Structure of quasi-free-standing graphene on the SiC (0001) surface prepared by the rapid cooling method

A systematic structural study of epitaxial graphene samples on the SiC (0001) surface has been performed by the surface x-ray diffraction method, which is a non-contact technique. For samples with only a buffer layer, one layer graphene, and multilayer graphene, the distances between the buffer layer and the surface Si atoms were found to be 2.3 angstrom. This value is the same as reported values. For quasi-free-standing graphene samples prepared by the rapid cooling method [Bao et al., Phys. Rev. Lett. 117, 205501 (2016)], there was no buffer layer and the distance between the quasi-free-standing graphene and the surface Si atoms was 3.5 angstrom, which is significantly shorter than the value in hydrogen-intercalated graphene and slightly longer than the interplane distance in graphite. The Si occupancy deviated from unity within 1nm of the SiC surface. The depth profile of the Si occupancy showed little sample dependence, and it was reproduced by a simple atomistic model based on random hopping of Si atoms.