Surface migration of atoms is a fundamental process in the self-organization of nanostructures. By manipulating the surface (i.e. lowering the reactivity) the growth of clusters can be drastically altered. We present a comparison of the diffusion and nucleation of Fe atoms on various noble metal passivated Si(111) surfaces.
By depositing small amounts (< 1 ML) of noble metals (Cu, Ag, Au) on a Si(111)-7x7 surface, a variety of Si surface reconstructions can be induced. Subsequent evaporation of Fe atoms (~ 1ML) leads to the formation of nanoprecipitates. Their structural properties are investigated with scanning tunneling microscopy (STM), while Fe-Si phase formation is studied with conversion electron Mössbauer spectroscopy (CEMS).
Both Ag and Cu induced surface reconstructions (R3xR3 and "5x5" resp.) give rise to an increased mobility of the Fe atoms, leading to distinct islands. Our STM and CEMS measurements reveal the existence of silicide islands on the surface: the CsCl-FeSi phase is found at a temperature below 500°C. For the Cu covered surface an explicit transformation into a Si rich silicide phase is observed above 500°C. From a detailed investigation of the deposition temperature and rate dependence of the island size, height and distribution, the activation energy, diffusion coefficient and critical island size for the Cu and Ag systems are determined.
The Au covered surfaces behave quite differently from the Ag and Cu covered surfaces. Firstly, the Au–5x2 reconstructed surface shows no enhanced diffusion and deposition of Fe leads to a rough (amorphous) silicide layer (similar to deposition on the bare Si–7x7 surface). Secondly, for the Au–6x6 surface an enhanced diffusion is observed and small (compared to Ag or Cu covered surfaces) islands are formed on the surface. Thirdly, the Au-R3xR3 reconstruction exhibits phase boundaries which limit the Fe diffusion between domains of different orientation. The density of boundaries critically depends on the Au coverage and by modulating it, the surface mobility is influenced and the island distribution can be controlled.
In this contribution we will show the strong dependence of the Fe diffusion and nucleation processes on the Si surface reconstruction.
|