Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session D40: Semiconductors: Electronic Structure |
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Sponsoring Units: FIAP Chair: Hai Ping Cheng, University of Florida Room: Colorado Convention Center 503 |
Monday, March 5, 2007 2:30PM - 2:42PM |
D40.00001: First-Principles Dielectric Spectra of Silicon: THz through UV H.M. Lawler, S. Dalosto, Z.H. Levine, E.L. Shirley, J.J. Rehr We present an implementation of the GW-Bethe-Salpeter-equation approach to first-principles calculations of dielectric response based in part on input from the plane-wave, pseudopotential code ABINIT. This work, together with lattice dynamical calculations, aims to develop versatile codes capable of calculating dielectric spectra in insulators for the full spectral range from THz to the UV. Below the bandgap, lattice vibrations absorb light in the THz range. These spectra are generally composed of sharp infrared-active features (absent by symmetry in silicon); weak, temperature dependent continuum effects from IR-active-multiphonon state hybridization; and contributions to the macroscopic polarization directly from multiphonon states. Above the bandgap, density-functional band structures are taken as a starting point for the inclusion of many-body interactions within the GW-BSE approximation. Emphasis will be on treating the excitionic effects and non-zero-momentum application of the modern theory of polarization with ABINIT. [Preview Abstract] |
Monday, March 5, 2007 2:42PM - 2:54PM |
D40.00002: Symmetry in optical properties: Silicon temperature dependent dielectric function M.J.G. Lee, A. Shkrebtii, W. Richter, M. Drago, G. Nicholls, Z.A. Ibrahim The dielectric function of the indirect gap semiconductor bulk Si has been measured experimentally in the temperature range 300 K to 1270 K, and has been modeled theoretically in the range 0 K to 1300 K. The observed low temperature optical peak at 1.1 eV is below the direct optical gap of the ideal lattice (about 3.0 eV). We attribute this peak to zero-point lattice vibrations which, by reducing the translational symmetry, allow direct optical transitions below the direct gap of the ideal lattice even at 0 K. A lattice dynamical calculation in which zero-point vibrations are included gives a good account of the temperature dependencies of the energies and the widths of the peaks in the observed dielectric function of bulk Si over the whole temperature range. We conclude that the dielectric function of bulk Si is very sensitive to the breaking of translational symmetry by the thermal motions of the atoms, and that zero-point vibrations play an essential role in a quantitative description of the dielectric function of bulk Si. The research was supported by Sfb 296 and NSERC Discovery Grants. [Preview Abstract] |
Monday, March 5, 2007 2:54PM - 3:06PM |
D40.00003: \textit{Ab initio} Optical Absorption by A Simple and Efficient Method: Single Excitation Configuration Interaction After Downfolding Kazuma Nakamura, Yoshihide Yoshimoto, Ryotaro Arita, Masatoshi Imada, Shinji Tsuneyuki We present a simple and efficient \textit{ab initio} method for calculating electronic excited states and optical absorption spectra of solids. The method is based on a single-excitation configuration-interaction calculation after downfolding to model Hamiltonians represented by maximally-localized Wannier functions. Single-excitation configurations are crucially important in evaluating a linear absorption, because they can describe a so-called excitonic effect; interactions in electron-hole pairs generated by excitations. A test was performed for a semiconductor GaAs, and detailed analyses for the resulting spectra and single-excitation many-body wavefunctions are presented. This work is supported by a Grant-in-Aid for Scientific Research in Priority Areas, ``Development of New Quantum Simulators and Quantum Design'' (No. 17064004) of the Ministry of Education, Culture, Sports, Science and Technology, Japan. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D40.00004: Band-offsets of semiconductor heterostructures using hybrid density functionals Amita Wadehra, John W. Wilkins, Richard G. Hennig, Gustavo E. Scuseria The performance of novel devices, e.g., HEMTs based on semiconductor heterostructures, depend strongly on their conduction and valence band-offsets. However, conventional density functional theory based on LDA and GGA fails for narrow-gap semiconductors such as InAs, predicting it to be a metal. An accurate treatment of such systems requires self-consistent DFT calculations with hybrid functionals such as B3LYP and HSE. B3LYP success for a wide variety of atoms and molecules is computationally challenging to translate to solids. We compare band gaps and band offsets of strained and unstrained InAs/InP and InAs/AlAs heterostructures calculated with these hybrid functionals. Our preliminary results agree well with experimental and other theoretical investigations. [Preview Abstract] |
Monday, March 5, 2007 3:18PM - 3:30PM |
D40.00005: B3LYP Works for Semiconductors Heterostructures Jeremy Nicklas, John Wilkins The B3LYP functional with Gaussian basis functions gives reliable valence band offsets (VBO) of heterostructures involving, for example, GaN, AlN, and InN. The density of states of the inner bulk layers in the heterostructure estimates the valence band offset while confirming the bulk bandgaps. The VBO is calculated for the cubic and hexagonal structures with a range of lattice constants. For example, cubic-AlN and cubic-GaN heterostructure with 8+8 (001) layers show decreasing VBO with increasing (001) planar lattice: 0.98 eV for 4.38 \AA, 0.97 eV for 4.45 \AA, and 0.85 eV for 4.52 \AA. These are consistent with previous GWA calculations [1]. [1] D. Cociorva, W. G. Aulbur and J. W. Wilkins, Solid State Communications 124, 63-66 (2002). [Preview Abstract] |
Monday, March 5, 2007 3:30PM - 3:42PM |
D40.00006: Analysis of core-valence linearization in $G_{\mathrm{0}}W_{\mathrm{0}}$ calculations Ricardo Gomez-Abal, X. Li, M. Scheffler, C. Ambrosch-Draxl In recent years the $GW$ approach, typically applied as the first order correction to the Kohn-Sham (KS) eigenenergies ($G_{\mathrm{0}}W_{\mathrm{0}}$ approximation), has substantially improved the description of single-particle excitations in weakly correlated semiconductors and insulators. Most of the existing codes are based on the pseudopotential (PP) method. It is well known from ground-state calculations that the linear treatment of the core-valence exchange-correlation interaction is not always valid. However, in the $G_{\mathrm{0}}W_{\mathrm{0}}$-PP scheme, the self-energy is computed from the (pseudo-)valence states only, keeping the core-valence interaction at the KS level. There is no justification for such a ``core-valence linearization'' of the dynamical self-energy, a highly non-linear functional of the total density. Nevertheless, $G_{\mathrm{0}}W_{\mathrm{0}}$-PP results usually agree better with experiments than the all-electron ones. In this talk we analyze the reasons for this disturbing discrepancy and the validity of the ``core-valence linearization'' in the $G_{\mathrm{0}}W_{\mathrm{0}}$-PP scheme. Calculations are performed using our own all-electron $G_{\mathrm{0}}W_{\mathrm{0}}$ code, based on the Wien2k implementation of the FP-(L)APW+lo method. We compare our all-electron results with those obtained by computing the self-energy from the valence states only as well as with $G_{\mathrm{0}}W_{\mathrm{0}}$-PP calculations for selected materials (e.g. Si, NaCl,...). [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D40.00007: Band gap bowing in large size-mismatched II-VI alloys Su-Huai Wei, Chang-Youn Moon, Y.Z. Zhu, G.D. Chen Band gap bowing coefficients in large size-mismatched II-VI alloys M$^{II}$X$^{VI}_{1-x}$O$_x$ with M$^{II}$=Zn and Cd, and X$^{VI}$=S, Se, and Te in the zinc-blend structure are calculated using first-principles methods. We show that in these systems, the bowing coefficients are large and composition-dependent. The bowing coefficients increase as the size and chemical mismatch between the constituents increase. The bowing coefficients for the Zn alloys are larger than the corresponding Cd alloys, but smaller than the corresponding III-V alloys. We show that these results can be explained by the size and atomic eigenvalue differences between the constituents and the resulting band offsets and isovalent defect levels in these systems. Our results are compared with recent experimental data. [Preview Abstract] |
Monday, March 5, 2007 3:54PM - 4:06PM |
D40.00008: Large band gap bowing in CuAgGaS2 chalcopyrite semiconductors and its effect on optical parameters Chandrima Mitra, Walter Lambrecht CuxAg$_{1-x}$GaS$_2$ chalcopyrite semiconductors have been found to exhibit a large band gap bowing. Here we use full-potential linearized muffin-tin orbital calculations in the local density approximation of density functional theory to study the electronic band structure of these materials. The randomness in the alloy is treated by the special quasirandom structures. Some layered ordered compounds are studied as well. We find the band gap to depend strongly on the c/a ratio which varies nonlinearly with concentration, in agreement with experimental data by Matsushita et al. The bowing coefficient is found to be 0.74. We also calculate the indices of refraction and their dependence on concentration. Band gap corrections are adjusted using direct shifts to the conduction bands and adjusted for the end compounds. We find that the nonlinear behaviour of the band gap also leads to a non-linear behaviour of the index of refraction with x. [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D40.00009: Optical properties of semiconductor alloys and superlattices HyeJung Kim, Yia-Chung Chang, Y.D. Kim Optical properties of semiconductor alloys and superlattices are studied using a full-potential linear augmented-Slater-type orbital (LASTO) method. The LASTO method is highly efficient for dealing with large supercells to model alloys and superlattices in reasonable time frame. Bulk properties such as band structures, equilibrium lattice constants and bulk moduli are in good agreement with existing data. We calculate optical bowing parameters and dielectric functions of zincblende semiconductors and our calculations are compared with ellipsometric measurements for InAs$_{x}$P$_{1-x}$, Zn$_{x}$Cd$_{1-x}$Se, Al$_{x}$Ga$_{1-x}$Sb alloys and (GaSb)$_{m}$(AlSb)$_{n}$ superlattices. To model alloys A$_{x}$B$_{1-x}$C, we consider 8-atom supercells of configurations B$_{4}$C$_{4}$, A$_{1}$B$_{3}$C$_{4}$, A$_{2}$B$_{2}$C$_{4}$, A$_{3}$B$_{1}$C$_{4}$, A$_{4}$C$_{4}$ (i.e., ratio of A to B atom equals 0, 0.25, 0.5, 0.75, 1). Atoms are allowed to relax to reach equilibrium positions. Complex dielectric functions are obtained after adding empirical GW corrections and semi-empirical excitonic effects. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D40.00010: First principles exploration of the possibility of high mobility phases and alloys of group IV semiconductors. Jay Sau, Marvin Cohen At present it is known that in some cases the mobility of Si can be increased through alloying of Si with Ge and straining Si epitaxially. Here we examine the possibility of higher mobility group IV semiconductors. Currently it appears that Ge based devices might become popular in the near future due to the higher mobility of Ge. Using Density Functional Theory and GW quasiparticle corrections together with k.p theory and EPM, we examine how strain and alloying Sn can be used to increase the mobility of Ge related semiconductors. In this study we account for alloy scattering due to Sn impurities in Ge using first principles calculations. We find that the effect of alloy scattering is not prohibitively large. Recently CVD based methods [Kouvetakis et al. APL, 81, 2992(2002)] have been developed to fabricate these alloys making such studies of increased technological relevance. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D40.00011: Band Edge Energies and Band Gaps of Quaternary GaInAsSb Alloys Rita Magri, Alex Zunger, Herbert Kroemer Quaternary alloys without a common atom such as (Ga,In)-(As,Sb) pose a difficult combinatorially design problem in that there are many different atomic configurations even when the system is constrained to be lattice-matched on a substrate. Using an atomistic pseudopotential approach\footnote{R. Magri, A. Zunger, H. Kroemer, JAP 98, 043701 (2005)} we have calculated the band edge energies of this quaternary random alloys as a function of Ga/In (x) and As/Sb (y) compositions assuming lattice-matching to either GaSb or InAs. The alloy is represented by a large supercell with random atomic occupations and atomic positions relaxed via the atomistic VFF functional. We find upwards bowing for both the conduction and valence band edge energies. On GaSb, the transition from staggered to broken-gap lineup is found to occur at x = 0.81 and y = 0.92, while on InAs it occurs at x = 0.59 and y = 0.62. We show that at the usual growth temperatures this quaternary alloy is not random but tends to exhibit an increased number of Ga-Sb and In-As bonds and a reduced number of In-Sb and Ga-As bonds. This effect brings the calculated band gaps in better agreement with experimental data. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D40.00012: Inverse Band Structure determination of optical properties of (In,Ga)(As,Sb) alloys Paulo Piquini, Alex Zunger InAs/GaSb superlattices and quantum wells present interesting band structure properties due to the overlap between the conduction band minimum of bulk InAs and the valence band maximum of bulk GaSb, which allow electrons to be transfered from the GaSb region to the InAs region. In long period (GaSb)$_n$/(InAs)$_n$ superlattices (SL's) one thus has a negative band gap that becomes positive as the period n decreases, due to the quantum-confinement. Using a supercell approach and calculating the electronic structure via the plane-wave empirical pseudopotential method we search for superlattice period and orientation (lattice matched to either GaSb and InAs) that gives a target band gaps in the mid infra-red region, e.g, 300 meV. This is performed by using the Inverse Band Structure approach, where a target value is first stated and a genetic algorithm search of the alloy configuration space is conducted. We study different compositions of In$_x$Ga$_{(1-x)}$As$_y$Sb$_{(1-y)}$ alloys and compare the results to those obtained by simpler $\vec k \cdot \vec p$ approaches. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D40.00013: First-principles investigation of the band gap evolution in (Pb,Sn)Te Xing Gao, Murray Daw The electronic structure of Sn-doped PbTe is interesting because of the so-called band inversion in its two end members, PbTe and SnTe [1,2]. Although, the electronic structure of these two compounds have been extensively studied by first-principles calculations, to our best knowledge, there are no direct first-principles calculations of the band gap evolution through the full range of alloying. We report a study of the electronic structure of this material through the full range of alloy content, combining SQS [3] and LDA. Our results show that disorder plays an important role in the electronic structure of this alloy. \newline [1] Dimmock, Melngailis, and Strauss, Phys. Rev. Lett. 16, 1193 (1966). \newline [2] Tung, and Cohen, Phys. Rev. 180, 823 (1969). \newline [3] Wei and Zunger, Phys. Rev. B 55, 13605 (1997). [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D40.00014: Structure, Electronic Structure and Phonons of ZnSnN2 Tula R. Paudel , Walter R. L. Lambrecht The structure and electronic structure of ZnSnN$_2$ was calculated using density functional theory in the local density density approximation (LDA) and the linearized muffin-tin orbital method (LMTO). The wurtzite lattice constant is found to be slightly greater then that of GaN. The system is found to have small direct band gap of 0.07eV at $\Gamma$ in LDA. The phonon frequencies at the $\Gamma$ were calculated by linear response theory and were labeled according to the symmetry. To the best of our knowledge this compound is studied for the first time both experimentally and theoretically. The stability of the compound will be discussed and the structural electronic and vibrational properties will be compared with other members of the Zn-IV-N$_2$ (IV=Si,Ge) compounds. [Preview Abstract] |
Monday, March 5, 2007 5:18PM - 5:30PM |
D40.00015: Electronic structure of SmN, DyN, and GdN B. Ruck, S. Granville, A.R.H. Preston, D.H. Housden, H.J. Trodahl, A. Bittar, J.E. Downes, K.E. Smith, P. Larson, W.R.L. Lambrecht The rare-earth nitrides lie on the boundary between metals and insulators, and as such present an exciting challenge to both experiment and theory. Incorporating the localized and correlated 4f electrons into band structure calculations is difficult, while the propensity of the rare-earth nitrides to oxidize in atmosphere impedes experimental studies. Here we present nitrogen K-edge x-ray absorption and emission measurements from SmN and DyN films grown in-situ at the synchrotron beamline, supported by resistivity and magnetization results. The x-ray data show a clear gap between the occupied and unoccupied states, implying that the materials are semiconducting. The density of states are in excellent agreement with band structure calculations performed in the LSDA+U scheme, as long as a rigid 1 eV upward shift is applied to the conduction band. [Preview Abstract] |
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