Influence of Local Coordination on the Electronic Structure of Ga₂O₃

Ga₂O₃ is a key ultra-wide band gap oxide offering a rich structural space, with a number of accessible polymorphs. Its most stable form is monoclinic β-Ga₂O₃ , but other polymorphs, including hexagonal α-Ga₂O₃ , cubic γ-Ga₂O₃ , and orthorhombic ε(κ)-Ga₂O₃ , are of increasing interest. Although this wealth of possible structures opens opportunities to control and tune structure, electronic structure, and ultimately physical characteristics, the polymorphs beyond β-Ga₂O₃ remain comparatively unexplored. Experimental results are particularly scarce due to current limitations in producing high-quality materials and the lack of theoretical results for the more structurally complex polymorphs.<br/><br/>This study presents an in-depth exploration of the relationship between local coordination environments and the electronic structure of the α, β, γ, and ε(κ) polymorphs of Ga₂O₃ . The samples investigated are either bulk single crystals or epitaxial films grown using molecular beam epitaxy (MBE) or atomic layer deposition (ALD), selecting the highest quality samples available for each of the polymorphs. We report high-resolution valence band spectra from soft and hard X-ray photoelectron spectroscopy (SXPS and HAXPES) which are directly compared to theoretical partial and total electronic densities of states as calculated within the framework of density functional theory (DFT). Both deep and shallow core level spectra are also compared to DFT results to explore the influence of structure, rather than solely the oxidation state, on the core level behavior. X-ray absorption spectroscopy (XAS) is used to probe the unoccupied states and in combination with SXPS is used to gain an estimate of the changes in the band gaps of the polymorphs. For the highly disordered γ-Ga₂O₃ polymorph, we combine DFT with machine learning to screen one million potential structures of γ-Ga₂O₃ to develop a robust atomistic model. We can clearly show that only the lowest energy structural models are able to describe the experimental results from SXPS and HAXPES, providing a new avenue to understand disordered oxides.<br/><br/>Ultimately, this work presents a systematic and comprehensive study of the electronic structure of Ga₂O₃ polymorphs, providing an insight into electronic trends and their relationship to crystal structure. This comparative study helps to discern trends between the different structures and advances our understanding of this polymorphic material. It lays the foundation for further exploration of Ga₂O₃ in applications beyond its β phase.