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Czochralski grown crystalline silicon: how to minimize and use material with structure loss
2003 (English)Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

The loss of full structure or single structure in silicon crystals (SL) causes great costs from losses of material and reduced quality. The price ranges from around 200$/kg for an Ok crystal to around 20$/kg for recycled material. In average about 15 kg may be lost from a SL ingot. This implies that small improvements can have a relatively large effect. An extensive study was carried out, in order to, come up with ideas of how to minimize SL, and find better use for SL material, in the local production of Czochralski grown ingots. With a stricter procedure for the growth of the neck or necking, the number of SL ingots may be reduced. A stable cooling system of the furnaces, where the Water Temperature and Water Flow, be hold at constant levels is likely to remove SL for a number of ingots. A dark room with high intensity light for investigation of slip for cut-away of the SL block, would ease the examination considerably and costs be reduced. The selling of SL blocks for solar cell manufacturing could lower the costs considerably. A price above that of SL material sold for recycling would be sheer gain. These proposals require relatively moderate investments or none at all, but require working hours to be set aside. To arrive at these proposals, data from parameters, during the growth of the crystals, were analysed and compared between SL growth and full structure (Ok) growth. The results from the Seedlift or pull rate of the ingot indicate that it may be possible to distinguish differences in the growth of ingots, as early as, in the necking stage, for a number of ingots. The Water Temperature showed abnormal behaviour for a number of SL ingot as compared to Ok ingots. Harvested ingots were studied with respect to crown and shoulder morphology. In order to succeed in the growth of an ingot, there are indications that the shape of the shoulder is important. The accuracy of the 30 mm cut, where the SL block is cut away, from the rest of the ingot, varied in relation to the point where slip could be detected. This implies that a better method for examining slip on the surface of the ingot directly after it has been harvested from the grower is desirable. From an SL block of material, aimed for the electronic industry (el grade), with relatively high resistivity, solar cells were made. The high resistivity solar cells reached efficiency of ca 12%, which was at the level with or higher than for the multi-crystalline wafers manufactured with the same procedure. The single crystalline wafers had a quality, high enough to be used for manufacturing while having a clear potential for higher efficiency. Ingots were cut, etched and checked for defects. In the examined material, the generation of defects decreases with a second degree polynomial from the point were SL is initiated, during the growth, and up into the sound part of the ingot. A theoretical thought experiment model for the creation of large defects, such as EPD is presented in Theory Of Defects. The use of a PV scanner for characterization of smaller defects would here have been desirable in order to see the interaction between the smaller defects and the larger visible defects.

Place, publisher, year, edition, pages
Keyword [en]
Technology, Physics, Czochralski, Defects, Crystalline material, Solar, cells, Structure loss
Keyword [sv]
URN: urn:nbn:se:ltu:diva-53053ISRN: LTU-EX--03/116--SELocal ID: a2043c6f-c719-4ed8-8792-29ef188d9534OAI: diva2:1026426
Subject / course
Student thesis, at least 30 credits
Educational program
Engineering Physics, master's level
Validerat; 20101217 (root)Available from: 2016-10-04 Created: 2016-10-04Bibliographically approved

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