Theory of the adatom-induced reconstruction of the SiC(0001)root 3x root 3 surface

Based on first-principles total-energy calculations we have determined the chemical identity and adsorption site of the adatoms observed recently in scanning tunneling microscopy on the SiC(0001)root 3 x root 3 surface. The calculations indicate that Si adatoms are preferred over C adatoms for the entire allowed range of Si and C chemical potentials. In addition, we find that the adatoms prefer the T-4 site over the H-3 Site for both Si and C. Based on these results we propose a model for the C-rich 6 root 3 x 6 root 3 reconstruction.