Characterization of polycrystalline Si silicon sheet grown by die casting combined with the Bridgman technique
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Characterization of polycrystalline Si silicon sheet grown by die casting combined with the Bridgman technique K. Saito, T. Iida, D. Akimoto, A. Nose, Y. Takanashi, S. Sakuragi1, H. Nanba1, G. Sakuragi1, and T. Shimazaki1 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 Die-casting growth was used for manufacturing the multicrystalline silicon sheet with a size of 100 x 120 x 0.5 mm. During the growth, incorporation of contaminants such as iron, cobalt, nickel and chromium was well suppressed. The average etch-pit density values ranged from 1x104 cm-2 to 4x106 cm-2 for growth rates of 5 to 60 mm/h, respectively. Measurement of minority-carrier lifetime bye microwave-photoconductivity-decay (µ-PCD) method was 0.5 µs for as-grown specimens, suggesting that defects and residual strain exist in the grown sheet. Moreover, post heat treatment at 1473 K reduced the etch-pit density and improved carrier lifetime up to 2.2 µs. INTRODUCTION During the past decade, photovoltaic power-generation systems have become widespread as consumer products. However, the reduction of the fabrication cost of their solar-cell module has not followed the plan as laid out. This seems mainly because of the low utilization coefficient of the crystal wafers grown for the photovoltaic cells. Namely, in the case of the cast method, which is the current dominant fabrication technique for the wafer production, more than 50% of the grown ingot is discarded during the sawing process. To avoid the slicing of the grown crystal, methods for forming direct sheet from molten silicon have been tried. Accordingly, edge-defined film-fed growth (EFG) and string ribbon methods, promising direct sheet formation, have been developed and have successfully made solar cells achieving conversion efficiency of 16 to 18% [1]. As one more promising growth technique for preparing multicrystalline silicon wafers, die-casting from molten silicon has been developed. This is based on “shaped crystal technology” that can grow crystals of various shapes in a crucible. This technique produces low-cost silicon due to the high utilization of the silicon feedstock and the absence of ingot sawing. Since molten silicon is one of the most reactive elements, it reacts with almost all elements and compounds resulting in stable ceramics, alloys, and many kinds of compounds. An important aspect of the die-cast growth method is to obtain a non-wetting (non-reactive) chemical condition for the molten silicon during growth at elevated temperature. We report here the results of a basic examination performed on multicrystalline silicon sheet by die-cast growth method.
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EXPERIMENTAL Crystal growth Multicrystalline silicon wafers were grown in a modified Bridgman electric furnace using the preparation sequences of the die-casting growth process as shown in Fig. 1. In brief, semiconductor-grade raw silico
