Electronic properties of the graphene/6H-SiC(000$\overline{1}$) interface: A first-principles study

Using calculations from first principles, we show how the structural and electronic properties of epitaxial graphene on 6H-SiC(000$\overline{1}$) are determined by the geometry and the chemical functionalization of the interface region. We also demonstrate that these properties can be correctly captured only if a proper treatment of the van der Waals interactions is included in the theoretical description based on density functional theory. Our results reproduce the experimentally observed $n$-type doping of monolayer epitaxial graphene and prove the possibility of opening a sizable (150 meV) energy gap in the bilayer case under special growth conditions. Depending on the details of the bonding at the interface, we are able to interpret recent experimental observations and provide a clear insight into the mechanisms of charge transfer and interface stability.

Figure 1

(a) Top view and (b) side view of the ${(2\times 2)}_C$ reconstructed model for the SiC(000$\overline{1}$) surface with a Si adatom in the H3 position. (b) shows all three possible high-symmetry positions T4, H3, and top for the Si adatom. Red (darker gray) atoms are Si adatoms, black atoms are Si in SiC bulk, and yellow (light gray) atoms are C in SiC bulk. The ${(2\times 2)}_C$ unit cell is depicted by the dashed lines in the top view (a).

Figure 2

The electronic band structures (along $\Gamma$KM path) of epitaxial graphene on the C face of SiC in the Si adatom surface reconstructed model, calculated with (a) LDA, (b) GGA, and (c) GGA(D). Fermi level is set to 0 eV.

Figure 3

The electronic band structures (along $\Gamma KM$ path) of graphene on the C face of SiC with different interfacial passivation: (a) unpassivated, (b) half-passivated, and (c) fully passivated. The upper panel shows the atomic configuration of the three interface configurations. Fermi level is set to 0 eV.

Figure 4

The electron density of the states localized on the interface at the Dirac point for (a) unpassivated, (b) half-passivated, and (c) fully passivated systems. Blue isosurfaces correspond to $0.15\times {10}^{-3}$ electrons.

Figure 5

Electronic band structures (along $\Gamma KM$ path) of graphene bilayers on the ($2\times 2$)${}_c$ SiC(000$\overline{1}$) surface with (a), (b) unpassivated, (c), (d) half-passivated, and (e), (f) full-passivated interface. The upper [(a), (c), (e)] and lower [(b), (d), (f)] panels show the electronic bands for $AB$ and $AA$ stackings, respectively.