The AlN layer thickness dependent coherent epitaxial growth, stress and hardness in NbN/AlN nanostructured multilayer films

NbN/AlN nano-multilayer films with AlN layer thickness (lAlN) ranging from 2.2 to 12.2 nm have been deposited on Si(100) substrate by reactive magnetron sputtering in Ar/N2 mixtures. The lAlN dependent structural and mechanical properties for resulting NbN/AlN multilayers have been evaluated by means of low-angle X-ray reflectivity, X-ray diffraction, transmission electron microscope, pole figure measurements and nanoindentation tests. The finding is that at small lAlN both hexagonal wurtzite-AlN(0002) and face-centered cubic (fcc) AlN(111) are coherent with the fcc NbN(111), and the increase of lAlN can promote coherent growth of fcc-NbN(111)/w-AlN(0002) due to minimization of total energy and formation of a strong NbN(111)/AlN(0002) fiber texture. The hardness of all NbN/AlN multilayers lies in between 32.7 and 37.5 GPa with increasing lAlN from 2.2 to 12.2 nm, which is obviously higher than that calculated by using a simple rule of mixture, showing that the remarkable hardness enhancement implements at a wide range of lAlN from 2.2 to 12.2 nm for NbN/AlN multilayer system. The high-hardness value in a wide range of lAlN can be mainly attributed to Koehler mechanism and structural barriers (fcc/hexagonal) to dislocation motion between NbN and AlN layers.