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

NbN x layers were deposited by reactive magnetron sputtering on MgO(001) substrates in 0.67 Pa pure N 2 at T s = 600–1000 °C. T s ≥ 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 T s ≤ 800 °C, but become nitrogen deficient with x = 0.81 and 0.91 for T s = 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 NbN 0.81 (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 NbN 0.98 (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 c 11 = 641 GPa, c 12 = 140 GPa, and c 44 = 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. [ABSTRACT FROM AUTHOR]