Peculiar covalent bonding of C₆₀/6H-SiC(0 0 0 1)-(3 × 3) probed by photoelectron spectroscopy

High resolution photoemission with synchrotron radiation was used to study the interface formation of a thin layer of C₆₀ on 6H-SiC(0 0 0 1)-(3 × 3), characterized by protruding Si-tetramers. The results show that C₆₀ is chemisorbed by orbital hybridization between the highest-occupied molecular orbital (HOMO) and the p _z orbital of Si adatom at the apex of the tetramers. The covalent nature of the bonding was inferred from core level as well as valence band spectra. The Si 2p spectra reveal that a large fraction (at least 45%) of the Si adatoms remain unbound despite the reactive character of the associated dangling bonds. This is consistent with a model in which each C₆₀ is attached to the substrate through a single covalent C₆₀-Si bond. A binding energy shift of the core levels associated with sub-surface Si or C atoms indicates a decrease of the SiC band bending caused by a charge transfer from the C₆₀ molecules to the substrate via the formation of donor-like interface states.