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

When annealed at elevated temperatures under vacuum, silicon carbide surfac es show a tendency towards graphitization. Using the sensitivity of empty c onduction-band states dispersion towards the Structural quality of the over layer, we have used angular-resolved inverse photoemission spectroscopy (KR IPES) to monitor the progressive formation of crystalline graphite on 6H-Si C(0001) surfaces. The KRIPES spectra obtained after annealing at 1300 degre es C are characteristic of azimuthally oriented, graphite multilayers of ve ry good single-crystalline quality. For lower annealing temperatures, the o rdered interface already presents most of the fingerprints of graphite as s oon as 1080 degrees C. The observation of unshifted pi* states, which revea ls a very weak interaction with the substrate, is consistent with the growt h of a van der Waals heteroepitaxial graphite lattice on top of silicon car bide, 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 characte ristic of the (root 3x root 3)R30 degrees reconstruction. These adatoms red uce 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 vacanci es in the C-rich subsurface layers of the SiC substrate. This strongly pert urbed substrate can be viewed as a diamondlike phase which acts as a precur sor to graphite formation by collapse of several layers. In this framework, previously published soft x-ray photoemission spectra find a natural expla nation. [S0163-1829(98)06347-4].