Abstract
This work presents β-Si3N4 as being the main nucleation site for multicrystalline silicon grown by directional solidification in crucibles coated with Si3N4-based coatings. We argue for initial nucleation occurring on the largest β-Si3N4 particles at very small undercoolings, in accordance with the free-growth model, with an increasing nucleation potency for smaller particles as the nucleation undercooling increases. A nearly continuous layer of large β-Si3N4 particles forms between the α-Si3N4 coating and the solidified silicon, confirming a transformation from α-to β-Si3N4 in the presence of liquid silicon. The layer appears to be in contact with the solidified ingot only at localized positions, and the apparent accumulation of parallel twins and other grain boundaries suggest that nucleation has occurred at these positions. We suggest that rapidly growing dendrites have a pronounced effect on the grain structure of conventionally grown multicrystalline silicon, and that their unpredictable occurrence limits the grain refinement potential for the most common growth conditions. However, it is also suggested that a certain degree of refinement can be achieved by utilizing a uniform distribution of large Si3N4 particles together with a slow cooling procedure. By engineering the size of the Si3N4 particles one should therefore be able to control the initial grain structure and to a certain degree tailor the final grain structure of the ingot.