  Understanding and optimizing the properties of photoactive two-dimensional
(2D) Van der Waals solids are crucial for developing optoelectronics
applications. Here we present a detailed investigation of layer dependent
photoconductive behavior of InSe based field-effect transistors (FETs). InSe
based FETs with five different channel thickness (t, 20 nm < t < 100 nm) were
investigated with a continuous laser source of {\lambda} = 658 nm (1.88 eV)
over a wide range of illumination power of 22.8 nW < P < 1.29 {\mu}W. All the
devices studied, showed signatures of photogating, however, our investigations
suggest that the photoresponsivities are strongly dependent on the thickness of
the conductive channel. A correlation between the field-effect mobility
({\mu}FE) values (as a function of channel thickness, t) and photoresponsivity
(R) indicates that in general R increases with increasing {\mu}FE (decreasing
t) and vice versa. The maximum responsivity of ~ 7.84 A/W and ~ 0.59 A/W was
obtained for the device with t = 20 nm and t = 100 nm respectively. These
values could substantially increase under the application of a gate voltage.
The structure-property correlation-based studies presented here indicate the
possibility of tuning the optical properties of InSe based photo-FETs for a
variety of applications related to photodetector and/or active layers in solar
cells.
