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

Abstract

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 SiOxNyHz known composition by elastic recoil detection analysis (ERDA). The optical gap EG, the slope B of Tauc’s plot (αhν)1/2=B(hν−EG), and the reciprocal slope Eo of the Urbach tail present strong correlation in SiOxNyHz and SiNyHz 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 SiOx, (y+z) in SiNyHz, and (x+y+z) in SiOxNyHz . 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.