The Experts below are selected from a list of 258 Experts worldwide ranked by ideXlab platform
Chris G Van De Walle - One of the best experts on this subject based on the ideXlab platform.
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first principles calculations for Point Defects in solids
Reviews of Modern Physics, 2014Co-Authors: Christoph Freysoldt, Blazej Grabowski, Tilmann Hickel, Jorg Neugebauer, Georg Kresse, Anderson Janotti, Chris G Van De WalleAbstract:Point Defects and impurities strongly affect the physical properties of materials and have a decisive impact on their performance in applications. First-principles calculations have emerged as a powerful approach that complements experiments and can serve as a predictive tool in the identification and characterization of Defects. The theoretical modeling of Point Defects in crystalline materials by means of electronic-structure calculations, with an emphasis on approaches based on density functional theory (DFT), is reviewed. A general thermodynamic formalism is laid down to investigate the physical properties of Point Defects independent of the materials class (semiconductors, insulators, and metals), indicating how the relevant thermodynamic quantities, such as formation energy, entropy, and excess volume, can be obtained from electronic structure calculations. Practical aspects such as the supercell approach and efficient strategies to extrapolate to the isolated-defect or dilute limit are discussed. Recent advances in tractable approximations to the exchange-correlation functional ($\mathrm{DFT}+U$, hybrid functionals) and approaches beyond DFT are highlighted. These advances have largely removed the long-standing uncertainty of defect formation energies in semiconductors and insulators due to the failure of standard DFT to reproduce band gaps. Two case studies illustrate how such calculations provide new insight into the physics and role of Point Defects in real materials.
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native Point Defects in zno
Physical Review B, 2007Co-Authors: Anderson Janotti, Chris G Van De WalleAbstract:We have performed a comprehensive first-principles investigation of native Point Defects in ZnO based on density functional theory within the local density approximation (LDA) as well as the $\mathrm{LDA}+U$ approach for overcoming the band-gap problem. Oxygen deficiency, manifested in the form of oxygen vacancies and zinc interstitials, has long been invoked as the source of the commonly observed unintentional $n$-type conductivity in ZnO. However, contrary to the conventional wisdom, we find that native Point Defects are very unlikely to be the cause of unintentional $n$-type conductivity. Oxygen vacancies, which have most often been cited as the cause of unintentional doping, are deep rather than shallow donors and have high formation energies in $n$-type ZnO (and are therefore unlikely to form). Zinc interstitials are shallow donors, but they also have high formation energies in $n$-type ZnO and are fast diffusers with migration barriers as low as $0.57\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$; they are therefore unlikely to be stable. Zinc antisites are also shallow donors but their high formation energies (even in Zn-rich conditions) render them unlikely to be stable under equilibrium conditions. We have, however, identified a different low-energy atomic configuration for zinc antisites that may play a role under nonequilibrium conditions such as irradiation. Zinc vacancies are deep acceptors and probably related to the frequently observed green luminescence; they act as compensating centers in $n$-type ZnO. Oxygen interstitials have high formation energies; they can occur as electrically neutral split interstitials in semi-insulating and $p$-type materials or as deep acceptors at octahedral interstitial sites in $n$-type ZnO. Oxygen antisites have very high formation energies and are unlikely to exist in measurable concentrations under equilibrium conditions. Based on our results for migration energy barriers, we calculate activation energies for self-diffusion and estimate defect-annealing temperatures. Our results provide a guide to more refined experimental studies of Point Defects in ZnO and their influence on the control of $p$-type doping.
Anderson Janotti - One of the best experts on this subject based on the ideXlab platform.
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first principles calculations for Point Defects in solids
Reviews of Modern Physics, 2014Co-Authors: Christoph Freysoldt, Blazej Grabowski, Tilmann Hickel, Jorg Neugebauer, Georg Kresse, Anderson Janotti, Chris G Van De WalleAbstract:Point Defects and impurities strongly affect the physical properties of materials and have a decisive impact on their performance in applications. First-principles calculations have emerged as a powerful approach that complements experiments and can serve as a predictive tool in the identification and characterization of Defects. The theoretical modeling of Point Defects in crystalline materials by means of electronic-structure calculations, with an emphasis on approaches based on density functional theory (DFT), is reviewed. A general thermodynamic formalism is laid down to investigate the physical properties of Point Defects independent of the materials class (semiconductors, insulators, and metals), indicating how the relevant thermodynamic quantities, such as formation energy, entropy, and excess volume, can be obtained from electronic structure calculations. Practical aspects such as the supercell approach and efficient strategies to extrapolate to the isolated-defect or dilute limit are discussed. Recent advances in tractable approximations to the exchange-correlation functional ($\mathrm{DFT}+U$, hybrid functionals) and approaches beyond DFT are highlighted. These advances have largely removed the long-standing uncertainty of defect formation energies in semiconductors and insulators due to the failure of standard DFT to reproduce band gaps. Two case studies illustrate how such calculations provide new insight into the physics and role of Point Defects in real materials.
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native Point Defects in zno
Physical Review B, 2007Co-Authors: Anderson Janotti, Chris G Van De WalleAbstract:We have performed a comprehensive first-principles investigation of native Point Defects in ZnO based on density functional theory within the local density approximation (LDA) as well as the $\mathrm{LDA}+U$ approach for overcoming the band-gap problem. Oxygen deficiency, manifested in the form of oxygen vacancies and zinc interstitials, has long been invoked as the source of the commonly observed unintentional $n$-type conductivity in ZnO. However, contrary to the conventional wisdom, we find that native Point Defects are very unlikely to be the cause of unintentional $n$-type conductivity. Oxygen vacancies, which have most often been cited as the cause of unintentional doping, are deep rather than shallow donors and have high formation energies in $n$-type ZnO (and are therefore unlikely to form). Zinc interstitials are shallow donors, but they also have high formation energies in $n$-type ZnO and are fast diffusers with migration barriers as low as $0.57\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$; they are therefore unlikely to be stable. Zinc antisites are also shallow donors but their high formation energies (even in Zn-rich conditions) render them unlikely to be stable under equilibrium conditions. We have, however, identified a different low-energy atomic configuration for zinc antisites that may play a role under nonequilibrium conditions such as irradiation. Zinc vacancies are deep acceptors and probably related to the frequently observed green luminescence; they act as compensating centers in $n$-type ZnO. Oxygen interstitials have high formation energies; they can occur as electrically neutral split interstitials in semi-insulating and $p$-type materials or as deep acceptors at octahedral interstitial sites in $n$-type ZnO. Oxygen antisites have very high formation energies and are unlikely to exist in measurable concentrations under equilibrium conditions. Based on our results for migration energy barriers, we calculate activation energies for self-diffusion and estimate defect-annealing temperatures. Our results provide a guide to more refined experimental studies of Point Defects in ZnO and their influence on the control of $p$-type doping.
Wolfgang Windl - One of the best experts on this subject based on the ideXlab platform.
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Native Point Defects in Boron Arsenide
arXiv: Materials Science, 2019Co-Authors: Yaxian Wang, Wolfgang WindlAbstract:We present $ab$ $initio$ results for structure and energetics of native Point Defects in BAs. We find that antisites are the constitutional Defects in B-rich and As-rich material, while B$_\mathrm{As}$ antisites and B vacancies dominate stoichiometry. As experimental validation, we examine the effect of these Point Defects on the thermal conductivity in non-stoichiometric BAs. With the thermodynamically correct Point Defects treated as mass Defects, extrapolation of experimental results for non-stoichiometric samples to perfect crystals confirm our predicted antisite Defects, while showing discrepancies for previously proposed constitutional Defects.
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Native Point Defects in binary InP semiconductors
Journal of Materials Science, 2012Co-Authors: Rohan Mishra, Oscar D. Restrepo, Ashutosh Kumar, Wolfgang WindlAbstract:We present a holistic method to identify stable Point Defects in InP and the position of their defect states within the experimental band gap using density functional theory. We have calculated the formation energy of the different charge neutral native Point Defects for both stoichiometric and non-stoichiometric InP by determining the chemical potentials of In and P within the compound correctly from thermodynamic considerations. For stoichiometric InP, we predict phosphorous vacancies and phosphorus antisites to be most probable, among the neutral Defects. For In-rich and P-rich compositions, we find indium and phosphorous antisites to be most stable, respectively, when neglecting charges. We then present a careful analysis to identify the defect levels associated with each Point defect within the experimental band gap and compare it with existing experiments. By comparing calculations with different cell sizes and with varying band gaps from different exchange–correlation functionals (GGA vs. hybrid functional), we examine the dependence of the defect states on cell size and position of the excited states and analyze their nature and expected position in real systems along with the resulting charges on the Defects. Finally, we include a recipe to approximate the Fermi level dependence of the chemical potential of charged Defects in binary compounds, allowing calculation of their formation energies. Considering charges, the dominant Point Defects for stoichiometric InP are +4 and +2 charged indium and phosphorous antisites for Fermi energies 1.1 eV, respectively. For non-stoichiometric InP, the respective antisites are constitutional Defects in their minimum-energy charge states, depending on the Fermi level.
Kazuhiro Otsuka - One of the best experts on this subject based on the ideXlab platform.
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Universal symmetry property of Point Defects in crystals
Physical review letters, 2000Co-Authors: Xiaobing Ren, Kazuhiro OtsukaAbstract:Point Defects (vacancies, solute atoms, and disorder) are ubiquitous in crystalline solids. With in situ transmission electron microscopy we find clear evidence for the existence of a universal symmetry property of Point Defects; i.e., the symmetry of short-range order of Point Defects follows the crystal symmetry when in equilibrium. We further show that this symmetry-conforming property can lead to various interesting effects including ``aging-induced microstructure memory'' and the associated ``time-dependent two-way shape memory.''
P Sherwood - One of the best experts on this subject based on the ideXlab platform.
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Point Defects in zno
Faraday Discussions, 2007Co-Authors: Alexey A Sokol, Samuel A French, Stefan T Bromley, Richard C A Catlow, Hubertus Van Dam, P SherwoodAbstract:We have investigated intrinsic Point Defects in ZnO and extended this study to Li, Cu and Al impurity centres. Atomic and electronic structures as well as defect energies have been obtained for the main oxidation states of all Defects using our embedded cluster hybrid quantum mechanical/molecular mechanical approach to the treatment of localised states in ionic solids. With these calculations we were able to explain the nature of a number of experimentally observed phenomena. We show that in zinc excess materials the energetics of zinc interstitial are very similar to those for oxygen vacancy formation. Our results also suggest assignments for a number of bands observed in photoluminescence and other spectroscopic studies of the material.
