Control of tunnel oxide thickness in Si-nanocrystal array memories obtained by ion implantation and its impact in writing speed and volatility

The injection and storage of charge in Si nanocrystals obtained by ion implantation and annealing have been studied for different tunnel oxide thicknesses. The energy of the ions was kept fixed at 15 keV, which is compatible with most ion implanters used in Si technology, and the distance between the Si nanocrystals and the substrate was controlled by using gate oxides with different thicknesses. The processing conditions were adjusted for precipitating all the Si excess and for having Si–SiO2 interfaces free of defects. Consequently, reliable structures were obtained working in the direct tunneling injection regime, which show unprecedented longer retention times. Furthermore, it is shown that by changing only the oxide thickness it is possible to engineer devices with a tradeoff between writing speed and retention time.