Modeling of convective eeat and mass transfer processes in crystal growth of silicon for photovoltaic applications

In this contribution a coupled local - global modeling approach is presented to compute the convective heat and C, N and O transfer and the formation of SiC and Si3N4 precipitates during crystallization of multicrystalline silicon ingots. Therefore, a local species transfer model of the hot zone region was developed and coupled to a global thermal model of the whole growth furnace. In a laboratory scale crystal growth facility multicrystalline silicon ingots with a diameter of 6 cm and a length of 4-5 cm were directionally solidified in Si3N4 coated silica crucibles. The experimentally obtained data (C, N and O distributions) were compared with numerical results. It will be shown that the amount of contaminants in the silicon ingot is mainly influenced by the initial feed stock quality, the Si3N4 coating, the incorporation of CO and the evaporation of SiO over the free melt surface. It will be demonstrated that melt convection plays an important role in the transport of the species in the silicon melt and that convection can explain the experimentally observed O, C, and N distributions as well as the formation of the SiC and Si3N4 precipitates.