The theory of the preferential sputtering of alloys, including the role of Gibbsian segregation

Preferential sputtering of alloys leads, not to a profile showing a simple depletion of one component starting at the surface, but to a more complicated situation in which the outermost atom layer shows a composition spike of one component while a depleted region begins at atom layer two. At steady state, the composition spike is governed by a conservation relation which equates the ratio of the sputtered fluxes (taking into account both mass and binding effects) to the ratio of bulk compositions. The steady-state second-atom-layer composition is determined by a relation similar to that for equilibrium Gibbsian segregation, but with the segregation ratio governed by a kinetic quantity rather than a thermodynamic one. Beyond the second atom layer there is a composition profile ideally describable (for bombardments at or below ambient temperature) by ion-beam mixing theory, but which can be shown also to admit a tractable diffusion approximation. (Auth.)