Size and surface effects in porous silicon studied by X-ray absorption spectroscopy

X-ray absorption spectroscopy (XAS) at the Si-K absorption edge has been applied to study the influence of size and chemical-physical surface effects on the optical properties of porous silicon (PS). The comparison between XAS spectra obtained by monitoring both total electron yield (TEY) and photoluminescence yield (PLY) has allowed us to discriminate between light emitting and non-emitting sites in PS. Thanks to partial PLY-XAS measurements and their site selectivity, we have been able to detect the presence of energy levels inside the band gap of the Si nanocrystallites. The origin of these radiative states is related to the surface passivation, whose influence becomes particularly significant at decreasing sizes.