Investigation of iron contamination of seed crystals and its impact on lifetime distribution in Quasimono silicon ingots

The use of seed plates during directional solidification (DS) of Quasimono silicon ingots causes additional yield losses compared to standard multi-crystalline ingots due to an increased area of low minority carrier lifetime ("red-zone") in the bottom region. This effect is attributed in literature mainly to iron impurities which are contaminating the seed crystal during heat up and afterwards the as-grown ingot during solidification. However, the contamination mechanisms itself are still not completely understood. Therefore, in this work the seed contamination mechanisms by iron and their effect on the lifetime distribution in the bottom region of Quasimono silicon ingots were investigated. For this purpose special crystal growth experiments in a laboratory-scale DS furnace were carried out by using diffusion barriers at the crucible/seed and seed/melt interfaces in order to separate the different contamination paths. The results show that the iron diffusion path from the crucible into the seed plates plays an important role. But in addition to this it will be demonstrated that an even more important iron contamination path is by gas phase transport from furnace parts via the furnace atmosphere to the seed crystals.