- Email: [email protected]
Interpretation of the temperature dependence of the strong visible photoluminescence of porous silicon
Thin Solid Films 255 (1995) 254-257
!
Interpretation
dependence of the strong visible of porous silicon
of the temperature photoluminescence S. Finkbeiner,
Max-Planck-Institut
fiir Festkiirperforschung,
J. Weber
Heisenbergstrasse
I, 70569 Stuttgarl, German)
Abstract
The temperature dependence of the strong visible photoluminescence (550-850 nm) is studied in differently prepared porous silicon samples. The variation in the photoluminescence intensity with temperature is a result of a decrease in the radiative decay time and an increase in the non-radiative recombination process with increasing temperatures. The strong visible photoluminescence is interpreted by a recombination of singlet and triplet excitons. The singlet-triplet splitting is comparable for all samples but depends on the detection wavelength and on sample preparation. We present similar data for the recombination process in siloxene which supports the model of a common origin of the strong visible photoluminescence in these very differently prepared
samples. Keywords:
Luminescence;
Optical spectroscopy; Silicon
1. Introduction
The great fascination about the strong visible photoluminescence (PL) of porous Si has initiated recently many studies on the specific recombination process in this material. In particular, time-resolved measurements reveal a complicated non-exponential decay which varies with detection wavelength and sample preparation [l-4]. The experimental results are in most cases interpreted to support the quantum confinement model originally proposed by Canham [5]. We have performed a detailed study of the temperature dependence of the PL intensity. It will be shown that only part of the decay process is characteristic of the recombination process. A comparison of this characteristic decay in differently prepared porous Si samples and also in siloxene supports our model of a common origin of the PL in these very differently prepared samples.
2. Experimental
details
In this study we have used differently prepared porous Si samples and several siloxene powder samples; for details of the sample preparation techniques see Ref. [6]. The temperature-dependent measurements 0040-6090/95/$9.50 0 1995 ~ SSDI 0040-6090(94)05666-8
Elsevier
Science S.A. All rights
reserved
were performed in an exchange gas cryostat, allowing us to vary the temperature from 9 K to room temperature (RT). For lower temperatures, the samples were immersed in liquid He in a bath cryostat. The Ar+-laser beam was modulated with an acoustooptical modulator (time resolution, 20 ns; light modulation on-off, better than 104). The decay was detected with a GaAs photomultiplier and analyzed with a multichannel scaler.
3. Experimental
results
Fig. 1 gives a typical example of the temperature variation in the strong visible PL in porous Si. Starting from low temperatures, the PL intensity increases (factor of 2-5) up to around T = 100 K. At higher temperatures the intensity decreases again (factor of 2-50) and the PL maxima exhibit a sample-dependent red shift. The inset of Fig. 1 gives the total PL intensity as function of temperature. We have previously investigated [6] the time decay of the recombination process after pulsed-laser excitation. A strong non-exponential behavior was found directly after the laser pulse and for long delay times after the pulse an exponential tail could be detected. The non-
S. Finkbeiner, J. Weber / Thin Solid Films 255 (1995) 254-257
2.0
1.6
Time
2.4
Energy ( eV ) Fig. I. PL spectra of porous silicon at different temperatures (i,, = 457.9 nm; 2 mW cm-‘). The inset shows the temperature dependence of the integrated intensity (n-type Si; 50 mA cm-‘; 10 min).
exponential part of the decay depends on sample preparation and excitation power but, at low and high temperatures, the exponential tail depends only on detection wavelength and sample temperature. Several approaches have been proposed in the literature to fit the decay curves. In Fig. 2, we present a typical decay curve together with three fitting curves: Z(f) rexp(-:)+a
exp(-t)
t Ii Of) x exp ~0 IO1 exp(th) I(t)
255
( ms )
Fig. 2. PL transient of porous silicon (sample (i,, = 457.9 nm; 2 mW cm -*: i,,,, = 660 nm): 0. type Si; 20 mA cm ‘; 20 min); , Eq. ( I ):
PS426) at measurement
IO K (p-
, Eq. (2); .-- -,
Eq. (3). Table I Parameters porous Si
for functions
used in Fig. 2 to tit the transient
x= 1.9 (I =6.13 /j = 0.65
Bimolecular Multiexponential Kohlrausch
decay
of
r,] = 793x ns r, = 2058 jr_s r,, = 7624 us r(, = 1923 us
(1) (2) (3)
A simple multiexponential fit, Eq. (1), with two exponents contains three fitting parameters and gives a reasonable description of the exponential part of the decay. Several groups have proposed the Kohlrausch formula (Eq. (2)) to be an appropriate fit to the decay curve. However, this fit does not give a satisfactory description of the characteristic exponential decay as shown in Fig. 2. A bimolecular recombination process [6] according to Eq. (3) is well suited to describe the entire decay (see Fig. 2) and gives a particularly good description of the exponential part of the decay. Table 1 lists the fitting parameters for the three different formulae. In the following we shall study only the characteristic exponential part of the decay. For each sample the decay time r,, was determined at different wavelengths and temperatures (Fig. 3). At low temperatures T = lo-20 K, the decay times r0 are in the range of 10 ms, weakly depending on the detection wavelength (see also Ref. [6]). There is a drastic change in the decay time at higher temperatures which occurs at different temperatures depending on the detection wavelength: At RT all decay times are again of the
10 Temperature Fig. 3. Temperature-dependent decay wavelengths: C. TI, 0, measurements; Fig. 1).
100
(K) time T() ibr ditrerent detection . tits (same sample as for
same order of magnitude; their variation with wavelength, however, is larger than at low temperatures. The temperature dependence of ?(I is characteristic of a two-level system with an energetically lower forbidden transition (lifetime rn) and a faster dipole-allowed transition at higher energies (lifetime ra). The change in decay time r0 results from the thermal population of the two levels, and examples for such systems can be found in many excitonic systems with singlet and triplet states (see for example Ref. [ 71). We use the following expression [8] to describe the temperature dependence of ro: 1 fg s,,(T) = 58
exp(-AE/kT)
1 + gy exp( - AE/kT)
(4)
256
S. Finkbeiner, J. Weber / Thin Solid Films 255 (1995) 254-257
Table 2 Parameters
of Eq. (4) used in Fig. 3
E. (nm)
% (ps)
AE (meV)
Y = reir,
600 700 800
7205 8473 9407
31.15 16.3 8.6
212.3 128.4 62.1
where g = i is the ratio of degeneracy of the two exciton states, y = 7,/z,, and BE denotes the splitting of the triplet and singlet exciton. The lines in Fig. 3 give the best fit to the data using the above expression for T,(T) and Table 2 summaiizes the fitting parameters. Apparently, the only parameters which show a clear dependence on detection wavelength are the singlet-triplet splittings AE of the exciton and the lifetime zA of the singlet exciton. A singlet-triplet exciton has already been proposed in Ref. [2] to explain the temperaturedependent decay. Although their definition of the decay time 7 (l/e time) is quite different from our zU, the values of their energy splittings BE and decay times z are similar to the data given in Table 2. From the temperature dependence of r0 and the PL intensity Z, we determine the concentration of excitons via n(T) K Z(T)Q(T). Fig. 4 gives the concentration for different detection wavelengths. A characteristic drop in n(T) is found at around 100 K, with the exact temperature depending strongly on the detection wavelength. We explain the decrease by increased non-radiative recombination of the excitons. We find the following activation energy (as a function of the detection wavelength) for this process, given by the slope of the concentration curves: E, (600 nm) = 37.1 meV; E, (700 nm) = 14.2 meV; E,( 800 nm) = 8.8 meV. In Fig. 5 we compare the temperature dependence of differently prepared porous Si samples and annealed siloxene. For fixed detection wavelength, the same qualitative behavior is found for all samples. The only
%.5[ , 0
, , 0.04 Reciprocal
, , , , , 1 0.12
0.08 Temperature
( K-’ )
Fig. 4. Exciton concentration n vs. inverse temperature for different detection wavelengths (I_,, = 457.9 nm; 4 mW cm-*) (same sample as for Fig. 1).
10
1
100
Temperature
1000
(K )
Fig. 5. Comparison of the temperature-dependent decay time Q for different porous silicon samples (PS426 (same sample as for Fig. 2) and PSI (same sample as for Fig. 1)) and annealed siloxane (ark, Wiihler’s conclusions, annealed for IO min in air); (>.,, = 457.9 nm; 2 mW cm-*. , i.de,= 660 nm).
Table 3 Parameters of the fit to Eq. (4) for different annealed siloxane shown in Fig. 5 Tl
ansi PS426 PSI
(PSI
7566 7939 8235
porous
Si samples
and
AE (meV)
;’ = %/YA
30 12.3 24.6
66 160 133
quantitative difference in the different curves is the singlet-triplet splitting and the lifetime of the singlet state 7A (Table 3).
4. Discussion The temperature dependence of the strong visible PL in porous Si is determined by two different processes: a decrease in the radiative decay time z,(T) and a reduction in the concentration of excitons with increasing temperature. Both processes are thermally activated and, within the accuracy of the experiment, the activation energies BE and E, are identical. We interpret our results by an excitonic decay, where the exciton consists of two levels: a low-lying triplet state with long lifetime and an excited singlet state with shorter lifetime. The thermal population of the two states leads to the temperature-dependent radiative decay time. Apparently, the exciton in the singlet state is able to diffuse to non-radiative recombination centers which leads to a strong reduction in the exciton concentration at increased temperatures. We have compared the temperature dependence of the PL in porous Si and annealed siloxene samples. Surprisingly, we find that the temperature dependences
S. Finkheiner,
J. Weber 1 Thin Solid Films 255 (1995) 2544257
of the decay time and the exciton concentration are almost identical. Therefore a very similar recombination process is responsible for the strong visible PL in these very different materials.
Acknowledgments
We would like to thank M. Rosenbauer in preparing the siloxene samples, and M. Stutzmann and H. J. Queisser for helpful discussions. The technical assistance of W. Heinz is gratefully acknowledged. This work was partly supported by the German Minister of Research and Technology (Contract 01 BM 210/6).
251
References
[II
J. C. Vial, A. Bsiesy, F. Gaspard, R. H&no. M. Ligeon, F. Muller, R. Romestain and R. M. Macfarlane, Phys. Rev. B, 45 (1992) 14 171. M. J. Kane and D. LLJ P. D. J. Calcott. K. J. Nash, L. T. Canham. Brumhead. J. Phys.: Condens. Mutter. 5 ( 1993) L91. W. L. Wilson. S. A. Ipri. G. E. [31Y. H. Xie, M. S. Hybertsen, Carver, W. L. Brown, E. Dons, B. E. Weir, A. R. Kortan, G. P. Watson and A. J. Liddle, PhJs. Rea. B, 49 (1994) 5386. [41 G. Mauckner, K. Thonke, T. Baier, T. Walter and R. Sauer. J. Appl. Phrs., 75 (1994) 4167. [51 L. T. Canham, Appl. Phys. Lrtt., 57 ( 1990) 1046. [61 S. Finkbeiner. J. Weber. M. Rosenbauer and M. Stutzmann. J. Lumin.. 57 ( 1993) 231. t71 J. Weber, W. Schmid and R. Sauer. Phys. Rec. B. 21 ( 1980) 2401. [sl J. D. Cuthbert and D. G. Thomas. Phj,.v. Rcr,.. 154 ( 1967) 763.
!
Interpretation
dependence of the strong visible of porous silicon
of the temperature photoluminescence S. Finkbeiner,
Max-Planck-Institut
fiir Festkiirperforschung,
J. Weber
Heisenbergstrasse
I, 70569 Stuttgarl, German)
Abstract
The temperature dependence of the strong visible photoluminescence (550-850 nm) is studied in differently prepared porous silicon samples. The variation in the photoluminescence intensity with temperature is a result of a decrease in the radiative decay time and an increase in the non-radiative recombination process with increasing temperatures. The strong visible photoluminescence is interpreted by a recombination of singlet and triplet excitons. The singlet-triplet splitting is comparable for all samples but depends on the detection wavelength and on sample preparation. We present similar data for the recombination process in siloxene which supports the model of a common origin of the strong visible photoluminescence in these very differently prepared
samples. Keywords:
Luminescence;
Optical spectroscopy; Silicon
1. Introduction
The great fascination about the strong visible photoluminescence (PL) of porous Si has initiated recently many studies on the specific recombination process in this material. In particular, time-resolved measurements reveal a complicated non-exponential decay which varies with detection wavelength and sample preparation [l-4]. The experimental results are in most cases interpreted to support the quantum confinement model originally proposed by Canham [5]. We have performed a detailed study of the temperature dependence of the PL intensity. It will be shown that only part of the decay process is characteristic of the recombination process. A comparison of this characteristic decay in differently prepared porous Si samples and also in siloxene supports our model of a common origin of the PL in these very differently prepared samples.
2. Experimental
details
In this study we have used differently prepared porous Si samples and several siloxene powder samples; for details of the sample preparation techniques see Ref. [6]. The temperature-dependent measurements 0040-6090/95/$9.50 0 1995 ~ SSDI 0040-6090(94)05666-8
Elsevier
Science S.A. All rights
reserved
were performed in an exchange gas cryostat, allowing us to vary the temperature from 9 K to room temperature (RT). For lower temperatures, the samples were immersed in liquid He in a bath cryostat. The Ar+-laser beam was modulated with an acoustooptical modulator (time resolution, 20 ns; light modulation on-off, better than 104). The decay was detected with a GaAs photomultiplier and analyzed with a multichannel scaler.
3. Experimental
results
Fig. 1 gives a typical example of the temperature variation in the strong visible PL in porous Si. Starting from low temperatures, the PL intensity increases (factor of 2-5) up to around T = 100 K. At higher temperatures the intensity decreases again (factor of 2-50) and the PL maxima exhibit a sample-dependent red shift. The inset of Fig. 1 gives the total PL intensity as function of temperature. We have previously investigated [6] the time decay of the recombination process after pulsed-laser excitation. A strong non-exponential behavior was found directly after the laser pulse and for long delay times after the pulse an exponential tail could be detected. The non-
S. Finkbeiner, J. Weber / Thin Solid Films 255 (1995) 254-257
2.0
1.6
Time
2.4
Energy ( eV ) Fig. I. PL spectra of porous silicon at different temperatures (i,, = 457.9 nm; 2 mW cm-‘). The inset shows the temperature dependence of the integrated intensity (n-type Si; 50 mA cm-‘; 10 min).
exponential part of the decay depends on sample preparation and excitation power but, at low and high temperatures, the exponential tail depends only on detection wavelength and sample temperature. Several approaches have been proposed in the literature to fit the decay curves. In Fig. 2, we present a typical decay curve together with three fitting curves: Z(f) rexp(-:)+a
exp(-t)
t Ii Of) x exp ~0 IO1 exp(th) I(t)
255
( ms )
Fig. 2. PL transient of porous silicon (sample (i,, = 457.9 nm; 2 mW cm -*: i,,,, = 660 nm): 0. type Si; 20 mA cm ‘; 20 min); , Eq. ( I ):
PS426) at measurement
IO K (p-
, Eq. (2); .-- -,
Eq. (3). Table I Parameters porous Si
for functions
used in Fig. 2 to tit the transient
x= 1.9 (I =6.13 /j = 0.65
Bimolecular Multiexponential Kohlrausch
decay
of
r,] = 793x ns r, = 2058 jr_s r,, = 7624 us r(, = 1923 us
(1) (2) (3)
A simple multiexponential fit, Eq. (1), with two exponents contains three fitting parameters and gives a reasonable description of the exponential part of the decay. Several groups have proposed the Kohlrausch formula (Eq. (2)) to be an appropriate fit to the decay curve. However, this fit does not give a satisfactory description of the characteristic exponential decay as shown in Fig. 2. A bimolecular recombination process [6] according to Eq. (3) is well suited to describe the entire decay (see Fig. 2) and gives a particularly good description of the exponential part of the decay. Table 1 lists the fitting parameters for the three different formulae. In the following we shall study only the characteristic exponential part of the decay. For each sample the decay time r,, was determined at different wavelengths and temperatures (Fig. 3). At low temperatures T = lo-20 K, the decay times r0 are in the range of 10 ms, weakly depending on the detection wavelength (see also Ref. [6]). There is a drastic change in the decay time at higher temperatures which occurs at different temperatures depending on the detection wavelength: At RT all decay times are again of the
10 Temperature Fig. 3. Temperature-dependent decay wavelengths: C. TI, 0, measurements; Fig. 1).
100
(K) time T() ibr ditrerent detection . tits (same sample as for
same order of magnitude; their variation with wavelength, however, is larger than at low temperatures. The temperature dependence of ?(I is characteristic of a two-level system with an energetically lower forbidden transition (lifetime rn) and a faster dipole-allowed transition at higher energies (lifetime ra). The change in decay time r0 results from the thermal population of the two levels, and examples for such systems can be found in many excitonic systems with singlet and triplet states (see for example Ref. [ 71). We use the following expression [8] to describe the temperature dependence of ro: 1 fg s,,(T) = 58
exp(-AE/kT)
1 + gy exp( - AE/kT)
(4)
256
S. Finkbeiner, J. Weber / Thin Solid Films 255 (1995) 254-257
Table 2 Parameters
of Eq. (4) used in Fig. 3
E. (nm)
% (ps)
AE (meV)
Y = reir,
600 700 800
7205 8473 9407
31.15 16.3 8.6
212.3 128.4 62.1
where g = i is the ratio of degeneracy of the two exciton states, y = 7,/z,, and BE denotes the splitting of the triplet and singlet exciton. The lines in Fig. 3 give the best fit to the data using the above expression for T,(T) and Table 2 summaiizes the fitting parameters. Apparently, the only parameters which show a clear dependence on detection wavelength are the singlet-triplet splittings AE of the exciton and the lifetime zA of the singlet exciton. A singlet-triplet exciton has already been proposed in Ref. [2] to explain the temperaturedependent decay. Although their definition of the decay time 7 (l/e time) is quite different from our zU, the values of their energy splittings BE and decay times z are similar to the data given in Table 2. From the temperature dependence of r0 and the PL intensity Z, we determine the concentration of excitons via n(T) K Z(T)Q(T). Fig. 4 gives the concentration for different detection wavelengths. A characteristic drop in n(T) is found at around 100 K, with the exact temperature depending strongly on the detection wavelength. We explain the decrease by increased non-radiative recombination of the excitons. We find the following activation energy (as a function of the detection wavelength) for this process, given by the slope of the concentration curves: E, (600 nm) = 37.1 meV; E, (700 nm) = 14.2 meV; E,( 800 nm) = 8.8 meV. In Fig. 5 we compare the temperature dependence of differently prepared porous Si samples and annealed siloxene. For fixed detection wavelength, the same qualitative behavior is found for all samples. The only
%.5[ , 0
, , 0.04 Reciprocal
, , , , , 1 0.12
0.08 Temperature
( K-’ )
Fig. 4. Exciton concentration n vs. inverse temperature for different detection wavelengths (I_,, = 457.9 nm; 4 mW cm-*) (same sample as for Fig. 1).
10
1
100
Temperature
1000
(K )
Fig. 5. Comparison of the temperature-dependent decay time Q for different porous silicon samples (PS426 (same sample as for Fig. 2) and PSI (same sample as for Fig. 1)) and annealed siloxane (ark, Wiihler’s conclusions, annealed for IO min in air); (>.,, = 457.9 nm; 2 mW cm-*. , i.de,= 660 nm).
Table 3 Parameters of the fit to Eq. (4) for different annealed siloxane shown in Fig. 5 Tl
ansi PS426 PSI
(PSI
7566 7939 8235
porous
Si samples
and
AE (meV)
;’ = %/YA
30 12.3 24.6
66 160 133
quantitative difference in the different curves is the singlet-triplet splitting and the lifetime of the singlet state 7A (Table 3).
4. Discussion The temperature dependence of the strong visible PL in porous Si is determined by two different processes: a decrease in the radiative decay time z,(T) and a reduction in the concentration of excitons with increasing temperature. Both processes are thermally activated and, within the accuracy of the experiment, the activation energies BE and E, are identical. We interpret our results by an excitonic decay, where the exciton consists of two levels: a low-lying triplet state with long lifetime and an excited singlet state with shorter lifetime. The thermal population of the two states leads to the temperature-dependent radiative decay time. Apparently, the exciton in the singlet state is able to diffuse to non-radiative recombination centers which leads to a strong reduction in the exciton concentration at increased temperatures. We have compared the temperature dependence of the PL in porous Si and annealed siloxene samples. Surprisingly, we find that the temperature dependences
S. Finkheiner,
J. Weber 1 Thin Solid Films 255 (1995) 2544257
of the decay time and the exciton concentration are almost identical. Therefore a very similar recombination process is responsible for the strong visible PL in these very different materials.
Acknowledgments
We would like to thank M. Rosenbauer in preparing the siloxene samples, and M. Stutzmann and H. J. Queisser for helpful discussions. The technical assistance of W. Heinz is gratefully acknowledged. This work was partly supported by the German Minister of Research and Technology (Contract 01 BM 210/6).
251
References
[II
J. C. Vial, A. Bsiesy, F. Gaspard, R. H&no. M. Ligeon, F. Muller, R. Romestain and R. M. Macfarlane, Phys. Rev. B, 45 (1992) 14 171. M. J. Kane and D. LLJ P. D. J. Calcott. K. J. Nash, L. T. Canham. Brumhead. J. Phys.: Condens. Mutter. 5 ( 1993) L91. W. L. Wilson. S. A. Ipri. G. E. [31Y. H. Xie, M. S. Hybertsen, Carver, W. L. Brown, E. Dons, B. E. Weir, A. R. Kortan, G. P. Watson and A. J. Liddle, PhJs. Rea. B, 49 (1994) 5386. [41 G. Mauckner, K. Thonke, T. Baier, T. Walter and R. Sauer. J. Appl. Phrs., 75 (1994) 4167. [51 L. T. Canham, Appl. Phys. Lrtt., 57 ( 1990) 1046. [61 S. Finkbeiner. J. Weber. M. Rosenbauer and M. Stutzmann. J. Lumin.. 57 ( 1993) 231. t71 J. Weber, W. Schmid and R. Sauer. Phys. Rec. B. 21 ( 1980) 2401. [sl J. D. Cuthbert and D. G. Thomas. Phj,.v. Rcr,.. 154 ( 1967) 763.