Double effects of atomic hydrogen anneal on the microstructure of hydrogenated amorphous silicon films

Hydrogenated amorphous silicon (a-Si:H) films have been deposited with pure silane and then annealed with atomic hydrogen at lower temperature (T_s=170 °C) in a novel, hot wire assisted microwave electron cyclotron resonant chemical vapor deposition system (HW-MWECR-CVD). The experimental results showed that the total hydrogen concentration (C_H) in the film decreased with H₂ flux increase in the atomic hydrogen anneal (AHA) step, but it does not obviously change, keeping at about 3% when H₂ flux increased to a higher value. This is due to double effects of AHA, the crystallization effect at lower substrate temperature and the rehydrogenation effect in low hydrogen concentration films. Furthermore, it was proposed that with increasing R(R=1/{F_hydrogen/(F_silane+F_hydrogen)}, in which F_silane is the silane gas flux in first film synthesis stage and F_hydrogen is the hydrogen gas flux during the annealing step, respectively.), polyhydrides such as SiH_x (x=2 or 3) turn into monohydrides SiH, which results in reducing the network defects, improving the film microstructure and decreasing the optical band gap from 1.644 to 1.557 eV.