Initial growth stage of CaF2 on Si(111)-7x7 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 degrees C, islands of characteristic shape, with steps arranged in the [1(1) over bar0$] direction, is observed on the Si(111) surface. This island formation initially occurs both at steps and on the flats of Si terraces, At a higher temperature of around 680 degrees C, a submonolayer grows epitaxially from Si step edges, exhibiting a well- ordered row-like structure along the [1(1) over bar0$] direction, The empty state image of this row-like structure has a 3 x 1 periodicity at 680 degrees C, Further deposition of CaF2 results in covering the Si surface uniformly with the heteroepitaxial monolayers.