Surface phonons of the Si(111)-(7x7) reconstruction observed by Raman spectroscopy.

We have studied the surface phonon modes of the reconstructed Si(111)-(7×7) surface by polarized Raman spectroscopy. Six surface vibration modes are observed in the frequency range between 62.5 and 420.0 cm-1. The mode frequencies agree very well with reported calculation results. This enables their attribution to calculated eigenmodes, whose elongation patterns are dominated by specific atomic sites: the two most characteristic novel fingerprints of the (7×7) reconstruction are sharp Raman peaks from localized adatom vibrations, located at 250.9 cm-1, and collective vibrations of the adatoms and first- and second-layer atoms, located at 420.0 cm-1. While the sharp localized adatom vibration peak is a substantial refinement of an earlier broad spectral structure from electron energy-loss spectroscopy, no spectroscopic features were reported before in the collective-vibration frequency region. Furthermore, we observe in-plane wagging vibrations in the range from 110 to 140 cm-1, and finally the backfolded acoustic Rayleigh wave at 62.5 cm-1, which coincides with helium atom scattering data. Moreover, the Raman peak intensities of the surface phonons show a mode-specific dependence on the polarization directions of incident and scattered light. From this polarization dependence the relevant symmetry components in the Raman scattering process (A i and/or E symmetry) are deduced for each mode. [ABSTRACT FROM AUTHOR]