Interference phenomena observed by X-ray diffraction in nanocrystalline thin films

An increase of the X-ray diffraction line broadening with increasing diffraction angle was observed experimentally in nanocrystalline thin films. Such a change of the line width is usually related to the microstrain in the sample, which, however, contradicts the assumptions that the microstrain is relatively low in nanocrystalline materials and that the line broadening is caused mainly by small crystallite size. For nanocrystalline thin films, the observed changes in the diffraction line broadening are explained by a partial coherence of adjacent crystallites, which is stronger at low diffraction angles than at high diffraction angles. Furthermore, it is found that the degree of coherence of the adjacent crystallites depends on their size and preferred orientation. Smaller crystallites show better coherence, because the corresponding reciprocal-lattice points are broadened compared with those related to large crystallites. A strong preferred orientation improves further the coherence of the adjacent crystallites. Theoretical results are confirmed by experimental data obtained on nanocrystalline (Ti,Al)N thin films using a combination of glancing-angle X-ray diffraction, high-resolution transmission electron microscopy and X-ray texture analysis.