Exciton-assisted electron tunneling in van der Waals heterostructures

The control of elastic and inelastic electron tunneling relies on materials with well defined interfaces. Van der Waals materials made of two-dimensional constituents form an ideal platform for such studies. Signatures of acoustic phonons and defect states have been observed in current-to-voltage ($I-V$) measurements. These features can be explained by direct electron-phonon or electron-defect interactions. Here, we use a novel tunneling process that involves excitons in transition metal dichalcogenides (TMDs). We study tunnel junctions consisting of graphene and gold electrodes separated by hexagonal boron nitride (hBN) with an adjacent TMD monolayer and observe prominent resonant features in $I-V$ measurements. These resonances appear at bias voltages that correspond to TMD exciton energies. By placing the TMD outside of the tunneling pathway, we demonstrate that this phonon-exciton mediated tunneling process does not require any charge injection into the TMD. This work demonstrates the appearance of optical modes in electrical transport measurements and introduces a new functionality for optoelectronic devices based on van der Waals materials.