The influence of preferential enhanced diffusion on composition changes in sputtered binary solids

The preferential sputter enrichment or depletion of a species in a binary solid is a well established experimental observation. This composition change is usually found to occur over a depth similar to that of the penetration depth of the sputtering projectile. This paper therefore considers a model for preferential sputtering based upon a preferentially enhanced diffusion of one species over a finite depth in the solid which acts as a continuous atomic supply to the surface. Exact solutions are determined for steady state surface and depth distributions of the composition ratio of the two species for the time dependent behaviour of these concentration distributions. The results are compared with models which assume equal diffusion enhancement of both species over both limited and infinite depth and with experimental studies. (author)