Efficient Carrier-to-exciton Conversion in Field Emission Tunnel Diodes based on MIS-type van der Waals Heterostack

Van der Waals (vdW) heterostructures comprising of two-dimensional (2D) crystals offer promising prospects for realizing ultrathin electronic and photonic devices with precisely tailored functionalities. Efficient excitonic electroluminescence (EL) is a fundamental requirement for the realization of practical excitonic devices. We report on efficient carrier-to-exciton conversion and planer electroluminescence from tunnel diodes based on a metal-insulator-semiconductor van der Waals heterostack consisting of few-layer graphene (FLG), hexagonal boron nitride (hBN), and monolayer tungsten disulfide (WS₂). These devices exhibit excitonic electroluminescence with extremely low threshold current density of a few pA μm^-2. Our observations indicate that the efficiency of our devices is not limited by carrier-to-exciton conversion efficiency but by the inherent exciton-to-photon yield of the material. The device characteristics indicate that the light emission is triggered by injection of hot minority carriers (holes) to n-doped WS₂ by Fowler-Nordheim tunneling and that hBN serves as an efficient hole-transport and electron-blocking layer.