Electronic states of the clean Ge(001) surface near Fermi energy

Electronic states of the clean Ge(001) surface are studied by angle-resolved photoemission spectroscopy and standing wave observation with scanning tunneling microscopy. The bottom of the surface conduction band is $0.3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ above the top of the valence band, and thermally filled with increasing temperature above $470\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Its dispersion in the direction perpendicular to the dimer axis is much larger than that in the Ge dimer-axis direction. The valence band top at $\overline{\ensuremath{\Gamma}}$ is a bulk state, and surface resonances exist near this point. These features are consistent with the band calculation based on the density functional theory for the $c(4\ifmmode\times\else\texttimes\fi{}2)$ surface.