Stabilities of adsorption sites and charge transfers at the K/Si(001) surface.

Abstract

Various adsorption models of a K atom on the Si(001) surface including the top-site model, and the charge transfer between K and Si have been exhaustively investigated using accurate ab initio self-consistent unrestricted Hartree-Fock total-energy calculations with the Hay-Wadt effective core potentials. Several cluster models of bare and hydrogen-bonded silicon atoms used to simulate the different possible adsorption sites on the Si(001) surface have been examined. The structural relaxation of the substrate on adsorption of K atoms has been accounted for by optimizing all the atomic positions of the adsorbed and substrate atoms. Also, the effects of electron correlation have been included by invoking the concepts of fourth-order many-body perturbation theory and these are found to be crucial in determining the preferred adsorption sites. Population analyses up to the many-body perturbation theory at the second level of all the models studied consistently show that the charge transferred from the K atom to Si lies between 0.12e and 0.40e. This charge transfer combined with the values of the K-Si bond lengths obtained in our calculations tend to suggest a polarized covalent bonding for the K/Si(001) system.