Comparison of Kinetic MC Simulations and EFSTEM Observations of Phase Separation in Si Implanted Thin SiO2 Films

Studies on the ion beam synthesis of narrow Si nanocrystal (NC) layers in thin SiO2 films are presented. Very low-energy Si+ implantation into gate oxides for MOS transistors followed by thermal annealing allows for the fabrication of novel Si NC floating gate based non-volatile charge storage devices.
Small and isolated Si NCs at high density are required to obtain a large threshold voltage shift of the memory transistor. However, former work shows that the characterization of the Si NCs embedded in SiO2 by conventional Transmission Electron Microscopy (TEM) is difficult. It requires careful con-siderations and special imaging conditions [1] due to the weak contrast be-tween Si and SiO2 .
In this contribution, Energy Filtered Scanning Transmission Electron Micros-copy (EFSTEM) investigations on the morphology of phase separated Si in SiO2 are presented, which overcome the contrast limitations of the conven-tional TEM. Furthermore, a comparison of the observed Si pattern with pre-dictions of kinetic lattice Monte Carlo (MC) simulations [2] is performed. The Si precipitates were synthesized by 1 keV Si+ implantation into 10 nm thick SiO2 and by furnace annealing in N2 (or N2 + O2). Varying fluences from 5E15 to 2E16 Si+ cm-2 were used in order to adjust the Si excess in the SiO2. For these conditions, dynamical binary collision simulations (TRIDYN) of high-fluence implantation were combined with kinetic Monte Carlo simula-tions of NC formation by phase separations. For low Si excess, NCs are pre-dicted to form by nucleation, growth and Ostwald ripening. On the other hand, at high Si excess, phase separation proceeds via spinodal decomposition, were elongated NCs are found in our computer experiment. At even higher flu-ences, structural percolation occurs and a random connected Si mesh forms. Thus, the morphology of the phase separated Si changes with increasing ion fluence from isolated, spherical NCs to percolated structures as observed by EFSTEM. The pattern of the phase separated Si predicted by kinetic Monte Carlo simulations and observed by Electron Microscopy agree remarkably well.