Deposition of boron doped a-Si:H films by a novel combination of rf glow discharge technique and filament heating: enhancement of doping efficiency

Abstract

A novel variation of plasma enhanced chemical vapour deposition in the form of insertion of a heated filament in the plasma was applied to a SiH₄+1% B₂H₆(in H₂)+H₂ gas mixture for the deposition of boron doped hydrogenated amorphous silicon thin films. Current levels giving filament temperatures below and above the thermionic emission level of the filament material were used to separate the effect of thermal dissociation of B₂H₆ from that of the production of atomic hydrogen in combination with B₂H₆ dissociation. Control runs without filament heating but accounting for rise in substrate temperature because of the heated coil were also carried out. The films were characterised by electrical and optical measurements, infrared vibrational spectroscopy and secondary ion mass spectroscopy. A significant enhancement in the doping efficiency of boron was obtained by this technique and thus was found not to be an artifact of increase in substrate temperature. Nor could it be attributed to the effect of atomic hydrogen only.