Structure evolution during growth of epitaxial NbN films on Al2O3 (0006) deposited by magnetron sputtering and its impact on electrical properties

Abstract

We explore the growth of high-quality epitaxial NbN films on Al₂O₃ (0006) (Sapphire c-plane). The study investigates the structural and electrical characteristics as a function of film thickness. Pole figures analyses reveal the formation of epitaxial growth with two sets of domains, attributed to the differing symmetries of the cubic NbN and rhombohedral Sapphire. We identified the epitaxial relationship as $\text{(0001)[2}\overline{\text{1}}\overline{\text{1}}\text{0]}$ || $\text{(111)[1}\overline{\text{1}}\text{0]}$. Medium and high-resolution X-ray diffractometry shows clear Kiessig fringes, which become asymmetric for thicknesses greater than 18 nm. The asymmetry is inferred to arise from relaxed domains dispersed in the film in low concentrations near the substrate-film interface and then become more widespread with increased thickness based on dynamical XRD simulation and dark field transition electron microscopy. All films display a rocking curve consisting of an intense sharp peak, accompanied by a broader peak that intensifies as the thickness increases. The sharp peak reflects a highly ordered, long-range correlated structure within NbN, while the broadened peak indicates regions of slightly misoriented NbN, likely related to strain relaxation in local domains. X-ray reflectivity indicates that the mass density is higher than bulk NbN for the thinnest films, it decreases as the films grow thicker and stabilizes at 7.8 g·cm⁻³ for films thicker than ∼ 70 nm. As thickness increases, the lattice constant decreases due to relaxation, which also impacts the electrical resistivity. The strained films exhibit slightly lower resistivity than the relaxed films, likely due to the higher crystallinity of the thinner films.