Tunneling in graphene–topological insulator hybrid devices

Hybrid graphene--topological insulator (TI) devices were fabricated using a mechanical transfer method and studied via electronic transport. Devices consisting of bilayer graphene (BLG) under the TI Bi₂Se₃ exhibit differential conductance characteristics which appear to be dominated by tunneling, roughly reproducing the Bi₂Se₃ density of states. Similar results were obtained for BLG on top of Bi₂Se₃, with tenfold greater conductance consistent with a larger contact area due to better surface conformity. The devices further show evidence of inelastic phonon-assisted tunneling processes involving both Bi₂Se₃ and graphene phonons. These processes favor phonons which compensate for momentum mismatch between the TI Γ and graphene K,K^' points. Finally, the utility of these tunnel junctions is demonstrated on a density-tunable BLG device, where the charge neutrality point is traced along the energy-density trajectory. This trajectory is used as a measure of the ground-state density of states.