Electronic and atomic structures of the Ag induced 3×3 superstructure on Ge(111)

The Ag/Ge(111) 3×3 surface together with Ag/Si(111) 3×3 constitutes a set of surfaces that is ideally suited for fundamental studies related to low dimensional physics. We here focus on the atomic and electronic structures of the two-dimensional 3×3 superstructure induced by Ag on Ge(111), a surface that is significantly less studied than the Si counterpart. Extensive information on the surface band structure obtained by angle resolved photoelectron spectroscopy (ARPES) is presented, complemented by atomic information from scanning tunneling microscopy (STM). The results reveal new findings that are important for the understanding of the Ag induced 3×3 structure, acting as a prototype for semiconductor/metal interfaces. i) We have identified a new occupied surface band near the M¯-point of the 3×3 surface Brillouin zone. ii) The Ag/Ge(111) 3×3 surface exhibits a partially occupied surface band, S1, with a parabolic-like shape at Γ¯. At low temperature (LT) this band splits into two bands, S1U and S1D. The identification of two bands is significantly different from the case of Ag/Si(111) 3×3 for which just one band has been reported. Besides these specific results, our extensive ARPES study reveals four surface bands at room temperature (RT), while five surface bands were identified at ≈ 100 K (LT). Room temperature empty state STM images show, depending on the tunneling bias, both honeycomb and hexagonal periodicities which are consistent with the honeycomb chained trimer and the in-equivalent trimer models, respectively.