Nucleation kinetics of Si quantum dots on SiO2

The formation of Si quantum dots on SiO2 by chemical vapor deposition of SiH4 has been investigated in the range from the submonolayer to the complete coverage with Si. In order to investigate the very early stages of the nucleation process of Si on SiO2, the energy filtered transmission electron microscopy has been chosen as the main characterization technique, because of the high spatial resolution typical of the transmission electron microscopy analysis, coupled to the compositional information obtained by the electron energy loss spectroscopy. The plan view configuration has been used to measure the dot size distributions down to dimensions of about 1 nm, and in cross section to evaluate the dot wetting angle. For all the several experimental conditions, a wetting angle distribution has been obtained and has shown to be centered at about 90degrees. Data on the dot size distributions are shown and discussed in the framework of a continuous nucleation model, which has been implemented to take into account the dot coalescence process contribution. Through the comparison with experimental data the relevant thermodynamic parameters of the process have been evaluated: the critical radius, the free energy barrier for nucleation, and the concentration of nucleation sites.