Epitaxial growth of cubic WC<i>_y</i>(001) thin films

Tungsten carbide films were sputter-deposited onto MgO(001) substrates at 400 °C in 5 mTorr Ar-CH₄ gas mixtures as a function of the CH₄ fraction f_CH4= 0.4 - 6%. High resolution transmission electron microscopy (TEM) and x-ray diffraction ω-2θ scans, ω-rocking curves, and reciprocal space maps on d = 10 nm thick layers indicate epitaxial growth of rock-salt WC_y with a cube-on-cube epitaxial relationship: (001)_WC || (001)_MgO and [100]_WC || [100]_MgO. The measured out-of-plane coherence length matches the film thickness for d = 10 nm but remains nearly unaffected by a 60-fold increase in layer thickness to d = 600 nm. This suggests a critical thickness for epitaxial breakdown of ~ 10 nm, associated with the nucleation of misoriented grains as also observed by TEM. The relaxed lattice constant increases monotonically from 0.419 to 0.425 nm with increasing f_CH4. Comparing these measured lattice parameters with first-principle predictions indicates a C-to-W ratio y = 0.47 - 0.68. However, composition measurements using energy-dispersive x-ray spectroscopy and Rutherford backscattering spectrometry yield measured C-to-W ratios of 0.57-1.25, suggesting that a considerable fraction (18 – 46 %) of C does not incorporate in the cubic phase but likely forms amorphous carbon.