Size and Location Control of Si Nanocrystals at Ion Beam Synthesis in Thin SiO2 Films

Binary collision simulations of high-fluence 1 keV Si⁺ ion implantation into 8 nm thick SiO₂ films on (001)Si were combined with kinetic Monte Carlo simulations of Si nanocrystal (NC) formation by phase separation during annealing. For nonvolatile memory applications, these simulations help to control size and location of NCs. For low concentrations of implanted Si, NCs form via nucleation, growth and Ostwald ripening, whereas for high concentrations Si separates by spinodal decomposition. In both regimes, a self-adjusted oxide layer denuded of NCs forms at the SiO₂/Si interface, which has just the right thickness for NC charging by direct electron tunneling. However, only in the nucleation regime the width of the tunneling oxide and the mean NC diameter remain constant during annealing. This stability originates in the competition of Ostwald ripening and Si loss to the Si/SiO₂ interface. Based on the process simulations, its is predicted that the technological demands on the NC synthesis for nonvolatile memories are fulfilled best in the nucleation regime.