Epitaxial NbCxN1−x(001) layers: Growth, mechanical properties, and electrical resistivity

Abstract

NbCxN1−x layers were deposited on MgO(001) by reactive magnetron co-sputtering from Nb and graphite targets in 5mTorr pure N2 at Ts=600–1000°C. The anion-to-Nb ratio of 1.09±0.05 is independent of Ts and indicates a nearly stoichiometric rock–salt structure Nb(N,C) solid solution. In contrast, the C-to-N ratio increases from 0.20–0.59 for Ts=600–1000°C, which is attributed to a low C sticking probability at high N surface coverage at low Ts. Layers grown at Ts≥700°C are epitaxial single-crystals with a cube-on-cube relationship to the substrate, (001)NbCN||(001)MgO and [100]NbCN||[100]MgO, as determined from X-ray diffraction θ–2θ and ϕ-scans. Reciprocal space mapping on a NbC0.37N0.63 layer deposited at Ts=1000°C indicates an in-plane compressive strain of −0.4% and a relaxed lattice constant of 4.409±0.009Å. The lattice constant of NbCxN1−x increases with x, consistent with a linear increase predicted by first-principles density functional calculations. The calculated bulk modulus, 307GPa for NbN and 300GPa for NbC, is nearly independent of x. Similarly, c11 increases slightly from 641 to 666GPa, but c12 decreases considerably from 140 to 117GPa, and c44 more than doubles from 78 to 171GPa as x increases from 0 to 1, indicating a transition from ductile NbN to brittle NbC. This also results in an increase in the predicted isotropic elastic modulus from 335 to 504GPa, which is in good agreement with the measured 350±12GPa for NbCxN1−x(001) with x=0.19–0.31. The hardness H=22±2GPa of epitaxial NbCxN1−x layers is nearly independent of x=0.19–0.37 and Ts=700–1000°C, but is reduced to H=18.2±0.8GPa for the nanocrystalline layer deposited at Ts=600°C. The electrical resistivity decreases strongly with increasing Ts<800°C, due to increasing crystalline quality, and is 262±21μΩ-cm at room temperature and 299±22μΩ-cm at 77K for Ts≥800°C, indicating weak carrier localization due to the random distribution of C atoms on anion sites.