An x-ray diffraction study of the reconstructions induced by Sn and Pb on Ge(111) surfaces

This report describes surface x-ray diffraction studies of Ge(111) surfaces covered by submonolayers of Sn and Pb. The first part is a brief review of the properties of the ''clean'' Si(111) and Ge(111) surfaces, of the properties of the Sn- and Pb-covered Ge(111) surfaces, and of the surface x-ray scattering technique. Part two of the report concerns x-ray scattering measurements on the α-phase of the Ge(111)√ 3x√ 3-Sn and the Ge(111)√3x√ 3-Pb reconstructions. The structure factor analysis shows a significant relaxation in the Ge substrate induced by the Sn/Pb adatoms. The registry of the adatoms is determined from the analysis of the integer-order reflections. The intensity profiles of the fractional-order Bragg rods display a pronounced variation, which is explained by subsurface relaxation extending four atomic layers into the bulk. Part three of the report describes the structure of the 7x7 and 5x5 reconstructions that are observed after deposition of submonolayers of Sn on the Ge(111) surface and subsequent annealing. The diffraction patterns for both structures show considerable similarity with the pattern for Si(111)7x7, and the analysis shows that the Dimer-Adatom-Stacking-fault (DAS) model also is applicable to the Sn-induced Ge(111)7x7 and Ge(111)5x5 structures. The adatoms are identified to be Sn. The results of the refinement of the atomic coordinates show that the atoms in the upper five atomic layers are displaced from their ideal positions. The displacements around the adatoms are similar to the displacements of the Ge(111)√3x3-Sn surface. Furthermore, the observed relaxations are in good agreement with the predictions of total-energy calculations. A series of elastic strain calculations by the Keating model have been carried out. It is demonstrated that this model gives a good description of the atomic relaxations of the Ge(111)√3x√3-Sn (Pb), Ge(111)7x7-Sn, and Ge(111)5x5-Sn surfaces. 127 refs. (author)