The (111)-(3 x 3) phase was analyzed by using scanning tunnelling microscopy, low-energy electron diffraction holography, density functional theory, and conventional low-energy electron diffraction. A single adatom per unit cell, which was found in scanning tunnelling microscopy, acted as a beam-splitter for the holographic inversion of discrete low-energy electron diffraction spot intensities. The resultant 3-dimensional image guided further detailed low-energy electron diffraction and density functional theory analyses which identified a Si tetramer on a twisted Si adlayer, with clover-like rings. This twist model, with one dangling bond left per unit cell, represented a novel (n x n) reconstruction mechanism for group-IV (111) surfaces.

U.Starke, J.Schardt, J.Bernhardt, M.Franke, K.Reuter, H.Wedler, K.Heinz, J.Furthmüller, P.Käckell, F.Bechstedt: Physical Review Letters, 1998, 80[4], 758-61