The effects of Ar and He dilution of silane plasmas on the microstructure of a-Si:H detected by small-angle X-ray scattering

The small-angle x-ray scattering (SAXS) technique was used to detect microstructure in a-Si:H prepared using Ar and He diluted SiH₄ plasmas. One of the series of films was peepared using different amounts of Ar in an ArSiH₄, 70 MHz VHF plasma while all other deposition parameters were nominally fixed. With the increase of Ar dilution above 60%, the SAXS increased and became dependent on the angle at which the samples was titled with respect to the incident x-ray beam, consistent with the appearance of columnar-like microstructure. A lowering of the photo/dark conductivity ratio, a broadening of the Urbach edge and increased sub-bandgap absorption all accompanied the enhanced SAXS. A second series of films was prepared with and without He dilution of a standard 13.56 MHz SiH₄ plasma. As in the case of the Ar dilution, the SAXS was found to increase with the use of the He dilution gas. However, it is not evident that the addition of He to the plasma caused the increased microstructure since higher RF powers and chamber pressures were used in order to increase the deposition rate. Increasing the substrate temperature up to 320°C allowed for an increase in the deposition rate to 12 Å/s with a decrease in the SAXS. The scattering centers produced in the He dilution study are significantly smaller than those of the Ar dilution study and there is less tendency for an oriented microstructure.