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Precursor lifetime estimation by ultraviolet laser modulation of a hydrogenated amorphous silicon growth surface
applied surface s c i e n c e ELSEVIER
Applied Surface Science 79/80 (1994) 250-254
Precursor lifetime estimation by ultraviolet laser modulation of a hydrogenated amorphous silicon growth surface Atsushi Suzuki *, Gautam Ganguly, Akihisa Matsuda Nonequilibrium Materials Section, Electrotechnical Laboratory. 1-1-4 Umezono. Tsukuba. lbaraki 305, Japan
(Received 13 October 1993; accepted for publication 17 November 19931
Abstract The reaction lifetime of the film precursors on a no-plasma surface of hydrogenated amorphous silicon (a-Si: H) has been evaluated by periodic deposition and ultraviolet (UV) laser irradiation. The reaction lifetime of the precursors on the no-plasma surface was estimated to be less than 1 s from the change of photoconductivity in the resulting films as a function of the delay time between deposition and UV laser irradiation. A comparison of the lifetimes of the precursors on the plasma-exposed and no-plasma surface of a-Si : H leads to the conclusion that the reactivities of the precursors are comparable on both surfaces. However, UV laser irradiation encrgizes the precursors which results in an enhancement of the dangling-bond-termination reaction rate.
1. Introduction T h e r e a c t i o n lifetime of film p r e c u r s o r s is an i m p o r t a n t factor for t h e surface r e a c t i o n s in the g r o w t h p r o c e s s o f h y d r o g e n a t e d a m o r p h o u s silicon (a-Si : H) which is used as the active m a t e r i a l in solar cells, thin film transistors, etc. T h e film quality of a - S i : H is d o m i n a t e d by the density o f d a n g l i n g b o n d s which are a d e f e c t that works as a r e c o m b i n a t i o n c e n t e r for p h o t o - c r e a t e d e l e c t r o n hole pairs, A c c o r d i n g to t h e surface diffusion m o d e l [1], t h e d e n s i t y of d a n g l i n g - b o n d s in the resulting a - S i : H film is d e t e r m i n e d by a b a l a n c e between the rates of dangling bond creation and
* Corresponding author. Fax: (+ 811 298 58 5425.
t e r m i n a t i o n r e a c t i o n s by the film p r e c u r s o r s (ex, S i l l 3 radical) on the film growth surface in plasma-enhanced chemical-vapor deposition ( P E C V D ) . T h e p r e c u r s o r s which c a n n o t t e r m i n a t e d a n g l i n g b o n d s m a y be c o n s u m e d by the a b s t r a c t i o n r e a c t i o n with h y d r o g e n on the growth surface. N a m e l y , the lifetime of p r e c u r s o r s is d e t e r m i n e d by the rate of a b s t r a c t i o n r e a c t i o n and d a n g l i n g - b o n d - t e r m i n a t i o n reaction. This m e a n s that the lifetime o f the p r e c u r s o r s is influe n c e d by the ratio of t h e s e two r e a c t i o n s on the growth surface. T h e r e f o r e , m e a s u r e m e n t of the lifetime o f the film p r e c u r s o r s u n d e r d i f f e r e n t d e p o s i t i o n c o n d i t i o n s is n e c e s s a r y to quantify the r a t e s of surface reactions, which a r e r e q u i r e d to d e t e r m i n e g r o w t h c o n d i t i o n s to o b t a i n the highquality a - S i : H films. In-situ ultraviolet ( U V ) laser i r r a d i a t i o n of the film growth surface d u r i n g P E C V D (in-situ UV-
0169-4332/94/$07.00 ~') 1994 Elsevier Science B.V. All rights reserved SSDI 0l 69-4332(94)00059-A
A. Suzuki et al. /Applied Surface Science 79 / 80 (1994) 250-254
PECVD technique) results in an effect equivalent to an increase of the substrate temperature [2], which enhances the dangling-bond-termination reaction by energizing film precursors. As a consequence, the in-situ UV-PECVD technique improves the photoconductivity from 10 -9 to 10 -5 S / c m in a-Si:H films prepared at low substrate temperatures [3]. Utilizing the effect of improvement of the photoconductivity by in-situ UVPECVD, we have estimated the reaction lifetime of film precursors on the surface during the plasma deposition period (plasma-exposed surface) [4]. We separated the in-situ UV-PECVD method into the plasma deposition period and the UV laser irradiation period by time. The UV laser irradiation was carried out during the interval between successive deposition periods by intermittent PECVD (post-deposition irradiation). The post-deposition irradiation procedure resuited that the photoconductivity was mostly improved at the shortest deposition period and decreased with increase the one cycle deposition period. These results can be explained by the existence of the film precursors which can contribute the enhancement of dangling-bond-termination reaction through energizing by UV laser irradiation on the plasma-exposed surface even after the plasma is turned off. Namely, each film precursor has a lifetime on the plasma-exposed surface without UV laser irradiation (rg). ~-g can be estimated to be 10- ~ s from the change of the photoconductivity against the one cycle deposition period in the post-deposition irradiation procedure. Since the growth surface is exposed to a plasma, energetic species like ions, excited molecules and electrons from the plasma may affect the surface reactions. Therefore, the effect of the existence of a plasma for the surface reactions needs to be experimentally examined. In this work, we have evaluated the lifetime of film precursors on the surface without plasma (no-plasma surface) of a-Si:H by the modification of the post-deposition irradiation procedure outlined above. The reactivity of the growth surface and the effect of UV laser irradiation are discussed by comparison of the lifetime on the plasma-exposed and no-plasma surface.
251
2. Experimental We have tried two kinds of the modified postdeposition irradiation procedures. Sequences of one cycle in the post-deposition irradiation and the modified procedures are shown in Fig. 1. As reported previously [4], the photoconductivity in the resulting a-Si:H can be improved up to 2 × 10 6 S / c m for a equal deposition and irradiation period of 5 s using the post-deposition irradiation procedure shown in Fig. la. In this study, the a-Si:H film is deposited by a helium (100 sccm)diluted silane (5 sccm) intermittent plasma for 5 s corresponding to the monolayer growth time. Other experimental conditions for PECVD and a UV excimer laser were identical to those de-
(a) Post-deposition irradiation 5S
5S
KrF laser irradiation
Plasmadeposition
D
Time
(b) Delayed irradiation , 5s
KrFlaser irradiation
I
#
5sD_
~ ~ ,
Plasmadeposition
Time
(c) Irradiation-time variation 5s KrF laser irradiation
Plasmadeposition Time Fig. 1. Schematic diagrams of one cycle during post-deposition irradiation procedure (a), the delayed irradiation procedure (b) and the irradiation-time variation procedure (c). The delay time (t D) and UV laser irradiation time (t L) were varied while keeping other parameters fixed in (b) and (c), respectively.
A. Suzuki et al. /Applied Surf:ace Science 7 9 / 8 0 (1994) 250 254
252
scribed elsewhere [2-4]. A glass substrate was used for m e a s u r e m e n t of the photoconductivity. The substrate t e m p e r a t u r e was set at room temperature (RT). In the first modified procedure (Fig. lb), a delay time (t D) is inserted between the deposition period and the irradiation period in the post-deposition irradiation procedure (delayed irradiation). The deposited a - S i : H layer at the deposition period is irradiated by a UV laser for 5 s after a delay time, t D. From the change of the photoconductivity when changing t D, the reaction lifetime of the film precursors on the no-plasma surface of a - S i : H without U V laser irradiation (%) will be evaluated. In the second modified procedure (Fig. lc), we have varied the U V laser irradiation time (t c) in the post-deposition irradiation procedure (irradiation-time variation), t c was varied from 0 to 30 s with t D = 0. Each sequence is controlled by the apparatus whose diagram is shown in Fig. 2. The intermittent plasma is generated by a sequential signal generator (Dainichi Shinkuu Co.). The delay time t D is provided by a p u l s e / f u n c t i o n generator (lwatsu Co., SG-4511) which is triggered by a sequential signal generator after each 5 s deposition. The p u l s e / f u n c t i o n generator also switches the KrF excimer laser (248 nm) for periodic laser Radio frequency = Trigger ( 13.56 MHz) generator
[
Sequential signal generator Trigger
Radio frequency power amplifier Hcliuln-dilutcd Jsilanc plasma
48 nm ] 10 mJ/cm2 ' J 100 ttz
Pulse / function generator
~Trigger KrF excimer laser
PECVD chamber Fig. 2. Block diagram of the apparatus used in this study.
E ¢,.) >,
1 0 .6(
>
b
"0 0
1 0 .7
0 , , , ~
err
. . . .
1
i
. . . .
2
i
3
. . . .
i
. . . .
4
i
. . . .
5
Delay time (s) Fig. 3. AMI (ll)O m W / c m 2) photoconductivity for films obtained using the delayed irradiation procedure as a function of delay time (tD).
irradiation of t L S. The KrF laser is operated at a repetition rate of 100 Hz and a fluence of 10 m J / c m 2. Using these procedures, each cycle is rel~eated until the total film thickness reaches 5000 A. The photoconductivity in the resulting a - S i : H films was measured by the AM1 light source with an illumination intensity of 100 m W / c m 2.
3. Results and discussion
The room t e m p e r a t u r e photoconductivity (AM1, 100 m W / c m 2) in the as-deposited a-Si: H films prepared by the delayed irradiation procedure is plotted as a function of t D in Fig. 3. The photoconductivity decreases rapidly up to 1 s and saturates to the value of 2 × 10 -v S / c m at t D > 1 s. The photoconductivity of 2 × l(1-7 S / c m is comparable to the value in a - S i : H prepared by continuous deposition at R T and post-deposition UV laser irradiation for the same time as the deposition time [4]. It should be noted that the photoconductivity of 2 × 10 -7 S / c m is comparable to the value for films p r e p a r e d at a substrate temperature of 100°C by a conventional P E C V D without UV laser irradiation, but further experiments will be required to identify the causes of improvement up to 2 × 10 7 S / c m . Therefore, we will restrict the discussion to the change of photoconductivity against t D in the region where the photoconductivity is higher than 2 X 10 7 S / cm.
A. Suzuki et al. /Applied Surface Science 79/80 (1994) 250-254
The improvement of the photoconductivity above 2 × 10 - 7 S / c m is due to the suppression of the dangling bond density in the resulting a - S i : H caused by the energized precursors [3]. During the plasma deposition, the number of the precursors on the plasma-exposed surface is determined by the balance between the supply from the plasma and the c o n s u m p t i o n by the dangling-bond-termination and the hydrogen-abstraction reactions. Since the precursors can survive on the plasma-exposed surface without U V laser irradiation for Zg, some part of the precursors are still active at the end of the deposition period. At t t ) = 0 s, all surviving precursors can contribute to the enhancement of the danglingbond-termination reaction when energized by the U V laser irradiation. However, when t D > 0 s, the number of precursors decreases with t o because no further supply arrives from the plasma during t D. The precursors are consumed by the reactions with hydrogen or dangling bond within their lifetime on the no-plasma surface without U V laser irradiation (%). This leads to a decrease of the number of the precursors which can contribute to the enhancement of the danglingbond-termination reaction rate at the start of U V laser irradiation, after t o. As a consequence, the improvement of photoconductivity in the resulting a-Si : H films p r e p a r e d by the delayed irradiation becomes smaller for t o > 0 s. From the t o dependence of the photoconductivity in the region below 1 s, ~0 can be evaluated to be less than 1 s on assuming a first-order reaction process of the precursors. We defined % as the lifetime of film precursors on the no-plasma surface without U V laser irradiation. In other words, % means a quenching time of the film precursors on the growth surface after a plasma is turned off. The result shows that the lifetimes of film precursors (~-g) are not so long even on the no-plasma surface. Fig. 4 shows the photoconductivity in the aS i : H films p r e p a r e d by the irradiation-time variation procedure plotted against t L. At t L = 0 (no U V laser irradiation case), the photoconductivity in the resulting a - S i : H is the same as that for a continuous P E C V D without U V laser irradiation at RT.
E O
1 0 "5
' ' ' 1
. . . .
I
. . . .
253
I . . . .
~ . . . .
i . . . .
~ ' ' '
10"6
;3 "0 ,r= 0 0 0 ..c O.
1 0 ">
1 0 .8
1
0.,t
No UV laser irradiation ,I
. . . .
5
I , , , , I , , , , I
10
15
. . . .
20
I . . . .
25
I . . ,
30
35
Laser irradiation time (s) Fig. 4. AM1 (100 m W / c m 2) photoconductivity for films obtained from the irradiation-time variation procedure plotted against UV laser irradiation time (tL).
The photoconductivity starts to improve with U V laser irradiation. As explained before, considering that film precursors have a lifetime on the plasma-exposed surface (~-g), the surviving precursors can enhance the dangling-bondtermination reaction rate if they are energized by U V laser irradiation and therefore, improve the photoconductivity. The result seems to show that the number of precursors which can terminate dangling bonds increases with t L. However, this might be explained by the increase of the temperature at the start of the each cycle of deposition. When the substrate temperature increases, the number of contributing precursors for the termination reaction increases significantly. In the present irradiation-time variation procedure, there is no interval between the U V laser irradiation and the next 5 s deposition. Therefore, the heat generated by the U V laser irradiation may still be stored on the surface at the start of the next deposition due to the low thermal conductivity (0.01 W / c m . deg) of the glass substrate. Though the heat is reduced during the next deposition period, this raised temperature at the initial stage of the deposition period may enhance the dangling-bond-termination reaction rate. If an interval time is inserted between the U V laser irradiation period and the next deposition period, the surface temperature would decrease back to R T at the start of the deposition period. Such other modified procedures will clarify the effect of temperature rise for the increase of the photoconductivity up to t e = 5 s.
254
A. Suzuki et al. /Applied Surface Science 7 9 / 8 0 (1994) 250-254
The photoconductivity in the a-Si: H films prepared by the irradiation-time variation procedure saturates at a value of about 2 x 10 ~ S / c m for t L > 5 s. This value is less than that obtained with continuous in-situ U V - P E C V D because the number of the precursors at the start of UV laser irradiation on the growth surface is small compared to those that arrive on the surface during the entire period in the case of in-situ UVPECVD. Therefore, the saturation of the photoconductivity up to 2 × 10 6 S / c m is explained by the limitation of the number of the film precursors at the start of the UV laser irradiation.
4. Conclusions In conclusion, variation of the photoconductivity in a-Si : H films as functions of the delay time and post-deposition irradiation-time has been interpreted in terms of the termination reaction enhanced by the precursors on the film growth surface. The lifetime of film precursors on the
no-plasma surface of a - S i : H is estimated to bc below 1 s without UV laser irradiation at RT. Therefore, it is found that the lifetime of the film precursors is not so long even when no energy is supplied from the plasma. UV laser irradiation energizes the film precursor on the no-plasma surface, which contributes the improvement of the photoconductivity.
Acknowledgement The authors would like to thank Dr. N. Hata for helpful discussions.
References [1] G. Ganguly and A. Matsuda, Phys. Rev. B 47 (1993) 3661. [2] A. Suzuki, N. Hata and A. Matsuda, J. Non-Crys. Solids 164 166 (1993) 51. [3] A. Suzuki, Y. Toyoshima, P.J. McElheny and A. Matsuda, Jpn. J. Appl. Phys. 30 (1991) L790. [4] A. Suzuki, G. Ganguly and A. Matsuda, Appl. Phys. gett. 63 (1993) 2806.
Applied Surface Science 79/80 (1994) 250-254
Precursor lifetime estimation by ultraviolet laser modulation of a hydrogenated amorphous silicon growth surface Atsushi Suzuki *, Gautam Ganguly, Akihisa Matsuda Nonequilibrium Materials Section, Electrotechnical Laboratory. 1-1-4 Umezono. Tsukuba. lbaraki 305, Japan
(Received 13 October 1993; accepted for publication 17 November 19931
Abstract The reaction lifetime of the film precursors on a no-plasma surface of hydrogenated amorphous silicon (a-Si: H) has been evaluated by periodic deposition and ultraviolet (UV) laser irradiation. The reaction lifetime of the precursors on the no-plasma surface was estimated to be less than 1 s from the change of photoconductivity in the resulting films as a function of the delay time between deposition and UV laser irradiation. A comparison of the lifetimes of the precursors on the plasma-exposed and no-plasma surface of a-Si : H leads to the conclusion that the reactivities of the precursors are comparable on both surfaces. However, UV laser irradiation encrgizes the precursors which results in an enhancement of the dangling-bond-termination reaction rate.
1. Introduction T h e r e a c t i o n lifetime of film p r e c u r s o r s is an i m p o r t a n t factor for t h e surface r e a c t i o n s in the g r o w t h p r o c e s s o f h y d r o g e n a t e d a m o r p h o u s silicon (a-Si : H) which is used as the active m a t e r i a l in solar cells, thin film transistors, etc. T h e film quality of a - S i : H is d o m i n a t e d by the density o f d a n g l i n g b o n d s which are a d e f e c t that works as a r e c o m b i n a t i o n c e n t e r for p h o t o - c r e a t e d e l e c t r o n hole pairs, A c c o r d i n g to t h e surface diffusion m o d e l [1], t h e d e n s i t y of d a n g l i n g - b o n d s in the resulting a - S i : H film is d e t e r m i n e d by a b a l a n c e between the rates of dangling bond creation and
* Corresponding author. Fax: (+ 811 298 58 5425.
t e r m i n a t i o n r e a c t i o n s by the film p r e c u r s o r s (ex, S i l l 3 radical) on the film growth surface in plasma-enhanced chemical-vapor deposition ( P E C V D ) . T h e p r e c u r s o r s which c a n n o t t e r m i n a t e d a n g l i n g b o n d s m a y be c o n s u m e d by the a b s t r a c t i o n r e a c t i o n with h y d r o g e n on the growth surface. N a m e l y , the lifetime of p r e c u r s o r s is d e t e r m i n e d by the rate of a b s t r a c t i o n r e a c t i o n and d a n g l i n g - b o n d - t e r m i n a t i o n reaction. This m e a n s that the lifetime o f the p r e c u r s o r s is influe n c e d by the ratio of t h e s e two r e a c t i o n s on the growth surface. T h e r e f o r e , m e a s u r e m e n t of the lifetime o f the film p r e c u r s o r s u n d e r d i f f e r e n t d e p o s i t i o n c o n d i t i o n s is n e c e s s a r y to quantify the r a t e s of surface reactions, which a r e r e q u i r e d to d e t e r m i n e g r o w t h c o n d i t i o n s to o b t a i n the highquality a - S i : H films. In-situ ultraviolet ( U V ) laser i r r a d i a t i o n of the film growth surface d u r i n g P E C V D (in-situ UV-
0169-4332/94/$07.00 ~') 1994 Elsevier Science B.V. All rights reserved SSDI 0l 69-4332(94)00059-A
A. Suzuki et al. /Applied Surface Science 79 / 80 (1994) 250-254
PECVD technique) results in an effect equivalent to an increase of the substrate temperature [2], which enhances the dangling-bond-termination reaction by energizing film precursors. As a consequence, the in-situ UV-PECVD technique improves the photoconductivity from 10 -9 to 10 -5 S / c m in a-Si:H films prepared at low substrate temperatures [3]. Utilizing the effect of improvement of the photoconductivity by in-situ UVPECVD, we have estimated the reaction lifetime of film precursors on the surface during the plasma deposition period (plasma-exposed surface) [4]. We separated the in-situ UV-PECVD method into the plasma deposition period and the UV laser irradiation period by time. The UV laser irradiation was carried out during the interval between successive deposition periods by intermittent PECVD (post-deposition irradiation). The post-deposition irradiation procedure resuited that the photoconductivity was mostly improved at the shortest deposition period and decreased with increase the one cycle deposition period. These results can be explained by the existence of the film precursors which can contribute the enhancement of dangling-bond-termination reaction through energizing by UV laser irradiation on the plasma-exposed surface even after the plasma is turned off. Namely, each film precursor has a lifetime on the plasma-exposed surface without UV laser irradiation (rg). ~-g can be estimated to be 10- ~ s from the change of the photoconductivity against the one cycle deposition period in the post-deposition irradiation procedure. Since the growth surface is exposed to a plasma, energetic species like ions, excited molecules and electrons from the plasma may affect the surface reactions. Therefore, the effect of the existence of a plasma for the surface reactions needs to be experimentally examined. In this work, we have evaluated the lifetime of film precursors on the surface without plasma (no-plasma surface) of a-Si:H by the modification of the post-deposition irradiation procedure outlined above. The reactivity of the growth surface and the effect of UV laser irradiation are discussed by comparison of the lifetime on the plasma-exposed and no-plasma surface.
251
2. Experimental We have tried two kinds of the modified postdeposition irradiation procedures. Sequences of one cycle in the post-deposition irradiation and the modified procedures are shown in Fig. 1. As reported previously [4], the photoconductivity in the resulting a-Si:H can be improved up to 2 × 10 6 S / c m for a equal deposition and irradiation period of 5 s using the post-deposition irradiation procedure shown in Fig. la. In this study, the a-Si:H film is deposited by a helium (100 sccm)diluted silane (5 sccm) intermittent plasma for 5 s corresponding to the monolayer growth time. Other experimental conditions for PECVD and a UV excimer laser were identical to those de-
(a) Post-deposition irradiation 5S
5S
KrF laser irradiation
Plasmadeposition
D
Time
(b) Delayed irradiation , 5s
KrFlaser irradiation
I
#
5sD_
~ ~ ,
Plasmadeposition
Time
(c) Irradiation-time variation 5s KrF laser irradiation
Plasmadeposition Time Fig. 1. Schematic diagrams of one cycle during post-deposition irradiation procedure (a), the delayed irradiation procedure (b) and the irradiation-time variation procedure (c). The delay time (t D) and UV laser irradiation time (t L) were varied while keeping other parameters fixed in (b) and (c), respectively.
A. Suzuki et al. /Applied Surf:ace Science 7 9 / 8 0 (1994) 250 254
252
scribed elsewhere [2-4]. A glass substrate was used for m e a s u r e m e n t of the photoconductivity. The substrate t e m p e r a t u r e was set at room temperature (RT). In the first modified procedure (Fig. lb), a delay time (t D) is inserted between the deposition period and the irradiation period in the post-deposition irradiation procedure (delayed irradiation). The deposited a - S i : H layer at the deposition period is irradiated by a UV laser for 5 s after a delay time, t D. From the change of the photoconductivity when changing t D, the reaction lifetime of the film precursors on the no-plasma surface of a - S i : H without U V laser irradiation (%) will be evaluated. In the second modified procedure (Fig. lc), we have varied the U V laser irradiation time (t c) in the post-deposition irradiation procedure (irradiation-time variation), t c was varied from 0 to 30 s with t D = 0. Each sequence is controlled by the apparatus whose diagram is shown in Fig. 2. The intermittent plasma is generated by a sequential signal generator (Dainichi Shinkuu Co.). The delay time t D is provided by a p u l s e / f u n c t i o n generator (lwatsu Co., SG-4511) which is triggered by a sequential signal generator after each 5 s deposition. The p u l s e / f u n c t i o n generator also switches the KrF excimer laser (248 nm) for periodic laser Radio frequency = Trigger ( 13.56 MHz) generator
[
Sequential signal generator Trigger
Radio frequency power amplifier Hcliuln-dilutcd Jsilanc plasma
48 nm ] 10 mJ/cm2 ' J 100 ttz
Pulse / function generator
~Trigger KrF excimer laser
PECVD chamber Fig. 2. Block diagram of the apparatus used in this study.
E ¢,.) >,
1 0 .6(
>
b
"0 0
1 0 .7
0 , , , ~
err
. . . .
1
i
. . . .
2
i
3
. . . .
i
. . . .
4
i
. . . .
5
Delay time (s) Fig. 3. AMI (ll)O m W / c m 2) photoconductivity for films obtained using the delayed irradiation procedure as a function of delay time (tD).
irradiation of t L S. The KrF laser is operated at a repetition rate of 100 Hz and a fluence of 10 m J / c m 2. Using these procedures, each cycle is rel~eated until the total film thickness reaches 5000 A. The photoconductivity in the resulting a - S i : H films was measured by the AM1 light source with an illumination intensity of 100 m W / c m 2.
3. Results and discussion
The room t e m p e r a t u r e photoconductivity (AM1, 100 m W / c m 2) in the as-deposited a-Si: H films prepared by the delayed irradiation procedure is plotted as a function of t D in Fig. 3. The photoconductivity decreases rapidly up to 1 s and saturates to the value of 2 × 10 -v S / c m at t D > 1 s. The photoconductivity of 2 × l(1-7 S / c m is comparable to the value in a - S i : H prepared by continuous deposition at R T and post-deposition UV laser irradiation for the same time as the deposition time [4]. It should be noted that the photoconductivity of 2 × 10 -7 S / c m is comparable to the value for films p r e p a r e d at a substrate temperature of 100°C by a conventional P E C V D without UV laser irradiation, but further experiments will be required to identify the causes of improvement up to 2 × 10 7 S / c m . Therefore, we will restrict the discussion to the change of photoconductivity against t D in the region where the photoconductivity is higher than 2 X 10 7 S / cm.
A. Suzuki et al. /Applied Surface Science 79/80 (1994) 250-254
The improvement of the photoconductivity above 2 × 10 - 7 S / c m is due to the suppression of the dangling bond density in the resulting a - S i : H caused by the energized precursors [3]. During the plasma deposition, the number of the precursors on the plasma-exposed surface is determined by the balance between the supply from the plasma and the c o n s u m p t i o n by the dangling-bond-termination and the hydrogen-abstraction reactions. Since the precursors can survive on the plasma-exposed surface without U V laser irradiation for Zg, some part of the precursors are still active at the end of the deposition period. At t t ) = 0 s, all surviving precursors can contribute to the enhancement of the danglingbond-termination reaction when energized by the U V laser irradiation. However, when t D > 0 s, the number of precursors decreases with t o because no further supply arrives from the plasma during t D. The precursors are consumed by the reactions with hydrogen or dangling bond within their lifetime on the no-plasma surface without U V laser irradiation (%). This leads to a decrease of the number of the precursors which can contribute to the enhancement of the danglingbond-termination reaction rate at the start of U V laser irradiation, after t o. As a consequence, the improvement of photoconductivity in the resulting a-Si : H films p r e p a r e d by the delayed irradiation becomes smaller for t o > 0 s. From the t o dependence of the photoconductivity in the region below 1 s, ~0 can be evaluated to be less than 1 s on assuming a first-order reaction process of the precursors. We defined % as the lifetime of film precursors on the no-plasma surface without U V laser irradiation. In other words, % means a quenching time of the film precursors on the growth surface after a plasma is turned off. The result shows that the lifetimes of film precursors (~-g) are not so long even on the no-plasma surface. Fig. 4 shows the photoconductivity in the aS i : H films p r e p a r e d by the irradiation-time variation procedure plotted against t L. At t L = 0 (no U V laser irradiation case), the photoconductivity in the resulting a - S i : H is the same as that for a continuous P E C V D without U V laser irradiation at RT.
E O
1 0 "5
' ' ' 1
. . . .
I
. . . .
253
I . . . .
~ . . . .
i . . . .
~ ' ' '
10"6
;3 "0 ,r= 0 0 0 ..c O.
1 0 ">
1 0 .8
1
0.,t
No UV laser irradiation ,I
. . . .
5
I , , , , I , , , , I
10
15
. . . .
20
I . . . .
25
I . . ,
30
35
Laser irradiation time (s) Fig. 4. AM1 (100 m W / c m 2) photoconductivity for films obtained from the irradiation-time variation procedure plotted against UV laser irradiation time (tL).
The photoconductivity starts to improve with U V laser irradiation. As explained before, considering that film precursors have a lifetime on the plasma-exposed surface (~-g), the surviving precursors can enhance the dangling-bondtermination reaction rate if they are energized by U V laser irradiation and therefore, improve the photoconductivity. The result seems to show that the number of precursors which can terminate dangling bonds increases with t L. However, this might be explained by the increase of the temperature at the start of the each cycle of deposition. When the substrate temperature increases, the number of contributing precursors for the termination reaction increases significantly. In the present irradiation-time variation procedure, there is no interval between the U V laser irradiation and the next 5 s deposition. Therefore, the heat generated by the U V laser irradiation may still be stored on the surface at the start of the next deposition due to the low thermal conductivity (0.01 W / c m . deg) of the glass substrate. Though the heat is reduced during the next deposition period, this raised temperature at the initial stage of the deposition period may enhance the dangling-bond-termination reaction rate. If an interval time is inserted between the U V laser irradiation period and the next deposition period, the surface temperature would decrease back to R T at the start of the deposition period. Such other modified procedures will clarify the effect of temperature rise for the increase of the photoconductivity up to t e = 5 s.
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The photoconductivity in the a-Si: H films prepared by the irradiation-time variation procedure saturates at a value of about 2 x 10 ~ S / c m for t L > 5 s. This value is less than that obtained with continuous in-situ U V - P E C V D because the number of the precursors at the start of UV laser irradiation on the growth surface is small compared to those that arrive on the surface during the entire period in the case of in-situ UVPECVD. Therefore, the saturation of the photoconductivity up to 2 × 10 6 S / c m is explained by the limitation of the number of the film precursors at the start of the UV laser irradiation.
4. Conclusions In conclusion, variation of the photoconductivity in a-Si : H films as functions of the delay time and post-deposition irradiation-time has been interpreted in terms of the termination reaction enhanced by the precursors on the film growth surface. The lifetime of film precursors on the
no-plasma surface of a - S i : H is estimated to bc below 1 s without UV laser irradiation at RT. Therefore, it is found that the lifetime of the film precursors is not so long even when no energy is supplied from the plasma. UV laser irradiation energizes the film precursor on the no-plasma surface, which contributes the improvement of the photoconductivity.
Acknowledgement The authors would like to thank Dr. N. Hata for helpful discussions.
References [1] G. Ganguly and A. Matsuda, Phys. Rev. B 47 (1993) 3661. [2] A. Suzuki, N. Hata and A. Matsuda, J. Non-Crys. Solids 164 166 (1993) 51. [3] A. Suzuki, Y. Toyoshima, P.J. McElheny and A. Matsuda, Jpn. J. Appl. Phys. 30 (1991) L790. [4] A. Suzuki, G. Ganguly and A. Matsuda, Appl. Phys. gett. 63 (1993) 2806.