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Photoinduced volume expansion and contraction in a-Si:H films
Journal of Non-Crystalline Solids 299–302 (2002) 516–520 www.elsevier.com/locate/jnoncrysol
Photoinduced volume expansion and contraction in a-Si:H films N. Yoshida a,*, Y. Sobajima a, H. Kamiguchi a, T. Iida a, T. Hatano a, H. Mori a, Y. Nakae a, M. Itoh b, A. Masuda b, H. Matsumura b, S. Nonomura a a b
Environmental and Renewable Energy Systems Division, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Tatsunokuchi, Ishikawa 923-1292, Japan
Abstract Photoinduced volume expansion and contraction in hydrogenated amorphous silicon (a-Si:H) films have been studied. Photoinduced volume contraction seems to occur in a-Si:H films having hydrogen contents less than 5 at.%. The wavelength dependence of the photoinduced volume expansion indicates that one of driving forces of this phenomenon may be relaxation energy of photoexcited carriers. The reciprocity relation between light intensity and illumination time does not hold both in photoinduced volume expansion and photodegradation. The mechanism of the photoinduced volume changes is also discussed. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 61.43.Dq; 71.23.Cq
1. Introduction A practical use of hydrogenated amorphous silicon (a-Si:H) films for solar cells is now in progress. However, the photodegradation [1] have been still unresolved. On the other hand, after Fritzsche’s proposal in 1995 [2], experimental evidences for photostructural changes in a-Si:H films have been reported by several groups [3–6]. We have previously reported photoinduced volume expansion [4,5] in a-Si:H films prepared by plasma enhanced chemical vapor deposition (PECVD) method. Following similarities between
*
Corresponding author. Tel.: +81-58 293 2695; fax: +81-58 230 1109. E-mail address: [email protected] (N. Yoshida).
the photoinduced volume expansion and the photodegradation (the photoinduced defect creation) have been pointed out. One is the time constant of time evolution in these phenomena, and the other is a recovery of both phenomena by thermal annealing at 200 °C for 1 h. It has been suggested that there is some connection [4,5] between the mechanisms of these phenomena. However, the mechanism of these phenomena is not known. Furthermore, we have recently found photoinduced volume contraction [7] in a-Si:H film prepared by catalytic chemical vapor deposition (Cat-CVD or Hot-wire CVD) method. In this study, therefore, photoinduced volume expansion and contraction in a-Si:H films has been extensively investigated. The correlation between the photoinduced volume change and the photodegradation is also experimentally investigated.
0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 2 ) 0 0 9 3 4 - 1
N. Yoshida et al. / Journal of Non-Crystalline Solids 299-302 (2002) 516–520
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2. Experimental Silica glass with dimensions of 2 20 mm2 and a thickness of 0.1 mm was used as substrates for experiments on photoinduced volume change. Silica glass, Corning 7059 glass and c-Si substrates with dimensions of 7 20 mm2 and a thickness of 0.5 mm were also used as substrates for measurements of bandgap energies, conductivities and hydrogen contents of the films, respectively. a-Si:H films were prepared by PECVD and CatCVD methods. For PECVD, a hydrogen dilution ratio (r ¼ H2 =SiH4 ) of 4 and a total gas pressure of 1 Torr were used. The rf (13.56 MHz) power was 15 W. Substrate temperatures were 50–450 °C. For Cat-CVD, r of 0.2–0.4, total gas pressures of 5–7 mTorr, filament temperatures of 1660–1760 °C and substrate temperatures of 220–400 °C were used. The thickness of a-Si:H films was 0.4–1 lm. For estimating the magnitude of photoinduced volume changes in a-Si:H films, the laser optical lever method was used. The detail of this technique is described in Refs. [4,5,7]. An Arþ laser and a dye laser with intensities of 50–700 mW/cm2 was used for giving rise to the photoinduced volume change in a-Si:H films. An intermittent light illumination with a light-on and -off period of 5 min was performed to avoid thermal effects in the sample by a light soaking.
3. Results Fig. 1 shows samples of the output of a position sensitive detector on the experiment of the photoinduced volume change in a-Si:H films. Data in light-on and -off regime correspond to thermal expansion and the photoinduced volume change [4,5], respectively. As shown in the figure, the output at light-off regime (a) increases and (b) decreases with time. These correspond to (a) photoinduced volume expansion [4,5] and (b) photoinduced volume contraction [7]. Fig. 2 shows time evolution of (a) photoinduced volume expansion and (b) contraction in a-Si:H films using the data in Fig. 1. The vertical axis DV =V represents the magnitude of relative volume changes estimated from the Stoney’s equation
Fig. 1. PSD outputs on the experiment of the photoinduced volume change in a-Si:H films prepared by Cat-CVD. Data in light-on and -off regime (shown at upper left) correspond to thermal expansion and photoinduced volume change [4,5,7], respectively. The data of (a) and (b) correspond to photoinduced volume expansion [4,5] and contraction [7], respectively.
[4,5,7]. Positive and negative values of the axis indicate volume expansion and contraction, respectively. To investigate the mechanism of photoinduced volume change, experiments on the wavelength dependence have been performed. PECVD films were employed as samples. An Arþ laser and a dye laser with an intensity of 150 mW/cm2 were used for inducing the phenomenon. The result is shown in Fig. 3. It is found that the magnitude of the photoinduced volume expansion becomes greater with increasing photon energy in the same absorbed photon numbers.
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Fig. 2. Photoinduced volume expansion (a) and photoinduced volume contraction (b) in a-Si:H films prepared by Cat-CVD [7]. The magnitude of the experimental error is included in the symbol for data.
Fig. 3. The wavelength dependence of photoinduced volume expansion in a-Si:H films. The wavelength and the corresponding photon energy are indicated in the figure. The magnitude of the experimental error is included in the symbol for data.
The light-intensity dependence of the photoinduced volume change and the photodegradation in a-Si:H films have been investigated to clarify the correlation between both phenomena. The light intensity was varied from 50 to 700 mW/cm2 . The result is shown in Fig. 4. The horizontal axis shows the product of light intensity and illumination time, which corresponds to the incident photon number. For photodegradation, the data for photoinduced defect creation [8] are used. It is found that both phenomena have light-intensity dependence. That is, the reciprocity relation between light intensity and illumination time does not hold. It is also found that the DV =V and the number of the induced defect by 700 mW/cm2 become smaller than those by 400 mW/cm2 at the same incident photon numbers. There may happen reconstruction of Si lattice due to a strong light intensity. From these results, it is confirmed that there exists correlations between photoinduced volume expansion and the photoinduced defect creation (the photodegradation), adding to previous results (see Section 1) [4,5].
To investigate the origin for photoinduced volume expansion and contraction in Cat-CVD films, Fig. 5 shows hydrogen-content dependence of the photoinduced volume change. The hydrogen content was estimated from the integral intensity of IR absorption at 630 cm1 . The data indicate the value of DV =V after light illumination of 2 h. The DV =V seems to increase with H content. However, the data scatter a lot. Alternatively, Fig. 6 shows bandgap-energy (estimated from the Tauc plot) dependence of DV =V using the same films as in Fig. 5. It is found that DV =V increases with bandgap energy. Accordingly, it seems that a-Si:H films with H content less than 5 at.% contract due to light illumination. 1 Note 1
Note that the H content (Fig. 5) and the bandgap energy (Fig. 6) were measured using a-Si:H films deposited on c-Si and quartz, respectively. And, DV =V was measured using a-Si:H films deposited on quartz. Furthermore, the bandgap energy in a-Si:H films depends almost monotonically on the H content [9,10]. Hence, Fig. 6 seems to represent the H-content dependence of DV =V more truly.
N. Yoshida et al. / Journal of Non-Crystalline Solids 299-302 (2002) 516–520
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Fig. 5. Hydrogen-content dependence of the photoinduced volume changes in a-Si:H films prepared by Cat-CVD. The magnitude of the experimental error is included in the symbol for data.
Fig. 4. The light-intensity dependence of (a) photoinduced volume expansion and (b) photoinduced defect creation [6] in aSi:H films. Light intensities used are indicated in the figure. The dimensions of the sample (b) [6] are 0.5 cm2 and a thickness of 3 lm. The magnitude of the experimental error is included in the symbol for data.
that the initial stress of the film may be the origin for the sign (expansion or contraction) of the photoinduced volume change as pointed out in Ref. [7]. Also note that the sign does not depend on initial defect densities of a-Si:H films [7]. In addition to a-Si:H films prepared by CatCVD method, photoinduced volume contraction in a-Si:H films prepared by PECVD was observed in this study. Fig. 7 shows the data for DV =V in an a-Si:H film prepared with a substrate temperature of 50 °C. First, the film expands due to light illumination (Fig. 4(a): the photoinduced volume expansion [4,5]). After annealing of this film at 200 °C for 1 h in a vacuum of 106 Torr, then, the film exhibits photoinduced volume con-
Fig. 6. Bandgap dependence of the photoinduced volume changes in a-Si:H films prepared by Cat-CVD.
traction (Fig. 4(b)). Note that the hydrogen content of this film changed from 25 to 12 at.% by the annealing. It seems to be important to investigate the correlation between the sign of the photoinduced volume change and the existence of the photodegradation. It is found (not shown) that photodegradation occurs in all samples exhibiting photoinduced volume contraction [7]. The detail is shown in Ref. [7].
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N. Yoshida et al. / Journal of Non-Crystalline Solids 299-302 (2002) 516–520
be hydrogen content (Figs. 5 and 6) and the initial stress [7] of the film. Alternatively, hydrogen structures [6] may dominate the sign of the photoinduced volume change in a-Si:H films. Experiments for the detail is now in progress. 5. Conclusions The photoinduced volume change in a-Si:H films has been studied. The possibility is pointed out that the driving force for the photoinduced volume change may be the relaxation energy of photoexcited carriers. It is confirmed that the correlation exists between the photoinduced volume change and the photodegradation. The sign of the photoinduced volume change seems to be determined by hydrogen content and the initial stress of a-Si:H films. Fig. 7. Photoinduced volume expansion (a) and photoinduced volume contraction (b) in an a-Si:H film prepared by PECVD at a substrate temperature of 50 °C. Thermal annealing of 200 °C for 1 h in a vacuum of 106 Torr was performed between experiments (a) and (b). The magnitude of the experimental error is included in the symbol for data.
4. Discussion The mechanism of the photoinduced volume change is discussed. From the result of Fig. 3, the relaxation energy of photoexcited carriers can be a candidate for driving force of photostructural changes in a-Si:H films. That is, the energy may localise at a part of Si network consisting of several atoms. Then, the part of Si network can shift to other metastable states and this results in the photoinduced volume change. From the result of Figs. 5–7, it is assumed that there exist, at least, two kinds of structures responsible for the photoinduced volume expansion and contraction in a-Si:H films. It is also assumed that these photoinduced phenomena occur competitively. And, the larger one can be observed in experiments as photoinduced volume expansion or contraction. The photodegradation may occur at the structure exhibiting the photoinduced volume expansion. The origin for the sign of the photoinduced volume change (expansion and contraction) may
Acknowledgements This study was financially supported by NEDO as a part of the New Sunshine Program of METI, Kawasaki Steel 21st Century Foundation, The Saijiro Endo Foundation for Science and Technology, The Mazda Foundation’s Research Grant The Koshiyana Research Grant and The Ogawa Science and Technology Foundation. References [1] H. Fritzsche, Solid State Commun. 94 (1995) 953. [2] D.L. Staebler, C.R. Wronski, Appl. Phys. Lett. 31 (1977) 292. [3] K. Shimizu, T. Shiba, T. Tabuchi, H. Okamoto, Jpn. J. Appl. Phys. 36 (1997) 29. [4] T. Gotoh, S. Nonomura, M. Nishio, S. Nitta, M. Kondo, A. Matsuda, Appl. Phys. Lett. 72 (1998) 2978. [5] S. Nonomura, N. Yoshida, T. Gotoh, T. Sakamoto, M. Kondo, A. Matsuda, S. Nitta, J. Non-Cryst. Solids 266– 269 (2000) 474. [6] D. Han, J. Baugh, G. Yue, Phys. Rev. B 62 (2000) 7169. [7] T. Hatano, Y. Nakae, H. Mori, K. Ohkado, N. Yoshida, S. Nonomura, M. Itoh, A. Masuda, H. Matsumura, Thin Solid Films 395 (2001) 84. [8] M. Stutzmann, W.B. Jackson, C.C. Tsai, Phys. Rev. B 32 (1985) 23. [9] S. Yamasaki, Philos. Mag. B 56 (1987) 79. [10] H. Shirai, D. Das, J. Hanna, I. Shimizu, Appl. Phys. Lett. 59 (1991) 1096.
Photoinduced volume expansion and contraction in a-Si:H films N. Yoshida a,*, Y. Sobajima a, H. Kamiguchi a, T. Iida a, T. Hatano a, H. Mori a, Y. Nakae a, M. Itoh b, A. Masuda b, H. Matsumura b, S. Nonomura a a b
Environmental and Renewable Energy Systems Division, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Tatsunokuchi, Ishikawa 923-1292, Japan
Abstract Photoinduced volume expansion and contraction in hydrogenated amorphous silicon (a-Si:H) films have been studied. Photoinduced volume contraction seems to occur in a-Si:H films having hydrogen contents less than 5 at.%. The wavelength dependence of the photoinduced volume expansion indicates that one of driving forces of this phenomenon may be relaxation energy of photoexcited carriers. The reciprocity relation between light intensity and illumination time does not hold both in photoinduced volume expansion and photodegradation. The mechanism of the photoinduced volume changes is also discussed. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 61.43.Dq; 71.23.Cq
1. Introduction A practical use of hydrogenated amorphous silicon (a-Si:H) films for solar cells is now in progress. However, the photodegradation [1] have been still unresolved. On the other hand, after Fritzsche’s proposal in 1995 [2], experimental evidences for photostructural changes in a-Si:H films have been reported by several groups [3–6]. We have previously reported photoinduced volume expansion [4,5] in a-Si:H films prepared by plasma enhanced chemical vapor deposition (PECVD) method. Following similarities between
*
Corresponding author. Tel.: +81-58 293 2695; fax: +81-58 230 1109. E-mail address: [email protected] (N. Yoshida).
the photoinduced volume expansion and the photodegradation (the photoinduced defect creation) have been pointed out. One is the time constant of time evolution in these phenomena, and the other is a recovery of both phenomena by thermal annealing at 200 °C for 1 h. It has been suggested that there is some connection [4,5] between the mechanisms of these phenomena. However, the mechanism of these phenomena is not known. Furthermore, we have recently found photoinduced volume contraction [7] in a-Si:H film prepared by catalytic chemical vapor deposition (Cat-CVD or Hot-wire CVD) method. In this study, therefore, photoinduced volume expansion and contraction in a-Si:H films has been extensively investigated. The correlation between the photoinduced volume change and the photodegradation is also experimentally investigated.
0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 2 ) 0 0 9 3 4 - 1
N. Yoshida et al. / Journal of Non-Crystalline Solids 299-302 (2002) 516–520
517
2. Experimental Silica glass with dimensions of 2 20 mm2 and a thickness of 0.1 mm was used as substrates for experiments on photoinduced volume change. Silica glass, Corning 7059 glass and c-Si substrates with dimensions of 7 20 mm2 and a thickness of 0.5 mm were also used as substrates for measurements of bandgap energies, conductivities and hydrogen contents of the films, respectively. a-Si:H films were prepared by PECVD and CatCVD methods. For PECVD, a hydrogen dilution ratio (r ¼ H2 =SiH4 ) of 4 and a total gas pressure of 1 Torr were used. The rf (13.56 MHz) power was 15 W. Substrate temperatures were 50–450 °C. For Cat-CVD, r of 0.2–0.4, total gas pressures of 5–7 mTorr, filament temperatures of 1660–1760 °C and substrate temperatures of 220–400 °C were used. The thickness of a-Si:H films was 0.4–1 lm. For estimating the magnitude of photoinduced volume changes in a-Si:H films, the laser optical lever method was used. The detail of this technique is described in Refs. [4,5,7]. An Arþ laser and a dye laser with intensities of 50–700 mW/cm2 was used for giving rise to the photoinduced volume change in a-Si:H films. An intermittent light illumination with a light-on and -off period of 5 min was performed to avoid thermal effects in the sample by a light soaking.
3. Results Fig. 1 shows samples of the output of a position sensitive detector on the experiment of the photoinduced volume change in a-Si:H films. Data in light-on and -off regime correspond to thermal expansion and the photoinduced volume change [4,5], respectively. As shown in the figure, the output at light-off regime (a) increases and (b) decreases with time. These correspond to (a) photoinduced volume expansion [4,5] and (b) photoinduced volume contraction [7]. Fig. 2 shows time evolution of (a) photoinduced volume expansion and (b) contraction in a-Si:H films using the data in Fig. 1. The vertical axis DV =V represents the magnitude of relative volume changes estimated from the Stoney’s equation
Fig. 1. PSD outputs on the experiment of the photoinduced volume change in a-Si:H films prepared by Cat-CVD. Data in light-on and -off regime (shown at upper left) correspond to thermal expansion and photoinduced volume change [4,5,7], respectively. The data of (a) and (b) correspond to photoinduced volume expansion [4,5] and contraction [7], respectively.
[4,5,7]. Positive and negative values of the axis indicate volume expansion and contraction, respectively. To investigate the mechanism of photoinduced volume change, experiments on the wavelength dependence have been performed. PECVD films were employed as samples. An Arþ laser and a dye laser with an intensity of 150 mW/cm2 were used for inducing the phenomenon. The result is shown in Fig. 3. It is found that the magnitude of the photoinduced volume expansion becomes greater with increasing photon energy in the same absorbed photon numbers.
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N. Yoshida et al. / Journal of Non-Crystalline Solids 299-302 (2002) 516–520
Fig. 2. Photoinduced volume expansion (a) and photoinduced volume contraction (b) in a-Si:H films prepared by Cat-CVD [7]. The magnitude of the experimental error is included in the symbol for data.
Fig. 3. The wavelength dependence of photoinduced volume expansion in a-Si:H films. The wavelength and the corresponding photon energy are indicated in the figure. The magnitude of the experimental error is included in the symbol for data.
The light-intensity dependence of the photoinduced volume change and the photodegradation in a-Si:H films have been investigated to clarify the correlation between both phenomena. The light intensity was varied from 50 to 700 mW/cm2 . The result is shown in Fig. 4. The horizontal axis shows the product of light intensity and illumination time, which corresponds to the incident photon number. For photodegradation, the data for photoinduced defect creation [8] are used. It is found that both phenomena have light-intensity dependence. That is, the reciprocity relation between light intensity and illumination time does not hold. It is also found that the DV =V and the number of the induced defect by 700 mW/cm2 become smaller than those by 400 mW/cm2 at the same incident photon numbers. There may happen reconstruction of Si lattice due to a strong light intensity. From these results, it is confirmed that there exists correlations between photoinduced volume expansion and the photoinduced defect creation (the photodegradation), adding to previous results (see Section 1) [4,5].
To investigate the origin for photoinduced volume expansion and contraction in Cat-CVD films, Fig. 5 shows hydrogen-content dependence of the photoinduced volume change. The hydrogen content was estimated from the integral intensity of IR absorption at 630 cm1 . The data indicate the value of DV =V after light illumination of 2 h. The DV =V seems to increase with H content. However, the data scatter a lot. Alternatively, Fig. 6 shows bandgap-energy (estimated from the Tauc plot) dependence of DV =V using the same films as in Fig. 5. It is found that DV =V increases with bandgap energy. Accordingly, it seems that a-Si:H films with H content less than 5 at.% contract due to light illumination. 1 Note 1
Note that the H content (Fig. 5) and the bandgap energy (Fig. 6) were measured using a-Si:H films deposited on c-Si and quartz, respectively. And, DV =V was measured using a-Si:H films deposited on quartz. Furthermore, the bandgap energy in a-Si:H films depends almost monotonically on the H content [9,10]. Hence, Fig. 6 seems to represent the H-content dependence of DV =V more truly.
N. Yoshida et al. / Journal of Non-Crystalline Solids 299-302 (2002) 516–520
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Fig. 5. Hydrogen-content dependence of the photoinduced volume changes in a-Si:H films prepared by Cat-CVD. The magnitude of the experimental error is included in the symbol for data.
Fig. 4. The light-intensity dependence of (a) photoinduced volume expansion and (b) photoinduced defect creation [6] in aSi:H films. Light intensities used are indicated in the figure. The dimensions of the sample (b) [6] are 0.5 cm2 and a thickness of 3 lm. The magnitude of the experimental error is included in the symbol for data.
that the initial stress of the film may be the origin for the sign (expansion or contraction) of the photoinduced volume change as pointed out in Ref. [7]. Also note that the sign does not depend on initial defect densities of a-Si:H films [7]. In addition to a-Si:H films prepared by CatCVD method, photoinduced volume contraction in a-Si:H films prepared by PECVD was observed in this study. Fig. 7 shows the data for DV =V in an a-Si:H film prepared with a substrate temperature of 50 °C. First, the film expands due to light illumination (Fig. 4(a): the photoinduced volume expansion [4,5]). After annealing of this film at 200 °C for 1 h in a vacuum of 106 Torr, then, the film exhibits photoinduced volume con-
Fig. 6. Bandgap dependence of the photoinduced volume changes in a-Si:H films prepared by Cat-CVD.
traction (Fig. 4(b)). Note that the hydrogen content of this film changed from 25 to 12 at.% by the annealing. It seems to be important to investigate the correlation between the sign of the photoinduced volume change and the existence of the photodegradation. It is found (not shown) that photodegradation occurs in all samples exhibiting photoinduced volume contraction [7]. The detail is shown in Ref. [7].
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N. Yoshida et al. / Journal of Non-Crystalline Solids 299-302 (2002) 516–520
be hydrogen content (Figs. 5 and 6) and the initial stress [7] of the film. Alternatively, hydrogen structures [6] may dominate the sign of the photoinduced volume change in a-Si:H films. Experiments for the detail is now in progress. 5. Conclusions The photoinduced volume change in a-Si:H films has been studied. The possibility is pointed out that the driving force for the photoinduced volume change may be the relaxation energy of photoexcited carriers. It is confirmed that the correlation exists between the photoinduced volume change and the photodegradation. The sign of the photoinduced volume change seems to be determined by hydrogen content and the initial stress of a-Si:H films. Fig. 7. Photoinduced volume expansion (a) and photoinduced volume contraction (b) in an a-Si:H film prepared by PECVD at a substrate temperature of 50 °C. Thermal annealing of 200 °C for 1 h in a vacuum of 106 Torr was performed between experiments (a) and (b). The magnitude of the experimental error is included in the symbol for data.
4. Discussion The mechanism of the photoinduced volume change is discussed. From the result of Fig. 3, the relaxation energy of photoexcited carriers can be a candidate for driving force of photostructural changes in a-Si:H films. That is, the energy may localise at a part of Si network consisting of several atoms. Then, the part of Si network can shift to other metastable states and this results in the photoinduced volume change. From the result of Figs. 5–7, it is assumed that there exist, at least, two kinds of structures responsible for the photoinduced volume expansion and contraction in a-Si:H films. It is also assumed that these photoinduced phenomena occur competitively. And, the larger one can be observed in experiments as photoinduced volume expansion or contraction. The photodegradation may occur at the structure exhibiting the photoinduced volume expansion. The origin for the sign of the photoinduced volume change (expansion and contraction) may
Acknowledgements This study was financially supported by NEDO as a part of the New Sunshine Program of METI, Kawasaki Steel 21st Century Foundation, The Saijiro Endo Foundation for Science and Technology, The Mazda Foundation’s Research Grant The Koshiyana Research Grant and The Ogawa Science and Technology Foundation. References [1] H. Fritzsche, Solid State Commun. 94 (1995) 953. [2] D.L. Staebler, C.R. Wronski, Appl. Phys. Lett. 31 (1977) 292. [3] K. Shimizu, T. Shiba, T. Tabuchi, H. Okamoto, Jpn. J. Appl. Phys. 36 (1997) 29. [4] T. Gotoh, S. Nonomura, M. Nishio, S. Nitta, M. Kondo, A. Matsuda, Appl. Phys. Lett. 72 (1998) 2978. [5] S. Nonomura, N. Yoshida, T. Gotoh, T. Sakamoto, M. Kondo, A. Matsuda, S. Nitta, J. Non-Cryst. Solids 266– 269 (2000) 474. [6] D. Han, J. Baugh, G. Yue, Phys. Rev. B 62 (2000) 7169. [7] T. Hatano, Y. Nakae, H. Mori, K. Ohkado, N. Yoshida, S. Nonomura, M. Itoh, A. Masuda, H. Matsumura, Thin Solid Films 395 (2001) 84. [8] M. Stutzmann, W.B. Jackson, C.C. Tsai, Phys. Rev. B 32 (1985) 23. [9] S. Yamasaki, Philos. Mag. B 56 (1987) 79. [10] H. Shirai, D. Das, J. Hanna, I. Shimizu, Appl. Phys. Lett. 59 (1991) 1096.