Theoretical aspects of atomic mixing by ion beams

Abstract

Collisional mixing by ion bombardment of dilute impurities in homogeneous matrices is treated theoretically. Relocation profiles are determined explicitly for ion-impurity knockon events (recoil implantation) as well as isotropic cascade mixing. Mean shifts and variances are evaluated for anisotropic cascade mixing. Both matrix and impurity knockon events are taken into account. Widely different fluence dependencies are found for the calculated half-widths of relocation profiles for different mechanisms, while the variance, when defined at all, is always proportional to fluence. Recoil implantation may give rise to a considerable mean shift of an impurity marker, but causes negligible peak shift up to quite high fluences. Isotropic cascade mixing receives a substantial contribution from recoils with energy far above threshold; this contribution is increasingly important with increasing fluence. Matrix relocation dominates measurable mixing profiles for heavy impurities in light matrices, while impurity relocation dominates in the opposite case. A Pt marker in Si bombarded by 300 keV Xe, is shown to move toward the surface with a mean rate comparable to the rate of surface erosion due to sputtering. The spread due to matrix relocation in this system is shown to exceed the spread due to impurity relocation by a factor of ∼ 20 and turns out to be in good agreement with experimental results.