Structural, optical and photoluminescence properties of Ga₂O₃ thin films deposited by vacuum thermal evaporation

In this study, Gallium oxide (Ga₂O₃) thin films were deposited by vacuum thermal evaporation technique. The effects of annealing temperature on the structural, optical and photoluminescence (PL) properties of Ga₂O₃ thin films were investigated in detail. The Ga₂O₃ thin films annealed at lower temperatures were amorphous, while those annealed at above 800 °C were nanocrystalline. The annealing temperature increasing from 400 to 1100 °C results in the band-gap increase from 4.70 to 5.13 eV due to the reduction in density of defect states. Under the excitation at 250 nm, the PL spectra of thin films consisted of a UV emission (300 nm) and two blue emissions (400 nm and 438 nm). The UV and blue luminescence intensities increased as the annealing temperature was increased from 400 to 1100 °C. The blue luminescence is suggested to originate from the recombination of trapped electrons in the donor with trapped holes in the acceptor, where the origin of the donor should be an double ionized oxygen vacancy (V_O ⁺⁺) and the acceptor should be a gallium vacancy (V_Ga) or gallium-oxygen vacancy pair (V_Ga-V_O ⁺⁺).