Structure of Ge(111)√3 × √3R30°-Au determined by surface x-ray diffraction

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Abstract
The structure of the Ge(111)√3 × √3 R30°-Au reconstruction has been measured with surface x-ray diffraction. In-plane and out-of-plane scattering show that the basic structural unit is a trimer of Au atoms embedded in a single layer of Ge with small relaxations of the underlying substrate down to the second complete bilayer. The Au-Au intermetallic distance is 2.81 Å, which is close to the bulk value of Au. The model is similar to the √3 × √3 R30° reconstructions formed by Au on Si(111) and Ag on Ge(111). These are characterized by an inward relaxation of the metal atoms from the threefold-hollow H3 sites towards the fourfold T4 site, in contrast to the reconstruction formed by Ag on Si(111), which exhibits an outward relaxation. The difference in behavior can be understood in terms of the relative bond energies.