Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session B12: Focus Session: Dopants and Defects in Semiconductors: Compound Semiconductors I |
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Sponsoring Units: DMP Chair: Su-Huai Wei, National Renewable Energy Laboratory Room: D223/224 |
Monday, March 21, 2011 11:15AM - 11:27AM |
B12.00001: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 11:27AM - 11:39AM |
B12.00002: Optimized basis sets for coarse-grained electronic structure calculations of point defects Bj\"orn Lange, Christoph Freysoldt, J\"org Neugebauer Density-functional theory is a powerful tool to study the properties of point defects in the supercell approach. Yet, the size limitations make a description of the extended tails of defect states, especially for shallow defects, cumbersome. Atomic orbital basis sets are the method of choice to coarse-grain electronic structure calculations, but are in general not flexible enough for describing the unusual bonding situations, which occur in point defects. We employ a newly developed method that, based on a variational principle, allows to generate small atomic basis sets which optimally mimic the Kohn-Sham wavefunctions with a plane-wave basis set. We show that these basis sets accurately reproduce the underlying plane-wave calculation. We analyze how the atomic orbitals close to the defect are modified in comparison to their bulk counterparts. We are able to extend basis sets generated from small supercells and to reproduce the bandstructure of larger cells. Using this approach we construct and solve a reliable sparse model Hamiltonian for a shallow defect test system containing $10^3 .. 10^4$ atoms. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B12.00003: Strain-Enhanced Doping in Semiconductors: Effects of Dopant Size and Charge State Junyi Zhu, Feng Liu, Gerald Stringfellow, Su-Huai Wei When a semiconductor host is doped by a foreign element, it is inevitable that a volume change will occur in the doped system. This volume change depends on both the size and charge state difference between the dopant and the host element. Unlike the ``common expectation'' that if the host is deformed to the same size as the dopant, then the formation energy of the dopant would reach a minimum, our first-principles calculations discovered that when an external hydrostatic strain is applied, the change of the impurity formation energy is monotonic: it decreases if the external hydrostatic strain is applied in the same direction as the volume change. This effect also exists when a biaxial strain is applied. A simple strain model is proposed to explain this unusual behavior, and we suggest that strain could be used to significantly improve the doping solubility in semiconductor systems. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:27PM |
B12.00004: Defects and Carrier Compensation in CdTe Invited Speaker: CdTe is a very useful semiconductor material for its radiation detection and thin-film solar cell applications. Good carrier mobility and lifetime are needed for CdTe since efficient carrier collection is essential for the success of both applications. On the other hand, high resistivity is required for radiation detection for suppressing dark current and device noise. This is in contrast to CdTe-based solar cells, in which low resistivity is desired. In this talk, I will discuss the properties of native defects and impurities in CdTe with emphasis on carrier compensation and its implications in radiation detection and solar cell applications. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B12.00005: n-type doping in $Cu_{2}O$ by halogen impurities: a first-principles study Qiong Bai, Meng Tao, Qiming Zhang The present work focuses on first-principles calculations on n- type doping by F, Cl, and Br impurities in $Cu_{2}O$ under solution-grown environments. From the formation energy point of view, the substitution of oxygen in $Cu_{2}O$ is favored over the interstitial sites. The electronic structures after doping are carefully studied. [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B12.00006: Diffusion of ion implanted indium in ZnO crystals Faisal Yaqoob, Mengbing Huang, David Look We report on diffusion behaviors for ion implanted indium atoms in ZnO crystals. A c-plane ZnO crystal was implanted with In ions for four different energies (40, 100, 200, and 350 keV, respectively) and doses (8.0$\times $10$^{13}$, 1.2$\times $10$^{14}$, 1.6$\times $10$^{14}$ and 6.5$\times $10$^{14}$ /cm$^{2}$, respectively), resulting in a uniform concentration profile of In from surface to the depth $\sim $ 150 nm. The samples were annealed for 30 minutes at temperatures between 700-1000 \r{ }C with an argon or oxygen gas flow. The distributions of In atoms, either aligned or nonaligned along the crystalline directions, were measured by Rutherford backscattering combined with ion channeling. The diffusivities for nonaligned (interstitial) and aligned In atoms varied with annealing temperature via the Arrhenius relationship. The diffusion activation energies (E$_{a})$ for aligned In atoms were lower than those for interstitial In atoms, e.g., for annealing in an Ar gas, E$_{a} \quad \sim $ 0.61 eV for $<$1010$>$ aligned In atoms and E$_{a} \quad \sim $ 1.1 eV for interstitial In atoms between $<$1010$>$ atomic rows. Furthermore, the diffusion activation energies were affected by the gas species used during annealing, e.g., for annealing in an O$_{2}$ gas, E$_{a} \quad \sim $ 0.39 eV for $<$1010$>$ In atoms and E$_{a} \quad \sim $ 0.79 eV for interstitial In atoms between $<$1010$>$ atomic rows. These experimental results will be compared with first-principle calculations for In diffusion in ZnO crystals. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B12.00007: Surface effects in Co-doped ZnO nanocrystals Aline L. Schoenhalz, Gustavo M. Dalpian Semiconducting nanostructures have received high attention by scientific community due to their unusual properties and wide rage of possible applications. In this scale, the understanding of the surface effects of the material is fundamental to explain its properties. By using the Density Functional Theory within the Local Density Approximation, we report on the effects of the surface on the magnetic properties of Co-doped ZnO nanocrystals. For bulk ZnO, it is well known that the most stable magnetic interaction between Co impurities is antiferromagnetic. This is also the case for saturated nanostructures, where surface effects are not taken into account. However, when surface effects are considered, the interaction between transition metal impurities becomes ferromagnetic. We will discuss the interaction between surface and impurity states, comparing our results to experimental findings. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B12.00008: Vacancy-assisted migration of group-III impurities in ZnO Daniel Steiauf, John L. Lyons, Anderson Janotti, Chris G. Van de Walle Zinc oxide is a wide-band-gap material used as transparent conductor. As grown it often shows n-type conductivity, probably due to impurity contamination. High electron concentrations can be achieved by intentional doping with group-III elements, a process that usually involves annealing. It is thus important to understand the diffusion properties of the dopants. We perform first-principles calculations for the vacancy-assisted migration process of Al, Ga and In in ZnO, using both standard density functionals and hybrid functionals to correct the underestimated band gap. Indium induces the largest distortions in the lattice and has the highest formation energy. Its migration barrier to a neighboring Zn vacancy is the lowest. Al shows the highest barrier and thus has the best thermal stability. From the calculated migration barriers and formation energies, we determine diffusion activation energies and estimate annealing temperatures. The results are compared with recent experiments. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B12.00009: Improved LDA+U model for band gap corrected ZnO defect calculations Adisak Boonchun, Walter Lambrecht The local density approximation (LDA) is known to fail dramatically for point defects in ZnO. In the case of the oxygen vacancy, the one electron level of the 1+ charge state lies above the conduction band and leads to improper filling of the levels. Different points of view on how to implement a-posterior gap corrections still leave large uncertainty on the position of the defect levels. For the Zn-vacancy, LDA leads incorrectly to a delocalized wave function of the hole on all four neighbors. Our approach is to apply LDA+U corrections to various orbitals, O-sp and Zn-spd. The U-parameters which lead to orbital shifts $V_i=U_i(1/2-n_i)$ are adjusted to quasiparticle self-consistent GW (QSGW) calculations of the band structure, including the shifts of the band structure relative to the LDA one on an absolute scale. With this improved LDA+U model, good agreement is obtained for the minimum gap, the conduction band mass and the valence and conduction band shifts separately. The structural properties of ZnO also remain intact. When applied to the oxygen vacancy, we find the 2+/0 transition level in good agreement with recent hybrid functional calculations. Applications of the same LDA+U model to the Zn-vacancy are in progress and show that localization of the wave function on two oxygen neighbors is obtained. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B12.00010: Hybrid density functional study of gallium in ZnO Denis Demchenko The properties of interstitial and substitutional gallium impurities as well as their complexes in wurtzite ZnO are modeled using hybrid density functional theory. This approach reproduces experimental band gap and avoids any artificial gap corrections necessary when using LDA/GGA/LDA+U methods. We find that the lattice relaxations at the LDA/GGA level can also introduce large errors to the defect formation energies (up to 1.5 eV). The error is particularly large in cases where shallow occupied defect levels are formed and LDA can produce incorrect charges. The correct defect structure therefore should be obtained by relaxation using hybrid DFT method. We find both interstitial and substitutional Ga forming donor-like defect states, with substitutional Ga being energetically favorable. Acceptor-like Zn vacancy has high formation energy in the isolated state but exhibits strong preference to form defect complexes with both substitutional and interstitial gallium. (Ga$_i$-V$_{Zn}$) complex acts as a shallow donor, while (Ga$_{Zn}-V_{Zn}$) is a deep acceptor. The low formation energies of both complexes suggests an appreciable degree of self-compensation. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B12.00011: Study of defects in TlBr, InI as potential semiconductor radiation detectors Koushik Biswas, Mao-Hua Du Group III-halides such as TlBr and InI are receiving considerable attention for application in room temperature radiation detector devices. It is however, essential that these detector materials have favorable defect properties which enable good carrier transport when operating under an external bias voltage. We have studied the properties of native defects of InI and Tlbr and several important results emerge: (1) Schottky defects are the dominant low-energy defects in both materials that can potentially pin the Fermi level close to midgap, leading to high resistivity; (2) native defects in TlBr are benign in terms of electron trapping. However, anion-vacancy in InI induces a deep electron trap similar to the $F$-centers in alkali halides. This can reduce electron mobility-lifetime product in InI; (3) low diffusion barriers of vacancies and ionic conductivity could be responsible for the observed polarization phenomenon in both materials at room temperature. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B12.00012: Information-based screens for deep traps in semiconducting materials Kim Ferris, Kunal Shah, Dumont Jones The key to a successful materials search is the ability to suggest promising materials and a priori eliminate unfruitful inquiry. For semiconducting radiation detection materials, performance is characterized by several key properties; band gap, density, electron mobility, and carrier lifetime. The material's proclivity to form defects is critical, as even simple antisite and vacancy defects can be sufficiently deep to affect effective carrier lifetime and mobility. We have developed a new model for defect formation proclivity, leveraging prior defect models (van Vechten and Feichter) and our information-based work. Our approach is based upon classification of materials chemistry and properties consistent with high concentrations of particular defects (e.g. antisites and vacancies). One issue is that nearly any charged local defect can potentially form a deep trap, so the screen must cover different defect types. Second, the screening model for new materials cannot rely on generally unknown factors such as 3D crystal geometry. The resulting model is intended to provide design guidance on expected defect behavior for candidate detection materials for which there is little or no prior information. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B12.00013: Ab-Initio analysis of TlBr: limiting the ionic current without degrading the electronic one Cedric Rocha Leao, Vincenzo Lordi Although TlBr in principle presents all the theoretical requirements for making high resolution room temperature radiation detectors, practical applications of TlBr have proven to be nonviable due to the polarization that is observed in the crystal after relatively short periods of operation. This polarization, that is believed to be caused by accumulation of oppositely charged ionic species at the ends of the crystal, results in an electric field that opposes that of the applied bias, counter-acting its effect. In this work, we use state of the art quantum modeling to benchmark the theoretical limits for the performance of TlBr as a radiation detector, showing that the best experimental reports demonstrate near-ideal electronic characteristics. We then propose a model to inhibit the detrimental ionic current in the material without impacting the excellent properties of the electronic current. Prepared by LLNL under Contract DE-AC52-07NA27344. [Preview Abstract] |
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