Dislocation densities and alternating strain fields in CrN/AlN nanolayers

Nitride multilayered structures have received substantial attention in practical applications and fundamental research owing to their exceptional mechanical properties. Here, the microstructures of the CrN/AlN multilayers with a nominal bilayer period (Λ) of 6.0, 5.5, and 2.0 nm were carefully characterized by spherical aberration corrected (C_S-corrected) transmission electron microscopy (TEM). Detailed high-resolution TEM (HRTEM) studies, combined with quantitative atomic displacement measurements, reveal that the dislocation density is quite high in CrN/AlN multilayer, and it appears a higher number in CrN than in AlN layers. A strong inter-diffusion across the layers is observed for small bilayer period. The interplanar spacings oscillate, with a wavelength corresponding to the bilayer period and increasing amplitude for decreasing bilayer periods. Correspondingly, the strains oscillate and their magnitude generally increases with decreasing bilayer period. These observations are discussed in detail with the hardness enhancement observed in the CrN/AlN multilayers.