The influence of growth rate on the formation and avoidance of C and N related precipitates during directional solidification of multi crystalline silicon

SiC and Si3N4 precipitates in multi-crystalline (mc) silicon for photovoltaic application have detrimental effects on wafer sawing process and solar cell performance. In this study the influence of the growth rate on the incorporation of carbon and nitrogen and the SiC/Si3N4 precipitate formation during directional solidification of mc-silicon was investigated. Cylindrical silicon ingots with 6 cm diameter and 4–5 cm length were grown in a laboratory scale vertical gradient freeze (VGF) furnace at different growth rates R (R=0.2, 1.0 and 2.2 cm/h) and characterized by infrared-transmission (IR-TM), FTIR-spectroscopy and lateral photovoltage scanning (LPS). The results show that the growth rate R is influencing the shape of the phase boundary, the distribution of carbon and nitrogen in the silicon melt and crystal, and the formation of SiC and Si3N4 precipitates. It will be shown that an improved crystallization process with increased convective transport in the melt leads to precipitate-free crystals even at high growth rates.