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 1400 °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 °C. The observation of unshifted π^* 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√3×6√3)R30° symmetry. The growth of the first graphene sheet proceeds on top of adatoms characteristic of the (√3×√3)R30° 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.