(3×3)R30°reconstruction of the6H−SiC(0001) surface: A simpleT4Si adatom structure solved by grazing-incidence x-ray diffraction

The atomic structure of the $(\sqrt{3}\times \sqrt{3})$ $R30\mathrm{}°$ reconstructed surface of $6H-\mathrm{SiC}$ (0001) silicon terminated, is solved by grazing-incidence x-ray diffraction in ultrahigh vacuum. The simple adatom structure with one silicon atom per reconstructed unit cell sitting in a $T4\mathrm{}$ site over the top SiC bilayer is the only one compatible with the data, thus ruling our other models involving Si adatoms in $H3\mathrm{}$ site, Si trimers, C adatoms, or Si vacancies, and supporting the most recent theoretical predictions. The predominance 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 reconstructed surface but preserves a unique atomic structure within the $(\sqrt{3}\times \sqrt{3})$ $R30\mathrm{}°$ ordered domains.