Adsorption and diffusion of Si Atoms on the Ca/Si(111) 2 × 1 Surface

Recent theoretical research has led to a proposal that the silicon structure with a periodic Seiwatz defect has a direct band gap. If such a defect structure is to be realized, a surface with the Seiwatz chain structure is required, which is seen in the Ca/Si(111) 2 × 1 surface. We present our first-principles calculation on the absorption and the diffusion of Si atoms on the Ca/Si(111) 2 × 1 surface, which is important in understanding the initial stage of the silicon epitaxial growth on the surface. Extensive searches for one, two and three Si adatoms provide the most stable adsorption structures. A single Si adatom favors the site surrounded by three Si adatoms and two Ca atoms. For two Si adatoms, the adatom dimer is formed parallel to the Seiwatz chain. With one more adatom, a linear chain is formed along the Seiwatz chain. The calculated potential-energy surface (PES) for a single Si adatom shows that the diffusion energy barriers are 1.0 eV and 1.8 eV parallel and perpendicular to the Seiwatz chain on the surface, respectively. The anisotropic adatom diffusion and formation of linear adatom clusters suggest Si epitaxial growth on the Ca/Si(111) 2 × 1 surface.