  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 Bi$_2$Se$_3$ exhibit
differential conductance characteristics which appear to be dominated by
tunneling, roughly reproducing the Bi$_2$Se$_3$ density of states. Similar
results were obtained for BLG on top of Bi$_2$Se$_3$, with 10-fold 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 Bi$_2$Se$_3$ and graphene phonons. These
processes favor phonons which compensate for momentum mismatch between the TI
$\Gamma$ 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.
