Crystal growth of photovoltaic polycrystalline Si 1-x Ge x by die-casting growth
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Crystal growth of photovoltaic polycrystalline Si1-xGex by die-casting growth H. Hirahara, T. Iida, Y. Sugiyama, T. Baba, Y. Takanashi, and S. Sakuragi1 Department of Materials Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda-shi, Chiba 278-8510, Japan 1 Union Material Inc., 1640 Oshido-jyoudai, Tone-Machi, Kitasouma, Ibaraki 300-1602, Japan ABSTRACT Coin-shaped multicrystalline Si1-xGex crystals were grown using a Brigdman method combined with die-casting growth. Si1-xGex alloy is known as a candidate material for producing Auger generation, which creates more than one electron/hole pair per absorbed photon. Since Si1-xGex alloy shows a complete series of solid solutions, precipitating crystals with a certain composition of silicon or germanium by conventional selective growth methods is burdensome. Using die-casting combined with Bridgman growth brought about Si1-xGex precipitation in a form completely different from that predicted by the Si-Ge phase diagram. By combining this growth with subsequent heat treatment of the precipitated Si1-xGex sample, Si1-xGex (x= 0.5 ± 3 %) could be obtained. Indirect band-gap energy was estimated by measuring room-temperature optical absorption coefficient of the grown samples. INTRODUCTION With the spread of solar-cell power generation, the demand for solar cells with higher conversion efficiency is increasing. Currently, prospective candidates for highly efficient (≥20%) solar-cell materials are seen to be gallium arsenide (GaAs) and chalcopyrite-type compounds such as copper indium diselenide (CuInSe2). However, these materials are not environmentally friendly photovoltaic semiconductors. That is, first, arsenic and selenium exhibit toxicity when they are emitted; second, indium is a scarce element. It is necessary that the materials which can be used for energy-conversion applications should be environmentally friendly semiconductors, namely, they possess the advantages of abundance of their constituent elements in the earth’s crust and non-toxicity of their processing by-products. Alloys of silicon and germanium (Si1-xGex) are important materials not only for microelectronic devices but also for solid-state power generators such as solar cells and thermoelectric devices. Chemical stability, mechanical strength at elevated temperatures, and a close match of the n-/p-type alloys in terms of their thermal and electrical characteristics enable better device operation with no noticeable variation in efficiency. Accordingly, Si1-xGex has been studied as a possible high-conversion-efficiency photovoltaic material. Such alloys can attain a conversion efficiency of 43% when the germanium content is about 60%. This is because of the carrier-multiplication mechanism that comes from the Auger generation [1]. However, because the Si-Ge system shows a complete series of solid solutions with a phase relationship, it is not easy to precipitate crystals possessing a certain composition of silicon or germanium selectively by using
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conventional Bridg
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