Adsorption of water on Si(001)-2 × 1 and Si(111)-7 × 7 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 Si 2p core-level and of the valence band during the adsorption of water on Si(001)-2 × 1 and Si(111)-7 × 7 surfaces, at two substrate temperatures (300 and 90 K). In all cases, water reacts with silicon. We have followed the decay of Si 2p 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 SiSi 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 × 7 has been investigated: both valence band and Si 2p spectra indicate clearly that the rest-atoms are less reactive than the adatoms.