A new form of impurity cluster in casting quasi-single crystalline silicon

Abstract

The casting quasi-single crystalline silicon technology is very promising in the development of high quality and low cost crystals for manufacturing photovoltaic devices. In this study, we find a new type of impurity cluster named as “black spot” in the casting quasi-single crystalline, which is different from the “shadows” caused by SiC/Si3N4 particles discovered in previous research. The “black spot” is systematically studied and characterized using inductively coupled plasma mass spectrometry (ICP-MS), photoluminescence (PL), and infrared (IR) detectors. The test results show that the “black spot” is composed of metal impurities. According to our analysis, the formation mechanism of the “black spot” may be caused by the accumulation of the metal impurities in certain region of the solid/liquid interface. These clustering impurities are not absorbed by enough grain boundaries in the casting quasi-single crystalline after being solidified into the crystal and thus exhibit as “black spot”. An optimization method is proposed in the direction of numerical simulation and verified by experiments to avoid the formation of the “black spot”.