Ethylene on Si(001)-2x1 and Si(111)-7x7: X-ray photoemission spectroscopy with synchrotron radiation

The adsorption of ethylene on Si(001)-2 x 1 and Si(111)-7 x 7 has been studied at 300 K using core-level (Si 2p, C Is) and valence-band x-ray photoemission with synchrotron radiation; Dangling bonds play a very different role in the chemisorption process on these two surfaces. While for Si(001)-2 x 1 the quenching of the surface states "monitors" the molecule uptake, this is not the case for Si(111)-7 x 7: the adsorption process continues well after all surface states have disappeared, and carbon coverages greater than that deduced from the rest-atom/adatom bridge model are obtained. An alternative model is proposed, in which molecules are bonded directly to the rest plane. The thermal stability of both surfaces covered by ethylene is also studied. We relate the formation of carbon-rich layers at 820-860 K to the strong changes in the electronic structure of the silicon surface. [S0163-1829(98)02840-9].