Distinguishing anisotropic from isotropic transport in ion mixing of metal/oxide bilayers

Abstract

We have characterized the interaction of isotropic and anisotropic atomic transport mechanisms in the mixing of Ni/S'02/S' multilayers induced by irradiation with Xe at fluencesof 0.01–10 X 10^(15) cm^(−2). In the work reported here, the fluence dependence of net metal transport into the underlying layers was measured with different thicknesses of Si02 and different sample temperatures during irradiation (−196 to 50°C). There is a linear dependence at low fluences. The initial slope depends on sample temperature. A^(1/2) behavior dominates at high fluences for irradiation temperatures much below 500°C. For thin SiO_2 layers (< 20 nm), the cross-over point depends on the SiO_2 thickness. These results are readily interpreted in terms of competition between the flux of injected atoms and diffusion of the accumulating metal. Metal appears to be the dominant moving species in these systems. The initial linear dependence was not observed in samples without SiO_2, proving that the effect is not due to cascade overlap. The injected atoms are most likely to be secondary recoils. We conclude from our experiments that chemical reaction kinetics are more important than reaction enthalpies in determining net atomic transport in metals-SiO_2 ternary systems, although the latter affects final phase formation.