Calcium fluoride on Si(001): adsorption mechanisms and epitaxial growth modes

Growth of CaF2 on Si(001) is studied as a function of the substrate temperature during deposition for coverages from fraction of a monolayer (ML) up to several monolayers. Structural and morphological studies using atomic force microscopy, low-energy electron diffraction, and reflection high-energy electron diffraction are combined with measurements of core-level photoemission and x-ray absorption. Bonding between CaF2 molecules and Si(001) substrates is followed by monitoring core-level shifts and x-ray absorption line shape. It is found that a dissociative reaction occurs at high deposition temperatures (similar to 750 degrees C), giving rise to a 1-ML-thick uniform wetting layer, which is bonded with the substrate through Ca atoms. This wetting layer changes the surface periodicity from double domain 2x1+1x2 to single domain 3x1. Three-dimensional CaF2 elongated islands develop on top of the wetting layer, with their (110) planes parallel to the Si surface plane. At temperatures below 600 degrees C no dissociative reaction takes place for CaF2; nanodimensional islands develop in the form of rectangular-based huts. The crystallographic orientation of these islands is parallel to that of the Si(001) substrate. The data are compared to results obtained on CaF2 deposited on Si(111).