Study On The Removal Of Impurities Al, Ca From Polysilicon By Directional Solidification And Electron Beam Melting

With the continuous development of world economy and technology, the dependency on and the requirement for the energy increases continuously. In order to ensure the stability of economic development, the main developed countries in the world have promoted energy security to the national strategic height. However, restricted by the issues such as the reserves of conventional energy and carbon emission, energy shortage gradually becomes obvious year by year. To solve energy security problem, many countries pay more and more attention to the development and application of new energy. Under this background, the metallurgical method used for preparing solar grade silicon has drawn attention of many counties for its environmental protection and low cost. The metallurgical method is a comprehensive technology consisting of directional solidification, electron beam melting, acid cleaning, slag refining and so on.As one of the harmful metal impurities in silicon, Aluminum and Calcium deteriorate the electrical properties of silicon, such as the electrical resistivity and minority carrier lifetime. To meet the performance requirement of solar cells, the content of Al in silicon need to be reduced to less than1.0×10-5wt.%. Due to the segregation effect of A1and Ca in silicon, they can be removed by directional solidification. However, the segregation coefficient of Al and Ca are2.0×10-3and8.0×10-3,respectively, which are relatively large, the content of Al and Ca after directional solidification once cannot meet the requirement of solar cells. As a consequence, multiple directional solidification is used to reduce the content of Al and Ca to a low level, which improves the cost. At the same time, Al and Ca have higher saturated vapor pressure than silicon so that they can be removed by evaporating from the free surface of the molten silicon under a high temperature and high vacuum environment. However, both the evaporation reactions of Al and Ca are first order reactions, so the removal efficiency of impurities will decrease as time goes on. If the silicon raw material undergoes a acid cleaning process as pretreatment before melting, the removal effect will be better, but it will also cause environmental problem. So far, the removal of Al and Ca is still an important issue for the preparation of solar grade silicon materials.In this paper, based on the deeply analysis of the segregation effect and evaporation behavior of impurities in silicon, several melting methods are combined organically for removing impurities Al and Ca. According to compare these melting methods, the effect of melting conditions to removal efficiency is discussed to seek a melting method with high efficiency, low cost and environmentally friendly. It is found that the content of A1in the upper part of the silicon ingot is smaller than1×10-4wt.%and that of Ca is about1×10-4.%after once directional solidification. During electron melting process, the contents of Al and Ca decrease logarithmically as melting time extends under a certain power, which is up to low than0.6×10-4wt.%and0.5×10-4wt.%, respectively after1800s under a power of21kW. After electron beam melting, the silicon ingot preprocessed directional solidification has higher impurity removal efficiency with the same melting time or power. With the same total time, including melting time and solidification time, the silicon ingot obtained by controlling the reduction mode of the electron beam current has higher impurity removal efficiency.