Coupling of Metallurgical Method to Remove Impurities in Solar Grade Polycrystalline Silicon

doi:10.15541/jim20160324

Abstract

Abstract:

Preparation of 6N grade polycrystalline silicon materials using raw materials of industrial silicon was explored by medium melting, directional solidification and electron beam melting. The contents of impurities B and P in the samples are both lower than 0.20 ppmw, while the total content of metal impurity (TM) is less than 0.23 ppmw. During the process of removing B by medium melting, a large proportion of Al and Ca is simultaneously removed through the redox reaction. During the process of electron beam melting, volatile impurities including P, Al and Ca are further effectively removed by using saturated vapor pressure. Meanwhile, other metal impurities are removed by decreasing beam power, which induces the occurrence of directional solidification. Coupling of metallurgical methods reduces the purification process of polycrystalline silicon, and provides the technical support for continuous and large-scale production.

Key words: industrial silicon, medium melting, directional solidification, electron beam melting

CLC Number:

TF89

LI Peng-Ting, WANG Kai, JIANG Da-Chuan, REN Shi-Qiang, TAN-Yi, AN Guang-Ye, ZHANG Lei, GUO Xiao-Liang, WANG Feng. Coupling of Metallurgical Method to Remove Impurities in Solar Grade Polycrystalline Silicon[J]. Journal of Inorganic Materials, 2017, 32(3): 281-286.

Figures/Tables 13

Fig. 1 Schematic diagram of two melting equipments(a) Directional solidification melting equipment; (b) Electron beam melting equipment

Table1 Component test results of industrial silicon (ppmw)

Element	B	P	Al	Ca	Fe	Mg	Na	TM
Concentration	1.55	6.67	149.15	154.00	300.73	0.60	0.84	1059.06

Fig. 2 Flowchart of 6N grade polycrystalline silicon prepared by metallurgical method

Table 2 Component test results of silicon after medium melting (ppmw)

Element	B	P	Al	Ca	Fe	Mg	Na	TM
First	0.60	6.84	0.67	15.96	461.00	1.21	563.36	1154.92
Second	0.26	7.61	0.36	2.38	525.00	4.68	468.16	967.46
Third	0.21	8.13	0.46	0.56	918.00	8.18	352.36	811.32

Table3 Component test results of silicon ingot after directional solidification (ppmw)

Solidification ratio	B	P	Al	Ca	Fe	Mg	Na	TM
0.98	0.22	7.84	9.00	29.16	3986.00	29.69	5.76	8131.96
0.90	0.18	6.33	2.61	13.64	393.68	8,17	2.06	746.46
0.86	0.16	5.96	0.13	0.18	0.36	0.13	0.20	2.29
0.78	0.15	5.16	0.21	0.68	0.31	0.08	0.45	1.09
0.62	0.12	5.19	0.21	0.13	0.42	0.10	0.20	0.97
0.42	0.11	5,87	0.20	0.30	0.27	0.07	0.28	0.67
0.22	0.12	4.96	0.16	0.24	0.21	0.08	0.16	0.56
0.06	0.08	4.13	0.08	0.61	0.26	0.07	0.17	0.53
0.02	0.11	4.46	0.09	0.18	0.21	0.06	0.19	0.47

Fig. 3 Distribution of TM concentration after directional solidification process

Fig. 4 Microstructure of the top of silicon ingot

Fig. 5 Change of impurity content in metallurgical process

Table 4 Component test results of silicon after electron beam melting (ppmw)

Element	B	P	Al	Ca	Fe	Mg	Na	TM
Concentration	0.180	0.190	0.019	0.027	0.026	0.003	0.008	0.228

Fig. 6 Ellingham diagram of oxide

Fig. 7 Schematic diagram of the concentration distribution of the solid-liquid interface

Fig. 8 Vapor pressure curves of silicon and impurities

Fig. 9 Schematic diagram of electron beam melting coupling directional solidification of removing impurities

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