Relative orientation of the constituents on the degree of crystallographic coherence in AlN/TiN superlattices

AlN/TiN multilayer thin films with the layer thickness ranging from 1 nm to 50 nm were synthesized at 400 °C using a dual-cathode magnetron sputtering. Two series of samples were prepared under conditions of epitaxially matching orientations and far from the epitaxy to study the possible influence of the relative orientation of the constituents on the formation of coherent structures and hardness enhancement of superlattices. Both XRD and TEM observations showed that in large composition modulation period films each constituent grows under its own growth kinetic, leading to the formation of randomly oriented nanocrystalline layers. Decreasing progressively the layer thickness to beneath ≈ 10 nm favours the alignment of (002) basal plane of h-AlN on (111) plane of c-TiN, and results in development of strong (111) texture, prerequisite for stabilisation of c-AlN and the formation of epitaxially coherent structures. The increase of hardness coincides with the contraction of TiN lattice and transition of structures from a randomly oriented nanocrystalline layers to highly (111) textured superlattices. When the relative orientation of the constituents is far from the epitaxy relation, a partial crystallographic coherence of nanocrystalline domains can be obtained with (200) texture, leading to moderate enhancement of hardness.