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Special Quasi-ordered Structures: role of short-range order in the semiconductor alloy (GaN)$_{1-x}$(ZnO)$_x$  [PDF]
Jian Liu,Maria V. Fernández-Serra,Philip B. Allen
Physics , 2015,
Abstract: This paper studies short-range order (SRO) in the semiconductor alloy (GaN)$_{1-x}$(ZnO)$_x$. Monte Carlo simulations performed on a density functional theory (DFT)-based cluster expansion model show that the heterovalent alloys exhibit strong SRO because of the energetic preference for the valence-matched nearest-neighbor Ga-N and Zn-O pairs. To represent the SRO-related structural correlations, we introduce the concept of Special Quasi-ordered Structure (SQoS). Subsequent DFT calculations reveal dramatic influence of SRO on the atomic, electronic and vibrational properties of the (GaN)$_{1-x}$(ZnO)$_x$ alloy. Due to the enhanced statistical presence of the energetically unfavored Zn-N bonds with the strong Zn3$d$-N2$p$ repulsion, the disordered alloys exhibit much larger lattice bowing and band-gap reduction than those of the short-range ordered alloys. Inclusion of lattice vibrations stabilizes the disordered alloy.
Temperature and composition dependence of short-range order and entropy, and statistics of bond length: the semiconductor alloy (GaN)$_{1-x}$(ZnO)$_x$  [PDF]
Jian Liu,Luana S. Pedroza,Carissa Misch,Maria V. Fernández-Serra,Philip B. Allen
Physics , 2013, DOI: 10.1088/0953-8984/26/27/274204
Abstract: We present total energy and force calculations on the (GaN)$_{1-x}$(ZnO)$_{x}$ alloy. Site-occupancy configurations are generated by Monte Carlo (MC) simulations, based on a cluster expansion (CE) model proposed in a previous study. Surprisingly large local atomic coordinate relaxations are found by density-functional calculations using a 432-atom periodic supercell, for three representative configurations at $x=0.5$. These are used to generate bond length distributions. The configurationally averaged composition- and temperature-dependent short-range order (SRO) parameters of the alloys are discussed. Entropy is approximated in terms of pair distribution statistics and thus related to SRO parameters. This approximate entropy is compared with accurate numerical values from MC. An empirical model for the dependence of bond length on local chemical environments is proposed.
Luminescence of ZnO heterolayers prepared by isovalent substitution on ZnTe substrates
Makhniy V.P.,Slyotov M.M.,Khusnutdinov S.V.
Ukrainian Journal of Physical Optics , 2008,
Abstract: The optical properties of undoped zinc oxide layers prepared by isovalent substitution technique are discussed. The singularities observed in the reflection and luminescence spectra occurring at 3.2 and 3.265 eV are related to the energy distances between the conduction band and the valence subbands, the basic one and that split due to spin-orbit interaction. The emission bands at the energies less than 3.2 eV are formed by transitions of charge carriers with the participation of local centres.
Comparison of strong coupling regimes in bulk GaAs, GaN and ZnO semiconductor microcavities  [PDF]
Stéphane Faure,T. Guillet,P. Lefebvre,T. Bretagnon,B. Gil
Physics , 2008, DOI: 10.1103/PhysRevB.78.235323
Abstract: Wide bandgap semiconductors are attractive candidates for polariton-based devices operating at room temperature. We present numerical simulations of reflectivity, transmission and absorption spectra of bulk GaAs, GaN and ZnO microcavities, in order to compare the particularities of the strong coupling regime in each system. Indeed the intrinsic properties of the excitons in these materials result in a different hierarchy of energies between the valence-band splitting, the effective Rydberg and the Rabi energy, defining the characteristics of the exciton-polariton states independently of the quality factor of the cavity. The knowledge of the composition of the polariton eigenstates is central to optimize such systems. We demonstrate that, in ZnO bulk microcavities, only the lower polaritons are good eigenstates and all other resonances are damped, whereas upper polaritons can be properly defined in GaAs and GaN microcavities.
Photoluminescence of a ZnO/GaN Heterostructure Interface

LIU Shu-Jian,YU Qing-Xuan,WANG Jian,LIAO Yuan,LI Xiao-Guang,

中国物理快报 , 2009,
Abstract: Growth of a ZnO/GaN heterostructure is carried out using pulsed laser deposition. By etching the ZnO layer from the ZnO/GaN structure, the photoluminescence (PL) of the associated GaN layer shows that the donor-acceptor luminescence of GaN shifts to about 3.27eV, which is consistent with the electroluminescence (EL) of n-ZnO/p-GaN already reported. XPS shows that oxygen diffuses into the GaN crystal lattice from the surface to 20nm depth. The PL spectra at different temperatures and excitation densities show that oxygen plays the role of potential fluctuation. The associated PL results of the interface in these LEDs could be helpful to understand the mechanism of EL spectra for ZnO/GaN p-n junctions.
Valence Bond Entanglement Entropy  [PDF]
Fabien Alet,Sylvain Capponi,Nicolas Laflorencie,Matthieu Mambrini
Physics , 2007, DOI: 10.1103/PhysRevLett.99.117204
Abstract: We introduce for SU(2) quantum spin systems the Valence Bond Entanglement Entropy as a counting of valence bond spin singlets shared by two subsystems. For a large class of antiferromagnetic systems, it can be calculated in all dimensions with Quantum Monte Carlo simulations in the valence bond basis. We show numerically that this quantity displays all features of the von Neumann entanglement entropy for several one-dimensional systems. For two-dimensional Heisenberg models, we find a strict area law for a Valence Bond Solid state and multiplicative logarithmic corrections for the Neel phase.
A Ga-doped ZnO transparent conduct layer for GaN-based LEDs

Liu Zhen,Wang Xiaofeng,Yang Hu,Duan Yao,Zeng Yiping,

半导体学报 , 2010,
Abstract: An 8 μ m thick Ga-doped ZnO (GZO) film grown by metal-source vapor phase epitaxy was deposited on a GaN-based light-emitting diode (LED) to substitute for the conventional ITO as a transparent conduct layer (TCL). Electroluminescence spectra exhibited that the intensity value of LED emission with a GZO TCL is markedly improved by 23.6% as compared to an LED with an ITO TCL at 20 mA. In addition, the forward voltage of the LED with a GZO TCL at 20 mA is higher than that of the conventional LED. To investigate the reason for the increase of the forward voltage, X-ray photoelectron spectroscopy was performed to analyze the interface properties of the GZO/p-GaN heterojunction. The large valence band offset (2.24± 0.21 eV) resulting from the formation of Ga2O3 in the GZO/p-GaN interface was attributed to the increase of the forward voltage.
The Valence Bond Glass phase  [PDF]
M. Tarzia,G. Biroli
Physics , 2008, DOI: 10.1209/0295-5075/82/67008
Abstract: We show that a new glassy phase can emerge in presence of strong magnetic frustration and quantum fluctuations. It is a Valence Bond Glass. We study its properties solving the Hubbard-Heisenberg model on a Bethe lattice within the large $N$ limit introduced by Affleck and Marston. We work out the phase diagram that contains Fermi liquid, dimer and valence bond glass phases. This new glassy phase has no electronic or spin gap (although a pseudo-gap is observed), it is characterized by long-range critical valence bond correlations and is not related to any magnetic ordering. As a consequence it is quite different from both valence bond crystals and spin glasses.
High spin states of cation vacancies in GaP, GaN, AlN, BN, ZnO and BeO: A first principles study  [PDF]
O. Volnianska,P. Boguslawski
Physics , 2011, DOI: 10.1103/PhysRevB.83.205205
Abstract: High spin states of cation vacancies in GaP, GaN, AlN, BN, ZnO and BeO were analyzed by first principles calculations. The spin-polarized vacancy-induced level is located in the band gap in GaP, ZnO and BeO. In the nitrides, the stronger exchange coupling forces the vacancy states to be resonant with valence bands, forbids formation of positively charged vacancies in GaN and BN, and allows Al vacancy in p-AlN to assume the highest possible S=2 spin state. The shape of the spin density, isotropic in the zinc blende structure, has a pronounced directional character in the wurtzite structure. Stability of spin polarization of the vacancy states is determined by spin polarization energies of anions, as well as by interatomic distances between the vacancy neighbors, and thus is given by both the lattice constant of the host and the atomic relaxations around the vacancy. Implications for experiment are discussed.
Valence band photoemission from the GaN(0001) surface  [PDF]
T. Strasser,C. Solterbeck,F. Starrost,W. Schattke
Physics , 1999, DOI: 10.1103/PhysRevB.60.11577
Abstract: A detailed investigation by one-step photoemission calculations of the GaN(0001)-(1x1) surface in comparison with recent experiments is presented in order to clarify its structural properties and electronic structure. The discussion of normal and off-normal spectra reveals through the identified surface states clear fingerprints for the applicability of a surface model proposed by Smith et al. Especially the predicted metallic bonds are confirmed. In the context of direct transitions the calculated spectra allow to determine the valence band width and to argue in favor of one of two theoretical bulk band structures. Furthermore a commonly used experimental method to fix the valence band maximum is critically tested.
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