坩埚表面Si₃N₄改性对定向凝固提纯多晶硅性能影响研究

【作者】 张聪；

【导师】 马文会；

【作者基本信息】 昆明理工大学 ， 有色金属冶金， 2012， 硕士

【摘要】 在目前,随着化石能源的枯竭,新能源的开发与利用已成为世界研究的热点。光能以其储量巨大、来源广泛、环境友好等优点被越来越多的研究与利用。硅材料作为光伏产业的重要原料,每年的消耗量非常巨大。通常在太阳电池材料中所用的硅材料分单晶硅和多晶硅两种,与单晶硅相比,多晶硅以其品质高、成本低等优点,在光伏产业中占据了大部分市场。在太阳能级多晶硅的诸多制备方法中,冶金法中的定向凝固提纯新工艺具有成本低、工艺简单、建设投资少、对环境的污染小等优点受到广泛关注,迄今为止已经取得了很大的成就与进展。但该工艺所用坩埚中含有较多的杂质,这些杂质与晶粒间的晶界和位错、缺陷等交互作用,严重影响了多晶硅材料的质量和后期太阳电池的转换效率。减少石墨坩埚中杂质对硅料的污染是获得高品质太阳能级多晶硅的关键之一。为了提高定向凝固提纯多晶硅材料的性能,本文采用高纯氮化硅对坩埚表面进行改性,利用金相显微镜,X射线衍射(XRD),扫描电镜(SEM),二次离子质谱仪(SIMS),四探针电阻测试仪等检测设备,研究了定向凝固过程中不同冷凝速率条件下,坩埚表面改性对小试样品中宏观生长形貌、缺陷密度、位错密度、晶界类型及数量、晶粒尺寸、电学性能的影响。同时还利用不同纯度的原料,不同种类的氮化硅涂层在最佳冷凝速率条件下对冶金法多晶硅铸锭的电学性能影响,取得了以下进展：坩埚表面改性可明显改善定向凝固提纯多晶硅的综合性能。宏观生长形貌体现在晶体垂直于底部生长。当冷凝速率为20gm/s时,所得多晶硅铸锭的性能最佳,其中,铸锭中部切片晶粒尺寸由633.2gm提高到921μm,晶体垂直生长的趋势也明显增强；硅片表面的点缺陷密度降低为7.8×103个/cm2；位错密度呈“V”型分布,其数量由原来的5～40×104个/cm2降低到1-30×103个/cm2；晶粒间晶界类型均为大角度晶界,并且以R和CSL中的∑3类型晶界为主。其中∑3(111)类型晶界约为67.4%,∑3(211)类型晶界约为12.2%；晶体的择优生长面为(111),(311)、(422)、(533)等非择优生长取向得到了很好的消除；少子寿命由原来的0.81μs提高到1.89μs；电阻率最高值由原来的110mΩ·cm提高到227mΩ·cm。除此之外,本文还对不同纯度的原料及不同类型氮化硅涂层坩埚在冷凝速率为20μm/s时所得铸锭的电学性能做了比较验证。结果表明：纯度最高的1号原料在本实验室自制的经表面改性处理的坩埚内提纯后的电学性能最高,少子寿命值最高为1.89μs、电阻率最高为227mΩ·cm,由此可以得出,杂质含量的高低为多晶硅材料电学性能最主要的影响因素；坩埚经烧结后所得多晶硅铸锭的电学性能得到了明显提高,少子寿命值最高为1.89μs、电阻率最高为227mΩ·cm。更多还原

【Abstract】 With the depletion of fossil energy, the development and utilization of new energy has been becoming a hotspot of the whole world. The research and utilization of photoenergy is attracting more and more attention for its many advantages, such as huge reserves, easy accessibility, environmental friendliness, etc. The annual consumption of silicon is very large as the main raw material of the photovoltaic industry. Usually, the silicon used in solar cell can be classified into monocrystalline silicon and multicrystalline silicon. Compared with monocrystalline silicon, multicrystalline silicon occupy most of the photovoltaic market for its high quality and low cost. A new metallurgical method of directional solidification purification process for solar-grade multicrystalline silicon production has attracted many attentions for its low cost, simple process, less investment, and environment pollution, etc., and great achievements and progresses have been made. However, the crucibles used in this metallurgical process usually contain many different impurities, the interaction of these impurities with the grain boundaries dislocations and defects of the multicrystalline silicon would decrease quality of the material, and finally the conversion efficiency of the solar cell. So the reduction of the pollution from the impurities is one of the keys to high-quality solar-grade multicrystalline silicon.In order to improve the performance of the multicrystalline silicon material prepared by directional solidification purification, the surface modified graphite crubcibles by high-purity Si3N4were used in the progress. The affect of the surface modification on the many properties was investigated at different condensation rates, such as the morphology, density of defects, density of dislocation, boundary type, grain size and electronic properties, with the characterization means of optical microscopy, x-Ray diffraction, SEM, secondary ion mass spectroscopy (SIMS) and glow discharge mass spectroscopy (GDMS). And the affects of the purity of the raw materials and the types of Si3N4were also studied at the optimal condensation rate. The main results as follows:The surface modification can significantly improve the silicon’s performance.. The grains all grew normal to the bottom plate of the crucible. When the condensation rate was 20μm/s, the best performances of multicrystalline silicon ingots were obtained. The grain size from the central part of the ingots slice grew up from633.2μm to921μm, and the trend of the growth orientation was significantly enhanced; the density of point defects on the slice surface reduced to7.8×103/cm2, and the dislocation density shown a "V" shaped distribution with the lowest value of1～30×103/cm2. The grain boundaries belonged to the large-angle type, and the main type was of∑3in the R and CSL. The∑3(111) type accounted for about67.4%, and∑3(211) type accounted for about12.2%; preferential growth surfaces of the crystal were (111),(311),(422), and(533). Other non-preferential growth orientation has been perfectly eliminated. The average minority carrier lifetime increased from the0.81μs to1.89μs; the maximum resistivity increased from110mΩ·cm to227mQ-cm.In addition, the affects of the purity degree of raw materials and the types of Si3N4at the condensation rate of20μm/s was studied as the control experiment. The results showed that:the electrical properties from the raw material No.1with the highest purity degree got the best, the highest average minority carrier lifetime approached1.89μs, and the resistivity approached227mΩ·cm. It thus came to a conclusion that the level of impurities in the raw material was the most important factor on the electrical properties; the electrical properties of the ingots all increased with the modification of Si3N4to graphite crucibles.更多还原