Photoemission study of the valence band of Pb monolayers on Ge(111)

Monolayers of Pb have been grown epitaxially on clean Ge(111) surfaces. The metal overlayer reorders the surface and results in a (√3 × √3 )R30° unit cell. The valence-band dispersions of this metal-semiconductor interface system have been measured with use of angle-resolved photoemission spectroscopy with synchrotron radiation. The Pb/Ge(111) overlayer remains semiconducting, as reflected in the valence-band density of states at the Fermi level. Two lead-induced surface states are found, which are related to surface states seen in the clean, reconstructed Ge(111)-c(2×8). The binding energies of these states, relative to the valence-band maximum, are 0.45 and 1.2 eV. A small bandwidth of 0.3 eV is found for the deeper state. The similarity of the Pb-induced surface state bands to those of clean, reconstructed Ge(111)-c(2×8) suggests that there is a corresponding similarity in surface geometric structure. These experimental findings support an adatom model of the clean Ge reconstruction in the same way that Si(111)(√3 × √3 )-metal overlayer structures have supported the adatom model for Si(111)-(7×7). A structure for Pb/Ge(111) incorporating Pb adatoms on threefold-coordinated $T_4$ or $H_3$ sites is proposed.