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Effect of thermal annealing on minority-carrier lifetimes in multicrystalline Si wafers
Solar Energy Materials & Solar Cells 65 (2001) 459}463
E!ect of thermal annealing on minority-carrier lifetimes in multicrystalline Si wafers M. Mimura*, S. Ishikawa, T. Saitoh Department of Electrical and Electronic Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Nakacho, Koganei, Tokyo 184-8588, Japan
Abstract E!ects of high-temperature annealing conditions on minority-carrier-lifetime variation have been investigated for multicrystalline Si wafers. Lifetime recovery peaks appear obviously at temperatures over 10003C for the multicrystalline Si wafers, which is the same as for FZ and CZ single wafers. Bulk lifetimes degrade after a further long-time annealing in oxygen. This suggests that there exists the "lling of vacancies with interstitial Si atoms generated at the Si surfaces during high-temperature annealing. 2001 Elsevier Science B.V. All rights reserved. Keywords: Thermal annealing; Lifetimes; Multicrystalline Si wafers
1. Introduction Multicrystalline Si wafers have become prevalent in recent photovoltaic market. However, it needs further quality improvement for highly e$cient and low-cost solar cells by understanding the behaviors of impurities and defects in the multicrystalline Si wafers in more detail. Our previous work reported that di!usion length of minority-carriers was also a!ected by recombination centers except for Fe relating to nitrogen-vacancy complexes [1]. Takano et al. showed that the nitrogen-vacancy complexes exist during high-temperature annealing from the experiments on generation and annihilation of deep levels measured by deep-level transient spectroscopy for n-type single-crystalline Si wafers [2]. This phenomenon was also con"rmed in p-type, 1 k) cm, FZ Si single wafers by indicating the improvement of the bulk lifetimes after oxidation [3]. That might be useful for quality improvement of the multicrystalline Si
* Corresponding author. He is now with Kyocera Corp. 0927-0248/01/$ - 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 1 2 7 - 6
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M. Mimura et al. / Solar Energy Materials & Solar Cells 65 (2001) 459}463
wafers. In this paper, this phenomenon was applied to investigate the e!ect of thermal annealing on carrier lifetimes for multicrystalline Si wafers. 2. Experimental Samples in this experiment were p-type,1 ) cm, multicrystalline Si wafers provided by Kyocera Corporation and Bayer Solar Corporation. Sliced damage of the wafers was removed in an HNO /HF(20 : 1) solution. After removal of native oxide in HF(5%) solution, the wafers were oxidized in a dry O ambient at 9403C, 10003C and 10503C. Annealing times were from 0.5 to 4 h. For comparison, some wafers were annealed in N ambient at 9403C. p-type CZ, 2 ) cm, Si wafers were also oxidized in a dry O atmosphere at 10003C. After annealing, wafers were rapidly cooled and Si oxide was removed in an HF(5%) solution. Minority-carrier lifetimes were measured by a microwave photoconductivity decay method using a pulse laser of 904 nm. A chemical passivation (CP) technique using a solution of iodine in ethanol was used to reduce surface recombination velocity at wafer surfaces to obtain bulk lifetimes [4]. 3. Results and discussion Firstly, the e!ect of annealing conditions in oxygen on bulk lifetimes was investigated using p-type, FZ single Si wafers. As indicated in Fig. 1, bulk lifetimes recovered at annealing times of 1 and 1.5 h at annealing temperatures of 10503C and 10003C, respectively, and then degraded at a prolonged annealing. This lifetime recovery is considered by "lling vacancies with interstitial Si atoms generated at the Si surfaces in oxygen ambient.
Fig. 1. Variation of minority-carrier lifetimes with oxidation time for p-type, 1 k) cm, FZ Si single wafers.
M. Mimura et al. / Solar Energy Materials & Solar Cells 65 (2001) 459}463
461
Before a similar experiment for multicrystalline Si wafers, the e!ect of ambient gases on bulk lifetimes was examined at an annealing temperature of 9403C. As shown in Fig. 2, lifetimes of the multicrystalline wafers annealed in N ambient were lower than those annealed in O . In particular, the lifetime values in N ambient decreased drastically in short annealing time. This result is the same as that for p-type singlecrystalline Si wafers (1 k) cm), indicating that the lifetime degradation might be due to the generation of new recombination centers probably relating to nitrogen-vacancy complexes [5]. The same phenomenon occurred in the multicrystalline Si wafers. It should be pointed out that a longer oxidation caused reduction of lifetimes. It seems that appropriate interstitial Si formed in the oxidation process "lls the vacancies, whereas excess interstitial Si might recreate new recombination centers. Then, lifetime variations for the multicrystalline Si wafers were examined at various temperatures and times in O ambient. As shown in Fig. 3, lifetime variation of
Fig. 2. E!ect of ambient gas on lifetime variation for multicrystalline Si wafers annealed at 9403C.
Fig. 3. Variation of minority-carrier-lifetime ratio with oxidation time for multicrystalline Si wafers.
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M. Mimura et al. / Solar Energy Materials & Solar Cells 65 (2001) 459}463
Fig. 4. Comparison of lifetime variation for low-resistivity multicrystalline and CZ Si wafers annealed at 10003C in O
minority-carriers included recovery at annealing times of 1, 1.5 and 1 h at temperatures of 9403C, 10003C and 10503C, respectively. The lifetime recovery at 10003C was the highest though the values did not exceed the initial values. The peak lifetimes at 10503C and at 10003C appeared at annealing times which were the same as singlecrystalline wafers in Fig. 1. In addition, lifetimes for the multicrystalline Si wafers annealed at 10003C reduced to only 20% in spite of high-temperature annealing. The similar recovery of bulk lifetimes was also observed at 10003C for lowresistivity CZ and multicrystalline Si wafers as shown in Fig. 4. The oxidation time to obtain the peaks was 1.5 h for both the wafers, which is the same result as those shown in Figs. 1 and 3.
4. Conclusion Generally, bulk lifetimes of multicrystalline and also single-crystalline Si wafers annealed in N ambient tended to decrease with annealing time. However, lifetime recovery appeared at annealing times of 1 and 1.5 h at 9403C}10503C. This suggests that there exists the "lling of vacancies with interstitial Si atoms generated at the Si surfaces during high-temperature annealing.
Acknowledgements This work was supported by New Energy and Industrial Technology Development Organization as a part of the New Sunshine Program under the Ministry of International Trade and Industry, Japan. The authors would acknowledge to Kyocera Corporation for providing us multicrystalline Si wafers.
M. Mimura et al. / Solar Energy Materials & Solar Cells 65 (2001) 459}463
463
References [1] S. Ishikawa, S. Inoue, T. Saitoh, Proceedings of the 15th European Photovoltaic Solar Energy Conference, Vienna, 1998, p. 1314. [2] N. Fuma, K. Tashiro, K. Kakumoto, Y. Takano, Jpn. J. Appl. Phys. 35 (1996) 1993. [3] K. Yoshioka, S. Ishikawa, M. Mimira, T. Saitoh, Technical digest of 11th International Photovoltaic Science and Engineering Conference, Hokkaido, 1999, p. 559. [4] T.S. HoraH nyi, T. Pavelka, P. TuK ttoK , Appl. Surface Sci. 63 (1993) 306. [5] S. Ishikawa, private communications.
E!ect of thermal annealing on minority-carrier lifetimes in multicrystalline Si wafers M. Mimura*, S. Ishikawa, T. Saitoh Department of Electrical and Electronic Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Nakacho, Koganei, Tokyo 184-8588, Japan
Abstract E!ects of high-temperature annealing conditions on minority-carrier-lifetime variation have been investigated for multicrystalline Si wafers. Lifetime recovery peaks appear obviously at temperatures over 10003C for the multicrystalline Si wafers, which is the same as for FZ and CZ single wafers. Bulk lifetimes degrade after a further long-time annealing in oxygen. This suggests that there exists the "lling of vacancies with interstitial Si atoms generated at the Si surfaces during high-temperature annealing. 2001 Elsevier Science B.V. All rights reserved. Keywords: Thermal annealing; Lifetimes; Multicrystalline Si wafers
1. Introduction Multicrystalline Si wafers have become prevalent in recent photovoltaic market. However, it needs further quality improvement for highly e$cient and low-cost solar cells by understanding the behaviors of impurities and defects in the multicrystalline Si wafers in more detail. Our previous work reported that di!usion length of minority-carriers was also a!ected by recombination centers except for Fe relating to nitrogen-vacancy complexes [1]. Takano et al. showed that the nitrogen-vacancy complexes exist during high-temperature annealing from the experiments on generation and annihilation of deep levels measured by deep-level transient spectroscopy for n-type single-crystalline Si wafers [2]. This phenomenon was also con"rmed in p-type, 1 k) cm, FZ Si single wafers by indicating the improvement of the bulk lifetimes after oxidation [3]. That might be useful for quality improvement of the multicrystalline Si
* Corresponding author. He is now with Kyocera Corp. 0927-0248/01/$ - 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 1 2 7 - 6
460
M. Mimura et al. / Solar Energy Materials & Solar Cells 65 (2001) 459}463
wafers. In this paper, this phenomenon was applied to investigate the e!ect of thermal annealing on carrier lifetimes for multicrystalline Si wafers. 2. Experimental Samples in this experiment were p-type,1 ) cm, multicrystalline Si wafers provided by Kyocera Corporation and Bayer Solar Corporation. Sliced damage of the wafers was removed in an HNO /HF(20 : 1) solution. After removal of native oxide in HF(5%) solution, the wafers were oxidized in a dry O ambient at 9403C, 10003C and 10503C. Annealing times were from 0.5 to 4 h. For comparison, some wafers were annealed in N ambient at 9403C. p-type CZ, 2 ) cm, Si wafers were also oxidized in a dry O atmosphere at 10003C. After annealing, wafers were rapidly cooled and Si oxide was removed in an HF(5%) solution. Minority-carrier lifetimes were measured by a microwave photoconductivity decay method using a pulse laser of 904 nm. A chemical passivation (CP) technique using a solution of iodine in ethanol was used to reduce surface recombination velocity at wafer surfaces to obtain bulk lifetimes [4]. 3. Results and discussion Firstly, the e!ect of annealing conditions in oxygen on bulk lifetimes was investigated using p-type, FZ single Si wafers. As indicated in Fig. 1, bulk lifetimes recovered at annealing times of 1 and 1.5 h at annealing temperatures of 10503C and 10003C, respectively, and then degraded at a prolonged annealing. This lifetime recovery is considered by "lling vacancies with interstitial Si atoms generated at the Si surfaces in oxygen ambient.
Fig. 1. Variation of minority-carrier lifetimes with oxidation time for p-type, 1 k) cm, FZ Si single wafers.
M. Mimura et al. / Solar Energy Materials & Solar Cells 65 (2001) 459}463
461
Before a similar experiment for multicrystalline Si wafers, the e!ect of ambient gases on bulk lifetimes was examined at an annealing temperature of 9403C. As shown in Fig. 2, lifetimes of the multicrystalline wafers annealed in N ambient were lower than those annealed in O . In particular, the lifetime values in N ambient decreased drastically in short annealing time. This result is the same as that for p-type singlecrystalline Si wafers (1 k) cm), indicating that the lifetime degradation might be due to the generation of new recombination centers probably relating to nitrogen-vacancy complexes [5]. The same phenomenon occurred in the multicrystalline Si wafers. It should be pointed out that a longer oxidation caused reduction of lifetimes. It seems that appropriate interstitial Si formed in the oxidation process "lls the vacancies, whereas excess interstitial Si might recreate new recombination centers. Then, lifetime variations for the multicrystalline Si wafers were examined at various temperatures and times in O ambient. As shown in Fig. 3, lifetime variation of
Fig. 2. E!ect of ambient gas on lifetime variation for multicrystalline Si wafers annealed at 9403C.
Fig. 3. Variation of minority-carrier-lifetime ratio with oxidation time for multicrystalline Si wafers.
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M. Mimura et al. / Solar Energy Materials & Solar Cells 65 (2001) 459}463
Fig. 4. Comparison of lifetime variation for low-resistivity multicrystalline and CZ Si wafers annealed at 10003C in O
minority-carriers included recovery at annealing times of 1, 1.5 and 1 h at temperatures of 9403C, 10003C and 10503C, respectively. The lifetime recovery at 10003C was the highest though the values did not exceed the initial values. The peak lifetimes at 10503C and at 10003C appeared at annealing times which were the same as singlecrystalline wafers in Fig. 1. In addition, lifetimes for the multicrystalline Si wafers annealed at 10003C reduced to only 20% in spite of high-temperature annealing. The similar recovery of bulk lifetimes was also observed at 10003C for lowresistivity CZ and multicrystalline Si wafers as shown in Fig. 4. The oxidation time to obtain the peaks was 1.5 h for both the wafers, which is the same result as those shown in Figs. 1 and 3.
4. Conclusion Generally, bulk lifetimes of multicrystalline and also single-crystalline Si wafers annealed in N ambient tended to decrease with annealing time. However, lifetime recovery appeared at annealing times of 1 and 1.5 h at 9403C}10503C. This suggests that there exists the "lling of vacancies with interstitial Si atoms generated at the Si surfaces during high-temperature annealing.
Acknowledgements This work was supported by New Energy and Industrial Technology Development Organization as a part of the New Sunshine Program under the Ministry of International Trade and Industry, Japan. The authors would acknowledge to Kyocera Corporation for providing us multicrystalline Si wafers.
M. Mimura et al. / Solar Energy Materials & Solar Cells 65 (2001) 459}463
463
References [1] S. Ishikawa, S. Inoue, T. Saitoh, Proceedings of the 15th European Photovoltaic Solar Energy Conference, Vienna, 1998, p. 1314. [2] N. Fuma, K. Tashiro, K. Kakumoto, Y. Takano, Jpn. J. Appl. Phys. 35 (1996) 1993. [3] K. Yoshioka, S. Ishikawa, M. Mimira, T. Saitoh, Technical digest of 11th International Photovoltaic Science and Engineering Conference, Hokkaido, 1999, p. 559. [4] T.S. HoraH nyi, T. Pavelka, P. TuK ttoK , Appl. Surface Sci. 63 (1993) 306. [5] S. Ishikawa, private communications.