  Due to the excellent electrical transport properties and optoelectronic
performance, thin indium selenide (InSe) has recently attracted attention in
the field of 2D semiconducting materials. However, the mechanism behind the
photocurrent generation in thin InSe photodetectors remains elusive. Here, we
present a set of experiments aimed at explaining the strong scattering in the
photoresponsivity values reported in the literature for thin InSe
photodetectors. By performing optoelectronic measurements on thin InSe-based
photodetectors operated under different environmental conditions we find that
the photoresponsivity, the response time and the photocurrent power dependency
are strongly correlated in this material. This observation indicates that the
photogating effect plays an imporant role for thin InSe flakes, and it is the
dominant mechanism in the ultra-high photoresponsivity of pristine InSe
devices. In addition, when exposing the pristine InSe photodetectors to the
ambient environment we observe a fast and irreversible change in the
photoresponse, with a decrease in the photoresponsivity accompanied by an
increase of the operating speed. We attribute this photodetector performance
change (upon atmospheric exposure) to the decrease in the density of the traps
present in InSe, due to the passivation of selenium vacancies by atmospheric
oxygen species. This passivation is accompanied by a downward shift of the InSe
Fermi level and by a decrease of the Fermi level pinning, which leads to an
increase of the Schottky barrier between Au and InSe. Our study reveals the
important role of traps induced by defects in tailoring the properties of
devices based on 2D materials and offers a controllable route to design and
functionalize thin InSe photodetectors to realize devices with either ultrahigh
photoresposivity or fast operation speed.
