Removal of phosphorus in molten silicon by electron beam candle melting

doi:10.1016/j.matlet.2012.03.031

Materials Letters

Volume 78, 1 July 2012, Pages 4-7

https://doi.org/10.1016/j.matlet.2012.03.031 Get rights and content

Abstract

In the current study, a new method for the removal of phosphorus in molten silicon, electron beam candle melting (EBCM), is proposed and discussed. The proposed method combines the characteristics of electron beam melting (EBM) and the high saturated vapor pressure of P in molten Si. Simulation results show the existence of three typical temperature distributions and morphologies of the molten pool that correspond to the different radii of the electron beam circular pattern at constant power. The critical molten pool with the maximum surface area and the minimum depth was determined. EBCM resulting in the critical molten pool was proven to be more effective in the removal of P in molten Si compared with EBM. In addition, the energy utilization ratio was enhanced.

Highlights

► As a new method, EBCM is proposed and discussed. ► It combines the feature of EBM and the saturated vapor pressure of P in molten Si. ► A molten pool with maximum surface area and minimum depth exists during EBCM. ► Compared with EBM, EBCM is more effective in the removal of P in molten Si. ► Compared with EBM, EBCM has a higher energy utilization ratio.

Introduction

Given the rapid development of the photovoltaic industry, the requirements for solar-grade Si have increased dramatically in recent years. Due to yield and cost limitations, traditional methods cannot meet the requirements of the photovoltaic market; therefore, metallurgical methods have been given more attention, thanks to their low cost, low energy consumption, and minimal environmental impact. The main impurities in Si that affect its electrical characteristics include the doping element P. P can be removed effectively at high temperature and high vacuum due to its relatively high saturated vapor pressure. Takahiro et al. [1] investigated the thermodynamic properties of P in molten Si. Pires et al. [2] confirmed the technical feasibility of purifying Si by the EBM method. Hanazawa et al. [3] thought that the P removal rate was controlled by free evaporation from the molten Si surface.

However, the wide application of EBM in industrial production is limited by issues such as the low energy utilization ratio caused by the heat exchange between the water-cooled copper crucible and the heat radiation from the molten Si surface during refining.

The removal of P from molten Si mainly depends on the surface area and the depth of the molten pool at constant temperature and vacuum conditions [4]. Increasing the surface area of the molten pool and decreasing the depth of the molten pool can increase the removal of P and the energy utilization ratio.

In the current study, a new method is proposed for the removal of P from molten Si based on the above considerations. The process consists of several steps. First, a low-energy electron beam is used to melt the surface of the columned Si ingot after directional solidification. Irradiation is maintained for several minutes until a desired amount of P is removed. Electron beam energy is then increased to melt the top edge of the Si ingot. Liquid Si with low P content flows down the sides of the ingot sidewall into the receptacle. The above processes are repeated until the P content in the whole ingot is reduced to the target value. This process resembles the melting of a candle, hence its name.

In the present paper, the efficiency of P removal and energy utilization rate using EBCM are evaluated and compared with those using EBM.

Section snippets

Description of EBCM

As shown in Fig. 1, the radius of the circular electron beam pattern is r (Fig. 1a). The beam pattern in a small area can be regarded to propagate in a straight line (Fig. 1b). The area directly irradiated by the electron beam is exposed to the highest temperature; thus, the Si in this area melts first. The continuous electron beam irradiation causes heat energy to transfer from the area to the surroundings. As shown in Fig. 1c, a molten pool with width d_m forms when the energy transfer is

Experimental

One 70 × 58 × 73 mm³ Si ingot weighing 729 g was prepared by directional solidification for use in EBCM. The P content of the ingot was 1.44 × 10^− 2 wt%, as obtained by inductively coupled plasma mass spectrometry. The ingot was placed in the water-cooled copper crucible, and the chamber was evacuated to 2 × 10^− 3 Pa. During EBCM, the critical molten pool was established by adjusting the experiment parameters to fit the size of the Si ingot. The beam pattern was circular, with a diameter of approximately 25

Results and discussion

Fig. 3a and b shows the Si ingot morphologies before and after EBCM. No splashing occurred during EBCM. The ingot had a button shape and high surface luster. However, because the ingot was cubic and the electron beam pattern was circular, part of the Si on the edge did not melt completely. This problem could be solved by using a cylindrical ingot.

The Si ingot morphologies after EBM at 9, 15, and 21 kW are shown in Fig. 3c–e. The button-shaped ingots show a protuberance on the top surface, and

Conclusions

EBCM combines the characteristics of EBM and the high saturated vapor pressure of P in molten Si. A critical molten pool with the maximum surface area and the minimum depth could effectively remove P in molten Si is confirmed through simulations. For same accumulated removal rate of P, EBCM requires a shorter timeframe or lower power compared with EBM; therefore, the energy utilization ratio using EBCM is higher.

Acknowledgments

The authors gratefully acknowledge financial support by the National Natural Science Foundation of China (Grant No. 51074032).

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