Joseph T. Schick

First-principles study of As interstitials in GaAs: Convergence, relaxation, and formation energy

Convergence of density-functional supercell calculations for defect formation energies, charge transition levels, localized defect state properties, and defect atomic structure and relaxation is investigated using the arsenic split interstitial in GaAs as an example. Supercells containing up to 217 atoms and a variety of

Enhanced charge ordering transition in doped CaFeO3 through steric templating

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Gallium interstitial contributions to diffusion in gallium arsenide

-space sampling schemes are considered. It is shown that a good description of the localized defect state dispersion and charge state transition levels requires at least a 217-atom supercell, although the defect structure and atomic relaxations can be well converged in a 65-atom cell. Formation energies are calculated for the As split interstitial, Ga vacancy, and As antisite defects in GaAs, taking into account the dependence upon chemical potential and Fermi energy. It is found that equilibrium concentrations of As interstitials will be much lower than equilibrium concentrations of As antisites in As-rich, n-type, or semi-insulating GaAs.

Coupling between octahedral rotations and local polar displacements in WO3/ReO3 superlattices

We report a density functional theory investigation of B-site doped CaFeO3, a prototypical charge ordered perovskite. At 290 K, CaFeO3 undergoes a metal-insulator transition and a charge disproportionation reaction 2Fe 4+ →Fe 5+ +Fe 3+ . We observe that when Zr dopants occupy a (001) layer, the band gap of the resulting solid solution increases to 0.93 eV due to a two-dimensional Jahn-Teller-type distortion, where FeO6 cages on the xy plane elongate along x and y alternatively between neighboring Fe sites. Furthermore, we show that the rock-salt ordering of the Fe 5+ and Fe 3+ cations can be enhanced when the B-site dopants are arranged in a (111) plane due to a collective steric effect that facilitates the size discrepancy between the Fe 5+ O6 and Fe 3+ O6 octahedra and therefore gives rise to a larger band gap. The enhanced charge disproportionation in these solid solutions is verified by rigorously calculating the oxidation states of the Fe cations with different octahedral cage sizes. We therefore predict that the corresponding transition temperature will increase due to the enhanced charge ordering and larger band gap. The compositional, structural, and electrical relationships exploited in this paper can be extended to a variety of perovskites and nonperovskite oxides, providing guidance in the structural manipulation of electrical properties of functional materials.

Thermoelectric figure of merit of a material with caged structure and rattler atoms

A new diffusion path is identified for gallium interstitials, which involves lower barriers than the barriers for previously identified diffusion paths [K. Levasseur-Smith and N. Mousseau, J. Appl. Phys. 103, 113502 (2008), P. A. Schultz and O. A. von Lilienfeld, Modelling and Simulation in Materials Science and Engineering 17, 084007 (2009)] for the charge states which dominate diffusion over most of the available range of Fermi energies. This path passes through the ⟨110⟩ gallium-gallium split interstitial configuration, and has a particularly low diffusion barrier of 0.35 eV for diffusion in the neutral charge state. As a part of this work, the character of the charge states for the gallium interstitials which are most important for diffusion is investigated, and it is shown that the last electron bound to the neutral interstitial occupies a shallow hydrogenic bound state composed of conduction band states for the hexagonal interstitial and both tetrahedral interstitials. How to properly account for th...

Arsenic interstitials and interstitial complexes in low-temperature grown GaAs

We model short-period superlattices of WO3 and ReO3 with first-principles calculations. In fully relaxed superlattices we observe that octahedral tilts about an axis in the planes of the superlattices do not propagate from one material, despite the presence of the corner-shared oxygen atoms. However, we find that octahedral rotation is enhanced within WO3 layers in cases in which strain couples with native antiferroelectric displacements of tungsten within their octahedral cages. Resulting structures remain antiferroelectric with low net global polarization. Thermodynamic analysis reveals that superlattices with sufficiently thick ReO3 layers, the absolute number being three or more layers and the Re fraction 50%, tend to be more stable than the separated material phases and also show enhanced octahedral rotations in the WO3 layers.

Antisite-Related Defects in GaAs Grown at Low Temperatures.

A model is proposed to calculate the thermoelectric figure of merit of a framework crystal containing rattler atoms in cages. Such systems are expected to behave like a Phonon Glass and Electron Crystal (PGEC). The model resembles an effective Anderson model for a correlated system. The dispersion of the electronic energies and of the phonon frequencies in the system is calculated exactly. These results are used to evaluate the electronic and the thermal transport coefficients, which in turn give the temperature dependence of the thermoelectric figure of merit. Explicit calculation of the thermoelectric figure of merit for a one-dimensional case shows that the lattice thermal conductivity plays an important role in providing a peak structure to its temperature dependence. The results are presented for three different electronic dispersions and it is found that the room temperature figure of merit attains the highest value for the tight binding case. The calculation provides guidelines for designing a better thermoelectric material for use as a refrigerator.