Si nanocrystals by ultra-low energy ion implantation for non-volatile memory applications

In nanocrystal (nc) metal-oxide-semiconductor (MOS) memory structures, a fine control of nc location and population is required for pinpointing the optimal device architectures. In this work, we show how to manipulate and control the depth-position, size and surface density of two dimensional (2D) arrays of Si ncs embedded in thin (<10 nm) SiO2 layers, fabricated by ultra-low energy (typically 1 keV) ion implantation and subsequent annealing. The influence of implantation and annealing conditions on the nc characteristics (e.g. size, density) and the charge storage properties of associated MOS structures is reported with particular emphasis upon the effect of annealing in N2-diluted–O2 gas mixture. The latter annealing conditions restore the integrity of the oxide and allow for the fabrication of non-volatile memory devices operating at low-gate voltages. Annealing in diluted oxygen has also an effect on the population of silicon ncs. Their evolution has been studied as a function of the annealing duration under N2 + O2 at 900 °C. An extended spherical Deal–Grove model for the self-limiting oxidation of embedded Si ncs has been carried out. It shows that stress effects, due to the deformation of the oxide, slows down the chemical oxidation rate and leads to a self-limiting oxide growth. The model predictions are in agreement with the experimental results.