Investigation on Quartz Crucibles for Monocrystalline Silicon Ingots for Solar Cells
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This thesis focuses on the bubble formation and growth in the quartz crucibles used to produce Czochralski (Cz) monocrystalline silicon ingots for solar cells production. Several experiments have been conducted to investigate how much the bubbles grow during the crystal pulling process, and which parameters contribute to their growth. Two different crucible types have been investigated, before use in the Cz process, after use and after several heat-treatments. From these crucibles, samples have been cut from three different positions along the crucible wall. These samples have been investigated by X-ray tomography, and the size, distribution and volume of the bubbles inside the crucible wall have been measured. It is observed that the bubbles grow in size and quantity during the Cz process. More bubbles are also detected closer to the inner part of the crucible wall after use in the Cz-process. Small bubbles that are below the detection limit of the X-ray equipment were analysed by scanning electron microscope. The high-temperature heat-treated samples formed a thick cristobalite-layer and many cracks. The lower temperature heat-treated samples formed less cristobalite and less cracks. Comparison was done between the two different crucible types and between the different heat-treatments. The different positions in the samples were also compared. These results indicate that there are some differences in quality between different crucible producers and that bubble formation and growth are significantly affected by the Cz process parameters, such as temperature and time.