Metal-insulator transition in Si(111)-(4 x 1)/(8 x 2)-In studied by optical spectroscopy

Measurements of the surface vibrational modes and optical response of Si(111)-(4x1)/(8x2)-In are compiled and a comparison to ab initio calculations performed within DFT-LDA formalism is given. Surface resonant Raman spectroscopy allows identifying a number of surface phonons with high spectral precision. The phase transition of the (4x1)(8x2) surface structure is found to be accompanied by characteristic changes of the surface phonons, which are discussed with respect to various structural models suggested. The optical anisotropy of the (8x2) phase shows that the anisotropic Drude tail of the (4x1) phase is replaced by two peaks at 0.50 and 0.72eV. The spectroscopic signatures of the (4x1) and (8x2) phases agree with a metalinsulator transition. The mid-IR-anisotropic optical response of the insulating (8x2) phase is interpreted in terms of electronic single particle excitations between surface electronic bands related to the In-nanowire surface. Comparison of the measured optical transitions with DFT ab initio calculations for the hexagon model and the trimer model of the (8x2) structure shows evidence for the existence of the hexagon structure.