ADSORPTION OF WATER ON SI(001)-2X1 AND SI(111)-7X7 SURFACES AT 90 AND 300 K - A SI 2P CORE-LEVEL AND VALENCE-BAND STUDY WITH SYNCHROTRON-RADIATION

We have studied by soft X-ray photoemission (hv = 145 eV) the modifications of the Si2p core-level and of the valence band during the adsorption of water on si(001)-2 X 1 and si(111)-7 X 7 surfaces, at two substrate temperatures (300 and 90 K). In all cases, water reacts with silicon. We have followed the decay of Si2p and valence-band surface states, as well as the growth of oxidation states, with increasing exposures to H2O. The breaking of H2O into -H and -OH fragments and their attachment to the triply coordinated surface atoms (silicon dimers on Si(001), adatoms and rest-atoms on Si(111)) is not the only reactional mechanism. Oxygen atoms are also inserted into Si-Si bonds, so that oxidation offers an alternative channel to H2O dissociation and chemisorption. For both orientations, surface oxidation is facilitated at low temperature. The relative reactivity of the dangling-bonds of Si(111)-7 X 7 has been investigated: both valence band and Si2p spectra indicate clearly that the rest-atoms are less reactive than the adatoms.