Nitrogen diffusion in stainless steel during irradiation with mass-selected low-energy N+ ion beams

Nitrogen diffusion in stainless steel at elevated temperatures during low-energy N ion implantation is studied. Stainless steels in the temperature range from room temperature (RT) to 600 °C are irradiated by isotopically mass selected ∼1 keV 15N+ ion beams (less than several 10 μA/cm2) in an ultrahigh vacuum chamber. The depth profile of implanted N atoms as a function of substrate temperature is investigated by nuclear reaction analysis (NRA) in order to evaluate the diffusion coefficient and activation energy of N atoms in the stainless steel. The results demonstrate that the N diffusion during irradiation with low current density (less than several 10 μA/cm2) N ions is faster than that during a postanneal process after N ion irradiation at RT and is slower than that during higher current density (∼mA/cm2) N implantation previously reported. The activation energy of the N diffusion in this study is ∼1 eV, which is comparable to that of the ion irradiation experiment using higher current density beams, e.g., ∼mA/cm2, and is lower than that of the postanneal process. The N diffusion mechanism associated with ion irradiation in stainless steel is discussed.