Molecular beam epitaxial growth of thin CaF 2 films on vicinal Si(111) surfaces

Abstract

The growth of CaF 2 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 CaF 2 grows in a layer-by-layer fashion for the first two monolayers which gives rise to RHEED intensity oscillations. Beyond two monolayers, relatively thick CaF 2 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 CaF 2 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 CaF 2 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.