Interfacial role in room-temperature diffusion of Au into Si substrates

X-ray reflectivity is used in tracking the diffusion of Au into Si(001) substrates with time at room temperature. It has been observed that the diffusion of Au into Si substrates strongly depends on the initial pretreatment conditions of Si surface. In particular, there is very little diffusion for the untreated Si surface, while the Si surface pretreated with HF seems to be prone to strong diffusion and the surface further pretreated with Br shows diffusion in between. Such different diffusion and apparent non-Fickian-type time dependence in the diffusion can be quantitatively explained by Fickian diffusion of Au through changing unblocked interfacial layer. The growth of the blocking (oxide) layer with time essentially prevents further diffusion through those areas, and the growth of that layer is directly related to the surface stability due to the surface pretreatment and/or passivation conditions, which gives a control in the formation of diffusion-induced Au-Si nanolayer of different widths and compositions. The morphology and evolution of the top surface, mapped with atomic force microscopy and scanning electron microscopy, further helped to verify and understand such differences.