in the dependence of the g factor with temperature (T). Peak-to-peak linewidth decreases with increasing temperature, reflecting that motional narrowing limits the hyperfine and dipolar broadening in this polymer. EDMR spectra could only be observed above 250 K in accordance to EPR results. Surface and bulk transport could be separated and their analysis reflected the effect of magnetic interaction with oxygen.

  2. Guided Evolution of Bulk Metallic Glass Nanostructures: A Platform for Designing 3D Electrocatalytic Surfaces.

    PubMed

    Doubek, Gustavo; Sekol, Ryan C; Li, Jinyang; Ryu, Won-Hee; Gittleson, Forrest S; Nejati, Siamak; Moy, Eric; Reid, Candy; Carmo, Marcelo; Linardi, Marcelo; Bordeenithikasem, Punnathat; Kinser, Emily; Liu, Yanhui; Tong, Xiao; Osuji, Chinedum O; Schroers, Jan; Mukherjee, Sundeep; Taylor, André D

    2016-03-09

    Electrochemical devices such as fuel cells, electrolyzers, lithium-air batteries, and pseudocapacitors are expected to play a major role in energy conversion/storage in the near future. Here, it is demonstrated how desirable bulk metallic glass compositions can be obtained using a combinatorial approach and it is shown that these alloys can serve as a platform technology for a wide variety of electrochemical applications through several surface modification techniques. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  3. Ultrafast magneto-photocurrents in GaAs: Separation of surface and bulk contributions

    SciTech Connect

    Schmidt, Christian B. Priyadarshi, Shekhar; Bieler, Mark; Tarasenko, Sergey A.

    2015-04-06

    We induce ultrafast magneto-photocurrents in a GaAs crystal employing interband excitation with femtosecond laser pulses at room temperature and non-invasively separate surface and bulk contributions to the overall current response. The separation between the different symmetry contributions is achieved by measuring the simultaneously emitted terahertz radiation for different sample orientations. Excitation intensity and photon energy dependences of the magneto-photocurrents for linearly and circularly polarized excitations reveal an involvement of different microscopic origins, one of which is the inverse spin Hall effect. Our experiments are important for a better understanding of the complex momentum-space carrier dynamics in magnetic fields.

  4. Bulk and surface electromagnetic response of metallic metamaterials to convection electrons

    NASA Astrophysics Data System (ADS)

    So, Jin-Kyu; Jang, Kyu-Ha; Park, Gun-Sik; Garcia-Vidal, F. J.

    2011-08-01

    The electromagnetic response of three-dimensional metallic metamaterials with isotropic effective index of refraction to fast-moving electrons is studied by numerical simulations. The considered metamaterials can support Cerenkov radiation [P. A. Cherenkov, Dokl. Akad. Nauk SSSR 2, 451 (1934)], and their effective dielectric behavior is confirmed by the detailed angular dependence of the generated radiation cone on the kinetic energy of electrons. Moreover, in addition to the predicted bulk modes, surface electromagnetic excitation is observed in a specific type of metamaterials and its dispersion is sensitive to the thickness of the subwavelength rods.

  5. Ba(Zn1−2xMnxCux)2As2: A Bulk Form Diluted Ferromagnetic Semiconductor with Mn and Cu Codoping at Zn Sites

    PubMed Central

    Man, Huiyuan; Guo, Shengli; Sui, Yu; Guo, Yang; Chen, Bin; Wang, Hangdong; Ding, Cui; Ning, F.L.

    2015-01-01

    We report the synthesis and characterization of a bulk form diluted magnetic semiconductor Ba(Zn1−2xMnxCux)2As2 with the crystal structure identical to that of “122” family iron based superconductors and the antiferromagnet BaMn2As2. No ferromagnetic order occurs with (Zn, Mn) or (Zn, Cu) substitution in the parent compound BaZn2As2. Only when Zn is substituted by both Mn and Cu simultaneously, can the system undergo a ferromagnetic transition below TC ~ 70 K, followed by a magnetic glassy transition at Tf  ~ 35 K. AC susceptibility measurements for Ba(Zn0.75Mn0.125Cu0.125)2As2 reveal that Tf strongly depends on the applied frequency with and a DC magnetic field dependence of , demonstrating that a spin glass transition takes place at Tf. As large as −53% negative magnetoresistance has been observed in Ba(Zn1−2xMnxCux)2As2, enabling its possible application in memory devices. PMID:26492957

  6. Ba(Zn(1-2x)MnxCux)2As2: A Bulk Form Diluted Ferromagnetic Semiconductor with Mn and Cu Codoping at Zn Sites.

    PubMed

    Man, Huiyuan; Guo, Shengli; Sui, Yu; Guo, Yang; Chen, Bin; Wang, Hangdong; Ding, Cui; Ning, F L

    2015-10-23

    We report the synthesis and characterization of a bulk form diluted magnetic semiconductor Ba(Zn(1-2x)MnxCux)2As2 (0.025 ≤ x ≤ 0.2) with the crystal structure identical to that of "122" family iron based superconductors and the antiferromagnet BaMn2As2. No ferromagnetic order occurs with (Zn, Mn) or (Zn, Cu) substitution in the parent compound BaZn2As2. Only when Zn is substituted by both Mn and Cu simultaneously, can the system undergo a ferromagnetic transition below TC ~ 70 K, followed by a magnetic glassy transition at Tf  ~ 35 K. AC susceptibility measurements for Ba(Zn0.75Mn0.125Cu0.125)2As2 reveal that Tf strongly depends on the applied frequency with [formula in text] and a DC magnetic field dependence of [formula in text], demonstrating that a spin glass transition takes place at Tf. As large as -53% negative magnetoresistance has been observed in Ba(Zn(1-2x)MnxCux)2As2, enabling its possible application in memory devices.

  7. Graded morphology in bulk-heterojunction solar cells based on colloidal semiconductor nanostructures: Directed charge-separation and facile carrier transport

    NASA Astrophysics Data System (ADS)

    Dasgupta, Uttiya; Pal, Amlan J.

    2016-08-01

    We introduce density gradient of p- and n-type compound semiconductor nanostructures in bulk-heterojunction (BHJ) solar cells. The graded BHJs (GBHJs) were formed with the p-type copper-zinc-tin-sulfide (Cu2ZnSnS4) nanoparticles and n-type Bi2S3 nanorods, both of which were based on nontoxic and earth-abundant elements and were grown at a moderate reaction temperature (180 °C). The concentration gradient of the nanostructures in the GBHJs provided a preferred directionality of nano-depletion regions for directed charge separation and also a controlled vertical segregation for ever-increasing carrier-transport pathways during the charge-extraction process. A comparison of solar cell characteristics having a bilayer, a BHJ, and a GBHJ structure is being presented. The performance of the thickness-optimized devices for highest efficiency shows that the GBHJ offered an improved short-circuit current as compared to the bilayer and the BHJ structures and a higher fill-factor as compared to the BHJ device. The overall energy conversion efficiency (η) of GBHJ exceeded that of the other two heterojunctions. The advantages of GBHJ structures in yielding an improved η have been explained through an increased exciton dissociation process along with a lower carrier recombination as compared to the bilayer and the BHJ structures, respectively. Series and shunt resistances, which were derived from current-voltage characteristics and impedance spectroscopy, supported such analyses.

  8. K and Mn co-doped BaCd{sub 2}As{sub 2}: A hexagonal structured bulk diluted magnetic semiconductor with large magnetoresistance

    SciTech Connect

    Yang, Xiaojun; Zhang, Pan; Jiang, Hao; Luo, Yongkang; Chen, Qian; Feng, Chunmu; Tao, Qian; Cao, Guanghan; Xu, Zhu-An; Li, Yuke; Cao, Chao; Dai, Jianhui

    2013-12-14

    A bulk diluted magnetic semiconductor was found in the K and Mn co-doped BaCd{sub 2}As{sub 2} system. Different from recently reported tetragonal ThCr{sub 2}Si{sub 2}-structured II-II-V based (Ba,K)(Zn,Mn){sub 2}As{sub 2}, the Ba{sub 1−y}K{sub y}Cd{sub 2−x}Mn{sub x}As{sub 2} system has a hexagonal CaAl{sub 2}Si{sub 2}-type structure with the Cd{sub 2}As{sub 2} layer forming a honeycomb-like network. The Mn concentration reaches up to x ∼ 0.4. Magnetization measurements show that the samples undergo ferromagnetic transitions with Curie temperature up to 16 K. With low coercive field of less than 10 Oe and large magnetoresistance of about −70%, the hexagonal structured Ba{sub 1−y}K{sub y}Cd{sub 2−x}Mn{sub x}As{sub 2} can be served as a promising candidate for spin manipulations.

  9. Nature of Catalytic Active Sites Present on the Surface of Advanced Bulk Tantalum Mixed Oxide Photocatalysts

    SciTech Connect

    Phivilay, Somphonh; Puretzky, Alexander A; Domen, Kazunari Domen; Wachs, Israel

    2013-01-01

    The most active photocatalyst system for water splitting under UV irradiation (270 nm) is the promoted 0.2%NiO/NaTaO3:2%La photocatalyst with optimized photonic efficiency (P.E.) of 56%, but fundamental issues about the nature of the surface catalytic active sites and their involvement in the photocatalytic process still need to be clarified. This is the first study to apply cutting edge surface spectroscopic analyses to determine the surface nature of tantalum mixed oxide photocatalysts. Surface analysis with HR-XPS (1-3nm) and HS-LEIS (0.3nm) spectroscopy indicates that the NiO and La2O3 promoters are concentrated in the surface region of the bulk NaTaO3 phase. The La2O3 is concentrated on the NaTaO3 outermost surface layers while NiO is distributed throughout the NaTaO3 surface region (1-3nm). Raman and UV-vis spectroscopy revealed that the bulk molecular and electronic structures, respectively, of NaTaO3 were not modified by the addition of the La2O3 and NiO promoters, with La2O3 resulting in a slightly more ordered structure. Photoluminescence (PL) spectroscopy reveals that the addition of La2O3 and NiO produces a greater number of electron traps resulting in the suppression of the recombination of excited electrons/holes. In contrast to earlier reports, the La2O3 is only a textural promoter (increasing the BET surface area ~7x by stabilizing smaller NaTaO3 particles), but causes a ~3x decrease in the specific photocatalytic TORs ( mol H2/m2/h) rate because surface La2O3 blocks exposed catalytic active NaTaO3 sites. The NiO promoter was found to be a potent electronic promoter that enhances the NaTaO3 surface normalized TORs by a factor of ~10-50 and TOF by a factor of ~10. The level of NiO promotion is the same in the absence and presence of La2O3 demonstrating that there is no promotional synergistic interaction between the NiO and La2O3 promoters. This study demonstrates the important contributions of the photocatalyst surface properties to the fundamental

  10. Surface reactions during the atomic layer deposition of high-kappa dielectrics on III-V semiconductor surfaces

    NASA Astrophysics Data System (ADS)

    Ye, Liwang

    The quality of the dielectric/semiconductor interface is one of the most critical parameters for the fabrication of high-speed and low-power-consumption III-V semiconductor based metal-oxide-semiconductor field effect transistors (MOSFETs), as it determines the device performance. This dissertation contains investigations of the deposition and interface of binary oxide films on GaAs(100) and InAs(100) surfaces aiming at understanding the removal of the surface native oxides during certain atomic layer deposition (ALD) processes. To accomplish that, two complementary experimental approaches have been used. Initially, films were deposited in a conventional ALD reactor and characterized ex situ using spectroscopic ellipsometry (SE), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), and atomic force microscopy (AFM). The systems examined were Ta2O 5 on GaAs(100) surfaces from pentakis(dimethylamino) tantalum (Ta(N(CH 3)2)5, PDMAT) and TiO2 on GaAs(100) and InAs(100) surfaces from tetrakis(dimethylamino) titanium (Ti(N(CH 3)2)4, TDMAT). For these systems, deposition at the optimal ALD temperature resulted in practically sharp interfaces. Indium oxides were found to diffuse through ~ 6 nm of TiO2 film and accumulate on the topmost film layer. For the ALD of Ta2O5 on GaAs(100) surfaces, native oxide removal was enhanced at deposition temperatures above the ALD window; for ALD of TiO2 on both GaAs(100) and InAs(100) surfaces, native oxide removal was enhanced as the deposition temperatures increased up to 250 A°C, while oxidation of the interface was observed for deposition above 300 A°C due to the formation of noncontinuous films. To elucidate the surface reactions occurring during the deposition, an in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy apparatus was constructed and used to investigate the surface reactions during the ALD of TiO2 and HfO2 on GaAs(100) surfaces. The

  11. Surface hydrodynamics of viscoelastic fluids and soft solids: Surfing bulk rheology on capillary and Rayleigh waves.

    PubMed

    Monroy, Francisco

    2017-07-18

    From the recent advent of the new soft-micro technologies, the hydrodynamic theory of surface modes propagating on viscoelastic bodies has reinvigorated this field of technology with interesting predictions and new possible applications, so recovering its scientific interest very limited at birth to the academic scope. Today, a myriad of soft small objects, deformable meso- and micro-structures, and macroscopically viscoelastic bodies fabricated from colloids and polymers are already available in the materials catalogue. Thus, one can envisage a constellation of new soft objects fabricated by-design with a functional dynamics based on the mechanical interplay of the viscoelastic material with the medium through their interfaces. In this review, we recapitulate the field from its birth and theoretical foundation in the latest 1980s up today, through its flourishing in the 90s from the prediction of extraordinary Rayleigh modes in coexistence with ordinary capillary waves on the surface of viscoelastic fluids, a fact first confirmed in experiments by Dominique Langevin and me with soft gels [Monroy and Langevin, Phys. Rev. Lett. 81, 3167 (1998)]. With this observational discovery at sight, we not only settled the theory previously formulated a few years before, but mainly opened a new field of applications with soft materials where the mechanical interplay between surface and bulk motions matters. Also, new unpublished results from surface wave experiments performed with soft colloids are reported in this contribution, in which the analytic methods of wave surfing synthetized together with the concept of coexisting capillary-shear modes are claimed as an integrated tool to insightfully scrutinize the bulk rheology of soft solids and viscoelastic fluids. This dedicatory to the figure of Dominique Langevin includes an appraisal of the relevant theoretical aspects of the surface hydrodynamics of viscoelastic fluids, and the coverage of the most important experimental

  12. Passive microwave sensing of soil moisture content: Soil bulk density and surface roughness

    NASA Technical Reports Server (NTRS)

    Wang, J. R.

    1982-01-01

    Microwave radiometric measurements over bare fields of different surface roughnesses were made at the frequencies of 1.4 GHz, 5 GHz, and 10.7 GHz to study the frequency dependence as well as the possible time variation of surface roughness. The presence of surface roughness was found to increase the brightness temperature of soils and reduce the slope of regression between brightness temperature and soil moisture content. The frequency dependence of the surface roughness effect was relatively weak when compared with that of the vegetation effect. Radiometric time series observation over a given field indicated that field surface roughness might gradually diminish with time, especially after a rainfall or irrigation. This time variation of surface roughness served to enhance the uncertainty in remote soil moisture estimate by microwave radiometry. Three years of radiometric measurements over a test site revealed a possible inconsistency in the soil bulk density determination, which turned out to be an important factor in the interpretation of radiometric data.

  13. Structural and electronic properties of bulk and low-index surfaces of zincblende PtC

    NASA Astrophysics Data System (ADS)

    Gokhan Sensoy, Mehmet; Toffoli, Daniele; Ustunel, Hande

    2017-03-01

    Transition metal carbides have been extensively used in diverse applications over the past decade. Their versatility is in part thanks to their unique bonding, which displays a mixture of ionic, metallic and covalent character. While the bulk structure of zincblende (ZB) PtC has been investigated several times, a detailed understanding of the electronic and structural properties of its low-index surfaces is lacking. In this work, we present an ab initio investigation of the properties of five crystallographic ZB PtC surfaces (Pt/C-terminated PtC(1 0 0), PtC(1 1 0) and Pt/C-terminated PtC(1 1 1)). Upon geometry optimization, both polar and nonpolar surfaces undergo a mild interlayer relaxation, without extensive reconstructions. Calculated vacancy formation energies indicate facile C removal on the (1 1 1) surface while Pt-vacancy formation is endothermic. Finally, atomic O adsorption energies on all surfaces reveal a high affinity of the C-terminated surfaces towards this species.

  14. Surface-and bulk-properties of EPDM rubber modified by electron beam irradiation

    NASA Astrophysics Data System (ADS)

    Majumder, Papiya Sen; Bhowmick, Anil K.

    1999-01-01

    Electron beam initiated grafting of trimethylol propane triacrylate (TMPTA) onto ethylene propylene diene monomer (EPDM) has been carried out over a wide range of irradiation doses (0-200 kGy) using a fixed concentration (10%) of TMPTA. The samples have been both surface and bulk modified. Infrared (IR) studies indicate increased peak absorbances at 1730, 1260, 1120 and 1019 cm -1 upto 50 kGy and hence increased CO and C-O-C concentrations. The results are further supported by X-ray photoelectron spectroscopy (XPS) studies. The surface energy of EPDM increases from 46.5 to 60.7 mJ/m 2 on irradiation of the surface modified samples to 50 kGy dose, due to increased contribution of γSAB and γS(-). The results have been explained with the help of IR and XPS data. The values of tensile strength of the surface modified samples have not changed very significantly, while the moduli values have increased at the cost of the elongation at break. DMTA studies have shown changes in Tg and tan δmax on modification of the surface. The surface morphology of the modified and irradiated samples reveals acrylate flow marks at high magnification.

  15. The role of bandgap energy excess in surface emission of terahertz radiation from semiconductors

    NASA Astrophysics Data System (ADS)

    Alfaro-Gomez, M.; Castro-Camus, E.

    2017-01-01

    We use a Monte-Carlo model to simulate semi-classical photo-carrier dynamics of InAs, InGaAs, and GaAs that leads to terahertz emission. We compare the emission power of all three semiconductors as a function of excitation photon energy finding that the carrier excess excitation energy is more relevant to explain their performance difference than their mobilities. We conclude that ballistic transport after photoexcitation is the dominant mechanism for terahertz emission instead of diffusion driven or surface field driven charge separation, which were traditionally considered the most relevant mechanisms.

  16. Strong Coupling between Surface Plasmons and Excitons in an Organic Semiconductor

    NASA Astrophysics Data System (ADS)

    Bellessa, J.; Bonnand, C.; Plenet, J. C.; Mugnier, J.

    2004-07-01

    We report on the observation of a strong coupling between a surface plasmon and an exciton. Reflectometry experiments are performed on an organic semiconductor, namely, cyanide dye J aggregates, deposited on a silver film. The dispersion lines present an anticrossing that is the signature of a strong plasmon-exciton coupling. Mixed states are formed in a similar way as microcavities polaritons. The Rabi splitting characteristic of this coupling reaches 180meV at room temperature. The emission of the low energy plasmon-exciton mixed state has been observed and is largely shifted from the uncoupled emission.

  17. General aspects of the vapor growth of semiconductor crystals - A study based on DFT simulations of the NH3/NH2 covered GaN(0001) surface in hydrogen ambient

    NASA Astrophysics Data System (ADS)

    Kempisty, Paweł; Strak, Paweł; Sakowski, Konrad; Krukowski, Stanisław

    2014-03-01

    Vapor growth of semiconductors is analyzed using recently obtained dependence of the adsorption energy on the electron charge transfer between the surface adsorbed species and the bulk [Krukowski et al. J. Appl. Phys. 114 (2013) 063507, Kempisty et al. ArXiv1307.5778 (2013)]. Ab initio calculations were performed to study the physical properties of GaN(0001) surface in ammonia-rich conditions, i.e. covered by mixture of NH3 molecules and NH2 radicals. The Fermi level is pinned at valence band maximum (VBM) and conduction band minimum (CBM) for full coverage by NH3 molecules and NH2 radicals, respectively. For the crossover content of ammonia of about 25% monolayer (ML), the Fermi level is unpinned. It was shown that hydrogen adsorption energy depends on the doping in the bulk for the unpinned Fermi level, i.e. for this coverage. Surface structure thermodynamic and mechanical stability criteria are defined and compared. Mechanical stability of the coverage of such surfaces was checked by determination of the desorption energy of hydrogen molecules. Thermodynamic stability analysis indicates that initially equilibrium hydrogen vapor partial pressure steeply increases with NH3 content to attain the crossover NH3/NH2 coverage, i.e. the unpinned Fermi level condition. For such condition the entire range of experimentally accessible pressures belongs showing that vapor growth of semiconductor crystals occurs predominantly for unpinned Fermi level at the surface, i.e. for flat bands. Accordingly, adsorption energy of most species depends on the doping in the bulk that is based on the possible molecular scenario explaining dependence of the growth and the doping of semiconductor crystals on the doping in the bulk.

  18. Manifestation of a Second Dirac Surface State and Bulk Bands in THz Radiation from Topological Insulators

    PubMed Central

    Tu, Chien-Ming; Yeh, Tien-Tien; Tzeng, Wen-Yen; Chen, Yi-Ru; Chen, Hsueh-Ju; Ku, Shin-An; Luo, Chih-Wei; Lin, Jiunn-Yuan; Wu, Kaung-Hsiung; Juang, Jenh-Yih; Kobayashi, Takayoshi; Cheng, Cheng-Maw; Tsuei, Ku-Ding; Berger, Helmuth; Sankar, Raman; Chou, Fang-Cheng

    2015-01-01

    Topological insulators (TIs) are interesting quantum matters that have a narrow bandgap for bulk and a Dirac-cone-like conducting surface state (SS). The recent discovered second Dirac surface state (SS) and bulk bands (BBs) located ~1.5 eV above the first SS are important for optical coupling in TIs. Here, we report on the time-domain measurements of THz radiation generated from TIs n-type Cu0.02Bi2Se3 and p-type Bi2Te3 single crystals by ultrafast optical pulse excitation. The observed polarity-reversal of the THz pulse originated from transient current is unusual, and cannot be reconciled with the photo-Dember effect. The second SS and BBs are found to be indispensable for the explanation of the unusual phenomenon. Thanks to the existence of the second SS and BBs, TIs manifest an effective wide band gap in THz generation. The present study demonstrates that time-domain THz spectroscopy provide rich information of the optical coupling and the electronic structure of TIs. PMID:26370337

  19. Interplay of surface energy and bulk thermodynamic forces in ordered block copolymer droplets.

    SciTech Connect

    Hur, Su-Mi; Onses, M. Serdar; Ramirez-Hernandez, Abelardo; Nealey, Paul F.; Rogers, John A.; de Pablo, Juan J.

    2015-07-14

    The wetting state of a simple liquid on a solid substrate, as summarized by Young's equation, is dictated by the interfacial energies of the different phases that coexist in the system. For simple fluids, rotational symmetry gives rise to symmetric droplets around the axis perpendicular to the substrate. This is not the case for nanostructured fluids, such as block copolymers, where the inherent thermodynamic ordering forces compete with surface tension. This competition is particularly important in nanoscale droplets, where the size of the droplets is a small multiple of the natural periodicity of the block copolymer in the bulk. In the nanoscale regime, droplet shape and internal structure arise from a subtle interplay between interfacial and bulk contributions to the free energy. In this work, we examine the consequences of surface-polymer interaction energies on droplet morphology through a concerted simulation and experimental effort. When the block copolymer is deposited on a neutral substrate we find non-circular arrangements with perpendicular domains. However, when a preferential substrate is used, the resulting morphology depends on droplet size. In large droplets, we observe bottle cap-shaped structures with a ring of perpendicular domains along the perimeter, while small droplets exhibit stripes of perpendicular domains.

  20. Discerning the Location and Nature of Coke Deposition from Surface to Bulk of Spent Zeolite Catalysts

    PubMed Central

    Devaraj, Arun; Vijayakumar, Murugesan; Bao, Jie; Guo, Mond F.; Derewinski, Miroslaw A.; Xu, Zhijie; Gray, Michel J.; Prodinger, Sebastian; Ramasamy, Karthikeyan K.

    2016-01-01

    The formation of carbonaceous deposits (coke) in zeolite pores during catalysis leads to temporary deactivation of catalyst, necessitating regeneration steps, affecting throughput, and resulting in partial permanent loss of catalytic efficiency. Yet, even to date, the coke molecule distribution is quite challenging to study with high spatial resolution from surface to bulk of the catalyst particles at a single particle level. To address this challenge we investigated the coke molecules in HZSM-5 catalyst after ethanol conversion treatment by a combination of C K-edge X-ray absorption spectroscopy (XAS), 13C Cross polarization-magic angle spinning nuclear magnetic resonance (CP-MAS NMR) spectroscopy, and atom probe tomography (APT). XAS and NMR highlighted the aromatic character of coke molecules. APT permitted the imaging of the spatial distribution of hydrocarbon molecules located within the pores of spent HZSM-5 catalyst from surface to bulk at a single particle level. 27Al NMR results and APT results indicated association of coke molecules with Al enriched regions within the spent HZSM-5 catalyst particles. The experimental results were additionally validated by a level-set–based APT field evaporation model. These results provide a new approach to investigate catalytic deactivation due to hydrocarbon coking or poisoning of zeolites at an unprecedented spatial resolution. PMID:27876869

  1. Discerning the Location and Nature of Coke Deposition from Surface to Bulk of Spent Zeolite Catalysts

    NASA Astrophysics Data System (ADS)

    Devaraj, Arun; Vijayakumar, Murugesan; Bao, Jie; Guo, Mond F.; Derewinski, Miroslaw A.; Xu, Zhijie; Gray, Michel J.; Prodinger, Sebastian; Ramasamy, Karthikeyan K.

    2016-11-01

    The formation of carbonaceous deposits (coke) in zeolite pores during catalysis leads to temporary deactivation of catalyst, necessitating regeneration steps, affecting throughput, and resulting in partial permanent loss of catalytic efficiency. Yet, even to date, the coke molecule distribution is quite challenging to study with high spatial resolution from surface to bulk of the catalyst particles at a single particle level. To address this challenge we investigated the coke molecules in HZSM-5 catalyst after ethanol conversion treatment by a combination of C K-edge X-ray absorption spectroscopy (XAS), 13C Cross polarization-magic angle spinning nuclear magnetic resonance (CP-MAS NMR) spectroscopy, and atom probe tomography (APT). XAS and NMR highlighted the aromatic character of coke molecules. APT permitted the imaging of the spatial distribution of hydrocarbon molecules located within the pores of spent HZSM-5 catalyst from surface to bulk at a single particle level. 27Al NMR results and APT results indicated association of coke molecules with Al enriched regions within the spent HZSM-5 catalyst particles. The experimental results were additionally validated by a level-set–based APT field evaporation model. These results provide a new approach to investigate catalytic deactivation due to hydrocarbon coking or poisoning of zeolites at an unprecedented spatial resolution.

  2. Skyrmions in quasi-2D chiral magnets with broken bulk and surface inversion symmetry

    NASA Astrophysics Data System (ADS)

    Rowland, James; Banerjee, Sumilan; Randeria, Mohit

    2015-03-01

    Most theoretical studies of skyrmions have focused on chiral magnets with broken bulk inversion symmetry, stabilized by easy-axis anisotropy. Recently, we considered 2D systems with broken surface inversion and showed that skyrmion crystals are more stable than in 3D, pointing out the importance of easy-plane anisotropy. In the present work we investigate quasi-2D systems which break both bulk and surface inversion symmetry. The Landau-Ginzburg free energy functional thus contains two Dzyloshinskii-Moriya terms of strength DD and DR arising from Dresselhaus and Rashba spin-orbit coupling respectively. We trace the evolution of the phase diagram as DD /DR is varied, and find that skyrmions are increasingly destabilized with respect to the cone phase as DD increases relative to DR. We find an evolution from vortex-like skyrmions in the pure Dresselhaus limit to hedgehog-like skyrmions in the pure Rashba limit. We discuss the relevance of these results to existing experiments and the prospects of tuning the ratio of Dresselhaus and Rashba spin-orbit coupling via film thickness and strain. Supported by NSF DMR-1410364 (J.R. and M.R.) and DOE-BES DE-SC0005035 (S.B.)

  3. Characterizing Surfaces of the Wide Bandgap Semiconductor Ilmenite with Scanning Probe Microcopies

    NASA Technical Reports Server (NTRS)

    Wilkins, R.; Powell, Kirk St. A.

    1997-01-01

    Ilmenite (FeTiO3) is a wide bandgap semiconductor with an energy gap of about 2.5eV. Initial radiation studies indicate that ilmenite has properties suited for radiation tolerant applications, as well as a variety of other electronic applications. Two scanning probe microscopy methods have been used to characterize the surface of samples taken from Czochralski grown single crystals. The two methods, atomic force microscopy (AFM) and scanning tunneling microscopy (STM), are based on different physical principles and therefore provide different information about the samples. AFM provides a direct, three-dimensional image of the surface of the samples, while STM give a convolution of topographic and electronic properties of the surface. We will discuss the differences between the methods and present preliminary data of each method for ilmenite samples.

  4. Formation and evolution of ripples on ion-irradiated semiconductor surfaces

    SciTech Connect

    Kang, M.; Wu, J. H.; Ye, W.; Jiang, Y.; Robb, E. A.; Chen, C.; Goldman, R. S.

    2014-02-03

    We have examined the formation and evolution of ripples on focused-ion-beam (FIB) irradiated compound semiconductor surfaces. Using initially normal-incidence Ga{sup +} FIB irradiation of InSb, we tuned the local beam incidence angle (θ{sub eff}) by varying the pitch and/or dwell time. For single-pass FIB irradiation, increasing θ{sub eff} induces morphological evolution from pits and islands to ripples to featureless surfaces. Multiple-pass FIB irradiation of the rippled surfaces at a fixed θ{sub eff} leads to island formation on the ripple crests, followed by nanorod (NR) growth. This ripple-NR transition provides an alternative approach for achieving dense arrays of NRs.

  5. Analytical models for the surface potential and electrical field distribution of bulk-silicon RESURF devices

    NASA Astrophysics Data System (ADS)

    Sun, Weifeng; Shi, Longxing

    2004-05-01

    In this paper, two 2-D analytical models for the surface potential and electrical field distribution along the drift region of the bulk-silicon RESURF lateral-extended drain MOS (LEDMOS) have been presented, which can be applied in the off- and on-states. The models give an approximate but explicit influence on surface potential and electrical field distributions in terms of the structure and fabricated process parameters, such as the length of the poly gate-field-plate, the implanted dose of the drift region, the diffusion time of the drift region and the substrate doping concentration etc. An effectual way to gain the optimum high-voltage device is also proposed. The analytical results are well supported by the simulation results obtained by Medici.

  6. Analysis of surface, subsurface, and bulk hydrogen in ZnO using nuclear reaction analysis

    SciTech Connect

    Traeger, F.; Kauer, M.; Woell, Ch.; Rogalla, D.; Becker, H.-W.

    2011-08-15

    Hydrogen concentrations in ZnO single crystals exposing different surfaces have been determined to be in the range of (0.02-0.04) at.% with an error of {+-}0.01 at.% using nuclear reaction analysis. In the subsurface region, the hydrogen concentration has been determined to be higher by up to a factor of 10. In contrast to the hydrogen in the bulk, part of the subsurface hydrogen is less strongly bound, can be removed by heating to 550 deg. C, and reaccommodated by loading with atomic hydrogen. By exposing the ZnO(1010) surface to water above room temperature and to atomic hydrogen, respectively, hydroxylation with the same coverage of hydrogen is observed.

  7. Experimental study on the surface characteristics of Pd-based bulk metallic glass

    NASA Astrophysics Data System (ADS)

    Zhang, Xiang; Sun, Bingli; Zhao, Na; Li, Qian; Hou, Jianhua; Feng, Weina

    2014-12-01

    The metallic glass has many unique and desirable physical and chemical characteristics for their long-range disordered atomic structure, among them the interfacial properties of the metallic glasses are crucial for their applications and manufacturing. In this work, the contact wetting angles between the polymer melts and Pd40Cu30Ni10P20 bulk metallic glass (Pd-BMG) with four kinds of roughness were analyzed. Experiments show the order of four polymers wettability on Pd-BMG was PP > HDPE > COC > PC. The surface free energy of Pd-BMG was estimated by Owens-Wendt method using the contact angles of three testing liquids. Neumann method was also used to further evidence the surface free energy of Pd-BMG comparing with PTFE, mold steels NAK80 and LKM2343ESR. The results provide theoretical and technical supports for the fabrication of metallic glass micro mold and the parameter optimization of polymer micro injection molding.

  8. Differentiation of Surface and Bulk Conductivities in Topological Insulators via Four-Probe Spectroscopy.

    PubMed

    Durand, Corentin; Zhang, X-G; Hus, Saban M; Ma, Chuanxu; McGuire, Michael A; Xu, Yang; Cao, Helin; Miotkowski, Ireneusz; Chen, Yong P; Li, An-Ping

    2016-04-13

    We show a new method to differentiate conductivities from the surface states and the coexisting bulk states in topological insulators using a four-probe transport spectroscopy in a multiprobe scanning tunneling microscopy system. We derive a scaling relation of measured resistance with respect to varying interprobe spacing for two interconnected conduction channels to allow quantitative determination of conductivities from both channels. Using this method, we demonstrate the separation of 2D and 3D conduction in topological insulators by comparing the conductance scaling of Bi2Se3, Bi2Te2Se, and Sb-doped Bi2Se3 against a pure 2D conductance of graphene on SiC substrate. We also quantitatively show the effect of surface doping carriers on the 2D conductance enhancement in topological insulators. The method offers a means to understanding not just the topological insulators but also the 2D to 3D crossover of conductance in other complex systems.

  9. Ultrafast optical studies of surface reaction processes at semiconductor interfaces. Progress report

    SciTech Connect

    Miller, R.J.D.

    1994-10-01

    The DOE funded research has focused on the development of novel non-linear optical methods for the in situ study of surface reaction dynamics. In particular, the work has concentrated on interfacial charge transfer processes as this is the simplest of all surface reactions, i.e., no bonds are broken and the reaction is derived from nuclear repolarization. Interfacial charge transfer forms the basis for a number of important solar energy conversion strategies. In these studies, semiconductor liquid junctions provide a convenient system in which the interfacial charge transfer can be optically initiated. The all-optical approach necessitates that the dynamics of the charge transfer event itself be put in the proper context of the operating photophysical processes at the surface. There are at least four dynamical processes that are coupled in determining the overall rate of electron flux across the interface. In the limit that interfacial charge transfer approaches strong coupling, the time scale for transport of even field accelerated carriers within the space charge region becomes comparable to the charge transfer dynamics. The transport component needs to be convolved to probes of the carrier population at the surface. The other two dynamical processes, carrier thermalization and surface state trapping, determine the states which ultimately serve as the donor levels to the solution acceptor distribution. In terms of the hot carrier model, these latter two processes compete with direct unthermalized charge transfer. There is a fifth dynamical process which also needs consideration: the solvent modes that are coupled to the reaction coordinate. Ultimately, the dynamics of solvent relaxation determine the upper limit to the charge transfer process. Different optical techniques have been developed to follow all the above dynamical processes in which a real time view of charge transfer dynamics at semiconductor surfaces is emerging. These results are discussed here.

  10. Modulating semiconductor surface electronic properties by inorganic peptide-binders sequence design.

    PubMed

    Matmor, Maayan; Ashkenasy, Nurit

    2012-12-19

    The use of proteins and peptides as part of biosensors and electronic devices has been the focus of intense research in recent years. However, despite the fact that the interface between the bioorganic molecules and the inorganic matter plays a significant role in determining the properties of such devices, information on the electronic properties of such interfaces is sparse. In this work, we demonstrate that the identity and position of single amino acid in short inorganic binding protein-segments can significantly modulate the electronic properties of semiconductor surfaces on which they are bound. Specifically, we show that the introduction of tyrosine or tryptophan, both possessing an aromatic side chain which higher occupied molecular orbitals are positioned in proximity to the edge of GaAs valence band, to the sequence of a peptide that binds to GaAs (100) results in changes of both the electron affinity and surface potential of the semiconductor. These effects were found to be more pronounced than the effects induced by the same amino acids once bound on the surface in a head-tail configuration. Furthermore, the relative magnitude of each effect was found to depend on the position of the modification in the sequence. This sequence dependent behavior is induced both indirectly by changes in the peptide surface coverage, and directly, probably, due to changes in the orientation and proximity of the tyrosine/tryptophan side group with respect to the surface due to the preferred conformation the peptide adopts on the surface. These studies reveal that despite the use of short protein oligomers and aiming at a non-natural-electronic task, the well-known relations between the proteins' structure and function is preserved. Combining the ability to tune the electronic properties at the interface with the ability to direct the growth of inorganic materials makes peptides promising building blocks for the construction of novel hybrid electronic devices and biosensors.

  11. Theoretical characterization of divacancies at the surface and in bulk MgO

    NASA Astrophysics Data System (ADS)

    Ojamäe, Lars; Pisani, Cesare

    1998-12-01

    Two types of divacancy at the (001) surface of MgO are theoretically studied and compared with the corresponding defect in the bulk: the pit, where a surface magnesium and the oxygen ion underneath are removed, and the tub, where both removed ions are at the surface. All calculations have been performed by means of the EMBED program which adopts an embedded-cluster approach in the frame of the Hartree-Fock (HF) approximation [C. Pisani F. Corà, R. Nada, and R. Orlando, Comput. Phys. Commun. 82, 139 (1994); C. Pisani and U. Birkenheuer, ibid. 96, 152 (1996)]; the semi-infinite host crystal for the study of the surface defects has been simulated with a four-layer slab. The energy released on formation of the divacancy from the two charged isolated vacancies is very high, almost 300 kcal/mol. The tub divacancy is the most stable, both as a neutral and as a singly charged defect. For the paramagnetic center (one electron trapped in the cavity), spin density data are provided and discussed with reference to results from electron paramagnetic resonance experiments and molecular cluster calculations [E. Giamello M. C. Paganini, D. Murphy, A. M. Ferrari, and G. Pacchioni, J. Phys. Chem. 101, 971 (1997)]. It is suggested that the tub divacancy is a common defect, if not the most common, at the highly dehydrated MgO surface.

  12. Mechanism to generate a two-dimensional electron gas at the surface of the charge-ordered semiconductor BaBiO3.

    PubMed

    Vildosola, Verónica; Güller, Francisco; Llois, Ana María

    2013-05-17

    In this Letter, we find by means of first-principles calculations a new physical mechanism to generate a two-dimensional electron gas, namely, the breaking of charge ordering at the surface of a charge-ordered semiconductor due to the incomplete oxygen environment of the surface ions. The emergence of the 2D gas is independent of the presence of oxygen vacancies or polar discontinuities; this is a self-doping effect. This mechanism might apply to many charge-ordered systems, in particular, we study the case of BaBiO(3)(001). Our calculations show that the outer layer of the Bi-terminated simulated surface turns more cubiclike and metallic while the inner layers remain in the insulating monoclinic state that the system present in the bulk form. On the other hand, the metallization does not occur for the Ba termination, a fact that makes this system appealing for nanostructuring. Finally, in view of the bulk properties of this material under doping, this particular finding sets another possible route for future exploration: the potential scenario of 2D superconductivity at the BaBiO(3) surface.

  13. Mechanism to Generate a Two-Dimensional Electron Gas at the Surface of the Charge-Ordered Semiconductor BaBiO3

    NASA Astrophysics Data System (ADS)

    Vildosola, Verónica; Güller, Francisco; Llois, Ana María

    2013-05-01

    In this Letter, we find by means of first-principles calculations a new physical mechanism to generate a two-dimensional electron gas, namely, the breaking of charge ordering at the surface of a charge-ordered semiconductor due to the incomplete oxygen environment of the surface ions. The emergence of the 2D gas is independent of the presence of oxygen vacancies or polar discontinuities; this is a self-doping effect. This mechanism might apply to many charge-ordered systems, in particular, we study the case of BaBiO3(001). Our calculations show that the outer layer of the Bi-terminated simulated surface turns more cubiclike and metallic while the inner layers remain in the insulating monoclinic state that the system present in the bulk form. On the other hand, the metallization does not occur for the Ba termination, a fact that makes this system appealing for nanostructuring. Finally, in view of the bulk properties of this material under doping, this particular finding sets another possible route for future exploration: the potential scenario of 2D superconductivity at the BaBiO3 surface.

  14. Degradation Potential of Bulk Versus Incrementally Applied and Indirect Composites: Color, Microhardness, and Surface Deterioration.

    PubMed

    El Gezawi, M; Kaisarly, D; Al-Saleh, H; ArRejaie, A; Al-Harbi, F; Kunzelmann, K H

    This study investigated the color stability and microhardness of five composites exposed to four beverages with different pH values. Composite discs were produced (n=10); Filtek Z250 (3M ESPE) and Filtek P90 (3M ESPE) were applied in two layers (2 mm, 20 seconds), and Tetric N-Ceram Bulk Fill (TetricBF, Ivoclar Vivadent) and SonicFill (Kerr) were applied in bulk (4 mm) and then light cured (40 seconds, Ortholux-LED, 1600 mW/cm(2)). Indirect composite Sinfony (3M ESPE) was applied in two layers (2 mm) and cured (Visio system, 3M ESPE). The specimens were polished and tested for color stability; ΔE was calculated using spectrophotometer readings. Vickers microhardness (50 g, dwell time=45 seconds) was assessed on the top and bottom surfaces at baseline, 40 days of storage, subsequent repolishing, and 60 days of immersion in distilled water (pH=7.0), Coca-Cola (pH=2.3), orange juice (pH=3.75), or anise (pH=8.5) using scanning electron microscopy (SEM). The materials had similar ΔE values (40 days, p>0.05), but TetricBF had a significantly greater ΔE than P90 or SF (40 days). The ΔE was less for P90 and TetricBF than for Z250, SonicFill, and Sinfony (60 days). Repolishing and further immersion significantly affected the ΔE (p<0.05) except for P90. All composites had significantly different top vs bottom baseline microhardnesses. This was insignificant for the Z250/water, P90/orange juice (40 days), and Sinfony groups (40 and 60 days). Immersion produced variable time-dependent deterioration of microhardness in all groups. Multivariate repeated measures analysis of variance with post hoc Bonferroni tests were used to compare the results. ΔE and microhardness changes were significantly inversely correlated at 40 days, but this relationship was insignificant at 60 days (Pearson test). SEM showed degradation (40 days) that worsened (60 days). Bulk-fill composites differ regarding color-stability and top-to-bottom microhardness changes compared with those of other

  15. Bulk and surface characterization of In2O3(001) single crystal

    SciTech Connect

    Hagleitner, D.; Jacobson, Peter; Blomberg, Sara; Schulte, Karina; Lundgren, Edvin; Kubicek, Markus; Fleig, Jürgen; Kubel, Frank; Puls, Christoph; Limbeck, Andreas; Hutter, Herbert; Boatner, Lynn A; Schmid, M.; Diebold, U.

    2012-01-01

    A comprehensive bulk and surface investigation of high-quality In2O3(001) single crystals is reported. The transparent-yellow, cube-shaped single crystals were grown using the flux method. ICP-MS measurements reveal small residues of Pb, Mg and Pt in the crystals. Four-point-probe measurements show a resistivity of 2 0.5 105 cm, which translates into a carrier concentration of 1012 cm-3. The results from X-ray diffraction (XRD) measurements revise the lattice constant to 10.1150(5) from the previously accepted value of 10.117 . Scanning Tunneling Microscopy (STM) images of a reduced (sputtered/annealed) surface show a step height of 5 , which indicates a preference for one type of surface termination. A combination of low-energy ion scattering (LEIS) and atomically resolved STM indicates an indium-terminated surface with small islands of 2.5 height, which corresponds to a strongly distorted indium lattice. Scanning Tunneling Spectroscopy (STS) reveals a pronounced surface state at the Fermi Level (EF). Photoelectron Spectroscopy (PES) shows additional, deep-lying band gap states, which can be removed by exposure of the surface to activated oxygen. Oxidation also results in a shoulder at the O 1s core level at a higher binding energy, possibly indicative of a surface peroxide species. A downward band bending of 0.4 eV and an upward band bending of ~0.1 eV is observed for the reduced and oxidized surfaces, respectively.

  16. Periodic density functional theory calculations of bulk and the (010) surface of goethite

    PubMed Central

    Kubicki, James D; Paul, Kristian W; Sparks, Donald L

    2008-01-01

    Background Goethite is a common and reactive mineral in the environment. The transport of contaminants and anaerobic respiration of microbes are significantly affected by adsorption and reduction reactions involving goethite. An understanding of the mineral-water interface of goethite is critical for determining the molecular-scale mechanisms of adsorption and reduction reactions. In this study, periodic density functional theory (DFT) calculations were performed on the mineral goethite and its (010) surface, using the Vienna Ab Initio Simulation Package (VASP). Results Calculations of the bulk mineral structure accurately reproduced the observed crystal structure and vibrational frequencies, suggesting that this computational methodology was suitable for modeling the goethite-water interface. Energy-minimized structures of bare, hydrated (one H2O layer) and solvated (three H2O layers) (010) surfaces were calculated for 1 × 1 and 3 × 3 unit cell slabs. A good correlation between the calculated and observed vibrational frequencies was found for the 1 × 1 solvated surface. However, differences between the 1 × 1 and 3 × 3 slab calculations indicated that larger models may be necessary to simulate the relaxation of water at the interface. Comparison of two hydrated surfaces with molecularly and dissociatively adsorbed H2O showed a significantly lower potential energy for the former. Conclusion Surface Fe-O and (Fe)O-H bond lengths are reported that may be useful in surface complexation models (SCM) of the goethite (010) surface. These bond lengths were found to change significantly as a function of solvation (i.e., addition of two extra H2O layers above the surface), indicating that this parameter should be carefully considered in future SCM studies of metal oxide-water interfaces. PMID:18477389

  17. Surface Plasmon Instability Leading to Emission of Radiation in Hybrid Semiconductors

    NASA Astrophysics Data System (ADS)

    Gumbs, Godfrey; Iurov, Andrii; Huang, Danhong; Pan, Wei

    2015-03-01

    An energy conversion approach from a dc electric field to a terahertz wave based on a new generation of hybrid semiconductors by combining two-dimensional (2D) crystalline layers and a thick conducting material is proposed with possible applications as a source of coherent radiation. The hybrid nano-structure may consist of a single or pair of sheets of graphene, silicene or a 2D electron gas as would occur at a semiconductor hetero-interface. When an electric current is passed through a layer, we discover that the low-frequency plasmons may become unstable beyond a critical wave vector qc. However, there is no instability for a single driven layer far from the conductor and the instability of an isolated pair of 2D layers occurs only at ultra long wavelengths. To bring in frequency agility for this spontaneous radiation, we manipulate the surface-plasmon induced instability, which leads to the emission of radiation (spiler), to occur at shorter wavelengths by choosing the conductor electron density, layer separation, distances of layers from the conductor surface and the driving-current strength. Applications of terahertz radiation from spiler for chemical analysis, security scanning, medical imaging and telecommunications are expected.

  18. Cascaded exciton energy transfer in a monolayer semiconductor lateral heterostructure assisted by surface plasmon polariton.

    PubMed

    Shi, Jinwei; Lin, Meng-Hsien; Chen, I-Tung; Mohammadi Estakhri, Nasim; Zhang, Xin-Quan; Wang, Yanrong; Chen, Hung-Ying; Chen, Chun-An; Shih, Chih-Kang; Alù, Andrea; Li, Xiaoqin; Lee, Yi-Hsien; Gwo, Shangjr

    2017-06-26

    Atomically thin lateral heterostructures based on transition metal dichalcogenides have recently been demonstrated. In monolayer transition metal dichalcogenides, exciton energy transfer is typically limited to a short range (~1 μm), and additional losses may be incurred at the interfacial regions of a lateral heterostructure. To overcome these challenges, here we experimentally implement a planar metal-oxide-semiconductor structure by placing a WS2/MoS2 monolayer heterostructure on top of an Al2O3-capped Ag single-crystalline plate. We find that the exciton energy transfer range can be extended to tens of microns in the hybrid structure mediated by an exciton-surface plasmon polariton-exciton conversion mechanism, allowing cascaded exciton energy transfer from one transition metal dichalcogenides region supporting high-energy exciton resonance to a different transition metal dichalcogenides region in the lateral heterostructure with low-energy exciton resonance. The realized planar hybrid structure combines two-dimensional light-emitting materials with planar plasmonic waveguides and offers great potential for developing integrated photonic and plasmonic devices.Exciton energy transfer in monolayer transition metal dichalcogenides is limited to short distances. Here, Shi et al. fabricate a planar metal-oxide-semiconductor structure and show that exciton energy transfer can be extended to tens of microns, mediated by an exciton-surface-plasmon-polariton-exciton conversion mechanism.

  19. Is the surface oxygen exchange rate linked to bulk ion diffusivity in mixed conducting Ruddlesden-Popper phases?

    PubMed

    Tomkiewicz, Alex C; Tamimi, Mazin A; Huq, Ashfia; McIntosh, Steven

    2015-01-01

    The possible link between oxygen surface exchange rate and bulk oxygen anion diffusivity in mixed ionic and electronic conducting oxides is a topic of great interest and debate. While a large body of experimental evidence and theoretical analyses support a link, observed differences between bulk and surface composition of these materials are hard to reconcile with this observation. This is further compounded by potential problems with simultaneous measurement of both parameters. Here we utilize separate techniques, in situ neutron diffraction and pulsed isotopic surface exchange, to examine bulk ion mobility and surface oxygen exchange rates of three Ruddlesden-Popper phases, general form A(n-1)A(2)'B(n)O(3n+1), A(n-1)A(2)'B(n)X(3n+1); LaSrCo(0.5)Fe(0.5)O(4-δ) (n = 1), La(0.3)Sr(2.7)CoFeO(7-δ) (n = 2) and LaSr3Co(1.5)Fe(1.5)O(10-δ) (n = 3). These measurements are complemented by surface composition determination via high sensitivity-low energy ion scattering. We observe a correlation between bulk ion mobility and surface exchange rate between materials. The surface exchange rates vary by more than one order of magnitude with high anion mobility in the bulk of an oxygen vacancy-rich n = 2 Ruddlesden-Popper material correlating with rapid oxygen exchange. This is in contrast with the similar surface exchange rates which we may expect due to similar surface compositions across all three samples. We conclude that experimental limitations lead to inherent convolution of surface and bulk rates, and that surface exchange steps are not likely to be rate limiting in oxygen incorporation.

  20. Is the surface oxygen exchange rate linked to bulk ion diffusivity in mixed conducting Ruddlesden–Popper phases?

    DOE PAGES

    Tomkiewicz, Alex C.; Tamimi, Mazin A.; Huq, Ashfia; ...

    2015-03-02

    There is a possible link between oxygen surface exchange rate and bulk oxygen anion diffusivity in mixed ionic and electronic conducting oxides; it is a topic of great interest and debate. While a large body of experimental evidence and theoretical analyses support a link, observed differences between bulk and surface composition of these materials are hard to reconcile with this observation. This is further compounded by potential problems with simultaneous measurement of both parameters. Here we utilize separate techniques, in situ neutron diffraction and pulsed isotopic surface exchange, to examine bulk ion mobility and surface oxygen exchange rates of threemore » Ruddlesden-Popper phases, general form An-1A2'BnO3n+1, An-1A2'BnX3n+1; LaSrCo0.5Fe0.5O4-δ (n = 1), La0.3Sr2.7CoFeO7-δ (n = 2) and LaSr3Co1.5Fe1.5O10-δ (n = 3). These measurements are complemented by surface composition determination via high sensitivity-low energy ion scattering. We observe a correlation between bulk ion mobility and surface exchange rate between materials. The surface exchange rates vary by more than one order of magnitude with high anion mobility in the bulk of an oxygen vacancy-rich n = 2 Ruddlesden-Popper material correlating with rapid oxygen exchange. Furthermore this is in contrast with the similar surface exchange rates which we may expect due to similar surface compositions across all three samples. This paper conclude that experimental limitations lead to inherent convolution of surface and bulk rates, and that surface exchange steps are not likely to be rate limiting in oxygen incorporation.« less

  1. Comparison of the bulk and surface properties of ceria and zirconia by ab initio investigations

    SciTech Connect

    Gennard, S.; Cora, F.; Catlow, C.R.A.

    1999-11-18

    Quantum mechanical (QM) calculations were made at a periodic Hartree-Fock (HF) level, on the bulk and surface properties of cubic CeO{sub 2} and ZrO{sub 2}. The authors have investigated the M-O bonding features, and established the high degree of ionicity of both materials, which is greater for CeO{sub 2} than ZrO{sub 2}. The calculated values for the C{sub 11}, C{sub 12}, and C{sub 44} elastic constants, are in close agreement with experiment; an extended oxygen basis set, containing d-orbital polarization functions, is essential to model accurately the symmetry lowering during the C{sub 44} distortion. In the surface studies, they have calculated the surface energies of the {l{underscore}brace}011{r{underscore}brace} and {l{underscore}brace}111{r{underscore}brace} faces of both ceria and zirconia. QM results are compared with interatomic potential-based (IP) methods to assess the accuracy of the latter. IP methods were found to provide a correct estimate of the surface relaxations and the correct order of stability of the two faces examined, with the energy difference between the {l{underscore}brace}011{r{underscore}brace} and the {l{underscore}brace}111{r{underscore}brace} surfaces being approximately 1 J/m{sup 2}, as in the QM study. However, IP methods do not always discriminate adequately between the properties of the two materials. Finally, geometric and electronic relaxations in the {l{underscore}brace}111{r{underscore}brace} surface are confined to the outermost oxygen ions, while in the {l{underscore}brace}011{r{underscore}brace} slabs they are more important and extend to the subsurface layers in a columnar way. The unsaturation of the surface ions in the {l{underscore}brace}011{r{underscore}brace} face may have important implications for the catalytic activity of the materials.

  2. The drift-diffusion interpretation of the electron current within the organic semiconductor characterized by the bulk single energy trap level

    NASA Astrophysics Data System (ADS)

    Cvikl, B.

    2010-01-01

    The closed solution for the internal electric field and the total charge density derived in the drift-diffusion approximation for the model of a single layer organic semiconductor structure characterized by the bulk shallow single trap-charge energy level is presented. The solutions for two examples of electric field boundary conditions are tested on room temperature current density-voltage data of the electron conducting aluminum/tris(8-hydroxyquinoline aluminum/calcium structure [W. Brütting et al., Synth. Met. 122, 99 (2001)] for which jexp∝Va3.4, within the interval of bias 0.4 V≤Va≤7. In each case investigated the apparent electron mobility determined at given bias is distributed within a given, finite interval of values. The bias dependence of the logarithm of their lower limit, i.e., their minimum values, is found to be in each case, to a good approximation, proportional to the square root of the applied electric field. On account of the bias dependence as incorporated in the minimum value of the apparent electron mobility the spatial distribution of the organic bulk electric field as well as the total charge density turn out to be bias independent. The first case investigated is based on the boundary condition of zero electric field at the electron injection interface. It is shown that for minimum valued apparent mobilities, the strong but finite accumulation of electrons close to the anode is obtained, which characterize the inverted space charge limited current (SCLC) effect. The second example refers to the internal electric field allowing for self-adjustment of its boundary values. The total electron charge density is than found typically to be of U shape, which may, depending on the parameters, peak at both or at either Alq3 boundary. It is this example in which the proper SCLC effect is consequently predicted. In each of the above two cases, the calculations predict the minimum values of the electron apparent mobility, which substantially

  3. Magnetic modification at sub-surface of FeRh bulk by energetic ion beam irradiation

    SciTech Connect

    Koide, T.; Iwase, A.; Uno, H.; Sakane, H.; Sakamaki, M.; Amemiya, K.; Matsui, T.

    2015-05-07

    Ferromagnetic layered structure has been made at sub-surface of the antiferromagnetic FeRh bulk samples by high energy He ion beam irradiation. In accordance with the Transport of Ions in Matter simulation, such ion beam can effectively deposit the elastic collision energy in several μm regions in the depth from the surface. Measurement with a superconducting quantum interference device reveals the irradiated samples to be ferromagnetic. Assuming that only the part the energy deposited can be modified to be ferromagnetic, the corresponding irradiation induced magnetization is consistent with the data that we previously reported. On the other hand, the X-ray magnetic circular dichroism (XMCD) spectra for the irradiated samples are totally unchanged as those for the unirradiated samples. Since XMCD signal in total emission yield method is considered to be surface sensitive with a typical probing depth of several nm, the surface magnetic state is maintained to be antiferromagnetic. By utilizing these phenomena, three-dimensional magnetic patterning of FeRh can be realized, which may potentially be used for future magnetic exchange device application such as nano-scale sensors and memories.

  4. Exchange interactions of CaMnO3 in the bulk and at the surface

    NASA Astrophysics Data System (ADS)

    Keshavarz, S.; Kvashnin, Y. O.; Rodrigues, D. C. M.; Pereiro, M.; Di Marco, I.; Autieri, C.; Nordström, L.; Solovyev, I. V.; Sanyal, B.; Eriksson, O.

    2017-03-01

    We present electronic and magnetic properties of CaMnO3 (CMO) as obtained from ab initio calculations. We identify the preferable magnetic order by means of density functional theory plus Hubbard U calculations and extract the effective exchange parameters (Ji j's) using the magnetic force theorem. We find that the effects of geometrical relaxation at the surface as well as the change of crystal field are very strong and are able to influence the lower-energy magnetic configuration. In particular, our analysis reveals that the exchange interaction between the Mn atoms belonging to the surface and the subsurface layers is very sensitive to the structural changes. An earlier study [A. Filippetti and W. E. Pickett, Phys. Rev. Lett. 83, 4184 (1999), 10.1103/PhysRevLett.83.4184] suggested that this coupling is ferromagnetic and gives rise to the spin-flip (SF) process on the surface of CMO. In our work, we confirm their finding for an unrelaxed geometry, but once the structural relaxations are taken into account, this exchange coupling changes its sign. Thus, we suggest that the surface of CMO should have the same G -type antiferromagnetic order as in the bulk. Finally, we show that the suggested SF can be induced in the system by introducing an excess of electrons.

  5. Periodic DFT modeling of bulk and surface properties of MgCl2.

    PubMed

    Credendino, Raffaele; Busico, Vincenzo; Causà, Mauro; Barone, Vincenzo; Budzelaar, Peter H M; Zicovich-Wilson, Claudio

    2009-08-14

    MgCl(2) is the preferred support for the industrial Ziegler-Natta catalysts, and is believed to act as a template for the epitactic chemisorption of the active Ti species. As the first step of a thorough computational modeling of these systems, we studied the bulk and surface structure of the ordered alpha and beta phases of MgCl(2) by means of periodic DFT (B3LYP) methods using localized basis sets. The layer structure of both phases was reproduced satisfactorily with the inclusion of a (small) empirical dispersion correction ("DFT-D") as a practical method to describe the attraction between the layers. Surface models were studied on slabs with adequate thickness. It appears that various surfaces exposing 5-coordinated Mg are very similar in energy and are the lowest non-trivial surfaces. Cuts exposing 4-coordinated Mg are significantly less stable; both kinetic and equilibrium models of crystal growth indicate that they should normally not be formed to a significant extent. "Nano-ribbons" of single, flat chains of MgCl(2), sometimes proposed as components of the disordered delta phase, were also evaluated, but are predicted to be unstable to rearrangement. Implications for the role of MgCl(2) as catalyst support are discussed.

  6. Magnetic modification at sub-surface of FeRh bulk by energetic ion beam irradiation

    NASA Astrophysics Data System (ADS)

    Koide, T.; Uno, H.; Sakane, H.; Sakamaki, M.; Amemiya, K.; Iwase, A.; Matsui, T.

    2015-05-01

    Ferromagnetic layered structure has been made at sub-surface of the antiferromagnetic FeRh bulk samples by high energy He ion beam irradiation. In accordance with the Transport of Ions in Matter simulation, such ion beam can effectively deposit the elastic collision energy in several μm regions in the depth from the surface. Measurement with a superconducting quantum interference device reveals the irradiated samples to be ferromagnetic. Assuming that only the part the energy deposited can be modified to be ferromagnetic, the corresponding irradiation induced magnetization is consistent with the data that we previously reported. On the other hand, the X-ray magnetic circular dichroism (XMCD) spectra for the irradiated samples are totally unchanged as those for the unirradiated samples. Since XMCD signal in total emission yield method is considered to be surface sensitive with a typical probing depth of several nm, the surface magnetic state is maintained to be antiferromagnetic. By utilizing these phenomena, three-dimensional magnetic patterning of FeRh can be realized, which may potentially be used for future magnetic exchange device application such as nano-scale sensors and memories.

  7. Cultivable bacteria from bulk water, aggregates, and surface sediments of a tidal flat ecosystem

    NASA Astrophysics Data System (ADS)

    Stevens, Heike; Simon, Meinhard; Brinkhoff, Thorsten

    2009-04-01

    Most-probable-number (MPN) dilution series were used to enumerate and isolate bacteria from bulk water, suspended aggregates, the oxic layer, and the oxic-anoxic transition zone of the sediment of a tidal flat ecosystem in the southern North Sea. The heterotrophic aerobic bacteria were able to grow on agar-agar, alginate, cellulose, chitin, dried and ground Fucus vesiculosus, Marine Broth 2216, palmitate, and starch. MPN counts of bulk water and aggregate samples ranged between 0.18 × 101 and 1.1 × 106 cells per milliliter and those of the sediment surface and the transition zone between 0.8 × 101 and 5.1 × 107 cells per gram dry weight. Marine Broth and F. vesiculosus yielded the highest values of all substrates tested and corresponded to 2.3-32% of 4,6-diamidinophenyl indole cell counts. Strains of seven phylogenetic classes were obtained: Actinobacteria, Bacilli, α- and γ-Proteobacteria, Sphingobacteria, Flavobacteria, and Planctomycetacia. Only with agar-agar as substrate could organisms of all seven classes be isolated.

  8. One-dimensional quantum matter: gold-induced nanowires on semiconductor surfaces.

    PubMed

    Dudy, L; Aulbach, J; Wagner, T; Schäfer, J; Claessen, R

    2017-09-15

    Interacting electrons confined to only one spatial dimension display a wide range of unusual many-body quantum phenomena, ranging from Peierls instabilities to the breakdown of the canonical Fermi liquid paradigm to even unusual spin phenomena. The underlying physics is not only of tremendous fundamental interest, but may also have bearing on device functionality in future micro- and nanoelectronics with lateral extensions reaching the atomic limit. Metallic adatoms deposited on semiconductor surfaces may form self-assembled atomic nanowires, thus representing highly interesting and well-controlled solid-state realizations of such 1D quantum systems. Here we review experimental and theoretical investigations on a few selected prototypical nanowire surface systems, specifically Ge(0 0 1)-Au and Si(hhk)-Au, and the search for 1D quantum states in them. We summarize the current state of research and identify open questions and issues.

  9. Molecular-scale shear response of the organic semiconductor β -DBDCS (100) surface

    NASA Astrophysics Data System (ADS)

    Álvarez-Asencio, Rubén; Moreno-Ramírez, Jorge S.; Pimentel, Carlos; Casado, Santiago; Matta, Micaela; Gierschner, Johannes; Muccioli, Luca; Yoon, Seong-Jun; Varghese, Shinto; Park, Soo Young; Gnecco, Enrico; Pina, Carlos M.

    2017-09-01

    In this work we present friction-force microscopy (FFM) lattice-resolved images acquired on the (100) facet of the semiconductor organic oligomer (2 Z ,2'Z )-3 , 3' -(1,4-phenylene)bis(2-(4-butoxyphenyl)acrylonitrile) (β -DBDCS) crystal in water at room temperature. Stick-slip contrast, lateral contact stiffness, and friction forces are found to depend strongly on the sliding direction due to the anisotropic packing of the molecular chains forming the crystal surface along the [010] and [001] directions. The anisotropy also causes the maximum value of the normal force applicable before wearing to increase by a factor of 3 when the scan is performed along the [001] direction on the (100) face. Altogether, our results contribute to achieving a better understanding of the molecular origin of friction anisotropy on soft crystalline surfaces, which has been often hypothesized but rarely investigated in the literature.

  10. Enhanced electron-phonon coupling for a semiconductor charge qubit in a surface phonon cavity

    PubMed Central

    Chen, J. C. H.; Sato, Y.; Kosaka, R.; Hashisaka, M.; Muraki, K.; Fujisawa, T.

    2015-01-01

    Electron-phonon coupling is a major decoherence mechanism, which often causes scattering and energy dissipation in semiconductor electronic systems. However, this electron-phonon coupling may be used in a positive way for reaching the strong or ultra-strong coupling regime in an acoustic version of the cavity quantum electrodynamic system. Here we propose and demonstrate a phonon cavity for surface acoustic waves, which is made of periodic metal fingers that constitute Bragg reflectors on a GaAs/AlGaAs heterostructure. Phonon band gap and cavity phonon modes are identified by frequency, time and spatially resolved measurements of the piezoelectric potential. Tunneling spectroscopy on a double quantum dot indicates the enhancement of phonon assisted transitions in a charge qubit. This encourages studying of acoustic cavity quantum electrodynamics with surface phonons. PMID:26469629

  11. The Effect of Bulk Traps on the InP (Indium Phosphide) Accumulation Type MISFET (Metal-Insulator-Semiconductor Field-Effect Transistor),

    DTIC Science & Technology

    1985-01-01

    S. Blakemore , Semiconductor Statistics , p. 118, Pergamon Press, New York, 1962. 18. E.W. Williams, W. Elder, M.G. Astles, M. Wehh, J.R. Mullin, B...the fabrication of metal-insulator- semiconductor (MIS) field-effect transistor (FET) because of its large saturated drift velocity for electrons and the...favorable conditions present at its oxide- semiconductor interface. While efforts to construct MISFETs on gallium arsenide were unsuccessful due to

  12. PASSIVATION OF SEMICONDUCTOR SURFACES FOR IMPROVED RADIATION DETECTORS: X-RAY PHOTOEMISSION ANALYSIS

    SciTech Connect

    Nelson, A; Conway, A; Reinhardt, C; Ferreira, J; Nikolic, R; Payne, S

    2007-12-10

    Surface passivation of device-grade radiation detector materials was investigated using x-ray photoelectron spectroscopy in combination with transport property measurements before and after various chemical treatments. Specifically Br-MeOH (2% Br), KOH with NH{sub 4}F/H{sub 2}O{sub 2} and NH{sub 4}OH solutions were used to etch, reduce and oxidize the surface of Cd{sub (1-x)}Zn{sub x}Te semiconductor crystals. Scanning electron microscopy was used to evaluate the resultant microscopic surface morphology. Angle-resolved high-resolution photoemission measurements on the valence band electronic structure and core lines were used to evaluate the surface chemistry of the chemically treated surfaces. Metal overlayers were then deposited on these chemically treated surfaces and the I-V characteristics measured. The measurements were correlated to understand the effect of interface chemistry on the electronic structure at these interfaces with the goal of optimizing the Schottky barrier height for improved radiation detector devices.

  13. Single-molecule surface studies of fibrinogen and DNA on semiconductors

    NASA Astrophysics Data System (ADS)

    Kong, Xianhua

    Understanding of protein adsorption onto non-biological substrates is of fundamental interest in science, but also has great potential technological applications in medical devices and biosensors. This study explores the non-specific interaction, at the single molecule level, of a blood protein and DNA with semiconductor surfaces through the use of a custom built, non rastering electron emission microscope and a scanning probe microscope. The specifics and history of electron emission are described as well as the equipment used in this study. The protein examined in this study is human plasma fibrinogen, which plays an important role in haemostatis and thrombosis, and deoxyribonucleic acid (DNA) is also studied. A novel technique for determining the photothreshold of biomolecules on single molecule level is developed and applied to fibrinogen molecules adsorbed on oxidized silicon surfaces, using photo-electron emission microscopy (PEEM). Three theoretical models are employed and compared to analyze the experimental photothreshold data. The non-specific adsorption of human plasma fibrinogen on oxidized p- and n- type silicon (100) surfaces is investigated to characterize both hydrophobic interactions and electrostatic forces. The experimental results indicate that hydrophobic interactions are one of the driving forces for protein adsorption and the electrostatic interactions also play a role in the height of the fibrinogen molecules adsorbed on the surface. PEEM images establish a photo threshold of 5.0 +/- 0.2 eV for fibrinogen on both n-type and p-type Si (100) surfaces. We suggest that the photothreshold results from surface state associated Fermi level (EF) pinning and there exists negative charge transfer from the adsorbed fibrinogen onto the p-type silicon substrates, while on n-type silicon substrates negative charge is transferred in the opposite direction. The adsorption of deoxyribonucleic acid (DNA) on mica and silicon is studied in liquid and ambient

  14. Charge transfer during low energy metal/semiconductor ion-surface interactions

    NASA Astrophysics Data System (ADS)

    Chen, Xiaojian

    Ion-surface charge exchange is a central process in many surface analysis techniques and technical processes. Previous ion scattering studies have extensively investigated the interactions between alkali/noble gas ions and surfaces. Investigations of the interactions between metal/semiconductor ions and metal surfaces have seldom been reported, although they are of central importance in understanding processes involving the removal material from metal/semiconductor surfaces. This dissertation uses low energy ion scattering and direct recoil to reveal charge transfer mechanisms between metal/semiconductor atomic particles and clean and adsorb ate-covered Al and Si surfaces. All involved experiments were performed in ultra-high vacuum. Charge transfer between an Al atom and an Al surface is studied by producing energetic recoiled Al from a Al(100) surface via Xe+ bombardment. The measured neutral fractions of the recoils show that resonant charge transfer (RCT) is a key mechanism. The ion formation of recoiled and sputtered atoms can be both described by RCT. The characteristic difference between recoiled and sputtered atoms is interpreted as due to different surface conditions at the time of ion emission. 1˜5 keV Si+ ions were incident on atomically clean Al(100) surface. All scattered Si was neutralized, while Al ions were found in multi-charged states. In contrast to the traditional sputtering studies, there is an abnormally high yield of Al2+ and Al3+. The multiply charged ions are attributed to the charge promotion of Al 2p level during the electronically nearly-symmetric Si-Al collision, and subsequent shake-off processes. This mechanism is supported by the energy dependence of Al ions as well as ion induced Auger electrons. Si+ ions were scattered from submonolayers of Cs deposited onto Al(100). Because of the high ionization energy of Si, resonant charge transfer would be expected to completely neutralize the scattered projectiles. In contrast, a

  15. Multimode filter composed of single-mode surface acoustic wave/bulk acoustic wave resonators

    NASA Astrophysics Data System (ADS)

    Huang, Yulin; Bao, Jingfu; Tang, Gongbin; Wang, Yiling; Omori, Tatsuya; Hashimoto, Ken-ya

    2017-07-01

    This paper discusses the possibility of realizing multimode filters composed of multiple single-mode resonators by using radio frequency surface and bulk acoustic wave (SAW/BAW) technologies. First, the filter operation and design principle are given. It is shown that excellent filter characteristics are achievable by combining multiple single-mode resonators with identical capacitance ratios provided that their resonance frequencies and clamped capacitances are set properly. Next, the effect of balun performance is investigated. It is shown that the total filter performance is significantly degraded by balun imperfections such as the common-mode rejection. Then, two circuits are proposed to improve the common-mode rejection, and their effectiveness is demonstrated.

  16. Control of bulk superconductivity in a BCS superconductor by surface charge doping via electrochemical gating

    NASA Astrophysics Data System (ADS)

    Piatti, E.; Daghero, D.; Ummarino, G. A.; Laviano, F.; Nair, J. R.; Cristiano, R.; Casaburi, A.; Portesi, C.; Sola, A.; Gonnelli, R. S.

    2017-04-01

    The electrochemical gating technique is a powerful tool to tune the surface conduction properties of various materials by means of pure charge doping, but its efficiency is thought to be hampered in materials with a good electronic screening. We show that, if applied to a metallic superconductor (NbN thin films), this approach allows the observation of reversible enhancements or suppressions of the bulk superconducting transition temperature, which vary with the thickness of the films. These results are interpreted in terms of a proximity effect, and indicate that the effective screening length depends on the induced charge density, becoming much larger than that predicted by a standard screening theory at very high electric fields.

  17. Correlated electronic structure of Fe in bulk Cs and on a Cs surface

    NASA Astrophysics Data System (ADS)

    Costa, M.; Thunström, P.; Di Marco, I.; Bergman, A.; Klautau, A. B.; Lichtenstein, A. I.; Katsnelson, M. I.; Eriksson, O.

    2013-03-01

    We have investigated the spectral properties of Fe impurities in a Cs host, for both surface and bulk systems, by means of a combination of density functional theory in the local density approximation and dynamical mean-field theory (LDA + DMFT). The effective impurity model arising in LDA + DMFT was solved via two different techniques, i.e., the Hubbard I approximation and the exact diagonalization. It is shown that noticeable differences can be seen in the unoccupied part of the spectrum for different positions of Fe atoms in the host, despite the fact that hybridization between Fe d-states and Cs is low. Our calculations show good agreement with the experimental photoemission spectra reported by Carbone [Carbone, Veronese, Moras, Gardonio, Grazioli, Zhou, Rader, Varykhalov, Krull, Balashov, Mugarza, Gambardella, Lebègue, Eriksson, Katsnelson, and Lichtenstein, Phys. Rev. Lett.PRLTAO0031-900710.1103/PhysRevLett.104.117601 104, 117601 (2010)].

  18. Unconventional bulk three-dimensional Fermi surface in Kondo insulating SmB6

    NASA Astrophysics Data System (ADS)

    Tan, Beng

    We report the observation of a paradoxical insulator with a bulk state which is electrically insulating and simultaneously yields quantum oscillations typical of good metals. We present high field measurements of conductivity and magnetic torque in high purity single crystals of the Kondo insulator SmB6 which reveal an activated behavior characteristics of an insulator with an energy gap at the Fermi energy in the former and quantum oscillation of frequencies characteristics of a large three-dimensional conduction electron Fermi surface similar to the metallic rare earth hexaborides such as PrB6 and LaB6 in the latter. The quantum oscillations observed in the magnetic torque measurements are characteristic of an unconventional Fermi liquid - the amplitude strongly increases at low temperatures in a stark contrast to the saturating Lifshitz-Kosevich behavior in conventional metallic states.

  19. Systematic study of transport via surface and bulk states in Bi2Te3 topological insulator

    NASA Astrophysics Data System (ADS)

    de Castro, S.; Peres, M. L.; Chitta, V. A.; Gratens, X.; Soares, D. A. W.; Fornari, C. I.; Rappl, P. H. O.; Abramof, E.; Oliveira, N. F., Jr.

    2016-07-01

    We performed magnetoresistance measurements on Bi2Te3 thin film in the temperature range of T = 1.2-4.0 K and for magnetic fields up to 2 T. The curves exhibited anomalous behavior for temperatures below 4.0 K. Different temperature intervals revealed electrical transport through different conductive channels with clear signatures of weak antilocalization. The magnetoresistance curves were explained using the Hikami-Larkin-Nagaoka model and the 2D Dirac modified model. The comparison between the parameters obtained from the two models revealed the transport via topological surface states and bulk states. In addition, a superconductive like transition is observed for the lowest temperatures and we suggest that this effect can be originated from the misfit dislocations caused by strain, giving rise to a superconductive channel between the interface of the film and the substrate.

  20. Effects of ion bombardment on bulk GaAs photocathodes with different surface-cleavage planes

    NASA Astrophysics Data System (ADS)

    Liu, Wei; Zhang, Shukui; Stutzman, Marcy; Poelker, Matt

    2016-10-01

    Bulk GaAs samples with different surface cleave planes were implanted with 100 and 10 000 V hydrogen ions inside an ultrahigh vacuum test apparatus to simulate ion back-bombardment of the photocathode inside a DC high voltage photogun. The photocathode yield, or quantum efficiency, could easily be recovered following implantation with 100 V hydrogen ions but not for 10 000 V ions. Moreover, the implantation damage with 10 000 V hydrogen ions was more pronounced for GaAs photocathode samples with (100) and (111A) cleave planes, compared to the photocathode with (110) cleave plane. This result is consistent with enhanced ion channeling for the (110) cleave plane compared to the other cleave planes, with ions penetrating deeper into the photocathode material beyond the absorption depth of the laser light and beyond the region of the photocathode where the photoemitted electrons originate.

  1. Effects of ion bombardment on bulk GaAs photocathodes with different surface-cleavage planes

    DOE PAGES

    Liu, Wei; Zhang, Shukui; Stutzman, Marcy; ...

    2016-10-24

    Bulk GaAs samples with different surface cleave planes were implanted with 100 and 10 000 V hydrogen ions inside an ultrahigh vacuum test apparatus to simulate ion back-bombardment of the photocathode inside a DC high voltage photogun. The photocathode yield, or quantum efficiency, could easily be recovered following implantation with 100 V hydrogen ions but not for 10 000 V ions. Moreover, the implantation damage with 10 000 V hydrogen ions was more pronounced for GaAs photocathode samples with (100) and (111A) cleave planes, compared to the photocathode with (110) cleave plane. Lastly, this result is consistent with enhanced ionmore » channeling for the (110) cleave plane compared to the other cleave planes, with ions penetrating deeper into the photocathode material beyond the absorption depth of the laser light and beyond the region of the photocathode where the photoemitted electrons originate.« less

  2. The bulk, surface and corner free energies of the square lattice Ising model

    NASA Astrophysics Data System (ADS)

    Baxter, R. J.

    2017-01-01

    We use Kaufman’s spinor method to calculate the bulk, surface and corner free energies {f}{{b}},{f}{{s}},{f}{{s}}\\prime ,{f}{{c}} of the anisotropic square lattice zero-field Ising model for the ordered ferromagnetic case. For {f}{{b}},{f}{{s}},{f}{{s}}\\prime our results of course agree with the early work of Onsager, McCoy and Wu. We also find agreement with the conjectures made by Vernier and Jacobsen (VJ) for the isotropic case. We note that the corner free energy f c depends only on the elliptic modulus k that enters the working, and not on the argument v, which means that VJ’s conjecture applies for the full anisotropic model. The only aspect of this paper that is new is the actual derivation of f c, but by reporting all four free energies together we can see interesting structures linking them.

  3. Modelling bulk surface resistance from MODIS time series data to estimate actual regional evapotranspiration

    NASA Astrophysics Data System (ADS)

    Autovino, Dario; Minacapilli, Mario; Provenzano, Giuseppe

    2015-04-01

    Estimation of actual evapotraspiration by means of Penman-Monteith (P-M) equation requires the knowledge of the so-called 'bulk surface resistance', rc,act, representing the vapour flow resistance through the transpiring crop and evaporating soil surface. The accurate parameterization of rc,act still represents an unexploited topic, especially in the case of heterogeneous land surface. In agro-hydrological applications, the P-M equation commonly used to evaluate reference evapotranspiration (ET0) of a well-watered 'standardized crop' (grass or alfalfa), generally assumes for the bulk surface resistance a value of 70 s m-1. Moreover, specific crop coefficients have to be used to estimate maximum and/or actual evapotranspiration based on ET0. In this paper, a simple procedure for the indirect estimation of rc,act as function of a vegetation index computed from remote acquisition of Land Surface Temperature (LST), is proposed. An application was carried out in an irrigation district located near Castelvetrano, in South-West of Sicily, mainly cultivated with olive groves, in which actual evapotranspiration fluxes were measured during two years (2010-2011) by an Eddy Covariance flux tower (EC). Evapotranspiration measurements allowed evaluating rc,actbased on the numerical inversion of the P-M equation. In the same study area, a large time series of MODIS LST data, characterized by a spatial resolution of 1x1 km and a time step of 8-days, was also acquired for the period from 2000 to 2014. A simple Vegetation Index Temperatures (VTI), with values ranging from 0 to 1, was computed using normalized LST values. Evapotranspiration fluxes measured in 2010 were used to calibrate the relationship between rc,act and VTI, whereas data from 2011 were used for its validation. The preliminary results evidenced that, for the considered crop, an almost constant value of rc,act, corresponding to about 250 s m-1, can be considered typical of periods in which the crop is well

  4. Electronic and optical properties of titanium nitride bulk and surfaces from first principles calculations

    SciTech Connect

    Mehmood, Faisal; Pachter, Ruth Murphy, Neil R.; Johnson, Walter E.

    2015-11-21

    Prediction of the frequency-dependent dielectric function of thin films poses computational challenges, and at the same time experimental characterization by spectroscopic ellipsometry remains difficult to interpret because of changes in stoichiometry and surface morphology, temperature, thickness of the film, or substrate. In this work, we report calculations for titanium nitride (TiN), a promising material for plasmonic applications because of less loss and other practical advantages compared to noble metals. We investigated structural, electronic, and optical properties of stoichiometric bulk TiN, as well as of the TiN(100), TiN(110), and TiN(111) outermost surfaces. Density functional theory (DFT) and many-body GW methods (Green's (G) function-based approximation with screened Coulomb interaction (W)) were used, ranging from G{sub 0}W{sub 0}, GW{sub 0} to partially self-consistent sc-GW{sub 0}, as well as the GW-BSE (Bethe-Salpeter equation) and time-dependent DFT (TDDFT) methods for prediction of the optical properties. Structural parameters and the band structure for bulk TiN were shown to be consistent with previous work. Calculated dielectric functions, plasma frequencies, reflectivity, and the electron energy loss spectrum demonstrated consistency with experiment at the GW{sub 0}-BSE level. Deviations from experimental data are expected due to varying experimental conditions. Comparison of our results to spectroscopic ellipsometry data for realistic nanostructures has shown that although TDDFT may provide a computationally feasible level of theory in evaluation of the dielectric function, application is subject to validation with GW-BSE calculations.

  5. Electronic and optical properties of titanium nitride bulk and surfaces from first principles calculations

    NASA Astrophysics Data System (ADS)

    Mehmood, Faisal; Pachter, Ruth; Murphy, Neil R.; Johnson, Walter E.

    2015-11-01

    Prediction of the frequency-dependent dielectric function of thin films poses computational challenges, and at the same time experimental characterization by spectroscopic ellipsometry remains difficult to interpret because of changes in stoichiometry and surface morphology, temperature, thickness of the film, or substrate. In this work, we report calculations for titanium nitride (TiN), a promising material for plasmonic applications because of less loss and other practical advantages compared to noble metals. We investigated structural, electronic, and optical properties of stoichiometric bulk TiN, as well as of the TiN(100), TiN(110), and TiN(111) outermost surfaces. Density functional theory (DFT) and many-body GW methods (Green's (G) function-based approximation with screened Coulomb interaction (W)) were used, ranging from G0W0, GW0 to partially self-consistent sc-GW0, as well as the GW-BSE (Bethe-Salpeter equation) and time-dependent DFT (TDDFT) methods for prediction of the optical properties. Structural parameters and the band structure for bulk TiN were shown to be consistent with previous work. Calculated dielectric functions, plasma frequencies, reflectivity, and the electron energy loss spectrum demonstrated consistency with experiment at the GW0-BSE level. Deviations from experimental data are expected due to varying experimental conditions. Comparison of our results to spectroscopic ellipsometry data for realistic nanostructures has shown that although TDDFT may provide a computationally feasible level of theory in evaluation of the dielectric function, application is subject to validation with GW-BSE calculations.

  6. Surface chemistry of bulk nanocrystalline pure iron and electrochemistry study in gas-flow physiological saline.

    PubMed

    Nie, F L; Zheng, Y F

    2012-07-01

    Conventional microcrystalline pure iron (MC-Fe) becomes a new candidate as biodegradable metals, which has the insufficient physical feature and inferior biodegradation behavior. Novel bulk nanocrystalline pure iron (NC-Fe) was fabricated via equal channel angular pressing technique in the present work to overcome these problems. The contact angle test with water and glycerol droplets shows a smaller angle (though >90°) of NC-Fe than that of MC-Fe, which implies a lower surface energy of NC-Fe. The surface roughness of NC-Fe increased greatly than that of MC-Fe. A further comparative study of corrosion and electrochemistry performance between NC-Fe and its original MC-Fe was investigated in physiological saline with different dissolved oxygen concentration, aiming to in vitro simulate the corrosion process of coronary stent occurred in physiological environment. The electrochemical impedance spectra analysis and anodic polarization measurements indicated that the NC-Fe exhibited higher corrosion resistance than that of the MC-Fe; meanwhile obvious enhanced corrosion resistance with the decrement of dissolved oxygen concentration was observed. Related equivalent circuit model and surface reconstruction process were further discussed, and the degradation mechanism of the MC-Fe and NC-Fe were finally established. Copyright © 2012 Wiley Periodicals, Inc.

  7. Energy-level matching of Fe(III) ions grafted at surface and doped in bulk for efficient visible-light photocatalysts.

    PubMed

    Liu, Min; Qiu, Xiaoqing; Miyauchi, Masahiro; Hashimoto, Kazuhito

    2013-07-10

    Photocatalytic reaction rate (R) is determined by the multiplication of light absorption capability (α) and quantum efficiency (QE); however, these two parameters generally have trade-off relations. Thus, increasing α without decreasing QE remains a challenging issue for developing efficient photocatalysts with high R. Herein, using Fe(III) ions grafted Fe(III) doped TiO2 as a model system, we present a novel method for developing visible-light photocatalysts with efficient R, utilizing the concept of energy level matching between surface-grafted Fe(III) ions as co-catalysts and bulk-doped Fe(III) ions as visible-light absorbers. Photogenerated electrons in the doped Fe(III) states under visible-light efficiently transfer to the surface grafted Fe(III) ions co-catalysts, as the doped Fe(III) ions in bulk produced energy levels below the conduction band of TiO2, which match well with the potential of Fe(3+)/Fe(2+) redox couple in the surface grafted Fe(III) ions. Electrons in the surface grafted Fe(III) ions efficiently cause multielectron reduction of adsorbed oxygen molecules to achieve high QE value. Consequently, the present Fe(III)-FexTi1-xO2 nanocomposites exhibited the highest visible-light R among the previously reported photocatalysts for decomposition of gaseous organic compounds. The high R can proceed even under commercial white-light emission diode irradiation and is very stable for long-term use, making it practically useful. Further, this efficient method could be applied in other wide-band gap semiconductors, including ZnO or SrTiO3, and may be potentially applicable for other photocatalysis systems, such as water splitting, CO2 reduction, NOx removal, and dye decomposition. Thus, this method represents a strategic approach to develop new visible-light active photocatalysts for practical uses.

  8. Photoreactions of freons on semiconductor surfaces in the presence of air

    SciTech Connect

    Weaver, S.; Fette, M.; Mills, G.

    1995-12-31

    The reactions of trichlorofluoromethane (CFC-11), dichlorodifluoromethane (CFC-12) and 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113) initiated on illuminated semiconductor, were studied in systems containing air. Photoreduction of the freons was observed in aqueous suspensions of TiO{sub 2} particles with formation of halide ions. With formate or acetate ions as hole scavengers the rates of reaction increased in the order: CFC-113 {approximately} CFC-12 < CFC-11. Surprisingly, reduction of the CFC molecules was also detected in solutions containing nitrate ions as hole scavenger; the reaction rates followed the trend: acetate < nitrate < formate. In addition, photoreduction of the freons occurred in the absence of solvent, that is, when the gaseous CFC molecules adsorbed on the surface of illuminated semiconductors. Chloride and fluoride ions were generated in a ratio of 3/1 during the solid-gas reaction of CFC-11, indicating that the radical formed in the initial reduction step of the freon decayed via reactions with oxygen.

  9. Formation of Ideal Rashba States on Layered Semiconductor Surfaces Steered by Strain Engineering

    DOE PAGES

    Ming, Wenmei; Wang, Z. F.; Zhou, Miao; ...

    2015-12-10

    Spin splitting of Rashba states in two-dimensional electron system provides a mechanism of spin manipulation for spintronics applications. However, Rashba states realized experimentally to date are often outnumbered by spin-degenerated substrate states at the same energy range, hindering their practical applications. Here, by density functional theory calculation, we show that Au one monolayer film deposition on a layered semiconductor surface β-InSe(0001) can possess “ideal” Rashba states with large spin splitting, which are completely situated inside the large band gap of the substrate. The position of the Rashba bands can be tuned over a wide range with respect to the substratemore » band edges by experimentally accessible strain. Furthermore, our nonequilibrium Green’s function transport calculation shows that this system may give rise to the long-sought strong current modulation when made into a device of Datta-Das transistor. Similar systems may be identified with other metal ultrathin films and layered semiconductor substrates to realize ideal Rashba states.« less

  10. Formation of Ideal Rashba States on Layered Semiconductor Surfaces Steered by Strain Engineering

    SciTech Connect

    Ming, Wenmei; Wang, Z. F.; Zhou, Miao; Yoon, Mina; Liu, Feng

    2015-12-10

    Spin splitting of Rashba states in two-dimensional electron system provides a mechanism of spin manipulation for spintronics applications. However, Rashba states realized experimentally to date are often outnumbered by spin-degenerated substrate states at the same energy range, hindering their practical applications. Here, by density functional theory calculation, we show that Au one monolayer film deposition on a layered semiconductor surface β-InSe(0001) can possess “ideal” Rashba states with large spin splitting, which are completely situated inside the large band gap of the substrate. The position of the Rashba bands can be tuned over a wide range with respect to the substrate band edges by experimentally accessible strain. Furthermore, our nonequilibrium Green’s function transport calculation shows that this system may give rise to the long-sought strong current modulation when made into a device of Datta-Das transistor. Similar systems may be identified with other metal ultrathin films and layered semiconductor substrates to realize ideal Rashba states.

  11. Theory of Covalent Adsorbate Frontier Orbital Energies on Functionalized Light-Absorbing Semiconductor Surfaces.

    PubMed

    Yu, Min; Doak, Peter; Tamblyn, Isaac; Neaton, Jeffrey B

    2013-05-16

    Functional hybrid interfaces between organic molecules and semiconductors are central to many emerging information and solar energy conversion technologies. Here we demonstrate a general, empirical parameter-free approach for computing and understanding frontier orbital energies - or redox levels - of a broad class of covalently bonded organic-semiconductor surfaces. We develop this framework in the context of specific density functional theory (DFT) and many-body perturbation theory calculations, within the GW approximation, of an exemplar interface, thiophene-functionalized silicon (111). Through detailed calculations taking into account structural and binding energetics of mixed-monolayers consisting of both covalently attached thiophene and hydrogen, chlorine, methyl, and other passivating groups, we quantify the impact of coverage, nonlocal polarization, and interface dipole effects on the alignment of the thiophene frontier orbital energies with the silicon band edges. For thiophene adsorbate frontier orbital energies, we observe significant corrections to standard DFT (∼1 eV), including large nonlocal electrostatic polarization effects (∼1.6 eV). Importantly, both results can be rationalized from knowledge of the electronic structure of the isolated thiophene molecule and silicon substrate systems. Silicon band edge energies are predicted to vary by more than 2.5 eV, while molecular orbital energies stay similar, with the different functional groups studied, suggesting the prospect of tuning energy alignment over a wide range for photoelectrochemistry and other applications.

  12. Direct imaging of electron recombination and transport on a semiconductor surface by femtosecond time-resolved photoemission electron microscopy

    SciTech Connect

    Fukumoto, Keiki Yamada, Yuki; Koshihara, Shin-ya; Onda, Ken

    2014-02-03

    Much effort has been devoted to the development of techniques to probe carrier dynamics, which govern many semiconductor device characteristics. We report direct imaging of electron dynamics on semiconductor surfaces by time-resolved photoemission electron microscopy using femtosecond laser pulses. The experiments utilized a variable-repetition-rate femtosecond laser system to suppress sample charging problems. The recombination of photogenerated electrons and the lateral motion of the electrons driven by an external electric field on a GaAs surface were visualized. The mobility was estimated from a linear relationship between the drift velocity and the potential gradient.

  13. Surface Chemistry and Transport Properties of II-VI Semiconductor Nanowires

    NASA Astrophysics Data System (ADS)

    Paudel, Pravin

    Semiconductor nanowires have been widely studied due to their unique properties such as width comparable to critical length-scales, high aspect ratio, and high carrier mobility. These unique properties make them a suitable candidate for various optical and electronic devices like photovoltaics, photodetectors, and field effect transistors. The nanowire surface plays an important role in the performance of these devices because of their high surface to volume ratio. The larger surface area of nanowires may provide better charge separation than planar heterostructures in photovoltaics by providing shorter distance to move for carrier before separation, however, presence of surface states may lead to the recombination of photo-generated carriers, limiting the amount of charge separation. In order to remove these surface states, ligands can be attached to the surface of nanowires. In this work CdS and CdSe nanowires are grown through the high temperature Vapor-Liquid-Solid (VLS) process. VLS process yields single crystalline, low defect nanowires with controllable length and diameter. In order to show ligand binding on nanowire surface, CdS nanowires were treated with a dye-labeled polymer. Fluorescence microscopy and spectroscopy were used to confirm ligand binding. Fluorescence microscopy can also be used to show the kinetics of ligand binding on nanowire surfaces. In order to control the electronic properties of the nanowire surfaces, nanowires were treated with solution phase and vapor phase reagents. Photoluminescence measurements and transport measurements were performed before and after the chemical treatment to see the consequences of ligand binding on the optical and electronic properties of nanowires.

  14. Surface energy-mediated construction of anisotropic semiconductor wires with selective crystallographic polarity

    NASA Astrophysics Data System (ADS)

    Sohn, Jung Inn; Hong, Woong-Ki; Lee, Sunghoon; Lee, Sanghyo; Ku, Jiyeon; Park, Young Jun; Hong, Jinpyo; Hwang, Sungwoo; Park, Kyung Ho; Warner, Jamie H.; Cha, Seungnam; Kim, Jong Min

    2014-07-01

    ZnO is a wide band-gap semiconductor with piezoelectric properties suitable for opto-electronics, sensors, and as an electrode material. Controlling the shape and crystallography of any semiconducting nanomaterial is a key step towards extending their use in applications. Whilst anisotropic ZnO wires have been routinely fabricated, precise control over the specific surface facets and tailoring of polar and non-polar growth directions still requires significant refinement. Manipulating the surface energy of crystal facets is a generic approach for the rational design and growth of one-dimensional (1D) building blocks. Although the surface energy is one basic factor for governing crystal nucleation and growth of anisotropic 1D structures, structural control based on surface energy minimization has not been yet demonstrated. Here, we report an electronic configuration scheme to rationally modulate surface electrostatic energies for crystallographic-selective growth of ZnO wires. The facets and orientations of ZnO wires are transformed between hexagonal and rectangular/diamond cross-sections with polar and non-polar growth directions, exhibiting different optical and piezoelectrical properties. Our novel synthetic route for ZnO wire fabrication provides new opportunities for future opto-electronics, piezoelectronics, and electronics, with new topological properties.

  15. Surface energy-mediated construction of anisotropic semiconductor wires with selective crystallographic polarity

    PubMed Central

    Sohn, Jung Inn; Hong, Woong-Ki; Lee, Sunghoon; Lee, Sanghyo; Ku, JiYeon; Park, Young Jun; Hong, Jinpyo; Hwang, Sungwoo; Park, Kyung Ho; Warner, Jamie H.; Cha, SeungNam; Kim, Jong Min

    2014-01-01

    ZnO is a wide band-gap semiconductor with piezoelectric properties suitable for opto-electronics, sensors, and as an electrode material. Controlling the shape and crystallography of any semiconducting nanomaterial is a key step towards extending their use in applications. Whilst anisotropic ZnO wires have been routinely fabricated, precise control over the specific surface facets and tailoring of polar and non-polar growth directions still requires significant refinement. Manipulating the surface energy of crystal facets is a generic approach for the rational design and growth of one-dimensional (1D) building blocks1234. Although the surface energy is one basic factor for governing crystal nucleation and growth of anisotropic 1D structures, structural control based on surface energy minimization has not been yet demonstrated56789. Here, we report an electronic configuration scheme to rationally modulate surface electrostatic energies for crystallographic-selective growth of ZnO wires. The facets and orientations of ZnO wires are transformed between hexagonal and rectangular/diamond cross-sections with polar and non-polar growth directions, exhibiting different optical and piezoelectrical properties. Our novel synthetic route for ZnO wire fabrication provides new opportunities for future opto-electronics, piezoelectronics, and electronics, with new topological properties. PMID:25017476

  16. Surface energy-mediated construction of anisotropic semiconductor wires with selective crystallographic polarity.

    PubMed

    Sohn, Jung Inn; Hong, Woong-Ki; Lee, Sunghoon; Lee, Sanghyo; Ku, JiYeon; Park, Young Jun; Hong, Jinpyo; Hwang, Sungwoo; Park, Kyung Ho; Warner, Jamie H; Cha, SeungNam; Kim, Jong Min

    2014-07-14

    ZnO is a wide band-gap semiconductor with piezoelectric properties suitable for opto-electronics, sensors, and as an electrode material. Controlling the shape and crystallography of any semiconducting nanomaterial is a key step towards extending their use in applications. Whilst anisotropic ZnO wires have been routinely fabricated, precise control over the specific surface facets and tailoring of polar and non-polar growth directions still requires significant refinement. Manipulating the surface energy of crystal facets is a generic approach for the rational design and growth of one-dimensional (1D) building blocks. Although the surface energy is one basic factor for governing crystal nucleation and growth of anisotropic 1D structures, structural control based on surface energy minimization has not been yet demonstrated. Here, we report an electronic configuration scheme to rationally modulate surface electrostatic energies for crystallographic-selective growth of ZnO wires. The facets and orientations of ZnO wires are transformed between hexagonal and rectangular/diamond cross-sections with polar and non-polar growth directions, exhibiting different optical and piezoelectrical properties. Our novel synthetic route for ZnO wire fabrication provides new opportunities for future opto-electronics, piezoelectronics, and electronics, with new topological properties.

  17. [Spatial variation characteristics of surface soil water content, bulk density and saturated hydraulic conductivity on Karst slopes].

    PubMed

    Zhang, Chuan; Chen, Hong-Song; Zhang, Wei; Nie, Yun-Peng; Ye, Ying-Ying; Wang, Ke-Lin

    2014-06-01

    Surface soil water-physical properties play a decisive role in the dynamics of deep soil water. Knowledge of their spatial variation is helpful in understanding the processes of rainfall infiltration and runoff generation, which will contribute to the reasonable utilization of soil water resources in mountainous areas. Based on a grid sampling scheme (10 m x 10 m) and geostatistical methods, this paper aimed to study the spatial variability of surface (0-10 cm) soil water content, soil bulk density and saturated hydraulic conductivity on a typical shrub slope (90 m x 120 m, projected length) in Karst area of northwest Guangxi, southwest China. The results showed that the surface soil water content, bulk density and saturated hydraulic conductivity had different spatial dependence and spatial structure. Sample variogram of the soil water content was fitted well by Gaussian models with the nugget effect, while soil bulk density and saturated hydraulic conductivity were fitted well by exponential models with the nugget effect. Variability of soil water content showed strong spatial dependence, while the soil bulk density and saturated hydraulic conductivity showed moderate spatial dependence. The spatial ranges of the soil water content and saturated hydraulic conductivity were small, while that of the soil bulk density was much bigger. In general, the soil water content increased with the increase of altitude while it was opposite for the soil bulk densi- ty. However, the soil saturated hydraulic conductivity had a random distribution of large amounts of small patches, showing high spatial heterogeneity. Soil water content negatively (P < 0.01) correlated with the bulk density and saturated hydraulic conductivity, while there was no significant correlation between the soil bulk density and saturated hydraulic conductivity.

  18. Magnetic order in a frustrated two-dimensional atom lattice at a semiconductor surface.

    PubMed

    Li, Gang; Höpfner, Philipp; Schäfer, Jörg; Blumenstein, Christian; Meyer, Sebastian; Bostwick, Aaron; Rotenberg, Eli; Claessen, Ralph; Hanke, Werner

    2013-01-01

    Two-dimensional electron systems, as exploited for device applications, can lose their conducting properties because of local Coulomb repulsion, leading to a Mott-insulating state. In triangular geometries, any concomitant antiferromagnetic spin ordering can be prevented by geometric frustration, spurring speculations about 'melted' phases, known as spin liquid. Here we show that for a realization of a triangular electron system by epitaxial atom adsorption on a semiconductor, such spin disorder, however, does not appear. Our study compares the electron excitation spectra obtained from theoretical simulations of the correlated electron lattice with data from high-resolution photoemission. We find that an unusual row-wise antiferromagnetic spin alignment occurs that is reflected in the photoemission spectra as characteristic 'shadow bands' induced by the spin pattern. The magnetic order in a frustrated lattice of otherwise non-magnetic components emerges from longer-range electron hopping between the atoms. This finding can offer new ways of controlling magnetism on surfaces.

  19. One-step photoembossing for submicrometer surface relief structures in liquid crystal semiconductors.

    PubMed

    Liedtke, Alicia; Lei, Chunhong; O'Neill, Mary; Dyer, Peter E; Kitney, Stuart P; Kelly, Stephen M

    2010-06-22

    We report a new single-step method to directly imprint nanometer-scale structures on photoreactive organic semiconductors. A surface relief grating is spontaneously formed when a light-emitting, liquid crystalline, and semiconducting thin film is irradiated by patterned light generated using a phase mask. Grating formation requires no postannealing nor wet etching so there is potential for high-throughput fabrication. The structured film is cross-linked for robustness. Gratings deeper than the original film thickness are made with periods as small as 265 nm. Grating formation is attributed to mass transfer, enhanced by self-assembly, from dark to illuminated regions. A photovoltaic device incorporating the grating is discussed.

  20. Implementation of Surface Acoustic Wave Vapor Sensor Using Complementary Metal-Oxide-Semiconductor Amplifiers

    NASA Astrophysics Data System (ADS)

    Chiu, Chia-Sung; Chang, Ching-Chun; Ku, Chia-Lin; Peng, Kang-Ming; Jeng, Erik S.; Chen, Wen-Lin; Huang, Guo-Wei; Wu, Lin-Kun

    2009-04-01

    A surface acoustic wave (SAW) vapor sensor is presented in this work. A SAW delay line oscillator on quartz substrate with the high gain complementary metal-oxide-semiconductor (CMOS) amplifier using a two-poly-two-metal (2P2M) 0.35 µm process was designed. The gain of the CMOS amplifier and its total power consumption are 20 dB and 70 mW, respectively. The achieved phase noise of this SAW oscillator is -150 dBc/Hz at 100 kHz offset. The sensing is successfully demonstrated by a thin poly(epichlorohydrin) (PECH) polymer film on a SAW oscillator with alcohol vapor. This two-in-one sensor unit includes the SAW device and the CMOS amplifier provides designers with comprehensive model for using these components for sensor circuit fabrication. Furthermore it will be promising for future chemical and biological sensing applications.

  1. Long- and short-period nanostructure formation on semiconductor surfaces at different ambient conditions

    SciTech Connect

    Ganeev, R. A.; Baba, M.; Kuroda, H.; Ozaki, T.

    2010-05-15

    We present the results of studies of nanoripples formation during interaction of the 800 nm, 120, and 35 fs pulses with semiconductor surfaces. Simultaneous appearance of the ripples with the period (700 nm) close to the wavelength of interacting radiation and considerably smaller period (180 nm) was achieved. We discuss the experimental conditions for the formation of these nanoripples (incidence angle, polarization, number of shots, etc.). We show a decisive role of surrounding medium on the quality of nanoripples formation. The self-organization of high-quality nanoripples was clearly shown in the case of dense surrounding medium (methanol), while in the case of insufficient amount of surrounding material (i.e., at different vacuum conditions), the quality of ripples considerably decreased.

  2. Broadband Surface Plasmon Lasing in One-dimensional Metallic Gratings on Semiconductor.

    PubMed

    Kim, Seung-Hyun; Han, Won Seok; Jeong, Tae-Young; Lee, Hyang-Rok; Jeong, H; Lee, D; Shim, Seung-Bo; Kim, Dai-Sik; Ahn, Kwang Jun; Yee, Ki-Ju

    2017-08-11

    We report surface plasmon (SP) lasing in metal/semiconductor nanostructures, where one-dimensional periodic silver slit gratings are placed on top of an InGaAsP layer. The SP nature of the lasing is confirmed from the emission wavelength governed by the grating period, polarization analysis, spatial coherence, and comparison with the linear transmission. The excellent performance of the device as an SP source is demonstrated by its tunable emission in the 400-nm-wide telecom wavelength band at room temperature. We show that the stimulated emission enhanced by the Purcell effect enables successful SP lasing at high energies above the gap energy of the gain. We also discuss the dependence of the lasing efficiency on temperature, grating dimension, and type of metal.

  3. Fixing the energy scale in scanning tunneling microscopy on semiconductor surfaces.

    PubMed

    Münnich, Gerhard; Donarini, Andrea; Wenderoth, Martin; Repp, Jascha

    2013-11-22

    In scanning tunneling experiments on semiconductor surfaces, the energy scale within the tunneling junction is usually unknown due to tip-induced band bending. Here, we experimentally recover the zero point of the energy scale by combining scanning tunneling microscopy with Kelvin probe force spectroscopy. With this technique, we revisit shallow acceptors buried in GaAs. Enhanced acceptor-related conductance is observed in negative, zero, and positive band-bending regimes. An Anderson-Hubbard model is used to rationalize our findings, capturing the crossover between the acceptor state being part of an impurity band for zero band bending and the acceptor state being split off and localized for strong negative or positive band bending, respectively.

  4. Van der Waals Interactions of Organic Molecules on Semiconductor and Metal Surfaces: a Comparative Study

    NASA Astrophysics Data System (ADS)

    Li, Guo; Cooper, Valentino; Cho, Jun-Hyung; Tamblyn, Isaac; Du, Shixuan; Neaton, Jeffrey; Gao, Hong-Jun; Zhang, Zhenyu

    2012-02-01

    We present a comparative investigation of vdW interactions of the organic molecules on semiconductor and metal surfaces using the DFT method implemented with vdW-DF. For styrene/H-Si(100), the vdW interactions reverse the effective intermolecular interaction from repulsive to attractive, ensuring preferred growth of long wires as observed experimentally. We further propose that an external E field and the selective creation of Si dangling bonds can drastically improve the ordered arrangement of the molecular nanowires [1]. For BDA/Au(111), the vdW interactions not only dramatically enhances the adsorption energies, but also significantly changes the molecular configurations. In the azobenzene/Ag(111) system, vdW-DF produces superior predictions for the adsorption energy than those obtained with other vdW corrected DFT approaches, providing evidence for the applicability of the vdW-DF method [2].

  5. Optimizing electrically pumped vertical extended cavity surface emitting semiconductor lasers (E-VECSELs)

    NASA Astrophysics Data System (ADS)

    McInerney, John G.; Mooradian, Aram

    2011-03-01

    The future evolution of photonics, for a wide spectrum of applications ranging from established optical telecommunications to emerging opportunities such as biotechnology, reprographics and projection displays, will depend on availability of compact, rugged, efficient and inexpensive lasers which deliver high power, good beam quality, excellent wavelength stability, low noise and long lifetime in the near infrared and visible regions. This combination is not readily available from either of the traditional classes of semiconductor laser, edge-emitters and vertical cavity surface emitters (VCSELs). Here we describe a novel class of laser based on geometry similar to VCSELs but controlled by an extended coupled cavity. These devices are scalable to high powers while maintaining fundamental spatial mode performance, a feature that is essential to efficient coupling into a single mode optical fibre or waveguide, or long range propagation in free space. They are also ideally suited to mode locking, gain-switching and intracavity frequency conversion, among other applications.

  6. Governing Equations of Tissue Modelling and Remodelling: A Unified Generalised Description of Surface and Bulk Balance.

    PubMed

    Buenzli, Pascal R

    2016-01-01

    Several biological tissues undergo changes in their geometry and in their bulk material properties by modelling and remodelling processes. Modelling synthesises tissue in some regions and removes tissue in others. Remodelling overwrites old tissue material properties with newly formed, immature tissue properties. As a result, tissues are made up of different "patches", i.e., adjacent tissue regions of different ages and different material properties, within evolving boundaries. In this paper, generalised equations governing the spatio-temporal evolution of such tissues are developed within the continuum model. These equations take into account nonconservative, discontinuous surface mass balance due to creation and destruction of material at moving interfaces, and bulk balance due to tissue maturation. These equations make it possible to model patchy tissue states and their evolution without explicitly maintaining a record of when/where resorption and formation processes occurred. The time evolution of spatially averaged tissue properties is derived systematically by integration. These spatially-averaged equations cannot be written in closed form as they retain traces that tissue destruction is localised at tissue boundaries. The formalism developed in this paper is applied to bone tissues, which exhibit strong material heterogeneities due to their slow mineralisation and remodelling processes. Evolution equations are proposed in particular for osteocyte density and bone mineral density. Effective average equations for bone mineral density (BMD) and tissue mineral density (TMD) are derived using a mean-field approximation. The error made by this approximation when remodelling patchy tissue is investigated. The specific signatures of the time evolution of BMD or TMD during remodelling events are exhibited. These signatures may provide a way to detect remodelling events at lower, unseen spatial resolutions from microCT scans.

  7. Governing Equations of Tissue Modelling and Remodelling: A Unified Generalised Description of Surface and Bulk Balance

    PubMed Central

    Buenzli, Pascal R.

    2016-01-01

    Several biological tissues undergo changes in their geometry and in their bulk material properties by modelling and remodelling processes. Modelling synthesises tissue in some regions and removes tissue in others. Remodelling overwrites old tissue material properties with newly formed, immature tissue properties. As a result, tissues are made up of different “patches”, i.e., adjacent tissue regions of different ages and different material properties, within evolving boundaries. In this paper, generalised equations governing the spatio-temporal evolution of such tissues are developed within the continuum model. These equations take into account nonconservative, discontinuous surface mass balance due to creation and destruction of material at moving interfaces, and bulk balance due to tissue maturation. These equations make it possible to model patchy tissue states and their evolution without explicitly maintaining a record of when/where resorption and formation processes occurred. The time evolution of spatially averaged tissue properties is derived systematically by integration. These spatially-averaged equations cannot be written in closed form as they retain traces that tissue destruction is localised at tissue boundaries. The formalism developed in this paper is applied to bone tissues, which exhibit strong material heterogeneities due to their slow mineralisation and remodelling processes. Evolution equations are proposed in particular for osteocyte density and bone mineral density. Effective average equations for bone mineral density (BMD) and tissue mineral density (TMD) are derived using a mean-field approximation. The error made by this approximation when remodelling patchy tissue is investigated. The specific signatures of the time evolution of BMD or TMD during remodelling events are exhibited. These signatures may provide a way to detect remodelling events at lower, unseen spatial resolutions from microCT scans. PMID:27043309

  8. Scanning tunnelling spectroscopy and manipulation of double-decker phthalocyanine molecules on a semiconductor surface

    NASA Astrophysics Data System (ADS)

    Pan, Yi; Kanisawa, Kiyoshi; Ishikawa, Naoto; Fölsch, Stefan

    2017-09-01

    A scanning tunnelling microscope (STM) operated at 5 K was used to study dysprosium biphthalocyanine (DyPc2) molecules adsorbed on the inert III-V semiconductor surface InAs(1 1 1)A. Orbital imaging and scanning tunnelling spectroscopy measurements reveal that the molecular electronic structure remains largely unperturbed, indicating a weak molecule-surface binding. The molecule adsorbs at the In vacancy site of the (2  ×  2)-reconstructed surface and is highly sensitive to current-induced excitations leading to random rotational fluctuations. Owing to the weak surface binding, individual molecules can be precisely repositioned and arranged by the STM tip via attractive tip-molecule interaction. In this way, DyPc2 dimers of well-defined internal structure can be assembled which exist in two conformations of equivalent appearance. A binary switching between these two conformers can be induced by injecting electrons into one of the two molecules. The conformational change of the dimer proceeds via a concerted molecular rotation and minor lateral displacement. The synchronised switching observed here is attributed to steric interactions between the two molecules constituting the dimer.

  9. Study of interface formation on the cleavage surfaces of A3{B}6 layered semiconductors

    NASA Astrophysics Data System (ADS)

    Galiy, P. V.; Nenchuk, T. M.; Stakhira, J. M.

    2001-01-01

    The adsorption activity of In4Se3, In4Se3(Ag), InSe, GaSe and TlGaSe2 semiconductor crystal interlayer cleavage surfaces relatively to N2, O2, CO gases and water vapour has been studied by Auger electron spectroscopy and mass spectrometry. It has been determined that atomically clean layered crystal surfaces do not adsorb N2 and water vapour but reveal a low activity with respect to O2. The kinetics of CO adsorption on surfaces obtained by cleavage in an UHV have been investigated. Indium and gallium selenides adsorb CO with the tendency increasing in the sequence GaSe→TlGaSe2→ InSe→In4Se3 crystals; In4Se3 is essentially more active than the others. The adsorption model with dissociations of the CO molecule and carbon adsorption resulting from the layered structure and peculiarities in the electron-energy spectra of the crystals and their surfaces is discussed with the In4Se3 crystal serving as example.

  10. Epitaxial growth of large-gap quantum spin Hall insulator on semiconductor surface

    PubMed Central

    Zhou, Miao; Ming, Wenmei; Liu, Zheng; Wang, Zhengfei; Li, Ping; Liu, Feng

    2014-01-01

    Formation of topological quantum phase on a conventional semiconductor surface is of both scientific and technological interest. Here, we demonstrate epitaxial growth of 2D topological insulator, i.e., quantum spin Hall state, on Si(111) surface with a large energy gap, based on first-principles calculations. We show that the Si(111) surface functionalized with one-third monolayer of halogen atoms [Si(111)-3×3-X (X = Cl, Br, I)] exhibiting a trigonal superstructure provides an ideal template for epitaxial growth of heavy metals, such as Bi, which self-assemble into a hexagonal lattice with high kinetic and thermodynamic stability. Most remarkably, the Bi overlayer is atomically bonded to but electronically decoupled from the underlying Si substrate, exhibiting isolated quantum spin Hall state with an energy gap as large as ∼0.8 eV. This surprising phenomenon originates from an intriguing substrate-orbital-filtering effect, which critically selects the orbital composition around the Fermi level, leading to different topological phases. In particular, the substrate-orbital-filtering effect converts the otherwise topologically trivial freestanding Bi lattice into a nontrivial phase; and the reverse is true for Au lattice. The underlying physical mechanism is generally applicable, opening a new and exciting avenue for exploration of large-gap topological surface/interface states. PMID:25246584

  11. Semiconductor sensors

    NASA Technical Reports Server (NTRS)

    Gatos, Harry C. (Inventor); Lagowski, Jacek (Inventor)

    1977-01-01

    A semiconductor sensor adapted to detect with a high degree of sensitivity small magnitudes of a mechanical force, presence of traces of a gas or light. The sensor includes a high energy gap (i.e., .about. 1.0 electron volts) semiconductor wafer. Mechanical force is measured by employing a non-centrosymmetric material for the semiconductor. Distortion of the semiconductor by the force creates a contact potential difference (cpd) at the semiconductor surface, and this cpd is determined to give a measure of the force. When such a semiconductor is subjected to illumination with an energy less than the energy gap of the semiconductors, such illumination also creates a cpd at the surface. Detection of this cpd is employed to sense the illumination itself or, in a variation of the system, to detect a gas. When either a gas or light is to be detected and a crystal of a non-centrosymmetric material is employed, the presence of gas or light, in appropriate circumstances, results in a strain within the crystal which distorts the same and the distortion provides a mechanism for qualitative and quantitative evaluation of the gas or the light, as the case may be.

  12. Surface zone articular chondrocytes modulate the bulk and surface mechanical properties of the tissue-engineered cartilage.

    PubMed

    Peng, Gordon; McNary, Sean M; Athanasiou, Kyriacos A; Reddi, A Hari

    2014-12-01

    The central hypothesis of functional tissue engineering is that an engineered construct can serve as a viable replacement tissue in vivo by replicating the structure and function of native tissue. In the case of articular cartilage, this requires the reproduction of the bulk mechanical and surface lubrication properties of native hyaline cartilage. Cartilage tissue engineering has primarily focused on achieving the bulk mechanical properties of native cartilage such as the compressive aggregate modulus and tensile strength. A scaffold-free self-assembling process has been developed that produces engineered cartilage with compressive properties approaching native tissue levels. Thus, the next step in this process is to begin addressing the friction coefficient and wear properties of these engineered constructs. The superficial zone protein (SZP), also known as lubricin or PRG4, is a boundary mode lubricant that is synthesized by surface zone (SZ) articular chondrocytes. Under conditions of high loading and low sliding speeds, SZP reduces friction and wear at the articular surface. The objective of this investigation was to determine whether increasing the proportion of SZ chondrocytes in cartilage constructs, in the absence of external stimuli such as growth factors and mechanical loading, would enhance the secretion of SZP and improve their frictional properties. In this study, cartilage constructs were engineered through a self-assembling process with varying ratios of SZ and middle zone (MZ) chondrocytes (SZ:MZ): 0:100, 25:75, 50:50, 75:25, and 100:0. Constructs containing different ratios of SZ and MZ chondrocytes did not significantly differ in the glycosaminoglycan composition or compressive aggregate modulus. In contrast, tensile properties and collagen content were enhanced in nearly all constructs containing greater amounts of SZ chondrocytes. Increasing the proportion of SZ chondrocytes had the hypothesized effect of improving the synthesis and secretion

  13. Roles of Bulk and Surface Chemistry in the Oxygen Exchange Kinetics and Related Properties of Mixed Conducting Perovskite Oxide Electrodes

    PubMed Central

    Perry, Nicola H.; Ishihara, Tatsumi

    2016-01-01

    Mixed conducting perovskite oxides and related structures serving as electrodes for electrochemical oxygen incorporation and evolution in solid oxide fuel and electrolysis cells, respectively, play a significant role in determining the cell efficiency and lifetime. Desired improvements in catalytic activity for rapid surface oxygen exchange, fast bulk transport (electronic and ionic), and thermo-chemo-mechanical stability of oxygen electrodes will require increased understanding of the impact of both bulk and surface chemistry on these properties. This review highlights selected work at the International Institute for Carbon-Neutral Energy Research (I2CNER), Kyushu University, set in the context of work in the broader community, aiming to characterize and understand relationships between bulk and surface composition and oxygen electrode performance. Insights into aspects of bulk point defect chemistry, electronic structure, crystal structure, and cation choice that impact carrier concentrations and mobilities, surface exchange kinetics, and chemical expansion coefficients are emerging. At the same time, an understanding of the relationship between bulk and surface chemistry is being developed that may assist design of electrodes with more robust surface chemistries, e.g., impurity tolerance or limited surface segregation. Ion scattering techniques (e.g., secondary ion mass spectrometry, SIMS, or low energy ion scattering spectroscopy, LEIS) with high surface sensitivity and increasing lateral resolution are proving useful for measuring surface exchange kinetics, diffusivity, and corresponding outer monolayer chemistry of electrodes exposed to typical operating conditions. Beyond consideration of chemical composition, the use of strain and/or a high density of active interfaces also show promise for enhancing performance. PMID:28773978

  14. Theory of surface second-harmonic generation for semiconductors including effects of nonlocal operators

    NASA Astrophysics Data System (ADS)

    Anderson, Sean M.; Tancogne-Dejean, Nicolas; Mendoza, Bernardo S.; Véniard, Valérie

    2015-02-01

    We formulate a theoretical approach of surface second-harmonic generation from semiconductor surfaces based on the length gauge and the electron density operator. Within the independent particle approximation, the nonlinear second-order surface susceptibility tensor χa b c(-2 ω ;ω ,ω ) is calculated, including in one unique formulation (i) the scissors correction, needed to have the correct value of the energy band gap, (ii) the contribution of the nonlocal part of the pseudopotentials, routinely used in ab initio band-structure calculations, and (iii) the derivation for the inclusion of the cut function, used to extract the surface response. The first two contributions are described by spatially nonlocal quantum-mechanical operators and are fully taken into account in the present formulation. As a test case of the approach, we calculate χx x x(-2 ω ;ω ,ω ) for the clean Si (001 )2 ×1 reconstructed surface. The effects of the scissors correction and of the nonlocal part of the pseudopotentials are discussed in surface nonlinear optics. The scissors correction shifts the spectrum to higher energies though the shifting is not rigid and mixes the 1 ω and 2 ω resonances, and has a strong influence in the line shape. The effects of the nonlocal part of the pseudopotentials keeps the same line shape of | χ2×1 x x x(-2 ω ;ω ,ω ) | , but reduces its value by 15%-20%. Therefore the inclusion of the three aforementioned contributions is very important and makes our scheme unprecedented and opens the possibility to study surface second-harmonic generation with more versatility and providing more accurate results.

  15. Surface passivation and interface properties of bulk GaAs and epitaxial-GaAs/Ge using atomic layer deposited TiAlO alloy dielectric.

    PubMed

    Dalapati, G K; Chia, C K; Tan, C C; Tan, H R; Chiam, S Y; Dong, J R; Das, A; Chattopadhyay, S; Mahata, C; Maiti, C K; Chi, D Z

    2013-02-01

    High quality surface passivation on bulk-GaAs substrates and epitaxial-GaAs/Ge (epi-GaAs) layers were achieved by using atomic layer deposited (ALD) titanium aluminum oxide (TiAlO) alloy dielectric. The TiAlO alloy dielectric suppresses the formation of defective native oxide on GaAs layers. X-ray photoelectron spectroscopy (XPS) analysis shows interfacial arsenic oxide (As(x)O(y)) and elemental arsenic (As) were completely removed from the GaAs surface. Energy dispersive X-ray diffraction (EDX) analysis and secondary ion mass spectroscopy (SIMS) analysis showed that TiAlO dielectric is an effective barrier layer for reducing the out-diffusion of elemental atoms, enhancing the electrical properties of bulk-GaAs based metal-oxide-semiconductor (MOS) devices. Moreover, ALD TiAlO alloy dielectric on epi-GaAs with AlGaAs buffer layer realized smooth interface between epi-GaAs layers and TiAlO dielectric, yielding a high quality surface passivation on epi-GaAs layers, much sought-after for high-speed transistor applications on a silicon platform. Presence of a thin AlGaAs buffer layer between epi-GaAs and Ge substrates improved interface quality and gate dielectric quality through the reduction of interfacial layer formation (Ga(x)O(y)) and suppression of elemental out-diffusion (Ga and As). The AlGaAs buffer layer and TiAlO dielectric play a key role to suppress the roughening, interfacial layer formation, and impurity diffusion into the dielectric, which in turn largely enhances the electrical property of the epi-GaAs MOS devices.

  16. Effect of surface charge and s-wave component on (001) surface and bulk states of YBa2Cu3O7-δ

    NASA Astrophysics Data System (ADS)

    Pasanai, K.

    2017-01-01

    The local densities of the (001) surface and bulk states (LDOS) of thin YBa2Cu3O7-δ films were studied based on the self-consistent tight binding model. The CuO-chain and CuO2-plane layers were considered to be a coupling between bands, and then the input surface charge density and the s-wave sub-dominant component were included in the material. In the calculation process, the surface and bulk states were determined, for comparison. It was found that the size of the superconducting gap of a plane layer at the surface increased with an increasing input of the surface charge density, but the LDOS in the bulk were not affected much by this change. When the s-wave component was included in the material, it affected several peaks in the LDOS. This caused some peaks to split while some other peaks became sharper.

  17. Valence instability in the bulk and at the surface of the antiferromagnet SmRh2Si2

    NASA Astrophysics Data System (ADS)

    Chikina, A.; Generalov, A.; Kummer, K.; Güttler, M.; Antonov, V. N.; Kucherenko, Yu.; Kliemt, K.; Krellner, C.; Danzenbächer, S.; Kim, T.; Dudin, P.; Geibel, C.; Laubschat, C.; Vyalikh, D. V.

    2017-04-01

    Using resonant angle-resolved photoemission spectroscopy and electron band-structure calculations, we explore the electronic structure and properties of Sm atoms at the surface and in the bulk of the antiferromagnet SmRh2Si2 . We show that the Sm atoms reveal weak mixed-valent behavior both in the bulk and at the surface. Although trivalent 4 f emission strongly dominates, a small divalent 4 f signal near the Fermi energy can be clearly resolved for surface and bulk Sm atoms. This behavior is quite different to most other Sm-based materials which typically experience a surface valence transition to a divalent state of Sm atoms at the surface. This phenomenon is explained in analogy to the isostructural Ce compound, where strong 4 f hybridization stabilizes mixed-valent ground state both in the bulk and at the surface, and which were described in the light of the single-impurity Anderson model. Implications for other RE Rh2Si2 (RE = rare-earth elements) compounds are discussed.

  18. Bulk chirality effect for symmetric bistable switching of liquid crystals on topologically self-patterned degenerate anchoring surface.

    PubMed

    Park, Min-Kyu; Joo, Kyung-Il; Kim, Hak-Rin

    2017-06-26

    We demonstrate a bistable switching liquid crystal (LC) mode utilizing a topologically self-structured dual-groove surface for degenerated easy axes of LC anchoring. In our study, the effect of the bulk elastic distortion of the LC directors on the bistable anchoring surface is theoretically analyzed for balanced bistable states based on a free energy diagram. By adjusting bulk LC chirality, we developed ideally symmetric and stable bistable anchoring and switching properties, which can be driven by a low in-plane pulsed field of about 0.7 V/µm. The fabricated device has a contrast ratio of 196:1.

  19. Influence of Surface and Bulk Water Ice on the Reactivity of a Water-forming Reaction

    NASA Astrophysics Data System (ADS)

    Lamberts, Thanja; Kästner, Johannes

    2017-09-01

    On the surface of icy dust grains in the dense regions of the interstellar medium, a rich chemistry can take place. Due to the low temperature, reactions that proceed via a barrier can only take place through tunneling. The reaction {{H}}+{{{H}}}2{{{O}}}2\\longrightarrow {{{H}}}2{{O}}+{OH} is such a case with a gas-phase barrier of ∼26.5 kJ mol‑1. Still, the reaction is known to be involved in water formation on interstellar grains. Here, we investigate the influence of a water ice surface and of bulk ice on the reaction rate constant. Rate constants are calculated using instanton theory down to 74 K. The ice is taken into account via multiscale modeling, describing the reactants and the direct surrounding at the quantum mechanical level with density functional theory (DFT), while the rest of the ice is modeled on the molecular mechanical level with a force field. We find that H2O2 binding energies cannot be captured by a single value, but rather they depend on the number of hydrogen bonds with surface molecules. In highly amorphous surroundings, the binding site can block the routes of attack and impede the reaction. Furthermore, the activation energies do not correlate with the binding energies of the same sites. The unimolecular rate constants related to the Langmuir–Hinshelwood mechanism increase as the activation energy decreases. Thus, we provide a lower limit for the rate constant and argue that rate constants can have values up to two order of magnitude larger than this limit.

  20. Nonlinear dependence between the surface reflectance and the duty-cycle of semiconductor nanorod array.

    PubMed

    Pai, Yi-Hao; Lin, Yu-Chan; Tsai, Jai-Lin; Lin, Gong-Ru

    2011-01-31

    The nonlinear dependence between the duty-cycle of semiconductor nanorod array and its surface reflectance minimization is demonstrated. The duty-cycle control on thin-SiO2 covered Si nanorod array is performed by O(2-) plasma pre-etching the self-assembled polystyrene nanosphere array mask with area density of 4 × 10(8) rod/cm(-2). The 120-nm high SiO2 covered Si nanorod array is obtained after subsequent CF4/O2 plasma etching for 160 sec. This results in a tunable nanorod diameter from 445 to 285 nm after etching from 30 to 80 sec, corresponding to a varying nanorod duty-cycle from 89% to 57%. The TM-mode reflection analysis shows a diminishing Brewster angle shifted from 71° to 54° with increasing nanorod duty-cycle from 57% to 89% at 532 nm. The greatly reduced small-angle reflectance reveals a nonlinear trend with enlarging duty-cycle, leading to a minimum surface reflectance at nanorod duty-cycle of 85%. Both the simulation and experiment indicate that such a surface reflectance minimum is even lower than that of a uniformly SiO2 covered Si substrate on account of its periodical nanorod array architecture with tuned duty-cycle.

  1. Semiconductor nanowires and nanotubes: effects of size and surface-to-volume ratio.

    PubMed

    Pan, Hui; Feng, Yuan Ping

    2008-11-25

    The electronic properties of semiconductor (SiC, GaN, BN, ZnO, ZnS, and CdS) nanowires and nanotubes were investigated using first-principles calculations based on density functional theory and generalized gradient approximation. Different size or surface-to-volume ratio dependences were found for the II-VI (ZnO, ZnS, and CdS) and IV-IV (SiC) and III-V (GaN and BN) nanostructures. For SiC, GaN, and BN nanostructures, the band gap decreases with the increase of the surface-to-volume ratio or the reduction of the diameter, while for ZnO, ZnS, and CdS nanostructures, the band gap increases with the increase of surface-to-volume ratio or the reduction of the diameter. The mechanism is attributed to the competition between the interaction from dangling p-like and sigma states and the quantum confinement effect.

  2. Mobility of large clusters on a semiconductor surface: Kinetic Monte Carlo simulation results

    NASA Astrophysics Data System (ADS)

    M, Esen; A, T. Tüzemen; M, Ozdemir

    2016-01-01

    The mobility of clusters on a semiconductor surface for various values of cluster size is studied as a function of temperature by kinetic Monte Carlo method. The cluster resides on the surface of a square grid. Kinetic processes such as the diffusion of single particles on the surface, their attachment and detachment to/from clusters, diffusion of particles along cluster edges are considered. The clusters considered in this study consist of 150-6000 atoms per cluster on average. A statistical probability of motion to each direction is assigned to each particle where a particle with four nearest neighbors is assumed to be immobile. The mobility of a cluster is found from the root mean square displacement of the center of mass of the cluster as a function of time. It is found that the diffusion coefficient of clusters goes as D = A(T)Nα where N is the average number of particles in the cluster, A(T) is a temperature-dependent constant and α is a parameter with a value of about -0.64 < α < -0.75. The value of α is found to be independent of cluster sizes and temperature values (170-220 K) considered in this study. As the diffusion along the perimeter of the cluster becomes prohibitive, the exponent approaches a value of -0.5. The diffusion coefficient is found to change by one order of magnitude as a function of cluster size.

  3. Surface sensitivity of four-probe STM resistivity measurements of bulk ZnO correlated to XPS

    NASA Astrophysics Data System (ADS)

    Lord, Alex M.; Evans, Jonathan E.; Barnett, Chris J.; Allen, Martin W.; Barron, Andrew R.; Wilks, Steve P.

    2017-09-01

    Multi-probe instruments based on scanning tunnelling microscopy (STM) are becoming increasingly common for their ability to perform nano- to atomic-scale investigations of nanostructures, surfaces and in situ reactions. A common configuration is the four-probe STM often coupled with in situ scanning electron microscopy (SEM) that allows precise positioning of the probes onto surfaces and nanostructures enabling electrical and scanning experiments to be performed on highly localised regions of the sample. In this paper, we assess the sensitivity of four-probe STM for in-line resistivity measurements of the bulk ZnO surface. The measurements allow comparisons to established models that are used to relate light plasma treatments (O and H) of the surfaces to the resistivity measurements. The results are correlated to x-ray photoelectron spectroscopy (XPS) and show that four-probe STM can detect changes in surface and bulk conduction mechanisms that are beyond conventional monochromatic XPS.

  4. Surface sensitivity of four-probe STM resistivity measurements of bulk ZnO correlated to XPS.

    PubMed

    Lord, Alex M; Evans, Jonathan E; Barnett, Chris J; Allen, Martin W; Barron, Andrew R; Wilks, Steve P

    2017-09-27

    Multi-probe instruments based on scanning tunnelling microscopy (STM) are becoming increasingly common for their ability to perform nano- to atomic-scale investigations of nanostructures, surfaces and in situ reactions. A common configuration is the four-probe STM often coupled with in situ scanning electron microscopy (SEM) that allows precise positioning of the probes onto surfaces and nanostructures enabling electrical and scanning experiments to be performed on highly localised regions of the sample. In this paper, we assess the sensitivity of four-probe STM for in-line resistivity measurements of the bulk ZnO surface. The measurements allow comparisons to established models that are used to relate light plasma treatments (O and H) of the surfaces to the resistivity measurements. The results are correlated to x-ray photoelectron spectroscopy (XPS) and show that four-probe STM can detect changes in surface and bulk conduction mechanisms that are beyond conventional monochromatic XPS.

  5. Circularly Symmetric, Distributed-Feedback Structures for Surface-Emitting Semiconductor Lasers.

    NASA Astrophysics Data System (ADS)

    Erdogan, Turan

    1992-01-01

    This thesis explores a fundamentally unique, two -dimensional optical resonator which confines the propagation of circular waves through a distributed-feedback (DFB) process. In addition to an investigation of the unique physics associated with the interaction of circular waves with radially periodic structures, a theoretical and experimental analysis of a novel, circularly symmetric surface-emitting semiconductor laser is carried out. The intent of this analysis is to show that it is possible for a semiconductor laser to directly produce low-divergence, spectrally narrow emission that is circularly symmetric in cross-section. Such emission requires lasing in a single radial and azimuthal mode in a resonator that is much larger than an optical wavelength in two dimensions. A coupled-amplitude theory is developed that describes the coupling of both cylindrical waves and circular guided modes along the radial direction. The angular nature of propagation is characterized by a discrete set of azimuthal modes, which remain uncoupled in a circularly symmetric periodic structure. The theory is applied to the classical problem of a radiating source surrounded by a circular, periodic structure. It is found that a substantial region near the center of the structure can exist, wherein the spontaneous emission from sources at all possible locations is either strongly enhanced or inhibited. Furthermore, the theory is applied to the problem of a circular DFB laser. It is shown that the behavior of the modes associated with the radial direction is very similar to that of a linear DFB laser, while the modes associated with the azimuthal dimension are very densely spaced in frequency and threshold gain. The results suggest that it should be difficult to achieve lasing in a single azimuthal mode. The experimental analysis emphasizes the possibility of exciting circular modes in optically pumped semiconductor lasers. An analysis of the fabrication of concentric-circle gratings by

  6. "Liquid-liquid-solid"-type superoleophobic surfaces to pattern polymeric semiconductors towards high-quality organic field-effect transistors.

    PubMed

    Wu, Yuchen; Su, Bin; Jiang, Lei; Heeger, Alan J

    2013-12-03

    Precisely aligned organic-liquid-soluble semiconductor microwire arrays have been fabricated by "liquid-liquid-solid" type superoleophobic surfaces directed fluid drying. Aligned organic 1D micro-architectures can be built as high-quality organic field-effect transistors with high mobilities of >10 cm(2) ·V(-1) ·s(-1) and current on/off ratio of more than 10(6) . All these studies will boost the development of 1D microstructures of organic semiconductor materials for potential application in organic electronics. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  7. Peptide-functionalized semiconductor surfaces: strong surface electronic effects from minor alterations to backbone composition.

    PubMed

    Matmor, Maayan; Lengyel, George A; Horne, W Seth; Ashkenasy, Nurit

    2017-02-22

    The use of non-canonical amino acids is a powerful way to control protein structure. Here, we show that subtle changes to backbone composition affect the ability of a dipeptide to modify solid surface electronic properties. The extreme sensitivity of the interactions to the peptide structure suggests potential applications in improving the performance of electronic devices.

  8. From bulk to plasmonic nanoparticle surfaces: the behavior of two potent therapeutic peptides, octreotide and pasireotide.

    PubMed

    Hernández, Belén; López-Tobar, Eduardo; Sanchez-Cortes, Santiago; Coïc, Yves-Marie; Baron, Bruno; Chenal, Alexandre; Kruglik, Sergei G; Pflüger, Fernando; Cohen, Régis; Ghomi, Mahmoud

    2016-09-21

    Octreotide and pasireotide are two cyclic somatostatin analogues with an important clinical use in the treatment and diagnosis of neuroendocrine tumors. Herein, by the combined use of several techniques (UV-visible absorption, fluorescence, circular dichroism, ζ-potential, transmission electron microscopy, Raman scattering, surface-enhanced Raman scattering, and quantum mechanical calculations) we have followed the structural dynamics of these analogues in the bulk, as well as their binding sites on plasmonic (gold and silver) colloids. In contrast to the previously derived conclusions, the two peptides seem to possess completely different conformational features. Octreotide, a cyclic octapeptide, is formed by a moderately flexible type-II'β-turn maintained by a deformable disulfide linkage. Pasireotide, in which the cyclic character is made possible by peptide bonds, manifests a rigid backbone formed by two oppositely placed tight turns of different types, i.e.γ-turn and type-I β-turn. Owing to their cationic character, both analogues induce aggregation of negatively charged gold and silver colloids. Nevertheless, despite their notable structural differences, both peptides bind onto gold nanoparticles through their unique d-Trp residue. In contrast, their binding to silver colloids seems to be of electrostatic nature, as formed through monodentate or bidentate ionic pairs.

  9. Bismuth-induced surface structure and morphology in III-V semiconductors

    NASA Astrophysics Data System (ADS)

    Duzik, Adam J.

    2015-04-01

    Bi is the largest group V element and has a number of advantages in III-V semiconductor properties, such as bandgap reduction, spin-orbit coupling, a preserved electron mobility over III-V-N materials, and nearly ideal surfactant properties resulting in a surface smoothing effect on GaAs. However, the mechanism for this behavior is not well understood. Insight on the mechanism is obtained through study of the Bi-terminated GaAs surface morphology and atomic reconstructions produced via molecular beam epitaxy (MBE). Experimental scanning tunneling microscopy (STM) characterization of the Bi/GaAs surface reveal disordered (1x3), (2x3), and (4x3) reconstructions, often sharing the same reflective high-energy electron diffraction (RHEED) patterns. Roughness on the micron length scale decreases as the step widen, attributed to the concurrent increase of opposite direction step edges on the nanometer length scale. Corresponding cluster expansion, density functional theory (DFT), and Monte Carlo simulations all point to the stability of the disordered (4x3) reconstruction at finite temperature as observed in experimental STM. The effects of incorporated Bi are determined through epitaxial GaSbBi growth on GaSb with various Ga:Sb:Bi flux ratios. Biphasic surface droplets are observed with sub-droplets, facets, and substrate etching. Despite the rough growth front, X-ray diffraction (XRD) and Rutherford backscatter (RBS) measurements show significant Bi incorporation of up to 12% into GaSb, along with a concurrent increase of background As concentration. This is attributed to a strain auto-compensation effect. Bi incorporation of up to 10% is observed for the highest Bi fluxes while maintaining low surface droplet density.

  10. Molecular dynamics simulation of self-diffusion processes in titanium in bulk material, on grain junctions and on surface.

    PubMed

    Sushko, Gennady B; Verkhovtsev, Alexey V; Yakubovich, Alexander V; Schramm, Stefan; Solov'yov, Andrey V

    2014-08-21

    The process of self-diffusion of titanium atoms in a bulk material, on grain junctions and on surface is explored numerically in a broad temperature range by means of classical molecular dynamics simulation. The analysis is carried out for a nanoscale cylindrical sample consisting of three adjacent sectors and various junctions between nanocrystals. The calculated diffusion coefficient varies by several orders of magnitude for different regions of the sample. The calculated values of the bulk diffusion coefficient correspond reasonably well to the experimental data obtained for solid and molten states of titanium. Investigation of diffusion in the nanocrystalline titanium is of a significant importance because of its numerous technological applications. This paper aims to reduce the lack of data on diffusion in titanium and describe the processes occurring in bulk, at different interfaces and on surface of the crystalline titanium.

  11. Is the surface oxygen exchange rate linked to bulk ion diffusivity in mixed conducting Ruddlesden–Popper phases?

    SciTech Connect

    Tomkiewicz, Alex C.; Tamimi, Mazin A.; Huq, Ashfia; McIntosh, Steven

    2015-03-02

    There is a possible link between oxygen surface exchange rate and bulk oxygen anion diffusivity in mixed ionic and electronic conducting oxides; it is a topic of great interest and debate. While a large body of experimental evidence and theoretical analyses support a link, observed differences between bulk and surface composition of these materials are hard to reconcile with this observation. This is further compounded by potential problems with simultaneous measurement of both parameters. Here we utilize separate techniques, in situ neutron diffraction and pulsed isotopic surface exchange, to examine bulk ion mobility and surface oxygen exchange rates of three Ruddlesden-Popper phases, general form An-1A2'BnO3n+1, An-1A2'BnX3n+1; LaSrCo0.5Fe0.5O4-δ (n = 1), La0.3Sr2.7CoFeO7-δ (n = 2) and LaSr3Co1.5Fe1.5O10-δ (n = 3). These measurements are complemented by surface composition determination via high sensitivity-low energy ion scattering. We observe a correlation between bulk ion mobility and surface exchange rate between materials. The surface exchange rates vary by more than one order of magnitude with high anion mobility in the bulk of an oxygen vacancy-rich n = 2 Ruddlesden-Popper material correlating with rapid oxygen exchange. Furthermore this is in contrast with the similar surface exchange rates which we may expect due to similar surface compositions across all three samples. This paper conclude that experimental limitations lead to inherent convolution of surface and bulk rates, and that surface exchange steps are not likely to be rate limiting in oxygen incorporation.

  12. adwTools Developed: New Bulk Alloy and Surface Analysis Software for the Alloy Design Workbench

    NASA Technical Reports Server (NTRS)

    Bozzolo, Guillermo; Morse, Jeffrey A.; Noebe, Ronald D.; Abel, Phillip B.

    2004-01-01

    A suite of atomistic modeling software, called the Alloy Design Workbench, has been developed by the Computational Materials Group at the NASA Glenn Research Center and the Ohio Aerospace Institute (OAI). The main goal of this software is to guide and augment experimental materials research and development efforts by creating powerful, yet intuitive, software that combines a graphical user interface with an operating code suitable for real-time atomistic simulations of multicomponent alloy systems. Targeted for experimentalists, the interface is straightforward and requires minimum knowledge of the underlying theory, allowing researchers to focus on the scientific aspects of the work. The centerpiece of the Alloy Design Workbench suite is the adwTools module, which concentrates on the atomistic analysis of surfaces and bulk alloys containing an arbitrary number of elements. An additional module, adwParams, handles ab initio input for the parameterization used in adwTools. Future modules planned for the suite include adwSeg, which will provide numerical predictions for segregation profiles to alloy surfaces and interfaces, and adwReport, which will serve as a window into the database, providing public access to the parameterization data and a repository where users can submit their own findings from the rest of the suite. The entire suite is designed to run on desktop-scale computers. The adwTools module incorporates a custom OAI/Glenn-developed Fortran code based on the BFS (Bozzolo- Ferrante-Smith) method for alloys, ref. 1). The heart of the suite, this code is used to calculate the energetics of different compositions and configurations of atoms.

  13. Semiconductor ohmic contact

    NASA Technical Reports Server (NTRS)

    Hawrylo, F. Z.; Kressel, H.

    1977-01-01

    Contact formed on p-type surface of semiconductor laser has several advantages: highly conductive degenerate region and narrow band gap provides surface for good metal-to-semiconductor contact; lattice parameter of GaAs is 5.6533 A; improved lattice match eases interface strain which reduces interface cracking of semiconductor material.

  14. Effect of the photoreaction solvent on surface and bulk properties of poly(lactic acid) and poly(hydroxyalkanoate) films.

    PubMed

    Rasal, Rahul M; Bohannon, Bradley G; Hirt, Douglas E

    2008-05-01

    The major objective of this research was to modify the surface properties of poly(lactic acid) (PLA) and poly(hydroxyalkanoate) (PHA) films by using a sequential two-step photografting approach. In step 1, benzophenone was photografted on the film surface and in step 2, hydrophilic monomers acrylamide and acrylic acid were photopolymerized from the film surfaces. Another objective was to study the effect of the reaction solvent in step 2 on surface and bulk properties of these films. ATR-FTIR spectroscopy and water contact angle goniometry were used to characterize the resultant film surfaces. When ethanol was used as the solvent in step 2, there was significant penetration of the monomers into the films, and bulk properties such as toughness and percent elongation at break were drastically affected. The penetration of these monomers into the bulk was characterized using transmission FTIR microspectroscopy. Microtomed sections of the surface-modified films were placed in a diamond compression cell to perform the FTIR microspectroscopic analyses. The observed monomer penetration into the films was successfully reduced by using water instead of ethanol in step 2, and resultant films showed higher toughness and percent elongation at break than films surface-modified using ethanol as a solvent in step 2.

  15. Unveiling and controlling the electronic structure of oxidized semiconductor surfaces: Crystalline oxidized InSb(100)(1 Ã-- 2)-O

    NASA Astrophysics Data System (ADS)

    Lâng, J. J. K.; Punkkinen, M. P. J.; Tuominen, M.; Hedman, H.-P.; Vähä-Heikkilä, M.; Polojärvi, V.; Salmi, J.; Korpijärvi, V.-M.; Schulte, K.; Kuzmin, M.; Punkkinen, R.; Laukkanen, P.; Guina, M.; Kokko, K.

    2014-07-01

    The exothermic nature of oxidation causes nearly all semiconductor applications in various fields like electronics, medicine, photonics, and sensor technology to acquire an oxidized semiconductor surface part during the application manufacturing. The significance of understanding and controlling the atomic scale properties of oxidized semiconductor surfaces is expected to increase even further with the development of nanoscale semiconductor crystals. The nature of oxidized semiconductor layers is, however, hard to predict and characterize as they are usually buried and amorphous. To shed light on these issues, we pursue a different approach based on oxidized III-V semiconductor layers that are crystalline. We present a comprehensive characterization of oxidized crystalline InSb(100)(1×2)-O layers by ab initio calculations, photoelectron spectroscopy, scanning tunneling microscopy, and spectroscopy, and demonstrate the electronic band structures of different oxidized phases of the semiconductor, which elucidate the previous contradictory semiconductor-oxidation effects. At 0.5 monolayer (ML) oxidation, oxygen atoms tend to occupy subsurface Sb sites, leading to metallic states in the semiconductor band gap, which arise from top dimers. When the oxidation is increased to the 1.0-2.0 ML concentration, oxygen occupies also interstitial sites, and the insulating band structure without gap states is stabilized with unusual occupied In dangling bonds. In contrast, the 2.5-3.0 ML oxide phases undergo significant changes toward a less ordered structure. The findings suggest a methodology for manipulating the electronic structure of oxidized semiconductor layers.

  16. Investigation of preparation methods on surface/bulk structural relaxation and glass fragility of amorphous solid dispersions.

    PubMed

    Ke, Peng; Hasegawa, Susumu; Al-Obaidi, Hisham; Buckton, Graham

    2012-01-17

    The objective of this study was to investigate the effect of preparation methods on the surface/bulk molecular mobility and glass fragility of solid dispersions. Solid dispersions containing indomethacin and PVP K30 were chosen as the model system. An inverse gas chromatography method was used to determine the surface structural relaxation of the solid dispersions and these data were compared to those for bulk relaxation obtained by DSC. The values of τ(β) for the surface relaxation were 4.6, 7.1 and 1.8h for melt quenched, ball milled and spray dried solid dispersions respectively, compared to 15.6, 7.9 and 9.8h of the bulk. In all systems, the surface had higher molecular mobility than the bulk. The glass fragility of the solid dispersions was also influenced by the preparation methods with the most fragile system showing the best stability. The zero mobility temperature (T(0)) was used to correlate with the physical stability of the solid dispersions. Despite having similar T(g) (65°C), the T(0) of the melt quenched, ball milled and spray dried samples were 21.6, -4.2 and 16.7°C respectively which correlated well with their physical stability results. Therefore, T(0) appears to be a better indicator than T(g) for predicting stability of amorphous materials.

  17. Factors influencing soil-surface bulk density on oak savanna rangeland in the southern Sierra Nevada foothills

    Treesearch

    Dennis M. Dudley; Kenneth W. Tate; Neil K. McDougald; Melvin R. George

    2002-01-01

    The objectives of this study were to compare soil-surface bulk density between rangeland pastures not grazed since 1935, 1975, and 1995 to grazed areas with a 15-year record of light (>1,000 lbs ac-1 RDM), moderate (600-800 lbs ac-1 RDM), and heavy (-1 RDM) grazing by beef cattle; and...

  18. PRECISION CLEANING OF SEMICONDUCTOR SURFACES USING CARBON DIOXIDE-BASED FLUIDS

    SciTech Connect

    J. RUBIN; L. SIVILS; A. BUSNAINA

    1999-07-01

    The Los Alamos National Laboratory, on behalf of the Hewlett-Packard Company, is conducting tests of a closed-loop CO{sub 2}-based supercritical fluid process, known as Supercritical CO{sub 2} Resist Remover (SCORR). We have shown that this treatment process is effective in removing hard-baked, ion-implanted photoresists, and appears to be fully compatible with metallization systems. We are now performing experiments on production wafers to assess not only photoresist removal, but also residual surface contamination due to particulate and trace metals. Dense-phase (liquid or supercritical) CO{sub 2}, since it is non-polar, acts like an organic solvent and therefore has an inherently high volubility for organic compounds such as oils and greases. Also, dense CO{sub 2} has a low-viscosity and a low dielectric constant. Finally, CO{sub 2} in the liquid and supercritical fluid states can solubilize metal completing agents and surfactants. This combination of properties has interesting implications for the removal not only of organic films, but also trace metals and inorganic particulate. In this paper we discuss the possibility of using CO{sub 2} as a precision-cleaning solvent, with particular emphasis on semiconductor surfaces.

  19. A scale-bridging modeling approach for anisotropic organic molecules at patterned semiconductor surfaces

    NASA Astrophysics Data System (ADS)

    Kleppmann, Nicola; Klapp, Sabine H. L.

    2015-02-01

    Hybrid systems consisting of organic molecules at inorganic semiconductor surfaces are gaining increasing importance as thin film devices for optoelectronics. The efficiency of such devices strongly depends on the collective behavior of the adsorbed molecules. In the present paper, we propose a novel, coarse-grained model addressing the condensed phases of a representative hybrid system, that is, para-sexiphenyl (6P) at zinc-oxide (ZnO). Within our model, intermolecular interactions are represented via a Gay-Berne potential (describing steric and van-der-Waals interactions) combined with the electrostatic potential between two linear quadrupoles. Similarly, the molecule-substrate interactions include a coupling between a linear molecular quadrupole to the electric field generated by the line charges characterizing ZnO(10-10). To validate our approach, we perform equilibrium Monte Carlo simulations, where the lateral positions are fixed to a 2D lattice, while the rotational degrees of freedom are continuous. We use these simulations to investigate orientational ordering in the condensed state. We reproduce various experimentally observed features such as the alignment of individual molecules with the line charges on the surface, the formation of a standing uniaxial phase with a herringbone structure, as well as the formation of a lying nematic phase.

  20. Efficient power extraction in surface-emitting semiconductor lasers using graded photonic heterostructures.

    PubMed

    Xu, Gangyi; Colombelli, Raffaele; Khanna, Suraj P; Belarouci, Ali; Letartre, Xavier; Li, Lianhe; Linfield, Edmund H; Davies, A Giles; Beere, Harvey E; Ritchie, David A

    2012-07-17

    Symmetric and antisymmetric band-edge modes exist in distributed feedback surface-emitting semiconductor lasers, with the dominant difference being the radiation loss. Devices generally operate on the low-loss antisymmetric modes, although the power extraction efficiency is low. Here we develop graded photonic heterostructures, which localize the symmetric mode in the device centre and confine the antisymmetric modes close to the laser facet. This modal spatial separation is combined with absorbing boundaries to increase the antisymmetric mode loss, and force device operation on the symmetric mode, with elevated radiation efficiency. Application of this concept to terahertz quantum cascade lasers leads to record-high peak-power surface emission (>100 mW) and differential efficiencies (230 mW A(-1)), together with low-divergence, single-lobed emission patterns, and is also applicable to continuous-wave operation. Such flexible tuning of the radiation loss using graded photonic heterostructures, with only a minimal influence on threshold current, is highly desirable for optimizing second-order distributed feedback lasers.