Investigations On Boron Removal From Polycrystalline Silicon Using Metallurgical Method

With the rapid development of science and technology, energy dependence has been becoming increasingly serious ever since the Industrial Revolution. Especially in the21th century, the energy crisis and deterioration of the environment to human sounded the alarm. Searching for a new energy alternative to substitute traditional energy sources has become a common goal all over the world. Solar energy, as one of the new energies, has been given extensive national attention and application due to its wide distribution, safe use and pollution-free. As the main form of solar applications, photovoltaic is the most important object to study and explore. The United States, Japan, Germany and other developed countries initiated a PV development program in succession. China also promulgated the "Golden Sun Demonstration Project" in2009to develop solar power generation project. By the end of2011, the total installed capacity was increased from300MW to3GW.However, as the main raw material used for solar cells, high-purity poly-silicon is in serious shortage and can’t meet the requirement of solar power generation technology with rapid development. The most important issue is to look for a new technology to fill up the gap of this raw material. As a low-cost, low energy consumption and low pollution method, metallurgy method is an effective way for high purity poly-silicon preparation. The key feature of metallurgical method is targeted for effective removal of metal, phosphorus (P) and boron (B) impurities in the polycrystalline silicon, resulting in a6N purity level required by solar power generation.During the development of metallurgy technology for poly-silicon purification, many new technologies are constantly raised. Both the removal of metal and P impurities have made significant breakthroughs and formed stable process routes. In contrast, the removal of B still needs to be improved. There are a lot of researches on metallurgical method to remove B impurity in the poly-silicon, but methods that can meet the requirements of industrial production are still in lack. It’s urgent to find a reliable method to remove B impurity, leading the metallurgical method to a large industrial scale.This thesis starts from the characteristics of silicon and B impurity. Based on the removal of B by oxidation, methods of pickling oxide and slag treatment were investigated. The factors that affect B impurity removal by oxidation were studied according to thermodynamics and reaction kinetics respectively. Slag refining was conducted in atmosphere which is closer to the pattern of industrial production, in the hope of providing more reliable data for the determination of the process route. After summarization of the experimental data, conclusions were presented as follow.1) Under the temperature of900-1200℃, it is effective to remove B impurity from poly-silicon if the silicon powder with a particle size of17-65fim is thermal oxidation processed for1-10h. The higher is the temperature, the longer is the heat treatment time and the smaller is for the size of silicon powder, the more effective is for the B removal. Under vapor oxidizing conditions, the growth rate of oxide film on the surface of the silicon powder is faster than that of dry oxidation.Under atmospheric conditions, after heat treatment for the silicon surface, B impurity will redistributed between the Si and SiOi layer due to the segregation effect. In the P-type region, B impurity diffuses to the SiOi. leading to the close concentration of donor and acceptor impurity which is reflected in the increase of the resistivity.2) Under vacuum conditions for the NaiO-CaO-SiOi slag system, the lowest value of LB is with the basicity of0.8. Whether lo increase or decrease the basicity will benefit the purification effect. The LB value reaches to the highest of5.81when the basicity is1.21. As alkaline oxide, the addition of NaO can increase the range of alkalinity range; silicon to slag ratio will directly affect the separation of silicon and the slag and the B removal efficiency. The larger is the silicon to slag ratio, the better is the separation effect. The separation of silicon and slag can’t be achieved when the silicon to slag ratio is less than2. The directional solidification process can not only get better separation of silicon and slag, but also can effectively remove metal impurities. B impurity closest to the interface of silicon and slag has the best removal effect. With the increasing of the distance from the interface, LB value decreased gradually and was minimized at the bottom of the crucible; the separation effect of the silicon and slag can be effectively improved through secondary slag treatment, to obtain a smooth silicon slag interlace and a higher purity of silicon.3) Under atmospheric conditions for CaF2-Al2O3-CaO-SiO2slag system. B removal effect is the best with the removal rate of82.8%when the optical basicity is0.597. Whether to increase or decrease the optical basicity will reduce the effect of B removal. Because the factors that affect the LB value is mutually restrained due to the adjustment of basicity which is not conductive to the removal of B; with the increasing of melting time, the B efficiency is gradually increased. But after the melting time is over60min. the trend starts lo become gentle and the best B removal effect reaches4.3ppmw with a removal rate of82.8%at120min: the mass transfer of B impurity is the limiting condition of the entire process; the CaF2addition can effectively improve the viscosity of the molten slag, while excess of CaF2will decrease the B removal efficiency. It’s difficult to meet the B content requirement of solar grade poly-silicon through slag treatment for once, and it is rational to achieve the target of0.3ppmw by slag treatment for twice without improving much cost.