Overstoichiometric low-energy nitrogen implantations into silicon--formation of gas nanobubbles vs. silicon nanowhiskers.

Accelerator-based ion implantation can be used to produce stoichiometric ratios beyond thermodynamic equilibrium. In the studies reported here, single crystalline silicon wafer material was implanted with high fluences of nitrogen. The implantations, performed with 15N at 5 keV/ion, resulted in the formation of highly swollen nitrogen rich surfaces that incorporated up to 63 at.% nitrogen. The implanted specimens were subsequently annealed with electron beams at high temperature, typically 1150 degrees C for moderately short periods of time (15 s) to investigate the formation of silicon nanowhiskers. However, it was observed that nanowhiskers, the formation of which can be expected by comparable understoichiometric implantations, did not appear. Although the shallow implantations created ultrathin silicon nitride films with typical thickness of 25 nm, laterally swollen areas of 400 +/- 50 nm were observed with atomic force microscopy operated in supersonic mode.