THE GE(001) (2 X-1) RECONSTRUCTION - ASYMMETRIC DIMERS AND MULTILAYER RELAXATION OBSERVED BY GRAZING-INCIDENCE X-RAY-DIFFRACTION

Grazing incidence X-ray diffraction has been used to analyze in detail the atomic structure of the (2 x 1) reconstruction of the Ge(001) surface involving far reaching subsurface relaxations. Two kinds of disorder models, a statistical and a dynamical were taken into account for the data analysis, both indicating substantial disorder along the surface normal. This can only be correlated to asymmetric dimers. Considering a statistical disorder model assuming randomly oriented dimers the analysis of 13 symmetrically independent in-plane fractional order reflections and of four fractional order reciprocal lattice rods up to the maximum attainable momentum transfer q(z) = 3c* (c* = 1.77 x 10(-1) angstrom-1) indicates the formation of asymmetric dimers characterized by a dimer bondlength of RD = 2.46(5) angstrom as compared to the bulk bonding length of R = 2.45 angstrom. The dimer height of DELTAz = 0.74(15) angstrom corresponds to a dimer buckling angle of 17(4)-degrees. The data refinement using anisotropic thermal parameters leads to a bonding length of R(D) = 2.44(4) angstrom and to a large anisotropy of the root mean-square vibration amplitudes of the dimer atoms ([u2/11])1/2 = 0.25 angstrom, ([u2/22])1/2 = 0.14 angstrom, ([u2/33])1/2 = 0.50 angstrom). We have evidence for lateral and vertical displacements of the Ge atoms down to the tenth layer below the surface.