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Effects of dopant and impurity incorporation on metastable light-induced defect formation
Solar Cells. 21 (1987) 431 - 438
431
E F F E C T S OF DOPANT AND IMPURITY INCORPORATION ON METAS TAB L E LIGHT-INDUCED D E F E C T FO RMA T IO N * W. B. JACKSON, M. STUTZMANN and C. C. TSAI Xerox Palo Alto Research Center, Palo Alto, CA 94304 (U.S.A.)
(Received May 1986; accepted July 3, 1986)
Summary In this paper we summarize recent measurements of the effects of impurities and dopants on metastability in h y d r o g e n a t e d am orphous silicon. Using a new m e t h o d of electron spin resonance transient spectroscopy, the distribution of defect annealing energies of samples containing significant concentrations of oxygen, nitrogen and carbon are compared with ultrahigh vacuum intrinsic material. Although the impurities cause significant alterations in the annealing energy distribution, deuteration does not change the creation rate or the annealing distribution. The addition of phosphorus d o p a n t atoms causes significant changes in the num ber of the metastable defects, and hyperfine measurements indicate that light induces metastable changes in the phosphorus d o n o r concentration. Possible models for this effect are discussed.
1. I n t r o d u c t i o n Light-induced metastable defect creation is an i m p o r t a n t problem in the study of hydrogenated amorphous silicon (a-Si:H). Because the defects reduce carrier lifetimes and shift the Fermi level, there is significant technological interest in understanding the origin of such metastable effects. Previous work has de m ons t r at ed that the primary effect of illumination is to create dangling bond defects by band-to-band recombination [1 - 3]. The creation rate was more or less i n d e p e n d e n t of impurity level for small impurity concentrations, suggesting that the defect form at i on process depends only on silicon and/ or hydrogen in the intrinsic material [4]. Concentrations of oxygen, nitrogen and carbon above 1 at.% however, were found to increase both the stable and metastable defect densities [ 4 - 6] and to cause significant changes in solar cell performance [1, 5]. F u r t h e r m o r e , in phosphorus-doped films, the magnitude of the photo-induced changes in *Paper presented at the 7th Photovoltaic Advanced Research and Development Project Review Meeting, Denver, CO, U.S.A., May 13, 1986. 0379-6787/87/$3.50
© Elsevier Sequoia/Printed in The Netherlands
432 subgap absorption indicated that the number of dangling bond defects increased proportionately to the doping-induced defect density in phosphorus-doped material [7 ]. In this paper we examine the effects of various impurities on the metastable defect changes in a-Si:H to gain further understanding of the defect formation process. In particular, using a new technique of electron spin resonance (ESR) transient spectroscopy, we examine the distribution of annealing energies for oxygen-, nitrogen- and carbon-induced defects. To examine the role of hydrogen in the defect creation process, the annealing process in deuterated films is also investigated. Finally, some new metastable changes of the donor levels in phosphorus-doped samples were discovered which have implications for solar cell performance and other experiments measuring the metastable changes in phosphorus-doped material. A model is proposed to explain the results.
2. Experimental details The primary experimental method used to examine the effects of impurities on metastable defect creation is ESR transient spectroscopy. In this technique the spin densities of illuminated samples with increased density of dangling bond defects are measured as a function of time for various annealing temperatures. The decrease in the total spin population at a given time t ( i . e . the derivative with respect to time) is due to the annealing of those defects with an annealing rate R(TA) such that 1 = tR(TA)
= tu o
exp
(1)
where v0 is the attempt frequency, T A is the annealing temperature and E is the activation energy of the defect. Solving for E, one obtains E = kT A
ln(v0t )
(2)
Hence, an approximation of the distribution of annealing energies may be obtained by taking the derivative of the ESR decay with respect to k T A ln(v0t), and the a t t e m p t frequency can be determined by measuring decays at different annealing energies [4, 8]. The distribution of annealing energies in intrinsic material was investigated by illuminating samples deposited under ultrahigh vacuum conditions for up to 16 h. The impurity levels for oxygen, nitrogen and carbon were on the order of 10 ~s cm -~. The samples were annealed within the ESR cavity, and the spin density was measured using a double-modulation scheme [4]. 3. Intrinsic metastable defects The resulting ESR transient spectra are presented in Fig. 1. The distribution has a peak at 1.1 eV and is about 0.3 eV wide; it looks very similar to
433
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(a)
0,, I 0.5
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I
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(b)
Fig. l ( a ) Normalized n u m b e r of metastable spins vs. k T A ln(v0t) for h y d r o g e n a t e d ( - - - - - - ) and deuterated ( ) films (P0 = 101° s - l ) • (b) Distribution of annealing energies obtained from (a).
the energy distribution of singly occupied dangling bonds relative to the conduction band. This fact suggests the possibility that the rate-limiting step for annealing is excitation of an electron from a dangling bond. Since it has been proposed that recombination causes hydrogen motion and subsequent dangling bond formation, the substitution of deuterium for hydrogen would provide information about the mechanism for hydrogen motion [9]. A sample containing only deuterium exhibits a distribution of annealing energies virtually identical with that found for hydrogencontaining samples. Furthermore, no significant change was found for the creation rate. This observation of identical creation and annealing rates eliminates hydrogen tunneling as a mechanism in dangling bond formation as well as recombination at the Si--H bond by multiphonon emission [10]. An example of a possible mechanism consistent with the above results is depicted in Fig. 2. The recombination of an electron-hole pair occurs at a weak Si--Si bond, causing the transfer of a back-bonded hydrogen to the
H v
(a)
(b)
Fig. 2. A possible hydrogen-switching reaction responsible for increased dangling bonds: (a) for the annealed state; (b) after the r e c o m b i n a t i o n event.
434 weak Si--Si b o n d position, resulting in t w o dangling silicon bonds. The annealing process is the reverse. The removal of an electron from a dangling b o n d causes the back bonds to assume a planar sp2 configuration. This configuration causes the h y d r o g e n atom to move toward its annealed state position and enables the h y d r o g e n atom to move out of the weak Si--Si b o n d i n g region, allowing re-formation of the broken bond.
4. Impurity-induced metastable defects The spectra for the different impurities are presented in Fig. 3. There are distinct differences b e t w e e n the spectra for the impurity-induced defects and those for the intrinsic material. The annealing energy distributions of nitrogen-doped samples begin at lower energies and e x t e n d to quite high i 08
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Fig. 3. Distributions of annealing energies in a-Si:H for different impurities: (a) ultrahigh vacuum material(P0= 101°s l);(b)4at.%N(P0= 101°s-1);(c)20at.% O(v o= 10 ss-l).
435 energies (Fig. 3(b)). The distribution of annealing energies is t herefore quite broad. One might e xpe c t t ha t a similar broad distribution of defects may occur in a m o r p h o u s silicon nitride and at the a-Si:H-silicon nitride interface. Such metastable changes can cause metastable threshold voltage shifts if a transistor is illuminated or if the traps can be field induced. In a similar manner, metastable defects in oxygen-containing films also exhibit a wide range of annealing energies (Fig. 3(c)). Some of the centers induced by oxygen have such large activation energies that it is difficult to separate the metastable changes from the stable background. Finally, carbon-induced defects have a distribution of low activation energies extending from 0.5 to 0.8 eV and a low prefactor (106 Hz). Consequently, carbon-induced centers anneal at comparatively low temperatures. The fact t hat these impurity-related spectra are significantly different from the spectra of intrinsic specimens is additional evidence that the metastable defects in intrinsic material are a result of silicon a n d / o r hydrogen r econs t r uct i on alone. This result supports the impurity studies which indicated t hat impurities do not play a role in metastable defect formation.
5. Metastable changes in phosphorus-doped samples It has recently been shown that the density of occupied phosphorus donors (P4 °) as well as the density of occupied c o n d u c t i o n band tail states in phosphorus-doped material could be determined using hyperfine measurements [11]. By measuring the P4° and Si4 ° density for the annealed state as well as for the illuminated state, we have found that the density P4° increases on illumination, whereas the density of paramagnetic c o n d u c t i o n band tail states and the Fermi level decreases (Table 1). This remarkable result demonstrates that light substantially increases the n u m b e r of fourfold phosphorus atoms, since the decrease in Fermi level would otherwise lead to a decrease in the occupied phosphorus d o n o r levels {Fig. 4). The n u m b e r of increased d o n o r levels estimated f r om ESR is within a factor of 2 of the estimated increase in density of dangling bonds for similarly doped material. Although the precise mechanism is not known, the results are consisten t with a model t hat is similar to that proposed for the creation of TABLE 1 Metastable spin density changes in annealed phosphorus-doped samples after 16 h of illumination Doping level
Conduction band tail density
(cm a)
change (cm a)
(em 3)
10-4
--4.5 × 10 is --1.6 X 1016
2 )< 1015 2 X 1016
10 2
Occupied donor density
436
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Fig. 4. The changes in the band tail and defect states as a result of illumination for phosphorus doping levels of 10 4 cm 3 and higher: - - , before illumination: . . . . , after illumination. /
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Fig. 5. The possible hydrogen-switching reaction P3°+ Si4° ~ Si 3 + P4* responsible for the increase in donor levels on illumination or rapid quenching: (a) dark; (b) illuminated. d a n g l i n g b o n d s in u n d o p e d m a t e r i a l (Fig. 2) w h i c h involves silicon [ 9 ] . In this r e a c t i o n , a h y d r o g e n a t o m b o n d e d t o a silicon a t o m a d j a c e n t t o t h e t h r e e f o l d p h o s p h o r u s a t o m m o v e s t o t h e p h o s p h o r u s a t o m , m a k i n g it f o u r f o l d c o o r d i n a t e d a n d leaving b e h i n d a d a n g l i n g b o n d on t h e silicon a t o m (Fig. 5 ). S u c h an e v e n t is e q u i v a l e n t t o t h e r e a c t i o n P 9 + Si4 ° ~
P4 + + Si3
This r e a c t i o n is c o n s i s t e n t w i t h t h a t p r o p o s e d f o r d o p i n g - i n d u c e d d e f e c t s d u r i n g d e p o s i t i o n a n d , in f a c t , r e p r e s e n t s a s p e c i f i c i m p l e m e n t a t i o n of t h e g e n e r a l p r o c e s s [ 1 2 ] . This e v e n t d o e s n o t m o v e t h e F e r m i level s i g n i f i c a n t l y , since t h e F e r m i level o c c u r s b e t w e e n P4 ° a n d t h e Si3 levels. D u r i n g i l l u m i n a t i o n , t h e p r o c e s s s h o w n in Fig. 2 also o c c u r s , w h i c h d o e s l o w e r t h e F e r m i level. W h e n t h e c o n c e n t r a t i o n o f d o n o r levels is s i m i l a r t o t h e i n c r e a s e d
437
number of broken weak Si--Si bonds, the Fermi level moves significantly. For heavier doping, despite the increase in dangling bond density from the reaction of Fig. 2, the Fermi level does not decrease since the reaction in Fig. 5 dominates. This explains the small changes in the dark conductivity for both lightly doped and heavily doped samples, whereas intermediately doped samples exhibit a large charge [3]. It should be mentioned that the mechanisms in Fig. 5 create equal numbers of dangling bonds and donor levels. Another possible hydrogen-switching reaction in which the hydrogen atom breaks the Si--P bond may also occur. This reaction, equivalent to the reaction P4 ° + Si4 ~
P3 ° + Si 3-
creates a P4 ° center while destroying an Si3 center, and would explain why discrepancies between the donor levels determined by hyperfine and dangling bonds density, as measured by absorption [ 7 ].
6. Conclusion The results presented in this paper indicate that the metastability of both undoped and doped material is most likely to be a consequence of the presence of hydrogen in the amorphous silicon lattice and is not because of impurities, since the characteristic energies of impurity-induced metastable defects are significantly different. The striking similarity between the lightinduced metastable defects, doping-induced and field-induced defects suggests a c o m m o n origin related to the motion of hydrogen through the material. The microscopic mechanisms proposed in this paper are possible microscopic origins of this c o m m o n connection.
Acknowledgments We would like to thank R. Street and J. Kakalios for helpful discussions. This work was supported in part by Solar Energy Research Institute Contract XB-3-03112-1.
References 1 D. L. Staebler and C. Wronski, Appl. Phys. Lett., 31 (1977) 292. Z. E. Smith and S. Wagner, Appl. Phys. Lett., 46 (1985) 1078. C. Lee, W. D. Ohlsen and P. C. Taylor, Phys. Rev. B, 31 (1985) 100. 2 H. Dersch, J. Stuke and J. Beichler, Appl. Phys. Lett., 38 (1980) 456. 3 H. Fritzsche, Sol. Energy Mater., 3 (1980) 447. 4 M. Stutzmann, W. B. Jackson and C. C. Tsai, Phys. Rev. B, 34 (1986) 63. 5 R. S. Crandall, Phys. Rev. B, 24 (1981) 7457. R. S. Crandall, D. E. Carlson, A. Catalano and H. A. Weakliem, Appl. Phys. Lett., 44 (1984) 1078.
438 6 N. Nakamura, S. Tsuda, T. Takahama, M. Nishikuni, K. Watanabe, M. Ohnishi and Y. Kuwano, in P. C. Taylor and S. G. Bishop (eds.), Optical Effects in Amorphous Semiconductors, Snowbird, UT, 1984, No. 120, AIP, New York, 1984, p. 303. 7 A. Skumanich, N. M. Amer and W. B. Jackson, Phys. Rev. B, 31 (1985) 2263. 8 W. B. Jackson, M. Stutzmann and C. C. Tsai, Phys. Rev. B, 34 (1986) 54. 9 M. Stutzmann, W. B. Jackson and C. C. Tsai, Phys. Rev. B, 32 (1985) 23. 10 M. Stutzmann, W. B. Jackson and C. C. Tsai, Appl. Phys. Lett., 48 (1986) 62. 11 M. Stutzmann and R. A. Street, Phys. Rev. Lett., 54 (1985) 1836. 12 R. A. Street, Phys. Rev. Lett., 49 (1982) 1187.
431
E F F E C T S OF DOPANT AND IMPURITY INCORPORATION ON METAS TAB L E LIGHT-INDUCED D E F E C T FO RMA T IO N * W. B. JACKSON, M. STUTZMANN and C. C. TSAI Xerox Palo Alto Research Center, Palo Alto, CA 94304 (U.S.A.)
(Received May 1986; accepted July 3, 1986)
Summary In this paper we summarize recent measurements of the effects of impurities and dopants on metastability in h y d r o g e n a t e d am orphous silicon. Using a new m e t h o d of electron spin resonance transient spectroscopy, the distribution of defect annealing energies of samples containing significant concentrations of oxygen, nitrogen and carbon are compared with ultrahigh vacuum intrinsic material. Although the impurities cause significant alterations in the annealing energy distribution, deuteration does not change the creation rate or the annealing distribution. The addition of phosphorus d o p a n t atoms causes significant changes in the num ber of the metastable defects, and hyperfine measurements indicate that light induces metastable changes in the phosphorus d o n o r concentration. Possible models for this effect are discussed.
1. I n t r o d u c t i o n Light-induced metastable defect creation is an i m p o r t a n t problem in the study of hydrogenated amorphous silicon (a-Si:H). Because the defects reduce carrier lifetimes and shift the Fermi level, there is significant technological interest in understanding the origin of such metastable effects. Previous work has de m ons t r at ed that the primary effect of illumination is to create dangling bond defects by band-to-band recombination [1 - 3]. The creation rate was more or less i n d e p e n d e n t of impurity level for small impurity concentrations, suggesting that the defect form at i on process depends only on silicon and/ or hydrogen in the intrinsic material [4]. Concentrations of oxygen, nitrogen and carbon above 1 at.% however, were found to increase both the stable and metastable defect densities [ 4 - 6] and to cause significant changes in solar cell performance [1, 5]. F u r t h e r m o r e , in phosphorus-doped films, the magnitude of the photo-induced changes in *Paper presented at the 7th Photovoltaic Advanced Research and Development Project Review Meeting, Denver, CO, U.S.A., May 13, 1986. 0379-6787/87/$3.50
© Elsevier Sequoia/Printed in The Netherlands
432 subgap absorption indicated that the number of dangling bond defects increased proportionately to the doping-induced defect density in phosphorus-doped material [7 ]. In this paper we examine the effects of various impurities on the metastable defect changes in a-Si:H to gain further understanding of the defect formation process. In particular, using a new technique of electron spin resonance (ESR) transient spectroscopy, we examine the distribution of annealing energies for oxygen-, nitrogen- and carbon-induced defects. To examine the role of hydrogen in the defect creation process, the annealing process in deuterated films is also investigated. Finally, some new metastable changes of the donor levels in phosphorus-doped samples were discovered which have implications for solar cell performance and other experiments measuring the metastable changes in phosphorus-doped material. A model is proposed to explain the results.
2. Experimental details The primary experimental method used to examine the effects of impurities on metastable defect creation is ESR transient spectroscopy. In this technique the spin densities of illuminated samples with increased density of dangling bond defects are measured as a function of time for various annealing temperatures. The decrease in the total spin population at a given time t ( i . e . the derivative with respect to time) is due to the annealing of those defects with an annealing rate R(TA) such that 1 = tR(TA)
= tu o
exp
(1)
where v0 is the attempt frequency, T A is the annealing temperature and E is the activation energy of the defect. Solving for E, one obtains E = kT A
ln(v0t )
(2)
Hence, an approximation of the distribution of annealing energies may be obtained by taking the derivative of the ESR decay with respect to k T A ln(v0t), and the a t t e m p t frequency can be determined by measuring decays at different annealing energies [4, 8]. The distribution of annealing energies in intrinsic material was investigated by illuminating samples deposited under ultrahigh vacuum conditions for up to 16 h. The impurity levels for oxygen, nitrogen and carbon were on the order of 10 ~s cm -~. The samples were annealed within the ESR cavity, and the spin density was measured using a double-modulation scheme [4]. 3. Intrinsic metastable defects The resulting ESR transient spectra are presented in Fig. 1. The distribution has a peak at 1.1 eV and is about 0.3 eV wide; it looks very similar to
433
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I
I
i
0.6 /
I
i
,
I
i
]
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i
1 I
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0.8
0.4
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~ x\\
Z
i
//
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\\\
0.2
._.E
0.2 0
,,
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~13 Z
0.8
1.0 kTAIn (rot)(eV)
(a)
0,, I 0.5
1.2
I
/ x I \1% 1.0 ACTIVATION ENERGY (eV)
I
1.5
(b)
Fig. l ( a ) Normalized n u m b e r of metastable spins vs. k T A ln(v0t) for h y d r o g e n a t e d ( - - - - - - ) and deuterated ( ) films (P0 = 101° s - l ) • (b) Distribution of annealing energies obtained from (a).
the energy distribution of singly occupied dangling bonds relative to the conduction band. This fact suggests the possibility that the rate-limiting step for annealing is excitation of an electron from a dangling bond. Since it has been proposed that recombination causes hydrogen motion and subsequent dangling bond formation, the substitution of deuterium for hydrogen would provide information about the mechanism for hydrogen motion [9]. A sample containing only deuterium exhibits a distribution of annealing energies virtually identical with that found for hydrogencontaining samples. Furthermore, no significant change was found for the creation rate. This observation of identical creation and annealing rates eliminates hydrogen tunneling as a mechanism in dangling bond formation as well as recombination at the Si--H bond by multiphonon emission [10]. An example of a possible mechanism consistent with the above results is depicted in Fig. 2. The recombination of an electron-hole pair occurs at a weak Si--Si bond, causing the transfer of a back-bonded hydrogen to the
H v
(a)
(b)
Fig. 2. A possible hydrogen-switching reaction responsible for increased dangling bonds: (a) for the annealed state; (b) after the r e c o m b i n a t i o n event.
434 weak Si--Si b o n d position, resulting in t w o dangling silicon bonds. The annealing process is the reverse. The removal of an electron from a dangling b o n d causes the back bonds to assume a planar sp2 configuration. This configuration causes the h y d r o g e n atom to move toward its annealed state position and enables the h y d r o g e n atom to move out of the weak Si--Si b o n d i n g region, allowing re-formation of the broken bond.
4. Impurity-induced metastable defects The spectra for the different impurities are presented in Fig. 3. There are distinct differences b e t w e e n the spectra for the impurity-induced defects and those for the intrinsic material. The annealing energy distributions of nitrogen-doped samples begin at lower energies and e x t e n d to quite high i 08
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07 09 11 13 A C T I V A T I O N E N E R G Y (eV)
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Fig. 3. Distributions of annealing energies in a-Si:H for different impurities: (a) ultrahigh vacuum material(P0= 101°s l);(b)4at.%N(P0= 101°s-1);(c)20at.% O(v o= 10 ss-l).
435 energies (Fig. 3(b)). The distribution of annealing energies is t herefore quite broad. One might e xpe c t t ha t a similar broad distribution of defects may occur in a m o r p h o u s silicon nitride and at the a-Si:H-silicon nitride interface. Such metastable changes can cause metastable threshold voltage shifts if a transistor is illuminated or if the traps can be field induced. In a similar manner, metastable defects in oxygen-containing films also exhibit a wide range of annealing energies (Fig. 3(c)). Some of the centers induced by oxygen have such large activation energies that it is difficult to separate the metastable changes from the stable background. Finally, carbon-induced defects have a distribution of low activation energies extending from 0.5 to 0.8 eV and a low prefactor (106 Hz). Consequently, carbon-induced centers anneal at comparatively low temperatures. The fact t hat these impurity-related spectra are significantly different from the spectra of intrinsic specimens is additional evidence that the metastable defects in intrinsic material are a result of silicon a n d / o r hydrogen r econs t r uct i on alone. This result supports the impurity studies which indicated t hat impurities do not play a role in metastable defect formation.
5. Metastable changes in phosphorus-doped samples It has recently been shown that the density of occupied phosphorus donors (P4 °) as well as the density of occupied c o n d u c t i o n band tail states in phosphorus-doped material could be determined using hyperfine measurements [11]. By measuring the P4° and Si4 ° density for the annealed state as well as for the illuminated state, we have found that the density P4° increases on illumination, whereas the density of paramagnetic c o n d u c t i o n band tail states and the Fermi level decreases (Table 1). This remarkable result demonstrates that light substantially increases the n u m b e r of fourfold phosphorus atoms, since the decrease in Fermi level would otherwise lead to a decrease in the occupied phosphorus d o n o r levels {Fig. 4). The n u m b e r of increased d o n o r levels estimated f r om ESR is within a factor of 2 of the estimated increase in density of dangling bonds for similarly doped material. Although the precise mechanism is not known, the results are consisten t with a model t hat is similar to that proposed for the creation of TABLE 1 Metastable spin density changes in annealed phosphorus-doped samples after 16 h of illumination Doping level
Conduction band tail density
(cm a)
change (cm a)
(em 3)
10-4
--4.5 × 10 is --1.6 X 1016
2 )< 1015 2 X 1016
10 2
Occupied donor density
436
P DONOR BAN
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,,..x ,'//
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/
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Fig. 4. The changes in the band tail and defect states as a result of illumination for phosphorus doping levels of 10 4 cm 3 and higher: - - , before illumination: . . . . , after illumination. /
/
--0,,, p C51( N
s~
P Q
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Fig. 5. The possible hydrogen-switching reaction P3°+ Si4° ~ Si 3 + P4* responsible for the increase in donor levels on illumination or rapid quenching: (a) dark; (b) illuminated. d a n g l i n g b o n d s in u n d o p e d m a t e r i a l (Fig. 2) w h i c h involves silicon [ 9 ] . In this r e a c t i o n , a h y d r o g e n a t o m b o n d e d t o a silicon a t o m a d j a c e n t t o t h e t h r e e f o l d p h o s p h o r u s a t o m m o v e s t o t h e p h o s p h o r u s a t o m , m a k i n g it f o u r f o l d c o o r d i n a t e d a n d leaving b e h i n d a d a n g l i n g b o n d on t h e silicon a t o m (Fig. 5 ). S u c h an e v e n t is e q u i v a l e n t t o t h e r e a c t i o n P 9 + Si4 ° ~
P4 + + Si3
This r e a c t i o n is c o n s i s t e n t w i t h t h a t p r o p o s e d f o r d o p i n g - i n d u c e d d e f e c t s d u r i n g d e p o s i t i o n a n d , in f a c t , r e p r e s e n t s a s p e c i f i c i m p l e m e n t a t i o n of t h e g e n e r a l p r o c e s s [ 1 2 ] . This e v e n t d o e s n o t m o v e t h e F e r m i level s i g n i f i c a n t l y , since t h e F e r m i level o c c u r s b e t w e e n P4 ° a n d t h e Si3 levels. D u r i n g i l l u m i n a t i o n , t h e p r o c e s s s h o w n in Fig. 2 also o c c u r s , w h i c h d o e s l o w e r t h e F e r m i level. W h e n t h e c o n c e n t r a t i o n o f d o n o r levels is s i m i l a r t o t h e i n c r e a s e d
437
number of broken weak Si--Si bonds, the Fermi level moves significantly. For heavier doping, despite the increase in dangling bond density from the reaction of Fig. 2, the Fermi level does not decrease since the reaction in Fig. 5 dominates. This explains the small changes in the dark conductivity for both lightly doped and heavily doped samples, whereas intermediately doped samples exhibit a large charge [3]. It should be mentioned that the mechanisms in Fig. 5 create equal numbers of dangling bonds and donor levels. Another possible hydrogen-switching reaction in which the hydrogen atom breaks the Si--P bond may also occur. This reaction, equivalent to the reaction P4 ° + Si4 ~
P3 ° + Si 3-
creates a P4 ° center while destroying an Si3 center, and would explain why discrepancies between the donor levels determined by hyperfine and dangling bonds density, as measured by absorption [ 7 ].
6. Conclusion The results presented in this paper indicate that the metastability of both undoped and doped material is most likely to be a consequence of the presence of hydrogen in the amorphous silicon lattice and is not because of impurities, since the characteristic energies of impurity-induced metastable defects are significantly different. The striking similarity between the lightinduced metastable defects, doping-induced and field-induced defects suggests a c o m m o n origin related to the motion of hydrogen through the material. The microscopic mechanisms proposed in this paper are possible microscopic origins of this c o m m o n connection.
Acknowledgments We would like to thank R. Street and J. Kakalios for helpful discussions. This work was supported in part by Solar Energy Research Institute Contract XB-3-03112-1.
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