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Picosecond to microsecond decay of photoinduced absorption in hydrogenated amorphous silicon
Journal of Non-Crystalline Solids 77 & 78 (1985) 551-554 North-Holland, Amsterdam
551
PICOSECOND TO MICROSECOND DECAY OF PHOTOINDUCED ABSORPTION IN HYDROGENATED AMORPHOUS SILICON
Dale M. ROBERTS. Joseph F. PALMER. and Terry L. GUSTAFSON The Standard Oil Company (Ohio), Corporate Research Center, 4440 Warrensville Center Road, Warrensville Heights, Ohio 44128
We measured picosecond photoinduced absorption in hydrogentated amorphous silicon using independently tunable pump and probe wavelengths. On a picosecond time scale we observed a change in the optical thickness, A(nd), in addition to a change in the electronic absorption. A=.
1. INTRODUCTION The study of the transient behavior of photogenerated carriers is helping to elucidate the mechanisms for carrier relaxation in amorphous silicon (a-Si) materials.
We are
using photoinduced absorption (PA) as a probe of the dynamics of the excited carriers. Recently. several workers have observed the PA dynamics of unhydrogenated a-Si, 1'2 and doped and undoped hydrogenated amorphous silicon (a-Si:H) 3-6 using single wavelength picosecond and sub-picosecond excitation.
These data show evidence for a
correlation between the dynamics and the defect density of the material. In this work we present picosecond PA results on a-Si:H that were obtained using independently tunable pump and probe lasers, The spectral dependence of the dynamics provided an additional parameter for separating the competing carrier relaxation processes observed in PA.
Specifically. on a picosecond time scale we observed a change in the
optical thickness, h(nd), in addition to a change in the electronic absorption. An. 2. EXPERIMENTAL The details of the experimental apparatus have been presented elsewhere.7
Briefly, a
mode locked argon ion laser pumped two synchronously pumped cavity dumped dye lasers to produce independently tunable pump and probe pulses. The probe beam was directed along a variable delay and combined colinearly with the pump beam. The light transmitted through the sample was spectrally resolved with a prism; the probe beam was detected with a photodiode.
The cross correlation between the pump and probe
pulses gave a time resolution of -15 ps. at a repetition period of 2 /~s. Pulse energies were 6.2 and 3.0 nJ for pump and probe pulses, respectively. 0022-3093/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
The pulses were focused
552
D.M. Roberts et aL / Picosecond to microsecond decay o f photoinduced absorption
through a 100 mm focal length lens to a spot size of ~50 microns, with the pump being slightly larger in diameter at the focus than the probe. We detected the increase in absorption using a time modulation technique in order to eliminate the thermal background that is present with mechanical chopping. 7
In this
scheme the time position of the probe pulse is modulated with respect to the pump pulse. The two modulation states were probe before pump (by 26 ns) and probe after pump (by the variable delay).
In this way both the pump and probe pulses are present
at the sample each pulse repetition period.
This time modulation was accomplished by
electronically switching the delay setting in the cavity dumper driver electronics,
The
picosecond PA signal was detected synchronously using the electronic switching rate as the reference. The sample used in this work was deposited in a capacitively coupled rf glow discharge system.
The sample was assesed as having a low defect density on the basis
of the steady state photoluminescence intensity,
3. RESULTS The sample we used was not uniform in thickness over the area we studied, but rather it was wedge shaped. As we changed the sample
I 0
I I DELAY
I [ (nsee)
[ !
position along the axis of increasing
I I 0
I I DELAY
I I (eeeo)
I 1
thickness with only the probe beam on the sample we observed an
"£ =
interferogram, with sucessive peaks corresponding to a sample thickness change of ~/2.
This is shown in the
center of Figure 1.
Note that the
I 2|0
I ,100
I
I " POSITION
I (mill)
I
I 400
I 420
peaks are narrower as position increases, indicating that the sample is increasing in thickness super-linearly with position.
Measurements of the
DELAY
[Rlee)
f
0
DELAY
(~ae©)
1
induced absorption decay at different parts of a single interferogram cycle gave different decay shapes, whereas corresponding points along different interferogram cycles gave the same shapes. This is shown in A. B. C,
FIGURE 1 Decay of photoinduced absorption at various positions in the I0 interferogram. Decays in A. B, C. and D correspond to the positions indicated in the center interferogram. (See text for details.)
D.M. Roberts et al. / Picosecond to microsecond decay o f photoinduced absorption
and D of Figure 1.
553
This suggests that the differences in the shapes of the induced
absorption decays were not due to sample inhomogenaities, but rather were related to an interferometric effect. Figure 2 shows the results of induced absorption measurements as a function of sample position (thickness) for various fixed delay times.
Note that there are two
the decrease in peak amplitude with time. and the change position with time. The decay in the peak amplitude is related to the decay in
effects present in the data: in peak
induced absorption, t,~,; the change in peak position is related to an increase in optical thickness. A(nd).
These parameters are related to the measured changes in transmission
using the equation which describes the transmission of light through the interferometer formed by the sample, substrate system. 8 Pump: 2 . 1 1 • V
FIGURE 2
Intensity of photoinduced absorption as a function of sample position for delay times of-100. 20. 100. 500. 2000 ps.
-
I
I
280
t
320
I
t
I
ao0
t
400
I
440
POSITION (mils)
4. DISCUSSION The most important result is that single wavelength PA measurements can be distorted according to the position on the interferometer fringe where the decay was recorded (Figure 1).
Since the fringe position is determined by probe wavelength as
well as optical thickness (nd). even if the sample were fiat the results of a single wavelength measurement could be distorted.
Models for carrier transport based on the
form of the decay of induced absorption must take these interferometric effects into account in order to be meaningful and accurate. We modeled the induced absorption using numerical differentiation of an equation for T O developed by O'Connor. 8
The results of the modeling indicate that the cycle
average of AI is linear in Aa.
We performed a cycle average of our data and found that
•a(t)
is not fit well either by a power law decay, or by a single exponential decay.
554
D.M. Roberts et al. / Picosecond to microsecond decay o f photoinduced absorption
While more work will be required to determine the best functional form for these decays, we note that the 1/e time for the cycle averaged data is -1 ns.
Another
implication of these results is that if the change in interferometer peak position were appreciably faster (e.g.. in another semiconductor material) the change in optical thickness could include a complete )L/2 cycle or more during the measured decay.
In a
single wavelength experiment this would appear as an oscillation in the transmitted light on top of a generally decaying induced absorption.
The rate at which the interferogram
peaks change may be related to s-ample temperature and thickness, as well as material properties. Using the same apparatus we have measured the PA decay over several microseconds. 7
These measurements are consistent with the observation that -50% of
the induced absorption amplitude decays in the first few nanoseconds. Measurements on other samples with higher defect densities show a smaller amplitude of this fast component in the induced absorption decay. We plan to investigate these phenomena further. ACKNOWLEDGEMENTS We want to thank R.W. Collins and H. Scher for their many and thoughtful contributions to this work.
We also appreciate the help of D.A. Chernoff, H.L. Fang.
and R.L. Swofford.
REFERENCES 1) J. Kuhl. E.O. Gobel. Th. Pfeiffer, and A. Jonietz. Appl. Phys. A 34. 105(1984). 2) Z. Vardeny. J. Strait, and J. Tauc. Appl. Phys. Lett. 4._22.580(1983). 3) J. Tauc. Hydrolzented Amorphous Silicon~ Part B. J, Pankove. Eel.. (Academic Press. New York, 1984), 4) Z. Vardeny. J. Non-Cryst. Solids. 5_99& 60. 317(1983). 5) Z. Varcleny. J. Strait. D. Pfost. J. Tauc. and B. Abeles. Phys. Rev. Lett. 4_.88.
1132(1982). 6) J. Strait, Ph.D. Thesis. (Brown University, May. 1985)0 pp. 75. 7) D,M. Roberts. J.F. Palmer. and T.L. Gustafson, Spectroscopic Characterization Techniques for Semiconductor Technology II. F.H. Pollak. Ed.. Proc. SPIE 524. 106(1985). 8) P. O'Connor. Ph.D. Thesis. (Brown University. June. 1980). pp. 87.
551
PICOSECOND TO MICROSECOND DECAY OF PHOTOINDUCED ABSORPTION IN HYDROGENATED AMORPHOUS SILICON
Dale M. ROBERTS. Joseph F. PALMER. and Terry L. GUSTAFSON The Standard Oil Company (Ohio), Corporate Research Center, 4440 Warrensville Center Road, Warrensville Heights, Ohio 44128
We measured picosecond photoinduced absorption in hydrogentated amorphous silicon using independently tunable pump and probe wavelengths. On a picosecond time scale we observed a change in the optical thickness, A(nd), in addition to a change in the electronic absorption. A=.
1. INTRODUCTION The study of the transient behavior of photogenerated carriers is helping to elucidate the mechanisms for carrier relaxation in amorphous silicon (a-Si) materials.
We are
using photoinduced absorption (PA) as a probe of the dynamics of the excited carriers. Recently. several workers have observed the PA dynamics of unhydrogenated a-Si, 1'2 and doped and undoped hydrogenated amorphous silicon (a-Si:H) 3-6 using single wavelength picosecond and sub-picosecond excitation.
These data show evidence for a
correlation between the dynamics and the defect density of the material. In this work we present picosecond PA results on a-Si:H that were obtained using independently tunable pump and probe lasers, The spectral dependence of the dynamics provided an additional parameter for separating the competing carrier relaxation processes observed in PA.
Specifically. on a picosecond time scale we observed a change in the
optical thickness, h(nd), in addition to a change in the electronic absorption. An. 2. EXPERIMENTAL The details of the experimental apparatus have been presented elsewhere.7
Briefly, a
mode locked argon ion laser pumped two synchronously pumped cavity dumped dye lasers to produce independently tunable pump and probe pulses. The probe beam was directed along a variable delay and combined colinearly with the pump beam. The light transmitted through the sample was spectrally resolved with a prism; the probe beam was detected with a photodiode.
The cross correlation between the pump and probe
pulses gave a time resolution of -15 ps. at a repetition period of 2 /~s. Pulse energies were 6.2 and 3.0 nJ for pump and probe pulses, respectively. 0022-3093/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
The pulses were focused
552
D.M. Roberts et aL / Picosecond to microsecond decay o f photoinduced absorption
through a 100 mm focal length lens to a spot size of ~50 microns, with the pump being slightly larger in diameter at the focus than the probe. We detected the increase in absorption using a time modulation technique in order to eliminate the thermal background that is present with mechanical chopping. 7
In this
scheme the time position of the probe pulse is modulated with respect to the pump pulse. The two modulation states were probe before pump (by 26 ns) and probe after pump (by the variable delay).
In this way both the pump and probe pulses are present
at the sample each pulse repetition period.
This time modulation was accomplished by
electronically switching the delay setting in the cavity dumper driver electronics,
The
picosecond PA signal was detected synchronously using the electronic switching rate as the reference. The sample used in this work was deposited in a capacitively coupled rf glow discharge system.
The sample was assesed as having a low defect density on the basis
of the steady state photoluminescence intensity,
3. RESULTS The sample we used was not uniform in thickness over the area we studied, but rather it was wedge shaped. As we changed the sample
I 0
I I DELAY
I [ (nsee)
[ !
position along the axis of increasing
I I 0
I I DELAY
I I (eeeo)
I 1
thickness with only the probe beam on the sample we observed an
"£ =
interferogram, with sucessive peaks corresponding to a sample thickness change of ~/2.
This is shown in the
center of Figure 1.
Note that the
I 2|0
I ,100
I
I " POSITION
I (mill)
I
I 400
I 420
peaks are narrower as position increases, indicating that the sample is increasing in thickness super-linearly with position.
Measurements of the
DELAY
[Rlee)
f
0
DELAY
(~ae©)
1
induced absorption decay at different parts of a single interferogram cycle gave different decay shapes, whereas corresponding points along different interferogram cycles gave the same shapes. This is shown in A. B. C,
FIGURE 1 Decay of photoinduced absorption at various positions in the I0 interferogram. Decays in A. B, C. and D correspond to the positions indicated in the center interferogram. (See text for details.)
D.M. Roberts et al. / Picosecond to microsecond decay o f photoinduced absorption
and D of Figure 1.
553
This suggests that the differences in the shapes of the induced
absorption decays were not due to sample inhomogenaities, but rather were related to an interferometric effect. Figure 2 shows the results of induced absorption measurements as a function of sample position (thickness) for various fixed delay times.
Note that there are two
the decrease in peak amplitude with time. and the change position with time. The decay in the peak amplitude is related to the decay in
effects present in the data: in peak
induced absorption, t,~,; the change in peak position is related to an increase in optical thickness. A(nd).
These parameters are related to the measured changes in transmission
using the equation which describes the transmission of light through the interferometer formed by the sample, substrate system. 8 Pump: 2 . 1 1 • V
FIGURE 2
Intensity of photoinduced absorption as a function of sample position for delay times of-100. 20. 100. 500. 2000 ps.
-
I
I
280
t
320
I
t
I
ao0
t
400
I
440
POSITION (mils)
4. DISCUSSION The most important result is that single wavelength PA measurements can be distorted according to the position on the interferometer fringe where the decay was recorded (Figure 1).
Since the fringe position is determined by probe wavelength as
well as optical thickness (nd). even if the sample were fiat the results of a single wavelength measurement could be distorted.
Models for carrier transport based on the
form of the decay of induced absorption must take these interferometric effects into account in order to be meaningful and accurate. We modeled the induced absorption using numerical differentiation of an equation for T O developed by O'Connor. 8
The results of the modeling indicate that the cycle
average of AI is linear in Aa.
We performed a cycle average of our data and found that
•a(t)
is not fit well either by a power law decay, or by a single exponential decay.
554
D.M. Roberts et al. / Picosecond to microsecond decay o f photoinduced absorption
While more work will be required to determine the best functional form for these decays, we note that the 1/e time for the cycle averaged data is -1 ns.
Another
implication of these results is that if the change in interferometer peak position were appreciably faster (e.g.. in another semiconductor material) the change in optical thickness could include a complete )L/2 cycle or more during the measured decay.
In a
single wavelength experiment this would appear as an oscillation in the transmitted light on top of a generally decaying induced absorption.
The rate at which the interferogram
peaks change may be related to s-ample temperature and thickness, as well as material properties. Using the same apparatus we have measured the PA decay over several microseconds. 7
These measurements are consistent with the observation that -50% of
the induced absorption amplitude decays in the first few nanoseconds. Measurements on other samples with higher defect densities show a smaller amplitude of this fast component in the induced absorption decay. We plan to investigate these phenomena further. ACKNOWLEDGEMENTS We want to thank R.W. Collins and H. Scher for their many and thoughtful contributions to this work.
We also appreciate the help of D.A. Chernoff, H.L. Fang.
and R.L. Swofford.
REFERENCES 1) J. Kuhl. E.O. Gobel. Th. Pfeiffer, and A. Jonietz. Appl. Phys. A 34. 105(1984). 2) Z. Vardeny. J. Strait, and J. Tauc. Appl. Phys. Lett. 4._22.580(1983). 3) J. Tauc. Hydrolzented Amorphous Silicon~ Part B. J, Pankove. Eel.. (Academic Press. New York, 1984), 4) Z. Vardeny. J. Non-Cryst. Solids. 5_99& 60. 317(1983). 5) Z. Varcleny. J. Strait. D. Pfost. J. Tauc. and B. Abeles. Phys. Rev. Lett. 4_.88.
1132(1982). 6) J. Strait, Ph.D. Thesis. (Brown University, May. 1985)0 pp. 75. 7) D,M. Roberts. J.F. Palmer. and T.L. Gustafson, Spectroscopic Characterization Techniques for Semiconductor Technology II. F.H. Pollak. Ed.. Proc. SPIE 524. 106(1985). 8) P. O'Connor. Ph.D. Thesis. (Brown University. June. 1980). pp. 87.