Crystallization and re-melting of Si_1-xGe_x alloy semiconductor during rapid cooling

Si_0.7Ge_0.3 was prepared by rapid cooling (∼330 °C/min)to get the fine grain structure for high thermoelectric performance. The same material was prepared under slow cooling (1 °C/min)for comparative analysis. The results were out of our expectation as the EBSD pattern revealed almost same grain structure of both samples. To know the reason for the similar grain structures, crystal growth of Si_0.7Ge_0.3 sample was in-situ observed under rapid and slow cooling. During rapid cooling, initially grown fine dendrites were completely re-melted and recrystallized on further cooling. No re-melting was occurred in Si under rapid cooling. Due to the growth of Si-rich Si_1-xGe_x dendrites at high temperature the melt becomes Ge richer thereby initially grown fine dendrites were re-melted to reach equilibrium liquidus composition. Moreover, the similar grain structures of Si_1-xGe_x under rapid cooling as well as slow cooling was originated from the re-melting of initially grown fine dendrites and recrystallization at relatively low temperature.