  Kinetic 3D lattice Monte Carlo studies are presented on Si nanocrystal (NC)
formation by phase separation in 1 keV Si implanted thin SiO2 films. The
simulation start from Si depth profiles calculated using the dynamic,
high-fluence binary collision code TRIDYN. From the initial Si supersaturation,
NCs are found to form either by nucleation, growth and Ostwald ripening at low
Si concentrations. Or at higher concentrations, non-spherical, elongated Si
structures form by spinodal decomposition, which spheroidize by interface
minimization during longer annealing. In both cases, the close SiO2/Si
interface is a strong sink for diffusing Si atoms. The NCs align above a thin
NC free oxide layer at the SiO2/Si interface. Hence, the width of this zone
denuded of NCs has just the right thickness for NC charging by direct electron
tunneling, which is crucial for non-volatile memory applications. Moreover, the
competition of Ostwald ripening and Si loss to the interface leads at low Si
concentrations (nucleation regime) to a constant width of the denuded zone and
a constant mean NC size over a long period of annealing.
