Fragmentation of faceted dendrite in solidification of undercooled B-doped Si melts

Abstract

The fragmentation of the faceted dendrite of B-doped Si solidified from the undercooled melt was investigated using an electromagnetic levitator. The 〈110〉 dendrites, which grew at ΔT <∼100 K, never fragmented because they were composed of {111} planes with the lowest interface energy. On the other hand, the 〈100〉 dendrites, which grew at ΔT>∼100 K, showing fourfold axial symmetry, broke up into small pieces at undercoolings of more than 200 K. It was suggested that the capillary force acts on the interface with a relatively high energy to break up the dendrite into small pieces, since the 〈100〉 dendrites are composed of {110} and {100} planes with interface energies larger than that of the {111} plane. Moreover, striations of concentric circles formed by the segregation of B revealed that the remaining melt solidifies from the surface toward the center to engulf the fragmented dendrites. This solidification process seems different from those of typical metallic materials, in which the fragmented dendrites are randomly distributed throughout the sample and the remaining liquid solidifies from the fragmented dendrites. This solidification characteristic was discussed in relation to the influence of electromagnetic force on the microstructure of Si.