Optical second-harmonic generation was applied to the study of clean Si(111)-(2 x 1) and 7 x 7 surfaces. By an analysis of polarization dependences of the two reconstructions, the symmetry properties of the surfaces were determined. The 2 x 1 to 7 x 7 surface phase transformation during thermal annealing had also been monitored in real time by the second-harmonic technique.

Study of Si(111) Surfaces by Optical Second-Harmonic Generation - Reconstruction and Surface Phase Transformation. T.F.Heinz, M.M.T.Loy, W.A.Thompson: Physical Review Letters, 1985, 54, 63-6

7 x 7 Reconstruction of Si (111)