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Influencing the crystallization behaviour of multi crystalline silicon in a R&D furnace

Iven Kupka 1Christian Reimann 2Jan Seebeck 2Toni Lehmann 1Ulrike Wunderwald 1Jochen Friedrich 2Kaspars Dadzis 3Frieder Kropfgans 3Lamin Sylla 3

1. Technologiezentrum Halbleitermaterialien, Freiberg 09599, Germany
2. Fraunhofer Institut IISB, Schottkystr. 10, Erlangen 91058, Germany
3. SolarWorld Innovations GmbH, Freiberg 09599, Germany


For a further improvement of the industrial crystallization process of multi crystalline silicon for PV application it is necessary to gain a fundamental understanding of the initial nucleation process of silicon on the crucible bottom, the subsequent grain selection process and the final grain growth behaviour over the crystal height. These phenomena have to be understood to decrease the amount of recombination active regions especially in form of harmful dislocation clusters and therefore to increase the resulting solar cell performance.
For this reason a R&D crystallization furnace was developed at Fraunhofer THM, which offers different ways to influence the grain structure and the defect formation during directional solidification of multi crystalline silicon ingots in G1 crucibles.
It is well known that on the one hand the nucleation process can be affected by the thermal conditions occurring during the start of crystallization. On the other hand the thermal field can be influence by the use of a time-dependent magnetic field in an elegant way. Therefore, we studied the nucleation and subsequent grain growth process under the action of a special configuration of a time-dependent magnetic field. Based on 3D numerical simulations [1] a heater configuration consisting of 3 side heaters with a standard AC power supply (fixed frequency of 50 Hz) was implemented in a new R&D furnace which allows the solidification of multicrystalline silicon ingots in G1 crucibles. Different Lorentz forces, which can vary in direction and magnitude, are generated in the silicon melt by changing the phase shift between the side heaters. This leads to various axial and radial temperature gradients caused by various convection conditions in the melt. Experimental and numerical investigations show clearly that the highest Lorentz force density, generated with a phase shift at the side heaters of 0°, leads to a strong melt mixing and small temperature gradients, whereas a small Lorentz force density, generated with a phase shift at the side heaters of 180° increases the temperature gradients in the melt because of lower melt mixing.
It will be shown that the initial nucleus formation is affected by the used convection conditions. In addition to the level of melt mixing the influence of the cooling conditions and the properties of the crucible bottom on the initial grain growth will be addressed.


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Related papers

Presentation: Poster at 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17, General Session 4, by Iven Kupka
See On-line Journal of 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17

Submitted: 2013-03-28 16:28
Revised:   2013-03-28 16:41