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Possibility of hydrogen migration in photoinduced defect creation process of a-Si:H
North-HollandJ°urnal of Non-Crystalline Solids 164-166 (1993) 227-230
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Possibility of hydrogen migration in photoinduced defect creation process of a-Si:H Michio Kondo a and Kazuo Morigaki b Institute for Solid State Physics, University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan The possibility of hydrogen migration accompanying photoinduced defect creation has been proposed from optically detected electron nuclear double resonance (ODENDOR) measurements. The matrixODENDOR signal due to the dipole-dipole interaction between trapped holes and hydrogen nuclei changes its line shape upon light exposure. A similar effect is induced by the application of pressure. The change in the line shape is discussed, taking into account the hydrogen migration and other effects. 1. INTRODUCTION It has been controversial whether hydrogen migration accompanies thephotocreation processof a dangling bond, i.e. the Staebler-Wronski (SW) effect. The activation energy for the thermal annealing of the light-induced defects is similar to that for hydrogen diffusion [1]. In the hydrogen evolution experiment, hydrogen starts to evolve at 200 N 300 °C depending on the sample preparation conditions [2], while the light-induced defects are annealed out at a similar temperature (- 150 °C). These suggest that hydrogen migration is involved in the annealing process of the SW effect, Similarly, it has been suggested that hydrogen is also involved in the light-induced defect creation from the correlation between hydrogen content and the degree of the SW effect [3-5]. Based on these arguments, models have been proposed for the SW effect [6-10] where hydrogen migration plays an important role in the generation and annihilation processes of the SW effect. Some experimental evidence 111-121 has been reported to support these models. To test the validity of the model for the SW effect, it is crucial to confirm experimentally the participation of hydrogen in the defect creation, On the other hand, based on thc Si-Si bond breaking model, the origin of bond breaking is considered to be a strained or weak Si-Si bond owing to the structural metastability in the amorphous network. This metastability is related to hydrogen, because the network flexibility is increased as the coordination number of Si is reduced by Si-H bondings. Structural metastability has been investigated from the pressure induced effect in a-Si:H. Below
the critical pressure for transition to a metallic phase, band gap narrowing and photoluminescence (PL) quenching due to pressure have been reported [13-14]. Weinstein [131 observed strong PL quenching, while Wilkinson et al. [14] did not. Since both experiments were performed using a diamond anvil cell, they could not confirm dangling bond formation during and after the application of pressure. We have pertormed an ESR experiment for a sample after the application of pressure of 5 GPa, and have confirmed dangling bond creation due to pressure similar to the SW effect [15]. Although our result supports Weinstein's conjecture, the discrepancywith the othergroupisstill unclear. In light induced phenomena, a transition from the ground state to the metastable excited state is induced by phonons emitted by nonradiative recombination, while in the pressure induced phenomenon, the transition is induced by thermal phonons because the barrier height is reduced by the increase of the internal energy of the ground state due to pressure. Relating to the hydrogen migration in the SW effect, it is an interesting question whether a similar migration occurs in the pressure induced process or not. We have performed optically detected electron nuclear double resonance (ODENDOR) measurements in order to clarify the microscopic mechanism of an electron-hole recombination involved in the light-induced and pressureinduced phenomena mentioned above. It is well accepted that the electron-hole recombination induces the SW effect. As reported before [16], a photoexcited hole is self-trapped at a Si-Si bond adjacent to a Si-H bond, obtained from the matrix-ENDOR signal due to a dipole-dipole interaction between
a) Present address: Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba-shi, Ibaraki 305, Japan. b) Present address: Faculty of Engineering, Yamaguchi University, Tokiwadai, Ube 755, Japa-n 0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved.
M. Kondo, K. Morigaki / Possibility of hydrogen migration
228
Table 1. Linewidths and intensities for the broad and narrow components in the matrix ODENDOR signal. F1 and I"2 are the line widths of the broad and narrow components, respectively. Their intensities are A1 and A2, respectively. RTF designates the room-temperature-fatigued sample. Sample #1021 #1021 RTF #1027 # 1027 RTF
F 1 (kHz) 280 200 250 250 ,
,
• • * Light-Induced . As d e p o s i t e d
,~ ,-~
F2 (kHz) 72 50 44 not observed ,
"%'.
Sample #1027 7 K
~ ¢~ ~ ~
A1/(AI+A2) 0.73 0.72 0.6 - 1.0
• Light-induced + As d e p o s i t e d 0
Sample #1021 7K
Pressure
o .
I
-0.4
I
I
|
a
!
|
¢
|
!
|
-0.2
O 0.2 0.4 Af (MHz) F i g u r e I. M a t r i x O D E N D O R s p e c t r a for t h e a s d e p o s i t e d s a m p l e a n d light e x p o s e d s a m p l e s p r e p a r e d o n a 2 3 0 °C s u b s t r a t e . T h e s i g n a l i n t e n s i t i e s are n o t n o r m a l ~ e d , the hole and hydrogen nuclei. In the ODENDOR measurement, hydrogen around the hole is observed under optical excitation. Dangling bond formation is considered to occur at a trapped site of the hole. As a consequence, it is expected that hydrogen involved in the dangling bond creation process effectively contributes to the ODENDOR signal, In this paper, we present the first observation of a change in the ODENDOR signal due to light exposure and the application of pressure, 2. E X P E R I M E N T A L RESULTS
PROCEDURE
AND
Since the experimental procedure has been reported elsewhere [17], an outline is presented here. A sample is mounted in the ENDOR cavity for applying microwave and RF magnetic field, and
-0.4
-0.2
O A f (MHz)
0.2
0.4
F i g u r e 2. M a t r i x - O D E N D O R s p e c t r a for t h e a s d e p o s i t e d (cross), light e x p o s e d (closed triangle} a n d p r e s s u r e t r e a t e d (open circle) s a m p l e s . is excited by argon-ion laser light (~ = 514.5 nm). Emitted light is monitored by a Ge pin diode. Samples used in this work were prepared under two different conditions; one is on a 250 °C substrate (#1021), ant the other is on a 230 °C substrate (#1027). The dangling bond density is 8 x 1015 cm -3 /'or sample #1021 and 3 x 1016 cm -3 for sample #1027. The light exposure was carried out using 500 W Xe lamp at room temperature. The pressure effect was also examined for the same sample as used in Ref. 15. Under weak microwave intensity, a matrix ENDOR signal due to dipole-dipole interaction between the hole and hydrogen nuclei is dominantly observed. The matrix-ENDOR signal is centered at the resonant frequency of a free proton, while the line width is broader than the NMR of a free proton due to the dipole-dipole interaction. As reported
M. Kondo, K. Morigaki / Possibility of hydrogen migration before [16], the signal consists of two components, a narrow component and a broad one. The narrow component arises from a distant hydrogen, while the broad one arises from a hydrogen near the trapped hole. If the hole is trapped in the vicinity of the clustered hydrogen, the ratio of the broad component to the narrow component in intensity is enhanced due to the nearby hydrogen cluster, Therefore, we can obtain information on the local distribution of hydrogen around the hole by this method. As mentioned in the previous section, if hydrogen migration occurs during the SW effect, one expects to observe a change in the line shape of the matrix-ENDOR signal. A similar possibility is also examined for the application of pressure, The line shapes for a sample prepared at 230 °C before and after the light exposure are shown in Fig.1. The width and the ratio of the two components' intensities are listed in Table 1. Light exposure reduces the intensity of the narrow component, while the broad component is unchanged, This change is recovered by annealing at 150 °C. On the other hand, for a sample prepared at 250 °C, the intensity of the broad component is reduced by the light exposure as shown in Fig. 2. This change is also recovered by the thermal annealing. The line shape after the pressure application is also shown in Fig. 2. The result is very similar to that of the light exposed sample. As reported before [15], the pressure-induced defect shows a very similar thermal annealing behaviour, 3. DISCUSSION We discuss possible explanations for the change in the line shape of the matrix-ODENDOR signal. First, we consider the origin of the intensity decrease of the narrow component by light exposure. As mentioned in section 2, the narrow component is due to hydrogen distant from the trapped hole. NMR of such distant hydrogen influences the hole spin through mutual dipolar interaction between 1H nuclei [18], and consequently the luminescence intensity is increased at the NMR frequency of distant 1H nuclei. When the dangling bonds interact with 1H nuclei, however, the spin diffusion may be interrupted, so that the luminescence intensity is not affected by NMR of distant 1H nuclei,
229
Secondly, we consider the origin of the intensity decrease of the broad component. The broad component arises from I H nuclei being located at sites near the hole spin. As has already been reported [16], holes are easily self-trapped in weak SiSi bonds close to clustered hydrogen, because the network around clustered hydrogen becomes flexible. Thus, such clustered hydrogen contributes dominantly to the broad component. Light exposure breaks the weak Si-Si bonds near clustered hydrogen and consequently hydrogen migrates from the hole trapping site. However, the migrating hydrogen and also the remaining hydrogen accompany dangling bonds, so that these hydrogens cannot participate in the ODENDOR owing to strong interaction with their nearby dangling bonds. Furthermore, even if holes are self-trapped in the weak Si-Si bonds around the clustered hydrogen having a nearby dangling bond, radiative recombination at the self-trapped holes is suppressed by nonradiative recombination at the nearby dangling bond. As a result, light exposure reduces the intensity of the broad component. The two origins mentioned above compete with each other; either the narrow component decreases or the broad component decreases in intensity. When both origins operate, their relative importance should be considered. The first origin may be applied to the case for sample #1027. This sample contains dangling bonds of 3 x 1016 cm-3. The photocreated dangling bonds are also of this order of magnitude. This leads to the first origin for the intensity decrease of the narrow component. On the other hand, sample #1021 contains fewer dangling bonds than sample #1027. The second origin rather than the first origin seems to operate to decrease the intensity of the broad component. The clustered hydrogen content is less in sample #1021 than in sample #1027. Such hydrogen, however, contributes to thc broad component, so that hydrogen migration from its clustered region by the light exposure causes the broad component to be reduced in intensity. Thus, this is the case for sample #1021, i.e., the second origin becomes more important than the first origin. We have discussed two possibilities for the intensity decrease in each component of the matrix ODENDOR signal. Other possibilities associated with hydrogen migration, however, cannot be exeluded. For example, it has been considered that, under light exposure, hydrogen is released from a
230
M. Kondo, K. Morigala" / Possibility of hydrogen migration
certain Si-H bond, migrates through interstitial sites and finally terminates a dangling bond to form a Sill bond and to leave a dangling bond at the initial site. This process makes no difference to the dangling bond density, but it makes hydrogen migrate. Thus, when hydrogen migrates from its clustered region, this process increases the number of isolated hydrogen without accompanying dangling bonds, so that the narrow component increases in intensity relative to the broad component. Finally, we discuss the pressure-induced defect creation process. As mentioned before, the application of pressure induces a similar defect creation to the light exposure. The resultant change in the ODENDOR line shape is almost identical to the light-induced one. The pressure application experiment has been done only for sample #1021. According to the hydrogen migration model for the SW effect, a similar hydrogen migration is expected for the pressure-induced process. The main difference between two processes is the energy distribution of phonons, because in the pressure induced process the energy distribution of the phonon is almost in the thermal equilibrium state, while in the light induced case it is in a nonequilibrium state due to the emission of phonons associated with nonradiative electron-hole recombination. In the bond breaking model, the broken bond creates two intimate dangling bonds initially, and subsequently these dangling bonds separate from each other through a diffusion process at room temperature [9]. However, how the weak Si-Si bonds are broken by the application of pressure is an open question. We have proposed one possibility that the bond breaking is induced by strain around a void which is compressed easily by pressure [15]. The above discussion is only of a qualitative nature. A quantitarive consideration remains for the future. In summary, we have observed a matrixODENDOR signal for a-Si:H prepared under two different conditions. The different changes in the line shape have been observed after light exposure for the two samples. A similar line shape change has been observed due to the application of pressure. It is proposed that probable explanations for the line shape change are light-induced hydrogen migration, and also the effect of dangling bonds on the spin diffusion.
ACKNOWLEDGEMENTS The authors would like to thank Drs. A. Matsuda and S. Yamasaki for fruitful discussions. They are also indebted to Prof. T. Yagi for useful discussions on the pressure effect. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
D.L. Staebler and C. R. Wronski, J. Appl. Phys. 51 (1980)3262. W. Beyer and H. Wagner, J. Appl. Phys. 53 (1982) 8745. M. Ohsawa et al., Jpn. J. Appl. Phys. 24 (1985) L838. N. Nakamura et al., Jpn. J. Appl. Phys. 28(1989) 1762. Y. Sano, K. Morigaki and I. Hirabayashi, Solid State Commun. 43 (1982) 439. R.E. Norberg, P.A Fedders, J. Badart, R. Corey and Y. W. Kim, Mat. Res. Soc. Symp. Proc. 219 (1991) 223. P.V. Santos, N. M. Johnson and R. A. Street, Mat. Res. Soc. Symp. Proc. 258 (1992) 353. W.B. Jackson and J. Kakalios, Phys. Rev. B37 (1988) 1020. K. Morigaki, Jpn. J. Appl. Phys. 27 (1988) 163. M. Stutzmann, W. B. Jackson and C. C. Tsai, Phys. Rev. B32 (1985) 23. S.B. Zhang, W. B. Jackson and D. J. Chadi, Phys. Rev. Lett. 65 (1990) 2575. R.A. Street and K. Winer, Phys. Rev. B 40. (1989)5236. B.A. Weinstein, Phys. Rev. B40 (1981) 787. V.A. Wilkinson, D. J. Dunstan, P. G. LeComber, R. A. G. Gibson, Phil. Mag. B. 59 (1989) 37. M. Kondo, W. Utsumi, T. Yagi, and K. Morigaki, J. Non-Cryst. Solids 137 &138 (1991) 275. M. Kondo and K. Morigaki, J. Non-Cryst. Solids 137 &138 (1991) 247. M. Kondo and K. Morigaki, J. Non-Cryst. Solids 114 (1989)408. J. Lambe, N. Laurance, E. C. Mclrvine, and R.W. Terhune, Phys. Rev. 122 (1961) 1161.
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~IlI~~
Possibility of hydrogen migration in photoinduced defect creation process of a-Si:H Michio Kondo a and Kazuo Morigaki b Institute for Solid State Physics, University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan The possibility of hydrogen migration accompanying photoinduced defect creation has been proposed from optically detected electron nuclear double resonance (ODENDOR) measurements. The matrixODENDOR signal due to the dipole-dipole interaction between trapped holes and hydrogen nuclei changes its line shape upon light exposure. A similar effect is induced by the application of pressure. The change in the line shape is discussed, taking into account the hydrogen migration and other effects. 1. INTRODUCTION It has been controversial whether hydrogen migration accompanies thephotocreation processof a dangling bond, i.e. the Staebler-Wronski (SW) effect. The activation energy for the thermal annealing of the light-induced defects is similar to that for hydrogen diffusion [1]. In the hydrogen evolution experiment, hydrogen starts to evolve at 200 N 300 °C depending on the sample preparation conditions [2], while the light-induced defects are annealed out at a similar temperature (- 150 °C). These suggest that hydrogen migration is involved in the annealing process of the SW effect, Similarly, it has been suggested that hydrogen is also involved in the light-induced defect creation from the correlation between hydrogen content and the degree of the SW effect [3-5]. Based on these arguments, models have been proposed for the SW effect [6-10] where hydrogen migration plays an important role in the generation and annihilation processes of the SW effect. Some experimental evidence 111-121 has been reported to support these models. To test the validity of the model for the SW effect, it is crucial to confirm experimentally the participation of hydrogen in the defect creation, On the other hand, based on thc Si-Si bond breaking model, the origin of bond breaking is considered to be a strained or weak Si-Si bond owing to the structural metastability in the amorphous network. This metastability is related to hydrogen, because the network flexibility is increased as the coordination number of Si is reduced by Si-H bondings. Structural metastability has been investigated from the pressure induced effect in a-Si:H. Below
the critical pressure for transition to a metallic phase, band gap narrowing and photoluminescence (PL) quenching due to pressure have been reported [13-14]. Weinstein [131 observed strong PL quenching, while Wilkinson et al. [14] did not. Since both experiments were performed using a diamond anvil cell, they could not confirm dangling bond formation during and after the application of pressure. We have pertormed an ESR experiment for a sample after the application of pressure of 5 GPa, and have confirmed dangling bond creation due to pressure similar to the SW effect [15]. Although our result supports Weinstein's conjecture, the discrepancywith the othergroupisstill unclear. In light induced phenomena, a transition from the ground state to the metastable excited state is induced by phonons emitted by nonradiative recombination, while in the pressure induced phenomenon, the transition is induced by thermal phonons because the barrier height is reduced by the increase of the internal energy of the ground state due to pressure. Relating to the hydrogen migration in the SW effect, it is an interesting question whether a similar migration occurs in the pressure induced process or not. We have performed optically detected electron nuclear double resonance (ODENDOR) measurements in order to clarify the microscopic mechanism of an electron-hole recombination involved in the light-induced and pressureinduced phenomena mentioned above. It is well accepted that the electron-hole recombination induces the SW effect. As reported before [16], a photoexcited hole is self-trapped at a Si-Si bond adjacent to a Si-H bond, obtained from the matrix-ENDOR signal due to a dipole-dipole interaction between
a) Present address: Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba-shi, Ibaraki 305, Japan. b) Present address: Faculty of Engineering, Yamaguchi University, Tokiwadai, Ube 755, Japa-n 0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved.
M. Kondo, K. Morigaki / Possibility of hydrogen migration
228
Table 1. Linewidths and intensities for the broad and narrow components in the matrix ODENDOR signal. F1 and I"2 are the line widths of the broad and narrow components, respectively. Their intensities are A1 and A2, respectively. RTF designates the room-temperature-fatigued sample. Sample #1021 #1021 RTF #1027 # 1027 RTF
F 1 (kHz) 280 200 250 250 ,
,
• • * Light-Induced . As d e p o s i t e d
,~ ,-~
F2 (kHz) 72 50 44 not observed ,
"%'.
Sample #1027 7 K
~ ¢~ ~ ~
A1/(AI+A2) 0.73 0.72 0.6 - 1.0
• Light-induced + As d e p o s i t e d 0
Sample #1021 7K
Pressure
o .
I
-0.4
I
I
|
a
!
|
¢
|
!
|
-0.2
O 0.2 0.4 Af (MHz) F i g u r e I. M a t r i x O D E N D O R s p e c t r a for t h e a s d e p o s i t e d s a m p l e a n d light e x p o s e d s a m p l e s p r e p a r e d o n a 2 3 0 °C s u b s t r a t e . T h e s i g n a l i n t e n s i t i e s are n o t n o r m a l ~ e d , the hole and hydrogen nuclei. In the ODENDOR measurement, hydrogen around the hole is observed under optical excitation. Dangling bond formation is considered to occur at a trapped site of the hole. As a consequence, it is expected that hydrogen involved in the dangling bond creation process effectively contributes to the ODENDOR signal, In this paper, we present the first observation of a change in the ODENDOR signal due to light exposure and the application of pressure, 2. E X P E R I M E N T A L RESULTS
PROCEDURE
AND
Since the experimental procedure has been reported elsewhere [17], an outline is presented here. A sample is mounted in the ENDOR cavity for applying microwave and RF magnetic field, and
-0.4
-0.2
O A f (MHz)
0.2
0.4
F i g u r e 2. M a t r i x - O D E N D O R s p e c t r a for t h e a s d e p o s i t e d (cross), light e x p o s e d (closed triangle} a n d p r e s s u r e t r e a t e d (open circle) s a m p l e s . is excited by argon-ion laser light (~ = 514.5 nm). Emitted light is monitored by a Ge pin diode. Samples used in this work were prepared under two different conditions; one is on a 250 °C substrate (#1021), ant the other is on a 230 °C substrate (#1027). The dangling bond density is 8 x 1015 cm -3 /'or sample #1021 and 3 x 1016 cm -3 for sample #1027. The light exposure was carried out using 500 W Xe lamp at room temperature. The pressure effect was also examined for the same sample as used in Ref. 15. Under weak microwave intensity, a matrix ENDOR signal due to dipole-dipole interaction between the hole and hydrogen nuclei is dominantly observed. The matrix-ENDOR signal is centered at the resonant frequency of a free proton, while the line width is broader than the NMR of a free proton due to the dipole-dipole interaction. As reported
M. Kondo, K. Morigaki / Possibility of hydrogen migration before [16], the signal consists of two components, a narrow component and a broad one. The narrow component arises from a distant hydrogen, while the broad one arises from a hydrogen near the trapped hole. If the hole is trapped in the vicinity of the clustered hydrogen, the ratio of the broad component to the narrow component in intensity is enhanced due to the nearby hydrogen cluster, Therefore, we can obtain information on the local distribution of hydrogen around the hole by this method. As mentioned in the previous section, if hydrogen migration occurs during the SW effect, one expects to observe a change in the line shape of the matrix-ENDOR signal. A similar possibility is also examined for the application of pressure, The line shapes for a sample prepared at 230 °C before and after the light exposure are shown in Fig.1. The width and the ratio of the two components' intensities are listed in Table 1. Light exposure reduces the intensity of the narrow component, while the broad component is unchanged, This change is recovered by annealing at 150 °C. On the other hand, for a sample prepared at 250 °C, the intensity of the broad component is reduced by the light exposure as shown in Fig. 2. This change is also recovered by the thermal annealing. The line shape after the pressure application is also shown in Fig. 2. The result is very similar to that of the light exposed sample. As reported before [15], the pressure-induced defect shows a very similar thermal annealing behaviour, 3. DISCUSSION We discuss possible explanations for the change in the line shape of the matrix-ODENDOR signal. First, we consider the origin of the intensity decrease of the narrow component by light exposure. As mentioned in section 2, the narrow component is due to hydrogen distant from the trapped hole. NMR of such distant hydrogen influences the hole spin through mutual dipolar interaction between 1H nuclei [18], and consequently the luminescence intensity is increased at the NMR frequency of distant 1H nuclei. When the dangling bonds interact with 1H nuclei, however, the spin diffusion may be interrupted, so that the luminescence intensity is not affected by NMR of distant 1H nuclei,
229
Secondly, we consider the origin of the intensity decrease of the broad component. The broad component arises from I H nuclei being located at sites near the hole spin. As has already been reported [16], holes are easily self-trapped in weak SiSi bonds close to clustered hydrogen, because the network around clustered hydrogen becomes flexible. Thus, such clustered hydrogen contributes dominantly to the broad component. Light exposure breaks the weak Si-Si bonds near clustered hydrogen and consequently hydrogen migrates from the hole trapping site. However, the migrating hydrogen and also the remaining hydrogen accompany dangling bonds, so that these hydrogens cannot participate in the ODENDOR owing to strong interaction with their nearby dangling bonds. Furthermore, even if holes are self-trapped in the weak Si-Si bonds around the clustered hydrogen having a nearby dangling bond, radiative recombination at the self-trapped holes is suppressed by nonradiative recombination at the nearby dangling bond. As a result, light exposure reduces the intensity of the broad component. The two origins mentioned above compete with each other; either the narrow component decreases or the broad component decreases in intensity. When both origins operate, their relative importance should be considered. The first origin may be applied to the case for sample #1027. This sample contains dangling bonds of 3 x 1016 cm-3. The photocreated dangling bonds are also of this order of magnitude. This leads to the first origin for the intensity decrease of the narrow component. On the other hand, sample #1021 contains fewer dangling bonds than sample #1027. The second origin rather than the first origin seems to operate to decrease the intensity of the broad component. The clustered hydrogen content is less in sample #1021 than in sample #1027. Such hydrogen, however, contributes to thc broad component, so that hydrogen migration from its clustered region by the light exposure causes the broad component to be reduced in intensity. Thus, this is the case for sample #1021, i.e., the second origin becomes more important than the first origin. We have discussed two possibilities for the intensity decrease in each component of the matrix ODENDOR signal. Other possibilities associated with hydrogen migration, however, cannot be exeluded. For example, it has been considered that, under light exposure, hydrogen is released from a
230
M. Kondo, K. Morigala" / Possibility of hydrogen migration
certain Si-H bond, migrates through interstitial sites and finally terminates a dangling bond to form a Sill bond and to leave a dangling bond at the initial site. This process makes no difference to the dangling bond density, but it makes hydrogen migrate. Thus, when hydrogen migrates from its clustered region, this process increases the number of isolated hydrogen without accompanying dangling bonds, so that the narrow component increases in intensity relative to the broad component. Finally, we discuss the pressure-induced defect creation process. As mentioned before, the application of pressure induces a similar defect creation to the light exposure. The resultant change in the ODENDOR line shape is almost identical to the light-induced one. The pressure application experiment has been done only for sample #1021. According to the hydrogen migration model for the SW effect, a similar hydrogen migration is expected for the pressure-induced process. The main difference between two processes is the energy distribution of phonons, because in the pressure induced process the energy distribution of the phonon is almost in the thermal equilibrium state, while in the light induced case it is in a nonequilibrium state due to the emission of phonons associated with nonradiative electron-hole recombination. In the bond breaking model, the broken bond creates two intimate dangling bonds initially, and subsequently these dangling bonds separate from each other through a diffusion process at room temperature [9]. However, how the weak Si-Si bonds are broken by the application of pressure is an open question. We have proposed one possibility that the bond breaking is induced by strain around a void which is compressed easily by pressure [15]. The above discussion is only of a qualitative nature. A quantitarive consideration remains for the future. In summary, we have observed a matrixODENDOR signal for a-Si:H prepared under two different conditions. The different changes in the line shape have been observed after light exposure for the two samples. A similar line shape change has been observed due to the application of pressure. It is proposed that probable explanations for the line shape change are light-induced hydrogen migration, and also the effect of dangling bonds on the spin diffusion.
ACKNOWLEDGEMENTS The authors would like to thank Drs. A. Matsuda and S. Yamasaki for fruitful discussions. They are also indebted to Prof. T. Yagi for useful discussions on the pressure effect. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
D.L. Staebler and C. R. Wronski, J. Appl. Phys. 51 (1980)3262. W. Beyer and H. Wagner, J. Appl. Phys. 53 (1982) 8745. M. Ohsawa et al., Jpn. J. Appl. Phys. 24 (1985) L838. N. Nakamura et al., Jpn. J. Appl. Phys. 28(1989) 1762. Y. Sano, K. Morigaki and I. Hirabayashi, Solid State Commun. 43 (1982) 439. R.E. Norberg, P.A Fedders, J. Badart, R. Corey and Y. W. Kim, Mat. Res. Soc. Symp. Proc. 219 (1991) 223. P.V. Santos, N. M. Johnson and R. A. Street, Mat. Res. Soc. Symp. Proc. 258 (1992) 353. W.B. Jackson and J. Kakalios, Phys. Rev. B37 (1988) 1020. K. Morigaki, Jpn. J. Appl. Phys. 27 (1988) 163. M. Stutzmann, W. B. Jackson and C. C. Tsai, Phys. Rev. B32 (1985) 23. S.B. Zhang, W. B. Jackson and D. J. Chadi, Phys. Rev. Lett. 65 (1990) 2575. R.A. Street and K. Winer, Phys. Rev. B 40. (1989)5236. B.A. Weinstein, Phys. Rev. B40 (1981) 787. V.A. Wilkinson, D. J. Dunstan, P. G. LeComber, R. A. G. Gibson, Phil. Mag. B. 59 (1989) 37. M. Kondo, W. Utsumi, T. Yagi, and K. Morigaki, J. Non-Cryst. Solids 137 &138 (1991) 275. M. Kondo and K. Morigaki, J. Non-Cryst. Solids 137 &138 (1991) 247. M. Kondo and K. Morigaki, J. Non-Cryst. Solids 114 (1989)408. J. Lambe, N. Laurance, E. C. Mclrvine, and R.W. Terhune, Phys. Rev. 122 (1961) 1161.