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Crystal/melt interface morphology at grain boundaries of multicrystalline silicon

Kozo Fujiwara ,  Masaya Ishii ,  Kensaku Maeda ,  Haruhiko Koizumi ,  Jun Nozawa ,  Satoshi Uda 

Institute for Materials Research, Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai 980-8577, Japan


         The morphology of the crystal/melt interface during unidirectional solidifications affects the structures of materials. The generation of crystal defects, such as dislocations, twin boundaries, and grain boundaries, and the segregation of impurities are related to the morphology of crystal/melt interface. With the increasing importance of engineering metal-impurity nanodefects in multicrystalline silicon (mc-Si) to improve solar cell properties, the distributions of metal impurities in mc-Si wafers have been extensively investigated. A high metal impurity concentration has been shown to exist at grain boundaries without low coincidence site lattice (CSL) indices. However, there have been no convincing explanations why and how metal impurities segregate at grain boundaries owing to the lack of information on the effect of grain boundaries on atomic partitioning at a crystal/melt interface. In this study, we report how grain boundaries affect crystal/melt interface morphology on the basis of in situ observation.

          We successfully observed the grain boundaries at the crystal/melt interfaces during the unidirectional solidification of mc-Si. It was clearly shown that the morphology of the crystal/melt interface at grain boundaries is related to the grain boundary characteristics. Sharp and smooth grooves were formed at the crystal/melt interfaces at Σ 9, Σ 27 and random grain boundaries, whereas no grooves were formed at Σ 3boundaries. We explain the groove formation in mc-Si by considering the anisotropic crystal/melt interfacial energy of Si, which fully explains the reason why Σ3 boundary does not form a groove, and why the other grain boundaries form grooves at the crystal/melt interface. From these findings, we can explain why the degree of impurity segregation depends on the grain boundary characteristics by considering the impurity partitioning at the grooves at the crystal/melt interface.

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Related papers

Presentation: Oral at 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17, Topical Session 5, by Kozo Fujiwara
See On-line Journal of 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17

Submitted: 2013-03-30 00:41
Revised:   2013-03-30 00:42