Fundamental aspects of ion beam surface modification: defect production and migration processes

Ion beam modification of metals is generating increasing scientific interest not only because it has exciting technological potential but also because it has raised fundamental questions concerning radiation-induced diffusion processes. In addition to the implanted species, several defect production and migration mechanisms contribute to changes in the near-surface composition of an alloy during ion bombardment, e.g. atoms exchange positions via displacements and replacement sequences, preferential sputtering effects arise, and radiation-enhanced diffusion and radiation-induced segregation occur. The last two defect migration mechanisms are of particular significance since they can alter the composition to depths which are much greater than the implanted ion range. By altering various parameters such as irradiation temperature, ion mass, energy and current density and by changing the initial alloying distributions, a rich variety of near-surface composition profiles can be created. We have utilized changes in ion mass and energy and irradiation temperature to distinguish defect production from defect migration effects. Experimental results are presented which provide a guide to the relative efficiencies of different mechanisms under various irradiation conditions.