EVIDENCE OF NEAR-SURFACE LOCALIZATION OF EXCITED ELECTRONIC STATES INCRYSTALLINE SI

Surface- and interface-related spectra, obtained either directly by te chniques such as reflectance-difference (-anisotropy) spectroscopy or indirectly by subtracting pseudodielectric function spectra obtained e llipsometrically on surfaces with different chemical termination, exhi bit features related to energy derivatives of the bulk dielectric func tion. We argue that these spectra provide direct evidence that the exc itations involved are localized both in space and time. These data une quivocally indicate that critical point energies obtained from above-b and-gap ellipsometric or reflectrometric optical spectra are not neces sarily equal to bulk values, and that surface chemical and structural termination is at least one contributing factor. Present surface-optic al calculations do not include these effects, which may explain, in pa rt, remaining discrepancies between theory and experiment. (C) 1997 Am erican Vacuum Society.