'Freeing' Graphene from its Substrate: Observing Intrinsic Velocity Saturation with Rapid Electrical Pulsing

Fifty years of continued transistor downscaling has yielded nanodevices whose operation is strongly influenced by high-field phenomena, prominent among which is drift-velocity saturation (DVS) arising from optical-phonon emission by "hot" electrons or holes. While this phenomenon has long been studied in conventional semiconductors, the emergence of graphene as a candidate for use in future nanoelectronics makes it vital to understand the origins of DVS in this novel material. Although the large optical-phonon energies of graphene promise high saturation velocities, experiments reveal much lower values that are degraded by Joule heating of the supporting substrate. Here we show a strategy to overcome this problem, using nanosecond pulsing to drive graphene's hot carriers on time scales much faster than those on which substrate heating can occur. In this way we observe, for the first time, the inherent velocity-saturation characteristics of graphene, independent of the influence of its (SiO2) substrate. Resultant saturation velocities (approaching 108 cm/s near the Dirac point) exceed those reported for suspended graphene and for devices on boron nitride substrates, with corresponding current densities (~109 A/cm2 ) reaching those found in carbon nanotubes and graphene- on-diamond transistors. By "freeing" graphene from the influence of its substrate in this manner, we therefore reveal a pathway to achieving the superior electrical performance promised by this material.