High-temperature graphitization of the 6H-SiC (0001) face

Abstract

Silicon carbide surfaces annealed at high temperature under vacuum tend to graphitize. The gradual graphitization of the 6H-SiC (0001̄) face (carbon termination) has been studied by angle-resolved inverse photoemission spectroscopy (KRIPES). The initial growth of graphite occurs on a 2×2 reconstruction which is best observed by LEED after annealing at 1050°C. Graphite begins to grow at temperatures as low as 1080°C, above which a hexagonal array of tangentially elongated diffraction spots appears superposed with the 2×2 spots of the substrate. The observation by KRIPES of a fingerprint of the three-dimensional character of graphite reveals the growth of several layers as low as 1150°C. Above 1200°C, the elongated spots join to form closed diffraction rings whose brightest parts define a hexagonal lattice corresponding to graphite. The graphite layers grown on the C face are thus essentially azimuthally disordered, although a significant fraction of the film keeps a preferred orientation, where the graphite lattice basis vectors are rotated by 30° with respect to the basis vectors of the SiC lattice. KRIPES reveals that the first graphite layer is strongly bound to the C face since the π∗ states are lacking at low carbon coverage. This strong interaction explains why graphite is not free to orient at the surface, which is best viewed as a mosaic of small crystalline domains with azimuthal disorder, similar to the case of HOPG. A rehybridization of the graphite π∗ states with occupied orbitals of the substrate is inferred from an observed increase in the density of states in the vicinity of the Fermi level.