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

Abstract

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 of the gas flow generated by different kinds of gas showers and compare them to experimental results. The effect of multiple filament geometry on the uniformity and the quality of the a-Si:H films is presented. The number of filaments, the distance between the filaments and the distance between the substrate and the filament grid were systematically varied. Using a special gas shower with ∼2000 holes of sub-millimeter diameter in a 30 cm×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 (σPh/σD=2×105, Nd(PDS)=6.8×1016/cm3) with a thickness uniformity of ±2.5% on a circular area with a diameter of 20 cm, grown at a deposition rate of rd≈6 Å/s. The optimum 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, respectively, in different deposition systems. Although there were airbreaks at both the p/i and i/n interface, we could fabricate small area solar cells (0.08–0.8 cm2 active area) with initial efficiencies of η=(6.4±0.8)% over an area of 20 cm×20 cm.