Photocarrier transport reconstruction and dramatical performance enhancement in ultrawide-bandgap ε-Ga2O3 photodetectors via surface defect passivation

The semiconductor surface is a major platform dictating carrier transport and recombination, in particular for planar photoconductive devices such as metal-semiconductor-metal (MSM) photodetectors. Here we demonstrate the integration of an ultrathin Al2O3 capping layer to effectively passivate the surface defects of epitaxial ε-Ga2O3 films and substantially enhance the photodetector performance. Incorporating an Al2O3 layer is found to significantly reduce the dark current of ε-Ga2O3 MSM photodetectors by more than three orders of magnitude, and simultaneously enhance the photocurrent and the response speed. With an ultrathin Al2O3 surface layer, the Al2O3/ε-Ga2O3 MSM photodetector achieves a superhigh responsivity of >104 A/W, a detectivity of >1016 Jones with a photo-to-dark ratio of >107, an UV–vis rejection ratio of >104 and millisecond response time for continuous-wave illumination. The surface passivation mechanism is analyzed by combined X-ray photoemission spectroscopy, secondary ion mass-spectroscopy and numerical calculations. It is revealed that the Al2O3 capping layer passivates the surface defects that are in the form of reduced tin species and oxygen vacancies, and reconstructs a faster surface transport channel for more efficient photocarrier collection. This study expands the design concept of surface passivation with ultrawide bandgap semiconductors for developing efficient deep ultraviolet photodetectors.