“Freeing” Graphene from Its Substrate: Observing Intrinsic Velocity Saturation with Rapid Electrical Pulsing

Abstract

Rapid (nanosecond-scale) electrical pulsing is used to study drift-velocity saturation in graphene field-effect devices. In these experiments, high-field pulses are utilized to drive graphene's carriers on time scales much faster than that on which energy loss to the underlying substrate can occur, thereby allowing the observation of the highest saturation velocities reported to date. In a dramatic departure from the behavior exhibited by conventional metals and semiconductors, as the electron or hole density is reduced toward the charge-neutrality point, the drift velocity is found to reach values comparable to the Fermi velocity itself. Corresponding current densities are as large as 10(9) A/cm(2), similar to the values reported for carbon nanotubes and for graphene-on-diamond transistors. In essence, our approach of rapid pulsing allows us to "free" graphene from the deleterious influence of its substrate, revealing a pathway to achieve the superior electrical performance promised by this material. The usefulness of this approach is not merely limited to graphene but should extend also to a broad variety of two-dimensional semiconductors.

Nonequilibrium excitations and transport of Dirac electrons in electric-field-driven graphene

“…This effect can be considered a solid-state analog of the Schwinger effect [23][24][25][26] . However, the main consequence from the theory on the current-voltage relation, I ∼ V α with the exponent α greater than 1, has been inconsistent with majority of the measured dc transport in graphene 6,8,27 where the current saturation gradually crosses over to a linear or marginally sublinear I-V relation (α ≈ 1).…”

“…Recently, the phenomena of current saturation have attracted intense interest. 7,8,10,11,36 It is found that the current saturation in graphene at high electric field depends on the optical phonon frequency ω ph and equilibrium current carrier density n eq . The former is a parameter independent of electric control, and the latter is controlled through the chemical potential µ.…”

“…The calculation confirms the semi-classical behaviors away from the Dirac point, predicted by the Boltzmann transport theory. In the charge-neutrality limit, the excitation population is linearly proportional to the electric field in the presence of the coupling to optical phonons, while the drift velocity saturates to about 50% of the Fermi velocity 8 . Despite the reversed role of the charge excitation and the drift velocity in the Drude arXiv:1802.00099v2 [cond-mat.mes-hall] 20 May 2018 metal, an apparent Ohmic relation I ∝ V is established.…”

Transient Response of h-BN-Encapsulated Graphene Transistors: Signatures of Self-Heating and Hot-Carrier Trapping

“…The key result we demonstrate is significant immunity of transistor action to hot-carrier trapping, providing a marked improvement over graphene-on-SiO 2 devices, whose operation is known to be negatively impacted by such trapping at high fields. 17−23 We also identify the dynamics of self-heating in the graphene/h-BN devices and discuss how this influences transistor performance. Overall, our results establish the excellent immunity of the graphene/h-BN system to hot-carrier degradation and highlight its value as a specific materials combination for the realization of active devices.…”

In Situ Monitoring of the Thermal-Annealing Effect in a Monolayer of MoS2

“…[6][7][8] Therefore, the answer to a question like how the substrate will impact the carrier saturation velocity of a 2D material is unlikely to be unique. [9][10][11] Furthermore, it is unclear how thermal annealing or unintended thermal stress during temperature dependent measurements will affect the material properties, and how will be the interplays of above mentioned "intrinsic" (substrate) and extrinsic perturbations responding to the annealing.…”