Monte Carlo study of fluence dependent mixing and sputtering of isotopic targets under ion bombardment

Abstract

The collisional mixing of isotopic targets under ion bombardment is studied theoretically. The system 5 keV Ar + → Ge is treated as a case study. We employ a two-step algorithm: By Monte Carlo simulation, the target atom relocation in an ion-induced collision cascade is calculated; in the second step, the evolution of the concentration profiles in the target with fluence is computed numerically, using the Monte Carlo data for relocation, and relaxation as input. The steady-state concentration profile calculated shows a depletion of the light isotope close to the target surface, and an enrichment further inside. The Monte Carlo data depend only mildly on the displacement threshold used, on electronic stopping and on the fate of the implanted Ar gas atoms (evaporation from the surface or incorporation). In comparison with experiment, the sputter preferentiality shows a somewhat too strong dependence on fluence. Full quantitative agreement between simulation and experiment can only be achieved if an initial stoichiometry deviation in the near-surface region of the target from the natural isotope abundance ratio is assumed.