Graphene-substrate interaction on 6H-SiC(000(1)over bar): A scanning tunneling microscopy study

The early stage of graphene formation on the 6H-SiC(000(1)over bar) surface is investigated by scanning tunneling microscopy. Islands made of a single graphitic plane form above regions of the substrate that show either a (2x2)(C) or a (3x3) reconstruction. The orientations of the single-layer domains present a broad distribution of rotation angles with respect to the substrate. The atomic structures of the (3x3) and (2x2)(C) substrate reconstructions are preserved under the first carbon layer. Low bias images reveal a graphitic structure, which indicates that the interaction between the first carbon plane and the SiC surface is comparatively much weaker on the C face than on the Si face of the substrate. The coupling is stronger on the (2x2)(C) surface reconstruction than on the (3x3) one, where an almost ideal graphene structure is found close to the Fermi level.