Ion irradiation effects on WNxOy thin films

We have investigated ion irradiation effects on modifications of electronic- and atomic-structures of WN_xO_y films on C-plane-cut-sapphire (C-Al₂O₃) substrate. Rutherford backscattering spectroscopy (RBS) of 1.8 MeV He⁺ ions leads to the composition, x = 1.1 ± 0.1 and y = 0.4 ± 0.2. X-ray diffraction (XRD) with Cu-K_α radiation shows a strong peak at the diffraction angle (2θ) ≈37° and a weak peak at ∼78°. These peaks are assigned as hexagonal-WN and no peaks were observed other than WN and C-Al₂O₃ (2θ = 41.7°). It is found that the electrical resistivity of unirradiated film (25 × 10⁻³ Ωcm) is reduced to 3 × 10⁻⁴ and 2 × 10⁻³ Ωcm by 100 MeV Xe ion irradiation at 4 × 10¹⁴ cm⁻² and 100 keV N at 10¹⁶ cm⁻², respectively, and that the resistivity of unirradiated film increases with decreasing temperature from RT to 30 K like semiconductor and irradiated film shows very weak temperature dependence. It also appears that the XRD intensity decreases by more than a factor of 10 under these ion irradiation, and lattice expansion of ∼1% at low ion fluence turns into lattice compaction for further ion irradiation. Optical absorption monotonically decreases with increasing the wavelength from 200 to 2500 nm, implying that the bandgap does not exist in the region of 0.5–6 eV and little change in absorption is observed by the ion irradiation.