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24 January 1980 Laser Growth Of Silicon Ribbon
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Carbon Dioxide and YAG lasers have been used to establish a narrow molten zone in a pre-formed polycrystalline silicon ribbon. To achieve the narrow melt across the width of the ribbon, the laser is focussed and scanned across the ribbon by means of mirror scanners. Large grained "macrocrystalline" silicon ribbon is then drawn from the molten zone at growth rates up to 12.5 cm/min. Photovoltaic cells fabricated on this material have shown conversion efficiencies up to 11.3%. The actual absorbed laser power which is required for ribbon growth of interest, e.g. .15 mm thick, 7.5 cm wide, 7.5 cm/min. growth velocity, is only a few hundred watts. Unfortunately, the reflectivity of liquid silicon at 10.6 μm is very high (>90%) so that multikilowatt CO2 lasers will be needed. Nd:YAG lasers offer about a five-fold improvement in laser coupling to liquid silicon but this advantage is offset by the lower efficiency and higher operating cost of Nd:YAG lasers. The effective laser coupling to the melt can be essentially doubled by using spherical reflectors to re-image the reflected laser beam back onto the molten zone. The emissivity change which occurs upon melting, while disadvantageous for laser power requirements, is a real benefit for melt zone control since this produces an inherent stabilizing effect.
© (1980) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
R. W. Gurtler and A. Baghdadi "Laser Growth Of Silicon Ribbon", Proc. SPIE 0198, Laser Applications in Materials Processing, (24 January 1980);

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