Characterization of Polycrystalline Tungsten Surfaces Irradiated with Nitrogen Ions by X-ray Photoelectron Spectroscopy

Abstract

Polycrystalline tungsten surfaces were irradiated at room temperature with two kinds of nitrogen ions—N⁺ and N₂⁺—at 2.5 keV by using an ion beam apparatus. Results of X-ray photoelectron spectroscopy (XPS) experiments performed using synchrotron radiation at SPring-8 showed that upon irradiation of the tungsten sample with either kind of ion, the full widths at half maximum (FWHM) of the W 4f_7/2 and W 4f_5/2 peaks broadened and the peaks at 35.8 eV and 37.8 eV—which correspond to WO₃ binding energies—increased slightly; this indicated the formation of tungsten nitride at the subsurface below the interface. The N 1s spectra of tungsten after nitrogen ion beam irradiation were decomposed into four component peaks. The positions of these component peaks were observed to be the same as those of the standard tungsten oxynitride W_0.62(N_0.62O_0.38), which exhibited W₂N peaks in X-ray diffraction analysis. The main decomposed peaks at 397.3 eV and 398.1 eV were attributed to W-N bonds and W-N-O bonds, respectively. The variation of the intensity ratio of the N 1s peak at 397.3 eV to the W 4f doublet peaks (corresponding to W-N bonds) as a function of the escape depth, which was measured by angle-resolved XPS, apparently followed a normal distribution for the irradiated samples. This indicates that the W-N bond density of the tungsten surface irradiated with N₂⁺ ions is higher than that of the surface irradiated with N⁺ ions and also that the N₂⁺ ions penetrate slightly deeper than the N⁺ ions.