Silicon nitride coating and crucible - Effects of using upgraded materials in the casting of multicrystalline silicon ingots

A method for removal of metallic contaminants from commercial Si3N4-powders has been developed. The method is based on acid leaching and shows promising results. Analyses show that the content of both intermetallic compunds as Fe, Ti and Al as well as 0 are significantly reduced. Clean, synthetic fused silica crucibles have been used along with normal sintered ones based on natural quartz. The crucibles were coated with normal, commercial silicon nitride powder and with purified Si3N4 in a cleanest possible environment. The crucibles were then used as vessels for directional solidification of multicrystalline silicon in a pilot scale furnace. The average lifetime of minority charge carriers in the cast silicon was determined by quasi steady state photoconductance (QSSPC)from the bottom to the top of the ingots. These varied in a systematic way, so that the materials cast in the pure environment had significantly higher values than the materials cast with conventional coating- and crucible materials. The maximum values for the lifetimes in the individual ingots varied from 7 to 135 mu s. Copyright (c) 2007 John Wiley & Sons, Ltd.