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Removal of aluminum from silicon by electron beam melting with exponential decreasing power
Separation and Purification Technology 152 (2015) 32–36
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Removal of aluminum from silicon by electron beam melting with exponential decreasing power Shuang Shi a,b, Yi Tan a,b,⇑, Dachuan Jiang a,b, Shiqiang Qin a,b, Xiaoliang Guo a,b, H.M. Noor ul Huda Khan Asghar a,b,c a b c
School of Materials Science and Engineering, Dalian University of Technology, Dalian 116023, China Key Laboratory for Solar Energy Photovoltaic System of Liaoning Province, Dalian 116023, China Department of Physics, Balochistan University of Information Technology, Engineering & Management Sciences, Quetta, Pakistan
a r t i c l e
i n f o
Article history: Received 8 April 2015 Received in revised form 23 July 2015 Accepted 3 August 2015 Available online 4 August 2015 Keywords: Silicon Purification Electron beam melting Directional solidification Aluminum removal
a b s t r a c t Aluminum is one of the main impurities in silicon, which can be separated and eliminated by electron beam melting. However, high removal efficiency can be obtained only by increasing melt temperature or extending refining time, resulting in high energy consumption. In this work, the directional solidification of silicon was achieved by electron beam with exponential decreasing power, considering that aluminum has both characteristics of segregation and evaporation. The distributions of aluminum show increasing trend from the bottom to the top of the electron beam melted silicon ingot, which is the same as that after traditional directional solidification. The removal efficiency is improved by the coupling of segregation and evaporation. Compared with traditional electron beam melting, the loss of silicon reduced by more than 52% and the energy consumption reduced by more than 54%. This method is more effective to remove aluminum from silicon with low energy consumption. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the photovoltaic industry is developing rapidly due to the energy shortage, increasing by over 30% per year [1]. As a key raw material for solar cells, crystalline silicon occupies more than 90% of the global photovoltaic market [2,3]. As a consequence of the dramatic increasing requirement for silicon materials, the current focus is to improve the efficiency of solar cells while still using cost-effective, high-throughput, and large-scale processes. The metallurgical route is a promising way for the purification of metallurgical-grade silicon (MG-Si) to solar-grade silicon (SoG-Si) [4,5]. Based on the difference of physical properties between silicon and impurities, several refining steps are used to remove different impurities in turn on the premise that silicon do not participate in chemical reactions. Electron beam melting is one of the key procedures in metallurgical route, which has been proved to be an effective method to remove volatile impurities from silicon [6,7], especially for phosphorus [8–10]. Under a high temperature and high vacuum condition provided by electron beam equipment, impurity elements
with high saturated vapor pressure can be evaporated from molten silicon to gaseous phase, and then removed by the vacuum system. As a major impurity in silicon, aluminum deteriorates the electrical properties, such as electrical resistivity, minority carrier lifetime and then photoelectric conversion efficiency [11,12], so it must be removed to a fairly low level to meet the performance requirement of solar cells. However, the removal efficiency depends upon the temperature and refining time [13], so it requires to raise the temperature or extend the refining time to obtain higher removal efficiency, resulting in a large energy consumption. In this paper, the directional solidification of silicon was achieved by electron beam with exponential decreasing power. Based on the distribution of aluminum in silicon ingot, the removal efficiency and mechanism were discussed. Meanwhile, the evaporation loss of silicon and the energy consumption during the whole process were also evaluated by comparing with the traditional electron beam melting process.
2. Directional solidification of silicon induced by electron beam ⇑ Corresponding author at: School of Materials Science and Engineering, Dalian University of Technology, No. 2 Linggong Road, Ganjingzi District, Dalian 116023, China. E-mail address: [email protected] (Y. Tan). http://dx.doi.org/10.1016/j.seppur.2015.08.002 1383-5866/Ó 2015 Elsevier B.V. All rights reserved.
In our previous work [14], the directional solidification of silicon induced by electron beam has been achieved by controlling the decreasing rate of beam power, and the simulation results
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S. Shi et al. / Separation and Purification Technology 152 (2015) 32–36
show that the solidification rate is uniform with an exponential decreasing rate. Fig. 1 shows the schematic diagram of directional solidification of silicon induced by electron beam. A unidirectional temperature gradient always exists in the molten pool, because the surface of the molten pool is heated by electron beam and the bottom contacts directly with the water-cooled copper crucible, so that the melt solidifies from the bottom to the top. The segregation coefficient of aluminum is smaller than 1, so the removal efficiency of aluminum is considered to be improved with the action of segregation behavior. 3. Experimental 3.1. Experimental apparatus Experiments were conducted in an SEBM-30A-type electron beam melting furnace. It consists of a chamber with the volume of 1 m3, an electron beam gun with an accelerating voltage of 30 kV and a maximum power of 30 kW, a water-cooled copper crucible, a circulation water cooling system and two independent vacuum systems. The crucible has a button shape and a maximum of 2 kg silicon can be melted in it.
Fig. 2. Power changes with time during the whole electron beam melting process.
Table 1 Melting conditions in electron beam melting experiment.
3.2. Materials and pretreatment MG-Si with an initial purity of 99.8 wt.% was used as the raw material, in which the initial content of aluminum was 1.71 101 wt.%. Prior to the processing by electron beam melting, it was washed sufficiently in alcohol by a supersonic wave cleaner to remove oil, organic compounds and possible extraneous impurities adhered to the surfaces. After that, it was put in a drying oven for 2 h.
Melting power (kW) Refining time (s) Solidification time (s) Pressure (Pa) Scanning pattern
Sample 1
Sample 2
Sample 3
18 900 Instantaneous 5 102– 5 103 Circle
18 900 2700 5 102– 5 103 Circle
18 900 3900 5 102– 5 103 Circle
3.3. Experimental procedure 600 g silicon was placed inside the water-cooled crucible, thereafter the chamber was evacuated to a pressure less than 5 102 Pa and the gun chamber was evacuated to a pressure less than 5 103 Pa. The electron beam was then irradiated on the surface of the silicon with a circular scanning pattern to ensure it was heated homogeneously and steadily. As shown in Fig. 2, the beam power increased gradually to 9 kW so that the silicon could be melted completely. The refining time was started once silicon was melted completely in the crucible. The melt was then maintained for 900 s with the beam power of 18 kW. After that, the power was stopped instantaneously or reduced exponentially. The experimental parameters are given in Table 1.
3.4. Testing and characterization The obtained silicon ingots were cut along the vertical cross section. It was polished and etched with 20% NaOH solution for 20 min so that the morphologies of the crystal can be observed clearly. The resistivity distribution on the cross section was measured by KDY-1 four-point probe resistivity tester. Five samples were chosen in the central region at 0%, 25%, 50%, 75%, and 100% of the ingot height for chemical analysis. As for each sample, the aluminum concentration was determined by inductively coupled plasma optical emission spectroscopy (ICP-MS).
Fig. 1. Schematic diagram of directional solidification of silicon induced by electron beam.
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4. Results and discussion 4.1. Solidification rate During a general electron beam melting process, the molten pool is in a stable state under the action of heating by electron beam and cooling by water-cooled copper crucible. When it came to the pre-defined refining time, the beam was closed to end the melting process. Once the beam power is decreased, the silicon melt begins to solidify; however, the silicon solidifies completely when the beam power is decreased to some critical value but not 0. The critical power depends on the mass of the silicon and the cooling condition. Under this experimental condition, the value related to 600 g silicon is determined as 5.1 kW, shown as the dashed line in Fig. 2. Thus, the solidification time for each sample is the time when the power decreased from 18 kW to 5.1 kW, which are determined as 45 min and 65 min for sample 2 and sample 3, respectively. The solidification rates can be obtained through the solidification time and the height of the ingot as 9.82 106 m/s and 6.67 106 m/s, respectively. As for sample 1, the power was stopped instantaneously, so it is difficult to obtain its solidification rate. However, it is considered to be far larger than the others’ so that the impurities have no time to diffuse sufficiently. 4.2. Morphologies of silicon ingots Morphologies of the surfaces and vertical cross sections of the ingots with different cooling rates are shown in Fig. 3. The ingot has a rough surface when the power was stopped instantaneously, as shown in Fig. 3a. Under this condition, the region where the molten silicon contacted with the water-cooled copper crucible solidified firstly from bottom to top; meanwhile, the surface of the melt also began to solidify from top to bottom because a large amount of heat was taken away by radiation. After solidifying completely, there was a dividing line existing inside the ingot. Fine columnar crystallites can be observed in the area below the dividing line and coarse columnar crystallites can be observed in the area above the dividing line. The different grain morphology depends on the nucleation condition and solidification rate in the two areas. If the power was stopped with a reduction rate less than some critical value, the region where the molten silicon contacted with the water-cooled copper crucible also solidified firstly. Under this condition, the molten silicon is strongly cooled at the bottom by the water-cooled copper crucible and still heated at the top by the gradual lowering electron beam to form a unidirectional temperature gradient, resulting in the melt solidifying from the bottom to the top. As shown in Fig. 3b–c, the ingot has a smooth surface, where is the final solidification area. Fine columnar crystallites growing from bottom to top can be observed clearly in the vertical cross section, indicating a classical structure after directional solidification. 4.3. Resistivity distribution The resistivity distribution of silicon depends on the impurity distribution on some level, which can reflect the crystal growth process. It is relatively homogeneous in the case of solidification instantaneously and all of the values are around 2.5 X cm, as shown in Fig. 4a, indicating the homogeneous distribution of impurity. Silicon solidified very fast when the power was closed instantaneously so that the impurities have no time to diffuse sufficiently. As a consequence, the segregation behavior of impurity is not obvious. The resistivity values of some areas are far larger than the other areas, resulting from some cracks on the cross
section forming by internal stress after solidification. The resistivity distribution show a decreasing trend from the bottom to the top in the case of solidification induced by electron beam with exponential decreasing power, as shown in Fig. 4b–c. Impurity with low segregation coefficient will segregate at the solid–liquid interface and enrich into the liquid during directional solidification, leading to the decreasing trend of resistivity distribution. 4.4. Distribution of aluminum in silicon ingot Aluminum is one of the main impurities in silicon. The saturated vapor pressure of aluminum is larger than that of silicon, so it can be removed under a high temperature and high vacuum environment provided by electron beam melting [13]; meanwhile, the segregation coefficient of aluminum is smaller than 1, which means it can be removed by directional solidification. As a consequence, the removal efficiency and distribution of aluminum are considered to depend on both two removal mechanisms. The distributions of aluminum along growth direction of the ingot are shown in Fig. 5. The contents of aluminum are uniform in the case of instantaneous solidification. During melting process, a large part of aluminum evaporates from the melt free surface into the gas phase, and then is extracted by the vacuum system. The removal efficiency depends on the temperature, time and melt geometry. In this experiment, the aluminum content decreases from 1.71 101 wt.% to 1.14 101 wt.% with the refining time of 900 s. As for the ingot in the case of directional solidification induced by gradual lowering electron beam, the content of aluminum shows an obvious increasing trend. During solidification process, aluminum atoms segregate at the solid–liquid interface and concentrate into the liquid phase; meanwhile, aluminum atoms in the liquid phase also evaporate from melt free surface into the gas phase because the temperature of the un-solidified silicon is still above the melting point. It can be predicted with consideration of the segregation from silicon crystal to silicon melt as well as evaporation from melt to gaseous phase [15]:
C s ¼ keff C 0 ð1 f s Þ
keff þd
D 1 L-G v
ð1Þ
where Cs is the content of impurity incorporated into the solid phase, C0 is the initial content in the liquid at solidification beginning, fs is the solidified fraction, and dL-G is the free surface boundary layer thickness. keff is the effective segregation coefficient, depending on the equilibrium segregation coefficient k0, the growth rate v (m/s), the diffusion coefficient of impurity in molten silicon D (m2/s) and the solid–liquid interface boundary layer thickness dS-L (m), which can be expressed as [16]:
keff ¼
k0 k0 þ ð1 k0 Þ expðv dS-L =DÞ
ð2Þ
This exponent includes two characteristic items: keff and D/ dL-G v are suggested to be related to the effect of segregation and evaporation, respectively. The aluminum content after melting process is regarded as the initial value for the subsequent solidification process. Given that k0(Al) = 2 103 [17], D(Al) = 1.13 108 m2/s [18], dL-G = 8 103 m [16] as well as the solidification rate value mentioned above, and dS-L is set as 4 103 m [19], the aluminum distribution in silicon ingot can be simulated by Eq. (1), as shown in Fig. 5. The theoretical curves show agreement to the measured data. 4.5. Comparison with traditional electron beam melting Aluminum can be removed from silicon by traditional electron beam melting. The content of aluminum reduces with the increase
S. Shi et al. / Separation and Purification Technology 152 (2015) 32–36
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Fig. 3. Morphologies of the surfaces and vertical sections of the ingots with different solidification rates ((a) Instantaneous solidification; (b) 2700 s; (c) 3900 s).
Fig. 4. Resistivity distribution on the vertical cross sections of the ingots with different solidification rates ((a) Instantaneous solidification; (b) 2700 s; (c) 3900 s).
Fig. 5. Distributions of aluminum along growth direction in electron beam melted silicon ingot.
Fig. 6. Comparison of removal efficiencies of aluminum with two different solidification modes.
of power or refining time. The removal efficiency can be expressed by
removal efficiency in different positions of the ingot after solidification induced by electron beam, and the dashed lines represent the removal efficiency after instantaneous solidification with different refining time. It is obvious that the removal efficiency of the former is higher than that of the latter in most area of the ingot except the top. In the end stage of solidification, the diffusibility and evaporability of aluminum decrease sharply with the decrease of melt temperature. As a consequence, it is concentrated and preserved at the top of the ingot, which will be cut to obtain purified silicon ingot. Besides, the evaporation loss of silicon and the energy consumption are also considered, as shown in Table 2. The results show that, compared with traditional electron beam melting, the
g¼1
Cs C0
ð3Þ
A larger power leads to higher removal efficiency with the same refining time, but higher energy consumption. Note that aluminum in silicon has both evaporation behavior and segregation behavior, so it is considered to be removed more efficiency by coupling of the two removal mechanisms during the solidification process induced by electron beam. Fig. 6 shows the removal efficiency of aluminum with two different solidification modes. The curves represent the
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S. Shi et al. / Separation and Purification Technology 152 (2015) 32–36
Table 2 Loss of silicon and energy consumption by two different solidification modes. Experiment condition Refining 3600 s + Instantaneous solidification Refining 4800 s + Instantaneous solidification Refining 900 s + Solidification 2700 s Refining 900 s + Solidification 3900 s
Loss of silicon Energy consumption (g) (kJ) 7.80
48,600
10.38
70,200
3.48 4.98
22,271 31,248
loss of silicon reduced by more than 52% and the energy consumption reduced by more than 54%. The energy consumption is equal to the integral of beam power over refining time, so the decreasing of beam power leads to energy saving. Meanwhile, the temperature of the melt surface also decreased so that the evaporation of silicon was inhibited.
5. Conclusions The directional growth of silicon was achieved by controlling the electron beam with exponential decreasing power. During this process, the removal of aluminum can be improved by the influence of segregation on its evaporation. Compared with the traditional electron beam melting, the removal efficiency of aluminum is higher, indicating that it is a more effective way to remove impurity with high saturated vapor pressure and low segregation coefficient in silicon. During the whole process, the evaporation loss of silicon reduced by more than 52%, and the energy consumption also reduced by more than 54%.
Acknowledgements The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant No. U1137601 and 51304033) and Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20130041110004).
References [1] B. Gao, K. Kakimoto, Three-dimensional analysis of dislocation multiplication in single-crystal silicon under accurate control of cooling history of temperature, J. Cryst. Growth 396 (2014) 7–13. [2] M.D. Johnston, M. Barati, Calcium and titanium as impurity getter metals in purification of silicon, Sep. Purif. Technol. 107 (2013) 129–134. [3] X. Gu, X.G. Yu, D.R. Yang, Low-cost solar grade silicon purification process with Al–Si system using a powder metallurgy technique, Sep. Purif. Technol. 77 (2011) 33–39. [4] T. Schutz-Kuchly, V. Sanzone, Y. Veschetti, N-type solar-grade silicon purified via the metallurgical route: characterisation and fabrication of solar cells, Prog. Photovoltaics 21 (2013) 1214–1221. [5] F. Tanay, S. Dubois, N. Enjalbert, J. Veirman, Low temperature-coefficient for solar cells processed from solar-grade silicon purified by metallurgical route, Prog. Photovoltaics 19 (2011) 966–972. [6] T. Ikeda, M. Maeda, Purification of metallurgical silicon for solar-grade silicon by electron beam button melting, ISIJ Int. 32 (1992) 635–642. [7] J.S.C. Pires, J. Otubo, A.F.B. Braga, P.R. Mei, The purification of metallurgical grade silicon by electron beam melting, J. Mater. Process. Technol. 169 (2005) 16–20. [8] H. Sasaki, Y. Kobashi, T. Nagai, M. Maeda, Application of electron beam melting to the removal of phosphorus from silicon: toward production of solar-grade silicon by metallurgical processes, Adv. Mater. Sci. Eng. 2013 (2013) 1–8 857196. [9] Y. Tan, X.L. Guo, S. Shi, W. Dong, D.C. Jiang, Study on the removal process of phosphorus from silicon by electron beam melting, Vacuum 93 (2013) 65–70. [10] S. Shi, W. Dong, X. Peng, D.C. Jiang, Y. Tan, Evaporation and removal mechanism of phosphorus from the surface of silicon melt during electron beam melting, Appl. Surf. Sci. 266 (2013) 344–349. [11] A.A. Istratov, T. Buonassisi, M.D. Pickett, M. Heuer, E.R. Weber, Control of metal impurities in ‘‘dirty” multicrystalline silicon for solar cells, Mater. Sci. Eng. B 134 (2006) 282–286. [12] S.S. Zheng, T.A. Engh, M. Tangstad, X.T. Luo, Separation of phosphorus from silicon by induction vacuum refining, Sep. Purif. Technol. 82 (2011) 128–137. [13] X. Peng, W. Dong, Y. Tan, D.C. Jiang, Removal of aluminum from metallurgical grade silicon by an electron beam melting, Vacuum 86 (2011) 471–475. [14] Y. Tan, S. Shi, X.L. Guo, D.C. Jiang, W. Dong, S.Q. Ren, Effect of cooling rate on solidification of electron beam melted silicon ingots, Vacuum 89 (2013) 12–16. [15] Z.Q. Xi, J. Tang, H. Deng, D. Yang, D.L. Que, A model for distribution of oxygen in multicrystalline silicon ingot grown by directional solidification, Sol. Energy Mater. Sol. Cells 91 (2007) 1688–1691. [16] J.A. Burton, R.C. Prim, W.P. Slichter, The distribution of solute in crystals grown from the melt. Part 1. theoretical, J. Chem. Phys. 21 (1987) 1987–1991. [17] F.A. Trumbore, Solid solubility of impurity elements in germanium and silicon, Bell Syst. Tech. J. 39 (1960) 205–233. [18] J. Safarian, M. Tangstad, Vacuum refining of molten silicon, Metall. Mater. Trans. B 43 (2012) 1427–1445. [19] N. Yuge, Y. Sakaguchi, H. Terashima, F. Aratani, Purification of silicon by directional solidification, J. Jpn. Inst. Met. 61 (1997) 1094–1100.
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Removal of aluminum from silicon by electron beam melting with exponential decreasing power Shuang Shi a,b, Yi Tan a,b,⇑, Dachuan Jiang a,b, Shiqiang Qin a,b, Xiaoliang Guo a,b, H.M. Noor ul Huda Khan Asghar a,b,c a b c
School of Materials Science and Engineering, Dalian University of Technology, Dalian 116023, China Key Laboratory for Solar Energy Photovoltaic System of Liaoning Province, Dalian 116023, China Department of Physics, Balochistan University of Information Technology, Engineering & Management Sciences, Quetta, Pakistan
a r t i c l e
i n f o
Article history: Received 8 April 2015 Received in revised form 23 July 2015 Accepted 3 August 2015 Available online 4 August 2015 Keywords: Silicon Purification Electron beam melting Directional solidification Aluminum removal
a b s t r a c t Aluminum is one of the main impurities in silicon, which can be separated and eliminated by electron beam melting. However, high removal efficiency can be obtained only by increasing melt temperature or extending refining time, resulting in high energy consumption. In this work, the directional solidification of silicon was achieved by electron beam with exponential decreasing power, considering that aluminum has both characteristics of segregation and evaporation. The distributions of aluminum show increasing trend from the bottom to the top of the electron beam melted silicon ingot, which is the same as that after traditional directional solidification. The removal efficiency is improved by the coupling of segregation and evaporation. Compared with traditional electron beam melting, the loss of silicon reduced by more than 52% and the energy consumption reduced by more than 54%. This method is more effective to remove aluminum from silicon with low energy consumption. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the photovoltaic industry is developing rapidly due to the energy shortage, increasing by over 30% per year [1]. As a key raw material for solar cells, crystalline silicon occupies more than 90% of the global photovoltaic market [2,3]. As a consequence of the dramatic increasing requirement for silicon materials, the current focus is to improve the efficiency of solar cells while still using cost-effective, high-throughput, and large-scale processes. The metallurgical route is a promising way for the purification of metallurgical-grade silicon (MG-Si) to solar-grade silicon (SoG-Si) [4,5]. Based on the difference of physical properties between silicon and impurities, several refining steps are used to remove different impurities in turn on the premise that silicon do not participate in chemical reactions. Electron beam melting is one of the key procedures in metallurgical route, which has been proved to be an effective method to remove volatile impurities from silicon [6,7], especially for phosphorus [8–10]. Under a high temperature and high vacuum condition provided by electron beam equipment, impurity elements
with high saturated vapor pressure can be evaporated from molten silicon to gaseous phase, and then removed by the vacuum system. As a major impurity in silicon, aluminum deteriorates the electrical properties, such as electrical resistivity, minority carrier lifetime and then photoelectric conversion efficiency [11,12], so it must be removed to a fairly low level to meet the performance requirement of solar cells. However, the removal efficiency depends upon the temperature and refining time [13], so it requires to raise the temperature or extend the refining time to obtain higher removal efficiency, resulting in a large energy consumption. In this paper, the directional solidification of silicon was achieved by electron beam with exponential decreasing power. Based on the distribution of aluminum in silicon ingot, the removal efficiency and mechanism were discussed. Meanwhile, the evaporation loss of silicon and the energy consumption during the whole process were also evaluated by comparing with the traditional electron beam melting process.
2. Directional solidification of silicon induced by electron beam ⇑ Corresponding author at: School of Materials Science and Engineering, Dalian University of Technology, No. 2 Linggong Road, Ganjingzi District, Dalian 116023, China. E-mail address: [email protected] (Y. Tan). http://dx.doi.org/10.1016/j.seppur.2015.08.002 1383-5866/Ó 2015 Elsevier B.V. All rights reserved.
In our previous work [14], the directional solidification of silicon induced by electron beam has been achieved by controlling the decreasing rate of beam power, and the simulation results
33
S. Shi et al. / Separation and Purification Technology 152 (2015) 32–36
show that the solidification rate is uniform with an exponential decreasing rate. Fig. 1 shows the schematic diagram of directional solidification of silicon induced by electron beam. A unidirectional temperature gradient always exists in the molten pool, because the surface of the molten pool is heated by electron beam and the bottom contacts directly with the water-cooled copper crucible, so that the melt solidifies from the bottom to the top. The segregation coefficient of aluminum is smaller than 1, so the removal efficiency of aluminum is considered to be improved with the action of segregation behavior. 3. Experimental 3.1. Experimental apparatus Experiments were conducted in an SEBM-30A-type electron beam melting furnace. It consists of a chamber with the volume of 1 m3, an electron beam gun with an accelerating voltage of 30 kV and a maximum power of 30 kW, a water-cooled copper crucible, a circulation water cooling system and two independent vacuum systems. The crucible has a button shape and a maximum of 2 kg silicon can be melted in it.
Fig. 2. Power changes with time during the whole electron beam melting process.
Table 1 Melting conditions in electron beam melting experiment.
3.2. Materials and pretreatment MG-Si with an initial purity of 99.8 wt.% was used as the raw material, in which the initial content of aluminum was 1.71 101 wt.%. Prior to the processing by electron beam melting, it was washed sufficiently in alcohol by a supersonic wave cleaner to remove oil, organic compounds and possible extraneous impurities adhered to the surfaces. After that, it was put in a drying oven for 2 h.
Melting power (kW) Refining time (s) Solidification time (s) Pressure (Pa) Scanning pattern
Sample 1
Sample 2
Sample 3
18 900 Instantaneous 5 102– 5 103 Circle
18 900 2700 5 102– 5 103 Circle
18 900 3900 5 102– 5 103 Circle
3.3. Experimental procedure 600 g silicon was placed inside the water-cooled crucible, thereafter the chamber was evacuated to a pressure less than 5 102 Pa and the gun chamber was evacuated to a pressure less than 5 103 Pa. The electron beam was then irradiated on the surface of the silicon with a circular scanning pattern to ensure it was heated homogeneously and steadily. As shown in Fig. 2, the beam power increased gradually to 9 kW so that the silicon could be melted completely. The refining time was started once silicon was melted completely in the crucible. The melt was then maintained for 900 s with the beam power of 18 kW. After that, the power was stopped instantaneously or reduced exponentially. The experimental parameters are given in Table 1.
3.4. Testing and characterization The obtained silicon ingots were cut along the vertical cross section. It was polished and etched with 20% NaOH solution for 20 min so that the morphologies of the crystal can be observed clearly. The resistivity distribution on the cross section was measured by KDY-1 four-point probe resistivity tester. Five samples were chosen in the central region at 0%, 25%, 50%, 75%, and 100% of the ingot height for chemical analysis. As for each sample, the aluminum concentration was determined by inductively coupled plasma optical emission spectroscopy (ICP-MS).
Fig. 1. Schematic diagram of directional solidification of silicon induced by electron beam.
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S. Shi et al. / Separation and Purification Technology 152 (2015) 32–36
4. Results and discussion 4.1. Solidification rate During a general electron beam melting process, the molten pool is in a stable state under the action of heating by electron beam and cooling by water-cooled copper crucible. When it came to the pre-defined refining time, the beam was closed to end the melting process. Once the beam power is decreased, the silicon melt begins to solidify; however, the silicon solidifies completely when the beam power is decreased to some critical value but not 0. The critical power depends on the mass of the silicon and the cooling condition. Under this experimental condition, the value related to 600 g silicon is determined as 5.1 kW, shown as the dashed line in Fig. 2. Thus, the solidification time for each sample is the time when the power decreased from 18 kW to 5.1 kW, which are determined as 45 min and 65 min for sample 2 and sample 3, respectively. The solidification rates can be obtained through the solidification time and the height of the ingot as 9.82 106 m/s and 6.67 106 m/s, respectively. As for sample 1, the power was stopped instantaneously, so it is difficult to obtain its solidification rate. However, it is considered to be far larger than the others’ so that the impurities have no time to diffuse sufficiently. 4.2. Morphologies of silicon ingots Morphologies of the surfaces and vertical cross sections of the ingots with different cooling rates are shown in Fig. 3. The ingot has a rough surface when the power was stopped instantaneously, as shown in Fig. 3a. Under this condition, the region where the molten silicon contacted with the water-cooled copper crucible solidified firstly from bottom to top; meanwhile, the surface of the melt also began to solidify from top to bottom because a large amount of heat was taken away by radiation. After solidifying completely, there was a dividing line existing inside the ingot. Fine columnar crystallites can be observed in the area below the dividing line and coarse columnar crystallites can be observed in the area above the dividing line. The different grain morphology depends on the nucleation condition and solidification rate in the two areas. If the power was stopped with a reduction rate less than some critical value, the region where the molten silicon contacted with the water-cooled copper crucible also solidified firstly. Under this condition, the molten silicon is strongly cooled at the bottom by the water-cooled copper crucible and still heated at the top by the gradual lowering electron beam to form a unidirectional temperature gradient, resulting in the melt solidifying from the bottom to the top. As shown in Fig. 3b–c, the ingot has a smooth surface, where is the final solidification area. Fine columnar crystallites growing from bottom to top can be observed clearly in the vertical cross section, indicating a classical structure after directional solidification. 4.3. Resistivity distribution The resistivity distribution of silicon depends on the impurity distribution on some level, which can reflect the crystal growth process. It is relatively homogeneous in the case of solidification instantaneously and all of the values are around 2.5 X cm, as shown in Fig. 4a, indicating the homogeneous distribution of impurity. Silicon solidified very fast when the power was closed instantaneously so that the impurities have no time to diffuse sufficiently. As a consequence, the segregation behavior of impurity is not obvious. The resistivity values of some areas are far larger than the other areas, resulting from some cracks on the cross
section forming by internal stress after solidification. The resistivity distribution show a decreasing trend from the bottom to the top in the case of solidification induced by electron beam with exponential decreasing power, as shown in Fig. 4b–c. Impurity with low segregation coefficient will segregate at the solid–liquid interface and enrich into the liquid during directional solidification, leading to the decreasing trend of resistivity distribution. 4.4. Distribution of aluminum in silicon ingot Aluminum is one of the main impurities in silicon. The saturated vapor pressure of aluminum is larger than that of silicon, so it can be removed under a high temperature and high vacuum environment provided by electron beam melting [13]; meanwhile, the segregation coefficient of aluminum is smaller than 1, which means it can be removed by directional solidification. As a consequence, the removal efficiency and distribution of aluminum are considered to depend on both two removal mechanisms. The distributions of aluminum along growth direction of the ingot are shown in Fig. 5. The contents of aluminum are uniform in the case of instantaneous solidification. During melting process, a large part of aluminum evaporates from the melt free surface into the gas phase, and then is extracted by the vacuum system. The removal efficiency depends on the temperature, time and melt geometry. In this experiment, the aluminum content decreases from 1.71 101 wt.% to 1.14 101 wt.% with the refining time of 900 s. As for the ingot in the case of directional solidification induced by gradual lowering electron beam, the content of aluminum shows an obvious increasing trend. During solidification process, aluminum atoms segregate at the solid–liquid interface and concentrate into the liquid phase; meanwhile, aluminum atoms in the liquid phase also evaporate from melt free surface into the gas phase because the temperature of the un-solidified silicon is still above the melting point. It can be predicted with consideration of the segregation from silicon crystal to silicon melt as well as evaporation from melt to gaseous phase [15]:
C s ¼ keff C 0 ð1 f s Þ
keff þd
D 1 L-G v
ð1Þ
where Cs is the content of impurity incorporated into the solid phase, C0 is the initial content in the liquid at solidification beginning, fs is the solidified fraction, and dL-G is the free surface boundary layer thickness. keff is the effective segregation coefficient, depending on the equilibrium segregation coefficient k0, the growth rate v (m/s), the diffusion coefficient of impurity in molten silicon D (m2/s) and the solid–liquid interface boundary layer thickness dS-L (m), which can be expressed as [16]:
keff ¼
k0 k0 þ ð1 k0 Þ expðv dS-L =DÞ
ð2Þ
This exponent includes two characteristic items: keff and D/ dL-G v are suggested to be related to the effect of segregation and evaporation, respectively. The aluminum content after melting process is regarded as the initial value for the subsequent solidification process. Given that k0(Al) = 2 103 [17], D(Al) = 1.13 108 m2/s [18], dL-G = 8 103 m [16] as well as the solidification rate value mentioned above, and dS-L is set as 4 103 m [19], the aluminum distribution in silicon ingot can be simulated by Eq. (1), as shown in Fig. 5. The theoretical curves show agreement to the measured data. 4.5. Comparison with traditional electron beam melting Aluminum can be removed from silicon by traditional electron beam melting. The content of aluminum reduces with the increase
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Fig. 3. Morphologies of the surfaces and vertical sections of the ingots with different solidification rates ((a) Instantaneous solidification; (b) 2700 s; (c) 3900 s).
Fig. 4. Resistivity distribution on the vertical cross sections of the ingots with different solidification rates ((a) Instantaneous solidification; (b) 2700 s; (c) 3900 s).
Fig. 5. Distributions of aluminum along growth direction in electron beam melted silicon ingot.
Fig. 6. Comparison of removal efficiencies of aluminum with two different solidification modes.
of power or refining time. The removal efficiency can be expressed by
removal efficiency in different positions of the ingot after solidification induced by electron beam, and the dashed lines represent the removal efficiency after instantaneous solidification with different refining time. It is obvious that the removal efficiency of the former is higher than that of the latter in most area of the ingot except the top. In the end stage of solidification, the diffusibility and evaporability of aluminum decrease sharply with the decrease of melt temperature. As a consequence, it is concentrated and preserved at the top of the ingot, which will be cut to obtain purified silicon ingot. Besides, the evaporation loss of silicon and the energy consumption are also considered, as shown in Table 2. The results show that, compared with traditional electron beam melting, the
g¼1
Cs C0
ð3Þ
A larger power leads to higher removal efficiency with the same refining time, but higher energy consumption. Note that aluminum in silicon has both evaporation behavior and segregation behavior, so it is considered to be removed more efficiency by coupling of the two removal mechanisms during the solidification process induced by electron beam. Fig. 6 shows the removal efficiency of aluminum with two different solidification modes. The curves represent the
36
S. Shi et al. / Separation and Purification Technology 152 (2015) 32–36
Table 2 Loss of silicon and energy consumption by two different solidification modes. Experiment condition Refining 3600 s + Instantaneous solidification Refining 4800 s + Instantaneous solidification Refining 900 s + Solidification 2700 s Refining 900 s + Solidification 3900 s
Loss of silicon Energy consumption (g) (kJ) 7.80
48,600
10.38
70,200
3.48 4.98
22,271 31,248
loss of silicon reduced by more than 52% and the energy consumption reduced by more than 54%. The energy consumption is equal to the integral of beam power over refining time, so the decreasing of beam power leads to energy saving. Meanwhile, the temperature of the melt surface also decreased so that the evaporation of silicon was inhibited.
5. Conclusions The directional growth of silicon was achieved by controlling the electron beam with exponential decreasing power. During this process, the removal of aluminum can be improved by the influence of segregation on its evaporation. Compared with the traditional electron beam melting, the removal efficiency of aluminum is higher, indicating that it is a more effective way to remove impurity with high saturated vapor pressure and low segregation coefficient in silicon. During the whole process, the evaporation loss of silicon reduced by more than 52%, and the energy consumption also reduced by more than 54%.
Acknowledgements The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant No. U1137601 and 51304033) and Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20130041110004).
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