NUCLEATION AND GROWTH-MORPHOLOGY IN HETEROEPITAXY OF CAF2 ON SI(111) - A STUDY WITH SCANNING-TUNNELING-MICROSCOPY

Scanning tunneling microscopy (STM) was used to investigate nucleation and growth phenomena in the heteroepitaxy of calcium fluoride (CaF2) on a Si(111) surface. CaF2 was evaporated at room temperature on Si(111), and annealed to about 450 degrees C. Subsequently, characteristically, shaped islands, with steps arranged in the [1 (1) over bar 0] direction, were observed both on a terrace and a step edge of Si(111). After annealing this surface to about 600 degrees C, STM images clearly showed that these islands underwent a structural transformation to well-ordered heteroepitaxial layers which consisted of rows running along the [1 (1) over bar 0] direction and grew from Si step edges. These findings show that at a high temperature the preferential nucleation sites are distributed on the Si step edges and that CaF2 molecules have sufficient surface diffusion length to form the heteroepitaxial layers at the step edges.