Depth-dependent interdiffusion in a Si/Fe/Si trilayer was studied at sub-nm depth-resolution by using X-ray standing waves. The high depth-selectivity of the present technique permitted independent measurement of diffusion at the two Fe interfaces: Fe on Si and Si on Fe. This yielded the intriguing result that the Fe diffusivities at the two interfaces were not symmetrical. The values of the activation energy at the two interfaces were comparable, but the pre-exponential factors were different; suggesting different diffusion mechanisms operated at the two interfaces. This counter-intuitive result was explained in terms of asymmetrical structures of the interfaces, as revealed by depth-selective conversion electron Mössbauer spectroscopy. The asymmetry was, in turn, attributed to peculiarities occurring during Si/Fe/Si layer formation by sputter-deposition, due to the difference between the surface free energies of Fe and Si.

Asymmetric Diffusion at the Interfaces in Fe/Si Multilayers. A.Gupta, D.Kumar, V.Phatak: Physical Review B, 2010, 81[15], 155402