On the Phosphorus Evaporation from Liquid Silicon by Knudsen Effusion Mass Spectrometry

Abstract

Silicon refining for solar applications is intensively on demand, and removal of phosphorus from Si is one of the most challenging steps. Evaporation of P from liquid Si in a vacuum refining process is the most efficient method for P removal from Si, and this research deals with the insight mechanisms of P evaporation from liquid Si. In this research, the gaseous species evaporating from the dilute liquid solutions of phosphorus in silicon were studied experimentally, and it was shown that phosphorus evaporates in the form of P, P2, P4, P3, SiP, Si2P, Si3P, and SiP2 at elevated temperatures. Except P and P2, the other molecules were detected experimentally for the first time, and Si3P was detected as a new compound in the gas phase. Knudsen effusion mass spectrometry technique was applied to characterize the evaporation of phosphorus from liquid Si samples containing 100, 1250, and 3000 ppmw phosphorus. The evaporation of phosphorus from liquid Si was studied by isothermal and polythermal experiments, up to 1840 °C. The vapor pressures of various P-containing molecules (P, P2, P4, SiP, Si2P) at 1442 °C were measured as a function of phosphorus fraction in liquid silicon. Results indicated that a major part of the phosphorus evaporates in the form of silicon phosphides and P4, especially when the sample temperature exceeds 1750 °C. When initial phosphorus was 100 ppmw, about 71 pct of phosphorus evaporation was by means of silicon phosphides and P4. The mechanisms of phosphorus evaporation from liquid Si are proposed, which depend on the melt composition and temperature. It occurs through phosphorus species evaporation independently or via the decomposition of transient silicon phosphides at the surface or through the direct evaporation of silicon phosphides at the melt surface.

Experimental Study on the Interaction Between Al and P in Si Solution

“…Upon increasing the pull-down speed, a large amount of P was enriched at the top of the sample, and more P diffused to the surface of the silicon melt under the action of an electromagnetic force. Hoseinpur A [25] et al used electromagnetic induction heating to stir molten silicon, which accelerated the diffusion of impurities in the silicon melt and obtained a large mass transfer coe cient. According to the mathematical model of phosphorus removal by vacuum directional solidi cation, Zhang [26] et al found that the segregation on the upper surface of silicon melt during this process concentrated phosphorus and facilitated phosphorus volatilization.…”