We investigated the pattern formation mechanism of a periodically faceted crystal-melt interface during the crystallization of Si by in situ observation It was directly proved that spacing between the reentrants of adjacent zigzag facets increases with the unification of adjacent facets when a facet with a higher growth velocity catches up with the one with a lower growth velocity The spacing becomes stable after unification and the stable spacing was found to increase with increase in growth velocity The experimental results was discussed by taking the negative temperature gradient in front of the growth interface into account (C) 2010 Elsevier B V All rights reserved