Optical Properties and electronic structure of non-d0 perovskite oxide epitaxial films and heterostructures

Abstract

Perovskite oxide materials provide a chemically diverse pseudo cubic ABO3 platform that possesses a large number of technologically relevant physical properties. It is possible grow complex heterostructures in addition to the nearly infinite stable chemical combinations. With this vast possibility for tuning and combining properties, it is not unlikely that perovskite oxides could change the face of many electronic and energy devices. However, analysis of the optical properties of non-d0 perovskite oxides is quite lacking and thus the electronic structure and important device parameters of many systems are not well characterized. This thesis is focused on understanding the coupling between composition, electronic structure and optical absorption in complex oxides. Modeling the absorption with Tauc plots is the standard way to determine the band gap energy (EG), a critical parameter for high performance photovoltaics, and other optical devices. Accurately modeling the absorption in complex oxides has not previously been studied in depth, leading to significant uncertainty in the limited work that has been conducted. Additionally, most of the prior work on the optical properties of perovskite oxides has been conducted on relatively simple systems with no d-electrons in the valence band (d0). Using LaFeO3 (LFO) (d5) as a model system, the optical absorption has been calculated and used to determine the most appropriate Tauc model. The results were applied to epitaxial films of LFO to determine a more accurate experimental band gap energy. Building on this knowledge, the effects of epitaxial strain, cation vacancies, A- and B-site substitution, and oxygen vacancies, on the optical and structural properties of LaFeO3-related compounds were investigated, providing new insights for band gap engineering in perovskite oxides.%%%%Ph.D., Materials Science – Drexel University, 2015