Adatom-induced variations of the atomic and electronic structures of Si(111)√3 x √3-Ag:: A first-principles study

Using a first-principles calculation method, we study the changes in the atomic and electronic structures of the Si(111)root 3x root 3-Ag surface (hereafter root 3-Ag) via doping of extra Ag adatoms. We present a structural model for the adatom-induced root 21x root 21 superstructure (root 21-Ag), which has three Ag adatoms immersed into the substrate Ag layer within a unit cell. The present structural model reproduces well the measured scanning-tunneling-microscopy images as well as the electronic band structure measured by angle-resolved photoelectron spectroscopy. We find out that the complex band structure seen on the root 21-Ag phase basically arises from the band folding of the original surface bands of root 3-Ag. The extra Ag adatoms doped on root 3-Ag modify only the band alignment without any additional adatom-induced surface state. The almost unoccupied two-dimensional free-electron-like band, generally called S-1, at pristine root 3-Ag is gradually filled and shifted downward with an increase in the dopant coverage. As this shifted S-1 band crosses other surface bands, it loses its free-electron nature.