Role of layer thickness and field-effect mobility on photoresponsivity of indium selenide (InSe)-based phototransistors

Understanding and optimizing the properties of photoactive two-dimensional (2D) Van der Waals solids is crucial for developing optoelectronics applications. The main goal of this work is to present a detailed investigation of layer dependent photoconductive behavior of indium selenide (InSe)-based field-effect transistors (FETs). InSe-based FETs with five different channel thicknesses (t, 20 nm < t < 100 nm) were investigated with a continuous laser source of λ = 658 nm (1.88 eV) over a wide range of illumination power (Peff) of 22.8 nW < P < 1.29 μW. 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 (μFE) values (as a function of channel thickness, t) and photoresponsivity (R) indicates that in general R increases with increasing μFE (decreasing t) and vice versa. Maximum responsivities of ∼7.84 A/W and ∼0.59 A/W were obtained the devices 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.