Photogating WS₂ Photodetectors Using Embedded WSe₂ Charge Puddles

Performance of 2D photodetectors is often predominated by charge traps that offer an effective photogating effect. The device features an ultrahigh gain and responsivity, but at the cost of a retarded temporal response due to the nature of long-lived trap states. In this work, we devise a gain mechanism that originates from massive charge puddles formed in the type-II 2D lateral heterostructures. This concept is demonstrated using graphene-contacted WS₂ photodetectors embedded with WSe₂ nanodots. Upon light illumination, photoexcited carriers are separated by the built-in field at the WSe₂/WS₂ heterojunctions (HJs), with holes trapped in the WSe₂ nanodots. The resulting WSe₂ hole puddles provide a photoconductive gain, as electrons are recirculating during the lifetime of holes that remain trapped in the puddles. The WSe₂/WS₂ HJ photodetectors exhibit a responsivity of 3 × 10² A/W with a gain of 7 × 10² electrons per photon. Meanwhile, the zero-gate response time is reduced by 5 orders of magnitude as compared to the prior reports for the graphene-contacted pristine WS₂ monolayer and WS₂/MoS₂ heterobilayer photodetectors due to the ultrafast intralayer excitonic dynamics in the WSe₂/WS₂ HJs.