Structural and physical properties of NbO₂ and Nb₂O₅ thin films prepared by magnetron sputtering

NbO_x thin films have been deposited on silicon (100) and quartz substrates by magnetron sputtering using a metallic Nb target in an optimized argon–oxygen atmosphere. This work investigates the dependence of structure–property relations on the key deposition parameters towards establishing optimum deposition conditions for the growth of NbO_x polycrystalline films. It is found that a sputtering condition corresponding to DC power density 9.87 W cm⁻², a substrate temperature of 720 °C, low gas pressures of 8 mtorr, a target to substrate distance of 45 mm gives thin films with good homogeneity and a high degree of crystallinity in the case of both NbO₂ and Nb₂O₅. X-ray diffraction (XRD) and Raman spectroscopy confirmed the tetragonal phase of NbO₂ and orthorhombic phase of Nb₂O₅ for similar deposition temperatures. Scanning electron microscopy (SEM) observations indicate that NbO₂ has a unique nanoslice structure while Nb₂O₅ has a flake-like structure. The optical transmittance of the films has been investigated and found to be dependent on the oxygen gas content during deposition; the optical transmittance decreases with increasing O₂ gas content. Optical constants of the films were calculated by fitting a suitable thin film transmittance model to experimental transmittance spectra using a modified Swanepoel technique. The nanohardness and stress in the films were measured by nanoindentation and an optical profilometer respectively. Nanohardness and stress in the film show no large dependence on the oxygen gas content except at high oxygen gas content. The nanohardness value of NbO₂ films is approximately 6 GPa, and the Young's modulus is 150 GPa. The Nb₂O₅ films exhibit a nanohardness of 5.8–13 GPa and a Young's modulus of 137–161 GPa.