A study on estimation of localized states and its reduction in microcrystalline silicon films 〜An application of resonant photothenmal bending spectroscopy to hetero-structured films〜

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In this study, optical absorption spectra of hydrogenated microcrystalline silicon (μc-Si : H) films have been measured by resonant photothermal bending spectroscopy (R-PBS) to elucidate the origin of extra low energy absorption at photon energy of 0.7〜1.2 eV. The samples were prepared by plasma enhanced chemical vapor deposition method with source gases of SiH_4 and H_2.First, we found that the low energy absorption was observed for the film prepared with a back pressure 〜10^<-3> Torr of a chamber before the deposition of the film. The absorption coefficient a at a photon energy of 1 eV was 〜10^2 cm^<-1>. On the other hand, the extra absorption was not observed for the film prepared with a back pressure 〜10^<-6> Torr of the chamber. The absorption coefficient a at a photon energy of 1 eV was 〜10^1 cm^<-1>. Furthermore, the extra absorption was not observed in the spectra measured by constant photocurrent method. From these results, it is suggested that the origin of the extra absorption at photon energy of 0.7〜1.2 eV in μc-Si : H films is localized states related to oxygen impurities existing at grain boundaries.The R-PBS technique was applied to measuring optical absorption spectra of microcrystalline 3C-SiC : H (μc-3C-SiC : H) films. The sample was prepared by hot-wire CVD (catalytic CVD) method. We found that the spectra ranged above photon energy of 2.2 eV reflect the indirect transition due to 3C-SiC crystallites in the film. On the other hand, the absorption below 2.2 eV seems to originate from optical transition at localized states due to impurities.