Interaction, growth, and ordering of epitaxial graphene on SiC{0001} surfaces: A comparative photoelectron spectroscopy study

Thermally induced growth of graphene on the two polar surfaces of 6H-SiC is investigated with emphasis on the initial stages of growth and interface structure. The experimental methods employed are angle-resolved valence band photoelectron spectroscopy, soft x-ray induced core-level spectroscopy, and low-energy electron diffraction. On the Si-terminated (0001) surface, the (6 root 3x6 root 3)R30 degrees reconstruction is the precursor of the growth of graphene and it persists at the interface upon the growth of few layer graphene (FLG). The (6 root 3x6 root 3)R30 degrees structure is a carbon layer with graphene-like atomic arrangement covalently bonded to the substrate where it is responsible for the azimuthal ordering of FLG on SiC(0001). In contrast, the interaction between graphene and the C-terminated (000 (1) over bar) surface is much weaker, which accounts for the low degree of order of FLG on this surface. A model for the growth of FLG on SiC{0001} is developed, wherein each new graphene layer is formed at the bottom of the existing stack rather than on its top. This model yields, in conjunction with the differences in the interfacial bonding strength, a natural explanation for the different degrees of azimuthal order observed for FLG on the two surfaces.