Multiple Czochralski growth of silicon crystals from a single crucible

Abstract

Several economic analyses on the effectiveness of the Czochralski crystal growth process to meet the cost goals of terrestrical photovoltaic arrays have indicated that serial ingot growth with periodic melt replenishment is required. This study demonstrates the feasibility of one such approach. A novel Czochralski grower utilizing the latest technology has been designed and built for th low-cost production of 100 kg of single crystal silicon ingots from a single crucible. The advanced features of the grower include installation of a properly functioning isolation valve which separates the growth chamber from the pull chamber, a hot melt replenishment system, a dopant fixture, capacity to accomodate 35 cm diameter crucible, and a cable mechanism of pulling and simultaneously weighing over 30 kg of silicon crystal. After the growth of a crystal, the recharge system lowers the polysilicon feed stock as it melts into the residual melt in the same crucible, and a weight measuring device in the recharge system allows correct amount of recharge to be melted-off to replenish the melt. Several ingots have been grown to evaluate and to demonstrate the capabilities of the grower. For example, a 22 kg single crystal has been obtained for the first time from a 30 cm diameter by 22 cm high crucible. Multiple silicon ingots of total weight of 100 kg from up to four successive melts have been grown by sequentially replenishing the crucible with polysilicon feedstock. Results of the performance of the solar cells fabricated from silicon crystals grown by using the grower are in good agreement with a calculation due to Davis et al. based on an impurity effects model. The growth technique presented here is expected to have a significant impact on the nation's low-cost silicon solar array program.