CZ和CCZ法生长太阳能硅晶体的数值模拟对比研究
摘自IWMCG-8会议COMPARISION OF CZ/CCZ SILICON INGOT GROWTHPROCESS FOR SOLAR APPLICATIONBY NUMERICAL SIMULATIONSCZ和CCZ法生长太阳能硅晶体的数值模拟对比研究X.L. Deng, K. Cao, J. Wang, Z.H. Wang, L. Wu*State Key Laboratory of Advanced Special Steel, Shanghai University,Shanghai 200072, ChinaThere will be strong demand of cost-effective solar cells with high efficiency based on monocrystaline N type silicon wafers into the future , and it is assumed that the Continuous Czochralski (CCZ) single crystal growth method would be a prospectiveway to grow high quality and low cost silicon ingot for high efficiency N type solar cells. As an improved conventional Czochralski (CZ) process, the origin of the CCZ technique can be traced back to as early as the 1950s, and different variants have been developed in the past decades. However there are still challenges remaining to be solved for CCZ process in order to fully take advantages of this technique, such as long enough lifetime crucibles suitable for multiple ingot growth, impurities introduced by feeding system and materials, lower growth rate compared with CZ etc.
In this paper, one CZ hotzone and one CCZ hotzone targeted for 8inch silicon ingot are designed in order to investigate the respective process differences. These two hotzones are exactly the same in size except that the CCZ one has one additional internal crucible. A serial of simulations by FEMAG software are performed with the same corresponding operation conditions, and comparison is conducted based on the results obtained.
Figure 1 presents the global temperature distribution of CZ and CCZ growth processes at crystal length 200 mm taking into account the gas convection. All operation conditions are chosen to be exactly the same for both processes. The computed heater powers for the two growth processes are 52.82 kW for CZ and 54.08 kW for CCZ. It can be observed clearly from Figure 2 and Figure 3 that the temperature distribution in the silicon melt in CCZ process is more uniform and stable beneath the growing crystal, and that the temperature gradient growing along the crystal surface is slightly smaller than that of CZ process. In the meantime, for CCZ process the silicon melt convection underneath the crystal is damped and the interaction between the main vortex and the vortex along the inner crucible is suppressed, although the vertex along the outer crucible is much stronger than that of CZ process.
Figure 4 compares the melt/crystal interface shape and the temperature gradient along the melt/crystal interface of the two processes under various crucible/crystal rotation rates. It is found that for CCZ process under all specified crucible/crystal rotation rates, the melt/crystal interface shape is more concave to the crystal than that of CZ process. Correspondingly, the temperature gradient along the melt/crystal interface for CCZ process is much lower than that of CZ process, which implies that for the CCZ growth process, the growth rate is lower in order to maintain the same melt/crystal interface as that of CZ process
References: Intl. Tech. Roadmap for Photovoltaic (ITRPV) 2014 Results, http://www.itrpv.net/ I.H. Jafri, V. Prasad, A.P. Anselmo, K.P. Gupta, J. Cryst. Growth, 1995, 154:280 .
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