Optical properties of vacancies in aluminum oxide (α−Al₃O₃) from first principles

Abstract

We employ first-principles calculations based on hybrid density functional theory to investigate the structural and optical properties of the oxygen vacancy (V₀) and aluminum vacancy (VAl) in α-Al₂O₃, the most stable (corundum) phase of alumina. Our calculations facilitate the identification of experimental excitation and luminescence spectra with specific electronic transitions at the vacancy sites. The absorption line shape for excitation of an electron at V₀⁰ to the conduction-band minimum (CBM) is in excellent agreement with the 6.1 eV band detected by optical absorption spectroscopy, and we find that the 5.9 eV absorption band is generated by an internal electron transition at V₀⁰. We confirm that the slowly decaying 3.0 eV emission band of the F center is due to a triplet-singlet transition at V₀⁰. Our calculations also reveal that the 4.8 eV/5.4 eV absorption and 3.8 eV emission bands assigned to the F⁺ center are generated by internal transitions at V₀⁺¹. The line shape for the excitation of an electron from V₀⁺¹ to the CBM agrees well with an absorption band at 6.4 eV, while the recombination of an electron with V₀⁺² also produces luminescence around 3.8 eV, but with a considerably broader line shape. For Al vacancies, we confirm that the most prevalent configuration in α-A₂O₃ is a split-vacancy configuration VAl,s, and we calculate migration barriers for different directions in the corundum crystal. We predict absorption and emission spectra for the excitation and recombination of an electron localized at VAl⁻³ and VAl,s⁻³ sites with the CBM. The line shapes of the two VAl configurations overlap and are considerably broader than the spectra corresponding to V₀