Structure of Si(111)-7x7 II.

The previously proposed island-dimer model of the Si(111)-7x7 surface [E. G. McRae, Surf. Sci. 124, 106 (1983)] is modified to conform to wurzite-like surface stacking sequences implied by Rutherford backscattering results. The (7x7) surface is considered to have a triangular checkerboard structure in which the layer stacking sequences ...AaBbA and ...AaCcA alternate in neighboring triangular subunits or in which neighboring subunits are double-layer islands and troughs with local stacking ...AaCcA. The topological requirements of joining double-layers across the subunit boundaries lead to the prediction of a deep (6.3 angstrom) hole at the subunit apex and of alternating dimers and deep holes along the subunit sides.