Initial growth stage of CaF₂ on Si(111)-7 × 7 studied by high temperature UHV-STM

The initial stages of CaF₂ growth on Si(111) at a high substrate temperature have been studied, in situ, with an ultra high vacuum scanning tunneling microscope (UHV-STM) from submonolayer range up to a monolayer. The STM images directly demonstrate that the initial growth mode changes from a three-dimensional island formation to a wetting heteroepitaxial layer growth with increasing substrate temperature. At a substrate temperature of about 470°C, islands of characteristic shape, with steps arranged in the [11̄0] direction, is observed on the Si(111) surface. This island formation initially occurs both at steps and on the flats of Si terraces. At a higher temperature of around 680°C, a submonolayer grows epitaxially from Si step edges, exhibiting a well- ordered row-like structure along the [11̄0] direction. The empty state image of this row-like structure has a 3 × 1 periodicity at 680°C. Further deposition of CaF₂ results in covering the Si surface uniformly with the heteroepitaxial monolayers.