Photoluminescence and Raman studies of nanocrystalline silicon enclosed in a SiO₂ shell

Spherical silicon-nanocrystals prepared by the thermal evaporation method were examined using Raman and photoluminescence measurements. The Raman analysis indicates that the content of the crystalline silicon decayed exponentially over time, which explains the reduction in size of the silicon-nanocrystals caused by oxidation. The visible photoluminescence spectra of silicon-nanocrystals are about the same. Although the size of the samples varied significantly, the temporal variation of the photoluminescence spectra was negligible, except for the very first day when samples were taken out from the growth chamber. Based on the Raman and photoluminescence results, we conclude that the red photoluminescence of silicon-nanocrystals near 680 nm is mainly attributed to defect states residing at the Si/SiO₂ interfaces.