Pronounced photogating effect in atomically thin WSe₂ with a self-limiting surface oxide layer

The photogating effect is a photocurrent generation mechanism that leads to marked responsivity in two-dimensional (2D) semiconductor-based devices. A key step to promote the photogating effect in a 2D semiconductor is to integrate it with a high density of charge traps. Here, we show that self-limiting surface oxides on atomically thin WSe₂ can serve as effective electron traps to facilitate p-type photogating. By examining the gate-bias-induced threshold voltage shift of a p-type transistor based on single-layer WSe₂ with surface oxide, the electron trap density and the trap rate of the oxide are determined to be >10¹² cm^-2 and >10¹⁰ cm^-2 s^-1, respectively. White-light illumination on an oxide-covered 4-layer WSe₂ transistor leads to the generation of photocurrent, the magnitude of which increases with the hole mobility. During illumination, the photocurrent evolves on a timescale of seconds, and a portion of the current persists even after illumination. These observations indicate that the photogenerated electrons are trapped deeply in the surface oxide and effectively gate the underlying WSe₂. Owing to the pronounced photogating effect, the responsivity of the oxide-covered WSe₂ transistor is observed to exceed 3000 A/W at an incident optical power of 1.1 nW, suggesting the effectiveness of surface oxidation in facilitating the photogating effect in 2D semiconductors.