Formation, evolution and photoluminescence properties of Si nanoclusters

Si nanoclusters embedded in SiO2 have been produced by thermal annealing in the 900-1250 C range of SiOx films or of SiO2/Si/SiO2 multilayers, both prepared by plasma enhanced chemical vapour deposition. The structural properties of these systems have been investigated by energy filtered transmission electron microscopy. This technique, due to its capability to detect Si nanoclusters independently of the presence of a crystalline phase, has evidenced a relevant contribution of amorphous nanostructures, not detectable by using the more conventional dark field transmission electron microscopy technique. By also taking into account this contribution, an accurate quantitative description of the evolution of the samples upon thermal annealing has been accomplished. The temperatures at which the formation of amorphous and crystalline Si nanoclusters starts have been determined. Furthermore, the nanocluster mean radius and density and the crystalline fraction have been determined as a function of the annealing temperature. Finally, the optical and the structural properties of the two systems have been compared, leading to the demonstration that the photoluminescence properties are determined by both the amorphous and crystalline clusters.