Modelling of the transition from a planar faceted front to equiaxed growth: Application to photovoltaic polycrystalline silicon

Recent experiments under X-ray examination have shown that a transition from planar front to equiaxed growth is likely to occur in the case of faceted interfaces. Such a transition is suspected to be at the origin of the sudden occurrence of deleterious small grains, among large columnar grains, observed in photovoltaic silicon ingots.A model is presented for the occurrence of equiaxed grains observed ahead of a planar faceted interface. Simple expressions are obtained which predict when the equiaxed structures should develop, in the case of rough (thermal dendrite) and of faceted equiaxed grains. These models provide a semi-quantitative basis to discuss the Faceted front to Equiaxed structure Transition (FET). Then, it is applied to the case-study of photovoltaic silicon. Further developments are proposed to improve the model.