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Electron tunneling spectroscopy measurements on van der Waalsheterostructures consisting of metal and graphene (or graphite) electrodesseparated by atomically thin hexagonal boron nitride tunnel barriers arereported. The tunneling conductance dI/dV at low voltages is relatively weak,with a strong enhancement reproducibly observed to occur at around |V| ~ 50 mV.While the weak tunneling at low energies is attributed to the absence ofsubstantial overlap, in momentum space, of the metal and graphene Fermisurfaces, the enhancement at higher energies signals the onset of inelasticprocesses in which phonons in the heterostructure provide the momentumnecessary to link the Fermi surfaces. Pronounced peaks in the second derivativeof the tunnel current, are observed at voltages where known phonon modes in thetunnel junction have a high density of states. In addition, features in thetunneling conductance attributed to single electron charging of nanometer-scaledefects in the boron nitride are also observed in these devices. The smallelectronic density of states of graphene allows the charging spectra of thesedefect states to be electrostatically tuned, leading to Coulomb diamonds in thetunneling conductance.

Signatures of phonon and defect-assisted tunneling in planar metal-hexagonal boron nitride-graphene junctions