THERMALLY-INDUCED EVOLUTION OF CODEPOSITED CO-SI LAYERS ON SI(100) SURFACES

The formation of ultrathin epitaxial layers of CoSi2 on Si(100) surfac es is studied by means of valence-band and core-level photoemission sp ectroscopy with synchrotron radiation, low-energy electron diffraction (LEED) and Auger electron spectroscopy. The CoSi2 films are prepared by the template method in which an amorphous layer with CoSi2 stoichio metry is coevaporated on top of a 2.6-3 monolayer thick Co overlayer a t room temperature. The amorphous layer does not react to CoSi,. Annea ling to 300 degrees C leads to the crystallization of the layer demons trated by the formation of a (root 2x root 2)R45 degrees LEED pattern. This layer consists of disilicide in CaF2 structure with some contrib ution from another silicide phase. This surface is stable at least up to 460 degrees C. After further annealing above 460 degrees C the disi licide disappears, at least from the sub-surface layer, and the surfac e is covered by clean Si patches and patches of the second silicide ph ase. The new silicide phase is characterized by a (2 root 2x root 2)R4 5 degrees LEED pattern and has a different electronic structure and Si 2p binding-energy shifts. A comparison with previous experimental resu lts and existing theoretical electronic structure calculations suggest s that the new silicide phase can be identified as adamantane disilici de.