(root 3 x root 3) R30 degrees reconstruction of the 6H-SiC (0001) surface:A simple T4 Si adatom structure solved by grazing-incidence x-ray diffraction

The atomic structure of the (root 3 x root 3) R30 degrees reconstructed sur face of 6H-SiC (0001) silicon terminated, is solved by grazing-incidence x- ray diffraction in ultrahigh vacuum. The simple adatom structure with one s ilicon atom per reconstructed unit cell sitting in a T4 site over the top S IC bilayer is the only one compatible with the data, thus ruling our other models involving Si adatoms in H3 site, Si trimers, C adatoms, or Si vacanc ies, and supporting the most recent theoretical predictions. The predominan ce on the surface of three bilayer steps with an A-type termination of the bulk stacking is fully confirmed. In addition, it is shown that varying the preparation conditions changes the density of faulted boundaries in the re constructed surface but preserves a unique atomic structure within the (roo t 3 x root 3) R30 degrees ordered domains. [S0163-1829(99)00519-6].