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Reversibility of the light-induced saturation and annealing of defects in a-Si: H
Material Letters I3 North-Holland
( 1992) 279-283
Reversibility of the light-induced saturation and annealing of defects in a-Si: H H. Gleskova
‘, P.A. Morin,
J. Bullock and S. Wagner
Department ofElectrical Engineering, Engineering Quadrangle, Princeton Univewty, Princeton, NJ 08544, USA Received
28 October
199
1;in final form 26 January
1992
Results on the reversibility of the light-induced saturation and dark- and light-annealing of the deep-level defects in a-S: H films are presented. The value of the saturated defect density (N,,) in three samples obtained after initial light-soaking was observed to decrease upon cyclic light-annealing and saturation. This drop in N,, was observed in two samples after the second illumination while in another sample the drop was observed only after a series of experiments carried out to determine the temperature dependence of N,,. Both light and elevated temperature are responsible for the observed decrease in N,,,. Annealing in the dark at 25’ C and light-induced annealing at 35°C also were observed.
1. Introduction Light-induced degradation of the properties of amorphous silicon has been studied extensively since 1977 [ 11. Metastable defects in a-Si: H can be induced by illumination [ 11, charge accumulation [ 2 1, current flow [ 31 or rapid thermal quenching from high temperatures [ 41. The light-induced degradation reaches a saturated value in agreement with a proposal made by Redfield and Bube [ 5 1. This saturated defect density (N,,,) is a result of a balance between defect generation and annealing [ 61. An improvement in solar-cell characteristics using cycles of light-soaking and thermal annealing has recently been observed [ 7,8 1. This observation raises the question if light only generates the metastable defects in amorphous silicon or whether it can also anneal them. Very recently, room temperature annealing of the sub-band-gap absorption in a-Si: H films during exposure to 1 MeV protons was observed [ 9 1. Current-induced defect annihilation [3] and light-induced annealing [ 10 ] in a-Si : H p-i-n devices have also been reported. This paper reports the results of a study of the reI On the leave from the Department of Solid State Physics, Mathematics and Physics Faculty, Comenius University, 842 I5 Bratislava, Czechoslovakia. 0167-577x/92/$
05.00 0 1992 Elsevier
Science Publishers
versibility of the light-induced saturation and the dark- and light-annealing of defects in a-Si: H films. We seek to determine if it is possible to reduce N,,, through cyclic light-soaking and light-annealing.
2. Experimental The film used in this experiment was undoped aSi: H grown by dc glow discharge decomposition [ 111 of pure silane on Coming 7059 glass at a temperature of 250°C. In this study we measured four adjoining samples from the same substrate. To avoid any nonuniformity in the films all samples were taken from the middle of the 2.5x7.6 cm2 substrate. The thickness of the film determined from the interference fringes in the optical transmittance measurement was 1.5 pm and the (Taut) optical gap was 1.69 eV. Other initial parameters of the film are: dark conductivity (300K): 1.5~10~“Scm-‘;darkconductivity activation energy: 0.83 eV; photoconductivity (at a carrier generation rate of 5.8 x 1019 cme3 s-l): l.l~lO-~ S cm-‘; Urbach energy: 53 meV; defect state density: 2.3x lOI cme3. The light source for the light-soaking experiments was either a xenon-arc lamp with a water filter (referred to below as the “white light”), or 647.1 nm light from a high-power Kr-ion laser. The full spec-
B.V. All rights reserved.
279
Volume 13, number 45
MATERIALS LETTERS
trum power density of the white light was 3 W cme2. The Kr-ion laser was focused to a beam of 2 W crne2 power density. In some experiments with the white light, a bandpass filter of 650 & 35 nm was used to give uniformly absorbed red light. In this case the source, referred to below as the “red light”, had a power density of 0.27 W cmp2. During light-soaking with the Kr-ion laser we used the experimental arrangement described in ref. [ 12 1. For light-soaking experiments with the white-light source the samples were mounted on a thermoelectric heater/cooler enclosed in a dry nitrogen atmosphere. To avoid a temperature difference between the film and the heater [ 121, the films faced the heater/cooler and were illuminated through the glass substrate. The temperature of the sample was kept stable to within 2°C of the set temperature. The defect density (N,) was measured with the constant photocurrent method (CPM) [ 13 ] on a coplanar electrode structure by converting the integrated subgap absorption intensity to N, using a calibration factor [ 141. To avoid introducing error in the evaluation of the CPM spectra, after each measurement the new and old spectra were overlayed for direct comparison. This is a convenient and accurate method for observing relative changes in the defect density [ 15 1. The initial state of the samples was the as-deposited state. Samples J114-1 and J 114-2 were lightsoaked with the Kr-ion laser at room temperature (35 “C). Samples J114-3 and J 114-5 were exposed to the white-light source for 5 h and then saturated with the Kr-ion laser at room temperature. Initial and saturated parameters of all samples are listed in table 1. Throughout the remainder of this paper we shall identify samples J114-1, J 114-2, J 114-3 and J114-5 as samples 1, 2, 3 and 5, respectively.
3. Results and discussion After reaching saturation, samples 1 and 2 were kept in the dark at a temperature of about 25 ‘C for 23 days. After this time the defect density of both samples had dropped from a saturated value of 8.5 x 1OL6to 6.5 x lOI cmp3. Both samples were saturated again with the Kr-ion laser at 3 5 ‘C and again reached the initial saturated value. Subsequently 280
April 1992
Table 1 The initial and saturated defect densities and photoconductivities for all samples used in the experiments. The photoconductivity carrier generation rate was 5.8~ lOI cmm3 s-’ Sample
Jll4-1
initial saturated initial saturated initial saturated initial saturated
J 114-2 J 114-3 J 114-5
1 x
IO”1
1
N, (cm-‘)
% (a-’
2.1x1015 8.6x lOI 2.4x 10” 8.4x lOI 2.3x lOI 8.6X lOI 2.6x lOI 8.8X lOI
1.1 x10-5 2.3x lo-’ 1.2x 1OW 2.5 x lo-’ 1.4x 1OW 2.8 x lO-7 1.1 x10-5 2.6x lo-’
cm-‘)
I J
114-i
I
0
b
Z4.6
Kwon
laser,
2 W/cm*.
35’C
E
I
dark, 2 5’C, 23 days
E
3
Kr-ion
laser,
2 W/cm’,
5 5%.
1 hour
5
Kr-ion
laser.
2 W/cm’,
75’C,
ihour
4 x lOi TIME
[arb. units]
Fig. 1. Changes in the defect density of sample 1 after cyclic lightannealing and light-soaking. The first full circle represents the saturated defect density after soaking with the Kr-ion laser. Other full circles represent the saturated defect density after soaking with the Kr-ion laser at 35°C after light-annealing at elevated temperatures.
sample 2 was set aside and the defect density of sample 1 was reduced twice by first saturating at elevated temperature, then saturating at 35°C always with the Kr-ion laser. The changes of the defect density of sample 1 are shown in fig. 1. The full circles in this graph represent resaturation to the maximum value of iV,, with the Kr-ion laser at room temperature (fig. 1, segments 2,4, 6). The second open circle represents the defect density after 1 h of illumination with the Kr-ion laser at a temperature of 55’ C. Segment 5 represents the decrease in defect density during 1 h of illumination with the Kr-ion laser at 75°C. It is surprising to observe that both samples 1 and 2 show annealing at a low temperature (25 “C) in
the dark. This is the first observation of room-temperature annealing of N,,,. In addition, sample 1 was found to be very sensitive to changes in temperature during illumination. However, no irreversible change in N,,, was observed after this sequence of experiments. Sample 5, on the other hand, was treated in the following manner. After saturation with the white light and the Kr-ion laser light, sample 5 was immediately exposed to the white light at 3 W cm-* power density for 5.5 h at a temperature of 110°C. Then the sample was saturated at room temperature with the Kr-ion laser. We repeated this cycle of exposure to the white light followed by saturation with the Kr-ion laser light twice. The changes of the defect density are shown in fig. 2. The full circles in this figure again represent saturation of the sample with the Kr-ion laser. However, the saturated defect density after the second lint-soa~ng was lower than after the first light-soaking. Repetition of this cycle had no further effect on the saturated defect density. A different light-annealing and light-soaking cycle was performed on sample 3. After saturation, sample 3 was exposed to the white ligth for 2 h. Then this sample was exposed to the red light for 2 h. At this point the defect density was found to have decreased to one half of the initial defect density (see fig. 3, segment 1). Subsequently the sample was saturated with the full spectrum of the white light (fig. 3, segment 2) and then with the I&ion laser (fig. 3, seg’
1 x10”...
mi 0
’
r ’
0
I ii,_J\r:..i,;f J
.
: 3
114.5
6 kv B
1x10’6.
1.3.5
white
2,4,6
Kr-ion
April 1992
MATERIALS LETTERS
Volume 13, number 4,5
light. laser,
3 W/cm’,
1 1 O’C, 5.5 hours
2 W/cm’.
I -
’ TIME
35’C
*
’
’
-
[arb. units]
Fig. 2. Changes in the defect density of sample 5 after cyclic lightannealing and light-soaking. The first full circle represents the saturated defect density with both the white light and the Kr-ion laser at 35°C. Other full circles represent the saturated defect density after soaking with the I&ion laser at 35°C following lightannealing at 110°C.
o‘E 0
J
0
114-3
--.-?A
3 -,*
z
1
2
,I'.
.T
o--
iij /'
B L
1
red light 650*35
nm, 0.27
TIME
[arb.
W/cm*,
35%2
hours
units]
Fig. 3. Changes in the defect density of sample 3 after cyclic lightannealing and lift-soaki~. The first full circle represents the saturated defect density with both the white light and Kr-ion laser at 35°C. The second full circle represents the saturated defect density after soaking with the IQ-ion laser at 35°C following room temperature light-annealing.
ment 31, because the power of the white-light source useful for the creation of the defects is lower than that of the IQ--ion laser. This sample was kept at a temperature of 35°C during each illumination. The changes of the defect density of this sample are shown in fig. 3. After the last saturation with the Kr-ion laser the saturated value again was found to be lower than after the first saturation. The reduction in N,,, is in very good agreement with that of sample 5 (fig. 2 ). We would like to call the attention of the reader to fig. 3, segment 1. A decrease in the saturated defect density at higher temperature during illumination has previously been reported [ 12,16 1. This decrease was found to set in between 75 and lOO”C, dependent on the sample. In all reported cases it was unknown whether this decrease was due to thermal annealing alone or associated to some degree with light-induced annealing. In the case of our sample (sample 2), the defect density dropped from 8.7x lOI to 5.0~ lOI cmP3 after 2 h of illumination at 35°C. Note that samples 1 and 2 showed some dark annealing at room temperature (see above), but this annealing was very slow. Therefore, the defects in sample 3 must have been annealed mostly by light. From a comparison of the behavior of sample 1 (fig. 1) and samples 3 (fig. 3) and 5 (fig. 2) it might seem that the difference in the behavior of the samples depends on sample history (for the first lightsoaking we used different sources of light ). To clarify this point we measured the temperature depen281
Volume
13, number
45
MATERIALS
dence of N,,, and the photoconductivity at saturation of samples 1 and 3 (see figs. 4 ‘and 5). The sequence of experiments is marked on the figures. Saturation at each temperature was verified before illumination at the next temperature. The saturated defect density of sample 1 decreases slowly with increasing temperature from 35 to 120°C. The defect density of sample 3 shows a slight initial increase before decreasing in a manner similar to sample 1. After saturation at 120°C the samples were saturated at 10°C. Both samples have lower saturated values at 10’ C than at 35 ” C.The saturated defect density after illumination at 10°C showed no change after 7 h of light-soaking with the Kr-ion laser. The total illumination time was 11 h. On the basis of the stretched-
0
20
40
60
80
100
120
140
TEMPERATURE [“Cl
Fig. 4. Temperature dependence of the saturated defect density for samples 1 and 3. The numbers ( 1 to 6) below the data points indicate the sequence of the measurements.
LETTERS
April 1992
exponential model [ 5 ] or the defect pool model [ 17 ] we would expect a similar or higher value of saturated defect density at 10°C than at 35°C. However, in both samples we observed a lower N,,,. The samples used in this experiment have different histories of saturation treatment. Before this experiment sample 1 was exposed only to the Kr-ion laser, while sample 3 was exposed to both the whitelight source and the Kr-ion laser. Prior to the temperature dependence measurements, sample 3 had been held at a constant temperature of 35 ‘C while sample 1 had been light-annealed at several different temperatures. Therefore the observed drop in defect density is independent of the sample’s history of saturation treatment. Morel and co-workers [ 81 explained the “stabilization” of solar cells after a light-soak/dark anneal cycling procedure by network modification caused by the application of thermal stress. However, in their study a much higher dark annealing temperature was used than was used in our experiments during illumination. Even if we accept the hypothesis that initially some hydrogen is incorporated in unfavorable bonding configurations which can be removed by applying thermal stress, this does not explain the decrease in the defect density of sample 3 because in all our experiments the sample was held at a low temperature (see fig. 3 ). In fig. 5 we show the dependence of the photoconductivity at saturation on temperature. The increase in a,,, with temperature from 35 to 120’ C correlates with the decrease in the saturated defect density in this region. However, the reduced $h at 10°C (after step 6) does not correlate with the decrease in N,, at this temperature. We ascribe this drop in a,,, to a change in the distribution of defect states.
4. Conclusion
0
20
40
60
80
100
120
140
TEMPERATURE [“Cl
Fig. 5. Temperature dependence of the photoconductivity at saturation of samples 1 and 3. The numbers ( 1 to 6) below the data points indicate the sequence of the measurements.
282
We have presented the results of a study of the reversibility of the light-induced saturation and darkand light-annealing of defects in a-S: H films. We used four samples with two different histories of initial saturation treatment. Samples 1 and 2 show some dark annealing at a temperature of about 25 “C. Sample 1 shows no change in saturated defect density after three cycles of light-annealing and light-
Volume
13, number
45
MATERIALS
soaking. On the other hand samples 3 and 5 show a small decrease in the saturated defect density after the first light-annealing and re-light-soaking which is independent of the specific path used during lightannealing. Finally, the measurement of the saturated defect density step by step from 35 up to 120°C and then at 10°C shows a decrease in the defect density at 10°C in two samples with different sample history. The difference in sample history may explain why the absolute drop of the saturated defect density from 35 to 10°C for these two samples is not equal. Our experiment showed that during the cycle of light-soaking/light-annealing the saturated defect density of the a-Si : H films can be lowered. In two samples this drop appeared after the second saturation while in one sample simply light-soaking and light-annealing did not change the saturated defect density. Finally, in this sample the drop was observed after saturation at 120°C and then at 10°C. This observed decrease in N,,, may partially explain the improvement of solar cells after cycles of exposure to light-soaking and dark annealing. From these experiments we conclude that both light and elevated temperature during illumination are responsible for this effect.
Acknowledgement
April 1992
LETTERS
References
[ 11 D.E. Staebler and C.R. Wronski, (1977)
[2] W.B. Jackson 6217.
and M.D. Moyer,
31
Phys. Rev. B 36 (1987)
[ 31R.A. Street, Appl. Phys. Letters 59 ( 199 1) 1084. [4] R.A. Street, J. Kakalios, Rev. B 35 (1987)
C.C. Tsai and T.M. Hayes, Phys.
1316.
[ 51D. Redtield and R.H. Bube, Appl. Phys. Letters 54 ( 1989) 1037. [6] P.V. Santos, W.B. Jackson 234 (1991) 51.
and R.A. Street, AIP Conf. Proc.
[ 71 W.A. Nevin, H. Yamagishi, Y. Tawada,
J. Non-Cryst.
K. Asaoka,
M. Yamaguchi
and
Solids 115 ( 1989) 48.
[ 81 D.L. Morel, P. Yan and K. Blaker, Tech. Dig. 1st Sunshine Workshop,
Feb. 6-8,1990,
(New Energy Foundation,
Shizuoka, Japan, ed. M. Konagai Tokyo, 1990) p. 15.
19 I J.S. Payson, S. Abdulaziz, Y. Li and J.R. Woodyard, Space Photovolt. Research and Tech.- 1989, Proc. Conf. NASA Lewis Research
Center,
NASA Conference [lo]
A.E. Delahoy
Cleveland,
Publication
and T. Tonon,
Ohio, Nov. 7-9,
1989.
3 107 (NASA, 199 1 ) p. 389. AIP Conf. Proc.
157 (1987)
263. [ 111 D. Slobodin, S. Aljishi, R. Schwarz and S. Wagner, Res. Sot. Symp. Proc. 49 ( 1985) 153. [ 121 M. Isomura 106.
and S. Wagner,
Mater.
AIP Conf. Proc. 234 ( 1991)
[ 131 M. Vanecek, J. Kocka, J. Stuchlik, 2. Kosicek, 0. Stika and A. Triska, Solar Energy Mater. 8 ( 1983) 411. [ 141 Z.E. Smith, V. Chu, K. Shepard,
The authors thank M. Isomura, N. Hata and T.J. McMahon for helpful discussions. This research is supported by the Amorphous Thin Film Solar Cell Program of the Electric Power Research Institute.
Appl. Phys. Letters
292.
S. Aljishi, D. Slobodin,
J.
Kolodzey, S. Wagner and T.L. Chu, Appl. Phys. Letters 50 (1987) 1521. [ 15 I N.W. Wang, X. Xu and S. Wagner, (1991) 186.
AIP Conf. Proc. 234
[ 161 V. Chu, P.A. Morin, N.W. Wang, D.S. Shen and S. Wagner, to be published. [ 171 N. Hata, M. Isomura
and S. Wagner, to be published.
283
( 1992) 279-283
Reversibility of the light-induced saturation and annealing of defects in a-Si: H H. Gleskova
‘, P.A. Morin,
J. Bullock and S. Wagner
Department ofElectrical Engineering, Engineering Quadrangle, Princeton Univewty, Princeton, NJ 08544, USA Received
28 October
199
1;in final form 26 January
1992
Results on the reversibility of the light-induced saturation and dark- and light-annealing of the deep-level defects in a-S: H films are presented. The value of the saturated defect density (N,,) in three samples obtained after initial light-soaking was observed to decrease upon cyclic light-annealing and saturation. This drop in N,, was observed in two samples after the second illumination while in another sample the drop was observed only after a series of experiments carried out to determine the temperature dependence of N,,. Both light and elevated temperature are responsible for the observed decrease in N,,,. Annealing in the dark at 25’ C and light-induced annealing at 35°C also were observed.
1. Introduction Light-induced degradation of the properties of amorphous silicon has been studied extensively since 1977 [ 11. Metastable defects in a-Si: H can be induced by illumination [ 11, charge accumulation [ 2 1, current flow [ 31 or rapid thermal quenching from high temperatures [ 41. The light-induced degradation reaches a saturated value in agreement with a proposal made by Redfield and Bube [ 5 1. This saturated defect density (N,,,) is a result of a balance between defect generation and annealing [ 61. An improvement in solar-cell characteristics using cycles of light-soaking and thermal annealing has recently been observed [ 7,8 1. This observation raises the question if light only generates the metastable defects in amorphous silicon or whether it can also anneal them. Very recently, room temperature annealing of the sub-band-gap absorption in a-Si: H films during exposure to 1 MeV protons was observed [ 9 1. Current-induced defect annihilation [3] and light-induced annealing [ 10 ] in a-Si : H p-i-n devices have also been reported. This paper reports the results of a study of the reI On the leave from the Department of Solid State Physics, Mathematics and Physics Faculty, Comenius University, 842 I5 Bratislava, Czechoslovakia. 0167-577x/92/$
05.00 0 1992 Elsevier
Science Publishers
versibility of the light-induced saturation and the dark- and light-annealing of defects in a-Si: H films. We seek to determine if it is possible to reduce N,,, through cyclic light-soaking and light-annealing.
2. Experimental The film used in this experiment was undoped aSi: H grown by dc glow discharge decomposition [ 111 of pure silane on Coming 7059 glass at a temperature of 250°C. In this study we measured four adjoining samples from the same substrate. To avoid any nonuniformity in the films all samples were taken from the middle of the 2.5x7.6 cm2 substrate. The thickness of the film determined from the interference fringes in the optical transmittance measurement was 1.5 pm and the (Taut) optical gap was 1.69 eV. Other initial parameters of the film are: dark conductivity (300K): 1.5~10~“Scm-‘;darkconductivity activation energy: 0.83 eV; photoconductivity (at a carrier generation rate of 5.8 x 1019 cme3 s-l): l.l~lO-~ S cm-‘; Urbach energy: 53 meV; defect state density: 2.3x lOI cme3. The light source for the light-soaking experiments was either a xenon-arc lamp with a water filter (referred to below as the “white light”), or 647.1 nm light from a high-power Kr-ion laser. The full spec-
B.V. All rights reserved.
279
Volume 13, number 45
MATERIALS LETTERS
trum power density of the white light was 3 W cme2. The Kr-ion laser was focused to a beam of 2 W crne2 power density. In some experiments with the white light, a bandpass filter of 650 & 35 nm was used to give uniformly absorbed red light. In this case the source, referred to below as the “red light”, had a power density of 0.27 W cmp2. During light-soaking with the Kr-ion laser we used the experimental arrangement described in ref. [ 12 1. For light-soaking experiments with the white-light source the samples were mounted on a thermoelectric heater/cooler enclosed in a dry nitrogen atmosphere. To avoid a temperature difference between the film and the heater [ 121, the films faced the heater/cooler and were illuminated through the glass substrate. The temperature of the sample was kept stable to within 2°C of the set temperature. The defect density (N,) was measured with the constant photocurrent method (CPM) [ 13 ] on a coplanar electrode structure by converting the integrated subgap absorption intensity to N, using a calibration factor [ 141. To avoid introducing error in the evaluation of the CPM spectra, after each measurement the new and old spectra were overlayed for direct comparison. This is a convenient and accurate method for observing relative changes in the defect density [ 15 1. The initial state of the samples was the as-deposited state. Samples J114-1 and J 114-2 were lightsoaked with the Kr-ion laser at room temperature (35 “C). Samples J114-3 and J 114-5 were exposed to the white-light source for 5 h and then saturated with the Kr-ion laser at room temperature. Initial and saturated parameters of all samples are listed in table 1. Throughout the remainder of this paper we shall identify samples J114-1, J 114-2, J 114-3 and J114-5 as samples 1, 2, 3 and 5, respectively.
3. Results and discussion After reaching saturation, samples 1 and 2 were kept in the dark at a temperature of about 25 ‘C for 23 days. After this time the defect density of both samples had dropped from a saturated value of 8.5 x 1OL6to 6.5 x lOI cmp3. Both samples were saturated again with the Kr-ion laser at 3 5 ‘C and again reached the initial saturated value. Subsequently 280
April 1992
Table 1 The initial and saturated defect densities and photoconductivities for all samples used in the experiments. The photoconductivity carrier generation rate was 5.8~ lOI cmm3 s-’ Sample
Jll4-1
initial saturated initial saturated initial saturated initial saturated
J 114-2 J 114-3 J 114-5
1 x
IO”1
1
N, (cm-‘)
% (a-’
2.1x1015 8.6x lOI 2.4x 10” 8.4x lOI 2.3x lOI 8.6X lOI 2.6x lOI 8.8X lOI
1.1 x10-5 2.3x lo-’ 1.2x 1OW 2.5 x lo-’ 1.4x 1OW 2.8 x lO-7 1.1 x10-5 2.6x lo-’
cm-‘)
I J
114-i
I
0
b
Z4.6
Kwon
laser,
2 W/cm*.
35’C
E
I
dark, 2 5’C, 23 days
E
3
Kr-ion
laser,
2 W/cm’,
5 5%.
1 hour
5
Kr-ion
laser.
2 W/cm’,
75’C,
ihour
4 x lOi TIME
[arb. units]
Fig. 1. Changes in the defect density of sample 1 after cyclic lightannealing and light-soaking. The first full circle represents the saturated defect density after soaking with the Kr-ion laser. Other full circles represent the saturated defect density after soaking with the Kr-ion laser at 35°C after light-annealing at elevated temperatures.
sample 2 was set aside and the defect density of sample 1 was reduced twice by first saturating at elevated temperature, then saturating at 35°C always with the Kr-ion laser. The changes of the defect density of sample 1 are shown in fig. 1. The full circles in this graph represent resaturation to the maximum value of iV,, with the Kr-ion laser at room temperature (fig. 1, segments 2,4, 6). The second open circle represents the defect density after 1 h of illumination with the Kr-ion laser at a temperature of 55’ C. Segment 5 represents the decrease in defect density during 1 h of illumination with the Kr-ion laser at 75°C. It is surprising to observe that both samples 1 and 2 show annealing at a low temperature (25 “C) in
the dark. This is the first observation of room-temperature annealing of N,,,. In addition, sample 1 was found to be very sensitive to changes in temperature during illumination. However, no irreversible change in N,,, was observed after this sequence of experiments. Sample 5, on the other hand, was treated in the following manner. After saturation with the white light and the Kr-ion laser light, sample 5 was immediately exposed to the white light at 3 W cm-* power density for 5.5 h at a temperature of 110°C. Then the sample was saturated at room temperature with the Kr-ion laser. We repeated this cycle of exposure to the white light followed by saturation with the Kr-ion laser light twice. The changes of the defect density are shown in fig. 2. The full circles in this figure again represent saturation of the sample with the Kr-ion laser. However, the saturated defect density after the second lint-soa~ng was lower than after the first light-soaking. Repetition of this cycle had no further effect on the saturated defect density. A different light-annealing and light-soaking cycle was performed on sample 3. After saturation, sample 3 was exposed to the white ligth for 2 h. Then this sample was exposed to the red light for 2 h. At this point the defect density was found to have decreased to one half of the initial defect density (see fig. 3, segment 1). Subsequently the sample was saturated with the full spectrum of the white light (fig. 3, segment 2) and then with the I&ion laser (fig. 3, seg’
1 x10”...
mi 0
’
r ’
0
I ii,_J\r:..i,;f J
.
: 3
114.5
6 kv B
1x10’6.
1.3.5
white
2,4,6
Kr-ion
April 1992
MATERIALS LETTERS
Volume 13, number 4,5
light. laser,
3 W/cm’,
1 1 O’C, 5.5 hours
2 W/cm’.
I -
’ TIME
35’C
*
’
’
-
[arb. units]
Fig. 2. Changes in the defect density of sample 5 after cyclic lightannealing and light-soaking. The first full circle represents the saturated defect density with both the white light and the Kr-ion laser at 35°C. Other full circles represent the saturated defect density after soaking with the I&ion laser at 35°C following lightannealing at 110°C.
o‘E 0
J
0
114-3
--.-?A
3 -,*
z
1
2
,I'.
.T
o--
iij /'
B L
1
red light 650*35
nm, 0.27
TIME
[arb.
W/cm*,
35%2
hours
units]
Fig. 3. Changes in the defect density of sample 3 after cyclic lightannealing and lift-soaki~. The first full circle represents the saturated defect density with both the white light and Kr-ion laser at 35°C. The second full circle represents the saturated defect density after soaking with the IQ-ion laser at 35°C following room temperature light-annealing.
ment 31, because the power of the white-light source useful for the creation of the defects is lower than that of the IQ--ion laser. This sample was kept at a temperature of 35°C during each illumination. The changes of the defect density of this sample are shown in fig. 3. After the last saturation with the Kr-ion laser the saturated value again was found to be lower than after the first saturation. The reduction in N,,, is in very good agreement with that of sample 5 (fig. 2 ). We would like to call the attention of the reader to fig. 3, segment 1. A decrease in the saturated defect density at higher temperature during illumination has previously been reported [ 12,16 1. This decrease was found to set in between 75 and lOO”C, dependent on the sample. In all reported cases it was unknown whether this decrease was due to thermal annealing alone or associated to some degree with light-induced annealing. In the case of our sample (sample 2), the defect density dropped from 8.7x lOI to 5.0~ lOI cmP3 after 2 h of illumination at 35°C. Note that samples 1 and 2 showed some dark annealing at room temperature (see above), but this annealing was very slow. Therefore, the defects in sample 3 must have been annealed mostly by light. From a comparison of the behavior of sample 1 (fig. 1) and samples 3 (fig. 3) and 5 (fig. 2) it might seem that the difference in the behavior of the samples depends on sample history (for the first lightsoaking we used different sources of light ). To clarify this point we measured the temperature depen281
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dence of N,,, and the photoconductivity at saturation of samples 1 and 3 (see figs. 4 ‘and 5). The sequence of experiments is marked on the figures. Saturation at each temperature was verified before illumination at the next temperature. The saturated defect density of sample 1 decreases slowly with increasing temperature from 35 to 120°C. The defect density of sample 3 shows a slight initial increase before decreasing in a manner similar to sample 1. After saturation at 120°C the samples were saturated at 10°C. Both samples have lower saturated values at 10’ C than at 35 ” C.The saturated defect density after illumination at 10°C showed no change after 7 h of light-soaking with the Kr-ion laser. The total illumination time was 11 h. On the basis of the stretched-
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TEMPERATURE [“Cl
Fig. 4. Temperature dependence of the saturated defect density for samples 1 and 3. The numbers ( 1 to 6) below the data points indicate the sequence of the measurements.
LETTERS
April 1992
exponential model [ 5 ] or the defect pool model [ 17 ] we would expect a similar or higher value of saturated defect density at 10°C than at 35°C. However, in both samples we observed a lower N,,,. The samples used in this experiment have different histories of saturation treatment. Before this experiment sample 1 was exposed only to the Kr-ion laser, while sample 3 was exposed to both the whitelight source and the Kr-ion laser. Prior to the temperature dependence measurements, sample 3 had been held at a constant temperature of 35 ‘C while sample 1 had been light-annealed at several different temperatures. Therefore the observed drop in defect density is independent of the sample’s history of saturation treatment. Morel and co-workers [ 81 explained the “stabilization” of solar cells after a light-soak/dark anneal cycling procedure by network modification caused by the application of thermal stress. However, in their study a much higher dark annealing temperature was used than was used in our experiments during illumination. Even if we accept the hypothesis that initially some hydrogen is incorporated in unfavorable bonding configurations which can be removed by applying thermal stress, this does not explain the decrease in the defect density of sample 3 because in all our experiments the sample was held at a low temperature (see fig. 3 ). In fig. 5 we show the dependence of the photoconductivity at saturation on temperature. The increase in a,,, with temperature from 35 to 120’ C correlates with the decrease in the saturated defect density in this region. However, the reduced $h at 10°C (after step 6) does not correlate with the decrease in N,, at this temperature. We ascribe this drop in a,,, to a change in the distribution of defect states.
4. Conclusion
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Fig. 5. Temperature dependence of the photoconductivity at saturation of samples 1 and 3. The numbers ( 1 to 6) below the data points indicate the sequence of the measurements.
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We have presented the results of a study of the reversibility of the light-induced saturation and darkand light-annealing of defects in a-S: H films. We used four samples with two different histories of initial saturation treatment. Samples 1 and 2 show some dark annealing at a temperature of about 25 “C. Sample 1 shows no change in saturated defect density after three cycles of light-annealing and light-
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soaking. On the other hand samples 3 and 5 show a small decrease in the saturated defect density after the first light-annealing and re-light-soaking which is independent of the specific path used during lightannealing. Finally, the measurement of the saturated defect density step by step from 35 up to 120°C and then at 10°C shows a decrease in the defect density at 10°C in two samples with different sample history. The difference in sample history may explain why the absolute drop of the saturated defect density from 35 to 10°C for these two samples is not equal. Our experiment showed that during the cycle of light-soaking/light-annealing the saturated defect density of the a-Si : H films can be lowered. In two samples this drop appeared after the second saturation while in one sample simply light-soaking and light-annealing did not change the saturated defect density. Finally, in this sample the drop was observed after saturation at 120°C and then at 10°C. This observed decrease in N,,, may partially explain the improvement of solar cells after cycles of exposure to light-soaking and dark annealing. From these experiments we conclude that both light and elevated temperature during illumination are responsible for this effect.
Acknowledgement
April 1992
LETTERS
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