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 m icrostructure in a-Si:H films prepared using Ar and He diluted SiH4 pl asmas. One of the series of films was prepared using different amounts of Ar in an Ar-SiH4, 70 MHz VHF plasma while all other deposition par ameters were nominally fixed. With the increase of Ar dilution above 6 0%, the SAXS increased and became dependent on the angle at which the sample was tilted 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 in creased sub-bandgap absorption all accompanied the enhanced SAXS. A se cond series of films was prepared with and without He dilution of a st andard 13.56 MHz SiH4 plasma. As in the case of the Ar dilution, the S AXS 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 i ncreased microstructure since higher RF powers and chamber pressures w ere used in order to increase the deposition rate. Increasing the subs trate temperature up to 320 degrees C allowed for an increase in the d eposition rate to 12 Angstrom/s with a decrease in the SAXS. The scatt ering centers produced in the He dilution study are significantly smal ler than those of the Ar dilution study and there is less tendency for an oriented microstructure.