- Email: [email protected]
Lifetime improvement of photovoltaic silicon crystals grown by Czochralski technique using “liquinert” quartz crucibles
Journal of Crystal Growth 438 (2016) 76–80
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Lifetime improvement of photovoltaic silicon crystals grown by Czochralski technique using “liquinert” quartz crucibles Tetsuo Fukuda a,n, Yukichi Horioka b, Nobutaka Suzuki a, Masaaki Moriya a, Katsuto Tanahashi a, Shalamujiang Simayi a, Katsuhiko Shirasawa a, Hidetaka Takato a a Fukushima Renewable Energy Institute, National Institute of Advanced Industrial Science and Technology (AIST), 2-2-9 Machiike-dai, Koriyama, Fukushima 963-0215, Japan b Future Technology Business Research Institute Co. Ltd., 605 Tokatsu Techno Plaza, 5-4-6 Kashiwanoha, Kashiwa, Chiba 277-0882, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 22 June 2015 Received in revised form 24 December 2015 Accepted 27 December 2015 Communicated by Chung wen Lan Available online 7 January 2016
We succeeded in growing CZ monocrystalline silicon crystals with a longer lifetime than previously achieved. The MCZ technique was not used; instead, we employed melt-phobic quartz crucibles in a conventional CZ furnace. The improved lifetime is the result of reduced carbon incorporation into the growing crystals due to the suppression of SiO evaporation from the melt in the melt-phobic crucible. The melt-phobic effect of our crucibles has the potential to control the convection of molten silicon. & 2016 Elsevier B.V. All rights reserved.
Keywords: A2. Czocharlski method B2. Semiconducting silicon B3. Solar cells
1. Introduction In photovoltaic (PV) industry, conversion efficiency has been recognized as a key metric of solar cell technologies. Since crystalline silicon currently accounts for over 85% of all PV cell substrates, the superior one has been developed to achieve higher efficiency than before. Multicrystalline silicon growth techniques have made dramatic progress, such as in reducing the silicon nitride [1] and increasing the absorption of dislocations by shrinking the grain size [2], resulting in higher conversion efficiency of multicrystalline-based PV cells. On the other hand, the Czochralski (CZ) monocrystalline silicon growth technique has been considered to be a field with mature technologies. Currently, large-diameter CZ silicon crystals with the highest quality (such as long lifetime) are obtained by employing the magnetic-field-applied CZ (MCZ) technique, which makes it possible to grow low oxygen content silicon by suppressing the thermal convection of the melt, and then reducing the dissolution of the quartz crucible. n
Corresponding author. Tel.: þ 81 29 849 1547; fax: þ81 24 963 0824. E-mail addresses: [email protected] (T. Fukuda), [email protected] (Y. Horioka), [email protected] (N. Suzuki), [email protected] (M. Moriya), [email protected] (K. Tanahashi), [email protected] (S. Simayi), [email protected] (K. Shirasawa), [email protected] (H. Takato). http://dx.doi.org/10.1016/j.jcrysgro.2015.12.039 0022-0248/& 2016 Elsevier B.V. All rights reserved.
However, compared with the conventional CZ system, MCZ has several issues. First, there is an inevitable cost increase in growing silicon due to the use of superconductive magnets, a cryogenic refrigerator system, liquid helium and its infusion equipment. Secondly, the MCZ system strictly requires magnetic shielding because the maximum magnetic field reaches several thousand gauss (G) whereas the threshold limit value is 600 G for the whole body on an 8-h time weighted average, according to ACGIH threshold limit values for continuous exposure to static magnetic fields (ACGIH: American Conference of Governmental Industrial Hygienists [3]). Here, we report that it is possible to grow large-diameter p-type CZ monocrystalline silicon crystals with a longer lifetime than that conventionally achieved, without applying a magnetic field. The newly-developed CZ technique employs special quartz “liquinert (liquid inert)” crucibles [4, 5], the inner surfaces of which are treated to have a melt-phobic effect resulting in liquid inert surfaces [6]. These new crucibles are expected to suppress the dissolution of the quartz crucible during CZ silicon growth.
2. Experiments We grew boron (B)-doped monocrystalline silicon crystals having a diameter of 200 mm using a CZ puller in which everything but the liquinert crucible consists of conventional fixtures (heater, shield, and crucible support in the furnace) and
T. Fukuda et al. / Journal of Crystal Growth 438 (2016) 76–80
components. In this paper, we use the term “liquinert-crucible silicon” for the silicon grown from the melt contained in the liquinert crucible. We cut and shaped liquinert-silicon crystals into blocks having a cross section of 156 156 mm2 pseudo square and length of 210–250 mm. Next, we sliced these blocks into 200 μm-thick wafers using an electrodeposited-diamond multi-wire saw. After slicing, some sample wafers were cleaned in an ultrasonic bath filled with hot water, then chemically cleaned by surface-acting agents. We also prepared commercially available p-type conventional CZ wafers. After removing 10 μm-thick layers from both surfaces by KOH aqueous solution, we performed RCA cleaning, subsequent removal of natural oxide film by HF aqueous solution and then chemical passivation with quinhydrone–methanol aqueous solution. We measured the bulk lifetime values at five points on wafers taken from both conventional and liquinert-crucible silicon using the microwave photoconductive decay method (μ-PCD; Semilab PV-2000A). Phosphorus diffusion gettering (PDG) treatment was not performed as has been well done to improve lifetime values [7–9]. The concentration of oxygen and carbon impurities was measured by Fourier transform infrared spectroscopy (FTIR) or secondary ion mass spectroscopy (SIMS) and that of metal impurities (Fe, Cu, Cr, and Ni) by induction coupled plasma mass spectroscopy (ICP-MS). Resistivity was determined using the four-point probe measurement method.
3. Results Fig. 1 shows the position (framed in red) where post-growth crucible samples were taken to observe the boundary between the initial melt and the inner wall. Fig. 2 shows a comparison of the cross-sectional views in conventional (1) and liquinert (2) crucibles after the completion of growth. Fig. 2 (2) shows that the dissolution of crucible surface is suppressed because the contact interface below the initial melt curve remains almost unchanged. Fig. 3 is direct proof that the liquid-inert layer has a meltphobic effect. On the surface of the liquid-inert layer, a lump of multicrystalline silicon was heated and a silicon droplet was
77
obtained. After holding for 20 min, the droplet gradually solidified, allowing the growth of a U-shaped horn to absorb thermal expansion, and finally, a small solidified droplet was obtained as shown in the figure. Fig. 4 shows the dependence of oxygen concentration on solidification fraction for conventional and liquinert-crucible silicon. The oxygen concentration of the liquinert-cucible silicon is lower than that of conventional silicon due to the melt-phobic effect of our new cucible.
Wall thickness
Wall thickness
Initial melt surface curve
1mm
1mm Liquinert layer
Dissolved quartz wall
(1)
(2)
Fig. 2. The comparison of the cross-sectional views in conventional (1) and liquinert (2) crucibles after the completion of growth.
Silicon lump Liquinert layer
Quartz plate
Heater
10mm Fig. 1. The position (framed in red) where post-growth crucible samples were taken. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(1)
(2)
Fig. 3. The direct proof that the liquid-inert layer has a melt-phobic effect.
78
T. Fukuda et al. / Journal of Crystal Growth 438 (2016) 76–80
Fig. 5 shows the impurity concentrations (oxygen and carbon) of the liquinert-crucible silicon in (1) and conventional silicon MCZ in (2). Table 1 denotes the resistivity values of all sample wafers. In these samples, the oxygen concentration in both the liquinertcrucible silicon and conventional silicon MCZ is almost equal, whereas the carbon content in the liquinert-crucible silicon is lower (below the background level of 5 1014 cm 3) than that of 1.60
Conventional Liquinert
Oxygen concentration (× 1018cm-3)
1.40 1.20 1.00 0.80
conventional silicon MCZ. Note that the oxygen content measured in our study consists of both interstitial and precipitated oxygen atoms, different from the content measured by FTIR spectroscopy, which is frequently used to measure interstitial oxygen atoms. Bulk concentrations of metal impurities (Fe, Cu, Cr, and Ni) were below the background level (each o5 ng/g) for all sample wafers. Fig. 6 shows the results of the five-point lifetime measurement in the liquinert-crucible silicon and conventional silicon MCZ and CZ without phosphorus diffusion gettering. Position C is just the center of each sample wafer. Positions 1, 2, 3, and 4 are at intervals of 45° on the circumference of the circle having a radius of 45 mm from the center. Although the lifetime values of the conventional wafers vary from 258–384 μs, those of liquinert-crucible silicon are above 700 μs, twice to three times as large as those of the conventional silicon.
0.60
4. Discussion 0.40 0.20 0.00 0
20
40
60
80
100
Solidification fraction (%) Fig. 4. The dependence of oxygen concentrations on solidification fractions in both the conventional silicon and the liquinert-crucible silicon. Table 1 The resistivity values of all sample wafers. Sample wafers
Resistivity [p-type] (Ω cm)
Liquinert-crucible silicon Conventional silicon 1 Conventional silicon 2
2.6–2.8 2.8–2.9 2.3–2.5
Based on the comparison of carbon content in the liquinertcrucible silicon and conventional silicon MCZ (Fig. 5), we believe that the improvement in lifetime values is mainly the result of carbon reduction. Also, Higasa et al. [10] pointed out that carbon reduction is very effective in improving the lifetime of as-grown CZ silicon. Since it is well known that carbon in CZ silicon can become nucleation sites for oxygen precipitates, which degrade the lifetime [11,12], our results (Fig. 5) suggest that the amount of as-grown oxygen precipitates is lower in the liquinert-crucible silicon than in conventional silicon. In the conventional silicon MCZ, the lifetime value at the center is remarkably lower than those in the surrounding area. As described above, given that oxygen precipitates degrade the lifetime, the decrease in the conventional silicon MCZ suggests that the density of as-grown oxygen precipitates is higher in the center than in the periphery.
Fig. 5. The oxygen and carbon concentrations of the liquinert-crucible silicon (1) and conventional silicon (MCZ) in (2).
T. Fukuda et al. / Journal of Crystal Growth 438 (2016) 76–80
79
concentration of the melt OðmÞ is expressed as OðmÞ ¼ OðmD Þ þ OðmA Þ; where OðmD Þ is the concentration of oxygen from the dissolution of the crucible and OðmA Þ from the ambience, respectively. The continuity equation of oxygen flux is expressed as follows: RD þ RA ¼ RE þ RS ;
ð1Þ
where RD is the amount of oxygen dissolved from the crucible wall into the melt, RA that of oxygen absorbed by the melt in the form of CO from the ambience, RE that of oxygen evaporated from the melt surface, and RS that of oxygen incorporated into the growing silicon. Here, all variables in Eq. (1) are defined per unit area and unit time. Let the prime ð0 Þ denote each variable in growth by a liquinert crucible, that is, R0D þ R0A ¼ R0E þ R0S
ð2Þ
where again, all variables in Eq. (2) are defined per unit area and unit time. From the considerations described in the Appendix, it is assumed that RA {RD and R0A {R0D , and hence, Eqs. (1) and (2) are approximately expressed as RD RE þ RS ;
ð3Þ
and R0D R0E þ R0S
ð4Þ
Since our experimental results show that the oxygen content of the liquinert-crucible silicon is almost the same as that of the conventional silicon (Fig. 5), we can put R0S RS . Therefore, Fig. 6. The results of the five-point lifetime measurement in the liquinert-crucible silicon, conventional silicon (MCZ) and (CZ) without phosphorus diffusion gettering. Position C is just the center of each sample wafer. Positions 1, 2, 3, and 4 are at intervals of 45 on the circumference of the circle having a radius of 45 mm from the center.
R0D RD R0E RE
From the melt-phobic effect of the liquinert crucible, we can assume that R0 D o RD . Using this inequality and Eq. (5), we have R0E oRE :
Molten silicon Quartz crucible
RE(SiO) RA(CO)
Graphite crucible R S (O)
R D (O)
Graphite heater Graphite shield Fig. 7. Schematic drawing of impurity flow in CZ ambient.
Here, we consider the reason for the lower carbon content in the liquinert-crucible silicon. As is shown in Fig. 7, in the CZ growth furnace, silicon monoxide (SiO) evaporates from the melt surface just after the oxygen in the melt combines with molten silicon at the interface of the melt/argon ambience. Since the source of oxygen is both the dissolution of the quartz crucible and the absorption of carbon monoxide (CO) synthesized by the reaction between evaporated SiO and graphite fixtures (heater, shield, and crucible support in the CZ furnace) [13,14], the oxygen
ð5Þ
ð6Þ
Since Eq. (6) means that the evaporation rate of SiO from the melt in the liquinert crucible is lower than that in the conventional crucible, we believe that the amount of synthesized CO is smaller in the liquinert crucible ambience than that in the conventional ambience. This is why the carbon content of the liquinert-crucible silicon is lower than that of the conventional silicon.
5. Conclusion We conclude that the CZ technique employing liquinert crucibles makes it possible to grow monocrystalline silicon crystals with a greatly improved lifetime compared to using the conventional CZ technique and that this improvement is mainly due to the carbon reduction. Finally, we note that a study should be done for thermal and forced convection including the melt-phobic effect at the melt/ crucible interface. Since the diameter fluctuation of as-grown liquinert-crucible silicon is smaller than that of conventional silicon, the melt convection would be less active in a liquinert crucible than in a conventional crucible. We consider that the liquinert crucible will yield important scientific and technologic benefits for the fluid dynamics of molten silicon.
Appendix Since the absorbed oxygen from the ambience is in the state of CO, the contents of absorbed oxygen and carbon in the melt are the same.
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T. Fukuda et al. / Journal of Crystal Growth 438 (2016) 76–80
1. In conventional CZ ambience, the concentration of incorporated carbon in growing crystals is in the order of 2 1015 cm 3 (Fig. 4) and the segregation coefficient of carbon is 0.07 [15]. Hence, the concentration of carbon in the melt CðmA Þ and absorbed oxygen in the melt OðmA Þ can be estimated as CðmA Þ ¼ OðmA Þ
2 1015 0:07 3 1016
cm 3 :
On the other hand, since the segregation coefficient of oxygen is considered to be almost 1 [16], the oxygen in the melt OðmÞ can also be estimated as 1E18 cm 3, thirty times higher than that of OðmA Þ. This means that the oxygen absorbed by the melt from the ambience OðmA Þ is significantly lower than the oxygen OðmD Þ from the crucible, that is to say, RA {RD : 2. In the liquinert-crucible CZ ambience, the concentration of carbon in the melt C 0 ðmA Þ and absorbed oxygen in the melt O0 ðmA Þ can be estimated as C 0 ðmA Þ ¼ O0 ðmA Þ o
5 1014 0:07 ¼ 0:7 1016
cm 3 :
The oxygen O0 ðmÞ ¼ O0 ðmD Þ þ O0 ðmA Þ in the liquinert crucible can also be estimated as 1018 cm 3, two orders of magnitude higher than O0 ðmA Þ. This means that the oxygen absorbed by the melt from the ambience O0 ðmA Þ is significantly lower than that of the oxygen from the crucible O0 ðmD Þ, that is to say, R0A {R0D :
References [1] A.-K. Soiland, G. Halvorsen, K. Friestad, A.H. Amundsen, J.O. Odden, T. Ulset, Proceedings of the 29th EUPVSEC, 2014, pp. 536–541. [2] Z. Xia, K. Guo, H. Sun, H. Pan, J. Li, R. Liu, H. Cui, R. Sidhu, B. Zhang, G. Xing, Proceedings of the 29th EUPVSEC, 2014, pp. 542–547. [3] 〈http://pub.extranet.fsu.edu/sites/safety/safetywiki/Wiki%20Pages/ACGIH% 20Threshold%20Limit%20Values%20for%20Continuous%20Exposure%20to% 20Static%20Magnetic%20Fields.aspx〉. [4] S. Sakuragi, T. Shimasaki, G. Sakuragi, H. Nanba, Proceedings of the 19th EUPVSEC, 2004, pp. 1197–1200. [5] S. Sakuragi, Proceedings of the 19th EUPVSEC, 2004, pp. 1201–1204. [6] JP-4854814 (in Japanese). [7] M. Kivambe, D.M. Powell, S. Castellanos, M.A. Jensen, A.E. Morishige, K. Nakajima, K. Morishita, R. Murai, T. Buonassisi, J. Cryst. Growth 407 (2014) 31. [8] S. Joonwichien, I. Takahashi, S. Matsushima, N. Usami, Energy Procedia 55 (2014) 203. [9] I. Takahashi, S. Joonwichien, K. Kutsukake, S. Matsushima, I. Yonenaga and N. Usami, Proceedings of the 40th IEEE Photovoltaic Specialists Conference, 2014, p. 3017. [10] M. Higasa, Y. Nagai, S. Nakagawa, and K. Kashima, Abstr. No. 20a-A20-3, 75th Jpn. Soc. Appl. Phys. Autumn Meeting, 2014. [11] J.D. Murphy, K. Bothe, M. Olimo, V.V. Voronkov, R.J. Falster, J. Appl. Phys. 110 (2011) 053713. [12] M. Higasa, S. Nakagawa, K. Kashima, and Y. Nagai, Abstr. No. 12p-A18-6, 62nd Jpn. Soc. Appl. Phys. Spring Meeting, 2015. [13] T. Fukuda, M. Koizuka, A. Ohsawa, J. Electrochem. Soc. 141 (1994) 2216. [14] D.E. Bornside, R.A. Brown, T. Fujiwara, H. Fujiwara, T. Kubo, J. Electrochem. Soc. 142 (1995) 2790. [15] T. Nozaki, Y. Yatsurugi, N. Akiyama, J. Elctrochem. Soc. 117 (1970) 1566. [16] H. Harada, T. Itoh, N. Ozawa, and T. Abe, Electrochem Soc. Extended Abstr. 851, Toronto, Canada, May 12–17, 1985, p. 282.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Lifetime improvement of photovoltaic silicon crystals grown by Czochralski technique using “liquinert” quartz crucibles Tetsuo Fukuda a,n, Yukichi Horioka b, Nobutaka Suzuki a, Masaaki Moriya a, Katsuto Tanahashi a, Shalamujiang Simayi a, Katsuhiko Shirasawa a, Hidetaka Takato a a Fukushima Renewable Energy Institute, National Institute of Advanced Industrial Science and Technology (AIST), 2-2-9 Machiike-dai, Koriyama, Fukushima 963-0215, Japan b Future Technology Business Research Institute Co. Ltd., 605 Tokatsu Techno Plaza, 5-4-6 Kashiwanoha, Kashiwa, Chiba 277-0882, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 22 June 2015 Received in revised form 24 December 2015 Accepted 27 December 2015 Communicated by Chung wen Lan Available online 7 January 2016
We succeeded in growing CZ monocrystalline silicon crystals with a longer lifetime than previously achieved. The MCZ technique was not used; instead, we employed melt-phobic quartz crucibles in a conventional CZ furnace. The improved lifetime is the result of reduced carbon incorporation into the growing crystals due to the suppression of SiO evaporation from the melt in the melt-phobic crucible. The melt-phobic effect of our crucibles has the potential to control the convection of molten silicon. & 2016 Elsevier B.V. All rights reserved.
Keywords: A2. Czocharlski method B2. Semiconducting silicon B3. Solar cells
1. Introduction In photovoltaic (PV) industry, conversion efficiency has been recognized as a key metric of solar cell technologies. Since crystalline silicon currently accounts for over 85% of all PV cell substrates, the superior one has been developed to achieve higher efficiency than before. Multicrystalline silicon growth techniques have made dramatic progress, such as in reducing the silicon nitride [1] and increasing the absorption of dislocations by shrinking the grain size [2], resulting in higher conversion efficiency of multicrystalline-based PV cells. On the other hand, the Czochralski (CZ) monocrystalline silicon growth technique has been considered to be a field with mature technologies. Currently, large-diameter CZ silicon crystals with the highest quality (such as long lifetime) are obtained by employing the magnetic-field-applied CZ (MCZ) technique, which makes it possible to grow low oxygen content silicon by suppressing the thermal convection of the melt, and then reducing the dissolution of the quartz crucible. n
Corresponding author. Tel.: þ 81 29 849 1547; fax: þ81 24 963 0824. E-mail addresses: [email protected] (T. Fukuda), [email protected] (Y. Horioka), [email protected] (N. Suzuki), [email protected] (M. Moriya), [email protected] (K. Tanahashi), [email protected] (S. Simayi), [email protected] (K. Shirasawa), [email protected] (H. Takato). http://dx.doi.org/10.1016/j.jcrysgro.2015.12.039 0022-0248/& 2016 Elsevier B.V. All rights reserved.
However, compared with the conventional CZ system, MCZ has several issues. First, there is an inevitable cost increase in growing silicon due to the use of superconductive magnets, a cryogenic refrigerator system, liquid helium and its infusion equipment. Secondly, the MCZ system strictly requires magnetic shielding because the maximum magnetic field reaches several thousand gauss (G) whereas the threshold limit value is 600 G for the whole body on an 8-h time weighted average, according to ACGIH threshold limit values for continuous exposure to static magnetic fields (ACGIH: American Conference of Governmental Industrial Hygienists [3]). Here, we report that it is possible to grow large-diameter p-type CZ monocrystalline silicon crystals with a longer lifetime than that conventionally achieved, without applying a magnetic field. The newly-developed CZ technique employs special quartz “liquinert (liquid inert)” crucibles [4, 5], the inner surfaces of which are treated to have a melt-phobic effect resulting in liquid inert surfaces [6]. These new crucibles are expected to suppress the dissolution of the quartz crucible during CZ silicon growth.
2. Experiments We grew boron (B)-doped monocrystalline silicon crystals having a diameter of 200 mm using a CZ puller in which everything but the liquinert crucible consists of conventional fixtures (heater, shield, and crucible support in the furnace) and
T. Fukuda et al. / Journal of Crystal Growth 438 (2016) 76–80
components. In this paper, we use the term “liquinert-crucible silicon” for the silicon grown from the melt contained in the liquinert crucible. We cut and shaped liquinert-silicon crystals into blocks having a cross section of 156 156 mm2 pseudo square and length of 210–250 mm. Next, we sliced these blocks into 200 μm-thick wafers using an electrodeposited-diamond multi-wire saw. After slicing, some sample wafers were cleaned in an ultrasonic bath filled with hot water, then chemically cleaned by surface-acting agents. We also prepared commercially available p-type conventional CZ wafers. After removing 10 μm-thick layers from both surfaces by KOH aqueous solution, we performed RCA cleaning, subsequent removal of natural oxide film by HF aqueous solution and then chemical passivation with quinhydrone–methanol aqueous solution. We measured the bulk lifetime values at five points on wafers taken from both conventional and liquinert-crucible silicon using the microwave photoconductive decay method (μ-PCD; Semilab PV-2000A). Phosphorus diffusion gettering (PDG) treatment was not performed as has been well done to improve lifetime values [7–9]. The concentration of oxygen and carbon impurities was measured by Fourier transform infrared spectroscopy (FTIR) or secondary ion mass spectroscopy (SIMS) and that of metal impurities (Fe, Cu, Cr, and Ni) by induction coupled plasma mass spectroscopy (ICP-MS). Resistivity was determined using the four-point probe measurement method.
3. Results Fig. 1 shows the position (framed in red) where post-growth crucible samples were taken to observe the boundary between the initial melt and the inner wall. Fig. 2 shows a comparison of the cross-sectional views in conventional (1) and liquinert (2) crucibles after the completion of growth. Fig. 2 (2) shows that the dissolution of crucible surface is suppressed because the contact interface below the initial melt curve remains almost unchanged. Fig. 3 is direct proof that the liquid-inert layer has a meltphobic effect. On the surface of the liquid-inert layer, a lump of multicrystalline silicon was heated and a silicon droplet was
77
obtained. After holding for 20 min, the droplet gradually solidified, allowing the growth of a U-shaped horn to absorb thermal expansion, and finally, a small solidified droplet was obtained as shown in the figure. Fig. 4 shows the dependence of oxygen concentration on solidification fraction for conventional and liquinert-crucible silicon. The oxygen concentration of the liquinert-cucible silicon is lower than that of conventional silicon due to the melt-phobic effect of our new cucible.
Wall thickness
Wall thickness
Initial melt surface curve
1mm
1mm Liquinert layer
Dissolved quartz wall
(1)
(2)
Fig. 2. The comparison of the cross-sectional views in conventional (1) and liquinert (2) crucibles after the completion of growth.
Silicon lump Liquinert layer
Quartz plate
Heater
10mm Fig. 1. The position (framed in red) where post-growth crucible samples were taken. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(1)
(2)
Fig. 3. The direct proof that the liquid-inert layer has a melt-phobic effect.
78
T. Fukuda et al. / Journal of Crystal Growth 438 (2016) 76–80
Fig. 5 shows the impurity concentrations (oxygen and carbon) of the liquinert-crucible silicon in (1) and conventional silicon MCZ in (2). Table 1 denotes the resistivity values of all sample wafers. In these samples, the oxygen concentration in both the liquinertcrucible silicon and conventional silicon MCZ is almost equal, whereas the carbon content in the liquinert-crucible silicon is lower (below the background level of 5 1014 cm 3) than that of 1.60
Conventional Liquinert
Oxygen concentration (× 1018cm-3)
1.40 1.20 1.00 0.80
conventional silicon MCZ. Note that the oxygen content measured in our study consists of both interstitial and precipitated oxygen atoms, different from the content measured by FTIR spectroscopy, which is frequently used to measure interstitial oxygen atoms. Bulk concentrations of metal impurities (Fe, Cu, Cr, and Ni) were below the background level (each o5 ng/g) for all sample wafers. Fig. 6 shows the results of the five-point lifetime measurement in the liquinert-crucible silicon and conventional silicon MCZ and CZ without phosphorus diffusion gettering. Position C is just the center of each sample wafer. Positions 1, 2, 3, and 4 are at intervals of 45° on the circumference of the circle having a radius of 45 mm from the center. Although the lifetime values of the conventional wafers vary from 258–384 μs, those of liquinert-crucible silicon are above 700 μs, twice to three times as large as those of the conventional silicon.
0.60
4. Discussion 0.40 0.20 0.00 0
20
40
60
80
100
Solidification fraction (%) Fig. 4. The dependence of oxygen concentrations on solidification fractions in both the conventional silicon and the liquinert-crucible silicon. Table 1 The resistivity values of all sample wafers. Sample wafers
Resistivity [p-type] (Ω cm)
Liquinert-crucible silicon Conventional silicon 1 Conventional silicon 2
2.6–2.8 2.8–2.9 2.3–2.5
Based on the comparison of carbon content in the liquinertcrucible silicon and conventional silicon MCZ (Fig. 5), we believe that the improvement in lifetime values is mainly the result of carbon reduction. Also, Higasa et al. [10] pointed out that carbon reduction is very effective in improving the lifetime of as-grown CZ silicon. Since it is well known that carbon in CZ silicon can become nucleation sites for oxygen precipitates, which degrade the lifetime [11,12], our results (Fig. 5) suggest that the amount of as-grown oxygen precipitates is lower in the liquinert-crucible silicon than in conventional silicon. In the conventional silicon MCZ, the lifetime value at the center is remarkably lower than those in the surrounding area. As described above, given that oxygen precipitates degrade the lifetime, the decrease in the conventional silicon MCZ suggests that the density of as-grown oxygen precipitates is higher in the center than in the periphery.
Fig. 5. The oxygen and carbon concentrations of the liquinert-crucible silicon (1) and conventional silicon (MCZ) in (2).
T. Fukuda et al. / Journal of Crystal Growth 438 (2016) 76–80
79
concentration of the melt OðmÞ is expressed as OðmÞ ¼ OðmD Þ þ OðmA Þ; where OðmD Þ is the concentration of oxygen from the dissolution of the crucible and OðmA Þ from the ambience, respectively. The continuity equation of oxygen flux is expressed as follows: RD þ RA ¼ RE þ RS ;
ð1Þ
where RD is the amount of oxygen dissolved from the crucible wall into the melt, RA that of oxygen absorbed by the melt in the form of CO from the ambience, RE that of oxygen evaporated from the melt surface, and RS that of oxygen incorporated into the growing silicon. Here, all variables in Eq. (1) are defined per unit area and unit time. Let the prime ð0 Þ denote each variable in growth by a liquinert crucible, that is, R0D þ R0A ¼ R0E þ R0S
ð2Þ
where again, all variables in Eq. (2) are defined per unit area and unit time. From the considerations described in the Appendix, it is assumed that RA {RD and R0A {R0D , and hence, Eqs. (1) and (2) are approximately expressed as RD RE þ RS ;
ð3Þ
and R0D R0E þ R0S
ð4Þ
Since our experimental results show that the oxygen content of the liquinert-crucible silicon is almost the same as that of the conventional silicon (Fig. 5), we can put R0S RS . Therefore, Fig. 6. The results of the five-point lifetime measurement in the liquinert-crucible silicon, conventional silicon (MCZ) and (CZ) without phosphorus diffusion gettering. Position C is just the center of each sample wafer. Positions 1, 2, 3, and 4 are at intervals of 45 on the circumference of the circle having a radius of 45 mm from the center.
R0D RD R0E RE
From the melt-phobic effect of the liquinert crucible, we can assume that R0 D o RD . Using this inequality and Eq. (5), we have R0E oRE :
Molten silicon Quartz crucible
RE(SiO) RA(CO)
Graphite crucible R S (O)
R D (O)
Graphite heater Graphite shield Fig. 7. Schematic drawing of impurity flow in CZ ambient.
Here, we consider the reason for the lower carbon content in the liquinert-crucible silicon. As is shown in Fig. 7, in the CZ growth furnace, silicon monoxide (SiO) evaporates from the melt surface just after the oxygen in the melt combines with molten silicon at the interface of the melt/argon ambience. Since the source of oxygen is both the dissolution of the quartz crucible and the absorption of carbon monoxide (CO) synthesized by the reaction between evaporated SiO and graphite fixtures (heater, shield, and crucible support in the CZ furnace) [13,14], the oxygen
ð5Þ
ð6Þ
Since Eq. (6) means that the evaporation rate of SiO from the melt in the liquinert crucible is lower than that in the conventional crucible, we believe that the amount of synthesized CO is smaller in the liquinert crucible ambience than that in the conventional ambience. This is why the carbon content of the liquinert-crucible silicon is lower than that of the conventional silicon.
5. Conclusion We conclude that the CZ technique employing liquinert crucibles makes it possible to grow monocrystalline silicon crystals with a greatly improved lifetime compared to using the conventional CZ technique and that this improvement is mainly due to the carbon reduction. Finally, we note that a study should be done for thermal and forced convection including the melt-phobic effect at the melt/ crucible interface. Since the diameter fluctuation of as-grown liquinert-crucible silicon is smaller than that of conventional silicon, the melt convection would be less active in a liquinert crucible than in a conventional crucible. We consider that the liquinert crucible will yield important scientific and technologic benefits for the fluid dynamics of molten silicon.
Appendix Since the absorbed oxygen from the ambience is in the state of CO, the contents of absorbed oxygen and carbon in the melt are the same.
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T. Fukuda et al. / Journal of Crystal Growth 438 (2016) 76–80
1. In conventional CZ ambience, the concentration of incorporated carbon in growing crystals is in the order of 2 1015 cm 3 (Fig. 4) and the segregation coefficient of carbon is 0.07 [15]. Hence, the concentration of carbon in the melt CðmA Þ and absorbed oxygen in the melt OðmA Þ can be estimated as CðmA Þ ¼ OðmA Þ
2 1015 0:07 3 1016
cm 3 :
On the other hand, since the segregation coefficient of oxygen is considered to be almost 1 [16], the oxygen in the melt OðmÞ can also be estimated as 1E18 cm 3, thirty times higher than that of OðmA Þ. This means that the oxygen absorbed by the melt from the ambience OðmA Þ is significantly lower than the oxygen OðmD Þ from the crucible, that is to say, RA {RD : 2. In the liquinert-crucible CZ ambience, the concentration of carbon in the melt C 0 ðmA Þ and absorbed oxygen in the melt O0 ðmA Þ can be estimated as C 0 ðmA Þ ¼ O0 ðmA Þ o
5 1014 0:07 ¼ 0:7 1016
cm 3 :
The oxygen O0 ðmÞ ¼ O0 ðmD Þ þ O0 ðmA Þ in the liquinert crucible can also be estimated as 1018 cm 3, two orders of magnitude higher than O0 ðmA Þ. This means that the oxygen absorbed by the melt from the ambience O0 ðmA Þ is significantly lower than that of the oxygen from the crucible O0 ðmD Þ, that is to say, R0A {R0D :
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