First Principles Study of Oxides : Bulk, Interfaces and Defects

Abstract

In the recent years, oxides have been the focus of numerous theoretical and experimental studies. This is because of a wide variety of exotic physical phenomenon, such as multiferroicity, charge ordering, metal-insulator transitions, high-Tc superconductivity etc that have been observed in these materials. Moreover, most oxides are earth-abundant, stable, non-toxic and easy to produce in a wide range of environmental conditions. As a result, they have also been used in a variety of technological applications. In this thesis, we study bulk oxides, interfaces between different oxides, and defects in bulk oxides. We use first-principles methods to calculate different properties of these systems as discussed below. These state-of-the- art methods based on density functional theory (for ground-state properties) and many-body perturbation theory (for excited-state properties) have been shown to predict properties that are in excellent agreement with experiments. Our study of bulk properties of oxides is motivated by the possibility of constructing an efficient all-oxide solar cell. We explore two ferroelectric transition metal oxides, YMnO3 and Zn2Mo3O8, as potential candidates for photoabsorbers. We calculate the electronic structure and optical properties of these materials and compare our results with available experiments. A technologically and fundamentally interesting phenomenon at oxide interfaces is the formation of a two-dimensional electron gas (2DEG). We propose a novel oxide heterostructure system, consisting of two materials with chemical formula A2Mo3O8 (A = Zn, Mg, Cd), which has the potential to host a 2DEG. Our calculations predict the formation of 2DEG at this interface with electron densities and localization comparable to that of other well-known 2DEG systems. In the last part of the thesis, we investigate the electronic structure and optical properties of the oxygen vacancies (F-centers) in -alumina. -Alumina or sapphire is a widely used and well-studied material. We propose a modi fication of the existing method for calculation of defect charge transition levels (CTLs) in solids. Using this modi fication we calculate CTLs for F-centers in -alumina. We show that our modi fication improves the accuracy of the results signifi cantly. Furthermore, we calculate excited state properties of these F-centers to understand and explain photoluminescence experiments performed on these systems