Highly Sensitive InSe Nanosheet Ultraviolet Photodetector Realized Via an Asymmetric Fabry–Pérot Structure

Abstract Due to the wavelength‐dependent absorption, narrow bandgap semiconductors show potential application in ultraviolet (UV) photodetection by thinning. However, the limited absorption in the thinner layer retards their application in high‐performance photodetection. In this paper, the fabrication of an asymmetric Fabry–Pérot (F–P) structure is reported by planar stacking of InSe layer, Al ₂ O ₃ spacer, and Al back reflector. For the selective absorption enhancement in UV region, the Al ₂ O ₃ thickness is optimized to be 39 nm by finite element method (FEM). The device presents a peak response at 265 nm illumination, showing the photo‐to‐dark current ratio ( I _light / I _dark ) of 719. Responsivity ( R ) and photoconductive gain ( G ) reach 11.7 A W ⁻¹ and 54.7 at the weak light intensity of 0.03 mW cm ⁻² , respectively, which are two orders of magnitude higher than those of the device without the asymmetric F–P structure, revealing the remarkably enhanced absorption. This work shows well compatibility with the manufacturing processes for 2D semiconductor‐based photodetectors and provides a simple and versatile strategy for the application of 2D narrow bandgap semiconductors in high‐performance UV photodetectors.