MOMENTUM-SPACE IMAGES OF SURFACE DIMERS ON GAAS(001)-(2X4) BY HIGH-ENERGY AUGER AND X-RAY PHOTOELECTRON DIFFRACTION

We have obtained k-space images of As dimers in the surface layer of GaAs(001)-c(2 X 8)/(2 X 4) from the two-dimensional intensity maps of high-energy, core-level Auger, and x-ray photoelectron emission. The dimers are revealed as a major splitting in a small-angle scattering peak that is common to Ga 3d, 3p, 3s, and L3M4,5M4,5 intensity maps. This peak is of a rather complex origin, but the majority component is forward-focusing of Ga Auger and photoelectron waves generated in the fourth layer by As atoms in the surface layer. The splitting is caused by a change in symmetry in the surface layer resulting from dimer formation. Simple internuclear emitter-to-scatterer geometric modeling cannot be used to obtain the dimer bond length from the angles at which the two split-off features occur. Such modeling, which generates a reasonable quantitative estimate of the surface-structural parameters when the emitter and scatterer are nearest neighbors, fails in the present case because of the complex origin of this peak. Rather, quantum-mechanical scattering calculations carried out at either a kinematical or dynamical level are required to extract accurate surface-structural parameters. Fourier inversions of the angular intensity maps, which are essentially holograms of the surface, do not produce well-resolved dimer images in real space. This result is due to substantial contributions to the intensity map from emitters other than the one that produces the split peak in k space.