Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session B44: Focus Session: Defects in Semiconductors: Computational Methods |
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Sponsoring Units: DMP FIAP Chair: Joongoo Kang, National Renewable Energy Laboratory Room: Mile High Ballroom 4C |
Monday, March 3, 2014 11:15AM - 11:27AM |
B44.00001: Free-carrier effects on electronic and optical properties of binary oxide semiconductors Andre Schleife, Claudia Roedl While there is persistent interest in oxides, e.g., for semiconductor technology or optoelectronics, it remains difficult to achieve $n$-type and $p$-type doping of one and the same material. At the same time, higher and higher conductivities are reported for both types of doping individually. Hence, it is important to understand the corresponding influence of free carriers on electronic structure and optical properties. Modern electronic-structure calculations, based on hybrid exchange-correlation functionals and the $GW$ approximation, were performed for $n$-type (ZnO, CdO, SnO$_2$) and $p$-type (MnO, NiO) binary oxides. We use these results to analyze the influence of free carriers by computing contributions that increase (Burstein-Moss shift) or reduce (electron-electron interaction and ionized-impurity scattering) the band gaps as a function of free-carrier concentration. We also compute the carrier-concentration dependence of effective electron and hole masses and compare to experimental data. For $n$-type ZnO we compute optical absorption spectra by means of a recent extension of the Bethe-Salpeter framework. This allows us to take excitonic effects as well as the influence of free carriers on the electron-hole interaction into account. [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B44.00002: Bound and Unbound: Stabilizing the Anionic States of Adamantane through Functionalization Zachary Pozun, Vamsee Voora, Michael Falcetta, Kenneth Jordan Adamantane is the simplest diamondoid, which are nanostructures of carbon with a diamond-like cage structure. Although diamondoids have been reported to have negative electron affinities (EA),\footnote{N. D. Drummond, A. J. Williamson, R. J. Needs, and G. Galli. Phys. Rev. Lett., 95, 096801 (2005).} calculating an EA for an unbound anion is not straightforward because the localized anion state is strongly coupled to the continuum. In order to determine the energy and lifetime of temporary anions, we apply the stabilization method, where the exponents of the diffuse basis functions centered on the carbon atoms are scaled during equation-of-motion calculations; the energies of the anionic states are relatively insensitive to the scaling factor as compared to the continuum states.\footnote{J. S.-Y. Chao, M. F. Falcetta, and K. D. Jordan. J. Chem. Phys., 93(2), 1125-1134 (1990)} We use this method in order to identify temporary anion states and their associated energies and lifetimes. We also demonstrate that these states can be tuned in energy based substituting the adamantane cage with substituents that either withdraw or donate charge into the carbon-carbon backbone. Thus, the unique optoelectric properties of small diamondoids can be properly calculated and easily tuned. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B44.00003: A bound exciton model of acceptors in semiconductors Yong Zhang, Jianwei Wang We point out that the electronic structure of an acceptor bears a close similarity to that of an isoelectronic impurity bound exciton with a larger electronegativity (known as ``acceptor-like bound exciton'') [Hopfield et al., PRL 17, 312(1966)], and to some extent to that of a free exciton in a semiconductor. Instead of using only one quantity \textbf{\textit{acceptor binding energy}} E$_{\mathrm{A}}$ (based on Coulomb interaction) when dealing with the electronic transitions involving an acceptor, another quantity \textbf{\textit{impurity binding energy}} E$_{\mathrm{I}}$, depending on the atomic orbital difference, is usually more important in the transition processes. E$_{\mathrm{I}}$ resembles the role of the electron bound state or conduction band edge, whereas E$_{\mathrm{A}}$ resembles the hole or exciton binding energy, respectively, in the isoelectronic impurity or free exciton case. Furthermore, instead of viewing the acceptor impurity as a ``shallow impurity'' and isoelectronic impurity as a ``deep impurity'', it would be more appropriate to view for both impurity types that the bare electron bound state involves a localized potential, and the ionized impurity has a long-range Coulomb potential. A first-principles calculation of the total energy difference yields approximately E$_{\mathrm{I}}$ -- E$_{\mathrm{A}}$, but the energy needed to generate free holes is in fact E$_{\mathrm{I}}$. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:27PM |
B44.00004: First principle prediction of shallow defect level binding energies and deep level nonradiative recombination rates Invited Speaker: Linwang Wang Accurate calculation of defect level energies in semiconductors and their carrier capturing rate is an important issue in ab initio prediction of semiconductor properties. In this talk, I will present our result work in ab initio shallow level calculation [1] and deep level caused nonradiative recombination rate calculation [2]. In the shallow acceptor level calculation, a large system up to 64,000 atoms needs to be used to properly describe the weakly bounded hole wave functions. The single particle Hamiltonian of that system is patched from bulk potential and central potential. Furthermore, GW calculation is used to correct the one site potential of the impurity atom. The resulting binding energy agrees excellently with the experiments within 10 meV. To calculate the nonradiative decay rate, the electron-phonon coupling constants in the defect system are calculated all at once using a new variational algorithm. Multiphonon process formalism is used to calculate the nonradiative decay rate. It is found that the transition is induced by the electron and the optical phonon coupling, but the energy conservation is mostly satisfied by the acoustic phonons. The new algorithm allows fast calculation of such nonradiative decay rate for any defect levels, as well as other multiphonon processes in nanostructures. \\[4pt] [1] G. Zhang, A. Canning, N. Gronbech-Jensen, S. Derenzo, L.W. Wang, ``Shallow impurity level calculations in semiconductors using ab initio method,'' Phys. Rev. Lett 110, 166404 (2013). \\[0pt] [2] L. Shi, L.W. Wang, ``Ab initio calculations of deep level carrier nonradiative recombination rates in bulk semiconductors,'' Phys. Rev. Lett. 109, 245501 (2012). [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B44.00005: First-principles theory of radiative and nonradiative carrier capture rates at defects in semiconductors Audrius Alkauskas, Cyrus E. Dreyer, John L. Lyons, Qimin Yan, Chris G. Van de Walle We have developed a first-principles approach to calculate radiative and nonradiative carrier capture coefficients (cross sections) at defects in semiconductors. The methodology is based on the use of hybrid density functionals that provide an excellent description of both bulk and defect properties. As test cases, we applied the methodology to selected defects in GaN and ZnO. We have obtained excellent agreement with experimental results in the few cases where they are available. For deep acceptors, radiative electron capture cross sections are of the order 10$^{\mathrm{-5}}$ {\AA}$^{\mathrm{2}}$, while nonradiative hole capture cross sections are in the range 1-500 {\AA}$^{\mathrm{2}}$. Our results will (i) be very helpful for identifying the microscopic origin of defects in GaN and ZnO; (ii) provide fundamental insights into the origin of traps in electronic devices based on these materials; and (iii) help finding and controlling the centers responsible for Shockley-Read-Hall recombination in nitride optoelectronic devices. This work was supported by DOE. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B44.00006: Recombination driven vacancy motion - a mechanism of memristive switching in oxides Xiao Shen, Yevgeniy S. Puzyrev, Sokrates T. Pantelides Wide-band gap oxides with high O deficiencies are attractive memristive materials for applications. However, the details of the defect dynamics remain elusive, especially regarding what drives the defect motion to form the conducting state. While the external field is often cited as the driving force, we report an investigation of memristive switching in polycrystalline ZnO and propose a new mechanism [1]. Using results from density functional theory calculations, we show that the motion of O vacancies during switching to the conductive state is not driven by the electric field, but by recombination of carriers at these vacancies, which transfers energy to the defects and greatly enhances their diffusion. Such mechanism originates from the large structural change of O vacancies upon capturing electrons. In addition, contrary to the hypothesis that memristive switching in polycrystalline materials is facilitated by the defect motion along the grain boundary (GB), we show in our system the vacancies move perpendicular to the GB, attaching and detaching from it during the switching process. We call it recombination driven vacancy breathing.\\[4pt] [1] X. Shen, Y. S. Puzyrev, and S. T. Pantelides, MRS Commun. 3, 167 (2013). [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B44.00007: The impact of +U term on the electronic structure of Mn and Fe ions and of the gallium vacancy in GaN: GGA+U calculations Piotr Boguslawski, Oksana Volnianska, Tomasz Zakrzewski Band structure of solids is commonly calculated in the Local Density Approximation or the Generalized Gradient Approximation to the Density Functional Theory. Their known failure is the underestimation of the band gap. Within LDA or GGA, the approach of semi-empirical character that leads to correct band gaps consists in adding the +U term for particular atomic orbitals. While the impact of the +U term on bands of an ideal crystal was extensively discussed, its impact on the electronic structure of defects is less understood. Here, we systematically analysed how the +U term affects the properties of the gallium vacancy V:Ga, and of the Mn and Fe transition metal (TM) ions in GaN. The +U term was treated as a free parameter, and it was applied to p(N) and d(TM) orbitals. The results of GGA+U calculations were compared to available experimental data. U(N)=4 eV reproduces well the gap of GaN. We find that the +U terms strongly affect the electronic structure of Mn, Fe, and V:Ga. Surprisingly, however, for U=0, the energies of the gap levels induced by these centers, and of the intra-center optical transitions, agree well with experiment. In contrast, for U(N)=U(TM)=4 eV, these energies are in substantial disagreement with experimental values by about 1-2 eV. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B44.00008: Relating the defect band gap and the density functional band gap Peter Schultz, Arthur Edwards Density functional theory (DFT) is an important tool to probe the physics of materials. The Kohn-Sham (KS) gap in DFT is typically (much) smaller than the observed band gap for materials in nature, the infamous ``band gap problem.'' Accurate prediction of defect energy levels is often claimed to be a casualty---the band gap defines the energy scale for defect levels. By applying rigorous control of boundary conditions in size-converged supercell calculations, however, we compute defect levels in Si and GaAs with accuracies of $\sim$0.1 eV, across the full gap, unhampered by a band gap problem. Using GaAs as a theoretical laboratory, we show that the defect band gap---the span of computed defect levels---is insensitive to variations in the KS gap (with functional and pseudopotential), these KS gaps ranging from 0.1 to 1.1 eV. The defect gap matches the experimental 1.52 eV gap. The computed defect gaps for several other III-V, II-VI, I-VII, and other compounds also agree with the experimental gap, and show no correlation with the KS gap. Where, then, is the band gap problem? This talk presents these results, discusses why the defect gap and the KS gap are distinct, implying that current understanding of what the ``band gap problem'' means---and how to ``fix'' it---need to be rethought. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B44.00009: Convergence of density and hybrid functional defect calculations for compound semiconductors Stephan Lany, Haowei Peng, David Scanlon, Vladan Stevanovic, Julien Vidal, Graeme Watson Recent revisions of defect formation energy calculations based on band-gap corrected hybrid functionals have raised concerns about the validity of earlier results based on standard density functionals, and about the reliability of the theoretical prediction of electrical properties in semiconductor materials in general. We show here that a close agreement between the two types of functionals can be achieved by determining appropriate values for the electronic and atomic reference energies, thereby mitigating uncertainties associated with the choice of the underlying functional. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B44.00010: From Light Impurity Doping to Complete Cation Exchange in Semiconductor Nanocrystals: The Role of Coulomb Interactions Steven Erwin, Florian Ott, David Norris Cation exchange is a reversible chemical reaction used to create new materials by replacing one type of cation with another, usually from solution. We have developed an atomistic model describing cation exchange in semiconductor nanocrystals. The model uses a small set of results obtained from DFT calculations for Ag-doped CdSe. From these we constructed a kinetic Monte Carlo model to address finite temperatures and time scales beyond the reach of DFT. Our simulations span a wide range of Ag concentrations, from light doping to full cation exchange. Thus our model provides a single conceptual framework in which these two phenomena can be understood as limiting endpoints. The results of the simulations are consistent with several experimentally observed aspects of both phenomena. An unexpected finding of our simulations is that the Coulomb interaction plays a central, but changing, role as the Ag concentration varies from light doping to fully cation exchanged. For example, if the Coulomb interaction is strongly screened then cation exchange is suppressed or stopped. When only moderately screened, Coulomb effects play an unanticipated but important role for both doping and cation exchange. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B44.00011: Electronic structure and van der Waals interactions in the stability and mobility of point defects in semiconductors Wang Gao, Alexandre Tkatchenko Point defects are abundant in materials, and significantly affect the electronic, optical, and magnetic properties of solids. However, our understanding of the stability and mobility of point defects remains incomplete, despite decades of intensive work on the subject. In the framework of density-functional theory, Perdew-Burke-Ernzerhof functional underestimates formation energies by 0.7 eV due to the electron self-interaction error, while Heyd-Scuseria-Ernzerhof (HSE) functional yields formation energies in better agreement with high-level many-body methods, but often overestimates migration barriers by up to 0.4 eV. Using HSE coupled with screened long-range vdW interactions [1], we demonstrate that HSE+vdW can accurately describe both the formation energies and migration barriers of point defects. The inclusion of vdW interactions largely changes the transition state geometries, and brings migration barrier into close agreement with experimental values for six different defects. For multiatom vacancies and point defects in heavier semiconductors, vdW energy plays an increasingly larger role [2]. [1] G. X. Zhang {\em et al.}, PRL {\bf 107}, 245501 (2011); A. Tkatchenko, {\em et al.}, PRL {\bf 108}, 236402 (2012). [2] W. Gao {\em et al.}, PRL {\bf 111}, 045501 (2013). [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B44.00012: spds* Tight-Binding Model for Transition Metal Dopants in SiC Victoria R. Kortan, C\"{u}neyt \c{S}ahin, Michael E. Flatt\'e SiC is a well known, wide-band-gap semiconductor with excellent chemical, thermal and mechanical stability. These traits make it an attractive material for high temperature, hostile environment, high power and high frequency device design[1]. A necessary step in the development of SiC technology is the understanding and subsequent control of point defects[2]. In addition to altering optoelectronic properties, single dopants can add effects dependent on the specific dopant species. In particular the d-states of transition metal dopants have been predicted to allow the control of the single Ni spin state with the application of strain in diamond [3] and single Fe dopants in GaAs have a core transition that can be manipulated by a STM and produce a decrease in tunneling current[4]. Here we choose a first and second nearest neighbor spds* tight-binding model to calculate the electronic trends and defect wavefunctions of transition metal dopants in 3C-SiC. Additionally we calculate the exchange interaction between pairs of dopants.\\[4pt] [1] H. Morko\c{c}, et al, J. of Appl. Phys. 76, 1363 (1994).\\[0pt] [2] S. Greulich-Weber, Phys. Stat. Sol. (a) 162, 95 (1997).\\[0pt] [3] T. Chanier, et al, EPL 99, 67006 (2012).\\[0pt] [4] J. Bocquel, et al, Phys. Rev. B 87, 075421 (2013). [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B44.00013: Simulated doping of Si from first principles using pseudo-atoms Ofer Sinai, Leeor Kronik Semiconductor doping is a process of fundamental importance to semiconductor physics and solid-state electronics, but cannot be explicitly simulated from first-principles due to the huge system size needed for most doping scenarios. We examine the efficacy of the simulation of doping in silicon by the inclusion of ``pseudo atoms'' with fractional nuclear charge, introduced via specially-constructed pseudopotentials. These provide a net charge carrier concentration matching an arbitrary chosen doping level, at no increase of the computational cost. By extending this approach to consider minute deviations from the integer charge, we demonstrate that the electron Fermi level can be set to any value within the forbidden gap, at minimal perturbation of the electronic structure. Beyond the bulk scenario, we successfully simulate the development of the space-charge region in a heavily-doped p-n junction and examine the doping-dependence of the work function of the hydrogen-passivated (semiconducting) Si(111) surface. [Preview Abstract] |
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