5-FOLD TWINNED SILICON-CRYSTALS GROWN IN AN AL-16 WT-PERCENT SI MELT

A crystallographic study of a five-branched primary silicon crystal grown from & hypcroutectic A1-Si melt indicates that such crystals grow from a twinned decahedral nucleus. This nucleus morphology has been observed in experiments on vapour deposition of f.c.c. metals. The decahedron is an assembly of five silicon tetrahedra in twinned orientation, giving a particle containing five twin planes with a common ⟨110⟩ axis and with a surface composed exclusively of low-energy {111} facets. As the equi-axed particle grows it becomes unstable and each twin plane becomes the centre of a faceted branch growing by the twin plane re-entrant growth (TPRE) mechanism. A model for. the transition from equi-axed to branched growth is proposed, and it is suggested that, in a sodium-modified alloy, the primary silicon particles remain equi-axed because the sodium reduces the growth anisotropy required for the transition. © 1979 Taylor & Francis Group, LLC.