Molecular beam epitaxial growth of thin CaF2 films on vicinal Si(111) surfaces

The growth of CaF2 on Si(111) 7 × 7 surface at ∼ 770°C and ∼ 250°C on both ∼ 2° and ∼ 0.5° off-normal, vicinal substrates (titled towards the [11 -2] azimuth) has been studied using RHEED, AFM and SEM. For growth at ∼ 770°C on both substrates, the CaF2 grows in a layer-by-layer fashion for the first two monolayers which gives rise to RHEED intensity oscillations. Beyond two monolayers, relatively thick CaF2 islands nucleate at the Si step edges (which are bunched into step bands) and grow laterally with constant height (∼ 5 nm for ∼ 2° and ∼ 2 nm for ∼ 0.5° miscut substrates) along the Si step edges and eventually form a flat overlayer. The height of the CaF2 islands appears to be determined by the height of the Si step bunches which, in turn, is determined by the substrate miscut angle. For growth at ∼ 250°C, the integral diffraction spot RHEED intensity decreases exponentially for the first ∼ 2 monolayers of growth and then oscillates for ∼ 6 periods. In addition, the non-integral diffraction spots, corresponding to the Si(111) 7 × 7 pattern, are not fully removed until the integral diffraction spot intensity starts to oscillate at ∼ 3 monolayers. These results indicate that the CaF2 initially grows by nucleation and coalescence of 2D islands which are 2 to 3 monolayers high, followed by a multilayer-by-multilayer mode which leads to a featureless, flat surface morphology.