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

Abstract

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. Xray 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 a 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 171-437 μΩ-cm at room temperature and 155-646 μΩ-cm at 77 K, suggesting carrier localization due to disorder from vacancies and crystalline defects.

Vacancy-induced mechanical stabilization of cubic tungsten nitride

“…Transition metal nitrides have gained considerable interest due to their excellent mechanical properties, thermal stability, and corrosion resistance, [1][2][3][4][5][6][7][8][9] and are widely used as hard wear-resistant coatings, diffusion barriers, and protective decorative coatings. 2,3,6,[8][9][10][11][12][13] While TiN is the most studied transition metal nitride and is used in all of the above applications, WN is relatively unexplored and its processing-structure-property relationships as well as its phase stability is not fully established yet.…”

“…2,3,6,[8][9][10][11][12][13] While TiN is the most studied transition metal nitride and is used in all of the above applications, WN is relatively unexplored and its processing-structure-property relationships as well as its phase stability is not fully established yet. WN has potentially promising properties as its six valence electrons result in a high valence electron density with 3 electrons per formula unit occupying metal d-bands.…”

“…However, the large range of elastic moduli is particularly fascinating and requires a detailed understanding of the effect of vacancies on the mechanical properties, since the only difference between the NbO and the NaCl structure is the density of vacancies. Previous studies have investigated the effect of cation and anion vacancies on the elastic properties of NbN, 41 HfN, 42 and TiN, 43 and on stabilizing metastable structures of in a lower total energy and, in turn, a negative shear modulus. However, the introduction of vacancy pairs on 5% of anion and cation sites causes mechanical stabilization, with the most stable cubic WN phase, the NbO phase, having 25% of vacancies resulting in a positive C 44 = 175 GPa and E = 561 GPa.…”

Epitaxial NbC N1−(001) layers: Growth, mechanical properties, and electrical resistivity

“…They have gained interest due to their relatively high superconducting transition temperatures of T c = 17.3 K [7] and 12 K [8], respectively, with potential applications in superconducting electronics [9,10]. They are also promising as protective coating materials due to their high chemical stability [11], high melting points of 2204 [12] and 3600°C [13], respectively, and high hardness with reported values that range from 7-48.5 GPa for NbN [14][15][16][17][18][19] and from 11.3-36.8 GPa for NbC [20,21]. Both NbN and NbC exhibit metallic conductivity with reported room temperature resistivities ρ NbN = 88-750 μΩ-cm [19,22,23] and ρ NbC = 250-1200 μΩ-cm [24].…”

“…That is, y = 1−x and the layer can be labeled as NbC x N 1−x . This approach of using stoichiometric epitaxial single-crystal layers to determine the mechanical properties has already been applied to a range of transition metal nitrides including TiN(001) [38], ScN(001) [39], TaN(001) [40][41][42], HfN(001) [43,44], NbN(001) [19], CrN(001) [45], and CeN(001) [46].…”