Chemical and structural aspects of CoPt silicide nanostructures grown on Si(100)

We investigate the growth of CoPt-based nanostructures on thermally oxidized Si(100) substrates by ultrahigh-vacuum coevaporation at 50, 400, and $650°C$ substrate temperatures. The thermal evolution of the bimetallic deposits has been studied by x-ray photoelectron spectroscopy (XPS) as well as x-ray diffraction (XRD) in grazing incidence. This study reveals a fast migration of the deposited Co and Pt towards the $SiO2$ underlayer, which promotes silicide formation at substrate temperatures higher than $400°C$⁠. The XPS spectra of Pt and Co present shifts characteristic of the metal silicide, from $71to72.6eV$ and from $778to778.5eV$⁠, respectively. Moreover, structural investigations on the sample prepared at a substrate temperature of $650°C$ by symmetric XRD and by high resolution transmission electron microscopy reveal the growth of mixed CoPt silicide nanostructures via an endotaxial solid-state mechanism. It has been found that the extremely low deposition rate together with the high temperature during the deposition and the presence of $SiO2$ layer favor the disilicide formation, preserving the initial equiatomic CoPt ratio in the silicide islands.