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

TL;DR

This study investigates electron tunneling in metal-hexagonal boron nitride-graphene heterostructures, revealing phonon-assisted inelastic processes and defect-related charging effects through detailed spectroscopy.

Contribution

It provides new insights into phonon and defect-assisted tunneling mechanisms in van der Waals heterostructures with atomically thin barriers.

Findings

Enhanced tunneling at ~50 mV linked to phonon modes

Observation of Coulomb diamonds from defect charging

Identification of inelastic phonon-assisted tunneling features

Abstract

Electron tunneling spectroscopy measurements on van der Waals heterostructures consisting of metal and graphene (or graphite) electrodes separated by atomically thin hexagonal boron nitride tunnel barriers are reported. 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 of substantial overlap, in momentum space, of the metal and graphene Fermi surfaces, the enhancement at higher energies signals the onset of inelastic processes in which phonons in the heterostructure provide the momentum necessary to link the Fermi surfaces. Pronounced peaks in the second derivative of the tunnel current, are observed at voltages where known phonon modes in the tunnel junction have a high density of states. In addition, features in the tunneling conductance attributed to single electron charging of nanometer-scale defects in the boron nitride are also observed in these devices. The small electronic density of states of graphene allows the charging spectra of these defect states to be electrostatically tuned, leading to Coulomb diamonds in the tunneling conductance.