Segregation, precipitation and dislocation generation between seeds in directionally solidified mono-like silicon for photovoltaic applications

Abstract

The generation of structural defects in directionally solidified mono-like silicon on a pavement of seeds has been investigated by synchrotron X-ray imaging, micro-FTIR mapping and electronic techniques. In particular, we analyse the region where the liquid Si penetrates between two seeds and we correlate the segregation and precipitation of impurities with the generation of cascades of dislocations during crystal growth. The solidified silicon grows epitaxially on the seeds without creating any distortion at the interface; however, due to the relative misorientation between the two seeds a highly and inhomogeneously distorted sub-grain boundary is created. Locally distorted zones, in particular linked to precipitates, are detected along and near the sub-grain boundary. The precipitates mainly consist of Si, C, N and O. Dislocations generated in these distorted zones propagate away from the sub-grain boundary towards the un-melted portions of the seeds, but they are blocked by barriers of precipitates formed at the positions of the initial seed surfaces. Higher in the ingot, bunches of dislocations propagate and multiply in the bulk.