Stress induced preferred orientation and phase transition for ternary WCxNy thin films

We deposit ternary WCxNy thin films on Si (1 0 0) substrates at 500 °C using direct current (DC) reactive magnetron sputtering in a mixture of CH4/N2/Ar discharge, and explore the effects of substrate bias (Vb) on the intrinsic stress, preferred orientation and phase transition for the obtained films by virtue of X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and selective area electron diffraction (SAED). We find that with increasing the absolute value of Vb up to 200 V the carbon (x) and nitrogen (y) atom concentrations of WCxNy films keep almost constant with the values of 0.75 and 0.25, respectively. The XPS and SAED results, combined with the density-functional theory (DFT) calculations on the electronic structure of WC0.75N0.25, show our obtained WCxNy films are single-phase of carbonitrides. Furthermore, we find that the compressive stress sharply increases with increasing the absolute value of Vb, which leads to a pronounced change in the preferred orientation and phase structure for the film, in which a phase transition from cubic β-WCxNy to hexagonal α-WCxNy occurs as Vb is in the range of −40 to −120 V. In order to reveal the relationship between the stress and phase transition as well as preferred orientation, the DFT calculations are used to obtain the elastic constants for β-WCxNy and α-WCxNy. The calculated results show that the preferred orientation is dependent on the competition between strain energy and surface energy, and the phase transition can be attributed to a decrease in the strain energy.