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