多晶硅真空定向凝固过程热质迁移及其对位错增殖的影响和调控研究

Improving the removal of impurities while realizing efficient ingot casting has recently been the focus and difficulty in preparing solar-grade polycrystalline silicon (SoG-Si) using vacuum directional solidification method. Based on the numerical simulation combined with experiments, a proper multi-field coupling mathematical model of heat transfer, melt flow, mass transfer and thermal stress is established, so as to study the heat-transfer characteristics during the growth of silicon crystal and the flowing behavior of silicon melts driven by various forces. The research aims to figure out the continuously migrating laws of the segregation, diffusion and volatilization process of impurities under the effect of thermal current. Then the restrictive link during the impurity removal process can be determined and meanwhile some improved measures will be proposed accordingly. The research studies the initial nucleation of silicon crystal and the formation and propagation of crystal defects such as dislocation during the growing process of silicon crystals. Besides, the influence laws of crystal defect density and its distribution on the electrical properties of silicon crystals are studied as well. Furthermore, the research discusses the collaborative behaviors of preferable orientation growth of polycrystalline silicon and the impurity removal. Additionally, the influence laws of materials characteristics, equipment structure and technological conditions on the removal efficiency of impurity and the quality of silicon crystals are studied. Eventually, the optimal matching principles of efficiently removing impurities and effectively improving the quality of silicon crystals can be obtained via the optimization and control of the vacuum directional solidification process. The research will reveal the synergistic effect mechanism of purification and growth of silicon crystals, breaking through the limitations on impurity removal and ingot casting in a single method, and realizing the effective impurity removal and efficient ingot casting, which will provide theoretical and practical support for the preparation of solar-grade polycrystalline silicon at lower costs and in shorter process as well.

如何在深度除杂的同时实现高效铸锭是目前利用真空定向凝固法制备太阳能级多晶硅技术研究的热点和难点。本项目采用数值计算与实验相结合的方法，建立真空定向凝固过程传热、流动、传质及热应力耦合数学模型，研究硅晶体生长过程的传热特性和多种驱动力下硅熔体的流动行为规律；研究热流作用下杂质分凝、扩散及挥发过程的连续迁移规律及影响因素，确定除杂过程的限制性环节，并提出改善措施；研究硅晶体初始形核、生长过程位错等缺陷的形成、增殖机理，探明晶体缺陷密度及分布对其电学性能的影响规律；研究硅晶体择优生长和杂质净化的协同行为，探讨物料特性、设备结构及工艺条件对杂质去除效率和晶体质量的影响规律，通过过程优化和调控，获得深度除杂和提高晶体质量的最佳匹配参数。本课题揭示提纯和晶体生长之间的协同作用机理，突破单一方法在除杂和铸锭上的限度，达到深度除杂和高效铸锭的目的，为低成本、短流程制备太阳能级多晶硅技术提供理论和应用支撑。

利用定向凝固技术去除硅中杂质、获取特定生长取向的多晶硅铸锭是冶金法生产多晶硅铸锭中必不可少的过程。真空定向凝固工艺通过杂质分凝、挥发、控制硅熔体凝固过程中晶体生长取向，达到除杂、消除缺陷的目的。.本项目首先建立多晶硅真空定向凝固过程中的温度场-流场-应力等多物理场耦合模型，研究了熔体Marangoni对流和流动行为对多晶硅传热特性的影响机制，并对Marangoni效应下不同拉速对温度梯度、固液界面形状影响研究，发现Marangoni效应增强了硅溶体的流动和对流换热系数，对其传热特性有显著影响。流速的增大使硅熔体的温度分布更加的均匀，径向温度梯度减小，导致了固液界面更加的平坦，有利于晶体柱状晶的生长。在拉速为10μｍ/ｓ时，硅锭质量较好，XRD检测发现，在整个凝固的过程中晶体（１１１）面是占绝对的生长优势。实验验证了模拟结果的可靠性。其次研究了保温温度对多晶硅真空定向凝固过程中铸锭内部的晶体生长取向、杂质去除率及杂质分布形态的影响。得出了拉速10μm/s、保温0.5h、保温温度1730K的条件下形成的多晶硅铸锭内部晶体生长取向、除杂效果、杂质分布形态等各项指标最优。另外，通过建立的多晶硅真空定向凝固过程的多物理场耦合热质迁移模型，研究了Marangoni对流对多晶硅铸锭质量的影响以及硅熔体中Al杂质元素扩散迁移的特性，并测试了样品的宏观形貌、杂质富集特性、晶粒尺寸以及少子寿命。在Marangoni对流作用下，硅熔体的温度梯度降低了4.8%-9.9%，硅固体的温度梯度降低了2.1%-2.6%，平均应力和最大应力都得到了一定程度的降低。硅熔体流动强度大于10-2m/s时硅熔体流动会显著增强杂质Al的输运，当凝固结束后，杂质Al最终在硅锭顶部靠近侧壁的位置形成富集。.本项目研究共发表论文9篇，其中外文期刊SCI收录6篇（其中JCR一区论文一篇，二区论文2篇），中文核心期刊论文3篇；云南省优秀硕士论文奖论文1 篇；申请国家发明专利3项、授权使用新型专利1项。3名科研骨干晋升职称，协助培养博士研究生1人，硕士研究生5人；参加国内外学术交流16人次。