Uniform epitaxial CoSi2 layers have been grown in situ on a (100) Si substrate at temperatures above 600 °C by reactive chemical-vapor deposition of cyclopentadienyl dicarbonyl cobalt, Co(η5-C5H5)(CO)2. Co-rich phases such as Co2Si and CoSi were suppressed during cobalt metallorganic chemical-vapor deposition at substrate temperatures above 500 °C. A thin carbon layer was found on the top of the epitaxial CoSi2 layer grown on the Si substrate due to incomplete decomposition of the cobalt metallorganic source and diffusion of Co into the Si substrate. In spite of the existence of a surface carbon layer, an ion channeling minimum yield, χmin, of 8% in Rutherford backscattering/channeling spectrometry has been achieved in the epitaxial layer, indicating a nearly perfect epitaxial order. The carbon pileup on the surface of the CoSi2 layer at the initial stage of Co deposition seems to play the role of a cobalt diffusion barrier, avoiding the formation of Co-rich phases. © 1999 American Institute of Physics.