Multi-dot floating-gates for nonvolatile semiconductor memories: Their ion beam synthesis and morphology

Scalability and performance of current flash memories can be improved substantially by replacing the floating polycrystalline-silicon gate by a layer of Si dots. Here, we present both experimental and theoretical studies on ion beam synthesis of multi-dot layers consisting of Si nanocrystals (NCs) embedded in the gate oxide. Former studies have suffered from the weak Z contrast between Si and SiO2 in transmission electron microscopy (TEM). This letter maps Si plasmon losses with a scanning TEM equipped with a parallel electron energy loss spectroscopy system. Kinetic Monte Carlo simulations of Si phase separation have been performed and compared with Si plasmon maps. Predicted and measured Si morphologies agree remarkably well, both change with increasing ion fluence from isolated NCs to spinodal pattern. However, the predicted fluences are lower than the experimental ones. We identify as the main reason of this discrepancy the partial oxidation of implanted Si by atmospheric humidity, which penetrates into the as-implanted SiO2.