Comparison of the Charge Storage Properties of Ion Beam Sythesized Si and Ge Nanoclusters in Thin Gate Oxides

Semiconductor nanoclusters (NCs) embedded in thin gate dielectrics are of fundamental interest for future integrated memory concepts. In this contribution we compare the properties of Ge and Si NCs embedded in thin SiO2 films (d = 20 nm) with respect to their charge storage behaviour. Those NCs were prepared by ion beam synthesis combining low energy ion implantation (6-18 keV, 5x1015-2x1016cm-2) with subsequent rapid thermal processing (950 or 1050°C, 30-120 s) choosing identical ion ranges and peak concentrations for both elements. Electrical characterization based on CV, IU, pulsed Ct and It measurements were performed at MOS capacitors. The investigations reveal significant differences for Ge and Si NC containing gate oxides. Ge NCs near the Si/SiO2 interface are characterized by fast charging / decharging and a flatband voltage shift of several volts. In contrast, Si implanted oxides show memory effects with a smaller flatband voltage shift, but considerably improved data retention. In both cases charging is possible for electrical field strength of 4-6 MV/cm, well below the Fowler-Nordheim tunnelling region. The varying electrical properties correlate with differences in the spatial distribution of ion beam synthesized Ge and Si NC in thin SiO2 films, which were investigated by XTEM and RBS. The change of spatial distribution during annealing is due to different thermodynamics of NC evolution and chemical processes especially in thin SiO2 films.