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Sample records for modulation doped heterostructures

  1. Electron mobility in modulation-doped heterostructures

    NASA Technical Reports Server (NTRS)

    Walukiewicz, W.; Ruda, H. E.; Lagowski, J.; Gatos, H. C.

    1984-01-01

    A model for electron mobility in a two-dimensional electron gas confined in a triangular well was developed. All major scattering processes (deformation potential and piezoelectric acoustic, polar optical, ionized impurity, and alloy disorder) were included, as well as intrasubband and intersubband scattering. The model is applied to two types of modulation-doped heterostructures, namely GaAs-GaAlAs and In(0.53)Ga(0.47)As-Al(0.52)In(0.48)As. In the former case, phonons and remote ionized impurities ultimately limit the mobility, whereas in the latter, alloy disorder is a predominant scattering process at low temperatures. The calculated mobilities are in very good agreement with recently reported experimental characteristics for both GaAs-Ga(1-x)Al(x)As and In(0.53)Ga(0.47)As-Al(0.52)In(0.48)As modulation-doped heterostructures.

  2. Analysis of energy states in modulation doped multiquantum well heterostructures

    NASA Technical Reports Server (NTRS)

    Ji, G.; Henderson, T.; Peng, C. K.; Huang, D.; Morkoc, H.

    1990-01-01

    A precise and effective numerical procedure to model the band diagram of modulation doped multiquantum well heterostructures is presented. This method is based on a self-consistent iterative solution of the Schroedinger equation and the Poisson equation. It can be used rather easily in any arbitrary modulation-doped structure. In addition to confined energy subbands, the unconfined states can be calculated as well. Examples on realistic device structures are given to demonstrate capabilities of this procedure. The numerical results are in good agreement with experiments. With the aid of this method the transitions involving both the confined and unconfined conduction subbands in a modulation doped AlGaAs/GaAs superlattice, and in a strained layer InGaAs/GaAs superlattice are identified. These results represent the first observation of unconfined transitions in modulation doped multiquantum well structures.

  3. Photo-induced Modulation Doping in Graphene/Boron nitride Heterostructures

    NASA Astrophysics Data System (ADS)

    Velasco, Jairo, Jr.; Ju, Long; Hwang, Edwin; Kahn, Salman; Nosiglia, Casey; Tsai, Hsin-Zon; Yang, Wei; Zhang, Guangyu; Taniguchi, Takashi; Watanabe, Kenji; Zhang, Yuanbo; Crommie, Michael; Zettl, Alex; Wang, Feng

    2014-03-01

    Van der Waals heterostructures (VDH) provide an exciting new platform for materials engineering, where a variety of layered materials with different electrical, optical and mechanical responses can be stacked together to enable new physics and novel functionalities. We report an emerging optoelectronic phenomenon (i.e. photo-induced modulation doping) in the graphene-boron nitride VDH (G/BN heterostructure). We find it enables flexible and repeatable writing and erasing of charge doping in graphene with optical light. We show that the photo-induced modulation doping maintains the remarkable carrier mobility of the G/BN heterostructure, and it can be used to generate spatially varying doping profiles like pn junctions. Our work contributes towards understanding light matter interactions in VDHs, and introduces a simple technique for creating inhomogeneous doping in high mobility graphene devices. J. Velasco Jr. acknowledges support from UC President's Postdoctoral Fellowship.

  4. Controllable GMR device in a δ-doped, magnetically and electrically modulated, GaAs /Alx Ga1-x As heterostructure

    NASA Astrophysics Data System (ADS)

    Shen, Li-Hua; Zhang, Gui-Lian; Yang, Duan-Chui

    2016-09-01

    We report on a theoretical study of giant magnetoresistance (GMR) effect in a δ-doped GaAs /Alx Ga1-x As heterostructure modulated by two stripes of ferromagnetic metal and a stripe of Schottky metal in parallel configuration. The δ-doping dependent transmission and conductance of the device are calculated. It is shown that a considerable giant magnetoresistance (GMR) effect exists in this structure. It is also shown that the magnetoresistance ratio (MR) can be switched by the δ-doping. The underlying physical mechanism of the results is analysed in light of δ-doping-dependent tunneling process in the device.

  5. Modulation-Doped SrTiO3/SrTi1-xZrxO3 Heterostructures

    NASA Astrophysics Data System (ADS)

    Kajdos, Adam Paul

    surface reconstruction from (1x1) to (2x1) to c(4x4) is correlated with a change from mixed SrO/TiO2 to pure TiO2 surface termination. It is argued that optimal cation stoichiometry is achieved for growth conditions within the XRD-defined growth window that result in a c(4x4) surface lattice. The development of a doped perovskite oxide semiconductor with a suitable conduction band offset is then discussed as the next necessary step towards realizing modulation-doped heterostructures. The SrTixZr1-x O3 solid solution is investigated for this purpose, with a focus on optimizing cation stoichiometry to allow for controlled doping. In particular, the hybrid MBE growth of SrTixZr1-xO3 thin films is explored using a metal-organic precursor for Zr, zirconium tert-butoxide (ZTB). The successful generation of 2DEGs by modulation doping of SrTiO3 is then demonstrated in SrTiO3/La:SrTi0.95Zr0.05O 3 heterostructures, and the electronic structure is studied by Shubnikov-de Haas analysis using multiple-subband models.

  6. Complex quantum transport in a modulation doped strained Ge quantum well heterostructure with a high mobility 2D hole gas

    NASA Astrophysics Data System (ADS)

    Morrison, C.; Casteleiro, C.; Leadley, D. R.; Myronov, M.

    2016-09-01

    The complex quantum transport of a strained Ge quantum well (QW) modulation doped heterostructure with two types of mobile carriers has been observed. The two dimensional hole gas (2DHG) in the Ge QW exhibits an exceptionally high mobility of 780 000 cm2/Vs at temperatures below 10 K. Through analysis of Shubnikov de-Haas oscillations in the magnetoresistance of this 2DHG below 2 K, the hole effective mass is found to be 0.065 m0. Anomalous conductance peaks are observed at higher fields which deviate from standard Shubnikov de-Haas and quantum Hall effect behaviour due to conduction via multiple carrier types. Despite this complex behaviour, analysis using a transport model with two conductive channels explains this behaviour and allows key physical parameters such as the carrier effective mass, transport, and quantum lifetimes and conductivity of the electrically active layers to be extracted. This finding is important for electronic device applications, since inclusion of highly doped interlayers which are electrically active, for enhancement of, for example, room temperature carrier mobility, does not prevent analysis of quantum transport in a QW.

  7. Study of photoconductivity in AlxGa1 - xAs/GaAs modulation-doped heterostructures

    NASA Astrophysics Data System (ADS)

    Peng, Zhongling; Saku, Tadashi; Horikoshi, Yoshiji

    1996-04-01

    Temperature-dependent Hall effect measurements under different illumination conditions are performed on AlxGa1-xAs/GaAs heterostructures doped with both Si and Sn. For a Sn-doped AlxGa1-xAs/GaAs heterostructure with x=0.35, two distinct DX center levels are observed directly without exposure to light. The two-dimensional electron gas concentration measured under illumination decreases from the value due to persistent photoconductivity to a value less than that measured in the dark when the excitation photon energy is larger than the band gap of the AlGaAs barrier. This decrease occurs only at temperatures below 90 K. This negative photoconductivity is explained by taking into account the partial freeze-out of electrons into the shallow DX centers and the transfer of holes photogenerated in the barrier into the channel region. No such phenomena are observed in Sn-doped heterostructures with x <0.35 or in the Si-doped heterostructures. The reasons for this are also investigated.

  8. Optical investigation of InAs quantum dots inserted in AlGaAs/GaAs modulation doped heterostructure

    SciTech Connect

    Khmissi, H.; Baira, M.; Bouzaieene, L.; Saidi, F.; Maaref, H.; Sfaxi, L.; Bru-Chevallier, C.

    2011-03-01

    Optical properties of InAs quantum dots (QDs) inserted in AlGaAs/GaAs modulation doped heterostructure are investigated. To study the effect of carrier transfer behavior on the luminescence of self-assembled quantum dots, a series of sample has been prepared using molecular beam epitaxy (Riber 32 system) in which we have varied the thickness separating the delta dopage and the InAs quantum dots layer. Photoluminescence spectra show the existence of two peaks that can be attributed to transition energies from the ground state (E{sub 1}-HH{sub 1}) and the first excited state (E{sub 2}-HH{sub 2}). Two antagonist effects have been observed, a blue shift of the emission energies result from electron transferred from the AlGaAs/GaAs heterojunction to the InAs quantum dots and a red shift caused by the quantum confined Stark effect due to the internal electric field existing In the AlGaAs/GaAs heterojunction.

  9. Electrostatic doping in oxide heterostructures.

    NASA Astrophysics Data System (ADS)

    Demkov, Alexander A.; Lee, Jaekwang; Sai, Na

    2009-03-01

    Recent experiments on perovskite heterostructures grown by methods ranging from molecular beam epitaxy to pulsed laser deposition suggest the existence of two dimensional electron gas of high mobility at the oxide/oxide interface, and even a possibility of a superconducting state. Both p-type and n-type interfaces have been reported. However, the origin of charge in these insulating materials is still under debate. We report a first-principles study of several heterostructures where we employ the internal filed in a polar oxide LaAlO3 to demonstrate the possibility of the electrostatic doping, an effect similar to a well known polar catastrophe in e.g., III-V semiconductors. We use density functional theory at the LDA+U level. We mainly focus on the electronic structure of the oxide/oxide junctions. The results of our calculations suggest that once the critical thickness of the aluminate layer is reached the internal electric field is sufficient to produce the electrostatic doping. We will discuss simple estimates for the temperature of the superconducting transition and the role of oxygen-related defects such as vacancies in the electronic structure and thermodynamic stability of these fascinating oxide structures.

  10. Molecular-beam epitaxial growth and characterization of modulation-doped field-effect transistor heterostructures using InAs/GaAs superlattice channels

    SciTech Connect

    Baeta Moreira, M.V.; Py, M.A.; Ilegems, M.

    1993-05-01

    The molecular-beam epitaxial growth conditions of (N + 1)(InAs){sub m}/N(GaAsw){sub n} short period superlattices (SPSs) on GaAs substrates have been optimized. Hall electrical properties measured by the van der Pauw method were compared to low-temperature photoluminescence (77 K PL) spectra of GaAs/SPS/AlGaAs modulation-doped field-effect transistor-type heterostructures. By using these two characterization methods, the influences of the growth temperature T{sub s}, of the SPS channel thickness d{sub ch} and of its average indium composition y{sub m} were studied. Interesting correlations were established between their optical and their transport properties measured at 77 K either in the dark or under white-light illumination. The thickness m of the InAs layers was varied in the range 0.57 to 1.7 and sharp optimum properties were obtained slightly above m=1 monolayer.

  11. Negative terahertz conductivity in remotely doped graphene bilayer heterostructures

    SciTech Connect

    Ryzhii, V.; Ryzhii, M.; Mitin, V.; Shur, M. S.; Otsuji, T.

    2015-11-14

    Injection or optical generation of electrons and holes in graphene bilayers (GBLs) can result in the interband population inversion enabling the terahertz (THz) radiation lasing. The intraband radiative processes compete with the interband transitions. We demonstrate that remote doping enhances the indirect interband generation of photons in the proposed GBL heterostructures. Therefore, such remote doping helps to surpass the intraband (Drude) absorption, and results in large absolute values of the negative dynamic THz conductivity in a wide range of frequencies at elevated (including room) temperatures. The remotely doped GBL heterostructure THz lasers are expected to achieve higher THz gain compared with previously proposed GBL-based THz lasers.

  12. Effect of Split Gate Size on the Electrostatic Potential and 0.7 Anomaly within Quantum Wires on a Modulation-Doped GaAs /AlGaAs Heterostructure

    NASA Astrophysics Data System (ADS)

    Smith, L. W.; Al-Taie, H.; Lesage, A. A. J.; Thomas, K. J.; Sfigakis, F.; See, P.; Griffiths, J. P.; Farrer, I.; Jones, G. A. C.; Ritchie, D. A.; Kelly, M. J.; Smith, C. G.

    2016-04-01

    We study 95 split gates of different size on a single chip using a multiplexing technique. Each split gate defines a one-dimensional channel on a modulation-doped GaAs /AlGaAs heterostructure, through which the conductance is quantized. The yield of devices showing good quantization decreases rapidly as the length of the split gates increases. However, for the subset of devices showing good quantization, there is no correlation between the electrostatic length of the one-dimensional channel (estimated using a saddle-point model) and the gate length. The variation in electrostatic length and the one-dimensional subband spacing for devices of the same gate length exceeds the variation in the average values between devices of different lengths. There is a clear correlation between the curvature of the potential barrier in the transport direction and the strength of the "0.7 anomaly": the conductance value of the 0.7 anomaly reduces as the barrier curvature becomes shallower. These results highlight the key role of the electrostatic environment in one-dimensional systems. Even in devices with clean conductance plateaus, random fluctuations in the background potential are crucial in determining the potential landscape in the active device area such that nominally identical gate structures have different characteristics.

  13. High-Current-Density Vertical-Tunneling Transistors from Graphene/Highly Doped Silicon Heterostructures.

    PubMed

    Liu, Yuan; Sheng, Jiming; Wu, Hao; He, Qiyuan; Cheng, Hung-Chieh; Shakir, Muhammad Imran; Huang, Yu; Duan, Xiangfeng

    2016-06-01

    Scalable fabrication of vertical-tunneling transistors is presented based on heterostructures formed between graphene, highly doped silicon, and its native oxide. Benefiting from the large density of states of highly doped silicon, the tunneling transistors can deliver a current density over 20 A cm(-2) . This study demonstrates that the interfacial native oxide plays a crucial role in governing the carrier transport in graphene-silicon heterostructures.

  14. Ferroelectric memristor based on Pt/BiFeO3/Nb-doped SrTiO3 heterostructure

    NASA Astrophysics Data System (ADS)

    Hu, Zhongqiang; Li, Qian; Li, Meiya; Wang, Qiangwen; Zhu, Yongdan; Liu, Xiaolian; Zhao, Xingzhong; Liu, Yun; Dong, Shuxiang

    2013-03-01

    We report a continuously tunable resistive switching behavior in Pt/BiFeO3/Nb-doped SrTiO3 heterostructure for ferroelectric memristor application. The resistance of this memristor can be tuned up to 5 × 105% by applying voltage pulses at room temperature, which exhibits excellent retention and anti-fatigue characteristics. The observed memristive behavior is attributed to the modulation effect of the ferroelectric polarization reversal on the width of depletion region and the height of potential barrier of the p-n junction formed at the BiFeO3/Nb-doped SrTiO3 interface.

  15. Electronic properties of embedded graphene: doped amorphous silicon/CVD graphene heterostructures

    NASA Astrophysics Data System (ADS)

    Arezki, Hakim; Boutchich, Mohamed; Alamarguy, David; Madouri, Ali; Alvarez, José; Cabarrocas, Pere Roca i.; Kleider, Jean-Paul; Yao, Fei; Lee, Young Hee

    2016-10-01

    Large-area graphene film is of great interest for a wide spectrum of electronic applications, such as field effect devices, displays, and solar cells, among many others. Here, we fabricated heterostructures composed of graphene (Gr) grown by chemical vapor deposition (CVD) on copper substrate and transferred to SiO2/Si substrates, capped by n- or p-type doped amorphous silicon (a-Si:H) deposited by plasma-enhanced chemical vapor deposition. Using Raman scattering we show that despite the mechanical strain induced by the a-Si:H deposition, the structural integrity of the graphene is preserved. Moreover, Hall effect measurements directly on the embedded graphene show that the electronic properties of CVD graphene can be modulated according to the doping type of the a-Si:H as well as its phase i.e. amorphous or nanocrystalline. The sheet resistance varies from 360 Ω sq-1 to 1260 Ω sq-1 for the (p)-a-Si:H/Gr (n)-a-Si:H/Gr, respectively. We observed a temperature independent hole mobility of up to 1400 cm2 V-1 s-1 indicating that charge impurity is the principal mechanism limiting the transport in this heterostructure. We have demonstrated that embedding CVD graphene under a-Si:H is a viable route for large scale graphene based solar cells or display applications.

  16. Electronic properties of embedded graphene: doped amorphous silicon/CVD graphene heterostructures.

    PubMed

    Arezki, Hakim; Boutchich, Mohamed; Alamarguy, David; Madouri, Ali; Alvarez, José; Cabarrocas, Pere Roca I; Kleider, Jean-Paul; Yao, Fei; Hee Lee, Young

    2016-10-12

    Large-area graphene film is of great interest for a wide spectrum of electronic applications, such as field effect devices, displays, and solar cells, among many others. Here, we fabricated heterostructures composed of graphene (Gr) grown by chemical vapor deposition (CVD) on copper substrate and transferred to SiO2/Si substrates, capped by n‑ or p-type doped amorphous silicon (a-Si:H) deposited by plasma-enhanced chemical vapor deposition. Using Raman scattering we show that despite the mechanical strain induced by the a-Si:H deposition, the structural integrity of the graphene is preserved. Moreover, Hall effect measurements directly on the embedded graphene show that the electronic properties of CVD graphene can be modulated according to the doping type of the a-Si:H as well as its phase i.e. amorphous or nanocrystalline. The sheet resistance varies from 360 Ω sq(-1) to 1260 Ω sq(-1) for the (p)-a-Si:H/Gr (n)-a-Si:H/Gr, respectively. We observed a temperature independent hole mobility of up to 1400 cm(2) V(-1) s(-1) indicating that charge impurity is the principal mechanism limiting the transport in this heterostructure. We have demonstrated that embedding CVD graphene under a-Si:H is a viable route for large scale graphene based solar cells or display applications. PMID:27506254

  17. Pressure-Induced Charge Transfer Doping of Monolayer Graphene/MoS2 Heterostructure.

    PubMed

    Pandey, Tribhuwan; Nayak, Avinash P; Liu, Jin; Moran, Samuel T; Kim, Joon-Seok; Li, Lain-Jong; Lin, Jung-Fu; Akinwande, Deji; Singh, Abhishek K

    2016-08-01

    A unique way of achieving controllable, pressure-induced charge transfer doping in the graphene/MoS2 heterostructure is proposed. The charge transfer causes an upward shift in the Dirac point with respect to Fermi level at a rate of 15.7 meV GPa(-1) as a function of applied hydrostatic pressure, leading to heavy p-type doping in graphene. The doping was confirmed by I2D /IG measurements. PMID:27323330

  18. Graphene/CdTe heterostructure solar cell and its enhancement with photo-induced doping

    SciTech Connect

    Lin, Shisheng Chen, Hongsheng; Li, Xiaoqiang; Zhang, Shengjiao; Wang, Peng; Xu, Zhijuan; Zhong, Huikai; Wu, Zhiqian

    2015-11-09

    We report a type of solar cell based on graphene/CdTe Schottky heterostructure, which can be improved by surface engineering as graphene is atomic thin. By coating a layer of ultrathin CdSe quantum dots onto graphene/CdTe heterostructure, the power conversion efficiency is increased from 2.08% to 3.10%. Photo-induced doping is mainly accounted for this enhancement, as evidenced by field effect transport, Raman, photoluminescence, and quantum efficiency measurements. This work demonstrates a feasible way of improving the performance of graphene/semiconductor heterostructure solar cells by combining one dimensional with two dimensional materials.

  19. Electrical property studies on chemically processed polypyrolle/aluminum doped ZnO based hybrid heterostructures

    NASA Astrophysics Data System (ADS)

    Mohan Kumar, G.; Ilanchezhiyan, P.; Madhan Kumar, A.; Yuldashev, Sh. U.; Kang, T. W.

    2016-04-01

    A hybrid structure based on p-type polypyrolle (PPy) and n-type aluminum (Al) doped ZnO nanorods was successfully constructed. The effect of Al doping on material properties of wurtzite structured ZnO were studied using several analytical techniques. To establish the desired hybrid structure, pyrrole monomers were polymerized on hydrothermally grown Al doped ZnO nanorods by chemical polymerization. The current-voltage characteristics on the fabricated PPy/Al doped ZnO heterostructures were found to exhibit excellent rectifying characteristics under dark and illumination conditions. The obtained results augment the prescribed architecture to be highly suitable for high-sensitivity optoelectronic applications.

  20. Indirect doping effects from impurities in MoS2/h-BN heterostructures

    NASA Astrophysics Data System (ADS)

    Gillen, Roland; Robertson, John; Maultzsch, Janina

    2014-08-01

    We performed density functional theory calculations on heterostructures of single layers of hexagonal BN and MoS2 to assess the effect of doping in the BN sheet and of interstitial Na atoms on the electronic properties of the adjacent MoS2 layer. Our calculations predict that n doping of the boron nitride subsystem by oxygen, carbon, and sulfur impurities causes noticeable charge transfer into the conduction band of the MoS2 sheet, while p doping by beryllium and carbon leaves the molybdenum disulphide layer largely unaffected. Intercalated sodium atoms lead to a significant increase of the interlayer distance in the heterostructure and to a metallic ground state of the MoS2 subsystem. The presence of such n dopants leads to a distinct change of valence-band and conduction-band offsets, suggesting that doped h-BN remains a suitable substrate and gate material for applications of n-type MoS2.

  1. Tc Enhancement in Electron-Doped Cuprate Heterostructures

    NASA Astrophysics Data System (ADS)

    Bach, P. L.; Jin, K.; Zhang, X. H.; Greene, R. L.; Grupel, U.; Zohar, E.; Diamant, E.; Dagan, Y.; Smadici, S.; Abbamonte, P.

    2011-03-01

    Multilayer thin films of La 2-x Ce x Cu O4 (LCCO) and Pr 2-x Ce x Cu O4 (PCCO) were fabricated as superlattices of different dopings. Pairing over-doped and under-doped (or un-doped) layers is found to increase Tc significantly above that of the single-phase films corresponding to the under- or over-doped layers. We report transport measurements on these mulitlayer films and discuss possible mechanisms for the Tc enhancement. This work was supported by the US-Israel Binational Science Foundation Grant #2006385 and the Center for Nanophysics and Advanced Materials (CNAM).

  2. Conduction electrons in acceptor-doped GaAs/GaAlAs heterostructures: a review

    NASA Astrophysics Data System (ADS)

    Zawadzki, Wlodek; Raymond, Andre; Kubisa, Maciej

    2016-05-01

    We review magneto-optical and magneto-transport effects in GaAs/GaAlAs heterostructures doped in GaAlAs barriers with donors, providing two-dimensional (2D) electron gas (2DEG) in GaAs quantum wells (QWS), and additionally doped with smaller amounts of acceptors (mostly Be atoms) in the vicinity of 2DEG. One may also deal with residual acceptors (mostly C atoms). The behavior of such systems in the presence of a magnetic field differs appreciably from those doped in the vicinity of 2DEG with donors. Three subjects related to the acceptor-doped heterostructures are considered. First is the problem of bound states of conduction electrons confined to the vicinity of negatively charged acceptors by the joint effect of a QW and an external magnetic field parallel to the growth direction. A variational theory of such states is presented, demonstrating that an electron turning around a repulsive center has discrete energies above the corresponding Landau levels. Experimental evidence for the discrete electron energies comes from the work on interband photo-magneto-luminescence, intraband cyclotron resonance and quantum magneto-transport (the Quantum Hall and Shubnikov-de Haas effects). An electron rain-down effect at weak electric fields and a boil-off effect at strong electric fields are introduced. It is demonstrated, both theoretically and experimentally, that a negatively charged acceptor can localize more than one electron. The second subject describes experiment and theory of asymmetric quantized Hall and Shubnikov-de Haas plateaus in acceptor-doped GaAs/GaAlAs heterostructures. It is shown that the main features of the plateau asymmetry can be attributed to asymmetric density of Landau states in the presence of acceptors. However, at high magnetic fields, the rain-down effect is also at work. The third subject deals with the so-called disorder modes (DMs) in the cyclotron resonance of conduction electrons. The DMs originate from random distributions of negatively

  3. Interfacial control of oxygen vacancy doping and electrical conduction in thin film oxide heterostructures

    PubMed Central

    Veal, Boyd W.; Kim, Seong Keun; Zapol, Peter; Iddir, Hakim; Baldo, Peter M.; Eastman, Jeffrey A.

    2016-01-01

    Oxygen vacancies in proximity to surfaces and heterointerfaces in oxide thin film heterostructures have major effects on properties, resulting, for example, in emergent conduction behaviour, large changes in metal-insulator transition temperatures or enhanced catalytic activity. Here we report the discovery of a means of reversibly controlling the oxygen vacancy concentration and distribution in oxide heterostructures consisting of electronically conducting In2O3 films grown on ionically conducting Y2O3-stabilized ZrO2 substrates. Oxygen ion redistribution across the heterointerface is induced using an applied electric field oriented in the plane of the interface, resulting in controlled oxygen vacancy (and hence electron) doping of the film and possible orders-of-magnitude enhancement of the film's electrical conduction. The reversible modified behaviour is dependent on interface properties and is attained without cation doping or changes in the gas environment. PMID:27283250

  4. Half Metallic Digital Ferromagnetic Heterostructure Composed of a (Delta)-doped Layer of Mn in Si

    SciTech Connect

    Qian, M C; Fong, C Y; Liu, K; Pickett, W E; Pask, J E; Yang, L H

    2006-05-30

    The authors propose and investigate the properties of a digital ferromagnetic heterostructure (DFH) consisting of a {delta}-doped layer of Mn in Si, using ab initio electronic-structure methods. They find that (1) ferromagnetic order of the Mn layer is energetically favorable relative to antiferromagnetic, and (2) the heterostructure is a two-dimensional half metallic system. The metallic behavior is contributed by three majority-spin bands originating from hybridized Mn-d and nearest-neighbor Si-p states, and the corresponding carriers are responsible for the ferromagnetic order in the Mn layer. The minority-spin channel has a calculated semiconducting gap of 0.25 eV. Analysis of the total and partial densities of states, band structure, Fermi surfaces and associated charge density reveals the marked two-dimensional nature of the half metallicity. The band lineup is found to be favorable for retaining the half metal character to near the Curie temperature (T{sub C}). Being Si based and possibly having a high T{sub C} as suggested by an experiment on dilutely doped Mn in Si, the heterostructure may be of special interest for integration into mature Si technologies for spintronic applications.

  5. Modulation of over 10{sup 14} cm{sup −2} electrons in SrTiO{sub 3}/GdTiO{sub 3} heterostructures

    SciTech Connect

    Boucherit, M.; Shoron, O.; Polchinski, C.; Jackson, C. A.; Cain, T. A.; Buffon, M. L. C.; Stemmer, S.; Rajan, S.

    2014-05-05

    We demonstrate charge modulation of over 10{sup 14} cm{sup −2} electrons in a two-dimensional electron gas formed in SrTiO{sub 3}/GdTiO{sub 3} inverted heterostructure field-effect transistors. Increased charge modulation was achieved by reducing the effect of interfacial region capacitances through thick SrTiO{sub 3} cap layers. Transport and device characteristics of the heterostructure field-effect transistors were found to match a long channel field effect transistor model. SrTiO{sub 3} impurity doped metal–semiconductor field effect transistors were also demonstrated with excellent pinch-off and current density exceeding prior reports. The work reported here provides a path towards oxide-based electronics with extreme charge modulation exceeding 10{sup 14} cm{sup −2}.

  6. Intrinsic Spin-Orbit Coupling in Superconducting Delta-Doped SrTiO3 Heterostructures

    SciTech Connect

    Bell, Christopher

    2011-08-19

    We report the violation of the Pauli limit due to intrinsic spin-orbit coupling in SrTiO{sub 3} heterostructures. Via selective doping down to a few nanometers, a two-dimensional superconductor is formed, geometrically suppressing orbital pair-breaking. The spin-orbit scattering is exposed by the robust in-plane superconducting upper critical field, exceeding the Pauli limit by a factor of 4. Transport scattering times several orders of magnitude higher than for conventional thin film superconductors enables a new regime to be entered, where spin-orbit coupling effects arise non-perturbatively.

  7. Magnetization switching through giant spin-orbit torque in a magnetically doped topological insulator heterostructure

    NASA Astrophysics Data System (ADS)

    Fan, Yabin; Upadhyaya, Pramey; Kou, Xufeng; Lang, Murong; Takei, So; Wang, Zhenxing; Tang, Jianshi; He, Liang; Chang, Li-Te; Montazeri, Mohammad; Yu, Guoqiang; Jiang, Wanjun; Nie, Tianxiao; Schwartz, Robert N.; Tserkovnyak, Yaroslav; Wang, Kang L.

    2014-07-01

    Recent demonstrations of magnetization switching induced by in-plane current in heavy metal/ferromagnetic heterostructures (HMFHs) have drawn great attention to spin torques arising from large spin-orbit coupling (SOC). Given the intrinsic strong SOC, topological insulators (TIs) are expected to be promising candidates for exploring spin-orbit torque (SOT)-related physics. Here we demonstrate experimentally the magnetization switching through giant SOT induced by an in-plane current in a chromium-doped TI bilayer heterostructure. The critical current density required for switching is below 8.9 × 104 A cm-2 at 1.9 K. Moreover, the SOT is calibrated by measuring the effective spin-orbit field using second-harmonic methods. The effective field to current ratio and the spin-Hall angle tangent are almost three orders of magnitude larger than those reported for HMFHs. The giant SOT and efficient current-induced magnetization switching exhibited by the bilayer heterostructure may lead to innovative spintronics applications such as ultralow power dissipation memory and logic devices.

  8. Intraband absorption in self-assembled Ge-doped GaN/AlN nanowire heterostructures.

    PubMed

    Beeler, M; Hille, P; Schörmann, J; Teubert, J; de la Mata, M; Arbiol, J; Eickhoff, M; Monroy, E

    2014-03-12

    We report the observation of transverse-magnetic-polarized infrared absorption assigned to the s-p(z) intraband transition in Ge-doped GaN/AlN nanodisks (NDs) in self-assembled GaN nanowires (NWs). The s-p(z) absorption line experiences a blue shift with increasing ND Ge concentration and a red shift with increasing ND thickness. The experimental results in terms of interband and intraband spectroscopy are compared to theoretical calculations of the band diagram and electronic structure of GaN/AlN heterostructured NWs, accounting for their three-dimensional strain distribution and the presence of surface states. From the theoretical analysis, we conclude that the formation of an AlN shell during the heterostructure growth applies a uniaxial compressive strain which blue shifts the interband optical transitions but has little influence on the intraband transitions. The presence of surface states with density levels expected for m-GaN plane charge-deplete the base of the NWs but is insufficient to screen the polarization-induced internal electric field in the heterostructures. Simulations show that the free-carrier screening of the polarization-induced internal electric field in the NDs is critical to predicting the photoluminescence behavior. The intraband transitions, on the other hand, are blue-shifted due to many-body effects, namely, the exchange interaction and depolarization shift, which exceed the red shift induced by carrier screening. PMID:24502703

  9. Manipulable GMR Effect in a δ-Doped Magnetically Confined Semiconductor Heterostructure

    NASA Astrophysics Data System (ADS)

    Jiang, Ya-Qing; Lu, Mao-Wang; Huang, Xin-Hong; Yang, Shi-Peng; Tang, Qiang

    2016-06-01

    A giant magnetoresistance (GMR) device formed by depositing two parallel nanosized ferromagnetic strips on top of a semiconductor heterostructure has been proposed theoretically (Zhai et al. in Phys Rev B 66:125305, 2002). For the sake of manipulating its performance, we introduce a tunable δ-potential into this device with the help of atomic-layer doping techniques such as molecular beam epitaxy (MBE) or metal-organic chemical-vapor deposition. We investigate theoretically the impact of such δ-doping on the magnetoresistance ratio (MR) of the GMR device. We find that, although the δ-doping is embedded in the device, a considerable GMR effect still exists due to the significant difference in electronic transmission between parallel (P) and antiparallel (AP) configurations. Moreover, the calculated results show that the MR of the GMR device varies sensitively with the weight and/or position of the δ-doping. Thus, the GMR device can be controlled by changing the δ-doping to obtain an adjustable GMR device for magnetoelectronics applications.

  10. Optically pumped terahertz wave modulation in MoS2-Si heterostructure metasurface

    NASA Astrophysics Data System (ADS)

    Zheng, Wei; Fan, Fei; Chen, Meng; Chen, Sai; Chang, Sheng-Jiang

    2016-07-01

    An optically pumped terahertz (THz) modulator based on a MoS2-Si heterostructure metasurface are fabricated and investigated in this paper. The THz wave modulation in MoS2 metasurface has been demonstrated by THz time domain spectroscopy experiment and numerical simulation, which can reach over 90% under the continuous wave laser pumping of 4W/cm2 power density. Importantly, the catalysis of photocarrier generation in MoS2-Si heterostructure has been proved by the comparsion between the modulation depth of metasurface with and without MoS2 nanosheet under the same pumping power, and we found that the strcuture of metasurface and polariztion direction can also influence the photocarrier density in MoS2 metasurface. This novel THz modulator based on 2D material has a high effective modulation on THz waves under a low pumping power, which has a bright potential in THz applications.

  11. Giant magnetoresistance in a two-dimensional electron gas modulated by magnetic barriers and the δ-doping

    NASA Astrophysics Data System (ADS)

    Zhang, Lan-Lan; Lu, Mao-Wang; Yang, Shi-Peng; Tang, Qiang

    2016-10-01

    We theoretically investigate the modulation of the δ-doping to a semiconductor-based giant magnetoresistance (GMR) device, which can be realized experimentally by depositing two parallel ferromagnetic (FM) stripes on top and bottom of a GaAs /AlxGa1-xAs heterostructure. It is shown that a considerable GMR effect still exists in this device with the δ-doping. It is also shown that the magnetoresistance ratio (MR) depends on not only the weight but also the position of the δ-doping. These interesting results will be useful in understanding and designing structurally-controllable GMR devices for magnetic information storage.

  12. Visualization of Photo-induced Doping patterns in Graphene/Boron Nitride Heterostructures via Scanning Tunneling Microscopy

    NASA Astrophysics Data System (ADS)

    Velasco, Jairo, Jr.; Ju, Long; Wong, Dillon; Lee, Juwon; Kahn, Salman; Tsai, Hsin-Zon; Germany, Chad; Taniguchi, Takashi; Watanabe, Kenji; Zettl, Alex; Wang, Feng; Crommie, Mike

    2015-03-01

    Photo-induced doping in graphene-boron nitride (G/BN) heterostructures enables flexible and repeatable writing and erasing of charge doping in graphene using optical irradiation. So far, however, this phenomenon has been explored using spatially averaging probes such as electron transport, and there have been no local studies into the underlying microscopic behavior. Here we report a combined scanning tunneling microscopy (STM) and optoelectronic measurement scheme that has been utilized to investigate the microscopic mechanisms at work in this process. We will discuss the latest experimental progress towards the visualization of light-induced charge doping patterns on G/BN heterostructures via STM. J.V.J. acknowledges support from the UC President's Postdoctoral Fellowship.

  13. Van der Waals heterostructure of phosphorene and graphene: tuning the Schottky barrier and doping by electrostatic gating.

    PubMed

    Padilha, J E; Fazzio, A; da Silva, Antônio J R

    2015-02-13

    In this Letter, we study the structural and electronic properties of single-layer and bilayer phosphorene with graphene. We show that both the properties of graphene and phosphorene are preserved in the composed heterostructure. We also show that via the application of a perpendicular electric field, it is possible to tune the position of the band structure of phosphorene with respect to that of graphene. This leads to control of the Schottky barrier height and doping of phosphorene, which are important features in the design of new devices based on van der Waals heterostructures.

  14. Investigation of As-doped ZnO films synthesized via thermal annealing of ZnSe/GaAs heterostructures

    NASA Astrophysics Data System (ADS)

    Maksimov, O.; Liu, B. Z.

    2008-06-01

    We synthesized ZnO films via oxidative annealing of ZnSe/GaAs heterostructures and investigated their structural and optical properties. Films were polycrystalline, c-axis oriented and exhibited superior optical properties. In addition, we detected nanometer-size As clusters into the ZnO film and a Ga xO y layer at the ZnO/GaAs interface. Formation of an interfacial layer can prevent use of this technique for p-type doping and complicates identification of the origin of p-type response in the annealed ZnO/GaAs heterostructures.

  15. Highly Luminescent Heterostructured Copper-Doped Zinc Sulfide Nanocrystals for Application in Cancer Cell Labeling.

    PubMed

    Ang, Huixiang; Bosman, Michel; Thamankar, Ramesh; Zulkifli, Muhammad Faizal B; Yen, Swee Kuan; Hariharan, Anushya; Sudhaharan, Thankiah; Selvan, Subramanian Tamil

    2016-08-18

    The structural characteristics of the seed-mediated synthesis of heterostructured CuS-ZnS nanocrystals (NCs) and Cu-doped ZnS (ZnS:Cu) NCs synthesized by two different protocols are compared and analyzed. At high Cu dopant concentrations, segregated subclusters of ZnS and CuS are observed. The photoluminescence quantum yield of ZnS:Cu NCs is about 50-80 %; a value much higher than that of ZnS NCs (6 %). Finally, these NCs are coated with a thin silica shell by using (3-mercaptopropyl)triethoxysilane in a reverse microemulsion to make them water soluble. Cytotoxicity experiments show that these silica-coated NCs have greatly reduced toxicity on both cancerous HeLa and noncancerous Chinese hamster ovary cells. The labeling of cancerous HeLa cells is also demonstrated. PMID:27146419

  16. Half-metallic digital ferromagnetic heterostructure composed of a delta-doped layer of Mn in Si.

    PubMed

    Qian, M C; Fong, C Y; Liu, Kai; Pickett, W E; Pask, J E; Yang, L H

    2006-01-20

    We propose and investigate the properties of a digital ferromagnetic heterostructure consisting of a delta-doped layer of Mn in Si, using ab initio electronic-structure methods. We find that (i) ferromagnetic order of the Mn layer is energetically favorable relative to antiferromagnetic, and (ii) the heterostructure is a two-dimensional half-metallic system. The metallic behavior is contributed by three majority-spin bands originating from hybridized Mn-d and nearest-neighbor Si-p states, and the corresponding carriers are responsible for the ferromagnetic order in the Mn layer. The minority-spin channel has a calculated semiconducting gap of 0.25 eV. The band lineup is found to be favorable for retaining the half-metal character to near the Curie temperature. This kind of heterostructure may be of special interest for integration into mature Si technologies for spintronic applications.

  17. Film size-dependent voltage-modulated magnetism in multiferroic heterostructures

    PubMed Central

    Hu, J.-M.; Shu, L.; Li, Z.; Gao, Y.; Shen, Y.; Lin, Y. H.; Chen, L. Q.; Nan, C. W.

    2014-01-01

    The electric-voltage-modulated magnetism in multiferroic heterostructures, also known as the converse magnetoelectric (ME) coupling, has drawn increasing research interest recently owing to its great potential applications in future low-power, high-speed electronic and/or spintronic devices, such as magnetic memory and computer logic. In this article, based on combined theoretical analysis and experimental demonstration, we investigate the film size dependence of such converse ME coupling in multiferroic magnetic/ferroelectric heterostructures, as well as exploring the interaction between two relating coupling mechanisms that are the interfacial strain and possibly the charge effects. We also briefly discuss some issues for the next step and describe new device prototypes that can be enabled by this technology. PMID:24421375

  18. Electric-field-modulated nonvolatile resistance switching in VO₂/PMN-PT(111) heterostructures.

    PubMed

    Zhi, Bowen; Gao, Guanyin; Xu, Haoran; Chen, Feng; Tan, Xuelian; Chen, Pingfan; Wang, Lingfei; Wu, Wenbin

    2014-04-01

    The electric-field-modulated resistance switching in VO2 thin films grown on piezoelectric (111)-0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 (PMN-PT) substrates has been investigated. Large relative change in resistance (10.7%) was observed in VO2/PMN-PT(111) hererostructures at room temperature. For a substrate with a given polarization direction, stable resistive states of VO2 films can be realized even when the applied electric fields are removed from the heterostructures. By sweeping electric fields across the heterostructure appropriately, multiple resistive states can be achieved. These stable resistive states result from the different stable remnant strain states of substrate, which is related to the rearrangements of ferroelectric domain structures in PMN-PT(111) substrate. The resistance switching tuned by electric field in our work may have potential applications for novel electronic devices. PMID:24634978

  19. Carrier Density Modulation in Ge Heterostructure by Ferroelectric Switching

    SciTech Connect

    Ponath, Patrick; Fredrickson, Kurt; Posadas, Agham B.; Ren, Yuan; Vasudevan, Rama K.; Okatan, Mahmut Baris; Jesse, Stephen; Aoki, Toshihiro; McCartney, Martha; Smith, David J.; Kalinin, Sergei V.; Lai, Keji; Demkov, Alexander A.

    2015-01-14

    The development of nonvolatile logic through direct coupling of spontaneous ferroelectric polarization with semiconductor charge carriers is nontrivial, with many issues, including epitaxial ferroelectric growth, demonstration of ferroelectric switching, and measurable semiconductor modulation. Here we report a true ferroelectric field effect carrier density modulation in an underlying Ge(001) substrate by switching of the ferroelectric polarization in the epitaxial c-axis-oriented BaTiO3 (BTO) grown by molecular beam epitaxy (MBE) on Ge. Using density functional theory, we demonstrate that switching of BTO polarization results in a large electric potential change in Ge. Aberration-corrected electron microscopy confirms the interface sharpness, and BTO tetragonality. Electron-energy-loss spectroscopy (EELS) indicates the absence of any low permittivity interlayer at the interface with Ge. Using piezoelectric force microscopy (PFM), we confirm the presence of fully switchable, stable ferroelectric polarization in BTO that appears to be single domain. Using microwave impedance microscopy (MIM), we clearly demonstrate a ferroelectric field effect.

  20. Carrier density modulation in a germanium heterostructure by ferroelectric switching.

    PubMed

    Ponath, Patrick; Fredrickson, Kurt; Posadas, Agham B; Ren, Yuan; Wu, Xiaoyu; Vasudevan, Rama K; Okatan, M Baris; Jesse, S; Aoki, Toshihiro; McCartney, Martha R; Smith, David J; Kalinin, Sergei V; Lai, Keji; Demkov, Alexander A

    2015-01-14

    The development of non-volatile logic through direct coupling of spontaneous ferroelectric polarization with semiconductor charge carriers is nontrivial, with many issues, including epitaxial ferroelectric growth, demonstration of ferroelectric switching and measurable semiconductor modulation. Here we report a true ferroelectric field effect-carrier density modulation in an underlying Ge(001) substrate by switching of the ferroelectric polarization in epitaxial c-axis-oriented BaTiO3 grown by molecular beam epitaxy. Using the density functional theory, we demonstrate that switching of BaTiO3 polarization results in a large electric potential change in Ge. Aberration-corrected electron microscopy confirms BaTiO3 tetragonality and the absence of any low-permittivity interlayer at the interface with Ge. The non-volatile, switchable nature of the single-domain out-of-plane ferroelectric polarization of BaTiO3 is confirmed using piezoelectric force microscopy. The effect of the polarization switching on the conductivity of the underlying Ge is measured using microwave impedance microscopy, clearly demonstrating a ferroelectric field effect.

  1. Carrier Density Modulation in Ge Heterostructure by Ferroelectric Switching

    DOE PAGES

    Ponath, Patrick; Fredrickson, Kurt; Posadas, Agham B.; Ren, Yuan; Vasudevan, Rama K.; Okatan, Mahmut Baris; Jesse, Stephen; Aoki, Toshihiro; McCartney, Martha; Smith, David J.; et al

    2015-01-14

    The development of nonvolatile logic through direct coupling of spontaneous ferroelectric polarization with semiconductor charge carriers is nontrivial, with many issues, including epitaxial ferroelectric growth, demonstration of ferroelectric switching, and measurable semiconductor modulation. Here we report a true ferroelectric field effect carrier density modulation in an underlying Ge(001) substrate by switching of the ferroelectric polarization in the epitaxial c-axis-oriented BaTiO3 (BTO) grown by molecular beam epitaxy (MBE) on Ge. Using density functional theory, we demonstrate that switching of BTO polarization results in a large electric potential change in Ge. Aberration-corrected electron microscopy confirms the interface sharpness, and BTO tetragonality. Electron-energy-lossmore » spectroscopy (EELS) indicates the absence of any low permittivity interlayer at the interface with Ge. Using piezoelectric force microscopy (PFM), we confirm the presence of fully switchable, stable ferroelectric polarization in BTO that appears to be single domain. Using microwave impedance microscopy (MIM), we clearly demonstrate a ferroelectric field effect.« less

  2. Ferroelectricity-induced resistive switching in Pb(Zr0.52Ti0.48)O3/Pr0.7Ca0.3MnO3/Nb-doped SrTiO3 epitaxial heterostructure

    NASA Astrophysics Data System (ADS)

    Md. Sadaf, Sharif; Mostafa Bourim, El; Liu, Xinjun; Hasan Choudhury, Sakeb; Kim, Dong-Wook; Hwang, Hyunsang

    2012-03-01

    We investigated the effect of a ferroelectric Pb(Zr0.52Ti0.48)O3 (PZT) thin film on the generation of resistive switching in a stacked Pr0.7Ca0.3MnO3 (PCMO)/Nb-doped SrTiO3 (Nb:STO) heterostructure forming a p-n junction. To promote the ferroelectric effect, the thin PZT active layer was deposited on an epitaxially grown p-type PCMO film on a lattice-matched n-type Nb:STO single crystal. It was concluded that the observed resistive switching behavior in the all-perovskite Pt/PZT/PCMO/Nb:STO heterostructure was related to the modulation of PCMO/Nb:STO p-n junction's depletion width, which was caused either by the PZT ferroelectric polarization field effect, the electrochemical drift of oxygen ions under an electric field, or both simultaneously.

  3. Doping modulated carbon nanotube synapstors for a spike neuromorphic module.

    PubMed

    Shen, Alex Ming; Kim, Kyunghyun; Tudor, Andrew; Lee, Dongwon; Chen, Yong

    2015-04-01

    A doping-modulated carbon nanotube (CNT) electronic device, called a "synapstor," emulates the function of a biological synapse. The CNT synapstor has a field-effect transistor structure with a random CNT network as its channel. An aluminium oxide (Al2 O3 ) film is deposited over half of the CNT channel in the synapstor, converting the covered part of the CNT from p-type to n-type, forming a p-n junction in the CNT channel and increasing the Schottky barrier between the n-type CNT and its metal contact. This scheme significantly improves the postsynaptic current (PSC) from the synapstor, extends the tuning range of the plasticity, and reduces the power consumption of the CNT synapstor. A spike neuromorphic module is fabricated by integrating the CNT synapstors with a Si-based "soma" circuit. Spike parallel processing, memory, and plasticity functions of the module are demonstrated. The module could potentially be integrated and scaled up to emulate a biological neural network with parallel high-speed signal processing, low power consumption, memory, and learning capabilities. PMID:25423906

  4. Correspondence between MOS and modulation-doped structures

    NASA Astrophysics Data System (ADS)

    Pierret, R. F.; Lundstrom, M. S.

    1984-03-01

    There is currently considerable interest in the development of modulation-doped field-effect transistors suitable for high-speed applications. A promising version of the modulation-doped FET consists of a Schottky-barrier contact atop a thin Al(x)Ga(1-x)As layer on a lightly doped GaAs underlayer. It is pointed out that for a n-AlGaAs/p-GaAs structure, the conduction band discontinuity at the AlGaAs-GaAs interface gives rise to an inversion layer at the GaAs surface. The present paper is concerned with the physical correspondence between n-AlGaAs/p-GaAs modulation-doped structures and MOS structures. It is shown that certain key modulation-doped relationships can be obtained directly from MOSFET relationships.

  5. Simulation of the effective concentration profiles in InGaAs/GaAs heterostructures containing δ-doped layers

    SciTech Connect

    Khazanova, S. V. Degtyarev, V. E.; Tikhov, S. V.; Baidus, N. V.

    2015-01-15

    InGaAs/GaAs heterostructures containing quantum wells and δ-doped layers are studied theoretically and experimentally. On the basis of the procedure of self-consistently solving the Schrödinger equation and Poisson equation, the differential capacitance and the apparent electron concentration profiles are numerically calculated for structures with different mutual arrangements of the quantum well and the δ layer. The results of the calculations are compared with the result of analyzing the experimental capacitance-voltage characteristics of the structures. The systematic features of the behavior of the apparent concentration profiles and capacitance-voltage characteristics in relation to the geometric properties of the structure, the temperature, and the doping level are established.

  6. High-performance modulation-doped AlGaAs/InGaAs thermopiles for uncooled infrared FPA application

    NASA Astrophysics Data System (ADS)

    Abe, M.; Abe, Y.; Kogushi, N.; Ang, K. S.; Hofstetter, R.; Wang, H.; Ng, G. I.

    2013-07-01

    Novel thermopile based on modulation doped AlGaAs/InGaAs heterostructures is proposed and developed for the first time, for uncooled infrared FPA (Focal Plane Array) image sensor application. The high responsivity with the high speed response time are designed to be 4900 V/W with 110 μs under the 2 μm design rule. Based on integrated HEMT-MEMS technology, the 32 × 32 matrix FPA is fabricated to demonstrate its enhanced performances by black body measurement. The technology presented here demonstrates the potential of this approach for low-cost uncooled infrared FPA image sensor application.

  7. Polarization-dependent interfacial coupling modulation of ferroelectric photovoltaic effect in PZT-ZnO heterostructures.

    PubMed

    Pan, Dan-Feng; Bi, Gui-Feng; Chen, Guang-Yi; Zhang, Hao; Liu, Jun-Ming; Wang, Guang-Hou; Wan, Jian-Guo

    2016-03-08

    Recently, ferroelectric perovskite oxides have drawn much attention due to potential applications in the field of solar energy conversion. However, the power conversion efficiency of ferroelectric photovoltaic effect currently reported is far below the expectable value. One of the crucial problems lies in the two back-to-back Schottky barriers, which are formed at the ferroelectric-electrode interfaces and blocking most of photo-generated carriers to reach the outside circuit. Herein, we develop a new approach to enhance the ferroelectric photovoltaic effect by introducing the polarization-dependent interfacial coupling effect. Through inserting a semiconductor ZnO layer with spontaneous polarization into the ferroelectric ITO/PZT/Au film, a p-n junction with strong polarization-dependent interfacial coupling effect is formed. The power conversion efficiency of the heterostructure is improved by nearly two orders of magnitude and the polarization modulation ratio is increased about four times. It is demonstrated that the polarization-dependent interfacial coupling effect can give rise to a great change in band structure of the heterostructure, not only producing an aligned internal electric field but also tuning both depletion layer width and potential barrier height at PZT-ZnO interface. This work provides an efficient way in developing highly efficient ferroelectric-based solar cells and novel optoelectronic memory devices.

  8. Polarization-dependent interfacial coupling modulation of ferroelectric photovoltaic effect in PZT-ZnO heterostructures.

    PubMed

    Pan, Dan-Feng; Bi, Gui-Feng; Chen, Guang-Yi; Zhang, Hao; Liu, Jun-Ming; Wang, Guang-Hou; Wan, Jian-Guo

    2016-01-01

    Recently, ferroelectric perovskite oxides have drawn much attention due to potential applications in the field of solar energy conversion. However, the power conversion efficiency of ferroelectric photovoltaic effect currently reported is far below the expectable value. One of the crucial problems lies in the two back-to-back Schottky barriers, which are formed at the ferroelectric-electrode interfaces and blocking most of photo-generated carriers to reach the outside circuit. Herein, we develop a new approach to enhance the ferroelectric photovoltaic effect by introducing the polarization-dependent interfacial coupling effect. Through inserting a semiconductor ZnO layer with spontaneous polarization into the ferroelectric ITO/PZT/Au film, a p-n junction with strong polarization-dependent interfacial coupling effect is formed. The power conversion efficiency of the heterostructure is improved by nearly two orders of magnitude and the polarization modulation ratio is increased about four times. It is demonstrated that the polarization-dependent interfacial coupling effect can give rise to a great change in band structure of the heterostructure, not only producing an aligned internal electric field but also tuning both depletion layer width and potential barrier height at PZT-ZnO interface. This work provides an efficient way in developing highly efficient ferroelectric-based solar cells and novel optoelectronic memory devices. PMID:26954833

  9. Polarization-dependent interfacial coupling modulation of ferroelectric photovoltaic effect in PZT-ZnO heterostructures

    PubMed Central

    Pan, Dan-Feng; Bi, Gui-Feng; Chen, Guang-Yi; Zhang, Hao; Liu, Jun-Ming; Wang, Guang-Hou; Wan, Jian-Guo

    2016-01-01

    Recently, ferroelectric perovskite oxides have drawn much attention due to potential applications in the field of solar energy conversion. However, the power conversion efficiency of ferroelectric photovoltaic effect currently reported is far below the expectable value. One of the crucial problems lies in the two back-to-back Schottky barriers, which are formed at the ferroelectric-electrode interfaces and blocking most of photo-generated carriers to reach the outside circuit. Herein, we develop a new approach to enhance the ferroelectric photovoltaic effect by introducing the polarization-dependent interfacial coupling effect. Through inserting a semiconductor ZnO layer with spontaneous polarization into the ferroelectric ITO/PZT/Au film, a p-n junction with strong polarization-dependent interfacial coupling effect is formed. The power conversion efficiency of the heterostructure is improved by nearly two orders of magnitude and the polarization modulation ratio is increased about four times. It is demonstrated that the polarization-dependent interfacial coupling effect can give rise to a great change in band structure of the heterostructure, not only producing an aligned internal electric field but also tuning both depletion layer width and potential barrier height at PZT-ZnO interface. This work provides an efficient way in developing highly efficient ferroelectric-based solar cells and novel optoelectronic memory devices. PMID:26954833

  10. Polarization-dependent interfacial coupling modulation of ferroelectric photovoltaic effect in PZT-ZnO heterostructures

    NASA Astrophysics Data System (ADS)

    Pan, Dan-Feng; Bi, Gui-Feng; Chen, Guang-Yi; Zhang, Hao; Liu, Jun-Ming; Wang, Guang-Hou; Wan, Jian-Guo

    2016-03-01

    Recently, ferroelectric perovskite oxides have drawn much attention due to potential applications in the field of solar energy conversion. However, the power conversion efficiency of ferroelectric photovoltaic effect currently reported is far below the expectable value. One of the crucial problems lies in the two back-to-back Schottky barriers, which are formed at the ferroelectric-electrode interfaces and blocking most of photo-generated carriers to reach the outside circuit. Herein, we develop a new approach to enhance the ferroelectric photovoltaic effect by introducing the polarization-dependent interfacial coupling effect. Through inserting a semiconductor ZnO layer with spontaneous polarization into the ferroelectric ITO/PZT/Au film, a p-n junction with strong polarization-dependent interfacial coupling effect is formed. The power conversion efficiency of the heterostructure is improved by nearly two orders of magnitude and the polarization modulation ratio is increased about four times. It is demonstrated that the polarization-dependent interfacial coupling effect can give rise to a great change in band structure of the heterostructure, not only producing an aligned internal electric field but also tuning both depletion layer width and potential barrier height at PZT-ZnO interface. This work provides an efficient way in developing highly efficient ferroelectric-based solar cells and novel optoelectronic memory devices.

  11. Manipulating Surface-induced Ferromagnetism in Modulation-doped Topological Insulators

    NASA Astrophysics Data System (ADS)

    Kou, Xufeng; He, Liang; Lang, Murong; Fan, Yabin; Jiang, Ying; Wang, Yong; Xiu, Faxian; Wang, Kang; Device Research Laboratory Team; CenterElectron Microscopy; State Key Laboratory of Silicon Materials Collaboration; ECE Department Collaboration

    2013-03-01

    The manipulation of topological surface states is a key to realize applicable devices of topological insulators. In addition to the direct engineering of time-reversal-symmetry protected surface states, recent work suggests that various physical responses can be obtained from surface helical states by integrating additional ferromagnetism or superconductivity to the original topological order. Here, we report the coexistence and tunability of bulk carrier density-independent and surface-mediated electrically controllable ferromagnetisms in modulation-doped Crx(BiySb1-y)2Te3 epitaxial thin films. We demonstrate for the first time a dramatic enhancement of surface-induced magnetization on TI / Cr-TI bilayer devices. The surface magneto-electric effects can be either enhanced significantly or completely switched-off, by tuning the separation of the surface from the magnetic impurities. The electric-field-modulated ferromagnetism in our modulation-doped TI hetero-structures is fundamentally important for the realization of the quantum anomalous Hall Effect as well as the axion electromagnetic dynamics, and thus provides a new approach for spintronics applications. The authors would also like to acknowledge helpful discussions with Dr. Alexei Fedorov and Dr. Mathew Marcus from the Advanced Light Source at Berkeley.

  12. DX center analysis in Sn-doped AlGaAs layer of double heterostructures

    NASA Astrophysics Data System (ADS)

    Kaniewski, J.; Kaniewska, M.; Ždánský, K.

    1987-12-01

    Capacitance as well as photovoltage methods have been used to analyze deep centers in an n-type AlGaAs:Sn layer of double heterostructures. It is suggested that the trap with thermal activation energy equal to ΔE2=0.33±0.02 eV is associated with the L minimum of AlGaAs and could be interpreted as a DX center related to Sn. The observed changes of deep center concentration in double heterostructures are due to different Al contents within the depletion region.

  13. Identification of photoluminescence bands in AlGaAs/InGaAs/GaAs PHEMT heterostructures with donor-acceptor-doped barriers

    SciTech Connect

    Gulyaev, D. V. Zhuravlev, K. S.; Bakarov, A. K.; Toropov, A. I.

    2015-02-15

    The photoluminescence of AlGaAs/InGaAs/GaAs pseudomorphic high-electron mobility transistor heterostructures with donor-acceptor-doped AlGaAs barriers is studied. It is found that the introduction of additional p{sup +}-doped AlGaAs layers into the design brings about the appearance of new bands in the photoluminescence spectra. These bands are identified as resulting from transitions (i) in donor-acceptor pairs in doped AlGaAs layers and (ii) between the conduction subband and acceptor levels in the undoped InGaAs quantum well.

  14. Charge movement in a GaN-based hetero-structure field effect transistor structure with carbon doped buffer under applied substrate bias

    SciTech Connect

    Pooth, Alexander; Uren, Michael J.; Cäsar, Markus; Kuball, Martin; Martin, Trevor

    2015-12-07

    Charge trapping and transport in the carbon doped GaN buffer of a GaN-based hetero-structure field effect transistor (HFET) has been investigated under both positive and negative substrate bias. Clear evidence of redistribution of charges in the carbon doped region by thermally generated holes is seen, with electron injection and capture observed during positive bias. Excellent agreement is found with simulations. It is shown that these effects are intrinsic to the carbon doped GaN and need to be controlled to provide reliable and efficient GaN-based power HFETs.

  15. Growth and Magneto-transport Characterization of Double-doped InGaAs/InAlAs Heterostructures with High Indium Compositions

    SciTech Connect

    Akabori, M.; Morimoto, K.; Wei, W.; Iwase, H.; Yamada, S.

    2011-12-23

    We investigated double-doped InGaAs/InAlAs heterostructures with high indium compositions. The heterostructures were grown by molecular beam epitaxy on GaAs(001) with metamorphic step-graded buffer layers. The magneto-transport characterization was performed by using Hall-bar devices. We observed non-monotonic magneto-resistance oscillations, which indicate a single two-dimensional electron gas (2DEG) with multiple-subband occupation or a 2DEG bilayer. We also observed weak-antilocalization in all samples, which is an evidence of spin-orbit coupling.

  16. Room-temperature light emission from an airbridge double-heterostructure microcavity of Er-doped Si photonic crystal

    NASA Astrophysics Data System (ADS)

    Wang, Yue; An, Jun-ming; Wu, Yuan-da; Hu, Xiong-wei

    2016-01-01

    We experimentally demonstrate an efficient enhancement of luminescence from two-dimensional (2D) hexagonal photonic crystal (PC) airbridge double-heterostructure microcavity with Er-doped silicon (Si) as light emitters on siliconon-insulator (SOI) wafer at room temperature. A single sharp resonant peak at 1 529.6 nm dominates the photoluminescence (PL) spectrum with the pumping power of 12.5 mW. The obvious red shift and the degraded quality factor (Q-factor) of resonant peak appear with the pumping power increasing, and the maximum measured Q-factor of 4 905 is achieved at the pumping power of 1.5 mW. The resonant peak is observed to shift depending on the structural parameters of PC, which indicates a possible method to control the wavelength of enhanced luminescence for Si-based light emitters based on PC microcavity.

  17. Electron mobility and drift velocity in selectively doped InAlAs/InGaAs/InAlAs heterostructures

    SciTech Connect

    Vasil'evskii, I. S. Galiev, G. B.; Klimov, E. A.; Pozela, K.; Pozela, J.; Juciene, V.; Suziedelis, A.; Zurauskiene, N.; Kersulis, S.; Stankevic, V.

    2011-09-15

    An increase in the electron mobility and drift velocity in high electric fields in quantum wells of selectively doped InAlAs/InGaAs/InAsAs heterostructures is obtained experimentally via controlling the composition of semiconductors forming the interface. The electron mobility at the interface in the In{sub 0.8}Ga{sub 0.2}As/In{sub 0.7}Al{sub 0.3}As metamorphic structure with a high molar fraction of In (0.7-0.8) is as high as 12.3 Multiplication-Sign 10{sup 3} cm{sup 2} V{sup -1} s{sup -1} at room temperature. An increase in the electron mobility by a factor of 1.1-1.4 is attained upon the introduction of thin (1-3 nm) InAs layers into a quantum well of selectively doped In{sub 0.53}Ga{sub 0.47}As/In{sub 0.52}Al{sub 0.48}As heterostructures. A maximal drift velocity attains 2.5 Multiplication-Sign 10{sup 7} cm/s in electric fields of 2-5 kV/cm. The threshold field F{sub th} for the intervalley {Gamma}-L electron transfer (the Gunn effect) in the InGaAs quantum well is higher than in the bulk material by a factor of 2.5-3. The effect of two- to threefold decrease in the threshold field F{sub th} in the InGaAs quantum well is established upon increasing the molar fraction of In in the InAlAs barrier, as well as upon the introduction of thin InAs inserts into the InGaAs quantum well.

  18. Transport in two-dimensional modulation-doped semiconductor structures

    NASA Astrophysics Data System (ADS)

    Das Sarma, S.; Hwang, E. H.; Kodiyalam, S.; Pfeiffer, L. N.; West, K. W.

    2015-05-01

    We develop a theory for the maximum achievable mobility in modulation-doped 2D GaAs-AlGaAs semiconductor structures by considering the momentum scattering of the 2D carriers by the remote ionized dopants, which must invariably be present in order to create the 2D electron gas at the GaAs-AlGaAs interface. The minimal model, assuming first-order Born scattering by random quenched remote dopant ions as the only scattering mechanism, gives a mobility much lower (by a factor of 3 or more) than that observed experimentally in many ultrahigh-mobility modulation-doped 2D systems, establishing convincingly that the model of uncorrelated scattering by independent random remote quenched dopant ions is often unable to describe the physical system quantitively. We theoretically establish that the consideration of spatial correlations in the remote dopant distribution can enhance the mobility by (up to) several orders of magnitudes in experimental samples. The precise calculation of the carrier mobility in ultrapure modulation-doped 2D semiconductor structures thus depends crucially on the unknown spatial correlations among the dopant ions in the doping layer which may manifest sample to sample variations even for nominally identical sample parameters (i.e., density, well width, etc.), depending on the details of the modulation-doping growth conditions.

  19. Control of Rewriteable Doping Patterns in Graphene/Boron Nitride Heterostructures

    NASA Astrophysics Data System (ADS)

    Kahn, Salman; Velasco, Jairo, Jr.; Wong, Dillon; Lee, Juwon; Tsai, Hsin Zon; Ju, Long; Jiang, Lili; Shi, Zhiwen; Ashby, Paul; Taniguchi, Takashi; Watanabe, Kenji; Zettl, Alex; Wang, Feng; Crommie, Michael

    Spatial control of charge doping in 2D materials is a promising technique for designing future electronic devices and understanding novel physics. Electrostatic gating and chemical doping are common methods to achieve control of charge doping in 2D materials. However, these approaches suffer from complicated fabrication processes that introduce impurities, change material properties irreversibly, and lack flexibility. Here, we introduce a new method for patterning rewriteable doping profiles with local interface charge transfer from defects in a tunable BN substrate into an adjacent layer of graphene. We characterize these spatial doping patterns through local probe and transport techniques. This technique enables many novel device designs for 2D materials, including atomically thin p-n junctions and rewriteable memory devices.

  20. Bolometric detection of magnetoplasma resonances in microwave absorption by two-dimensional electron systems based on doping layer conductivity measurements in GaAs/AlGaAs heterostructures

    SciTech Connect

    Dorozhkin, S. I. Sychev, D. V.; Kapustin, A. A.

    2014-11-28

    We have implemented a new bolometric method to detect resonances in magneto-absorption of microwave radiation by two-dimensional electron systems (2DES) in selectively doped GaAs/AlGaAs heterostructures. Radiation is absorbed by the 2DES and the thermally activated conductivity of the doping layer supplying electrons to the 2DES serves as a thermometer. The resonant absorption brought about by excitation of the confined magnetoplasma modes appears as peaks in the magnetic field dependence of the low-frequency impedance measured between the Schottky gate and 2DES.

  1. Control of Rewriteable Doping Patterns in Graphene/Boron Nitride Heterostructures

    NASA Astrophysics Data System (ADS)

    Kahn, Salman; Velasco, Jairo, Jr.; Wong, Dillon; Lee, Juwon; Tsai, Hsin Zon; Ju, Long; Jiang, Lili; Shi, Zhiwen; Ashby, Paul; Taniguchi, Takashi; Watanabe, Kenji; Zettl, Alex; Wang, Feng; Crommie, Michael

    Spatial control of charge doping in 2D materials is a prerequisite for designing future electronic devices and understanding novel physics. Electrostatic gating and chemical doping are two of the most common methods to achieve this. However, these approaches suffer from complicated fabrication processes that introduce impurities, change material properties irreversibly, and lack flexibility. Here we introduce a new method for patterning rewriteable doping profiles using an STM tip by way of local tip-voltage-induced ionization of defects in a BN substrate. We characterize these spatial doping patterns through local probe and transport techniques. This technique enables many novel device designs for 2D materials, including atomically thin p-n junctions and rewriteable memory devices.

  2. The modulation of grain boundary barrier in ZnMgO/ZnO heterostructure by surface polar liquid

    PubMed Central

    Ji, Xu; Zhu, Yuan; Chen, Mingming; Su, Longxing; Chen, Anqi; Gui, Xuchun; Xiang, Rong; Tang, Zikang

    2014-01-01

    Modulation of grain boundary barrier in ZnO layer by polar liquid, was investigated in ZnMgO/ZnO heterostructures grown by plasma-assisted molecular beam epitaxy. Traditionally, surface adsorbates can only affect the surface atoms or surface electronic states. However, it was found that the electronic conduction property of ZnO far from the surface could be tailored obviously by the polar liquid adsorbed on the ZnMgO surface. Physically, this phenomenon is supposed to be caused by the electrostatical couple between the liquid polarity and the grain boundary barrier in the ZnO layer through crystal polarization field. PMID:24566523

  3. Mixed Carrier Conduction in Modulation-doped Field Effect Transistors

    NASA Technical Reports Server (NTRS)

    Schacham, S. E.; Haugland, E. J.; Mena, R. A.; Alterovitz, S. A.

    1995-01-01

    The contribution of more than one carrier to the conductivity in modulation-doped field effect transistors (MODFET) affects the resultant mobility and complicates the characterization of these devices. Mixed conduction arises from the population of several subbands in the two-dimensional electron gas (2DEG), as well as the presence of a parallel path outside the 2DEG. We characterized GaAs/AlGaAs MODFET structures with both delta and continuous doping in the barrier. Based on simultaneous Hall and conductivity analysis we conclude that the parallel conduction is taking place in the AlGaAs barrier, as indicated by the carrier freezeout and activation energy. Thus, simple Hall analysis of these structures may lead to erroneous conclusions, particularly for real-life device structures. The distribution of the 2D electrons between the various confined subbands depends on the doping profile. While for a continuously doped barrier the Shubnikov-de Haas analysis shows superposition of two frequencies for concentrations below 10(exp 12) cm(exp -2), for a delta doped structure the superposition is absent even at 50% larger concentrations. This result is confirmed by self-consistent analysis, which indicates that the concentration of the second subband hardly increases.

  4. Study of resistive switching and magnetism modulation in the Pt/CoFe2O4/Nb:SrTiO3 heterostructures

    NASA Astrophysics Data System (ADS)

    Wang, Qiangwen; Zhu, Yongdan; Liu, Xiaolian; Zhao, Meng; Wei, Maocai; Zhang, Feng; Zhang, Yuan; Sun, Beilei; Li, Meiya

    2015-08-01

    CoFe2O4 (CFO) thin films are epitaxially grown on Nb doped (001) SrTiO3 (NSTO) single-crystal substrates by pulsed laser deposition to form Pt/CFO/NSTO heterostructures. These heterostructures exhibit typical bipolar resistive switching effect with maximum switching ratio of 5 × 104, multi-level resistance states, excellent retention, and anti-fatigue properties. When the resistance states of the heterostructures are switched between low resistance state and high resistance state upon applying bias voltages, the saturation magnetization of the CFO films shows corresponding changes associated with the resistive switching. These close correlations between the resistive switching and the magnetization can be attributed to the electrons filling into and releasing from the defect energy levels introduced by oxygen vacancies in the CFO film. These results show potential application in the multi-functional magnetoelectric sensor and non-volatile multi-level resistive switching memory.

  5. Photovoltaic response in pristine WSe{sub 2} layers modulated by metal-induced surface-charge-transfer doping

    SciTech Connect

    Wi, Sungjin; Chen, Mikai; Li, Da; Nam, Hongsuk; Meyhofer, Edgar; Liang, Xiaogan

    2015-08-10

    We obtained photovoltaic response in pristine multilayer WSe{sub 2} by sandwiching WSe{sub 2} between top and bottom metals. In this structure, the work-function difference between the top metal and WSe{sub 2} plays a critical role in generating built-in potentials and photovoltaic responses. Our devices with Zn as top metal exhibit photo-conversion efficiencies up to 6.7% under 532 nm illumination and external quantum efficiencies in the range of 40%–83% for visible light. This work provides a method for generating photovoltaic responses in layered semiconductors without detrimental doping or exquisite heterostructures, and also advances the physics for modulating the band structures of such emerging semiconductors.

  6. A delta-doped quantum well system with additional modulation doping

    PubMed Central

    2011-01-01

    A delta-doped quantum well with additional modulation doping may have potential applications. Utilizing such a hybrid system, it is possible to experimentally realize an extremely high two-dimensional electron gas (2DEG) density without suffering inter-electronic-subband scattering. In this article, the authors report on transport measurements on a delta-doped quantum well system with extra modulation doping. We have observed a 0-10 direct insulator-quantum Hall (I-QH) transition where the numbers 0 and 10 correspond to the insulator and Landau level filling factor ν = 10 QH state, respectively. In situ titled-magnetic field measurements reveal that the observed direct I-QH transition depends on the magnetic component perpendicular to the quantum well, and the electron system within this structure is 2D in nature. Furthermore, transport measurements on the 2DEG of this study show that carrier density, resistance and mobility are approximately temperature (T)-independent over a wide range of T. Such results could be an advantage for applications in T-insensitive devices. PMID:21711656

  7. Visible-light-driven photoelectrochemical and photocatalytic performances of Cr-doped SrTiO3/TiO2 heterostructured nanotube arrays

    PubMed Central

    Jiao, Zhengbo; Chen, Tao; Xiong, Jinyan; Wang, Teng; Lu, Gongxuan; Ye, Jinhua; Bi, Yingpu

    2013-01-01

    Well-aligned TiO2 nanotube arrays have become of increasing significance because of their unique highly ordered array structure, high specific surface area, unidirectional charge transfer and transportation features. However, their poor visible light utilization as well as the high recombination rate of photoexcited electron-hole pairs greatly limited their practical applications. Herein, we demonstrate the fabrication of visible-light-responsive heterostructured Cr-doped SrTiO3/TiO2 nanotube arrays by a simple hydrothermal method, which facilitate efficient charge separation and thus improve the photoelectrochemical as well as photocatalytic performances. PMID:24056587

  8. Beyond modulation doping: Engineering a semiconductor to be ambipolar, or making an ON-OFF-ON transistor

    SciTech Connect

    Gupta, K. Das; Croxall, A. F.; Zheng, B.; Sfigakis, F.; Farrer, I.; Nicoll, C. A.; Beere, H. E.; Ritchie, D. A.

    2014-04-24

    Semiconductors are traditionaly either p-type or n-type, meaning that the mobile charge carriers in them are either 'holes' in the valence band or electrons in the conduction band. Ambipolar conduction implies that the experimenter should be able to populate the same channel with either electrons or holes in a controlled manner. This has been shown to be possible in newer materials like Graphene and some organic semiconductors. 'Ambipolarity' can open up new device possibilities as well as new ways to study fundamental scattering mechanisms in semiconductors. However, achieving this in a conventional high mobility structure like a GaAs-AlGaAs heterostructure/quantum well requires new thinking. It was realized, that to do this modulation doping must be given up and techniques to make an undoped heterostructure conduct, must be developed first. Such structures have been developed by only a few groups worldwide. They are of great interest to low temperature physicists working with Quantum Hall states and mesoscopic/nano structures in the ballistic regime. We discuss the reason behind this interest and the analysis of scattering mechanisms in such structures. Finally very recent experimental success in developing fully gate controlled ambipolar structures where both electron and hole mobilites exceed 1 million cm{sup 2}/Vs at low temperatures (T∼1Kelvin) are discussed. Such gated ambipolar structures can be used to analyse scattering mechanisms in ultra-high mobility 2dimensional electron and hole gases in a way that is not possible using other techniques.

  9. The Electron Transport in a Nanostructure Modulated by the Magnetic Field and the δ-DOPING

    NASA Astrophysics Data System (ADS)

    Lu, Jian-Duo; Xu, Bin; Zheng, Wei

    2013-09-01

    We theoretically investigate the effect of the δ-doping on the electron transport in a magnetic nanostructure, which can be experimentally realized by depositing one ferromagnetic stripe on the top of a semiconductor heterostructure. We find that the position and the strength of the δ-doping as well as the distance between the two magnetic fields play an important role on the electron transport properties such as the transmission probability, the conductance and the spin polarization. These interesting results may be very helpful for analyzing the spin-dependent transport mechanism of the electron and making the new types of the spintronic devices.

  10. Chemical sensing by band modulation of a black phosphorus/molybdenum diselenide van der Waals hetero-structure

    NASA Astrophysics Data System (ADS)

    Feng, Zhihong; Chen, Buyun; Qian, Shuangbei; Xu, Linyan; Feng, Liefeng; Yu, Yuanyuan; Zhang, Rui; Chen, Jiancui; Li, Qianqian; Li, Quanning; Sun, Chongling; Zhang, Hao; Liu, Jing; Pang, Wei; Zhang, Daihua

    2016-09-01

    We report on a new chemical sensor based on black phosphorus/molybdenum diselenide van der Waals hetero-junctions. Due to the atomically thin nature of two-dimensional (2D) materials, surface adsorption of gas molecules can effectively modulate the band alignment at the junction interface, making the device a highly sensitive detector for chemical adsorptions. Compared to sensors made of homogeneous nanomaterials, the hetero-junction demonstrates considerably lower detection limit and higher sensitivity toward nitrogen dioxide. Kelvin probe force microscopy and finite element simulations have provided experimental and theoretical explanations for the enhanced performance, proving that chemical adsorption can induce significant changes in band alignment and carrier transport behaviors. The study demonstrates the potential of van der Waals hetero-junction as a new platform for sensing applications, and provides more insights into the interaction between gaseous molecules and 2D hetero-structures.

  11. Maximum drift velocity of electrons in selectively doped InAlAs/InGaAs/InAlAs heterostructures with InAs inserts

    SciTech Connect

    Silenas, A.; Pozela, Yu. Pozela, K.; Juciene, V.; Vasil'evskii, I. S.; Galiev, G. B.; Pushkarev, S. S.; Klimov, E. A.

    2013-03-15

    The dependence of the electron mobility and drift velocity on the growth conditions, thickness, and doping of an InAs insert placed at the center of the quantum well in a selectively doped InAlAs/InGaAs/InAlAs heterostructure has been investigated. Record enhancement of the maximum drift velocity to (2-4) Multiplication-Sign 10{sup 7} cm/s in an electric field of 5 Multiplication-Sign 10{sup 3} V/cm has been obtained in a 17-nm-wide quantum well with an undoped 4-nm-thick InAs insert. In the structures with additional doping of the InAs insert, which facilitates an increase in the density of electrons in the quantum well to 4.0 Multiplication-Sign 10{sup 12} cm{sup -2}, the maximum drift velocity is as high as 2 Multiplication-Sign 10{sup 7} cm/s in an electric field of 7 Multiplication-Sign 10{sup 3} V/cm.

  12. Si and Mg pair-doped interlayers for improving performance of AlGaN/GaN heterostructure field effect transistors grown on Si substrate

    NASA Astrophysics Data System (ADS)

    Ni, Yi-Qiang; He, Zhi-Yuan; Yao, Yao; Yang, Fan; Zhou, De-Qiu; Zhou, Gui-Lin; Shen, Zhen; Zhong, Jian; Zheng, Yue; Zhang, Bai-Jun; Liu, Yang

    2015-05-01

    We report a novel structure of AlGaN/GaN heterostructure field effect transistors (HFETs) with a Si and Mg pair-doped interlayer grown on Si substrate. By optimizing the doping concentrations of the pair-doped interlayers, the mobility of 2DEG increases by twice for the conventional structure under 5 K due to the improved crystalline quality of the conduction channel. The proposed HFET shows a four orders lower off-state leakage current, resulting in a much higher on/off ratio (˜ 109). Further temperature-dependent performance of Schottky diodes revealed that the inhibition of shallow surface traps in proposed HFETs should be the main reason for the suppression of leakage current. Project supported by the National Natural Science Foundation of China (Grant Nos. 51177175 and 61274039), the National Basic Research Project of China (Grant Nos. 2010CB923200 and 2011CB301903), the Ph.D. Program Foundation of Ministry of Education of China (Grant No. 20110171110021), the International Sci. & Tech. Collaboration Program of China (Grant No. 2012DFG52260), the National High-tech R&D Program of China (Grant No. 2014AA032606), the Science and Technology Plan of Guangdong Province, China (Grant No. 2013B010401013), and the Opened Fund of the State Key Laboratory on Integrated Optoelectronics (Grant No. IOSKL2014KF17).

  13. Tunnel-injection quantum dot deep-ultraviolet light-emitting diodes with polarization-induced doping in III-nitride heterostructures

    SciTech Connect

    Verma, Jai Islam, S. M.; Protasenko, Vladimir; Kumar Kandaswamy, Prem; Xing, Huili; Jena, Debdeep

    2014-01-13

    Efficient semiconductor optical emitters in the deep-ultraviolet spectral window are encountering some of the most deep rooted problems of semiconductor physics. In III-Nitride heterostructures, obtaining short-wavelength photon emission requires the use of wide bandgap high Al composition AlGaN active regions. High conductivity electron (n-) and hole (p-) injection layers of even higher bandgaps are necessary for electrical carrier injection. This approach requires the activation of very deep dopants in very wide bandgap semiconductors, which is a difficult task. In this work, an approach is proposed and experimentally demonstrated to counter the challenges. The active region of the heterostructure light emitting diode uses ultrasmall epitaxially grown GaN quantum dots. Remarkably, the optical emission energy from GaN is pushed from 365 nm (3.4 eV, the bulk bandgap) to below 240 nm (>5.2 eV) because of extreme quantum confinement in the dots. This is possible because of the peculiar bandstructure and band alignments in the GaN/AlN system. This active region design crucially enables two further innovations for efficient carrier injection: Tunnel injection of carriers and polarization-induced p-type doping. The combination of these three advances results in major boosts in electroluminescence in deep-ultraviolet light emitting diodes and lays the groundwork for electrically pumped short-wavelength lasers.

  14. Near-infrared electroluminescence from light-emitting devices based on Nd-doped TiO2/p+-Si heterostructures

    NASA Astrophysics Data System (ADS)

    Yang, Yang; Lv, Chunyan; Zhu, Chen; Li, Si; Ma, Xiangyang; Yang, Deren

    2014-05-01

    We report on near-infrared (NIR) electroluminescence (EL) from the light-emitting devices based on Nd-doped TiO2/p+-Si heterostructures. NIR emissions peaking at ˜910, 1090, and 1370 nm, originated from intra-4f transitions in Nd3+ ions, can be activated by a forward bias voltage as low as ˜5 V. Such NIR EL is triggered by the energy transferred from TiO2 host to Nd3+ ions. It is found that the coexistence of anatase and rutile phases in the TiO2 host enables the device to exhibit pronounced Nd-related EL without concurrent emission from the TiO2 host itself, quite other than the case of existing only anatase phase in TiO2 host. We tentatively suggest that the anatase/rutile interface states play important role in the energy transfer from TiO2 host to Nd3+ ions.

  15. Study of resistive switching and magnetism modulation in the Pt/CoFe{sub 2}O{sub 4}/Nb:SrTiO{sub 3} heterostructures

    SciTech Connect

    Wang, Qiangwen; Zhao, Meng; Wei, Maocai; Zhang, Feng; Zhang, Yuan; Sun, Beilei; Li, Meiya E-mail: myli@whu.edu.cn; Liu, Xiaolian

    2015-08-10

    CoFe{sub 2}O{sub 4} (CFO) thin films are epitaxially grown on Nb doped (001) SrTiO{sub 3} (NSTO) single-crystal substrates by pulsed laser deposition to form Pt/CFO/NSTO heterostructures. These heterostructures exhibit typical bipolar resistive switching effect with maximum switching ratio of 5 × 10{sup 4}, multi-level resistance states, excellent retention, and anti-fatigue properties. When the resistance states of the heterostructures are switched between low resistance state and high resistance state upon applying bias voltages, the saturation magnetization of the CFO films shows corresponding changes associated with the resistive switching. These close correlations between the resistive switching and the magnetization can be attributed to the electrons filling into and releasing from the defect energy levels introduced by oxygen vacancies in the CFO film. These results show potential application in the multi-functional magnetoelectric sensor and non-volatile multi-level resistive switching memory.

  16. Axial Ge/Si nanowire heterostructure tunnel FETs

    SciTech Connect

    Picraux, Sanuel T; Daych, Shadi A

    2010-01-01

    The vapor-liquid-solid (VLS) growth of semiconductor nanowires allows doping and composition modulation along their axis and the realization of axial 1 D heterostructures. This provides additional flexibility in energy band-edge engineering along the transport direction which is difficult to attain by planar materials growth and processing techniques. We report here on the design, growth, fabrication, and characterization of asymmetric heterostructure tunnel field-effect transistors (HTFETs) based on 100% compositionally modulated Si/Ge axial NWs for high on-current operation and low ambipolar transport behavior. We discuss the optimization of band-offsets and Schottky barrier heights for high performance HTFETs and issues surrounding their experimental realization. Our HTFET devices with 10 nm PECVD SiN{sub x} gate dielectric resulted in a measured current drive exceeding 100 {mu}A/{mu}m (I/{pi}D) and 10{sup 5} I{sub on}/I{sub off} ratios.

  17. Thermoelectric Properties of Complex Oxide Heterostructures

    NASA Astrophysics Data System (ADS)

    Cain, Tyler Andrew

    Thermoelectrics are a promising energy conversion technology for power generation and cooling systems. The thermal and electrical properties of the materials at the heart of thermoelectric devices dictate conversion efficiency and technological viability. Studying the fundamental properties of potentially new thermoelectric materials is of great importance for improving device performance and understanding the electronic structure of materials systems. In this dissertation, investigations on the thermoelectric properties of a prototypical complex oxide, SrTiO3, are discussed. Hybrid molecular beam epitaxy (MBE) is used to synthesize La-doped SrTiO3 thin films, which exhibit high electron mobilities and large Seebeck coefficients resulting in large thermoelectric power factors at low temperatures. Large interfacial electron densities have been observed in SrTiO3/RTiO 3 (R=Gd,Sm) heterostructures. The thermoelectric properties of such heterostructures are investigated, including the use of a modulation doping approach to control interfacial electron densities. Low-temperature Seebeck coefficients of extreme electron-density SrTiO3 quantum wells are shown to provide insight into their electronic structure.

  18. A review of nano-optics in metamaterial hybrid heterostructures

    SciTech Connect

    Singh, Mahi R.

    2014-03-31

    We present a review for the nonlinear nano-optics in quantum dots doped in a metamaterial heterostructure. The heterostructure is formed by depositing a metamaterial on a dielectric substrate and ensemble of noninteracting quantum dots are doped near the heterostructure interface. It is shown that there is enhancement of the second harmonic generation due to the surface plasmon polaritons field present at the interface.

  19. Scattering analysis of 2DEG mobility in undoped and doped AlGaN/AlN/GaN heterostructures with an in situ Si3N4 passivation layer

    NASA Astrophysics Data System (ADS)

    Atmaca, G.; Ardali, S.; Tiras, E.; Malin, T.; Mansurov, V. G.; Zhuravlev, K. S.; Lisesivdin, S. B.

    2016-04-01

    The scattering mechanisms limiting mobility for low-dimensional charge carriers in a two-dimensional electron gas (2DEG) in undoped and doped AlGaN/AlN/GaN heterostructures with and without Si3N4 passivation are investigated. Hall effect measurements were carried out at temperatures from 1.8 K to 262 K and at a fixed magnetic field of 1 T. A good consistency was found between the calculated and the experimental results. The effects of in situ Si3N4 passivation on the 2DEG mobility are also discussed with majority scattering mechanisms. Interface-related parameters including quantum well width, deformation potential constant and correlation length of interface roughness were obtained from the fits of the analytical expressions of scattering mechanisms and compared for each heterostructure. After in situ Si3N4 passivation, we found that the effect of the interface roughness scattering, which was the dominant scattering mechanism at low temperatures, on the 2DEG mobility was more effective in undoped and doped AlGaN/GaN heterostructures.

  20. Impact ionisation electroluminescence in planar GaAs-based heterostructure Gunn diodes: Spatial distribution and impact of doping non-uniformities

    NASA Astrophysics Data System (ADS)

    Montes Bajo, M.; Dunn, G.; Stephen, A.; Khalid, Ata; Cumming, D. R. S.; Oxley, C. H.; Glover, J.; Kuball, M.

    2013-03-01

    When biased in the negative differential resistance regime, electroluminescence (EL) is emitted from planar GaAs heterostructure Gunn diodes. This EL is due to the recombination of electrons in the device channel with holes that are generated by impact ionisation when the Gunn domains reach the anode edge. The EL forms non-uniform patterns whose intensity shows short-range intensity variations in the direction parallel to the contacts and decreases along the device channel towards the cathode. This paper employs Monte Carlo models, in conjunction with the experimental data, to analyse these non-uniform EL patterns and to study the carrier dynamics responsible for them. It is found that the short-range lateral (i.e., parallel to the device contacts) EL patterns are probably due to non-uniformities in the doping of the anode contact, illustrating the usefulness of EL analysis on the detection of such inhomogeneities. The overall decreasing EL intensity towards the anode is also discussed in terms of the interaction of holes with the time-dependent electric field due to the transit of the Gunn domains. Due to their lower relative mobility and the low electric field outside of the Gunn domain, freshly generated holes remain close to the anode until the arrival of a new domain accelerates them towards the cathode. When the average over the transit of several Gunn domains is considered, this results in a higher hole density, and hence a higher EL intensity, next to the anode.

  1. Modulating TiO2 photocatalyst by Al doping: Density functional theory approach

    NASA Astrophysics Data System (ADS)

    Zhao, Ya Fei; Li, Can; Lu, Song; Gong, Yin Yan; Niu, Leng Yuan; Liu, Xin Juan

    2016-06-01

    In this work, systematic study of the thermal stability, crystal structure and electronic properties of Al doped TiO2 were studied by the first principles calculations. The results showed that Al atoms preferentially occupying the interstitial site under Ti-rich condition, but substituting the Ti atom under O-rich condition. In contrast to pure TiO2, the values of VBM and CBM are reduced for Al substituting Ti doped mode, but increased for Al interstitial atom doped mode. Thus, we can modulate the preparation condition and dosage concentration for preparing the optimal photocatalyst.

  2. Electrical transport and resistance switching characteristics of BiFeO3/Nb:SrTiO3/GaAs heterostructure fabricated by pulsed laser deposition

    NASA Astrophysics Data System (ADS)

    Huang, W.; Yang, J. J.; Gao, G. Y.; Lei, Y.; Zhu, J.; Zeng, H. Z.; Zheng, F. G.; Hao, J. H.

    2014-08-01

    BiFeO3 thin films were epitaxially grown on (001) GaAs substrate by pulsed laser deposition with Nb doped SrTiO3 as a buffer layer. Piezoresponse force microscopy images exhibit effective ferroelectric switching of the heterostructure. The temperature-dependent current-voltage characteristics of the heterostructure reveal a resistance switching phenomenon and diode-like behavior with a rectifying ratio of 2 × 102 at the applied voltage of ±13.4 V. The electrical transport mechanism in the heterostructure has been illustrated by constructing the energy band structure. In addition, the resistance switching behavior in the heterostructure could be explained by the polarization modulation of the depletion region at the interface of the semiconductor and the ferroelectric layers.

  3. The modulation of Schottky barriers of metal-MoS2 contacts via BN-MoS2 heterostructures.

    PubMed

    Su, Jie; Feng, Liping; Zhang, Yan; Liu, Zhengtang

    2016-06-22

    Using first-principles calculations within density functional theory, we systematically studied the effect of BN-MoS2 heterostructure on the Schottky barriers of metal-MoS2 contacts. Two types of FETs are designed according to the area of the BN-MoS2 heterostructure. Results show that the vertical and lateral Schottky barriers in all the studied contacts, irrespective of the work function of the metal, are significantly reduced or even vanish when the BN-MoS2 heterostructure substitutes the monolayer MoS2. Only the n-type lateral Schottky barrier of Au/BN-MoS2 contact relates to the area of the BN-MoS2 heterostructure. Notably, the Pt-MoS2 contact with n-type character is transformed into a p-type contact upon substituting the monolayer MoS2 by a BN-MoS2 heterostructure. These changes of the contact natures are ascribed to the variation of Fermi level pinning, work function and charge distribution. Analysis demonstrates that the Fermi level pinning effects are significantly weakened for metal/BN-MoS2 contacts because no gap states dominated by MoS2 are formed, in contrast to those of metal-MoS2 contacts. Although additional BN layers reduce the interlayer interaction and the work function of the metal, the Schottky barriers of metal/BN-MoS2 contacts still do not obey the Schottky-Mott rule. Moreover, different from metal-MoS2 contacts, the charges transfer from electrodes to the monolayer MoS2, resulting in an increment of the work function of these metals in metal/BN-MoS2 contacts. These findings may prove to be instrumental in the future design of new MoS2-based FETs with ohmic contact or p-type character.

  4. Charge carrier separation in modulation doped coaxial semiconductor nanowires.

    PubMed

    Nduwimana, A; Wang, Xiao-Qian

    2009-01-01

    Complementary doped donor and acceptor dipoles effectively generate confinement potentials for carriers across a p-type/intrinsic/n-type coaxial nanowire due to the lineup of charge neutrality level. In order to verify this physical picture, we employ first-principles density functional theory to study the confinement of electrons and holes in complementary boron (p-type) and phosphorus (n-type) doped coaxial silicon nanowires. An analysis of the charge density distributions reveals that the electrons and holes are spatially separated in core and outer shell regions, respectively, in conformity with a type-II lineup of band structures.

  5. Magnetic modulation doping in topological insulators toward higher-temperature quantum anomalous Hall effect

    SciTech Connect

    Mogi, M. Yoshimi, R.; Yasuda, K.; Kozuka, Y.; Tsukazaki, A.; Takahashi, K. S.; Kawasaki, M.; Tokura, Y.

    2015-11-02

    Quantum anomalous Hall effect (QAHE), which generates dissipation-less edge current without external magnetic field, is observed in magnetic-ion doped topological insulators (TIs) such as Cr- and V-doped (Bi,Sb){sub 2}Te{sub 3}. The QAHE emerges when the Fermi level is inside the magnetically induced gap around the original Dirac point of the TI surface state. Although the size of gap is reported to be about 50 meV, the observable temperature of QAHE has been limited below 300 mK. We attempt magnetic-Cr modulation doping into topological insulator (Bi,Sb){sub 2}Te{sub 3} films to increase the observable temperature of QAHE. By introducing the rich-Cr-doped thin (1 nm) layers at the vicinity of both the surfaces based on non-Cr-doped (Bi,Sb){sub 2}Te{sub 3} films, we have succeeded in observing the QAHE up to 2 K. The improvement in the observable temperature achieved by this modulation-doping appears to be originating from the suppression of the disorder in the surface state interacting with the rich magnetic moments. Such a superlattice designing of the stabilized QAHE may pave a way to dissipation-less electronics based on the higher-temperature and zero magnetic-field quantum conduction.

  6. Quantum confinement of crystalline silicon nanotubes with nonuniform wall thickness: Implication to modulation doping

    NASA Astrophysics Data System (ADS)

    Yan, Binghai; Zhou, Gang; Zeng, Xiao Cheng; Wu, Jian; Gu, Bing-Lin; Duan, Wenhui

    2007-09-01

    First-principles calculations of crystalline silicon nanotubes (SiNTs) show that nonuniformity in wall thickness can cause sizable variation in the band gap as well as notable shift in the optical absorption spectrum. A unique quantum confinement behavior is observed: the electronic wave functions of the valence band maximum and conduction band minimum are due mainly to atoms located in the thicker side of the tube wall. This is advantageous to spatially separate the doping impurities from the conducting channel in doped SiNTs. Practically, the performance of the SiNT-based transistors may be substantially improved by selective p /n doping in the thinner side of the tube wall in the spirit of modulation doping.

  7. Gating versus doping: Quality parameters of two-dimensional electron systems in undoped and doped GaAs/AlGaAs heterostructures

    NASA Astrophysics Data System (ADS)

    Peters, S.; Tiemann, L.; Reichl, C.; Wegscheider, W.

    2016-07-01

    We present an experimental study of the scattering mechanisms in a two-dimensional electron system which is either fully induced by the field effect or resulting from remote doping. The quality criteria—the electron mobility, the quantum scattering time, and the number and development of certain fractional quantum Hall states—are analyzed and compared. By eliminating the scattering off remote ionized impurities (RI) in undoped systems, we can identify the density regimes most susceptible to RI scattering and their impact on the formation of fractional quantum Hall states.

  8. Gate-tunable high mobility remote-doped InSb/In1-xAlxSb quantum well heterostructures

    NASA Astrophysics Data System (ADS)

    Yi, Wei; Kiselev, Andrey A.; Thorp, Jacob; Noah, Ramsey; Nguyen, Binh-Minh; Bui, Steven; Rajavel, Rajesh D.; Hussain, Tahir; Gyure, Mark F.; Kratz, Philip; Qian, Qi; Manfra, Michael J.; Pribiag, Vlad S.; Kouwenhoven, Leo P.; Marcus, Charles M.; Sokolich, Marko

    2015-04-01

    Gate-tunable high-mobility InSb/In1-xAlxSb quantum wells (QWs) grown on GaAs substrates are reported. The QW two-dimensional electron gas (2DEG) channel mobility in excess of 200 000 cm2/V s is measured at T = 1.8 K. In asymmetrically remote-doped samples with an HfO2 gate dielectric formed by atomic layer deposition, parallel conduction is eliminated and complete 2DEG channel depletion is reached with minimal hysteresis in gate bias response of the 2DEG electron density. The integer quantum Hall effect with Landau level filling factor down to 1 is observed. A high-transparency non-alloyed Ohmic contact to the 2DEG with contact resistance below 1 Ω.mm is achieved at 1.8 K.

  9. Refractive index modulation in polymer film doped with diazo Meldrum's acid

    NASA Astrophysics Data System (ADS)

    Zanutta, Alessio; Villa, Filippo; Bertarelli, Chiara; Bianco, Andrea

    2016-08-01

    Diazo Meldrum's acid undergoes a photoreaction induced by UV light and it is used as photosensitizer in photoresists. Upon photoreaction, a change in refractive index occurs, which makes this system interesting for volume holography. We report on the sublimation effect at room temperature and the effect of photoirradiation on the refractive index in thin films of CAB (Cellulose acetate butyrate) doped with different amount of diazo Meldrum's acid. A net modulation of the refractive index of 0.01 is achieved with 40% of doping ratio together with a reduction of the film thickness.

  10. Resonant tunneling modulation in quasi-2D Cu2O/SnO2 p-n horizontal-multi-layer heterostructure for room temperature H2S sensor application

    PubMed Central

    Cui, Guangliang; Zhang, Mingzhe; Zou, Guangtian

    2013-01-01

    Heterostructure material that acts as resonant tunneling system is a major scientific challenge in applied physics. Herein, we report a resonant tunneling system, quasi-2D Cu2O/SnO2 p-n heterostructure multi-layer film, prepared by electrochemical deposition in a quasi-2D ultra-thin liquid layer. By applying a special half-sine deposition potential across the electrodes, Cu2O and SnO2 selectively and periodically deposited according to their reduction potentials. The as-prepared heterostructure film displays excellent sensitivity to H2S at room temperature due to the resonant tunneling modulation. Furthermore, it is found that the laser illumination could enhance the gas response, and the mechanism with laser illumination is discussed. It is the first report on gas sensing application of resonant tunneling modulation. Hence, heterostructure material act as resonant tunneling system is believed to be an ideal candidate for further improvement of room temperature gas sensing. PMID:23409241

  11. Efficient and Hysteresis-Free Field Effect Modulation of Ambipolarly Doped Vanadium Dioxide Nanowires

    NASA Astrophysics Data System (ADS)

    Peng, Xingyue; Yang, Yiming; Hou, Yasen; Travaglini, Henry C.; Hellwig, Luke; Hihath, Sahar; van Benthem, Klaus; Lee, Kathleen; Liu, Weifeng; Yu, Dong

    2016-05-01

    The subpicosecond metal-insulator phase transition in vanadium dioxide (VO2 ) has attracted extensive attention with potential applications in ultrafast Mott transistors, which are based on electric-field-induced phase transition. However, the development of VO2 -based transistors lags behind, owing to inefficient and hysteretic gate modulation. Here we report ambipolar doping and strong field effects free of hysteresis in single-crystal VO2 nanowires synthesized via catalyst-free chemical vapor deposition. The ambipolarly doped VO2 nanowires are achieved by controlling the oxygen vacancy density during the synthesis and show strong gate effects because of their relatively low doping level. Both the doping type of the nanowires and the band-bending direction at the metal-insulator domain walls are reversibly switched by electrochemical gating, as revealed by scanning photocurrent microscopy. Furthermore, we eliminate the hysteresis in gate sweep via a hybrid gating method, which combines the merits of liquid-ionic and solid gating. The capability of efficient field effect modulation of ambipolar conduction and band alignment offers opportunities on understanding the phase transition mechanism and enables electronic applications based on VO2 .

  12. Impact of the modulation doping layer on the ν = 5/2 anisotropy

    DOE PAGES

    Shi, X.; Pan, W.; Baldwin, K. W.; West, K. W.; Pfeiffer, L. N.; Tsui, D. C.

    2015-03-30

    We have carried out a systematic study of the tilted magnetic field induced anisotropy at the Landau level filling factor ν = 5/2 in a series of high quality GaAs quantum wells, where the setback distance (d) between the modulation doping layer and the GaAs quantum well is varied from 33 to 164 nm. We have observed that in the sample of the smallest d, electronic transport is anisotropic when the in-plane magnetic field (Bip) is parallel to the [1–10] crystallographic direction, but remains more or less isotropic when Bip // [110]. In contrast, in the sample of largest d,more » electronic transport is anisotropic in both crystallographic directions. Lastly, our results clearly show that the modulation doping layer plays an important role in the tilted field induced ν = 5/2 anisotropy.« less

  13. Impact of the modulation doping layer on the ν = 5/2 anisotropy

    SciTech Connect

    Shi, X.; Pan, W.; Baldwin, K. W.; West, K. W.; Pfeiffer, L. N.; Tsui, D. C.

    2015-03-30

    We have carried out a systematic study of the tilted magnetic field induced anisotropy at the Landau level filling factor ν = 5/2 in a series of high quality GaAs quantum wells, where the setback distance (d) between the modulation doping layer and the GaAs quantum well is varied from 33 to 164 nm. We have observed that in the sample of the smallest d, electronic transport is anisotropic when the in-plane magnetic field (Bip) is parallel to the [1–10] crystallographic direction, but remains more or less isotropic when Bip // [110]. In contrast, in the sample of largest d, electronic transport is anisotropic in both crystallographic directions. Lastly, our results clearly show that the modulation doping layer plays an important role in the tilted field induced ν = 5/2 anisotropy.

  14. Intersubband scattering in modulation-doped Si two-dimensional electron gases

    NASA Astrophysics Data System (ADS)

    Su, Yi-Hsin; Li, Jiun-Yun; Rokhinson, Leonid; Sturm, James

    A bilayer of modulation doped two-dimensional electron gas (2DEG) is of great interest to probe Coulomb drag. For bottom-doped Si 2DEGs, impurity scattering due to poor phosphorus (P) turn-off results in low carrier mobility. Here we demonstrate a record-high electron mobility of 470,000 cm2/V-s at 0.3 K in a bottom-doped 2DEG, comparable to that in top-doped structures. The power-law exponent of mobility vs. density was also evaluated for different P turn-off slopes. With fast turn-off, the power is 1.5, indicative of dominant remote doping scattering. The power decreases with slower P turn-off due to the enhanced scattering from the segregated P atoms. Further, for the first time, we report the second subband occupancy and intersubband scattering in a single Si quantum well, supported by the Shubnikov-de Haas oscillation data.

  15. Experimental Performance of a Single-Mode Ytterbium-doped Fiber Ring Laser with Intracavity Modulator

    NASA Technical Reports Server (NTRS)

    Numata, Kenji; Camp, Jordan

    2012-01-01

    We have developed a linearly polarized Ytterbium-doped fiber ring laser with a single longitudinal mode output at 1064 run. A fiber-coupled intracavity phase modulator ensured mode-hop free operation and allowed fast frequency tuning. The fiber laser was locked with high stability to an iodine-stabilized laser, showing a frequency noise suppression of a factor approx 10 (exp 5) at 1 mHz

  16. Factors affecting light-induced excess conductivity in doping-modulated amorphous silicon superlattices

    SciTech Connect

    Su, F.; Levine, S.; Vanier, P.E.; Kampas, F.J.

    1986-03-15

    Doping-modulated amorphous silicon semiconducting films which exhibit the phenomenon of light-induced excess conductivity (LEC) have been made by silane glow discharge in a single-chamber system. This phenomenon shows a strong dependence on substrate temperature and process gas composition. The LEC effect decreases for very small and very large layer thickness. There also seems to be an optimum defect density for producing large effects.

  17. The design of Cu-doped ZnO thermoelectric module (simulation study)

    NASA Astrophysics Data System (ADS)

    Hadi, Syamsul; Suratwan, Agus; Kurniawan, Agus; Budiana, Eko Prasetya; Suyitno

    2016-03-01

    The p-type semiconductor of Cu-doped ZnO-based thermoelectric material has already been synthesized and studied as an energy harvester. The next challenge is manufacturing the thermoelectric module in the development of thermoelectric as an eco-friendly material in the future. This research aims to investigate the effect of thermoelectric geometric design on the electrical output power and voltage and to recommend the most appropriate thermoelectric geometric design. The design of thermoelectric generator (TEG) includes the determinations of dimension (width, length, and height), number of modules, and semiconductor materials. The simulation used the coupled-field analysis of ANSYS APDL 14.5 in the steady state condition. The p- and n- type thermoelectric material used Cu-doped ZnO and Al-doped ZnO, respectively. The width of element and the number of thermoelectric module were varied to obtain a thermoelectric design, which produces the largest current, power, and voltage. The result of research shows that the t hermoelectric generator with the element widths of 0.94 mm, 1.125 mm, 1.05 mm, and 1.2 mm generates the largest power output and voltage, namely: 0.32 W and 0.89 V, 0.38 W and 0.98 V, 0.45 W and 1.06 V, and 0.52 W and 1.13 V, respectively.

  18. Electric-field modulation of photoinduced effect in phase-separated Pr0.65 (Ca0.75Sr0.25)0.35MnO3/PMN-PT heterostructure

    NASA Astrophysics Data System (ADS)

    Li, W.; Dong, X. L.; Wang, S. H.; Jin, K. X.

    2016-08-01

    In this letter, we report the photoinduced effect modulated by different electric fields in the Pr0.65 (Ca0.75Sr0.25)0.35MnO3/0.7PbMg1/3Nb2/3O3-0.3PbTiO3 heterostructure. The film exhibits a decrease in the resistance up to five orders of magnitude by enhancing applied electric fields, combined with an electric-field-induced insulator-to-metal transition. More interestingly, a reversible bistability arises in the photoinduced change in resistance at T < 80 K as the voltages are increased. The results can be attributed to the phase separation in manganites, which provides a prototype of photoelectric conversion for electric-field modulation of all-oxide heterostructures.

  19. Tunable Electrical and Optical Characteristics in Monolayer Graphene and Few-Layer MoS2 Heterostructure Devices.

    PubMed

    Rathi, Servin; Lee, Inyeal; Lim, Dongsuk; Wang, Jianwei; Ochiai, Yuichi; Aoki, Nobuyuki; Watanabe, Kenji; Taniguchi, Takashi; Lee, Gwan-Hyoung; Yu, Young-Jun; Kim, Philip; Kim, Gil-Ho

    2015-08-12

    Lateral and vertical two-dimensional heterostructure devices, in particular graphene-MoS2, have attracted profound interest as they offer additional functionalities over normal two-dimensional devices. Here, we have carried out electrical and optical characterization of graphene-MoS2 heterostructure. The few-layer MoS2 devices with metal electrode at one end and monolayer graphene electrode at the other end show nonlinearity in drain current with drain voltage sweep due to asymmetrical Schottky barrier height at the contacts and can be modulated with an external gate field. The doping effect of MoS2 on graphene was observed as double Dirac points in the transfer characteristics of the graphene field-effect transistor (FET) with a few-layer MoS2 overlapping the middle part of the channel, whereas the underlapping of graphene have negligible effect on MoS2 FET characteristics, which showed typical n-type behavior. The heterostructure also exhibits a strongest optical response for 520 nm wavelength, which decreases with higher wavelengths. Another distinct feature observed in the heterostructure is the peak in the photocurrent around zero gate voltage. This peak is distinguished from conventional MoS2 FETs, which show a continuous increase in photocurrent with back-gate voltage. These results offer significant insight and further enhance the understanding of the graphene-MoS2 heterostructure.

  20. Self-Assembly-Induced Alternately Stacked Single-Layer MoS2 and N-doped Graphene: A Novel van der Waals Heterostructure for Lithium-Ion Batteries.

    PubMed

    Zhao, Chenyang; Wang, Xu; Kong, Junhua; Ang, Jia Ming; Lee, Pooi See; Liu, Zhaolin; Lu, Xuehong

    2016-01-27

    In this article, a simple self-assembly strategy for fabricating van der Waals heterostructures from isolated two-dimensional atomic crystals is presented. Specifically, dopamine (DOPA), an excellent self-assembly agent and carbon precursor, was adsorbed on exfoliated MoS2 monolayers through electrostatic interaction, and the surface-modified monolayers self-assembled spontaneously into DOPA-intercalated MoS2. The subsequent in situ conversion of DOPA to highly conductive nitrogen-doped graphene (NDG) in the interlayer space of MoS2 led to the formation of a novel NDG/MoS2 nanocomposite with well-defined alternating structure. The NDG/MoS2 was then studied as an anode for lithium-ion batteries (LIBs). The results show that alternating arrangement of NDG and MoS2 triggers synergistic effect between the two components. The kinetics and cycle life of the anode are greatly improved due to the enhanced electron and Li(+) transport as well as the effective immobilization of soluble polysulfide by NDG. A reversible capacity of more than 460 mAh/g could be delivered even at 5 A/g. Moreover, the abundant voids created at the MoS2-NDG interface also accommodate the volume change during cycling and provide additional active sites for Li(+) storage. These endow the NDG/MoS2 heterostructure with low charge-transfer resistance, high sulfur reservation, and structural robustness, rendering it an advanced anode material for LIBs.

  1. Surface transfer doping induced effective modulation on ambipolar characteristics of few-layer black phosphorus.

    PubMed

    Xiang, Du; Han, Cheng; Wu, Jing; Zhong, Shu; Liu, Yiyang; Lin, Jiadan; Zhang, Xue-Ao; Ping Hu, Wen; Özyilmaz, Barbaros; Neto, A H Castro; Wee, Andrew Thye Shen; Chen, Wei

    2015-03-12

    Black phosphorus, a fast emerging two-dimensional material, has been configured as field effect transistors, showing a hole-transport-dominated ambipolar characteristic. Here we report an effective modulation on ambipolar characteristics of few-layer black phosphorus transistors through in situ surface functionalization with caesium carbonate (Cs2CO3) and molybdenum trioxide (MoO3), respectively. Cs2CO3 is found to strongly electron dope black phosphorus. The electron mobility of black phosphorus is significantly enhanced to ~27 cm(2) V(-1) s(-1) after 10 nm Cs2CO3 modification, indicating a greatly improved electron-transport behaviour. In contrast, MoO3 decoration demonstrates a giant hole-doping effect. In situ photoelectron spectroscopy characterization reveals significant surface charge transfer occurring at the dopants/black phosphorus interfaces. Moreover, the surface-doped black phosphorus devices exhibit a largely enhanced photodetection behaviour. Our findings coupled with the tunable nature of the surface transfer doping scheme ensure black phosphorus as a promising candidate for further complementary logic electronics.

  2. Surface transfer doping induced effective modulation on ambipolar characteristics of few-layer black phosphorus

    NASA Astrophysics Data System (ADS)

    Xiang, Du; Han, Cheng; Wu, Jing; Zhong, Shu; Liu, Yiyang; Lin, Jiadan; Zhang, Xue-Ao; Ping Hu, Wen; Özyilmaz, Barbaros; Neto, A. H. Castro; Wee, Andrew Thye Shen; Chen, Wei

    2015-03-01

    Black phosphorus, a fast emerging two-dimensional material, has been configured as field effect transistors, showing a hole-transport-dominated ambipolar characteristic. Here we report an effective modulation on ambipolar characteristics of few-layer black phosphorus transistors through in situ surface functionalization with caesium carbonate (Cs2CO3) and molybdenum trioxide (MoO3), respectively. Cs2CO3 is found to strongly electron dope black phosphorus. The electron mobility of black phosphorus is significantly enhanced to ~27 cm2 V-1 s-1 after 10 nm Cs2CO3 modification, indicating a greatly improved electron-transport behaviour. In contrast, MoO3 decoration demonstrates a giant hole-doping effect. In situ photoelectron spectroscopy characterization reveals significant surface charge transfer occurring at the dopants/black phosphorus interfaces. Moreover, the surface-doped black phosphorus devices exhibit a largely enhanced photodetection behaviour. Our findings coupled with the tunable nature of the surface transfer doping scheme ensure black phosphorus as a promising candidate for further complementary logic electronics.

  3. Axial Ge/Si nanowire heterostructure tunnel FETs.

    SciTech Connect

    Dayeh, Shadi A.; Gin, Aaron V.; Huang, Jian Yu; Picraux, Samuel Thomas

    2010-03-01

    Axial Ge/Si heterostructure nanowires (NWs) allow energy band-edge engineering along the axis of the NW, which is the charge transport direction, and the realization of asymmetric devices for novel device architectures. This work reports on two significant advances in the area of heterostructure NWs and tunnel FETs: (i) the realization of 100% compositionally modulated Si/Ge axial heterostructure NWs with lengths suitable for device fabrication and (ii) the design and implementation of Schottky barrier tunnel FETs on these NWs for high-on currents and suppressed ambipolar behavior. Initial prototype devices with 10 nm PECVD SiN{sub x} gate dielectric resulted in a very high current drive in excess of 100 {micro}A/{micro}m (I/{pi}D) and 10{sup 5} I{sub on}/I{sub off} ratios. Prior work on the synthesis of Ge/Si axial NW heterostructures through the VLS mechanism have resulted in axial Si/Si{sub 1-x}Ge{sub x} NW heterostructures with x{sub max} {approx} 0.3, and more recently 100% composition modulation was achieved with a solid growth catalyst. In this latter case, the thickness of the heterostructure cannot exceed few atomic layers due to the slow axial growth rate and concurrent radial deposition on the NW sidewalls leading to a mixture of axial and radial deposition, which imposes a big challenge for fabricating useful devices form these NWs in the near future. Here, we report the VLS growth of 100% doping and composition modulated axial Ge/Si heterostructure NWs with lengths appropriate for device fabrication by devising a growth procedure that eliminates Au diffusion on the NW sidewalls and minimizes random kinking in the heterostructure NWs as deduced from detailed microscopy analysis. Fig. 1 a shows a cross-sectional SEM image of epitaxial Ge/Si axial NW heterostructures grown on a Ge(111) surface. The interface abruptness in these Ge/Si heterostructure NWs is of the order of the NW diameter. Some of these NWs develop a crystallographic kink that is {approx

  4. Coherent atomic and electronic heterostructures of single-layer MoS2.

    PubMed

    Eda, Goki; Fujita, Takeshi; Yamaguchi, Hisato; Voiry, Damien; Chen, Mingwei; Chhowalla, Manish

    2012-08-28

    Nanoscale heterostructures with quantum dots, nanowires, and nanosheets have opened up new routes toward advanced functionalities and implementation of novel electronic and photonic devices in reduced dimensions. Coherent and passivated heterointerfaces between electronically dissimilar materials can be typically achieved through composition or doping modulation as in GaAs/AlGaAs and Si/NiSi or heteroepitaxy of lattice matched but chemically distinct compounds. Here we report that single layers of chemically exfoliated MoS(2) consist of electronically dissimilar polymorphs that are lattice matched such that they form chemically homogeneous atomic and electronic heterostructures. High resolution scanning transmission electron microscope (STEM) imaging reveals the coexistence of metallic and semiconducting phases within the chemically homogeneous two-dimensional (2D) MoS(2) nanosheets. These results suggest potential for exploiting molecular scale electronic device designs in atomically thin 2D layers.

  5. Surface plasmons on a doped graphene sheet with periodically modulated conductivity

    NASA Astrophysics Data System (ADS)

    Ben Rhouma, M.; Oueslati, M.; Guizal, B.

    2016-08-01

    We propose a model taking into account the periodic spatial modulation of a doped graphene sheet conductivity when it is placed in the vicinity of a dielectric grating. We then compute the absorption and study the excitation of surface plasmons on such a structure. We show that it is possible to excite surface modes leading to very high absorption. Our findings could be of interest in the design of graphene-based plasmonic devices and sensors working in the infra-red and the terahertz regions of the electromagnetic spectrum.

  6. Ultrafast terahertz modulation characteristic of tungsten doped vanadium dioxide nanogranular film revealed by time-resolved terahertz spectroscopy

    SciTech Connect

    Xiao, Yang; Zhai, Zhao-Hui; Zhu, Li-Guo E-mail: huangwanxia@scu.edu.cn; Li, Jun; Peng, Qi-Xian; Li, Ze-Ren; Shi, Qi-Wu; Huang, Wan-Xia E-mail: huangwanxia@scu.edu.cn; Yue, Fang; Hu, Yan-Yan

    2015-07-20

    The ultrafast terahertz (THz) modulation characteristic during photo-induced insulator-to-metal transition (IMT) of undoped and tungsten (W)-doped VO{sub 2} film was investigated at picoseconds time scale using time-resolved THz spectroscopy. W-doping slows down the photo-induced IMT dynamic processes (both the fast non-thermal process and the slow metallic phase propagation process) in VO{sub 2} film and also reduces the pump fluence threshold of photo-induced IMT in VO{sub 2} film. Along with the observed broadening of phase transition temperature window of IMT in W-doped VO{sub 2}, we conclude that W-doping prevents metallic phase domains from percolation. By further extracting carrier properties from photo-induced THz conductivity at several phase transition times, we found that the electron-electron correlation during IMT is enhanced in W-doped VO{sub 2}.

  7. Metalorganic chemical vapor phase epitaxy of narrow-band distributed Bragg reflectors realized by GaN:Ge modulation doping

    NASA Astrophysics Data System (ADS)

    Berger, Christoph; Lesnik, Andreas; Zettler, Thomas; Schmidt, Gordon; Veit, Peter; Dadgar, Armin; Bläsing, Jürgen; Christen, Jürgen; Strittmatter, André

    2016-04-01

    We report on metalorganic vapor phase epitaxy (MOVPE) of distributed Bragg reflectors (DBR) applying a periodic modulation of the GaN doping concentration only. The doping modulation changes the refractive index of GaN via the Burstein-Moss-effect. MOVPE growth of highly doped GaN:Ge and modulation of the dopant concentration by at least two orders of magnitude within few nanometers is required to achieve a refractive index contrast of 2-3%. Such modulation characteristic is achieved despite the presence of Ge memory effects and incorporation delay. We realized DBRs with up to 100 layer pairs by combining GaN:Ge with a nominal doping concentration of 1.6×1020 cm-3 as low-refractive index material with unintentionally doped GaN as high-refractive index layer. Scanning transmission electron microscope images reveal DBR structures with abrupt interfaces and homogenous layer thicknesses in lateral and vertical direction. Reflectance measurements of DBRs designed for the blue and near UV-spectral region show a narrow stopband with a maximum reflectivity of 85% at 418 nm and even 95% at 370 nm. InGaN/GaN multi-quantum well structures grown on top of such DBRs exhibit narrow emission spectra with linewidths below 3 nm and significantly increased emission intensity.

  8. Different pulse pattern generation by frequency detuning in pulse modulated actively mode-locked ytterbium doped fiber laser

    NASA Astrophysics Data System (ADS)

    Chen, He; Chen, Sheng-Ping; Si, Lei; Zhang, Bin; Jiang, Zong-Fu

    2015-10-01

    We report the results of our recent experimental investigation of the modulation frequency detuning effect on the output pulse dynamics in a pulse modulated actively mode-locked ytterbium doped fiber laser. The experimental study shows the existence of five different mode-locking states that mainly depend on the modulation frequency detuning, which are: (a) amplitude-even harmonic/fundamental mode-locking, (b) Q-switched harmonic/fundamental mode-locking, (c) sinusoidal wave modulation mode, (d) pulses bundle state, and (e) noise-like state. A detailed experimental characterization of the output pulses dynamics in each operating mode is presented.

  9. Design of lateral heterostructure from arsenene and antimonene

    NASA Astrophysics Data System (ADS)

    Sun, Qilong; Dai, Ying; Ma, Yandong; Yin, Na; Wei, Wei; Yu, Lin; Huang, Baibiao

    2016-09-01

    Lateral heterostructures fabricated by two-dimensional building blocks have opened up exciting realms in material science and device physics. Identifying suitable materials for creating such heterostructures is urgently needed for the next-generation devices. Here, we demonstrate a novel type of seamless lateral heterostructures with excellent stabilities formed within pristine arsenene and antimonene. We find that these heterostructures could possess direct and reduced energy gaps without any modulations. Moreover, the highly coveted type-II alignment and the high carrier mobility are also identified, marking the enhanced quantum efficiency. The tensile strain can result in efficient bandgap engineering. Besides, the proposed critical condition for favored direct energy gaps would have a guiding significance on the subsequent works. Generally, our predictions not only introduce new vitality into lateral heterostructures, enriching available candidate materials in this field, but also highlight the potential of these lateral heterostructures as appealing materials for future devices.

  10. Ytterbium-doped fiber ultrashort pulse generator based on self-phase modulation and alternating spectral filtering.

    PubMed

    Regelskis, Kęstutis; Želudevičius, Julijanas; Viskontas, Karolis; Račiukaitis, Gediminas

    2015-11-15

    We numerically and experimentally demonstrate a Yb-doped fiber ultrashort pulse generator based on self-phase modulation and alternating spectral filtering, operating at a wavelength of 1060 nm and providing a stable ultrashort pulse train. Pulses with energy up to 2.8 nJ were generated experimentally and were limited only by available pump power. PMID:26565848

  11. Isoelectronic tungsten doping in monolayer MoSe2 for carrier type modulation

    DOE PAGES

    Li, Xufan; Lin, Ming -Wei; Basile, Leonardo; Hus, Saban M.; Puretzky, Alexander A.; Lee, Jaekwang; Kuo, Yen -Chien; Chang, Lo -Yueh; Wang, Kai; Idrobo, Juan C.; et al

    2016-07-06

    Doping and alloying are effective ways to engineer the band structure and modulate the optoelectronic functionality of monolayer transition metal dichalcogenides (TMDs). In this work, we explore the synthesis and electronic properties of monolayer Mo1-xWxSe2 (0 < x < 0.18) alloys with almost 100% alloying degree. The isoelectronic substitutional doping of tungsten for molybdenum in the monolayer MoSe2 is shown to suppress its intrinsically n-type conduction behavior, with p-type conduction gradually emerging to become dominant with increasing W concentration in the alloys. Atomic resolution Z-contrast electron microscopy show that W is shown to substitute directly for Mo without the introductionmore » of noticeable vacancy or interstitial defects, however with randomly-distributed W-rich regions ~2 nm in diameter. Scanning tunneling microscopy/spectroscopy measurements reveal that these W-rich regions exhibit a local band structure with the valence band maximum (VBM) closer to the Fermi level as compared with the Mo-rich regions in the monolayer Mo1-xWxSe2 crystal. These localized upshifts of the VBM in the local band structure appear responsible for the overall p-type behavior observed for the monolayer Mo1-xWxSe2 crystals. Stacked monolayers of n-type MoSe2 and p-type Mo1-xWxSe2 were demonstrated to form atomically thin, vertically stacked p n homojunctions with gate-tunable characteristics, which appear useful for future optoelectronic applications. Lastly, these results indicate that alloying with isoelectronic dopant atoms appears to be an effective and advantageous alternate strategy to doping or alloying with electron donors or acceptors in two-dimensional TMDs.« less

  12. Characterization of Si (sub X)Ge (sub 1-x)/Si Heterostructures for Device Applications Using Spectroscopic Ellipsometry

    NASA Technical Reports Server (NTRS)

    Sieg, R. M.; Alterovitz, S. A.; Croke, E. T.; Harrell, M. J.; Tanner, M.; Wang, K. L.; Mena, R. A.; Young, P. G.

    1993-01-01

    Spectroscopic ellipsometry (SE) characterization of several complex Si (sub X)Ge (sub 1-x)/Si heterostructures prepared for device fabrication, including structures for heterojunction bipolar transistors (HBT), p-type and n-type heterostructure modulation doped field effect transistors, has been performed. We have shown that SE can simultaneously determine all active layer thicknesses, Si (sub X)Ge (sub 1-x) compositions, and the oxide overlayer thickness, with only a general knowledge of the structure topology needed a priori. The characterization of HBT material included the SE analysis of a Si (sub X)Ge (sub 1-x) layer deeply buried (600 nanometers) under the silicon emitter and cap layers. In the SE analysis of n-type heterostructures, we examined for the first time a silicon layer under tensile strain. We found that an excellent fit can be obtained using optical constants of unstrained silicon to represent the strained silicon conduction layer. We also used SE to measure lateral sample homogeneity, providing quantitative identification of the inhomogeneous layer. Surface overlayers resulting from prior sample processing were also detected and measured quantitatively. These results should allow SE to be used extensively as a non-destructive means of characterizing Si (sub X)Ge (sub 1-x)/Si heterostructures prior to device fabrication and testing.

  13. Dynamic model of pulsed laser generators based on multi-junction N-p-N-i-P heterostructures

    NASA Astrophysics Data System (ADS)

    Slipchenko, Sergey; Podoskin, Alexsandr; Soboleva, Olga; Pikhtin, Nikita; Tarasov, Il'ya; Yuferev, Valentin

    2016-03-01

    This communication presents a dynamic model of a multi-junction heterostructure that combines the functions of a fast current switch and a high-efficiency laser emitter. Approaches to designing a multi-junction heterostructure with faster switching (rise and decay times of about 1 ns) and higher peak current (>10 A) are considered. It is shown that an important role is played in the dynamics of the injection drive currents of the laser part by the modulation by excess carriers in the lightly doped base and collector regions of the N-p-N transistor part. As a result, a field domain is formed, which serves as a virtual emitter of electrons and holes via impact ionization.

  14. Mobility modulation in inverted delta doped coupled double quantum well structure

    NASA Astrophysics Data System (ADS)

    Sahoo, N.; Sahu, T.

    2016-10-01

    We have studied the modulation of electron mobility μ as a function of the electric field perpendicular to the interface plane Fp in a GaAs/AlGaAs double quantum well structure near the resonance of subband states. The functional dependence of μ on Fp exhibits a minimum near the anticrossing of subband states leading to an oscillatory behavior of μ. We show that the oscillatory enhancement of μ becomes more pronounced with increase in the difference between the doping concentrations in the side barriers. The oscillation of μ also increases by varying the widths of the two wells through shifting of the position of the middle barrier. It is interesting to show that the oscillation of μ is always larger when there is doping in barrier towards the substrate side compared to that of the surface side due to the difference in the influence of the interface roughness scattering potential. Further, broadening of the central barrier width increases the peaks of the oscillation of μ mostly due to the changes in the ionized impurity scattering potential. Our results can be utilized for the performance enhancement of quantum well field effect transistor devices.

  15. Periodic Modulation of the Doping Level in Striped MoS₂ Superstructures.

    PubMed

    Zhou, Xiebo; Shi, Jianping; Qi, Yue; Liu, Mengxi; Ma, Donglin; Zhang, Yu; Ji, Qingqing; Zhang, Zhepeng; Li, Cong; Liu, Zhongfan; Zhang, Yanfeng

    2016-03-22

    Although the recently discovered monolayer transition metal dichalcogenides exhibit novel electronic and optical properties, fundamental physical issues such as the quasiparticle bandgap tunability and the substrate effects remain undefined. Herein, we present the report of a quasi-one-dimensional periodically striped superstructure for monolayer MoS2 on Au(100). The formation of the unique striped superstructure is found to be mainly modulated by the symmetry difference between MoS2 and Au(100) and their lattice mismatch. More intriguingly, we find that the monolayer MoS2 is heavily n-doped on the Au(100) facet with a bandgap of 1.3 eV, and the Fermi level is upshifted by ∼0.10 eV on the ridge (∼0.2 eV below the conduction band) in contrast to the valley regions (∼0.3 eV below the conduction band) of the striped patterns after high-temperature sample annealing process. This tunable doping effect is considered to be caused by the different defect densities over the ridge/valley regions of the superstructure. Additionally, an obvious bandgap reduction is observed in the vicinity of the domain boundary for monolayer MoS2 on Au(100). This work should therefore inspire intensive explorations of adlayer-substrate interactions, the defects, and their effects on band-structure engineering of monolayer MoS2. PMID:26913990

  16. Gibbs-Thomson Effect in Planar Nanowires: Orientation and Doping Modulated Growth.

    PubMed

    Shen, Youde; Chen, Renjie; Yu, Xuechao; Wang, Qijie; Jungjohann, Katherine L; Dayeh, Shadi A; Wu, Tom

    2016-07-13

    Epitaxy-enabled bottom-up synthesis of self-assembled planar nanowires via the vapor-liquid-solid mechanism is an emerging and promising approach toward large-scale direct integration of nanowire-based devices without postgrowth alignment. Here, by examining large assemblies of indium tin oxide nanowires on yttria-stabilized zirconia substrate, we demonstrate for the first time that the growth dynamics of planar nanowires follows a modified version of the Gibbs-Thomson mechanism, which has been known for the past decades to govern the correlations between thermodynamic supersaturation, growth speed, and nanowire morphology. Furthermore, the substrate orientation strongly influences the growth characteristics of epitaxial planar nanowires as opposed to impact at only the initial nucleation stage in the growth of vertical nanowires. The rich nanowire morphology can be described by a surface-energy-dependent growth model within the Gibbs-Thomson framework, which is further modulated by the tin doping concentration. Our experiments also reveal that the cutoff nanowire diameter depends on the substrate orientation and decreases with increasing tin doping concentration. These results enable a deeper understanding and control over the growth of planar nanowires, and the insights will help advance the fabrication of self-assembled nanowire devices. PMID:27254592

  17. Photoluminescence in modulation-doped GaAs/Ga1-xAlxAs heterojunctions

    NASA Astrophysics Data System (ADS)

    Shen, J. X.; Oka, Y.; Hu, C. Y.; Ossau, W.; Landwehr, G.; Friedland, K.-J.; Hey, R.; Ploog, K.; Weimann, G.

    1999-03-01

    We investigated the physical properties of modulation-doped GaAs/Ga1-xAlxAs heterojunctions by photoluminescence. We found two types of transitions and attributed them to the space direct excitons in the GaAs flat band region and the space indirect excitons near the heterojunction notch. We propose the vertical transport of photoexcited carriers prior to the actual exciton formation under the influence of the two-dimensional carrier gas. Therefore, the photoluminescence intensities of the two types of excitons are correlated. The vertical transport model explains successfully the experimental results of the optically detected cyclotron resonance, the photoluminescence intensity oscillation with magnetic field and the long rise time of the photoluminescence intensity for the excitonic transitions.

  18. Inversion of spin dependent photocurrent at Fe3O4/modulation doped GaAs heterointerfaces

    NASA Astrophysics Data System (ADS)

    Shirahata, Y.; Wada, E.; Itoh, M.; Taniyama, T.

    2011-04-01

    We demonstrate inversion of the spin dependent photocurrent across an Fe3O4/modulation doped GaAs interface under optical spin orientation condition. The spin dependent photocurrent for fully epitaxial Fe3O4/GaAs and Fe/GaAs interfaces clearly show the opposite magnetic field dependence, where the spin filtering efficiency for the Fe3O4/GaAs decreases with increasing magnetic field. The results clearly indicate that the spin polarization of the Fe3O4 layer has the opposite sign to that of Fe at the Fermi energy, consistent with theoretical predictions, and the result is a consequence of the atomically flat Fe3O4/GaAs interface we obtained.

  19. Direct modulation of an ultra-long doped fiber external cavity semiconductor laser at multiples of the cavity resonant frequency

    NASA Astrophysics Data System (ADS)

    Liu, Runnan; Wu, Ke; Kashyap, Raman

    2007-06-01

    The doped fiber external cavity semiconductor laser (DFECL) has been reported with a simple structure, high power, narrow linewidth, and stable wavelength. The DFECL is mostly suitable to be an optical carrier generator for external modulation or microwave optical generation. Because of mode locking, the DFECL, with saturable absorber in its external cavity, has the possibility to be direct modulated at its multiples of cavity resonant frequency. The useful modulation frequency of the laser can be increased significantly. In this paper, we present experimental results about the transmission response of direct modulation of an ultra-long DFECL, and the modulated microwave signal transmission at the frequency of the 22 nd. multiple of the cavity resonant frequency. Modulated narrow bandwidth microwave signals at 2.4GHz were transmitted by this DFECL. The received RF spectrum has no obvious distortion for a 10MHz narrow band microwave signal and, all the resonant and harmonic frequencies in the 0~2.5GHz region are 50 dB lower than the transmitted wave. The results show that narrowband modulated microwave can be transmitted at high frequency by the long DFECL; even through the cavity round-trip frequency is very low. We conclude that this ultra-long doped fiber external cavity semiconductor laser can be used for narrowband wireless communication with direct modulation.

  20. Effect of doping on the far-infrared intersubband transitions in nonpolar m-plane GaN/AlGaN heterostructures

    NASA Astrophysics Data System (ADS)

    Lim, C. B.; Ajay, A.; Bougerol, C.; Lähnemann, J.; Donatini, F.; Schörmann, J.; Bellet-Amalric, E.; Browne, D. A.; Jiménez-Rodríguez, M.; Monroy, E.

    2016-04-01

    This paper assesses the effects of Si doping on the properties of nonpolar m-plane GaN/AlGaN quantum wells (QWs) designed for intersubband (ISB) absorption in the far-infrared spectral range. For doping levels up to 3 × 1012 cm-2, structural analysis reveals uniform QWs with abrupt interfaces and no epitaxially induced defects. Cathodoluminescence spectroscopy confirms the homogeneity of the multiple QWs along the growth direction. Increasing the doping density in the QWs from 1 × 1011 cm-2 to 3 × 1012 cm-2 induces a broadening of the photoluminescence as well as a reduction of the exciton localization energy in the alloy. Also, enhancement of the ISB absorption is observed, along with a blue shift and widening of the absorption peak. The magnitude of the ISB absorption saturates for doping levels around 1 × 1012 cm-2, and the blue shift and broadening increase less than theoretically predicted for the samples with higher doping levels. This is explained by the presence of free carriers in the excited electron level due to the increase of the Fermi level energy.

  1. Magnesium Doping Controlled Density and Mobility of Two-Dimensional Electron Gas in MgxZn1-xO/ZnO Heterostructures

    NASA Astrophysics Data System (ADS)

    Falson, Joseph; Maryenko, Denis; Kozuka, Yusuke; Tsukazaki, Atsushi; Kawasaki, Masashi

    2011-09-01

    The magnesium content in MgxZn1-xO/ZnO heterostructures grown by molecular beam epitaxy enables the careful control of the carrier density of the two-dimensional electron system down to 5.6×1010 cm-2 while retaining a mobility of 200,000 cm2 V-1 s-1 when pursuing magnesium concentrations as low as x = 0.0038. By selecting an optimum magnesium content (x˜0.01), the mobility is enhanced to over 700,000 cm2 V-1 s-1 due to reduced impurity levels associated with the use of pure distilled ozone and avoiding interface roughness scattering. This control technique allows access to the coherent transport region with strong electron-electron interaction.

  2. Control of ballistic electrons in (AlGa)As/GaAs heterostructures by means of superconducting niobium gate structures

    NASA Astrophysics Data System (ADS)

    Schäpers, T.; Müller, F.; Förster, A.; Lengeler, B.; Lüth, H.

    1994-03-01

    Superconducting gate structures on a modulation doped (AlGa)As/GaAs heterostructure were used to control a ballistic electron beam within the high mobility 2DEG. A local and variable magnetic field was produced by superconducting current loops. The magnetic field induced by the loops was analysed by means of an external magnetic field as well as by an electron interference experiment. The maximum field obtained by two superconducting loops was 0.4 mT. In addition, a gate structure containing a superconducting ring between two opposite split gates was prepared in order to detect magnetic flux quantisation.

  3. Spatial modulation spectroscopy for imaging and quantitative analysis of single dye-doped organic nanoparticles inside cells.

    PubMed

    Devadas, Mary Sajini; Devkota, Tuphan; Guha, Samit; Shaw, Scott K; Smith, Bradley D; Hartland, Gregory V

    2015-06-01

    Imaging of non-fluorescent nanoparticles in complex biological environments, such as the cell cytosol, is a challenging problem. For metal nanoparticles, Rayleigh scattering methods can be used, but for organic nanoparticles, such as dye-doped polymer beads or lipid nanoparticles, light scattering does not provide good contrast. In this paper, spatial modulation spectroscopy (SMS) is used to image single organic nanoparticles doped with non-fluorescent, near-IR croconaine dye. SMS is a quantitative imaging technique that yields the absolute extinction cross-section of the nanoparticles, which can be used to determine the number of dye molecules per particle. SMS images were recorded for particles within EMT-6 breast cancer cells. The measurements allowed mapping of the nanoparticle location and the amount of dye in a single cell. The results demonstrate how SMS can facilitate efforts to optimize dye-doped nanoparticles for effective photothermal therapy of cancer.

  4. Multifunctional Oxide Heterostructures

    SciTech Connect

    Tsymbal, E Y; Dagotto, Elbio R; Eom, Professor Chang-Beom; Ramesh, Ramamoorthy

    2012-01-01

    This book is devoted to the rapidly developing field of oxide thin-films and heterostructures. Oxide materials combined with atomic-scale precision in a heterostructure exhibit an abundance of macroscopic physical properties involving the strong coupling between the electronic, spin, and structural degrees of freedom, and the interplay between magnetism, ferroelectricity, and conductivity. Recent advances in thin-film deposition and characterization techniques made possible the experimental realization of such oxide heterostructures, promising novel functionalities and device concepts.

  5. Spatial modulation spectroscopy for imaging and quantitative analysis of single dye-doped organic nanoparticles inside cells

    NASA Astrophysics Data System (ADS)

    Devadas, Mary Sajini; Devkota, Tuphan; Guha, Samit; Shaw, Scott K.; Smith, Bradley D.; Hartland, Gregory V.

    2015-05-01

    Imaging of non-fluorescent nanoparticles in complex biological environments, such as the cell cytosol, is a challenging problem. For metal nanoparticles, Rayleigh scattering methods can be used, but for organic nanoparticles, such as dye-doped polymer beads or lipid nanoparticles, light scattering does not provide good contrast. In this paper, spatial modulation spectroscopy (SMS) is used to image single organic nanoparticles doped with non-fluorescent, near-IR croconaine dye. SMS is a quantitative imaging technique that yields the absolute extinction cross-section of the nanoparticles, which can be used to determine the number of dye molecules per particle. SMS images were recorded for particles within EMT-6 breast cancer cells. The measurements allowed mapping of the nanoparticle location and the amount of dye in a single cell. The results demonstrate how SMS can facilitate efforts to optimize dye-doped nanoparticles for effective photothermal therapy of cancer.Imaging of non-fluorescent nanoparticles in complex biological environments, such as the cell cytosol, is a challenging problem. For metal nanoparticles, Rayleigh scattering methods can be used, but for organic nanoparticles, such as dye-doped polymer beads or lipid nanoparticles, light scattering does not provide good contrast. In this paper, spatial modulation spectroscopy (SMS) is used to image single organic nanoparticles doped with non-fluorescent, near-IR croconaine dye. SMS is a quantitative imaging technique that yields the absolute extinction cross-section of the nanoparticles, which can be used to determine the number of dye molecules per particle. SMS images were recorded for particles within EMT-6 breast cancer cells. The measurements allowed mapping of the nanoparticle location and the amount of dye in a single cell. The results demonstrate how SMS can facilitate efforts to optimize dye-doped nanoparticles for effective photothermal therapy of cancer. Electronic supplementary information (ESI

  6. The effects of the porous buffer layer and doping with dysprosium on internal stresses in the GaInP:Dy/por-GaAs/GaAs(100) heterostructures

    SciTech Connect

    Seredin, P. V.; Gordienko, N. N.; Glotov, A. V.; Zhurbina, I. A.; Domashevskaya, E. P.; Arsent'ev, I. N. Shishkov, M. V.

    2009-08-15

    In structures with a porous buffer layer, residual internal stresses caused by a mismatch between the crystal-lattice parameters of the epitaxial GaInP alloy and the GaAs substrate are redistributed to the porous layer that acts as a buffer and is conducive to disappearance of internal stresses. Doping of the epitaxial layer with dysprosium exerts a similar effect on the internal stresses in the film-substrate structure.

  7. Charge modulation of magnetization in X-doped MgO nanotube clusters (X=C, N)

    NASA Astrophysics Data System (ADS)

    Wang, Su-Fang; Chen, Li-Yong; Zhang, Tao; Xie, You

    2016-02-01

    First-principles calculations based on density functional theory are performed to study the magnetic and electronic properties of X-doped 8×7 MgO nanotube clusters (X=C, N). The N dopant easily occupies the O-site at the edge of MgO nanotube, embracing neutral or charged defect state, and induces notable magnetization in N-doped MgO tubular cluster. More important, this p-electron magnetization can be significantly modulated as the charged state of the defect changes. Regarding C doping, impurity atom readily substitute the Mg atom located at the edge of MgO nanotube to form neutral defect, and net magnetization is found to be zero. The calculated electron densities of states show that the O-site N doping at the edge greatly narrows or even destroys band-gap, while it enlarges somewhat for the Mg-site C doping at the edge. The results are likely to stimulate a promising class of materials for various applications ranging from spintronics to magneto-optics.

  8. Transparent conductivity modulation of ZnO by group-IVA doping

    NASA Astrophysics Data System (ADS)

    Liu, J.; Fan, X. F.; Sun, C. Q.; Zhu, W.

    2016-04-01

    We examined the effect of group-IVA doping on the electronic structure and transmittance of ZnO using first-principle calculations. All these doped ZnO materials are found to perform n-type conductive behavior. Si-doped ZnO and Pb-doped ZnO are found to have larger optical band gap than those of Ge-doped ZnO and Sn-doped ZnO. The transmittance of Si-doped ZnO is found to be high in both UV and visible region. The enhancement of UV region transmittance can be attributed to the enhanced optical band gap, while the reduction of visible region transmittance is due to the intraband optical transition.

  9. Characteristics of InAs/AlGaAs self-organized quantum dot modulation doped field effect transistors

    NASA Astrophysics Data System (ADS)

    Phillips, J.; Kamath, K.; Brock, T.; Bhattacharya, P.

    1998-06-01

    We have investigated the dc characteristics of InGaAs/AlGaAs modulation doped field effect transistors in which a layer of self-organized InAs quantum dots is inserted adjacent to the pseudomorphic quantum well channel. Distinct steps and a negative differential resistance are observed in the current-voltage characteristics at room temperature and lower temperatures. These are attributed to conduction through the bound states in the quantum dots.

  10. Electronic properties of phosphorene/graphene heterostructures: Effect of external electric field

    NASA Astrophysics Data System (ADS)

    Kaur, Sumandeep; Kumar, Ashok; Srivastava, Sunita; Tankeshwar, K.

    2016-05-01

    We report the electronic properties of electrically gated heterostructures of black and blue phosphorene with graphene. The heterostructure of blue phosphorene with graphene is energetically more favorable than black phospherene/graphene. However, both are bonded by weak interlayer interactions. Graphene induces the Dirac cone character in both heterostructure which shows tunabilities with external electric field. It is found that Dirac cone get shifted depending on the polarity of external electric field that results into the so called self induced p-type or n-type doping effect. These features have importance in the fabrication of nano-electronic devices based on the phosphorene/graphene heterostructures.

  11. Anisotropic modulation of magnetic properties and the memory effect in a wide-band (011)-Pr0.7Sr0.3MnO3/PMN-PT heterostructure.

    PubMed

    Zhao, Ying-Ying; Wang, Jing; Kuang, Hao; Hu, Feng-Xia; Liu, Yao; Wu, Rong-Rong; Zhang, Xi-Xiang; Sun, Ji-Rong; Shen, Bao-Gen

    2015-01-01

    Memory effect of electric-field control on magnetic behavior in magnetoelectric composite heterostructures has been a topic of interest for a long time. Although the piezostrain and its transfer across the interface of ferroelectric/ferromagnetic films are known to be important in realizing magnetoelectric coupling, the underlying mechanism for nonvolatile modulation of magnetic behaviors remains a challenge. Here, we report on the electric-field control of magnetic properties in wide-band (011)-Pr0.7Sr0.3MnO3/0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 heterostructures. By introducing an electric-field-induced in-plane anisotropic strain field during the cooling process from room temperature, we observe an in-plane anisotropic, nonvolatile modulation of magnetic properties in a wide-band Pr0.7Sr0.3MnO3 film at low temperatures. We attribute this anisotropic memory effect to the preferential seeding and growth of ferromagnetic (FM) domains under the anisotropic strain field. In addition, we find that the anisotropic, nonvolatile modulation of magnetic properties gradually diminishes as the temperature approaches FM transition, indicating that the nonvolatile memory effect is temperature dependent. By taking into account the competition between thermal energy and the potential barrier of the metastable magnetic state induced by the anisotropic strain field, this distinct memory effect is well explained, which provides a promising approach for designing novel electric-writing magnetic memories. PMID:25909177

  12. Anisotropic modulation of magnetic properties and the memory effect in a wide-band (011)-Pr0.7Sr0.3MnO3/PMN-PT heterostructure

    NASA Astrophysics Data System (ADS)

    Zhao, Ying-Ying; Wang, Jing; Kuang, Hao; Hu, Feng-Xia; Liu, Yao; Wu, Rong-Rong; Zhang, Xi-Xiang; Sun, Ji-Rong; Shen, Bao-Gen

    2015-04-01

    Memory effect of electric-field control on magnetic behavior in magnetoelectric composite heterostructures has been a topic of interest for a long time. Although the piezostrain and its transfer across the interface of ferroelectric/ferromagnetic films are known to be important in realizing magnetoelectric coupling, the underlying mechanism for nonvolatile modulation of magnetic behaviors remains a challenge. Here, we report on the electric-field control of magnetic properties in wide-band (011)-Pr0.7Sr0.3MnO3/0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 heterostructures. By introducing an electric-field-induced in-plane anisotropic strain field during the cooling process from room temperature, we observe an in-plane anisotropic, nonvolatile modulation of magnetic properties in a wide-band Pr0.7Sr0.3MnO3 film at low temperatures. We attribute this anisotropic memory effect to the preferential seeding and growth of ferromagnetic (FM) domains under the anisotropic strain field. In addition, we find that the anisotropic, nonvolatile modulation of magnetic properties gradually diminishes as the temperature approaches FM transition, indicating that the nonvolatile memory effect is temperature dependent. By taking into account the competition between thermal energy and the potential barrier of the metastable magnetic state induced by the anisotropic strain field, this distinct memory effect is well explained, which provides a promising approach for designing novel electric-writing magnetic memories.

  13. Electrochemically modulated liquid chromatography using a boron-doped diamond particle stationary phase.

    PubMed

    Muna, Grace W; Swope, Vernon M; Swain, Greg M; Porter, Marc D

    2008-11-14

    This paper reports on preliminary tests of the performance of boron-doped diamond powder (BDDP) as a stationary phase in electrochemically modulated liquid chromatography (EMLC). EMLC manipulates retention through changes in the potential applied (E(appl)) to a conductive packing. Porous graphitic carbon (PGC) has routinely been utilized as a material in EMLC separations. Herein the utility of BDDP as a stationary phase in EMLC was investigated and its stability, both compositionally and microstructurally, relative to PGC was compared. The results show that BDDP is stable over a wide range of E(appl) values (i.e., -1.2 to +1.2V vs. Ag/AgCl, sat'd NaCl). The data also reveal that electrostatics play a key role in the adsorption of the aromatic sulfonates on the BDDP stationary phase, and that these analytes are more weakly retained in comparison to the PGC support. The potential for this methodology to provide a means to advance the understanding of molecular adsorption and retention mechanisms on carbonaceous materials is briefly discussed.

  14. Parameters affecting light-induced excess conductivity in amorphous silicon doping-modulated multilayers

    SciTech Connect

    Su, F.C.; Levine, S.; Vanier, P.E.

    1986-01-01

    The phenomenon of light-induced excess conductivity (LEC) which occurs in a-Si:H npnp doping-modulated multilayers is found experimentally to be dependent on several different factors. The concentrations of the dopants in n-type and p-type layers affect the Fermi level position, the height of the barriers, and also the density of defects. These parameters are altered by different choices of inert gas diluent (Ar or He) and substrate temperature T/sub s/. For a given set of deposition conditions, the LEC effect can be maximized by varying the layer thickness. With undiluted silane at T/sub s/ = 250/sup 0/C, the effect was relatively small, reaching a maximum in relatively thick layers (540 A). The largest effects were obtained for films deposited from silane diluted in helium, using thinner (330 A) layers. However, for films deposited from silane diluted in argon, the magnitude of the effect and optimum layer thickness was intermediate (440 A). When T/sub s/ was varied, a minimum in LEC was found near 200 to 250/sup 0/C. The influence of internal field was examined by using nini, pipi and npnp multilayers. The internal field is a necessary factor to observe a large LEC effect. A compensated film shows a small LEC effect.

  15. Novel mesoporous P-doped graphitic carbon nitride nanosheets coupled with ZnIn2S4 nanosheets as efficient visible light driven heterostructures with remarkably enhanced photo-reduction activity

    NASA Astrophysics Data System (ADS)

    Chen, Wei; Liu, Tian-Yu; Huang, Ting; Liu, Xiao-Heng; Yang, Xu-Jie

    2016-02-01

    In this report, we rationally designed and fabricated P-C3N4/ZnIn2S4 nanocomposites by in situ immobilizing ZnIn2S4 nanosheets onto the surface of mesoporous P-doped graphite carbon nitrogen (P-C3N4) nanosheets in a mixed solvothermal environment; their application to the photoreduction of 4-nitroaniline was used to estimate the photocatalytic performance. Different to the template route, here the mesoporous P-C3N4 nanosheets were prepared with a template-free strategy. The as-fabricated P-C3N4/ZnIn2S4 nanocomposites were systematically characterized by analyzing the phase structure, chemical components, electronic and optical properties and separation of charge carrier pairs. More importantly, these P-C3N4/ZnIn2S4 heterostructures have been proven to be highly efficient visible light responsive photocatalysts for photo-reduction, and meanwhile exhibit excellent photo-stability during recycling runs. The sufficient evidence reveals that the significantly improved photocatalytic performance is mainly attributed to the more efficient charge carrier separation based on the construction of a close heterogeneous interface. This work may provide new insights into the utilization of P-C3N4/ZnIn2S4 nanocomposites as visible light driven photocatalysts for comprehensive organic transformations in the field of fine chemical engineering.In this report, we rationally designed and fabricated P-C3N4/ZnIn2S4 nanocomposites by in situ immobilizing ZnIn2S4 nanosheets onto the surface of mesoporous P-doped graphite carbon nitrogen (P-C3N4) nanosheets in a mixed solvothermal environment; their application to the photoreduction of 4-nitroaniline was used to estimate the photocatalytic performance. Different to the template route, here the mesoporous P-C3N4 nanosheets were prepared with a template-free strategy. The as-fabricated P-C3N4/ZnIn2S4 nanocomposites were systematically characterized by analyzing the phase structure, chemical components, electronic and optical properties and

  16. Variable electronic properties of lateral phosphorene-graphene heterostructures.

    PubMed

    Tian, Xiaoqing; Liu, Lin; Du, Yu; Gu, Juan; Xu, Jian-Bin; Yakobson, Boris I

    2015-12-21

    Phosphorene and graphene have a tiny lattice mismatch along the armchair direction, which can result in an atomically sharp in-plane interface. The electronic properties of the lateral heterostructures of phosphorene/graphene are investigated by the first-principles method. Here, we demonstrate that the electronic properties of this type of heterostructure can be highly tunable by the quantum size effects and the externally applied electric field (Eext). At strong Eext, Dirac Fermions can be developed with Fermi velocities around one order smaller than that of graphene. Undoped and hydrogen doped configurations demonstrate three drastically different electronic phases, which reveal the strongly tunable potential of this type of heterostructure. Graphene is a naturally better electrode for phosphorene. The transport properties of two-probe devices of graphene/phosphorene/graphene exhibit tunnelling transport characteristics. Given these results, it is expected that in-plane heterostructures of phosphorene/graphene will present abundant opportunities for applications in optoelectronic and electronic devices.

  17. Rapid thermal annealing and modulation-doping effects on InAs/GaAs quantum dots photoluminescence dependence on excitation power

    NASA Astrophysics Data System (ADS)

    Chaâbani, W.; Melliti, A.; Maaref, M. A.; Testelin, C.; Lemaître, A.

    2016-07-01

    The optical properties of p-doped and annealed InAs/GaAs quantum dots (QDs) was investigated by photoluminescence (PL) as a function of temperature and excitation power density (Pexc). At low-T, PL spectra of rapid thermal annealing (RTA) and p-modulation doped QDs show an energy blueshift and redshift, respectively. A superlinear dependence of integrated PL intensity on Pexc at high-T was found only for undoped QD. The superlinearity was suppressed by modulation-doping and RTA effects. A linear dependence of IPL at all temperatures and a decrease of the carrier-carrier Coulomb interaction at high-T was found after RTA.

  18. Gate-tunable high mobility remote-doped InSb/In{sub 1−x}Al{sub x}Sb quantum well heterostructures

    SciTech Connect

    Yi, Wei E-mail: MSokolich@hrl.com; Kiselev, Andrey A.; Thorp, Jacob; Noah, Ramsey; Nguyen, Binh-Minh; Bui, Steven; Rajavel, Rajesh D.; Hussain, Tahir; Gyure, Mark F.; Sokolich, Marko E-mail: MSokolich@hrl.com; Kratz, Philip; Qian, Qi; Manfra, Michael J.; Pribiag, Vlad S.; Kouwenhoven, Leo P.; Marcus, Charles M.

    2015-04-06

    Gate-tunable high-mobility InSb/In{sub 1−x}Al{sub x}Sb quantum wells (QWs) grown on GaAs substrates are reported. The QW two-dimensional electron gas (2DEG) channel mobility in excess of 200 000 cm{sup 2}/V s is measured at T = 1.8 K. In asymmetrically remote-doped samples with an HfO{sub 2} gate dielectric formed by atomic layer deposition, parallel conduction is eliminated and complete 2DEG channel depletion is reached with minimal hysteresis in gate bias response of the 2DEG electron density. The integer quantum Hall effect with Landau level filling factor down to 1 is observed. A high-transparency non-alloyed Ohmic contact to the 2DEG with contact resistance below 1 Ω·mm is achieved at 1.8 K.

  19. Modulating electronic, magnetic and chemical properties of MoS2 monolayer sheets by substitutional doping with transition metals

    NASA Astrophysics Data System (ADS)

    Ma, Dongwei; Ju, Weiwei; Li, Tingxian; Zhang, Xiwei; He, Chaozheng; Ma, Benyuan; Tang, Yanan; Lu, Zhansheng; Yang, Zongxian

    2016-02-01

    Based on first-principles calculations, the effects of substitutional doping with transition-metal (TM) atoms (Co, Ni, Ru, Rh, Pd, Ir, Pt and Au) were investigated on the electronic structure, magnetic property and chemical activity of the molybdenum disulfide (MoS2) monolayer sheet. It is found that all the considered TM atoms are strongly bonded to the sulfur defects. The magnetic properties of MoS2 monolayer sheets can be modulated by embedding TM atoms. The introduced spin magnetic moments are 1.00, 1.00, 1.00, 0.99, and 2.00μB, respectively, for Ir, Rh, Co, Au and Ru doping. The electronic properties of MoS2 monolayer sheets are also significantly changed due to the induced impurity states in the band gap. The chemical activity of the TM-doped MoS2 monolayer sheet (TM-MoS2) is significantly enhanced compared with the undoped sheet. Most TM-MoS2 can strongly adsorb and thus effectively activate the adsorbed O2. It is proposed that the partially occupied d orbitals of the doped TM atoms localized in the vicinity of the Fermi level play a crucial role in adsorbing and activating the adsorbed O2. The adsorption of O2 can in turn modify the electronic structures and magnetic properties of TM-MoS2.

  20. Electrical spin injection in modulation-doped GaAs from an in situ grown Fe/MgO layer

    SciTech Connect

    Shim, Seong Hoon; Kim, Hyung-jun; Koo, Hyun Cheol; Lee, Yun-Hi; Chang, Joonyeon

    2015-09-07

    We study spin accumulation in n-doped GaAs that were electrically injected from Fe via MgO using three-terminal Hanle measurement. The Fe/MgO/GaAs structures were prepared in a cluster molecular beam epitaxy that did not require the breaking of the vacuum. We found the crystal orientation relationship of epitaxial structures Fe[100]//MgO[110]//GaAs[110] without evident defects at the interface. Control of depletion width and interface resistance by means of modulation doping improves spin injection, leading to enhanced spin voltage (ΔV) of 6.3 mV at 10 K and 0.8 mV even at 400 K. The extracted spin lifetime and spin diffusion length of GaAs are 220 ps and 0.77 μm, respectively, at 200 K. MgO tunnel barrier grown in situ with modulation doping at the interface appears to be promising for spin injection into GaAs.

  1. Bandgap modulation and hydrogen storage with Cr-doped BN sheets

    NASA Astrophysics Data System (ADS)

    Ma, Shengqian

    2015-08-01

    The theoretical calculations indicate that the metal-doped boron nitride (BN) sheets are potential materials to store the hydrogen and tune the bandgap. It is all known that the BN sheet is a nonmagnetic wide-bandgap semiconductor. Using density function theory (DFT), the lattice parameters of Cr-doped BN sheets are optimized, which are still kept on two-dimensional (2D) planar geometry, and the bandgap and H2 storage are studied. The simulation results show that the H2 molecule can be easily absorbed by Cr-doped N in BN sheet. As the adsorption energy was greatly decreasing with the increasing number of Cr-doped N, B had an affinity for adsorption of H2. With the increase of Cr doping, the bandgap of Cr-doped BN sheet is decreasing. The bandgap decreases from 4.705 eV to 0.08 eV. So Cr-doped BN sheet is a promising material in storing H2 and tuning the bandgap.

  2. Control of threshold voltage in E-mode and D-mode GaN-on-Si metal-insulator-semiconductor heterostructure field effect transistors by in-situ fluorine doping of atomic layer deposition Al2O3 gate dielectrics

    NASA Astrophysics Data System (ADS)

    Roberts, J. W.; Chalker, P. R.; Lee, K. B.; Houston, P. A.; Cho, S. J.; Thayne, I. G.; Guiney, I.; Wallis, D.; Humphreys, C. J.

    2016-02-01

    We report the modification and control of threshold voltage in enhancement and depletion mode AlGaN/GaN metal-insulator-semiconductor heterostructure field effect transistors through the use of in-situ fluorine doping of atomic layer deposition Al2O3. Uniform distribution of F ions throughout the oxide thickness are achievable, with a doping level of up to 5.5 × 1019 cm-3 as quantified by secondary ion mass spectrometry. This fluorine doping level reduces capacitive hysteretic effects when exploited in GaN metal-oxide-semiconductor capacitors. The fluorine doping and forming gas anneal also induces an average positive threshold voltage shift of between 0.75 and 1.36 V in both enhancement mode and depletion mode GaN-based transistors compared with the undoped gate oxide via a reduction of positive fixed charge in the gate oxide from +4.67 × 1012 cm-2 to -6.60 × 1012 cm-2. The application of this process in GaN based power transistors advances the realisation of normally off, high power, high speed devices.

  3. Modulation doping of GaAs/AlGaAs core-shell nanowires with effective defect passivation and high electron mobility.

    PubMed

    Boland, Jessica L; Conesa-Boj, Sonia; Parkinson, Patrick; Tütüncüoglu, Gözde; Matteini, Federico; Rüffer, Daniel; Casadei, Alberto; Amaduzzi, Francesca; Jabeen, Fauzia; Davies, Christopher L; Joyce, Hannah J; Herz, Laura M; Fontcuberta i Morral, Anna; Johnston, Michael B

    2015-02-11

    Reliable doping is required to realize many devices based on semiconductor nanowires. Group III-V nanowires show great promise as elements of high-speed optoelectronic devices, but for such applications it is important that the electron mobility is not compromised by the inclusion of dopants. Here we show that GaAs nanowires can be n-type doped with negligible loss of electron mobility. Molecular beam epitaxy was used to fabricate modulation-doped GaAs nanowires with Al0.33Ga0.67As shells that contained a layer of Si dopants. We identify the presence of the doped layer from a high-angle annular dark field scanning electron microscopy cross-section image. The doping density, carrier mobility, and charge carrier lifetimes of these n-type nanowires and nominally undoped reference samples were determined using the noncontact method of optical pump terahertz probe spectroscopy. An n-type extrinsic carrier concentration of 1.10 ± 0.06 × 10(16) cm(-3) was extracted, demonstrating the effectiveness of modulation doping in GaAs nanowires. The room-temperature electron mobility was also found to be high at 2200 ± 300 cm(2) V(-1) s(-1) and importantly minimal degradation was observed compared with undoped reference nanowires at similar electron densities. In addition, modulation doping significantly enhanced the room-temperature photoconductivity and photoluminescence lifetimes to 3.9 ± 0.3 and 2.4 ± 0.1 ns respectively, revealing that modulation doping can passivate interfacial trap states.

  4. Cavity-less on-chip optomechanics using excitonic transitions in semiconductor heterostructures.

    PubMed

    Okamoto, Hajime; Watanabe, Takayuki; Ohta, Ryuichi; Onomitsu, Koji; Gotoh, Hideki; Sogawa, Tetsuomi; Yamaguchi, Hiroshi

    2015-01-01

    The hybridization of semiconductor optoelectronic devices and nanomechanical resonators provides a new class of optomechanical systems in which mechanical motion can be coupled to light without any optical cavities. Such cavity-less optomechanical systems interconnect photons, phonons and electrons (holes) in a highly integrable platform, opening up the development of functional integrated nanomechanical devices. Here we report on a semiconductor modulation-doped heterostructure-cantilever hybrid system, which realizes efficient cavity-less optomechanical transduction through excitons. The opto-piezoelectric backaction from the bound electron-hole pairs enables us to probe excitonic transition simply with a sub-nanowatt power of light, realizing high-sensitivity optomechanical spectroscopy. Detuning the photon energy from the exciton resonance results in self-feedback cooling and amplification of the thermomechanical motion. This cavity-less on-chip coupling enables highly tunable and addressable control of nanomechanical resonators, allowing high-speed programmable manipulation of nanomechanical devices and sensor arrays. PMID:26477487

  5. Optical modulation of guided mode resonance in the waveguide grating structure incorporated with azo-doped-poly(methylmethacrylate) cladding layer.

    PubMed

    Lin, Jian Hung; Huang, Yu Chung; DiepLai, Ngoc; Kan, Hung-Chih; Hsu, Chia Chen

    2012-01-01

    Optical modulation of guided mode resonance (GMR) is demonstrated in a waveguide grating structure (WGS) which contains a disperse-red1 (DR1)-doped poly(methylmethacrylate) (PMMA) cladding layer. The resonance wavelength of a GMR mode can be tuned by pumping the cladding layer with a 442 nm wavelength laser beam, because of photoinduced refractive index change in the layer. The resonance wavelength shifts to shorter wavelength side, and the shift increases with pumping power, up to a maximum shift of 5 nm. A detector was used to monitor the intensity of the light that was reflected from the WGS at the wavelengths of the GMR peak positions, and the WGS was found to exhibit optical modulation with a shortest switching time of less than 0.3s.

  6. Dynamic modulation of electronic properties of graphene by localized carbon doping using focused electron beam induced deposition

    NASA Astrophysics Data System (ADS)

    Kim, S.; Russell, M.; Henry, M.; Kim, S. S.; Naik, R. R.; Voevodin, A. A.; Jang, S. S.; Tsukruk, V. V.; Fedorov, A. G.

    2015-09-01

    We report on the first demonstration of controllable carbon doping of graphene to engineer local electronic properties of a graphene conduction channel using focused electron beam induced deposition (FEBID). Electrical measurements indicate that an ``n-p-n'' junction on graphene conduction channel is formed by partial carbon deposition near the source and drain metal contacts by low energy (<50 eV) secondary electrons due to inelastic collisions of long range backscattered primary electrons generated from a low dose of high energy (25 keV) electron beam (1 × 1018 e- per cm2). Detailed AFM imaging provides direct evidence of the new mechanism responsible for dynamic evolution of the locally varying graphene doping. The FEBID carbon atoms, which are physisorbed and weakly bound to graphene, diffuse towards the middle of graphene conduction channel due to their surface chemical potential gradient, resulting in negative shift of Dirac voltage. Increasing a primary electron dose to 1 × 1019 e- per cm2 results in a significant increase of carbon deposition, such that it covers the entire graphene conduction channel at high surface density, leading to n-doping of graphene channel. Collectively, these findings establish a unique capability of FEBID technique to dynamically modulate the doping state of graphene, thus enabling a new route to resist-free, ``direct-write'' functional patterning of graphene-based electronic devices with potential for on-demand re-configurability.We report on the first demonstration of controllable carbon doping of graphene to engineer local electronic properties of a graphene conduction channel using focused electron beam induced deposition (FEBID). Electrical measurements indicate that an ``n-p-n'' junction on graphene conduction channel is formed by partial carbon deposition near the source and drain metal contacts by low energy (<50 eV) secondary electrons due to inelastic collisions of long range backscattered primary electrons generated

  7. Ultraviolet-light-driven doping modulation in chemical vapor deposition grown graphene.

    PubMed

    Iqbal, M Z; Iqbal, M W; Khan, M F; Eom, Jonghwa

    2015-08-28

    The tuning of charge carrier density of graphene is an essential factor to achieve the integration of high-efficiency electronic and optoelectronic devices. We demonstrate the reversible doping in graphene using deep ultraviolet (UV) irradiation and treatment with O2 and N2 gases. The Dirac point shift towards a positive gate voltage of chemical vapor deposition grown graphene field-effect transistors confirms the p-type doping, which is observed under UV irradiation and treatment with O2 gas, while it restores its pristine state after treatment with N2 gas under UV irradiation. The emergence of an additional peak in the X-ray photoelectron spectra during UV irradiation and treatment with O2 gas represents the oxidation of graphene, and the elimination of this peak during UV irradiation and treatment with N2 gas reveals the restoration of graphene in its pristine state. The shift in the G and 2D bands in Raman spectra towards higher and then lower wavenumber also suggests p-type doping and then reversible doping in graphene. The controlled doping and its reversibility in large area grown graphene offer a new vision for electronic applications.

  8. Ultraviolet-light-driven doping modulation in chemical vapor deposition grown graphene.

    PubMed

    Iqbal, M Z; Iqbal, M W; Khan, M F; Eom, Jonghwa

    2015-08-28

    The tuning of charge carrier density of graphene is an essential factor to achieve the integration of high-efficiency electronic and optoelectronic devices. We demonstrate the reversible doping in graphene using deep ultraviolet (UV) irradiation and treatment with O2 and N2 gases. The Dirac point shift towards a positive gate voltage of chemical vapor deposition grown graphene field-effect transistors confirms the p-type doping, which is observed under UV irradiation and treatment with O2 gas, while it restores its pristine state after treatment with N2 gas under UV irradiation. The emergence of an additional peak in the X-ray photoelectron spectra during UV irradiation and treatment with O2 gas represents the oxidation of graphene, and the elimination of this peak during UV irradiation and treatment with N2 gas reveals the restoration of graphene in its pristine state. The shift in the G and 2D bands in Raman spectra towards higher and then lower wavenumber also suggests p-type doping and then reversible doping in graphene. The controlled doping and its reversibility in large area grown graphene offer a new vision for electronic applications. PMID:26198203

  9. Characterization of the transport properties of channel delta-doped structures by light-modulated Shubnikov-de Haas measurements

    NASA Technical Reports Server (NTRS)

    Mena, R. A.; Schacham, S. E.; Haugland, E. J.; Alterovitz, S. A.; Young, P. G.; Bibyk, S. B.; Ringel, S. A.

    1995-01-01

    The transport properties of channel delta-doped quantum well structures were characterized by conventional Hall effect and light-modulated Shubnikov-de Haas (SdH) effect measurements. The large number of carriers that become available due to the delta-doping of the channel, leads to an apparent degeneracy in the well. As a result of this degeneracy, the carrier mobility remains constant as a function of temperature from 300 K down to 1.4 K. The large amount of impurity scattering, associated with the overlap of the charge carriers and the dopants, resulted in low carrier mobilities and restricted the observation of the oscillatory magneto-resistance used to characterize the two-dimensional electron gas (2DEG) by conventional SdH measurements. By light-modulating the carriers, we were able to observe the SdH oscillation at low magnetic fields, below 1.4 tesla, and derive a value for the quantum scattering time. Our results for the ratio of the transport and quantum scattering times are lower than those previously measured for similar structures using much higher magnetic fields.

  10. Extreme mobility enhancement of two-dimensional electron gases at oxide interfaces by charge-transfer-induced modulation doping

    NASA Astrophysics Data System (ADS)

    Chen, Y. Z.; Trier, F.; Wijnands, T.; Green, R. J.; Gauquelin, N.; Egoavil, R.; Christensen, D. V.; Koster, G.; Huijben, M.; Bovet, N.; Macke, S.; He, F.; Sutarto, R.; Andersen, N. H.; Sulpizio, J. A.; Honig, M.; Prawiroatmodjo, G. E. D. K.; Jespersen, T. S.; Linderoth, S.; Ilani, S.; Verbeeck, J.; van Tendeloo, G.; Rijnders, G.; Sawatzky, G. A.; Pryds, N.

    2015-08-01

    Two-dimensional electron gases (2DEGs) formed at the interface of insulating complex oxides promise the development of all-oxide electronic devices. These 2DEGs involve many-body interactions that give rise to a variety of physical phenomena such as superconductivity, magnetism, tunable metal-insulator transitions and phase separation. Increasing the mobility of the 2DEG, however, remains a major challenge. Here, we show that the electron mobility is enhanced by more than two orders of magnitude by inserting a single-unit-cell insulating layer of polar La1-xSrxMnO3 (x = 0, 1/8, and 1/3) at the interface between disordered LaAlO3 and crystalline SrTiO3 produced at room temperature. Resonant X-ray spectroscopy and transmission electron microscopy show that the manganite layer undergoes unambiguous electronic reconstruction, leading to modulation doping of such atomically engineered complex oxide heterointerfaces. At low temperatures, the modulation-doped 2DEG exhibits Shubnikov-de Haas oscillations and fingerprints of the quantum Hall effect, demonstrating unprecedented high mobility and low electron density.

  11. Extreme mobility enhancement of two-dimensional electron gases at oxide interfaces by charge-transfer-induced modulation doping.

    PubMed

    Chen, Y Z; Trier, F; Wijnands, T; Green, R J; Gauquelin, N; Egoavil, R; Christensen, D V; Koster, G; Huijben, M; Bovet, N; Macke, S; He, F; Sutarto, R; Andersen, N H; Sulpizio, J A; Honig, M; Prawiroatmodjo, G E D K; Jespersen, T S; Linderoth, S; Ilani, S; Verbeeck, J; Van Tendeloo, G; Rijnders, G; Sawatzky, G A; Pryds, N

    2015-08-01

    Two-dimensional electron gases (2DEGs) formed at the interface of insulating complex oxides promise the development of all-oxide electronic devices. These 2DEGs involve many-body interactions that give rise to a variety of physical phenomena such as superconductivity, magnetism, tunable metal-insulator transitions and phase separation. Increasing the mobility of the 2DEG, however, remains a major challenge. Here, we show that the electron mobility is enhanced by more than two orders of magnitude by inserting a single-unit-cell insulating layer of polar La(1-x)Sr(x)MnO3 (x = 0, 1/8, and 1/3) at the interface between disordered LaAlO3 and crystalline SrTiO3 produced at room temperature. Resonant X-ray spectroscopy and transmission electron microscopy show that the manganite layer undergoes unambiguous electronic reconstruction, leading to modulation doping of such atomically engineered complex oxide heterointerfaces. At low temperatures, the modulation-doped 2DEG exhibits Shubnikov-de Haas oscillations and fingerprints of the quantum Hall effect, demonstrating unprecedented high mobility and low electron density. PMID:26030303

  12. Variable Temperature High-Frequency Response of Heterostructure Transistors

    NASA Astrophysics Data System (ADS)

    Laskar, Joy

    1992-01-01

    The development of high performance heterostructure transistors is essential for emerging opto-electronic integrated circuits (OEICs) and monolithic microwave integrated circuits (MMICs). Applications for OEICs and MMICs include the rapidly growing telecommunications and personal communications markets. The key to successful OEIC and MMIC chip sets is the development of high performance, cost-effective technologies. In this work, several different transistor structures are investigated to determine the potential for high speed performance and the physical mechanisms controlling the ultimate device operation. A cryogenic vacuum microwave measurement system has been developed to study the high speed operation of modulation doped field-effect transistors (MODFETs), doped channel metal insulator field-effect transistors (MISFETs), and metal semiconductor field-effect transistors (MESFETs). This study has concluded that the high field velocity and not the low field mobility is what controls high frequency operation of GaAs based field-effect transistors. Both Al_{rm x} Ga_{rm 1-x}As/GaAs and InP/In_{rm y}Ga _{rm 1-y}As heterostructure bipolar transistors (HBTs) have also been studied at reduced lattice temperatures to understand the role of diffusive transport in the Al_{rm x} Ga_{rm 1-x}As/GaAs HBT and nonequilibrium transport in the InP/In _{rm y}Ga_ {rm 1-y}As HBT. It is shown that drift/diffusion formulation must be modified to accurately estimate the base delay time in the conventional Al _{rm x}Ga_ {rm 1-x}As/GaAs HBT. The reduced lattice temperature operation of the InP/In_ {rm y}Ga_{rm 1-y}As HBT demonstrates extreme nonequilibrium transport in the neutral base and collector space charge region with current gain cut-off frequency exceeding 300 GHz, which is the fastest reported transistor to date. Finally, the MODFET has been investigated as a three-terminal negative differential resistance (NDR) transistor. The existence of real space transfer is confirmed by

  13. Two Carrier Analysis of Persistent Photoconductivity in Modulation-Doped Structures

    NASA Technical Reports Server (NTRS)

    Schacham, S. E.; Mena, R. A.; Haugland, E. J.; Alterovitz, S. A.

    1995-01-01

    A simultaneous fit of Hall and conductivity data gives quantitative results on the carrier concentration and mobility in both the quantum well and the parallel conduction channel. In this study this method was applied to reveal several new findings on the effect of persistent photoconductivity (PPC) on free-carrier concentrations and mobilities. The increase in the two-dimensional electron-gas (2DEG) concentration is significantly smaller than the apparent one derived from single carrier analysis of the Hall coefficient. In the two types of structures investigated, delta doped and continuously doped barrier, the apparent concentration almost doubles following illumination, while analysis reveals an increase of about 20% in the 2DEG. The effect of PPC on mobility depends on the structure. For the sample with a continuously doped barrier the mobility in the quantum well more than doubles. This increase is attributed to the effective screening of the ionized donors by the large electron concentration in the barrier. In the delta doped barrier sample the mobility is reduced by almost a factor of 2. This decrease is probably caused by strong coupling between the two wells, as is demonstrated by self-consistent analysis.

  14. Topological States of Heterostructures

    NASA Astrophysics Data System (ADS)

    Usanmaz, Demet; Nath, Pinku; Plata, Jose J.; Buongiorno Nardelli, Marco; Fornari, Marco; Curtarolo, Stefano

    Topological insulators (TIs) have exotic properties, such as having insulating behavior in the bulk and metallic states at the surface [1]. Observations of metallic states rely on the spin-orbit induced band inversion in bulk materials and are protected by time-reversal symmetry or crystal symmetry [ 2 ]. These remarkable characteristics of TIs give rise to various applications from spintronics to quantum computers. In order to broaden the range of applications of TIs and make it more effective, an exploration of high quality heterostructures are required. Creating heterostructures of TIs has recently demonstrated to be advantageous for controlling electronic properties [3]. Inspired by these interesting properties, we have investigated the topological interface states of heterostructures.

  15. Novel engineered compound semiconductor heterostructures for advanced electronics applications

    NASA Astrophysics Data System (ADS)

    Stillman, Gregory E.; Holonyak, Nick, Jr.; Coleman, James J.

    1992-06-01

    To provide the technology base that will enable SDIO capitalization on the performance advantages offered through novel engineered multiple-lavered compound semiconductor structures, this project has focussed on three specific areas: (1) carbon doping of AlGaAs/GaAs and InP/InGaAs materials for reliable high frequency heterojunction bipolar transistors; (2) impurity induced layer disordering and the environmental degradation of AlxGal-xAs-GaAs quantum-well heterostructures and the native oxide stabilization of AlxGal-xAs-GaAs quantum well heterostructure lasers; and (3) non-planar and strained-layer quantum well heterostructure lasers and laser arrays. The accomplishments in this three year research are reported in fifty-six publications and the abstracts included in this report.

  16. Novel nanowire heterostructures for nanoelectronic applications

    NASA Astrophysics Data System (ADS)

    Yan, Hao

    , subtractor and latch. These results show significant progress in the complexity of nanowire electronics, and demonstrate the great versatility of the CTNW crossbar array. Lastly, semiconductor/ferroelectric nanowire heterostructure is investigated for its application in nanoscale memories. Si/ZrO2/BaTiO3 core/shell/shell nanowire shows polarization-modulated conductance and the nanowire ferroelectric FET (NW-FeFET) shows excellent memory characteristics such as >106 ON/OFF ratio, 100ns switching and week-long stability. The down-scaling of the device is also studied, and shows great potential towards high-density information storage.

  17. Spin and charge modulations in a single-hole-doped Hubbard ladder: Verification with optical lattice experiments

    NASA Astrophysics Data System (ADS)

    Zhu, Zheng; Weng, Zheng-Yu; Ho, Tin-Lun

    2016-03-01

    We show that pronounced modulations in spin and charge densities can be induced by the insertion of a single hole in an otherwise half-filled two-leg Hubbard ladder. Accompanied with these modulations is a loosely bound structure of the doped charge with a spin-1/2, in contrast to the tightly bound case where such modulations are absent. These behaviors are caused by the interference of the Berry phases associated with a string of flipped spins (or "phase strings") left behind as a hole travels through a spin bath with a short-range antiferromagnetic order. The key role of the phase strings is also reflected in how the system responds to increasing spin polarization and the on-site repulsion, addition of a second hole, and increasing asymmetry between intra- and interchain hopping. Remarkably, all these properties persist down to ladders as short as ˜10 sites, as the smoking gun of the phase-string effect. They can therefore be studied in cold-atom experiments using the recently developed fermion microscope.

  18. Modulating doping and interface magnetism of epitaxial graphene on SiC(0001)

    NASA Astrophysics Data System (ADS)

    Pan, Zhou; Da-Wei, He

    2016-01-01

    On the basis of first principles calculations, we report that the type and density of charge carriers of epitaxial graphene on SiC(0001) can be deliberately controlled by decorating the buffer layer with specific atoms (i.e., F, Cl, O, or N). More importantly, a fine tuning of the doping behavior from intrinsic n-type to charge neutrality to p-type and interface magnetism is achieved via increasing the doping concentration of F atoms on the buffer layer. Our results suggest an interesting avenue to the application of epitaxial graphene in nanoscale electronic and spintronic devices. Project supported by the National Natural Science Foundation of China (Grant Nos. 61335006, 61378073, and 61527817), the Beijing Municipal Science and Technology Committee, China (Grant No. Z151100003315006), and Fundamental Research Funds for the Central Universities of Beijing Jiaotong University, China (Grant No. 2012YJS123).

  19. Screening for two selective androgen receptor modulators using gas chromatography-mass spectrometry in doping control analysis.

    PubMed

    Thevis, Mario; Kohler, Maxie; Schlörer, Nils; Fusshöller, Gregor; Schänzer, Wilhelm

    2008-01-01

    Selective androgen receptor modulators (SARMs) have become a major field of clinical research enabling the tissue-selective stimulation of androgen receptors. The treatment of debilitating diseases, osteoporosis and frailty are primary goals and promising results have been obtained from clinical trials. However, the potential for misuse of SARMs in sport is great and drug testing methods based on liquid chromatography were established for different classes including arylpropionamide-, 2-quinolinone- and bicyclic hydantoin-derived compounds. As gas chromatography and mass spectrometry (GC-MS) are still important analytical tools in sports drug testing, a method to determine 2-quinolinone- and bicyclic hydantoin-derived SARMs established. Spiked urine samples were subjected to routine doping control protocols including enzymatic hydrolysis, liquid-liquid extraction, concentration and derivatisation to trimethylsilylated analogues followed by GC-MS analysis. The method was validated for the items specificity, lower limit of detection (0.2-10 ng mL(-1)), recovery (83-85%), intraday and interday precision (9-15% and 13-18%, respectively), which demonstrates the suitability of conventional GC-MS systems to determine representatives of an emerging class of compounds in doping control specimens.

  20. Mass spectrometric characterization of urinary metabolites of the selective androgen receptor modulator andarine (S-4) for routine doping control purposes.

    PubMed

    Thevis, Mario; Thomas, Andreas; Fusshöller, Gregor; Beuck, Simon; Geyer, Hans; Schänzer, Wilhelm

    2010-08-15

    Selective androgen receptor modulators (SARMs) are potent anabolic agents with tissue-selective properties. Due to their potential misuse in elite sport, the World Anti-Doping Agency (WADA) has prohibited SARMs since 2008, and although no representative drug candidate has yet received full clinical approval, recent findings of SARMs illegally sold via the internet have further supported the need to efficiently test for these compounds in doping controls. In the present communication, the mass spectrometric characterization of urinary metabolites of the SARM Andarine (also referred to as S-4) compared with earlier in vitro and animal studies is reported. Liquid chromatography interfaced to high-resolution/high-accuracy (tandem) mass spectrometry was used to identify phase I and II metabolites, confirming the predicted target analytes for sports drug testing purposes including the glucuronic acid conjugates of the active drug, its monohydroxylated and/or deacetylated product, the hydrolysis product resulting from the removal of the compound's B-ring, as well as the sulfate of the monohydroxylated and the deacetylated phase I metabolite. The obtained data will support future efforts to effectively screen for and confirm the misuse of the non-approved drug candidate Andarine. PMID:20623476

  1. Photoluminescence related to the 2-dimensional electron gas in modulation doped GaN/AlGaN structures

    SciTech Connect

    Bergman, J.P.; Lundstroem, T.; Monemar, B.; Amano, H.; Akasaki, I.

    1996-11-01

    The authors report low temperature photoluminescence (PL) spectra related to a two-dimensional electron gas confined at a GaN/AlGaN heterointerface. The recombination between electrons confined in the bottom of the interface potential and photoexcited holes causes a broad PL emission about 50 meV below the bulk GaN exciton emission. A second emission, attributed to the recombination of electrons in the first excited level at the interface, is also observed close to the excitonic band gap in GaN. The data agrees with a self consistent calculation of the energy levels and the electron concentration at the interface. Similar PL data from a modulation doped AlGaN/GaN quantum well exhibit three PL emissions related to the 2D electron gas.

  2. Evaluation of modulating field of photoreflectance of surface-intrinsic-n+ type doped GaAs by using photoinduced voltage

    NASA Astrophysics Data System (ADS)

    Lee, W. Y.; Chien, J. Y.; Wang, D. P.; Huang, K. F.; Huang, T. C.

    2002-04-01

    Photoreflectance (PR) of surface-intrinsic-n+ type doped GaAs has been measured for various power densities of pump laser. The spectra exhibited many Franz-Keldysh oscillations, whereby the strength of electric field F in the undoped layer can be determined. The thus obtained Fs are subject to photovoltaic effect and are less than built-in field Fbi. In the previous work we have obtained the relation F≈Fbi-δF/2 when δF≪Fbi by using electroreflectance to simulate PR, where δF is the modulating field of the pump beam. In this work a method was devised to evaluate δF by using photoinduced voltages Vs and, hence, the relation can be verified by PR itself. The δFs obtained by Vs are also consistent with those of using imaginary part of fast Fourier transform of PR spectra.

  3. Thermoelectric properties of IV–VI-based heterostructures and superlattices

    SciTech Connect

    Borges, P.D.; Petersen, J.E.; Scolfaro, L.; Leite Alves, H.W.; Myers, T.H.

    2015-07-15

    Doping in a manner that introduces anisotropy in order to reduce thermal conductivity is a significant focus in thermoelectric research today. By solving the semiclassical Boltzmann transport equations in the constant scattering time (τ) approximation, in conjunction with ab initio electronic structure calculations, within Density Functional Theory, we compare the Seebeck coefficient (S) and figure of merit (ZT) of bulk PbTe to PbTe/SnTe/PbTe heterostructures and PbTe doping superlattices (SLs) with periodically doped planes. Bismuth and Thallium were used as the n- and p-type impurities, respectively. The effects of carrier concentration are considered via chemical potential variation in a rigid band approximation. The impurity bands near the Fermi level in the electronic structure of PbTe SLs are of Tl s- and Bi p-character, and this feature is independent of the doping concentration or the distance between impurity planes. We observe the impurity bands to have a metallic nature in the directions perpendicular to the doping planes, yet no improvement on the values of ZT is found when compared to bulk PbTe. For the PbTe/SnTe/PbTe heterostructures, the calculated S presents good agreement with recent experimental data, and an anisotropic behavior is observed for low carrier concentrations (n<10{sup 18} cm{sup −3}). A large value of ZT{sub ||} (parallel to the growth direction) of 3.0 is predicted for n=4.7×10{sup 18} cm{sup −3} and T=700 K, whereas ZT{sub p} (perpendicular to the growth direction) is found to peak at 1.5 for n=1.7×10{sup 17} cm{sup −3}. Both electrical conductivity enhancement and thermal conductivity reduction are analyzed. - Graphical abstract: Figure of merit for PbTe/SnTe/PbTe heterostructure along the [0 0 1] direction, P.D. Borges, J.E. Petersen, L. Scolfaro, H.W. Leite Alves, T.H. Myers, Improved thermoelectric properties of IV–VI-based heterostructures and superlattices. - Highlights: • Thermoelectric properties of IV

  4. Photoresponsive memory device based on Graphene/Boron Nitride heterostructure

    NASA Astrophysics Data System (ADS)

    Kahn, Salman; Velasco, Jairo, Jr.; Ju, Long; Wong, Dillon; Lee, Juwon; Tsai, Hsin Zon; Taniguchi, Takashi; Watanabe, Kenji; Zettl, Alex; Wang, Feng; Crommie, Michael

    2015-03-01

    Recent technological advancements have allowed the stacking of two dimensional layered material in order to create van der Waals heterostructures (VDH), enabling the design of novel properties by exploiting the proximal interaction between layers with different electronic properties. We report the creation of an optoelectronic memory device using a Graphene/Boron Nitride (hBN) heterostructure. Using the photo-induced doping phenomenon, we are able to spatially ``write'' a doping profile on graphene and ``read'' the profile through electrical transport and local probe techniques. We then utilize defect engineering to enhance the optoelectronic response of graphene and explore the effect of defects in hBN. Our work introduces a simple device architecture to create an optoelectronic memory device and contributes towards understanding the proximal effects of hBN on Graphene.

  5. The electrical conductivity of thin film donor doped hematite: from insulator to semiconductor by defect modulation.

    PubMed

    Engel, J; Tuller, H L

    2014-06-21

    Hematite or α-Fe2O3 has emerged as a highly promising photoanode candidate for photoelectrochemical cells. While significant improvements in its performance have recently been achieved, it remains unclear why the maximum photocurrents still remain well below their theoretical predictions. Here, we report, for the first time, a detailed correlation between the electrical conductivity of undoped and 1 atom% Ti doped hematite and the conditions under which it was annealed (20 ≤ T ≤ 800 °C and 10(-4) ≤ pO2 ≤ 1 atm). Hematite thin films grown by pulsed laser deposition onto sapphire single crystals were evaluated by impedance spectroscopy. Hematite's room temperature conductivity can be increased from ∼10(-11) S cm(-1) for undoped films by as much as nine orders of magnitude by doping with the Ti donor. Furthermore, by controlling the non-stoichiometry of Ti-doped hematite, one can tune its conductivity by up to five orders of magnitude. Depending on processing conditions, donor dopants in hematite may be compensated largely by electrons or by ionic defects (Fe vacancies). A defect model was derived to explain this phenomenon. In addition, a temperature independent value for the electron mobility of 0.01 cm(2) V(-1) s(-1) for a donor density of 4.0 × 10(20) cm(-3) (1% Ti) was derived. These results highlight the importance of carefully controlling photoanode processing conditions, even when operating within the material's extrinsic dopant regime, and more generally, provide a model for the electronic properties of semiconducting metal oxide photoanodes.

  6. Strain-modulated ferromagnetism and band gap of Mn doped Bi2Se3

    PubMed Central

    Qi, Shifei; Yang, Hualing; Chen, Juan; Zhang, Xiaoyang; Yang, Yingping; Xu, Xiaohong

    2016-01-01

    The quantized anomalous Hall effect (QAHE) have been theoretically predicted and experimentally confirmed in magnetic topological insulators (TI), but dissipative channels resulted by small-size band gap and weak ferromagnetism make QAHE be measured only at extremely low temperature (<0.1 K). Through density functional theory calculations, we systemically study of the magnetic properties and electronic structures of Mn doped Bi2Se3 with in-plane and out-of-plane strains. It is found that out-of-plane tensile strain not only improve ferromagnetism, but also enlarge Dirac-mass gap (up to 65.6 meV under 6% strain, which is higher than the thermal motion energy at room temperature ~26 meV) in the Mn doped Bi2Se3. Furthermore, the underlying mechanisms of these tunable properties are also discussed. This work provides a new route to realize high-temperature QAHE and paves the way towards novel quantum electronic device applications. PMID:27374782

  7. Strain-modulated ferromagnetism and band gap of Mn doped Bi2Se3.

    PubMed

    Qi, Shifei; Yang, Hualing; Chen, Juan; Zhang, Xiaoyang; Yang, Yingping; Xu, Xiaohong

    2016-01-01

    The quantized anomalous Hall effect (QAHE) have been theoretically predicted and experimentally confirmed in magnetic topological insulators (TI), but dissipative channels resulted by small-size band gap and weak ferromagnetism make QAHE be measured only at extremely low temperature (<0.1 K). Through density functional theory calculations, we systemically study of the magnetic properties and electronic structures of Mn doped Bi2Se3 with in-plane and out-of-plane strains. It is found that out-of-plane tensile strain not only improve ferromagnetism, but also enlarge Dirac-mass gap (up to 65.6 meV under 6% strain, which is higher than the thermal motion energy at room temperature ~26 meV) in the Mn doped Bi2Se3. Furthermore, the underlying mechanisms of these tunable properties are also discussed. This work provides a new route to realize high-temperature QAHE and paves the way towards novel quantum electronic device applications.

  8. Strain-modulated ferromagnetism and band gap of Mn doped Bi2Se3

    NASA Astrophysics Data System (ADS)

    Qi, Shifei; Yang, Hualing; Chen, Juan; Zhang, Xiaoyang; Yang, Yingping; Xu, Xiaohong

    2016-07-01

    The quantized anomalous Hall effect (QAHE) have been theoretically predicted and experimentally confirmed in magnetic topological insulators (TI), but dissipative channels resulted by small-size band gap and weak ferromagnetism make QAHE be measured only at extremely low temperature (<0.1 K). Through density functional theory calculations, we systemically study of the magnetic properties and electronic structures of Mn doped Bi2Se3 with in-plane and out-of-plane strains. It is found that out-of-plane tensile strain not only improve ferromagnetism, but also enlarge Dirac-mass gap (up to 65.6 meV under 6% strain, which is higher than the thermal motion energy at room temperature ~26 meV) in the Mn doped Bi2Se3. Furthermore, the underlying mechanisms of these tunable properties are also discussed. This work provides a new route to realize high-temperature QAHE and paves the way towards novel quantum electronic device applications.

  9. Generation of self-induced-transparency gap solitons by modulational instability in uniformly doped fiber Bragg gratings

    SciTech Connect

    Kalithasan, B.; Porsezian, K.; Senthilnathan, K.; Tchofo Dinda, P.

    2010-05-15

    We consider the continuous-wave (cw) propagation through a fiber Bragg grating that is uniformly doped with two-level resonant atoms. Wave propagation is governed by a system of nonlinear coupled-mode Maxwell-Bloch (NLCM-MB) equations. We identify modulational instability (MI) conditions required for the generation of ultrashort pulses in both anomalous and normal dispersion regimes. From a detailed linear stability analysis, we find that the atomic detuning frequency has a strong influence on the MI. That is, the atomic detuning frequency induces nonconventional MI sidebands at the photonic band gap (PBG) edges and near the PBG edges. Especially in the normal dispersion regime, MI occurs without any threshold condition, which is in contrast with that of conventional fiber Bragg gratings. We also perform a numerical analysis to solve the NLCM-MB equations. The numerical results of the prediction of both the optimum modulation wave number and the optimum gain agree well with that of the linear stability analysis. Another main result of the present work is the prediction of the existence of both bright and dark self-induced transparency gap solitons at the PBG edges.

  10. Multifunctional tunable multiwavelength erbium-doped fiber laser based on tunable comb filter and intensity-dependent loss modulation

    NASA Astrophysics Data System (ADS)

    Quan, Mingran; Li, Yuan; Tian, Jiajun; Yao, Yong

    2015-04-01

    A multiwavelength erbium-doped fiber laser based on tunable comb spectral filter and intensity-dependent loss modulation is proposed and experimentally demonstrated. The laser allows fine and multifunctional tunable operations of channel-spacing, peak-location, spectral-range, and wavelength-number. More specifically, channel-spacing switch from 0.4 nm to 0.2 nm and peak-location adjustment within half of free spectrum range are obtained via controlling the tunable comb filter. The wavelength-number and the spectral-range of the lasing lines can be accurately controlled by intensity-dependent loss modulation in the laser cavity, enabled by a power-symmetric nonlinear optical loop mirror. In addition, fine control over the wavelength-number at fixed spectral-range is realized by simply adjusting the pump power. More important, the tunable operation process for every type of specific parameter is individual, without influences for other output parameters. Such features of this fiber laser make it useful and convenient for the practical application.

  11. 27Al fourier-transform electron-spin-echo modulation of Cu 2+-doped zeolites A and X

    NASA Astrophysics Data System (ADS)

    Goldfarb, Daniella; Kevan, Larry

    Cu 2+-doped NaA, CaA, and NaX zeolites were studied using the electron-spin-echo modulation (ESEM) method. In both hydrated and dehydrated samples 27Al modulation has been observed. The time-domain ESEM traces were Fourier transformed and analyzed in the frequency domain. All FT-ESEM spectra of the hydrated samples showed a single peak at the Larmor frequency of 27Ai, indicating that the zeeman interaction is dominant and that the 27Al quadrupole and hyperfine interactions are relatively small. Considerable changes in the spectrum appear upon dehydration. Several frequencies significantly different from the Larmor frequency appear and the spectrum depends on the major cocation present. The major features of the spectra of the dehydrated zeolites could be theoretically reproduced, using exact diagonalization of the nuclear Hamiltonian, with relatively large isotropic hyperfine and quadrupole coupling constants. For example, in CuCaA and CuNaA zeolites the isotropic hyperfine constant is in the range of 0.2-0.5 and 0.8-1.0 MHz, respectively, with the quadrupole coupling constant in the range of 6-10 MHz for both.

  12. Heterostructures as a quantum optical klistron

    SciTech Connect

    Malov, Yu.A.

    1995-12-31

    THE beam of {open_quotes}hot{close_quotes} ballistic electrons which were first obtained experimentally in (1) is considered when passing through the heterostructures consisting of two potential barriers in barriers in the presence of FEL tuning in infra-red region. In the presence of the first barrier the electron beam in the FEL electromagnetic field can either absorb or emit the field quanta. The initial electron beam may split into states n=1 (absorption of one quantum), n=0 (the beam energy unchanged), and n=-1 (emission of one quantum). The interference of the states with n=0, n=1 and n=0, n=1 results in the initiation of two traveling modulation waves at electromagnetic frequency w. Beats between these waves in the region of the of their overlapping lead to a periodic dependence of the modulation from w. It has been found that the spontaneous coherent radiation (SCR) intensity oscillates with the period depending on FEL frequency w. It is possible to increase or to decrease the spectral intensity of SCR varying w. The suggested idea of modulation on the first barrier of heterostructures and the radiation of modulated electron beam on the second barrier is a scheme of an quantum optical klistron in infra-red region.

  13. Npn double heterostructure bipolar transistor with ingaasn base region

    DOEpatents

    Chang, Ping-Chih; Baca, Albert G.; Li, Nein-Yi; Hou, Hong Q.; Ashby, Carol I. H.

    2004-07-20

    An NPN double heterostructure bipolar transistor (DHBT) is disclosed with a base region comprising a layer of p-type-doped indium gallium arsenide nitride (InGaAsN) sandwiched between n-type-doped collector and emitter regions. The use of InGaAsN for the base region lowers the transistor turn-on voltage, V.sub.on, thereby reducing power dissipation within the device. The NPN transistor, which has applications for forming low-power electronic circuitry, is formed on a gallium arsenide (GaAs) substrate and can be fabricated at commercial GaAs foundries. Methods for fabricating the NPN transistor are also disclosed.

  14. Modulation of metal-insulator transitions by field-controlled strain in NdNiO3/SrTiO3/PMN-PT (001) heterostructures

    NASA Astrophysics Data System (ADS)

    Heo, Seungyang; Oh, Chadol; Eom, Man Jin; Kim, Jun Sung; Ryu, Jungho; Son, Junwoo; Jang, Hyun Myung

    2016-02-01

    The band width control through external stress has been demonstrated as a useful knob to modulate metal-insulator transition (MIT) in RNiO3 as a prototype correlated materials. In particular, lattice mismatch strain using different substrates have been widely utilized to investigate the effect of strain on transition temperature so far but the results were inconsistent in the previous literatures. Here, we demonstrate dynamic modulation of MIT based on electric field-controlled pure strain in high-quality NdNiO3 (NNO) thin films utilizing converse-piezoelectric effect of (001)-cut - (PMN-PT) single crystal substrates. Despite the difficulty in the NNO growth on rough PMN-PT substrates, the structural quality of NNO thin films has been significantly improved by inserting SrTiO3 (STO) buffer layers. Interestingly, the MIT temperature in NNO is downward shifted by ~3.3 K in response of 0.25% in-plane compressive strain, which indicates less effective TMI modulation of field-induced strain than substrate-induced strain. This study provides not only scientific insights on band-width control of correlated materials using pure strain but also potentials for energy-efficient electronic devices.

  15. Modulation of metal-insulator transitions by field-controlled strain in NdNiO3/SrTiO3/PMN-PT (001) heterostructures.

    PubMed

    Heo, Seungyang; Oh, Chadol; Eom, Man Jin; Kim, Jun Sung; Ryu, Jungho; Son, Junwoo; Jang, Hyun Myung

    2016-01-01

    The band width control through external stress has been demonstrated as a useful knob to modulate metal-insulator transition (MIT) in RNiO3 as a prototype correlated materials. In particular, lattice mismatch strain using different substrates have been widely utilized to investigate the effect of strain on transition temperature so far but the results were inconsistent in the previous literatures. Here, we demonstrate dynamic modulation of MIT based on electric field-controlled pure strain in high-quality NdNiO3 (NNO) thin films utilizing converse-piezoelectric effect of (001)-cut Pb(Mg(1/3)Nb(2/3)O3-(PbTiO3) (PMN-PT) single crystal substrates. Despite the difficulty in the NNO growth on rough PMN-PT substrates, the structural quality of NNO thin films has been significantly improved by inserting SrTiO3 (STO) buffer layers. Interestingly, the MIT temperature in NNO is downward shifted by ~3.3 K in response of 0.25% in-plane compressive strain, which indicates less effective TMI modulation of field-induced strain than substrate-induced strain. This study provides not only scientific insights on band-width control of correlated materials using pure strain but also potentials for energy-efficient electronic devices. PMID:26916618

  16. Modulation of metal-insulator transitions by field-controlled strain in NdNiO3/SrTiO3/PMN-PT (001) heterostructures

    PubMed Central

    Heo, Seungyang; Oh, Chadol; Eom, Man Jin; Kim, Jun Sung; Ryu, Jungho; Son, Junwoo; Jang, Hyun Myung

    2016-01-01

    The band width control through external stress has been demonstrated as a useful knob to modulate metal-insulator transition (MIT) in RNiO3 as a prototype correlated materials. In particular, lattice mismatch strain using different substrates have been widely utilized to investigate the effect of strain on transition temperature so far but the results were inconsistent in the previous literatures. Here, we demonstrate dynamic modulation of MIT based on electric field-controlled pure strain in high-quality NdNiO3 (NNO) thin films utilizing converse-piezoelectric effect of (001)-cut - (PMN-PT) single crystal substrates. Despite the difficulty in the NNO growth on rough PMN-PT substrates, the structural quality of NNO thin films has been significantly improved by inserting SrTiO3 (STO) buffer layers. Interestingly, the MIT temperature in NNO is downward shifted by ~3.3 K in response of 0.25% in-plane compressive strain, which indicates less effective TMI modulation of field-induced strain than substrate-induced strain. This study provides not only scientific insights on band-width control of correlated materials using pure strain but also potentials for energy-efficient electronic devices. PMID:26916618

  17. Air-gap heterostructures

    SciTech Connect

    Heyn, Ch.; Schmidt, M.; Schwaiger, S.; Stemmann, A.; Mendach, S.; Hansen, W.

    2011-01-17

    We demonstrate the fabrication of thin GaAs layers which quasi hover above the underlying GaAs substrate. The hovering layers have a perfect epitaxial relationship to the substrate crystal lattice and are connected to the substrate surface only by lattice matched nanopillars of low density. These air-gap heterostructures are created by combining in situ molecular beam epitaxy compatible self-assembled droplet-etching and ex situ selective wet-chemical etching.

  18. Magneto-transport analysis of an ultra-low-density two-dimensional hole gas in an undoped strained Ge/SiGe heterostructure

    DOE PAGES

    Laroche, D.; Huang, S. -H.; Chuang, Y.; Li, J. -Y.; Liu, C. W.; Lu, T. M.

    2016-06-06

    We report the magneto-transport, scattering mechanisms, and e ective mass analysis of an ultralow density two-dimensional hole gas capacitively induced in an undoped strained Ge/Si0:2Ge0:8 heterostructure. This fabrication technique allows hole densities as low as p 1:1 1010 cm² to be achieved, more than one order of magnitude lower than previously reported in doped Ge/SiGe heterostructures. The power-law exponent of the electron mobility versus density curve, / n , is found to be 0:29 over most of the density range, implying that background impurity scattering is the dominant scattering mechanism at intermediate densities in such devices. A charge migration modelmore » is used to explain the mobility decrease at the highest achievable densities. The hole e ective mass is deduced from the temperature dependence of Shubnikov-de Haas oscillations. At p 1:0 1011cm², the e ective mass m is 0:105 m0, which is signi cantly larger than masses obtained from modulation-doped Ge/SiGe two-dimensional hole gases.« less

  19. Magneto-transport analysis of an ultra-low-density two-dimensional hole gas in an undoped strained Ge/SiGe heterostructure

    NASA Astrophysics Data System (ADS)

    Laroche, D.; Huang, S.-H.; Chuang, Y.; Li, J.-Y.; Liu, C. W.; Lu, T. M.

    2016-06-01

    We report the magneto-transport, scattering mechanisms, and effective mass analysis of an ultra-low density two-dimensional hole gas capacitively induced in an undoped strained Ge/Si0.2Ge0.8 heterostructure. This fabrication technique allows hole densities as low as p ˜ 1.1 × 1010 cm-2 to be achieved, more than one order of magnitude lower than previously reported in doped Ge/SiGe heterostructures. The power-law exponent of the electron mobility versus density curve, μ ∝ nα, is found to be α ˜ 0.29 over most of the density range, implying that background impurity scattering is the dominant scattering mechanism at intermediate densities in such devices. A charge migration model is used to explain the mobility decrease at the highest achievable densities. The hole effective mass is deduced from the temperature dependence of Shubnikov-de Haas oscillations. At p ˜ 1.0 × 1011 cm-2, the effective mass m* is ˜0.105 m0, which is significantly larger than masses obtained from modulation-doped Ge/SiGe two-dimensional hole gases.

  20. Gate-Tunable Spin Transport and Giant Electroresistance in Ferromagnetic Graphene Vertical Heterostructures

    NASA Astrophysics Data System (ADS)

    Myoung, Nojoon; Park, Hee Chul; Lee, Seung Joo

    2016-04-01

    Controlling tunneling properties through graphene vertical heterostructures provides advantages in achieving large conductance modulation which has been known as limitation in lateral graphene device structures. Despite of intensive research on graphene vertical heterosturctures for recent years, the potential of spintronics based on graphene vertical heterostructures remains relatively unexplored. Here, we present an analytical device model for graphene-based spintronics by using ferromagnetic graphene in vertical heterostructures. We consider a normal or ferroelectric insulator as a tunneling layer. The device concept yields a way of controlling spin transport through the vertical heterostructures, resulting in gate-tunable spin-switching phenomena. Also, we revealed that a ‘giant’ resistance emerges through a ferroelectric insulating layer owing to the anti-parallel configuration of ferromagnetic graphene layers by means of electric fields via gate and bias voltages. Our findings discover the prospect of manipulating the spin transport properties in vertical heterostructures without use of magnetic fields.

  1. Gate-Tunable Spin Transport and Giant Electroresistance in Ferromagnetic Graphene Vertical Heterostructures.

    PubMed

    Myoung, Nojoon; Park, Hee Chul; Lee, Seung Joo

    2016-01-01

    Controlling tunneling properties through graphene vertical heterostructures provides advantages in achieving large conductance modulation which has been known as limitation in lateral graphene device structures. Despite of intensive research on graphene vertical heterosturctures for recent years, the potential of spintronics based on graphene vertical heterostructures remains relatively unexplored. Here, we present an analytical device model for graphene-based spintronics by using ferromagnetic graphene in vertical heterostructures. We consider a normal or ferroelectric insulator as a tunneling layer. The device concept yields a way of controlling spin transport through the vertical heterostructures, resulting in gate-tunable spin-switching phenomena. Also, we revealed that a 'giant' resistance emerges through a ferroelectric insulating layer owing to the anti-parallel configuration of ferromagnetic graphene layers by means of electric fields via gate and bias voltages. Our findings discover the prospect of manipulating the spin transport properties in vertical heterostructures without use of magnetic fields. PMID:27126101

  2. Gate-Tunable Spin Transport and Giant Electroresistance in Ferromagnetic Graphene Vertical Heterostructures

    PubMed Central

    Myoung, Nojoon; Park, Hee Chul; Lee, Seung Joo

    2016-01-01

    Controlling tunneling properties through graphene vertical heterostructures provides advantages in achieving large conductance modulation which has been known as limitation in lateral graphene device structures. Despite of intensive research on graphene vertical heterosturctures for recent years, the potential of spintronics based on graphene vertical heterostructures remains relatively unexplored. Here, we present an analytical device model for graphene-based spintronics by using ferromagnetic graphene in vertical heterostructures. We consider a normal or ferroelectric insulator as a tunneling layer. The device concept yields a way of controlling spin transport through the vertical heterostructures, resulting in gate-tunable spin-switching phenomena. Also, we revealed that a ‘giant’ resistance emerges through a ferroelectric insulating layer owing to the anti-parallel configuration of ferromagnetic graphene layers by means of electric fields via gate and bias voltages. Our findings discover the prospect of manipulating the spin transport properties in vertical heterostructures without use of magnetic fields. PMID:27126101

  3. Nitrogen-doped carbon nanoparticle modulated turn-on fluorescent probes for histidine detection and its imaging in living cells

    NASA Astrophysics Data System (ADS)

    Zhu, Xiaohua; Zhao, Tingbi; Nie, Zhou; Miao, Zhuang; Liu, Yang; Yao, Shouzhuo

    2016-01-01

    In this work, nitrogen-doped carbon nanoparticle (N-CNP) modulated turn-on fluorescent probes were developed for rapid and selective detection of histidine. The as synthesized N-CNPs exhibited high fluorescence quantum yield and excellent biocompatibility. The fluorescence of N-CNPs can be quenched selectively by Cu(ii) ions with high efficiency, and restored by the addition of histidine owing to the competitive binding of Cu(ii) ions and histidine that removes Cu(ii) ions from the surface of the N-CNPs. Under the optimal conditions, a linear relationship between the increased fluorescence intensity of N-CNP/Cu(ii) ion conjugates and the concentration of histidine was established in the range from 0.5 to 60 μM. The detection limit was as low as 150 nM (signal-to-noise ratio of 3). In addition, the as-prepared N-CNP/Cu(ii) ion nanoprobes showed excellent biocompatibility and were applied for a histidine imaging assay in living cells, which presented great potential in the bio-labeling assay and clinical diagnostic applications.In this work, nitrogen-doped carbon nanoparticle (N-CNP) modulated turn-on fluorescent probes were developed for rapid and selective detection of histidine. The as synthesized N-CNPs exhibited high fluorescence quantum yield and excellent biocompatibility. The fluorescence of N-CNPs can be quenched selectively by Cu(ii) ions with high efficiency, and restored by the addition of histidine owing to the competitive binding of Cu(ii) ions and histidine that removes Cu(ii) ions from the surface of the N-CNPs. Under the optimal conditions, a linear relationship between the increased fluorescence intensity of N-CNP/Cu(ii) ion conjugates and the concentration of histidine was established in the range from 0.5 to 60 μM. The detection limit was as low as 150 nM (signal-to-noise ratio of 3). In addition, the as-prepared N-CNP/Cu(ii) ion nanoprobes showed excellent biocompatibility and were applied for a histidine imaging assay in living cells, which

  4. Fermi-surface induced modulation in an optimally doped YBCO superconductor.

    SciTech Connect

    Liu, X.; Islam, Z.; Sinha, S. K.; Moss, S. C.; McQueeney, R. J.; Lang, J. C.; Welp, U.; Univ. of California at San Diego; Univ. of Houston; Iowa State Univ.

    2008-01-01

    We have observed a Fermi-surface (FS) induced lattice modulation in a YBa{sub 2}Cu{sub 3}O{sub 7-x} superconductor with a wave vector along CuO chains; i.e., q{sub 1} = (0,{delta},0). The value of {delta} {approx} 0.21 is twice the Fermi wave vector (2k{sub F}) along b* connecting nearly nested FS 'ridges'. The q{sub 1} modulation exists only within O-vacancy-ordered islands [characterized by q{sub 0} = (1/4,0,0)] and persists well above and below T{sub c}. Our results are consistent with the presence of a FS-induced charge-density wave.

  5. Probing interlayer interactions in WS2 -graphene van der Waals heterostructures

    NASA Astrophysics Data System (ADS)

    Chung, Ting Fung; Yuan, Long; Huang, Libai; Chen, Yong P.

    Two-dimensional crystals based van der Waals coupled heterostructures are of interest owing to their potential applications for flexible and transparent electronics and optoelectronics. The interaction between the 2D layered crystals at the interfaces of these heterostructures is crucial in determining the overall performance and is strongly affected by contamination and interfacial strain. We have fabricated heterostructures consisting of atomically thin exfoliated WS2 and chemical-vapor-deposited (CVD) graphene, and studied the interaction and coupling between the WS2 and graphene using atomic force microscopy (AFM), Raman spectroscopy and femtosecond transient absorption measurement (TAM). Information from Raman-active phonon modes allows us to estimate charge doping in graphene and interfacial strain on the crystals. Spatial imaging probed by TAM can be correlated to the heterostructure surface morphology measured by AFM and Raman maps of graphene and WS2, showing how the interlayer coupling alters exciton decay dynamics quantitatively.

  6. Imparting electroactivity to polycaprolactone fibers with heparin-doped polypyrrole: Modulation of hemocompatibility and inflammatory responses.

    PubMed

    Xiong, Gordon M; Yuan, Shaojun; Wang, Jun Kit; Do, Anh Tuan; Tan, Nguan Soon; Yeo, Kiat Seng; Choong, Cleo

    2015-09-01

    Hemocompatibility, anti-inflammation and anti-thrombogenicity of acellular synthetic vascular grafts remains a challenge in biomaterials design. Using electrospun polycaprolactone (PCL) fibers as a template, a coating of polypyrrole (PPy) was successfully polymerized onto the fiber surface. The fibers coated with heparin-doped PPy (PPy-HEP) demonstrated better electroactivity, lower surface resistivity (9-10-fold) and better anti-coagulation response (non-observable plasma recalcification after 30min vs. recalcification at 8-9min) as compared to fibers coated with pristine PPy. Red blood cell compatibility, measured by% hemolysis, was greatly improved on PPy-HEP-coated PCL in comparison to uncoated PCL (3.9±2.1% vs. 22.1±4.1%). PPy-HEP-coated PCL fibers also exhibited higher stiffness values (6.8±0.9MPa vs. 4.2±0.8MPa) as compared to PCL fibers, but similar tensile strengths. It was also observed that the application of a low alternating current led to a 4-fold reduction of platelet activation (as quantitated by CD62p expression) for the PPy-HEP-coated fibers as compared to non-stimulated conditions. In parallel, a reduction in the leukocyte adhesion to both pristine PPy-coated and PPy-HEP-coated fibers was observable with AC stimulation. Overall, a new strategy involving the use of hemocompatible conducting polymers and electrical stimulation to control thrombogenicity and inflammatory responses for synthetic vascular graft designs was demonstrated.

  7. Heating of the magnetic-ion spin system in modulation doped ZnMnSe/ZnBeSe quantum wells by means of photoexcitation.

    SciTech Connect

    Keller, D.; Astakhov, G. V.; Yakovlev, D. R.; Barrick, T.; Crooker, S. A.; Hansen, L.; Ossau, W.; Molenkamp, L. W.

    2002-01-01

    Heating of the spin system of magnetic ions by means of photoexcited carriers has been studied in modulation-doped (Zn,Mn)Se/(Zn,Be)Se quantum well structures with different electron densities varying from about 10{sup 9} to 5.5 x 10{sup 11} cm{sup -2}. The elevated temperature of the magnetic ions manifests in a reduced Zeeman splitting of the carriers already for low excitation densities. The efficiency of the heating decreases with increasing electron concentration.

  8. Enhanced photo-assisted electrical gating in vanadium dioxide based on saturation-induced gain modulation of erbium-doped fiber amplifier.

    PubMed

    Lee, Yong Wook; Kim, Bong-Jun; Choi, Sungyoul; Lee, Yong Wan; Kim, Hyun-Tak

    2009-10-26

    By incorporating saturation-induced gain modulation of an erbium-doped fiber amplifier (EDFA), we have demonstrated a high-speed photo-assisted electrical gating with considerably enhanced switching characteristics in a two-terminal device fabricated by using vanadium dioxide thin film. The gating operation was performed by illuminating the output light of the EDFA, whose transient gain was modulated by adjusting the chopping frequency of the input light down to 1 kHz, onto the device. In the proposed gating scheme, gated signals with a temporal duration of approximately 40 micros were successively generated at a repetition rate of 1 kHz. PMID:19997180

  9. Nitrogen-doped carbon nanoparticle modulated turn-on fluorescent probes for histidine detection and its imaging in living cells.

    PubMed

    Zhu, Xiaohua; Zhao, Tingbi; Nie, Zhou; Miao, Zhuang; Liu, Yang; Yao, Shouzhuo

    2016-01-28

    In this work, nitrogen-doped carbon nanoparticle (N-CNP) modulated turn-on fluorescent probes were developed for rapid and selective detection of histidine. The as synthesized N-CNPs exhibited high fluorescence quantum yield and excellent biocompatibility. The fluorescence of N-CNPs can be quenched selectively by Cu(II) ions with high efficiency, and restored by the addition of histidine owing to the competitive binding of Cu(II) ions and histidine that removes Cu(II) ions from the surface of the N-CNPs. Under the optimal conditions, a linear relationship between the increased fluorescence intensity of N-CNP/Cu(II) ion conjugates and the concentration of histidine was established in the range from 0.5 to 60 μM. The detection limit was as low as 150 nM (signal-to-noise ratio of 3). In addition, the as-prepared N-CNP/Cu(II) ion nanoprobes showed excellent biocompatibility and were applied for a histidine imaging assay in living cells, which presented great potential in the bio-labeling assay and clinical diagnostic applications. PMID:26730681

  10. Magnetooptical study of CdSe/ZnMnSe semimagnetic quantum-dot ensembles with n-type modulation doping

    SciTech Connect

    Reshina, I. I. Ivanov, S. V.

    2014-12-15

    Magnetic and polarization investigations of the photoluminescence and resonant electron spin-flip Raman scattering in ensembles of self-organized CdSe/ZnMnSe semimagnetic quantum dots with n-type modulation doping are carried out. It is demonstrated that exciton transitions contribute to the photoluminescence band intensity, along with the transitions of trions in the singlet state. In the Hanle-effect measurements, negative circular polarization in zero magnetic field is observed, which is related to the optical orientation of a trion heavy hole. The lifetime and spin-relaxation time of a heavy hole are estimated as ≤3 and ≤1 ps, respectively. Such short times are assumed to be due to Auger recombination with the excitation of an intrinsic transition in a Mn{sup 2+} ion. Investigations of the photoluminescence-maximum intensity and shift in a longitudinal magnetic field at the σ{sup −}σ{sup +} and σ{sup −}σ{sup −} polarizations reveal the pronounced spin polarization of electrons. Under resonant excitation conditions, a sharp increase in the photoluminescence-band maximum intensity at σ{sup −} excitation polarization over the σ{sup +} one is observed. The Raman scattering peak at the electron spin-flip transition is observed upon resonant excitation in a transverse magnetic field in crossed linear polarizations. This peak is shown to be a Brillouin function of a magnetic field.

  11. Coherent dynamics of Landau-Levels in modulation doped GaAs quantum wells at high magnetic fields

    NASA Astrophysics Data System (ADS)

    Liu, Cunming; Paul, Jagannath; Reno, John; McGill, Stephen; Hilton, David; Karaiskaj, Denis

    By using two-dimensional Fourier transform spectroscopy, we investigate the dynamics of Landau-Levels formed in modulation doped GaAs/AlGaAs quantum wells of 18 nm thickness at high magnetic fields and low temperature. The measurements show interesting dephasing dynamics and linewidth dependency as a function of the magnetic field. The work at USF and UAB was supported by the National Science Foundation under grant number DMR-1409473. The work at NHMFL, FSU was supported by the National Science Foundation under grant numbers DMR-1157490 and DMR-1229217. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Office of Basic Energy Sciences user facility. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under Contract No. DE-AC04-94AL85000.

  12. Variable electronic properties of lateral phosphorene-graphene heterostructures.

    PubMed

    Tian, Xiaoqing; Liu, Lin; Du, Yu; Gu, Juan; Xu, Jian-Bin; Yakobson, Boris I

    2015-12-21

    Phosphorene and graphene have a tiny lattice mismatch along the armchair direction, which can result in an atomically sharp in-plane interface. The electronic properties of the lateral heterostructures of phosphorene/graphene are investigated by the first-principles method. Here, we demonstrate that the electronic properties of this type of heterostructure can be highly tunable by the quantum size effects and the externally applied electric field (Eext). At strong Eext, Dirac Fermions can be developed with Fermi velocities around one order smaller than that of graphene. Undoped and hydrogen doped configurations demonstrate three drastically different electronic phases, which reveal the strongly tunable potential of this type of heterostructure. Graphene is a naturally better electrode for phosphorene. The transport properties of two-probe devices of graphene/phosphorene/graphene exhibit tunnelling transport characteristics. Given these results, it is expected that in-plane heterostructures of phosphorene/graphene will present abundant opportunities for applications in optoelectronic and electronic devices. PMID:26554700

  13. Imparting electroactivity to polycaprolactone fibers with heparin-doped polypyrrole: Modulation of hemocompatibility and inflammatory responses.

    PubMed

    Xiong, Gordon M; Yuan, Shaojun; Wang, Jun Kit; Do, Anh Tuan; Tan, Nguan Soon; Yeo, Kiat Seng; Choong, Cleo

    2015-09-01

    Hemocompatibility, anti-inflammation and anti-thrombogenicity of acellular synthetic vascular grafts remains a challenge in biomaterials design. Using electrospun polycaprolactone (PCL) fibers as a template, a coating of polypyrrole (PPy) was successfully polymerized onto the fiber surface. The fibers coated with heparin-doped PPy (PPy-HEP) demonstrated better electroactivity, lower surface resistivity (9-10-fold) and better anti-coagulation response (non-observable plasma recalcification after 30min vs. recalcification at 8-9min) as compared to fibers coated with pristine PPy. Red blood cell compatibility, measured by% hemolysis, was greatly improved on PPy-HEP-coated PCL in comparison to uncoated PCL (3.9±2.1% vs. 22.1±4.1%). PPy-HEP-coated PCL fibers also exhibited higher stiffness values (6.8±0.9MPa vs. 4.2±0.8MPa) as compared to PCL fibers, but similar tensile strengths. It was also observed that the application of a low alternating current led to a 4-fold reduction of platelet activation (as quantitated by CD62p expression) for the PPy-HEP-coated fibers as compared to non-stimulated conditions. In parallel, a reduction in the leukocyte adhesion to both pristine PPy-coated and PPy-HEP-coated fibers was observable with AC stimulation. Overall, a new strategy involving the use of hemocompatible conducting polymers and electrical stimulation to control thrombogenicity and inflammatory responses for synthetic vascular graft designs was demonstrated. PMID:25983317

  14. Analytical Model to Determine the Gate Leakage Current in In0.52Al0.48As/InxGa1-xAs Pseudomorphic Modulation Doped Field-Effect Transistors Caused by Thermionic Field Emission

    NASA Astrophysics Data System (ADS)

    Dickmann, Jürgen; Daembkes, Heinrich; Schildberg, Steffen; Fittng, Hans-Joachim; Ellrod, Peter; Tegude, Franz

    1994-04-01

    The gate leakage current of pulse doped In0.52Al0.48As/In xGa1- xAs pseudomorphic modulation doped field-effect transistors (MODFETs) is analysed by thermionic field emission theory. For the first time, a theoretically based investigation of the leakage current for this type of device is carried out. The influence of parameters of the layer structure design on the gate leakage current such as the thickness of the barrier layer or the doping concentration in the supply layer is predicted. The model adequately predicts the experimental decrease in leakage current with increased thickness of the barrier layer and reduced doping concentration.

  15. Preface: Heterostructure terahertz devices

    NASA Astrophysics Data System (ADS)

    Ryzhii, Victor

    2008-08-01

    The terahertz (THz) range of frequencies is borderline between microwave electronics and photonics. It corresponds to the frequency bands of molecular and lattice vibrations in gases, fluids, and solids. The importance of the THz range is in part due to numerous potential and emerging applications which include imaging and characterization, detection of hazardous substances, environmental monitoring, radio astronomy, covert inter-satellite communications, as well as biological and medical applications. During the last decades marked progress has been achieved in the development, fabrication, and practical implementation of THz devices and systems. This is primarily owing to the utilization of gaseous and free electron lasers and frequency converters using nonlinear optical phenomena as sources of THz radiation. However, such devices and hence the systems based on them are fairly cumbersome. This continuously stimulates an extensive search for new compact and efficient THz sources based on semiconductor heterostructures. Despite tremendous efforts lasting several decades, the so-called THz gap unbridged by semiconductor heterostructure electron and optoelectron devices still exists providing appropriate levels of power of the generated THz radiation. The invention and realization of quantum cascade lasers made of multiple quantum-well heterostructures already resulted in the partial solution of the problem in question, namely, in the successful coverage of the high-frequency portion of the THz gap (2-3 THz and higher). Further advancement to lower frequencies meets, perhaps, fundamental difficulties. All this necessitates further extensive theoretical and experimental studies of more or less traditional and novel semiconductor heterostructures as a basis for sources of THz radiation. This special issue includes 11 excellent original papers submitted by several research teams representing 14 institutions in Europe, America, and Asia. Several device concepts which

  16. Photoluminescence properties of modulation-doped In{sub x}Al{sub 1–x}As/In{sub y}Ga{sub 1–y}As/In{sub x}Al{sub 1–x}As structures with strained inas and gaas nanoinserts in the quantum well

    SciTech Connect

    Galiev, G. B.; Vasil’evskii, I. S.; Klimov, E. A.; Klochkov, A. N.; Lavruhin, D. V.; Pushkarev, S. S.; Maltsev, P. P.

    2015-09-15

    The photoluminescence spectra of modulation-doped InAlAs/InGaAs/InAlAs heterostructures with quantum wells containing thin strained InAs and GaAs inserts are investigated. It is established that the insertion of pair InAs layers and/ or a GaAs transition barriers with a thickness of 1 nm into a quantum well leads to a change in the form and energy position of the photoluminescence spectra as compared with a uniform In{sub 0.53}Ga{sub 0.47}As quantum well. Simulation of the band structure shows that this change is caused by a variation in the energy and wave functions of holes. It is demonstrated that the use of InAs inserts leads to the localization of heavy holes near the InAs layers and reduces the energy of optical transitions, while the use of GaAs transition barriers can lead to inversion of the positions of the light- and heavy-hole subbands in the quantum well. A technique for separately controlling the light- and heavy-hole states by varying the thickness and position of the GaAs and InAs inserts in the quantum well is suggested.

  17. Chemically modulated multiferroicity in Dy-doped Gd2Ti2O7

    NASA Astrophysics Data System (ADS)

    Lin, L.; Zhao, Z. Y.; Liu, D.; Xie, Y. L.; Dong, S.; Yan, Z. B.; Liu, J.-M.

    2013-05-01

    The ferroelectricity and magnetoelectric coupling of Gd2Ti2O7 and Gd2-xDyxTi2O7 with Dy3+ substitution of Gd3+ are investigated. For Gd2Ti2O7, a ferroelectric polarization emerges at ˜30 K and is only ˜1.80 μC/m2 at 2 K, while the magnetoelectric response is quite weak. The Dy3+ substitution in Gd2-xDyxTi2O7, however, results in significant enhancement of polarization with remarkable magnetoelectric response up to 35% at 2 K under a magnetic field of 9 T, suggesting the multiferroicity of Gd2-xDyxTi2O7. It is understood that the chemical modulation of the multiferroicity is basically related to the extreme sensitivity of the spin-spin interactions in this highly frustrated system.

  18. Enhancement of donor ionization in phosphorus-doped n-diamond

    NASA Astrophysics Data System (ADS)

    Koide, Yasuo

    2005-05-01

    In order to explore a possibility for controlling an electron concentration in phosphorus-doped n-diamond, electron and ionized-donor concentrations in n-diamond/cBN and n-diamond/AlN heterojunctions are analyzed by self-consistently solving Poisson and Schrödinger equations. Although the electron concentration is an order of 10 11 cm -3 at room temperature for single n-diamond with a donor concentration of 5 × 10 18 cm -3 and a compensation ratio of 0.01, a modulation-doping technique predicts to provide full ionization of phosphorus donor in the n-diamond/cBN heterostructure and generation of an electron concentration larger than 10 18 cm -3 at room temperature.

  19. Modulation of the thermodynamic, kinetic, and magnetic properties of the hydrogen monomer on graphene by charge doping.

    PubMed

    Huang, Liang Feng; Ni, Mei Yan; Zhang, Guo Ren; Zhou, Wang Huai; Li, Yong Gang; Zheng, Xiao Hong; Zeng, Zhi

    2011-08-14

    The thermodynamic, kinetic, and magnetic properties of the hydrogen monomer on doped graphene layers were studied by ab initio simulations. Electron doping heightens the diffusion potential barrier, while hole doping lowers it. However, both kinds of dopings heighten the desorption potential barrier. The underlying mechanism was revealed by investigating the effect of charge doping on the bond strength of graphene and on the electron transfer and the coulomb interaction between the hydrogen monomer and graphene. The kinetic properties of H and D monomers on doped graphene layers during both the annealing process (annealing time t(0) = 300 s) and the constant-rate heating process (heating rate α = 1.0 K/s) were simulated. Macroscopic diffusion of hydrogen monomers on graphene can be achieved when the doping-hole density reaches 5.0 × 10(13) cm(-2). Both electron and hole dopings linearly reduce the total magnetic moment and exchange splitting, which was explained by a simple exchange model. The laws found in this work had been generalized to explain many phenomena reported in literature. This study can further enhance the understanding of the interaction between hydrogen and graphene and was expected to be helpful in the design of hydrogenated-graphene-based devices. PMID:21842947

  20. Final Report for Award DE-SC0005403. Improved Electrochemical Performance of Strained Lattice Electrolytes via Modulated Doping

    SciTech Connect

    Hertz, Joshua L.; Prasad, Ajay K.

    2015-09-06

    The enclosed document provides a final report to document the research performed at the University of Delaware under Grant DE-SC0005403: Improved Electrochemical Performance of Strained Lattice Electrolytes via Modulated Doping. The ultimate goal of this project was to learn how to systematically strain the inter-atomic distance in thin ceramic films and how to use this newfound control to improve the ease by which oxygen ions can conduct through the films. Increasing the ionic conductivity of ceramics holds the promise of drastic improvements in the performance of solid oxide fuel cells, chemical sensors, gas permeation membranes, and related devices. Before this work, the experimental evidence advocating for strain-based techniques was often controversial and poorly characterized. Enabling much of this work was a new method to quickly create a very wide range of ceramic nanostructures that was established during the first phase of the project. Following this initial phase, we created a variety of promising nanostructured epitaxial films and multilayers with systematic variations in lattice mismatch and dopant content. Over the course of the work, a positive effect of tensile atomic strain on the oxygen conductivity was conclusively found using a few different forms of samples and experimental techniques. The samples were built by sputtering, an industrially scalable technique, and thus the technological implementation of these results may be economically feasible. Still, two other results consistently achieved over multiple efforts in this work give pause. The first of these results was that very specific, pristine surfaces upon which to build the nanostructures were strictly required in order to achieve measurable results. The second of these results was that compressively strained films with concomitant reductions in oxygen conductivity are much easier to obtain relative to tensile-strained films with increased conductivity.

  1. Epitaxial magnetic oxide heterostructures

    NASA Astrophysics Data System (ADS)

    Belenky, Land J.

    Perovskite oxides exhibit a range of physical properties including insulator, semiconductor, metal, superconductor, ferromagnet and many more. Interactions between order parameters result in new properties such as the multiferroic materials. The production of artificial layered epitaxial magnetic heterostructures motivates this research. This requires atomic layer controlled growth which depends on selection of materials for their structural and chemical compatibility, preparation of substrates to achieve well-defined surfaces at the atomic level and the development of a deposition and analysis technique capable of controlling growth' at this level. We have used a pulsed laser deposition system with in situ reflection high-energy electron diffraction to produce epitaxial magnetic oxide heterostructures on lattice-matched substrates and have investigated a number of magnetic interactions. We have demonstrated an unusual antiferromagnetic interfacial exchange coupling between epitaxial bilayers of La0.67Sr0.33MnO 3 and SrRuO3 grown on (001) SrTiO3 substrates. The sign and magnitude of the exchange field depends on the cooling field. By interrupting the charge transfer at the interface with a very thin insulating layer, we have demonstrated this exchange biasing effect is related to the spin-dependent band structures of the materials. We have investigated the structural and magnetic properties of epitaxial multilayers and superlattices of manganites. These materials exhibit colossal magnetoresistance and the Curie temperature can be adjusted over a range of 100 K. We have fabricated La0.67Sr0.33MnO3/La 0.82Ba0.18MnO3 superlattices with layers as thin as 8 unit cells (32A). These superlattices have magnetic transition temperatures above 350 K and coercivities of approximately 10 Oe. Deposition techniques can effectively control the out-of-plane dimension on the nanoscale but control or lateral dimensions has proven more challenging. We have fabricated magnetic

  2. Anomalous enhancement of the superconducting transition temperature of electron-doped La2-xCexCuO4 and Pr2-xCexCuO4 cuprate heterostructures

    NASA Astrophysics Data System (ADS)

    Jin, K.; Bach, P.; Zhang, X. H.; Grupel, U.; Zohar, E.; Diamant, I.; Dagan, Y.; Smadici, S.; Abbamonte, P.; Greene, R. L.

    2011-02-01

    The superconducting transition temperature Tc of multilayers of electron-doped cuprates, composed of underdoped (or undoped) and overdoped La2-xCexCuO4 (LCCO) and Pr2-xCexCuO4 (PCCO) thin films, is found to increase significantly with respect to the Tc of the corresponding single-phase films. By investigating the critical current density of superlattices with different doping levels and layer thicknesses, we find that the Tc enhancement is caused by a redistribution of charge over an anomalously large distance.

  3. MBE HgCdTe heterostructure detectors

    NASA Technical Reports Server (NTRS)

    Schulman, Joel N.; Wu, Owen K.

    1990-01-01

    HgCdTe has been the mainstay for medium (3 to 5 micron) and long (10 to 14 micron) wavelength infrared detectors in recent years. Conventional growth and processing techniques are continuing to improve the material. However, the additional ability to tailor composition and placement of doped layers on the tens of angstroms scale using molecular beam epitaxy (MBE) provides the opportunity for new device physics and concepts to be utilized. MBE-based device structures to be discussed here can be grouped into two categories: tailored conventional structures and quantum structures. The tailored conventional structures are improvements on familiar devices, but make use of the ability to create layers of varying composition, and thus band gap, at will. The heterostructure junction can be positioned independently of doping p-n junctions. This allows the small band gap region in which the absorption occurs to be separated from a larger band gap region in which the electric field is large and where unwanted tunneling can occur. Data from hybrid MBE/liquid phase epitaxy (LPE)/bulk structures are given. Quantum structures include the HgTe-CdTe superlattice, in which the band gap and transport can be controlled by alternating thin layers (tens of angstroms thick) of HgTe and CdTe. The superlattice has been shown to exhibit behavior which is non-alloy like, including very high hole mobilities, two-dimensional structure in the absorption coefficient, resonant tunneling, and anisotropic transport.

  4. Strain mediated coupling in magnetron sputtered multiferroic PZT/Ni-Mn-In/Si thin film heterostructure

    SciTech Connect

    Singh, Kirandeep; Kaur, Davinder; Singh, Sushil Kumar

    2014-09-21

    The strain mediated electrical and magnetic properties were investigated in PZT/Ni-Mn-In heterostructure deposited on Si (100) by dc/rf magnetron sputtering. X-ray diffraction pattern revealed that (220) orientation of Ni-Mn-In facilitate the (110) oriented tertragonal phase growth of PZT layer in PZT/Ni-Mn-In heterostructure. A distinctive peak in dielectric constant versus temperature plots around martensitic phase transformation temperature of Ni-Mn-In showed a strain mediated coupling between Ni-Mn-In and PZT layers. The ferroelectric measurement taken at different temperatures exhibits a well saturated and temperature dependent P-E loops with a highest value of P{sub sat}~55 μC/cm² obtained during martensite-austenite transition temperature region of Ni-Mn-In. The stress induced by Ni-Mn-In layer on upper PZT film due to structural transformation from martensite to austenite resulted in temperature modulated Tunability of PZT/Ni-Mn-In heterostructure. A tunability of 42% was achieved at 290 K (structural transition region of Ni-Mn-In) in these heterostructures. I-V measurements taken at different temperatures indicated that ohmic conduction was the main conduction mechanism over a large electric field range in these heterostructures. Magnetic measurement revealed that heterostructure was ferromagnetic at room temperature with a saturation magnetization of ~123 emu/cm³. Such multiferroic heterostructures exhibits promising applications in various microelectromechanical systems.

  5. Electric-field induced strain modulation of magnetization in Fe-Ga/Pb(Mg{sub 1/3}Nb{sub 2/3})-PbTiO{sub 3} magnetoelectric heterostructures

    SciTech Connect

    Zhang, Yue Wang, Zhiguang; Wang, Yaojin; Luo, Chengtao; Li, Jiefang; Viehland, Dwight

    2014-02-28

    Magnetostrictive Fe-Ga thin layers were deposited on 〈110〉-oriented Pb(Mg{sub 1/3}Nb{sub 2/3})-30%PbTiO{sub 3} (PMN-30%PT) substrates by pulsed laser deposition. The as-prepared heterostructures showed columnar arrays aligned in the out-of-plane direction. Transmission electron microscopy revealed nanocrystalline regions within the columnar arrays of the Fe-Ga film. The heterostructure exhibited a strong converse magnetoelectric coupling effect of up to 4.55 × 10{sup −7} s m{sup −1}, as well as an electric field tunability of the in-plane magnetic anisotropy. Furthermore, the remanent magnetization states of the Fe-Ga films can be reversibly and irreversibly changed by external electric fields, suggesting a promising and robust application in magnetic random access memories and spintronics.

  6. Zinc-oxide-based nanostructured materials for heterostructure solar cells

    SciTech Connect

    Bobkov, A. A.; Maximov, A. I.; Moshnikov, V. A. Somov, P. A.; Terukov, E. I.

    2015-10-15

    Results obtained in the deposition of nanostructured zinc-oxide layers by hydrothermal synthesis as the basic method are presented. The possibility of controlling the structure and morphology of the layers is demonstrated. The important role of the procedure employed to form the nucleating layer is noted. The faceted hexagonal nanoprisms obtained are promising for the fabrication of solar cells based on oxide heterostructures, and aluminum-doped zinc-oxide layers with petal morphology, for the deposition of an antireflection layer. The results are compatible and promising for application in flexible electronics.

  7. Engineering the Morphology and Configuration of Ternary Heterostructures for Improving Their Photocatalytic Activity.

    PubMed

    Li, Kui; Chen, Rong; Li, Shun-Li; Xie, Shuai-Lei; Cao, Xue-Li; Dong, Long-Zhang; Bao, Jian-Chun; Lan, Ya-Qian

    2016-02-01

    Heteronanomaterials composed of suitable semiconductors enable the direct conversion from solar power into clean and renewable energy. Ternary heterostructures with appropriate configuration and morphology possess rich and varied properties, especially for improving the photocatalytic activity and stability synchronously. However, suitable ternary heterostructure prototypes and facile while effective strategy for modulating their morphology and configuration are still scarce. Herein, various ternary ZnS-CdS-Zn(1-x)Cd(x)S heterostructures with tunable morphology (0 to 2 D) and semiconductor configurations (randomly distributed, interface mediated, and quantum dots sensitized core@shell heterostructures) were facilely synthesized via one-pot hydrothermal method resulting from the different molecular structures of the amine solvents. Semiconductor morphology, especially configuration of the ternary heterostructure, shows dramatic effect on their photocatalytic activity. The CdS sensitized porous Zn(1-x)CdxS@ZnS core@shell takes full advantage of ZnS, Zn(1-x)Cd(x)S and CdS and shows the maximal photocatalytic H2-production rate of 100.2 mmol/h/g and excellent stability over 30 h. This study provides some guidelines for the design and synthesis of high-performance ternary heterostructure via modulation of semiconductor configuration and morphology using one-pot method.

  8. Engineering the Morphology and Configuration of Ternary Heterostructures for Improving Their Photocatalytic Activity.

    PubMed

    Li, Kui; Chen, Rong; Li, Shun-Li; Xie, Shuai-Lei; Cao, Xue-Li; Dong, Long-Zhang; Bao, Jian-Chun; Lan, Ya-Qian

    2016-02-01

    Heteronanomaterials composed of suitable semiconductors enable the direct conversion from solar power into clean and renewable energy. Ternary heterostructures with appropriate configuration and morphology possess rich and varied properties, especially for improving the photocatalytic activity and stability synchronously. However, suitable ternary heterostructure prototypes and facile while effective strategy for modulating their morphology and configuration are still scarce. Herein, various ternary ZnS-CdS-Zn(1-x)Cd(x)S heterostructures with tunable morphology (0 to 2 D) and semiconductor configurations (randomly distributed, interface mediated, and quantum dots sensitized core@shell heterostructures) were facilely synthesized via one-pot hydrothermal method resulting from the different molecular structures of the amine solvents. Semiconductor morphology, especially configuration of the ternary heterostructure, shows dramatic effect on their photocatalytic activity. The CdS sensitized porous Zn(1-x)CdxS@ZnS core@shell takes full advantage of ZnS, Zn(1-x)Cd(x)S and CdS and shows the maximal photocatalytic H2-production rate of 100.2 mmol/h/g and excellent stability over 30 h. This study provides some guidelines for the design and synthesis of high-performance ternary heterostructure via modulation of semiconductor configuration and morphology using one-pot method. PMID:26835705

  9. Synthesis, fabrication and characterization of Ge/Si axial nanowire heterostructure tunnel FETs

    SciTech Connect

    Picraux, Samuel T; Dayeh, Shadi A

    2010-01-01

    Axial Ge/Si heterostructure nanowires allow energy band-edge engineering along the axis of the nanowire, which is the charge transport direction, and the realization of asymmetric devices for novel device architectures. This work reports on two advances in the area of heterostructure nanowires and tunnel FETs: (i) the realization of 100% compositionally modulated Si/Ge axial heterostructure nanowires with lengths suitable for device fabrication and (ii) the design and implementation of Schottky barrier tunnel FETs on these nanowires for high-on currents and suppressed ambipolar behavior. Initial prototype devices resulted in a current drive in excess of 100 {micro}A/{micro}m (I/{pi}D) and 10{sup 5} I{sub on}/I{sub off} ratios. These results demonstrate the potential of such asymmetric heterostructures (both in the semiconductor channel and metal-semiconductor barrier heights) for low-power and high performance electronics.

  10. Diffusion-Mediated Synthesis of MoS2/WS2 Lateral Heterostructures.

    PubMed

    Bogaert, Kevin; Liu, Song; Chesin, Jordan; Titow, Denis; Gradečak, Silvija; Garaj, Slaven

    2016-08-10

    Controlled growth of two-dimensional transition metal dichalcogenide (TMD) lateral heterostructures would enable on-demand tuning of electronic and optoelectronic properties in this new class of materials. Prior to this work, compositional modulations in lateral TMD heterostructures have been considered to depend solely on the growth chronology. We show that in-plane diffusion can play a significant role in the chemical vapor deposition of MoS2/WS2 lateral heterostructures leading to a variety of nontrivial structures whose composition does not necessarily follow the growth order. Optical, structural, and compositional studies of TMD crystals captured at different growth temperatures and in different diffusion stages suggest that compositional mixing versus segregation are favored at high and low growth temperatures, respectively. The observed diffusion mechanism will expand the realm of possible lateral heterostructures, particularly ones that cannot be synthesized using traditional methods.

  11. Diffusion-Mediated Synthesis of MoS2/WS2 Lateral Heterostructures.

    PubMed

    Bogaert, Kevin; Liu, Song; Chesin, Jordan; Titow, Denis; Gradečak, Silvija; Garaj, Slaven

    2016-08-10

    Controlled growth of two-dimensional transition metal dichalcogenide (TMD) lateral heterostructures would enable on-demand tuning of electronic and optoelectronic properties in this new class of materials. Prior to this work, compositional modulations in lateral TMD heterostructures have been considered to depend solely on the growth chronology. We show that in-plane diffusion can play a significant role in the chemical vapor deposition of MoS2/WS2 lateral heterostructures leading to a variety of nontrivial structures whose composition does not necessarily follow the growth order. Optical, structural, and compositional studies of TMD crystals captured at different growth temperatures and in different diffusion stages suggest that compositional mixing versus segregation are favored at high and low growth temperatures, respectively. The observed diffusion mechanism will expand the realm of possible lateral heterostructures, particularly ones that cannot be synthesized using traditional methods. PMID:27438807

  12. Analysis of nonlinear optical and dynamic gain effects of moderate-power, pulse-position-modulated, erbium-doped fiber amplifiers for deep-space applications.

    PubMed

    Yao, Haomin; Wright, Malcolm W; Marciante, John R

    2014-09-20

    Lasers for use in deep-space applications such as interplanetary optical communications employ multiwatt resonantly pumped dual-clad erbium-doped fiber amplifiers and the pulse-position modulation scheme. Nonlinear optical effects and dynamic gain effects often impair their performance and limit their operational range. These effects are analyzed theoretically and numerically with a time-dependent two-level propagation model, respectively. Self-phase modulation and stimulated Raman scattering are found to limit the usable data format space. In operational regimes free from nonlinear effects, dynamic gain effects such as the variation in the output pulse energy and square-pulse distortion are quantified. Both are found to primarily depend on the symbol duration and can be as large as 28% and 21%, respectively.

  13. Superconducting junctions from doped nonsuperconducting CuO2 layers

    NASA Astrophysics Data System (ADS)

    Loktev, V. M.; Pogorelov, Yu. G.

    2011-04-01

    The theoretical approach proposed recently to describe the redistribution of electronic charge in multilayered selectively doped systems is modified for a system with a finite number of layers. Special attention is payed to the case of a finite heterostructure made of copper-oxide layers which are all nonsuperconducting (including nonconducting) because the doping levels in them are beyond the characteristic interval for superconductivity. Specific finite structures and doping configurations are proposed to obtain atomically thin superconducting heterojunctions of different compositions.

  14. Ultrafast Band Structure Control of a Two-Dimensional Heterostructure.

    PubMed

    Ulstrup, Søren; Čabo, Antonija Grubišić; Miwa, Jill A; Riley, Jonathon M; Grønborg, Signe S; Johannsen, Jens C; Cacho, Cephise; Alexander, Oliver; Chapman, Richard T; Springate, Emma; Bianchi, Marco; Dendzik, Maciej; Lauritsen, Jeppe V; King, Phil D C; Hofmann, Philip

    2016-06-28

    The electronic structure of two-dimensional (2D) semiconductors can be significantly altered by screening effects, either from free charge carriers in the material or by environmental screening from the surrounding medium. The physical properties of 2D semiconductors placed in a heterostructure with other 2D materials are therefore governed by a complex interplay of both intra- and interlayer interactions. Here, using time- and angle-resolved photoemission, we are able to isolate both the layer-resolved band structure and, more importantly, the transient band structure evolution of a model 2D heterostructure formed of a single layer of MoS2 on graphene. Our results reveal a pronounced renormalization of the quasiparticle gap of the MoS2 layer. Following optical excitation, the band gap is reduced by up to ∼400 meV on femtosecond time scales due to a persistence of strong electronic interactions despite the environmental screening by the n-doped graphene. This points to a large degree of tunability of both the electronic structure and the electron dynamics for 2D semiconductors embedded in a van der Waals-bonded heterostructure. PMID:27267820

  15. Identification of selected in vitro generated phase-I metabolites of the steroidal selective androgen receptor modulator MK-0773 for doping control purposes.

    PubMed

    Lagojda, Andreas; Kuehne, Dirk; Krug, Oliver; Thomas, Andreas; Wigger, Tina; Karst, Uwe; Schänzer, Wilhelm; Thevis, Mario

    2016-01-01

    Research into developing anabolic agents for various therapeutic purposes has been pursued for decades. As the clinical utility of anabolic-androgenic steroids has been found to be limited because of their lack of tissue selectivity and associated off-target effects, alternative drug entities have been designed and are commonly referred to as selective androgen receptor modulators (SARMs). While most of these SARMs are of nonsteroidal structure, the drug candidate MK-0773 comprises a 4-aza-steroidal nucleus. Besides the intended therapeutic use, SARMs have been found to be illicitly distributed and misused as doping agents in sport, necessitating frequently updated doping control analytical assays. As steroidal compounds reportedly undergo considerable metabolic transformations, the phase-I metabolism of MK-0773 was simulated using human liver microsomal (HLM) preparations and electrochemical conversion. Subsequently, major metabolic products were identified and characterized employing liquid chromatography-high-resolution/high- accuracy tandem mass spectrometry with electrospray (ESI) and atmospheric pressure chemical ionization (APCI) as well as nuclear magnetic resonance (NMR) spectroscopy. MK-0773 produced numerous phase-I metabolites under the chosen in vitro incubation reactions, mostly resulting from mono- and bisoxygenation of the steroid. HLM yielded at least 10 monooxygenated species, while electrochemistry-based experiments resulted predominantly in three monohydroxylated metabolites. Elemental composition data and product ion mass spectra were generated for these analytes, ESI/APCI measurements corroborated the formation of at least two N-oxygenated metabolites, and NMR data obtained from electrochemistry-derived products supported structures suggested for three monohydroxylated compounds. Hereby, the hydroxylation of the A-ring located N- bound methyl group was found to be of particular intensity. In the absence of controlled elimination studies, the

  16. Characterization of equine urinary metabolites of selective androgen receptor modulators (SARMs) S1, S4 and S22 for doping control purposes.

    PubMed

    Hansson, Annelie; Knych, Heather; Stanley, Scott; Thevis, Mario; Bondesson, Ulf; Hedeland, Mikael

    2015-08-01

    Selective androgen receptor modulators, SARMs, constitute a class of compounds with anabolic properties but with few androgenic side-effects. This makes them possible substances of abuse and the World Anti-Doping Agency (WADA) has banned the entire class of substances. There have been several cases of illicit use of aryl propionamide SARMs in human sports and in 2013, 13 cases were reported. These substances have been found to be extensively metabolized in humans, making detection of metabolites necessary for doping control. SARMs are also of great interest to equine doping control, but the in vivo metabolite pattern and thus possible analytical targets have not been previously studied in this species. In this study, the urinary metabolites of the SARMs S1, S4, and S22 in horses were studied after intravenous injection, using ultra high performance liquid chromatography coupled to quadrupole time-of-flight mass spectrometry (UHPLC-QToF-MS). Eight different metabolites were found for SARM S1, nine for SARM S4, and seven for SARM S22. The equine urinary metabolite profiles differed significantly from those of humans. The parent compounds were only detected for SARMs S4 and S22 and only at the first sampling time point at 3 h post administration, making them unsuitable as target compounds. For all three SARMs tested, the metabolite yielding the highest response had undergone amide hydrolysis, hydroxylation and sulfonation. The resulting phase II metabolites (4-nitro-3-trifluoro-methyl-phenylamine sulfate for SARMs S1 and S4 and 4-cyano-3-trifluoro-methyl-phenylamine sulfate for SARM S22) are proposed as analytical targets for use in equine doping control.

  17. Identification of selected in vitro generated phase-I metabolites of the steroidal selective androgen receptor modulator MK-0773 for doping control purposes.

    PubMed

    Lagojda, Andreas; Kuehne, Dirk; Krug, Oliver; Thomas, Andreas; Wigger, Tina; Karst, Uwe; Schänzer, Wilhelm; Thevis, Mario

    2016-01-01

    Research into developing anabolic agents for various therapeutic purposes has been pursued for decades. As the clinical utility of anabolic-androgenic steroids has been found to be limited because of their lack of tissue selectivity and associated off-target effects, alternative drug entities have been designed and are commonly referred to as selective androgen receptor modulators (SARMs). While most of these SARMs are of nonsteroidal structure, the drug candidate MK-0773 comprises a 4-aza-steroidal nucleus. Besides the intended therapeutic use, SARMs have been found to be illicitly distributed and misused as doping agents in sport, necessitating frequently updated doping control analytical assays. As steroidal compounds reportedly undergo considerable metabolic transformations, the phase-I metabolism of MK-0773 was simulated using human liver microsomal (HLM) preparations and electrochemical conversion. Subsequently, major metabolic products were identified and characterized employing liquid chromatography-high-resolution/high- accuracy tandem mass spectrometry with electrospray (ESI) and atmospheric pressure chemical ionization (APCI) as well as nuclear magnetic resonance (NMR) spectroscopy. MK-0773 produced numerous phase-I metabolites under the chosen in vitro incubation reactions, mostly resulting from mono- and bisoxygenation of the steroid. HLM yielded at least 10 monooxygenated species, while electrochemistry-based experiments resulted predominantly in three monohydroxylated metabolites. Elemental composition data and product ion mass spectra were generated for these analytes, ESI/APCI measurements corroborated the formation of at least two N-oxygenated metabolites, and NMR data obtained from electrochemistry-derived products supported structures suggested for three monohydroxylated compounds. Hereby, the hydroxylation of the A-ring located N- bound methyl group was found to be of particular intensity. In the absence of controlled elimination studies, the

  18. Morphology-modulation of SnO2 Hierarchical Architectures by Zn Doping for Glycol Gas Sensing and Photocatalytic Applications

    PubMed Central

    Zhao, Qinqin; Ju, Dianxing; Deng, Xiaolong; Huang, Jinzhao; Cao, Bingqiang; Xu, Xijin

    2015-01-01

    The morphology of SnO2 nanospheres was transformed into ultrathin nanosheets assembled architectures after Zn doping by one-step hydrothermal route. The as-prepared samples were characterized in detail by various analytical techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and nitrogen adsorption-desorption technique. The Zn-doped SnO2 nanostructures proved to be the efficient gas sensing materials for a series of flammable and explosive gases detection, and photocatalysts for the degradation of methyl orange (MO) under UV irradiation. It was observed that both of the undoped and Zn-doped SnO2 after calcination exhibited tremendous gas sensing performance toward glycol. The response (S = Ra/Rg) of Zn-doped SnO2 can reach to 90 when the glycol concentration is 100 ppm, which is about 2 times and 3 times higher than that of undoped SnO2 sensor with and without calcinations, respectively. The result of photocatalytic activities demonstrated that MO dye was almost completely degraded (~92%) by Zn-doped SnO2 in 150 min, which is higher than that of others (MO without photocatalyst was 23%, undoped SnO2 without and with calcination were 55% and 75%, respectively). PMID:25597269

  19. Large anisotropic magnetoresistance across the Schottky interface in all oxide ferromagnet/semiconductor heterostructures

    NASA Astrophysics Data System (ADS)

    Li, P.; Guo, B. L.; Bai, H. L.

    2011-06-01

    Over 80% fourfold symmetric anisotropic magnetoresistance (AMR) across the interface is observed in epitaxial Fe3O4 (001)/Nb:SrTiO3 (001) heterostructures, while the twofold symmetric AMR across the interface in epitaxial Fe3O4 (111)/ZnO (0001) heterostructures is rather small. The large AMR across the interface is considered to be induced by the assistance of magnetocrystalline anisotropic energy for the transport electrons while the applied voltage bias is near the height of Schottky barrier, which is further verified by the fact that a larger critical current is needed for the maximum AMR in the Fe3O4 (001)/Nb:SrTiO3 (001) heterostructures with heavier Nb-doping.

  20. Novel Excitonic Solar Cells in Phosphorene-TiO2 Heterostructures with Extraordinary Charge Separation Efficiency.

    PubMed

    Zhou, Liujiang; Zhang, Jin; Zhuo, Zhiwen; Kou, Liangzhi; Ma, Wei; Shao, Bin; Du, Aijun; Meng, Sheng; Frauenheim, Thomas

    2016-05-19

    Constructing van der Waals heterostructures is an efficient approach to modulate the electronic structure, to advance the charge separation efficiency, and thus to optimize the optoelectronic property. Here, we theoretically investigated the phosphorene interfaced with TiO2(110) surface (1L-BP/TiO2) with a type-II band alignment, showing enhanced photoactivity. The 1L-BP/TiO2 excitonic solar cell (XSC) based on the 1L-BP/TiO2 exhibits large built-in potential and high power conversion efficiency (PCE), dozens of times higher than conventional solar cells, comparable to MoS2/WS2 XSC. The nonadiabatic molecular dynamics simulation shows the ultrafast electron transfer time of 6.1 fs, and slow electron-hole recombination of 0.58 ps, yielding >98% internal quantum efficiency for charge separation, further guaranteeing the practical PCE. Moreover, doping in phosphorene has a tunability on built-in potential, charge transfer, light absorbance, as well as electron dynamics, which greatly helps to optimize the optoelectronic efficiency of a XSC.

  1. Photodiode-like behavior and excellent photoresponse of vertical Si/monolayer MoS2 heterostructures.

    PubMed

    Li, Yang; Xu, Cheng-Yan; Wang, Jia-Ying; Zhen, Liang

    2014-11-26

    Monolayer transition metal dichalcogenides (TMDs) and their van der Waals heterostructures have been experimentally and theoretically demonstrated as potential candidates for photovoltaic and optoelectronic devices due to the suitable bandgap and excellent light absorption. In this work, we report the observation of photodiode behavior in (both n- and p- type) silicon/monolayer MoS2 vertical heterostructures. The photocurrent and photoresponsivity of heterostructures photodiodes were dependent both on the incident light wavelength and power density, and the highest photoresponsivity of 7.2 A/W was achieved in n-Si/monolayer MoS2 vertical heterostructures photodiodes. Compared with n-Si/MoS2 heterostructures, the photoresponsivity of p-Si/MoS2 heterostructure was much lower. Kelvin probe microscope (KFM) results demonstrated the more efficient separation of photogenerated excitons in n-Si/MoS2 than that in p-Si/MoS2. Coupling KFM results with band alignments of (p-, n-) Si/MoS2 heterostructures, the origins of photodiode-like phenomena of p-Si/MoS2 and n-Si/MoS2 have been unveiled, that is intrinsic built-in electric field in p-n junction, and modulated barrier height and width at the interface in n-n junction. Our work may benefit to the deep understanding of the integration of two-dimensional materials with more conventional three-dimensional semiconductors, and then contribute to the developments in the area of van der Waals heterostructures.

  2. Electro-optic phase modulation in light induced self-written waveguides propagated in a 5CB doped photopolymer.

    PubMed

    Jemal, Abdelmonem; Ben Belgacem, Mohamed; Kamoun, Saber; Gargouri, Mohamed; Honorat Dorkenoo, Kokou D; Barsella, Alberto; Mager, Loïc

    2013-01-28

    We present the inscription of a Light Induced Self-Written (LISW) waveguide in a 4-cyano-4'-pentylbipheny (5CB) doped photopolymer. The dynamic reorientation of the 5CB molecules in the material under applied electric field leads to birefringence in LISW waveguide and thus allows the control of the phase of the guided mode. PMID:23389136

  3. Assembly of quasicrystalline photonic heterostructures

    DOEpatents

    Grier, David G.; Roichman, Yael; Man, Weining; Chaikin, Paul Michael; Steinhardt, Paul Joseph

    2013-03-12

    A method and system for assembling a quasicrystalline heterostructure. A plurality of particles is provided with desirable predetermined character. The particles are suspended in a medium, and holographic optical traps are used to position the particles in a way to achieve an arrangement which provides a desired property.

  4. Assembly of quasicrystalline photonic heterostructures

    DOEpatents

    Grier, David G.; Roichman, Yael; Man, Weining; Chaikin, Paul Michael; Steinhardt, Paul Joseph

    2011-07-19

    A method and system for assembling a quasicrystalline heterostructure. A plurality of particles is provided with desirable predetermined character. The particles are suspended in a medium, and holographic optical traps are used to position the particles in a way to achieve an arrangement which provides a desired property.

  5. Effect of strain on voltage-controlled magnetism in BiFeO3-based heterostructures

    PubMed Central

    Wang, J. J.; Hu, J. M.; Yang, T. N.; Feng, M.; Zhang, J. X.; Chen, L. Q.; Nan, C. W.

    2014-01-01

    Voltage-modulated magnetism in magnetic/BiFeO3 heterostructures can be driven by a combination of the intrinsic ferroelectric-antiferromagnetic coupling in BiFeO3 and the antiferromagnetic-ferromagnetic exchange interaction across the heterointerface. However, ferroelectric BiFeO3 film is also ferroelastic, thus it is possible to generate voltage-induced strain in BiFeO3 that could be applied onto the magnetic layer across the heterointerface and modulate magnetism through magnetoelastic coupling. Here, we investigated, using phase-field simulations, the role of strain in voltage-controlled magnetism for these BiFeO3-based heterostructures. It is predicted, under certain condition, coexistence of strain and exchange interaction will result in a pure voltage-driven 180° magnetization reversal in BiFeO3-based heterostructures. PMID:24686503

  6. Dynamic in situ visualization of voltage-driven magnetic domain evolution in multiferroic heterostructures.

    PubMed

    Gao, Ya; Hu, Jia-Mian; Wu, Liang; Nan, C W

    2015-12-23

    Voltage control of magnetism in multiferroic heterostructures provides a promising solution to the excessive heating in spintronic devices. Direct observation of voltage-modulated magnetic domain evolution dynamics is desirable for studying the mechanism of the voltage control of magnetism at mesoscale, but has remained challenging. Here we explored a characterization method for the dynamic in situ evolution of pure voltage modulated magnetic domains in the heterostructures by employing the scanning Kerr microscopy function in the magneto optic Kerr effect system. The local magnetization reorientation of a Ni/PMN-PT heterostructure were characterized under sweeping applied voltage on the PMN-PT single crystal, and the results show that the magnetization rotation angle in the local regions is much greater than that obtained from macroscopic magnetization hysteresis loops.

  7. Effect of strain on voltage-controlled magnetism in BiFeO₃-based heterostructures.

    PubMed

    Wang, J J; Hu, J M; Yang, T N; Feng, M; Zhang, J X; Chen, L Q; Nan, C W

    2014-04-01

    Voltage-modulated magnetism in magnetic/BiFeO3 heterostructures can be driven by a combination of the intrinsic ferroelectric-antiferromagnetic coupling in BiFeO3 and the antiferromagnetic-ferromagnetic exchange interaction across the heterointerface. However, ferroelectric BiFeO3 film is also ferroelastic, thus it is possible to generate voltage-induced strain in BiFeO3 that could be applied onto the magnetic layer across the heterointerface and modulate magnetism through magnetoelastic coupling. Here, we investigated, using phase-field simulations, the role of strain in voltage-controlled magnetism for these BiFeO3-based heterostructures. It is predicted, under certain condition, coexistence of strain and exchange interaction will result in a pure voltage-driven 180° magnetization reversal in BiFeO3-based heterostructures.

  8. Dynamic in situ visualization of voltage-driven magnetic domain evolution in multiferroic heterostructures

    NASA Astrophysics Data System (ADS)

    Gao, Ya; Hu, Jia-Mian; Wu, Liang; Nan, C. W.

    2015-12-01

    Voltage control of magnetism in multiferroic heterostructures provides a promising solution to the excessive heating in spintronic devices. Direct observation of voltage-modulated magnetic domain evolution dynamics is desirable for studying the mechanism of the voltage control of magnetism at mesoscale, but has remained challenging. Here we explored a characterization method for the dynamic in situ evolution of pure voltage modulated magnetic domains in the heterostructures by employing the scanning Kerr microscopy function in the magneto optic Kerr effect system. The local magnetization reorientation of a Ni/PMN-PT heterostructure were characterized under sweeping applied voltage on the PMN-PT single crystal, and the results show that the magnetization rotation angle in the local regions is much greater than that obtained from macroscopic magnetization hysteresis loops.

  9. Dynamic in situ visualization of voltage-driven magnetic domain evolution in multiferroic heterostructures.

    PubMed

    Gao, Ya; Hu, Jia-Mian; Wu, Liang; Nan, C W

    2015-12-23

    Voltage control of magnetism in multiferroic heterostructures provides a promising solution to the excessive heating in spintronic devices. Direct observation of voltage-modulated magnetic domain evolution dynamics is desirable for studying the mechanism of the voltage control of magnetism at mesoscale, but has remained challenging. Here we explored a characterization method for the dynamic in situ evolution of pure voltage modulated magnetic domains in the heterostructures by employing the scanning Kerr microscopy function in the magneto optic Kerr effect system. The local magnetization reorientation of a Ni/PMN-PT heterostructure were characterized under sweeping applied voltage on the PMN-PT single crystal, and the results show that the magnetization rotation angle in the local regions is much greater than that obtained from macroscopic magnetization hysteresis loops. PMID:26613293

  10. Blue light emission from the heterostructured ZnO/InGaN/GaN

    PubMed Central

    2013-01-01

    ZnO/InGaN/GaN heterostructured light-emitting diodes (LEDs) were fabricated by molecular beam epitaxy and atomic layer deposition. InGaN films consisted of an Mg-doped InGaN layer, an undoped InGaN layer, and a Si-doped InGaN layer. Current-voltage characteristic of the heterojunction indicated a diode-like rectification behavior. The electroluminescence spectra under forward biases presented a blue emission accompanied by a broad peak centered at 600 nm. With appropriate emission intensity ratio, the heterostructured LEDs had potential application in white LEDs. Moreover, a UV emission and an emission peak centered at 560 nm were observed under reverse bias. PMID:23433236

  11. Graphene/Si-nanowire heterostructure molecular sensors

    PubMed Central

    Kim, Jungkil; Oh, Si Duk; Kim, Ju Hwan; Shin, Dong Hee; Kim, Sung; Choi, Suk-Ho

    2014-01-01

    Wafer-scale graphene/Si-nanowire (Si-NW) array heterostructures for molecular sensing have been fabricated by vertically contacting single-layer graphene with high-density Si NWs. Graphene is grown in large scale by chemical vapour deposition and Si NWs are vertically aligned by metal-assisted chemical etching of Si wafer. Graphene plays a key role in preventing tips of vertical Si NWs from being bundled, thereby making Si NWs stand on Si wafer separately from each other under graphene, a critical structural feature for the uniform Schottky-type junction between Si NWs and graphene. The molecular sensors respond very sensitively to gas molecules by showing 37 and 1280% resistance changes within 3.5/0.15 and 12/0.15 s response/recovery times under O2 and H2 exposures in air, respectively, highest performances ever reported. These results together with the sensor responses in vacuum are discussed based on the surface-transfer doping mechanism. PMID:24947403

  12. Photocatalytic enhancement of TiO2 by B and Zr co-doping and modulation of microstructure

    NASA Astrophysics Data System (ADS)

    Fu, Chengxin; Gong, Yinyan; Wu, Yitao; Liu, Jiaqi; Zhang, Zhen; Li, Can; Niu, Lengyuan

    2016-08-01

    Visible-light photodegradation test revealed that B and Zr co-doping can raise the photocatalytic ability of the undoped TiO2 by a fold. XRD crystallography and Raman phonon spectroscopy measurements suggest that the Zr4+ ions replace the Ti4+ ions while the B3+ ions occupy the interstitial sites, expanding the unit-cell volume and reducing crystallite size. The incorporation of interstitial boron dopants creates oxygen vacancies (Ovrad rad) and reduce Ti4+ to Ti3+ to form [Ovrad rad -Ti3+]+, which traps the carriers and prolongs carrier lifetime. Moreover, Zr4+ ions replace Ti4+ ions and form impurity levels, which could improve visible light response. The co-doped samples are benefited from both B interstitials and Zr substitutes.

  13. Room temperature broadband terahertz gains in graphene heterostructures based on inter-layer radiative transitions

    SciTech Connect

    Tang, Linlong; Du, Jinglei; Shi, Haofei Wei, Dongshan; Du, Chunlei

    2014-10-15

    We exploit inter-layer radiative transitions to provide gains to amplify terahertz waves in graphene heterostructures. This is achieved by properly doping graphene sheets and aligning their energy bands so that the processes of stimulated emissions can overwhelm absorptions. We derive an expression for the gain estimation and show the gain is insensitive to temperature variation. Moreover, the gain is broadband and can be strong enough to compensate the free carrier loss, indicating graphene based room temperature terahertz lasers are feasible.

  14. White electroluminescence of n-ZnO:Al/p-diamond heterostructure devices

    NASA Astrophysics Data System (ADS)

    Yang, Can; Wang, Xiao-Ping; Wang, Li-Jun; Pan, Xiu-Fang; Li, Song-Kun; Jing, Long-Wei

    2013-08-01

    An n-ZnO:Al/p-boron-doped diamond heterostructure electroluminescent device is produced, and a rectifying behavior can be observed. The electroluminescence spectrum at room temperature exhibits two visible bands centred at 450 nm-485 nm (blue emission) and 570 nm-640 nm (yellow emission). Light emission with a luminance of 15 cd/m2 is observed from the electroluminescent device at a forward applied voltage of 85 V, which is distinguished from white light by the naked eye.

  15. Modulating optical rectification, second and third harmonic generation of doped quantum dots: Interplay between hydrostatic pressure, temperature and noise

    NASA Astrophysics Data System (ADS)

    Ganguly, Jayanta; Saha, Surajit; Bera, Aindrila; Ghosh, Manas

    2016-10-01

    We examine the profiles of optical rectification (OR), second harmonic generation (SHG) and third harmonic generation (THG) of impurity doped QDs under the combined influence of hydrostatic pressure (HP) and temperature (T) in presence and absence of Gaussian white noise. Noise has been incorporated to the system additively and multiplicatively. In order to study the above nonlinear optical (NLO) properties the doped dot has been subjected to a polarized monochromatic electromagnetic field. Effect of application of noise is nicely reflected through alteration of peak shift (blue/red) and variation of peak height (increase/decrease) of above NLO properties as temperature and pressure are varied. All such changes again sensitively depends on mode of application (additive/multiplicative) of noise. The remarkable influence of interplay between noise strength and its mode of application on the said profiles has also been addressed. The findings illuminate fascinating role played by noise in tuning above NLO properties of doped QD system under the active presence of both hydrostatic pressure and temperature.

  16. Enhanced thermoelectric transport in modulation-doped GaN/AlGaN core/shell nanowires

    DOE PAGES

    Song, Erdong; Li, Qiming; Swartzentruber, Brian; Pan, Wei; Wang, George T.; Martinez, Julio A.

    2015-11-25

    The thermoelectric properties of unintentionally n-doped core GaN/AlGaN core/shell N-face nanowires are reported. We found that the temperature dependence of the electrical conductivity is consistent with thermally activated carriers with two distinctive donor energies. The Seebeck coefficient of GaN/AlGaN nanowires is more than twice as large as that for the GaN nanowires alone. However, an outer layer of GaN deposited onto the GaN/AlGaN core/shell nanowires decreases the Seebeck coefficient at room temperature, while the temperature dependence of the electrical conductivity remains the same. We attribute these observations to the formation of an electron gas channel within the heavily-doped GaN coremore » of the GaN/AlGaN nanowires. The room-temperature thermoelectric power factor for the GaN/AlGaN nanowires can be four times higher than the GaN nanowires. As a result, selective doping in bandgap engineered core/shell nanowires is proposed for enhancing the thermoelectric power.« less

  17. Enhanced thermoelectric transport in modulation-doped GaN/AlGaN core/shell nanowires

    SciTech Connect

    Song, Erdong; Li, Qiming; Swartzentruber, Brian; Pan, Wei; Wang, George T.; Martinez, Julio A.

    2015-11-25

    The thermoelectric properties of unintentionally n-doped core GaN/AlGaN core/shell N-face nanowires are reported. We found that the temperature dependence of the electrical conductivity is consistent with thermally activated carriers with two distinctive donor energies. The Seebeck coefficient of GaN/AlGaN nanowires is more than twice as large as that for the GaN nanowires alone. However, an outer layer of GaN deposited onto the GaN/AlGaN core/shell nanowires decreases the Seebeck coefficient at room temperature, while the temperature dependence of the electrical conductivity remains the same. We attribute these observations to the formation of an electron gas channel within the heavily-doped GaN core of the GaN/AlGaN nanowires. The room-temperature thermoelectric power factor for the GaN/AlGaN nanowires can be four times higher than the GaN nanowires. As a result, selective doping in bandgap engineered core/shell nanowires is proposed for enhancing the thermoelectric power.

  18. Enhanced thermoelectric transport in modulation-doped GaN/AlGaN core/shell nanowires.

    PubMed

    Song, Erdong; Li, Qiming; Swartzentruber, Brian; Pan, Wei; Wang, George T; Martinez, Julio A

    2016-01-01

    The thermoelectric properties of unintentionally n-doped core GaN/AlGaN core/shell N-face nanowires are reported. We found that the temperature dependence of the electrical conductivity is consistent with thermally activated carriers with two distinctive donor energies. The Seebeck coefficient of GaN/AlGaN nanowires is more than twice as large as that for the GaN nanowires alone. However, an outer layer of GaN deposited onto the GaN/AlGaN core/shell nanowires decreases the Seebeck coefficient at room temperature, while the temperature dependence of the electrical conductivity remains the same. We attribute these observations to the formation of an electron gas channel within the heavily-doped GaN core of the GaN/AlGaN nanowires. The room-temperature thermoelectric power factor for the GaN/AlGaN nanowires can be four times higher than the GaN nanowires. Selective doping in bandgap engineered core/shell nanowires is proposed for enhancing the thermoelectric power.

  19. Enhanced thermoelectric transport in modulation-doped GaN/AlGaN core/shell nanowires.

    PubMed

    Song, Erdong; Li, Qiming; Swartzentruber, Brian; Pan, Wei; Wang, George T; Martinez, Julio A

    2016-01-01

    The thermoelectric properties of unintentionally n-doped core GaN/AlGaN core/shell N-face nanowires are reported. We found that the temperature dependence of the electrical conductivity is consistent with thermally activated carriers with two distinctive donor energies. The Seebeck coefficient of GaN/AlGaN nanowires is more than twice as large as that for the GaN nanowires alone. However, an outer layer of GaN deposited onto the GaN/AlGaN core/shell nanowires decreases the Seebeck coefficient at room temperature, while the temperature dependence of the electrical conductivity remains the same. We attribute these observations to the formation of an electron gas channel within the heavily-doped GaN core of the GaN/AlGaN nanowires. The room-temperature thermoelectric power factor for the GaN/AlGaN nanowires can be four times higher than the GaN nanowires. Selective doping in bandgap engineered core/shell nanowires is proposed for enhancing the thermoelectric power. PMID:26606258

  20. Magnetooptical study of Zeeman effect in Mn modulation-doped InAs/InGaAs/InAlAs quantum well structures

    SciTech Connect

    Terent'ev, Ya. V.; Danilov, S. N.; Plank, H.; Loher, J.; Schuh, D.; Bougeard, D.; Weiss, D.; Ganichev, S. D.; Durnev, M. V.; Ivanov, S. V.; Tarasenko, S. A.; Rozhansky, I. V.; Yakovlev, D. R.

    2015-09-21

    We report on a magneto-photoluminescence (PL) study of Zeeman effect in Mn modulation-doped InAs/InGaAs/InAlAs quantum wells (QW). Two PL lines corresponding to the radiative recombination of photoelectrons with free and bound-on-Mn holes have been observed. In the presence of a magnetic field applied in the Faraday geometry, both lines split into two circularly polarized components. While temperature and magnetic field dependence of the splitting are well described by the Brillouin function, providing an evidence for exchange interaction with spin polarized manganese ions, the value of the splitting exceeds by two orders of magnitude the value of the giant Zeeman splitting estimated for the average Mn density in QW obtained by the secondary ion mass spectroscopy.

  1. Synchronization of two passively mode-locked erbium-doped fiber lasers by an acousto-optic modulator and grating scheme

    SciTech Connect

    Jiang, M.; Sha, W.; Rahman, L.; Barnett, B.C.; Andersen, J.K.; Islam, M.N.; Reddy, K.V.

    1996-06-01

    We synchronize two passively mode-locked erbium-doped fiber lasers by adjusting only the cavity length to correct both the repetition rate and the phase. The interlaser jitter is less than 6ps (1.3times the pulse width) and is extracted from the cross correlation of the two lasers. The lock can be maintained for extended periods of time. These results are obtained by use of a novel acousto-optic-modulator{endash}grating scheme, which provides an equivalent of 300 {mu}m in cavity length tuning with a bandwidth of 10 kHz. These parameters are 30 times the length and 10 times the bandwidth of a typical piezoelectric transducer. {copyright} {ital 1996 Optical Society of America.}

  2. In-plane magneto-photoluminescence studies of modulation-doped GaAs/AlGaAs coupled double quantum wells

    SciTech Connect

    KIM,YONGMIN; PERRY,C.H.; SIMMONS,JERRY A.; KLEM,JOHN F.

    2000-05-11

    In-plane magnetic field photoluminescence spectra from n series of n-type modulation doped GaAs/Al{sub 0.3}Ga{sub 0.7}As coupled double quantum wells show distinctive doublet structures related to the tunnel-split ground sub-level states. The magnetic field behavior of the upper transition from the antisymmetric state strongly depends on sample mobility. In a lower mobility sample, the transition energy displays an N-type kink with field (namely a maximum followed by a minimum), whereas higher mobility samples have a linear dependence. The former is attributed to a coupling mechanism due to homogeneous broadening of the electron and hole states. The results are in good agreement with recent theoretical calculations.

  3. Equally efficient interlayer exciton relaxation and improved absorption in epitaxial and nonepitaxial MoS2/WS2 heterostructures.

    PubMed

    Yu, Yifei; Hu, Shi; Su, Liqin; Huang, Lujun; Liu, Yi; Jin, Zhenghe; Purezky, Alexander A; Geohegan, David B; Kim, Ki Wook; Zhang, Yong; Cao, Linyou

    2015-01-14

    Semiconductor heterostructures provide a powerful platform to engineer the dynamics of excitons for fundamental and applied interests. However, the functionality of conventional semiconductor heterostructures is often limited by inefficient charge transfer across interfaces due to the interfacial imperfection caused by lattice mismatch. Here we demonstrate that MoS(2)/WS(2) heterostructures consisting of monolayer MoS(2) and WS(2) stacked in the vertical direction can enable equally efficient interlayer exciton relaxation regardless the epitaxy and orientation of the stacking. This is manifested by a similar 2 orders of magnitude decrease of photoluminescence intensity in both epitaxial and nonepitaxial MoS(2)/WS(2) heterostructures. Both heterostructures also show similarly improved absorption beyond the simple superimposition of the absorptions of monolayer MoS(2) and WS(2). Our result indicates that 2D heterostructures bear significant implications for the development of photonic devices, in particular those requesting efficient exciton separation and strong light absorption, such as solar cells, photodetectors, modulators, and photocatalysts. It also suggests that the simple stacking of dissimilar 2D materials with random orientations is a viable strategy to fabricate complex functional 2D heterostructures, which would show similar optical functionality as the counterpart with perfect epitaxy.

  4. Light-induced excess conductivity and the role of argon in the deposition of doping-modulated amorphous silicon superlattices

    SciTech Connect

    Su, F.; Levine, S.; Vanier, P.E.; Kampas, F.J.

    1985-09-15

    Amorphous silicon pnpn ... structures that exhibit the phenomenon of light-induced excess conductivity (LEC) have been deposited in a single-chamber glow discharge deposition system by simple control of gas flows. This phenomenon is negligible when the doped silane is undiluted but clearly evident when the silane is diluted in argon. Experiments were performed in which argon dilution was only used for specific regions of the structure. The LEC effect was found to occur if argon dilution was used during the deposition of any fraction of the superlattice layers. These experiments rule out mechanisms requiring phosphorus-boron defect complexes or interface states.

  5. Band-gap modulation via gallium substitution in cerium doped gadolinium aluminum garnet using a mixed fuel combustion approach

    SciTech Connect

    Tyagi, Mohit; Pitale, Shreyas S.; Ghosh, Manoranjan; Shinde, Seema

    2014-04-24

    Cerium doped Gadolinium garnets (Gd{sub 3}Al{sub x}Ga{sub 5−x}O{sub 12} where 0≤x≤5) are synthesized via combustion synthesis using mixture of urea and glycine fuels. A 4h Post annealing at 1400 oC is found to be necessary for pure phase formation. Lattice spacing variation as a result of partial or total Ga substitution at Al site was mapped by X-ray diffraction. Photoluminescence emission of Ce shifts as a consequence of Ga substitution and therefore suggests a local re-adjustment of crystal field around activator site.

  6. Spatially Correlated Disorder in Epitaxial van der Waals Heterostructures

    NASA Astrophysics Data System (ADS)

    Laanait, Nouamane; Zhang, Zhan; Schleputz, Christian; Liu, Ying; Wojcik, Michael; Myers-Ward, Rachael; Gaskill, D. Kurt; Fenter, Paul; Li, Lian

    The structural cohesion of van der Waals (vdW) heterostructures relies upon a cooperative balance between strong intra-layer bonded interactions and weak inter-layer coupling. The confinement of extended defects to within a single vdW layer and competing interactions introduced by epitaxial constraints could generate fundamentally new structural disorders. Here we report on the presence of spatially correlated and localized disorder states that coexist with the near perfect crystallographic order along the growth direction of epitaxial vdW heterostructure of Bi2Se3/graphene/SiC grown by molecular beam epitaxy. With the depth penetration of hard X-ray diffraction microscopy and high-resolution surface scattering, we imaged local structural configurations from the atomic to mesoscopic length scales, and found that these disorder states result as a confluence of atomic scale modulations in the strength of vdW layer-layer interactions and nanoscale boundary conditions imposed by the substrate. These findings reveal a vast landscape of novel disorder states that can be manifested in epitaxial vdW heterostructures. Supported by the Wigner Fellowship program at Oak Ridge Nat'l Lab.

  7. Study of energy band discontinuity in NiZnO/ZnO heterostructure using X-ray photoelectron spectroscopy

    NASA Astrophysics Data System (ADS)

    Dewan, Sheetal; Tomar, Monika; Goyal, Anshu; Kapoor, A. K.; Tandon, R. P.; Gupta, Vinay

    2016-05-01

    A heterostructure based on ZnO and Ni doped ZnO (NiZnO) thin films has been prepared on c-plane sapphire substrate by pulsed laser deposition technique. X-ray photo electron spectroscopy has been utilized to study the energy band discontinuities, i.e., valence band offset ( Δ E v ) and conduction band offset ( Δ E c ) at the interface of NiZnO and ZnO thin films. A type-II band alignment is identified at the interface of prepared heterostructure from the computed data, which is attractive for the realization of efficient optoelectronic devices.

  8. Dimensional magnetoplasma resonance detected by free-exciton photoluminescence in modulation-doped GaAs/AlxGa1-xAs heterojunctions

    NASA Astrophysics Data System (ADS)

    Ashkinadze, B. M.; Linder, E.; Umansky, V.

    2000-10-01

    We studied the effect of microwave (mw) irradiation on the low-temperature photoluminescence (PL) of high-quality, modulation-doped, wide GaAs/AlxGa1-xAs heterojunctions (HJ's) containing a two-dimensional electron gas (2DEG), in the density range of (0.9-4)×1011 cm-2. The PL arises from excitons that recombine radiatively in the GaAs buffer layer, far from the 2DEG which is confined close to the GaAs/AlxGa1-xAs interface. We observe that the exciton PL is affected by a mw heating of the 2DEG: the mw-induced PL intensity change increases with increasing 2DEG density as well as under a perpendicular magnetic field that corresponds to the 2DEG dimensional magnetoplasma resonance (DMPR) condition. Moreover, the exciton PL intensity shows a bistability at magnetic field strengths that are close to those observed in the DMPR mw absorption. The mw-induced PL modulation effects are interpreted as being due to the interaction of the excitons with low-energy, ballistically propagating acoustic phonons that are emitted by the mw-heated 2DEG. The exciton PL quenching is associated with an exciton drag by the phonon flux towards the opposite HJ interface where the excitons recombine nonradiatively. The rate of phonon emission is determined by the 2DEG state, and thus the exciton PL responds to the changes of the 2DEG parameters.

  9. Optimization of peak power of doubly Q-switched lasers with both an acousto-optic modulator and a Cr4+-doped saturable absorber

    SciTech Connect

    Li Dechun; Zhao Shengzhi; Li Guiqiu; Yang Kejian

    2006-08-01

    A doubly Q-switched laser can obtain a shorter pulse with a stable repetition rate and high pulse peak power, which has been experimentally proved. By taking into account the Gaussian spatial distributions of the intracavity photon density and the initial population-inversion density as well as the influence of the acousto-optic (AO) Q switch, we introduce the coupled rate equations for a doubly Q-switched laser with both an AO modulator and a Cr4+-doped saturable absorber. These coupled rate equations are solved numerically. The key parameters of an optimally coupled doubly Q-switched laser are determined based on maximizing the peak power, which include the optimal normalized coupling parameter, the optimal normalized saturable absorber parameters, and the normalized parameters of the AO Q switch. The optimal normalized peak power, the corresponding normalized energy, and the normalized pulse width are also given, and a group of general curves are generated for the first time to our knowledge. The curves can give us a good understanding of the dependence of the optimal key parameters on the parameters of the gain medium, the saturable absorber, the AO Q switch, the resonator, and the spatial distributions of the intracavity photon density. The optimal calculations for a diode-pumped Nd3+:YVO4 laser with both an AO modulator and a Cr4+:YAG saturable absorber are presented to demonstrate the use of the curves and the related formulas.

  10. Nanoscale Electrostructural Characterization of Compositionally Graded Al(x)Ga(1-x)N Heterostructures on GaN/Sapphire (0001) Substrate.

    PubMed

    Kuchuk, Andrian V; Lytvyn, Petro M; Li, Chen; Stanchu, Hryhorii V; Mazur, Yuriy I; Ware, Morgan E; Benamara, Mourad; Ratajczak, Renata; Dorogan, Vitaliy; Kladko, Vasyl P; Belyaev, Alexander E; Salamo, Gregory G

    2015-10-21

    We report on AlxGa1-xN heterostructures resulting from the coherent growth of a positive then a negative gradient of the Al concentration on a [0001]-oriented GaN substrate. These polarization-doped p-n junction structures were characterized at the nanoscale by a combination of averaging as well as depth-resolved experimental techniques including: cross-sectional transmission electron microscopy, high-resolution X-ray diffraction, Rutherford backscattering spectrometry, and scanning probe microscopy. We observed that a small miscut in the substrate orientation along with the accumulated strain during growth led to a change in the mosaic structure of the AlxGa1-xN film, resulting in the formation of macrosteps on the surface. Moreover, we found a lateral modulation of charge carriers on the surface which were directly correlated with these steps. Finally, using nanoscale probes of the charge density in cross sections of the samples, we have directly measured, semiquantitatively, both n- and p-type polarization doping resulting from the gradient concentration of the AlxGa1-xN layers. PMID:26431166

  11. Reconstructions at the Interface in Complex Oxide Heterostructures with Strongly Correlated Electrons

    NASA Astrophysics Data System (ADS)

    Gray, Benjamin

    Strongly correlated oxides exhibit a rich spectrum of closely competing orders near the localized-itinerant Mott insulator transition leaving their ground states ripe with instabilities susceptible to small perturbations such as lattice distortions, variation in stoichiometry, magnetic and electric fields, etc. As the field of interfacial engineering has matured, these underlying instabilities in the electronic structure of correlated oxides continue to be leveraged to manipulate existing phases or search for emergent ones. The central theme is matching materials across the interface with disparate physical, chemical, electronic, or magnetic structure to harness interfacial reconstructions in the strongly coupled charge, spin, orbital, and lattice degrees of freedom. In this dissertation, we apply the above paradigm to cuprate-manganite and cuprate-titanate interfaces. We examine ultrathin YBa2Cu3O7/La 2/3Ca1/3MnO3 multilayers, where interfacial charge reconstruction modulates the distribution of charge carriers within the superconducting planes and thereby act as dials to tune through the cuprate doping phase diagram. The ultrathin nature of the cuprate layers allows the reconstructed states to be resolved free of a bulk admixture. The depleted carriers are observed to directly enter the CuO2 planes. With increasing LCMO thickness, magnetic correlations are introduced, and coupling between interfacial Cu and Mn develops. The reconstructions in spin and electronic degrees of freedom found in cuprate-manganite heterostructures are expected to completely mask all other competing interactions. To this end, SrTiO3 is incorporated as a spacer material in cuprate-titanate multilayers to reveal the role of dimensionality, interlayer coupling, and broken translational symmetry. At the unit cell limit, a decrease in carrier concentration is found that directly correlates with underdoping from lost charge reservoir layers at the interface, while increased STO layer thickness

  12. Silicon-based silicon–germanium–tin heterostructure photonics

    PubMed Central

    Soref, Richard

    2014-01-01

    The wavelength range that extends from 1550 to 5000 nm is a new regime of operation for Si-based photonic and opto-electronic integrated circuits. To actualize the new chips, heterostructure active devices employing the ternary SiGeSn alloy are proposed in this paper. Foundry-based monolithic integration is described. Opportunities and challenges abound in creating laser diodes, optical amplifiers, light-emitting diodes, photodetectors, modulators, switches and a host of high-performance passive infrared waveguided components. PMID:24567479

  13. Light-induced excess conductivity and the role of argon in the deposition of doping-modulated amorphous silicon superlattices

    SciTech Connect

    Su, F.C.; Levine, S.; Vanier, P.E.; Kampas, F.J.

    1985-01-01

    Amorphous semiconductor superlattice structures consisting of alternating n-type and p-type doped layers of hydrogenated amorphous silicon (a-Si:H) have been made by silane glow discharge in a single chamber system. These multilayered films show the novel phenomenon of light-induced excess conductivity (LEC) associated with a metastable state having a lifetime of order of days. This report shows that the LEC effect is quite dependent on the specific details of the deposition parameters, namely dilution of the silane with inert gas, substrate temperature and layer thickness. In order to investigate the origin of the LEC effect, argon dilution was used for specific regions of the structure. This experiment shows that the slow states are distributed throughout the layers, and are not concentrated at the interfaces.

  14. Modulation of microstructure and optical properties of Mo-doped ZnO thin films by substrate temperature

    SciTech Connect

    Zhang, J.W.; He, G.; Li, T.S.; Liu, M.; Chen, X.S.; Liu, Y.M.; Sun, Z.Q.

    2015-05-15

    Highlights: • Mo-doped ZnO films are obtained by sputtering at various substrate temperatures. • High-quality MZO thin films with good crystalline have been obtained at 200 °C. • Deposition temperature affects the amount of defects in the crystalline structure - Abstract: Mo-doped ZnO(MZO) films were deposited on Si (1 1 1) substrates by radio frequency sputtering at different substrate temperatures of 200, 300 and 400 °C. The effect of the substrate temperature on the structural and optical properties of the MZO films has been investigated. X-ray diffraction results reveal that all the films are polycrystalline with a hexagonal wurtzite structure with a preferential orientation according to the direction (0 0 2) plane. The crystallinity and surface morphologies of the films are strongly dependent on the growth temperature, which in turn exerts a great effect on microstructural and optical properties of the MZO films. The optical absorption measurements show high ultraviolet (UV) absorbance property of MZO with sharp and intense absorption band in this region and the optical band gap (E{sub g}) are 3.18, 3.22, 3.25 and 3.21 eV for the films deposited at room temperature, 200, 300 and 400 °C. The photoluminescence (PL) intensity of a strong broad violet–blue emission from MZO nanostructures with increasing deposited temperature was also observed. X-ray photoelectron spectroscopy (XPS) was employed to investigate the surface chemical composition of growth products.

  15. Designed Synthesis of van der Waals Heterostructures: The Power of Kinetic Control.

    PubMed

    Alemayehu, Matti B; Falmbigl, Matthias; Ta, Kim; Ditto, Jeffrey; Medlin, Douglas L; Johnson, David C

    2015-12-14

    Selecting specific 2D building blocks and specific layering sequences of van der Waals heterostructures should allow the formation of new materials with designed properties for specific applications. Unfortunately, the synthetic ability to prepare such structures at will, especially in a manner that can be manufactured, does not exist. Herein, we report the targeted synthesis of new metal-semiconductor heterostructures using the modulated elemental-reactant technique to nucleate specific 2D building blocks, control their thickness, and avoid epitaxial structures with long-range order. The building blocks, VSe2 and GeSe2 , have different crystal structures, which inhibits cation intermixing. The precise control of this approach enabled us to synthesize heterostructures containing GeSe2 monolayers alternating with VSe2 structural units with specific sequences. The transport properties systematically change with nanoarchitecture and a charge-density wave-like transition is observed.

  16. Semiconductor-halfmetal-metal transition and magnetism of bilayer graphene nanoribbons/hexagonal boron nitride heterostructure

    NASA Astrophysics Data System (ADS)

    Ilyasov, V. V.; Meshi, B. C.; Nguyen, V. C.; Ershov, I. V.; Nguyen, D. C.

    2014-12-01

    The paper presents the results of ab initio study of electronic structure modulation and edge magnetism in the antiferromagnetic (AF) bilayer zigzag graphene nanoribbons (AF-BZGNR)/hexagonal boron nitride (h-BN(0001)) semiconductor heterostructure induced with transverse external electric field (Eext) and nanomechanical compression (extension), performed within the framework of the density functional theory using Grimme's DFT(PBE)-D2 scheme. For the first time we established critical values of Eext and interlayer distance in the bilayer for the BZGNR/h-BN(0001) heterostructure providing for semiconductor-halfmetal-metal phase transition for one of the electron spin configurations. We discovered the effect of preserved local magnetic moment (0.3μB) of edge carbon atoms of the lower (buffer) graphene nanoribbon during nanomechanical uniaxial compression (or extension) of the BZGNR/h-BN(0001) semiconductor heterostructure. It has been demonstrated that magnetic properties of the AF-BZGNR/h-BN(0001) semiconductor heterostructure can be controlled using Eext. In particular, the local magnetic moment of edge carbon atoms decreases by 10% at a critical value of the positive potential. We have established that local magnetic moments and band gaps can be altered in a wide range using nanomechanical uniaxial compression and Eext, thus making the AF-BZGNR/h-BN(0001) semiconductor heterostructure potentially promising for nanosensors, spin filters, and spintronics applications.

  17. Electronic reconstruction of doped Mott insulator heterojunctions

    NASA Astrophysics Data System (ADS)

    Charlebois, M.; Hassan, S. R.; Karan, R.; Dion, M.; Senechal, D.; Tremblay, A.-M. S.

    2012-02-01

    Correlated electron heterostructures became a possible alternative when thin-film deposition techniques achieved structures with a sharp interface transition [1]. Soon thereafter, Okamoto & Millis introduced the concept of ``electronic reconstruction'' [2]. We study here the electronic reconstruction of doped Mott insulator heterostructures based on a Cluster Dynamical Mean Field Theory (CDMFT) calculations of the Hubbard model in the limit where electrostatic energy dominates over the kinetic energy associated with transport across layers. The grand potential of individual layers is first computed within CDMFT and then the electrostatic potential energy is taken into account in the Hartree approximation. The charge reconstruction in an ensemble of stacked planes of different nature can lead to a distribution of electron charge and to transport properties that are unique to doped-Mott insulators.[4pt] [1] J. Mannhart, D. G. Schlom, Science 327, 1607 (2010).[0pt] [2] S. Okamoto and A. J. Millis, Nature 428, 630 (2004).

  18. Molecular-beam epitaxially grown InP/InGaAsP heterostructure for inversion-channel devices

    SciTech Connect

    Kovacic, S.J.; Robinson, B.J.; Swoger, J.H.

    1993-05-01

    A heterostructure, based on InP/InGaAsP alloys grown by gas source molecular-beam epitaxy, is presented which can be used to implement the inversion-channel family of devices. The double-heterostructure optoelectronic switch (DOES) is demonstrated for the first time in the InP/InGaAsP materials system and the current density versus voltage characteristics are presented. Numerical simulations are used to demonstrate the tolerances required in materials growth, doping, and layer thickness in order to successfully realize the heterostructure and the associated I-V characteristics of the DOES. In addition, material and interface quality are shown by simulations to affect the I-V characteristics of the InP/InGaAsP-based DOES. 13 refs., 4 figs.

  19. Anomalous Hall and spin Hall conductivities in three-dimensional ferromagnetic topological insulator/normal insulator heterostructures

    NASA Astrophysics Data System (ADS)

    Men'shov, Vladimir N.; Tugushev, Victor V.; Chulkov, Evgueni V.

    2016-05-01

    In this letter we theoretically demonstrate how an interface perturbation and size effect can be used to manipulate the transport properties of semiconductor heterostructures composed of a thin film of a three-dimensional topological insulator (TI) doped with magnetic impurities and sandwiched between topologically normal insulators. In the framework of a continual scheme, we argue that electron states of the TI film are strongly dominated by its thickness and magnetization as well as by an interface potential whose variation can lead to the modification of topological properties of the heterostructure. This opens diverse possibilities to efficiently tune intrinsic Hall conductivity in the system. We calculate a phase diagram of the heterostructure, which demonstrates a series of quantum transitions between distinct regimes of conductivity. We derive the anomalous Hall conductivity and the spin Hall conductivity dependences on the chemical potential. Applicability conditions of the used approach are also discussed.

  20. Ultimate photovoltage in perovskite oxide heterostructures with critical film thickness

    SciTech Connect

    Wang Cong; Jin Kuijuan; Zhao Ruiqiang; Lu Huibin; Guo Haizhong; Ge Chen; He Meng; Wang Can; Yang Guozhen

    2011-05-02

    One order larger photovoltage is obtained with critical thicknesses of La{sub 0.9}Sr{sub 0.1}MnO{sub 3} films in both kinds of heterostructures of La{sub 0.9}Sr{sub 0.1}MnO{sub 3}/SrTiO{sub 3} (0.8 wt % Nb-doped) and La{sub 0.9}Sr{sub 0.1}MnO{sub 3}/Si fabricated at various oxygen pressures. Our self-consistent calculation reveals that the critical thickness of the La{sub 0.9}Sr{sub 0.1}MnO{sub 3} film with the ultimate value of photovoltage is just the thickness of the depletion layer of La{sub 0.9}Sr{sub 0.1}MnO{sub 3} in both heterojunctions, respectively.

  1. Grey scale memory in an optically addressed spatial light modulator with a Lu(Pc)2 doped layer

    NASA Astrophysics Data System (ADS)

    Guéna, M.; Wu, Z. Y.; L'Her, M.; Pondaven, A.; Cadiou, C.

    1998-02-01

    An optically addressed spatial light modulator with electrolyte bisphtalocyanine of lutetium incorporated in the alignment layer is shown to be capable of memorizing a grey scale image. The relationship between the memorized grey scale and the illumination is shown. The temperature, the concentration of Lu(Pc)2 and the spontaneous polarization of liquid crystals are found to have the effect on the memorization rate.

  2. OPENING ADDRESS: Heterostructures in Semiconductors

    NASA Astrophysics Data System (ADS)

    Grimmeiss, Hermann G.

    1996-01-01

    Good morning, Gentlemen! On behalf of the Nobel Foundation, I should like to welcome you to the Nobel Symposium on "Heterostructures in Semiconductors". It gives me great pleasure to see so many colleagues and old friends from all over the world in the audience and, in particular, to bid welcome to our Nobel laureates, Prof. Esaki and Prof. von Klitzing. In front of a different audience I would now commend the scientific and technological importance of heterostructures in semiconductors and emphatically emphasise that heterostructures, as an important contribution to microelectronics and, hence, information technology, have changed societies all over the world. I would also mention that information technology is one of the most important global key industries which covers a wide field of important areas each of which bears its own character. Ever since the invention of the transistor, we have witnessed a fantastic growth in semiconductor technology, leading to more complex functions and higher densities of devices. This development would hardly be possible without an increasing understanding of semiconductor materials and new concepts in material growth techniques which allow the fabrication of previously unknown semiconductor structures. But here and today I will not do it because it would mean to carry coals to Newcastle. I will therefore not remind you that heterostructures were already suggested and discussed in detail a long time before proper technologies were available for the fabrication of such structures. Now, heterostructures are a foundation in science and part of our everyday life. Though this is certainly true, it is nevertheless fair to say that not all properties of heterostructures are yet understood and that further technologies have to be developed before a still better understanding is obtained. The organisers therefore hope that this symposium will contribute not only to improving our understanding of heterostructures but also to opening new

  3. 14th Workshop on Crystalline Silicon Solar Cells& Modules: Materials and Processes; Summary of Discussion Sessions

    SciTech Connect

    Sopori, B.; Tan, T.; Sinton, R.; Swanson, D.

    2004-10-01

    The 14th Workshop discussion sessions addressed funding needs for Si research and for R&D to enhance U.S. PV manufacturing. The wrap-up session specifically addressed topics for the new university silicon program. The theme of the workshop, Crystalline Silicon Solar Cells: Leapfrogging the Barriers, was selected to reflect the astounding progress in Si PV technology during last three decades, despite a host of barriers and bottlenecks. A combination of oral, poster, and discussion sessions addressed recent advances in crystal growth technology, new cell structures and doping methods, silicon feedstock issues, hydrogen passivation and fire through metallization, and module issues/reliability. The following oral/discussion sessions were conducted: (1) Technology Update; (2) Defects and Impurities in Si/Discussion; (3) Rump Session; (4) Module Issues and Reliability/Discussion; (5) Silicon Feedstock/Discussion; (6) Novel Doping, Cells, and Hetero-Structure Designs/Discussion; (7) Metallization/Silicon Nitride Processing/Discussion; (8) Hydrogen Passivation/Discussion; (9) Characterization/Discussion; and (10) Wrap-Up. This year's workshop lasted three and a half days and, for the first time, included a session on Si modules. A rump session was held on the evening of August 8, which addressed efficiency expectations and challenges of c Si solar cells/modules. Richard King of DOE and Daren Dance of Wright Williams& Kelly (formerly of Sematech) spoke at two of the luncheon sessions. Eleven students received Graduate Student Awards from funds contributed by the PV industry.

  4. Interlayer coupling enhancement in graphene/hexagonal boron nitride heterostructures by intercalated defects or vacancies

    SciTech Connect

    Park, Sohee; Park, Changwon; Kim, Gunn

    2014-04-07

    Hexagonal boron nitride (hBN), a remarkable material with a two-dimensional atomic crystal structure, has the potential to fabricate heterostructures with unusual properties. We perform first-principles calculations to determine whether intercalated metal atoms and vacancies can mediate interfacial coupling and influence the structural and electronic properties of the graphene/hBN heterostructure. Metal impurity atoms (Li, K, Cr, Mn, Co, and Cu), acting as extrinsic defects between the graphene and hBN sheets, produce n-doped graphene. We also consider intrinsic vacancy defects and find that a boron monovacancy in hBN acts as a magnetic dopant for graphene, whereas a nitrogen monovacancy in hBN serves as a nonmagnetic dopant for graphene. In contrast, the smallest triangular vacancy defects in hBN are unlikely to result in significant changes in the electronic transport of graphene. Our findings reveal that a hBN layer with some vacancies or metal impurities enhances the interlayer coupling in the graphene/hBN heterostructure with respect to charge doping and electron scattering.

  5. Enhanced thermoelectric properties in bulk nanowire heterostructure-based nanocomposites through minority carrier blocking.

    PubMed

    Yang, Haoran; Bahk, Je-Hyeong; Day, Tristan; Mohammed, Amr M S; Snyder, G Jeffrey; Shakouri, Ali; Wu, Yue

    2015-02-11

    To design superior thermoelectric materials the minority carrier blocking effect in which the unwanted bipolar transport is prevented by the interfacial energy barriers in the heterogeneous nanostructures has been theoretically proposed recently. The theory predicts an enhanced power factor and a reduced bipolar thermal conductivity for materials with a relatively low doping level, which could lead to an improvement in the thermoelectric figure of merit (ZT). Here we show the first experimental demonstration of the minority carrier blocking in lead telluride-silver telluride (PbTe-Ag2Te) nanowire heterostructure-based nanocomposites. The nanocomposites are made by sintering PbTe-Ag2Te nanowire heterostructures produced in a highly scalable solution-phase synthesis. Compared with Ag2Te nanowire-based nanocomposite produced in similar method, the PbTe-Ag2Te nanocomposite containing ∼5 atomic % PbTe exhibits enhanced Seebeck coefficient, reduced thermal conductivity, and ∼40% improved ZT, which can be well explained by the theoretical modeling based on the Boltzmann transport equations when energy barriers for both electrons and holes at the heterostructure interfaces are considered in the calculations. For this p-type PbTe-Ag2Te nanocomposite, the barriers for electrons, that is, minority carriers, are primarily responsible for the ZT enhancement. By extending this approach to other nanostructured systems, it represents a key step toward low-cost solution-processable nanomaterials without heavy doping level for high-performance thermoelectric energy harvesting.

  6. Aptameric Recognition-Modulated Electroactivity of Poly(4-Styrenesolfonic Acid)-Doped Polyaniline Films for Single-Shot Detection of Tetrodotoxin

    PubMed Central

    Fomo, Gertrude; Waryo, Tesfaye T.; Sunday, Christopher E.; Baleg, Abd A.; Baker, Priscilla G.; Iwuoha, Emmanuel I.

    2015-01-01

    The work being reported is the first electrochemical sensor for tetrodotoxin (TTX). It was developed on a glassy carbon electrodes (C) that was modified with poly(4-styrenesolfonic acid)-doped polyaniline film (PANI/PSSA). An amine-end functionalized TTX-binding aptamer, 5′-NH2-AAAAATTTCACACGGGTGCCTCGGCTGTCC-3′ (NH2-Apt), was grafted via covalent glutaraldehyde (glu) cross-linking. The resulting aptasensor (C//PANI+/PSSA-glu-NH2-Apt) was interrogated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in sodium acetate buffer (NaOAc, pH 4.8) before and after 30 min incubation in standard TTX solutions. Both CV and EIS results confirmed that the binding of the analyte to the immobilized aptamer modulated the electrochemical properties of the sensor: particularly the charge transfer resistance (Rct) of the PANI+/PSSA film, which served as a signal reporter. Based on the Rct calibration curve of the TTX aptasensor, the values of the dynamic linear range (DLR), sensitivity and limit of detection (LOD) of the sensor were determined to be 0.23–1.07 ng·mL−1 TTX, 134.88 ± 11.42 Ω·ng·mL−1 and 0.199 ng·mL−1, respectively. Further studies are being planned to improve the DLR as well as to evaluate selectivity and matrix effects in real samples. PMID:26370994

  7. The effects of the magnitude of the modulation field on electroreflectance spectroscopy of undoped-n+ type doped GaAs

    NASA Astrophysics Data System (ADS)

    Wang, D. P.; Huang, K. M.; Shen, T. L.; Huang, K. F.; Huang, T. C.

    1998-01-01

    The electroreflectance (ER) spectra of an undoped-n+ type doped GaAs has been measured at various amplitudes of modulating fields (δF). Many Franz-Keldysh oscillations were observed above the band gap energy, thus enabling the electric field (F) in the undoped layer to be determined. The F is obtained by applying fast Fourier transformation to the ER spectra. When δF is small, the power spectrum can be clearly resolved into two peaks, which corresponds to heavy- and light-hole transitions. When δF is less than ˜1/8 of the built-in field (Fbi˜77 420 V/cm), the F deduced from the ER is almost independent of δF. However, when larger than this, F is increased with δF. Also, when δF is increased to larger than ˜1/8 of Fbi, a shoulder appears on the right side of the heavy-hole peak of the power spectrum. The separation between the main peak and the shoulder of the heavy-hole peak becomes wider as δF becomes larger.

  8. Aptameric Recognition-Modulated Electroactivity of Poly(4-Styrenesolfonic Acid)-Doped Polyaniline Films for Single-Shot Detection of Tetrodotoxin.

    PubMed

    Fomo, Gertrude; Waryo, Tesfaye T; Sunday, Christopher E; Baleg, Abd A; Baker, Priscilla G; Iwuoha, Emmanuel I

    2015-09-08

    The work being reported is the first electrochemical sensor for tetrodotoxin (TTX). It was developed on a glassy carbon electrodes (C) that was modified with poly(4-styrenesolfonic acid)-doped polyaniline film (PANI/PSSA). An amine-end functionalized TTX-binding aptamer, 5'-NH₂-AAAAATTTCACACGGGTGCCTCGGCTGTCC-3' (NH₂-Apt), was grafted via covalent glutaraldehyde (glu) cross-linking. The resulting aptasensor (C//PANI⁺/PSSA-glu-NH2-Apt) was interrogated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in sodium acetate buffer (NaOAc, pH 4.8) before and after 30 min incubation in standard TTX solutions. Both CV and EIS results confirmed that the binding of the analyte to the immobilized aptamer modulated the electrochemical properties of the sensor: particularly the charge transfer resistance (Rct) of the PANI⁺/PSSA film, which served as a signal reporter. Based on the Rct calibration curve of the TTX aptasensor, the values of the dynamic linear range (DLR), sensitivity and limit of detection (LOD) of the sensor were determined to be 0.23-1.07 ng·mL(-1) TTX, 134.88 ± 11.42 Ω·ng·mL(-1) and 0.199 ng·mL(-1), respectively. Further studies are being planned to improve the DLR as well as to evaluate selectivity and matrix effects in real samples.

  9. Efficient Interlayer Relaxation and Transition of Excitons in Epitaxial and Non-epitaxial MoS2/WS2 Heterostructures

    DOE PAGES

    Yu, Yifei; Hu, Shi; Su, Liqin; Huang, Lujun; Liu, Yi; Jin, Zhenghe; Puretzky, Alexander A.; Geohegan, David B.; Kim, Ki Wook; Zhang, Yong; et al

    2014-12-03

    Semiconductor heterostructurs provide a powerful platform for the engineering of excitons. Here we report on the excitonic properties of two-dimensional (2D) heterostructures that consist of monolayer MoS2 and WS2 stacked epitaxially or non-epitaxially in the vertical direction. We find similarly efficient interlayer relaxation and transition of excitons in both the epitaxial and non-epitaxial heterostructures. This is manifested by a two orders of magnitude decrease in the photoluminescence and an extra absorption peak at low energy region of both heterostructures. The MoS2/WS2 heterostructures show weak interlayer coupling and essentially act as an atomic-scale heterojunction with the intrinsic band structures of themore » two monolayers largely preserved. They are particularly promising for the applications that request efficient dissociation of excitons and strong light absorption, including photovoltaics, solar fuels, photodetectors, and optical modulators. Our results also indicate that 2D heterostructures promise to provide capabilities to engineer excitons from the atomic level without concerns of interfacial imperfection.« less

  10. Efficient Interlayer Relaxation and Transition of Excitons in Epitaxial and Non-epitaxial MoS2/WS2 Heterostructures

    SciTech Connect

    Yu, Yifei; Hu, Shi; Su, Liqin; Huang, Lujun; Liu, Yi; Jin, Zhenghe; Puretzky, Alexander A.; Geohegan, David B.; Kim, Ki Wook; Zhang, Yong; Cao, Linyou

    2014-12-03

    Semiconductor heterostructurs provide a powerful platform for the engineering of excitons. Here we report on the excitonic properties of two-dimensional (2D) heterostructures that consist of monolayer MoS2 and WS2 stacked epitaxially or non-epitaxially in the vertical direction. We find similarly efficient interlayer relaxation and transition of excitons in both the epitaxial and non-epitaxial heterostructures. This is manifested by a two orders of magnitude decrease in the photoluminescence and an extra absorption peak at low energy region of both heterostructures. The MoS2/WS2 heterostructures show weak interlayer coupling and essentially act as an atomic-scale heterojunction with the intrinsic band structures of the two monolayers largely preserved. They are particularly promising for the applications that request efficient dissociation of excitons and strong light absorption, including photovoltaics, solar fuels, photodetectors, and optical modulators. Our results also indicate that 2D heterostructures promise to provide capabilities to engineer excitons from the atomic level without concerns of interfacial imperfection.

  11. EDITORIAL: Focus on Advanced Semiconductor Heterostructures for Optoelectronics

    NASA Astrophysics Data System (ADS)

    Amann, Markus C.; Capasso, Federico; Larsson, Anders; Pessa, Markus

    2009-12-01

    Semiconductor heterostructures are the basic materials underlying optoelectronic devices, particularly lasers and light-emitting diodes (LEDs). Made from various III-V-, II-VI-, SiGe- and other compound semiconductors, modern semiconductor devices are available for the generation, detection and modulation of light covering the entire ultra-violet to far-infrared spectral region. Recent approaches that introduced multilayer heterostructures tailored on the lower nanometre scale made possible artificial semiconductors with new properties, such as extended wavelength coverage, that enabled new applications. Together with ongoing progress on wide-gap semiconductors, the optical wavelengths accessible by semiconductor devices are steadily expanding towards the short-wavelength ultra-violet regime, as well as further into the far-infrared and terahertz spectral regions. It is the aim of this focus issue to present cutting-edge research topics on the most recent optoelectronic material and device developments in this field using advanced semiconductor heterostructures. Focus on Advanced Semiconductor Heterostructures for Optoelectronics Contents Theoretical and experimental investigations of the limits to the maximum output power of laser diodes H Wenzel, P Crump, A Pietrzak, X Wang, G Erbert and G Tränkle GaN/AlGaN intersubband optoelectronic devices H Machhadani, P Kandaswamy, S Sakr, A Vardi, A Wirtmüller, L Nevou, F Guillot, G Pozzovivo, M Tchernycheva, A Lupu, L Vivien, P Crozat, E Warde, C Bougerol, S Schacham, G Strasser, G Bahir, E Monroy and F H Julien Bound-to-continuum terahertz quantum cascade laser with a single-quantum-well phonon extraction/injection stage Maria I Amanti, Giacomo Scalari, Romain Terazzi, Milan Fischer, Mattias Beck, Jérôme Faist, Alok Rudra, Pascal Gallo and Eli Kapon Structural and optical characteristics of GaN/ZnO coaxial nanotube heterostructure arrays for light-emitting device applications Young Joon Hong, Jong-Myeong Jeon, Miyoung

  12. Distributed Bragg reflector laser for frequency modulated communication systems

    SciTech Connect

    Chraplyvy, A.R.; Koch, T.L.; Tkach, R.W.

    1990-02-27

    This patent describes a lightwave transmitter. It includes a distributed Bragg reflector laser and means for frequency modulating said laser. The laser comprises first and second semiconductor heterostructure regions.

  13. Mass spectrometric characterization of urinary metabolites of the selective androgen receptor modulator S-22 to identify potential targets for routine doping controls.

    PubMed

    Thevis, Mario; Thomas, Andreas; Möller, Ines; Geyer, Hans; Dalton, James T; Schänzer, Wilhelm

    2011-08-15

    Drugs that promote anabolic processes with limited undesirable effects are of considerable therapeutic interest; some notable examples include those for the treatment of cancer cachexia and muscle-wasting diseases. Anabolic properties are not only therapeutically beneficial to critically ill and debilitated patients, but are also desirable to athletes seeking artificial enhancements in endurance, strength and accelerated recovery. The use of anabolic agents in the clinical setting is being reconsidered with the emergence of a new class of drugs referred to as SARMs (selective androgen receptor modulators). SARMs have the potential to complement or even replace anabolic androgenic steroidal use with the benefit of a reduction of the undesirable side effects associated with steroid administration alone. Arylpropionamide-based SARMs such as andarine (S-4) and S-22 have shown promising therapeutic properties and have attracted the interest of elite and amateur athletes despite the absence of clinical approval, and evidence for trafficking and misuse in sport has been obtained by doping control authorities. In this communication, the elucidation of urinary metabolites of the SARM drug candidate S-22 is compared with earlier in vitro metabolism studies. Following oral administration of illicit S-22, urine samples were collected after 62 and 135 h and analyzed for the active drug and its major metabolic products. Liquid chromatography interfaced with high-resolution/high-accuracy (tandem) mass spectrometry was used to identify and/or confirm the predicted target analytes for sports drug testing purposes. S-22 was detected in both specimens accompanied by its glucuronic acid conjugate. This was the B-ring hydroxylated derivative of S-22 plus the corresponding glucuronide (with the phase-II metabolites being the more abundant analytes). In addition, the samples collected 62 h post-administration also contained the phase-I metabolite hydroxylated at the methyl residue (C-20

  14. Mass spectrometric characterization of urinary metabolites of the selective androgen receptor modulator S-22 to identify potential targets for routine doping controls.

    PubMed

    Thevis, Mario; Thomas, Andreas; Möller, Ines; Geyer, Hans; Dalton, James T; Schänzer, Wilhelm

    2011-08-15

    Drugs that promote anabolic processes with limited undesirable effects are of considerable therapeutic interest; some notable examples include those for the treatment of cancer cachexia and muscle-wasting diseases. Anabolic properties are not only therapeutically beneficial to critically ill and debilitated patients, but are also desirable to athletes seeking artificial enhancements in endurance, strength and accelerated recovery. The use of anabolic agents in the clinical setting is being reconsidered with the emergence of a new class of drugs referred to as SARMs (selective androgen receptor modulators). SARMs have the potential to complement or even replace anabolic androgenic steroidal use with the benefit of a reduction of the undesirable side effects associated with steroid administration alone. Arylpropionamide-based SARMs such as andarine (S-4) and S-22 have shown promising therapeutic properties and have attracted the interest of elite and amateur athletes despite the absence of clinical approval, and evidence for trafficking and misuse in sport has been obtained by doping control authorities. In this communication, the elucidation of urinary metabolites of the SARM drug candidate S-22 is compared with earlier in vitro metabolism studies. Following oral administration of illicit S-22, urine samples were collected after 62 and 135 h and analyzed for the active drug and its major metabolic products. Liquid chromatography interfaced with high-resolution/high-accuracy (tandem) mass spectrometry was used to identify and/or confirm the predicted target analytes for sports drug testing purposes. S-22 was detected in both specimens accompanied by its glucuronic acid conjugate. This was the B-ring hydroxylated derivative of S-22 plus the corresponding glucuronide (with the phase-II metabolites being the more abundant analytes). In addition, the samples collected 62 h post-administration also contained the phase-I metabolite hydroxylated at the methyl residue (C-20

  15. Heterostructures for Increased Quantum Efficiency in Nitride LEDs

    SciTech Connect

    Davis, Robert F.

    2010-09-30

    Task 1. Development of an advanced LED simulator useful for the design of efficient nitride-based devices. Simulator will contain graphical interface software that can be used to specify the device structure, the material parameters, the operating conditions and the desired output results. Task 2. Theoretical and experimental investigations regarding the influence on the microstructure, defect concentration, mechanical stress and strain and IQE of controlled changes in the chemistry and process route of deposition of the buffer layer underlying the active region of nitride-based blue- and greenemitting LEDs. Task 3. Theoretical and experimental investigations regarding the influence on the physical properties including polarization and IQE of controlled changes in the geometry, chemistry, defect density, and microstructure of components in the active region of nitride-based blue- and green-emitting LEDs. Task 4. Theoretical and experimental investigations regarding the influence on IQE of novel heterostructure designs to funnel carriers into the active region for enhanced recombination efficiency and elimination of diffusion beyond this region. Task 5. Theoretical and experimental investigations regarding the influence of enhanced p-type doping on the chemical, electrical, and microstructural characteristics of the acceptor-doped layers, the hole injection levels at Ohmic contacts, the specific contact resistivity and the IQE of nitride-based blue- and green-emitting LEDs. Development and optical and electrical characterization of reflective Ohmic contacts to n- and p-type GaN films.

  16. Nanocrystalline Fe-Fe2O3 particle-deposited N-doped graphene as an activity-modulated Pt-free electrocatalyst for oxygen reduction reaction

    NASA Astrophysics Data System (ADS)

    Dhavale, Vishal M.; Singh, Santosh K.; Nadeema, Ayasha; Gaikwad, Sachin S.; Kurungot, Sreekumar

    2015-11-01

    The size-controlled growth of nanocrystalline Fe-Fe2O3 particles (2-3 nm) and their concomitant dispersion on N-doped graphene (Fe-Fe2O3/NGr) could be attained when the mutually assisted redox reaction between NGr and Fe3+ ions could be controlled within the aqueous droplets of a water-in-oil emulsion. The synergistic interaction existing between Fe-Fe2O3 and NGr helped the system to narrow down the overpotential for the oxygen reduction reaction (ORR) by bringing a significant positive shift to the reduction onset potential, which is just 15 mV higher than its Pt-counterpart. In addition, the half-wave potential (E1/2) of Fe-Fe2O3/NGr is found to be improved by a considerable amount of 135 mV in comparison to the system formed by dispersing Fe-Fe2O3 nanoparticles on reduced graphene oxide (Fe-Fe2O3/RGO), which indicates the presence of a higher number of active sites in Fe-Fe2O3/NGr. Despite this, the ORR kinetics of Fe-Fe2O3/NGr are found to be shifted significantly to the preferred 4-electron-transfer pathway compared to NGr and Fe-Fe2O3/RGO. Consequently, the H2O2% was found to be reduced by 78.3% for Fe-Fe2O3/NGr (13.0%) in comparison to Fe-Fe2O3/RGO (51.2%) and NGr (41.0%) at -0.30 V (vs. Hg/HgO). This difference in the yield of H2O2 formed between the systems along with the improvements observed in terms of the oxygen reduction onset and E1/2 in the case of Fe-Fe2O3/NGr reveals the activity modulation achieved for the latter is due to the coexistence of factors such as the presence of the mixed valancies of iron nanoparticles, small size and homogeneous distribution of Fe-Fe2O3 nanoparticles and the electronic modifications induced by the doped nitrogen in NGr. A controlled interplay of these factors looks like worked favorably in the case of Fe-Fe2O3/NGr. As a realistic system level validation, Fe-Fe2O3/NGr was employed as the cathode electrode of a single cell in a solid alkaline electrolyte membrane fuel cell (AEMFC). The system could display an open

  17. Band Alignment and Minigaps in Monolayer MoS2-Graphene van der Waals Heterostructures.

    PubMed

    Pierucci, Debora; Henck, Hugo; Avila, Jose; Balan, Adrian; Naylor, Carl H; Patriarche, Gilles; Dappe, Yannick J; Silly, Mathieu G; Sirotti, Fausto; Johnson, A T Charlie; Asensio, Maria C; Ouerghi, Abdelkarim

    2016-07-13

    Two-dimensional layered MoS2 shows great potential for nanoelectronic and optoelectronic devices due to its high photosensitivity, which is the result of its indirect to direct band gap transition when the bulk dimension is reduced to a single monolayer. Here, we present an exhaustive study of the band alignment and relativistic properties of a van der Waals heterostructure formed between single layers of MoS2 and graphene. A sharp, high-quality MoS2-graphene interface was obtained and characterized by micro-Raman spectroscopy, high-resolution X-ray photoemission spectroscopy (HRXPS), and scanning high-resolution transmission electron microscopy (STEM/HRTEM). Moreover, direct band structure determination of the MoS2/graphene van der Waals heterostructure monolayer was carried out using angle-resolved photoemission spectroscopy (ARPES), shedding light on essential features such as doping, Fermi velocity, hybridization, and band-offset of the low energy electronic dynamics found at the interface. We show that, close to the Fermi level, graphene exhibits a robust, almost perfect, gapless, and n-doped Dirac cone and no significant charge transfer doping is detected from MoS2 to graphene. However, modification of the graphene band structure occurs at rather larger binding energies, as the opening of several miniband-gaps is observed. These miniband-gaps resulting from the overlay of MoS2 and the graphene layer lattice impose a superperiodic potential. PMID:27281693

  18. Band Alignment and Minigaps in Monolayer MoS2-Graphene van der Waals Heterostructures.

    PubMed

    Pierucci, Debora; Henck, Hugo; Avila, Jose; Balan, Adrian; Naylor, Carl H; Patriarche, Gilles; Dappe, Yannick J; Silly, Mathieu G; Sirotti, Fausto; Johnson, A T Charlie; Asensio, Maria C; Ouerghi, Abdelkarim

    2016-07-13

    Two-dimensional layered MoS2 shows great potential for nanoelectronic and optoelectronic devices due to its high photosensitivity, which is the result of its indirect to direct band gap transition when the bulk dimension is reduced to a single monolayer. Here, we present an exhaustive study of the band alignment and relativistic properties of a van der Waals heterostructure formed between single layers of MoS2 and graphene. A sharp, high-quality MoS2-graphene interface was obtained and characterized by micro-Raman spectroscopy, high-resolution X-ray photoemission spectroscopy (HRXPS), and scanning high-resolution transmission electron microscopy (STEM/HRTEM). Moreover, direct band structure determination of the MoS2/graphene van der Waals heterostructure monolayer was carried out using angle-resolved photoemission spectroscopy (ARPES), shedding light on essential features such as doping, Fermi velocity, hybridization, and band-offset of the low energy electronic dynamics found at the interface. We show that, close to the Fermi level, graphene exhibits a robust, almost perfect, gapless, and n-doped Dirac cone and no significant charge transfer doping is detected from MoS2 to graphene. However, modification of the graphene band structure occurs at rather larger binding energies, as the opening of several miniband-gaps is observed. These miniband-gaps resulting from the overlay of MoS2 and the graphene layer lattice impose a superperiodic potential.

  19. Methods for Doping Detection.

    PubMed

    Ponzetto, Federico; Giraud, Sylvain; Leuenberger, Nicolas; Boccard, Julien; Nicoli, Raul; Baume, Norbert; Rudaz, Serge; Saugy, Martial

    2016-01-01

    Over the past few years, the World Anti-Doping Agency (WADA) has focused its efforts on detecting not only small prohibited molecules, but also larger endogenous molecules such as hormones, in the view of implementing an endocrinological module in the Athlete Biological Passport (ABP). In this chapter, the detection of two major types of hormones used for doping, growth hormone (GH) and endogenous anabolic androgenic steroids (EAASs), will be discussed: a brief historical background followed by a description of state-of-the-art methods applied by accredited anti-doping laboratories will be provided and then current research trends outlined. In addition, microRNAs (miRNAs) will also be presented as a new class of biomarkers for doping detection. PMID:27348309

  20. Validation of a GC/MS method for the detection of two quinolinone-derived selective androgen receptor modulators in doping control analysis.

    PubMed

    Gerace, E; Salomone, A; Fasano, F; Costa, R; Boschi, D; Di Stilo, A; Vincenti, M

    2011-04-01

    Selective androgen receptor modulators (SARMs) represent an emerging class of drugs likely to be abused in sport. For clinical applications, these substances provide a promising alternative to testosterone-replacement therapies and their advantages include oral bioavailability, androgen receptor specificity, tissue selectivity, and the absence of steroid-related side effects. Although not yet commercially available, since January 2008 SARMs have been included on the prohibited list issued yearly by the World Anti-Doping Agency (WADA), so control laboratories need to update their procedures to detect either the parent drugs or their metabolites. Within this context, two quinolinone SARM models were synthesized and automatically characterized to update the existing routine screening procedures. The conditions for the new target analytes are compatible with the existing laboratory protocols used for both in-competition and out-of-competition controls and can be included in them. Validation parameters according to ISO 17025 and WADA guidelines were successfully determined. For analytical determinations, spiked urine samples were hydrolyzed and extracted at pH 9.6 with 10 mL of tert-butyl methyl ether. Then, the analytes were subsequently converted into trimethylsilyl derivatives and detected by gas chromatography-mass spectrometry. The absence of interferents, together with excellent repeatability of both retention times and the relative abundances of diagnostic ions, allowed proper identification of all SARM analytes. The analytes' quantification was linear up to 500 ng/mL and precision criteria were satisfied (coefficient of variation less than 25% at 10 ng/mL). The limits of detection were 1 ng/mL for both SARMs, whereas recovery values were between 95.5 and 99.3%. The validated method can be efficiently used for urine screening of the 2-quinolinone-derived SARMs tested.

  1. From gene engineering to gene modulation and manipulation: can we prevent or detect gene doping in sports?

    PubMed

    Fischetto, Giuseppe; Bermon, Stéphane

    2013-10-01

    During the last 2 decades, progress in deciphering the human gene map as well as the discovery of specific defective genes encoding particular proteins in some serious human diseases have resulted in attempts to treat sick patients with gene therapy. There has been considerable focus on human recombinant proteins which were gene-engineered and produced in vitro (insulin, growth hormone, insulin-like growth factor-1, erythropoietin). Unfortunately, these substances and methods also became improper tools for unscrupulous athletes. Biomedical research has focused on the possible direct insertion of gene material into the body, in order to replace some defective genes in vivo and/or to promote long-lasting endogenous synthesis of deficient proteins. Theoretically, diabetes, anaemia, muscular dystrophies, immune deficiency, cardiovascular diseases and numerous other illnesses could benefit from such innovative biomedical research, though much work remains to be done. Considering recent findings linking specific genotypes and physical performance, it is tempting to submit the young athletic population to genetic screening or, alternatively, to artificial gene expression modulation. Much research is already being conducted in order to achieve a safe transfer of genetic material to humans. This is of critical importance since uncontrolled production of the specifically coded protein, with serious secondary adverse effects (polycythaemia, acute cardiovascular problems, cancer, etc.), could occur. Other unpredictable reactions (immunogenicity of vectors or DNA-vector complex, autoimmune anaemia, production of wild genetic material) also remain possible at the individual level. Some new substances (myostatin blockers or anti-myostatin antibodies), although not gene material, might represent a useful and well-tolerated treatment to prevent progression of muscular dystrophies. Similarly, other molecules, in the roles of gene or metabolic activators [5-aminoimidazole-4

  2. Terahertz-range spontaneous emission under the optical excitation of donors in uniaxially stressed bulk silicon and SiGe/Si heterostructures

    SciTech Connect

    Zhukavin, R. Kh. Kovalevsky, K. A.; Orlov, M. L.; Tsyplenkov, V. V.; Bekin, N. A.; Yablonskiy, A. N.; Yunin, P. A.; Pavlov, S. G.; Abrosimov, N. V.; Hübers, H.-W.; Radamson, H. H.; Shastin, V. N.

    2015-01-15

    The results of measurements of the total terahertz-range photoluminescence of Group-V donors (phosphorus, antimony, bismuth, arsenic) in bulk silicon and SiGe/Si heterostructures depending on the excitation intensity are presented. The signal of bulk silicon was also measured as a function of uniaxial stress. The results of measurement of the dependence of the spontaneous emission intensity on the uniaxial stress is in rather good agreement with theoretical calculations of the relaxation times of excited states of donors in bulk silicon. Comparative measurements of the spontaneous emission from various strained heterostructures showed that the photoluminescence signal is caused by donor-doped silicon regions.

  3. Device Concepts Based on Spin-dependent Transmission in Semiconductor Heterostructures

    NASA Technical Reports Server (NTRS)

    Ting, David Z. - Y.; Cartoixa, X.

    2004-01-01

    We examine zero-magnetic-field spin-dependent transmission in nonmagnetic semiconductor heterostructures with structural inversion asymmetry (SIA) and bulk inversion asymmetry (BIA), and report spin devices concepts that exploit their properties. Our modeling results show that several design strategies could be used to achieve high spin filtering efficiencies. The current spin polarization of these devices is electrically controllable, and potentially amenable to highspeed spin modulation, and could be integrated in optoelectronic devices for added functionality.

  4. Single-hole transistor in p-type GaAs /AlGaAs heterostructures

    NASA Astrophysics Data System (ADS)

    Grbić, Boris; Leturcq, Renaud; Ensslin, Klaus; Reuter, Dirk; Wieck, Andreas D.

    2005-12-01

    A single-hole transistor is patterned in a p-type, C-doped GaAs /AlGaAs heterostructure by scanning probe oxidation lithography. Clear Coulomb blockade resonances have been observed at Thole=300mK. A charging energy of ˜1.5meV is extracted from Coulomb diamond measurements, in agreement with the lithographic dimensions of the dot. The absence of excited states in Coulomb diamond measurements, as well as the temperature dependence of Coulomb peak heights indicate that the dot is in the multilevel transport regime. Fluctuations in peak spacings larger than the estimated mean single-particle level spacing are observed.

  5. Magnetotransport measurements on modulation Si δ-doped pseudomorphic In 0.2Ga 0.8As/GaAs quantum wells

    NASA Astrophysics Data System (ADS)

    Babinski, Adam; Li, G.; Jagadish, C.

    1998-05-01

    The effect of doping concentration on electrical properties of Si δ-doped pseudomorphic In 0.2Ga 0.8As/GaAs quantum wells (QWs) was examined in this work. Magnetotransport measurements were carried out in the magnetic fields up to 12 T in the dark at the temperature of T=1.7 K. The results were analysed using the fast Fourier transform (FFT). It was found that one electron subband was occupied in the QW with the doping concentrations of 2.3×10 12 or 6.0×10 12 cm -2. The parallel conduction was present in both samples, which increased with an increase of Si δ-doping concentration. The well-developed plateaus arising from the quantised Hall effect were observed in the Si δ-doped In 0.2Ga 0.8As/GaAs QW with reduced parallel conduction. The two subbands remain occupied within the V-shaped potential well formed at the Si doping plane when the Si δ-doping concentration was high. The results of measurements in tilted magnetic fields, confirming the attribution of FFT peaks to the particular 2DEG systems, are also presented.

  6. The effect of split gate dimensions on the electrostatic potential and 0.7 anomaly within one-dimensional quantum wires on a modulation doped GaAs/AlGaAs heterostructure

    NASA Astrophysics Data System (ADS)

    Smith, L. W.; Al-Taie, H.; Lesage, A. A. J.; Thomas, K. J.; Sfigakis, F.; See, P.; Griffiths, J. P.; Farrer, I.; Jones, G. A. C.; Ritchie, D. A.; Kelly, M. J.; Smith, C. G.

    We use a multiplexing scheme to measure the conductance properties of 95 split gates of 7 different gate dimensions fabricated on a GaAs/AlGaAs chip, in a single cool down. The number of devices for which conductance is accurately quantized reduces as the gate length increases. However, even the devices for which conductance is accurately quantized in units of 2e2 / h show no correlation between the length of electrostatic potential barrier in the channel and the gate length, using a saddle point model to estimate the barrier length. Further, the strength of coupling between the gates and the 1D channel does not increase with gate length beyond 0.7 μm. The background electrostatic profile appears as significant as the gate dimension in determining device behavior. We find a clear correlation between the curvature of the electrostatic barrier along the channel and the strength of the ``0.7 anomaly'' which identifies the electrostatic length of the channel as the principal factor governing the conductance of the 0.7 anomaly. Present address: Wisconsin Institute for Quantum Information, University of Wisconsin-Madison, Madison, WI.

  7. Superfluid Stiffness and Tc Enhancement in Cuprate Heterostructures

    NASA Astrophysics Data System (ADS)

    Goren, Lilach

    The basic electronic correlations underlying the effect of high Tc superconductivity in cuprates, still elude a complete and unified theoretical description. This thesis deals with several central open questions regarding the crucial aspects that determine superconductivity in cuprates. A key question concerns the nature of the phase which is formed when superconductivity is destroyed. The origin of the 'pseudogap' in the density of states above Tc is not clear to this day. We address this question by investigating the destruction of superconductivity at T = 0 as current is applied. We design novel Gutzwiller projected variational states, that incorporate supercurrent in a d-wave BCS wave-function. We identify two different mechanisms which determine the critical current at which superconductivity is destroyed: at high hole doping [special characters omitted] it occurs when quasiparticle pockets completely destroy the gap in a BCS-like mechanism. In the underdoped regime the mechanism is bosonic, whereby the critical current is set by a maximal phase twist which destroys the superfluid stiffness with pairing still intact. This result is indicative of a pseudogapped 'normal' state which retains pairing correlations. Moreover, we find a dome shaped critical current as a function of doping, similar to Tc. A second question concerns the determination of Tc and in particular possible ways to increase it in cuprate heterostructures. We investigate two possible scenarios that are aimed at profiting from proximity between a largegap underdoped and a large carrier density overdoped cuprate material. In the first scenario we consider an underdoped-overdoped bilayer and find a possible Tc enhancement, assuming a relatively high interlayer coupling. In the second case, we investigate underdoped-overdoped in-plane inhomogeneity. There, the coupling is naturally high, and the proximity effect can be strong. For a microscopic doping inhomogeneity we find an enhancement of Tc

  8. Graphene oxide/graphene vertical heterostructure electrodes for highly efficient and flexible organic light emitting diodes

    NASA Astrophysics Data System (ADS)

    Jia, S.; Sun, H. D.; Du, J. H.; Zhang, Z. K.; Zhang, D. D.; Ma, L. P.; Chen, J. S.; Ma, D. G.; Cheng, H. M.; Ren, W. C.

    2016-05-01

    , and good compatibility with HIL materials (MoO3 in this work). Moreover, the conductivity of the heterostructure is not sacrificed compared to the pristine three-layer graphene electrodes, but is significantly higher than that of pristine two-layer graphene films. In addition to high flexibility, OLEDs with different emission colors based on the GO/G heterostructure TCEs show much better performance than those based on indium tin oxide (ITO) anodes. Green OLEDs with GO/G heterostructure electrodes have the maximum current efficiency and power efficiency, as high as 82.0 cd A-1 and 98.2 lm W-1, respectively, which are 36.7% (14.8%) and 59.2% (15.0%) higher than those with pristine graphene (ITO) anodes. These findings open up the possibility of using graphene for next generation high-performance flexible and wearable optoelectronics with high stability. Electronic supplementary information (ESI) available: XPS spectra, Raman spectra, sheet resistance and transmittance of graphene films with different numbers of layers and different ozone treatment times, doping effect of MoO3 on graphene and GO/G electrodes, performance of green OLEDs with different graphene anodes, a movie showing the flexibility of device. See DOI: 10.1039/c6nr01649a

  9. A novel ultra steep dynamically reconfigurable electrostatically doped silicon nanowire Schottky Barrier FET

    NASA Astrophysics Data System (ADS)

    Singh, Sangeeta; Sinha, Ruchir; Kondekar, P. N.

    2016-05-01

    In this paper, an ultra steep, symmetric and dynamically configurable, electrostatically doped silicon nanowire Schottky FET (E-SiNW-SB-FET) based on dopant-free technology is investigated. It achieves the ultra steep sub-threshold slope (SS) due to the cumulative effect of weak impact-ionization induced positive feedback and electrostatic modulation of Schottky barrier heights at both source and drain terminals. It consists of axial nanowire heterostructure (silicide-intrinsic silicon-silicide) with three independent all-around gates, two gates are polarity control gates for dynamically reconfiguring the device polarity by modulating the effective Schottky barrier heights and a control gate switches the device ON and OFF. The most interesting features of the proposed structure are simplified fabrication process as the state-of-the-art for ion implantation and high thermal budget no more required for annealing. It is highly immune to process variations, doping control issues and random dopant fluctuations (RDF) and there are no mobility degradation issues related to high doping. A calibrated 3-D TCAD simulation results exhibit the SS of 2 mV/dec for n-type E-SiNW-SB-FET and 9 mV/dec for p-type E-SiNW-SB-FET for about five decades of current. Further, it resolves all the reliability related issues of IMOS as hot electron effects are no more limiting our device performance. It offers significant drive current of the order of 10-5-10-4 A and magnificently high ION/IOFF ratio of ∼108 along with the inherent advantages of symmetric device structure for its circuit realization.

  10. Nanocrystalline Fe-Fe2O3 particle-deposited N-doped graphene as an activity-modulated Pt-free electrocatalyst for oxygen reduction reaction.

    PubMed

    Dhavale, Vishal M; Singh, Santosh K; Nadeema, Ayasha; Gaikwad, Sachin S; Kurungot, Sreekumar

    2015-12-21

    The size-controlled growth of nanocrystalline Fe-Fe2O3 particles (2-3 nm) and their concomitant dispersion on N-doped graphene (Fe-Fe2O3/NGr) could be attained when the mutually assisted redox reaction between NGr and Fe(3+) ions could be controlled within the aqueous droplets of a water-in-oil emulsion. The synergistic interaction existing between Fe-Fe2O3 and NGr helped the system to narrow down the overpotential for the oxygen reduction reaction (ORR) by bringing a significant positive shift to the reduction onset potential, which is just 15 mV higher than its Pt-counterpart. In addition, the half-wave potential (E1/2) of Fe-Fe2O3/NGr is found to be improved by a considerable amount of 135 mV in comparison to the system formed by dispersing Fe-Fe2O3 nanoparticles on reduced graphene oxide (Fe-Fe2O3/RGO), which indicates the presence of a higher number of active sites in Fe-Fe2O3/NGr. Despite this, the ORR kinetics of Fe-Fe2O3/NGr are found to be shifted significantly to the preferred 4-electron-transfer pathway compared to NGr and Fe-Fe2O3/RGO. Consequently, the H2O2% was found to be reduced by 78.3% for Fe-Fe2O3/NGr (13.0%) in comparison to Fe-Fe2O3/RGO (51.2%) and NGr (41.0%) at -0.30 V (vs. Hg/HgO). This difference in the yield of H2O2 formed between the systems along with the improvements observed in terms of the oxygen reduction onset and E1/2 in the case of Fe-Fe2O3/NGr reveals the activity modulation achieved for the latter is due to the coexistence of factors such as the presence of the mixed valancies of iron nanoparticles, small size and homogeneous distribution of Fe-Fe2O3 nanoparticles and the electronic modifications induced by the doped nitrogen in NGr. A controlled interplay of these factors looks like worked favorably in the case of Fe-Fe2O3/NGr. As a realistic system level validation, Fe-Fe2O3/NGr was employed as the cathode electrode of a single cell in a solid alkaline electrolyte membrane fuel cell (AEMFC). The system could display an open

  11. Nanocrystalline Fe-Fe2O3 particle-deposited N-doped graphene as an activity-modulated Pt-free electrocatalyst for oxygen reduction reaction.

    PubMed

    Dhavale, Vishal M; Singh, Santosh K; Nadeema, Ayasha; Gaikwad, Sachin S; Kurungot, Sreekumar

    2015-12-21

    The size-controlled growth of nanocrystalline Fe-Fe2O3 particles (2-3 nm) and their concomitant dispersion on N-doped graphene (Fe-Fe2O3/NGr) could be attained when the mutually assisted redox reaction between NGr and Fe(3+) ions could be controlled within the aqueous droplets of a water-in-oil emulsion. The synergistic interaction existing between Fe-Fe2O3 and NGr helped the system to narrow down the overpotential for the oxygen reduction reaction (ORR) by bringing a significant positive shift to the reduction onset potential, which is just 15 mV higher than its Pt-counterpart. In addition, the half-wave potential (E1/2) of Fe-Fe2O3/NGr is found to be improved by a considerable amount of 135 mV in comparison to the system formed by dispersing Fe-Fe2O3 nanoparticles on reduced graphene oxide (Fe-Fe2O3/RGO), which indicates the presence of a higher number of active sites in Fe-Fe2O3/NGr. Despite this, the ORR kinetics of Fe-Fe2O3/NGr are found to be shifted significantly to the preferred 4-electron-transfer pathway compared to NGr and Fe-Fe2O3/RGO. Consequently, the H2O2% was found to be reduced by 78.3% for Fe-Fe2O3/NGr (13.0%) in comparison to Fe-Fe2O3/RGO (51.2%) and NGr (41.0%) at -0.30 V (vs. Hg/HgO). This difference in the yield of H2O2 formed between the systems along with the improvements observed in terms of the oxygen reduction onset and E1/2 in the case of Fe-Fe2O3/NGr reveals the activity modulation achieved for the latter is due to the coexistence of factors such as the presence of the mixed valancies of iron nanoparticles, small size and homogeneous distribution of Fe-Fe2O3 nanoparticles and the electronic modifications induced by the doped nitrogen in NGr. A controlled interplay of these factors looks like worked favorably in the case of Fe-Fe2O3/NGr. As a realistic system level validation, Fe-Fe2O3/NGr was employed as the cathode electrode of a single cell in a solid alkaline electrolyte membrane fuel cell (AEMFC). The system could display an open

  12. Enhanced Surface-and-Interface Coupling in Pd-Nanoparticle-coated LaAlO3/SrTiO3 Heterostructures: Strong Gas- and Photo-Induced Conductance Modulation

    PubMed Central

    Kim, Haeri; Chan, Ngai Yui; Dai, Ji-yan; Kim, Dong-Wook

    2015-01-01

    Pd nanoparticle (NP) coated LaAlO3/SrTiO3 (LAO/STO) heterointerface exhibits more notable conductance (G) change while varying the ambient gas (N2, H2/N2, and O2) and illuminating with UV light (wavelength: 365 nm) than a sample without the NPs. Simultaneous Kelvin probe force microscopy and transport measurements reveal close relationships between the surface work function (W) and G of the samples. Quantitative analyses suggest that a surface adsorption/desorption-mediated reaction and redox, resulting in a band-alignment modification and charge-transfer, could explain the gas- and photo-induced conductance modulation at the LAO/STO interface. Such surface-and-interface coupling enhanced by catalytic Pd NPs is a unique feature, quite distinct from conventional semiconductor hetero-junctions, which enables the significant conductance tunability at ultrathin oxide heterointerfaces by external stimuli. PMID:25704566

  13. Enhanced Surface-and-Interface Coupling in Pd-Nanoparticle-coated LaAlO3/SrTiO3 Heterostructures: Strong Gas- and Photo-Induced Conductance Modulation

    NASA Astrophysics Data System (ADS)

    Kim, Haeri; Chan, Ngai Yui; Dai, Ji-Yan; Kim, Dong-Wook

    2015-02-01

    Pd nanoparticle (NP) coated LaAlO3/SrTiO3 (LAO/STO) heterointerface exhibits more notable conductance (G) change while varying the ambient gas (N2, H2/N2, and O2) and illuminating with UV light (wavelength: 365 nm) than a sample without the NPs. Simultaneous Kelvin probe force microscopy and transport measurements reveal close relationships between the surface work function (W) and G of the samples. Quantitative analyses suggest that a surface adsorption/desorption-mediated reaction and redox, resulting in a band-alignment modification and charge-transfer, could explain the gas- and photo-induced conductance modulation at the LAO/STO interface. Such surface-and-interface coupling enhanced by catalytic Pd NPs is a unique feature, quite distinct from conventional semiconductor hetero-junctions, which enables the significant conductance tunability at ultrathin oxide heterointerfaces by external stimuli.

  14. Nanocrystalline Fe-Fe2O3 particle-deposited N-doped graphene as an activity-modulated Pt-free electrocatalyst for oxygen reduction reaction

    NASA Astrophysics Data System (ADS)

    Dhavale, Vishal M.; Singh, Santosh K.; Nadeema, Ayasha; Gaikwad, Sachin S.; Kurungot, Sreekumar

    2015-11-01

    The size-controlled growth of nanocrystalline Fe-Fe2O3 particles (2-3 nm) and their concomitant dispersion on N-doped graphene (Fe-Fe2O3/NGr) could be attained when the mutually assisted redox reaction between NGr and Fe3+ ions could be controlled within the aqueous droplets of a water-in-oil emulsion. The synergistic interaction existing between Fe-Fe2O3 and NGr helped the system to narrow down the overpotential for the oxygen reduction reaction (ORR) by bringing a significant positive shift to the reduction onset potential, which is just 15 mV higher than its Pt-counterpart. In addition, the half-wave potential (E1/2) of Fe-Fe2O3/NGr is found to be improved by a considerable amount of 135 mV in comparison to the system formed by dispersing Fe-Fe2O3 nanoparticles on reduced graphene oxide (Fe-Fe2O3/RGO), which indicates the presence of a higher number of active sites in Fe-Fe2O3/NGr. Despite this, the ORR kinetics of Fe-Fe2O3/NGr are found to be shifted significantly to the preferred 4-electron-transfer pathway compared to NGr and Fe-Fe2O3/RGO. Consequently, the H2O2% was found to be reduced by 78.3% for Fe-Fe2O3/NGr (13.0%) in comparison to Fe-Fe2O3/RGO (51.2%) and NGr (41.0%) at -0.30 V (vs. Hg/HgO). This difference in the yield of H2O2 formed between the systems along with the improvements observed in terms of the oxygen reduction onset and E1/2 in the case of Fe-Fe2O3/NGr reveals the activity modulation achieved for the latter is due to the coexistence of factors such as the presence of the mixed valancies of iron nanoparticles, small size and homogeneous distribution of Fe-Fe2O3 nanoparticles and the electronic modifications induced by the doped nitrogen in NGr. A controlled interplay of these factors looks like worked favorably in the case of Fe-Fe2O3/NGr. As a realistic system level validation, Fe-Fe2O3/NGr was employed as the cathode electrode of a single cell in a solid alkaline electrolyte membrane fuel cell (AEMFC). The system could display an open

  15. WSe2 heterostructures with p-type multi-layer graphene contacts

    NASA Astrophysics Data System (ADS)

    Arefe, Ghidewon; Finney, Nathan; Seo, Dongjea; Kim, Young Duck; Chang, Damien; Cui, Xu; Kang, Kyung Nam; Jerng, Sahng-Kyoon; Chun, Seung Hyun; Yang, Eui-Hyeok; Hone, James

    Recent advances in 2D material research have opened up new opportunities to study fundamental physics and to imagine new applications for this advanced class of materials. 2D tungsten diselenide (WSe2) is a transition metal dichalcogenide (TMDC) semiconductor that is intrinsically p-type with great potential for advanced opto-electronic applications. WSe2 monolayers grown by CVD and highly p-doped PECVD multilayer graphene are used to construct 2D heterostructure p-type field effect transistors as a platform to study the potential for applications and novel physical phenomena. P-type graphene is being used to overcome the challenge of making good electrical contact to WSe2 that is Ohmic at low temperatures and to allow for the construction of an entirely 2D heterostructure. Electrical transport and novel optical effects will be studied in these WSe2 heterostructures that are fully encapsulated in hexagonal boron nitride (h-BN) in order to show greatly improved environmental stability and high mobility at low temperature due to the suppression of extrinsic scattering effects such as charge impurities, surface polar optical phonons, and absorbents from air.

  16. Atomically Sharp Interface in an h-BN-epitaxial graphene van der Waals Heterostructure

    PubMed Central

    Sediri, Haikel; Pierucci, Debora; Hajlaoui, Mahdi; Henck, Hugo; Patriarche, Gilles; Dappe, Yannick J.; Yuan, Sheng; Toury, Bérangère; Belkhou, Rachid; Silly, Mathieu G.; Sirotti, Fausto; Boutchich, Mohamed; Ouerghi, Abdelkarim

    2015-01-01

    Stacking various two-dimensional atomic crystals is a feasible approach to creating unique multilayered van der Waals heterostructures with tailored properties. Herein for the first time, we present a controlled preparation of large-area h-BN/graphene heterostructures via a simple chemical deposition of h-BN layers on epitaxial graphene/SiC(0001). Van der Waals forces, which are responsible for the cohesion of the multilayer system, give rise to an abrupt interface without interdiffusion between graphene and h-BN, as shown by X-ray Photoemission Spectroscopy (XPS) and direct observation using scanning and High-Resolution Transmission Electron Microscopy (STEM/HRTEM). The electronic properties of graphene, such as the Dirac cone, remain intact and no significant charge transfer i.e. doping, is observed. These results are supported by Density Functional Theory (DFT) calculations. We demonstrate that the h-BN capped graphene allows the fabrication of vdW heterostructures without altering the electronic properties of graphene. PMID:26585245

  17. Atomically Sharp Interface in an h-BN-epitaxial graphene van der Waals Heterostructure.

    PubMed

    Sediri, Haikel; Pierucci, Debora; Hajlaoui, Mahdi; Henck, Hugo; Patriarche, Gilles; Dappe, Yannick J; Yuan, Sheng; Toury, Bérangère; Belkhou, Rachid; Silly, Mathieu G; Sirotti, Fausto; Boutchich, Mohamed; Ouerghi, Abdelkarim

    2015-01-01

    Stacking various two-dimensional atomic crystals is a feasible approach to creating unique multilayered van der Waals heterostructures with tailored properties. Herein for the first time, we present a controlled preparation of large-area h-BN/graphene heterostructures via a simple chemical deposition of h-BN layers on epitaxial graphene/SiC(0001). Van der Waals forces, which are responsible for the cohesion of the multilayer system, give rise to an abrupt interface without interdiffusion between graphene and h-BN, as shown by X-ray Photoemission Spectroscopy (XPS) and direct observation using scanning and High-Resolution Transmission Electron Microscopy (STEM/HRTEM). The electronic properties of graphene, such as the Dirac cone, remain intact and no significant charge transfer i.e. doping, is observed. These results are supported by Density Functional Theory (DFT) calculations. We demonstrate that the h-BN capped graphene allows the fabrication of vdW heterostructures without altering the electronic properties of graphene. PMID:26585245

  18. Heterostructured TiO2 Nanorod@Nanobowl Arrays for Efficient Photoelectrochemical Water Splitting.

    PubMed

    Wang, Wenhui; Dong, Jingya; Ye, Xiaozhou; Li, Yang; Ma, Yurong; Qi, Limin

    2016-03-01

    Heterostructured TiO2 nanorod@nanobowl (NR@NB) arrays consisting of rutile TiO2 nanorods grown on the inner surface of arrayed anatase TiO2 nanobowls are designed and fabricated as a new type of photoanodes for photoelectrochemical (PEC) water splitting. The unique heterostructures with a hierarchical architecture are readily fabricated by interfacial nanosphere lithography followed by hydrothermal growth. Owing to the two-dimensionally arrayed structure of anatase nanobowls and the nearly radial alignment of rutile nanorods, the TiO2 NR@NB arrays provide multiple scattering centers and hence exhibit an enhanced light harvesting ability. Meanwhile, the large surface area of the NR@NB arrays enhances the contact with the electrolyte while the nanorods offer direct pathways for fast electron transfer. Moreover, the rutile/anatase phase junction in the NR@NB heterostructure improves charge separation because of the facilitated electron transfer. Accordingly, the PEC measurements of the TiO2 NR@NB arrays on the fluoride-doped tin oxide (FTO) substrate show significantly enhanced photocatalytic properties for water splitting. Under AM1.5G solar light irradiation, the unmodified TiO2 NR@NB array photoelectrode yields a photocurrent density of 1.24 mA cm(-2) at 1.23 V with respect to the reversible hydrogen electrode, which is almost two times higher than that of the TiO2 nanorods grown directly on the FTO substrate. PMID:26779803

  19. Heterostructured TiO2 Nanorod@Nanobowl Arrays for Efficient Photoelectrochemical Water Splitting.

    PubMed

    Wang, Wenhui; Dong, Jingya; Ye, Xiaozhou; Li, Yang; Ma, Yurong; Qi, Limin

    2016-03-01

    Heterostructured TiO2 nanorod@nanobowl (NR@NB) arrays consisting of rutile TiO2 nanorods grown on the inner surface of arrayed anatase TiO2 nanobowls are designed and fabricated as a new type of photoanodes for photoelectrochemical (PEC) water splitting. The unique heterostructures with a hierarchical architecture are readily fabricated by interfacial nanosphere lithography followed by hydrothermal growth. Owing to the two-dimensionally arrayed structure of anatase nanobowls and the nearly radial alignment of rutile nanorods, the TiO2 NR@NB arrays provide multiple scattering centers and hence exhibit an enhanced light harvesting ability. Meanwhile, the large surface area of the NR@NB arrays enhances the contact with the electrolyte while the nanorods offer direct pathways for fast electron transfer. Moreover, the rutile/anatase phase junction in the NR@NB heterostructure improves charge separation because of the facilitated electron transfer. Accordingly, the PEC measurements of the TiO2 NR@NB arrays on the fluoride-doped tin oxide (FTO) substrate show significantly enhanced photocatalytic properties for water splitting. Under AM1.5G solar light irradiation, the unmodified TiO2 NR@NB array photoelectrode yields a photocurrent density of 1.24 mA cm(-2) at 1.23 V with respect to the reversible hydrogen electrode, which is almost two times higher than that of the TiO2 nanorods grown directly on the FTO substrate.

  20. Influence of dislocations on indium diffusion in semi-polar InGaN/GaN heterostructures

    SciTech Connect

    Yin, Yao; Sun, Huabin; Chen, Peng; Sang, Liwen; Dierre, Benjamin; Sumiya, Masatomo; Sekiguchi, Takashi; Zheng, Youdou; Shi, Yi

    2015-05-15

    The spatial distribution of indium composition in InGaN/GaN heterostructure is a critical topic for modulating the wavelength of light emitting diodes. In this letter, semi-polar InGaN/GaN heterostructure stripes were fabricated on patterned GaN/Sapphire substrates by epitaxial lateral overgrowth (ELO), and the spatial distribution of indium composition in the InGaN layer was characterized by using cathodoluminescence. It is found that the indium composition is mainly controlled by the diffusion behaviors of metal atoms (In and Ga) on the surface. The diffusivity of metal atoms decreases sharply as migrating to the region with a high density of dislocations and other defects, which influences the distribution of indium composition evidently. Our work is beneficial for the understanding of ELO process and the further development of InGaN/GaN heterostructure based devices.

  1. Atomic thin titania nanosheet-coupled reduced graphene oxide 2D heterostructures for enhanced photocatalytic activity and fast lithium storage

    NASA Astrophysics Data System (ADS)

    Li, Dong Jun; Huang, Zhegang; Hwang, Tae Hoon; Narayan, Rekha; Choi, Jang Wook; Kim, Sang Ouk

    2016-03-01

    Realizing practical high performance materials and devices using the properties of 2D materials is of key research interest in the materials science field. In particular, building well-defined heterostructures using more than two different 2D components in a rational way is highly desirable. In this paper, a 2D heterostructure consisting of atomic thin titania nanosheets densely grown on reduced graphene oxide surface is successfully prepared through incorporating polymer functionalized graphene oxide into the novel TiO2 nanosheets synthesis scheme. As a result of the synergistic combination of a highly accessible surface area and abundant interface, which can modulate the physicochemical properties, the resultant heterostructure can be used in high efficiency visible light photocatalysis as well as fast energy storage with a long lifecycle. [Figure not available: see fulltext.

  2. Mesoscopic superconductor-semiconductor heterostructures

    NASA Astrophysics Data System (ADS)

    Klapwijk, T. M.

    1994-03-01

    A summary is given of recent results on carrier transport in mesoscopic conductors with superconducting electrodes. Three-dimensional transport in the diffusive limit is studied with crystalline silicon membranes sandwiched between two niobium electrodes or between one electrode superconducting and one normal. At low temperatures, inelastic scattering is negligible in the intermediate silicon layer. At finite voltages the distribution of electrons over the energies in the silicon is found to be strongly nonthermal, with details depending on the interplay between Andreev scattering and elastic scattering at the interfaces. At small voltages, well below the gap-voltage, transport is phase-coherent. A supercurrent is found if both electrodes are superconducting, provided the membrane is thin enough. If only one electrode is superconducting or for thick membranes an enhanced conductance is observed, which decreases with increasing voltage and magnetic field. Two-dimensional transport and the ballistic regime are being studied by using InAs- and GaAs-based heterostructures. Various interesting theoretical predictions have been made and some novel phenomena have been discovered experimentally. Interesting new phenomena have also been found in mesoscopic normal metals with superconducting electrodes.

  3. Multilayer heterostructures and their manufacture

    DOEpatents

    Hammond, Scott R; Reese, Matthew; Rupert, Benjamin; Miedaner, Alexander; Curtis, Clavin; Olson, Dana; Ginley, David S

    2015-11-04

    A method of synthesizing multilayer heterostructures including an inorganic oxide layer residing on a solid substrate is described. Exemplary embodiments include producing an inorganic oxide layer on a solid substrate by a liquid coating process under relatively mild conditions. The relatively mild conditions include temperatures below 225.degree. C. and pressures above 9.4 mb. In an exemplary embodiment, a solution of diethyl aluminum ethoxide in anhydrous diglyme is applied to a flexible solid substrate by slot-die coating at ambient atmospheric pressure, and the diglyme removed by evaporation. An AlO.sub.x layer is formed by subjecting material remaining on the solid substrate to a relatively mild oven temperature of approximately 150.degree. C. The resulting AlO.sub.x layer exhibits relatively high light transmittance and relatively low vapor transmission rates for water. An exemplary embodiment of a flexible solid substrate is polyethylene napthalate (PEN). The PEN is not substantially adversely affected by exposure to 150.degree. C

  4. Spin Transport in Semiconductor heterostructures

    SciTech Connect

    Domnita Catalina Marinescu

    2011-02-22

    The focus of the research performed under this grant has been the investigation of spin transport in magnetic semiconductor heterostructures. The interest in these systems is motivated both by their intriguing physical properties, as the physical embodiment of a spin-polarized Fermi liquid, as well as by their potential applications as spintronics devices. In our work we have analyzed several different problems that affect the spin dynamics in single and bi-layer spin-polarized two-dimensional (2D) systems. The topics of interests ranged from the fundamental aspects of the electron-electron interactions, to collective spin and charge density excitations and spin transport in the presence of the spin-orbit coupling. The common denominator of these subjects is the impact at the macroscopic scale of the spin-dependent electron-electron interaction, which plays a much more subtle role than in unpolarized electron systems. Our calculations of several measurable parameters, such as the excitation frequencies of magneto-plasma modes, the spin mass, and the spin transresistivity, propose realistic theoretical estimates of the opposite-spin many-body effects, in particular opposite-spin correlations, that can be directly connected with experimental measurements.

  5. Wafer bonded epitaxial templates for silicon heterostructures

    NASA Technical Reports Server (NTRS)

    Atwater, Harry A., Jr. (Inventor); Zahler, James M. (Inventor); Morral, Anna Fontcubera I (Inventor)

    2008-01-01

    A heterostructure device layer is epitaxially grown on a virtual substrate, such as an InP/InGaAs/InP double heterostructure. A device substrate and a handle substrate form the virtual substrate. The device substrate is bonded to the handle substrate and is composed of a material suitable for fabrication of optoelectronic devices. The handle substrate is composed of a material suitable for providing mechanical support. The mechanical strength of the device and handle substrates is improved and the device substrate is thinned to leave a single-crystal film on the virtual substrate such as by exfoliation of a device film from the device substrate. An upper portion of the device film exfoliated from the device substrate is removed to provide a smoother and less defect prone surface for an optoelectronic device. A heterostructure is epitaxially grown on the smoothed surface in which an optoelectronic device may be fabricated.

  6. Wafer bonded epitaxial templates for silicon heterostructures

    DOEpatents

    Atwater, Jr., Harry A.; Zahler, James M.; Morral, Anna Fontcubera I

    2008-03-11

    A heterostructure device layer is epitaxially grown on a virtual substrate, such as an InP/InGaAs/InP double heterostructure. A device substrate and a handle substrate form the virtual substrate. The device substrate is bonded to the handle substrate and is composed of a material suitable for fabrication of optoelectronic devices. The handle substrate is composed of a material suitable for providing mechanical support. The mechanical strength of the device and handle substrates is improved and the device substrate is thinned to leave a single-crystal film on the virtual substrate such as by exfoliation of a device film from the device substrate. An upper portion of the device film exfoliated from the device substrate is removed to provide a smoother and less defect prone surface for an optoelectronic device. A heterostructure is epitaxially grown on the smoothed surface in which an optoelectronic device may be fabricated.

  7. Manipulable MR effect in a δ-doped magnetic nanostructure

    NASA Astrophysics Data System (ADS)

    Kong, Yong-Hong; Jiang, Ya-Qing; Fu, Xi; Li, Ai-Hua

    2016-05-01

    A magnetoresistance (MR) device was proposed by depositing two nanosized ferromagnetic strips on top and bottom of the semiconductor heterostructure. For the sake of manipulating its performance, we introduce a tunable δ-potential into this device with the help of the atomic-layer doping technique such as molecular beam epitaxy (MBE) or metal-organic chemical-vapor deposition (MOCVD). We investigate theoretically the impact of the δ-doping on the magnetoresistance ratio (MMRR) of the MR device. Although the δ-doping is embedded in the device, a considerable MR effect still exists due to different transmissions for the electron across parallel (P) and antiparallel (AP) configurations. Moreover, its MMRR varies sensitively with the magnitude and/or position of the δ-doping. Such an MR device can be controlled by changing the δ-doping, resulting in an adjustable MR device for magnetoelectronics applications.

  8. Dual-donor (Zn(i) and V(O)) mediated ferromagnetism in copper-doped ZnO micron-scale polycrystalline films: a thermally driven defect modulation process.

    PubMed

    Hu, Liang; Huang, Jun; He, Haiping; Zhu, Liping; Liu, Shijiang; Jin, Yizheng; Sun, Luwei; Ye, Zhizhen

    2013-05-01

    The paper reports robust ferromagnetic Cu-doped ZnO micron-scale polycrystalline films via spin-coating using high-quality doped nanocrystals. A reliable magnetic response is observed in the 900 °C vacuum annealed film without any ferromagnetic contribution from other sources. Post-annealing treatment in terms of atmosphere and temperature can control the proportion of oxygen vacancies (V(O)) and zinc interstitials (Zn(i)) defects and further help to precisely regulate defect-related ferromagnetic behavior. Complex charge transfer processes derived from dual-donor (Zn(i) and V(O)) to Cu acceptor are revealed by photoluminescence (PL) and electron paramagnetic resonance (EPR) spectra. Based on the above, specific charge transfer (CT)-type Stoner splitting and indirect double-exchange mechanisms are proposed to understand the ferromagnetic origin. The improvable FM performance and annealing-specific modulation further indicate that a thermal driven process can delicately tailor the magnetic property of the transition metal ion-doped ZnO system.

  9. Complementary junction heterostructure field-effect transistor

    DOEpatents

    Baca, A.G.; Drummond, T.J.; Robertson, P.J.; Zipperian, T.E.

    1995-12-26

    A complimentary pair of compound semiconductor junction heterostructure field-effect transistors and a method for their manufacture are disclosed. The p-channel junction heterostructure field-effect transistor uses a strained layer to split the degeneracy of the valence band for a greatly improved hole mobility and speed. The n-channel device is formed by a compatible process after removing the strained layer. In this manner, both types of transistors may be independently optimized. Ion implantation is used to form the transistor active and isolation regions for both types of complimentary devices. The invention has uses for the development of low power, high-speed digital integrated circuits. 10 figs.

  10. Complementary junction heterostructure field-effect transistor

    DOEpatents

    Baca, Albert G.; Drummond, Timothy J.; Robertson, Perry J.; Zipperian, Thomas E.

    1995-01-01

    A complimentary pair of compound semiconductor junction heterostructure field-effect transistors and a method for their manufacture are disclosed. The p-channel junction heterostructure field-effect transistor uses a strained layer to split the degeneracy of the valence band for a greatly improved hole mobility and speed. The n-channel device is formed by a compatible process after removing the strained layer. In this manner, both types of transistors may be independently optimized. Ion implantation is used to form the transistor active and isolation regions for both types of complimentary devices. The invention has uses for the development of low power, high-speed digital integrated circuits.

  11. Heterostructures produced from nanosheet-based inks.

    PubMed

    Withers, F; Yang, H; Britnell, L; Rooney, A P; Lewis, E; Felten, A; Woods, C R; Sanchez Romaguera, V; Georgiou, T; Eckmann, A; Kim, Y J; Yeates, S G; Haigh, S J; Geim, A K; Novoselov, K S; Casiraghi, C

    2014-07-01

    The new paradigm of heterostructures based on two-dimensional (2D) atomic crystals has already led to the observation of exciting physical phenomena and creation of novel devices. The possibility of combining layers of different 2D materials in one stack allows unprecedented control over the electronic and optical properties of the resulting material. Still, the current method of mechanical transfer of individual 2D crystals, though allowing exceptional control over the quality of such structures and interfaces, is not scalable. Here we show that such heterostructures can be assembled from chemically exfoliated 2D crystals, allowing for low-cost and scalable methods to be used in device fabrication.

  12. High performance vertical tunneling diodes using graphene/hexagonal boron nitride/graphene hetero-structure

    SciTech Connect

    Hwan Lee, Seung; Lee, Jia; Ho Ra, Chang; Liu, Xiaochi; Hwang, Euyheon; Sup Choi, Min; Hee Choi, Jun; Zhong, Jianqiang; Chen, Wei; Jong Yoo, Won

    2014-02-03

    A tunneling rectifier prepared from vertically stacked two-dimensional (2D) materials composed of chemically doped graphene electrodes and hexagonal boron nitride (h-BN) tunneling barrier was demonstrated. The asymmetric chemical doping to graphene with linear dispersion property induces rectifying behavior effectively, by facilitating Fowler-Nordheim tunneling at high forward biases. It results in excellent diode performances of a hetero-structured graphene/h-BN/graphene tunneling diode, with an asymmetric factor exceeding 1000, a nonlinearity of ∼40, and a peak sensitivity of ∼12 V{sup −1}, which are superior to contending metal-insulator-metal diodes, showing great potential for future flexible and transparent electronic devices.

  13. Silicon-germanium nanowire tunnel-FETs with homo- and heterostructure tunnel junctions

    NASA Astrophysics Data System (ADS)

    Richter, S.; Blaeser, S.; Knoll, L.; Trellenkamp, S.; Fox, A.; Schäfer, A.; Hartmann, J. M.; Zhao, Q. T.; Mantl, S.

    2014-08-01

    Experimental results on tunneling field-effect transistors (TFETs) based on strained SiGe on SOI nanowire arrays are presented. A heterostructure SiGe/Si TFET with a vertical tunnel junction consisting of an in situ doped SiGe source and a Si channel with a minimum inverse subthreshold slope of 90 mV/dec is demonstrated. An increase in tunneling area results in higher on-current. The in situ doped heterojunction TFET shows great improvement compared to a homojunction SiGe on SOI nanowire design with implanted junctions. Temperature dependent measurements and device simulations are performed in order to analyze the tunnel transport mechanism in the devices.

  14. Challenges and opportunities of ZnO-related single crystalline heterostructures

    NASA Astrophysics Data System (ADS)

    Kozuka, Y.; Tsukazaki, A.; Kawasaki, M.

    2014-03-01

    Recent technological advancement in ZnO heterostructures has expanded the possibility of device functionalities to various kinds of applications. In order to extract novel device functionalities in the heterostructures, one needs to fabricate high quality films and interfaces with minimal impurities, defects, and disorder. With employing molecular-beam epitaxy and single crystal ZnO substrates, the density of residual impurities and defects can be drastically reduced and the optical and electrical properties have been dramatically improved for the ZnO films and heterostructures with MgxZn1-xO. Here, we overview such recent technological advancement from various aspects of application. Towards optoelectronic devices such as a light emitter and a photodetector in an ultraviolet region, the development of p-type ZnO and the fabrication of excellent Schottky contact, respectively, have been subjected to intensive studies for years. For the former, the fine tuning of the growth conditions to make MgxZn1-xO as intrinsic as possible has opened the possibilities of making p-type MgxZn1-xO through NH3 doping method. For the latter, conducting and transparent polymer films spin-coated on MgxZn1-xO was shown to give almost ideal Schottky junctions. The wavelength-selective detection can be realized with varying the Mg content. From the viewpoint of electronic devices, two-dimensional electrons confined at the MgxZn1-xO/ZnO interfaces are promising candidate for quantum devices because of high electron mobility and strong electron-electron correlation effect. These wonderful features and tremendous opportunities in ZnO-based heterostructures make this system unique and promising in oxide electronics and will lead to new quantum functionalities in optoelectronic devices and electronic applications with lower energy consumption and high performance.

  15. Graphene-based van der Waals heterostructures for emission and detection of terahertz radiation

    NASA Astrophysics Data System (ADS)

    Otsuji, Taiichi; Dubinov, Alexander; Aleshkin, Vladimir Y.; Svintsov, Dmitry; Ryzhii, Maxim; Boubanga Tombet, Stephane; Yadav, Deepika; Satou, Akira; Mitin, Vladimir; Shur, Michael S.; Ryzhii, Victor

    2016-04-01

    This paper reviews recent advances in the research of graphene-based van der Waals heterostructures for emission and detection of terahertz radiation. A gated double-graphene-layer (DGL) nanocapacitor is the core shell under consideration, in which a thin tunnel barrier layer is sandwiched by outer graphene layers at both sides. The DGL can support symmetric optical and anti-symmetric acoustic coupled plasmon modes in the GLs. The latter mode can modulate the band-offset between the GL, giving rise to modulation of the inter-GL-layer resonant tunneling. This can dramatically enhance the THz gain or responsivity via plasmon-assisted inter-GL resonant tunneling.

  16. Direct writing of in-plane-gated nanostructures by focused laser beam-induced doping

    NASA Astrophysics Data System (ADS)

    Baumgartner, P.; Wegscheider, W.; Bichler, M.; Groos, G.; Abstreiter, G.

    1998-07-01

    The fabrication technique of local doping with a focused laser beam is employed to fabricate electronic nanostructures. Zn-doped regions are used to fabricate in-plane electron channels in a high mobility GaAs/AlGaAs heterostructure. The operation of different devices, like quantum point contacts, single electron transistors or Aharonov-Bohm rings, is demonstrated. The coplanar gate geometry improves the performance of the devices in charge sensing applications.

  17. Electron and hole gas in modulation-doped GaAs/Al{sub 1-x}Ga{sub x}As radial heterojunctions

    SciTech Connect

    Bertoni, Andrea; Royo, Miquel; Mahawish, Farah; Goldoni, Guido

    2011-11-15

    We perform self-consistent Schroedinger-Poisson calculations with exchange and correlation corrections to determine the electron and hole gas in a radial heterojunction formed in a GaAs/AlGaAs core-multi-shell nanowire, which is either n- or p-doped. We show that the electron and hole gases can be tuned to different localizations and symmetries inside the core as a function of the doping density/gate potential. Contrary to planar heterojunctions, conduction electrons do not form a uniform 2D electron gas (2DEG) localized at the GaAs/AlGaAs interface, but rather show a transition between an isotropic, cylindrical distribution deep in the GaAs core (low doping) and a set of six tunnel-coupled quasi-1D channels at the edges of the interface (high doping). Holes, on the other hand, are much more localized at the GaAs/AlGaAs interface. At low doping, they present an additional localization pattern with six separated 2DEGs strips. The field generated by a back-gate may easily deform the electron or hole gas, breaking the sixfold symmetry. Single 2DEGs at one interface or multiple quasi-1D channels are shown to form as a function of voltage intensity, polarity, and carrier type.

  18. Optical investigations of InGaN heterostructures and GeSn nanocrystals for photonic and phononic applications: light emitting diodes and phonon cavities

    NASA Astrophysics Data System (ADS)

    Hafiz, Shopan din Ahmad

    InGaN heterostructures are at the core of blue light emitting diodes (LEDs) which are the basic building blocks for energy efficient and environment friendly modern white light generating sources. Through quantum confinement and electronic band structure tuning on the opposite end of the spectrum, Ge1-xSnx alloys have recently attracted significant interest due to its potential role as a silicon compatible infra-red (IR) optical material for photodetectors and LEDs owing to transition to direct bandgap with increasing Sn. This thesis is dedicated to establishing an understanding of the optical processes and carrier dynamics in InGaN heterostructures for achieving more efficient visible light emitters and terahertz generating nanocavities and in colloidal Ge1-xSnx quantum dots (QDs) for developing efficient silicon compatible optoelectronics. To alleviate the electron overflow, which through strong experimental evidence is revealed to be the dominating mechanism responsible for efficiency degradation at high injection in InGaN based blue LEDs, different strategies involving electron injectors and optimized active regions have been developed. Effectiveness of optimum electron injector (EI) layers in reducing electron overflow and increasing quantum efficiency of InGaN based LEDs was demonstrated by photoluminescence (PL) and electroluminescence spectroscopy along with numerical simulations. Increasing the two-layer EI thickness in double heterostructure LEDs substantially reduced the electron overflow and increased external quantum efficiency (EQE) by three fold. By incorporating delta p-doped InGaN barriers in multiple quantum well (MQW) LEDs, 20% enhancement in EQE was achieved due to improved hole injection without degrading the layer quality. Carrier diffusion length, an important physical parameter that directly affects the performance of optoelectronic devices, was measured in epitaxial GaN using PL spectroscopy. The obtained diffusion lengths at room

  19. Strain induced piezoelectric effect in black phosphorus and MoS2 van der Waals heterostructure

    PubMed Central

    Huang, Le; Li, Yan; Wei, Zhongming; Li, Jingbo

    2015-01-01

    The structural, electronic, transport and optical properties of black phosphorus/MoS2 (BP/MoS2) van der Waals (vdw) heterostructure are investigated by using first principles calculations. The band gap of BP/MoS2 bilayer decreases with the applied normal compressive strain and a semiconductor-to-metal transition is observed when the applied strain is more than 0.85 Å. BP/MoS2 bilayer also exhibits modulation of its carrier effective mass and carrier concentration by the applied compressive strain, suggesting that mobility engineering and good piezoelectric effect can be realized in BP/MoS2 heterostructure. Because the type-II band alignment can facilitate the separation of photo-excited electrons and holes, and it can benefit from the great absorption coefficient in ultra-violet region, the BP/MoS2 shows great potential to be a very efficient ultra-violet photodetector. PMID:26553370

  20. Tunable photoelectrochemical performance of Au/BiFeO3 heterostructure.

    PubMed

    Huang, Yen-Lin; Chang, Wei Sea; Van, Chien Nguyen; Liu, Heng-Jui; Tsai, Kai-An; Chen, Jhih-Wei; Kuo, Ho-Hung; Tzeng, Wen-Yen; Chen, Yi-Chun; Wu, Chung-Lin; Luo, Chih-Wei; Hsu, Yung-Jung; Chu, Ying-Hao

    2016-08-25

    Ferroelectric photoelectrodes, other than conventional semiconductors, are alternative photo-absorbers in the process of water splitting. However, the capture of photons and efficient transfer of photo-excited carriers remain as two critical issues in ferroelectric photoelectrodes. In this work, we overcome the aforementioned issues by decorating the ferroelectric BiFeO3 (BFO) surface with Au nanocrystals, and thus improving the photoelectrochemical (PEC) performance of BFO film. We demonstrate that the internal field induced by the spontaneous polarization of BFO can (1) tune the efficiency of the photo-excited carriers' separation and charge transfer characteristics in bare BFO photoelectrodes, and (2) modulate an extra optical absorption within the visible light region, created by the surface plasmon resonance excitation of Au nanocrystals to capture more photons in the Au/BFO heterostructure. This study provides key insights for understanding the tunable features of PEC performance, composed of the heterostructure of noble metals and ferroelectric materials. PMID:27533610

  1. Strain induced piezoelectric effect in black phosphorus and MoS2 van der Waals heterostructure.

    PubMed

    Huang, Le; Li, Yan; Wei, Zhongming; Li, Jingbo

    2015-01-01

    The structural, electronic, transport and optical properties of black phosphorus/MoS2 (BP/MoS2) van der Waals (vdw) heterostructure are investigated by using first principles calculations. The band gap of BP/MoS2 bilayer decreases with the applied normal compressive strain and a semiconductor-to-metal transition is observed when the applied strain is more than 0.85 Å. BP/MoS2 bilayer also exhibits modulation of its carrier effective mass and carrier concentration by the applied compressive strain, suggesting that mobility engineering and good piezoelectric effect can be realized in BP/MoS2 heterostructure. Because the type-II band alignment can facilitate the separation of photo-excited electrons and holes, and it can benefit from the great absorption coefficient in ultra-violet region, the BP/MoS2 shows great potential to be a very efficient ultra-violet photodetector. PMID:26553370

  2. Topological Phases in Perovskite Oxide Heterostructures

    NASA Astrophysics Data System (ADS)

    Kim, Rokyeon; Yu, Jaejun; Jin, Hosub

    Quantum spin Hall (QSH) insulator is a new state of matter characterized by gapless edge states and insulating bulk states. Because the edge states are topologically protected and therefore robust against non-magnetic perturbations, it has a potential to be utilized in spintronics devices. Quauntum vally Hall (QVH) phase, on the other hand, is another class of topological state exhibiting valley-contrasting Berry curvature and spin splitting, which could yield novel transport properties, such as valley Hall effect and valley spin Hall effect. We propose a new kind of perovskite (111) heterostructures which can host both QSH and QVH phases with appropriate choices of composing elements. By carrying out first-principles calculations, we demonstrate that a Dirac cone emerges in a particular choice of heterostructure, and a sizable spin-orbit coupling turns the system into the QSH phase. In addition, the QVH phase with different Berry phases and spin textures in each valley is shown to be realized in the heterostructure with broken inversion symmetry. We porpose that these perovskite heterostructures can provide a feasible platform for spintronics, valleytronics, and topological engineering of the two-dimensional electron system.

  3. Electric field modulation of ultra-high resonance frequency in obliquely deposited [Pb(Mg1/3Nb2/3)O3]0.68-[PbTiO3]0.32(011)/FeCoZr heterostructure for reconfigurable magnetoelectric microwave devices

    NASA Astrophysics Data System (ADS)

    Phuoc, Nguyen N.; Ong, C. K.

    2014-07-01

    The multiferroic heterostructure of FeCoZr/[Pb(Mg1/3Nb2/3)O3]0.68-[PbTiO3]0.32(011) (PMN-PT) prepared by oblique sputtering deposition technique shows a large electrical tunability of ultra-high ferromagnetic resonance frequency from 7.4 GHz to 12.3 GHz. Moreover, we experimentally demonstrate the possibility of realizing electrically reconfigurable magnetoelectric microwave devices with ultra-low power consumption by employing the heterostructure under different resetting electric fields through a reconfiguration process. In particular, the tunability of the FeCoZr/PMN-PT heterostructure from 8.2 GHz to 11.6 GHz can retain in a remanent state after releasing the resetting electric field. This suggests that the tunable microwave devices based on such heterostructures are permanently reconfigurable by simply using a trigger electric field double-pulse which requires much less energy than that of the conventional ones wherein an electric field needs to be constantly applied during operation.

  4. Tunneling through localized barrier states in superconducting heterostructures

    NASA Astrophysics Data System (ADS)

    Shaternik, V. E.; Shapovalov, A. P.; Suvorov, A. V.; Skoryk, N. A.; Belogolovskii, M. A.

    2016-05-01

    Thin film heterostructures composed of superconducting electrodes (molybdenum rhenium alloy) and a nanoscale silicon layer doped with tungsten, have been designed and experimentally studied. The current-voltage characteristics of junctions exhibiting local maxima of the current against the background of abrupt current increases for the first time, were measured in the voltage range of -800 to 800 mV, at temperatures of 4.2-8 K. The positions of these singularities, which are symmetrical with respect to zero voltage, varied from sample to sample within the range of 40-300 mV. With increasing temperature, they became blurred and completely vanished with the disappearance of superconductivity in the electrodes. The nature of the observed singularities is associated with the properties of electron tunneling through the impurity states localized in the semiconducting barrier. The use of a superconducting electrode enhances the interaction of the localized electron with the conduction electrons thanks to the root divergence in the density of electron states of a superconductor.

  5. Spin-torque generation in topological insulator based heterostructures

    NASA Astrophysics Data System (ADS)

    Fischer, Mark H.; Vaezi, Abolhassan; Manchon, Aurelien; Kim, Eun-Ah

    2016-03-01

    Heterostructures utilizing topological insulators exhibit a remarkable spin-torque efficiency. However, the exact origin of the strong torque, in particular whether it stems from the spin-momentum locking of the topological surface states or rather from spin-Hall physics of the topological-insulator bulk, remains unclear. Here, we explore a mechanism of spin-torque generation purely based on the topological surface states. We consider topological-insulator-based bilayers involving ferromagnetic metal (TI/FM) and magnetically doped topological insulators (TI/mdTI), respectively. By ascribing the key theoretical differences between the two setups to location and number of active surface states, we describe both setups within the same framework of spin diffusion of the nonequilibrium spin density of the topological surface states. For the TI/FM bilayer, we find large spin-torque efficiencies of roughly equal magnitude for both in-plane and out-of-plane spin torques. For the TI/mdTI bilayer, we elucidate the dominance of the spin-transfer-like torque. However, we cannot explain the orders of magnitude enhancement reported. Nevertheless, our model gives an intuitive picture of spin-torque generation in topological-insulator-based bilayers and provides theoretical constraints on spin-torque generation due to topological surface states.

  6. Probing charge transfer in a novel class of luminescent perovskite-based heterostructures composed of quantum dots bound to RE-activated CaTiO3 phosphors

    NASA Astrophysics Data System (ADS)

    Lewis, Crystal S.; Liu, Haiqing; Han, Jinkyu; Wang, Lei; Yue, Shiyu; Brennan, Nicholas A.; Wong, Stanislaus S.

    2016-01-01

    We report on the synthesis and structural characterization of novel semiconducting heterostructures composed of cadmium selenide (CdSe) quantum dots (QDs) attached onto the surfaces of novel high-surface area, porous rare-earth-ion doped alkaline earth titanate micron-scale spherical motifs, i.e. both Eu-doped and Pr-doped CaTiO3, composed of constituent, component nanoparticles. These unique metal oxide perovskite building blocks were created by a multi-pronged synthetic strategy involving molten salt and hydrothermal protocols. Subsequently, optical characterization of these heterostructures indicated a clear behavioral dependence of charge transfer in these systems upon a number of parameters such as the nature of the dopant, the reaction temperature, and particle size. Specifically, 2.7 nm diameter ligand-functionalized CdSe QDs were anchored onto sub-micron sized CaTiO3-based spherical assemblies, prepared by molten salt protocols. We found that both the Pr- and Eu-doped CaTiO3 displayed pronounced PL emissions, with maximum intensities observed using optimized lanthanide concentrations of 0.2 mol% and 6 mol%, respectively. Analogous experiments were performed on Eu-doped BaTiO3 and SrTiO3 motifs, but CaTiO3 still performed as the most effective host material amongst the three perovskite systems tested. Moreover, the ligand-capped CdSe QD-doped CaTiO3 heterostructures exhibited effective charge transfer between the two individual constituent nanoscale components, an assertion corroborated by the corresponding quenching of their measured PL signals.We report on the synthesis and structural characterization of novel semiconducting heterostructures composed of cadmium selenide (CdSe) quantum dots (QDs) attached onto the surfaces of novel high-surface area, porous rare-earth-ion doped alkaline earth titanate micron-scale spherical motifs, i.e. both Eu-doped and Pr-doped CaTiO3, composed of constituent, component nanoparticles. These unique metal oxide perovskite

  7. Airplane dopes and doping

    NASA Technical Reports Server (NTRS)

    Smith, W H

    1919-01-01

    Cellulose acetate and cellulose nitrate are the important constituents of airplane dopes in use at the present time, but planes were treated with other materials in the experimental stages of flying. The above compounds belong to the class of colloids and are of value because they produce a shrinking action on the fabric when drying out of solution, rendering it drum tight. Other colloids possessing the same property have been proposed and tried. In the first stages of the development of dope, however, shrinkage was not considered. The fabric was treated merely to render it waterproof. The first airplanes constructed were covered with cotton fabric stretched as tightly as possible over the winds, fuselage, etc., and flying was possible only in fine weather. The necessity of an airplane which would fly under all weather conditions at once became apparent. Then followed experiments with rubberized fabrics, fabrics treated with glue rendered insoluble by formaldehyde or bichromate, fabrics treated with drying and nondrying oils, shellac, casein, etc. It was found that fabrics treated as above lost their tension in damp weather, and the oil from the motor penetrated the proofing material and weakened the fabric. For the most part the film of material lacked durability. Cellulose nitrate lacquers, however were found to be more satisfactory under varying weather conditions, added less weight to the planes, and were easily applied. On the other hand, they were highly inflammable, and oil from the motor penetrated the film of cellulose nitrate, causing the tension of the fabric to be relaxed.

  8. Self-selection effects and modulation of TaOx resistive switching random access memory with bottom electrode of highly doped Si

    NASA Astrophysics Data System (ADS)

    Yu, Muxi; Fang, Yichen; Wang, Zongwei; Pan, Yue; Li, Ming; Cai, Yimao; Huang, Ru

    2016-05-01

    In this paper, we propose a TaOx resistive switching random access memory (RRAM) device with operation-polarity-dependent self-selection effect by introducing highly doped silicon (Si) electrode, which is promising for large-scale integration. It is observed that with highly doped Si as the bottom electrode (BE), the RRAM devices show non-linear (>103) I-V characteristic during negative Forming/Set operation and linear behavior during positive Forming/Set operation. The underling mechanisms for the linear and non-linear behaviors at low resistance states of the proposed device are extensively investigated by varying operation modes, different metal electrodes, and Si doping type. Experimental data and theoretical analysis demonstrate that the operation-polarity-dependent self-selection effect in our devices originates from the Schottky barrier between the TaOx layer and the interfacial SiOx formed by reaction between highly doped Si BE and immigrated oxygen ions in the conductive filament area.

  9. Electronic Properties of MoS2-WS2 Heterostructures Synthesized with Two-Step Lateral Epitaxial Strategy.

    PubMed

    Chen, Kun; Wan, Xi; Wen, Jinxiu; Xie, Weiguang; Kang, Zhiwen; Zeng, Xiaoliang; Chen, Huanjun; Xu, Jian-Bin

    2015-10-27

    Formation of heterojunctions of transition metal dichalcogenides (TMDs) stimulates wide interest in new device physics and technology by tuning optical and electronic properties of TMDs. TMDs heterojunctions are of scientific and technological interest for exploration of next generation flexible electronics. Herein, we report on a two-step epitaxial ambient-pressure CVD technique to construct in-plane MoS2-WS2 heterostructures. The technique has the potential to artificially control the shape and structure of heterostructures or even to be more potentially extendable to growth of TMD superlattice than that of one-step CVD technique. Moreover, the unique MX2 heterostructure with monolayer MoS2 core wrapped by multilayer WS2 is obtained by the technique, which is entirely different from MX2 heterostructures synthesized by existing one-step CVD technique. Transmission electron microscopy, Raman and photoluminescence mapping studies reveal that the obtained heterostructure nanosheets clearly exhibit the modulated structural and optical properties. Electrical transport studies demonstrate that the special MoS2 (monolayer)/WS2 (multilayer) heterojunctions serve as intrinsic lateral p-n diodes and unambiguously show the photovoltaic effect. On the basis of this special heterostructure, depletion-layer width and built-in potential, as well as the built-in electric field distribution, are obtained by KPFM measurement, which are the essential parameters for TMD optoelectronic devices. With further development in future studies, this growth approach is envisaged to bring about a new growth platform for two-dimensional atomic crystals and to create unprecedented architectures therefor.

  10. Probing charge transfer in a novel class of luminescent perovskite-based heterostructures composed of quantum dots bound to RE-activated CaTiO3 phosphors

    DOE PAGES

    Crystal S. Lewis; Wong, Stanislaus S.; Liu, Haiqing; Han, Jinkyu; Wang, Lei; Yue, Shiyu; Brennan, Nicholas A.

    2016-01-04

    We report on the synthesis and structural characterization of novel semiconducting heterostructures composed of cadmium selenide (CdSe) quantum dots (QDs) attached onto the surfaces of novel high-surface area, porous rare-earth-ion doped alkaline earth titanate micron-scale spherical motifs, i.e. both Eu-doped and Pr-doped CaTiO3, composed of constituent, component nanoparticles. These unique metal oxide perovskite building blocks were created by a multi-pronged synthetic strategy involving molten salt and hydrothermal protocols. Subsequently, optical characterization of these heterostructures indicated a clear behavioral dependence of charge transfer in these systems upon a number of parameters such as the nature of the dopant, the reaction temperature,more » and particle size. Specifically, 2.7 nm diameter ligand-functionalized CdSe QDs were anchored onto sub-micron sized CaTiO3-based spherical assemblies, prepared by molten salt protocols. We found that both the Pr- and Eu-doped CaTiO3 displayed pronounced PL emissions, with maximum intensities observed using optimized lanthanide concentrations of 0.2 mol% and 6 mol%, respectively. Analogous experiments were performed on Eu-doped BaTiO3 and SrTiO3 motifs, but CaTiO3 still performed as the most effective host material amongst the three perovskite systems tested. Furthermore, the ligand-capped CdSe QD-doped CaTiO3 heterostructures exhibited effective charge transfer between the two individual constituent nanoscale components, an assertion corroborated by the corresponding quenching of their measured PL signals.« less

  11. Properties of ferroelectric/ferromagnetic thin film heterostructures

    SciTech Connect

    Chen, Daming; Harward, Ian; Linderman, Katie; Economou, Evangelos; Celinski, Zbigniew; Nie, Yan

    2014-05-07

    Ferroelectric/ferromagnetic thin film heterostructures, SrBi{sub 2}Ta{sub 2}O{sub 9}/BaFe{sub 12}O{sub 19} (SBT/BaM), were grown on platinum-coated Si substrates using metal-organic decomposition. X-ray diffraction patterns confirmed that the heterostructures contain only SBT and BaM phases. The microwave properties of these heterostructures were studied using a broadband ferromagnetic resonance (FMR) spectrometer from 35 to 60 GHz, which allowed us to determine gyromagnetic ratio and effective anisotropy field. The FMR linewidth is as low as140 Oe at 58 GHz. In addition, measurements of the effective permittivity of the heterostructures were carried out as a function of bias electric field. All heterostructures exhibit hysteretic behavior of the effective permittivity. These properties indicate that such heterostructures have potential for application in dual electric and magnetic field tunable resonators, filters, and phase shifters.

  12. Heterostructures based on inorganic and organic van der Waals systems

    SciTech Connect

    Lee, Gwan-Hyoung; Lee, Chul-Ho; Zande, Arend M. van der; Han, Minyong; Cui, Xu; Arefe, Ghidewon; Hone, James; Nuckolls, Colin; Heinz, Tony F.; Kim, Philip

    2014-09-01

    The two-dimensional limit of layered materials has recently been realized through the use of van der Waals (vdW) heterostructures composed of weakly interacting layers. In this paper, we describe two different classes of vdW heterostructures: inorganic vdW heterostructures prepared by co-lamination and restacking; and organic-inorganic hetero-epitaxy created by physical vapor deposition of organic molecule crystals on an inorganic vdW substrate. Both types of heterostructures exhibit atomically clean vdW interfaces. Employing such vdW heterostructures, we have demonstrated various novel devices, including graphene/hexagonal boron nitride (hBN) and MoS{sub 2} heterostructures for memory devices; graphene/MoS{sub 2}/WSe{sub 2}/graphene vertical p-n junctions for photovoltaic devices, and organic crystals on hBN with graphene electrodes for high-performance transistors.

  13. Properties of ferroelectric/ferromagnetic thin film heterostructures

    NASA Astrophysics Data System (ADS)

    Chen, Daming; Harward, Ian; Linderman, Katie; Economou, Evangelos; Nie, Yan; Celinski, Zbigniew

    2014-05-01

    Ferroelectric/ferromagnetic thin film heterostructures, SrBi2Ta2O9/BaFe12O19 (SBT/BaM), were grown on platinum-coated Si substrates using metal-organic decomposition. X-ray diffraction patterns confirmed that the heterostructures contain only SBT and BaM phases. The microwave properties of these heterostructures were studied using a broadband ferromagnetic resonance (FMR) spectrometer from 35 to 60 GHz, which allowed us to determine gyromagnetic ratio and effective anisotropy field. The FMR linewidth is as low as140 Oe at 58 GHz. In addition, measurements of the effective permittivity of the heterostructures were carried out as a function of bias electric field. All heterostructures exhibit hysteretic behavior of the effective permittivity. These properties indicate that such heterostructures have potential for application in dual electric and magnetic field tunable resonators, filters, and phase shifters.

  14. In-situ Electric Field-Induced Modulation of Photoluminescence in Pr-doped Ba0.85Ca0.15Ti0.90Zr0.10O3 Lead-Free Ceramics

    NASA Astrophysics Data System (ADS)

    Sun, Hai Ling; Wu, Xiao; Chung, Tat Hang; Kwok, K. W.

    2016-06-01

    Luminescent materials with dynamic photoluminescence activity have aroused special interest because of their potential widespread applications. One proposed approach of directly and reversibly modulating the photoluminescence emissions is by means of introducing an external electric field in an in-situ and real-time way, which has only been focused on thin films. In this work, we demonstrate that real-time electric field-induced photoluminescence modulation can be realized in a bulk Ba0.85Ca0.15Ti0.90Zr0.10O3 ferroelectric ceramic doped with 0.2 mol% Pr3+, owing to its remarkable polarization reversal and phase evolution near the morphotropic phase boundary. Along with in-situ X-ray diffraction analysis, our results reveal that an applied electric field induces not only typical polarization switching and minor crystal deformation, but also tetragonal-to-rhombohedral phase transformation of the ceramic. The electric field-induced phase transformation is irreversible and engenders dominant effect on photoluminescence emissions as a result of an increase in structural symmetry. After it is completed in a few cycles of electric field, the photoluminescence emissions become governed mainly by the polarization switching, and thus vary reversibly with the modulating electric field. Our results open a promising avenue towards the realization of bulk ceramic-based tunable photoluminescence activity with high repeatability, flexible controllability, and environmental-friendly chemical process.

  15. In-situ Electric Field-Induced Modulation of Photoluminescence in Pr-doped Ba0.85Ca0.15Ti0.90Zr0.10O3 Lead-Free Ceramics

    PubMed Central

    Sun, Hai Ling; Wu, Xiao; Chung, Tat Hang; Kwok, K. W.

    2016-01-01

    Luminescent materials with dynamic photoluminescence activity have aroused special interest because of their potential widespread applications. One proposed approach of directly and reversibly modulating the photoluminescence emissions is by means of introducing an external electric field in an in-situ and real-time way, which has only been focused on thin films. In this work, we demonstrate that real-time electric field-induced photoluminescence modulation can be realized in a bulk Ba0.85Ca0.15Ti0.90Zr0.10O3 ferroelectric ceramic doped with 0.2 mol% Pr3+, owing to its remarkable polarization reversal and phase evolution near the morphotropic phase boundary. Along with in-situ X-ray diffraction analysis, our results reveal that an applied electric field induces not only typical polarization switching and minor crystal deformation, but also tetragonal-to-rhombohedral phase transformation of the ceramic. The electric field-induced phase transformation is irreversible and engenders dominant effect on photoluminescence emissions as a result of an increase in structural symmetry. After it is completed in a few cycles of electric field, the photoluminescence emissions become governed mainly by the polarization switching, and thus vary reversibly with the modulating electric field. Our results open a promising avenue towards the realization of bulk ceramic-based tunable photoluminescence activity with high repeatability, flexible controllability, and environmental-friendly chemical process. PMID:27339815

  16. Liquid chromatography-high resolution/ high accuracy (tandem) mass spectrometry-based identification of in vivo generated metabolites of the selective androgen receptor modulator ACP-105 for doping control purposes.

    PubMed

    Thevis, Mario; Thomas, Andreas; Piper, Thomas; Krug, Oliver; Delahaut, Philippe; Schänzer, Wilhelm

    2014-01-01

    Selective androgen receptor modulators (SARMs) represent an emerging class of therapeutics which have been prohibited in sport as anabolic agents according to the regulations of the World Anti-Doping Agency (WADA) since 2008. Within the past three years, numerous adverse analytical findings with SARMs in routine doping control samples have been reported despite missing clinical approval of these substances. Hence, preventive doping research concerning the metabolism and elimination of new therapeutic entities of the class of SARMs are vital for efficient and timely sports drug testing programs as banned compounds are most efficiently screened when viable targets (for example, characteristic metabolites) are identified. In the present study, the metabolism of ACP-105, a novel SARM drug candidate, was studied in vivo in rats. Following oral administration, urine samples were collected over a period of seven days and analyzed for metabolic products by Liquid chromatography-high resolution/high accuracy (tandem) mass spectrometry. Samples were subjected to enzymatic hydrolysis prior to liquid-liquid extraction and a total of seven major phase-I metabolites were detected, three of which were attributed to monohydroxylated and four to bishydroxylated ACP-105. The hydroxylation sites were assigned by means of diagnostic product ions and respective dissociation pathways of the analytes following positive or negative ionization and collisional activation as well as selective chemical derivatization. The identified metabolites were used as target compounds to investigate their traceability in a rat elimination urine samples study and monohydroxylated and bishydroxylated species were detectable for up to four and six days post-administration, respectively.

  17. Superconducting cuprate heterostructures for hot electron bolometers

    SciTech Connect

    Wen, B.; Yakobov, R.; Vitkalov, S. A.; Sergeev, A.

    2013-11-25

    Transport properties of the resistive state of quasi-two dimensional superconducting heterostructures containing ultrathin La{sub 2−x}Sr{sub x}CuO{sub 4} layers synthesized using molecular beam epitaxy are studied. The electron transport exhibits strong deviation from Ohm's law, δV∼γI{sup 3}, with a coefficient γ(T) that correlates with the temperature variation of the resistivity dρ/dT. Close to the normal state, analysis of the nonlinear behavior in terms of electron heating yields an electron-phonon thermal conductance per unit area g{sub e−ph}≈1 W/K cm{sup 2} at T = 20 K, one-two orders of magnitude smaller than in typical superconductors. This makes superconducting LaSrCuO heterostructures to be attractive candidate for the next generation of hot electron bolometers with greatly improved sensitivity.

  18. Low energy consumption spintronics using multiferroic heterostructures.

    PubMed

    Trassin, Morgan

    2016-01-27

    We review the recent progress in the field of multiferroic magnetoelectric heterostructures. The lack of single phase multiferroic candidates exhibiting simultaneously strong and coupled magnetic and ferroelectric orders led to an increased effort into the development of artificial multiferroic heterostructures in which these orders are combined by assembling different materials. The magnetoelectric coupling emerging from the created interface between the ferroelectric and ferromagnetic layers can result in electrically tunable magnetic transition temperature, magnetic anisotropy or magnetization reversal. The full potential of low energy consumption magnetic based devices for spintronics lies in our understanding of the magnetoelectric coupling at the scale of the ferroic domains. Although the thin film synthesis progresses resulted into the complete control of ferroic domain ordering using epitaxial strain, the local observation of magnetoelectric coupling remains challenging. The ability to imprint ferroelectric domains into ferromagnets and to manipulate those solely using electric fields suggests new technological advances for spintronics such as magnetoelectric memories or memristors.

  19. Anomalous spontaneous reversal in magnetic heterostructures.

    PubMed

    Li, Zhi-Pan; Eisenmenger, Johannes; Miller, Casey W; Schuller, Ivan K

    2006-04-01

    We observe a thermally induced spontaneous magnetization reversal of epitaxial ferromagnet/antiferromagnet heterostructures under a constant applied magnetic field. Unlike any other magnetic system, the magnetization spontaneously reverses, aligning antiparallel to an applied field with decreasing temperature. We show that this unusual phenomenon is caused by the interfacial antiferromagnetic coupling overcoming the Zeeman energy of the ferromagnet. A significant temperature hysteresis exists, whose height and width can be tuned by the field applied during thermal cycling. The hysteresis originates from the intrinsic magnetic anisotropy in the system. The observation of this phenomenon leads to open questions in the general understanding of magnetic heterostructures. Moreover, this shows that in general heterogeneous nanostructured materials may exhibit unexpected phenomena absent in the bulk.

  20. Resonant Tunneling in Double Bilayer Graphene Heterostructures

    NASA Astrophysics Data System (ADS)

    Fallahazad, Babak; Lee, Kayoung; Kang, Sangwoo; Xue, Jiamin; Larentis, Stefano; Corbet, Christopher; Kim, Kyounghwan; Movva, Hema; Taniguchi, Takashi; Watanabe, Kenji; Register, Leonard; Banerjee, Sanjay; Tutuc, Emanuel

    2015-03-01

    We present the realization and characterization of independently contacted and rotationally aligned double bilayer graphene heterostructures, that show gate-tunable tunneling resonances and negative differential resistance in their interlayer current-voltage characteristics. Our devices are fabricated by successively stacking mechanically exfoliated bilayer graphene and hexagonal boron nitride dielectric using a layer-by-layer transfer technique. The bilayers are rotationally aligned during the device fabrication by selecting flakes with straight edges, and using them as a reference for alignment. We determine the heterostructure energy band alignment at the tunneling resonance using the individual layer carrier densities, and including the chemical potential dependence on the carrier density. Our analysis show that the tunneling resonances occur when the charge neutrality points of the two bilayer graphene are energetically aligned, which suggests the resonances stem from the momentum conserving tunneling. This work has been supported by NRI-SWAN, ONR, and Intel.

  1. Highly efficient gate-tunable photocurrent generation in vertical heterostructures of layered materials.

    PubMed

    Yu, Woo Jong; Liu, Yuan; Zhou, Hailong; Yin, Anxiang; Li, Zheng; Huang, Yu; Duan, Xiangfeng

    2013-12-01

    Layered materials of graphene and MoS₂, for example, have recently emerged as an exciting material system for future electronics and optoelectronics. Vertical integration of layered materials can enable the design of novel electronic and photonic devices. Here, we report highly efficient photocurrent generation from vertical heterostructures of layered materials. We show that vertically stacked graphene-MoS₂-graphene and graphene-MoS₂-metal junctions can be created with a broad junction area for efficient photon harvesting. The weak electrostatic screening effect of graphene allows the integration of single or dual gates under and/or above the vertical heterostructure to tune the band slope and photocurrent generation. We demonstrate that the amplitude and polarity of the photocurrent in the gated vertical heterostructures can be readily modulated by the electric field of an external gate to achieve a maximum external quantum efficiency of 55% and internal quantum efficiency up to 85%. Our study establishes a method to control photocarrier generation, separation and transport processes using an external electric field.

  2. Single-crystal organic charge-transfer interfaces probed using Schottky-gated heterostructures.

    PubMed

    Gutiérrez Lezama, Ignacio; Nakano, Masaki; Minder, Nikolas A; Chen, Zhihua; Di Girolamo, Flavia V; Facchetti, Antonio; Morpurgo, Alberto F

    2012-09-01

    Organic semiconductors based on small conjugated molecules generally behave as insulators when undoped, but the heterointerfaces of two such materials can show electrical conductivity as large as in a metal. Although charge transfer is commonly invoked to explain the phenomenon, the details of the process and the nature of the interfacial charge carriers remain largely unexplored. Here we use Schottky-gated heterostructures to probe the conducting layer at the interface between rubrene and PDIF-CN(2) single crystals. Gate-modulated conductivity measurements demonstrate that interfacial transport is due to electrons, whose mobility exhibits band-like behaviour from room temperature to ~150 K, and remains as high as ~1 cm(2) V(-1) s(-1) at 30 K for the best devices. The electron density decreases linearly with decreasing temperature, an observation that can be explained quantitatively on the basis of the heterostructure band diagram. These results elucidate the electronic structure of rubrene/PDIF-CN(2) interfaces and show the potential of Schottky-gated organic heterostructures for the investigation of transport in molecular semiconductors. PMID:22820687

  3. Graphene diamond-like carbon films heterostructure

    SciTech Connect

    Zhao, Fang; Afandi, Abdulkareem; Jackman, Richard B.

    2015-03-09

    A limitation to the potential use of graphene as an electronic material is the lack of control over the 2D materials properties once it is deposited on a supporting substrate. Here, the use of Diamond-like Carbon (DLC) interlayers between the substrate and the graphene is shown to offer the prospect of overcoming this problem. The DLC films used here, more properly known as a-C:H with ∼25% hydrogen content, have been terminated with N or F moieties prior to graphene deposition. It is found that nitrogen terminations lead to an optical band gap shrinkage in the DLC, whilst fluorine groups reduce the DLC's surface energy. CVD monolayer graphene subsequently transferred to DLC, N terminated DLC, and F terminated DLC has then been studied with AFM, Raman and XPS analysis, and correlated with Hall effect measurements that give an insight into the heterostructures electrical properties. The results show that different terminations strongly affect the electronic properties of the graphene heterostructures. G-F-DLC samples were p-type and displayed considerably higher mobility than the other heterostructures, whilst G-N-DLC samples supported higher carrier densities, being almost metallic in character. Since it would be possible to locally pattern the distribution of these differing surface terminations, this work offers the prospect for 2D lateral control of the electronic properties of graphene layers for device applications.

  4. Voltage control of magnetism in multiferroic heterostructures

    PubMed Central

    Liu, Ming; Sun, Nian X.

    2014-01-01

    Electrical tuning of magnetism is of great fundamental and technical importance for fast, compact and ultra-low power electronic devices. Multiferroics, simultaneously exhibiting ferroelectricity and ferromagnetism, have attracted much interest owing to the capability of controlling magnetism by an electric field through magnetoelectric (ME) coupling. In particular, strong strain-mediated ME interaction observed in layered multiferroic heterostructures makes it practically possible for realizing electrically reconfigurable microwave devices, ultra-low power electronics and magnetoelectric random access memories (MERAMs). In this review, we demonstrate this remarkable E-field manipulation of magnetism in various multiferroic composite systems, aiming at the creation of novel compact, lightweight, energy-efficient and tunable electronic and microwave devices. First of all, tunable microwave devices are demonstrated based on ferrite/ferroelectric and magnetic-metal/ferroelectric composites, showing giant ferromagnetic resonance (FMR) tunability with narrow FMR linewidth. Then, E-field manipulation of magnetoresistance in multiferroic anisotropic magnetoresistance and giant magnetoresistance devices for achieving low-power electronic devices is discussed. Finally, E-field control of exchange-bias and deterministic magnetization switching is demonstrated in exchange-coupled antiferromagnetic/ferromagnetic/ferroelectric multiferroic hetero-structures at room temperature, indicating an important step towards MERAMs. In addition, recent progress in electrically non-volatile tuning of magnetic states is also presented. These tunable multiferroic heterostructures and devices provide great opportunities for next-generation reconfigurable radio frequency/microwave communication systems and radars, spintronics, sensors and memories. PMID:24421373

  5. Correlated Heterostructures for Efficient Solar Cells

    NASA Astrophysics Data System (ADS)

    Assmann, Elias; Aichhorn, Markus; Sangiovanni, Giorgio; Okamoto, Satoshi; Blaha, Peter; Bhandary, Sumanta; Held, Karsten

    Polar |non-polar oxide heterostructures such as LaAlO3 | SrTiO3 have become well-known for the many intriguing phenomena occurring at the interface, especially the internal potential gradient and the resulting 2d electron gas. We propose to make use of these unique systems as absorbing materials for high-efficiency solar cells. In particular, LaVO3 | SrTiO3 (i) has a direct band gap ~1.1 eV, nearly optimal for a solar cell; (ii) the internal potential gradient serves to efficiently separate the photo-generated electron-hole pairs and reduce recombination losses; (iii) the conducting interface offers a natural contact for charge-carrier extraction. Furthermore, (iv) oxide heterostructures afford the flexibility to combine layers with different gaps, e.g. LaVO3 with LaFeO3, in order to achieve even higher efficiencies with band-gap graded solar cells. We use density-functional theory and dynamical mean-field theory to study this strongly correlated heterostructure.

  6. Functional ferroic heterostructures with tunable integral symmetry.

    PubMed

    Becher, C; Trassin, M; Lilienblum, M; Nelson, C T; Suresha, S J; Yi, D; Yu, P; Ramesh, R; Fiebig, M; Meier, D

    2014-07-02

    The relation between symmetry and functionality was pinpointed by Pierre Curie who stated that it is the symmetry breaking that creates physical properties. This fundamental principle is nowadays used for engineering heterostructures whose integral symmetry leads to exotic phenomena such as one-way transparency. For switching devices, however, such symmetry-related functionalities cannot be used because the symmetry in conventional heterostructures is immutable once the material has been synthesized. Here we demonstrate a concept for post-growth symmetry control in PbZr0.2Ti0.8O3 and BiFeO3-based heterostructures. A conducting oxide is sandwiched between two ferroelectric layers, and inversion symmetry is reversibly switched on or off by layer-selective electric-field poling. The generalization of our approach to other materials and symmetries is discussed. We thus establish ferroic trilayer structures as device components with reversibly tunable symmetry and demonstrate their use as light emitters that can be activated and deactivated by applying moderate electric voltages.

  7. Graphene diamond-like carbon films heterostructure

    NASA Astrophysics Data System (ADS)

    Zhao, Fang; Afandi, Abdulkareem; Jackman, Richard B.

    2015-03-01

    A limitation to the potential use of graphene as an electronic material is the lack of control over the 2D materials properties once it is deposited on a supporting substrate. Here, the use of Diamond-like Carbon (DLC) interlayers between the substrate and the graphene is shown to offer the prospect of overcoming this problem. The DLC films used here, more properly known as a-C:H with ˜25% hydrogen content, have been terminated with N or F moieties prior to graphene deposition. It is found that nitrogen terminations lead to an optical band gap shrinkage in the DLC, whilst fluorine groups reduce the DLC's surface energy. CVD monolayer graphene subsequently transferred to DLC, N terminated DLC, and F terminated DLC has then been studied with AFM, Raman and XPS analysis, and correlated with Hall effect measurements that give an insight into the heterostructures electrical properties. The results show that different terminations strongly affect the electronic properties of the graphene heterostructures. G-F-DLC samples were p-type and displayed considerably higher mobility than the other heterostructures, whilst G-N-DLC samples supported higher carrier densities, being almost metallic in character. Since it would be possible to locally pattern the distribution of these differing surface terminations, this work offers the prospect for 2D lateral control of the electronic properties of graphene layers for device applications.

  8. Voltage control of magnetism in multiferroic heterostructures.

    PubMed

    Liu, Ming; Sun, Nian X

    2014-02-28

    Electrical tuning of magnetism is of great fundamental and technical importance for fast, compact and ultra-low power electronic devices. Multiferroics, simultaneously exhibiting ferroelectricity and ferromagnetism, have attracted much interest owing to the capability of controlling magnetism by an electric field through magnetoelectric (ME) coupling. In particular, strong strain-mediated ME interaction observed in layered multiferroic heterostructures makes it practically possible for realizing electrically reconfigurable microwave devices, ultra-low power electronics and magnetoelectric random access memories (MERAMs). In this review, we demonstrate this remarkable E-field manipulation of magnetism in various multiferroic composite systems, aiming at the creation of novel compact, lightweight, energy-efficient and tunable electronic and microwave devices. First of all, tunable microwave devices are demonstrated based on ferrite/ferroelectric and magnetic-metal/ferroelectric composites, showing giant ferromagnetic resonance (FMR) tunability with narrow FMR linewidth. Then, E-field manipulation of magnetoresistance in multiferroic anisotropic magnetoresistance and giant magnetoresistance devices for achieving low-power electronic devices is discussed. Finally, E-field control of exchange-bias and deterministic magnetization switching is demonstrated in exchange-coupled antiferromagnetic/ferromagnetic/ferroelectric multiferroic hetero-structures at room temperature, indicating an important step towards MERAMs. In addition, recent progress in electrically non-volatile tuning of magnetic states is also presented. These tunable multiferroic heterostructures and devices provide great opportunities for next-generation reconfigurable radio frequency/microwave communication systems and radars, spintronics, sensors and memories.

  9. Blue Phosphorene/MS2 (M = Nb, Ta) Heterostructures As Promising Flexible Anodes for Lithium-Ion Batteries.

    PubMed

    Peng, Qiong; Wang, Zhenyu; Sa, Baisheng; Wu, Bo; Sun, Zhimei

    2016-06-01

    The idea of forming van der Waals (vdW) heterostructures by integrating various two-dimensional materials breaks the limitation of the restricted properties of single material systems. In this work, the electronic structure modulation, stability, entire stress response and the Li adsorption properties of heterostructures by combining blue phosphorene (BlueP) and MS2 (M = Nb, Ta) together were systematically investigated using first-principles calculations based on vdW corrected density functional theory. We revealed that BlueP/MS2 vdW heterostructures possess good structural stability with negative formation energy, enhanced electrical conductivity, improved mechanical flexibility (ultimate strain >17%) and high-capacity (528.257 mAhg(-1) for BlueP/NbS2). The results suggest that BlueP/NbS2 and BlueP/TaS2 heterostructures are ideal candidates used as promising flexible electrode for high recycling rate and portable lithium-ion batteries, which satisfy the requirement of next-generation flexible energy storage and conversion devices. PMID:27165567

  10. Polarization-controlled tunable rectifying behaviors in highly oriented (K,Na)NbO3/LaNiO3 heterostructures on silicon

    NASA Astrophysics Data System (ADS)

    Xu, Hanni; Liu, Yi; Xu, Bo; Xia, Yidong; Wang, Genshui; Yin, Jiang; Liu, Zhiguo

    2016-09-01

    We report polarization-controlled tunable rectifying behaviors in (K,Na)NbO3 (KNN)/LaNiO3 (LNO) heterostructures on silicon. The heterostructure shows a forward diode behavior at both the high resistance state and the low resistance state. The amplitude dependent rectifying features are attributed to the ferroelectric modulation effect on both the width of the depletion region and the height of the potential barrier at the KNN/LNO interface. By controlling the domain configurations using the writing voltage, the rectifying behaviors can be regulated and immediate states can be tuned. Our work shows the potential applications of KNN films in ferroelectric memristors.

  11. General Considerations of the Electrostatic Boundary Conditions in Oxide Heterostructures

    SciTech Connect

    Higuchi, Takuya

    2011-08-19

    different families creates a host of electrostatic issues. They can be somewhat avoided if, as in many semiconductor heterostructures, only one family is used, with small perturbations (such as n-type or p-type doping) around them. However, for most transition metal oxides, this is greatly restrictive. For example, LaMnO{sub 3} and SrMnO{sub 3} are both insulators in part due to strong electron correlations, and only in their solid solution does 'colossal magnetoresistance' emerge in bulk. Similarly, the metallic superlattice shown in Fig. 1(c) can be considered a nanoscale deconstruction of (La,Sr)TiO{sub 3} to the insulating parent compounds. Therefore the aspiration to arbitrarily mix and match perovskite components requires a basic understanding of, and ultimately control over, these issues. In this context, here we present basic electrostatic features that arise in oxide heterostructures which vary the ionic charge stacking sequence. In close relation to the analysis of the stability of polar surfaces and semiconductor heterointerfaces, the variation of the dipole moment across a heterointerface plays a key role in determining its stability. Different self-consistent assignments of the unit cell are presented, allowing the polar discontinuity picture to be recast in terms of an equivalent local charge neutrality picture. The latter is helpful in providing a common framework with which to discuss electronic reconstructions, local-bonding considerations, crystalline defects, and lattice polarization on an equal footing, all of which are the subject of extensive current investigation.

  12. Theory of resonant tunneling in bilayer-graphene/hexagonal-boron-nitride heterostructures

    SciTech Connect

    Barrera, Sergio C. de la; Feenstra, Randall M.

    2015-03-02

    A theory is developed for calculating vertical tunneling current between two sheets of bilayer graphene separated by a thin, insulating layer of hexagonal boron nitride, neglecting many-body effects. Results are presented using physical parameters that enable comparison of the theory with recently reported experimental results. Observed resonant tunneling and negative differential resistance in the current–voltage characteristics are explained in terms of the electrostatically-induced band gap, gate voltage modulation, density of states near the band edge, and resonances with the upper sub-band. These observations are compared to ones from similar heterostructures formed with monolayer graphene.

  13. Self-acousto-optic modulation and orthogonality violation in the transverse modes of a broad-area Nd-doped yttrium-aluminum-garnet single-shot laser

    SciTech Connect

    Soler Rus, M. Odin; Cabrera-Granado, E.; Guerra Perez, J. M.

    2011-05-15

    Self-acousto-optic Raman-Nath modulation and nonorthogonal transversal modes are found in a broad-area Nd:YAG single-shot laser. The device is free from the thermal-induced effects previously related to orthogonality violation and the acousto-optic modulation comes from a shock wave produced by the discharge of the flash lamps that optically pump the laser. The experimental findings are reproduced by a general model of a class B laser.

  14. Vertical field effect tunneling transistor based on graphene-ultrathin Si nanomembrane heterostructures

    NASA Astrophysics Data System (ADS)

    Das, Tanmoy; Jang, Houk; Bok Lee, Jae; Chu, Hyunwoo; Kim, Seong Dae; Ahn, Jong-Hyun

    2015-12-01

    Graphene-based heterostructured vertical transistors have attracted a great deal of research interest. Herein we propose a Si-based technology platform for creating graphene/ultrathin semiconductor/metal (GSM) junctions, which can be applied to large-scale and low-power electronics compatible with a variety of substrates. We fabricated graphene/Si nanomembrane (NM)/metal vertical heterostructures by using a dry transfer technique to transfer Si NMs onto chemical vapor deposition-grown graphene layers. The resulting van der Waals interfaces between graphene and p-Si NMs exhibited nearly ideal Schottky barrier behavior. Due to the low density of states of graphene, the graphene/Si NM Schottky barrier height can be modulated by modulating the band profile in the channel region, yielding well-defined current modulation. We obtained a maximum current on/off ratio (Ion/Ioff) of up to ˜103, with a current density of 102 A cm-2. We also observed significant dependence of Schottky barrier height Δφb on the thickness of the Si NMs. We confirmed that the transport in these devices is dominated by the effects of the graphene/Si NM Schottky barrier.

  15. Effect of heterostructure design on carrier injection and emission characteristics of 295 nm light emitting diodes

    SciTech Connect

    Mehnke, Frank Kuhn, Christian; Stellmach, Joachim; Rothe, Mark-Antonius; Reich, Christoph; Ledentsov, Nikolay; Pristovsek, Markus; Wernicke, Tim; Kolbe, Tim; Lobo-Ploch, Neysha; Rass, Jens; Kneissl, Michael

    2015-05-21

    The effects of the heterostructure design on the injection efficiency and external quantum efficiency of ultraviolet (UV)-B light emitting diodes (LEDs) have been investigated. It was found that the functionality of the Al{sub x}Ga{sub 1−x}N:Mg electron blocking layer is strongly influenced by its aluminum mole fraction x and its magnesium doping profile. By comparing LED electroluminescence, quantum well photoluminescence, and simulations of LED heterostructure, we were able to differentiate the contributions of injection efficiency and internal quantum efficiency to the external quantum efficiency of UV LEDs. For the optimized heterostructure using an Al{sub 0.7}Ga{sub 0.3}N:Mg electron blocking layer with a Mg to group III ratio of 4% in the gas phase the electron leakage currents are suppressed without blocking the injection of holes into the multiple quantum well active region. Flip chip mounted LED chips have been processed achieving a maximum output power of 3.5 mW at 290 mA and a peak external quantum efficiency of 0.54% at 30 mA.

  16. Fluorinated graphene and hexagonal boron nitride as ALD seed layers for graphene-based van der Waals heterostructures

    NASA Astrophysics Data System (ADS)

    Guo, Hongwei; Liu, Yunlong; Xu, Yang; Meng, Nan; Wang, Hongtao; Hasan, Tawfique; Wang, Xinran; Luo, Jikui; Yu, Bin

    2014-09-01

    Ultrathin dielectric materials prepared by atomic-layer-deposition (ALD) technology are commonly used in graphene electronics. Using the first-principles density functional theory calculations with van der Waals (vdW) interactions included, we demonstrate that single-side fluorinated graphene (SFG) and hexagonal boron nitride (h-BN) exhibit large physical adsorption energy and strong electrostatic interactions with H2O-based ALD precursors, indicating their potential as the ALD seed layer for dielectric growth on graphene. In graphene-SFG vdW heterostructures, graphene is n-doped after ALD precursor adsorption on the SFG surface caused by vertical intrinsic polarization of SFG. However, graphene-h-BN vdW heterostructures help preserving the intrinsic characteristics of the underlying graphene due to in-plane intrinsic polarization of h-BN. By choosing SFG or BN as the ALD seed layer on the basis of actual device design needs, the graphene vdW heterostructures may find applications in low-dimensional electronics.

  17. Fluorinated graphene and hexagonal boron nitride as ALD seed layers for graphene-based van der Waals heterostructures.

    PubMed

    Guo, Hongwei; Liu, Yunlong; Xu, Yang; Meng, Nan; Wang, Hongtao; Hasan, Tawfique; Wang, Xinran; Luo, Jikui; Yu, Bin

    2014-09-01

    Ultrathin dielectric materials prepared by atomic-layer-deposition (ALD) technology are commonly used in graphene electronics. Using the first-principles density functional theory calculations with van der Waals (vdW) interactions included, we demonstrate that single-side fluorinated graphene (SFG) and hexagonal boron nitride (h-BN) exhibit large physical adsorption energy and strong electrostatic interactions with H2O-based ALD precursors, indicating their potential as the ALD seed layer for dielectric growth on graphene. In graphene-SFG vdW heterostructures, graphene is n-doped after ALD precursor adsorption on the SFG surface caused by vertical intrinsic polarization of SFG. However, graphene-h-BN vdW heterostructures help preserving the intrinsic characteristics of the underlying graphene due to in-plane intrinsic polarization of h-BN. By choosing SFG or BN as the ALD seed layer on the basis of actual device design needs, the graphene vdW heterostructures may find applications in low-dimensional electronics. PMID:25116064

  18. Fluorinated graphene and hexagonal boron nitride as ALD seed layers for graphene-based van der Waals heterostructures.

    PubMed

    Guo, Hongwei; Liu, Yunlong; Xu, Yang; Meng, Nan; Wang, Hongtao; Hasan, Tawfique; Wang, Xinran; Luo, Jikui; Yu, Bin

    2014-09-01

    Ultrathin dielectric materials prepared by atomic-layer-deposition (ALD) technology are commonly used in graphene electronics. Using the first-principles density functional theory calculations with van der Waals (vdW) interactions included, we demonstrate that single-side fluorinated graphene (SFG) and hexagonal boron nitride (h-BN) exhibit large physical adsorption energy and strong electrostatic interactions with H2O-based ALD precursors, indicating their potential as the ALD seed layer for dielectric growth on graphene. In graphene-SFG vdW heterostructures, graphene is n-doped after ALD precursor adsorption on the SFG surface caused by vertical intrinsic polarization of SFG. However, graphene-h-BN vdW heterostructures help preserving the intrinsic characteristics of the underlying graphene due to in-plane intrinsic polarization of h-BN. By choosing SFG or BN as the ALD seed layer on the basis of actual device design needs, the graphene vdW heterostructures may find applications in low-dimensional electronics.

  19. Graphene-based lateral heterostructure transistors exhibit better intrinsic performance than graphene-based vertical transistors as post-CMOS devices

    NASA Astrophysics Data System (ADS)

    Logoteta, Demetrio; Fiori, Gianluca; Iannaccone, Giuseppe

    2014-10-01

    We investigate the intrinsic performance of vertical and lateral graphene-based heterostructure field-effect transistors, currently considered the most promising options to exploit graphene properties in post-CMOS electronics. We focus on three recently proposed graphene-based transistors, that in experiments have exhibited large current modulation. Our analysis is based on device simulations including the self-consistent solution of the electrostatic and transport equations within the Non-Equilibrium Green's Function formalism. We show that the lateral heterostructure transistor has the potential to outperform CMOS technology and to meet the requirements of the International Technology Roadmap for Semiconductors for the next generation of semiconductor integrated circuits. On the other hand, we find that vertical heterostructure transistors miss these performance targets by several orders of magnitude, both in terms of switching frequency and delay time, due to large intrinsic capacitances, and unavoidable current/capacitance tradeoffs.

  20. Lateral and Vertical Two-Dimensional Layered Topological Insulator Heterostructures.

    PubMed

    Li, Yanbin; Zhang, Jinsong; Zheng, Guangyuan; Sun, Yongming; Hong, Seung Sae; Xiong, Feng; Wang, Shuang; Lee, Hye Ryoung; Cui, Yi

    2015-11-24

    The heterostructured configuration between two-dimensional (2D) semiconductor materials has enabled the engineering of the band gap and the design of novel devices. So far, the synthesis of single-component topological insulator (TI) 2D materials such as Bi2Se3, Bi2Te3, and Sb2Te3 has been achieved through vapor phase growth and molecular beam epitaxy; however, the spatial controlled fabrication of 2D lateral heterostructures in these systems has not been demonstrated yet. Here, we report an in situ two-step synthesis process to form TI lateral heterostructures. Scanning transmission electron microscopy and energy-dispersive X-ray mapping results show the successful spatial control of chemical composition in these as-prepared heterostructures. The edge-induced growth mechanism is revealed by the ex situ atomic force microscope measurements. Electrical transport studies demonstrate the existence of p-n junctions in Bi2Te3/Sb2Te3 heterostructures.

  1. Biomarker monitoring in sports doping control.

    PubMed

    Pottgiesser, Torben; Schumacher, Yorck Olaf

    2012-06-01

    Biomarker monitoring can be considered a new era in the effort against doping. Opposed to the old concept in doping control of direct detection of a prohibited substance in a biological sample such as urine or blood, the new paradigm allows a personalized longitudinal monitoring of biomarkers that indicate non-physiological responses independently of the used doping technique or substance, and may cause sanctioning of illicit practices. This review presents the development of biomarker monitoring in sports doping control and focuses on the implementation of the Athlete Biological Passport as the current concept of the World Anti Doping Agency for the detection of blood doping (hematological module). The scope of the article extends to the description of novel biomarkers and future concepts of application.

  2. Ohmic contact formation between metal and AlGaN/GaN heterostructure via graphene insertion

    NASA Astrophysics Data System (ADS)

    Sung Park, Pil; Reddy, Kongara M.; Nath, Digbijoy N.; Yang, Zhichao; Padture, Nitin P.; Rajan, Siddharth

    2013-04-01

    A simple method for the creation of Ohmic contact to 2D electron gas in AlGaN/GaN high electron-mobility transistors using Cr/graphene layer is demonstrated. A weak temperature dependence of this Ohmic contact observed in the range 77 to 300 K precludes thermionic emission or trap-assisted hopping as possible carrier-transport mechanisms. It is suggested that the Cr/graphene combination acts akin to a doped n-type semiconductor in contact with AlGaN/GaN heterostructure, and promotes carrier transport along percolating Al-lean paths through the AlGaN layer. This use of graphene offers a simple method for making Ohmic contacts to AlGaN/GaN heterostructures, circumventing complex additional processing steps involving high temperatures. These results could have important implications for the fabrication and manufacturing of AlGaN/GaN-based microelectronic and optoelectronic devices/sensors of the future.

  3. The 5th Engineering Foundation Conference: Advanced Heterostructure Transistors

    NASA Astrophysics Data System (ADS)

    1992-12-01

    The session on Heterostructure FET's concentrated on power devices. L. Eastman of Cornell University reported 5 W at 4 GHz with 70% efficiency and 15 dB of gain from a GE device, and 1 W at 4 GHz with 80% efficiency from Raytheon. J. Wolter described avalanche breakdown via DX centers in AlGaAs, and theoretical optimization of deep submicron HFET's for power handling was reported by M. Das. Silicon-germanium HBT's have made several improvements. K. Ismail of Cairo University and IBM showed how the Si/SiGe band lineup can be changed by strain relief, producing barriers to electrons as well as holes. Very high mobilities were reported, and the claim was made that SiGe devices at 77 K may be operationally equivalent to III-V devices at room temperature. This would clearly be important given the fabrication advantages of silicon-based technologies. Silicon-germanium HBT technology seemed to be too complex to insert into digital processes, but SiGe FET's may not be. Resonant-tunneling diodes (RTD's) have been combined into potentially multigigahertz shift-register circuits by G. Sollner at MIT Lincoln Laboratory. R. Behringer of AT&T reported experiments in optically controlled patterned growth of Na gratings using a technique that may be applicable to imaging In or Ga atoms during MBE growth. The consequences of optical phonon propagation in AlGaAs structures were discussed experimentally by G. Maracas of Motorola and L. Eastman of Cornell, and theoretically by K. W. Kim of the University of North Carolina. Interesting effects occur because optical phonons in GaAs cannot propagate in AlAs and conversely. HBT papers from III-V materials dwelt on materials issues nad centered on carbon doping and its associated strain and activation.

  4. High pressure optical studies of semiconductors and heterostructures. Final report

    SciTech Connect

    Chandrasekhar, H.R.

    1995-02-01

    The authors have studied the effects of hydrostatic pressure on the confined transitions in quantum well heterostructures, using lattice matched GaAs/Al{sub x}GaAs{sub 1{minus}x}As, strained layer narrow band gap GaSb/AlSb and In{sub x}Ga{sub 1{minus}x}As/GaAs, and strained layer wide gap Zn{sub 1{minus}x}Cd{sub x}Se/ZnSe as examples. Precise values of the energies, pressure coefficients and band alignments are determined. In strained epilayers the interfacial strains, deformation potential constants and compressibilities are deduced. Strain compensation, structural stability and phase transitions are probed. The authors have observed a novel type of Fano resonance of excitons in GaAs associated with the {Gamma} conduction band as they hybridize with the X and L continua via electron-phonon coupling. This effect is used to extract the intervalley electron-phonon deformation potential D{sub {Gamma}X} to be 10.7 {+-} 0.7 eV/{angstrom}. They have observed a new electron trap state in Al{sub 0.3}Ga{sub 0.7}As doped with silicon at pressure of 60 kbar. They postulate that this new trap state has a large lattice relaxation with the trap energy well above the X CB. These trap states may be present in all Al{sub x}Ga{sub x}As materials and may be dominant at large x values (0.7 < x < 1).

  5. Evidence of impurity assisted tunneling in SiGe/Si heterostructures

    SciTech Connect

    Zhukavin, R. Kh.; Bekin, N. A.; Lobanov, D. N.; Drozdov, M. N.; Drozdov, Yu. N.; Kozlov, D. V.; Pryakhin, D. A.; Shastin, V. N.; Shengurov, V. G.

    2013-12-04

    The investigation of vertical transport in delta-doped SiGe/Si heterostructures has been presented. The asymmetrical triple barrier structure was grown by MBE technique. The delta layer of boron impurity was placed into the center of narrower quantum well. The growth procedure was followed by conventional processing including photolithography and plasma etching and magnetron sputtering. SIMS and X-rays diagnostics have been used to control the desired structure. The conductance of the structure has been measured at liquid helium temperature and analyzed. All pronounced resonances have been identified. The resonant feature near 60 mV has to be attributed to impurity-assisted tunneling that is supported by calculation of binding energy of the acceptor in the narrower quantum well.

  6. Enhancing the thermoelectric figure of merit through the reduction of bipolar thermal conductivity with heterostructure barriers

    SciTech Connect

    Bahk, Je-Hyeong Shakouri, Ali

    2014-08-04

    In this paper, we present theoretically that the thermoelectric figure of merit for a semiconductor material with a small band gap can be significantly enhanced near the intrinsic doping regime at high temperatures via the suppression of bipolar thermal conductivity when the minority carriers are selectively blocked by heterostructure barriers. This scheme is particularly effective in nanostructured materials where the lattice thermal conductivity is lowered by increased phonon scatterings at the boundaries, so that the electronic thermal conductivity including the bipolar term is limiting the figure of merit zT. We show that zT can be enhanced to above 3 for p-type PbTe, and above 2 for n-type PbTe at 900 K with minority carrier blocking, when the lattice thermal conductivity is as low as 0.3 W/m K.

  7. Optical measurements of trap state density and minority carrier lifetime in GaAs heterostructures grown at varying rates

    NASA Astrophysics Data System (ADS)

    Haughn, Chelsea; Schmieder, Kenneth; Zide, Joshua; Barnett, Allen; Ebert, Chris; Opila, Robert; Doty, Matthew

    2013-03-01

    Semiconductor growth rates are a critical factor for production costs and can have a significant impact on electrical properties. We use time resolved photoluminescence (TRPL) to characterize the effective lifetime of carriers in gallium arsenide - indium gallium phosphide (GaAs/InGaP) double heterostructures grown at varying rates. We measure the PL decay time as a function of laser fluence and extract an approximate trap state density by fitting this data with the Shockely-Read-Hall model of carrier recombination. Using the approximate trap densities, we then calculate minority carrier lifetimes for a range of doping conditions. The results suggest that the increased density of trap states associated with a two-fold increase in growth rate are less limiting to carrier lifetime than doping at the levels required for devices. The techniques and analysis developed here can be applied for rapid, non-destructive quantification of trap state densities in materials grown under varying conditions.

  8. Electrical Measurements of the Extrinsic Spin Hall Effect in Fe/ In x Ga 1-xAs Heterostructures

    NASA Astrophysics Data System (ADS)

    Geppert, Chad; Garlid, Eric; Chan, Mun; Crowell, Paul; Hu, Qi; Palmstrøm, Chris

    2011-03-01

    We report on all-electrical measurements of the extrinsic spin Hall effect in Fe/ In x Ga 1-x As heterostructures with n -type channel doping (Si) and highly doped Schottky tunnel barriers. The spin Hall effect refers to the transverse spin current generated by application of a longitudinal unpolarized charge current. Complementary spin accumulation at opposing edges of the channel is detected via a Hanle effect in the voltage measured by pairs of ferromagnetic Hall contacts. The spin Hall conductivity is extracted by fitting the data to a drift-diffusion model incorporating spin precession and relaxation. Tuning the channel conductivity with applied bias allows the skew and side-jump contributions to be determined independently. The resulting magnitude is in agreement with models based on ionized impurity scattering. Further quantitative comparison to theoretical models is achieved by increasing the In concentration beyond previously reported values. Supported by ONR and NSF.

  9. Design and characteristics of strained InAs/InAlAs composite-channel heterostructure field-effect transistors

    NASA Astrophysics Data System (ADS)

    Lin, H.-K.; Kadow, C.; Bae, J.-U.; Rodwell, M. J. W.; Gossard, A. C.; Brar, B.; Sullivan, G.; Nagy, G.; Bergman, J.

    2005-01-01

    We report composite-channel heterostructure field-effect transistors (HFETs) with an InAs channel and an In0.9Al0.1As subchannel. The HFETs are grown on antimonide buffer layers. Two composite-channel structures with different planar Te doping schemes are designed, fabricated, and characterized. High radio-frequency transconductances of above 0.9 S/mm and ˜55GHz current gain cutoff frequencies are achieved in devices with 500 nm gates. Planar Te doping in the buffer layers reduces the high kink-effect currents otherwise found in InAs/AlSb HFETs, an effect which can be attributed to either increased breakdown field in the In0.9Al0.1As subchannel or to suppression of hole blocking in the buffer. The present limitations to device performance and suggested approaches for their elimination are discussed.

  10. van der Waals Heterostructures Grown by MBE

    NASA Astrophysics Data System (ADS)

    Hinkle, Christopher

    In this work, we demonstrate the high-quality MBE heterostructure growth of various layered 2D materials by van der Waals epitaxy (VDWE). The coupling of different types of van der Waals materials including transition metal dichalcogenide thin films (e.g., WSe2, WTe2, HfSe2) , insulating hexagonal boron nitride (h-BN), and topological insulators (e.g., Bi2Se3) allows for the fabrication of novel electronic devices that take advantage of unique quantum confinement and spin-based characteristics. The relaxed lattice-matching criteria of van der Waals epitaxy has allowed for high-quality heterostructure growth with atomically abrupt interfaces, allowing us to couple these materials based primarily on their band alignment and electronic properties. We will discuss the impact of sample preparation, surface reactivity, and lattice mismatch of various substrates (sapphire, graphene, TMDs, Bi2Se3) on the growth mode and quality of the films and will discuss our studies of substrate temperature and flux rates on the resultant growth and grain size. Structural and chemical characterization was conducted via reflection high energy electron diffraction (RHEED, X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning tunneling microscopy/spectroscopy (STM/S), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Experimentally determined band alignments have been determined and compared with first-principles calculations allowing the design of novel low-power logic and magnetic memory devices. Initial results from the electrical characterization of these grown thin films and some simple devices will also be presented. These VDWE grown layered 2D materials show significant potential for fabricating novel heterostructures with tunable band alignments and magnetic properties for a variety of nanoelectronic and optoelectronic applications.

  11. Pseudomorphic GeSn/Ge (001) heterostructures

    SciTech Connect

    Tonkikh, A. A.; Talalaev, V. G.; Werner, P.

    2013-11-15

    The synthesis of pseudomorphic GeSn heterostructures on a Ge (001) substrate by molecular-beam epitaxy is described. Investigations by transmission electron microscopy show that the GeSn layers are defect free and possess cubic diamondlike structure. Photoluminescence spectroscopy reveals interband radiative recombination in the GeSn quantum wells, which is identified as indirect transitions between the subbands of heavy electrons and heavy holes. On the basis of experimental data and modeling of the band structure of pseudomorphic GeSn compounds, the lower boundary of the bowing parameter for the indirect band gap is estimated as b{sub L} {>=} 1.47 eV.

  12. Magnetoelectroluminescence of organic heterostructures: Analytical theory and spectrally resolved measurements

    SciTech Connect

    Liu, Feilong; Kelley, Megan R.; Crooker, Scott A.; Nie, Wanyi; Mohite, Aditya D.; Ruden, P. Paul; Smith, Darryl L.

    2014-12-22

    The effect of a magnetic field on the electroluminescence of organic light emitting devices originates from the hyperfine interaction between the electron/hole polarons and the hydrogen nuclei of the host molecules. In this paper, we present an analytical theory of magnetoelectroluminescence for organic semiconductors. To be specific, we focus on bilayer heterostructure devices. In the case we are considering, light generation at the interface of the donor and acceptor layers results from the formation and recombination of exciplexes. The spin physics is described by a stochastic Liouville equation for the electron/hole spin density matrix. By finding the steady-state analytical solution using Bloch-Wangsness-Redfield theory, we explore how the singlet/triplet exciplex ratio is affected by the hyperfine interaction strength and by the external magnetic field. In order to validate the theory, spectrally resolved electroluminescence experiments on BPhen/m-MTDATA devices are analyzed. With increasing emission wavelength, the width of the magnetic field modulation curve of the electroluminescence increases while its depth decreases. Furthermore, these observations are consistent with the model.

  13. Magnetoelectroluminescence of organic heterostructures: Analytical theory and spectrally resolved measurements

    DOE PAGES

    Liu, Feilong; Kelley, Megan R.; Crooker, Scott A.; Nie, Wanyi; Mohite, Aditya D.; Ruden, P. Paul; Los Alamos National Lab.; Smith, Darryl L.; Los Alamos National Lab.

    2014-12-22

    The effect of a magnetic field on the electroluminescence of organic light emitting devices originates from the hyperfine interaction between the electron/hole polarons and the hydrogen nuclei of the host molecules. In this paper, we present an analytical theory of magnetoelectroluminescence for organic semiconductors. To be specific, we focus on bilayer heterostructure devices. In the case we are considering, light generation at the interface of the donor and acceptor layers results from the formation and recombination of exciplexes. The spin physics is described by a stochastic Liouville equation for the electron/hole spin density matrix. By finding the steady-state analytical solutionmore » using Bloch-Wangsness-Redfield theory, we explore how the singlet/triplet exciplex ratio is affected by the hyperfine interaction strength and by the external magnetic field. In order to validate the theory, spectrally resolved electroluminescence experiments on BPhen/m-MTDATA devices are analyzed. With increasing emission wavelength, the width of the magnetic field modulation curve of the electroluminescence increases while its depth decreases. Furthermore, these observations are consistent with the model.« less

  14. Band gap formation in graphene by in-situ doping

    SciTech Connect

    Park, Jeongho; Mitchel, W. C.; Brown, Gail J.; Grazulis, Lawrence; Smith, Howard E.; Pacley, Shanee D.; Boeckl, John J.; Eyink, Kurt G.; Mou, Shin; Tomich, David H.; Hoelscher, John E.; Elhamri, Said

    2011-05-16

    We report the formation of band gaps in as-grown stacks of epitaxial graphene with opposite doping. Control of in-situ doping during carbon source molecular beam epitaxy growth on SiC was achieved by using different carbon sources. Doping heterostructures were grown by stacking n-type material from a C{sub 60} source on p-type material from a graphite filament source. Activation energies for the resistivity and carrier concentration indicated band gaps up to 200 meV. A photoconductivity threshold was observed in the range of the electrical activation energies. Band gap formation is attributed to electric fields induced by spatially separated ionized dopants of opposite charge.

  15. In Situ TEM Study of Reversible and Irreversible Electroforming in Pt/Ti:NiO/Pt Heterostructures

    SciTech Connect

    D'Aquila, Kenneth; Liu, Yuzi; Iddir, Hakim; Petford-Long, Amanda K.

    2015-05-01

    Experimental verification of the microscopic origin of resistance switching in metal/oxide/metal heterostructures is needed for applications in non-volatile memory and neuromorphic computing. Numerous reports suggest that resistance switching in NiO is caused by local reduction of the oxide layer into nanoscale conducting filaments, but few reports have shown experimental evidence correlating electroforming with site-specific changes in composition. We have investigated the mechanisms of reversible and irreversible electroforming in 250–500 nm wide pillars patterned from a single Ta/Ti/Pt/Ti-doped NiO/Pt/Ta heterostructure and have shown that these can coexist within a single sample. We performed in situ transmission electron microscopy (TEM) electroform- ing and switching on each pillar to correlate the local electron transport behavior with microstructure and composition in each pillar. DFT calculations fitted to electron energy loss spectroscopy data showed that the Ti-doped NiO layer is partially reduced after reversible electroforming, with the formation of oxygen vacancies ordered into lines in the <110> direction. However, under the same probing conditions, adjacent pillars show irreversible electroforming caused by electromigration of metallic Ta to form a single bridge across the oxide layer. We propose that the different electroforming behaviors are related to microstructural variations across the sample and may lead to switching variability.

  16. Spatial Control of Laser-Induced Doping Profiles in Graphene on Hexagonal Boron Nitride.

    PubMed

    Neumann, Christoph; Rizzi, Leo; Reichardt, Sven; Terrés, Bernat; Khodkov, Timofiy; Watanabe, Kenji; Taniguchi, Takashi; Beschoten, Bernd; Stampfer, Christoph

    2016-04-13

    We present a method to create and erase spatially resolved doping profiles in graphene-hexagonal boron nitride heterostructures. The technique is based on photoinduced doping by a focused laser beam and does neither require masks nor photoresists. This makes our technique interesting for rapid prototyping of unconventional electronic device schemes, where the spatial resolution of the rewritable, long-term stable doping profiles is limited by only the laser spot size (≈600 nm) and the accuracy of sample positioning. Our optical doping method offers a way to implement and to test different, complex doping patterns in one and the very same graphene device, which is not achievable with conventional gating techniques. PMID:26986938

  17. Dielectric Genome of van der Waals Heterostructures.

    PubMed

    Andersen, Kirsten; Latini, Simone; Thygesen, Kristian S

    2015-07-01

    Vertical stacking of two-dimensional (2D) crystals, such as graphene and hexagonal boron nitride, has recently lead to a new class of materials known as van der Waals heterostructures (vdWHs) with unique and highly tunable electronic properties. Ab initio calculations should in principle provide a powerful tool for modeling and guiding the design of vdWHs, but in their traditional form such calculations are only feasible for commensurable structures with a few layers. Here we show that the dielectric properties of realistic, incommensurable vdWHs comprising hundreds of layers can be efficiently calculated using a multiscale approach where the dielectric functions of the individual layers (the dielectric building blocks) are computed ab initio and coupled together via the long-range Coulomb interaction. We use the method to illustrate the 2D-3D transition of the dielectric function of multilayer MoS2 crystals, the hybridization of quantum plasmons in thick graphene/hBN heterostructures, and to demonstrate the intricate effect of substrate screening on the non-Rydberg exciton series in supported WS2. The dielectric building blocks for a variety of 2D crystals are available in an open database together with the software for solving the coupled electrodynamic equations.

  18. Tuning Light Emission of a Pressure-Sensitive Silicon/ZnO Nanowires Heterostructure Matrix through Piezo-phototronic Effects.

    PubMed

    Chen, Mengxiao; Pan, Caofeng; Zhang, Taiping; Li, Xiaoyi; Liang, Renrong; Wang, Zhong Lin

    2016-06-28

    Based on white light emission at silicon (Si)/ZnO hetrerojunction, a pressure-sensitive Si/ZnO nanowires heterostructure matrix light emitting diode (LED) array is developed. The light emission intensity of a single heterostructure LED is tuned by external strain: when the applied stress keeps increasing, the emission intensity first increases and then decreases with a maximum value at a compressive strain of 0.15-0.2%. This result is attributed to the piezo-phototronic effect, which can efficiently modulate the LED emission intensity by utilizing the strain-induced piezo-polarization charges. It could tune the energy band diagrams at the junction area and regulate the optoelectronic processes such as charge carriers generation, separation, recombination, and transport. This study achieves tuning silicon based devices through piezo-phototronic effect. PMID:27276167

  19. Electronic structure of transferred graphene/h-BN van der Waals heterostructures with nonzero stacking angles by nano-ARPES.

    PubMed

    Wang, Eryin; Chen, Guorui; Wan, Guoliang; Lu, Xiaobo; Chen, Chaoyu; Avila, Jose; Fedorov, Alexei V; Zhang, Guangyu; Asensio, Maria C; Zhang, Yuanbo; Zhou, Shuyun

    2016-11-01

    In van der Waals heterostructures, the periodic potential from the Moiré superlattice can be used as a control knob to modulate the electronic structure of the constituent materials. Here we present a nanoscale angle-resolved photoemission spectroscopy (nano-ARPES) study of transferred graphene/h-BN heterostructures with two different stacking angles of 2.4° and 4.3° respectively. Our measurements reveal six replicas of graphene Dirac cones at the superlattice Brillouin zone (SBZ) centers. The size of the SBZ and its relative rotation angle to the graphene BZ are in good agreement with Moiré superlattice period extracted from atomic force microscopy (AFM) measurements. Comparison to the epitaxial graphene/h-BN with 0° stacking angles suggests that the interaction between graphene and h-BN decreases with increasing stacking angle. PMID:27604538

  20. Electronic structure of transferred graphene/h-BN van der Waals heterostructures with nonzero stacking angles by nano-ARPES

    NASA Astrophysics Data System (ADS)

    Wang, Eryin; Chen, Guorui; Wan, Guoliang; Lu, Xiaobo; Chen, Chaoyu; Avila, Jose; Fedorov, Alexei V.; Zhang, Guangyu; Asensio, Maria C.; Zhang, Yuanbo; Zhou, Shuyun

    2016-11-01

    In van der Waals heterostructures, the periodic potential from the Moiré superlattice can be used as a control knob to modulate the electronic structure of the constituent materials. Here we present a nanoscale angle-resolved photoemission spectroscopy (nano-ARPES) study of transferred graphene/h-BN heterostructures with two different stacking angles of 2.4° and 4.3° respectively. Our measurements reveal six replicas of graphene Dirac cones at the superlattice Brillouin zone (SBZ) centers. The size of the SBZ and its relative rotation angle to the graphene BZ are in good agreement with Moiré superlattice period extracted from atomic force microscopy (AFM) measurements. Comparison to the epitaxial graphene/h-BN with 0° stacking angles suggests that the interaction between graphene and h-BN decreases with increasing stacking angle.

  1. Tuning Light Emission of a Pressure-Sensitive Silicon/ZnO Nanowires Heterostructure Matrix through Piezo-phototronic Effects.

    PubMed

    Chen, Mengxiao; Pan, Caofeng; Zhang, Taiping; Li, Xiaoyi; Liang, Renrong; Wang, Zhong Lin

    2016-06-28

    Based on white light emission at silicon (Si)/ZnO hetrerojunction, a pressure-sensitive Si/ZnO nanowires heterostructure matrix light emitting diode (LED) array is developed. The light emission intensity of a single heterostructure LED is tuned by external strain: when the applied stress keeps increasing, the emission intensity first increases and then decreases with a maximum value at a compressive strain of 0.15-0.2%. This result is attributed to the piezo-phototronic effect, which can efficiently modulate the LED emission intensity by utilizing the strain-induced piezo-polarization charges. It could tune the energy band diagrams at the junction area and regulate the optoelectronic processes such as charge carriers generation, separation, recombination, and transport. This study achieves tuning silicon based devices through piezo-phototronic effect.

  2. Fluorescence enhancement by heterostructure colloidal photonic crystals with dual stopbands.

    PubMed

    Li, Heng; Wang, Jingxia; Liu, Feng; Song, Yanlin; Wang, Rongming

    2011-04-01

    In this work, we present a facile approach on the remarkable enhancement of fluorescent signal by heterostructure colloidal photonic crystals (PCs) with dual stopbands. The intensity of fluorescent medium on heterostructure PCs with dual stopbands overlapping the excitation wavelength and the emission wavelength of fluorescent medium can be up to 162-fold enhancement in comparison to that on the control sample. Otherwise, parameters of heterostructure PC films such as film thickness or stacking order have important effects on fluorescent signals. The method will be of great significance for developing the highly sensitive fluorescence-based detection.

  3. Gene doping.

    PubMed

    Harridge, Stephen D R; Velloso, Cristiana P

    2008-01-01

    Gene doping is the misuse of gene therapy to enhance athletic performance. It has recently been recognised as a potential threat and subsequently been prohibited by the World Anti-Doping Agency. Despite concerns with safety and efficacy of gene therapy, the technology is progressing steadily. Many of the genes/proteins which are involved in determining key components of athletic performance have been identified. Naturally occurring mutations in humans as well as gene-transfer experiments in adult animals have shown that altered expression of these genes does indeed affect physical performance. For athletes, however, the gains in performance must be weighed against the health risks associated with the gene-transfer process, whereas the detection of such practices will provide new challenges for the anti-doping authorities.

  4. Doping-Induced Interband Gain in InAs/AlSb Quantum Wells

    NASA Technical Reports Server (NTRS)

    Kolokolov, K. I.; Ning, C. Z.

    2005-01-01

    A paper describes a computational study of effects of doping in a quantum well (QW) comprising a 10-nm-thick layer of InAs sandwiched between two 21-nm-thick AlSb layers. Heretofore, InAs/AlSb QWs have not been useful as interband gain devices because they have type-II energy-band-edge alignment, which causes spatial separation of electrons and holes, thereby leading to weak interband dipole matrix elements. In the doping schemes studied, an interior sublayer of each AlSb layer was doped at various total areal densities up to 5 X 10(exp 12) / square cm. It was found that (1) proper doping converts the InAs layer from a barrier to a well for holes, thereby converting the heterostructure from type II to type I; (2) the resultant dipole matrix elements and interband gains are comparable to those of typical type-I heterostructures; and (3) dipole moments and optical gain increase with the doping level. Optical gains in the transverse magnetic mode can be almost ten times those of other semiconductor material systems in devices used to generate medium-wavelength infrared (MWIR) radiation. Hence, doped InAs/AlSb QWs could be the basis of an alternative material system for devices to generate MWIR radiation.

  5. Non-redox modulated fluorescence strategy for sensitive and selective ascorbic acid detection with highly photoluminescent nitrogen-doped carbon nanoparticles via solid-state synthesis.

    PubMed

    Zhu, Xiaohua; Zhao, Tingbi; Nie, Zhou; Liu, Yang; Yao, Shouzhuo

    2015-08-18

    Highly photoluminescent nitrogen-doped carbon nanoparticles (N-CNPs) were prepared by a simple and green route employing sodium alginate as a carbon source and tryptophan as both a nitrogen source and a functional monomer. The as-synthesized N-CNPs exhibited excellent water solubility and biocompatibility with a fluorescence quantum yield of 47.9%. The fluorescence of the N-CNPs was intensively suppressed by the addition of ascorbic acid (AA). The mechanism of the fluorescence suppression of the N-CNPs was investigated, and the synergistic action of the inner filter effect (IFE) and the static quenching effect (SQE) contributed to the intensive fluorescence suppression, which was different from those reported for the traditional redox-based fluorescent probes. Owing to the spatial effect and hydrogen bond between the AA and the groups on the N-CNP surface, excellent sensitivity and selectivity for AA detecting was obtained in a wide linear relationship from 0.2 μM to 150 μM. The detection limit was as low as 50 nM (signal-to-noise ratio of 3). The proposed sensing systems also represented excellent sensitivity and selectivity for AA analysis in human biological fluids, providing a valuable platform for AA sensing in clinic diagnostic and drug screening. PMID:26202861

  6. Field effect controlled ferromagnetism in transition metal doped ZnO

    NASA Astrophysics Data System (ADS)

    Bellingeri, E.; Pellegrino, L.; Biasotti, M.; Pallecchi, I.; Canu, G.; Gerbi, A.; Vignolo, M.; Siri, A. S.; Marré, D.; Rusponi, S.; Lehnert, A.; Nolting, F.

    2008-02-01

    The ability to externally control the properties of magnetic materials would be highly desirable both from fundamental and technological point of views. In this respect, dilute magnetic semiconductor (DMS), in which a fraction of atoms of the nonmagnetic semiconductor host is replaced by magnetic ions, have recently attracted broad interest for their potential application in spintronics. In this work, we focused on transition metal (TM) (Co, Mn and Cu) doped Zinc oxide (ZnO) because room temperature ferromagnetism was both theoretically predicted and experimentally observed. However, the origin of such ferromagnetism, in particular whether it is a signature of a true DMS behaviour (long range magnetic interaction between the doping ions) or it arises from the formation of secondary phases, segregation or clustering is still under debate. Measuring the dependence of the magnetic properties on the carrier concentration can clarify the underlying physics. The samples were characterized by resistivity, Hall effect, magnetoresistance, Seebeck effect, synchrotron X-ray adsorption spectra (XAS) and magnetic dichroism (XMD) while modulating the carrier density by electric field. The insulating-gate field-effect transistor structures are realized in ZnO/Strontium Titanate (SrTiO 3) heterostructures by pulsed laser deposition. These devices offers the capability to modulate the carrier density of a probe accessible (light, AFM tip, ...) channel, by more than 5 orders of magnitude (from ~10 15 to ~10 20 e -/cm 3, estimated by Hall effect measurements under FE). The Co and Mn films measured by DC SQUID magnetometer result ferromagnetic and anomalous Hall effect was observed at low temperature but nor ferromagnetic nor antiferromagnetic signal was detectable in the XMD spectra. Cu doped films are insulating and nonmagnetic. Photo Emission Electron Microscopy (x-PEEM) and magnetic force microscopy (MFM) showed that the sample are homogeneus and no clustering of TM were detected

  7. Ge/Si core/multi shell heterostructure FETs

    SciTech Connect

    Picraux, Samuel T; Dayeh, Shadi A

    2010-01-01

    Concentric heterostructured materials provide numerous design opportunities for engineering strain and interfaces, as well as tailoring energy band-edge combinations for optimal device performance. Key to the realization of such novel device concepts is the complete understanding and full control over their growth, crystal structure, and hetero-epitaxy. We report here on a new route for synthesizing Ge/Si core/multi-shell heterostructure nanowires that eliminate Au seed diffusion on the nanowire sidewalls by engineering the interface energy density difference. We show that such control over core/shell synthesis enable experimental realization of heterostructure FET devices beyond those available in the literature with enhanced transport characteristics. We provide a side-by-side comparison on the transport properties of Ge/Si core/multi-shell nanowires grown with and without Au diffusion and demonstrate heterostructure FETs with drive currents that are {approx} 2X higher than record results for p-type FETs.

  8. Quantum dot nanoscale heterostructures for solar energy conversion.

    PubMed

    Selinsky, Rachel S; Ding, Qi; Faber, Matthew S; Wright, John C; Jin, Song

    2013-04-01

    Quantum dot nanoscale semiconductor heterostructures (QDHs) are a class of materials potentially useful for integration into solar energy conversion devices. However, realizing the potential of these heterostructured systems requires the ability to identify and synthesize heterostructures with suitably designed materials, controlled size and morphology of each component, and structural control over their shared interface. In this review, we will present the case for the utility and advantages of chemically synthesized QDHs for solar energy conversion, beginning with an overview of various methods of heterostructured material synthesis and a survey of heretofore reported materials systems. The fundamental charge transfer properties of the resulting materials combinations and their basic design principles will be outlined. Finally, we will discuss representative solar photovoltaic and photoelectrochemical devices employing QDHs (including quantum dot sensitized solar cells, or QDSSCs) and examine how QDH synthesis and design impacts their performance.

  9. 2D materials and van der Waals heterostructures.

    PubMed

    Novoselov, K S; Mishchenko, A; Carvalho, A; Castro Neto, A H

    2016-07-29

    The physics of two-dimensional (2D) materials and heterostructures based on such crystals has been developing extremely fast. With these new materials, truly 2D physics has begun to appear (for instance, the absence of long-range order, 2D excitons, commensurate-incommensurate transition, etc.). Novel heterostructure devices--such as tunneling transistors, resonant tunneling diodes, and light-emitting diodes--are also starting to emerge. Composed from individual 2D crystals, such devices use the properties of those materials to create functionalities that are not accessible in other heterostructures. Here we review the properties of novel 2D crystals and examine how their properties are used in new heterostructure devices.

  10. Effects of substrate misorientation and background impurities on electron transport in molecular-beam-epitaxial grown GaAs/AlGaAs modulation-doped quantum-well structures

    NASA Technical Reports Server (NTRS)

    Radulescu, D. C.; Wicks, G. W.; Schaff, W. J.; Calawa, A. R.; Eastman, L. F.

    1987-01-01

    The effects of substrate misorientation off the (001) plane and of background impurities on electron transport in MBE-grown GaAs/AlGaAs modulation-doped superlattice-buffered quantum-well structures were investigated. Low-field transport data were obtained on GaAs/AlGaAs structures grown on substrates oriented 0, 2, 4, and 6.5 deg off the (001) plane towards either (111)A or (111)B. It is shown that the low-field two-dimensional electron gas (2DEG) mobility is a function of the angle and direction of the substrate orientation, and that the 2DEG mobility is a function of the direction of the applied electric field in the GaAs quantum well. The anisotropy in the 2DEG mobility is also a function of the tilt angle and tilt azimuth direction of the substrate from the (001) plane. In addition, it is shown that the amount of interface scattering from the inverted interface is a sensitive function of the amount of background impurities in the MBE machine.

  11. Low-noise behavior of InGaAs quantum-well-structured modulation-doped FET's from 10 to the -2nd to 10 to the 8 Hz

    NASA Technical Reports Server (NTRS)

    Liu, Shih-Ming J.; Das, Mukunda B.; Peng, Chin-Kun; Klem, John; Henderson, Timothy S.

    1986-01-01

    Equivalent gate noise voltage spectra of 1-micron gate-length modulation-doped FET's with pseudomorphic InGaAs quantum-well structure have been measured for the frequency range of 0.01 Hz to 100 MHz and commpared with the noise spectra of conventional AlGaAs/GaAs MODFET's and GaAs MESFET's. The prominent generation-recombination (g-r) noise bulge commonly observed in the vicinity of 10 kHz in conventional MODFET's at 300 K does not appear in the case of the new InGaAs quantum-well MODFET. Instead, its noise spectra indicate the presence of low-intensity multiple g-r noise components superimposed on a reduced 1/f noise. The LF noise intensity in the new device appears to be the lowest among those observed in any MODFET or MESFET. The noise spectra at 82 K in the new device represent nearly true 1/f noise. This unusual low-noise behavior of the new structure suggests the effectiveness of electron confinement in the quantum well that significantaly reduces electron trapping in the n-AlGaAs, and thus eliminates the g-r noise bulge observed in conventional MODFET's.

  12. Basic properties, growth and preparation methods of group IV heterostructures

    NASA Astrophysics Data System (ADS)

    Kasper, E.

    This document is part of subvolume C3 'Optical Properties' of volume 34 'Semiconductor quantum structures' of Landolt-Börnstein, Group III, Condensed Matter, on the optical properties of quantum heterostructures based on group IV semiconductors. It discusses basic properties, growth and preparation methods of group IV heterostructures, including epitaxial growth processes, lattice mismatch and its implication on critical thickness and interface structure, and virtual substrates and strain relaxation.

  13. Method and apparatus for selectively annealing heterostructures using microwave

    NASA Technical Reports Server (NTRS)

    Atwater, Harry A. (Inventor); Brain, Ruth A. (Inventor); Barmatz, Martin B. (Inventor)

    1998-01-01

    The present invention discloses a process for selectively annealing heterostructures using microwaves. A heterostructure, comprised of a material having higher microwave absorption and a material having lower microwave absorption, is exposed to microwaves in the cavity. The higher microwave absorbing material absorbs the microwaves and selectively heats while the lower microwave absorbing material absorbs small amounts of microwaves and minimally heats. The higher microwave absorbing material is thereby annealed onto the less absorbing material which is thermally isolated.

  14. Method and apparatus for selectively annealing heterostructures using microwaves

    NASA Technical Reports Server (NTRS)

    Atwater, Harry A. (Inventor); Brain, Ruth A. (Inventor); Barmatz, Martin B. (Inventor)

    1998-01-01

    The present invention discloses a process for selectively annealing heterostructures using microwaves. A heterostructure, comprised of a material having higher microwave absorption and a material having lower microwave absorption, is exposed to microwaves in the cavity. The higher microwave absorbing material absorbs the microwaves and selectively heats while the lower microwave absorbing material absorbs small amounts of microwaves and minimally heats. The higher microwave absorbing material is thereby annealed onto the less absorbing material which is thermally isolated.

  15. Giant switchable Rashba effect in oxide heterostructures

    DOE PAGES

    Zhong, Zhicheng; Si, Liang; Zhang, Qinfang; Yin, Wei-Guo; Yunoki, Seiji; Held, Karsten

    2015-03-01

    One of the most fundamental phenomena and a reminder of the electron’s relativistic nature is the Rashba spin splitting for broken inversion symmetry. Usually this splitting is a tiny relativistic correction. Interfacing ferroelectric BaTiO₃ and a 5d (or 4d) transition metal oxide with a large spin-orbit coupling, Ba(Os,Ir,Ru)O₃, we show that giant Rashba spin splittings are indeed possible and even controllable by an external electric field. Based on density functional theory and a microscopic tight binding understanding, we conclude that the electric field is amplified and stored as a ferroelectric Ti-O distortion which, through the network of oxygen octahedra, inducesmore » a large (Os,Ir,Ru)-O distortion. The BaTiO₃/Ba(Os,Ru,Ir)O₃ heterostructure is hence the ideal test station for switching and studying the Rashba effect and allows applications at room temperature.« less

  16. Tunnelling in van der Waals heterostructures

    NASA Astrophysics Data System (ADS)

    Mishchenko, Artem; Novoselov, Kostya; Geim, Andre; Eaves, Laurence; Falko, Vladimir

    When graphene and other conductive two-dimensional (2D) materials are separated by an atomically thin insulating 2D crystal, quantum mechanical tunnelling leads to appreciable current between two 2D conductors due to the overlap of their wavefunctions. These tunnel devices demonstrate interesting physics and potential for applications: such effects as resonant tunnelling, negative differential conductance, light emission and detection have already been demonstrated. In this presentation we will outline the current status and perspectives of tunnelling transistors based on 2D materials assembled into van der Waals heterostructures. Particularly, we will present results on mono- and bilayer graphene tunnelling, tunnelling in 2D crystal-based quantum wells, and tunnelling in superconducting 2D materials. Such effects as momentum and chirality conservation, phonon- and impurity-assisted tunnelling will also be discussed. Finally, we will ponder the implications of discovered effects for practical applications.

  17. Tunable hyperbolic metamaterials utilizing phase change heterostructures

    SciTech Connect

    Krishnamoorthy, Harish N. S.; Menon, Vinod M.; Zhou, You; Ramanathan, Shriram; Narimanov, Evgenii

    2014-03-24

    We present a metal-free tunable anisotropic metamaterial where the iso-frequency surface is tuned from elliptical to hyperbolic dispersion by exploiting the metal-insulator phase transition in the correlated material vanadium dioxide (VO{sub 2}). Using VO{sub 2}-TiO{sub 2} heterostructures, we demonstrate the transition in the effective dielectric constant parallel to the layers to undergo a sign change from positive to negative as the VO{sub 2} undergoes the phase transition. The possibility to tune the iso-frequency surface in real time using external perturbations such as temperature, voltage, or optical pulses creates new avenues for controlling light-matter interaction.

  18. Rashba-Edelstein Magnetoresistance in Metallic Heterostructures

    NASA Astrophysics Data System (ADS)

    Nakayama, Hiroyasu; Kanno, Yusuke; An, Hongyu; Tashiro, Takaharu; Haku, Satoshi; Nomura, Akiyo; Ando, Kazuya

    2016-09-01

    We report the observation of magnetoresistance originating from Rashba spin-orbit coupling (SOC) in a metallic heterostructure: the Rashba-Edelstein (RE) magnetoresistance. We show that the simultaneous action of the direct and inverse RE effects in a Bi /Ag /CoFeB trilayer couples current-induced spin accumulation to the electric resistance. The electric resistance changes with the magnetic-field angle, reminiscent of the spin Hall magnetoresistance, despite the fact that bulk SOC is not responsible for the magnetoresistance. We further found that, even when the magnetization is saturated, the resistance increases with increasing the magnetic-field strength, which is attributed to the Hanle magnetoresistance in this system.

  19. Rashba-Edelstein Magnetoresistance in Metallic Heterostructures.

    PubMed

    Nakayama, Hiroyasu; Kanno, Yusuke; An, Hongyu; Tashiro, Takaharu; Haku, Satoshi; Nomura, Akiyo; Ando, Kazuya

    2016-09-01

    We report the observation of magnetoresistance originating from Rashba spin-orbit coupling (SOC) in a metallic heterostructure: the Rashba-Edelstein (RE) magnetoresistance. We show that the simultaneous action of the direct and inverse RE effects in a Bi/Ag/CoFeB trilayer couples current-induced spin accumulation to the electric resistance. The electric resistance changes with the magnetic-field angle, reminiscent of the spin Hall magnetoresistance, despite the fact that bulk SOC is not responsible for the magnetoresistance. We further found that, even when the magnetization is saturated, the resistance increases with increasing the magnetic-field strength, which is attributed to the Hanle magnetoresistance in this system. PMID:27661708

  20. Vertical Transport in Ferroelectric/Superconductor Heterostructures

    NASA Astrophysics Data System (ADS)

    Begon-Lours, Laura; Trastoy, Juan; Bernard, Rozenn; Jacquet, Eric; Carretero, Cecile; Bouzehouane, Karim; Fusil, Stephane; Garcia, Vincent; Xavier, Stephane; Girod, Stephanie; Deranlot, Cyrile; Bibes, Manuel; Barthelemy, Agnes; Villegas, Javier E.

    2015-03-01

    We study electric field-effects in superconducting films by measuring vertical transport in ferroelectric/superconductor heterostructures. These are based on ultrathin (4 to 8 nm thick) BiFeO3-Mn grown on YBa2Cu3O7 by pulsed laser deposition. Nanoscale contacts are defined on the BiFeO3 via a series of nanofabrication steps which include e-beam lithography, metal deposition (Nb or Co capped with Pt) and lift-off. Conductive-tip atomic force microscopy and piezoresponse force microscopy are used to characterize the transport across the ferroelectric barrier as a function of its polarization (up/down). The observed electro-resistance, measured at various temperatures, allows studying the different electric-field screening in the normal and superconducting states. Work supported by DIM Oxymore.

  1. Optical properties of SbSI heterostructures

    NASA Astrophysics Data System (ADS)

    Toroń, B.; Nowak, M.; Grabowski, A.; Kepńiska, M.; Szala, J.; Rzychoń, T.

    2012-10-01

    The antimony sulfoiodide (SbSI) single crystal being a ferroelectric semiconductor has a large number of interesting properties. Based on SbSI single crystal a new type of heterostructures has been produced. For the first time diodes, transistors and thyristors composed of SbSI/Sb2S3 heterojunctions have been fabricated by CO2 laser irradiation of selected sections of SbSI single crystals. Treated sections are composed of amorphous antimony (III) sulphide (Sb2S3) with energy gap 0.3 eV smaller (in room temperature) than that of SbSI. The structural optical, electrical and photoelectrical characteristics of produced devices have been investigated.

  2. Giant switchable Rashba effect in oxide heterostructures

    SciTech Connect

    Zhong, Zhicheng; Si, Liang; Zhang, Qinfang; Yin, Wei-Guo; Yunoki, Seiji; Held, Karsten

    2015-03-01

    One of the most fundamental phenomena and a reminder of the electron’s relativistic nature is the Rashba spin splitting for broken inversion symmetry. Usually this splitting is a tiny relativistic correction. Interfacing ferroelectric BaTiO₃ and a 5d (or 4d) transition metal oxide with a large spin-orbit coupling, Ba(Os,Ir,Ru)O₃, we show that giant Rashba spin splittings are indeed possible and even controllable by an external electric field. Based on density functional theory and a microscopic tight binding understanding, we conclude that the electric field is amplified and stored as a ferroelectric Ti-O distortion which, through the network of oxygen octahedra, induces a large (Os,Ir,Ru)-O distortion. The BaTiO₃/Ba(Os,Ru,Ir)O₃ heterostructure is hence the ideal test station for switching and studying the Rashba effect and allows applications at room temperature.

  3. Novel superconducting proximized heterostructures for ultrafast photodetection

    NASA Astrophysics Data System (ADS)

    Pepe, G. P.; Parlato, L.; Marrocco, N.; Pagliarulo, V.; Peluso, G.; Barone, A.; Tafuri, F.; Uccio, U. Scotti di; Miletto, F.; Radovic, M.; Pan, D.; Sobolewski, Roman

    2009-11-01

    Weak ferromagnet/superconductor (F/S) proximity bilayers have been fabricated and characterized for basic physics studies concerning the ultrafast carrier dynamics in layered materials. The normalized reflectivity change (Δ R/ R) as a function of the time delay between the pump and the probe laser beams has been measured in F/S heterostructures formed by a low critical temperature superconductor (Nb) with a NiCu overlayer, and a high Tc superconductor (YBCO) covered by Au/NiCu and manganite (LSMO) overlayers. The attention is mainly focused to the investigation of nonequilibrium excitation dynamics inside different bilayers in the low temperature region. The presence of the weak ferromagnetic overlayer produces faster optical relaxation times with respect to sole superconducting films. The results are promising in view of potential applications as ultrafast kinetic inductance superconducting photodetectors as confirmed by preliminary time-resolved photoimpedance experiments on both Nb and NiCu(21 nm)/Nb(70 nm) samples.

  4. Collective mode splitting in hybrid heterostructures

    NASA Astrophysics Data System (ADS)

    Ramírez, Juan Gabriel; de la Venta, J.; Wang, Siming; Saerbeck, Thomas; Basaran, Ali C.; Batlle, X.; Schuller, Ivan K.

    2016-06-01

    We report on a drastic change of the Ni collective magnetization dynamics when incorporated into a Ni /V2O3 heterostructure. Two, unexpected, well-defined Ni ferromagnetic resonance (FMR) modes are observed in the coexistence region of the first-order V2O3 structural phase transition (SPT). The phase coexistence across the V2O3 SPT can explain the presence of the two resonance fields but not their anticrossing and large linewidth broadenings. Our results imply a strong coupling between the lattice dynamics of the strongly correlated oxide (V2O3 ) and the magnon modes of the ferromagnet (Ni) in this hybrid. This and additional experiments on Ni grown on SrTi O3 , a prototypical second-order phase transition oxide, imply that these effects require the presence of first-order transitions in the oxides.

  5. Magnetoelectric imaging of multiferroic heterostructures (Presentation Recording)

    NASA Astrophysics Data System (ADS)

    Ghidini, Massimo; Lesaine, Arnaud; Zhu, Bonan; Moya, Xavier; Yan, Wenjing; Crossley, Sam; Nair, Bhasi; Mansell, Rhodri; Cowburn, Russell P.; Barnes, Crispin H. W.; Kronast, Florian; Valencia, Sergio; Maccherozzi, Francesco; Dhesi, Sarnjeet S.; Mathur, Neil

    2015-09-01

    Electrical control of magnetism has been demonstrated in multiferroic compounds and ferromagnetic semiconductors, but electrical switching of a substantial net magnetization at room temperature has not been demonstrated in these materials. This goal has instead been achieved in heterostructures comprising ferromagnetic films in which electrically driven magnetic changes arise due to strain or exchange bias from ferroic substrates, or due to charge effects induced by a gate. However, previous work focused on electrical switching of an in-plane magnetization or involved the assistance of applied magnetic fields. In heterostructures made of juxtaposed ferroelectric and ferromagnetic layers, we have shown electrical control with no applied magnetic field of the perpendicular magnetization of small features [1] and of magnetic stripe domains patterns [2]. Here we investigate Ni81Fe19 films on ferroelectric substrates with and without buffer layers of Cu, whose presence precludes charge-mediated coupling. Ni81Fe19 has virtually zero magnetostriction, but sufficiently thin films show large magnetostriction, and thus, on increasing film thickness through the threshold for zero magnetostriction, we have seeked the crossover from charge- to strain-mediated coupling. We will then show that strain associated with the motion of 90°- ferroelectric domain walls in a BaTiO3 substrate, can switch the magnetization of an array of overlying single-domain Ni dots. [1] M. Ghidini, R. Pellicelli, J. L. Prieto, X. Moya, J. Soussi, J. Briscoe, S. Dunn and N. D. Mathur, Nature Communications 4 (2013) 1453. [2] M. Ghidini, F.Maccherozzi, X. Moya, L. C. Phillips, W.Yan, J. Soussi, N. Métallier, M.Vickers, , N. -J.Steinke, R. Mansell, C. H. W. Barnes, S. S. Dhesi, and N. D. Mathur, Adv. Mater.doi: 10.1002/adma.201404799 (2015).

  6. A 980 nm pseudomorphic single quantum well laser for pumping erbium-doped optical fiber amplifiers

    NASA Technical Reports Server (NTRS)

    Larsson, A.; Forouhar, S.; Cody, J.; Lang, R. J.; Andrekson, P. A.

    1990-01-01

    The authors have fabricated ridge waveguide pseudomorphic InGaAs/GaAs/AlGaAs GRIN-SCH SQW (graded-index separate-confinement-heterostructure single-quantum-well) lasers, emitting at 980 nm, with a maximum output power of 240 mW from one facet and a 22 percent coupling efficiency into a 1.55-micron single-mode optical fiber. These lasers satisfy the requirements on efficient and compact pump sources for Er3+-doped fiber amplifiers.

  7. SABRE: A search for dark matter and a test of the DAMA/LIBRA annual-modulation result using thallium-doped sodium-iodide scintillation detectors

    NASA Astrophysics Data System (ADS)

    Shields, Emily Kathryn

    Ample evidence has been gathered demonstrating that the majority of the mass in the universe is composed of non-luminous, non-baryonic matter. Though the evidence for dark matter is unassailable, its nature and properties remain unknown. A broad effort has been undertaken by the physics community to detect dark-matter particles through direct-detection techniques. For over a decade, the DAMA/LIBRA experiment has observed a highly significant (9.3sigma) modulation in the scintillation event rate in their highly pure NaI(Tl) detectors, which they use as the basis of a claim for the discovery of dark-matter particles. However, the dark-matter interpretation of the DAMA/LIBRA modulation remains unverified. While there have been some recent hints of dark matter in the form of a light Weakly-Interacting Massive Particle (WIMP) from the CoGeNT and CDMS-Si experiments, when assuming a WIMP dark-matter model, several other experiments, including the LUX and XENON noble-liquid experiments, the KIMS CsI(Tl) experiment, and several bubble chamber experiments, conflict with DAMA/LIBRA. However, these experiments use different dark-matter targets and cannot be compared with DAMA/LIBRA in a model-independent way. The uncertainty surrounding the dark-matter model, astrophysical model, and nuclear-physics effects makes it necessary for a new NaI(Tl) experiment to directly test the DAMA/LIBRA result. The Sodium-iodide with Active Background REjection (SABRE) experiment seeks to provide a much-needed model-independent test of the DAMA/LIBRA modulation by developing highly pure crystal detectors with very low radioactivity and deploying them in an active veto detector that can reject key backgrounds in a dark-matter measurement. This work focuses on the efforts put forward by the SABRE collaboration in developing low-background, low-threshold crystal detectors, designing and fabricating a liquid-scintillator veto detector, and simulating the predicted background spectrum for a dark

  8. Noncontacting laser photocarrier radiometric depth profilometry of harmonically modulated band bending in the space-charge layer at doped SiO{sub 2}-Si interfaces

    SciTech Connect

    Mandelis, Andreas; Batista, Jerias; Gibkes, Juergen; Pawlak, Michael; Pelzl, Josef

    2005-04-15

    Laser infrared photocarrier radiometry (PCR) was used with a harmonically modulated low-power laser pump and a superposed dc superband-gap optical bias (a secondary laser beam) to control and monitor the space-charge-layer (SCL) width in oxidized p-Si-SiO{sub 2} and n-Si-SiO{sub 2} interfaces (wafers) exhibiting charged interface-state related band bending. Applying the theory of PCR-SCL dynamics [A. Mandelis, J. Appl. Phys. 97, 083508 (2005)] to the experiments yielded various transport parameters of the samples as well as depth profiles of the SCL exhibiting complete ( p-type Si) or partial (n-type Si) band flattening, to a degree controlled by widely different minority-carrier capture cross section at each interface. The uncompensated charge density at the interface was also calculated from the theory.

  9. Electrical Measurement of the Direct Spin Hall Effect in Fe/GaAs Heterostructures

    NASA Astrophysics Data System (ADS)

    Garlid, E. S.; Hu, Q.; Chan, M. K.; Palmstrøm, C. J.; Crowell, P. A.

    2010-03-01

    A handful of recent experiments, all of which have used optical techniques, have investigated the spin Hall effect in semiconductors. We report on an all-electrical measurement showing evidence of the spin Hall effect in epitaxial (100) Fe/GaAs heterostructures with a channel doping (Si) of n =5x10^16 cm-3 and highly doped Schottky tunnel barriers (n^+=5x10^18 cm-3). Multiple devices were fabricated on a single chip with the distance of the ferromagnetic (FM) electrodes from the channel edge varying between 2 and 10 μm. Devices were first characterized by performing non-local spin valve and Hanle measurements. Hall measurements were then performed with an unpolarized current flowing down the GaAs channel parallel to the magnetic easy axis of the FM electrodes. A magnetic field was applied along the FM hard axis to induce precession of the out-of-plane spins at the channel edges into the FM easy axis. We investigate the bias and temperature dependence of the resulting Hanle signal, and find that it is consistent with an extrinsic spin Hall effect. The data suggests that ionized impurity scattering is the dominant contribution to the spin Hall conductivity in these samples. Supported by ONR and NSF.

  10. Relevance of the selective oestrogen receptor modulators tamoxifen, toremifene and clomiphene in doping field: endogenous steroids urinary profile after multiple oral doses.

    PubMed

    Mazzarino, Monica; Braganò, Maria Cristina; de la Torre, Xavier; Molaioni, Francesco; Botrè, Francesco

    2011-11-01

    The present study was performed to investigate the influence of the intake of selective oestrogen receptor modulators on the urinary endogenous steroids profile. For this purpose the circadian variability of luteinizing hormone, follicle-stimulating hormone, testosterone, 5α-androstan-3α,17β-diol, 5β-androstan-3α,17β-diol, epitestosterone, 4-androstenedione, androsterone and etiocholanolone were measured on eight subjects (four males and four females) by gas chromatography-mass spectrometry and chemiluminescent immunometric assay techniques before and after oral administration of multiple doses of either tamoxifen (80 mg for 2 days) or toremifene (120 mg for 2 days) or clomiphene (100 mg for 2 days). The individual baseline variability of the steroids studied was set up by collecting the urine samples every 3 h, for 3 days prior to the treatment; whereas the evaluation of the effects of the oral administration of multiple doses of selective oestrogen receptor modulators on the steroid urinary profile was assessed by collecting urine samples every three hours for at least five days from the first administration. The results of our measurements showed that, only in male subjects, the relative urinary concentrations of testosterone, epitestosterone and 4-androstenedione were significantly altered generally after the second day of drug administration. While no significant effects were recorded in both sexes on the luteinizing hormone, follicle-stimulating hormone, androsterone, etiocholanolone, 5α-androstan-3α,17β-diol and 5β-androstan-3α,17β-diol urinary levels and on testosterone/epitestosterone, 5α-androstan-3α,17β-diol/5β-androstan-3α,17β-diol and androsterone/etiocholanolone ratios.

  11. Superconductivity-induced magnetization depletion in a ferromagnet through an insulator in a ferromagnet-insulator-superconductor hybrid oxide heterostructure.

    PubMed

    Prajapat, C L; Singh, Surendra; Paul, Amitesh; Bhattacharya, D; Singh, M R; Mattauch, S; Ravikumar, G; Basu, S

    2016-05-21

    Coupling between superconducting and ferromagnetic states in hybrid oxide heterostructures is presently a topic of intense research. Such a coupling is due to the leakage of the Cooper pairs into the ferromagnet. However, tunneling of the Cooper pairs though an insulator was never considered plausible. Using depth sensitive polarized neutron reflectivity we demonstrate the coupling between superconductor and magnetic layers in epitaxial La2/3Ca1/3MnO3 (LCMO)/SrTiO3/YBa2Cu3O7-δ (YBCO) hybrid heterostructures, with SrTiO3 as an intervening oxide insulator layer between the ferromagnet and the superconductor. Measurements above and below the superconducting transition temperature (TSC) of YBCO demonstrate a large modulation of magnetization in the ferromagnetic layer below the TSC of YBCO in these heterostructures. This work highlights a unique tunneling phenomenon between the epitaxial layers of an oxide superconductor (YBCO) and a magnetic layer (LCMO) through an insulating layer. Our work would inspire further investigations on the fundamental aspect of a long range order of the triplet spin-pairing in hybrid structures.

  12. Si-doped GaAs/AlGaAs TJS laser by MBE

    SciTech Connect

    Mitsunaga, K.; Fujiwara, K.; Nunoshita, M.; Nakayama, T.

    1984-04-01

    The effect of high temperature annealing on the properties of silicon-doped GaAs/AlGaAs double heterostructure (DH) grown by molecular beam expitaxy (MBE)= and its application to the fabrication of transverse junction stripe (TJS) lasers are reported. In spite of the amphoteric nature of Si, it was found that the high temperature annealing gave little influence on the electrical and optical quality of the n-type DH wafer. The TJS laser using Si-doped GaAs/AlGaAs wafer has been oscillated cw at room temperature and exhibited low threshold current of 30 mA and high quantum efficiency of 60%.

  13. Optical spectroscopy of nanoscale and heterostructured oxides

    NASA Astrophysics Data System (ADS)

    Senty, Tess R.

    Through careful analysis of a material's properties, devices are continually getting smaller, faster and more efficient each day. Without a complete scientific understanding of material properties, devices cannot continue to improve. This dissertation uses optical spectroscopy techniques to understand light-matter interactions in several oxide materials with promising uses mainly in light harvesting applications. Linear absorption, photoluminescence and transient absorption spectroscopy are primarily used on europium doped yttrium vanadate nanoparticles, copper gallium oxide delafossites doped with iron, and cadmium selenide quantum dots attached to titanium dioxide nanoparticles. Europium doped yttrium vanadate nanoparticles have promising applications for linking to biomolecules. Using Fourier-transform infrared spectroscopy, it was shown that organic ligands (benzoic acid, 3-nitro 4-chloro-benzoic acid and 3,4-dihydroxybenzoic acid) can be attached to the surface of these molecules using metal-carboxylate coordination. Photoluminescence spectroscopy display little difference in the position of the dominant photoluminescence peaks between samples with different organic ligands although there is a strong decrease in their intensity when 3,4-dihydroxybenzoic acid is attached. It is shown that this strong quenching is due to the presence of high-frequency hydroxide vibrational modes within the organic linker. Ultraviolet/visible linear absorption measurements on delafossites display that by doping copper gallium oxide with iron allows for the previously forbidden fundamental gap transition to be accessed. Using tauc plots, it is shown that doping with iron lowers the bandgap from 2.8 eV for pure copper gallium oxide, to 1.7 eV for samples with 1 -- 5% iron doping. Using terahertz transient absorption spectroscopy measurements, it was also determined that doping with iron reduces the charge mobility of the pure delafossite samples. A comparison of cadmium selenide

  14. Doping control analysis of tricyclic tetrahydroquinoline-derived selective androgen receptor modulators using liquid chromatography/electrospray ionization tandem mass spectrometry.

    PubMed

    Thevis, Mario; Kohler, Maxie; Thomas, Andreas; Schlörer, Nils; Schänzer, Wilhelm

    2008-08-01

    Selective androgen receptor modulators represent an emerging class of therapeutics to counteract various diseases such as osteoporosis and muscle wasting. Numerous drug candidates have been developed and investigated including a group that comprises a tricyclic tetrahydroquinoline nucleus such as 2-methyl-2-(8-nitro-3a,4,5,9b-tetrahydro-3H-cyclopenta[c]chinolin-4-yl)propan-1-ol. Due to their novelty and medicinal purpose, these compounds also possess great potential for misuse in sports, and studies on the mass spectrometric behavior of three synthesized model substances and drug candidates were conducted to provide information on typical dissociation pathways following electrospray ionization and collision-induced dissociation. Product ion mass spectra derived from protonated molecules were studied using high resolution/high accuracy orbitrap mass spectrometry, and characteristic fragmentation routes and product ions were elucidated. Major and general findings include the elimination of a hydroxyl radical from [M+H](+), the elimination of the 2-substituted side chain, and the gas-phase rearrangement of the investigated tricyclic tetrahydroquinolines to 6-nitroquinoline yielding a common product ion at m/z 175. Knowledge of these dissociation pathways supports the identification of related substances as well as metabolic products, which is of utmost importance to drug testing laboratories. The compounds were implemented into existing screening procedures, and detection limits (0.2-0.6 ng/mL), recoveries (92-97%), and intraday and interday precision (<22%) were evaluated.

  15. Atomic Ordering in InGaN Alloys within Nanowire Heterostructures.

    PubMed

    Woo, Steffi Y; Bugnet, Matthieu; Nguyen, Hieu P T; Mi, Zetian; Botton, Gianluigi A

    2015-10-14

    Ternary III-nitride based nanowires (NWs) are promising for optoelectronic applications by offering advantageous design and control over composition, structure, and strain. Atomic-level chemical ordering in wurtzite InGaN alloys along the c-plane direction with a 1:1 periodicity within InGaN/GaN NW heterostructures was investigated by scanning transmission electron microscopy. Atomic-number-sensitive imaging contrast was used to simultaneously assign the In-rich and Ga-rich planes and determine the crystal polarity to differentiate unique sublattice sites. The nonrandom occupation of the c-planes in the InGaN alloys is confirmed by the occurrence of additional superlattice spots in the diffraction pattern within the ternary alloy. Compositional modulations in the ordered InGaN was further studied using atomic-resolution elemental mapping, outlining the substantial In-enrichment. Confirming the preferential site occupation of In-atoms provides experimental validation for the previous theoretical model of ordered InGaN alloys in bulk epilayers based on differences in surface site energy. Therefore, this study strongly suggests that atomic ordering in InGaN has a surface energetics-induced origin. Optimization of atomic ordering, in particular in III-nitride NW heterostructures, could be an alternative design tool toward desirable structural and compositional properties for various device applications operating at longer visible wavelengths.

  16. Negative differential resistance in boron nitride graphene heterostructures: physical mechanisms and size scaling analysis.

    PubMed

    Zhao, Y; Wan, Z; Xu, X; Patil, S R; Hetmaniuk, U; Anantram, M P

    2015-01-01

    Hexagonal boron nitride (hBN) is drawing increasing attention as an insulator and substrate material to develop next generation graphene-based electronic devices. In this paper, we investigate the quantum transport in heterostructures consisting of a few atomic layers thick hBN film sandwiched between graphene nanoribbon electrodes. We show a gate-controllable vertical transistor exhibiting strong negative differential resistance (NDR) effect with multiple resonant peaks, which stay pronounced for various device dimensions. We find two distinct mechanisms that are responsible for NDR, depending on the gate and applied biases, in the same device. The origin of first mechanism is a Fabry-Pérot like interference and that of the second mechanism is an in-plane wave vector matching when the Dirac points of the electrodes align. The hBN layers can induce an asymmetry in the current-voltage characteristics which can be further modulated by an applied bias. We find that the electron-phonon scattering suppresses the first mechanism whereas the second mechanism remains relatively unaffected. We also show that the NDR features are tunable by varying device dimensions. The NDR feature with multiple resonant peaks, combined with ultrafast tunneling speed provides prospect for the graphene-hBN-graphene heterostructure in the high-performance electronics. PMID:25991076

  17. Thickness dependence of voltage-driven magnetization switching in FeCo/PI/piezoelectric actuator heterostructures

    NASA Astrophysics Data System (ADS)

    Cui, B. S.; Guo, X. B.; Wu, K.; Li, D.; Zuo, Y. L.; Xi, L.

    2016-03-01

    Strain mediated magnetization switching of ferromagnetic/substrate/piezoelectric actuator heterostructures has become a hot issue due to the advantage of low-power consumption. In this work, Fe65Co35 thin films were deposited on a flexible polyamides (PI) substrate, which has quite low Young’s module (~4 GPa for PI as compared to ~180 GPa for Si) and benefits from complete transfer of the strain from the piezoelectric actuator to magnetic thin films. A complete 90° transition of the magnetic easy axis was realized in 50 nm thick FeCo films under the voltage of 70 V, while a less than 90° rotation angle of the magnetic easy axis direction was observed in other samples, which was ascribed to the distribution of the anisotropy field and/or the orthogonal misalignment between stress induced anisotropy and original uniaxial anisotropy. A model considering two uniaxial anisotropies with orthogonal arrangement was used to quantitatively understand the observed results and the linear-like voltage dependent anisotropy field, especially for 10 nm FeCo films, in which the switching mechanism along the easy axis direction can be explained by the domain wall depinning model. It indicates that the magnetic domain-wall movement velocity may be controlled by strain through tuning the energy barrier of the pinning in heterostructures. Moreover, voltage-driven 90° magnetization switching with low-power consumption was achieved in this work.

  18. Ultrahigh sensitivity and gain white light photodetector based on GaTe/Sn:CdS nanoflake/nanowire heterostructures.

    PubMed

    Zhou, Weichang; Zhou, Yong; Peng, Yuehua; Zhang, Yong; Yin, Yanling; Tang, Dongsheng

    2014-11-01

    Optoelectronic diode based on PN heterostructure is one of the most fundamental device building blocks with extensive applications. Here we reported the fabrication and optoelectronic properties of GaTe/Sn : CdS nanoflake/nanowire PN heterojunction photodetectors. With high quality contacts between metal electrodes and Sn : CdS or GaTe, the electrical measurement of GaTe/Sn : CdS hybrid heterojunction under dark condition demonstrates an excellent diode characteristic with well-defined current rectification behavior. The photocurrent increases drastically under LED white light as well as red, green, UV illumination. The on-off ratio of current is about 100 for forward bias and 3000 for reverse bias, which clearly indicates the ultrahigh sensitivity of the heterostructure photodetector to white light. The responsivity and optical gain are determined to be 607 A W(-1) and (1.06-2.16) × 10(5)%, which is higher than previous reports of single GaTe or CdS nanostructures. Combination the Ids-Vds curves under different illumination power with energy band diagrams, we assign that both the light modulation effect under forward and reverse bias and the surface molecular oxygen adsorption/desorption mechanism are dominant to the electrical transport behavior of GaTe/Sn : CdS heterojunction. This heterostructure photodetector also shows good stability and fast response speed. Both the high photosensibility and fast response time described in the present study suggest strongly that the GaTe/Sn : CdS hybrid heterostructure is a promising candidate for photodetection, optical sensing and switching devices.

  19. Spin filtering in a δ-doped magnetic-electric-barrier nanostructure

    SciTech Connect

    Li, Shuai; Lu, Mao-Wang Jiang, Ya-Qing; Chen, Sai-Yan

    2014-09-15

    We report a theoretical study on spin-polarized transport in a δ-doped magnetic-electric-barrier nanostructure, which can be realized in experiments by depositing two ferromagnetic stripes on top and bottom of a semiconductor heterostructure under an applied voltage and by using atomic layer doping technique. The spin-polarized behavior of the electron in this device is found to be quite sensitive to the δ-doping. One can conveniently tune the degree of the electron spin polarization by adjusting the weight and/or position of the δ-doping. Thus, the involved nansosystem can be employed as a controllable spin filter, which may be helpful for exploiting new spin-polarized source for spintronics applications.

  20. Enhanced optoelectronic performances of vertically aligned hexagonal boron nitride nanowalls-nanocrystalline diamond heterostructures

    PubMed Central

    Sankaran, Kamatchi Jothiramalingam; Hoang, Duc Quang; Kunuku, Srinivasu; Korneychuk, Svetlana; Turner, Stuart; Pobedinskas, Paulius; Drijkoningen, Sien; Van Bael, Marlies K.; D’ Haen, Jan; Verbeeck, Johan; Leou, Keh-Chyang; Lin, I-Nan; Haenen, Ken

    2016-01-01

    Field electron emission (FEE) properties of vertically aligned hexagonal boron nitride nanowalls (hBNNWs) grown on Si have been markedly enhanced through the use of nitrogen doped nanocrystalline diamond (nNCD) films as an interlayer. The FEE properties of hBNNWs-nNCD heterostructures show a low turn-on field of 15.2 V/μm, a high FEE current density of 1.48 mA/cm2 and life-time up to a period of 248 min. These values are far superior to those for hBNNWs grown on Si substrates without the nNCD interlayer, which have a turn-on field of 46.6 V/μm with 0.21 mA/cm2 FEE current density and life-time of 27 min. Cross-sectional TEM investigation reveals that the utilization of the diamond interlayer circumvented the formation of amorphous boron nitride prior to the growth of hexagonal boron nitride. Moreover, incorporation of carbon in hBNNWs improves the conductivity of hBNNWs. Such a unique combination of materials results in efficient electron transport crossing nNCD-to-hBNNWs interface and inside the hBNNWs that results in enhanced field emission of electrons. The prospective application of these materials is manifested by plasma illumination measurements with lower threshold voltage (370 V) and longer life-time, authorizing the role of hBNNWs-nNCD heterostructures in the enhancement of electron emission. PMID:27404130

  1. Experimental investigation of boron diffusion in silicon/silicon germanium heterostructures

    NASA Astrophysics Data System (ADS)

    Kuo, Paohua

    Silicon (Si) is the most widely used semiconductor material in the electronics industry today. Incorporating germanium (Ge) into silicon adds bandgap engineering capability to the advanced technology base and manufacturing economies of silicon. Many researchers have recently demonstrated the potential of Si 1-xGex alloys for extending the performance limits of silicon-based devices including bipolar transistors as well as field-effect transistors. One of the most studied devices is the n-Si/p-Si1-x Gex/n-Si heterojunction bipolar transistor which includes a boron-doped Si1-xGex base layer. For optimal device and fabrication process design, it is important to have an understanding of boron diffusion in the Si/Si1-xGex heterostructure. The objectives of this work are the experimental characterization and modeling of boron diffusion in Si/Si1-xGex heterostructures. Boron diffusion in Si/Si1-xGex was characterized as a function of various processing parameters (anneal temperature, time, and ambient) and materials parameters (germanium content x, boron concentration, and macroscopic strain). Boron diffusivity in strained Si1-xGe x was observed, in general, to decrease with increasing germanium content for all annealing conditions and over large ranges of boron concentration investigated. Furthermore, boron diffusion was shown to depend primarily on germanium content rather than macroscopic strain. The segregation phenomenon across a Si/Si1-xGex heterointerface was also characterized and models for boron diffusion in Si1-x Gex are proposed.

  2. Enhanced optoelectronic performances of vertically aligned hexagonal boron nitride nanowalls-nanocrystalline diamond heterostructures.

    PubMed

    Sankaran, Kamatchi Jothiramalingam; Hoang, Duc Quang; Kunuku, Srinivasu; Korneychuk, Svetlana; Turner, Stuart; Pobedinskas, Paulius; Drijkoningen, Sien; Van Bael, Marlies K; D' Haen, Jan; Verbeeck, Johan; Leou, Keh-Chyang; Lin, I-Nan; Haenen, Ken

    2016-01-01

    Field electron emission (FEE) properties of vertically aligned hexagonal boron nitride nanowalls (hBNNWs) grown on Si have been markedly enhanced through the use of nitrogen doped nanocrystalline diamond (nNCD) films as an interlayer. The FEE properties of hBNNWs-nNCD heterostructures show a low turn-on field of 15.2 V/μm, a high FEE current density of 1.48 mA/cm(2) and life-time up to a period of 248 min. These values are far superior to those for hBNNWs grown on Si substrates without the nNCD interlayer, which have a turn-on field of 46.6 V/μm with 0.21 mA/cm(2) FEE current density and life-time of 27 min. Cross-sectional TEM investigation reveals that the utilization of the diamond interlayer circumvented the formation of amorphous boron nitride prior to the growth of hexagonal boron nitride. Moreover, incorporation of carbon in hBNNWs improves the conductivity of hBNNWs. Such a unique combination of materials results in efficient electron transport crossing nNCD-to-hBNNWs interface and inside the hBNNWs that results in enhanced field emission of electrons. The prospective application of these materials is manifested by plasma illumination measurements with lower threshold voltage (370 V) and longer life-time, authorizing the role of hBNNWs-nNCD heterostructures in the enhancement of electron emission. PMID:27404130

  3. Enhanced optoelectronic performances of vertically aligned hexagonal boron nitride nanowalls-nanocrystalline diamond heterostructures

    NASA Astrophysics Data System (ADS)

    Sankaran, Kamatchi Jothiramalingam; Hoang, Duc Quang; Kunuku, Srinivasu; Korneychuk, Svetlana; Turner, Stuart; Pobedinskas, Paulius; Drijkoningen, Sien; van Bael, Marlies K.; D' Haen, Jan; Verbeeck, Johan; Leou, Keh-Chyang; Lin, I.-Nan; Haenen, Ken

    2016-07-01

    Field electron emission (FEE) properties of vertically aligned hexagonal boron nitride nanowalls (hBNNWs) grown on Si have been markedly enhanced through the use of nitrogen doped nanocrystalline diamond (nNCD) films as an interlayer. The FEE properties of hBNNWs-nNCD heterostructures show a low turn-on field of 15.2 V/μm, a high FEE current density of 1.48 mA/cm2 and life-time up to a period of 248 min. These values are far superior to those for hBNNWs grown on Si substrates without the nNCD interlayer, which have a turn-on field of 46.6 V/μm with 0.21 mA/cm2 FEE current density and life-time of 27 min. Cross-sectional TEM investigation reveals that the utilization of the diamond interlayer circumvented the formation of amorphous boron nitride prior to the growth of hexagonal boron nitride. Moreover, incorporation of carbon in hBNNWs improves the conductivity of hBNNWs. Such a unique combination of materials results in efficient electron transport crossing nNCD-to-hBNNWs interface and inside the hBNNWs that results in enhanced field emission of electrons. The prospective application of these materials is manifested by plasma illumination measurements with lower threshold voltage (370 V) and longer life-time, authorizing the role of hBNNWs-nNCD heterostructures in the enhancement of electron emission.

  4. Doping-Induced Tunable Wettability and Adhesion of Graphene.

    PubMed

    Ashraf, Ali; Wu, Yanbin; Wang, Michael Cai; Yong, Keong; Sun, Tao; Jing, Yuhang; Haasch, Richard T; Aluru, Narayana R; Nam, SungWoo

    2016-07-13

    We report that substrate doping-induced charge carrier density modulation leads to the tunable wettability and adhesion of graphene. Graphene's water contact angle changes by as much as 13° as a result of a 300 meV change in doping level. Upon either n- or p-type doping with subsurface polyelectrolytes, graphene exhibits increased hydrophilicity. Adhesion force measurements using a hydrophobic self-assembled monolayer-coated atomic force microscopy probe reveal enhanced attraction toward undoped graphene, consistent with wettability modulation. This doping-induced wettability modulation is also achieved via a lateral metal-graphene heterojunction or subsurface metal doping. Combined first-principles and atomistic calculations show that doping modulates the binding energy between water and graphene and thus increases its hydrophilicity. Our study suggests for the first time that the doping-induced modulation of the charge carrier density in graphene influences its wettability and adhesion. This opens up unique and new opportunities for the tunable wettability and adhesion of graphene for advanced coating materials and transducers.

  5. Organic heterostructures based on arylenevinylene oligomers deposited by MAPLE

    NASA Astrophysics Data System (ADS)

    Socol, M.; Preda, N.; Vacareanu, L.; Grigoras, M.; Socol, G.; Mihailescu, I. N.; Stanculescu, F.; Jelinek, M.; Stanculescu, A.; Stoicanescu, M.

    2014-05-01

    Organic heterostructures were fabricated by matrix assisted pulsed laser evaporation (MAPLE) method using arylenevinylene oligomers based on triphenylamine (P78)/carbazole (P13) group and tris(8-hydroxyquinolinato)aluminum salt (Alq3). Optical properties of the organic multilayer structures were characterized by spectroscopic techniques: FTIR, UV-vis and photoluminescence (PL). A good transparency (over 60%) was remarked for the structures with two organic layers in the 550-800 nm range. Photoluminescence (PL) spectra proved that the emission characteristics of the materials have been preserved. I-V characteristics of (ITO/oligomer/Alq3/Al and ITO/Alq3/Al) heterostructures were symmetrically while rectifying properties of these heterostructures have not been observed. A comparison between the heterostructures made of layers with different thickness reveals that the higher current (8 × 10-6 A at 1 V) was obtained for the ITO/P78/Alq3/Al heterostructure, which is characterized by a larger thickness of the double organic layer. AFM measurements revealed a similar topography while RMS values of the reported structures depend on the organic material.

  6. Photoconductivity of Graphene in Proximity to La AlO3 /Sr TiO3 Heterostructures: Phenomenon and Photosensor Applications

    NASA Astrophysics Data System (ADS)

    Cheng, Long; Fan, Xiaodong; Wei, Laiming; Lu, Juanjuan; Liang, Haixing; Qi, Ji; Zeng, Changgan

    2016-07-01

    The proximal coupling between graphene and transition-metal-oxide heterostructures may integrate their unique features and further generate emergent states. Using the photoconductivity of graphene as an effective probe, we demonstrate the existence of a built-in polar field within the La AlO3 layer of the La AlO3 /Sr TiO3 heterostructures for both conducting and insulating La AlO3 /Sr TiO3 interfaces. Such a polar field is a prerequisite for the validity of the electronic reconstruction mechanism for the interfacial conductivity. The built-in polar field is reflected by the hole doping in the graphene in proximity to the La AlO3 /Sr TiO3 induced by pulsed deep-ultraviolet illumination regardless of the graphene's carrier type. These photoresponse characteristics also render the graphene /La AlO3 /Sr TiO3 hybrid system a convenient deep-ultraviolet sensor. Moreover, we design an efficient broad-spectrum photodetector benefiting from the large in-plane electric field in graphene across the boundary between the graphene/La AlO3 /Sr TiO3 and graphene/Sr TiO3 . Our findings may provide clues to the design of photosensors based on the hybrid structures of graphene and oxide heterostructures.

  7. Optical modulators with 2D layered materials

    NASA Astrophysics Data System (ADS)

    Sun, Zhipei; Martinez, Amos; Wang, Feng

    2016-04-01

    Light modulation is an essential operation in photonics and optoelectronics. With existing and emerging technologies increasingly demanding compact, efficient, fast and broadband optical modulators, high-performance light modulation solutions are becoming indispensable. The recent realization that 2D layered materials could modulate light with superior performance has prompted intense research and significant advances, paving the way for realistic applications. In this Review, we cover the state of the art of optical modulators based on 2D materials, including graphene, transition metal dichalcogenides and black phosphorus. We discuss recent advances employing hybrid structures, such as 2D heterostructures, plasmonic structures, and silicon and fibre integrated structures. We also take a look at the future perspectives and discuss the potential of yet relatively unexplored mechanisms, such as magneto-optic and acousto-optic modulation.

  8. Van der Waals heterostructures and devices

    NASA Astrophysics Data System (ADS)

    Liu, Yuan; Weiss, Nathan O.; Duan, Xidong; Cheng, Hung-Chieh; Huang, Yu; Duan, Xiangfeng

    2016-09-01

    Two-dimensional layered materials (2DLMs) have been a central focus of materials research since the discovery of graphene just over a decade ago. Each layer in 2DLMs consists of a covalently bonded, dangling-bond-free lattice and is weakly bound to neighbouring layers by van der Waals interactions. This makes it feasible to isolate, mix and match highly disparate atomic layers to create a wide range of van der Waals heterostructures (vdWHs) without the constraints of lattice matching and processing compatibility. Exploiting the novel properties in these vdWHs with diverse layering of metals, semiconductors or insulators, new designs of electronic devices emerge, including tunnelling transistors, barristors and flexible electronics, as well as optoelectronic devices, including photodetectors, photovoltaics and light-emitting devices with unprecedented characteristics or unique functionalities. We review the recent progress and challenges, and offer our perspective on the exploration of 2DLM-based vdWHs for future application in electronics and optoelectronics.

  9. Superconducting heterostructures: from antipinning to pinning potentials

    NASA Astrophysics Data System (ADS)

    Carreira, S. J.; Chiliotte, C.; Bekeris, V.; Rosen, Y. J.; Monton, C.; Schuller, Ivan K.

    2014-08-01

    We study vortex lattice dynamics in a heterostructure that combines two type-II superconductors: a niobium film and a dense triangular array of submicrometric vanadium (V) pillars. Magnetic ac susceptibility measurements reveal a sudden increase in ac penetration, related to an increase in vortex mobility above a magnetic field, {{H}^{*}}\\left( T \\right), that decreases linearly with temperature. Additionally, temperature independent matching effects that occur when the number of vortices in the sample is an integer of the number of V pillars, strongly reduce vortex mobility, and were observed for the first and second matching fields, {{H}_{1}} and {{H}_{2}}. The angular dependence of {{H}_{1}}, {{H}_{2}} and {{H}^{*}}\\left( T \\right) shows that matching is determined by the normal applied field component, while {{H}^{*}}\\left( T \\right) is independent of the applied field orientation. This important result identifies {{H}^{*}}\\left( T \\right) with the critical field boundary for the normal to superconducting transition of V pillars. Below {{H}^{*}}\\left( T \\right), superconducting V pillars repel vortices, and the array becomes an ‘antipinning’ landscape that is more effective in reducing vortex mobility than the ‘pinning’ landscape of the normal V sites above {{H}^{*}}\\left( T \\right). Matching effects are observed both below and above {{H}^{*}}\\left( T \\right), implying the presence of ordered vortex configurations for ‘antipinning’ or ‘pinning’ arrays.

  10. Heterostructure optimization for increasing LED efficiency

    NASA Astrophysics Data System (ADS)

    Rabinovich, Oleg; Legotin, Sergey; Didenko, Sergey; Yakimov, Evgeniy; Osipov, Yuriy; Fedorchenko, Irina

    2016-05-01

    Computer simulations were performed to increase the quantum efficiency of LED by optimizing the nanoheterostructure (NH). Furthermore, the InGaN and AlGaP NHs for LEDs were optimized. On the basis of the optimum NH, ways to further increase the efficiency and the influence of impurities and indium atoms doped into barriers between quantum wells were investigated. The optimum impurity and indium atom concentrations to achieve higher flux were determined.

  11. Transfer matrix theory of monolayer graphene/bilayer graphene heterostructure superlattice

    SciTech Connect

    Wang, Yu

    2014-10-28

    We have formulated a transfer matrix method to investigate electronic properties of graphene heterostructure consisting of monolayer graphene and bilayer counterpart. By evaluating transmission, conductance, and band dispersion, we show that, irrespective of the different carrier chiralities in monolayer graphene and bilayer graphene, superlattice consisting of biased bilayer graphene barrier and monolayer graphene well can mimic the electronic properties of conventional semiconductor superlattice, displaying the extended subbands in the quantum tunneling regime and producing anisotropic minigaps for the classically allowed transport. Due to the lateral confinement, the lowest mode has shifted away from the charge neutral point of monolayer graphene component, opening a sizeable gap in concerned structure. Following the gate-field and geometry modulation, all electronic states and gaps between them can be externally engineered in an electric-controllable strategy.

  12. Engineered Mott ground state in a LaTiO3+δ/LaNiO3 heterostructure

    DOE PAGES

    Cao, Yanwei; Liu, Xiaoran; Kareev, M.; Choudhury, D.; Middey, S.; Meyers, D.; Kim, J. -W.; Ryan, P. J.; Freeland, J. W.; Chakhalian, J.

    2016-01-21

    In pursuit of creating cuprate-like electronic and orbital structures, artificial heterostructures based on LaNiO3 have inspired a wealth of exciting experimental and theoretical results. However, to date there is a very limited experimental understanding of the electronic and orbital states emerging from interfacial charge transfer and their connections to the modified band structure at the interface. Towards this goal, we have synthesized a prototypical superlattice composed of a correlated metal LaNiO3 and a doped Mott insulator LaTiO3+δ, and investigated its electronic structure by resonant X-ray absorption spectroscopy combined with X-ray photoemission spectroscopy, electrical transport and theory calculations. The heterostructure exhibitsmore » interfacial charge transfer from Ti to Ni sites, giving rise to an insulating ground state with orbital polarization and eg orbital band splitting. Here, our findings demonstrate how the control over charge at the interface can be effectively used to create exotic electronic, orbital and spin states.« less

  13. Interface designed MoS2/GaAs heterostructure solar cell with sandwich stacked hexagonal boron nitride.

    PubMed

    Lin, Shisheng; Li, Xiaoqiang; Wang, Peng; Xu, Zhijuan; Zhang, Shengjiao; Zhong, Huikai; Wu, Zhiqian; Xu, Wenli; Chen, Hongsheng

    2015-01-01

    MoS2 is a layered two-dimensional semiconductor with a direct band gap of 1.8 eV. The MoS2/bulk semiconductor system offers a new platform for solar cell device design. Different from the conventional bulk p-n junctions, in the MoS2/bulk semiconductor heterostructure, static charge transfer shifts the Fermi level of MoS2 toward that of bulk semiconductor, lowering the barrier height of the formed junction. Herein, we introduce hexagonal boron nitride (h-BN) into MoS2/GaAs heterostructure to suppress the static charge transfer, and the obtained MoS2/h-BN/GaAs solar cell exhibits an improved power conversion efficiency of 5.42%. More importantly, the sandwiched h-BN makes the Fermi level tuning of MoS2 more effective. By employing chemical doping and electrical gating into the solar cell device, PCE of 9.03% is achieved, which is the highest among all the reported monolayer transition metal dichalcogenide based solar cells. PMID:26458358

  14. All Chemical Vapor Deposition Growth of MoS2:h-BN Vertical van der Waals Heterostructures.

    PubMed

    Wang, Shanshan; Wang, Xiaochen; Warner, Jamie H

    2015-05-26

    Vertical van der Waals heterostructures are formed when different 2D crystals are stacked on top of each other. Improved optical properties arise in semiconducting transition metal dichalcogenide (TMD) 2D materials, such as MoS2, when they are stacked onto the insulating 2D hexagonal boron nitride (h-BN). Most work to date has required mechanical exfoliation of at least one of the TMDs or h-BN materials to form these semiconductor:insulator structures. Here, we report a direct all-CVD process for the fabrication of high-quality monolayer MoS2:h-BN vertical heterostructured films with isolated MoS2 domains distributed across 1 cm. This is enabled by the use of few-layer h-BN films that are more robust against decomposition than monolayer h-BN during the MoS2 growth process. The MoS2 domains exhibit different growth dynamics on the h-BN surfaces compared to bare SiO2, confirming that there is strong interaction between the MoS2 and underlying h-BN. Raman and photoluminescence spectroscopies of CVD-grown MoS2 are compared to transferred MoS2 on both types of substrates, and our results show directly grown MoS2 on h-BN films have smaller lattice strain, lower doping level, cleaner and sharper interfaces, and high-quality interlayer contact.

  15. Photoinduced charge transfer in ZnO/Cu(2)O heterostructure films studied by surface photovoltage technique.

    PubMed

    Jiang, Tengfei; Xie, Tengfeng; Zhang, Yu; Chen, Liping; Peng, Linlin; Li, Haiyan; Wang, Dejun

    2010-12-21

    ZnO/Cu(2)O heterostructure films were prepared by a two-step electrodeposition method in aqueous solution on fluorine-doped tin oxide (FTO) substrates. Scanning electron microscopy (SEM), X-ray powder diffraction (XRD) and UV-vis transmission measurements were utilized to characterize the films. Surface photovoltage (SPV) technique was used to investigate the process of photoinduced charge transfer. The results show that there is an electric field located at the interface between ZnO and Cu(2)O film and the photoinduced electrons in Cu(2)O film inject into ZnO under the effect of interfacial electric field with visible light irradiation. While under ultraviolet light illumination, the photoinduced electrons in Cu(2)O film accumulate at the surface of Cu(2)O film instead of injecting into ZnO under the action of surface built-in electric field of Cu(2)O film. The work function measurements confirm that the direction of interfacial electric field is from ZnO to Cu(2)O. These results are help to future design of high performance heterostructure photovoltaic devices.

  16. Emission properties of InGaAs/GaAs heterostructures with quantum wells and dots after irradiation with neutrons

    SciTech Connect

    Baidus, N. V.; Vikhrova, O. V. Zvonkov, B. N.; Malysheva, E. I.; Trufanov, A. N.

    2015-03-15

    The effect of neutron radiation on the luminescence of InGaAs/GaAs heterostructures with quantum wells and quantum dots is studied. It is found that neutron radiation results both in the formation of defects and in the radiation-induced annealing of growth-related defects. Quantum dots are more stable to neutron radiation in comparison with quantum wells. It is shown that the layer of InGaAs/GaAs quantum dots located near the surface is less sensitive to irradiation with neutrons compared with a similar layer located in the bulk. In the first case, one can observe an increase in the photoluminescence and electroluminescence intensities after irradiation with neutrons, which is related to the effects of radiation-induced annealing. The pronounced effect of elastic strains in the InGaAs/GaAs quantum wells on the extent of quenching of the photoluminescence intensity upon irradiation with neutrons is revealed. In heterostructures with quantum wells, the effect of radiation-induced annealing manifests itself in a shift of the photoluminescence peak to longer wavelengths as a result of a decrease in elastic strains upon irradiation with neutrons. Doping of the GaAs buffer layer with silicon also reduces the value of this spectral shift.

  17. New planar process for implementation of p-on-n HgCdTe heterostructure infrared photodectors

    NASA Astrophysics Data System (ADS)

    Bahir, Gad; Garber, Valery; Dust, Arie

    2003-01-01

    We report on a new, simple process to fabricate planar Hg1-xCdyTe/Hg1-xCdxTe heterostructure photodiodes with p-on-n configuration. The material used for this demonstration was a double-layer p-on-n heterostructure that was grown by a liquid-phase-epitaxy technique. The p-on-n planar devices consisted of an arsenic-doped p-type cap epilayer on top of a long-wavelength IR n-type active epilayer. The ion-beam-miling p-type conversion effect was used to delineate the active device element, and to isolate the planar device. Detailed analysis of the current characteristics of these diodes as a funciton of temperatuer, show that they have high performance, and that their dark current is diffusion-limited down to 60 K. The results show that over a wide range of cut-off wavelengths, the RoA product values are close to the theoretical limit. Light Beam Induced Current technique was used to characterize the lateral and vertical dimensions of the ion beam milling induced junction. Electro-optic properties of a 2D array of small diodes with a 40-μm pitch are presented, and demonstrate the potential of the new process for implementation of 2D arrays.

  18. Engineered Mott ground state in a LaTiO3+δ/LaNiO3 heterostructure

    PubMed Central

    Cao, Yanwei; Liu, Xiaoran; Kareev, M.; Choudhury, D.; Middey, S.; Meyers, D.; Kim, J.-W.; Ryan, P. J.; Freeland, J.W.; Chakhalian, J.

    2016-01-01

    In pursuit of creating cuprate-like electronic and orbital structures, artificial heterostructures based on LaNiO3 have inspired a wealth of exciting experimental and theoretical results. However, to date there is a very limited experimental understanding of the electronic and orbital states emerging from interfacial charge transfer and their connections to the modified band structure at the interface. Towards this goal, we have synthesized a prototypical superlattice composed of a correlated metal LaNiO3 and a doped Mott insulator LaTiO3+δ, and investigated its electronic structure by resonant X-ray absorption spectroscopy combined with X-ray photoemission spectroscopy, electrical transport and theory calculations. The heterostructure exhibits interfacial charge transfer from Ti to Ni sites, giving rise to an insulating ground state with orbital polarization and eg orbital band splitting. Our findings demonstrate how the control over charge at the interface can be effectively used to create exotic electronic, orbital and spin states. PMID:26791402

  19. Interface designed MoS2/GaAs heterostructure solar cell with sandwich stacked hexagonal boron nitride

    PubMed Central

    Lin, Shisheng; Li, Xiaoqiang; Wang, Peng; Xu, Zhijuan; Zhang, Shengjiao; Zhong, Huikai; Wu, Zhiqian; Xu, Wenli; Chen, Hongsheng

    2015-01-01

    MoS2 is a layered two-dimensional semiconductor with a direct band gap of 1.8 eV. The MoS2/bulk semiconductor system offers a new platform for solar cell device design. Different from the conventional bulk p-n junctions, in the MoS2/bulk semiconductor heterostructure, static charge transfer shifts the Fermi level of MoS2 toward that of bulk semiconductor, lowering the barrier height of the formed junction. Herein, we introduce hexagonal boron nitride (h-BN) into MoS2/GaAs heterostructure to suppress the static charge transfer, and the obtained MoS2/h-BN/GaAs solar cell exhibits an improved power conversion efficiency of 5.42%. More importantly, the sandwiched h-BN makes the Fermi level tuning of MoS2 more effective. By employing chemical doping and electrical gating into the solar cell device, PCE of 9.03% is achieved, which is the highest among all the reported monolayer transition metal dichalcogenide based solar cells. PMID:26458358

  20. Engineered Mott ground state in a LaTiO3+δ/LaNiO3 heterostructure

    NASA Astrophysics Data System (ADS)

    Cao, Yanwei; Liu, Xiaoran; Kareev, M.; Choudhury, D.; Middey, S.; Meyers, D.; Kim, J.-W.; Ryan, P. J.; Freeland, J. W.; Chakhalian, J.

    2016-01-01

    In pursuit of creating cuprate-like electronic and orbital structures, artificial heterostructures based on LaNiO3 have inspired a wealth of exciting experimental and theoretical results. However, to date there is a very limited experimental understanding of the electronic and orbital states emerging from interfacial charge transfer and their connections to the modified band structure at the interface. Towards this goal, we have synthesized a prototypical superlattice composed of a correlated metal LaNiO3 and a doped Mott insulator LaTiO3+δ, and investigated its electronic structure by resonant X-ray absorption spectroscopy combined with X-ray photoemission spectroscopy, electrical transport and theory calculations. The heterostructure exhibits interfacial charge transfer from Ti to Ni sites, giving rise to an insulating ground state with orbital polarization and eg orbital band splitting. Our findings demonstrate how the control over charge at the interface can be effectively used to create exotic electronic, orbital and spin states.

  1. Postsynthesis of h‐BN/Graphene Heterostructures Inside a STEM

    PubMed Central

    Tizei, Luiz H. G.; Sato, Yohei; Lin, Yung‐Chang; Yeh, Chao‐Hui; Chiu, Po‐Wen; Terauchi, Masami; Iijima, Sumio

    2015-01-01

    Combinations of 2D materials with different physical properties can form heterostructures with modified electrical, mechanical, magnetic, and optical properties. The direct observation of a lateral heterostructure synthesis is reported by epitaxial in‐plane graphene growth from the step‐edge of hexagonal BN (h‐BN) within a scanning transmission electron microscope chamber. Residual hydrocarbon in the chamber is the carbon source. The growth interface between h‐BN and graphene is atomically identified as largely N–C bonds. This postgrowth method can form graphene nanoribbons connecting two h‐BN domains with different twisting angles, as well as isolated carbon islands with arbitrary shapes embedded in the h‐BN layer. The electronic properties of the vertically stacked h‐BN/graphene heterostructures are investigated by electron energy‐loss spectroscopy (EELS). Low‐loss EELS analysis of the dielectric response suggests a robust coupling effect between the graphene and h‐BN layers. PMID:26618896

  2. Electron scattering times in ZnO based polar heterostructures

    SciTech Connect

    Falson, J.; Kozuka, Y.; Smet, J. H.; Arima, T.; Tsukazaki, A.; Kawasaki, M.

    2015-08-24

    The remarkable historic advances experienced in condensed matter physics have been enabled through the continued exploration and proliferation of increasingly richer and cleaner material systems. In this work, we report on the scattering times of charge carriers confined in state-of-the-art MgZnO/ZnO heterostructures displaying electron mobilities in excess of 10{sup 6} cm{sup 2}/V s. Through an examination of low field quantum oscillations, we obtain the effective mass of charge carriers, along with the transport and quantum scattering times. These times compare favorably with high mobility AlGaAs/GaAs heterostructures, suggesting the quality of MgZnO/ZnO heterostructures now rivals that of traditional semiconductors.

  3. Spontaneous stacking faults in van der Waals heterostructures

    NASA Astrophysics Data System (ADS)

    Boussinot, G.

    2016-08-01

    The rapid developments in the manipulation of two-dimensional monoatomic layers such as graphene or h-BN allow one to create heterostructures consisting of possibly many chemically different layers, stacked owing to van der Waals attraction. We propose a Frenkel-Kontorova model including a transverse degree of freedom in order to describe local deformations in these heterostructures. We study the case where two dissimilar monolayers are alternatively stacked, and find that stacking faults may emerge spontaneously for a large enough number of stacked layers as a result of the competition between adhesion and elastic energies. This symmetry-breaking transition should become of fundamental importance for the description of three-dimensional van der Waals heterostructures as soon as a precise control on the lattice orientation of the van der Waals layers is achieved.

  4. Orbital engineering in symmetry-breaking polar heterostructures.

    PubMed

    Disa, Ankit S; Kumah, Divine P; Malashevich, Andrei; Chen, Hanghui; Arena, Dario A; Specht, Eliot D; Ismail-Beigi, Sohrab; Walker, F J; Ahn, Charles H

    2015-01-16

    We experimentally demonstrate a novel approach to substantially modify orbital occupations and symmetries in electronically correlated oxides. In contrast to methods using strain or confinement, this orbital tuning is achieved by exploiting charge transfer and inversion symmetry breaking using atomically layered heterostructures. We illustrate the technique in the LaTiO_{3}-LaNiO_{3}-LaAlO_{3} system; a combination of x-ray absorption spectroscopy and ab initio theory reveals electron transfer and concomitant polar fields, resulting in a ∼50% change in the occupation of Ni d orbitals. This change is sufficiently large to remove the orbital degeneracy of bulk LaNiO_{3} and creates an electronic configuration approaching a single-band Fermi surface. Furthermore, we theoretically show that such three-component heterostructuring is robust and tunable by choice of insulator in the heterostructure, providing a general method for engineering orbital configurations and designing novel electronic systems.

  5. Selective spin transport through a quantum heterostructure: Transfer matrix method

    NASA Astrophysics Data System (ADS)

    Dey, Moumita; Maiti, Santanu K.

    2016-09-01

    In the present work, we propose that a one-dimensional quantum heterostructure composed of magnetic and non-magnetic (NM) atomic sites can be utilized as a spin filter for a wide range of applied bias voltage. A simple tight-binding framework is given to describe the conducting junction where the heterostructure is coupled to two semi-infinite one-dimensional NM electrodes. Based on transfer matrix method, all the calculations are performed numerically which describe two-terminal spin-dependent transmission probability along with junction current through the wire. Our detailed analysis may provide fundamental aspects of selective spin transport phenomena in one-dimensional heterostructures at nanoscale level.

  6. Two-dimensional heterostructures: fabrication, characterization, and application

    DOE PAGES

    Wang, Hong; Liu, Fucai; Fu, Wei; Fang, Zheyu; Zhou, Wu; Liu, Zheng

    2014-08-13

    Two-dimensional (2D) materials such as graphene, hexagonal boron nitrides (hBN), and transition metal dichalcogenides (TMDs, e.g., MoS2) have attracted considerable attention in the past few years because of their novel properties and versatile potential applications. These 2D layers can be integrated into a monolayer (lateral 2D heterostructure) or a multilayer stack (vertical 2D heterostructure). The resulting artificial 2D structures provide access to new properties and applications beyond their component 2D atomic crystals and hence, they are emerging as a new exciting field of research. Lastly, in this article, we review recent progress on the fabrication, characterization, and applications of variousmore » 2D heterostructures.« less

  7. Two-dimensional heterostructures: fabrication, characterization, and application

    SciTech Connect

    Wang, Hong; Liu, Fucai; Fu, Wei; Fang, Zheyu; Zhou, Wu; Liu, Zheng

    2014-08-13

    Two-dimensional (2D) materials such as graphene, hexagonal boron nitrides (hBN), and transition metal dichalcogenides (TMDs, e.g., MoS2) have attracted considerable attention in the past few years because of their novel properties and versatile potential applications. These 2D layers can be integrated into a monolayer (lateral 2D heterostructure) or a multilayer stack (vertical 2D heterostructure). The resulting artificial 2D structures provide access to new properties and applications beyond their component 2D atomic crystals and hence, they are emerging as a new exciting field of research. Lastly, in this article, we review recent progress on the fabrication, characterization, and applications of various 2D heterostructures.

  8. Identifying microscopic mechanisms for hole traps in nitride heterostructures

    NASA Astrophysics Data System (ADS)

    Lyons, John; Gordon, Luke; Janotti, Anderson; van de Walle, Chris G.

    2014-03-01

    Some recent designs of nitride semiconductor devices employ heterostructures (such as N-face high-electron-mobility transistors) in which the electronic Fermi level is established near the valence-band maximum due to the influence of polarization fields. In many of these heterostructures, the presence of hole-trapping centers is thought to adversely affect device performance. This behavior has been observed in many different types of devices, and its physical origin remains unknown. Using first-principles calculations based on a hybrid functional, we investigate possible origins for this phenomenon. We explore both intrinsic defect candidates as well as impurities. With Schrödinger-Poisson simulations, we then investigate how the behavior of these species and their spatial distribution within the heterostructure layers is reflected in the performance of nitride semiconductor devices. This work was supported by the DEFINE MURI.

  9. High-frequency dynamics of hybrid oxide Josephson heterostructures

    NASA Astrophysics Data System (ADS)

    Komissinskiy, P.; Ovsyannikov, G. A.; Constantinian, K. Y.; Kislinski, Y. V.; Borisenko, I. V.; Soloviev, I. I.; Kornev, V. K.; Goldobin, E.; Winkler, D.

    2008-07-01

    We summarize our results on Josephson heterostructures Nb/Au/YBa2Cu3Ox that combine conventional (S) and oxide high- Tc superconductors with a dominant d -wave symmetry of the superconducting order parameter (D). The heterostructures were fabricated on (001) and (1 1 20) YBa2Cu3Ox films grown by pulsed laser deposition. The structural and surface studies of the (1 1 20) YBa2Cu3Ox thin films reveal nanofaceted surface structure with two facet domain orientations, which are attributed as (001) and (110)-oriented surfaces of YBa2Cu3Ox and result in S/D(001) and S/D(110) nanojunctions formed on the facets. Electrophysical properties of the Nb/Au/YBa2Cu3Ox heterostructures are investigated by the electrical and magnetic measurements at low temperatures and analyzed within the faceting scenario. The superconducting current-phase relation (CPR) of the heterostructures with finite first and second harmonics is derived from the Shapiro steps, which appear in the I-V curves of the heterostructures irradiated at frequencies up to 100 GHz. The experimental positions and amplitudes of the Shapiro steps are explained within the modified resistive Josephson junction model, where the second harmonic of the CPR and capacitance of the Josephson junctions are taken into account. We experimentally observe a crossover from a lumped to a distributed Josephson junction limit for the size of the heterostructures smaller than Josephson penetration depth. The effect is attributed to the variations of the harmonics of the superconducting CPR across the heterojunction, which may give rise to splintered vortices of magnetic flux quantum. Our investigations of parameters and phenomena that are specific for superconductors having d -wave symmetry of the superconducting order parameter may be of importance for applications such as high-frequency detectors and novel elements of a possible quantum computer.

  10. Boron-doped superlattices and Bragg mirrors in diamond

    SciTech Connect

    Fiori, A.; Bousquet, J.; Eon, D.; Omnès, F.; Bustarret, E.; Bellet-Amalric, E.

    2014-08-25

    A periodic modulation of the boron doping level of single crystal diamond multilayers over more than three orders of magnitude during epitaxial growth by microwave plasma-enhanced chemical vapor deposition is shown to yield Bragg mirrors in the visible. The thicknesses and doping level of the individual layers were controlled by in situ spectroscopic ellipsometry, enabling to tune the reflectance peak to the wavelength range of diamond color centers, such as NV{sup 0} or NV{sup −}. The crystalline quality, periodicity, and sharpness of the doping transitions in these doping superlattices over tens of periods were confirmed by high resolution X-ray diffraction.

  11. Doping Scheme of Semiconducting Atomic Chains

    NASA Technical Reports Server (NTRS)

    Toshishige, Yamada; Saini, Subhash (Technical Monitor)

    1998-01-01

    Atomic chains, precise structures of atomic scale created on an atomically regulated substrate surface, are candidates for future electronics. A doping scheme for intrinsic semiconducting Mg chains is considered. In order to suppress the unwanted Anderson localization and minimize the deformation of the original band shape, atomic modulation doping is considered, which is to place dopant atoms beside the chain periodically. Group I atoms are donors, and group VI or VII atoms are acceptors. As long as the lattice constant is long so that the s-p band crossing has not occurred, whether dopant atoms behave as donors or acceptors is closely related to the energy level alignment of isolated atomic levels. Band structures are calculated for Br-doped (p-type) and Cs-doped (n-type) Mg chains using the tight-binding theory with universal parameters, and it is shown that the band deformation is minimized and only the Fermi energy position is modified.

  12. Highly conductive coaxial SnO(2)-In(2)O(3) heterostructured nanowires for Li ion battery electrodes.

    PubMed

    Kim, Dong-Wan; Hwang, In-Sung; Kwon, S Joon; Kang, Hae-Yong; Park, Kyung-Soo; Choi, Young-Jin; Choi, Kyoung-Jin; Park, Jae-Gwan

    2007-10-01

    Novel SnO(2)-In(2)O(3) heterostructured nanowires were produced via a thermal evaporation method, and their possible nucleation/growth mechanism is proposed. We found that the electronic conductivity of the individual SnO(2)-In(2)O(3) nanowires was 2 orders of magnitude better than that of the pure SnO(2) nanowires, due to the formation of Sn-doped In(2)O(3) caused by the incorporation of Sn into the In(2)O(3) lattice during the nucleation and growth of the In(2)O(3) shell nanostructures. This provides the SnO(2)-In(2)O(3) nanowires with an outstanding lithium storage capacity, making them suitable for promising Li ion battery electrodes.

  13. Manipulation of emission energy in GaAs/AlGaAs core-shell nanowires with radial heterostructure

    SciTech Connect

    Barbosa, B. G.; Arakaki, H.; Souza, C. A. de; Pusep, Yu. A.

    2014-03-21

    Photoluminescence was studied in GaAs/AlGaAs nanowires (NWs) with different radial heterostructures. We demonstrated that manipulation of the emission energy may be achieved by appropriate choice of the shell structure. The emission at highest energy is generated in the NWs with tunneling thin AlGaAs inner shell and thin GaAs outer shell due to recombination of the photoexcited electrons confined in the outer shell with the holes in the core. Lower energy emission was shown to occur in the NWs with thick outer shell grown in the form of a short-period GaAs/AlGaAs multiple quantum well structure. In this case, the tunneling probability through the multiple quantum wells controls the energy emitted by the NWs. The doping of core results in dominated low energy emission from the GaAs core.

  14. Inverse spin galvanic effect in topological-insulator based heterostructures

    NASA Astrophysics Data System (ADS)

    Rodriguez-Vega, Martin; Schwiete, Georg; Sinova, Jairo; Rossi, Enrico

    2015-03-01

    We study the inverse spin galvanic effect in heterostructures formed by a layer of a three dimensional strong topological insulator (TI) and a magnetic material. We consider different configurations for the heterostructure and for the contacts. We carefully treat the effect on the TI bands of the proximity of a magnetic material and take into account both intra-band and inter-band contributions to the current-induced spin polarization of the TI surface states. Finally, we discuss the relevance of our results for recent experiments. Work supported by ONR-N00014-13-1-0321, ACS-PRF # 53581-DNI5, and the Jeffress Memorial Trust.

  15. Confinement of Surface Plasmon Polaritons by Heterostructures of Plasmonic Crystals.

    PubMed

    Saito, Hikaru; Mizuma, Shohei; Yamamoto, Naoki

    2015-10-14

    Square lattice plasmonic crystals (SQ-PlCs) composed of silver pillars generate large bandgaps for surface plasmon polaritons (SPPs). SPP confinement is demonstrated using one- and two-dimensional heterostructures of SQ-PlCs comprised of cylindrical pillars with different diameters in a common square lattice. Two kinds of localized modes are observed to appear in the heterostructures by photon map imaging using cathodoluminescence (CL) technique combined with a scanning transmission electron microscopy (STEM). Angle-resolved CL spectroscopy reveals contrasting characteristics of the two localized modes in their emission distributions, indicating that they originate from the band-edge A and E modes of the matrix SQ-PlC.

  16. Vertical-cavity in-plane heterostructures: Physics and applications

    SciTech Connect

    Taghizadeh, Alireza; Mørk, Jesper; Chung, Il-Sug

    2015-11-02

    We show that in-plane (lateral) heterostructures realized in vertical cavities with high contrast grating reflectors can be used to significantly modify the anisotropic dispersion curvature, also interpreted as the photon effective mass. This design freedom enables exotic configurations of heterostructures and many interesting applications. The effects of the anisotropic photon effective mass on the mode confinement, mode spacing, and transverse modes are investigated. As a possible application, the method of boosting the speed of diode lasers by engineering the photon-photon resonance is discussed. Based on this platform, we propose a system of two laterally coupled cavities, which shows the breaking of parity-time symmetry in vertical cavity structures.

  17. Voltage impulse induced bistable magnetization switching in multiferroic heterostructures

    NASA Astrophysics Data System (ADS)

    Nan, T. X.; Zhou, Z. Y.; Lou, J.; Liu, M.; Yang, X.; Gao, Y.; Rand, S.; Sun, N. X.

    2012-03-01

    We report on voltage impulse induced reversible bistable magnetization switching in FeGaB/lead zirconate titanate (PZT) multiferroic heterostructures at room temperature. This was realized through strain-mediated magnetoelectric coupling between ferroelectric PZT and ferromagnetic FeGaB layer. Two reversible and stable voltage-impulse induced mechanical strain states were obtained in the PZT by applying an electric field impulse with its amplitude smaller than the electric coercive field, which led to reversible voltage impulse induced bistable magnetization switching. These voltage impulse induced bistable magnetization switching in multiferroic heterostructures provides a promising approach to power efficient bistable magnetization switching that is crucial for information storage.

  18. Ion-implanted planar-buried-heterostructure diode laser

    DOEpatents

    Brennan, Thomas M.; Hammons, Burrell E.; Myers, David R.; Vawter, Gregory A.

    1991-01-01

    A Planar-Buried-Heterostructure, Graded-Index, Separate-Confinement-Heterostructure semiconductor diode laser 10 includes a single quantum well or multi-quantum well active stripe 12 disposed between a p-type compositionally graded Group III-V cladding layer 14 and an n-type compositionally graded Group III-V cladding layer 16. The laser 10 includes an ion implanted n-type region 28 within the p-type cladding layer 14 and further includes an ion implanted p-type region 26 within the n-type cladding layer 16. The ion implanted regions are disposed for defining a lateral extent of the active stripe.

  19. Gate-Tunable Resonant Tunneling in Double Bilayer Graphene Heterostructures

    NASA Astrophysics Data System (ADS)

    Fallahazad, Babak; Lee, Kayoung; Kang, Sangwoo; Xue, Jiamin; Larentis, Stefano; Corbet, Christopher; Kim, Kyounghwan; Movva, Hema C. P.; Taniguchi, Takashi; Watanabe, Kenji; Register, Leonard F.; Banerjee, Sanjay K.; Tutuc, Emanuel

    2015-01-01

    We demonstrate gate-tunable resonant tunneling and negative differential resistance in the interlayer current-voltage characteristics of rotationally aligned double bilayer graphene heterostructures separated by hexagonal boron-nitride (hBN) dielectric. An analysis of the heterostructure band alignment using individual layer densities, along with experimentally determined layer chemical potentials indicates that the resonance occurs when the energy bands of the two bilayer graphene are aligned. We discuss the tunneling resistance dependence on the interlayer hBN thickness, as well as the resonance width dependence on mobility and rotational alignment.

  20. Improving photoelectrochemical performance by building Fe{sub 2}O{sub 3} heterostructure on TiO{sub 2} nanorod arrays

    SciTech Connect

    Cao, Chunlan; Hu, Chenguo; Shen, Weidong; Wang, Shuxia; Song, Sihong; Wang, Mingjun

    2015-10-15

    Highlights: • Fe{sub 2}O{sub 3}@TiO{sub 2} heterostructure was fabricated by two-step method. • The photoelectrochemical properties were studied upon visible light irradiation. • Fe{sub 2}O{sub 3}@TiO{sub 2} heterostructure shows superior photoelectrochemical property. • A possible mechanism for enhanced photoelectrochemical property was put forward. - Abstract: Fe{sub 2}O{sub 3}@TiO{sub 2} heterostructure nanorod arrays were synthesized on a fluorine-doped tin oxide conductive (FTO) glass substrate via two-step method for improving photoelectrochemical activity of TiO{sub 2}. The TiO{sub 2} nanorod arrays on FTO substrate were first prepared by hydrothermal method and then Fe{sub 2}O{sub 3} nanoparticles were coated onto the surface of TiO{sub 2} nanorod arrays through chemical bath deposition. The heterojunction yielded a photocurrent density of 39.75 μA cm{sup −2} at a bias potential of 0 V (vs. Ag/AgCl) under visible light irradiation, which is 2.2 times as much as that produced by the pure TiO{sub 2} nanorod arrays. The enhanced photoelectrochemical activity is attributed to the extension of the light response range and efficient separation of photogenerated carriers. Our results have demonstrated the advantage of the novel Fe{sub 2}O{sub 3}@TiO{sub 2} heterojunction and will provide a new path to the fabrication of heterostructural materials.

  1. Doping and musculoskeletal system: short-term and long-lasting effects of doping agents.

    PubMed

    Nikolopoulos, Dimitrios D; Spiliopoulou, Chara; Theocharis, Stamatios E

    2011-10-01

    Doping is a problem that has plagued the world of competition and sports for ages. Even before the dawn of Olympic history in ancient Greece, competitors have looked for artificial means to improve athletic performance. Since ancient times, athletes have attempted to gain an unfair competitive advantage through the use of doping substances. A Prohibited List of doping substances and methods banned in sports is published yearly by the World Anti-Doping Agency. Among the substances included are steroidal and peptide hormones and their modulators, stimulants, glucocorticosteroids, β₂-agonists, diuretics and masking agents, narcotics, and cannabinoids. Blood doping, tampering, infusions, and gene doping are examples of prohibited methods indicated on the List. Apart from the unethical aspect of doping, as it abrogates fair-play's principle, it is extremely important to consider the hazards it presents to the health and well-being of athletes. The referred negative effects for the athlete's health have to do, on the one hand, by the high doses of the performance-enhancing agents and on the other hand, by the relentless, superhuman strict training that the elite or amateur athletes put their muscles, bones, and joints. The purpose of this article is to highlight the early and the long-lasting consequences of the doping abuse on bone and muscle metabolism.

  2. Scalable solution-phase epitaxial growth of symmetry-mismatched heterostructures on two-dimensional crystal soft template

    PubMed Central

    Lin, Zhaoyang; Yin, Anxiang; Mao, Jun; Xia, Yi; Kempf, Nicholas; He, Qiyuan; Wang, Yiliu; Chen, Chih-Yen; Zhang, Yanliang; Ozolins, Vidvuds; Ren, Zhifeng; Huang, Yu; Duan, Xiangfeng

    2016-01-01

    Epitaxial heterostructures with precisely controlled composition and electronic modulation are of central importance for electronics, optoelectronics, thermoelectrics, and catalysis. In general, epitaxial material growth requires identical or nearly identical crystal structures with small misfit in lattice symmetry and parameters and is typically achieved by vapor-phase depositions in vacuum. We report a scalable solution-phase growth of symmetry-mismatched PbSe/Bi2Se3 epitaxial heterostructures by using two-dimensional (2D) Bi2Se3 nanoplates as soft templates. The dangling bond–free surface of 2D Bi2Se3 nanoplates guides the growth of PbSe crystal without requiring a one-to-one match in the atomic structure, which exerts minimal restriction on the epitaxial layer. With a layered structure and weak van der Waals interlayer interaction, the interface layer in the 2D Bi2Se3 nanoplates can deform to accommodate incoming layer, thus functioning as a soft template for symmetry-mismatched epitaxial growth of cubic PbSe crystal on rhombohedral Bi2Se3 nanoplates. We show that a solution chemistry approach can be readily used for the synthesis of gram-scale PbSe/Bi2Se3 epitaxial heterostructures, in which the square PbSe (001) layer forms on the trigonal/hexagonal (0001) plane of Bi2Se3 nanoplates. We further show that the resulted PbSe/Bi2Se3 heterostructures can be readily processed into bulk pellet with considerably suppressed thermal conductivity (0.30 W/m·K at room temperature) while retaining respectable electrical conductivity, together delivering a thermoelectric figure of merit ZT three times higher than that of the pristine Bi2Se3 nanoplates at 575 K. Our study demonstrates a unique epitaxy mode enabled by the 2D nanocrystal soft template via an affordable and scalable solution chemistry approach. It opens up new opportunities for the creation of diverse epitaxial heterostructures with highly disparate structures and functions. PMID:27730211

  3. Electronic transport in graphene-based heterostructures

    SciTech Connect

    Tan, J. Y.; Avsar, A.; Balakrishnan, J.; Taychatanapat, T.; O'Farrell, E. C. T.; Eda, G.; Castro Neto, A. H.; Koon, G. K. W.; Özyilmaz, B.; Watanabe, K.; Taniguchi, T.

    2014-05-05

    While boron nitride (BN) substrates have been utilized to achieve high electronic mobilities in graphene field effect transistors, it is unclear how other layered two dimensional (2D) crystals influence the electronic performance of graphene. In this Letter, we study the surface morphology of 2D BN, gallium selenide (GaSe), and transition metal dichalcogenides (tungsten disulfide (WS{sub 2}) and molybdenum disulfide (MoS{sub 2})) crystals and their influence on graphene's electronic quality. Atomic force microscopy analysis shows that these crystals have improved surface roughness (root mean square value of only ∼0.1 nm) compared to conventional SiO{sub 2} substrate. While our results confirm that graphene devices exhibit very high electronic mobility (μ) on BN substrates, graphene devices on WS{sub 2} substrates (G/WS{sub 2}) are equally promising for high quality electronic transport (μ ∼ 38 000 cm{sup 2}/V s at room temperature), followed by G/MoS{sub 2} (μ ∼ 10 000 cm{sup 2}/V s) and G/GaSe (μ ∼ 2200 cm{sup 2}/V s). However, we observe a significant asymmetry in electron and hole conduction in G/WS{sub 2} and G/MoS{sub 2} heterostructures, most likely due to the presence of sulphur vacancies in the substrate crystals. GaSe crystals are observed to degrade over time even under ambient conditions, leading to a large hysteresis in graphene transport making it a less suitable substrate.

  4. Coupling Magnetism to Electricity in Multiferroic Heterostructures

    SciTech Connect

    Ramesh, R

    2012-02-15

    Complex perovskite oxides exhibit a rich spectrum of functional responses, including magnetism, ferroelectricity, highly correlated electron behavior, superconductivity, etc. The basic materials physics of such materials provide the ideal playground for interdisciplinary scientific exploration. Over the past decade we have been exploring the science of such materials (for example, colossal magnetoresistance, ferroelectricity, etc) in thin-film form by creating epitaxial heterostructures and nanostructures. Among the large number of materials systems, there exists a small set of materials which exhibit multiple order parameters; these are known as multiferroics. Using our work in the field of ferroelectric and ferromagnetic oxides as the background, we are now exploring such materials as epitaxial thin films, as well as nanostructures. Specifically, we are studying the role of thin-film growth, heteroepitaxy, and processing on the basic properties as well as the magnitude of the coupling between the order parameters. In our work we are exploring the switchability of the antiferromagnetic order using this coupling. What is the importance of this work? Antiferromagnets are pervasive in the recording industry. They are used as exchange biasing layers in magnetic tunnel junctions, etc. However, to date there has been no antiferomagnet that is electrically tunable. We believe that the multiferroic BiFeO3 is one compound where this can be observed at room temperature. The next step is to explore the coupling of a ferromagnet to this antiferromagnet through the exchange biasing concept. Ultimately, this will give us the opportunity to switch the magnetic state in a ferromagnet (and therefore the spin polarization direction) by simply applying an electric field to the underlying antiferromagnetic ferroelectric. In this talk, I will describe our progress to date on this exciting possibility.

  5. Orbital Reconstruction Enhanced Exchange Bias in La0.6Sr0.4MnO3/Orthorhombic YMnO3 Heterostructures

    PubMed Central

    Zheng, Dongxing; Jin, Chao; Li, Peng; Wang, Liyan; Feng, Liefeng; Mi, Wenbo; Bai, Haili

    2016-01-01

    The exchange bias in ferromagnetic/multiferroic heterostructures is usually considered to originate from interfacial coupling. In this work, an orbital reconstruction enhanced exchange bias was discovered. As La0.6Sr0.4MnO3 (LSMO) grown on YMnO3 (YMO) suffers a tensile strain (a > c), the doubly degenerate eg orbital splits into high energy 3z2 − r2 and low energy x2 − y2 orbitals, which makes electrons occupy the localized x2 − y2 orbital and leads to the formation of antiferromagnetic phase in LSMO. The orbital reconstruction induced antiferromagnetic phase enhances the exchange bias in the LSMO/YMO heterostructures, lightening an effective way for electric-field modulated magnetic moments in multiferroic magnetoelectric devices. PMID:27090614

  6. Orbital Reconstruction Enhanced Exchange Bias in La0.6Sr0.4MnO3/Orthorhombic YMnO3 Heterostructures

    NASA Astrophysics Data System (ADS)

    Zheng, Dongxing; Jin, Chao; Li, Peng; Wang, Liyan; Feng, Liefeng; Mi, Wenbo; Bai, Haili

    2016-04-01

    The exchange bias in ferromagnetic/multiferroic heterostructures is usually considered to originate from interfacial coupling. In this work, an orbital reconstruction enhanced exchange bias was discovered. As La0.6Sr0.4MnO3 (LSMO) grown on YMnO3 (YMO) suffers a tensile strain (a > c), the doubly degenerate eg orbital splits into high energy 3z2 ‑ r2 and low energy x2 ‑ y2 orbitals, which makes electrons occupy the localized x2 ‑ y2 orbital and leads to the formation of antiferromagnetic phase in LSMO. The orbital reconstruction induced antiferromagnetic phase enhances the exchange bias in the LSMO/YMO heterostructures, lightening an effective way for electric-field modulated magnetic moments in multiferroic magnetoelectric devices.

  7. Broadband Photovoltaic Detectors Based on an Atomically Thin Heterostructure.

    PubMed

    Long, Mingsheng; Liu, Erfu; Wang, Peng; Gao, Anyuan; Xia, Hui; Luo, Wei; Wang, Baigeng; Zeng, Junwen; Fu, Yajun; Xu, Kang; Zhou, Wei; Lv, Yangyang; Yao, Shuhua; Lu, Minghui; Chen, Yanfeng; Ni, Zhenhua; You, Yumeng; Zhang, Xueao; Qin, Shiqiao; Shi, Yi; Hu, Weida; Xing, Dingyu; Miao, Feng

    2016-04-13

    van der Waals junctions of two-dimensional materials with an atomically sharp interface open up unprecedented opportunities to design and study functional heterostructures. Semiconducting transition metal dichalcogenides have shown tremendous potential for future applications due to their unique electronic properties and strong light-matter interaction. However, many important optoelectronic applications, such as broadband photodetection, are severely hindered by their limited spectral range and reduced light absorption. Here, we present a p-g-n heterostructure formed by sandwiching graphene with a gapless band structure and wide absorption spectrum in an atomically thin p-n junction to overcome these major limitations. We have successfully demonstrated a MoS2-graphene-WSe2 heterostructure for broadband photodetection in the visible to short-wavelength infrared range at room temperature that exhibits competitive device performance, including a specific detectivity of up to 10(11) Jones in the near-infrared region. Our results pave the way toward the implementation of atomically thin heterostructures for broadband and sensitive optoelectronic applications. PMID:26886761

  8. Analysis of Critical Dimensions for Nanowire Core-Multishell Heterostructures

    NASA Astrophysics Data System (ADS)

    Yan, Xin; Fan, Shuyu; Zhang, Xia; Ren, Xiaomin

    2015-10-01

    Critical dimensions for nanowire core-multishell heterostructures are analyzed by using finite-element method based on the energy equilibrium criteria. Results show that the nanowire core-shell heterostructure can sufficiently reduce the strain in the shell and increase the critical shell thickness. The critical dimensions for the nanowire core-multishell heterostructure are determined by the stress fields generated at two heterointerfaces. For thin barrier, the critical dimensions decrease as the core radius increases, while when the barrier is thick enough, the critical dimensions show an increase with the increase of core radius conversely. This can be attributed to a competition between the lattice mismatch and strain distribution, which dominate the critical dimensions alternatively. Two critical quantum well thicknesses are obtained in the nanowire core-multishell heterostructure. Below the dislocation-free critical thickness, the structure will be coherent regardless of the barrier thickness. While above the dislocation-unavoidable thickness, dislocations are always energetically favored. In the dislocation-controllable region between the two critical thicknesses, coherent structure can be obtained via controlling the well and barrier thicknesses. The results are in good agreement with the experimental data and may serve as guidance for the design of coherent nanowire core-multishell quantum well structures and devices.

  9. Interface formation in monolayer graphene-boron nitride heterostructures.

    PubMed

    Sutter, P; Cortes, R; Lahiri, J; Sutter, E

    2012-09-12

    The ability to control the formation of interfaces between different materials has become one of the foundations of modern materials science. With the advent of two-dimensional (2D) crystals, low-dimensional equivalents of conventional interfaces can be envisioned: line boundaries separating different materials integrated in a single 2D sheet. Graphene and hexagonal boron nitride offer an attractive system from which to build such 2D heterostructures. They are isostructural, nearly lattice-matched, and isoelectronic, yet their different band structures promise interesting functional properties arising from their integration. Here, we use a combination of in situ microscopy techniques to study the growth and interface formation of monolayer graphene-boron nitride heterostructures on ruthenium. In a sequential chemical vapor deposition process, boron nitride grows preferentially at the edges of existing monolayer graphene domains, which can be exploited for synthesizing continuous 2D membranes of graphene embedded in boron nitride. High-temperature growth leads to intermixing near the interface, similar to interfacial alloying in conventional heterostructures. Using real-time microscopy, we identify processes that eliminate this intermixing and thus pave the way to graphene-boron nitride heterostructures with atomically sharp interfaces.

  10. Ab initio simulations of pseudomorphic silicene and germanene bidimensional heterostructures

    NASA Astrophysics Data System (ADS)

    Debernardi, Alberto; Marchetti, Luigi

    2016-06-01

    Among the novel two-dimensional (2D) materials, silicene and germanene, which are two honeycomb crystal structures composed of a monolayer of Si and Ge, respectively, have attracted the attention of material scientists because they combine the advantages of the new 2D ultimate-scaled electronics with their compatibility with industrial processes presently based on Si and Ge. We envisage pseudomorphic lateral heterostructures based on ribbons of silicene and germanene, which are the 2D analogs of conventional 3D Si/Ge superlattices and quantum wells. In spite of the considerable lattice mismatch (˜4 % ) between free-standing silicene and germanene, our ab initio simulations predict that, considering striped 2D lateral heterostructures made by alternating silicene and germanene ribbons of constant width, the silicene/germanene junction remains pseudomorphic—i.e., it maintains lattice-matched edges—up to critical ribbon widths that can reach some tens of nanometers. Such critical widths are one order of magnitude larger than the critical thickness measured in 3D pseudomorphic Si/Ge heterostructures and the resolution of state-of-the-art lithography, thus enabling the possibility of lithography patterned silicene/germanene junctions. We computed how the strain produced by the pseudomorphic growth modifies the crystal structure and electronic bands of the ribbons, providing a mechanism for band-structure engineering. Our results pave the way for lithography patterned lateral heterostructures that can serve as the building blocks of novel 2D electronics.

  11. High frequency GaAlAs modulator and photodetector for phased array antenna applications

    NASA Technical Reports Server (NTRS)

    Claspy, P. C.; Chorey, C. M.; Hill, S. M.; Bhasin, K. B.

    1988-01-01

    A waveguide Mach-Zehnder electro-optic modulator and an interdigitated photoconductive detector designed to operate at 820 nm, fabricated on different GaAlAs/GaAs heterostructure materials, are being investigated for use in optical interconnects in phased array antenna systems. Measured optical attenuation effects in the modulator are discussed and the observed modulation performance up to 1 GHz is presented. Measurements of detector frequency response are described and results presented.

  12. Ultrafast magnetization dynamics in ferromagnetic thin films and heterostructures

    NASA Astrophysics Data System (ADS)

    Guan, Yongfeng

    With the development of magnetic information storage technology, especially when data rates approach 1 GHz and above, new insight into the magnetization dynamics in ferromagnetic materials becomes a more pressing need. In this thesis, our recent studies of the ultrafast magnetization dynamics in ferromagnetic thin films and heterostructures using various measurement techniques are presented. We present our static transmission-mode x-ray magnetic circular dichroism (XMCD) characterization of element-specific moments in ferromagnetic thin films. Sum rules analysis are further used to extract the projected element-specific spin and orbital moments. A very low projected Tb moment in the 6% Tb-doped Ni81Fe19 thin film, which nonetheless reverses with low applied fields, indicates a sperimagnetic alignment with respect to the Fe and Ni elements in the alloy. The nearly unchanged orbital-to-spin moment ratio of Fe over the measured range of 0 ≤ x ≤ 0.15 in the Fe1- xVx thin films, compatible with known magnetization behavior as well as spectroscopic splitting g-factor data in the alloy by means of a two-sublattice model, confirms that the very low Gilbert damping attained through the introduction of V into epitaxial Fe1-xVx thin films does not result from the reduction of orbital moment content in the alloy. We also present our synchrotron-based development of time-resolved x-ray magnetic circular dichroism (TR-XMCD) technique. With this technique, we have demonstrated the first element- and layer-resolved magnetization dynamics with temporal resolution of 2--5 ps and angular resolution down to 0.1°. Coupled motion of Fe and Ni moments is verified in Ni81Fe 19 thin film, indicating a strong exchange coupling between Fe and Ni in the alloy. The influence of weak ferromagnetic interlayer coupling, difficult to identify in conventional FMR measurement, is clearly revealed in a pseudo-spin-valve structure of Ni81Fe19/Cu/Co93Zr7. Lagged phase behavior is observed between

  13. Sheet resistance under Ohmic contacts to AlGaN/GaN heterostructures

    SciTech Connect

    Hajłasz, M.; Donkers, J. J. T. M.; Sque, S. J.; Heil, S. B. S.; Gravesteijn, D. J.; Rietveld, F. J. R.; Schmitz, J.

    2014-06-16

    For the determination of specific contact resistance in semiconductor devices, it is usually assumed that the sheet resistance under the contact is identical to that between the contacts. This generally does not hold for contacts to AlGaN/GaN structures, where an effective doping under the contact is thought to come from reactions between the contact metals and the AlGaN/GaN. As a consequence, conventional extraction of the specific contact resistance and transfer length leads to erroneous results. In this Letter, the sheet resistance under gold-free Ti/Al-based Ohmic contacts to AlGaN/GaN heterostructures on Si substrates has been investigated by means of electrical measurements, transmission electron microscopy, and technology computer-aided design simulations. It was found to be significantly lower than that outside of the contact area; temperature-dependent electrical characterization showed that it exhibits semiconductor-like behavior. The increase in conduction is attributed to n-type activity of nitrogen vacancies in the AlGaN. They are thought to form during rapid thermal annealing of the metal stack when Ti extracts nitrogen from the underlying semiconductor. The high n-type doping in the region between the metal and the 2-dimensional electron gas pulls the conduction band towards the Fermi level and enhances horizontal electron transport in the AlGaN. Using this improved understanding of the properties of the material underneath the contact, accurate values of transfer length and specific contact resistance have been extracted.

  14. Sheet resistance under Ohmic contacts to AlGaN/GaN heterostructures

    NASA Astrophysics Data System (ADS)

    Hajłasz, M.; Donkers, J. J. T. M.; Sque, S. J.; Heil, S. B. S.; Gravesteijn, D. J.; Rietveld, F. J. R.; Schmitz, J.

    2014-06-01

    For the determination of specific contact resistance in semiconductor devices, it is usually assumed that the sheet resistance under the contact is identical to that between the contacts. This generally does not hold for contacts to AlGaN/GaN structures, where an effective doping under the contact is thought to come from reactions between the contact metals and the AlGaN/GaN. As a consequence, conventional extraction of the specific contact resistance and transfer length leads to erroneous results. In this Letter, the sheet resistance under gold-free Ti/Al-based Ohmic contacts to AlGaN/GaN heterostructures on Si substrates has been investigated by means of electrical measurements, transmission electron microscopy, and technology computer-aided design simulations. It was found to be significantly lower than that outside of the contact area; temperature-dependent electrical characterization showed that it exhibits semiconductor-like behavior. The increase in conduction is attributed to n-type activity of nitrogen vacancies in the AlGaN. They are thought to form during rapid thermal annealing of the metal stack when Ti extracts nitrogen from the underlying semiconductor. The high n-type doping in the region between the metal and the 2-dimensional electron gas pulls the conduction band towards the Fermi level and enhances horizontal electron transport in the AlGaN. Using this improved understanding of the properties of the material underneath the contact, accurate values of transfer length and specific contact resistance have been extracted.

  15. Metal versus insulator behavior in ultrathin SrTiO3-based heterostructures

    NASA Astrophysics Data System (ADS)

    Bjaalie, Lars; Janotti, Anderson; Himmetoglu, Burak; Van de Walle, Chris G.

    2016-07-01

    Complex-oxide interfaces can give rise to two-dimensional electron gases (2DEGs) with extremely high densities: for SrTiO3/GdTiO3 (STO/GTO), a density of 1/2 electron per unit-cell area is found within the STO. In this work we use first-principles calculations to study GTO/STO/GTO heterostructures, where both interfaces contribute electrons to the STO. We find that for a thick STO layer the electrons from the interfaces delocalize over multiple TiO2 planes. Once the STO thickness is reduced below three layers, we find that the electrons localize on every other interfacial Ti atom, leading to an insulating phase. We attribute this localization to the combination of high electron density and distortions at the interface. This is further confirmed by a model of the transition based on electron doping of bulk STO, allowing for the same type of distortions as at the interface with GTO. These findings elucidate previous observations [P. Moetakef et al., Phys. Rev. B 86, 201102 (2012), 10.1103/PhysRevB.86.201102], but our proposed physical mechanisms are general and should apply to other complex-oxide interfaces as well.

  16. Rational growth of branched nanowire heterostructures with synthetically encoded properties and function.

    PubMed

    Jiang, Xiaocheng; Tian, Bozhi; Xiang, Jie; Qian, Fang; Zheng, Gengfeng; Wang, Hongtao; Mai, Liqiang; Lieber, Charles M

    2011-07-26

    Branched nanostructures represent unique, 3D building blocks for the "bottom-up" paradigm of nanoscale science and technology. Here, we report a rational, multistep approach toward the general synthesis of 3D branched nanowire (NW) heterostructures. Single-crystalline semiconductor, including groups IV, III-V, and II-VI, and metal branches have been selectively grown on core or core/shell NW backbones, with the composition, morphology, and doping of core (core/shell) NWs and branch NWs well controlled during synthesis. Measurements made on the different composition branched NW structures demonstrate encoding of functional p-type/n-type diodes and light-emitting diodes (LEDs) as well as field effect transistors with device function localized at the branch/backbone NW junctions. In addition, multibranch/backbone NW structures were synthesized and used to demonstrate capability to create addressable nanoscale LED arrays, logic circuits, and biological sensors. Our work demonstrates a previously undescribed level of structural and functional complexity in NW materials, and more generally, highlights the potential of bottom-up synthesis to yield increasingly complex functional systems in the future.

  17. Rational growth of branched nanowire heterostructures with synthetically encoded properties and function

    PubMed Central

    Jiang, Xiaocheng; Tian, Bozhi; Xiang, Jie; Qian, Fang; Zheng, Gengfeng; Wang, Hongtao; Mai, Liqiang; Lieber, Charles M.

    2011-01-01

    Branched nanostructures represent unique, 3D building blocks for the “bottom-up” paradigm of nanoscale science and technology. Here, we report a rational, multistep approach toward the general synthesis of 3D branched nanowire (NW) heterostructures. Single-crystalline semiconductor, including groups IV, III–V, and II–VI, and metal branches have been selectively grown on core or core/shell NW backbones, with the composition, morphology, and doping of core (core/shell) NWs and branch NWs well controlled during synthesis. Measurements made on the different composition branched NW structures demonstrate encoding of functional p-type/n-type diodes and light-emitting diodes (LEDs) as well as field effect transistors with device function localized at the branch/backbone NW junctions. In addition, multibranch/backbone NW structures were synthesized and used to demonstrate capability to create addressable nanoscale LED arrays, logic circuits, and biological sensors. Our work demonstrates a previously undescribed level of structural and functional complexity in NW materials, and more generally, highlights the potential of bottom-up synthesis to yield increasingly complex functional systems in the future. PMID:21730174

  18. Optics and Optoelectronics of Two-dimensional Semiconducting Monolayers and Heterostructures

    NASA Astrophysics Data System (ADS)

    Ross, Jason Solomon

    Until recently, the physics of truly two-dimensional (2D) excitons could only be explored theoretically. Following the discovery of graphene, many 2D materials were quickly identified and isolated, one system being the semiconducting Group VI-B transition metal dichalcogenides (TMDs). These semiconductors are the first air-stable materials that are atomically thin (three atomics thick), and yet can be produced in arbitrarily large lateral sheets. They have a direct band gap in which confinement leads to large spatial overlap of electrons and holes resulting in strongly coupled excitonic transitions that dominate light-matter interactions. The direct band-gap of monolayer TMDs occurs at the corners of the hexagonal Brillouin zone, referred to as the K valleys. Entirely unique to these materials, excitons in adjacent K valleys selectively couple to light of opposite circular polarization, i.e. the K (K') valley is selective to right (left) circularly polarized photons. This property offers the possible realization of novel devices that will manipulate the valley index, known as valleytronics. Further, creating a stacked heterostructure (HS) of two TMD monolayers of different molecular species can exhibit type-II band alignment leading to the first atomically sharp built-in p-n junction and a bright interlayer exciton with long lifetimes. Being flat 2D sheets, it is easy to couple these materials to nearby systems such as microfabricated electrodes and photonic crystal cavities allowing for unique modulation and device schemes. Here, I employ both optical and electronic techniques to study the unique physics of 2D excitons in TMDs as well as demonstrate some of their first optoelectronic and valleytronic devices. The most notable achievement is perhaps the first demonstrations of both atomically thin and 2D heterostructure light emitting diodes and photovoltaic devices. Other breakthroughs include the first demonstration of exciton charging tunability in a 2D system

  19. Theoretical investigations on the N-polar GaN/AlxGa1-xN/GaN heterostructures: Considering the existence of both two-dimensional hole and electron gases

    NASA Astrophysics Data System (ADS)

    Yan, Junda; Wang, Quan; Wang, Xiaoliang; Feng, Chun; Xiao, Hongling; Liu, Shiming; Gong, Jiamin; Liu, Fengqi; Li, Baiquan

    2016-09-01

    In this paper, a theoretical study of N-polar GaN/AlxGa1-xN/GaN heterostructures is conducted systematically. The dependence of two-dimensional hole gas (2DHG, at the bottom AlxGa1-xN/GaN interface) and electron gas (2DEG, at the upper GaN/AlxGa1-xN interface) sheet densities on variables, such as GaN top layer thickness, AlxGa1-xN back barrier thickness, and Al content, is investigated. The effect of n-type doping in AlxGa1-xN and δ-doping concentrations in the GaN buffer on 2DHG and 2DEG sheet densities is also presented. For the unintentionally doped structure, the 2DHG-induced electric field E2DHG provides the only driving force in the 2DEG formation. Thus, in order to get high dense of 2DEG in such heterostructure, 2DEG and 2DHG shall coexist through proper AlxGa1-xN back barrier design. While for intentionally doped structure (with the n-type doped AlxGa1-xN back barrier layer or δ-doped GaN buffer layer), ionized donor-induced electric field Edonor is another driving force for 2DEG. The obtained insight offers indications for the structure design of the N-polar GaN/AlxGa1-xN/GaN high electron mobility transistors.

  20. Rebound effect of IMT properties by different doping form in Si-doped vanadium dioxide films

    NASA Astrophysics Data System (ADS)

    Wu, Xuefei; Wu, Zhiming; Liu, Zhijun; Ji, Chunhui; Huang, Zehua; Su, Yuanjie; Gou, Jun; Wang, Jun; Jiang, Yadong

    2016-09-01

    Vanadium dioxide is a promising material for THz modulations due to its remarkable insulator-to-metal transition (IMT) properties. Silicon-doped VO2 films, exhibiting excellent IMT properties with giant modulation amplitude and tunable phase transition temperature, greatly adapt in this area. In this paper, we report on a rebound effect of the IMT in Si-doped VO2 films. As the silicon dopants are increasingly introduced into VO2 films, the IMT is first tuned to lower temperature and then is anomalously shifted to higher temperature. This rebound effect is confirmed by crystal structure, valence concentration, and surface morphology. We attribute this rebound behavior to the interstitial and substitutive doping of Si atoms. Due to their distinct impactions on the crystallite, IMT properties of the VO2 films are depressed initially and recovered later.

  1. Van der Waals epitaxial double heterostructure: InAs/single-layer graphene/InAs.

    PubMed

    Hong, Young Joon; Yang, Jae Won; Lee, Wi Hyoung; Ruoff, Rodney S; Kim, Kwang S; Fukui, Takashi

    2013-12-17

    Van der Waals (vdW) epitaxial double heterostructures have been fabricated by vdW epitaxy of InAs nanostructures on both sides of graphene. InAs nanostructures diametrically form on/underneath graphene exclusively along As-polar direction, indicating polarity inversion of the double heterostructures. First-principles and density functional calculations demonstrate how and why InAs easily form to be double heterostructures with polarity inversion.

  2. Synthesis and applications of heterostructured semiconductor nanocrystals

    NASA Astrophysics Data System (ADS)

    Khon, Elena

    Semiconductor nanocrystals (NCs) have been of great interest to researchers for several decades due to their unique optoelectronic properties. These nanoparticles are widely used for a variety of different applications. However, there are many unresolved issues that lower the efficiency and/or stability of devices which incorporate these NCs. Our research is dedicated to addressing these issues by identifying potential problems and resolving them, improving existing systems, generating new synthetic strategies, and/or building new devices. The general strategies for the synthesis of different nanocrystals were established in this work, one of which is the colloidal growth of gold domains onto CdS semiconductor nanocrystals. Control of shape and size was achieved simply by adjusting the temperature and the time of the reaction. Depending on the exact morphology of Au and CdS domains, fabricated nano-composites can undergo evaporation-induced self-assembly onto a substrate, which is very useful for building devices. CdS/Au heterostructures can assemble in two different ways: through end-to-end coupling of Au domains, resulting in the formation of one-dimensional chains; and via side-by-side packing of CdS nanorods, leading to the onset of two-dimensional superlattices. We investigated the nature of exciton-plasmon interactions in Au-tipped CdS nanorods using femtosecond transient absorption spectroscopy. The study demonstrated that the key optoelectronic properties of electrically coupled metal and semiconductor domains are significantly different from those observed in systems with weak inter-domain coupling. In particular, strongly-coupled nanocomposites promote mixing of electronic states at semiconductor-metal domain interfaces, which causes a significant suppression of both plasmon and exciton carrier excitations. Colloidal QDs are starting to replace organic molecules in many different applications, such as organic light emmiting diods (OLEDs), due to their

  3. Vertical Field-Effect Transistor Based on Graphene-Transition Metal Dichalcogenides Heterostructures

    NASA Astrophysics Data System (ADS)

    Kumar, Jatinder; Chien, Hui-Chun; Bellus, Matthew Z.; Sicilian, David L.; Aubin, Davis St.; Chiu, Hsin-Ying; Physics and Astronomy, University of Kansas Team

    2014-03-01

    The remarkable properties of graphene has made it possible to create transistors just few atoms thick. A new development was that the other two-dimensional materials can be stacked on it with atomic layer precision, creating numerous heterostructures on demand. Here, novel vertical field-effect transistor composed of graphene- transition metal dichalcogenides (TMDs) heterostructures is fabricated and characterized at various temperatures. Due to ultrathin nature of these transistors, they present the ultimate limit for electron transport in heterostructures. Tunneling and thermionic transport characteristics are studied among different graphene-TMDs heterostructures. Their electronic properties have been investigated and can be used in vast range of devices.

  4. System-size dependent band alignment in lateral two-dimensional heterostructures

    NASA Astrophysics Data System (ADS)

    Leenaerts, O.; Vercauteren, S.; Schoeters, B.; Partoens, B.

    2016-06-01

    The electronic band alignment in semiconductor heterostructures is a key factor for their use in electronic applications. The alignment problem has been intensively studied for bulk systems but is less well understood for low-dimensional heterostructures. In this work we investigate the alignment in two-dimensional lateral heterostructures. First-principles calculations are used to show that the electronic band offset depends crucially on the width and thickness of the heterostructure slab. The particular heterostructures under study consist of thin hydrogenated and fluorinated diamond slabs which are laterally joined together. Two different limits for the band offset are observed. For infinitely wide heterostructures the vacuum potential above the two materials is aligned leading to a large step potential within the heterostructure. For infinitely thick heterostructure slabs, on the other hand, there is no potential step in the heterostructure bulk, but a large potential step in the vacuum region above the heterojunction is observed. The band alignment in finite systems depends on the particular dimensions of the system. These observations are shown to result from an interface dipole at the heterojunction that tends to align the band structures.

  5. Inherent Enhancement of Electronic Emission from Hexaboride Heterostructure

    NASA Astrophysics Data System (ADS)

    Voss, Johannes; Vojvodic, Aleksandra; Chou, Sharon H.; Howe, Roger T.; Abild-Pedersen, Frank

    2014-08-01

    Based on a nonequilibrium Green's-function approach to the calculation of emission currents from first principles, we show that cathodes consisting of LaB6/BaB6 superlattices can yield an order-of-magnitude higher thermionic current densities than pure LaB6 cathodes. Because of a 0.46-eV lowering of the work function, such a heterostructure cathode could thus be operated at significantly lower temperatures. Neither the stability nor the magnitude of electronic tunneling coefficients is compromised in the superlattice system as compared to pure LaB6, which is in contrast to the generally reduced stability and large dipole barriers in the case of adsorbate-induced lowering of the work function. The heterostructure could thus be used as a cathode material that at the same time is stable and has emission properties superior to those of pure LaB6.

  6. Band structure engineering in topological insulator based heterostructures.

    PubMed

    Menshchikova, T V; Otrokov, M M; Tsirkin, S S; Samorokov, D A; Bebneva, V V; Ernst, A; Kuznetsov, V M; Chulkov, E V

    2013-01-01

    The ability to engineer an electronic band structure of topological insulators would allow the production of topological materials with tailor-made properties. Using ab initio calculations, we show a promising way to control the conducting surface state in topological insulator based heterostructures representing an insulator ultrathin films on the topological insulator substrates. Because of a specific relation between work functions and band gaps of the topological insulator substrate and the insulator ultrathin film overlayer, a sizable shift of the Dirac point occurs resulting in a significant increase in the number of the topological surface state charge carriers as compared to that of the substrate itself. Such an effect can also be realized by applying the external electric field that allows a gradual tuning of the topological surface state. A simultaneous use of both approaches makes it possible to obtain a topological insulator based heterostructure with a highly tunable topological surface state.

  7. Intrinsic disorder in graphene on transition metal dichalcogenide heterostructures

    NASA Astrophysics Data System (ADS)

    Yankowitz, Matthew; Larentis, Stefano; Kim, Kyounghwam; Xue, Jiamin; McKenzie, Devin; Huang, Shengqiang; Paggen, Marina; Ali, Mazhar; Cava, Robert; Tutuc, Emanuel; Leroy, Brian J.

    2015-03-01

    Recently, semiconducting materials in the transition metal dichalcogenide (TMD) family have gained great popularity for use in novel graphene-based heterostructure devices such as tunneling transistors, highly efficient flexible photovoltaic devices, and nonvolatile memory cells. TMDs have also been explored as alternatives to hexagonal boron nitride (hBN) as substrates for pristine graphene devices. However, their quality has thus far been significantly worse than comparable hBN devices. We examine graphene on numerous TMD substrates (MoS2, WS2, WSe2, MoTe2) with scanning tunneling microscopy and spectroscopy and find that point and line defects intrinsic to all TMD crystals (both of natural and synthetic origin) result in scattering of electrons in graphene. Our findings suggest that the quality of graphene on TMD heterostructures is limited by the intrinsic crystalline quality of the TMDs.

  8. Millimeter-wave and optoelectronic applications of heterostructure integrated circuits

    NASA Technical Reports Server (NTRS)

    Pavlidis, Dimitris

    1991-01-01

    The properties are reviewed of heterostructure devices for microwave-monolithic-integrated circuits (MMICs) and optoelectronic integrated circuits (OICs). Specific devices examined include lattice-matched and pseudomorphic InAlAs/InGaAs high-electron mobility transistors (HEMTs), mixer/multiplier diodes, and heterojunction bipolar transistors (HBTs) developed with a number of materials. MMICs are reviewed that can be employed for amplification, mixing, and signal generation, and receiver/transmitter applications are set forth for OICs based on GaAs and InP heterostructure designs. HEMTs, HBTs, and junction-FETs can be utilized in combination with PIN, MSM, and laser diodes to develop novel communication systems based on technologies that combine microwave and photonic capabilities.

  9. An envelope function formalism for lattice-matched heterostructures

    NASA Astrophysics Data System (ADS)

    Van de Put, Maarten L.; Vandenberghe, William G.; Magnus, Wim; Sorée, Bart

    2015-08-01

    The envelope function method traditionally employs a single basis set which, in practice, relates to a single material because the k · p matrix elements are generally only known in a particular basis. In this work, we defined a basis function transformation to alleviate this restriction. The transformation is completely described by the known inter-band momentum matrix elements. The resulting envelope function equation can solve the electronic structure in lattice matched heterostructures without resorting to boundary conditions at the interface between materials, while all unit-cell averaged observables can be calculated as with the standard envelope function formalism. In the case of two coupled bands, this heterostructure formalism is equivalent to the standard formalism while taking position dependent matrix elements.

  10. Quantum Anomalous Hall Effect in Graphene-based Heterostructure.

    PubMed

    Zhang, Jiayong; Zhao, Bao; Yao, Yugui; Yang, Zhongqin

    2015-01-01

    Quantum anomalous Hall (QAH) effect, with potential applications in low-power-consumption electronics, is predicted in the heterostructure of graphene on the (001) surface of a real antiferromagnetic insulator RbMnCl3, based on density-functional theory and Wannier function methods. Due to the interactions from the substrate, a much large exchange field (about 280 meV) and an enhanced Rashba spin-orbit coupling are induced in graphene, leading to a topologically nontrivial QAH gap opened in the system. The avenues of enhancing the nontrivial gap are also proposed, from which nearly a gap one order large is achieved. Our work demonstrates that this graphene-based heterostructure is an appropriate candidate to be employed to experimentally observe the QAH effect and explore the promising applications.

  11. Tuning the dynamic exchange interaction in ferromagnet/semiconductor heterostructures

    NASA Astrophysics Data System (ADS)

    Ou, Yu-Sheng; Chiu, Yi-Hsin; Harmon, N. J.; Odenthal, Patrick; Sheffield, Matthew; Chilcote, Michael; Kawakami, R. K.; Flatté, M. E.; Johnston-Halperin, E.

    2015-09-01

    We investigate the impact of tunnel barrier thickness on electron spin dynamics in Fe/MgO/GaAs heterostructures using spin-resolved optical pump-probe spectroscopy. Comparison of the Larmor frequency between thick and thin MgO barriers reveals a four-fold variation in exchange coupling strength, and investigation of the inhomogeneous dephasing time, T2*, argues that inhomogeneity in the local effective hyperfine field dominates free-carrier spin relaxation across the entire range of barrier thickness. These results provide additional evidence to support the theory of hyperfine-dominated spin relaxation in GaAs at low temperature and in the presence of an externally applied magnetic field. Further, this work lays the foundation for engineering both the exchange coupling and the free carrier spin dynamics in ferromagnet/semiconductor heterostructures, allowing for the exploration of dissipation and transport in the regime of dynamically-driven spin pumping.

  12. Charge transfer and emergent phenomena of oxide heterostructures

    NASA Astrophysics Data System (ADS)

    Chen, Hanghui

    Charge transfer is a common phenomenon at oxide interfaces. We use first-principles calculations to show that via heterostructuring of transition metal oxides, the electronegativity difference between two dissimilar transition metal ions can lead to high level of charge transfer and induce substantial redistribution of electrons and ions. Notable examples include i) enhancing correlation effects and inducing a metal-insulator transition; ii) tailoring magnetic structures and inducing interfacial ferromagnetism; iii) engineering orbital splitting and inducing a non-cuprate single-orbital Fermi surface. Utilizing charge transfer to induce emergent electronic/magnetic/orbital properties at oxide interfaces is a robust approach. Combining charge transfer with quantum confinement and expitaxial strain provides an appealing prospect of engineering electronic structure of artificial oxide heterostructures. This research was supported by National Science Foundation under Grant No. DMR-1120296.

  13. Persistent hysteresis in graphene-mica van der Waals heterostructures.

    PubMed

    Mohrmann, Jens; Watanabe, Kenji; Taniguchi, Takashi; Danneau, Romain

    2015-01-01

    We report the study of electronic transport in graphene-mica van der Waals heterostructures. We have designed various graphene field-effect devices in which mica is utilized as a substrate and/or gate dielectric. When mica is used as a gate dielectric we observe a very strong positive gate voltage hysteresis of the resistance, which persists in samples that were prepared in a controlled atmosphere down to even millikelvin temperatures. In a double-gated mica-graphene-hBN van der Waals heterostructure, we found that while a strong hysteresis occurred when mica was used as a substrate/gate dielectric, the same graphene sheet on mica substrate no longer showed hysteresis when the charge carrier density was tuned through a second gate with the hBN dielectric. While this hysteretic behavior could be useful for memory devices, our findings confirm that the environment during sample preparation has to be controlled strictly.

  14. Carrier and photon dynamics in a topological insulator Bi{sub 2}Te{sub 3}/GaN type II staggered heterostructure

    SciTech Connect

    Chaturvedi, P.; Chouksey, S.; Banerjee, D.; Ganguly, S.; Saha, D.

    2015-11-09

    We have demonstrated a type-II band-aligned heterostructure between pulsed laser deposited topological insulator bismuth telluride and metal organic-chemical-vapour deposited GaN on a sapphire substrate. The heterostructure shows a large valence band-offset of 3.27 eV as determined from x-ray photoelectron spectroscopy, which is close to the bandgap of GaN (3.4 eV). Further investigation using x-ray diffraction, Raman spectroscopy, and energy-dispersive x-ray spectrum reveals the stoichiometric and material properties of bismuth telluride on GaN. Steady state photon emission from GaN is found to be modulated by the charge transfer process due to diffusion across the junction. The time constant involved with the charge transfer process is found to be 0.6 ns by transient absorption spectroscopy. The heterostructure can be used for designing devices with different functionalities and improving the performance of the existing devices on GaN.

  15. Strain engineering of magnetic state in vacancy-doped phosphorene

    NASA Astrophysics Data System (ADS)

    Ren, Jie; Zhang, Chunxiao; Li, Jin; Guo, Zhixin; Xiao, Huaping; Zhong, Jianxin

    2016-09-01

    Inducing and manipulating the magnetism in two-dimensional materials play an important role for the development of the next-generation spintronics. In this letter, the effects of the biaxial strain on magnetic properties of vacancy-doped phosphorene are investigated using first-principles calculation. We find although only SV956 doping induces magnetism for unstrained phosphorene, the biaxial strain induces nonzero magnetic moment for SV5566 and DVa doped phosphorene. The biaxial strain also modulates the magnetic state for SV956, SV5566 and DVa doped phosphorene. The local magnetic moment derives from the spin polarization of the dangling bonds near the vacancy. The biaxial strain influences the local bonding configuration near the vacancy which determines the presence of dangling bonds, and then modulates the magnetic state. Our findings promise the synergistic effect of strain engineering and vacancy decoration is an effective method for the operation of phosphorene-based spintronic devices.

  16. Nonadiabatic chemoelectron energy conversion in heterostructures for hydrogen power engineering

    NASA Astrophysics Data System (ADS)

    Grankin, V. P.; Grankin, D. V.

    2015-12-01

    A relationship describing the probability of electron excitation in a solid crystal by the energy of a chemical reaction on its surface has been found. It is established that the probability of electron excitation in this reaction exponentially increases with decreasing energy of electron transition in the solid. A method of nonadiabatic chemoelectron energy conversion in heterostructures for hydrogen power engineering based on Schottky diodes is proposed and the efficiency of this method is calculated.

  17. Method for forming monolayer graphene-boron nitride heterostructures

    DOEpatents

    Sutter, Peter Werner; Sutter, Eli Anguelova

    2016-08-09

    A method for fabricating monolayer graphene-boron nitride heterostructures in a single atomically thin membrane that limits intermixing at boundaries between graphene and h-BN, so as to achieve atomically sharp interfaces between these materials. In one embodiment, the method comprises exposing a ruthenium substrate to ethylene, exposing the ruthenium substrate to oxygen after exposure to ethylene and exposing the ruthenium substrate to borazine after exposure to oxygen.

  18. Spirals and Skyrmions in Two Dimensional Oxide Heterostructures

    NASA Astrophysics Data System (ADS)

    Li, Xiaopeng; Liu, W. Vincent; Balents, Leon

    2014-02-01

    We construct the general free energy governing long-wavelength magnetism in two dimensional oxide heterostructures, which applies irrespective of the microscopic mechanism for magnetism. This leads, in the relevant regime of weak but non-negligible spin-orbit coupling, to a rich phase diagram containing in-plane ferromagnetic, spiral, cone, and Skyrmion lattice phases, as well as a nematic state stabilized by thermal fluctuations.

  19. Topological properties and correlation effects in oxide heterostructures

    NASA Astrophysics Data System (ADS)

    Okamoto, Satoshi

    2015-03-01

    Transition-metal oxides (TMOs) have long been one of the main subjects of material science because of their novel functionalities such as high-Tc superconductivity in cuprates and the colossal magnetoresistance effect in manganites. In recent years, we have seen tremendous developments in thin film growth techniques with the atomic precision, resulting in the discovery of a variety of electronic states in TMO heterostructures. These developments motivate us to explore the possibility of novel quantum states of matter such as topological insulators (TIs) in TMO heterostructures. In this talk, I will present our systematic theoretical study on unprecedented electronic states in TMO heterostructures. An extremely simple but crucial observation is that, when grown along the [111] crystallographic axis, bilayers of perovskite TMOs form buckled honeycomb lattices of transition-metal ions, similar to graphene. Thus, with the relativistic spin-orbit coupling and proper band filling, two-dimensional TI states or spin Hall insulators are anticipated. Based on tight-binding modeling and density-functional theory calculations, possible candidate materials for TIs are identified. By means of the dynamical-mean-field theory and a slave-boson mean field theory, correlation effects, characteristics of TMOs, are also examined. I will further discuss future prospects in topological phenomena in TMO heterostructures and related systems. The author thanks D. Xiao, W. Zhu, Y. Ran, R. Arita, Y. Nomura and N. Nagaosa for their fruitful discussions and collaboration. This work is supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.

  20. Field effect and magnetically induced capacitive tuning in hole doped lanthanum(1-x) strontium(x) manganese oxide

    NASA Astrophysics Data System (ADS)

    Marton, Zsolt

    Electrostatic modulation of interface conduction between semiconductors and insulating oxides is the foundation of semiconductor technology. This field effect concept can be applied on complex oxides, such as high temperature superconductors and colossal magnetoresistive manganites, in order to create new electronic and magnetic phases. Competition and coexistence of multiple nanoscale phases make them exciting to study around phase transitions. This study on hole doped La1-xSrxMnO3 systems has a two-fold purpose. One is the demonstration of the field effect on La1-xSr xMnO3 (x = 0.125, 0.2, 0.3, 0.5) thin films. It is an important step towards electrostatic control of material properties, however, a challenging task because of their charge carrier densities of 0.01-1 hole/unit cell, a few orders of magnitude larger than in doped semiconductors. Control by linear dielectrics needs huge, constantly applied bias. Energy efficient tuning with low voltages requires highly polar ferroelectric. Pb(Zr0.2Ti 0.8)O3 was chosen, whose remanence provides 0.5 charge carrier/unit cell on the manganite/ferroelectric interface. La1-xSrxMnO 3/Pb(Zr0.2Ti0.8)O3 heterostructures were synthesized by pulsed laser epitaxy and remarkable conduction modifications were observed in the La1-xSrxMnO3. This can be a strong foundation of a new tool to research electronic oxides. The second purpose of this work is to utilize the phase separation in manganites. There has been extensive research on multiferroic materials, in which dielectric and magnetic responses are controlled by magnetic and electric field, respectively. In order to demonstrate magnetically tuned capacitance, insulating La7/8Sr1/8MnO3 was studied. Drastic capacitance change in magnetic field was shown through a phase transitions and explained in the framework of electronic phase separation. It makes this material eligible for high frequency magnetoelectric applications. Modulating charge carriers, mobility and magnetism in