Absorption in high-energy electron diffraction from non-centrosymmetric crystals

The influence of absorptive potentials on high-energy electron diffraction amplitudes is analysed, with an emphasis on effects which are specific to diffraction from non-centrosymmetric crystals. It is shown that the phase difference which then exists between the elastic and absorptive potential coefficients can give rise to a significant asymmetry between +g and -g reflections. This phase difference is calculated for a number of important III-V semiconductors. In many diffraction calculations it is assumed that the phase difference is zero - it is argued here that such calculations cannot be truly quantitative. The inclusion of absorption by perturbation theory is shown to be valid, but only if the change in the eigenvectors as well as the eigenvalues is taken into account. It is shown that within two-beam theory the +g, -g asymmetry in non-centrosymmetric crystals arises purely from changes in the eigenvectors.