Nucleation and growth of CoSi2 on Si(100) studied by scanning tunneling microscopy

The initial stages of CoSi2 formation on the Si(100) surface are investigated by scanning tunneling microscopy (STM). We find a quasi-periodical reconstruction of the Si surface for very low Co coverages of 0.01 ML which is similar to the Ni induced (2 x 8) structure. At higher Co coverage, in reactive deposition epitaxy, the formation of qualitatively different two and three-dimensional islands is observed. We have evidence that the former are Si terminated, with Co probably being positioned in substitutional sites beneath the island. The growth of the 3D CoSi2 islands is connected with substantial mass transport from the substrate into the islands to enable the silicide formation. Their elongated shape is attributed to strain and they occur in different epitaxial relations to the substrate. CoSi2 islands in (100) orientation are identified by the c(2 x 2) surface lattice with mixed Co and Si occupation. Simultaneous deposition of Co and Si up to 30 ML results in the formation of CoSi2 island clusters and a rough surface. The roughness exponent beta=0.66 is in agreement with an existing Monte-Carlo simulation.