Si(111)(7×7)-Ge and Si(111)(5×5)-Ge surfaces studied by angle-resolved electron-energy-loss spectroscopy

Very thin Ge layers on Si(111) are investigated with the use of angle-resolved electron-energy-loss spectroscopy and low-energy electron diffraction. Ge-derived (7×{}7) and (5×{}5) surfaces are identified by these measurements. New surface one-electron transitions are found at 1.4 eV on the Si(7×{}7)-Ge surface and at 1.4 and 9 eV on the Si(5×{}5)-Ge surface. Hydrogen adsorption and desorption measurements on these surfaces indicate that the transition at 1.4 eV are ascribable to the Ge dangling-bond surface state. The emergence of a 9-eV peak on the Si(5×{}5)-Ge surface suggests that the Si-Si back-bond surface states are replaced by those of Ge-Ge. We discuss the Ge growth mode on Si(111) in the light of our results and the previously reported results of total-energy calculations on several models.