A new reconstruction structure, having a (√3 x √3)R30° unit cell, was reported for the Si(111) surface. Analysis of low-energy electron-diffraction intensity spectra by multiple-scattering theory revealed a vacancy model. The bonds between the first- and second-layer atoms were of the sp2 type, resulting in a highly compressed surface bilayer. Measured intensity spectra of the clean and Ag-deposited (√3 x √2)R30° surfaces were very similar. This led to the conclusion that the Ag-Si interface was not sharp, and that the Ag atoms did not form a lattice structure which exhibited long-range ordering.

Observation and Structural Determination of (√3x√3)R30° Reconstruction of the Si(111) Surface. W.C.Fan, A.Ignatiev, H.Huang, S.Y.Tong: Physical Review Letters, 1989, 62, 1516-9