Gettering Effect in Low and High Density Structural Defect Regions of the Cast Multi-Crystalline-Silicon Wafer
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Gettering Effect in Low and High Density Structural Defect Regions of the Cast MultiCrystalline-Silicon Wafer Yongkook Park, Jinggang Lu, and G. A. Rozgonyi Materials Science and Engineering Department, North Carolina State University, Raleigh, NC, 27695-7907 ABSTRACT Two cast multi-crystalline silicon (mc-Si) sister wafers before and after solar cell processing were investigated to explore the gettering effect in low and high density structural defect regions. For the processed wafer, the minority carrier recombination lifetime was correlated to the structural defect distribution. For the low density region of as-grown wafer, Cr impurities were 3.28x1013cm-3 and they were reduced to 1.74x1012cm-3 for the processed wafer. The isolated Cr impurities dissolved from the precipitates at 9000C which is the typical gettering temperature in the solar cell processing are getterd effectively in the low density regions. Our current understanding for the gettering effect in the high density regions is that the segregated Cr impurities at defect sites are not released to the silicon matrix at 9000C resulting in the poor gettering effect. INTRODUCTION Multi-crystalline silicon solar cells occupy the largest portion of Si cell production and a considerable amount of mc-Si is composed of cast mc-Si. Drawbacks of cast mc-Si are that the nucleation of dislocations is caused by the excessive thermal stresses due to the vertical temperature distribution during the growth and that impurities are incorporated into the ingot using lower-grade Si feedstock, silica crucible, graphite die for supporting crucible, and coating layer such as silicon nitride on the inside wall of the crucible for discouraging sticking. The distribution of dislocations is inhomogeneous because thermal stress exceeds the critical resolved shear stress (CRSS) in grains of particular preferred orientations. Those with slip directions along the shear locally yield and relieve stress by local generation of dislocations [1]. Dislocations and grain boundaries (GBs) are decorated with incorporated impurities by precipitation due to the slow cooling rate of solidification and/or segregation. Based on these facts, mc-Si includes the low and high density structural defect and impurities in the form of isolated impurities, impurities complexes, or precipitates acting as recombination centers which deteriorate the minority carrier recombination lifetime significantly. For the sake of the enhancement in the lifetime, phosphorus diffusion gettering and hydrogenation processes are applied to the solar cell substrate material. However, most regions of the processed wafer still show the low minority carrier recombination lifetime which means that gettering and/or hydrogen processes are ineffective in the large portion of mc-Si wafer. In this study, we investigated what is responsible for the poor gettering effect for mc-Si wafer. EXPERIMENT Two cast mc-Si sister wafers having the same grain structure which are adjacent in the
brick were used for this study. One was
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