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Thin film poly-Si formation for solar cells by Flux method and Cat-CVD method
Solar Energy Materials & Solar Cells 69 (2001) 107}114
Thin "lm poly-Si formation for solar cells by Flux method and Cat-CVD method K. Niira, H. Hakuma, M. Komoda, K. Fukui, K. Shirasawa* Kyocera corporation, 10-1 Kawai, Gamo-cho, Gamo-gun, Shiga 529-1595, Japan
Abstract Two methods were examined for the formation of poly-Si "lms. One is #ux method and the other is Cat-CVD method. Flux method was used for forming poly-Si seed "lms on glass substrates covered with rear electrode. Poly-Si "lms of a few m grain size and of mainly (1 1 1) crystalline orientation were obtained at less than 6003C. To make the seed "lms function as BSF layer for solar cell, boron doping was applied and carrier concentration of 2;10/cm was obtained which is suitable for highly e$cient solar cells. Cat (catalytic)-CVD method was examined for forming poly-Si photo-active layers on the seed "lms. The "lms showed deposition gas pressure-dependent crystalline orientations and there was no amorphous incubation layer in (1 1 1) oriented "lms by Cat-CVD method when deposited on (1 1 1) oriented seed "lms prepared by Flux method. The electrical properties of the "lm are insu$cient at present, may be due to high defect density and the "lm structure which allows impurity contaminations of oxygen and carbon after "lm deposition. Although the "lm quality needs to be improved, poly-Si "lms whose crystal fraction is more than 85% were obtained at deposition rate of up to around 40 As /s. This result indicates high potential of Cat-CVD method for high throughput photo-active formation process necessary for low production cost thin "lm silicon solar cells. 2001 Elsevier Science B.V. All rights reserved. Keywords: Flux method; Cat-CVD method; Poly-Si "lm
1. Introduction Thin "lm poly-Si solar cells are expected to be the most promising candidates for next generation solar cells because of their potential for low production cost and use of low amounts of Si raw material.
* Corresponding author. E-mail address: [email protected] (K. Shirasawa). 0927-0248/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 0 ) 0 0 3 8 3 - 4
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K. Niira et al. / Solar Energy Materials & Solar Cells 69 (2001) 107}114
For low production cost, glass or stainless steel are suitable materials for coste!ective substrates. To use these substrates, low-temperature poly-Si "lm formation process (about less than 6003C) must be realized. For forming poly-Si "lms at low temperature, a variety of methods have been examined at many research institutes and companies [1}4]. We have been developing the #ux method and the Cat-CVD method [5}7]. The former is expected to be able to form large grain size poly-Si seed layer at a relatively low temperature in short process time, and the latter is expected to be able to form photo-active layer on the seed layer at high deposition rate and at a su$ciently low temperature.
2. Experiment 2.1. Formation of seed xlms by Flux method Fig. 1 shows typical Flux method process. Glass coated with rear electrode "lm was used as substrate and #ux material such as aluminum was deposited on the electrode by electron beam evaporation, then hydrogenated amorphous silicon (a-Si:H) "lms were deposited on the #ux layer by plasma-CVD method as starting material for the formation of poly-Si "lms. The typical thickness of amorphous silicon "lm and aluminum #ux "lm is about 300 and 200 nm, respectively. After heating at less than 6003C for the formation of poly-Si "lms, residual #ux was removed by acidic solu-
Fig. 1. Schematic process image of #ux method.
K. Niira et al. / Solar Energy Materials & Solar Cells 69 (2001) 107}114
109
Fig. 2. Schematic diagram of Cat-CVD apparatus.
tions. For boron doping, B H gas was mixed with SiH gas and H gas during the deposition of a-Si:H "lms. 2.2. Formation of photo-active xlms by Cat-CVD method Cat-CVD method was used to form poly-Si photo-active "lms. As a catalyzer, a tungsten (W) wire of 0.4}0.5 mm diameter was used and the catalyzer was preheated at around 25003C for a few minutes before deposition experiments. Poly-Si "lms were deposited at the catalyzer temperature of around 18003C by supplying DC power with the deposition parameter of gas pressure and SiH /H ratio, "xing substrate temperature at 2003C. The distance between catalyzer and substrate is about 5 cm. Fig. 2 shows schematic diagram of Cat-CVD apparatus.
3. Results and discussion 3.1. Films prepared by Flux method Fig. 3 shows cross-sectional TEM image of the "lm structure after the acidic solution treatment. Poly-Si "lms of a few m or large grain size were uniformly obtained on 10 cm;10 cm area size substrate and the "lms showed mainly (1 1 1) crystalline orientation. Fig. 4 shows carrier concentration of the "lm estimated from Hall measurements. Carrier concentration of 2;10/cm was obtained doping boron to a-Si:H starting material. The value is expected to be enough to make the seed layer function as a BSF layer necessary for highly e$cient solar cells.
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K. Niira et al. / Solar Energy Materials & Solar Cells 69 (2001) 107}114
Fig. 3. Cross sectional TEM image of poly-Si "lm obtained by #ux method.
Fig. 4. Boron doping e!ect for poly-Si seed "lm prepared by #ux method.
3.2. Films prepared by Cat-CVD method Table 1 shows main deposition conditions of Cat-CVD method. The crystal fraction of the "lm estimated from spectroscopic ellipsometry and the spin density measured by ESR analysis are also shown in the same table. Fig. 5 shows crystalline orientations of the "lm measured by XRD analysis. The "lm showed deposition pressure-dependent crystalline orientations which means crystalline orientation of photo-active layer by Cat-CVD method can be adjusted for the crystalline orientation of seed "lm to form better quality poly-Si photo-active "lm (low interface defect density, large grain size, and so on ). In the present case, because of the seed "lms prepared by Flux method are of (1 1 1) crystalline orientation, around 7 Pa deposition which allows to form (1 1 1) crystalline orientation "lms was carried out and it was con"rmed that there are no amorphous incubation layers in the "lms deposited on the seed "lms, from TEM image observation.
K. Niira et al. / Solar Energy Materials & Solar Cells 69 (2001) 107}114
111
Table 1 Deposition conditions and "lm properties of the "lm by Cat-CVD method Sample no. ¹sub (3C)
SiH (sccm)
H (sccm)
Press. (Pa)
¹ (3C)
X (%)
Spins (cm)(ESR)
601A 602A 602B 608A 608B 608C
5 10 20 5 10 20
100 100 100 100 100 100
0.95 0.65 0.70 7.0 7.0 7.0
.1800 .1800 .1800 .1800 .1800 .1800
81.5 85.4 0.0 55.8 77.9 81.8
1.5E18 1.6E18 1.1E18 2.1E17 3.3E17 7.0E17
2003C& 2003C& 2003C& 2003C& 2003C& 2003C&
Fig. 5. XRD pro"les of poly-Si "lms prepared by Cat-CVD method. Crystalline orientations are dependent on gas pressure.
Fig. 6 shows ESR spin density vs. deposition parameters. Although high crystalline fraction poly-Si "lms are obtained relatively easily, the spin density is higher than 1E16/cm order which is already obtained by conventional PECVD method. Because some impurity contaminations were suspected, SIMS analysis was carried out. Fig. 7 shows typical metal impurity concentrations of the "lm measured by SIMS analysis. W and Fe concentrations are about 1E15/cm or lower, which shows that the metal concentrations in the "lms are almost as low as device quality. But oxygen and carbon concentrations are too high for device quality as shown in Fig. 8. Because background residual gas pressure is less than 1E-4 Pa and W-catalyzer was su$ciently preheated (degassed), it seems to be unlikely to deduce that the contaminations of oxygen and carbon occurred during "lm deposition. So, the cause of the contaminations may be attributed to the "lm structure which allows di!usion of atmospheric gases like H O, O and CO into the "lm after deposition [6,7]. Fig. 9 shows deposition rate dependence of crystalline fraction and it was con"rmed that the poly-Si "lms of more than 85% crystalline fraction can be deposited at the
112
K. Niira et al. / Solar Energy Materials & Solar Cells 69 (2001) 107}114
Fig. 6. Defect density of the poly-Si "lms prepared by Cat-CVD method at various deposition conditions.
Fig. 7. Metal impurity concentrations of the poly-Si "lms prepared by Cat-CVD method. W(tungsten) catalyzer was preheated at 25003C before "lm deposition.
deposition rate at least up to 40 As /s. This result indicates high potential of Cat-CVD method for high throughput photo-active formation process necessary for low production cost thin "lm silicon solar cells.
K. Niira et al. / Solar Energy Materials & Solar Cells 69 (2001) 107}114
113
Fig. 8. Impurity concentrations of the poly-Si "lms prepared by Cat-CVD method. Data of conventional PECVD method were also shown for comparison.
Fig. 9. Crystalline fraction vs. deposition rate of Cat-CVD method. More than 85% crystalline fractions are maintained by at least up to 40 As /s.
4. Conclusions Poly-Si "lms of a few m grain size and of mainly (1 1 1) crystalline orientation were obtained at less than 6003C by #ux method. To make the seed "lms function as BSF
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K. Niira et al. / Solar Energy Materials & Solar Cells 69 (2001) 107}114
layer for solar cell, boron doping was examined and carrier concentration of 2;10/cm was obtained which is suitable for highly e$cient solar cells. Poly-Si "lms prepared by Cat-CVD method showed deposition gas-pressure dependent crystalline orientations and there was no amorphous incubation layer in (1 1 1) oriented "lms when deposited on (1 1 1) oriented seed "lms. Although it is necessary to reduce defect density and some contaminant elements, poly-Si "lms whose crystal fraction is more than 85% were obtained at deposition rates of up to around 40 As /s. Acknowledgements This work was supported by NEDO (New Energy and Industrial Technology Development Organization) as a part of the New Sunshine Program under Ministry of International Trade and Industry, Japan.
References [1] [2] [3] [4] [5] [6] [7]
K. Yamamoto et al., Second WCEPSEC, 1998, 1284}1289. J. Meier et al., Second WCEPSEC, 1998 pp. 375}380. J. Meier et al., Technical Digest of Eleventh PVSEC, Hokkaido, 1999, p. 221. O. Vetterl et al., Technical Digest of Eleventh PVSEC, Hokkaido, 1999, p. 233. H. Matsumura, Jpn. J. Appl. Phys. 37 (1998) 3175}3187. R.E.I. Schropp et al., Technical Digest of Eleventh PVSEC, Hokkaido, 1999, p. 929. R.E.I. Schropp, et al., Extended abstract of the International pre-workshop on Cat-CVD(HotWire)Process, Ishikawa, 1999, p. 37.
Thin "lm poly-Si formation for solar cells by Flux method and Cat-CVD method K. Niira, H. Hakuma, M. Komoda, K. Fukui, K. Shirasawa* Kyocera corporation, 10-1 Kawai, Gamo-cho, Gamo-gun, Shiga 529-1595, Japan
Abstract Two methods were examined for the formation of poly-Si "lms. One is #ux method and the other is Cat-CVD method. Flux method was used for forming poly-Si seed "lms on glass substrates covered with rear electrode. Poly-Si "lms of a few m grain size and of mainly (1 1 1) crystalline orientation were obtained at less than 6003C. To make the seed "lms function as BSF layer for solar cell, boron doping was applied and carrier concentration of 2;10/cm was obtained which is suitable for highly e$cient solar cells. Cat (catalytic)-CVD method was examined for forming poly-Si photo-active layers on the seed "lms. The "lms showed deposition gas pressure-dependent crystalline orientations and there was no amorphous incubation layer in (1 1 1) oriented "lms by Cat-CVD method when deposited on (1 1 1) oriented seed "lms prepared by Flux method. The electrical properties of the "lm are insu$cient at present, may be due to high defect density and the "lm structure which allows impurity contaminations of oxygen and carbon after "lm deposition. Although the "lm quality needs to be improved, poly-Si "lms whose crystal fraction is more than 85% were obtained at deposition rate of up to around 40 As /s. This result indicates high potential of Cat-CVD method for high throughput photo-active formation process necessary for low production cost thin "lm silicon solar cells. 2001 Elsevier Science B.V. All rights reserved. Keywords: Flux method; Cat-CVD method; Poly-Si "lm
1. Introduction Thin "lm poly-Si solar cells are expected to be the most promising candidates for next generation solar cells because of their potential for low production cost and use of low amounts of Si raw material.
* Corresponding author. E-mail address: [email protected] (K. Shirasawa). 0927-0248/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 0 ) 0 0 3 8 3 - 4
108
K. Niira et al. / Solar Energy Materials & Solar Cells 69 (2001) 107}114
For low production cost, glass or stainless steel are suitable materials for coste!ective substrates. To use these substrates, low-temperature poly-Si "lm formation process (about less than 6003C) must be realized. For forming poly-Si "lms at low temperature, a variety of methods have been examined at many research institutes and companies [1}4]. We have been developing the #ux method and the Cat-CVD method [5}7]. The former is expected to be able to form large grain size poly-Si seed layer at a relatively low temperature in short process time, and the latter is expected to be able to form photo-active layer on the seed layer at high deposition rate and at a su$ciently low temperature.
2. Experiment 2.1. Formation of seed xlms by Flux method Fig. 1 shows typical Flux method process. Glass coated with rear electrode "lm was used as substrate and #ux material such as aluminum was deposited on the electrode by electron beam evaporation, then hydrogenated amorphous silicon (a-Si:H) "lms were deposited on the #ux layer by plasma-CVD method as starting material for the formation of poly-Si "lms. The typical thickness of amorphous silicon "lm and aluminum #ux "lm is about 300 and 200 nm, respectively. After heating at less than 6003C for the formation of poly-Si "lms, residual #ux was removed by acidic solu-
Fig. 1. Schematic process image of #ux method.
K. Niira et al. / Solar Energy Materials & Solar Cells 69 (2001) 107}114
109
Fig. 2. Schematic diagram of Cat-CVD apparatus.
tions. For boron doping, B H gas was mixed with SiH gas and H gas during the deposition of a-Si:H "lms. 2.2. Formation of photo-active xlms by Cat-CVD method Cat-CVD method was used to form poly-Si photo-active "lms. As a catalyzer, a tungsten (W) wire of 0.4}0.5 mm diameter was used and the catalyzer was preheated at around 25003C for a few minutes before deposition experiments. Poly-Si "lms were deposited at the catalyzer temperature of around 18003C by supplying DC power with the deposition parameter of gas pressure and SiH /H ratio, "xing substrate temperature at 2003C. The distance between catalyzer and substrate is about 5 cm. Fig. 2 shows schematic diagram of Cat-CVD apparatus.
3. Results and discussion 3.1. Films prepared by Flux method Fig. 3 shows cross-sectional TEM image of the "lm structure after the acidic solution treatment. Poly-Si "lms of a few m or large grain size were uniformly obtained on 10 cm;10 cm area size substrate and the "lms showed mainly (1 1 1) crystalline orientation. Fig. 4 shows carrier concentration of the "lm estimated from Hall measurements. Carrier concentration of 2;10/cm was obtained doping boron to a-Si:H starting material. The value is expected to be enough to make the seed layer function as a BSF layer necessary for highly e$cient solar cells.
110
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Fig. 3. Cross sectional TEM image of poly-Si "lm obtained by #ux method.
Fig. 4. Boron doping e!ect for poly-Si seed "lm prepared by #ux method.
3.2. Films prepared by Cat-CVD method Table 1 shows main deposition conditions of Cat-CVD method. The crystal fraction of the "lm estimated from spectroscopic ellipsometry and the spin density measured by ESR analysis are also shown in the same table. Fig. 5 shows crystalline orientations of the "lm measured by XRD analysis. The "lm showed deposition pressure-dependent crystalline orientations which means crystalline orientation of photo-active layer by Cat-CVD method can be adjusted for the crystalline orientation of seed "lm to form better quality poly-Si photo-active "lm (low interface defect density, large grain size, and so on ). In the present case, because of the seed "lms prepared by Flux method are of (1 1 1) crystalline orientation, around 7 Pa deposition which allows to form (1 1 1) crystalline orientation "lms was carried out and it was con"rmed that there are no amorphous incubation layers in the "lms deposited on the seed "lms, from TEM image observation.
K. Niira et al. / Solar Energy Materials & Solar Cells 69 (2001) 107}114
111
Table 1 Deposition conditions and "lm properties of the "lm by Cat-CVD method Sample no. ¹sub (3C)
SiH (sccm)
H (sccm)
Press. (Pa)
¹ (3C)
X (%)
Spins (cm)(ESR)
601A 602A 602B 608A 608B 608C
5 10 20 5 10 20
100 100 100 100 100 100
0.95 0.65 0.70 7.0 7.0 7.0
.1800 .1800 .1800 .1800 .1800 .1800
81.5 85.4 0.0 55.8 77.9 81.8
1.5E18 1.6E18 1.1E18 2.1E17 3.3E17 7.0E17
2003C& 2003C& 2003C& 2003C& 2003C& 2003C&
Fig. 5. XRD pro"les of poly-Si "lms prepared by Cat-CVD method. Crystalline orientations are dependent on gas pressure.
Fig. 6 shows ESR spin density vs. deposition parameters. Although high crystalline fraction poly-Si "lms are obtained relatively easily, the spin density is higher than 1E16/cm order which is already obtained by conventional PECVD method. Because some impurity contaminations were suspected, SIMS analysis was carried out. Fig. 7 shows typical metal impurity concentrations of the "lm measured by SIMS analysis. W and Fe concentrations are about 1E15/cm or lower, which shows that the metal concentrations in the "lms are almost as low as device quality. But oxygen and carbon concentrations are too high for device quality as shown in Fig. 8. Because background residual gas pressure is less than 1E-4 Pa and W-catalyzer was su$ciently preheated (degassed), it seems to be unlikely to deduce that the contaminations of oxygen and carbon occurred during "lm deposition. So, the cause of the contaminations may be attributed to the "lm structure which allows di!usion of atmospheric gases like H O, O and CO into the "lm after deposition [6,7]. Fig. 9 shows deposition rate dependence of crystalline fraction and it was con"rmed that the poly-Si "lms of more than 85% crystalline fraction can be deposited at the
112
K. Niira et al. / Solar Energy Materials & Solar Cells 69 (2001) 107}114
Fig. 6. Defect density of the poly-Si "lms prepared by Cat-CVD method at various deposition conditions.
Fig. 7. Metal impurity concentrations of the poly-Si "lms prepared by Cat-CVD method. W(tungsten) catalyzer was preheated at 25003C before "lm deposition.
deposition rate at least up to 40 As /s. This result indicates high potential of Cat-CVD method for high throughput photo-active formation process necessary for low production cost thin "lm silicon solar cells.
K. Niira et al. / Solar Energy Materials & Solar Cells 69 (2001) 107}114
113
Fig. 8. Impurity concentrations of the poly-Si "lms prepared by Cat-CVD method. Data of conventional PECVD method were also shown for comparison.
Fig. 9. Crystalline fraction vs. deposition rate of Cat-CVD method. More than 85% crystalline fractions are maintained by at least up to 40 As /s.
4. Conclusions Poly-Si "lms of a few m grain size and of mainly (1 1 1) crystalline orientation were obtained at less than 6003C by #ux method. To make the seed "lms function as BSF
114
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layer for solar cell, boron doping was examined and carrier concentration of 2;10/cm was obtained which is suitable for highly e$cient solar cells. Poly-Si "lms prepared by Cat-CVD method showed deposition gas-pressure dependent crystalline orientations and there was no amorphous incubation layer in (1 1 1) oriented "lms when deposited on (1 1 1) oriented seed "lms. Although it is necessary to reduce defect density and some contaminant elements, poly-Si "lms whose crystal fraction is more than 85% were obtained at deposition rates of up to around 40 As /s. Acknowledgements This work was supported by NEDO (New Energy and Industrial Technology Development Organization) as a part of the New Sunshine Program under Ministry of International Trade and Industry, Japan.
References [1] [2] [3] [4] [5] [6] [7]
K. Yamamoto et al., Second WCEPSEC, 1998, 1284}1289. J. Meier et al., Second WCEPSEC, 1998 pp. 375}380. J. Meier et al., Technical Digest of Eleventh PVSEC, Hokkaido, 1999, p. 221. O. Vetterl et al., Technical Digest of Eleventh PVSEC, Hokkaido, 1999, p. 233. H. Matsumura, Jpn. J. Appl. Phys. 37 (1998) 3175}3187. R.E.I. Schropp et al., Technical Digest of Eleventh PVSEC, Hokkaido, 1999, p. 929. R.E.I. Schropp, et al., Extended abstract of the International pre-workshop on Cat-CVD(HotWire)Process, Ishikawa, 1999, p. 37.