Heteroepitaxial graphite on 6H-SiC(0001):: Interface formation through conduction-band electronic structure

When annealed at elevated temperatures under vacuum, silicon carbide surfaces show a tendency towards graphitization. Using the sensitivity of empty conduction-band states dispersion towards the Structural quality of the overlayer, we have used angular-resolved inverse photoemission spectroscopy (KRIPES) to monitor the progressive formation of crystalline graphite on 6H-SiC(0001) surfaces. The KRIPES spectra obtained after annealing at 1300 degrees C are characteristic of azimuthally oriented, graphite multilayers of very good single-crystalline quality. For lower annealing temperatures, the ordered interface already presents most of the fingerprints of graphite as soon as 1080 degrees C. The observation of unshifted pi* states, which reveals a very weak interaction with the substrate, is consistent with the growth of a van der Waals heteroepitaxial graphite lattice on top of silicon carbide, with a coincidence lattice of(6 root 3x6 root 3)R30 degrees symmetry. The growth of the first graphene sheet proceeds on top of adatoms characteristic of the (root 3x root 3)R30 degrees reconstruction. These adatoms reduce the chemical reactivity of the substrate. A strong feature located at 6.5 eV above the Fermi level is attributed to states derived from Si vacancies in the C-rich subsurface layers of the SiC substrate. This strongly perturbed substrate can be viewed as a diamondlike phase which acts as a precursor to graphite formation by collapse of several layers. In this framework, previously published soft x-ray photoemission spectra find a natural explanation. [S0163-1829(98)06347-4].