Influence of gas supply and filament geometry on the large-area depositionof amorphous silicon by hot-wire CVD

The scale-up of hot-wire CVD of a-Si:H based solar cells requires knowledge of how gas supply and filament geometry affect the deposition process. In this paper we report the important role of gas supply configuration and how it affects the uniformity of the film thickness. We show the calculation o f the gas flow generated by different kinds of gas showers and compare them to experimental results. The effect of multiple filament geometry on the u niformity and the quality of the a-Si:H films is presented. The number of f ilaments, the distance between the filaments and the distance between the s ubstrate and the filament grid were systematically varied. Using a special gas shower with similar to 2000 holes of sub-millimeter diameter in a 30 cm x 30 cm steel plate, and a filament grid consisting of six tantalum wires with a distance of 4 cm between the wires, we could achieve device quality a-Si:H films (sigma (Ph)/sigma (D) = 2 x 10(5), N-d(PDS) = 6.8 x 10(16)/cm( 3)) with a thickness uniformity of +/-2.5% on a circular area with a diamet er of 20 cm, grown at a deposition rate of r(d) = 6 Angstrom /s. The optimu m distance between the filament grid and the substrate was found to be 8.4 cm. This setup was used to deposit the intrinsic layers of a-Si:H pin solar cells. The p-layers and the n-layers were made by PECVD and HWCVD, respect ively, in different deposition systems. Although there were airbreaks at bo th the p/i and i/n interface, we could fabricate small area solar cells (0. 08-0.8 cm(2) active area) with initial efficiencies of eta = (6.4 +/- 0.8)% over an area of 20 cm x 20 cm. (C) 2001 Elsevier Science B.V. All rights r eserved.