Adatom-induced variations of the atomic and electronic structures of Si (111) 3×3-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) 3×3-Ag surface (hereafter 3-Ag) via doping of extra Ag adatoms. We present a structural model for the adatom-induced 21 × 21 superstructure (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 21 -Ag phase basically arises from the band folding of the original surface bands of 3-Ag. The extra Ag adatoms doped on 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 S1, at pristine 3-Ag is gradually filled and shifted downward with an increase in the dopant coverage. As this shifted S1 band crosses other surface bands, it loses its free-electron nature.