The initial stages of the high temperature (~ 700°C) MBE growth of CaF 2 on well-oriented Si(111) substrates have been studied by atomic force microscopy (AFM) and lateral force microscopy (LFM) in the atmosphere. At these temperatures CaF 2 molecules react with the silicon surface and form a CaF 1–Si interface layer. The CaF 1 layer covers the silicon surface completely before CaF 2 islands start to form, and it is stable in the atmospheric environment for some days. The subsequent growth on this interface layer results in multilayer CaF 2 islands (about 5 TL in height). The friction force experienced by the Si tip was found to be larger on the CaF 1 interface layer than that on the CaF 2 layer and allows a direct distinction between the CaF 1–Si interface layer and CaF 2. Two different types of islands are observed. One has a regular shape with a flat top surface; the other has an irregular shape and exhibits a 0.3–0.6-nm-high corrugation. We suppose that the formation of disturbed islands can be attributed to the dissociation of the CaF 1 interface at the growth temperature of 700°C. These results suggest that high growth temperatures (> 600°C) do not always result in the best CaF 2 film.
The structure and morphology of CaF2/Si(111) and Ge/CaF2/Si(111) layered structures with film thicknesses in the range of very few nanometers has been studied with synchrotron-based radiation. While the CaF2 film is grown via molecular beam epitaxy, the Ge film is fabricated by surfactant enhanced solid phase epitaxy with Sb as surfactant. The CaF2 film forms two laterally separated phases of relaxed CaF2 and pseudomorphic CaF2, respectively, although the film thickness is very homogeneous. The Ge film is completely relaxed and forms A-oriented parts as well as B-oriented parts, due to twinning. In spite of the large surface roughness of the Ge film, it completely wets CaF2/Si(111) also after annealing at 600 °C, due to the application of Sb during the annealing process.
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