Si nanoclusters embedded in ${\mathrm{SiO}}_2$ have been produced by thermal annealing of ${\mathrm{SiO}}_x$ films prepared by plasma enhanced chemical vapor deposition. The structural properties of the system have been investigated by energy filtered transmission electron microscopy (EFTEM). EFTEM has evidenced the presence of 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. In particular, the temperatures at which the nucleation of amorphous and crystalline Si nanoclusters starts have been determined. Furthermore, the nanocluster mean radius and density have been determined as a function of the annealing temperature. Finally, the optical and the structural properties of the system have been compared, to demonstrate that the photoluminescence properties of the system depend on both the amorphous and crystalline clusters.

Light-emitting silicon nanocrystals embedded in ${\mathrm{SiO}}_2$ have been investigated by x-ray absorption measurements in total electron and photoluminescence yields, by energy filtered transmission electron microscopy and by ab initio total energy calculations. Both experimental and theoretical results show that the interface between the silicon nanocrystals and the surrounding ${\mathrm{SiO}}_2$ is not sharp: an intermediate region of amorphous nature and variable composition links the crystalline Si with the amorphous stoichiometric ${\mathrm{SiO}}_2.$ This region plays an active role in the light-emission process.

In this letter, the role of amorphous Si clusters in the excitation of Er implanted in substoichiometric ${\mathrm{SiO}}_x$ films will be elucidated. It will be shown that the temperature of the ${\mathrm{SiO}}_x$ thermal process prior to Er implantation is crucial in determining the luminescence properties of the samples. In particular, the luminescence intensity at 1.54 μm is almost constant for ${\mathrm{SiO}}_x$ samples not annealed or pre-annealed at temperatures lower than $\text{800\hspace{0.05em}°}\mathrm{C},$ reaches the maximum at $\text{800\hspace{0.05em}°}\mathrm{C},$ and decreases at higher temperatures. The structural properties of these samples have been studied by energy filtered transmission electron microscopy. It will be shown that for annealing temperatures lower than $\text{1000\hspace{0.05em}°}\mathrm{C},$ only amorphous Si nanoclusters are present. We demonstrate that a large density of small amorphous Si clusters produces the best luminescence performance and enhances the fraction of optically active Er.