Huge photosensitivity gain combined with long photocurrent decay times in various polymorphs of Ga₂O₃: effects of carrier trapping with deep centers

Abstract The material system of ultra-wide bandgap Ga 2 O 3 has already shown great promise in the field of solar-blind photodetectors with high photoresponsivity, high photoresponsivity gain and low dark current. These promising results have been achieved on Ga 2 O 3 films of different polymorphs and by different methods, often not with particularly high crystalline quality. In fact, it would often seem the case that the lower the crystalline quality of the films, the higher the photosensitivity and its gain. This, however, is in most cases accompanied by unusually long photocurrent build-up and decay times. We show that the experimental results can be explained by models in which the high photosensitivity gain is related to the effects of holes being trapped by deep states, which, in Schottky diodes, results in a decrease of the Schottky barrier height with a consequent increase in the electron current, and in metal–semiconductor–metal (MSM) structures additionally gives rise to the usual gain increase due to the increased concentration and lifetime of electrons. We present and discuss models describing the effects in Ga 2 O 3 Schottky diodes, MSM structures, and unipolar and bipolar heterojunctions, and we propose possible candidates for the role of the hole traps in different Ga 2 O 3 polymorphs. We also discuss the existing results for the photocurrent build-up and decay times and offer possible explanations for the observed temperature dependences of the characteristic times where such data are present.