Removal of aluminum and calcium in multicrystalline silicon by vacuum induction melting and directional solidification

• Impurity behavior in Si was experimentally studied during refining and solidification process.
• The vacuum induction melting process was analyzed based on basic evaporation theory.
• The directional solidification was modeled by modified Scheil's equation, considering the effect of evaporation.
• The impurity distribution depends on the effect of segregation and evaporation.

A multicrystalline silicon ingot was obtained from metallurgical-grade silicon by vacuum induction melting and directional solidification. Based on the concentration distributions of aluminum and calcium along the growth direction, the removal mechanism of such impurities with both high saturated vapor pressures and low segregation coefficients is investigated. The results show that the removal of this type of impurities only depends on evaporation during vacuum induction melting process, thus their contents decrease significantly due to the strongly evaporation under the high temperature and high vacuum conditions. During subsequent directional solidification process, a model including both segregation and evaporation is used to simulate the concentration distribution. The results show that the impurity distribution is controlled by both two mechanisms in the initial stage of solidification and is mainly determined by segregation in the end stage due to the decrease of the diffusibility and evaporability of the impurity atoms.