Manipulating the Hydrogen-Bonding Configuration in ETP-CVD a-Si:H

The effect of ion bombardment on the relationship between the critical hydrogen concentration ccrit and the reactor pressure has been investigated for hydrogenated amorphous silicon (a-Si:H) deposited with the expanding thermal plasma-CVD (ETP-CVD) method. By increasing the reactor pressure, in particular above 0.24 mbar, the ionic cluster formation in the plasma can be increased, resulting in a decrease of ccrit. It is observed that this decrease of ccrit with increasing reactor pressure can not be compensated by ion bombardment at 14V biasing. Biasing with 20V however increases ccrit nearly up to the value obtained at low pressures. This observation indicates that the incorporation of ionic clusters formed at elevated reactor pressures can be reduced by substrate biasing, possibly due to break-up upon impact on the substrate surface or due to processes occurring in the secondary plasma close to the substrate. The onset of void formation in the film is found to depend on reactor pressure and substrate biasing, indicating that the maximum hydrogen solubility for ETP material might be affected by these deposition parameters.