The importance of convective heat and mass transfer for controlling material properties in ingot casting of multi-crystalline-silicon for photovoltaic applications

This contribution deals with the development of a species transfer model in the hot zone region of a laboratory scale crystal growth facility for the directional solidification of multi-crystalline silicon ingots with a diameter of 6 cm and a length of 4-5 cm in a Si3N4 coated silica crucible. A coupled local global modeling approach is used to compute the convective heat and mass transfer especially for C, N, O and the precipitate formation of SiC and Si3N4 during crystallization. The experimentally obtained data (C, N and O distributions) are 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