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Thickness dependence of photoelectrical properties of intrinsic amorphous silicon
Solar Cells, 23 (1988) 191 - 199
191
THICKNESS DEPENDENCE OF PHOTOELECTRICAL PROPERTIES OF INTRINSIC AMORPHOUS SILICON YA-GU JAMES YE* and W. A. ANDERSON
Department o f Electrical and Computer Engineering, State University o f N e w York at Buffalo, 217C Bonner Hall, Buffalo, N Y 14260 (U.S.A.) Y. S. TSUO
Solar Energy Research Institute, 1617 Cole Boulevard, Golden, CO 80401 (U.S.A.) (Received July 24, 1987; accepted September 17, 1987)
Summary The stable and transient photoconductivities Oph are measured in intrinsic amorphous silicon a ~ i : H of different thicknesses. The results show that the surface layer is more photosensitive than the bulk. The optical properties of a-Si:H with varying thickness of film are also determined. In a certain range of thickness of film, the absorption coefficient ~ and optical gap Eop t increase with reducing thickness. Both create the effect of increasing photoconductivity. Experiments indicate that thinner intrinsic samples have a higher activation energy Ea and lower dark conductivity OD, OWing to surface band bending and the existence of a surface layer. The short decay time of electrons, about 10 -s s, confirms the carrier loss from deep states, as revealed by the transient photoconductivity data. This result is also confirmed by photoluminescence.
1. Introduction This study is of current interest for solar cell applications because the density of states N(E) in the gap can be very low. When the density of states is very low, the surface and thickness effects caused by the surface and interface band bending of the thin intrinsic sample cannot be ignored [1 - 3]. This is proved by electrical transport measurements on thin films of a-Si:H. Rhodes [4] studied the electroluminescence (EL) and photoluminescence (PL) of a range of amorphous silicon (a-Si) samples including p - i - n junctions on stainless steel and indium tin oxide (ITO) glass substrates, in which the i-layer thickness varies. These results show an increase in relative EL efficiency with decreasing junction thickness [4]. *A visiting scholar from Shanghai Institute of Ceramics, Academia, China. 0379-6787/88/$3.50
© Elsevier Sequoia/Printed in The Netherlands
192 The a-Si:H solar cell is a thin film cell in which the efficiency has increased to over 11%. However, traditionally, thick samples of a-Si:H are always used to determine the cell's electrical properties which may provide data inconsistent with the use of thin samples. Also, for p - i - n amorphous silicon solar cells, the characteristics of each layer are very different. This paper provides the results of an investigation of the photoelectrical properties of the most important layer -- the intrinsic layer. To obtain the relationship between these properties, the following experiments have been performed: dark electrical conductivity OD, photoconductivity Oph in stable and transient experiments, dark conductivity vs. temperature ((~D-T), absorption coefficient ~, spectral response (SR), and optical gap Eop t. Finally, the data are analyzed and discussed.
2. Preparation and equipment
2.1. Preparation o f samples All the a-Si:H samples, except that for the PL experiment, were deposited at the Solar Energy Research Institute using a load-locked single-chamber r.f. glow
2.2. Equipment Stable photoconductivity oph was measured using an ELH light source, Keithley 233 power supply and Keithley 480 picoammeter. Transient photoconductivity measurements used a pulsed laser light source with k = 5320 A, a pulse time of about 15 ns, a determining voltage of 550 V, and a Tektronik 7834 storage oscilloscope as a recording instrument. Spectral response and absorption coefficient tests used the ELH lamp, with 100 mW cm -2 fight intensity, a GM-100-2 Schoeffel monochromator, and a Keithley 480 picoammeter.
193
Photoluminescence measurements used an argon laser (k = 488 nm) as the excitation source (with PL intensity of 225 mW). The excited sample emits a luminescence signal which is detected by a PbS detector through a monochromator. The signal is then amplified and recorded by a controlling computer. The determining temperature is 77 K.
3. Results and discussion
3.1. Photoconductivity The total photoconductivity O'ph may be divided into two parts: surface photoconductance Ophs and bulk photoconductance OphB. The former is independent of the film thickness, but the latter is proportional to the thickness. The relation of photocurrent iph and thickness of film d is shown in Fig. 1. It is shown that iph increases linearly with thickness d when d ~< 1 #m. When d > 1 pm, a smaller increase is seen. The dark conductivity a D and photoconductivity aph have a similar trend, as shown in Fig. 2. However, Oph of the thin sample (d < 0.5 #m) increases faster than that of the thick sample, because of the different Fermi level EF position with respect to the conduction band Ec at the surface and in the bulk [3]. If electron-hole pairs are created in the surface layer high field region where the band is bent, electrons and holes are likely to be more separated in space during excitation than they are in the bulk where a low field exists. Such a separation slows the recombination process and increases the photoconductivity. Thus
2.C
-4
-6
2.2 =. v
1.2
E
z.o
cv 1.8
-8
o.
c
:
o.e
1.6
o a_
==
0
-
C
0.4
0.0
2
0.4
0.8
1.2
1.6
2.0
Thickness, d(pm)
2.4
o.o o'.4 o:s
,'.2 'Thickness
,'.6 2'.0
i 2.4
(I,I m )
Fig. 1. Photocurrent iph (©) and peak of absorption coefficient ~p (/x) dependence of thickness d of a-Si:H film. Fig. 2. Dark conductivity aD (A) and photoconductivity 0"ph(©) dependence of thickness d of a-Si :H film.
194
Oph(d =
0.48 pm) > O p h ( d = 2.16 pm)
(1}
In Fig. 2, for d ~ 1 pm, the curve is less dependent on thickness. This shows that the photoconductivity is affected very little by the surface layer for thick samples. As the thickness of the a-Si:H film continues to decrease to 0.12 pm {Fig. 2), the photoconductivity significantly decreases. This is because of a higher density of states and more defects on the surface of a thinner film which produces the surface recombination. For very thin films, part of the incident light on the surface of the film is not fully absorbed because the film thickness is less than the absorption depth of light [5]. 3.2. Absorption coefficient and optical gap In general, iph and Oph increase with thickness, but there is one exception. In a certain range of thickness with low intensity of light, apa in a thin sample is higher than that of a thick sample (Fig. 3). Oph(d = 0.48 pm) < Oph(d = 0.12 pm)
(2)
2.16 pm) < aph(d = 0.98 pm)
(3)
Oph(d =
A higher activation energy in the thin (d < 0.5 pm) samples accounts for eqn. (2). When the thickness d of the film increases to 1 - 2 pm, the influence of the surface layer is neglected and aph mainly depends on d. From eqn. (3), if the film thickness is too large, the light penetration depth is less than the film thickness which causes a drop in oph for thicker films. Also,
o
IO 7
i
g
o
r5 9
3.2
%
2.8
2.4
2.0
1.6
1.2
0.8
Energy, hv (eV)
Fig. 3. The relation b e t w e e n the spectral r e s p o n s e (SR) and thickness d o f a-Si:H film:
v, 2.16 pm; ©, 0.98 pro;A, 0.48/gm; ~, 0.]2 pm.
195 under the a-Si:H process by glow discharge, the thick sample probably contains more defects and voids in a greater thickness. This also results in increasing recombination of photocarriers and a reduction in aph. Depending on illumination, the electrons and holes may be generated uniformly throughout the film such that Oph may be written as oph =
(4)
o~7#'reF
where Oph is the photoconductivity (~2 cm) 1, r/ is the quantum efficiency, # is the mobility of electrons (cm 2 V -1 s-l), v is the lifetime of electrons (s), e is the charge of electrons, F is the amount of incident light (photon cm -2), and ~ is the absorption coefficient of the film (cm 1). We can use eqn. (4) to obtain (~ from Oph. The r/pT product is assumed to be a constant, a reasonable assumption in a low absorption region. As indicated in Fig. 4, ~ is rapidly increasing with decreasing thickness of film, when d < 0.5 #m. Ol(d=0.12#m)
~
10 4 (cm - ] ) > Ol(d=0.4S#m)
~
].0 3 (cm -1)
(5)
Therefore, a higher a compensates for reduced d. However, the peak of the absorption spectrum moves to the high energy side (Fig. 4) and Eopt increases (Fig. 5) with decreasing thickness of film. In both cases, high energy photons are absorbed by the surface of a thin film. These are also transferred into higher iph and increased Oph [6]. 200 180
16( 14( 10 4
L.
12C
E o
v ¢1 2 <= _e o
19.®
IOC 8C
o I0 3
0
% %~
~
/
4C o
z Io
o.s ,.z
,'.s z:0
'
z'.8 3:z
o.s
s
r
,
2C 2.4
/
J JJ
t i
~a~'~
,
d
,'.o ,'.z
£nergy, hv (eV) Fig. 4. Absorption c o e f f i c i e n t ~ vs. hp c u r v e at d i f f e r e n t
,
:,
,'., ,.6
.' ,
,.s
Energy, h v ( e V ) thicknesses of a-Si:H
z:o z.z film: v
2.16/am; ©, 0.98/am; A, 0.48 /am; n, 0.12/am. 5. (ahp) 1j2 a n d o p t i c a l g a p Eop t vs. h p c u r v e ~z, 2.16/am; o, 0.98/am; A, 0.48/am; n, 0.12/am. Fig.
at different
thicknesses
of a-Si:H
film:
196 3.3. Transient p h o t o c u r r e n t and decay time The transient p h o t o c u r r e n t data of Fig. 6 show that iph decreases with decrasing thickness of film. This is because of localized states in t he space charge region resulting in fast r e c o m b i n a t i o n on the surface of a thin sample [7, 8]. The decay time of electrons Te decreases with decreasing thickness of the film, as shown in Fig. 7. The surface r e com bi nat i on effect created from the surface states seems to play the d o m i n a n t role [9, 10]. As the thickness of film is near or larger than 1 pm, the Te vs. d curve tends to be level. In that case, electron decay time does n o t vary with the thickness, because the penetration o f the light entering into the film is less than the thickness of the film.
_..~4o
2O
J
: 2oF I-
IO c ,~ i--
0
~ I°I 0i.4
0[8
IL2
I '.6
21.0
Thickness of Film (pm)
2.4
0.4
0'.8
1.2
I.
2 4
Thickness of Film (pro)
• Fig. 6. Transient photocurrent (iph)t dependence of thickness d of a-Si:H film. Fig. 7. Decay time of electron Te dependence of thickness d of a-Si:H film.
The Te data in the whole range of thickness are within one order of magnitude and very small. This fact can be explained as follows. Firstly, the p h o t o i n j e c t i o n time is much shorter in a transient pulse than it i s j n steady state, i.e. the total a m o u n t of p h o t o n s entering on the surface o f the film is e x t r e m e l y small. Then, the differences in Te among the samples is n o t obvious since r eco mb in at i on dominates. Secondly, the fast decay time of a b o u t 10 -s s, as in Fig. 7, is caused by the carrier loss to deep states, or t he carriers m a y tunnel directly into defect states with large capture cross-sections o f a b o u t 10 -15 cm -3 near midgap (Fig. 8) [11].
l----3ve
EC
Fig. 8. Schematic diagram of the trapped electron entering deep states in transient photoexcitation condition.
197
IOO a.
.:
'
8o 60
/
oi F_.C
~
'
40
k._,
ao
e
"-)PL ~= o-
EV (a)
'
,' /
0
0.6
0.8
I .0
I .2
Energy, hv
I .4
I .6
I .8
(eV)
(b)
Fig. 9. (a) E n e r g y b a n d s c h e m a t i c diagram and (b) p h o t o l u m i n e s c e n c e s p e c t r u m o f a-Si:H at d i f f e r e n t t h i c k n e s s e s o f a-Si:H film: - - , 0.02 p m ; . . . . , 1.0 p m .
3.4. Photoluminescence The photoluminescence experiment [12, 13] shows a stronger PL defect peak located in the low energy side 0.8 - 1.0 eV region for a thinner sample, as in Fig. 9. This indicates the dominance of recombination in deep energy levels for thin samples, and results in a fast decay time. The result is consistent with the transient photocurrent experiment, i.e. the carrier loss in deep states. As a consequence of fast recombination of photocarriers, iph would decrease exponentially with time with a specific thickness dependence not observed in this experiment. To show the exponential relationship between iph and r requires a very long recombination time and indicates a lack of deep states. This is consistent with that predicted by Street and Biegelsen [14]. 3.5. Dark conductivity dependence on temperature and activation energy The thinner films have lower dark conductivity OD and higher activation energy Ea, as shown in Fig. 10. As the thickness of the film is reduced to 0.12 #m, there is a turning point on the OD-T curve (T = 363 K) and t w o E a values, 0.99 eV and 0.43 eV, appear. This can be explained as follows. Firstly, we assume the simple model of increased band bending when the thickness of film is reduced step b y step as in Fig. 11. The band bending at the surface becomes larger with decreasing thickness of film. This results in the increase in activation energy E, from 0.70 eV to 0.79 eV, as in Figs. l l ( a ) and (b). Secondly, if the thickness of the film is reduced progressively to 0.12 pm, as in Fig. l l ( c ) , the bending extent of the band edge 'is increased to Ea = 0.99 eV and, because of the increased distortion of the crystal lattice (i.e. the extent of localized states), the activation energy E, decreases to a b o u t 0.43 eV. When the temperature is high (363 K), the carriers are probably thermalized to a position of higher energy level (Ea = 0.99 eV) whereas, when the temperature is below 363 K, the thermalization probability is
198
fT)~
T,~
t~ 7
C
,5 8
=< o
oc
I 59
w
-I
z
I .0 -¢
\ O.S
c>-l0
2.
213 L5 2:7 2:9 L
I
3'.3 3.5 :
0.6 0,0
0:4
0:8
1.2 '
I.6
2:0
2,4
Thickness of Film (~m)
I Oa/T(°K)
(a)
(b)
Fig. 10. (a) Dependence of electrical conductivity o D on temperature T at different thicknesses o f a-Si:H film: D, 2.16/~m; z:, 0.98 pm; o, 0.48 pro; ~7, 0 . 1 2 / / m . (b) D e p e n d e n c e o f the activation energy E a on thickness. E¢ E~=O.7eV . . . . . ~_.
d > I gm
•Ev
----~a=O. 79e v d:O.48pm
~ = 0 . 9 9 e V E' a = O. 43_qeV~ ~
d = O . I 2Urn
Fig. 11. Schematic diagram of the energy band edge in thin samples.
reduced and th e carriers m a y reach the lower energy level (Ea = 0.43 eV), causing th e OD-T curve in thin samples to have a turning point [15]. F r o m the evidence presented in this paper, we can suggest t hat a surface layer exists in glow
199
Acknowledgment We thank Mr. Rick Wallace and Y. S. Lee for their assistance, the laser group of Dr. H. S. Kwok for aiding in transient photoconductivity work and Dr. P. Ehrlich for use of a pulsed lamp source. References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
S. Hasegawa, S. Shimizu and Y, Kurata, Philos. Mag. B, 49 (5) (1984) 511. S. Hasegawa, S. Shimizu and Y, Kurata, Philos. Mag. B, 49 (5) (1984) 519. J. Solomon and M. H. Brodsky, J. Appl. Phys., 51 (8) (1981) 4548. A. J. Rhodes, J. Non-Cryst. Solids, 59 - 60 (1983) 365. R. Fischer, W. Rehn, J. Stuke and U. Voget-Grate, J. Non-Cryst. Solids, 35 - 36 (1980) 687. B. Abeles, C. R. Wronski, T. Tiedge and G. D. Cody, Solid State Commun., 36 (1980) 537. P. B. Kirby and W. Paul, Phys. Rev. B, 2 (1984) 826. R . S . Crandall, Sol. Cells, 2 (1980) 319. R. W. Collins and W. Paul, Phys. Rev. B, 25 (4) (1982) 2611. R. A. Street, Adv. Phys., 30 (1981) 629. G. Moddel and D. A. Anderson, Phys. Rev. B, 22 (1982) 1918. D. Engemann and R. Fisher, Proc. 5th Conf. Amorphous and Liquid Semiconductors, Stoke and Brenig, New York, 1974, p. 947. Y. G. Ye, R. Q. Yin, H. Y. Pan, P. S. Chou, J. K. Cheng and Z. J. Lin, Proc. 5th Conf. Semiconductor Physics, China, 1985, p. 247. R. A. Street and D. K. Biegelsen, Top. Appl. Phys., 56 (1984) 221. Y. G. Ye, Z. Q. Lu and C. Y. Wu, Acta Energial Solars Sinica, 1 (2) (1980) 123 - 131.
191
THICKNESS DEPENDENCE OF PHOTOELECTRICAL PROPERTIES OF INTRINSIC AMORPHOUS SILICON YA-GU JAMES YE* and W. A. ANDERSON
Department o f Electrical and Computer Engineering, State University o f N e w York at Buffalo, 217C Bonner Hall, Buffalo, N Y 14260 (U.S.A.) Y. S. TSUO
Solar Energy Research Institute, 1617 Cole Boulevard, Golden, CO 80401 (U.S.A.) (Received July 24, 1987; accepted September 17, 1987)
Summary The stable and transient photoconductivities Oph are measured in intrinsic amorphous silicon a ~ i : H of different thicknesses. The results show that the surface layer is more photosensitive than the bulk. The optical properties of a-Si:H with varying thickness of film are also determined. In a certain range of thickness of film, the absorption coefficient ~ and optical gap Eop t increase with reducing thickness. Both create the effect of increasing photoconductivity. Experiments indicate that thinner intrinsic samples have a higher activation energy Ea and lower dark conductivity OD, OWing to surface band bending and the existence of a surface layer. The short decay time of electrons, about 10 -s s, confirms the carrier loss from deep states, as revealed by the transient photoconductivity data. This result is also confirmed by photoluminescence.
1. Introduction This study is of current interest for solar cell applications because the density of states N(E) in the gap can be very low. When the density of states is very low, the surface and thickness effects caused by the surface and interface band bending of the thin intrinsic sample cannot be ignored [1 - 3]. This is proved by electrical transport measurements on thin films of a-Si:H. Rhodes [4] studied the electroluminescence (EL) and photoluminescence (PL) of a range of amorphous silicon (a-Si) samples including p - i - n junctions on stainless steel and indium tin oxide (ITO) glass substrates, in which the i-layer thickness varies. These results show an increase in relative EL efficiency with decreasing junction thickness [4]. *A visiting scholar from Shanghai Institute of Ceramics, Academia, China. 0379-6787/88/$3.50
© Elsevier Sequoia/Printed in The Netherlands
192 The a-Si:H solar cell is a thin film cell in which the efficiency has increased to over 11%. However, traditionally, thick samples of a-Si:H are always used to determine the cell's electrical properties which may provide data inconsistent with the use of thin samples. Also, for p - i - n amorphous silicon solar cells, the characteristics of each layer are very different. This paper provides the results of an investigation of the photoelectrical properties of the most important layer -- the intrinsic layer. To obtain the relationship between these properties, the following experiments have been performed: dark electrical conductivity OD, photoconductivity Oph in stable and transient experiments, dark conductivity vs. temperature ((~D-T), absorption coefficient ~, spectral response (SR), and optical gap Eop t. Finally, the data are analyzed and discussed.
2. Preparation and equipment
2.1. Preparation o f samples All the a-Si:H samples, except that for the PL experiment, were deposited at the Solar Energy Research Institute using a load-locked single-chamber r.f. glow
2.2. Equipment Stable photoconductivity oph was measured using an ELH light source, Keithley 233 power supply and Keithley 480 picoammeter. Transient photoconductivity measurements used a pulsed laser light source with k = 5320 A, a pulse time of about 15 ns, a determining voltage of 550 V, and a Tektronik 7834 storage oscilloscope as a recording instrument. Spectral response and absorption coefficient tests used the ELH lamp, with 100 mW cm -2 fight intensity, a GM-100-2 Schoeffel monochromator, and a Keithley 480 picoammeter.
193
Photoluminescence measurements used an argon laser (k = 488 nm) as the excitation source (with PL intensity of 225 mW). The excited sample emits a luminescence signal which is detected by a PbS detector through a monochromator. The signal is then amplified and recorded by a controlling computer. The determining temperature is 77 K.
3. Results and discussion
3.1. Photoconductivity The total photoconductivity O'ph may be divided into two parts: surface photoconductance Ophs and bulk photoconductance OphB. The former is independent of the film thickness, but the latter is proportional to the thickness. The relation of photocurrent iph and thickness of film d is shown in Fig. 1. It is shown that iph increases linearly with thickness d when d ~< 1 #m. When d > 1 pm, a smaller increase is seen. The dark conductivity a D and photoconductivity aph have a similar trend, as shown in Fig. 2. However, Oph of the thin sample (d < 0.5 #m) increases faster than that of the thick sample, because of the different Fermi level EF position with respect to the conduction band Ec at the surface and in the bulk [3]. If electron-hole pairs are created in the surface layer high field region where the band is bent, electrons and holes are likely to be more separated in space during excitation than they are in the bulk where a low field exists. Such a separation slows the recombination process and increases the photoconductivity. Thus
2.C
-4
-6
2.2 =. v
1.2
E
z.o
cv 1.8
-8
o.
c
:
o.e
1.6
o a_
==
0
-
C
0.4
0.0
2
0.4
0.8
1.2
1.6
2.0
Thickness, d(pm)
2.4
o.o o'.4 o:s
,'.2 'Thickness
,'.6 2'.0
i 2.4
(I,I m )
Fig. 1. Photocurrent iph (©) and peak of absorption coefficient ~p (/x) dependence of thickness d of a-Si:H film. Fig. 2. Dark conductivity aD (A) and photoconductivity 0"ph(©) dependence of thickness d of a-Si :H film.
194
Oph(d =
0.48 pm) > O p h ( d = 2.16 pm)
(1}
In Fig. 2, for d ~ 1 pm, the curve is less dependent on thickness. This shows that the photoconductivity is affected very little by the surface layer for thick samples. As the thickness of the a-Si:H film continues to decrease to 0.12 pm {Fig. 2), the photoconductivity significantly decreases. This is because of a higher density of states and more defects on the surface of a thinner film which produces the surface recombination. For very thin films, part of the incident light on the surface of the film is not fully absorbed because the film thickness is less than the absorption depth of light [5]. 3.2. Absorption coefficient and optical gap In general, iph and Oph increase with thickness, but there is one exception. In a certain range of thickness with low intensity of light, apa in a thin sample is higher than that of a thick sample (Fig. 3). Oph(d = 0.48 pm) < Oph(d = 0.12 pm)
(2)
2.16 pm) < aph(d = 0.98 pm)
(3)
Oph(d =
A higher activation energy in the thin (d < 0.5 pm) samples accounts for eqn. (2). When the thickness d of the film increases to 1 - 2 pm, the influence of the surface layer is neglected and aph mainly depends on d. From eqn. (3), if the film thickness is too large, the light penetration depth is less than the film thickness which causes a drop in oph for thicker films. Also,
o
IO 7
i
g
o
r5 9
3.2
%
2.8
2.4
2.0
1.6
1.2
0.8
Energy, hv (eV)
Fig. 3. The relation b e t w e e n the spectral r e s p o n s e (SR) and thickness d o f a-Si:H film:
v, 2.16 pm; ©, 0.98 pro;A, 0.48/gm; ~, 0.]2 pm.
195 under the a-Si:H process by glow discharge, the thick sample probably contains more defects and voids in a greater thickness. This also results in increasing recombination of photocarriers and a reduction in aph. Depending on illumination, the electrons and holes may be generated uniformly throughout the film such that Oph may be written as oph =
(4)
o~7#'reF
where Oph is the photoconductivity (~2 cm) 1, r/ is the quantum efficiency, # is the mobility of electrons (cm 2 V -1 s-l), v is the lifetime of electrons (s), e is the charge of electrons, F is the amount of incident light (photon cm -2), and ~ is the absorption coefficient of the film (cm 1). We can use eqn. (4) to obtain (~ from Oph. The r/pT product is assumed to be a constant, a reasonable assumption in a low absorption region. As indicated in Fig. 4, ~ is rapidly increasing with decreasing thickness of film, when d < 0.5 #m. Ol(d=0.12#m)
~
10 4 (cm - ] ) > Ol(d=0.4S#m)
~
].0 3 (cm -1)
(5)
Therefore, a higher a compensates for reduced d. However, the peak of the absorption spectrum moves to the high energy side (Fig. 4) and Eopt increases (Fig. 5) with decreasing thickness of film. In both cases, high energy photons are absorbed by the surface of a thin film. These are also transferred into higher iph and increased Oph [6]. 200 180
16( 14( 10 4
L.
12C
E o
v ¢1 2 <= _e o
19.®
IOC 8C
o I0 3
0
% %~
~
/
4C o
z Io
o.s ,.z
,'.s z:0
'
z'.8 3:z
o.s
s
r
,
2C 2.4
/
J JJ
t i
~a~'~
,
d
,'.o ,'.z
£nergy, hv (eV) Fig. 4. Absorption c o e f f i c i e n t ~ vs. hp c u r v e at d i f f e r e n t
,
:,
,'., ,.6
.' ,
,.s
Energy, h v ( e V ) thicknesses of a-Si:H
z:o z.z film: v
2.16/am; ©, 0.98/am; A, 0.48 /am; n, 0.12/am. 5. (ahp) 1j2 a n d o p t i c a l g a p Eop t vs. h p c u r v e ~z, 2.16/am; o, 0.98/am; A, 0.48/am; n, 0.12/am. Fig.
at different
thicknesses
of a-Si:H
film:
196 3.3. Transient p h o t o c u r r e n t and decay time The transient p h o t o c u r r e n t data of Fig. 6 show that iph decreases with decrasing thickness of film. This is because of localized states in t he space charge region resulting in fast r e c o m b i n a t i o n on the surface of a thin sample [7, 8]. The decay time of electrons Te decreases with decreasing thickness of the film, as shown in Fig. 7. The surface r e com bi nat i on effect created from the surface states seems to play the d o m i n a n t role [9, 10]. As the thickness of film is near or larger than 1 pm, the Te vs. d curve tends to be level. In that case, electron decay time does n o t vary with the thickness, because the penetration o f the light entering into the film is less than the thickness of the film.
_..~4o
2O
J
: 2oF I-
IO c ,~ i--
0
~ I°I 0i.4
0[8
IL2
I '.6
21.0
Thickness of Film (pm)
2.4
0.4
0'.8
1.2
I.
2 4
Thickness of Film (pro)
• Fig. 6. Transient photocurrent (iph)t dependence of thickness d of a-Si:H film. Fig. 7. Decay time of electron Te dependence of thickness d of a-Si:H film.
The Te data in the whole range of thickness are within one order of magnitude and very small. This fact can be explained as follows. Firstly, the p h o t o i n j e c t i o n time is much shorter in a transient pulse than it i s j n steady state, i.e. the total a m o u n t of p h o t o n s entering on the surface o f the film is e x t r e m e l y small. Then, the differences in Te among the samples is n o t obvious since r eco mb in at i on dominates. Secondly, the fast decay time of a b o u t 10 -s s, as in Fig. 7, is caused by the carrier loss to deep states, or t he carriers m a y tunnel directly into defect states with large capture cross-sections o f a b o u t 10 -15 cm -3 near midgap (Fig. 8) [11].
l----3ve
EC
Fig. 8. Schematic diagram of the trapped electron entering deep states in transient photoexcitation condition.
197
IOO a.
.:
'
8o 60
/
oi F_.C
~
'
40
k._,
ao
e
"-)PL ~= o-
EV (a)
'
,' /
0
0.6
0.8
I .0
I .2
Energy, hv
I .4
I .6
I .8
(eV)
(b)
Fig. 9. (a) E n e r g y b a n d s c h e m a t i c diagram and (b) p h o t o l u m i n e s c e n c e s p e c t r u m o f a-Si:H at d i f f e r e n t t h i c k n e s s e s o f a-Si:H film: - - , 0.02 p m ; . . . . , 1.0 p m .
3.4. Photoluminescence The photoluminescence experiment [12, 13] shows a stronger PL defect peak located in the low energy side 0.8 - 1.0 eV region for a thinner sample, as in Fig. 9. This indicates the dominance of recombination in deep energy levels for thin samples, and results in a fast decay time. The result is consistent with the transient photocurrent experiment, i.e. the carrier loss in deep states. As a consequence of fast recombination of photocarriers, iph would decrease exponentially with time with a specific thickness dependence not observed in this experiment. To show the exponential relationship between iph and r requires a very long recombination time and indicates a lack of deep states. This is consistent with that predicted by Street and Biegelsen [14]. 3.5. Dark conductivity dependence on temperature and activation energy The thinner films have lower dark conductivity OD and higher activation energy Ea, as shown in Fig. 10. As the thickness of the film is reduced to 0.12 #m, there is a turning point on the OD-T curve (T = 363 K) and t w o E a values, 0.99 eV and 0.43 eV, appear. This can be explained as follows. Firstly, we assume the simple model of increased band bending when the thickness of film is reduced step b y step as in Fig. 11. The band bending at the surface becomes larger with decreasing thickness of film. This results in the increase in activation energy E, from 0.70 eV to 0.79 eV, as in Figs. l l ( a ) and (b). Secondly, if the thickness of the film is reduced progressively to 0.12 pm, as in Fig. l l ( c ) , the bending extent of the band edge 'is increased to Ea = 0.99 eV and, because of the increased distortion of the crystal lattice (i.e. the extent of localized states), the activation energy E, decreases to a b o u t 0.43 eV. When the temperature is high (363 K), the carriers are probably thermalized to a position of higher energy level (Ea = 0.99 eV) whereas, when the temperature is below 363 K, the thermalization probability is
198
fT)~
T,~
t~ 7
C
,5 8
=< o
oc
I 59
w
-I
z
I .0 -¢
\ O.S
c>-l0
2.
213 L5 2:7 2:9 L
I
3'.3 3.5 :
0.6 0,0
0:4
0:8
1.2 '
I.6
2:0
2,4
Thickness of Film (~m)
I Oa/T(°K)
(a)
(b)
Fig. 10. (a) Dependence of electrical conductivity o D on temperature T at different thicknesses o f a-Si:H film: D, 2.16/~m; z:, 0.98 pm; o, 0.48 pro; ~7, 0 . 1 2 / / m . (b) D e p e n d e n c e o f the activation energy E a on thickness. E¢ E~=O.7eV . . . . . ~_.
d > I gm
•Ev
----~a=O. 79e v d:O.48pm
~ = 0 . 9 9 e V E' a = O. 43_qeV~ ~
d = O . I 2Urn
Fig. 11. Schematic diagram of the energy band edge in thin samples.
reduced and th e carriers m a y reach the lower energy level (Ea = 0.43 eV), causing th e OD-T curve in thin samples to have a turning point [15]. F r o m the evidence presented in this paper, we can suggest t hat a surface layer exists in glow
199
Acknowledgment We thank Mr. Rick Wallace and Y. S. Lee for their assistance, the laser group of Dr. H. S. Kwok for aiding in transient photoconductivity work and Dr. P. Ehrlich for use of a pulsed lamp source. References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
S. Hasegawa, S. Shimizu and Y, Kurata, Philos. Mag. B, 49 (5) (1984) 511. S. Hasegawa, S. Shimizu and Y, Kurata, Philos. Mag. B, 49 (5) (1984) 519. J. Solomon and M. H. Brodsky, J. Appl. Phys., 51 (8) (1981) 4548. A. J. Rhodes, J. Non-Cryst. Solids, 59 - 60 (1983) 365. R. Fischer, W. Rehn, J. Stuke and U. Voget-Grate, J. Non-Cryst. Solids, 35 - 36 (1980) 687. B. Abeles, C. R. Wronski, T. Tiedge and G. D. Cody, Solid State Commun., 36 (1980) 537. P. B. Kirby and W. Paul, Phys. Rev. B, 2 (1984) 826. R . S . Crandall, Sol. Cells, 2 (1980) 319. R. W. Collins and W. Paul, Phys. Rev. B, 25 (4) (1982) 2611. R. A. Street, Adv. Phys., 30 (1981) 629. G. Moddel and D. A. Anderson, Phys. Rev. B, 22 (1982) 1918. D. Engemann and R. Fisher, Proc. 5th Conf. Amorphous and Liquid Semiconductors, Stoke and Brenig, New York, 1974, p. 947. Y. G. Ye, R. Q. Yin, H. Y. Pan, P. S. Chou, J. K. Cheng and Z. J. Lin, Proc. 5th Conf. Semiconductor Physics, China, 1985, p. 247. R. A. Street and D. K. Biegelsen, Top. Appl. Phys., 56 (1984) 221. Y. G. Ye, Z. Q. Lu and C. Y. Wu, Acta Energial Solars Sinica, 1 (2) (1980) 123 - 131.