CRYSTAL GROWTH & DESIGN
Directional Solidification of Multicrystalline Silicon Using the Accelerated Crucible Rotation Technique
2008 VOL. 8, NO. 7 2525–2527
R. Bairava Ganesh,†,‡ Hitoshi Matsuo,† Yoshihiro Kangawa,†,§ Koji Arafune,| Yoshio Ohshita,| Masafumi Yamaguchi,| and Koichi Kakimoto*,†,§ Graduate School of Engineering, Kyushu UniVersity, 6-1, Kasuga-Koen, Kasuga, Fukuoka 816-8580, Japan, Crystal Growth Centre, Anna UniVersity, Chennai 600 025, India, Research Institute for Applied Mechanics, Kyushu UniVersity, 6-1, Kasuga-Koen, Kasuga, Fukuoka 816-8580, Japan, and Toyota Technological Institute, 2-12-1, Hisakata, Tempaku-ku, Nagoya 468-8511, Japan ReceiVed February 12, 2008; ReVised Manuscript ReceiVed March 17, 2008
ABSTRACT: Employing the accelerated crucible rotation technique, we grew multicrystalline silicon by the directional solidification process. The distribution of carbon concentration determined by Fourier transform infrared spectroscopy demonstrated that application of accelerated crucible rotation homogenized the carbon concentration in the grown ingot. Attempts were made to explain the effect of crucible rotation on homogenization of carbon concentration in terms of segregation phenomena. Moreover, growth striations induced by the crucible rotation were observed in the axial direction of the ingot.
1. Introduction In the photovoltaic industry, more than 90% of annual solar cell production is based on silicon in the form of single crystalline, multicrystalline wafer and thin film modules.1 The market share of multicrystalline silicon (mc-Si) has increased remarkably in recent years.2 The directional solidification (DS) process is a cost-effective technique for producing mc-Si. For reducing cost, low-quality feedstock, which contains many light impurities such as oxygen, carbon and nitrogen, is used in the DS process.3 The segregation of these impurities in mc-Si is a serious problem, which degrades the quality of the grown ingot.4–6 For less time consumption, the mc-Si is being grown by a rapid solidification process using high growth rates; hence, the concentration of impurities is high and inhomogeneous throughout the volume of the ingot.7 Among these impurities, carbon originates primarily from the feedstock, graphite crucible, insulation and heating elements in the furnace. Carbon is substitutionally dissolved in silicon crystals and tetrahedrally bonded to silicon atoms, resulting in precipitation in the form of silicon carbide (SiC) if the concentration exceeds the solubility limit.8 These SiC precipitates can cause severe ohmic shunts in solar cells.9 Furthermore, during the wafer sawing process, these precipitates in the wafer damage the wire saw.10 Therefore, an effective method for homogenizing the carbon concentration in mc-Si is needed. In this paper, we describe the directional solidification of mcSi by using the accelerated crucible rotation technique (ACRT). Fourier transform infrared (FTIR) spectroscopy was used to determine the carbon concentration in the grown ingot. The effects of the ACRT on the directional solidification of mc-Si and the homogenization of carbon concentration in the grown ingot are discussed. The growth striations induced by the ACRT are also discussed in detail. * Corresponding author: Mailing address: RIAM, Kyushu University, 6-1, Kasuga-Koen, Kasuga, Fukuoka 816-8580, Japan. Tel: +81-92-583-7741. Fax: +81- 92-583-7743. E-mail address:
[email protected]. † Graduate School of Engineering, Kyushu University. ‡ Anna University. § Research Institute for Applied Mechanics, Kyushu University. | Toyota Technological Institute.
Figure 1. ACRT cycle used in the present experiment.
2. Accelerated Crucible Rotation Technique Generally, the ACRT is used to produce forced convection in the melt and thereby enhance mixing of the melt.11 The basic idea is to transfer momentum of a crucible wall to the melt by modifying the rotation rate of a crucible as a function of time. As the rotation rate changes, an Ekman flow layer is built up and drives the melt flow either spin-up or spin-down.12 With the combination of spin-up and spin-down based on Ekman flow cycles, the ACRT has been widely used for mixing the melt and thus for enhancing mass transfer.13 The constitutional supercooling phenomenon in the melt growth may lead to morphological instability through interface breakdown. The ACRT eliminates the constitutional supercooling, thus restricting nucleation to one crystal or a few crystals in the case of nonseeded growth.14 It has been reported that crucible rotation in directional solidification of mc-Si is advantageous for reducing the SiC particle precipitation.15 In the present experiment, the duration of the ACRT was determined by calculating the spin-up time using the following equation:
τ )
1 0.5
E Ω
10.1021/cg800160g CCC: $40.75 2008 American Chemical Society Published on Web 05/28/2008
(1)
2526 Crystal Growth & Design, Vol. 8, No. 7, 2008
Figure 2. Temperature profile and heater movement during the growth of mc-Si.
where E and Ω are the Ekman number and maximum crucible rotation rate, respectively.16,17 The calculated values are E ) 0.00132, Ω ) 5 rpm and τ ) 329 s. However, for experimental feasibility we used 15 s as acceleration, holding and deceleration times, respectively, with maximum rotation rate of 5 rpm. The period of one ACRT cycle was 3 min, and the ACRT was applied 36 times with 15 min intervals during solidification. The applied ACRT cycle is shown in Figure 1.
3. Experimental Section The growth experiment was carried out in a directional solidification furnace at Toyota Technological Institute. Off-grade silicon feedstock with gallium dopant (1017 atoms/cm3) was placed in a cylindrical quartz crucible of 10 cm in diameter. The crucible had been coated with Si3N4, which prevents the ingot from sticking to the crucible. The materials were heated up to 1550 °C, just above the melting point of silicon, in an argon atmosphere inside the furnace. After melting, the temperature was lowered to 1450 °C. Then heater movement was initiated from
Bairava Ganesh et al. the bottom to the top of the furnace at a rate of 12 mm/h for solidification. The ACRT was applied simultaneously with the heater movement during solidification. After the heater had almost reached the top of the furnace, the temperature was lowered at a rate of 300 deg/h to room temperature. The entire growth cycle is shown in Figure 2. The grown ingot was sliced vertically parallel to the growth direction. The wafers were roughly etched with a solution of hydrofluoric acid and nitric acid in a ratio of 1:13 for 10 min to remove the sawing damage. To obtain a mirrorlike surface, the mc-Si wafers were polished with diamond particles of 0.5 and 0.3 µm in grain size, coordinated with water. Then, the wafers were polished with alumina powder particles of 0.1 µm in grain size and deionized water. Finally, each wafer was washed with acetone, methanol and water sequentially. After polishing, the wafers were etched for 3 min using the same etchant as that described above to remove the polishing damage. A JASCO (MFT-2000) FTIR spectrophotometer was used to measure the carbon concentration in silicon wafers. The measurement was done at room temperature in air atmosphere. The thickness of the samples for FTIR spectroscopy was 0.5 mm. The measurement range of wavenumber was set from 500 to 1200 cm-1 with a resolution of 4 cm-1. A nondoped Czochralski silicon sample was used as a reference. Both the test and reference samples were carefully prepared by keeping the thickness variation less than 0.005 mm with identical surface preparation. The two-phonon lattice bands of silicon at 610 cm-1 are superimposed on the band of carbon and affect the intensity of the carbon peak. Hence, these silicon lattice vibrations were subtracted using an FTIR spectrum of a pure Czochralski silicon sample. The optical absorption line at 605 cm-1, which is associated with the vibration of carbon in the substitutional position, was used for analyzing the carbon concentration.18 The carbon concentration was determined by ASTM standards.19
4. Results and Discussion In order to determine the effect of the ACRT on the homogenization of carbon concentration, the results were compared with those for another ingot grown under similar growth conditions without the ACRT. Figures 3 (a) and (b) show cross-sectional images of the ingot and carbon concentration distribution of mcSi grown with and without the ACRT, respectively. The carbon concentration distribution at the upper region of the ingot grown by the ACRT is not shown in Figure 3 (a). This is because the carrier concentration in the upper region is high, and we were therefore not able to measure the carbon concentration in this
Figure 3. (a) Photograph of the ACRT-grown mc-Si wafer with corresponding carbon concentration distribution. (b) Photograph of a conventionally grown mc-Si wafer with corresponding carbon concentration distribution.
Directional Solidification of Multicrystalline Si
Crystal Growth & Design, Vol. 8, No. 7, 2008 2527
nated by unsteady flow of the melt which makes back-melting.23 The formation of these striations is thought to be suppressed by choosing the proper interval of the ACRT. The ACRT can be implemented with the directional solidification process for the growth of large size mc-Si ingots. It can be achieved by using a suitable rotational assembly with the growth system. Moreover, a slow rotation rate would be enough to obtain better results in the industrial growth system.
5. Conclusion
Figure 4. Carbon concentration profiles in the axial direction of mc-Si grown by the ACRT and grown without the ACRT.
region. In Figure 3 (b), it can be seen the ingot grown without the ACRT has an inhomogeneous distribution of carbon concentration along radial and axial directions with a high local concentration in the central area. Along the radial direction, the variation of carbon concentration in Figure 3(a) is less than that in Figure 3 (b). The results imply that effective mixing of the melt by the ACRT resulted in homogenization of the carbon concentration along the radial direction, though a small variation along the radial direction due to the back-melting process during the ACRT was observed as shown in Figure 3 (a). Stirring by convection causes a relatively homogeneous distribution of composition in the interior part of the zone, whereas concentration gradients are formed in regions adjacent to the interfaces. Hence, the carbon concentration was high at the center and top part of the ingot grown without the ACRT. When the ACRT is used, the Ekman layer flow has a strong effect on the mixing of the melt. The combination of spin-up and spin-down processes changes the absolute value of the concentration of the species at the interface. Figure 4 shows the carbon concentration profiles in the axial direction of the mc-Si grown by the ACRT and that grown without the ACRT. In addition, during solidification, the imbalance between rejection of impurities from the growing crystal surface and mass transfer of impurities away from the crystal surface results in the formation of a concentration boundary layer in front of the solid-liquid interface. Application of the ACRT reduces the thickness of the impurity boundary layer by effective mixing and thus results in less radial segregation in the grown ingot. Another possible reason for the occurrence of radial segregation is incomplete mixing by convection.20 Application of the ACRT results in forced convection in a crucible, which effectively decreases the radial segregation. The ACRT has numerous advantages over conventional methods, but it also has some drawbacks. Due to the oscillatory nature of the ACRT, it can induce growth striations in the growing crystal. ACRT-induced growth striations have been reported by many authors.21,22 We observed growth striations induced by the ACRT in the axial direction of the grown ingot. Striation patterns were visible only at the time when the ACRT is switched on, possibly due to a sudden change in the crucible’s angular velocity. Due to the periodic fluctuation in the growth rate, the ingot experiences back-melting. Some of the missed striations could account for the back-melting. The striations were formed in accordance with the time difference between the ACRT cycles. Moreover, the missing striations are also origi-
Employing the ACRT technique, mc-Si was grown by the directional solidification process. Carbon concentration in the grown ingot was determined by using FTIR spectroscopy. The carbon concentration in the ingot grown by the ACRT was more homogeneous than that in a conventionally grown ingot. This implies that application of the ACRT resulted in the homogenization of carbon concentration in the grown ingot. Weak growth striations were observed in the axial direction of the ingot due to the oscillatory nature of the ACRT. Acknowledgment. One of the authors (R.B.G.) thanks the Ministry of Education, Culture, Sports, Science and Technology, Japan, for the award of Japanese Government Scholarship through the Ministry of Human Resource and Development, India. He is also thankful to Anna University, India, to carry out this work in Japan. This work was supported by a NEDO project, a Grant-in-Aid for Scientific Research (B) 06990707-0 from the Japanese Ministry of Education, Culture, Sports, Science and Technology.
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CG800160G
