Structure and stability of the interface between graphene and 6H-SiC(0 0 0 -1) (3 × 3): an STM and ab initio study

We examine in detail the structure and evolution upon annealing of the SiC(3 × 3) reconstruction which is known to be present at the interface between the SiC-C face substrate and the graphene layer for samples prepared in high vacuum. We use ab initio calculations to test the validity of proposed or classical structural models in comparison with scanning tunnelling microscopy (STM) images. We analyse the electronic structure of the bare surface and detect interface states which can pin the surface Fermi level. From a comparison of the signal coming from the bare and graphene-covered SiC(3 × 3) reconstruction we propose that the transparency of the graphene in high-bias STM images results from an enhancement of the local density of states of the interface plane by the graphene layer. We discuss the thermal stability of the SiC(3 × 3) surface, and show that it transforms more easily into the SiC(2 × 2)C reconstruction in the graphene-covered region than for the bare surface. This evolution generates both structural and electronic heterogeneities at the interface. (paper)