The solidification microstructure and crystal orientation were investigated for solar cell grade high purity multi-crystalline Si grown using a unidirectional solidification technique. The mechanism of twin growth on a re-entrant corner was also considered. A columnar structure was observed at solidification velocities of 1.25 to 30μm/s and a positive temperature gradient of 20K/cm in rod-like specimens grown in an electric resistance furnace. Over the solidification velocity range of 1.25 to 2.5μm/s, the grain size increased as solidification progressed, and large columnar grains contained many twin boundaries. However, the average grain size decreased as the solidification velocity increased and, at above a critical velocity of about 40μm/s, an equiaxed structure appeared in the central part of the specimens. Therefore, molten Si had to be solidified at a velocity below 2.5μm/s, where twins were always introduced into grains to obtain large columnar crystal grains. The undercooling for directional growth was less than 4K within the solidification range of 1.25 to 30μm/s. A model of 2-dimensional nucleation on the re-entrant corner was established, and the critical nucleus was estimated to be 70 to 80% of the radius of the general 2-dimensional nucleus. The nucleation undercooling on a surface containing twins also decreased to 70% of the general undercooling. The reduction of the critical radius and undercooling on the re-entrant corner could eventually influence the favored growth direction and the enlargement of the grain size.

Effect of Twin Growth on Unidirectional Solidification Control of Multicrystal Silicon for Solar Cells. H.Miyahara, S.Nara, M.Okugawa, K.Ogi: Materials Transactions, 2005, 46[5], 935-43