Research On Marangoni Convection And Impurity Migration Characteristics During Vacuum Directional Solidification Of Polycrystalline Silicon

The vacuum directional solidification process of polycrystalline silicon is a complex coupling process in which multiple physical fields interact and influence each other.The whole process involves a variety of physical processes such as heat transfer,melt flow,phase change,impurity diffusion,segregation,and stress proliferation.In this paper,the multi-physics coupling heat and mass transfer model of heat transfer-flow-mass transfer-thermal stress established during the vacuum directional solidification of polycrystalline silicon is used to simulate the influence of Marangoni convection on the quality of polycrystalline silicon ingots and the Al impurity elements in the silicon melt.The characteristics of diffusion and migration,and the macroscopic morphology,impurity enrichment characteristics,ingot grain size and minority carrier lifetime of polysilicon samples have been tested through experiments.Through the comparative study of simulation and experiment,a better process method for preparing polysilicon is obtained.The research work has certain guiding significance for the process improvement and parameter optimization of the vacuum directional solidification process of polysilicon.The results show that when the silicon melt is subjected to Marangoni convection,the average flow velocity and maximum flow velocity of the silicon melt increase by about 3 times and 4 times respectively.The strong flow enhancement brings about a great change in the heat transfer capacity of the silicon material and makes the temperature field more uniform.Under Marangoni convection,the temperature gradient of silicon melt decreased by 4.8%to 9.9%,and the temperature gradient of silicon solid decreased by 2.1%to 2.6%,and the average stress and maximum stress in ingot were reduced to some extent under Marangoni convection.The results of simulation and analysis of the experimental samples show that the stress,dislocation defects,impurity enrichment characteristics and grain size in the ingot will have a significant impact on the electrical properties of polysilicon ingots,especially the life of the minority.The results show that the flow strength of silicon melt is closely related to the transport effect of impurity al.When the flow strength of silicon melt is between 10^-3～10^-4m/s,the impurity transport caused by the flow of silicon melt is very small.When the flow strength of silicon melt is greater than 10^-2m/s,the flow of silicon melt will be significantly higher than 10^-2m/s When the directional solidification process is over,the impurity al finally forms enrichment near the side wall at the top of the ingot.