WATER-ADSORPTION KINETICS ON SI(111)-7X7 - DEFECT GENERATION MECHANISM

Hydrogen accumulation on the Si(111)-7 x 7 surface due to water adsorption at room temperature was investigated by the flash desorption technique. The water pressure varied from 10(-11) to 10(-7) Torr. The sticking coefficient S turns out to decrease from S = 0.08 at a water pressure of 10(-9) Torr to 0.009 at 10(-7) Torr, i.e. S does depend not only on exposure but on the exposure time itself. We suppose that the chemisorbed water might form a 2D phase of the Si, OH, H and O species. Probably, the phase starts at steps (fast adsorption stage). The number of Si atoms in the new phase layer should not necessarily be equal to the effective Si surface density in the clean (111)-7 x 7 layer. Surface diffusion of Si atoms to or from the growing chemisorbed phase (slow adsorption stage) determines the dependence of the sticking coefficient not only on exposure but also on time itself.