X-ray diffraction from inhomogeneous thin films of nanometre thickness: modelling and experiment

The diffraction of X-rays by a thin layer (of the order of a few nanometres) with a gradient in interplanar spacings is considered. It is shown that optical coherence over the film thickness leads to diffraction peak positions that no longer obey Bragg's law. Although a fitting of the diffracted intensity is indeed still possible, this has direct consequences on the applicability of more straightforward analysis methods, such as the sin²ψ method, which rely on diffraction peak positions. The intensity and peak position calculations are supported by a comparison with experimental data from a (001) Fe/GaAs thin (3 nm) epitaxic film.