Mechanisms of ion beam induced atomic mixing in solids

In the present paper, selected typical studies on the ion-beam mixing of bi- and multi-layer systems are reviewed. It is shown that by proper variation of the materials (atomic number, chemical affinity between top and bottom layer) and of the irradiation conditions (ion species, energy, fluence, target temperature), significant conclusions can be drawn concerning the relevant mixing mechanisms. It is found that low temperature ion-beam mixing of light systems (Z≤18) is of pure ballistic character, without any influence of chemical driving forces. For higher atomic numbers and low or medium mass ions, mixing occurs by chemically biased diffusion in spatially separated local thermal-spikes. For very heavy ions mixing effects, which are non-linear with respect to the deposited energy density, point to the formation of coherent global spikes along the ion path by the overlapping of local spikes. Very heavy ions might also be able to initiate thermal-spikes in otherwise ballistic systems (Z≤18) at their end of range (end-of-range spikes) by a high density of subcascades. Chemically guided motion of residual defects from the collision cascade seems to play a role for ion beam induced mixing only at elevated temperatures.