Effects of Post-deposition Annealing on Crystal Structures and UV Responses of Mist CVD Grown Amorphous Ga₂O₃ Thin Films

Abstract

This investigation examines the utilization of amorphous gallium oxide (a-Ga₂O₃) thin films, deposited via mist CVD, as deep-ultraviolet (UV-C) photodetectors, and assesses the influence of annealing treatment on their structural, optical, and UV photoresponse characteristics. Initially, a-Ga₂O₃ thin films were deposited on c-plane sapphire substrates at 260°C, followed by annealing at various temperatures in dry air. The crystal structure, surface morphology, and optical properties of these films were analyzed. Furthermore, Ni/Au electrodes were deposited to fabricate metal-semiconductor-metal (MSM) type photodetectors, and their responses to UV-A, UV-B, and UV-C light were evaluated. The findings indicated that annealing at temperatures exceeding 600°C induced the crystallization of Ga₂O₃, transitioning from a mixed phase of α, β, and κ phases to a predominantly β-phase structure. Annealing at temperatures above 800°C increased the optical bandgap, which is hypothesized to be due to the diffusion of Al atoms from the sapphire substrate. The a-Ga₂O₃ photodetectors exhibited responsiveness to UV-A, UV-B, and UV-C light, demonstrating persistent photoconductivity (PPC) after deactivation of the light source. On the other hand, annealing enhanced the selective response to UV-C light and eliminated the PPC phenomenon. This study suggests that annealing treatment of Mist-CVD grown a-Ga₂O₃ thin films is effective in enhancing the response performance of UV-C photodetectors.