Diffraction spot profile analysis for heteroepitaxial surfaces applied to the initial growth stages of CaF2 adlayers on Si(111)

We have studied the heteroepitaxial growth of CaF2 on Si(111) by high resolution low energy electron diffraction. Analysing spot profiles recorded during CaF2 deposition (SPA-LEED) we obtained information about the film morphology. The spots show a central spike and diffuse shoulder caused by atomic steps of the CaF2 film. In order to evaluate quantitatively the layer distribution from the intensity oscillations of the central spike at the out-of-phase condition we included the interference between substrate and adlayer in our analysis. Depositing CaF2 at 300°C we conclude from the central spike intensity that the nucleation of the second CaF2 layer starts immediately. We attribute this behaviour to the poor binding of the CaF2 molecules to the 7 × 7 substrate so that obviously the nucleation at surface defects (atomic steps, anti-phase boundaries) is favoured. The analysis of the central spike oscillations also reveals that the film grows in the multi layer growth mode at higher coverages.