Origin of Doping in Quasi-Free-Standing Graphene on Silicon Carbide

We explain the robust $p$-type doping observed for quasi-free-standing graphene on hexagonal silicon carbide by the spontaneous polarization of the substrate. This mechanism is based on a bulk property of SiC, unavoidable for any hexagonal polytype of the material and independent of any details of the interface formation. We show that sign and magnitude of the polarization are in perfect agreement with the doping level observed in the graphene layer. With this mechanism, models based on hypothetical acceptor-type defects as they are discussed so far are obsolete. The $n$-type doping of epitaxial graphene is explained conventionally by donorlike states associated with the buffer layer and its interface to the substrate that overcompensate the polarization doping.

Figure 1

Sketch and schematic band diagram for the epitaxial (a) and the quasi-free-standing (b) graphene interface with 6H-SiC(0001). Large and small circles represent silicon and carbon atoms, respectively, and the very small circles in (b) stand for hydrogen. The polarization vector inside and outside of the SiC is indicated at the bottom of the figure. Note that the negative polarization charge associated with the discontinuity of the polarization across the interface is not a real charge but a pseudo charge. In order to discern pseudo charge from real charge, it is marked by circles. It is not linked to electrons in real quantum-mechanical states (e.g., band states or defects). $D$ sketches the electrostatic potential between the SiC surface and the graphene layer anchored at the Dirac energy in the graphene layer. For the band bending, information from photoelectron spectroscopy [24] has been taken into account.