Investigation of the Formation and Phase Transition of Ge and Co Nanoparticles in a SiO2 matrix

The evolution of ion beam synthesized Co and Ge nanoclusters in a SiO2 matrix during annealing processes has been investigated by in-situ X-ray diffraction and ex-situ transmission electron microscopy. Remarkable differences has been found for Ge and Co clusters. For Ge implanted SiO2 films a clear influence of near-surface Ge oxidation and nanocluster melting has been established. Annealing at temperatures at around 1000°C leads to the formation of an ensemble of small (d~5 nm) nanocrystals. The cluster growth is mainly thermodynamically driven by classical diffusion limited Ostwald ripening. Contrary, for Co-implanted SiO2 films a drastic transition in nanoclusters evolution has been established. Within a relatively small temperature range at around 800°C an ensemble of small (d~4nm) amorphous Co clusters is transformed into a bimodal cluster distribution with large Co nanocrystals of 20-40 nm diameter near the surface. The influence of oxidation or nanocluster melting can be neglected.