Microstructural Defects of Device Quality Hot-Wire Cvd Poly-Silicon Films
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Some of the requirements of the poly-Si films for the solar cell application are that they should be intrinsic and should have low oxygen incorporation. The actual mechanism of oxygen incorporation into the material is not clear and the extent of oxygen incorporation depends on the type of deposition, growth process and purity of gases [1]. We have been able to achieve, by hot wire chemical vapour deposition (H-WCVD), device quality poly-Si:H films with very intrinsic nature (activation energy of 0.54 eV and band gap of 1.1 eV) [2]. These films were made at a low hydrogen dilution of silane gas. However, the level of oxygen incorporation can change with the change of deposition conditions. At high hydrogen dilution the poly-Si contains a large concentration of oxygen [3]. It is the aim of this paper to find a (possible) correlation between microstructural defects and the deposition condition, in particular the hydrogen dilution and investigate the oxygen incorporation mechanism. Moreover, the defect density in the material depends on the microstructural defects and the degree of passivation of internal surfaces of the voids. It is necessary to find a proper deposition regime where microstructural defects can be minimized maintaining good initial crystalline growth. In our earlier paper [3] we had proposed such a deposition scheme, where a profiled poly-Si was made integrating two regimes of growth i.e., high and low hydrogen dilution of silane gas. Our preliminary results obtained for actual thin film solar cells on a foreign substrate, i.e., complete n-i-p solar cells using thin n and
573 Mat. Res. Soc. Symp. Proc. Vol. 557 © 1999 Materials Research Society
p-layers fabricated on a stainless steel substrate, yielded an efficiency of 3.7% and a current density of 23.5 mA/cm 2 with a 1.5 pm thick poly-Si i-layer [3]. In this paper we present our latest solar cell characteristics and investigate the role of impurities and the structure of the cell as the limiting factors in the solar cell efficiencies. EXPERIMENTAL Poly-silicon films were deposited on 10 cm x 10 cm Coming 7059 glass and c-Si wafer substrates by HWCVD in one of the chambers of an ultra high vacuum multichamber system (PASTA). Details of the Hot Wire-set up and deposition process have been described elsewhere [4]. The thickness of the films was measured with a Dektak profilometer and with reflection/transmission measurements. Samples were characterised by laser Raman spectroscopy, Fourier transform infrared (FHIR) spectroscopy, hydrogen evolution, secondary ion mass spectrometry (SIMS) and electrical conductivity in the dark and in white light. Brightfield cross sectional transmission electron micrographs (XTEM) were made with a Philips CM30T electron microscope operated at 300kV. High-resolution electron micrographs (HREM) of the material were made to detect crystallinity, especially near the interfaces. N-i-p cells were made on various types of stainless steel (SS) substrates in the configuration SS/n-pcSi:H(PECVD)/i-poly-Si:H(HWCVD)/p-jic-Si:H(PE
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