Theoretical study of the hydrogen relay dissociation of water molecules onSi(001) surfaces

Important features of the dissociative adsorption of water molecules on a S i(001) clean surface are clarified by first-principles density functional c alculations. It is elucidated that the dissociation reactivity is more enha nced when the water molecule interacts with the surface not as a single mol ecule but as clusters. The mechanism of the dissociation is analyzed by the frontier orbital theory. The dissociation as a water dimer is found to be reasonable in terms of 'phase' and 'overlap' of the orbitals as well as the 'gap of energy levels'. The adsorption features are also investigated on S i(001) surfaces which are partially modified by the water fragments such as -OH and -H. It is found that the dissociation reactivity as a water dimer on a certain modified surface becomes higher than that on a clean surface. This provides a clue to the explanation of the experimental results that th e surface regions covered with Si-OH and Si-H species form two-dimensional island structures. (C) 1999 Elsevier Science B.V. All rights reserved.