Pronounced composition gradients in alloys under low-energy ion implantation at high fluence

The composition profile of a multicomponent material exposed to low-energy ion bombardment is exceedingly sensitive to recoil implantation. Preferential recoil implantation causes depletion of one or more species in the near-surface region, much like preferential sputtering. For an isotopic system, both processes are preferential in the lighter species but have opposite dependences on beam energy, the effect of recoil implantation being most pronounced near threshold. We have studied the problem for an isotopic system (Hg on Mo) on the basis of standard theory of atomic mixing. In order to isolate the effect of recoil implantation, both sputtering and cascade mixing have been assumed strictly stoichiometric. Pronounced composition gradients are predicted after high-fluence bombardment. It is shown that these gradients affect the sputter characteristics in the same direction as those induced by preferential sputtering. Comparison with low-energy high-fluence sputter experiments of Wehner and coworkers indicates that recoil implantation cannot be ignored in the interpretation but is not solely responsible for the observed effect.