  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.
