The Ge(001) (2 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 \xd7 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.\n\nConsidering 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 qz = 3c* (c* = 1.77 \xd7 10\u22121 \xc5\u22121) indicates the formation of asymmetric dimers characterized by R>D = 2.46(5) \xc5 as compared to the bulk bonding length of R = 2.45 \xc5. The dimer height of \u0394 Z = 0.74(15) \xc5 corresponds to a dimer buckling angle of 17(4)\xb0. The data refinement using anisotropic thermal parameters leads to a bonding length of RD = 2.44(4) \xc5 and to a large anisotropy of the root mean-square vibration amplitudes of the dimer atoms (u112) 1/2 = 0.25 \xc5, (u222)1/2 = 0.14 \xc5, (u332)1/2 = 0.50 \xc5). We have evidence for lateral and vertical disp tenth layer below the surface.