Structure of the Ge(111)-c(2 × 8) surface as determined from scattered X-ray intensities along crystal truncation rods

In an X-ray diffraction study of the c(2 × 8) reconstructed Ge(111) surface we have measured intensity profiles along integer-order rods of Bragg scattering perpendicular to the surface plane. The diffracted intensity profiles are explained in terms of a simple adatom model for the reconstruction. The adatoms are found to occupy T4 sites on a distorted bulk-like substrate. We rule out any model for c(2 × 8) reconstruction that is based on stacking faults in the substrate. For the adatom model we give best-fit values for the positions of the atoms at the surface and compare them with those obtained from total-energy and keating-energy minimisation calculations.