Cluster Model for the Interaction of K with Si(100)

Abstract

The adsorption of K on a Si(001)(2×l) surface is considered as a prototype of alkali metal over semiconductor surfaces. Two conflicting models have been proposed to explain experimental evidence. The first proposes an ionic bond between K and Si, and a complete donation of the K—4s electron that metallizes the surface [1] at coverages lower than one monolayer. The second one proposes a covalent bond K-Si, and the work function behaviour is explained by a coverage dependendent charge transfer from the adsórbate to the substrate [2]. With the aim of clarifying some aspects of this controversial matter, we have performed extensive calculations of the electronic structure of the K-Si system. These are based on an ab—initio unrestricted all-electron Hartree—Fock method, and are applied to K-Si clusters emulating various possible adsorption sites for the alkali over the Si(001)—(2×l) surface. We used three kinds of clusters: the sixfold hole between two parallel dimers (pedestal site), the cave between two adjacent rows of dimers (cave site), and the one over a dimer Si atom (on—top site). The spurious dangling bond of the clusters were saturated with H atoms at a distance of 1.48 Å from Si. We assume a symmetric reconstruction of the Si surface. We have used additional basis orbitals in the total energy calculations of the K and free Si surface to avoid the basis set superposition error in the bidding energy.