From amorphous to microcrystalline silicon films prepared by hydrogen dilution using the VHF (70 MHz) GD technique

The amorphous and microcrystalline silicon films have been prepared by hydrogen dilution from pure silane to silane concentrations ≥1.25%. At silane concentrations of less than 10%, a transition from the amorphous phase to the microcrystalline phase can be observed. X-ray diffraction spectroscopy indicates a preferential growth of the crystallites in the [220] direction. Additionally, the transition into the microcrystalline regime is accompanied by a shrinking of the optical gap, a reduction in hydrogen content and by a modified trend of the deposition rate. The observed changes in the infrared absorption modes indicate modifications in the hydrogen bonding and can be correlated with results known from monocrystalline silicon. Close to the transition zone, but still in the amorphous regime, the hydrogen content is increased, whereas the microstructure parameter reaches its smallest value. Precisely these films have a 0.06 eV higher optical gap and a reduced defect density by a factor of 4 as compared to a-Si:H layers prepared from pure silane.