Observation and structural determination of (√3×√3)R30° reconstruction of the Si(111) surface

A new reconstruction structure, having a {}3×{}{}3)R30^∘{} unit cell, is 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 are of the sp² type, resulting in a highly compressed surface bilayer. Measured intensity spectra of the clean and Ag-deposited ({}3×{}{}2)R30^∘{} surfaces are very similar. This leads us to conclude that the Ag-Si interface is not sharp and that the Ag atoms do not form a lattice structure that has long-range ordering.