Kinetics and Mechanism of Phosphorus Removal from Silicon in Vacuum Induction Refining

Abstract

Abstract. Vacuum induction refining is a process that can be applied to remove phosphorus from molten silicon for the production of solar grade silicon. Pure silicon was doped by phosphorus to make molten silicon containing around 17 ppmw phosphorus. The kinetics of phosphorus removal from this silicon was studied at 0.5 Pa through the application of vacuum induction refining. It was observed that vacuum removal of phosphorus occurs through a first-order reaction. The rate constants of phosphorus evaporation were determined as 2.28 10 6 m=s and 4.93 10 6 m=s at 1500 ı C and 1600 ı C, respectively. Moreover, an apparent activation energy 213.1 kJ=mol for phosphorus evaporation from molten silicon was calculated. It was found that mass transfer of phosphorus in the melt is not rate limiting in the inductively stirred silicon melt. The vacuum removal of phosphorus is mix-controlled by chemical reaction and gas phase mass transfer. Under medium vacuum conditions, the mass transfer in the gas phase is more rate-limiting than the chemical reaction at higher refining temperatures.

Silicon purification in vacuum induction furnace

“…This difference is due to the reaction be faster for higher levels of phosphorus. This phenomenon was observed by Safarian and colleagues (3) . According to them yet, the material with higher initial phosphorus content takes longer to achieve the same level of concentration to one of lower initial content.…”

Vacuum Evaporation of Pure Metals

“…The vacuum removal of phosphorus (P) from liquid silicon (Si) is a typical case in which very low concentrations of P such as 10 ppmw are eliminated from Si to achieve the concentrations required for the fabrication of silicon solar cells, i.e., below 0.1 ppmw. [1] Considering the above points, the vacuum evaporation of pure elements is studied in this paper as follows.…”

Numerical Modeling of Surface Waves Generated by Low Frequency Electromagnetic Field for Silicon Refinement Process

“…According to [11], the same relation as (1) is applicable for phosphorus removal; literature data is also shown in Table I. For phosphorus, temperature is a more important factor.…”