Unraveling the structural and electronic features of tungsten nitride (WNx) thin films

We report the investigation of the structural and electronic properties of tungsten nitride (WN_x) thin films, deposited using a reactive magnetron sputtering process utilizing different N₂ flow ratio (R_N2). The WN_x films were characterized using X-ray reflectivity (XRR), X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) taken at N K-edge and W L-edge. XRR measurements revealed that at R_N2 = 25 %, the film exhibited a density of 16.7 g/cm³ , closely matching the theoretical density of 17.1 g/cm³ for W₂N. XRD analysis identified a pure W₂N phase at R_N2 = 25 %, which transitions to a predominantly δ-WN phase at 50 %. XANES measurements at the N K-edge showed features characteristic of transition metal nitrides, indicating N and W orbital hybridization. XANES at the W L₃-edge showed no significant shift in absorption energy between R_N2 values of 17 % and 50 %, suggesting stable valence states across this range. EXAFS analysis further supported the XRD findings, revealing that the W-N and W-W bond distances at R_N2 = 25 % matches with those of bulk W₂N. At R_N2 = 50 %, the bond distances changed to values consistent with bulk δ-WN, confirming the transition to a δ-WN-dominant phase. Furthermore, the thermal stability of WN_x films with R_N2 from 17 % to 25 % was studied using XRD and XAS techniques. Films grown at 17 % R_N2 completely transformed to the α-W phase at 700 °C, while those grown at 22.5 % and 25 % exhibited a mixed phase of W₂N and α-W, with α-W being the dominant phase.