冶金法提纯多晶硅过程中氮化硅涂层的研究
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摘要
无论是在太阳能电池行业还是半导体工业中,硅都是该领域中基础材料。由于铸造多晶硅成本低廉,目前已经取代单晶硅成为光伏市场的主要原料。多晶硅太阳能电池已经占据光伏市场的50%以上。冶金法是制备太阳能级多晶硅的一种新方法,具有成本低,污染小的优点,其制备过程中需要使用坩埚。工业中通常使用的坩埚是石英或石墨坩埚,熔融硅与坩埚接触时不可避免的产生反应粘连,同时坩埚中的杂质也会进入硅熔体中。高浓度的杂质,如C、O、Fe等将使少子寿命显著降低,影响电池性能。通常在坩埚上涂敷涂层使熔体与坩埚壁隔离,来减少反应粘连使铸锭顺利脱模,并阻止坩埚中的杂质在铸造过程中进入硅中。
Si_3N_4由于其自扩散系数低,耐高温,化学稳定性好,通常被用来作为多晶硅铸造中的涂层材料。氮化硅涂层的制备方法很多,但通常工艺复杂,成本高。关于氮化硅涂层对多晶硅影响的研究也比较少。因此,有必要研究工艺简单成本较低的氮化硅涂层制备技术和氮化硅涂层与多晶硅性能之间的关系。本文利用扫描电镜,电子探针,少子寿命测试仪等设备,重点研究了氮化硅涂层的制备工艺与脱模效果的关系,熔炼过程中氮化硅涂层反应机制,氮化硅涂层对多晶硅中杂质、晶体结构和少子寿命的影响。
研究结果表明:1、水,无水乙醇,PVA水溶液和PVP乙醇溶液与氮化硅粉组合可制备涂层,其中脱模效果最好的组成为:wt8%PVP乙醇溶液与60wt%Si_3N_4。不经过预处理去除PVA、PVP添加剂的氮化硅涂层更利于脱模。真空熔炼时使用惰性气体增大压强防止氮化硅涂层的分解,有利于脱模。2、温度升高氮化硅涂层的分解加剧,N扩散进入硅熔体达饱和形成新Si_3N_4晶核并长大,最终在接触面处形成由大颗粒Si_3N_4组成的连续层。3、氮化硅涂层使铸锭中杂质含量降低。氮化硅涂层能够阻止坩埚中杂质如C、O、P等进入Si熔体,同时Si_3N_4的分解和新Si_3N_4晶核的形成促进涂层附近的C、Fe、Ca杂质沉淀。SiC沉淀以Si_3N_4晶核为异质核心形成并长大。Fe杂质依附在氮化硅颗粒表面不饱和键或沉积在新Si_3N_4晶核缺陷处形成微米级FeSi_2颗粒。纳米级Fe和Ca沉积物在不均匀分布在涂层附近的SiC缺陷内。4、涂层和硅熔体内的Si_3N_4和SiC沉淀促进孪晶和非孪晶形成并阻碍晶界扩展,使晶粒细化。5、氮化硅涂层显著提高少子寿命。氮化硅涂层促进附近杂质沉淀导致铸锭距离边缘较近的位置杂质较少。同时引起晶粒细化抑制横向定向凝固,导致中心和边缘位置少子寿命高于它们之间的区域。
Silicon is one of the important fundamental materials in the modern semiconductor and solar cells industry.Currently,casting multicrystaUine silicon(mc-Si) has replaced monocrystalline silicon as the main photovoltaic materials due to its lower production cost.In the present mc-Si solar cells have occupied about two-third of the PV-market.Physical Metallurgical Method is a new method to fabricate solar grade mc-Si for its low cost and pollution.In the process of mc-Si purification by Physical Metallurgical Method,crucibles, typically graphite or quartz crucibles have been used as support of melting silicon.However, in the casting process,there are reaction adhesions on the interface of melting silicon and crucible.Moreover,high density of impurity,such as C,O,Fe,Ca,which plays a crucial role on the degradation of mc-Si solar cells performance,are usually incorporated through the contact of the melt with the crucible walls.Usually coating has been spayed on the crucible wall as release agent to avoid melting silicon contacting with the crucible wall directly, eventually decrease the reaction and impurity diffusion from crucible.
High purity Si_3N_4 has been used as typical coating materials in the mc-Si casting process for its low self-diffusion coefficient,good high temperature resistance and chemical stability. There are kinds of methods to fabricate Si_3N_4 coating;however,most of them have high cost and complicated process.Meanwhile,at present there is less number of investigations about the effects of Si_3N_4 coating on the mc-Si performance.Therefore it is necessary to study low cost and uncomplicated method for Si_3N_4 coating and the relationships between Si_3N_4 coating and mc-Si performance.In this study,scanning electron microscope(SEM),electron probe microanalysis(EPMA),minority carrier lifetime meter and etc have been used to analyze the relationship between the releasing effect of Si_3N_4 coatings and technological parameters of the coatings,the reaction mechanism of Si_3N_4 in the casting process and the effect of Si_3N_4 coatings on the impurity,mc-Si crystalline structure and life time of minority carriers.
The results are as follows:
1.Four kinds of solutions,water,ethanol,PVA-water solution and PVP-ethanol solution were mixed with different ratios of Si_3N_4 powder to fabricate Si_3N_4 coatings.The Si_3N_4 coating fabricated by the 60wt%Si_3N_4 suspension liquid(8wt%PVP-ethanol solution mixed with Si_3N_4 powder) represent the best releasing molds.No pretreatment to remove PVA and PVP favor mold release Argon atmosphere decrease the decomposition of Si_3N_4 by increasing pressure then favor mold release.
2.In the melting process,the dissolution of Si_3N_4 coating are constantly increasing with the increasing temperature and eventually continuous layer consisted by large Si_3N_4 particles forms on the interface of Si/Si_3N_4 with continuous diffusion of N from coatings into the melting silicon.
3.Si_3N_4 coating can decrease impurity content in mc-Si.Si_3N_4 coating can prevent impurities in crucible diffusing into silicon as insulating layer and stimulate impurities(C,Fe, Ca) precipitating near the Si/Si_3N_4 interface.The new Si_3N_4 nucleus act as nucleation sites for the heterogeneous formation of SiC particles.The diffusion of Si_3N_4 and new Si_3N_4 nucleus provide microdefects for micron-size FeSi_2 particles precipitation between Si and Si_3N_4 crystalline boundaries in Si/Si_3N_4 mixed layer.Fe and Ca-rich nanoprecipitations have inhomogeneously precipitated in the new SiC microdefects in silicon phase.
4.SiC and Si_3N_4 particles,near the coating and in the silicon ingots,stimulate formation of twin and untwined Si crystal,meanwhile they also prevent silicon grain boundary from extending,resulting in the grain refinement.
5.Si_3N_4 coating have significantly increased the lifetime of minority carriers by decreasing impurity content.Impurity precipitating near the coating rather than diffusing into silicon further lead to declination of impurity content at the position next to the area near the coating in silicon ingots,and inhibit effect of directional solidification by promoting grain refinement.Therefore the lateral distribution of minority carriers' lifetime represent high near the edge and center area,and low in the area between edge and center area.
Si_3N_4由于其自扩散系数低,耐高温,化学稳定性好,通常被用来作为多晶硅铸造中的涂层材料。氮化硅涂层的制备方法很多,但通常工艺复杂,成本高。关于氮化硅涂层对多晶硅影响的研究也比较少。因此,有必要研究工艺简单成本较低的氮化硅涂层制备技术和氮化硅涂层与多晶硅性能之间的关系。本文利用扫描电镜,电子探针,少子寿命测试仪等设备,重点研究了氮化硅涂层的制备工艺与脱模效果的关系,熔炼过程中氮化硅涂层反应机制,氮化硅涂层对多晶硅中杂质、晶体结构和少子寿命的影响。
研究结果表明:1、水,无水乙醇,PVA水溶液和PVP乙醇溶液与氮化硅粉组合可制备涂层,其中脱模效果最好的组成为:wt8%PVP乙醇溶液与60wt%Si_3N_4。不经过预处理去除PVA、PVP添加剂的氮化硅涂层更利于脱模。真空熔炼时使用惰性气体增大压强防止氮化硅涂层的分解,有利于脱模。2、温度升高氮化硅涂层的分解加剧,N扩散进入硅熔体达饱和形成新Si_3N_4晶核并长大,最终在接触面处形成由大颗粒Si_3N_4组成的连续层。3、氮化硅涂层使铸锭中杂质含量降低。氮化硅涂层能够阻止坩埚中杂质如C、O、P等进入Si熔体,同时Si_3N_4的分解和新Si_3N_4晶核的形成促进涂层附近的C、Fe、Ca杂质沉淀。SiC沉淀以Si_3N_4晶核为异质核心形成并长大。Fe杂质依附在氮化硅颗粒表面不饱和键或沉积在新Si_3N_4晶核缺陷处形成微米级FeSi_2颗粒。纳米级Fe和Ca沉积物在不均匀分布在涂层附近的SiC缺陷内。4、涂层和硅熔体内的Si_3N_4和SiC沉淀促进孪晶和非孪晶形成并阻碍晶界扩展,使晶粒细化。5、氮化硅涂层显著提高少子寿命。氮化硅涂层促进附近杂质沉淀导致铸锭距离边缘较近的位置杂质较少。同时引起晶粒细化抑制横向定向凝固,导致中心和边缘位置少子寿命高于它们之间的区域。
Silicon is one of the important fundamental materials in the modern semiconductor and solar cells industry.Currently,casting multicrystaUine silicon(mc-Si) has replaced monocrystalline silicon as the main photovoltaic materials due to its lower production cost.In the present mc-Si solar cells have occupied about two-third of the PV-market.Physical Metallurgical Method is a new method to fabricate solar grade mc-Si for its low cost and pollution.In the process of mc-Si purification by Physical Metallurgical Method,crucibles, typically graphite or quartz crucibles have been used as support of melting silicon.However, in the casting process,there are reaction adhesions on the interface of melting silicon and crucible.Moreover,high density of impurity,such as C,O,Fe,Ca,which plays a crucial role on the degradation of mc-Si solar cells performance,are usually incorporated through the contact of the melt with the crucible walls.Usually coating has been spayed on the crucible wall as release agent to avoid melting silicon contacting with the crucible wall directly, eventually decrease the reaction and impurity diffusion from crucible.
High purity Si_3N_4 has been used as typical coating materials in the mc-Si casting process for its low self-diffusion coefficient,good high temperature resistance and chemical stability. There are kinds of methods to fabricate Si_3N_4 coating;however,most of them have high cost and complicated process.Meanwhile,at present there is less number of investigations about the effects of Si_3N_4 coating on the mc-Si performance.Therefore it is necessary to study low cost and uncomplicated method for Si_3N_4 coating and the relationships between Si_3N_4 coating and mc-Si performance.In this study,scanning electron microscope(SEM),electron probe microanalysis(EPMA),minority carrier lifetime meter and etc have been used to analyze the relationship between the releasing effect of Si_3N_4 coatings and technological parameters of the coatings,the reaction mechanism of Si_3N_4 in the casting process and the effect of Si_3N_4 coatings on the impurity,mc-Si crystalline structure and life time of minority carriers.
The results are as follows:
1.Four kinds of solutions,water,ethanol,PVA-water solution and PVP-ethanol solution were mixed with different ratios of Si_3N_4 powder to fabricate Si_3N_4 coatings.The Si_3N_4 coating fabricated by the 60wt%Si_3N_4 suspension liquid(8wt%PVP-ethanol solution mixed with Si_3N_4 powder) represent the best releasing molds.No pretreatment to remove PVA and PVP favor mold release Argon atmosphere decrease the decomposition of Si_3N_4 by increasing pressure then favor mold release.
2.In the melting process,the dissolution of Si_3N_4 coating are constantly increasing with the increasing temperature and eventually continuous layer consisted by large Si_3N_4 particles forms on the interface of Si/Si_3N_4 with continuous diffusion of N from coatings into the melting silicon.
3.Si_3N_4 coating can decrease impurity content in mc-Si.Si_3N_4 coating can prevent impurities in crucible diffusing into silicon as insulating layer and stimulate impurities(C,Fe, Ca) precipitating near the Si/Si_3N_4 interface.The new Si_3N_4 nucleus act as nucleation sites for the heterogeneous formation of SiC particles.The diffusion of Si_3N_4 and new Si_3N_4 nucleus provide microdefects for micron-size FeSi_2 particles precipitation between Si and Si_3N_4 crystalline boundaries in Si/Si_3N_4 mixed layer.Fe and Ca-rich nanoprecipitations have inhomogeneously precipitated in the new SiC microdefects in silicon phase.
4.SiC and Si_3N_4 particles,near the coating and in the silicon ingots,stimulate formation of twin and untwined Si crystal,meanwhile they also prevent silicon grain boundary from extending,resulting in the grain refinement.
5.Si_3N_4 coating have significantly increased the lifetime of minority carriers by decreasing impurity content.Impurity precipitating near the coating rather than diffusing into silicon further lead to declination of impurity content at the position next to the area near the coating in silicon ingots,and inhibit effect of directional solidification by promoting grain refinement.Therefore the lateral distribution of minority carriers' lifetime represent high near the edge and center area,and low in the area between edge and center area.
引文
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