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

Tungsten nitride layers, 1.45-mu m-thick, were deposited by reactive magnetron sputtering on MgO(001), MgO(111), and Al2O3(0001) in 20 mTorr N-2 at 700 degrees C. X-ray diffraction omega-2 theta scans, omega-rocking curves, phi 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 angstrom, 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 shearmoduli 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 x 10(-5) and 2.2 x 10-5 Omega-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. (C) 2015 Elsevier B.V. All rights reserved.