Influence of the spatial arrangement on the quantum confinement properties of Si nanocrystals

The room temperature photoluminescence of nanocrystalline Si/SiO2 superlattices (nc-SLs) and silicon nanocrystals (Si-nc) randomly dispersed inside an insulating matrix has been investigated. We have found that, for the same Si-nc mean size, photoluminescence (PL) spectra relative to nc-SLs are blueshifted with respect to those relative to Si-nc dispersed in SiO2. Furthermore, the characteristic stretched exponential decay time exhibited by disordered Si-nc systems evolves towards a single exponential behavior, characterized also by longer decay times, when a low nanocrystal density is obtained by strongly reducing the excess of Si in the films. As an extreme case, Si-nc arranged in ordered planar arrays, well separated by thin SiO2 layers, exhibit a single exponential behavior and decay times particularly long (about 200 mus) The above effects have been interpreted in terms of the lack of interaction among nanocrystals in nc-SLs, due to their large reciprocal distance, and to the absence of relevant non-radiative decay processes.