Influence of Post-Deposition Annealing in Oxygen Atmosphere on Crystalline Quality, Interfacial Properties, and Ultraviolet Photosensitivity of Ga2O3/GaN Heterojunctions

Heterogeneous integration of ultra-wide bandgap Ga2O3 with GaN represents a promising pathway to realize high-speed, self-powered solar-blind ultraviolet photodetectors by leveraging interfacial band alignment and enhanced carrier separation. However, post-growth thermal processing profoundly influences bulk defect chemistry and heterointerface stability, whose competitive mechanisms remain inadequately understood. Here, we investigate the influence of post-deposition annealing in an O2 atmosphere across 400–900 °C on the crystalline quality, interfacial integrity, and optoelectronic performance of ALD-grown Ga2O3/GaN heterostructures. Ellipsometry, high-resolution XRD, and XPS reveal that annealing at 700 °C triggers monoclinic β-phase recrystallization, promotes macroscopic film densification, and minimizes the oxygen vacancy ratio to 24.9% while preserving GaN underlayer integrity. Consequently, the photodetector annealed at 700 °C achieves an optimal photo-to-dark-current ratio of 6.5 × 104, a peak solar-blind responsivity of 23.3 A/W at 254 nm, and robust dynamic stability (τrise = 9.36 s, τdecay = 8.28 s). In contrast, annealing above 800 °C induces severe GaN lattice degradation and interfacial interdiffusion, activating unwanted parasitic UVA response and persistent photoconductivity. This work elucidates the critical thermal boundaries for Ga2O3/GaN heterojunction processing, providing practical design rules for high-selectivity solar-blind optoelectronic devices.