Recombination with larger than bandgap energy at centres on the surface of silicon microstructures

Previously it has been shown that the correlation between shifts in luminescence and Raman peak positions in various porous silicon samples cannot be explained by the size effect of the silicon microstructures, as postulated by the (physical) quantum confinement model. It is also unlikely that siloxene occurs in significant quantities in porous Si. In this contribution, a possibility is presented for radiative recombination of carriers on the surface of a silicon microstructure, to explain strong visible luminescence in porous silicon. Nanometre-size structures on the surfaces of silicon microcrystallites introduce localized resonances into the (bulk-like) bands of the crystallites, giving rise to larger-than-bandgap transitions. These surface structures act as electron- and hole-traps and, due to localization of the carriers, recombination is possible neglecting momentum selection rules. Results of semi-empirical calculations on model structures are presented.