Phonon and polarized reflectance spectra from Si(111)-(4x1)In: Evidence for a charge-density-wave driven phase transition

The (4x1)-->(4x"2") phase transition of the In terminated Si(111) surface around 120 K has been investigated by optical spectroscopy. Characteristic surface phonon modes of the 4x1 and 4x"2" phases are observed with Raman spectroscopy. Reflectance anisotropy spectroscopy (RAS) reveals a splitting of a large feature in the optical anisotropy at 2 eV on formation of the low temperature phase. The changes in the RAS and Raman spectra are discussed within the framework of the conflicting models of the phase transition. Charge density wave formation driven by a Peierls transition is favored over transitions involving significant structural modifications.