Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session F23: Focus Session: Dopants and Defects in Semiconductors IV |
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Sponsoring Units: DMP Chair: Beall Fowler, Lehigh University Room: 325 |
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F23.00001: The electronic structure of Group V dopants in silicon -- The requirements for a realistic DFT model Veronika Brazdova, David R. Bowler, Andrew J. Fisher Typical concentrations of Group V donors in Si wafers used in experiment are up to 10$^{18}$ cm$^{-3}$. In contrast, the simulation cell in a typical atomistic simulation would contain a few hundred Si atoms and one or two dopants. That is equivalent to concentrations on the order of at least 10$^{20}$ cm$^{-3}$. We investigate the effect of donor concentration on the electronic structure of doped bulk silicon in density functional simulations (DFT) using the linear scaling DFT code Conquest on very large cells, and the cell sizes required to model the metal-semiconductor transition correctly. [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F23.00002: Charged defect in GaSb by selective occupation in density functional theory Jianwei Wang, Yong Zhang In a density functional theory (DFT) approach, the transition energy of an acceptor-like defect is typically calculated by the total energy difference between E(N$+$1) and E(N), where N is the total number of the valence electrons of the defected system. Effectively, in this scheme, the hole in the valence band is simulated by a uniform positive background charge or a plane wave. A scheme closer to the reality would be to move one electron from the valence band maximum (VBM) to the defect level, because the VBM state usually is quite different from a plane wave. We apply this selective occupation scheme to a defect problem, an antisite defect of Ga on Sb in GaSb, and compare the results of two schemes with varying supercell size, using a pseudopotential DFT theory. [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F23.00003: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 8:36AM - 9:12AM |
F23.00004: Small polaron characteristics of the OH center in TiO$_{2}$ Invited Speaker: W. Beall Fowler Most insulating crystals have nearly-free-electron conduction bands and corresponding conduction properties, with the effective mass increased slightly by large-polaron effects. In TiO$_{2}$, the lowest conduction bands contain considerable admixture of Ti 3d states. In this case the conduction electrons become localized, or self-trapped, into small-polaron states [1], and their conduction properties differ considerably from the usual case. EPR experiments by Halliburton \textit{et al.} [2] have shown that this self-trapping is also present in association with point defects, namely substitutional F and interstitial H (which forms a bond with a lattice O). In each case the spin of the unpaired electron is localized on a nearest neighbor Ti. Infrared absorption experiments as a function of temperature on the OH center by Bekisli \textit{et al.} [3] have resolved apparent inconsistencies in the model used to fit earlier IR data. Through detailed analysis they have interpreted their results in terms of a small polaron model which involves several configurations corresponding to the localization of the OH electron on different Ti sites, each of which yields an IR line of slightly different frequency. These conclusions are supported by theoretical results in the literature and by our calculations using the CRYSTAL06 code [4] with a hybridized DFT Hamiltonian.\\[4pt] [1] A. Yidiz \textit{et al.}, J. Appl. Phys. \textbf{108}, 083701 (2010).\\[0pt] [2] S. Yang and L. E. Halliburton, Phys. Rev. B \textbf{81}, 035204 (2010); A. T. Brant \textit{et al.}, J. Appl. Phys. \textbf{110}, 053714 (2011).\\[0pt] [3] F. Bekisli \textit{et al.}, Phys. Rev. B \textbf{86}, 155208 (2012).\\[0pt] [4] R. Dovesi \textit{et al.}, \textit{Crystal06 User's Manual} (University of Torino, Torino, 2006). [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F23.00005: H and D centers in In$_{2}$O$_{3}$ studied by IR spectroscopy Weikai Yin, Kirby Smithe, Michael Stavola, W. Beall Fowler, L.A. Boatner Hydrogen has been predicted to be an important source of n-type conductivity in transparent conducting oxides (TCO's) [1]. We have used IR spectroscopy to investigate the properties of H (and D) in single crystals of the prototypical TCO, In$_{2}$O$_{3,}$ and to test the predictions of recent theory [2]. H (or D) introduces several O-H (or O-D) stretching lines and also the broad absorption arising from free carriers. We have used the vibrational properties of H- (and D-) containing centers as a probe of microscopic structure and as a strategy to monitor H-related reactions that occur upon annealing. [1] M. McCluskey \textit{et al.}, J. Mater. Res. \textbf{27}, 2190 (2012) [2] S. Limpijumnong \textit{et al.}, Phys. Rev. B \textbf{80}, 193202 (2009). [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F23.00006: Controlled introduction of defects in GaMnAs and GaBeAs thin films by ion-beam irradiation Marcelo Sant'Anna, Elis Sinnecker, Tatiana Rappoport, Mauricio Pires, Germano Penello, David Souza, Sergio Mello, Joaquim Mendes, Jacek Furdyna, Xinyu Liu The existence of interstitial Mn atoms, and other point defects, significantly modify magnetic and transport properties of Ga$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As. This opens a door to manipulate these properties in a controlled way by ion-beam irradiation of thin films. We study how the simultaneous lowering of hole concentration and increasing of disorder, introduced by ion-beam irradiation, affects the magnetization and conductivity of Ga$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As samples [1,2]. Highly doped Ga$_{\mathrm{1-x}}$Be$_{\mathrm{x}}$As is a material that can be produced with similar doping levels but that shows no ferromagnetism, acting as an interesting experimental standard for comparison of transport properties of Ga$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As. We irradiate Ga$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As and Ga$_{\mathrm{1-x}}$Be$_{\mathrm{x}}$As thin films with 2 MeV oxygen ion beams. Samples were grown by molecular beam epitaxy. Sheet resistance of the thin films was measured in situ in the irradiation chamber as a function of the incident dose.\\[4pt] [1] E. H. C. P. Sinnecker et al., Phys. Rev. B. 81, (2010) 245203.\\[0pt] [2] M. M. Sant'Anna, et al., Meth. in Phys. Res. B. 273 (2012) 72. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F23.00007: The role of d levels of substitutional magnetic impurities at the (110) GaAs surface M.R. Mahani, Anna Pertsova, Fhokrul Islam, C.M. Canali The study of the spin of individual transition-metal dopants in a semiconductor host is an emergent field known as magnetic solotronics, bearing exciting prospects for novel spintronics devices at the atomic scale. Advances in different STM based techniques allowed experimentalists to investigate substitutional dopants at a semiconductor surface with unprecedented accuracy and degree of details [1]. Theoretical studies based both on microscopic tight-binding (TB) models and DFT techniques have contributed in elucidating the experimental findings. In particular, for the case of Mn dopants on the (110) GaAs surface, TB models [2] have provided a quantitative description of the properties of the associated acceptor states. Most of these TB calculations ignore dealing explicitly with the Mn d-levels and treat the associated magnetic moment as a classical vector. However recent STM experiments [3] involving other TM impurities, such as Fe, reveal topographic features that might be related to electronic transitions within the d-level shell of the dopant. In this work we have included explicitly the d levels in the Hamiltonian. The parameters of the model have been extracted from DFT calculations. We have investigated the role that d levels play on the properties of the acceptor states of the doped GaAs(110) surface, and analyzed their implications for STM spectroscopy. [1] Yakunin et al., PRL 92, 216806 (2004), Kitchen et al., Nature 442, 436 (2006). [2] Tang et al., PRL 92, 047201 (2004), Strandberg et al., PRB 80, 024425 (2009). [3] J. Bocquel et al., arXiv:1203.6293v.1. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F23.00008: Optical measurements of trap state density and minority carrier lifetime in GaAs heterostructures grown at varying rates Chelsea Haughn, Kenneth Schmieder, Joshua Zide, Allen Barnett, Chris Ebert, Robert Opila, Matthew Doty Semiconductor growth rates are a critical factor for production costs and can have a significant impact on electrical properties. We use time resolved photoluminescence (TRPL) to characterize the effective lifetime of carriers in gallium arsenide - indium gallium phosphide (GaAs/InGaP) double heterostructures grown at varying rates. We measure the PL decay time as a function of laser fluence and extract an approximate trap state density by fitting this data with the Shockely-Read-Hall model of carrier recombination. Using the approximate trap densities, we then calculate minority carrier lifetimes for a range of doping conditions. The results suggest that the increased density of trap states associated with a two-fold increase in growth rate are less limiting to carrier lifetime than doping at the levels required for devices. The techniques and analysis developed here can be applied for rapid, non-destructive quantification of trap state densities in materials grown under varying conditions. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F23.00009: First-principles study on scattering potentials of defects on Ge(001) surfaces Tomoya Ono As new techniques for the nanoscale manipulation and modification of materials progress, the electron scattering properties of nanostructures are the focus of attention both experimentally and theoretically. The spatial maps of the local density of states obtained by scanning tunneling spectroscopy can give us the images of standing waves, which provide important information about the dispersion relation of the electron scattering process at the potential barrier. I examined the scattering potential of the Ge-Si and Ge-Sn dimers on Ge(001) surfaces using a first-principles calculation. By calculating the scattering wave functions, the standing waves in the spatial map of the local density of states are examined; the waves correspond to the image of the differential conductance obtained by scanning tunneling spectroscopy. The period of the standing wave and its phase shift agree with those obtained by the experiment. I found that the scattering potential acts as a barrier when the electronegativity of the upper atom of the dimer is larger than that of the lower atom, while it becomes a well in the opposite case. The scattering potential is related to the stabilization of the ? bands of the Ge(001) surface due to the difference in electronegativity between Ge and the impurity. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F23.00010: Why Cu diffuses fast in semiconductors? Jie Ma, Su-Huai Wei It is well-known that experimentally Cu diffuses fast in semiconductors and the fast diffusion plays an important role in many applications. However, the theoretical reason for the fast diffusion is still unclear. Using first-principles calculations, we compare the diffusion behavior between Cu and group-IA atoms in CdTe, and find that the fast diffusion of Cu can be explained by the existence of the symmetry-induced strong s-d coupling in the system, which lowers the energy significantly at the site usually consists the barrier for group-IA system. Due to this s-d coupling, the most stable doping site, diffusion pathway, and diffusion energy curve of Cu are different from those of group-IA atoms, and the diffusion barrier for Cu$+$ is usually larger than that for neutral Cu. The mechanism is expected to be general for all tetrahedral semiconductors. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F23.00011: Density functional study of the properties of Tl6SeI4 for radiation detection applications Koushik Biswas, Mao-Hua Du, David Singh The extra compositional freedom available in ternary compounds allow flexibility to tune their electronic and structural properties compared to the binary counterparts. Indeed, the Tl-based ternary semiconductor Tl6SeI4 is a promising candidate for radiation detectors. It has a band gap (1.86 eV) that is intermediate between those of Tl2Se (0.6 eV) and TlI (2.75 eV) and suitable for room temperature detectors. However, the flexibility in ternary semiconductors may come at the expense of more channels for defect formation and more complex defect chemistry, which need to be studied in details. To better understand the properties of Tl6SeI4$_{\mathrm{\thinspace }}$in relation to the radiation detection, we performed first-principles study of electronic structure, phase diagram, and dielectric, optical, and defect properties in Tl$_{\mathrm{6}}$SeI$_{\mathrm{4}}$.[1] We will discuss the properties of defects and their diffusion barriers in the context of resistivity and polarization phenomenon in Tl6SeI4. [1] K. Biswas, M.-H. Du, and D. J. Singh, \quad Phys. Rev. B \quad \textbf{86}, 144108 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F23.00012: Electronic Structure Engineering of Elpasolites for Brighter and Faster Scintillators Mao-Hua Du, Koushik Biswas Utilization of scintillator materials is one of the primary methods for radiation detection. Elpasolites are a large family of quaternary halides that have attracted considerable interest for their potential applications as $\gamma $-ray and neutron scintillators. However, many elpasolite scintillator materials currently under development suffer from low light yield and long scintillation decay time. The low light yield is partially due to a large band gap while the long scintillation decay time is a result of slow carrier transport to Ce dopants, where electrons and holes recombine to emit photons. We suggest that these problems may be mitigated by optimizing the band gap and carrier mobility by selecting constituent elements of proper electronegativity. For example, cations with lower electronegativity may lower the conduction band and increase the conduction band dispersion simultaneously, resulting in higher light yield and faster scintillation. First-principles calculations of electronic structure, small polarons, and Ce dopants in Cs$_{\mathrm{2}}$LiYCl$_{\mathrm{6}}$ and Cs$_{\mathrm{2}}$AgYCl$_{\mathrm{6}}$ compounds show that the strategy of manipulating electronegativity can lead to brighter and faster elpasolite-based scintillators. This work was supported by the U.S. DOE Office of Nonproliferation Research and Development NA22. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F23.00013: Hydrogen configurations at a high-angle grain boundary in yttria-stabilized zirconia Apostolos Marinopoulos Hydrogen is a common impurity in many technologically-relevant semiconductors and oxides. Being mobile and reactive it can form defect complexes with native defects or other extrinsic point defects. Ab initio calculations based on density-functional theory (DFT) have so far been instrumental in elucidating the tendency of hydrogen to form stable complexes with oxygen vacancies and acceptor dopants. The interaction of hydrogen with internal extended defects, such as grain boundaries, needs also to be addressed given the fact that metal oxides are commonly used in polycrystalline or nanocrystalline forms. The present DFT study aims to determine the type of hydrogen configurations that can exist at the core of a high-angle tilt grain boundary in yttria-stabilized zirconia (YSZ). The core is characterized by strong distortions for both anion and cation sublattices and lower ionic density and coordination numbers that lead to larger interstitial spaces at the interface with respect to the bulk. Formation energies and charge transition levels are determined and compared to those in the bulk YSZ where hydrogen was found to incorporate either at hydroxide-bond configurations or at interstitial sites with strong atomic character. [Preview Abstract] |
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