Optical and luminescent properties of undoped and rare-earth-doped Ga2O3 thin films deposited by spray pyrolysis

Spray pyrolysis was used to prepare polycrystalline thin films of undoped Ga2O3, as well as those doped separately with Eu3+, Tb3+ and Tm3+. The energy gap values of the as-grown films and those annealed at 600degreesC and 900degreesC were found to be 4.75, 4.48 and 4.44 eV, respectively. Films containing Eu3+ and Tb3+ exhibited red and green cathodoluminescence (CL), respectively. The CL of Ga2O3: Tm mainly showed a broad band in the blue-green region, which resulted from the emission by both the Ga2O3 host and Tm3+. The broad blue-green emission band was divided into three Gaussian peaks at 424 nm (2.92 eV), 497 nm (2.49 eV) and 526 nm (2.36 eV). The CL intensity of undoped Ga2O3 thin films depended on the annealing ambient and temperature, suggesting that it is associated with the presence of oxygen vacancies. Mechanisms responsible for the broad blue-green emission of undoped Ga2O3 thin films were explored.