Electronic and geometric investigations of the Ca/Si(1 1 1)-(5×2) surface

The atomic and electronic structures of the Ca/Si(1 1 1)-(5×2) surface have been theoretically investigated by using the pseudopotential method and the local density approximation (LDA) of the density functional theory. The geometrical model is based on the deposition of three Ca lines on the top of a combination of a honeycomb chain channel (HCC) and the Seiwatz (SZ) chain formed by Si-adatoms. The Ca adatoms lie on both H3 and T4 symmetry sites. This structural model produces a semiconducting surface band structure with a clear LDA energy gap of 0.56 eV. Ca-derived band gap states have been identified by comparing the band structures for the clean Si(1 1 1)-(5×2) and Ca/Si(1 1 1)-(5×2) surfaces. Significant charge transfer from the Ca adatoms to neighbouring Si atoms has been concluded by analysing electronic charge density and STM simulations. A discussion of the role of Ca adatoms in improving the chemical passivation of the Si(1 1 1) surface has been attempted by examining the orbital nature of states in the fundamental band gap of bulk Si.