Size-dependent correlation of the photoluminescence lifetime with Si suboxide states at Si nanocrystal/SiO₂ interfaces

Double layers of 28 nm SiO_x/2 nm SiO₂ have been prepared on Si wafers by ion beam sputtering deposition and subsequently annealed to form Si nanocrystals (NC_s) in the SiO_x layer. The photo-luminescence (PL) lifetime obtained by fitting the decay traces to a stretched exponential function shows a sharp decrease as the NC size (d) is reduced down to ̃3 nm, but below ̃3 nm, its rate of decrease slows down. This phenomenon is strongly related to the change in Si suboxide states at the Si NC/SiO₂ interfaces, as confirmed by synchrotron-radiation X-ray photoelectron spectroscopy spectra of the Si 2p core level for the Si NC layers. These results can be explained based on the fact that as d decreases, the PL decays are increasingly influenced by interface states such as Si-O or Si=O bonds.