Unraveling the Role of Interstitial Si–Vacancy Defect Complexes on Electronic Structure and Optical Properties of β‐Ga ₂ O ₃ : A First‐Principles Study

In this study, we investigate the electronic and optical properties of silicon‐doped β‐Ga 2 O 3 using first‐principles calculations. Four key defect configurations were analyzed: substitutional Si on a tetrahedral Ga site (Si GaI ), interstitial Si (Si i9 ), and the interstitial Si–Ga vacancy complexes Si i9 –1V GaI and Si i9 –2V GaI . We confirm that the substitutional Si GaI acts as a shallow donor, raising the Fermi level into the conduction band, which is consistent with experimental data. In contrast, the interstitial Si i9 introduces a midgap level and exhibits a smaller Bader charge compared to the substitutional case, deviating from the +4‐oxidation state typically observed experimentally. Crucially, complex formation with Ga vacancies stabilizes the interstitial species. The Si i9 –1V GaI complex retains n‐type behavior with a redshifted absorption edge. The Si i9 –2V GaI complex, however, introduces deeper states and a further reduced optical absorption edge below 4 eV. The comparable Bader charge and negative formation energy of these two complexes indicate that they can coexist with substitutional donors under implantation conditions. Our results provide novel insight into the mechanism behind the experimentally observed dual nature of Si in β‐Ga 2 O 3 .