A study on the Si(111)-√3x√3-Ag system. II. Interaction between substrade and adsorbate.

Local density band structure calculations are presented on the HCT model of the Si(111)-√3 × √3'-Ag surface. We analyze the bonding of the adsorbate to the substrate in this model, and compare this to the bonding in competing adsorption geometries. First we show that the missing top-layer silicon substrate in the HCT model is only 0.49 eV per surface atom higher in energy than the common intact double-layer (IDL) substrate. For the binding with the silver adatoms, small clusters are shown to be unsuitable to describe the HCT model. With slab calculations we find that the AgSi bonding in the HCT model is heteropolar in character, with considerable charge transfer from the silver adlayer to the silicon substrate. For adsorption on an IDL surface covalent bonding results. The calculated adsorption energy per silver atom in the HCT model is 3.05 eV, with respect to the reconstructed 7 × 7-Si(111) surface. For adsorption on the IDL substrate surface the corresponding energy is 2.70 eV. © 1995.