Comparison Study of $\beta $ -Ga 2 O 3 Photodetectors on Bulk Substrate and Sapphire

We fabricated $\beta $ -Ga 2 O 3 photodetectors on bulk substrate and sapphire. Bulk Ga 2 O 3 photodetector demonstrates the improved responsivity compared with the device on sapphire, due to the higher crystal quality in bulk material. Optical gain is achieved in both the devices. For the first time, we report that the Ga 2 O 3 photodetector epitaxially grown on sapphire achieves a blueshift of bandgap in comparison with bulk device. Based on the measured responsivity characteristics, the direct and indirect $E_{G}$ of bulk Ga 2 O 3 are 4.78 and 4.59 eV, respectively. The Ga 2 O 3 photodetector on sapphire exhibits a maximum cutoff wavelength at 253 nm, corresponding to the direct $E_{G}$ of 4.90 eV. The increment of $E_{G}$ in Ga 2 O 3 on sapphire over bulk material is attributed to the residual strain in the film. The time-dependent photoresponse of the devices suggests that Ga 2 O 3 on sapphire might have more oxygen vacancies than the bulk material.