Formation and ordering of Ge nanocrystals on SiO2

This paper describes the mechanisms of formation and ordering of Ge nanocrystals on a pre-patterned silica layer. In the first part, we investigate the nucleation and growth of Ge nanocrystals during thermal annealing of an amorphous Ge layer. In particular, morphological evolution of nanocrystals with different experimental parameters is analyzed. We show that nanocrystals exhibit a pseudoequilibrium shape independent on annealing conditions; their size and density being only controlled by the deposited thickness of the amorphous layer. This behavior is explained by a nucleus density saturation due to the presence of exclusion zones around critical nuclei. In the second part, we evidence a very nice ordering of Ge nanocrystals inside the focused ion beam (FIB) patterns of a thin silica layer. Preferential nucleation of nanocrystals inside the holes is mainly explained by energetic arguments. In particular, we find that surface free energy is dramatically reduced when nanocrystals are located inside the holes instead of on the flat top surface between the holes. From a kinetic side, preferential nucleation inside the FIB holes should also be favored due to the lower surface diffusion inside the holes.