Crystallization and bandgap variation of non-stoichiometric amorphous Ga2O3-x thin films during post-annealing process

The evolution of the structural and chemical properties of non-stoichiometric amorphous Ga2O3-x thin films during post-annealing in an atmospheric environment was investigated using scanning electron microscopy, X-ray diffraction, ambient pressure X-ray photoelectron spectroscopy (AP-XPS), and ultraviolet–visible light spectroscopy. The amorphous thin films were crystallized into a mixture of α and β phases that were epitaxial to the sapphire (0001) substrate. The AP-XPS results indicate that the non-stoichiometric state of the Ga2O3-x thin films is composed of metallic Ga, Ga2O, and Ga2O3. Ga and Ga2O are oxidized into Ga2O3, achieving a stoichiometric state during the post-annealing process. This gradually increases the optical bandgap from 4.16 eV for the as-grown sample to 4.81 eV for the sample annealed at 600 ℃. Our results support the relationship between the chemical composition and bandgap of non-stoichiometric Ga2O3-x films deposited by radio-frequency powder sputtering.