Hydrogenated amorphous silicon deposited at very high growth rates by an expanding Ar–H2–SiH4 plasma

The properties of hydrogenated amorphous silicon $\mathrm{(a-}\mathrm{Si:H})$ deposited at very high growth rates (6–80 nm/s) by means of a remote ${\mathrm{Ar–H}}_{\mathrm{2}}{\mathrm{–SiH}}_{\mathrm{4}}$ plasma have been investigated as a function of the ${\mathrm{H}}_{\mathrm{2}}$ flow in the ${\mathrm{Ar–H}}_{\mathrm{2}}$ operated plasma source. Both the structural and optoelectronic properties of the films improve with increasing ${\mathrm{H}}_{\mathrm{2}}$ flow, and $\mathrm{a-}\mathrm{Si:H}$ suitable for the application in solar cells has been obtained at deposition rates of 10 nm/s for high ${\mathrm{H}}_{\mathrm{2}}$ flows and a substrate temperature of 400 °C. The “optimized” material has a hole drift mobility which is about a factor of 10 higher than for standard $\mathrm{a-}\mathrm{Si:H}.$ The electron drift mobility, however, is slightly lower than for standard $\mathrm{a-}\mathrm{Si:H}.$ Furthermore, preliminary results on solar cells with intrinsic $\mathrm{a-}\mathrm{Si:H}$ deposited at 7 nm/s are presented. Relating the film properties to the ${\mathrm{SiH}}_{\mathrm{4}}$ dissociation reactions reveals that optimum film quality is obtained for conditions where H from the plasma source governs ${\mathrm{SiH}}_{\mathrm{4}}$ dissociation and where ${\mathrm{SiH}}_{\mathrm{3}}$ contributes dominantly to film growth. Conditions where ion-induced dissociation reactions of ${\mathrm{SiH}}_{\mathrm{4}}$ prevail and where the contribution of ${\mathrm{SiH}}_{\mathrm{3}}$ to film growth is much smaller lead to inferior film properties. A large contribution of very reactive (poly)silane radicals is suggested as the reason for this inferior film quality. Furthermore, a comparison with film properties and process conditions of other $\mathrm{a-}\mathrm{Si:H}$ deposition techniques is presented.