Low-energy positron diffraction from the (110) surfaces of GaAs and InP

The intensities of 16 nonequivalent beams of normally incident positrons diffracted from the (110) surfaces of GaAs and InP are reported. The sample temperature was approximately 110 K. The intensities were measured over the energy range 30 eV≤E≤200 eV. The atomic geometries of GaAs(110) and InP(110) were extracted from these intensities via their comparison with the predictions of a multiple-scattering model using the criterion of minimization of the x-ray R factor. The best-fit surface geometries resulting from these analyses are approximately bond-length-conserving top-layer and second-layer rotations characterized by the tilt angles (${\mathrm{\omega }}_1$=28.5°±2.5°, ${\mathrm{\omega }}_2$=-3.5°±3°) for GaAs(110) and (${\mathrm{\omega }}_1$=24.5°±1.5°, ${\mathrm{\omega }}_2$=-3.0°±3°) for InP(110). Comparable low-energy electron intensity data were obtained and analyzed for InP(110) leading to (${\mathrm{\omega }}_1$=${31}_{\mathrm{-}5\mathrm{°}}^{\mathrm{°}+3\mathrm{}\mathrm{°}}$ and ${\mathrm{\omega }}_2$=-3°±3°). Small changes (Δd≤0.07 Å) in the bond lengths associated with the top-layer species are characteristic of the best-fit structures, but of these, only a small contraction (Δd/d≤3%) of the bond between the top-layer cation and second-layer anion seems likely to lie outside the uncertainties inherent in the analysis.