Homogeneous in-plane WSe₂ P-N junctions for advanced optoelectronic devices

Conventional doping schemes of silicon (Si) microelectronics are incompatible with atomically thick two-dimensional (2D) transition metal dichalcogenides (TMDCs), which makes it challenging to construct high-quality 2D homogeneous p-n junctions. Herein, we adopt a simple yet effective plasma-treated doping method to seamlessly construct a lateral 2D WSe₂ p-n homojunction. WSe₂ with ambipolar transport properties was exposed to O₂ plasma to form WO on the surface in a self-limiting process that induces hole doping in the underlying WSe₂via electron transfer. Different electrical behaviors were observed between the as-exfoliated (ambipolar) region and the O₂ plasma-treated (p-doped) region under electrostatic modulation of the back-gate bias (V_BG), which produces a p-n in-plane homojunction. More importantly, a small contact resistance of 710 Ω μm with a p-doped region transistor mobility of ∼157 cm² V^-1 s^-1 was achieved due to the transformation of Schottky contact into Ohmic contact after plasma treatment. This effectively avoids Fermi-level pinning and significantly improves the performance of photodetectors. The resultant WSe₂ p-n junction device thus exhibits a high photoresponsivity of ∼7.1 × 10⁴ mA W^-1 and a superior external quantum efficiency of ∼228%. Also, the physical mechanism of charge transfer in the WSe₂ p-n homojunction was analyzed. Our proposed strategy offers a powerful route to realize low contact resistance and high photoresponsivity in 2D TMDC-based optoelectronic devices, paving the way for next-generation atomic-thickness optoelectronics.