Dynamical analysis of low-energy electron diffraction intensities from Al on GaP(110): The high-coverage A1P(110) limit

The measured intensities of 14 diffracted beams of low-energy (30E240 eV) electrons normally incident on A1P(110) are reported. The temperature of the A1P surface during the measurements was T=300 K. The surfaces were prepared by deposition of A1 on GaP(110) followed by annealing cycles and verification of surface stochiometry (to within 10%) via Auger-electron spectroscopy. Several crystals were examined in order to verify the reproducibility of the intensity data. The data were analyzed using a nonrelativistic, Hara-exchange electronion-core potential and an x-ray R-factor structure-analysis methodology. This analysis leads to the best-fit structure of A1P(110) being a rotation of the uppermost layer with the P rotated outward and the Al inward. The second layer also is reconstructed with the Al being displaced upward by 0.0350.05 relative to its position in the bulk and the P being displaced downward by an equal amount. The x-ray R factor for the best-fit structure is Rx=0.19. This structure gives a good visual description of the measured intensities. For bond-length-conserving top-layer rotations the angle between the plane of the surface A1P chains and the plane of truncated bulk surface is 1=27.5°3°, in accordance with two independent prior predictions. The "best-fit" R-factor structure corresponds to reduced relaxation parallel to the surface of the top-layer P and Al relative to the values expected for the bond-length-conserving structure.