Two-dimensional states localized in subsurface layers of Ge(111)

The origin of the two-dimensional surface states localized in subsurface regions of the Ge(111) substrate has been studied by density-functional-theory calculations, which were compared with the experimental results of angle-resolved photoelectron spectroscopy. For the Bi/Ge(111)-(root 3 x root 3)R30 degrees, Br/Ge(111)-(1x1), and Tl/Ge(111)-(1x1) surfaces, we found that the surface states are classified into three groups. The energy dispersion and the orbital character for each band implies the relationship between the subsurface states and the bulk heavy-hole, light-hole, and spin-orbit split-off bands. These results indicate that the subsurface states originate from the bulk bands that are perturbed due to the truncation of the three-dimensional periodicity at the surface.