Prof. Abdennaceur Karoui

       We chiefly used chemical etching and Nomarski microscopy, XRT, cross-section XRT , and LP-XRT, to reveal the micro-defect location as well as SEM and TEM imaging for detailed structural analysis of the defects. Also SEM-EBIC, Secondary Ion Mass Spectrometry (SIMS), micro-PCD, and Deep Level Transient Spectroscopy (DLTS). The raw material used by Astropower was recycled silicon and scraped wafers from microelectronics industry. Astropower has grown their silicon sheets on a belt furnace in a unique fashion. Melted silicon was dispensed on the furnace belt to form thin sheets; these were transported under the furnace zones on a graphite bed while the sheets remained shielded from the ambient with a pressurized nitrogen atmosphere. The formed multi-crystalline silicon sheets were rich in N, C, O, and other impurities, such as metals (remnants from the scrapped integrated circuit wafers). In this study we applied fundamental knowledge developed for NCZ silicon crystals through other research projects, to understand the defects in the silicon sheets. One result worth highlighting is the discovered extended ultra-clean denuded zones (DZ) formed in the bulk and near surface of these multi-crystalline silicon sheets. The DZ was dispersed over i) the vicinity of grain boundaries (GBs), where the DZ appeared unusually wide, and ii) the sheet sub-surface zone. This important result and the associated impurity gettering were explained theoretically through the modeling of the thermochemistry of atomic defects and light element impurity clusters.