Kinetics of hydrogen evolution and crystallization in hydrogenated amorphous silicon films studied by thermal analysis and Raman scattering

We observed the processes of hydrogen evolution and crystallization in hydrogenated amorphous silicon 0.5-7 μm thick films (deposited by dc glow discharge on molybdenum) by differential scanning calorimetry (DSC), Raman scattering and thermogravimetric analysis (TGA). Investigation was made as a function of doping, deposition temperature and film thickness. For all the films, an endothermic DSC peak was observed at 694 °C (onset). That this peak was at least partly due to hydrogen evolution was shown by TGA, which showed weight loss beginning at 694 °C, and by evolved gas analysis, which showed hydrogen evolution at 694 °C. This temperature (658-704 °C) increased with increasing heating rate (5-30 °C/min). Doping reduced this temperature from 694 to 625 °C for boron doping and to 675 °C for phosphorus doping. Hydrogen evolution kinetics and FTIR results suggest that the silicon-hydrogen bonding in the intrinsic film was a mixture of SiH and SiH₂, and was predominantly SiH in the phosphorous doped films and SiH₂ in the boron doped films. Crystallization was independent of silicon-hydrogen bonding in the as-deposited amorphous silicon film. It was bulk (not interface) induced. No exothermic DSC peak accompanied the crystallization. The film deposition temperature had little effect on the DSC result, but crystallization was enhanced by a higher deposition temperature.