Growth and mechanical properties of epitaxial NbN(001) films on MgO(001)

NbNx layers were deposited by reactive magnetron sputtering on MgO(001) substrates in 0.67 Pa pure N2 at Ts = 600-1000 °C. Ts ≥ 800 °C leads to epitaxial layers with a cube-on-cube relationship to the substrate: (001)NbN ||(001)MgO and [100]NbN ||[100]MgO. The layers are nearly stoichiometric with x = 0.95-0.98 for Ts ≤ 800 °C, but become nitrogen deficient with x = 0.81 and 0.91 for Ts = 900 and 1000 °C. X-ray diffraction reciprocal space maps indicate a small in-plane compressive strain of − 0.0008 ± 0.0004 for epitaxial layers, and a relaxed lattice constant that decreases from 4.372 Å for x = 0.81 to 4.363 Å for x = 0.98. This unexpected trend is attributed to increasing Nb and decreasing N vacancy concentrations, as quantified by first-principles calculations of the lattice parameter vs. point defect concentration, and consistent with the relatively small calculated formation energies for N and Nb vacancies of 1.00 and − 0.67 eV at 0 K and − 0.53 and 0.86 eV at 1073 K, respectively. The N-deficient NbN0.81(001) layer exhibits the highest crystalline quality with in-plane and out-of-plane x-ray coherence lengths of 4.5 and 13.8 nm, attributed to a high Nb-adatom diffusion on an N-deficient growth front. However, it also contains inclusions of hexagonal NbN grains which lead to a relatively high measured hardness H = 28.0 ± 5.1 GPa and elastic modulus E = 406 ± 70 GPa. In contrast, the nearly stoichiometric phase-pure epitaxial cubic NbN0.98(001) layer has a H = 17.8 ± 0.7 GPa and E = 315 ± 13 GPa. The latter value is slightly smaller than 335 and 361 GPa, the isotropic elastic modulus and the [100]-indentation modulus, respectively, predicted for NbN from the calculated c11 = 641 GPa, c12 = 140 GPa, and c44 = 78 GPa. The electrical resistivity ranges from 171 to 437 μΩ cm at room temperature and 155-646 μΩ cm at 77 K, suggesting carrier localization due to disorder from vacancies and crystalline defects.