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 into a SiO2 matrix during annealing processes has been investigated by X-ray diffraction and transmission electron microscopy. Remarkable differences have been found between Ge and Co clusters behaviour. For Ge implanted SiO2 films, a clear influence of near-surface Ge oxidation and nanoclusters melting has been established. Annealing at temperatures around 1000 deg. C leads to the formation of small (diameter ∼5 nm) nanocrystals. Classical Ostwald ripening mainly drives the clusters thermodynamical growth. On the contrary, for Co-implanted SiO2 films, a jump-like transition in nanoclusters evolution has been established at about 800 deg. C. A homogenous distribution of small (diameter ∼4 nm) amorphous clusters is transformed into a bimodal clusters profile, characterised by large (diameter between 20 and 40 nm) nanocrystals near the surface and a region of smaller clusters (diameter ∼7 nm) in depth. During Co nanoclusters formation the influence of nanoclusters melting can be neglected