Asymmetric diffusion at the interfaces in Fe/Si multilayers

Nanoscale diffusion at the interfaces in multilayers plays a vital role in controlling their physical properties for a variety of applications. In the present work depth-dependent interdiffusion in a Si/Fe/Si trilayer has been studied with subnanometer depth resolution using x-ray standing waves. High depth selectivity of the present technique allows one to measure diffusion at the two interfaces of Fe, namely, Fe on Si and Si on Fe independently, yielding an intriguing result that Fe diffusivity at the two interfaces is not symmetric. While the values of activation energy at the two interfaces are comparable, the main difference is found in the pre-exponent factor suggesting different mechanisms of diffusion at the two interfaces. This apparently counterintuitive result has been understood in terms of an asymmetric structure of the interfaces as revealed by depth selective conversion electron Mossbauer spectroscopy. This asymmetry is in turn explained by peculiarities during Si/Fe/Si layer formation by sputter deposition, in particular due to the difference between the surface free energies of Fe and Si.