High-Velocity Saturation in Graphene Encapsulated by Hexagonal Boron Nitride.

We measure drift velocity in monolayer graphene encapsulated by hexagonal boron nitride (hBN), probing its dependence on carrier density and temperature. Due to the high mobility (>5 × 104 cm2/V/s) of our samples, the drift velocity begins to saturate at low electric fields (∼0.1 V/μm) at room temperature. Comparing results to a canonical drift velocity model, we extract room-temperature electron saturation velocities ranging from 6 × 107 cm/s at a low carrier density of 8 × 1011 cm-2 to 2.7 × 107 cm/s at a higher density of 4.4 × 1012 cm-2. Such drift velocities are much higher than those in silicon (∼107 cm/s) and in graphene on SiO2, likely due to reduced carrier scattering with surface optical phonons whose energy in hBN (>100 meV) is higher than that in other substrates.