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Direct observation of substitutional Fe atoms in Si and SOI wafers at 1273 K
ARTICLE IN PRESS
Physica B 340–342 (2003) 605–608
Direct observation of substitutional Fe atoms in Si and SOI wafers at 1273 K Yutaka Yoshida*, Shigeru Ogawa, Kazuhiro Arikawa Shizuoka Institute of Science and Technology, 2200-2 Toyosawa, Fukuroi city, Shizuoka 437-8555, Japan
Abstract . Mossbauer absorber experiments on 57Fe deposited on CZ-Si are performed to study the lattice sites and the electronic states of Fe atoms in Si at 1273 K. Substitutional Fe atoms and Fe clusters are directly observed in Si at 1273 K. The Fe atoms are found to distribute dominantly in a region down to about 500 nm from the surface by SIMS . measurements performed after the whole Mossbauer experiments at high temperatures. The 57Fe diffusivity in Si at 1273 K can be estimated to be about 2 10 20 m2/s from the depth profile of the Fe atoms. r 2003 Published by Elsevier B.V. PACS: 71.55. i; 76.80.+y . Keywords: Fe diffusion in Si; Substitutional Fe; Gettering technique; Mossbauer spectroscopy
1. Introduction Diffusion and segregation of iron atoms in silicon have been one of the central problems in . silicon science and technology [1]. Mossbauer spectroscopy provides atomistic information on 57 Fe in Si via the hyperfine interactions between the 57Fe nucleus and electrons, so that this method is especially suitable to study Fe atoms on different lattice sites in Si and in precipitates such as Fe3Si, FeSi, and FeSi2 [2,3]. The jump frequencies of interstitial Fe diffusion in Si could be directly estimated from the line broadenings [4–6]. In addition, so far substitutional Fe atoms in the Si matrix have been found experimentally by . Mossbauer spectroscopy after implantation 57Fe or its mother isotopes such as 57Mn [5,7–9], i.e. in *Corresponding author. Fax: +0081-538-45-0110. E-mail address: [email protected] (Y. Yoshida). 0921-4526/$ - see front matter r 2003 Published by Elsevier B.V. doi:10.1016/j.physb.2003.09.125
processes, where excess vacancies must be created . by the slowing-down process of the Mossbauer . isotope. Recently, we have performed Mossbauer absorber experiments [10,11] on 57Fe-deposited on CZ-Si, FZ-Si and SOI (Si-on-Insulator) wafers at high temperatures, and substitutional Fe in Si, Fe clusters and FeSi2 have been directly observed at 1273 K. In the present report, we mainly investigate the distribution of Fe atoms in the samples after the high-temperature measurements.
2. Experimental P-type CZ-Si wafers with boron concentrations of 1015 and 1018 B/cm3 were cut into a size of 11 mm 11 mm 600 mm, and chemically etched by a 20% HF solution to remove the surface oxide. Stable 57Fe isotopes were deposited on the Si samples at room temperature under a vacuum
ARTICLE IN PRESS Y. Yoshida et al. / Physica B 340–342 (2003) 605–608
of 10 5 Pa. The 57Fe thicknesses were varied between 1.4 nm and 100 nm, leading to 57Fe concentrations of 43 and 392 at. ppm, respectively, when all 57Fe atoms are assumed to be homogeneously distributed as a solid solution in the samples. After 57Fe-deposition on a Si wafer, the specimen was fixed between a pair of Ta-holders, . and was set in a UHV furnace [1]. Mossbauer transmission spectra were measured (using a 100 mCi 57Co-in-Rh source) in a temperature range between 1273 and 300 K under a vacuum of 10 6 Pa. A series of high-temperature measurements on a sample was performed during increasing and decreasing specimen temperatures, and was continued typically for 3 months. Notice that the specimen temperature corresponds to the annealing temperature in the present experiments. The total concentration of 57Fe in a sample was determined by atomic absorption spectrometry using a graphite furnace after the whole measurements at high temperatures. The depth profiles of 57 Fe atoms were also measured on two samples by secondary ion microscopy (SIMS).
3. Results and discussion By the deposition 57Fe atoms on Si wafers, a-Fe was formed on the Si surface. During increasing specimen temperatures, different Fe–Si phases could be observed in the spectra below 1173 K, the results of which were partly reported already [10,11]. When the samples were annealed at 1273 K . for 1 week, Mossbauer spectra were simultaneously measured for 1–3 days. Final spectra of 57 Fe in CZ-Si (1015 B/cm3) with different 57Fe thicknesses between 3 and 100 nm consisted of a singlet and a doublet (Fig. 1), which could be assigned to ‘‘substitutional Fe’’ and ‘‘Fe clusters’’, respectively [11]. Similar spectra were also observed in CZ-Si (1018 B/cm3) and in SOI samples. Both components remained upon cooling even down to 300 K. The spectral fractions of the substitutional Fe component at 1273 K changed between 0.3 and 1.0 for different 57Fe thicknesses. An interstitial Fe component was also detected with a fraction of less than a few percent in the spectra of 3 nm-57Fe-deposited on Si between 473
100.0
99.5
57
Fe3nm
99.0 100.0
Nomalized counts
606
99.5
57
Fe3.5nm
99.0 100.0
99.5
57
Fe10nm
99.0 100
98
57
Fe100nm
96 -2
-1
0
1
2
Velocity mm/ s . Fig. 1. Mossbauer transmission spectra of 57Fe in CZ-Si 15 3 (10 B/cm ) at 1273 K for different 57Fe thicknesses between 3 nm and 100 nm.
and 673 K as a small shoulder, but will not be discussed further here. The 57Fe concentrations contained in the samples were at least two to three orders of magnitude higher than the Fe solubility at 1273 K reported so far [1]. Surprisingly, the simplest singlet spectra were obtained for 3 nm-57Fe-deposited on Si, which was accidentally contaminated with stainless steel after the 57Fedeposition. This indicates that the fraction of substitutional Fe atoms appears to be strongly influenced by the existence of Fe atoms themselves as well as other transition metals. After the whole measurements at high temperatures, the samples were further investigated by scanning electron microscopy (SEM), X-ray diffraction and SIMS, in order to study the distribution of Fe atoms in Si. Small dots with a diameter of a few micrometers could be seen in the SEM pictures, on the one hand, for the samples of 3.5–100 nm 57Fe-deposited on Si (Fig. 2(a)). On the other hand, for the 3 nm 57Fe-deposited sample, which was contaminated by stainless steel, a rather different image was found on the surface (Fig. 2(b)). Furthermore, small X-ray diffraction
ARTICLE IN PRESS Y. Yoshida et al. / Physica B 340–342 (2003) 605–608
607
1E21
(a)
3 nm57Fe 1.4 nm57Fe
Log C57Fe /atoms/cm
3
1E20
1E19
1E18
1E17 (b) 1E16 0.0
0.1
0.2 2
0.3
2
X /µm 57
Fig. 3. SIMS depth profiles of Fe as function of X2. The 57Fe diffusivity in Si at 1273 K can be roughly estimated to be 2 10 20 m2/s. The scanning area of the primary O+ 2 ions with an energy of 5.5 keV was 100 100mm, and the diameter of the analyzed area for secondary ions was 30 mm. 57
Fig. 2. SEM pictures obtained after high-temperature . Mossbauer experiments: (a) samples of 3.5–100 nm 57Fedeposited on Si; (b) 3 nm 57Fe-deposited sample contaminated by stainless steel.
peaks due to FeSi2 precipitates were observed on the surfaces for deposits with 10 and 100 nm 57Fe. . The Mossbauer spectra of these samples at 300 K also contained a small component of FeSi2 precipitates, but their fractions were always less than 5% in both samples. This indicates that the dominant 57Fe components should be due to substitutional Fe and Fe clusters. Fig. 3 presents the logarithm of the 57Fe concentrations as a function of the square of the depth, x, which were deduced from SIMS profiles of the 1.4 and 3 nm 57 Fe-deposited samples. Taking into account the total annealing times of the samples at 1273 K, i.e. 491 and 264 h, respectively, the diffusivity of
Fe in both Si samples can be estimated to be 2 10 20 m2/s by assuming a Gaussian distribution function. Since the measurements were also performed at different temperatures below 1273 K, the obtained diffusivity should be considered as only a rough estimation. It is, however, interesting to note that the value coincides with the selfdiffusion of Si at 1273 K.
4. Conclusions Substitutional Fe atoms and Fe clusters in Si were directly observed at 1273 K in M.ossbauer absorber experiments. The 57Fe atoms were distributed dominantly in a region down to about 500 nm from the surface. The depth profile of Fe atoms can be described by a Gaussian, yielding roughly 2 10 20 m2/s for the 57Fe diffusivity in Si at 1273 K. Since substitutional Fe atoms are supposed to be formed via a kinetic reaction between vancancies and fast-diffusing interstitial-Fe atoms,
ARTICLE IN PRESS 608
Y. Yoshida et al. / Physica B 340–342 (2003) 605–608
the vacancy concentration in the Si matrix may be strongly influenced by the metallic impurities. The present in-situ observations indicate the Fe atoms exist mainly on substitutional sites at 1273 K and that their concentration must correspond to the solubility of Fe in Si, which appears to be orders of magnitude higher than that reported before. To understand fully the solid solution of Fe atoms in Si, however, further experiments are necessary in order to clarify the distribution of substitutional Fe atoms and Fe clusters in the Si matrix.
Acknowledgements Prof. F. Shimura, Shizuoka Institute of Science and Technology, is gratefully acknowledged for fruitful discussions and for warm support during the whole project. I am heartily grateful to Emeritus Prof. M. Umeno, Osaka University, for his warm support during the last few years. Dr. T. Abe, Shin-Etsu Handotai Co., Ltd. is warmly acknowledged for supplying Si wafers. This work was partly supported by JSPS Research for the Future Program under the project of ‘‘Ultimate Characterization Techniques of SOI Wafer for the Nano-Scale LSI-Devices’’. Prof. S. Kishino, Himeji Institute of Technology, is acknowledged for his support. Drs. K. Yamada and M. Yamanaka, Toray-Research Centre, are acknowledged for SIMS measurements. I would like to express my sincere thanks to Prof. G. Weyer, Aarhus University, for valuable discussions and advices to the manuscript.
References [1] A. Istratov, H. Hieslmair, E.R. Weber, Appl. Phys. A 69 (1999) 13. [2] G. Langouche, in: G.L. Long, F. Grandjean (Eds.), . Mossbauer Spectroscopy Applied to Inorganic Chemistry, Vol. 3, Plenum Press, New York and London, 1989, pp. 445–512. [3] D.L. Williamson, L. Niesen, G. Weyer, R. Sielemann, G. Langouche, in: G. Langouche (Ed.), Hyperfine Interaction . of Defects in Semiconductors, Mossbauer Spectroscopy, Elsevier Science Publishers B.V., Amsterdam, London, New York, Tokyo, 1992, pp. 3 (Chapter 1). [4] P. Schwalbach, S. Laubach, M. Hartick, E. Kankeleit, B. Keck, M. Menningen, R. Sielemann, Phys. Rev. Lett. 64 (1990) 1274. [5] G. Weyer, A. Burchard, M. Fanciulli, V.N. Fedoseyev, H.P. Gunnlaugsson, V.I. Mishin, R. Sielemann, and the ISOLDE Collaboration, Physica B 273–274 (1999) 363. [6] H.P. Gunnlaugsson, G. Weyer, M. Dietrich and the ISOLDE Collaboration, M. Fanciulli, K. Bharuth-Ram, R. Sielemann, Appl. Phys. Lett. 80 (2002) 2657. [7] Y. Yoshida, Hyperfine Interactions 113 (1998) 183; Y. Kobayashi, Y. Yoshida, K. Hayakawa, K. Yukihira, J. Nakamura, H. H.aXlein, S. Nasu, N. Inabe, Y. Watanabe, A. Yoshida, M. Kase, A. Goto, S. Yano, E. Yagi, F. Ambe, Hyperfine Interactions (c) 3 (1998) 273. [8] Y. Yoshida, K. Kobayashi, K. Hayakawa, K. Yukihira, F. Shimura, A. Yoshida, X. Diao, H. Ogawa, Y. Yano, F. Ambe, Defect Diffusion Forum 194–199 (2001) 611. [9] Y. Yoshida, Y. Kobayashi, K. Hayakawa, K. Yukihira, F. Shimura, A. Yoshida, X. Diao, H. Ogawa, Y. Yano, F. Ambe, Hyperfine Interactions 141–142 (2002) 157. . [10] Y. Yoshida, F. Shimura, in: H.R. Huff, U. Gosele, H. Tsuya (Eds.), Proceedings of the Eighth International Symposium on Silicon Materials Science and Technology, Vol. 98-1, 1998, p. 984. [11] Y. Yoshida, in: B.O. Kolbesen, C.L. Claeys, P. Stallhofer, F. Tardif (Eds.), PV 2003-03 of ECS, ALTECH 2003 Analytical and Diagnostic Techniques for Semiconductor Materials, Devices, and Processes, p. 479.
Physica B 340–342 (2003) 605–608
Direct observation of substitutional Fe atoms in Si and SOI wafers at 1273 K Yutaka Yoshida*, Shigeru Ogawa, Kazuhiro Arikawa Shizuoka Institute of Science and Technology, 2200-2 Toyosawa, Fukuroi city, Shizuoka 437-8555, Japan
Abstract . Mossbauer absorber experiments on 57Fe deposited on CZ-Si are performed to study the lattice sites and the electronic states of Fe atoms in Si at 1273 K. Substitutional Fe atoms and Fe clusters are directly observed in Si at 1273 K. The Fe atoms are found to distribute dominantly in a region down to about 500 nm from the surface by SIMS . measurements performed after the whole Mossbauer experiments at high temperatures. The 57Fe diffusivity in Si at 1273 K can be estimated to be about 2 10 20 m2/s from the depth profile of the Fe atoms. r 2003 Published by Elsevier B.V. PACS: 71.55. i; 76.80.+y . Keywords: Fe diffusion in Si; Substitutional Fe; Gettering technique; Mossbauer spectroscopy
1. Introduction Diffusion and segregation of iron atoms in silicon have been one of the central problems in . silicon science and technology [1]. Mossbauer spectroscopy provides atomistic information on 57 Fe in Si via the hyperfine interactions between the 57Fe nucleus and electrons, so that this method is especially suitable to study Fe atoms on different lattice sites in Si and in precipitates such as Fe3Si, FeSi, and FeSi2 [2,3]. The jump frequencies of interstitial Fe diffusion in Si could be directly estimated from the line broadenings [4–6]. In addition, so far substitutional Fe atoms in the Si matrix have been found experimentally by . Mossbauer spectroscopy after implantation 57Fe or its mother isotopes such as 57Mn [5,7–9], i.e. in *Corresponding author. Fax: +0081-538-45-0110. E-mail address: [email protected] (Y. Yoshida). 0921-4526/$ - see front matter r 2003 Published by Elsevier B.V. doi:10.1016/j.physb.2003.09.125
processes, where excess vacancies must be created . by the slowing-down process of the Mossbauer . isotope. Recently, we have performed Mossbauer absorber experiments [10,11] on 57Fe-deposited on CZ-Si, FZ-Si and SOI (Si-on-Insulator) wafers at high temperatures, and substitutional Fe in Si, Fe clusters and FeSi2 have been directly observed at 1273 K. In the present report, we mainly investigate the distribution of Fe atoms in the samples after the high-temperature measurements.
2. Experimental P-type CZ-Si wafers with boron concentrations of 1015 and 1018 B/cm3 were cut into a size of 11 mm 11 mm 600 mm, and chemically etched by a 20% HF solution to remove the surface oxide. Stable 57Fe isotopes were deposited on the Si samples at room temperature under a vacuum
ARTICLE IN PRESS Y. Yoshida et al. / Physica B 340–342 (2003) 605–608
of 10 5 Pa. The 57Fe thicknesses were varied between 1.4 nm and 100 nm, leading to 57Fe concentrations of 43 and 392 at. ppm, respectively, when all 57Fe atoms are assumed to be homogeneously distributed as a solid solution in the samples. After 57Fe-deposition on a Si wafer, the specimen was fixed between a pair of Ta-holders, . and was set in a UHV furnace [1]. Mossbauer transmission spectra were measured (using a 100 mCi 57Co-in-Rh source) in a temperature range between 1273 and 300 K under a vacuum of 10 6 Pa. A series of high-temperature measurements on a sample was performed during increasing and decreasing specimen temperatures, and was continued typically for 3 months. Notice that the specimen temperature corresponds to the annealing temperature in the present experiments. The total concentration of 57Fe in a sample was determined by atomic absorption spectrometry using a graphite furnace after the whole measurements at high temperatures. The depth profiles of 57 Fe atoms were also measured on two samples by secondary ion microscopy (SIMS).
3. Results and discussion By the deposition 57Fe atoms on Si wafers, a-Fe was formed on the Si surface. During increasing specimen temperatures, different Fe–Si phases could be observed in the spectra below 1173 K, the results of which were partly reported already [10,11]. When the samples were annealed at 1273 K . for 1 week, Mossbauer spectra were simultaneously measured for 1–3 days. Final spectra of 57 Fe in CZ-Si (1015 B/cm3) with different 57Fe thicknesses between 3 and 100 nm consisted of a singlet and a doublet (Fig. 1), which could be assigned to ‘‘substitutional Fe’’ and ‘‘Fe clusters’’, respectively [11]. Similar spectra were also observed in CZ-Si (1018 B/cm3) and in SOI samples. Both components remained upon cooling even down to 300 K. The spectral fractions of the substitutional Fe component at 1273 K changed between 0.3 and 1.0 for different 57Fe thicknesses. An interstitial Fe component was also detected with a fraction of less than a few percent in the spectra of 3 nm-57Fe-deposited on Si between 473
100.0
99.5
57
Fe3nm
99.0 100.0
Nomalized counts
606
99.5
57
Fe3.5nm
99.0 100.0
99.5
57
Fe10nm
99.0 100
98
57
Fe100nm
96 -2
-1
0
1
2
Velocity mm/ s . Fig. 1. Mossbauer transmission spectra of 57Fe in CZ-Si 15 3 (10 B/cm ) at 1273 K for different 57Fe thicknesses between 3 nm and 100 nm.
and 673 K as a small shoulder, but will not be discussed further here. The 57Fe concentrations contained in the samples were at least two to three orders of magnitude higher than the Fe solubility at 1273 K reported so far [1]. Surprisingly, the simplest singlet spectra were obtained for 3 nm-57Fe-deposited on Si, which was accidentally contaminated with stainless steel after the 57Fedeposition. This indicates that the fraction of substitutional Fe atoms appears to be strongly influenced by the existence of Fe atoms themselves as well as other transition metals. After the whole measurements at high temperatures, the samples were further investigated by scanning electron microscopy (SEM), X-ray diffraction and SIMS, in order to study the distribution of Fe atoms in Si. Small dots with a diameter of a few micrometers could be seen in the SEM pictures, on the one hand, for the samples of 3.5–100 nm 57Fe-deposited on Si (Fig. 2(a)). On the other hand, for the 3 nm 57Fe-deposited sample, which was contaminated by stainless steel, a rather different image was found on the surface (Fig. 2(b)). Furthermore, small X-ray diffraction
ARTICLE IN PRESS Y. Yoshida et al. / Physica B 340–342 (2003) 605–608
607
1E21
(a)
3 nm57Fe 1.4 nm57Fe
Log C57Fe /atoms/cm
3
1E20
1E19
1E18
1E17 (b) 1E16 0.0
0.1
0.2 2
0.3
2
X /µm 57
Fig. 3. SIMS depth profiles of Fe as function of X2. The 57Fe diffusivity in Si at 1273 K can be roughly estimated to be 2 10 20 m2/s. The scanning area of the primary O+ 2 ions with an energy of 5.5 keV was 100 100mm, and the diameter of the analyzed area for secondary ions was 30 mm. 57
Fig. 2. SEM pictures obtained after high-temperature . Mossbauer experiments: (a) samples of 3.5–100 nm 57Fedeposited on Si; (b) 3 nm 57Fe-deposited sample contaminated by stainless steel.
peaks due to FeSi2 precipitates were observed on the surfaces for deposits with 10 and 100 nm 57Fe. . The Mossbauer spectra of these samples at 300 K also contained a small component of FeSi2 precipitates, but their fractions were always less than 5% in both samples. This indicates that the dominant 57Fe components should be due to substitutional Fe and Fe clusters. Fig. 3 presents the logarithm of the 57Fe concentrations as a function of the square of the depth, x, which were deduced from SIMS profiles of the 1.4 and 3 nm 57 Fe-deposited samples. Taking into account the total annealing times of the samples at 1273 K, i.e. 491 and 264 h, respectively, the diffusivity of
Fe in both Si samples can be estimated to be 2 10 20 m2/s by assuming a Gaussian distribution function. Since the measurements were also performed at different temperatures below 1273 K, the obtained diffusivity should be considered as only a rough estimation. It is, however, interesting to note that the value coincides with the selfdiffusion of Si at 1273 K.
4. Conclusions Substitutional Fe atoms and Fe clusters in Si were directly observed at 1273 K in M.ossbauer absorber experiments. The 57Fe atoms were distributed dominantly in a region down to about 500 nm from the surface. The depth profile of Fe atoms can be described by a Gaussian, yielding roughly 2 10 20 m2/s for the 57Fe diffusivity in Si at 1273 K. Since substitutional Fe atoms are supposed to be formed via a kinetic reaction between vancancies and fast-diffusing interstitial-Fe atoms,
ARTICLE IN PRESS 608
Y. Yoshida et al. / Physica B 340–342 (2003) 605–608
the vacancy concentration in the Si matrix may be strongly influenced by the metallic impurities. The present in-situ observations indicate the Fe atoms exist mainly on substitutional sites at 1273 K and that their concentration must correspond to the solubility of Fe in Si, which appears to be orders of magnitude higher than that reported before. To understand fully the solid solution of Fe atoms in Si, however, further experiments are necessary in order to clarify the distribution of substitutional Fe atoms and Fe clusters in the Si matrix.
Acknowledgements Prof. F. Shimura, Shizuoka Institute of Science and Technology, is gratefully acknowledged for fruitful discussions and for warm support during the whole project. I am heartily grateful to Emeritus Prof. M. Umeno, Osaka University, for his warm support during the last few years. Dr. T. Abe, Shin-Etsu Handotai Co., Ltd. is warmly acknowledged for supplying Si wafers. This work was partly supported by JSPS Research for the Future Program under the project of ‘‘Ultimate Characterization Techniques of SOI Wafer for the Nano-Scale LSI-Devices’’. Prof. S. Kishino, Himeji Institute of Technology, is acknowledged for his support. Drs. K. Yamada and M. Yamanaka, Toray-Research Centre, are acknowledged for SIMS measurements. I would like to express my sincere thanks to Prof. G. Weyer, Aarhus University, for valuable discussions and advices to the manuscript.
References [1] A. Istratov, H. Hieslmair, E.R. Weber, Appl. Phys. A 69 (1999) 13. [2] G. Langouche, in: G.L. Long, F. Grandjean (Eds.), . Mossbauer Spectroscopy Applied to Inorganic Chemistry, Vol. 3, Plenum Press, New York and London, 1989, pp. 445–512. [3] D.L. Williamson, L. Niesen, G. Weyer, R. Sielemann, G. Langouche, in: G. Langouche (Ed.), Hyperfine Interaction . of Defects in Semiconductors, Mossbauer Spectroscopy, Elsevier Science Publishers B.V., Amsterdam, London, New York, Tokyo, 1992, pp. 3 (Chapter 1). [4] P. Schwalbach, S. Laubach, M. Hartick, E. Kankeleit, B. Keck, M. Menningen, R. Sielemann, Phys. Rev. Lett. 64 (1990) 1274. [5] G. Weyer, A. Burchard, M. Fanciulli, V.N. Fedoseyev, H.P. Gunnlaugsson, V.I. Mishin, R. Sielemann, and the ISOLDE Collaboration, Physica B 273–274 (1999) 363. [6] H.P. Gunnlaugsson, G. Weyer, M. Dietrich and the ISOLDE Collaboration, M. Fanciulli, K. Bharuth-Ram, R. Sielemann, Appl. Phys. Lett. 80 (2002) 2657. [7] Y. Yoshida, Hyperfine Interactions 113 (1998) 183; Y. Kobayashi, Y. Yoshida, K. Hayakawa, K. Yukihira, J. Nakamura, H. H.aXlein, S. Nasu, N. Inabe, Y. Watanabe, A. Yoshida, M. Kase, A. Goto, S. Yano, E. Yagi, F. Ambe, Hyperfine Interactions (c) 3 (1998) 273. [8] Y. Yoshida, K. Kobayashi, K. Hayakawa, K. Yukihira, F. Shimura, A. Yoshida, X. Diao, H. Ogawa, Y. Yano, F. Ambe, Defect Diffusion Forum 194–199 (2001) 611. [9] Y. Yoshida, Y. Kobayashi, K. Hayakawa, K. Yukihira, F. Shimura, A. Yoshida, X. Diao, H. Ogawa, Y. Yano, F. Ambe, Hyperfine Interactions 141–142 (2002) 157. . [10] Y. Yoshida, F. Shimura, in: H.R. Huff, U. Gosele, H. Tsuya (Eds.), Proceedings of the Eighth International Symposium on Silicon Materials Science and Technology, Vol. 98-1, 1998, p. 984. [11] Y. Yoshida, in: B.O. Kolbesen, C.L. Claeys, P. Stallhofer, F. Tardif (Eds.), PV 2003-03 of ECS, ALTECH 2003 Analytical and Diagnostic Techniques for Semiconductor Materials, Devices, and Processes, p. 479.