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Lifetime distribution of the photoluminescence of a-Si:H and a-Si1−xCx:H
280
Journal of Non-Crystalline Solids 114 (1989) 280-282 North-Holland
LIFETIME DISTRIBUTION OF THE PHOTOLUMINESCENCE OF a-Si:H AND a-Sil-xCx:H M. BORT, R. CARIUS*, W. FUHS Fachbereich Physik, Universit~t Marburg, D-3550 Marburg, Renthof 5, FRG The lifetime distribution of the photoluminescence of a-Si:H and a-Sil-xCx:H has been studied by frequency resolved spectroscopy in the temperature range 10-100K. The experiments are performed on undoped and doped films and the defect density Nd is varied by electron bombardment and stepwise annealing. In undoped a-Si:H (low Nd) the lifetime distribution consists of a single broad structure which is dominated by a peak at r z 10-3s (excitation density 5. 1015cm-2sq). The maximum of this distribution shifts to shorter times when the excitation intensity increases, which is difficult to reconcile with geminate pair recombination. An increased temperature causes very similar changes: in the range 10--80K the peak position moves from 103s to 3.10-4s. Most surprisingly, the decrease of the PL-intensity due to additional defects is not connected with a shift of the lifetime distribution. We conclude that defects introduce a fast non-radiative channel which does not compete on the time scale of this experiment.
I. INTRODUCTION Two conventional techniques have been used to measure the lifetime distribution of excess carriers in a-Si:H: time resolved spectroscopy (TRS) 1 and frequency resolved spectroscopy (FRS) 2. The two methods are different in principle. In TRS, the sample is excited by a short light pulse and one obtains a lifetime distribution with the carrier density decaying during the experiment. FRS reveals the lifetime distribution G(r) under quasi---stationary conditions, namely at constant excess carrier density. In this paper we report a detailed study of FRS in a-Si:H and a-Sil-xCx:H which aims at elucidating the role of defects in the recombination process. Both undoped and doped films are studied and the defect density is varied by electron bombardment (2MeV, 4K) and stepwise annealing. The exciting light (1.96eV) was sine--wave--modulated by an acousto-optic modulator (10 Hz - 100 kHz) with the modulation amplitude amounting to about 20% of the bias light intensity. The signal (total luminescence intensity) was detected by setting the lock-in amplifier in exact quadrature to the exciting modulation (QFRS), which has been shown to give directly the lifetime distribution G(r) 2.
2.
RESULTS AND DISCUSSION In a-Si:H of low defect density the lifetime distribution (i.e. QFRS) consists of a broad structure centered near 3.10-3s at low excitation levels (Fig. 1). As in earlier studies 3'4 the distributions shift to 4,
I-
T=IO
El J
z
S ÷
(.9 u')
.**
i
\3
fl:: It_ 0.5c~ 1:3 txl U ._1
~E
nO z 0
I
10-6
I
FIGURE 1 Lifetime distributions of a-Si:H (low defect density) at various excitation intensities: (1) 5.1015cm-2s-1; (2) 1.1016cm-2s-1; (3) 5.1016cm2s-t; (4) 1.1017cm-2sq; (5) 5.1017cm-2s-1
* R. Carius, ISI-ST, KFA Jtilich, D-5170 Jiilich, FRG + This work was supported by the Bundesminister fiir Forschung und Technologie (BMFT). 0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)
I
10-~ 10-2 P L - L I F E T I M E I: (s)
M. Bort et M . / Photoluminescence of a-Si:H and a-Sil_~C=:H
shorter times with increasing intensity. For curve 5 the original data points are plotted. The lines are drawn taking into account that the width of the response function for a single lifetime is about 0.7 decades. In contradiction to the results of Searle 3, who found two distinct QFRS-peaks even in high efficiency a-Si:H samples, we observe an indication of a further increase of G(r) at short times only for samples of very high defect density (curve 4 and 5 in Fig. 4). The observed shift of the distribution at moderate light intensities is not consistent with the assumption of geminate pair recombination. In that model only at high carrier densities when the pairs overlap G(r) will depend on concentration 1. Fig. 2 shows that the data appear to follow the prediction of the distant pair model 4. We conclude from this behavior that for steady state experiments recombination does not occur between correlated pairs. With increasing temperature the lifetime spectra shift to shorter times (Fig. 3) as we expect if a non-radiative process competes with radiative
10-2-_
T= 10 K
'\\
\\ distant-pair \
z o
\ \
F-
X \
o12_ ,~. 10-3 LLI 13_
-p
- g%%-~,~:~.~
-\t~-
-,,
\\ '~\
16~
1015
,
,
1016
1017
~,
1018
1019
EXCITATIONINTENSITY r PHOTONS ~I cm 2 s FIGURE 2 Peak position of the G(r) spectra as a function of the excitation intensity. The broken lines give the prediction of the geminate pair] and distant-pair recombination models 4.
281
photons
Iex = 5"1016 crnZs D 0
I-
~
/ ~
o0 U_ 0.5C3 W
-
3
\
0
K
",~o K
III A \ \~110 K
..J w oc I
10-6
I
I
I
10-~ 10-2 PL-LIFETIME ~ (s)
FIGURE 3 G(7) spectra (normalized to the maximum of the 50 K---curve) at various temperatures. Excitation intensity: 5 • 1016cm-2s-1. recombination. As to the quantum efficiency 7/which is related to the height of the signal, Fig. 3 defines two temperature ranges: (1) T > 50 K; ~/decreases with increasing temperature and the shift of C(r) is accomplished by preferential quenching of long-living pairs 1,5. (2) T < 50 K; 9 and the signal height increase with temperature. For much higher excitation intensities, such behavior of r/has often been observed by others and has been associated with Auger recombination 1. However, for the low excitation intensity used in our experiment this process is very unlikely. The G(r) spectrum at 10 K suggests that the fall in ~/is predominantly caused by quenching of pairs with 7 < 10-4 s. The behavior for T > 50 K is in accordance with the general interpretation of the temperature quenchingl: the carriers become more mobile, and they diffuse to defects where they recombine non-radiatively. It is obvious that this affects predominantly pairs with long lifetimes and therefore there is only very little change of G(r) at short times. In order to study the influence of defects in more detail we have varied the defect density of undoped a-Si:H by electron bombardment (2 MeV, 4K) and
282
M. Bort et a l . / Photoluminescence of a-Si:H and a-Sil_~C~:H
13
1-
and Fig. 4). An increase of temperature at low Nd causes a competing non-radiative path which affects predominantly pairs with long lifetimes. In a defect-rich sample the temperature quenching is much enhanced. For instance, at N d > 1017 cm-3, the peak
T=IO K
~5
u3 u3
of the G(r) spectrum disappears completely if the temperature is raised only to 50K. This supports the
rr"
LL 0
0.5-
LU
>_
3
,__1
LLI tY I
10-6
I
10-~.
I
I
10-2
P L - L I F E T I M E ~{s)
FIGURE 4 G(r) spectra of a-Si:H at various defect densities: (1) 3.1015cm-3; (2) 4.1015cm-3; (3) 5.101%m-3; (4) 3. 1017cm-3; (5) 5.1017cm-3. Excitation intensity: 5.1015cm-2s -1. stepwise annealing between 300K and 450K. The defect density Nd was determined after each step by CPM-spectroscopy. As one expects, the PL intensity decreases with increasing Nd. This is in accordance with the assumption that the non-radiative process is tunneling of band tail electrons to dangling bonds (Do) 1. The most significant result in Fig. 4 is the absence of a peak shift of the G(r)-distribution. Due to the defect generation G(r) decreases uniformly in the time range 10-5s < r < 10-1s with increasing Nd. Thereby the peak remains located at 10-38. According to this result the non-radiative channel which is introduced by the defects does not compete with radiative recombination on the time scale of this experiment. We therefore conclude that at low temperature G(r) represents the distribution of the radiative lifetime. Capture into defects appears to occur rapidly during the thermalization process from higher tail states 6,7. We have also studied doped a-Si:H films where the defect density is enhanced by doping and found quite similar behavior as in Fig. 4. It is interesting to compare the quenching of the PL-intensity by temperature and by defects (Fig. 3
view that the temperature quenching occurs when with increasing temperature localized carriers become mobile and diffuse to defects. This process, which involves transport, then competes directly with radiative recombination, and is the more effective the larger Nd is. We have extended these investigations to a-Sii-xCx:H films where it is well known that the defect density increases with x. We find that in the range 0
R.A.Street, Adv. in Physics 30 (1981) 593.
2.
S.P. Depinna, D.J. Dunstan, Phil. Mag. B50 (1984) 579.
3.
T.M. Searle, H. Hopkinson, M. Edmeades, S. Kalem, I.G. Austin, R.A. Gibson in: "Disordered Semiconductors", eds. M.A. Kastner et al., (Plenum Press, 1987), pp 357-368.
4.
D.J. Dunstan, Phil. Mag. B52 (1984) 111.
5.
D.J. Dunstan, Phil. Mag. B49 (1984) 191.
6.
B.A. Wilson, A.M. Sergent, K.W. Wecht, A.J. Williams, and T.P. Kerwin, Phys. Rev. B30 (1984) 3320. W. Siebert, R. Carius,, W. Fuhs, K. Jahn phys. stat. sol.(b) 140 (1987) 311.
Journal of Non-Crystalline Solids 114 (1989) 280-282 North-Holland
LIFETIME DISTRIBUTION OF THE PHOTOLUMINESCENCE OF a-Si:H AND a-Sil-xCx:H M. BORT, R. CARIUS*, W. FUHS Fachbereich Physik, Universit~t Marburg, D-3550 Marburg, Renthof 5, FRG The lifetime distribution of the photoluminescence of a-Si:H and a-Sil-xCx:H has been studied by frequency resolved spectroscopy in the temperature range 10-100K. The experiments are performed on undoped and doped films and the defect density Nd is varied by electron bombardment and stepwise annealing. In undoped a-Si:H (low Nd) the lifetime distribution consists of a single broad structure which is dominated by a peak at r z 10-3s (excitation density 5. 1015cm-2sq). The maximum of this distribution shifts to shorter times when the excitation intensity increases, which is difficult to reconcile with geminate pair recombination. An increased temperature causes very similar changes: in the range 10--80K the peak position moves from 103s to 3.10-4s. Most surprisingly, the decrease of the PL-intensity due to additional defects is not connected with a shift of the lifetime distribution. We conclude that defects introduce a fast non-radiative channel which does not compete on the time scale of this experiment.
I. INTRODUCTION Two conventional techniques have been used to measure the lifetime distribution of excess carriers in a-Si:H: time resolved spectroscopy (TRS) 1 and frequency resolved spectroscopy (FRS) 2. The two methods are different in principle. In TRS, the sample is excited by a short light pulse and one obtains a lifetime distribution with the carrier density decaying during the experiment. FRS reveals the lifetime distribution G(r) under quasi---stationary conditions, namely at constant excess carrier density. In this paper we report a detailed study of FRS in a-Si:H and a-Sil-xCx:H which aims at elucidating the role of defects in the recombination process. Both undoped and doped films are studied and the defect density is varied by electron bombardment (2MeV, 4K) and stepwise annealing. The exciting light (1.96eV) was sine--wave--modulated by an acousto-optic modulator (10 Hz - 100 kHz) with the modulation amplitude amounting to about 20% of the bias light intensity. The signal (total luminescence intensity) was detected by setting the lock-in amplifier in exact quadrature to the exciting modulation (QFRS), which has been shown to give directly the lifetime distribution G(r) 2.
2.
RESULTS AND DISCUSSION In a-Si:H of low defect density the lifetime distribution (i.e. QFRS) consists of a broad structure centered near 3.10-3s at low excitation levels (Fig. 1). As in earlier studies 3'4 the distributions shift to 4,
I-
T=IO
El J
z
S ÷
(.9 u')
.**
i
\3
fl:: It_ 0.5c~ 1:3 txl U ._1
~E
nO z 0
I
10-6
I
FIGURE 1 Lifetime distributions of a-Si:H (low defect density) at various excitation intensities: (1) 5.1015cm-2s-1; (2) 1.1016cm-2s-1; (3) 5.1016cm2s-t; (4) 1.1017cm-2sq; (5) 5.1017cm-2s-1
* R. Carius, ISI-ST, KFA Jtilich, D-5170 Jiilich, FRG + This work was supported by the Bundesminister fiir Forschung und Technologie (BMFT). 0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)
I
10-~ 10-2 P L - L I F E T I M E I: (s)
M. Bort et M . / Photoluminescence of a-Si:H and a-Sil_~C=:H
shorter times with increasing intensity. For curve 5 the original data points are plotted. The lines are drawn taking into account that the width of the response function for a single lifetime is about 0.7 decades. In contradiction to the results of Searle 3, who found two distinct QFRS-peaks even in high efficiency a-Si:H samples, we observe an indication of a further increase of G(r) at short times only for samples of very high defect density (curve 4 and 5 in Fig. 4). The observed shift of the distribution at moderate light intensities is not consistent with the assumption of geminate pair recombination. In that model only at high carrier densities when the pairs overlap G(r) will depend on concentration 1. Fig. 2 shows that the data appear to follow the prediction of the distant pair model 4. We conclude from this behavior that for steady state experiments recombination does not occur between correlated pairs. With increasing temperature the lifetime spectra shift to shorter times (Fig. 3) as we expect if a non-radiative process competes with radiative
10-2-_
T= 10 K
'\\
\\ distant-pair \
z o
\ \
F-
X \
o12_ ,~. 10-3 LLI 13_
-p
- g%%-~,~:~.~
-\t~-
-,,
\\ '~\
16~
1015
,
,
1016
1017
~,
1018
1019
EXCITATIONINTENSITY r PHOTONS ~I cm 2 s FIGURE 2 Peak position of the G(r) spectra as a function of the excitation intensity. The broken lines give the prediction of the geminate pair] and distant-pair recombination models 4.
281
photons
Iex = 5"1016 crnZs D 0
I-
~
/ ~
o0 U_ 0.5C3 W
-
3
\
0
K
",~o K
III A \ \~110 K
..J w oc I
10-6
I
I
I
10-~ 10-2 PL-LIFETIME ~ (s)
FIGURE 3 G(7) spectra (normalized to the maximum of the 50 K---curve) at various temperatures. Excitation intensity: 5 • 1016cm-2s-1. recombination. As to the quantum efficiency 7/which is related to the height of the signal, Fig. 3 defines two temperature ranges: (1) T > 50 K; ~/decreases with increasing temperature and the shift of C(r) is accomplished by preferential quenching of long-living pairs 1,5. (2) T < 50 K; 9 and the signal height increase with temperature. For much higher excitation intensities, such behavior of r/has often been observed by others and has been associated with Auger recombination 1. However, for the low excitation intensity used in our experiment this process is very unlikely. The G(r) spectrum at 10 K suggests that the fall in ~/is predominantly caused by quenching of pairs with 7 < 10-4 s. The behavior for T > 50 K is in accordance with the general interpretation of the temperature quenchingl: the carriers become more mobile, and they diffuse to defects where they recombine non-radiatively. It is obvious that this affects predominantly pairs with long lifetimes and therefore there is only very little change of G(r) at short times. In order to study the influence of defects in more detail we have varied the defect density of undoped a-Si:H by electron bombardment (2 MeV, 4K) and
282
M. Bort et a l . / Photoluminescence of a-Si:H and a-Sil_~C~:H
13
1-
and Fig. 4). An increase of temperature at low Nd causes a competing non-radiative path which affects predominantly pairs with long lifetimes. In a defect-rich sample the temperature quenching is much enhanced. For instance, at N d > 1017 cm-3, the peak
T=IO K
~5
u3 u3
of the G(r) spectrum disappears completely if the temperature is raised only to 50K. This supports the
rr"
LL 0
0.5-
LU
>_
3
,__1
LLI tY I
10-6
I
10-~.
I
I
10-2
P L - L I F E T I M E ~{s)
FIGURE 4 G(r) spectra of a-Si:H at various defect densities: (1) 3.1015cm-3; (2) 4.1015cm-3; (3) 5.101%m-3; (4) 3. 1017cm-3; (5) 5.1017cm-3. Excitation intensity: 5.1015cm-2s -1. stepwise annealing between 300K and 450K. The defect density Nd was determined after each step by CPM-spectroscopy. As one expects, the PL intensity decreases with increasing Nd. This is in accordance with the assumption that the non-radiative process is tunneling of band tail electrons to dangling bonds (Do) 1. The most significant result in Fig. 4 is the absence of a peak shift of the G(r)-distribution. Due to the defect generation G(r) decreases uniformly in the time range 10-5s < r < 10-1s with increasing Nd. Thereby the peak remains located at 10-38. According to this result the non-radiative channel which is introduced by the defects does not compete with radiative recombination on the time scale of this experiment. We therefore conclude that at low temperature G(r) represents the distribution of the radiative lifetime. Capture into defects appears to occur rapidly during the thermalization process from higher tail states 6,7. We have also studied doped a-Si:H films where the defect density is enhanced by doping and found quite similar behavior as in Fig. 4. It is interesting to compare the quenching of the PL-intensity by temperature and by defects (Fig. 3
view that the temperature quenching occurs when with increasing temperature localized carriers become mobile and diffuse to defects. This process, which involves transport, then competes directly with radiative recombination, and is the more effective the larger Nd is. We have extended these investigations to a-Sii-xCx:H films where it is well known that the defect density increases with x. We find that in the range 0
R.A.Street, Adv. in Physics 30 (1981) 593.
2.
S.P. Depinna, D.J. Dunstan, Phil. Mag. B50 (1984) 579.
3.
T.M. Searle, H. Hopkinson, M. Edmeades, S. Kalem, I.G. Austin, R.A. Gibson in: "Disordered Semiconductors", eds. M.A. Kastner et al., (Plenum Press, 1987), pp 357-368.
4.
D.J. Dunstan, Phil. Mag. B52 (1984) 111.
5.
D.J. Dunstan, Phil. Mag. B49 (1984) 191.
6.
B.A. Wilson, A.M. Sergent, K.W. Wecht, A.J. Williams, and T.P. Kerwin, Phys. Rev. B30 (1984) 3320. W. Siebert, R. Carius,, W. Fuhs, K. Jahn phys. stat. sol.(b) 140 (1987) 311.