Role of Oxygen Vacancies in Modification of Electronic Structure of Bulk Oxides

Abstract

Emergent phenomena in oxides have been receiving a lot of experimental and theoretical interest in the recent decade. In general, oxides host a rich diversity of defects and several new emergent phenomena such as 2-DEG formation, Anomalous Hall Effect (AHE), Topological Hall Effect, etc., originate solely due to these defects. One of the most important defects in oxides is the oxygen vacancy. We present the modification of electronic structure in three technologically important oxide material KTaO3, ZrO2, and In2O3 due to the presence of oxygen vacancies. Bulk KTaO3 is a wide band-gap insulator showing quantum paraelectric behavior at low temperature. We use state-of-the-art density functional theory (DFT) to study ground state properties of bulk KTaO3 with defects. Our calculations show that oxygen vacancies in KTaO3 result in metallic behavior, momentum space spin texture, and formation of localized defect state in the band gap. Isolated oxygen vacancies in KTaO3 lead to spontaneous magnetization, which explains the experimentally observed AHE. Furthermore, our calculations give insight into the realization of non-trivial skyrmion-like spin textures and characteristic non-zero scalar spin chirality near oxygen vacancies. This leads to an extra contribution to the Hall measurement in KTaO3 apart from the ordinary Hall Effect and AHE. Our study also reveals that there is a tendency for clustering of oxygen vacancies. Charge transition levels (CTLs) play a crucial role in understanding and manipulating the electronic properties of materials. Using a combined DFT and GW formalism, we study the electronic structure and charge transition levels (CTLs) of oxygen vacancy defects in monoclinic Zirconia. The CTLs are calculated using two paths and employing electrostatic corrections due to localized charge at the defect site. We also describe a relaxation mechanism of atoms near an oxygen vacancy site. We show how one can obtain accurate CTLs using the DFT and GW method. The calculated CTLs agree well with the experimental results. Superior electrical and mechanical performance is desired in state-of-the-art display technology and next-generation printed electronics. In the last part of the thesis, we investigate the electronic structure modification of In2O3 due to the effect of strain. Inorganic/organic composite semiconductor devices based on In2O3 have shown ample performance in flexible electronic applications. Our calculations based on DFT find that elongation of the unit cell in one direction results in contraction of the cell in the other two directions. Further, this elongation and contraction of the cell leads to rotations of polyhedra formed by O atoms surrounding an In atom. However, even applying 2% strain, although quite large for a ceramic oxide system, the mobility of electrons in the hybrid semiconductor device remains within an identical range of that of pristine In2O3. Further, we investigate the role of oxygen vacancies in transforming the electronic structure of In2O3.