2019 Fiscal Year Annual Research Report
Investigating the mechanism of silicon crystal growth from melt by in situ observation system
| Project/Area Number |
19J11516
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| Research Institution | Tohoku University |
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Principal Investigator |
胡 寛侃 東北大学, 理学研究科, 特別研究員(DC2)
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| Project Period (FY) |
2019-04-25 – 2021-03-31
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| Keywords | Crystal growth from melt / Silicon / grain boundary / Crystal/melt interface / Twin boundary |
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| Outline of Annual Research Achievements |
The objective of this study is to understand the fundamental mechanism of crystal growth from melt during the solidification of multicrystalline silicon. We used our in situ observation system consisting of a furnace and a microscope with high-speed camera to directly observe the crystal/melt interface. In this year, we focused on following topics: the effect of grain boundaries on instability at the crystal/melt interface. In this study, we investigated the instability of a planar crystal/melt interface including frequent Σ3, small-angle and large-angle grain boundaries during the unidirectional growth of Si. The evolution of each crystal/melt interface including the appearance of grain boundaries, was examined at low and high growth velocities. The experimental observations show that grain boundaries can lead to local morphological transformations during the progression of the crystal/melt interface at high growth velocities. The data also confirmed that the critical growth velocity for the appearance of instability at a crystal/melt interface including grain boundaries is significantly lower than that for a single crystal Si/melt interface. The result demonstrate that a localized negative temperature gradient can form at a grain boundary and that such gradients readily initiate the morphological transformation.
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| Current Status of Research Progress |
Current Status of Research Progress
2: Research has progressed on the whole more than it was originally planned.
Reason
Grain boundaries in mc-Si are very important because lattice defects, which interact with minority carriers, are typically located at grain boundaries. Therefore, the development and control of grain boundaries in mc-Si ingots is very important. In this year we focused on the effect of grain boundaries on instability at the crystal/melt interface. The experimental observations show that grain boundaries can lead to local morphological transformations during the progression of the crystal/melt interface at high growth velocities. The result demonstrate that a localized negative temperature gradient can form at a grain boundary and that such gradients readily initiate the morphological transformation. We published our result as a journal in Materialia.
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| Strategy for Future Research Activity |
After understanding the influence of grain boundary on morphology of crystal/melt interface, we have found the local thermal field play an important role during solidification. In next year, we will directly study the temperature field at crystal/melt interface by using IR image equipped on our in situ observation system. The defect formation like twin grain boundaries, faceted-faceted groove, dendrite growth etc. are strongly dependent on the undercooling. Investigating the temperature profile during solidification,we can understand the effect of thermal field on defect formation and build up a fundamental model for crystal growth from melt. This study can help to optimize the growth process to obtain better performance ingots.
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