An electron microscopy study of the growth of Ge nanoparticles in SiO2

Ion implantation followed by high temperature annealing can be used to synthesize group IV semiconducting nanoparticles in ${\mathrm{SiO}}_{\mathrm{2}}.$ The density and the size distribution of these nanocrystals obviously depend on the implantation and annealing conditions. While their size can be measured by “classical” transmission electron microscopy techniques, their density cannot because no diffraction occurs in the amorphous matrix. In this letter, we use electron energy loss spectroscopy to overcome this problem. We have measured the evolution of the size distribution, the density, and the atomic fraction occupied by the Ge precipitates during annealing. We show that the nanocrystals grow in size and reduce their density, while the overall number of atoms they contain remains constant. This observation proves that the nanoparticles undergo a conservative ripening during annealing.