Received 17 November 2006; revised manuscript received 8 February 2007; published 8 May 2007X-ray reflectivity is used in tracking the diffusion of Au into Si 001 substrates with time at room tempera-ture. It has been observed that the diffusion ofAu into Si substrates strongly depends on the initial pretreatmentconditions of Si surface. In particular, there is very little diffusion for the untreated Si surface, while the Sisurface pretreated with HF seems to be prone to strong diffusion and the surface further pretreated with Brshows diffusion in between. Such different diffusion and apparent non-Fickian-type time dependence in thediffusion can be quantitatively explained by Fickian diffusion of Au through changing unblocked interfaciallayer. The growth of the blocking oxide layer with time essentially prevents further diffusion through thoseareas, and the growth of that layer is directly related to the surface stability due to the surface pretreatmentand/or passivation conditions, which gives a control in the formation of diffusion-induced Au-Si nanolayer ofdifferent widths and compositions. The morphology and evolution of the top surface, mapped with atomicforce microscopy and scanning electron microscopy, further helped to verify and understand such differences.DOI: 10.1103/PhysRevB.75.205411 PACS number s : 66.30.Pa, 68.35.Fx, 61.10.Kw, 68.37.Ps