Effect of Annealing on the Optical Properties of Gallium Oxide Thin Films

Ga2O3 is widely used in power devices, ultraviolet photodetectors and light emitting diodes. Defects, such as oxygen vacancies, gallium vacancies and gallium vacancy-oxygen vacancy pairs, play an important role in the optical property. Ga2O3 thin films are grown by electron beam evaporation and post-growth annealing is performed to improve the structural and optical quality. The effects of different annealing parameters (temperatures, atmospheres, durations and gas flow rates) on the optical properties of Ga2O3 are investigated. The structural property is continuously improved with the annealing temperature, while the photoluminescence (PL) intensity is the strongest in Ga2O3 annealed at 900°C. No PL is observed in the as-grown and low-temperature (<600°C) annealed Ga2O3. The PL intensities are significantly enhanced by annealing, especially in the blue–green region. Annealing under O2 at above 900°C eliminates the near ultraviolet emission, which is from the self-trapped excitons, while annealing under N2 or vacuum has no such effect. Among the samples annealed in vacuum, N2 and O2, the sample annealed in O2 shows the best structural and optical properties. For the annealing durations increasing from 0.5 to 1.5 h, the PL intensity first slightly increases and then decreases. The PL intensity of samples annealed in O2 with different flow rates shows similar trend.