Analysis of the primary process in isotope sputtering

Knockon sputtering from Mo isotopic mixtures has been studied by binary-collision and (in part) molecular-dynamics simulation. Differential and integrated partial sputter yields have been determined for homogeneous, mostly polycrystalline targets. Simulations have been carried out on Mo-103-Mo-92 and Mo-100-Mo-50 mixtures and accurate scaling relations with mass ratio have been established. The reliability of the simulation code was tested by computation of absolute elemental sputter yields, and the influence of key input parameters on these yields was determined. Relative sputter yields of isotopic mixtures do not depend sensitively on most of those parameters, with the notable exception of the target-target interatomic potential, the surface barrier, and the treatment of nonbinary collisions. Most of our simulations refer to Ar bombardment, and ion energies covered range from less than 100 eV to 100 keV. At high ion energies sputtering is found to be preferential in the light species in agreement with predictions from transport theory, and the magnitude of this effect appears consistent with the few available experimental results. A weak dependence of the yield ratio on emission angle is found which is identified as a surface scattering effect. At low ion energies very pronounced preferential sputtering is found which occasionally even may go in the opposite direction, i.e., preferential emission of the heavier species. This effect is governed by threshold processes and characterized by a predominant contribution from primary recoil atoms to the sputtered-particle flux. It is sensitive to the angle of incidence. Although the relative importance of threshold processes decreases rapidly with increasing energy their signatures remain visible at fairly high energies because isotope effects in higher generations of recoil atoms are weak. In the 1-5 keV range which is important for numerous applications and where most experimental and previous simulational work was carried out, the two types of processes compete. This complicates the analysis and explains why it has been difficult to reconcile discrepancies in the past.