Chain model of Si(111)2×1 surface: Optical properties and surface-state excitons

The optical properties of the chain model of Si(111)2×1 are computed both within a single-particle picture and with the inclusion of excitonic and local-field effects. The presence of bound exciton states depends on the magnitude of the screening of electrons in surface states. For weak screening the singlet exciton is strongly bound (~ 0.3 eV) and gives rise to a narrow Lorentzian peak which dominates the absorption spectrum. For strong screening, bound excitons are not present. The resulting absorption line shape is asymmetrical, in qualitative agreement with experiment, but almost twice as broad. Polarization-dependent reflectance measurements are suggested as an important test for the various models.