Water adsorption on cleaved silicon surfaces

Abstract

High-resolution electron-energy-loss spectroscopy together with x-ray photoelectron spectroscopy and low energy electron diffraction are used to study water adsorption on the cleaved 2×1 surface of Si. Adsorption studies at both 100 and 300 K were performed. At 300 K and during initial adsorption at 100 K the water is adsorbed principally by forming a dissociated chemisorption structure containing hydroxyl and hydride groups. At 300 K this phase saturates at low coverages, with a (1×1) LEED structure while at 100 K the sample will adsorb additional water. For exposures from about 1/3 to 2 L at 100 K an intermediate transitional adsorption layer forms which contains both molecular water and chemisorbed lines. Larger exposures form multiple layers of water (ice) and exhibit the loss features that have been reported for ice on other faces of Si. Very high resolution, about 5 meV, was achieved during the 300 K adsorption studies, allowing the identification of a small amount of molecular water. These phenomena for the cleavage face are quite complex and are different than those previously observed for water adsorption on the (100) surface where the 300 K adsorption consists simply of the dissociation of the water into the hydroxyl and monohydride species with no evidence for molecular water adsorption. In the latter case the adsorption is attributed to bonding with the dangling bonds. The adsorption on the cleavage surface is more disruptive of the surface, as evidenced by the observation of the dihydride species and the relaxation of the π-bonded chain structure characteristic of the cleaved surface into a more bulklike (1×1) structure.