Initial stages of CaF2/Si(1 1 1) epitaxy investigated by friction force microscopy

The initial stages of high temperature growth of CaF2 on Si(1 1 1) substrates have been studied by in situ scanning tunnelling microscopy and ex vacuo atomic force microscopy. At these temperatures, CaF2 molecules react with the silicon surface and form a CaF1–Si interface layer, which completely covers the surface before CaF2 start to grow. Using the material contrast ability of lateral force microscopy (LFM) a direct distinction between the CaF1–Si interface layer and CaF2 can be achieved. CaF2 decorates substrate steps from both sides in a non-uniform way; some terraces are nearly completely covered, some not at all. Additionally, isolated islands are observed on the terraces. The LFM signal shows a contrast between CaF2 and two types of CaF1. The CaF1 phase with the highest friction appears rough in AFM topography. This roughness contrast is also observed in UHV by STM. Thermal decomposition of parts of the CaF1 layer could be responsible for this effect.