Infrared spectroscopic study of SiO_x films produced by plasma enhanced chemical vapor deposition

We have studied the local atomic structure of silicon suboxide (SiO_x, x<2) thin films using infrared (IR) spectroscopy. The films were prepared by plasma enhanced chemical vapor deposition (PECVD) of silane (SiH₄) and nitrous oxide (N₂O) mixtures, which were then diluted with He. The IR spectra were found to vary significantly with the degree of He dilution. Films grown with no He showed SiN, NH, and SiH bonding groups in addition to the three characteristic vibrations of the Si–O–Si linkage. The addition of He reduced the strength of the SiN, NH, and SiH absorption bands, and resulted in systematic increases in the frequency of the Si–O–Si asymmetric stretching vibration. The frequency of this Si–O–Si stretching vibration scales linearly with the oxygen concentration from approximately 940 cm⁻¹ in oxygen doped amorphous silicon to 1075 cm⁻¹ in stoichiometric noncrystalline SiO₂. A deposition temperature of 350 °C and a He dilution of 50% gave a film composition close to SiO_1.9. We propose a model for the deposition process that emphasizes the role of the He dilution.