Elimination of impurities from the surface of silicon using hydrochloric and nitric acid

Acid leaching of silicon is insufficient in order to achieve solar grade silicon. Leaching of silicon previously purified by the copper gathering method can significantly reduce amount of impurities congregated in the Cu–Si intermetallic phase during solidification process. Two samples of 50 wt% Cu–50 wt% Si alloy were solidified at 0.5 and 1.0 °C/min cooling rate. They were treated with 10 vol% HNO3 and 5 vol% HCl and 7 vol% HNO3. The inductively Coupled Plasma Mass Spectrometry technique was employed to measure traces of impurities before and after the treatment. It was determined that the overall impurity level in purified silicon was reduced from 5277 ppmwt to 225.5 ppmwt. The samples cooled at 0.5 °C/min achieved lower impurities levels in all instances while the sample leached with 10% HNO3 produced the greatest reduction in impurity level. Scanning electron microscopy and Energy Dispersive X-Ray Spectroscopy analysis showed that the traces of Cu–Si intermetallic together with gathered impurities can be found only in the large silicon particles after the acid leaching treatment. In all instances, the surface of the silicon particles was free of impurities while Si yield was preserved at above 97%.