Hydrogenated amorphous silicon deposited at very high growth rates by an expanding Ar-H-2-SiH4 plasma

The properties of hydrogenated amorphous silicon (a-Si:H) deposited at very high growth rates (6-80 nm/s) by means of a remote Ar-H-2-SiH4 plasma have been investigated as a function of the H-2 flow in the Ar-H-2 operated pla sma source. Both the structural and optoelectronic properties of the films improve with increasing H-2 flow, and a-Si:H suitable for the application i n solar cells has been obtained at deposition rates of 10 nm/s for high H-2 flows and a substrate temperature of 400 degreesC. The "optimized" materia l has a hole drift mobility which is about a factor of 10 higher than for s tandard a-Si:H. The electron drift mobility, however, is slightly lower tha n for standard a-Si:H. Furthermore, preliminary results on solar cells with intrinsic a-Si:H deposited at 7 nm/s are presented. Relating the film prop erties to the SiH4 dissociation reactions reveals that optimum film quality is obtained for conditions where H from the plasma source governs SiH4 dis sociation and where SiH3 contributes dominantly to film growth. Conditions where ion-induced dissociation reactions of SiH4 prevail and where the cont ribution of SiH3 to film growth is much smaller lead to inferior film prope rties. A large contribution of very reactive (poly)silane radicals is sugge sted as the reason for this inferior film quality. Furthermore, a compariso n with film properties and process conditions of other a-Si:H deposition te chniques is presented. (C) 2001 American Institute of Physics.