We investigate band gaps, equilibrium structures, and phase stabilities of several bulk polymorphs of wide-gap oxide semiconductors ZnO, TiO 2 , ZrO 2 , and WO 3 . We are particularly concerned with assessing the performance of hybrid functionals built with the fraction of Hartree-Fock exact exchange obtained from the computed electronic dielectric constant of the material. We provide comparison with more standard density-functional theory and GW methods. We finally analyze the chemical reduction of TiO 2 into Ti 2 O 3 , involving a change in oxide stoichiometry. We show that the dielectric-dependent hybrid functional is generally good at reproducing both ground-state (lattice constants, phase stability sequences, and reaction energies) and excited-state (photoemission gaps) properties within a single, fully ab initio framework.

We investigate the behavior of oxygen vacancies in three different metal-oxide semiconductors\ud (rutile and anatase TiO2, monoclinic WO3, and tetragonal ZrO2) using a recently proposed hybrid\ud density-functional method in which the fraction of exact exchange is material-dependent but obtained\ud ab initio in a self-consistent scheme. In particular, we calculate charge-transition levels relative to the\ud oxygen-vacancy defect and compare computed optical and thermal excitation/emission energies with\ud the available experimental results, shedding light on the underlying excitation mechanisms and related\ud materials properties. We find that this novel approach is able to reproduce not only ground-state\ud properties and band structures of perfect bulk oxide materials but also provides results consistent\ud with the optical and electrical behavior observed in the corresponding substoichiometric defective\ud systems. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.493180

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We present ab initio calculations of electron energy loss spectroscopy in the reflection geometry ͑REELS͒ for the Si͑100͒ surface for which several experimental data are available. The standard surface models ͓p͑2 ϫ 1͒, c͑4 ϫ 2͒, and p͑2 ϫ 2͔͒ are structurally very similar in nature, and precise calculations are necessary to differentiate between them. Starting from optimized geometries we compute REELS spectra within the framework of the three-layer model. We adopt several methodologies to ensure a realistic model of the experiment, including a precise partitioning of the surface and bulk dielectric functions and a numerical integration over the detector aperture. We obtain good agreement with the various available experimental energy loss and reflectance anisotropy spectra. The calculations allow us to definitively rule out the presence of the p͑2 ϫ 1͒ reconstruction. We interpret the S 0 peak observed by Farrell et al. ͓Phys. Rev. B 30, 721 ͑1984͔͒ in high resolution REELS. Furthermore, we explain the observed dependence of the spectra on temperature by inferring the presence of dimer flipping at room temperature.

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