Adsorption of H2O on planar and stepped Si(100): Structural aspects

Abstract

The adsorption of water on two silicon surfaces [reconstructed planar (100)2×1 and single domain stepped (100)2×1 cut 5° towards (011)] was studied using low-energy electron diffraction and digital imaging electron stimulated desorption ion angular distributions (ESDIAD) as a function of temperature (145–700 K) and coverage. Water has been shown previously to chemisorb dissociatively to form surface OH groups. At 300 K the H+ ESDIAD pattern for the planar surface is a nearly symmetric halo, indicating that OH is oriented with its bond vector inclined away from the surface normal, while at low temperatures (≤200 K) a four-lobed pattern that preserves substrate symmetry is observed. This reversible temperature dependence is related to librations and rotations of the OH complexes. ESDIAD from the stepped surface exhibits a two-lobed pattern, with enhanced emission towards the steps, consistent with bonding of OH to single-domain terrace sites. An interpretation is presented based on the dimer model of the Si(100) reconstruction in which the OH bond axis azimuths are oriented nearly perpendicular to the dimer azimuths.