Optical properties of plasma-enhanced chemical vapor deposited silicon-oxynitride films

By transmission and reflection spectroscopy between 1.5 and 9.5 eV, at 300 and 100 K, we have obtained the absorption coefficient α of silicon oxynitride films grown by plasma-enhanced chemical vapor desposition from gas mixtures of SiH₄ and N₂O at 430 °C. IR, electron spin resonance (ESR), and transport measurements have been performed previously on this sample of SiO_xN_yH_z known composition by elastic recoil detection analysis (ERDA). The optical gap E_G, the slope B of Tauc's plot (αhν)^1/2=B(hν-E_G), and the reciprocal slope E_o of the Urbach tail present strong correlation in SiO_xN_yH_z and SiN_yH_z films. Below a percolation limit for oxygen, nitrogen, and hydrogen incorporation in the a-Si amorphous network, the relevant physical parameter for the composition is x in SiO_x, (y+z) in SiN_yH_z, and (x+y+z) in SiO_xN_yH_z . Thus, we find single curves for the variation with the composition of E04 (the energy at which α=10⁴ cm^-1), E₀, and 1/B. Above the percolation composition the optical absorption below the principal absorption edge is dominated by defect states in the gap. According to ESR and ERDA measurements, the absorption shoulder between 7 and 8 eV may be explained by the presence of OH^- or O⁺₃ diamagnetic centers in the insulator.