Epitaxial growth of cubic WCy(001) on MgO(001)

Abstract

Tungsten carbide films are sputter-deposited onto MgO(001) substrates at 400 °C in 5 mTorr Ar - CH4 gas mixtures with CH4 fractions fCH4 = 0.4–6%, yielding total C-to-W ratios x = 0.57–1.25 as determined by ion beam analyses and energy-dispersive X-ray spectroscopy. The C-to-W ratio y in the cubic WCy phase is smaller than x, ranging from y = 0.47–0.68, as determined from lattice constant ao measurements in combination with first-principle calculations that predict an increasing ao = (0.4053 + 0.0295 y) nm for y = 0.3–1.0. This suggests that the cubic phase is stabilized by carbon vacancies and that the layers contain amorphous C with a volume fraction increasing from 4% to 26% for fCH4 = 0.4–6%. X-ray diffraction (XRD) θ-2θ scans, ω-rocking curves, and reciprocal space maps in combination with high-resolution transmission electron microscopy (TEM) indicate the growth of epitaxial rock-salt structure WCy(001) layers with a cube-on-cube epitaxial relationship with the substrate: (001)WC || (001)MgO and [100]WC || [100]MgO. The measured XRD out-of-plane coherence length of 8 – 14 nm is nearly independent of the film thickness d = 10 or 600 nm, suggesting that growth beyond d = 10 nm leads to an epitaxial breakdown and the nucleation of misoriented hexagonal or orthorhombic W2C grains for fCH4 ≤ 1% and cubic nanocrystalline WCy grains for fCH4> 1%.