OPTICAL-RESPONSE OF SI(111)-7 X 7

The optical properties of the Si(111)-7 x 7 surface are studied by means of a surface differential reflectometer. The optical reflectivity of the clean surface is first compared to that of the native oxide-covered Si surface, then to that of the oxygen-exposed surface. The exact surface optical response function of the Si(111)-7 x 7 surface and its changes upon adsorption of oxygen are determined by use of an electromagnetic surface model. A main peak is observed at 3.9 eV and is interpreted as due to electronic transitions from the back-bond states of the Si adatoms to a bulk conduction band. Secondary maxima at lower energies, due to transitions involving the dangling bond states, are also observed. Indications on the oxygen chemisorption mode are finally deduced.