Room-temperature water adsorption on the Si(100) surface examined by UPS, XPS, and static SIMS

The adsorption of water on the reconstructed Si(100)-(2 x 1) surface at room temperature is studied using an integrated multi-technique approach. UPS, XPS, and static SIMS surface examination in a variable temperature mode of experimentation indicates dissociative adsorption of water to hydroxyl (OH) and hydride (H) groups saturating the single dangling bonds on adjacent dimer-pair Si atoms of the reconstructed surface. No evidence of molecular water is found. Upon heating the surface the hydroxyl decomposes to bridge-bonded oxygen (Si-O-Si) and additional silicon hydride groups. This is complete at ≈400°C. Above 400°C the surface loses hydrogen, presumably as H₂, and above 600°C the substoichiometric silicon oxide sublimes as SiO to return to the clean, reconstructed surface. The chemical state of oxygen on the surface is monitored as a function of temperature through the use of a Δ parameter (Δ = binding energy O(1s) - binding energy Si(2p)). These results are consistent with experimental evidence and surface chemistry first proposed by others. Assignment of the observed DOS features is made by comparison of the experimental DOS spectra to calculations for the system and for system analogs (SiOH radical, oxygen-saturated Si surface, hydrogen-saturated Si surface), and to another hydroxyl system (NaOH). Four major features of the saturated hydroxyl/hydride surface are found : 5 eV Si-H bond, Sisp³ + H1s character; 6.4 eV O 2p lone pair, π symmetry nonbonding orbital; 7.6 eV O-H bond, O 2p + H1s character, σ symmetry; 11.9 eV Si-OH bond, sp³ + O 2p character (binding energies relative to the Fermi level).