Growth mechanisms of modified eutectic silicon

Transmission electron microscopy has been used to study the growth mechanism of silicon in modified aluminium-silicon eutectic alloys. In agreement with earlier studies a high density of thin {1 1 1 } faults was observed in silicon modified by relatively large amounts of sodium or strontium. High-resolution microscopy showed that these faults were a mixture of thin twins and stacking faults, the inter-fault spacing being at least ten times the twin width. Because modified silicon is thought to grow by the twin re-entrant edge mechanism, the thin twins were examined in relation to the shape of the modified silicon dendrites and the solidified growth interfaces. It was concluded that the thin faults were not deformation twins resulting from either thermal contraction stresses or mechanical grinding during specimen preparation. No direct evidence was found to support the re-entrant edge mode of growth in modified silicon. Instead it is suggested that the interface may appear macroscopically non-faceted, as in the more general Lateral Microscopic Growth mechanisms.