On the role of atomic hydrogen during microcrystalline silicon thin-film deposition

The expanding thermal plasma, which is a promising technique for microcrystalline silicon (μc-Si:H) thin-film deposition because of its high growth rates over large areas, witnesses, so far, specific challenges in the deposition of device-grade μc-Si:H material. The μc-Si:H films show post-deposition oxidation, caused by an insufficient amount of (dense) amorphous tissue between the grains, resulting in low solar cell efficiencies. Atomic hydrogen, which is crucial for the formation of μc-Si:H films, is hypothesized to be responsible for this lack of amorphous tissue because of its ability to etch amorphous silicon (a-Si:H) by insertion in Si-Si bonds. Therefore, we studied the interaction of atomic H with thin a-Si:H films. Results show that etching does not compete with film growth, as etch rates are one order of magnitude lower than deposition rates. Furthermore, atomic H cannot be held responsible for the poor quality of amorphous tissue present in ETP-grown μc-Si:H, as the H up-take takes mainly place in divacancies.