Abstract
Silicon nitride is an alternative material to the widely used silica crucibles for directional solidification of mc-Si ingots, its main advantages being the reusability in successive castings and elimination for a source for oxygen contamination of the ingot. In this work, several ingots were cast in these crucibles and compared to reference ingots cast in silica crucibles. The thermal properties of the Si3N4 crucible differ from those of the SiO2 crucible and lead to a different thermal history during melting and casting. The oxygen contamination of the ingot was observed to depend mainly on the melting and holding temperature, rather than on the crucible material. The lowest oxygen concentration was observed in the ingots with the lowest melting temperature. However, the thermal properties of the Si3N4 crucible influence the oxygen profile along ingot height, with a faster decrease in the concentration with increasing ingot height. This is believed to be due to a different mechanism for oxygen transport compared to that of the silica crucibles. The concentration of dopants in the ingots showed that contamination from the Si3N4 crucible occurred, probably due to diffusion of B- and P-oxides into the Si melt.
