A thin-film sample is two-dimensionally formed on the surface of a substrate, and the film thickness is usually very small, about 1 μm or less. It can be difficult to use a conventional powder diffractometer to acquire high-quality diffraction data from a thin film because thin-film diffraction peaks are usually weak, and the background intensities caused by diffraction and scattering from the substrate are very high. It is known that preferred orientation or anisotropic lattice distortion can have a strong effect on physical properties of thin films, so that it is indispensable to reveal structural or textural properties of thin films using X-ray diffraction techniques with proper X-ray optics.
A description of X-ray optics for various diffraction methods have been given in Section 2.3 of the first article in this series. Generally speaking, X-ray diffraction techniques can be divided into two groups in terms of sample geometry; “out-of-plane” diffraction or “in-plane” diffraction. Out-of-plane diffraction is the most commonly used experimental technique for studying powder, bulk and thin-film materials.
This article describes the out-of-plane diffraction measurement techniques and their applications to the determinations of crystal structures of thin films.