Direct Photoemission Study of the Antibonding Surface-State Band on Ge(111)2×1

The cleaved Ge(111)2×1 surface of a heavily $n$-doped crystal has been studied with angle-resolved photoemission. A new partly filled surface-state band is observed at the Fermi level, only appearing close to the $\overline{J}-\overline{K}$ line in the 2×1 surface Brillouin zone. The observed emission corresponds to the dispersion minimum of the antibonding band in the $\pi$-bonded chain model. The direct band gap between the surface-state bands is found to be 0.5 eV, in good agreement with results from absorption measurements.