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

Mulligan

Surface and Coatings Technology

et al. 2016

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Tungsten nitride layers, 1.45-μm-thick, were deposited by reactive magnetron sputtering on MgO(001), MgO(111), and Al 2 O 3 (0001) in 20 mTorr N 2 at T s = 500-800 °C. All layers deposited at T s = 500-700 °C form a cubic phase, as determined by X-ray diffraction ω-2θ scans, and show an N-to-W ratio x that decreases from x = 1.21 to 0.83 with increasing T s = 500-700 °C, as measured by energy dispersive and photoelectron spectroscopies. T s = 500 and 600 °C yields polycrystalline predominantly 111 oriented β-WN on all substrates. In contrast, deposition at 700 °C results in epitaxial growth of β-WN(111) and β-WN(001) on MgO(111) and MgO(001), respectively, and a 111-preferred orientation on Al 2 O 3 (0001). T s = 800 °C causes nitrogen loss and WN x layers with primarily BCC W grains and x = 0.04-0.06. Density functional theory calculations indicate an increase in structural stability by the introduction of either W or N vacancies into the cubic rock-salt structure, reducing the formation energy per atom from 0.32 eV for the rock-salt structure to 0.09 eV for WN 0.75 and -0.07 eV for WN 1.33 , to -0.42 eV for stoichiometric WN in the NbO structure. The out-of-plane lattice constant decreases from 4.357-4.169 Å with increasing T s = 500-700 °C. Comparing these values with calculated latticeconstants indicates that the W vacancy concentration increases from 6-11% for T s = 500-600 °C to 11-18% for T s = 700 °C, while the N vacancy concentration also increases from negligible to 18-29%. The simultaneous increase of both vacancy types is attributed to thermally activated N 2 recombination and desorption and atomic rearrangement towards the thermodynamically favorable cubic NbO structure which contains 25% of both W and N vacancies. The measured elastic modulus ranges from 110-260 GPa for 500-700 °C and decreases with increasing Ncontent, and increases to 350 GPa for T s = 800 °C. The room temperature resistivity decreases with increasing T s = 500-700 °C from 4.5-1.1×10 3 µΩ-cm, indicating a resistivity decrease with decreasing nitrogen content and increasing crystalline quality and phase purity.

Section: Resultsmentioning

confidence: 69%

Section: Resultsmentioning

confidence: 99%

Cubic β-WN layers: Growth and properties vs N-to-W ratio

Mulligan

Surface and Coatings Technology

et al. 2016

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“…Thus, the MoN x layers exhibit a range of compositions from overstoichiometric for T s = 600-700 °C, to nearly stoichiometric for T s = 800 °C to understoichiometric for T s = 900-1000 °C. A similar decrease in the N-content with increasing growth temperature has previously been reported for various epitaxial transition metal nitrides including NbN x above 900 °C [10], CrN x above 730 °C [7], HfN x above 650 °C [14], WN x above 800 °C [20], and TaN x above 400 °C [65], and has been attributed to nitrogen vacancies that form due to a higher rate of nitrogen recombination and desorption at higher temperature, and the increasing importance of the entropy contribution to the free energy of the N 2 gas [10,66]. However, as discussed below, the deviation from stoichiometry in our MoN x layers is not only due to N-vacancies but also Mo-vacancies.…”

Section: Experimental and Computational Proceduresmentioning

confidence: 82%

Cation and anion vacancies in cubic molybdenum nitride

Journal of Alloys and Compounds

Gall

2017

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Epitaxial MoN x layers deposited on MgO(001) by reactive magnetron sputtering in 20 mTorr N 2 at T s = 600-1000 °C exhibit a cubic rock-salt type structure, a N-to-Mo ratio that decreases from x = 1.25-0.69 with increasing T s , and a lattice constant that simultaneously decreases from 4.26-4.16 Å. A combination of composition, thickness, lattice-constant, and atomic area-density measurements indicate that the rock-salt structure contains both anion and cation vacancies, with the Mo site occupancy Χ Mo decreasing from 0.89±0.06 to 0.70±0.04 while the N site occupancy Χ N increases from 0.60±0.04 to 0.88±0.04, as x increases from 0.69-1.25. Density functional calculations for over 200 cubic MoN x configurations confirm the energetic stability of both cation and anion vacancies and predict Χ Mo to decrease from 1.00 to 0.67 for x = 0.54-1.50, while Χ N increases from 0.50 to 1.00 for x = 0.50-1.36. The simulations are in good agreement with experiments and indicate a preference for a 75% site occupancy on both sublattices for compositions near stoichiometry, with Χ Mo = 0.75 for x = 1.00-1.22 and Χ N = 0.75 for x = 0.86-1.00. Correspondingly, cubic stoichiometric MoN is most stable in the NbO structure.

“…The two phase-pure epitaxial NbN x layers deposited at T s = 800 and 1000 °C exhibit nearly the same E = 315±13 and 319±19 GPa, respectively. This seems initially surprising because their N/Nb ratio of x = 0.98 and 0.91 is different, and the predicted indentation modulus M 100 increases considerably with an increasing N vacancy concentration, as shown in Table II [91], Ti 1-x W x N(001) [52], and Sc 1-x Al x N(001) [92], and (iv) random cation solid solutions for NbC x N 1-x (001) [93]. Consistent with this observation, we attribute the changes in ρ with deposition temperature of NbN x /MgO(001) layers primarily to carrier localization effects, which themselves are controlled by the crystalline quality and the vacancy concentrations.…”

Section: Resultsmentioning

confidence: 99%

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

Zhang

Surface and Coatings Technology

et al. 2016

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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.