Doctor of Philosophy
Dr. Tsun-Kong Sham
Gallium nitride – zinc oxide (GaN-ZnO) solid solutions have been realized as potential photocatalysts for overall water splitting due to a narrowing of the band-gap into the visible region of the solar spectrum. Although there has been much experimental and theoretical work on this novel class of semiconductors many questions about the nature of their electronic and chemical properties remain unanswered. In particular the mechanism of band-gap reduction in these materials is not clearly understood. It is also unclear what the local chemical environment of gallium, zinc, nitrogen, and oxygen is in both the surface and bulk of these solid solutions. These materials have unusual optical emission properties exhibiting broad visible emission bands that are substantially red-shifted from observed band-gap energies. Since crystallinity, electronic structure, defects, and local chemical environments (both bulk and surface) all affect photocatalytic efficiency further understanding of the aforementioned properties of these materials is necessary.
The focus of this thesis is on the analysis of GaN-ZnO nanostructures by X-ray absorption fine structure (XAFS), including the near edge region, X-ray absorption near edge structure (XANES) and the extended region, extended X-ray absorption fine structure (EXAFS) to further the understanding of the local chemical environment (both bulk and surface) and structure of these materials using tunable X-rays from a synchrotron light source. X-ray excited optical luminescence (XEOL) has also been used to study the optical emission and near-band-gap (NBG) properties of these materials.
From XANES it has been shown that there is local chemical disorder of gallium nitride and zinc oxide phases upon solid solution formation, but, there is also strong short-range order which is evident in the EXAFS. XEOL studies have revealed that optical luminescence observed from these materials is of complex origins including zinc-related acceptor levels in the bulk and defects in the complex surface oxide. Preliminary results from time-resolved XEOL experiments have provided evidence of two NBG emission components related to GaN and ZnO phases in the material respectively. These results suggest that band-gap reduction observed in these materials is due to repulsion between conduction band onsets at phase interfaces within a disordered solid solution.
Ward, Matthew J., "X-ray Absorption Fine Structure and X-ray Excited Optical Luminescence Studies of Gallium Nitride - Zinc Oxide Solid Solution Nanostructures" (2013). Electronic Thesis and Dissertation Repository. 1081.