Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session M45: Semiconductor Electronic Structure: Theory & Spectra II |
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Sponsoring Units: FIAP Chair: Alex Demkov, University of Texas at Austin Room: Mile High Ballroom 4D |
Wednesday, March 5, 2014 11:15AM - 11:27AM |
M45.00001: Ab-initio Calculations of Accurate Electronic Properties of ZnS Bethuel Khamala, Loushanda Franklin, Yuriy Malozovski, Anthony Stewart, Diola Bagayoko We present the results from \textit{ab-initio}, self consistent, local density approximation (LDA) calculations of the electronic and related properties of zinc-blende zinc sulphide (zb-ZnS). We employed the Ceperley and Alder LDA potential and the linear combination of atomic orbital (LCAO) formalism in our non-relativistic computations. The implementation of the LCAO formalism followed the Bagayoko, Zhao, and Williams method as enhanced by Ekuma and Franklin (BZW-EF). The BZW-EF method includes a methodical search for the optimal basis set that yields the minima of the occupied energies. This search entails increasing the size of the basis set and related modifications of angular symmetry and of radial orbitals. Our calculated, direct gap of 3.725 eV, at the $\Gamma $ point, is in excellent agreement with experiment. We have also calculated the total (DOS) and partial (pDOS) densities of states, electron and hole effective masses and total energies that agree very well with available, corresponding experimental results. Acknowledgement: This research is funded in part by the National Science Foundation (NSF) and the Louisiana Board of Regents, through LASiGMA [Award Nos. EPS- 1003897, NSF (2010-15)-RII-SUBR] and NSF HRD-1002541, the US Department of Energy -- National, Nuclear Security Administration (NNSA) (Award No. DE-NA0001861), LaSPACE, and LONI-SUBR. [Preview Abstract] |
Wednesday, March 5, 2014 11:27AM - 11:39AM |
M45.00002: Inverted band structure and excitons in halide perovskites Walter R.L. Lambrecht, Ling-yi Huang The halide perovskites CsSnX$_3$, X=I, Br, Cl have recently received attention for their application in photovoltaics. The high hole-mobility and optimal band gap (1.3 eV) of $\gamma$-CsSnI$_3$ make it attractive as both absorber and hole transport material in all solid-state Gr\"atzel cells. We present self-consistent $GW$-calculations of the electronic band structures of CsSn$I_3$, CsSnBr$_3$, CsSnCl$_3$ in different phases. The most important finding is that these materials have an ``inverted'' band structure in the sense that the CBM consists of $p$-like Sn states while the VBM consists of Sn-$s$ antibonding with halide $p$-states. The reasons for the location in {\bf k}-space of the direct gap and the nature of the band edge states are explained. The intra SnI$_6$ cluster bonding is the origin of various anomalies, such as the low hole mass, the anomalous temperature dependence of the gap with lattice expansion, the relative insensitivity to the anion, and the strong dipole allowed optical transitions. We show that when the phonon contribution to the screening is included, the exciton binding energy is two orders of magnitude smaller than with electronic screening only. The observed luminescence with high binding energy is argued to be due to a defect bound exciton. [Preview Abstract] |
Wednesday, March 5, 2014 11:39AM - 11:51AM |
M45.00003: Origin of the failed ensemble averaged rule for the band gaps of the disordered nonisovalent semiconductor alloys Jie Ma, Hui-Xiong Deng, Jun-Wei Luo, Su-Huai Wei Recent calculations show that the nonisovalent random alloy, such as Zn0.5Sn0.5P, has a band gap much smaller than their ordered phases; i.e., the band gap of random alloy is not the ensemble averaged value of the ordered structures, as observed in most isovalent semiconductor alloys and predicted by cluster expansion theory. We show that this abnormal behavior in nonisovalent alloys is caused by strong wavefunction localization of the band edge states. Moreover, we show that although the disordered phase of isovalent alloys are similar to the random phase, for nonisovalent alloys the disordered phase deviates significantly from the random phase, and the completely random phase is not achievable for nonisovalent alloys under equilibrium growth conditions. [Preview Abstract] |
Wednesday, March 5, 2014 11:51AM - 12:03PM |
M45.00004: An $O(h^{2})$ Coulomb singularity correction for the Bethe-Salpeter equation Daniel Aberg, Babak Sadigh, Andre Schleife, Tomas Oppelstrup We present an improved numerical correction, at no extra computational cost, for the Coulomb singularity in the Bethe Salpeter equation for bound excitonic states. This method leads to modifications of the off-diagonal matrix elements of the Bethe-Salpeter matrix with quadratic scaling of the asymptotic error. This method is particularly well suited for systems where hybrid Brillouin sampling schemes are ineffective, e.g., systems with an indirect fundamental band gap or large supercells containing defects. Numerical results are presented for the binding energy of the ground state excitons in the two-band model as well as the scintillator material CsI. [Preview Abstract] |
Wednesday, March 5, 2014 12:03PM - 12:15PM |
M45.00005: The effect of semicore electrons on the polarizability and band gaps in \textit{ab initio }planewave-pseudopotential (PW-PP) GW calculations Derek Vigil-Fowler, Brad Malone, Steven Louie Understanding the effect of semicore electrons on \textit{ab initiio }PW-PP GW calculations is currently of great interest due to the increasing importance of complex materials with active semicore electrons, e.g.,the transition metal dichalcogenides. While past research has found a significant effect due to the inclusion of semicore electrons, it did not fully explore the nature of the various deviations of traditional valence-only PW-PP GW calculations from calculations that include the semicore electrons. We study this issue in the simple system of the Si atom, where the effect is more easily isolated, and then extend our results to bulk Si, and other bulk systems. We present results showing the effect of semicore electrons on various contributions to the GW self energy, and discuss the nature of differences with the traditional PW-PP approach. We present methods to efficiently include the effect of semicore electrons in a hierarchy of computational cost and accuracy. [Preview Abstract] |
Wednesday, March 5, 2014 12:15PM - 12:27PM |
M45.00006: Electrical Analogues of Optical {\&} EELS Spectra: Silicon David Y. Smith, William Karstens We have explored an analogy between optical and electrical-circuit resonances that yields insight into single-particle and collective excitations. The analogy rests on the similarity of the differential equations for the Drude-Lorentz model of optics and the impedance of ac circuits. A parallel combination of capacitive (C) and inductive-capacitive (L-C) branches is a suitable circuit model. The L-C branches correspond to single-particle excitations. The C branch accounts for the electric-field term in the displacement, or equivalently the free-space susceptibility. Collective excitations represent combination resonances of the L-C and C branches. These excitations involve only \textit{internal} mesh currents that can flow in the absence of an \textit{external} (input) current. In this case, the admittance of the circuit is zero corresponding to the vanishing of the dielectric function at the plasmon resonance in optics (absent resistive losses). Circuit impedance corresponds to charged-particle energy loss. In contrast, circuit admittance (inverse impedance) corresponds to optical measurements. The interference of mesh currents in the circuit model plays the role of Coulomb screening in energy-loss spectra. [Preview Abstract] |
Wednesday, March 5, 2014 12:27PM - 12:39PM |
M45.00007: Zero-point motion effect on the bandgap of diamond: validation of codes Samuel Ponc\'e, Gabriel Antonius, Paul Boulanger, Elena Cannuccia, Andrea Marini, Michel C\^ot\'e, Xavier Gonze Verification and validation of codes, as well as new theoretical methods, are of utmost importance if one wants to provide reliable results. In this work we present a rigorous and careful study of all the quantities that enters into the calculation of the zero point motion renormalization of the direct band gap of diamond due to electron-phonon coupling. This study has been done within the Allen-Heine-Cardona (AHC) formalism as implemented into Abinit and Yambo on top of Quantum Espresso. We aim at quantifying the agreement between the codes for the different quantities of interest. This study shows that one can get less than $10^{-5}Ha/at$ differences on the total energy, 0.07 cm$^{-1}$ on the phonon frequencies, 0.5\% on the electron-phonon matrix elements and less than 4 meV on the zero-point motion renormalization. At the LDA level, the converged direct bandgap renormalization in diamond due to electron-phonon coupling in the AHC formalism is -409 meV (reduction of the band gap) [1]. \\[4pt] [1] S. Ponc\'e \textit{et al.}, arXiv:1309.0729 [cond-mat.mtrl-sci] and submitted for publication in Comput. Mat. Science (2013). [Preview Abstract] |
Wednesday, March 5, 2014 12:39PM - 12:51PM |
M45.00008: Carrier separation at intra-grain partial dislocation pairs in CdTe Yelong Wu, Chen Li, Stephen Pennycook, Wanjian Yin, Yanfa Yan, Mowafak Al-Jassim Using aberration corrected scanning transmission electron microscopy (STEM), we have determined the atomic configuration of CdTe intra-grain Shockley partial dislocation pairs. Counter-intuitively, density-functional theory calculations indicate that these partial dislocation pairs do not create deep states in the band gap, instead, they lead to local band bending that separates electrons and holes, therefore reducing undesirable carrier recombination. STEM images also show that the intra-grain partial dislocation pairs are seen to annihilate and regenerate under the influence of the electron beam. A systematical examination about the annihilation of the dislocation is done. Band calculations about the dislocation pairs with different core-core distance suggest that the band bending caused by the charge transfer between the cores, which helps to separate carriers and further avoid their recombination, becomes significant when the distance increases, but does not change when the distance is larger than a critical value, dc. [Preview Abstract] |
Wednesday, March 5, 2014 12:51PM - 1:03PM |
M45.00009: Electronic Structure of Perovskite Solid Solutions (SrTiO$_{3})_{\mathrm{1-x}}$(LaTiO$_{2}$N)$_{\mathrm{x}}$ Naoto Umezawa, Anderson Janotti Band gap engineering of oxide perovskite materials is of great interest for electronics and photocatalysis. In this study we demonstrate that the band gap of SrTiO$_{3}$ is narrowed by mixing it with the oxinitride LaTiO$_{2}$N. Using hybrid density functional calculations, we study the electronic structure of LaTiO$_{2}$N and (SrTiO$_{3})_{\mathrm{1-x}}$(LaTiO$_{2}$N)$_{\mathrm{x}}$ solid solutions. We show that the valence-band maximum (VBM) of (SrTiO$_{3})_{\mathrm{1-x}}$(LaTiO$_{2}$N)$_{\mathrm{x}}$ is raised as the LaTiO$_{2}$N concentration increases, while the conduction-band minimum (CBM) remains almost unchanged. This is explained by the atomic orbitals that composed the VBM and CBM in the two parent compounds: in LaTiO$_{2}$N the VBM is derived from N 2p states, which are higher in energy than the O 2p that composed the VBM in SrTiO$_{3}$. The band gap of (SrTiO$_{3})_{\mathrm{1-x}}$(LaTiO$_{2}$N)$_{\mathrm{x}}$ is quantified and discussed in terms of the valence- and conduction-band offsets of SrTiO$_{3}$/LaTiO$_{2}$N. [Preview Abstract] |
Wednesday, March 5, 2014 1:03PM - 1:15PM |
M45.00010: Electronic structure of p-type transparent conducting oxide CuAlO$_2$ Sung-Kwan Mo, Joonseok Yoon, Xiaosong Liu, Wanli Yang, Bongjin Mun, Honglyoul Ju CuAlO$_2$ is a prototypical p-type transparent conducting oxide. Despite its importance for potential applications and number of studies on its band structure and gap characteristics, experimental study on the momentum-resolved electronic structure has been lacking. We present angle-resolved photoemission data on single crystalline CuAlO$_2$ using synchrotron light source to reveal complete band structure. Complemented by the x-ray absorption and emission spectra, we also study band gap characteristics and compare them with theory. [Preview Abstract] |
Wednesday, March 5, 2014 1:15PM - 1:27PM |
M45.00011: Electronic and optical properties of tantalum pentoxide polymorphs from first principles calculations Jihang Lee, Emmanouil Kioupakis, Wei Lu Tantalum oxide has been extensively studied due to its attractive properties as dielectric films, anti-reflection coatings, and resistive switching memory. Although various crystalline structures of tantalum pentoxide (Ta$_{2}$O$_{5}$) have been reported, the structural and electronic/optical properties still remain a controversial issue. We investigate the electronic and optical properties of crystalline and amorphous Ta$_{2}$O$_{5}$ structures using first-principles calculations in the GW approximation. The calculated band gaps of the crystalline structures are too small to explain the experimental measurements. The amorphous structure exhibits a strong exciton binding energy and an optical band gap ($\sim$ 4eV) similar to experiment. We determine the atomic orbitals that form the conduction band of each polymorph and analyze the dependence of the band gap on the atomic geometry. Our results establish the connection between the underlying structure and the electronic and optical properties of Ta$_{2}$O$_{5}$. [Preview Abstract] |
Wednesday, March 5, 2014 1:27PM - 1:39PM |
M45.00012: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 1:39PM - 1:51PM |
M45.00013: Applicability check of ZnO crystals for device applications Mithun Bhowmick, Bruno Ullrich, David Ariza, Haowen Xi There has always been vital interest in wide-band gap semiconductors for their applicability in short-wavelength photonic devices and in electronic devices operating in high frequency regime. Historically, ZnO was never favored as a potential material for the above applications primarily because of difficulty in growing it. This situation, however, has improved drastically in the past decade thereby renewing the attention on this material system. Hence, ZnO is being proposed for potential light emitting devices in the blue and UV regions of electromagnetic spectrum. ZnO single crystals are also being considered for high power transistors. In this work, we present investigations of optical properties of pure (99.99{\%}) ZnO performing transmittance, reflectance, Raman, and photoluminescence measurements. The ZnO single crystals employed in this work, were obtained commercially. We present detailed analysis of the measured data through theoretical calculations. Our results identify the state-of-the-art application potential of commercially available ZnO, revealing its advantages and limitations when compared to similar materials such as GaN. [Preview Abstract] |
Wednesday, March 5, 2014 1:51PM - 2:03PM |
M45.00014: Real-time Approach for Core-hole Dynamics in X-ray Spectra Andrew Lee, John Rehr, Fernando Vila We present a real time method to calculate dynamical core hole effects in x-ray spectra using single-determinant wavefunctions to evaluate the time-correlation series. Starting with the system in the ground state, the x-ray field suddenly creates a particle-hole pair which is propagated in real-time in the presence of the dynamically screened core-hole potential. The approach is implemented in our software package RTXS\footnote{A. J. Lee, F. D. Vila, and J. J. Rehr, Phys. Rev. B \textbf{86}, 115107 (2012)}, a local time-correlation based program for the calculation of x-ray absorption and emission spectra. RTXS uses GPAW, a grid based electronic structure code, to calculate the time-evolution operator with a Crank-Nicolson algorithm and projector augmented wave transition matrix elements. This implementation builds in full-potential electronic structure and dynamical core-hole screening, resulting in a practical and generally applicable code. Recent improvements include electron-electron interactions which had been previously neglected. By introducing the single-determinant wavefunctions, we now model intrinsic and extrinsic interactions. The method is illustrated in a number of cases, including diamond and C60. [Preview Abstract] |
Wednesday, March 5, 2014 2:03PM - 2:15PM |
M45.00015: The First Picosecond after Sunlight Absorption in Si, GaAs, and CdTe from First-Principles Calculations Marco Bernardi, Jeffrey B. Neaton, Steven G. Louie Sunlight absorption in semiconducting materials generates out-of-equilibrium electron populations $-$ also known as hot carriers $-$ relaxing towards equilibrium through a host of scattering processes at the subpicosecond time scale. While such dissipation processes typically result in the loss of more than half of the energy associated with the absorbed sunlight, a microscopic understanding of this ultrafast regime is still missing. In this talk, we provide a detailed picture of the first picosecond after sunlight absorption in semiconductors of wide use in photovoltaics (PV) such as Si, GaAs, and CdTe. Our results are based on ab initio calculations combining density functional theory and the GW plus Bethe-Salpeter Equation (GW-BSE) approach together with electron-phonon interactions. We computed the lifetimes and k-space dependence of electron-electron and electron-phonon scattering events responsible for ultrafast solar energy dissipation. Using this information, we simulated the ultrafast dynamics of hot carriers using an empirical-parameter-free formulation of the Boltzmann equation. A clear understanding of hot carrier dynamics emerges for several materials of interest in PV, and novel engineering paradigms are suggested. [Preview Abstract] |
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