It was proposed that the milk-stool model for the 7 x 7 reconstruction of the silicon (111) crystal surface be modified by the addition of a silicon atom to the apex of the milk-stool and by taking the modified milk-stools to be bonded to a crystal surface layer of rest-atoms directly over atoms of the fourth layer as in a wurtzite lattice. It was postulated that the apical atom dangling bond electron of modified milk-stools was donated to surface rest-atoms and that a three-center bond develops between the apical atom and the two atoms directly below it on a “filled” surface site. These modifications were consistent with scanning tunnelling microscopy and helium ion scattering experiments, and provide a basis for understanding a wide range of chemical and physical properties of this surface.

Modified Milk-Stool on Wurtzite Layer Model for Si(111) 7 x 7 Surface Reconstruction. L.C.Snyder: Surface Science, 1984, 140[1], 101-7