Deposition of a-Si:H using a supersonically expanding argon plasma

Amorphous hydrogenated silicon (a-Si:H) is a material which is widely used in the field of solar cells and other optoelectronics. The only method available to produce high quality a-Si:H is by means of plasma enhanced chemical vapor deposition (PECVD). Radicals responsible for deposition diffuse from a glow discharge towards a substrate which is heated up to 800 K where a layer is grown with a speed of typically 0.3 nm s^-1. The deposition rate is limited because the transport is diffusion determined. An increase of this deposition rate and material efficiency can be expected if the radicals are transported towards the substrate using another transport mechanism. We have built an apparatus which uses a supersonically expanding cascaded are argon plasma in which silane (SiH₄) is injected to direct the radicals towards the substrate. It is expected that the deposition rate will be up to 100 times as high as in normal PECVD. A further advantage is that the larger spatial separation of the processes dissociation, transport and ionization allows more independent optimization of these three processes. First experiments show that the deposition rate is 7 nm s^-1 yielding a material with a refractive index of 3.7. Absorption spectroscopy shows only absorption by SiH stretching and bending modes. Further optimization and research is needed to explore the possibilities of this method.