Elastic constants of epitaxial cubic MoNx(001) layers

The N-to-Mo ratio x in epitaxial cubic molybdenum nitride γ-MoNx(001) layers was varied from x = 0.69-1.25 by adjusting the substrate temperature Ts = 1000-600 °C during reactive magnetron sputter deposition. X-ray diffraction, reflection, and reciprocal space mapping measurements in combination with Rutherford backscattering spectroscopy indicate that the layers exhibit negligible strain and that increasing x = 0.69-1.25 leads to an increasing lattice constant a = 4.16-4.26 Å and a decreasing density ρ = 8.7-6.8 g/cm3 which is primarily attributed to a decrease in the cation site occupation from 0.89 to 0.70. Time resolved pump-probe reflectivity measurements of the sound velocity parallel to the [001] growth direction indicate that increasing the nitrogen concentration leads to a velocity reduction, corresponding to a decrease in C11 from 502 ± 31 to 355 ± 30 GPa for x = 0.69-1.25. Similarly, the transverse surface acoustic wave velocity measured by surface Brillouin scattering spectroscopy decreases from 4.00-3.52 km/s, providing values for C44 from 100 ± 7 to 73 ± 3 GPa. The softening with increasing N concentration is primarily attributed to the decreasing Mo site occupancy, which causes a reduction in the valence electron concentration leading to a weakening of NMo bonds associated with hybridized N 2p - Mo 4d eg orbitals.