Epitaxial growth and properties of cubic WN on MgO(001), MgO(111), and Al2O3(0001)

Tungsten nitride layers, 1.45-μm-thick, were deposited by reactive magnetron sputtering on MgO(001), MgO(111), and Al2O3(0001) in 20 mTorr N2 at 700 °C. X-ray diffraction ω-2θ scans, ω-rocking curves, φ scans, and reciprocal space maps show that all layers exhibit a cubic rock salt structure, independent of their N-to-W ratio which ranges from x = 0.83-0.93, as determined by energy dispersive and photoelectron spectroscopies. Growth on MgO(001) leads to an epitaxial WN(001) layer which contains a small fraction of misoriented grains, WN(111)/MgO(111) is an orientation- and phase-pure single-crystal, and WN/Al2O3(0001) exhibits a 111-preferred orientation containing misoriented cubic WN grains as well as N-deficient BCC W. Layers on MgO(001) and MgO(111) with x = 0.92 and 0.83 have relaxed lattice constants of 4.214 ± 0.005 and 4.201 ± 0.031 Å, respectively, indicating a decreasing lattice constant with an increasing N-vacancy concentration. Nanoindentation provides hardness values of 9.8 ± 2.2, 12.5 ± 1.0, and 10.3 ± 0.4 GPa, and elastic moduli of 240 ± 40, 257 ± 13, and 242 ± 10 GPa for layers grown on MgO(001), MgO(111), and Al2O3(0001), respectively. Brillouin spectroscopy measurements yield shear moduli of 120 ± 2 GPa, 114 ± 2 GPa and 108 ± 2 GPa for WN on MgO(001), MgO(111) and Al2O3(0001), respectively, suggesting a WN elastic anisotropy factor of 1.6 ± 0.3, consistent with the indentation results. The combined analysis of the epitaxial WN(001) and WN(111) layers indicate Hill's elastic and shear moduli for cubic WN of 251 ± 17 and 99 ± 8 GPa, respectively. The resistivity of WN(111)/MgO(111) is 1.9 × 10− 5 and 2.2 × 10− 5 Ω-m at room temperature and 77 K, respectively, indicating weak carrier localization. The room temperature resistivities are 16% and 42% lower for WN/MgO(001) and WN/Al2O3(0001), suggesting a resistivity decrease with decreasing crystalline quality and phase purity.