Comparative Study of the Optical Properties of α‐, β‐, and κ‐Ga2O3

A systematic investigation of the optical properties of β-, α-, and κ-phase gallium oxide (Ga2O3) polymorphs is conducted by UV–vis spectrophotometry through the Swanepoel method and temperature-dependent photoluminescence. Using the same approach and apparatus allows similarities and differences between these three phases to be directly established. Differences between polymorphs are observed, including refractive indices of 1.89 (β), 2.00 (α), and 1.85 (κ) and optical bandgaps of 4.99 eV (β), 5.32 eV (α), and 4.87 eV (κ). In the luminescence studies, four emission peaks in each polymorph are revealed, located at different energies in the UV (3.1–3.9 eV), blue (2.7–3.0 eV), and green (2.2–2.6 eV) regions, with causes attributed to self-trapped holes, donor–acceptor pair transitions involving Ga and O vacancies (VGa, VO), Ga-O divacancies (VGa + VO), O interstitials (Oi), and H impurities (VGa–nH, Hi, Ho). In this systematic study, unique optical properties of the different Ga2O3 polymorphs are highlighted and it is warned that the commonly practiced analogy to β-Ga2O3 can lead to misinterpretations.