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

Abstract

Abstract 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 A for x = 0.81 to 4.363 A 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.