SLOW ADSORPTION-KINETICS OF H2O ON SI(111)7X7

The kinetics of H2O adsorption on Si(111)7 X 7 was studied at H2O vapor pressures of 2 X 10(-11) to 4 X 10(-8) Torr by the flash desorption technique. The surface coverage was found to depend not only on the surface exposure, i.e., on the P X t product, but also on the vapor pressure P or the exposure time t per se. Within the water vapor pressure interval mentioned above the sticking coefficient varied between 8 X 10(-2) and 9 X 10(-3), respectively. A possible mechanism includes creation of a 2D hydrosilicate phase forming islands on terraces between the steps and/or strips along the steps. Stoichiometry of this hydrosilicate phase should demand surface diffusion to or from the new 2D phase. The surface diffusion rate proportional-to t-1/2 may be responsible for the observed dependence of surface coverage both on the Pt product and on P or t in addition.