铸造多晶硅材料中氧缺陷的研究

【作者】 俞征峰；

【导师】 杨德仁；

【作者基本信息】 浙江大学 ， 材料物理与化学， 2004， 硕士

【摘要】 目前，铸造多晶硅材料已经取代直拉单晶硅成为最主要的太阳能电池材料，但是铸造多晶硅电池的转换效率略低于直拉单晶硅太阳能电池的转换效率，这主要是由于铸造多晶硅中存在着高密度的缺陷和高浓度的杂质，如晶界、位错、氧、碳等。其中，氧是最主要的有害杂质元素之一。在材料生长过程中或电池制备过程中，过饱和的氧会在硅中形成热施主，新施主和氧沉淀等，而这些缺陷会显著降低太阳能电池的转换效率。因此，研究铸造多晶硅中氧关缺陷有着很重要的意义。本文在综述前人工作的基础上，采用四探针电阻率测试仪，光学显微镜，透射电镜和傅立叶红外光谱测试仪等较系统地研究了铸造多晶硅中热施主和氧沉淀规律。 热施主实验发现，初始氧浓度高，位错密度高，碳含量低的样品中，所形成的热施主多。这说明碳对热施主的形成有抑制作用；位错对热施主的形成有促进作用；晶界对热施主的形成没有明显地影响，而且碳和初始氧浓度的影响最大。实验还发现，在650℃半小时退火处理可以消除一部分原生热施主。和单晶硅一样，450℃左右为铸造多晶硅的热施主的最佳形成温度。原生热施主对于随后的热施主的形成规律没有明显影响。与常规热处理工艺相比，快速热处理能有效消除铸造多晶硅中的热施主。而且RTP的温度越高，热施主的消除效果越好。 其次，研究了铸造多晶硅中氧沉淀的形成规律。结果表明，单步热处理工艺下铸造多晶硅中氧沉淀的形成规律和单晶硅的基本相似，高初始氧浓度的铸造多晶硅中所形成氧沉淀量较大，在1050℃温度附近单步退火热处理所形成的氧沉淀量最多。但是对比初始氧浓度相当的铸造多晶硅和直拉单晶硅，多晶硅中所形成氧沉淀的量明显高于直拉单晶硅中氧沉淀的量。其次，在含高密度位错的单晶硅中所形成氧沉淀的量远高于无位错单晶硅中氧沉淀的生成量，而有晶界的多晶硅中所形成氧沉淀生成量仅稍微高于无晶界单晶硅中氧沉淀生成量。以上结果说明铸造多晶硅中位错对氧沉淀的形成有明显的促进作用，而晶界则对氧沉淀的促进作用不是很显著。在两步退火实验中，我们发现低温预退火有助于高温下氧沉淀的生成。另外，与直拉单晶硅淀中氧沉淀规律不同是铸造多晶硅中的氧沉淀一般无二次缺陷，这与晶界和位错吸收自间隙硅原子从而促进氧沉淀有关。更多还原

【Abstract】 Currently, cast multicrystalline silicon has replaced monocrystalline silicon as the main photovoltaic materials. However, the efficiency of cast multicrystalline silicon solar cell is lower than that of monocrystalline silicon due to a high density of defects and a high concentration of impurities, such as grain boundaries, dislocations, carbon and oxygen. As one of the main and detrimental impurities, the super-saturation of oxygen may form thermal donors, new donors, precipitates during the crystal growth or the cell fabrication processes. Therefore, it is very important to investigate the behavior of thermal donors and oxygen precipitation in cast muliticrystalline silicon. On the basis of reviewing previous work, the formation behavior of thermal donors and oxygen precipitation in cast mulitcrystalline silicon was systemically studied by means of four-proble resistance measurement, Fourier transform infrared spectroscopy (FTIR), Transmission Electron Microscopy (TEM) and Optical Microscopy (OM).It was found that high concentration of thermal donors formed in the specimens with high concentration of oxygen, high density of dislocation and low concentration of carbon. The result above indicates that dislocations enhance the formation of thermal donors and grain boundaries hardly influence it, while carbon noticeably restrains thermal donors’ formation. It was also found that the pre-annealing at 650℃ for 30 minutes could effectively eliminate the as-grown thermal donors, but couldn’t affect the subsequent thermal donors’ formation. Like the formation behavior of thermal donors in monocrystalline silicon, the temperature of the maximum concentration of thermal donors is about 450℃. In comparison with classical annealing, rapid thermal annealing could eliminate thermal donors much more quickly.The behavior of oxygen precipitation in cast multicrystalline silicon was similar to that in Czochralski Silicon. The concentration of oxygen precipitates in both materials annealed at 1050℃ for 32 hours is highest, but the concentration in mc-Si is much higher than that in Cz-Si. It was also found that the concentration of oxygen precipitates in Cz-Si with high density of dislocation was higher than that in dislocation-free Cz-Si, which indicates that dislocation in silicon can noticeably enhance oxygen precipitation. And the role of grain boundaries of mc-Si is similar to that of dislocation for oxygen precipitation, but the enhancement of grain boundariesto oxygen precipitation is weaker. The experiment results of two-step annealing reveal that oxygen precipitation is enhanced after high temperature annealing following low temperature annealing. Being different from oxygen precipitates in monocrystalline silicon, the defects induced by oxygen precipitates hardly occure in cast multicrystalline silicon, which indicate that interstitial silicon atoms are absorbed by dislocations and grain boundaries.更多还原