Asymmetric diffusion at the interfaces in 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 sub-nanometer 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. It is faster at the Fe-on-Si interface. 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 counter-intuitive result has been understood in terms of an asymmetric structure of the interfaces as revealed by depth selective conversion electron Mossbauer spectroscopy. A difference in the surface free energies of Fe and Si can lead to such differences in the structure of the two interfaces.