Gallium oxide nanocrystals encapsulated in Al ₂ O ₃ film on a Si substrate have been synthesized by using ion implantation followed by thermal annealing at 800 °C or 900 °C. Formation of β-Ga ₂ O ₃ nanocrystals is confirmed by transmission electron microscopy. The photoresponse of a photodetector fabricated on the implanted layer by the deposition of interdigitated electrode pattern is estimated by analyzing the current-voltage characteristics. The ratio of photocurrent at the wavelength of 250 nm and a voltage of 4 V to dark current around ~15 is found for the sample annealed at 900 °C. The spectral dependence of photoresponse for this sample demonstrates excellent solar-blind ultraviolet characteristics in the wavelength range of 250-270 nm with a rejection ratio 42 at the bias of 1 V. At the bias of 4 V, the responsivity of 50 mA/μW upon illumination at 250 nm and a cutoff at 280 nm are observed for the 900 °C annealing. The sample annealed at 800 °C shows lower ultraviolet responsivity due to the high density of defects. The data are analyzed in relation to the generation of carriers in Ga ₂ O ₃ nanocrystals and their transport in the damaged Al ₂ O ₃ matrix. The reported results pave the way toward the fabrication of next generation high sensitive solar-blind photodetectors.