Atomic geometry of mixed Ge-Si dimers in the initial-stage growth of Ge on Si(001)2x1

We have investigated quantitatively the geometry of mixed Ge-Si dimers on a single domain Si(001)2x1 surface by azimuthal scanning core-level photoelectron diffraction. By analyzing Ge 3d diffraction patterns from Ge/Si(001) at 0.1 ML coverage using a concentric-shell multiple-scattering algorithm for photon energies of hv=90 and 136 eV, the bond length and tilt angle of the mixed Ge-Si dimer are determined to be 2.43+/-0.10 Angstrom and 31 degrees+/-2 degrees, respectively. It is also found that the mixed Ge-Si dimers are substitutional ones rather than ad-dimers. Another significant aspect of this work is that it demonstrates that photoelectron diffraction is able to probe the local environment of an adatom at submonolayer coverage even in the absence of long-range order.