Vacuum and Gas Refining of Silicon for Solar Cell Applications

Abstract

In this research, the vacuum and gas refining for P and B removal from Si to reach solar grade silicon (SoG – Si) are studied. The vacuum refining process is modeled by applying the Hertz – Knudsen – Langmuir evaporation formula. The new model can be used for multi-component systems, whose solvent composition changes over time of the refining process. Vacuum refining of P-containing Si that was alloyed with Al (20 wt. %) at 1400 and 1500 °C indicated enhanced P removal rates, and the kinetic model was verified by the obtained experimental results. Moreover, P removal from Si by vacuum refining technique is experimentally investigated over the temperature range of 1500 to 1900 °C. The empirical and theoretical kinetic models are applied to discuss the process rate. A unique behavior in Si – P system at ultra-high temperatures (temperatures above 1800 °C) was observed, leading to a significant increase in the refining rate. In addition, the P evaporation from Si was investigated by the Knudsen effusion mass spectrometry (KEMS) technique and a new theory for P evaporation based on the decomposition of transient silicon phosphides is presented. Considering the rate of P removal and the simultaneous Si loss to reach SoG – Si, 1800 °C is proposed as the most efficient temperature for the vacuum refining process. The effect of reduced pressures of various gases (Ar, He, and H2 20 – 65 Pa) in vacuum evaporation of P from liquid Si was studied. Results indicated on the selective evaporation of P over Si when hydrogen was in the chamber. Quantum chemistry was applied to study the interaction of the P and Si atoms with Ar, He, and H2. The theoretical results from quantum chemistry simulation were in good agreement with experimental results indicating the role of hydrogen on making the vacuum refining selective for P over Si evaporation. This contributes to the prevention of Si loss in the vacuum refining process. In addition, the effect of forced convection of the continues gas medium (over the Knudsen layer) on increasing the gas velocity was studied by applying an auxiliary evacuation tube above the melt surface, leading to prevention of condensation of evaporated species back to liquid Si surface and two times increase of the process rate.

Boron removal from Si by humidified hydrogen gas was studied by applying alumina and graphite crucibles in refining experiments. Effect of chamber atmosphere (Ar, He, and vacuum conditions) on rate of B removal was studied. Results from refining of Si with dry hydrogen in alumina crucibles indicated B removal, while in graphite crucible, dry hydrogen showed a very slow rate for B removal. The mechanism of B removal from Si by humidified hydrogen gases was studied by molecular beam mass spectroscopy (MBMS) technique leading to introduction a new mechanism for B removal with humidity, by means of HBOH compounds. The effect of melt interaction with crucible body was also studied by characterizing Si – B samples in graphite, alumina, and quartz boats, and results indicated on the evaporation of B in from of AlBO in alumina crucibles.

Combined vacuum, gas, and directional solidification process was carried out in lab scale and power consumption of process was theoretically discussed compared with other metallurgical techniques. With the obtained rate of processes, it is estimated that vacuum refining requires about 2.5 kWh / one kg Si while gas refining requires almost 13.13 kWh / one kg Si (both in experimental conditions reported in this thesis), without heat recovery from the process gas.