REACTION OF ULTRATHIN CO LAYERS WITH SI(111) AND SI(100) SURFACES

The initial stages of ultrathin epitaxial COSi2 formation on Si(111) a nd Si(100) surfaces were studied by core-level photoelectron spectrosc opy with synchrotron radiation. A high surface sensitivity was obtaine d for electrons with kinetic energy of approximately 30 eV, where the mean free path is estimated to be less than 2.5 angstrom. Our Si2p dat a show that on Si(111) a part of the Co atoms are in a CoSi2-like envi ronment at room temperature, the rest forms a Co-Si solid solution. Fo r a complete reaction, annealing to approximately 350-degrees-C is nee ded. In this way we obtain the Co-rich CoSi2(111)-C surface terminatio n free of pinholes which is stable up to at least 460-degrees-C. After annealing to approximately 550-degrees-C the Si-rich CoSi2(111)-S sur face termination containing some pinholes appears. Contrary to the Si( 111), Co deposition on Si(100) surface at room temperature does not le ad to CoSi2 but only to a Co-Si solid solution. For coverages below 2. 6 ML heating to approximately 460-degrees-C is necessary to obtain CoS i2, whereas for higher coverages heating to approximately 300-degrees- C is sufficient. This effect is attributed to a change of the Co diffu sion mechanism at room temperature. The CoSi2 films on the Si(100) sur face produced in this way always contain pinholes.