多晶硅生长过程固液界面、应力、位错及熔体流动的变化规律

Variation of Crystal-Melt Interface,Stress,Dislocation and Melt Flow during Multi-Crystalline Silicon Growth

定向凝固(DS)法生产的多晶硅锭是光伏产业最基础、最主要的功能性材料之一,但实际生产中多晶硅复杂的工艺仍需进行优化。采用有限体积法对定向凝固法多晶硅生长过程进行瞬态全局模拟,数值模拟过程中将温度场、应力场和流场进行耦合,以研究多晶硅生长过程中固液界面波动率、硅锭内应力、位错密度以及硅熔体流动的变化规律。结果表明,长晶初期固液界面波动率较大,为29.83%,而长晶中期和末期界面波动率仅为6%和3.434%;且熔体流速也对固液界面影响较大,通过降低靠近固液界面处的熔体流动速度有利于形成平坦的固液界面;在生长过程中硅锭中最大位错密度和最大应力值随长晶时间变化趋势大致相同,控制和优化长晶后期的工艺参数可以有效减少多晶硅生长过程中应力值和位错密度,为多晶硅定向凝固过程预测和控制提供一定的理论支持。

Photovoltaic power generation was a clean and sustainable new energy utilisation technology and improving efficiency and reducing costs has always been the direction of effort for the photovoltaic(PV)industry.The production of multi-crystalline silicon by directional solidification(DS)was the most basic and main functional material for the solar photovoltaic industry.However,during the DS process multi-crystalline silicon was prone to impurities,stresses,grain boundaries and dislocations,which severely limited its conversion efficiency.It’s necessary to explain the influence of the melt flow pattern on the thermal field,crystal-melt(c-m)interface and impurity transport during multi-crystalline silicon growth.Transient global simulation of multi-crystalline silicon growth by coupling temperature,stress and flow fields used the finite volume method in combination with the crystal growth software Crystal Growth Simulator(CGSim),crystal growth process took 25 h and the transients were calculated using temperature control.Modeled with reference to actual production multi-crystalline silicon ingot casting furnaces.The dimensions of the quartz crucible were 840 mm×840 mm×420 mm,the height of the graphite crucible was 382 mm,the thickness of the side walls was 25.5 mm,the thickness of the bottom was 25 mm,the capacity of the silicon raw material was approximately 450 kg,the temperature of the outer wall of the furnace was set at a constant room temperature of 300 K,the argon flow rate was set at 12.9 L·min^(-1)and the temperature at the c-m interface was set at 1685 K for the silicon melting point.The temperature distribution during crystal growth of the DS process was controlled used Fourier's fundamental law of heat transfer.The variation patterns of multi-crystalline silicon c-m interface morphology,c-m interface volatility,furnace power,maximum ingot stress,dislocation density and melt flow rate at different growth stages were analyzed.The c-m interface showed a"W"shape at 4 h,with an interfacial volatility of 29.83%.As the solidification process proceeded,the fluctuation rate at the c-m interface remained basically flat at around 6%,and gradually became flatter and the fluctuation rate increased at 20 h,and 3.434%at 24 h.1~6 h was the pre-growth phase,the power needed to be reduced to lower the temperature of the silicon melt to achieve the required subcooling for crystallisation.6~20 h was the smooth growth phase,during which the power rose steadily to maintain the interfacial energy required for crystal growth,and 20~25 h was the late growth phase,when the power needed to be reduced significantly to release the latent heat of crystallisation at the c-m interface.For regard to the relationship between dislocation density and stress,the maximum dislocation density and stress values in the ingots followed approximately the same trend throughout the growth process,with dislocation density and stress values increased slowly from 1 to 23 h,and increased sharply in the later stages of growth.From 4 to 16 h,two clockwise vortices at the top and bottom and one large counterclockwise vortex in the middle existed in the silicon melt,and the free surface vortices gradually disappeared after 16 h.The melt flow rate also had a large influence on the c-m interface,with a melt flow rate of 2.4952×10^(-3)m·s^(-1) in the bottom vortex at 4 h,decreased to about 1.3965×10^(-3)m·s^(-1) at 8~16 h,increased to 2.219×10^(-3)m·s^(-1)at 20 h.The melt flow rate decreased substantially at 24 h compared to the previous time,wihch was probably due to the excessive impurity particles in the melt,which impeded the melt flow and led to a rapid increased in polysilicon stress and dislocations in the late crystal growth phase.The results showed:(1)The c-m interface fluctuation at the early and late stages of crystal growth was large compared to the mid-term,with a maximum value of 29.83%and an interface fluctuation of around 6%at the mid-term of crystal growth;(2)During crystal growth,dislocation density and stress values in silicon ingots followed approximately the same trend over time,i.e.stress was the main factor causing dislocation proliferation.The maximum values of dislocation density and stress increased abruptly after 23 h of crystal growth,increased by 407%and 1014%,respectively,by the end of crystal growth.The stress and dislocation density were distributed in an irregular laminar pattern from the bottom center to the top edge of the ingot,with dislocation density increased rapidly at the edge and bottom of the ingot at around 24 h of growth,with values above 3.13×10^(7)m^(-2).Therefore,to reduce stress and dislocation density during multi-crystalline silicon growth,more attention should be paid to the control and optimisation of the later stages of growth;(3)In the DS process,the flow of molten silicon during crystal growth had a strong influence on the thermal field,the c-m interface and impurity transport.From 8 to 16 h of crystal growth,the melt flow velocity in the bottom vortex was the smallest at 1.3965×10^(-3)m·s^(-1),a reduction of 44%compared to 4 h.The reduced melt flow velocity near the c-m interface was conducive to the formation of a flat c-m interface.