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Transient quenching of CW luminescence in a-Si:H
~
Solid State Communications, Vol.54,No.6, pp.489-492, 1985. Printed in Great Britain.
0038-1098/85 $3.00 + .00 Pergamon Press Ltd.
TRANSIENT QUENCHING OF CW LUMINESCENCE IN a-Si:H
H. A. Stoddart and J. Tauc Department of Physics and Division of Engineering Brown University Providence, RI 02912
(Received 6 February 1985 by J. Tauc)
Experimental data on the influence of optical biasing on photoluminescence decays in a-Si:H at 20K are presented. It is shown that the decay at long times is strongly influenced by time dependent quenching of the CW photolumineseence. A simple model is proposed to analyze this effect.
function of time delay after excitation for various levels of biasing s. In order to make clear small differences between decays, the vertical scale has been greatly expanded. A useful picture which will be developed in the next section is to consider AI as a superposition of two terms. The dominant one, Air, associated with the recombination of carriers which were generated by the pulse excitation, is positive with a rapid recovery. In the absence of bias, this
INTRODUCTION The effects produced by optical biasing on the decay of photoluminescence (PL) in a-Si:H have recently come under investigation 1. In the absence of bias, it is common practice to analyze PL decays in terms of a distribution in carrier lifetimes2'a. This is possible because of the correspondence which exists between a log-log plot of PL intensity vs time and the distribution in lifetimes4. Such an approach in the case where bias is present cannot be taken. The problem lies in the fact that biasing produces luminescence of its own which adds to the total PL signal. We have found that this contribution is time dependent and can dominate the decay of PL at long times. An investigation of this contribution is the subject of this paper. EXPERIMENT There are four different measurements used to collect the data reported here. Two of these observe the decay of PL and of photo-induced absorption (PA) following a short excitation pulse. The other two measure steady-state PL and PA. The relevant experimental details are as follows: Pulse excitation was optically produced by a nitrogen laser pumping a dye laser tuned to 2.1 eV with 50pJ of energy per pulse. The pulses are of 10ns duration repeating every 1/10 second. All CW excitation including biasing was provided by an argon laser emitting at 2.5 eV. A tungsten filament lamp filtered to radiate below 1.2 eV provided the probe beam used to measure both transient and steadystate PA. Signal inteintenswas measured using a Judson Ge diode whose region of sensitivity lies between 0.7 and 1.4 eV. During PA measurements, a Si window was inserted in front of the detector to block sample luminescence. Sample temperature was held at 20K throughout each experiment. The transient signals were acquired and averaged using a waveform digitizer. Steady-state signals were measured using phase locked detection with a chopper frequency of 160Hz.
g o~ )--
.....
nr"
WI TH OU T
BIAS
rn nr" WITH BIAS
0
5
I0
15
t (msec) Fig. la.
RESULTS In Figure l a the observed change in PL intensity (AI) relative to the steady-state intensity (Iu) is plotted as a
489
Photoluminescence decay in a-Si:H at 20K. Bias intensities were 64 (highest curve), 127, 191, 254, and 382 mW/cm 2 (lowest curve). PL intensity AI was measured relative to the steady-state PL intensity.
TRANSIENT QUENCHING OF CW LUMINESCENCE
490
IN a-Si:H
Vol. 54, No. 6
is the only term present. The other term, Alb, associated with quenching of the bias generated luminescence is negative with a much slower decay. We propose that the mechanism responsible for this quenching is due to an enhancement of non-radiative recombination with transient carriers trapped in non-radiative recombination centers (NRC's). The reason for this interpretation is t h a t the time dependence of transient PA, which measures the total number of trapped carriers, matches the time dependence of AI b. Note that it is not possible to isolate AIt from knowledge of AI alone since AI b is not known. However, since AIt decays much more rapidly than AIb, at long times A I b ~ A I .
b..I _1 tj to >.. re,
WITHOUT
BIAS
Iz
v WITH
0
5
BIAS
I0
15
t (msec)
Fig. lb.
Decay curves produced by superposition of unbiased PL decay with biased PA decays from Figure 3 (see text). The scale of AI is the same in Figures l a and lb.
BIAS = 382
o~
mW/cm ~
~'~-~___.................................................... ~I~L
>_
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.........
.
. . . . . . . . .
nr .
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='' .............................................. i..Ii
The magnitude of AI in relation to the absolute luminescence intensity (I) is apparent from Figure 2 where we have plotted I as a function of time. The dashed lines indicate the steady-state luminescence intensities for the various levels of bias which all decays must eventually reach. It is not surprising that as I= is increased by addi: tional bias, the magnitude of AI b also increases. ANALYSIS In the situation where only the bias illumination is present, there exists a state of dynamic equilibrium in which the generation and recombination rates of carriers are equal. This establishes the steady-state luminescence intensity I u. Immediately following pulse excitation, there will be more carriers present than can be sustained. As the system relaxes back to equilibrium, some of these excess carriers will undergo radiative recombination giving rise to the increase in luminescence intensity which we have ascribed to AIt. Others will thermalize (probably by band-tail hopping) into deeper states s'7 which act as NRC's s. Since both these channels serve to decrease the number of excess carriers in luminescence states, they both contribute to the decay o f / k i t Of primary interest is how the buildup of thermalized carriers in NRC's effects the total luminescence intensity. We will consider times longer than the decay time of AIt but not so long that the occupation of NRC's has reached its equilibrium value. Band tail carriers generated by the bias will undergo recombination with partners in NRC's more frequently than they would during equilibrium. This reduces the efficiency of luminescence The result is a decrease in the overall luminescence intensity giving rise to the negative term Al b. The time dependence of AI b follows t h a t of the decay of the number of excess thermalized carriers. To understand this effect better, consider the following rate equation for the density of band-tail carriers in). The interaction between these carriers and those in NRC's is contained in the last term where we have represented the density of carriers in NRC's by the difference between the density of all trapped carriers (N) and n. d n / d t = G - br n2 - bnrn(N - n)
!
0 0
5
I0
....
ZERO
I
,
15
t (msec)
Fig. 2.
Absolute PL intensity decays. The dashed lines are steady state intensities.
(1)
Here G denotes the generation rate due to biasing and the second and third terms give the radiative and nonradiative recombination rate respectively. Expanding n and N about their steady-state values; n = n,s + An and N = N,, + AN, and retaining only 1st order terms leaves; ( d / d t + v)An = -bnrn,,AN
(2)
where v = 2(b r - bur)n,, + barN,s. Since AN decays more slowly than e-g, we have that at times longer than 1/v
491
TRANSIENT QUENCHING OF CW LUMINESCENCE IN a-Si:H
Vol. 54, No. 6
An = -(barn,,/v)AN
(3)
which, using brn 2 for the luminescence intensity gives AI b = -(2brbarn~/v)AN
(4)
Thus the negative term observed at Ions times is proportional to the excess density of trapped carriers. Since transient PA measures the change in the total number of trapped carriers °, this last expression implies t h a t the decay of AIb should have the same form as the decay of PA when measured under the same conditions (Figure 3). Since AIt does not change significantly with bias l, the total change in intensity measured in the absence of bias can be used to approximate AIt in the case of arbitrary bias. Thus we have fit the changes in luminescence intensity to a superposition of unbiased AI and the biased PA signals. The results of these fits shown in Figure l b are to be compared with Figure la.
The expression inside the parentheses in Equation (4) should be proportional to the scaling factor (c) which was used to fit the PA signals to AI. If it is assumed that b u r N n > > 2 ( b r - bnr)nn, then this factor reduces to c = 2 b r n ~ / N n ~ I n / N n. In Figure 4, as a function of bias intensity, are plotted the three measured quantities In, steady-state PA (assumed to be proportional to N,), and c
.J <~ ¢J to ynrf I-
as obtained from the previous fitsto AI. In addition, the quantity lu/N n is included for comparison. In spite of the simplicity of our model, the agreement between c and lja/Nn is quite good.
CONCLUSION Previous attempts at measuring the distribution in lifetimes of carriers in optically biased a-Si:H using photoluminescence have made the erroneous assumption that AI = Air and in doing so have been led to the conclusion that bias places an upper limit on carrier lifetimes. In fact, it is the negative contribution of the transiently quenched CW luminescence which is responsible for the rapid approach of AI to zero.
ACKNOWLEDGEMENTS W e thank B. Abeles for the samples and T. R. Kirst for technical assistance. The work was supported by the National Science Foundation grant D M R 82-09148; extensive use was made of the Optical Facility supported by the N S F Materials Research Laboratory Program at Brown University.
o3 z
WITHOUT BIAS >tw n-
rr
F-
nI
WITH BIAS
5
Fig. 3.
IO t(msec)
15
Decay of photoindueed absorption at 20K (relative change in transmission T). The biasing intensities were the same as in Figure la.
i
0 0
i
I00
i
I
i
200
I
300
i
400
BIAS (mw/cm =) Fig. 4.
Steady state PL (In), PA (Nn) and the ratio l n / N n as a h n c t i o n of bias intensity. The factor e scales the P A contribution (see text).
492
TRANSIENT QUENCHING OF CW LUMINESCENCE IN a-Si:H
Vol. 54, No. 6
REFERENCES E. Merk, D. J. Dunstan and W. Czaja, Phys. Bey. Letters 54. 250 (1985).
6.
R. A. Street, Adv. in Phys. ~ 593 (1981). B. A. Wilson, P. Hu, T. M. Jedju and J. P. Harbison, Phys. Rev. B 28. 5901 (1983). C. Tsan$ and R. A. Street, Phys. Rev. B 19. 3027 (1979).
7. 8.
See the rebuttal to Reference 1, Phys. Rev. Lett. 5_t, 251 (1985).
9.
D. J. Dunstan, F. Boulitrop, Phys. Rev. B ~ , 5945 (1984). D. Monroe, Phys. Rev. Lett. 54. 146 (1985). R. A. Street, D. K. Biegelscn, R. L. Wcisfield, Phys. Rcv. B 30, 5861 (1984). D. Pfost, Z. Vardeny and J. Taue, Phys. Rev. Lett. 376 (1984).
Solid State Communications, Vol.54,No.6, pp.489-492, 1985. Printed in Great Britain.
0038-1098/85 $3.00 + .00 Pergamon Press Ltd.
TRANSIENT QUENCHING OF CW LUMINESCENCE IN a-Si:H
H. A. Stoddart and J. Tauc Department of Physics and Division of Engineering Brown University Providence, RI 02912
(Received 6 February 1985 by J. Tauc)
Experimental data on the influence of optical biasing on photoluminescence decays in a-Si:H at 20K are presented. It is shown that the decay at long times is strongly influenced by time dependent quenching of the CW photolumineseence. A simple model is proposed to analyze this effect.
function of time delay after excitation for various levels of biasing s. In order to make clear small differences between decays, the vertical scale has been greatly expanded. A useful picture which will be developed in the next section is to consider AI as a superposition of two terms. The dominant one, Air, associated with the recombination of carriers which were generated by the pulse excitation, is positive with a rapid recovery. In the absence of bias, this
INTRODUCTION The effects produced by optical biasing on the decay of photoluminescence (PL) in a-Si:H have recently come under investigation 1. In the absence of bias, it is common practice to analyze PL decays in terms of a distribution in carrier lifetimes2'a. This is possible because of the correspondence which exists between a log-log plot of PL intensity vs time and the distribution in lifetimes4. Such an approach in the case where bias is present cannot be taken. The problem lies in the fact that biasing produces luminescence of its own which adds to the total PL signal. We have found that this contribution is time dependent and can dominate the decay of PL at long times. An investigation of this contribution is the subject of this paper. EXPERIMENT There are four different measurements used to collect the data reported here. Two of these observe the decay of PL and of photo-induced absorption (PA) following a short excitation pulse. The other two measure steady-state PL and PA. The relevant experimental details are as follows: Pulse excitation was optically produced by a nitrogen laser pumping a dye laser tuned to 2.1 eV with 50pJ of energy per pulse. The pulses are of 10ns duration repeating every 1/10 second. All CW excitation including biasing was provided by an argon laser emitting at 2.5 eV. A tungsten filament lamp filtered to radiate below 1.2 eV provided the probe beam used to measure both transient and steadystate PA. Signal inteintenswas measured using a Judson Ge diode whose region of sensitivity lies between 0.7 and 1.4 eV. During PA measurements, a Si window was inserted in front of the detector to block sample luminescence. Sample temperature was held at 20K throughout each experiment. The transient signals were acquired and averaged using a waveform digitizer. Steady-state signals were measured using phase locked detection with a chopper frequency of 160Hz.
g o~ )--
.....
nr"
WI TH OU T
BIAS
rn nr" WITH BIAS
0
5
I0
15
t (msec) Fig. la.
RESULTS In Figure l a the observed change in PL intensity (AI) relative to the steady-state intensity (Iu) is plotted as a
489
Photoluminescence decay in a-Si:H at 20K. Bias intensities were 64 (highest curve), 127, 191, 254, and 382 mW/cm 2 (lowest curve). PL intensity AI was measured relative to the steady-state PL intensity.
TRANSIENT QUENCHING OF CW LUMINESCENCE
490
IN a-Si:H
Vol. 54, No. 6
is the only term present. The other term, Alb, associated with quenching of the bias generated luminescence is negative with a much slower decay. We propose that the mechanism responsible for this quenching is due to an enhancement of non-radiative recombination with transient carriers trapped in non-radiative recombination centers (NRC's). The reason for this interpretation is t h a t the time dependence of transient PA, which measures the total number of trapped carriers, matches the time dependence of AI b. Note that it is not possible to isolate AIt from knowledge of AI alone since AI b is not known. However, since AIt decays much more rapidly than AIb, at long times A I b ~ A I .
b..I _1 tj to >.. re,
WITHOUT
BIAS
Iz
v WITH
0
5
BIAS
I0
15
t (msec)
Fig. lb.
Decay curves produced by superposition of unbiased PL decay with biased PA decays from Figure 3 (see text). The scale of AI is the same in Figures l a and lb.
BIAS = 382
o~
mW/cm ~
~'~-~___.................................................... ~I~L
>_
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.........
.
. . . . . . . . .
nr .
.
.
.
.
.
.
.
.
.
.
.
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.
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.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
='' .............................................. i..Ii
The magnitude of AI in relation to the absolute luminescence intensity (I) is apparent from Figure 2 where we have plotted I as a function of time. The dashed lines indicate the steady-state luminescence intensities for the various levels of bias which all decays must eventually reach. It is not surprising that as I= is increased by addi: tional bias, the magnitude of AI b also increases. ANALYSIS In the situation where only the bias illumination is present, there exists a state of dynamic equilibrium in which the generation and recombination rates of carriers are equal. This establishes the steady-state luminescence intensity I u. Immediately following pulse excitation, there will be more carriers present than can be sustained. As the system relaxes back to equilibrium, some of these excess carriers will undergo radiative recombination giving rise to the increase in luminescence intensity which we have ascribed to AIt. Others will thermalize (probably by band-tail hopping) into deeper states s'7 which act as NRC's s. Since both these channels serve to decrease the number of excess carriers in luminescence states, they both contribute to the decay o f / k i t Of primary interest is how the buildup of thermalized carriers in NRC's effects the total luminescence intensity. We will consider times longer than the decay time of AIt but not so long that the occupation of NRC's has reached its equilibrium value. Band tail carriers generated by the bias will undergo recombination with partners in NRC's more frequently than they would during equilibrium. This reduces the efficiency of luminescence The result is a decrease in the overall luminescence intensity giving rise to the negative term Al b. The time dependence of AI b follows t h a t of the decay of the number of excess thermalized carriers. To understand this effect better, consider the following rate equation for the density of band-tail carriers in). The interaction between these carriers and those in NRC's is contained in the last term where we have represented the density of carriers in NRC's by the difference between the density of all trapped carriers (N) and n. d n / d t = G - br n2 - bnrn(N - n)
!
0 0
5
I0
....
ZERO
I
,
15
t (msec)
Fig. 2.
Absolute PL intensity decays. The dashed lines are steady state intensities.
(1)
Here G denotes the generation rate due to biasing and the second and third terms give the radiative and nonradiative recombination rate respectively. Expanding n and N about their steady-state values; n = n,s + An and N = N,, + AN, and retaining only 1st order terms leaves; ( d / d t + v)An = -bnrn,,AN
(2)
where v = 2(b r - bur)n,, + barN,s. Since AN decays more slowly than e-g, we have that at times longer than 1/v
491
TRANSIENT QUENCHING OF CW LUMINESCENCE IN a-Si:H
Vol. 54, No. 6
An = -(barn,,/v)AN
(3)
which, using brn 2 for the luminescence intensity gives AI b = -(2brbarn~/v)AN
(4)
Thus the negative term observed at Ions times is proportional to the excess density of trapped carriers. Since transient PA measures the change in the total number of trapped carriers °, this last expression implies t h a t the decay of AIb should have the same form as the decay of PA when measured under the same conditions (Figure 3). Since AIt does not change significantly with bias l, the total change in intensity measured in the absence of bias can be used to approximate AIt in the case of arbitrary bias. Thus we have fit the changes in luminescence intensity to a superposition of unbiased AI and the biased PA signals. The results of these fits shown in Figure l b are to be compared with Figure la.
The expression inside the parentheses in Equation (4) should be proportional to the scaling factor (c) which was used to fit the PA signals to AI. If it is assumed that b u r N n > > 2 ( b r - bnr)nn, then this factor reduces to c = 2 b r n ~ / N n ~ I n / N n. In Figure 4, as a function of bias intensity, are plotted the three measured quantities In, steady-state PA (assumed to be proportional to N,), and c
.J <~ ¢J to ynrf I-
as obtained from the previous fitsto AI. In addition, the quantity lu/N n is included for comparison. In spite of the simplicity of our model, the agreement between c and lja/Nn is quite good.
CONCLUSION Previous attempts at measuring the distribution in lifetimes of carriers in optically biased a-Si:H using photoluminescence have made the erroneous assumption that AI = Air and in doing so have been led to the conclusion that bias places an upper limit on carrier lifetimes. In fact, it is the negative contribution of the transiently quenched CW luminescence which is responsible for the rapid approach of AI to zero.
ACKNOWLEDGEMENTS W e thank B. Abeles for the samples and T. R. Kirst for technical assistance. The work was supported by the National Science Foundation grant D M R 82-09148; extensive use was made of the Optical Facility supported by the N S F Materials Research Laboratory Program at Brown University.
o3 z
WITHOUT BIAS >tw n-
rr
F-
nI
WITH BIAS
5
Fig. 3.
IO t(msec)
15
Decay of photoindueed absorption at 20K (relative change in transmission T). The biasing intensities were the same as in Figure la.
i
0 0
i
I00
i
I
i
200
I
300
i
400
BIAS (mw/cm =) Fig. 4.
Steady state PL (In), PA (Nn) and the ratio l n / N n as a h n c t i o n of bias intensity. The factor e scales the P A contribution (see text).
492
TRANSIENT QUENCHING OF CW LUMINESCENCE IN a-Si:H
Vol. 54, No. 6
REFERENCES E. Merk, D. J. Dunstan and W. Czaja, Phys. Bey. Letters 54. 250 (1985).
6.
R. A. Street, Adv. in Phys. ~ 593 (1981). B. A. Wilson, P. Hu, T. M. Jedju and J. P. Harbison, Phys. Rev. B 28. 5901 (1983). C. Tsan$ and R. A. Street, Phys. Rev. B 19. 3027 (1979).
7. 8.
See the rebuttal to Reference 1, Phys. Rev. Lett. 5_t, 251 (1985).
9.
D. J. Dunstan, F. Boulitrop, Phys. Rev. B ~ , 5945 (1984). D. Monroe, Phys. Rev. Lett. 54. 146 (1985). R. A. Street, D. K. Biegelscn, R. L. Wcisfield, Phys. Rcv. B 30, 5861 (1984). D. Pfost, Z. Vardeny and J. Taue, Phys. Rev. Lett. 376 (1984).