The (7 x 7) surface was considered to have a triangular checkerboard structure in which the layer stacking sequences ⋯AaBbA and ⋯AaCcA alternate in neighbouring triangular subunits in conformity with ion back-scattering results. The topological requirements of joining double layers at the sub-unit boundaries lead to the prediction of arrays of dimers and of deep (6.3Å) holes which were consistent with results of electron diffraction and of tunnelling microscopy, respectively.

Surface Stacking Sequence and (7 x 7) Reconstruction at Si(111) Surfaces. E.G.McRae: Physical Review B, 1983, 28, 2305-7