Optimization of process parameters in a large-area hot-wire CVD reactor for the deposition of amorphous silicon (a-Si:H) for solar cell application with highly uniform material quality

Scale-up of a-Si:H-based thin film applications such as solar cells, entirely or partly prepared by hot-wire chemical vapor deposition (HWCVD), requires research on the deposition process in a large-area HWCVD system. The influence of gas supply and filament geometry on thickness uniformity has already been reported, but their influence on material quality is systematically studied for the first time. The optimization of deposition parameters for obtaining best material quality in our large-area HWCVD system resulted in an optimum filament temperature, T-fil approximate to 1600degreesC, pressure, p = 8 mTorr and silane flow, F(SiH4) = 100 sccm, keeping the substrate temperature at T-S = 200degreesC. A special gas supply (gas shower with tiny holes of uniform size) and a filament grid, consisting of six filaments with an interfilament distance, d(fil) = 4 cm were used. The optimum filament-to-substrate distance was found to be d(fil-S) = 8.4 cm. While studying the influence of different d(fil) and gas supply configurations on the material quality, the above-mentioned setup and parameters yield best results for both uniformity and material quality. With the setup mentioned, we could achieve device quality a-Si:H films with a thickness uniformity of +/-2.5% on a circular area of 20 cm in diameter. The material, grown at a deposition rate of r(d) approximate to 4 Angstrom/s, was characterized on nine positions of the 30 cm x 30 cm substrate area, and revealed reasonable uniformity of the opto-electronic properties, e.g photosensitivity, sigma(Ph)/sigma(D) = (2.46 +/- 0.7) x 10(5), microstructure factor, R = 0.17 +/- 0.05, defect densities, N-d(PDS) = (2.06 +/- 0.6) x 10(17) cm(-3) and N-d(CPM) = (2.05 +/- 0.5) x 10(16) cm(-3) (film properties are given as mean values and standard deviations). Finally, we fabricated pin solar cells, with the i-layer deposited on small-area p-substrates distributed over an area of 20 cm x 20 cm in this large-area deposition system, and achieved high uniformity of the cell parameters with initial efficiencies of eta = (6.1 +/- 0.2)% on the 20 cm x 20 cm area. (C) 2002 Elsevier Science B.V. All rights reserved.