Multicrystalline silicon (mc-Si) manufactured by a multi-stage solidification
control casting method has been characterized by the electron beam induced
current (EBIC) method. The average diffusion length of the ingot was over
250 µm, which was much longer than that of conventional mc-Si. The EBIC study revealed that
the electrical activities of grain boundaries (GBs) varied with the ingot position due to the
impurity contamination level. The main impurity detected was iron. The concentration of
iron in the central position was much lower than that at the bottom and top
positions. GBs in the central position showed no significant EBIC contrast at 300 K,
suggesting low contamination level. GBs in the top and bottom positions, however,
showed strong EBIC contrast at 300 K, suggesting high contamination level. At
100 K, a denuded zone with bright contrast developed around GBs in the top and
bottom positions. The existence of the denuded zone suggested that impurities were
gettered at the GBs. It was considered that the variation of the diffusion length
in the ingot was related to the variation of recombination activities of GBs in
the different positions, which mainly depended on the impurity contamination.
The solidification microstructure and crystal orientation have been investigated for solar cell grade high purity multicrystal silicon through a unidirectional solidification technique. The mechanism of the twin growth on a reentrant corner has been also discussed. A columnar structure is observed at solidification velocities of 1.25-30 mm/s and positive temperature gradient of 20 K/cm in the rod-like silicon specimens in an electric resistance furnace. In the solidification velocity range of 1.25-2.5 mm/s, the grain size enlarges as solidification progresses. Furthermore, large columnar grains contain many twin boundaries. However, the average grain size decreases as the solidification velocity increases and above the critical velocity around 40 mm/s, equiaxed structure appears at the central part of specimens. Therefore, molten silicon must be solidified at the velocity below 2.5 mm/s where twins are always introduced into grains to obtain large columnar crystal grains. The undercooling for directional growth is less than 4 K in the solidification velocity range of 1.25-30 mm/s. A model of two-dimensional nucleation on the reentrant corner was established, and the critical nucleus could be estimated to be 70 to 80% of the radius of the general two-dimensional nucleus. The nucleation undercooling on the surface containing twins also decreased to 70% of the general undercooling. The reduction of the critical radius and undercooling on the reentrant corner could eventually influence on the priority growth direction and the enlargement of the grain size.
The solidification microstructure and crystal orientation have been investigated for solar cell grade high purity polycrystalline silicon through a unidirectional solidification technique. In the solidification velocity range of 1.25 -2.5 X10 -6 m/s, the grain size enlarges as solidification progresses. Furthermore, large columnar grains contain many twin boundaries. However, in above the critical velocity around 40 X10 -6 m/s, equiaxed structure appears. A model of two-dimensional nucleation on the reentrant corner was established, and the critical nucleus could be estimated to be 70 % to 80 % of the radius of the general two-dimensional nucleus. The reduction of the critical radius and undercooling on the reentrant corner could influence on the priority growth direction and the enlargement of the grain size.
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