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Spinel ZnGa₂O₄ films were grown on c-plane sapphire substrates at the substrate temperature of 400 °C by radio-frequency magnetron sputtering. Post thermal annealing was employed at the annealing temperature of 700 °C in order to enhance their crystal quality. The effect of thermal annealing on the microstructural and optoelectronic properties of ZnGa₂O₄ films was systematically investigated in various ambiences, such as air, nitrogen, and oxygen. The X-ray diffraction patterns of annealed ZnGa₂O₄ films showed the crystalline structure to have (111) crystallographic planes. Transmission electron micrographs verified that ZnGa₂O₄ film annealed under air ambience possesses a quasi-single-crystalline structure. This ZnGa₂O₄ film annealed under air ambience exhibited a smooth surface, an excellent average transmittance above 82% in the visible region, and a wide bandgap of 5.05 eV. The oxygen vacancies under different annealing ambiences were revealed a substantial impact on the material and photodetector characteristics by X-ray photoelectron spectrum investigations. ZnGa₂O₄ film exhibits optimal performance as a metal-semiconductor-metal photodetector when annealed under air ambience. Under these conditions, ZnGa₂O₄ film exhibits a higher photo/dark current ratio of ~10⁴ order, as well as a high responsivity of 2.53 A/W at the bias of 5 V under an incident optical light of 240 nm. These results demonstrate that quasi-single-crystalline ZnGa₂O₄ films have significant potential in deep-ultraviolet applications.