Influence Of Marangoni Effect On Crystal Quality And Impurity Enrichment During Vacuum Directional Solidification Of Polysilico

Vacuum directional solidification method is a particularly important part in the preparation of polycrystalline silicon by metallurgical method,which involves heat transfer,melt flow,thermal stress,solid-liquid phase transition and impurity transport.This process is more complex,in an invisible,high temperature and vacuum environment,so numerical simulation has become an effective tool to study this process.In this paper,the Marangoni effect caused by the surface tension gradient of the melt is studied by establishing a global transient multi-physical field model of vacuum directional solidification of polycrystalline silicon.The effect of crystal quality and impurity enrichment and the law of impurity segregation and diffusion in the melt are studied.At the same time,the grain size,silicon ingot morphology,dislocation density,impurity content and existence form and resistivity value of the experimental samples are detected and analyzed.By comparing the simulation results with the experimental results,the method of improving the polycrystalline silicon ingot casting process is obtained,which has certain reference significance for improving the photoelectric efficiency of polycrystalline silicon solar cells.The results show that under the influence of Marangoni effect,the disturbance in the silicon melt is enhanced,the average flow velocity of the silicon melt increases by nearly three times,and the maximum flow velocity increases by four times or more.The decrease of the temperature gradient of the silicon melt is between 1.1%and 6.5%,and the decrease of the temperature gradient of the silicon solid is between 1.3%and 3.1%,which makes the thermal field of the silicon material more uniform during the whole casting process.The thermal stress is reduced by 8.7%-22.5%,and the stress range of the silicon ingot is about 3.36 MPa-96.7 MPa.The resistivity of the whole polycrystalline silicon increases first and then decreases along the axial direction.Combining the simulation results with the experimental sample test results,it can be seen that the crystal quality of polycrystalline silicon ingot is closely related to the factors such as grain growth,thermal stress,dislocation density and impurity content.In particular,thermal stress and impurity content have a significant effect on the resistivity of polycrystalline silicon.By establishing a Fe mass transfer model during directional solidification to study the effect of melt flow on the transport characteristics of Fe impurities,it can be seen that the transport characteristics of Fe impurities are closely related to the melt flow.It is found that when the flow rate of silicon melt reaches 10^-2 m/s,the strong flow in the melt will promote the transport of Fe impurity atoms.When the flow rate of silicon melt is in the order of 10^-4-10^-3 m/s,the weak flow in the melt has little effect on the transport of Fe impurity atoms.When the polysilicon ingot casting process ends,Fe impurities accumulate at the edge of the upper wall of the polysilicon ingot.