Direct production of silicon crystals for solar cells from the melt of silicon alloys using self-shape maintaining characteristics of a crystal

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The research was aimed to demonstrate the possibility of the new production process of plate or rod silicon crystals by crystal pulling from the melt of silicon-based alloys, which would be conducted at low temperature and need less energy consumption than usual processes. As model experiments, the crystal pulling was performed using Si-45wt%Ni alloy. The preferred orientation for silicon crystal growth at low undercooling was a <211> crystallographic direction and a seed crystal plate with a <211> direction was used in the experiments. When the melt was undercooled, facet dendrites grew along the free surface of the melt and thus it was difficult to grow a plate silicon crystal. When the melt was slightly superheated, silicon crystals epitaxially grown to the seed crystal were successfully pulled up under the conditions of superheating from one to five degrees Celsius and pulling velocity under 0.015 mm per minute. The obtained crystals were classified into three types of needle-, rod- and corn-shape-crystal in their shapes. Some of needle- and rod-crystals contained Si-Ni eutectics inside. The temperature distribution estimated in the seed and the growing crystal showed that the crystal was grown in thermally steady state. The wetting behavior of the melt to the crystal played an important role for the stable crystal pulling. The examination of the growth condition in detail showed that a silicon multi-crystal was stably grown in the range of growth velocity under 0.03 mm per minute. From the crystallographic examination revealed the re-entrant corner mechanism was dominant in the crystal pulling process. It was concluded that the crystal pulling from the melt of silicon-based alloys was practically possible. It was also suggested that a new solar cell system would be possible by combining a rod-crystal and a parabolic mirror.