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〜

利用共振光热弯曲光谱（R-PBS）测量了氢化微晶硅（μc-Si：H）薄膜在0.7 ~ 1.2eV光子能量处的光吸收谱，研究了其超低能吸收的原因。样品是用SiH_4和H_2为源气体的等离子体增强化学气相沉积法制备的。首先，我们发现在沉积前腔室的反压为10 ~ 10 μ Torr时所制备的薄膜的能量吸收很低<-3>。在1 eV的光子能量下的吸收系数α为1.10^2 cm-1<-1>。另一方面，对于在腔室的背压为10 μ Torr的情况下制备的膜，没有观察到额外的吸收<-6>。光子能量为1 eV时的吸收系数α为1.101cm-3<-1>。用恒光电流法测得的光谱中未观察到额外吸收。利用R-PBS技术测量了微晶3C-SiC：H（μc-3C-SiC：H）薄膜的光吸收谱。样品采用热丝CVD（catalytic CVD）法制备。我们发现，在2.2 eV以上的光子能量范围内的光谱反映了由于在薄膜中的3C-SiC微晶的间接跃迁。另一方面，低于2.2 eV的吸收似乎源于由于杂质而导致的局域态的光学跃迁。

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.