Microstructure and structure characteristics of cubic WN_x compounds

Microstructure and structure of cubic WN_x compounds were investigated by X-ray photoelectron spectroscopy (XPS), electron energy-loss spectroscopy (EELS), X-ray diffraction (XRD), transmission electron microscopy (TEM), and electron diffraction. The samples were thin films produced by reactive dc magnetron sputtering of tungsten at various N₂ partial pressures in an Ar-N₂ gas mixture. The composition and structure of the films were characterized by XPS, EELS, and XRD. It was found that a two-phase structure consisting of W₂N and bcc W was formed in the 0.12≤x≤0.28 range and a single-phase structure of W₂N was formed at higher values of x after annealing of as-deposited samples at 600°C or above. Cross-sectional TEM studies showed that all crystalline WN_x films had columnar microstructures. The average column width at stoichiometry of W₂N was ~15 nm, whereas the column grains were larger with decreasing value of x. Using the electron scattering data collected from a range of crystalline samples for calculating the pair distribution function (RDF) by Fourier transformation in real space, structural details of W-N and W-W bonding in W₂N have been obtained.