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Oxygen-related centers in multicrystalline silicon
Solar Energy Materials & Solar Cells 62 (2000) 37}42
Oxygen-related centers in multicrystalline silicon Deren Yang *, Liben Li , Xiangyang Ma , Ruixin Fan, Duanlin Que , H.J. Moeller State Key Lab of Silicon Material Science, Zhejiang University, Hangzhou 310027, People's Republic of China Institute for Experimental Physics, TU Bergakademi Freiberg, SilbermannStr.1, 09596 Freiberg, Germany
Abstract Oxygen and oxygen-related centers in cast multicrystalline silicon (mc-Si) have been investigated. The concentration pro"le and the map of oxygen in mc-Si revealed that oxygen with higher concentration occurred at the bottom area and at the edge area of mc-Si ingots. The oxygen concentration and its pro"le were partly dependent on the cooling progress. It was found that the thermal donors related to oxygen were generated in the area of higher oxygen concentration. The concentration of thermal donors can reach to 1;10/cm. Meanwhile, as-grown oxygen precipitates were observed at the bottom of mc-Si. 2000 Elsevier Science B.V. All rights reserved. Keywords: Multicrystalline silicon; Oxygen; Thermal donors
1. Introduction Photovoltaic industry is predominantly using crystalline silicon technology. Due to low cost, cast multicrystalline silicon is increasingly used in photovoltaic industry. However, the e$ciency of solar cells based on mc-Si is limited by grain boundaries, dislocations, microdefects and impurities [1]. Oxygen is an important and main impurity in mc-Si. It is mostly induced into mc-Si due to quartz crucible contamination. In Czochralski single-crystal silicon (CZ Si) used in microelectronics industry, oxygen has been extensively investigated in the past 40 years. It has been believed that the oxygen-related centers in CZ silicon, such as thermal donors [2,3] and oxygen precipitates [4,5], seriously a!ect the electrical and
* Corresponding author. Tel.: #86-571-799-1432; fax: #86-571-795-1954. E-mail address: [email protected] (D. Yang) 0927-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 9 9 ) 0 0 1 3 3 - 6
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D. Yang et al. / Solar Energy Materials & Solar Cells 62 (2000) 37}42
mechanical properties of silicon during device processes. In CZ silicon and in ribbon multicrystalline silicon used for solar cells, the e!ect of oxygen on the conversion e$ciency of solar cells has controversially been reported [6}12]. It was also found that in mc-Silicon annealed in the temperature of 600}10003C the electrically active donors linked to oxygen precipitates were formed [13,14]. However, due to the special manufacture process of mc-Si, the grain boundaries, dislocations and microdefects with high density are generated during cooling. The behaviors of the oxygen-related centers in as-grown mc-Si material, which has special thermal history, are di!erent from that in CZ silicon and in other silicon materials. In this paper, the oxygen-related centers in as-grown mc-Silicon material was studied by Fourier transmission infrared spectroscope (FTIR) and electron transmission microscope (TEM). It was found that oxygen with higher concentration occurred at the bottom area and at the edge area of mc-Si ingots. As-grown thermal donors with "ve species and oxygen precipitates were also observed at the bottom of mc-Si ingot.
2. Experiment Samples were cut from the di!erent positions in mc-Si ingots. The samples with the resistivity of about 1 )cm were polished mechanically on both sides. The sizes of samples were about 10;20;2 mm. Oxygen concentration was determined by a Bruker F113v FTIR at room temperature. The calibration factor for oxygen is about 3.14;10 cm\. The high-purity #oat-zone silicon in which oxygen concentration was lower than the detection limit was used as a reference. The samples were also measured by the FTIR at liquid-helium temperature (8 K). Oxygen precipitates in the samples were observed by a CM30 TEM.
3. Results and discussion Fig. 1 shows the oxygen concentration pro"le along with the crystal growth direction in a mc-Si. It can be seen that oxygen with higher concentration occurred at the bottom of the mc-Si ingot. Oxygen concentration gradually decreased from the bottom to the top of the mc-Si. The oxygen concentration pro"le from the edge area to the center of the mc-Si is given in Fig. 2. It is clear that at the edge oxygen concentration is higher. The oxygen concentration decreased from the edge to the center. While silicon is melted at high temperatures during crystal growth, the melted silicon can react with quartz crucibles. The reaction equation is as follows: Si#SiO P2SiO. (1) Some parts of SiO evaporate from the surface of the melted silicon. The rest of SiO is dissolved in the melted silicon. The equation is as follows: SiOPSi#O.
(2)
D. Yang et al. / Solar Energy Materials & Solar Cells 62 (2000) 37}42
39
Fig. 1. The oxygen concentration pro"le along with the direction of crystal growth in mc}Si.
Fig. 2. The oxygen concentration pro"le from the edge to the center of the mc-Si.
Finally, oxygen in the melted silicon is induced into mc-Si crystal. In comparison with CZ silicon, no mechanical constrained convection occurred during the growth of mc-Si crystal. The transport of oxygen in the melted silicon, which mainly depends on heat convection, is slow. Thus, at the bottom and at the edge of ingots the melted
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D. Yang et al. / Solar Energy Materials & Solar Cells 62 (2000) 37}42
silicon contacting quartz crucibles contains higher oxygen concentration. At the top and at the center of ingots where the melted silicon does not contact quartz crucibles, oxygen results from the transport of oxygen at the bottom and at the edge. Therefore, the silicon samples at the top and at the center of ingots contain lower oxygen concentration. On the other hand, the segregation coe$cient of oxygen in melted silicon is generally believed to be larger than 1.0. In the crystallized section of mc-Si ingots oxygen concentration is higher in comparison with in the melted silicon. During the growth of mc-Si crystal, the melted silicon at the bottom and at the edge is crystallized "rstly so as to the oxygen in such positions of ingots is higher (Figs. 1 and 2). Fig. 3 shows the FTIR spectrum in the far-infrared range of an as-grown mc-Si sample measured at low temperature (8 K). The sample was from the bottom of a mc-Si ingot. The thermal donors related to oxygen were observed in the spectrum. The thermal donors contained "ve species. The energy gap of each species is about 2 meV. The optical lines of the thermal donors in the mc-Si were the same as that in CZ silicon. The concentration of thermal donors can reach to 1;10/cm. Fig. 4 shows a TEM picture of the sample at the bottom of a mc-Si ingot. The picture indicates that oxygen precipitates were generated in the as-grown silicon. The oxygen precipitates were platelet and had no punched out dislocations around. Near the oxygen precipitates strain "eld existed. The edge length of the oxygen precipitates was about 100}300 nm.
Fig. 3. The FTIR spectrum of an as-grown mc-Silicon sample measured at low temperature (8 K).
D. Yang et al. / Solar Energy Materials & Solar Cells 62 (2000) 37}42
41
Fig. 4. The TEM photo of the sample at the bottom of a mc-Si ingot.
During cooling processes after the mc-Si crystal growth, the temperature declines successively from the melting point. The ingot is similar to be su!ered from heat treatments. Oxygen with higher concentration could accumulate together to form thermal donors and oxygen precipitates. In the temperature ranges of about 700}12003C during cooling, oxygen precipitates are generated at the bottom of mc-Si with higher oxygen concentration (Fig. 4). In the temperature ranges of about 350}5003C, thermal donors are produced at the bottom of mc-Si (Fig. 3).
4. Conclusions The oxygen-related centers in as-grown multicrystalline silicon used for solar cells has been studied. It was found that oxygen concentration decreased from the bottom section to the top section of ingots, and also decreased from the edge region to the center region of ingots. The "ve species thermal donors and the platelet oxygen precipitates were observed at the bottom section of mc-Si ingots.
Acknowledgements The author, Deren Yang, would like to appreciate the "nancial supports of the Chinese National Natural Science Fund (no. 59976035), the Research Fund for the Doctoral Program of Higher Education (RFDP) and the Chinese Excellent Younger Teacher Fund.
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D. Yang et al. / Solar Energy Materials & Solar Cells 62 (2000) 37}42
References [1] H.J. Moeller, Solid State Phenom. 47}48 (1996) 127. [2] M. Claybourn, R.C. Newman, Appl. Phys. Lett. 51 (1987) 2197. [3] S.A. McQuaid, M.J. Binns, C.A. Londos, J.H. Tucher, A.R. Brown, R.C. Newman, J. Appl. Phys. 77 (1995) 1427. [4] S.M. Hu, J. Appl. Phys. 51 (1980) 5945. [5] H. Ono, T. Ikarashi, S. Kimura, A. Tanikawa, J. Appl. Phys. 78 (1995) 4395. [6] K. Nauka, H.C. Gatos, J. Lagowski, Appl. Phys. Lett. 43 (1983) 241. [7] B.L. Sopori, Proceedings of the 20th IEEE Photovoltaic Specialist Conference, Las Vegas, NV 1988, p. 591. [8] P. Akhter, M.N. Zakir, A. Baig, Z.I. Zaidi, M. Ahmad, S. Hussain, I.A. Oazl, A. Mufti, Semicond. Sci. Technol. 6 (1991) 135. [9] C.J. Varker, J.D. Whit"eld, P.L. Fejes, Mater. Res. Soc. Symp. Proc. 14 (1983) 187. [10] F.V. Wald, in: Pinard, Kalbitzer (Eds.), Proceedings of Policrystalline, Microstalline and Amorphous Semiconductors, Les edition de Physique, France, 1985, p. 33. [11] B.R. Bathey, R.O. Bell, J.P. Kalejs, M. Prince, M.D. Rosenblum, R.W. Stormont, F.V. Wald, Proceedings of the 11th European Photovoltaic Solar Energy Conference, Montreux, 1992, p. 462. [12] J.M. Serra, R. Gamboa, A.M. Vallera, A. Eyer, W. Warta, Proceedings of the 12th European Photovoltaic Solar Energy Conference, Amsterdam, Netherland, 1994, p. 975. [13] J. Park, M. Duecher, H.J. Moeler, Proceedings of the 12th European Photovoltaic Solar Energy Conference, Amsterdam, Netherland, 1994 p. 996. [14] I.E. Reis, J. Chung, J. Park, H.J. Moeler, Proceedings of the 11th European Photovoltaic Energy Conference, Montreux, 1992, p. 499.
Oxygen-related centers in multicrystalline silicon Deren Yang *, Liben Li , Xiangyang Ma , Ruixin Fan, Duanlin Que , H.J. Moeller State Key Lab of Silicon Material Science, Zhejiang University, Hangzhou 310027, People's Republic of China Institute for Experimental Physics, TU Bergakademi Freiberg, SilbermannStr.1, 09596 Freiberg, Germany
Abstract Oxygen and oxygen-related centers in cast multicrystalline silicon (mc-Si) have been investigated. The concentration pro"le and the map of oxygen in mc-Si revealed that oxygen with higher concentration occurred at the bottom area and at the edge area of mc-Si ingots. The oxygen concentration and its pro"le were partly dependent on the cooling progress. It was found that the thermal donors related to oxygen were generated in the area of higher oxygen concentration. The concentration of thermal donors can reach to 1;10/cm. Meanwhile, as-grown oxygen precipitates were observed at the bottom of mc-Si. 2000 Elsevier Science B.V. All rights reserved. Keywords: Multicrystalline silicon; Oxygen; Thermal donors
1. Introduction Photovoltaic industry is predominantly using crystalline silicon technology. Due to low cost, cast multicrystalline silicon is increasingly used in photovoltaic industry. However, the e$ciency of solar cells based on mc-Si is limited by grain boundaries, dislocations, microdefects and impurities [1]. Oxygen is an important and main impurity in mc-Si. It is mostly induced into mc-Si due to quartz crucible contamination. In Czochralski single-crystal silicon (CZ Si) used in microelectronics industry, oxygen has been extensively investigated in the past 40 years. It has been believed that the oxygen-related centers in CZ silicon, such as thermal donors [2,3] and oxygen precipitates [4,5], seriously a!ect the electrical and
* Corresponding author. Tel.: #86-571-799-1432; fax: #86-571-795-1954. E-mail address: [email protected] (D. Yang) 0927-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 9 9 ) 0 0 1 3 3 - 6
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D. Yang et al. / Solar Energy Materials & Solar Cells 62 (2000) 37}42
mechanical properties of silicon during device processes. In CZ silicon and in ribbon multicrystalline silicon used for solar cells, the e!ect of oxygen on the conversion e$ciency of solar cells has controversially been reported [6}12]. It was also found that in mc-Silicon annealed in the temperature of 600}10003C the electrically active donors linked to oxygen precipitates were formed [13,14]. However, due to the special manufacture process of mc-Si, the grain boundaries, dislocations and microdefects with high density are generated during cooling. The behaviors of the oxygen-related centers in as-grown mc-Si material, which has special thermal history, are di!erent from that in CZ silicon and in other silicon materials. In this paper, the oxygen-related centers in as-grown mc-Silicon material was studied by Fourier transmission infrared spectroscope (FTIR) and electron transmission microscope (TEM). It was found that oxygen with higher concentration occurred at the bottom area and at the edge area of mc-Si ingots. As-grown thermal donors with "ve species and oxygen precipitates were also observed at the bottom of mc-Si ingot.
2. Experiment Samples were cut from the di!erent positions in mc-Si ingots. The samples with the resistivity of about 1 )cm were polished mechanically on both sides. The sizes of samples were about 10;20;2 mm. Oxygen concentration was determined by a Bruker F113v FTIR at room temperature. The calibration factor for oxygen is about 3.14;10 cm\. The high-purity #oat-zone silicon in which oxygen concentration was lower than the detection limit was used as a reference. The samples were also measured by the FTIR at liquid-helium temperature (8 K). Oxygen precipitates in the samples were observed by a CM30 TEM.
3. Results and discussion Fig. 1 shows the oxygen concentration pro"le along with the crystal growth direction in a mc-Si. It can be seen that oxygen with higher concentration occurred at the bottom of the mc-Si ingot. Oxygen concentration gradually decreased from the bottom to the top of the mc-Si. The oxygen concentration pro"le from the edge area to the center of the mc-Si is given in Fig. 2. It is clear that at the edge oxygen concentration is higher. The oxygen concentration decreased from the edge to the center. While silicon is melted at high temperatures during crystal growth, the melted silicon can react with quartz crucibles. The reaction equation is as follows: Si#SiO P2SiO. (1) Some parts of SiO evaporate from the surface of the melted silicon. The rest of SiO is dissolved in the melted silicon. The equation is as follows: SiOPSi#O.
(2)
D. Yang et al. / Solar Energy Materials & Solar Cells 62 (2000) 37}42
39
Fig. 1. The oxygen concentration pro"le along with the direction of crystal growth in mc}Si.
Fig. 2. The oxygen concentration pro"le from the edge to the center of the mc-Si.
Finally, oxygen in the melted silicon is induced into mc-Si crystal. In comparison with CZ silicon, no mechanical constrained convection occurred during the growth of mc-Si crystal. The transport of oxygen in the melted silicon, which mainly depends on heat convection, is slow. Thus, at the bottom and at the edge of ingots the melted
40
D. Yang et al. / Solar Energy Materials & Solar Cells 62 (2000) 37}42
silicon contacting quartz crucibles contains higher oxygen concentration. At the top and at the center of ingots where the melted silicon does not contact quartz crucibles, oxygen results from the transport of oxygen at the bottom and at the edge. Therefore, the silicon samples at the top and at the center of ingots contain lower oxygen concentration. On the other hand, the segregation coe$cient of oxygen in melted silicon is generally believed to be larger than 1.0. In the crystallized section of mc-Si ingots oxygen concentration is higher in comparison with in the melted silicon. During the growth of mc-Si crystal, the melted silicon at the bottom and at the edge is crystallized "rstly so as to the oxygen in such positions of ingots is higher (Figs. 1 and 2). Fig. 3 shows the FTIR spectrum in the far-infrared range of an as-grown mc-Si sample measured at low temperature (8 K). The sample was from the bottom of a mc-Si ingot. The thermal donors related to oxygen were observed in the spectrum. The thermal donors contained "ve species. The energy gap of each species is about 2 meV. The optical lines of the thermal donors in the mc-Si were the same as that in CZ silicon. The concentration of thermal donors can reach to 1;10/cm. Fig. 4 shows a TEM picture of the sample at the bottom of a mc-Si ingot. The picture indicates that oxygen precipitates were generated in the as-grown silicon. The oxygen precipitates were platelet and had no punched out dislocations around. Near the oxygen precipitates strain "eld existed. The edge length of the oxygen precipitates was about 100}300 nm.
Fig. 3. The FTIR spectrum of an as-grown mc-Silicon sample measured at low temperature (8 K).
D. Yang et al. / Solar Energy Materials & Solar Cells 62 (2000) 37}42
41
Fig. 4. The TEM photo of the sample at the bottom of a mc-Si ingot.
During cooling processes after the mc-Si crystal growth, the temperature declines successively from the melting point. The ingot is similar to be su!ered from heat treatments. Oxygen with higher concentration could accumulate together to form thermal donors and oxygen precipitates. In the temperature ranges of about 700}12003C during cooling, oxygen precipitates are generated at the bottom of mc-Si with higher oxygen concentration (Fig. 4). In the temperature ranges of about 350}5003C, thermal donors are produced at the bottom of mc-Si (Fig. 3).
4. Conclusions The oxygen-related centers in as-grown multicrystalline silicon used for solar cells has been studied. It was found that oxygen concentration decreased from the bottom section to the top section of ingots, and also decreased from the edge region to the center region of ingots. The "ve species thermal donors and the platelet oxygen precipitates were observed at the bottom section of mc-Si ingots.
Acknowledgements The author, Deren Yang, would like to appreciate the "nancial supports of the Chinese National Natural Science Fund (no. 59976035), the Research Fund for the Doctoral Program of Higher Education (RFDP) and the Chinese Excellent Younger Teacher Fund.
42
D. Yang et al. / Solar Energy Materials & Solar Cells 62 (2000) 37}42
References [1] H.J. Moeller, Solid State Phenom. 47}48 (1996) 127. [2] M. Claybourn, R.C. Newman, Appl. Phys. Lett. 51 (1987) 2197. [3] S.A. McQuaid, M.J. Binns, C.A. Londos, J.H. Tucher, A.R. Brown, R.C. Newman, J. Appl. Phys. 77 (1995) 1427. [4] S.M. Hu, J. Appl. Phys. 51 (1980) 5945. [5] H. Ono, T. Ikarashi, S. Kimura, A. Tanikawa, J. Appl. Phys. 78 (1995) 4395. [6] K. Nauka, H.C. Gatos, J. Lagowski, Appl. Phys. Lett. 43 (1983) 241. [7] B.L. Sopori, Proceedings of the 20th IEEE Photovoltaic Specialist Conference, Las Vegas, NV 1988, p. 591. [8] P. Akhter, M.N. Zakir, A. Baig, Z.I. Zaidi, M. Ahmad, S. Hussain, I.A. Oazl, A. Mufti, Semicond. Sci. Technol. 6 (1991) 135. [9] C.J. Varker, J.D. Whit"eld, P.L. Fejes, Mater. Res. Soc. Symp. Proc. 14 (1983) 187. [10] F.V. Wald, in: Pinard, Kalbitzer (Eds.), Proceedings of Policrystalline, Microstalline and Amorphous Semiconductors, Les edition de Physique, France, 1985, p. 33. [11] B.R. Bathey, R.O. Bell, J.P. Kalejs, M. Prince, M.D. Rosenblum, R.W. Stormont, F.V. Wald, Proceedings of the 11th European Photovoltaic Solar Energy Conference, Montreux, 1992, p. 462. [12] J.M. Serra, R. Gamboa, A.M. Vallera, A. Eyer, W. Warta, Proceedings of the 12th European Photovoltaic Solar Energy Conference, Amsterdam, Netherland, 1994, p. 975. [13] J. Park, M. Duecher, H.J. Moeler, Proceedings of the 12th European Photovoltaic Solar Energy Conference, Amsterdam, Netherland, 1994 p. 996. [14] I.E. Reis, J. Chung, J. Park, H.J. Moeler, Proceedings of the 11th European Photovoltaic Energy Conference, Montreux, 1992, p. 499.