Enhanced surface and interface diffusion in Ni-Bi bilayers by swift heavy ion irradiation

In this report, the effect of 100 MeV Au ion irradiation on diffusion in Ni/Bi bilayers has been studied. Normally, both of the layers across the interface need to reach their molten state to observe ion beam induced diffusion and mixing. However, we experimentally observed enhanced diffusion at the interface, though the thermal spike model calculations suggest that Ni does not reach its molten state at this energy regime. Spontaneous formation of a NiBi3 layer at the interface of Ni/Bi was found via the reaction-diffusion mechanism during the deposition. The evolution of porous like structures on the surface has been observed after irradiation of the films. This porous structure evolution was driven by the possible out-diffusion of Bi, as it reaches the molten state. The interface NiBi3 layer improves the mixing of the Ni and Bi layers and increases the ion fluences observed in the depth profiles obtained using cross-sectional transmission electron microscopy and Rutherford backscattering spectroscopy. Such an enhancement can be attributed to the combined swift heavy ion induced athermal effects and ballistic effects. The detailed mechanism of the ion induced surface and interface modifications were explained on the basis of thermal spike model calculations.