Crystal quality and strain state are crucial in the application of semiconductor epitaxial films. High-resolution X-ray reciprocal space mapping, available on Rigaku's SmartLab multipurpose diffractometer, provides the most reliable quantitative measure of both the crystal quality and strain state. For example, when AlN epilayers are grown on top of a sapphire (001) substrate, the lattice mismatch between the film and the substrate often causes strain relaxation.
The induced misfit dislocations are detrimental to the quality of the film. In the figure below, a symmetric reciprocal space map around the sapphire (006) and an asymmetric reciprocal space map around the sapphire (119) reciprocal space lattice points are shown. In the symmetric map, the AlN (002) peak appears much broader, indicating relatively poor film quality compared to the substrate. In the asymmetric map, the AlN (113) peak appears at an in-plane q-value differing from that of the sapphire (119) peak, suggesting partial strain relaxation of the film. It is clear that the relaxation induced misfit dislocations are responsible for the poor crystal quality of the AlN film.