High-resolution SEM images of germanium nanocrystals (Ge-nc) synthesized by ion implantation in fused silica samples annealed at temperatures below and above the melting point of Ge show a strong size-selective depth-distribution of nanostructures, as evidenced by the correlation between the dimension of the observed objects and the local concentration of implanted Ge measured by Rutherford backscattering spectroscopy (RBS). Whereas the Ge-nc nucleation seems to obey the Ostwald ripening process in samples annealed below 900 °C, Ge desorption effects, non-uniform in depth, in conjunction with the formation of large and spherical nanocavities, become dominant for annealing performed above the solid-liquid phase transition of Ge. Measurements for different annealing times at 1050 °C show two distinct processes in the Ge desorption dynamics: the first is related to direct Ge outgassing effects during the nucleation of Ge-nc, which occurs within the first minutes of the thermal annealing, while the second is due to the release of Ge from Ge-nc, associated with the formation of nanocavities. The formation rate of these nanocavities is more efficient at greater depth than in the vicinity of the sample surface. It appears to be strongly dependent on the local concentration of defects, responsible for the reduction of the Ge diffusion, and to be related to the breaking of Ge-O and Si-Ge bonds at the Ge-nc/SiO(2) interface.

Optical properties of silicon nanocrystals (Si-ncs) prepared by silicon implantation into silicon oxide have been investigated by photoluminescence measurements and spectroscopic ellipsometry. The dielectric function associated with Si-nc uniformly and nonuniformly depth distributed has been determined by means of the Tauc–Lorentz (TL) model, using the Bruggemann effective medium approximation. The evolution of the Si-nc sublayer dielectric response as a function of the ion fluence has been established for volume fractions of Si excess varying between 9.1% and 50.4%. Comparison between the depth profile of optical indices determined by ellipsometry and TRIM calculations shows that for implanted Si volume fraction lower than 30%, the center and the width of the optical index profile agree with the spatial distribution of the implanted Si when both the swelling and the ion sputtering effects are taken into account. This is also valid in systems having two separate Si-nc sublayers, where the geometric characterization of the optical index variations has been computed from a data extrapolation. For a volume fraction of 50.4%, where the ion implantation performed at high fluence can activate the oxygen depletion from the material surface, the spatial distribution of the optical refractive index is deeper and narrower than the Si excess profile.

Germanium nanocrystals (Ge-ncs) were synthesized by implantation of Ge ⁺ ions into the fused silica, followed by a thermal annealing at 1000 °C. High-resolution transmission electron microscopy was employed to characterize both the morphology of the formed Ge-ncs and the evolution of their depth-distribution as a function of annealing durations. The formation of nanocavities in the vicinity of nanocrystal/SiO ₂ interface is evidenced, as well as their influence on the release of the compressive stress exerted on Ge-ncs by surrounding SiO ₂ . Some Ge-ncs are found inside nanocavities, and can move into the implanted layer through a nanocavity-assisted diffusion mechanism. This finding sheds light on a new process that can explain the non-uniformity of the Ge-nanocrystal spatial distribution.