NEW PRECISE X-RAY STRUCTURE AMPLITUDES OF GaAs OBTAINED BY MEANS OF THE PENDELLÖSUNG METHOD. J. Stahn, M. Möhle, U. Pietsch, Institute of Solid State Physics, University of Potsdam, Germany

As reviewed in /l/ the electronic charge density of the semiconductor GaAs (space group F-43m) has been studied by various authors. Although the accuracy of the accumulated X-ray structure amplitudes |F_H| is in the order of l % some qualitative and quantitative discrepancies remain among the experimental data and in comparison with the outcome of pseudo-potential and density-functional calculations. In order to obtain highly precise data it is necessary to perform experiments which fulfil either the dynamical or the kinematical theory of X-ray diffraction. We measured some high order strong and medium intensity reflections in the range of 0.5 < sin[[theta]]/[[lambda]] < 0.8 Å^-1 by means of the Pendellösung method. In order to reduce the absorption the experiment was performed at [[lambda]] = 0.3 Å using the D3 beamline of HASYLAB (Hamburg, Germany). The pendellösung was obtained recording the integral intensity of a 500um thick wafer which was tilted step by step around the normal of the reflecting net plane. The dynamical effect appeared only at dislocation free regions of the wafer. Futhermore the exact knowledge of the wafer thickness, the degree of polarisation and the used wave length were necessary to determine the |F_H| with an accuracy better than 1%. The experimental data are verified using the computer code CRYSTAL92 which is based on the Hartree-Fock approach. The project is supported by the European Community under CHRX-CT93-155 and the BMBF under 05 647IPA.

/ l/ U. Pietsch & N. K. Hansen, Acta Cryst. B1996 submitted