Raman and IR study of narrow bandgap a-SiGe and μc-SiGe films deposited using different hydrogen dilution

Hydrogenated amorphous silicon-germanium (a-SiGe:H) films and n-i-p solar cells near the threshold of microcrystalline formation have been prepared by plasma enhanced chemical vapor deposition (PECVD) with a fixed germane to disilane ratio of 0.72 and a wide range of hydrogen dilution R_H=(H₂ flow)/(GeH₄+Si₂H₆ flow) values of 1.7, 10, 30, 50, 120, 180 and 240. The effects of RH on the structural properties of the films were investigated using Raman scattering and Fourier transform infrared (FTIR) absorption spectroscopy. It is found that H dilution causes the H content, especially that in SiH₂ configuration, in a-SiGe:H films to decrease and finally leads the films through amorphous to microcrystalline transition. The onset of the phase transition occurs at RH about 180, and the crystalline formation begins first in the Si-rich region. Light soaking tests on the solar cells demonstrate that the devices with higher RH exhibit higher stabilized efficiency after 1000 hours of 1 sun light soaking.