Ultraviolet Wavelength-Dependent Optoelectronic Properties in Two-Dimensional NbSe-WSe van der Waals Heterojunction-Based Field-Effect Transistors.

Atomically thin two-dimensional (2D) van der Waals (vdW) heterostructures are one of the very important research issues for stacked optoelectronic device applications. In this study, using the transferred and stacked NbSe-WSe films as electrodes and a channel, we fabricated the field-effect transistor (FET) devices based on 2D-2D vdW metal-semiconductor heterojunctions (HJs) and systematically studied their ultraviolet (UV) wavelength-dependent electrical and photoresponse properties. Upon the exposure to UV light with a wavelength of 365 nm, the NbSe-WSe vdW HJFET devices exhibited threshold voltage shift toward positive gate bias direction, a current increase, and a nonlinear photocurrent increase upon applying a gate bias due to the contribution of the photogenerated hole current. In contrast, for the 254 nm UV-irradiated FET devices, the drain current was decreased dramatically and the threshold voltage was negatively shifted. The time-resolved photoresponse properties showed that the device current after turning off the 254 nm UV light was completely and much more rapidly recovered compared with the case of the persistent photocurrent after turning off the 365 nm UV light. Interestingly, we found that the wettability of the WSe surface was changed with increasing irradiation time only after 254 nm UV irradiation. The measured wetting behavior on the WSe surface provided direct evidence that the experimentally observed UV-wavelength-dependent phenomena was attributed to the UV-induced dissociative adsorption of oxygen and water molecules, leading to the modulation of charge trap states on the photogenerated and intrinsic carriers in the p-type WSe channel. This study will help provide an understanding of the influence of environmental and electrical measurement conditions on the electrical and optical properties of 2D-2D vdW HJ devices for a variety of device applications through the stacking of 2D heterostructures.