Relative stability of $6H$-SiC${0001}$ surface terminations and formation of graphene overlayers by Si evaporation

We present density functional theory (DFT) calculations for 6H-SiC${0001}$ surfaces with different surface stackings and terminations. We compare the relative stability of different $(0001)$ and $(000bar1)$ surfaces in terms of their surface free energies. Removing surface and subsurface Si atoms, we simulate the formation of graphene and graphene-like overlayers by Si evaporation. We find that overlayers with a different nature of bonding are preferred at the two non-equivalent surface orientations. At $(0001)$, a chemically bonded, highly strained and buckled film is predicted. At $(000bar1)$, a van der Waals (vdW) bonded overlayer is preferred. We quantify the vdW binding and show that it can have a doping effect on electron behavior in the overlayer.