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Critical Point Spectroscopic Ellipsometry Analysis of Ultrawide Bandgap Materials
This thesis outlines a universal critical point model dielectric function approach developed to analyze ultrawide bandgap materials. In recent years, significant research attention has been applied to investigate ultrawide bandgap materials for applications such as near-ultraviolet optics and high power electronics. Many of these materials exhibit anisotropic properties which unlock the potential for new device designs. The polarization dependent nature of Mueller matrix spectroscopic ellipsometry permits an analysis an optical characterization of anisotropic electronic transitions. Here, visible to vacuum ultraviolet spectroscopic ellipsometry is implemented to determine the optical properties of selected ultrawide bandgap materials. A critical point model dielectric function approach is developed to determine essential information for future device designs including the dielectric function tensor, band-to-band transitions, high-frequency dielectric constant, and excitonic contributions. Importantly, our critical point model dielectric function approach can be applied to materials with a varying degree of symmetry. This is demonstrated for isotropic ZnGa2O4, uniaxial α-Ga2O3 and α-(Al1-xGax)2O3, and monoclinic β-(Al1-xGax)2O3. An elevated temperature spectroscopic ellipsometry analysis is performed on ZnGa2O4 to provide justification for the anharmonically broadened Lorentz function to be used in model analysis. Additionally, the strain and stress relationships on band-to-band transitions in β-Ga2O3 are investigated using density functional theory. We devise a set of linear equations to relate the strain parameters to the three lowest band-to-band transitions. The strain analysis is further expanded to include β-(Al1-xGax)2O3, where combining our spectroscopic ellipsometry and density functional theory calculations permits the discovery of the bandgap bowing.
Hilfiker, Matthew, "Critical Point Spectroscopic Ellipsometry Analysis of Ultrawide Bandgap Materials" (2022). ETD collection for University of Nebraska - Lincoln. AAI29167037.