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The crystal growth phenomena during melt growth processes of Si including the morphological transformation of crystal-melt interfaces and faceted dendrite growth will be discussed, because the fundamental understanding of crystal growth mechanisms is crucial to develop crystal growth technologies for obtaining high-quality multicrystalline Si (mc-Si) ingots for solar cells.The multicrystalline Si (mc-Si) which is produced by casting is one of the most important materials along with the Si single crystal (sc-Si) for the substrate of solar cells in the future, although other materials are being developed.Various types of defect included in an mc-Si ingot, such as grain boundaries, dislocations, sub-grain boundaries, and metallic impurities, affect the properties of solar cells.Therefore, there is increasing importance to control the macro-and micro-structures of mc-Si ingots.Therefore, we have investigated crystal growth mechanisms of Si during melt growth processes.An in-situ observation system, which consists of a furnace and a microscope, was used for investigating crystal growth behaviors from Si melts.The crystal-melt interface during unidirectional growth is clearly observed by using this system.It was shown that the morphology of the crystal-melt interface was changed from planar to zigzag facets when the crystal growth rate was higher than a critical value.Such a zigzag faceted interface is formed at high growth rate, because the perturbation introduced into a planar interface is amplified when the temperature gradient becomes negative owing to the latent heat of crystallization.It was also found that the segregation behavior of impurities at the crystal-melt interface is dependent on the morphology of the interface.When the interface becomes zigzag facets, the impurities locally segregate at the bottom of the zigzag facets.The growth behaviors of Si dendrite crystals were also observed.The unique features of dendrite crystals are available to grow a structure-controlled mc-Si ingot for solar cells.A mc-Si ingot with large-size grains oriented in specific orientation is obtained by the initiation of dendrite growth in the earlier stage of casting.