Effect of Cooling Rate during Thermal Processes on the Electrical Properties of Cast Multi-Crystalline Silicon

Photoluminescence (PL) imaging techniques and the minority carrier lifetime test system were employed to investigate the variation of the interstitial iron (Fe-i) concentration, the recombination activity of structural defects and the minority carrier lifetime of cast multicrystalline silicon (mc-Si) in response to the cooling rate after heating. The results showed that when the mc-Si wafers are heated to high-temperature (1000 degrees C) and then cooled to ambient temperature with different cooling rate, the Fe-i concentration, the number of recombination active dislocations and grain boundaries increased as the cooling rate rises while the minority carrier lifetime decreased. If cast mc-Si is heated followed by faster cooling at 30 degrees C/s, the Fe-i concentration increase by 223% and the electrical activity of grain boundaries, dislocations and intragrain increase significantly, that is to say, the whole wafer is heavily contaminated with metal impurities, and present extremely low minority carrier lifetime.