A CLUSTER STUDY OF THE STABILITIES OF ADSORPTION SITES AND CHARGE TRANSFERS AT THE NA SI(100) SURFACE/

The adsorption properties of sodium adatom on silicon (Si) surfaces ar e investigated using ab initio molecular orbital theory. Twenty-three bare silicon clusters are studied at the unrestricted Hartree-Fock (UH F) many-body perturbation theory (MP4) levels. Twenty-seven alkali ato m-silicon (AM-Si(n)) adsorbate systems are studied with adsorption sit es classified as on-top, open, and bridge sites. Adsorption distances are found by optimizing the distance between the alkali atom and Si(n) clusters at the UHF level and then chemisorption energies are calcula ted at the MP4 level. Electron correlation is found to have significan t effects on cluster stability and chemisorption energy. The chemisorp tion energies are smaller at the MP4 level than at the SCF level for m ost systems, and usually increase with increasing number of atoms in t he cluster. AM-Si bonds appear to be of mixed type.