4. Multiple layers

The reflectivity and specular scattering measurement methods described above for the study of single layers also apply to multilayers. In a multilayer system, each interface partially reflects the incident wave. The interference between these reflections appears as a system of bangs that can be observed during reflectivity measurements.

A particularly interesting case is that of periodic stacks of two materials. In this case, X-rays produce interference peaks analogous to those obtained by reflection on the atomic planes of crystals. Indeed, on a larger scale, but in the same way as the atomic planes of a crystal, alternating thin layers of two different materials modulate the electron density in the direction perpendicular to the substrate. An analogy can also be drawn with dielectric multilayers in the visible range, used as mirrors or interference filters. The...