Gate- versus defect-induced voltage drop and negative differential resistance in vertical graphene heterostructures

Abstract

To enable the computer-aided design of vertically stacked two-dimensional (2D) van der Waals (vdW) heterostructure devices, we here introduce a non-equilibrium first-principles simulation method based on the multi-space constrained-search density functional formalism. Applying it to graphene/few-layer hBN/graphene field-effect transistors, we show that the negative differential resistance (NDR) characteristics can be produced not only from the gating-induced mismatch between two graphene Dirac cones in energy-momentum space but from the bias-dependent energetic shift of defect levels. Specifically, for a carbon atom substituted for a nitrogen atom (C-N) within inner hBN layers, the increase of bias voltage is found to induce a self-consistent electron filling of in-gap C-N states, which in turn changes voltage drop profiles and produces symmetric NDR characteristics. With the C-N placed on outer hBN layers, however, the pinning of C-N states to nearby graphene significantly modifies device characteristics, demonstrating the critical impact of atomic details for 2D vdW devices.