Role of local structure in the optical and electronic properties of oxygen vacancies in different crystal phases of Ga2O3

We investigate the structural, electronic, and optical properties of oxygen vacancies (${\mathrm{V}}_{\mathrm{O}}$) in crystalline $\alpha , \beta$, and $\epsilon \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ using density functional theory (DFT) calculations with the PBE0-TC-LRC functional. Our results reveal that the charge transition levels (CTLs) associated with ${\mathrm{V}}_{\mathrm{O}}$ exhibit significant variations depending on the crystal phase and the coordination environment of surrounding atoms. In particular, ${\mathrm{V}}_{\mathrm{O}}\mathrm{s}$ surrounded by tetrahedral Ga atoms ($T\text{-Ga}$) exhibit deeper CTLs compared to those surrounded by octahedral Ga atoms ($O\text{-Ga}$). We also observe distinct atomic relaxations, with larger displacements of $T\text{-Ga}$ atoms compared to $O\text{-Ga}$ atoms in the vicinity of ${\mathrm{V}}_{\mathrm{O}}\mathrm{s}$. Using linear-response time-dependent DFT, we investigate the optical transitions of ${\mathrm{V}}_{\mathrm{O}}$ and identify two distinct types of transitions: defect state to conduction band state and valence band to defect state. These results can be used to better understand the optical properties of ${\mathrm{V}}_{\mathrm{O}}$ defects in ${\mathrm{Ga}}_2{\mathrm{O}}_3$ films.