Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session P11: Electronic Structure: Theory and Spectra I |
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Sponsoring Units: FIAP Chair: Chris Van Der Walle, University of California, Santa Barbara Room: D222 |
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P11.00001: Hybrid functional calculations for defects in TiO$_2$ Chris Van de Walle, Joel Varley, Anderson Janotti Density functional theory (DFT) has proven its value as an immensely powerful tool for assessing structural properties of defects in semiconductors or insulators. Frequently, however, information about electronic structure is required, i.e., the position of defect levels in the band gap. Since DFT in the LDA or GGA severely underestimates the band gap, the position of defect levels is subject to large error bars. Here we show that the use of hybrid functionals allows us to overcome this problem. We illustrate the power of the approach with the example of point defects in TiO$_2$, a material of high interest for electronics, optoelectronics, and photocatalysis. Unintentional n-type conductivity in TiO$_2$ has often been attributed to oxygen vacancies (V$_O$). We find that V$_O$ is indeed a shallow donor [1]. Our calculated formation energies allow us to assess whether vacancy concentrations are consistent with experimental observations of unintentional conductivity. \\[4pt] [1] A. Janotti, J. B. Varley, P. Rinke, N. Umezawa, G. Kresse, and C. G. Van de Walle, Phys. Rev. B 81, 085212 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P11.00002: Bandgap properties of amorphous TiO2 (aTiO2) M. Kylee Underwood, Binay Prasai, Bin Cai, David A. Drabold, James P. Lewis In photocatalytic and photovoltaic applications, TiO$_2$ is a convenient material due to its stability, abundance, and functionality. The natural bandgap of TiO$_2$ is very wide thus limits its usability to the UV region of the solar spectrum. Previous research has indicated that these limitations may be overcome by doping with anionic nonmetal elements such as carbon or nitrogen. However, both experimental and theoretical research suggests that these dopants tend to act more as recombination centers rather than truly enhance the functionality of TiO$_2$. Although naturally occurring in crystalline form, the initial processing of TiO$_2$ for the production of thin films and powders results in mostly amorphous materials and further processing locks the material into a particular crystalline or poly-crystalline form. The properties of anatase and rutile crystalline structures of bulk and nanophase TiO$_2$ have been studied in detail; however, amorphous TiO$_2$ (aTiO$_2$) lacks a similar depth of study. We show, through ab initio density functional theory calculations, that aTiO$_2$ exhibits a bandgap almost identical to crystalline TiO$_2$. We will further discuss our results for anionic nonmetal dopants in aTiO$_2$. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P11.00003: Evidences for Ti-N anchoring in organic dyes on TiO2 and its influence on photovoltaic performance Yang Jiao, Sheng Meng New metal-free organic dyes with a novel donor-pi-acceptor design produce efficiencies exceeding 10\% for dye-sensitized solar cells (DSSC) applications since 2010. Based on state-of- the-art electronic structure calculations and real time time- dependent density functional theory (TDDFT) simulations, we present consolidated evidences for novel Ti-N anchoring at the interface for such a broad group of new dyes, inferred from energetics, vibrational recognition, and electronic and optical data. This fact is contrary to what people usually believed and assumed in previous experiments and was largely ignored. We further demonstrate that the presence of interface Ti-N bonds largely benefit the electronic level alignment and photoelectron injection dynamics, greatly contributing to the improved efficiencies of DSSC based on cost-effective, environment-friendly organic dyes. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P11.00004: First Principles Study on Ta$_{2}$O$_{5}$ Polymorphs Yuning Wu, Hai-Ping Cheng, Lan Li Using density functional theory (DFT) with generalized gradient approximations (GGA) and the projector-augmented wave method, we have investigated structure, energetics, elastic tensors and mechanical properties of four crystalline forms of Ta$_{2}$O$_{5}$ with exact stoichiometry and a model amorphous structure. A virtual crystal potential has also been constructed to address partial oxygen occupancy and compared to models of explicit oxygen vacancies and the oxygen-rich system. Calculations show that mechanical properties of these polymorphs are highly anisotropic. By comparison with experimental data, we find that all crystalline phases and the simulated amorphous phase have Young's modulus higher than the amorphous thin film that is measured experimentally, but the variation among crystalline structures is as high a factor of 2. Electronic properties of three Ta$_{2}$O$_{5}$ polymorphs have been calculated using a hybrid DFT and Hartree-Fock functional method that improves gap size obtained by GGA. We suggest that further experimental measurements on tantala crystals are needed to understand physical properties of this import material. [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P11.00005: What is the G$^{0}$W$^{0}$ band gap of ZnO? M. Stankovski, G. Antonius, D. Waroquiers, A. Miglio, H. Dixit, P. Rinke, H. Jiang, M. Giantomassi, X. Gonze, M. C\^{o}t\'{e}, G.-M. Rignanese Recently, there has been considerable attention on ZnO as a candidate material for low-cost transparent conducting oxides. Even in its natural wurtzite bulk phase, it is numerically difficult to evaluate $G^{0}W^{0}$ quasiparticle (QP) corrections for ZnO. Therefore we have a wide range of theoretical QP gaps quoted in the literature (from $\sim\!\!1.6$~eV to $\sim\!\!3.6$~eV to be compared with $3.44$~eV experimentally). Typically, many approximations are used \textit{en route}. To find the correct theoretical gap, we have performed calculations of unprecedented accuracy. First, we study the $G^{0}W^{0}$ band gap given different ground-state DFT starting point approximations (LDA and GGA) and the effect of including scalar-relativistic corrections. Second, we present a study of results for norm-conserving pseudopotentials vs. all-electron techniques (both PAW and FP-LAPW). Four different plasmon-pole models are compared with the more accurate contour-deformation approach. Finally, a Hubbard U parameter for the 3d-states of Zn is shown to depend on the exact details of application. This work shows that the band-gap of ZnO is indeed underestimated in the $G^{0}W^{0}$ approach. [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P11.00006: Energies of formation and electronic band structure of Zn-IV-N$_{2}$ semiconductors Atchara Punya, Walter R.L. Lambrecht The II-IV-N$_{2}$ semiconductors are expected to have properties closely related to those of the III-N semiconductors. We focus on Zn-IV-N$_{2}$ semiconductors with the group IV-element Si, Ge and Sn. The formation energies of the compounds in this series were calculated by the full-potential linearized muffin-tin orbital method with LDA and GGA. Zero point motion corrections were included. Furthermore, the energies of formation of competing Zn$_{3}$N$_{2}$, Si$_{3}$N$_{4}$, Ge$_{3}$N$_{4}$, Sn$_{3}$N$_{4}$ compounds were also calculated to determine the allowed ranges of the chemical potentials of the elements where the compounds are stable at zero temperature. For comparison, we also calculated the energy of formation of GaN, which is found to be in good agreement with experimental values. All compounds in the series are found to have a large region of stability. The electronic band structures are calculated using the QSGW method. The band gaps span the region from 1.65 - 5.30 eV, increasing from ZnSnN$_{2}$ to ZnSiN$_{2}$, with the bandgap of ZnGeN$_{2}$ close to that of GaN. While ZnGeN$_{2}$ and ZnSnN$_{2}$ are direct band gaps semiconductors, ZnSiN$_{2}$ is found to have an indirect gap slightly smaller than its lowest direct gap. The states near the valence band maximum at Gamma are symmetry labeled and their splittings analyzed in terms of two crystal field parameters. Spin-orbit coupling is found to have negligible effect on these states. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P11.00007: Structural and Electronic properties of $\beta$-In$_{2}$X$_{3}$ (X = O, S, Se, Te) using \emph{ab initio} calculations S.V. Khare, S. Marsillac, N.S. Mangale, V. Gade Several III-VI body-centered tetragonal layered compounds belonging to space group I4$_{1}$/\emph{amd} have been a subject of interest recently because of their potential applications in high efficiency and environmentally friendly copper-indium-gallium-selenide (CIGS) solar cells and molecules. Here we have studied the structural, energetic, and electronic properties of four compounds $\beta$-In$_{2}$X$_{3}$ (X = O, S, Se, Te), in this space group. Using first principles computations, we have fully determined the lattice constants \emph{a} and \emph{c}, as well as 10 internal parameters that define this unique structure of primitive unit cells of 40 atoms. For $\beta$-In$_{2}$S$_{3}$ our computed values are found to be consistent with experimental measurements. The bulk modulus B, local electronic density of states (LDOS), total density of states (DOS), and band gap E$_{f}$ of these phases have been investigated. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P11.00008: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P11.00009: The surface passivation effects on the optical response of small CdTe quantum dots Osman Baris Malcioglu, Jean-Yves Raty In this work, the optical properties of various small-sized CdTe based quantum dots are investigated using time dependent density functional formalism. \texttt{turboTDDFT}, an implementation of the Lanczos-Liouville approach to linearized time-dependent density-functional theory, designed to simulate the optical spectra of molecular systems made of up to several hundreds atoms and distrubuted as a part of the open source \textsc{Quantum ESPRESSO} project is used. The response of the clusters at ambient temperature is estimated by performing averages of the optical spectra along the molecular dynamics trajectories. Different types of surface passivation schemes are considered in forming the quantum dot structures. Solvent effects on the surfaces that result from different passivation schemes are considered in detail using an explicit solvent approach. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P11.00010: First-principles study of the electronic structure of NiS and NiO Joaquin Noyola, Meng Tao, Qiming Zhang First-principles calculations of the electronic structure of NiS and NiO are performed. The exchange-correlation schemes of GGA, DFT+U and hybrid functional have been applied. The resulting band structures for each scheme are compared and analyzed to assess the reliability of the GGA, DFT+U, and hybrid functional. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P11.00011: Tight-binding based alloy scattering calculations in Si$_{1-x}$Ge$_{x}$ Saumitra Mehrotra, Abhijeet Paul, Gerhard Klimeck Role of alloy scattering in SiGe device performance has been up for debate since long time. The main source of confusion stems from the choice of alloy scattering potential parameter $\Delta $U$_{fit}$. We present a theoretical model within tight-binding representation for treating alloy scattering in SiGe devices. The approach is shown to inherently capture the alloy scattering potential parameter(s) which otherwise are experimentally fitted or determined from first principles calculations for different band edges. It is shown that both onsite (variation in atom type) and off-diagonal (variation in bond type) blocks are important in estimating the potential value. The extracted scattering potential is then used to estimate bulk alloy scattering limited mobility in atomistic SiGe representation. The results show good agreement for both n-type and p-type experimental bulk mobility values. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P11.00012: Predicting the Direct to Indirect Transition in III-V Alloys Jeremy Nicklas, John Wilkins The screened hybrid functional, HSE, used in density functional theory (DFT) has been gaining traction recently for its predictive powers of the band structure in bulk semiconductors. It is natural to assume that these accurate results would carry over to alloy semiconductors, but little work has been done to confirm this. We recently investigated the compositional dependence on the electronic band structure for a range of III-V semiconducting alloys (AlGaAs, InAlAs, AlInP, InGaP, and GaAsP) [1]. These alloys have a critical composition where the band gap crosses over from a direct band gap (having optoelectronic uses) to an indirect band gap (window layers in solar cells). A direct comparison of this critical composition is made between HSE and the standard density functional, PBE, revealing crossover compositions within 12\% atomic composition when compared to experiment while PBE overestimates by as much as 39\% atomic composition. Such results give merit that HSE is a reliable functional for tuning the electronic properties of semiconducting alloys.\\[4pt] [1] Jeremy W. Nicklas and John W. Wilkins, Appl. Phys. Lett. 97, 091902 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P11.00013: Hybrid DFT computes accurate band offsets of semiconductor alloy heterostructures Amita Wadehra, Jeremy Nicklas, John Wilkins Semiconductor alloy heterostructures are the backbone of optoelectronic devices. Among the most important parameters that determine the utility of heterostructure devices are the valence and conduction band offsets. Although DFT with standard functionals such as LDA or PBE does an acceptable job for valence band offsets, it fails to predict accurate conduction band offsets on its own due to the well-known band gap problem. We demonstrate the accuracy of HSE (Heyd-Scuseria-Ernzerhof) hybrid functional for computing the band gaps and band offsets of a broad selection of technologically important semiconductor alloys and their heterostructures, e.g., AlInAs/GaInAs, GaInP/AlGaAs, AlInP/GaInP [1]. The highlight of this study is the computation of conduction band offsets with a reliability that has eluded standard density functional theory. These results demonstrate predictive power of HSE for band engineering of relevant devices. \\[4pt] [1]. A. Wadehra, J. W. Nicklas and J. W. Wilkins, Appl. Phys. Lett. 97, 092119 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P11.00014: Electronic Structure of Random Alloys Chad Waxler, Byounghak Lee, Xavier Cartoix\'{a} We present a theoretical investigation of the evolution of the electronic properties of the random alloys as they undergo a transition from one pure crystal to another. For random substitutional alloys the Bloch wavevector is not a good quantum number due to the lack of translational invariance. In spite of this obvious fact the conventional methods used for random alloys calculations, e.g., Virtual Crystal Approximation and Coherent Potential Approximation, assume a medium that pertains the same symmetries of the parent compounds. The question we ask is how well the band structures from such effective medium theories agree with the real electronic structures. We address this issue using direct simulations of randomly distributed (Al,Ga)As and (In,Ga)P atom structures. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P11.00015: Atomic-scale evolution of interfacial electronic band alignment in epitaxial Gd$_{2}$O$_{3}$ on GaAs (100) B.C. Huang, Y.P. Chiu, M.C. Shih, J.Y. Shen, P. Chang, T.H. Chiang, C.S. Chang, M.L. Huang, M. Hong, J. Kwo Direct imaging of the atomic-scale configuration and interfacial electronic band alignment in epitaxial Gd$_{2}$O$_{3}$ high $\kappa $ oxides grown on GaAs (100) has been demonstrated using cross sectional scanning tunneling microscopy and spectroscopy. Measurements of the local density of states characteristics with atomic precision enabled us to determine the evolution of electronic properties in passivating the Gd$_{2}$O$_{3}$/GaAs hetero-interface. Close examinations suggested excellent electrical passivation at this interface, with low interfacial states and low leakage current density. In addition, from the local electronic states across the gate oxides, the spatial extent of the GaAs wavefunctions extended into the gate dielectric situates a minimum thickness of 0.8 nm for the Gd$_{2}$O$_{3}$ gate capacitance$_{.}$ [Preview Abstract] |
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