Thermodynamic behaviors of SiCl2 in silicon deposition by gas phase zinc reduction of silicon tetrachloride

The modified Siemens process, which is the major process of producing polycrystalline silicon through current technologies, is a high temperature, slow, semi-batch process and the product is expensive primarily due to the large energy consumption. Therefore, the zinc reduction process, which can produce solar-grade silicon in a cost effective manner, should be redeveloped for these conditions. The SiCl2 generation ratio, which stands for the degree of the side reactions, can be decomposed to SiCl4 and ZnCl2 in gas phase zinc atmosphere in the exit where the temperature is very low. Therefore, the lower SiCl2 generation ratio is profitable with lower power consumption. Based on the thermodynamic data for the related pure substances, the relations of the SiCl2 generation ratio and pressure, temperature and the feed molar ratio (nZn/nSiCl4)nZn/nSiCl4 are investigated and the graphs thereof are plotted. And the diagrams of KpΘ–T at standard atmosphere pressure have been plotted to account for the influence of temperature on the SiCl2 generation ratio. Furthermore, the diagram of KpΘ–T at different pressures have also been plotted to give an interpretation of the influence of pressure on the SiCl2 generation ratio. The results show that SiCl2 generation ratio increases with increasing temperature, and the higher pressure and excess gas phase zinc can restrict SiCl2 generation ratio. Finally, suitable operational conditions in the practical process of polycrystalline silicon manufacture by gas phase zinc reduction of SiCl4 have been established with 1200 K, 0.2 MPa and the feed molar ratio (nZn/nSiCl4)nZn/nSiCl4 of 4 at the entrance. Under these conditions, SiCl2 generation ratio is very low, which indicates that the side reactions can be restricted and the energy consumption is reasonable.

Graphical abstractThe surface of SiCl2 generation ratio with temperature and feeding ratio of gas phase zinc to H2 under 0.2 MPa SiCl2 generation ratio increases with increasing temperature when pressure and feed molar ratio of Zn to SiCl4 are kept as a constant and SiCl2 generation ratio increases slowly when the temperature is lower and becomes more quick when the temperature is higher. The higher feed molar ratio of gas phase zinc to SiCl4 can restrict the side reactions. Furthermore, the feed molar ratio of gas phase zinc to SiCl4 has greater effect on the SiCl2 generation ratio at higher temperature.Figure optionsDownload full-size imageDownload as PowerPoint slide