X-ray scattering from rotational disorder in epitaxial films: An unconventional mosaic crystal

Motivated by x-ray-scattering measurements performed on ErAs(001)/GaAs(001) and ${\mathrm{In}}_{0.7}$ ${\mathrm{Ga}}_{0.3}$P(001)/GaAs(001), we present a model that explains the origin of a narrow peak and diffuse scattering, which are frequently observed at Bragg reflections in epitaxial systems. Central to the model is a correlation length for mosaiclike rotational disorder that arises in lattice-mismatched epitaxial films. The adhesive force between the film and the substrate is found to play a crucial role and leads to a striking anisotropy in the line shapes.