In this study, defect analysis was conducted on ZnGa2O4 thin-film transistors of various thicknesses grown on sapphire substrates. The thickness of each ZnGa2O4 epilayer was controlled by adjusting its growth time. The electrical properties and physical characteristics were strongly related to epilayer thickness, which was also dependent on both crystallinity and the amount of oxygen vacancies in the thin film. Epilayer thickness was independent of thin-film surface roughness. The study demonstrated that an increase in the thickness of the epilayer can improve crystallinity and create more oxygen vacancies, which can serve as defect centers. If the epilayer is thin, then the film can be influenced by the dislocation of the epilayer from the sapphire substrate. The results suggests that the defects may occur because of crystallinity, oxygen vacancies, and dislocation of the ZnGa2O4 epilayer from the sapphire substrate. A ZnGa2O4 thin film with low resistance has high crystallinity and numerous oxygen vacancies. A trade-off exists between conductivity and defects in ZnGa2O4 epilayers. Moreover, the results demonstrate that conductivity in ZnGa2O4 epilayers is influenced more by the number of existing oxygen vacancies than by crystallinity. Two main regions trapping electrons, including the interface between the dielectric layer and ZnGa2O4 and the dislocation between ZnGa2O4 and the sapphire substrate, were proposed. The interfacial bonding configurations in ZnGa2O4 and sapphire heterostructures associated with different possible heterostructures were analyzed.

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