X-ray crystal truncation rod scattering: II. An effect of two-dimensional symmetry of the GaAs(001) surface

Abstract

The X-ray scattering, CTR scattering, from the (001) surface of the ZnS type structure was formulated by introducing the structure factor for the spiral chain connecting one atom with another extended along the [001] direction from the (001) surface. On the basis of this formula it is shown that the the intensity of the CTR scattering elongated from the 111 Bragg point from the (001) surface of GaAs is different from that elongated from the 1 1 1 Bragg point, because of the two-fold rotational symmetry of the (001) surface of this crystal around the [001] axis, while the intensities are the same for both Bragg points, being subject to the bulk symmetry. This intensity difference was confirmed by the experimental observation, although the observed difference is not as large as expected from the theoretical prediction for the ideal (001) surface of GaAs. From the analysis of the CTR scattering the surface roughness of a mechano-chemically polished (001) surface of GaAs was estimated to extend in eight atomic sheets near the surface. The lattice spacing for the first layer of the surface was found to be shrunk from an ideal spacing by about 3% and that for the second layer by about 1%.