The performance of silicon-based technology could be improved for almost half a century by making devices smaller and smaller. Because this approach will eventually come up against the intrinsic limitations of silicon, there is a constant search for alternatives. A material that has received much attention recently is graphene, a monolayer of graphite. Graphene is a two dimensional half metal comprising sp2-bonded carbon atoms arranged in a honeycomb structure. The high mobility of both electrons and holes in graphene makes it a promising starting material for high frequency transistors. For practical applications of such a material only one atom thick, the choice of a suitable substrate is non-trivial. Hexagonal boron nitride (h-BN) is a completely flat layered material with the same honeycomb structure as graphene with strong in-plane and weak out-of-plane interactions. Unlike graphene, it has a very large band gap so it is an ideal substrate and gate dielectric with which to build metal|h-BN|graphene field-effect devices.
We used first-principles density functional theory (DFT) calculations for Cu|h-BN|graphene stacks to study how the position of the Fermi level in graphene depends on the thickness of the h-BN layer and on a potential difference applied between Cu and graphene. We developed an analytical model that describes this doping very well, allowing us to identify the key parameters that govern the device behaviour. For ultrathin layers of h-BN layers, we predict an intrinsic doping of graphene that should be observable in experiment. It is dominated by novel interface terms that were evaluated from DFT calculations for the individual materials and for interfaces between h-BN and Cu or graphene.
Menno Bokdam, Petr A. Khomyakov, Geert Brocks, Zhicheng Zhong, and Paul J. Kelly, Electrostatic Doping of Graphene through Ultrathin Hexagonal Boron Nitride Films, Nano Letters 11, 4631-4635 (2011).
Figure 1. A single layer of graphene (Gr) separated from a copper electrode (Cu) by three layers of hexagonal boron nitride (BN). The dipole layers are visible as negative (blue) and positive (red) charge contours.