- Email: [email protected]
A new lifetime diagnostic system for photovoltaic materials
Solar Energy Materials & Solar Cells 95 (2011) 1985–1989
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
A new lifetime diagnostic system for photovoltaic materials Richard K. Ahrenkiel n, Donald J. Dunlavy Department of Metallurgical and Materials Engineering and Renewable Energy Materials Research Science and Engineering Center, 1500 Illinois Street, Colorado School of Mines, Golden, Colorado, 80401, USA
a r t i c l e in fo
abstract
Article history: Received 9 September 2009 Received in revised form 1 February 2010 Accepted 13 February 2010 Available online 19 March 2010
We have developed a new minority-carrier lifetime measurement apparatus for measuring the recombination lifetime in semiconductors. We describe the technique as transmission modulated photoconductive decay (TMPCD). This is a contactless, non-invasive technique that produces transient photoconductive lifetime data. The measurement procedure is very sensitive to small signals and has a superior time response for measurement of short carrier lifetimes. This technology has several advantages over resonant coupled photoconductive decay (RCPCD) and transient microwave reflection photoconductive decay (mPCD). The response time advantage provides a capability to measure very short lifetimes in thin film materials, such as nanocrystalline silicon films and nanowire composites. This is accomplished while maintaining a sensitivity that is at least comparable to RCPCD. The new technique has been successfully applied to silicon wafers, compound semiconductor thin films, nano-crystalline silicon films, and II–VI nanowires. & 2010 Elsevier B.V. All rights reserved.
Keywords: Carrier lifetime Recombination lifetime Semiconductors Photovoltiacs Thin film Photoconductive decay
1. Introduction
2. Current lifetime measurement techniques
For short minority-carrier lifetime applications, microwave reflection (mPCD) is currently used for measuring photoconductive transients [1,2] The time resolution is determined by the microwave detector response time and the laser pulse width. The mPCD sensitivity is somewhat limited in low-mobility, shortlifetime films, and does not always provide the desired data. The RCPCD technique is more sensitive than mPCD but has limitations in the ability to measure short lifetimes. The minimum lifetime that can be measured by RCPCD is about 40 ns, and depends on the quality factor (Q) of the given RCPCD measurement system. With the new technique, we have been able to measure carrier lifetimes in some non-conventional films that could not be measured with either of the other techniques. The measured response time of the TMPCD is appreciably faster than that of RCPCD. Recently, we have been able to measure transient photoconductivity in nanocrystalline silicon thin films. These films produced no signal in our RCPCD system, showing the improved performance of TMPCD. We have also used the technique to measure carrier lifetime in II–VI nanowires. This technology provides a promising new diagnostic that measures lifetime data for a wide variety of semiconductor materials.
There are a number of techniques that are currently in use for contactless measurement of the carrier lifetime in semiconducting and photovoltaic materials. A contactless, non-invasive technique is the only viable option for rapid and non-contaminating sample evaluation of carrier recombination.
n
Corresponding author: Tel.: + 1 303 273 3543; fax: + 1 393 988 8172. E-mail addresses: [email protected], [email protected] (R.K. Ahrenkiel).
0927-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.02.037
Transient microwave reflection measurements [1,2] have become an industry standard. This technique is especially powerful when combined with a lifetime mapping apparatus. Resonance coupled photoconductive decay (RCPCD), [3], was invented at NREL and is the primary lifetime tool used by the Measurements and Characterization group for silicon and other indirect bandgap PV materials. Quasi-steady state photoconductivity (QSSPC) [4] is the standard technique used in the wafer silicon community. Time-resolved photoluminescence (TRPL) [5,6] is a common technique used to measure the strongly, light-emitting semiconductors such as GaAs and related compounds. Many semiconducting materials can be characterized by at least one of the above techniques. All of the photoconductive decay techniques measure the mobility-excess carrier density product. Consequently, mobility variations with excess carrier density may corrupt the recombination lifetime determination if not corrected in the data analysis. The QSSPC technique (3) is designed for silicon but does not function for other PV materials. Microwave reflection is applicable to most materials, but works best with
1986
R.K. Ahrenkiel, D.J. Dunlavy / Solar Energy Materials & Solar Cells 95 (2011) 1985–1989
planar structures. Microwave mapping provides very useful lifetime maps of multi-crystalline silicon. Technique 2, (RCPCD), is used for measurements of most of the popular materials and is very sensitive to low-level signals. However, the lower limit for lifetime measurement is about 40 ns with our current apparatus. The RCPCD technology has become the primary method for measuring recombination lifetime at the National Renewable Energy Laboratory. Technique 4 (TRPL) is only applicable for direct bandgap materials, but is not applicable to silicon, germanium, and other indirect bandgap materials. This technique is applicable to very low mobility materials, in contrast to the above photoconductive technique. TRPL also measures the excess carrier decay directly, and is not influenced by mobility variations. All of these techniques are commercially available except for RCPCD. The RCPCD technique was developed at NREL [7–9] and has been shown to be very useful for materials with lifetimes larger than about 40–50 ns. However, it cannot measure lifetimes in the shorter lifetime range that are commonly found in compound semiconductor materials and thin film silicon. The RCPCD technique has a large dynamic range and allows measurement over several orders of injection level. By contrast,
mPCD is able to measure lifetimes in the ns range, but has a very small dynamic range and lower signal sensitivity [10]. None of the photoconductive techniques function well when a metal back contact is a component of the sample. This limits measurements on the thin films that are grown on metal substrates such as copper indium gallium disulfide (CIGS). Our objective in this work was to find a contactless technique with comparable sensitivity to RCPCD but with a much faster response to short lifetime materials. We have developed a technique that is described as transmission modulated photoconductive decay (TMPCD), and TMPCD appears to meet the above criteria. 2.1. Transmission modulated photoconductive decay (TMPCD) A schematic representation of the current basic TMPCD setup is shown in Fig. 1. Transmitting and receiving coil antennas are embedded in two adjacent metal enclosures and are isolated from each other by metal shielding. The transmitting antenna is driven by an oscillator at a frequency (f) that is adjustable. We have adjusted this frequency for the maximum signal that is observed with a silicon wafer bridging the two coils. The operating
LIGHT PULSE
SAMPLE RECEIVING COIL
TRANSMITTING COIL
METAL SHIELD
OSCILLATOR 500 MHZ
AMPLIFIER
PHASE SHIFTER
AMPLIFIER
AMPLIFIER
MIXER
V (mV)
TRANSIENT DIGITIZER
SPLITTER
t
IN
Fig. 1. Schematic diagram of the TMPCD apparatus.
R.K. Ahrenkiel, D.J. Dunlavy / Solar Energy Materials & Solar Cells 95 (2011) 1985–1989
1987
1
12°
SAMPLE
0.9
45° Transmission
0.8
0.7
0.6
0.5
TRANSMITTER
SHIELD
RECEIVER
Fig. 2. Ray tracing analysis of TMPCD operation. This neglects interference and diffraction effects.
frequency in these experiments was set at about 500 MHz. The peak response of the passive, receiving circuit is tuned to the transmitter frequency by means of a series capacitor. There is virtually no signal in the receiving circuit unless a sample bridges the two regions. The sample acts as a transmission line or waveguide to conduct the electromagnetic wave (EMW) to the receiving antenna. Fig. 2 shows a simple schematic of a classical EMW wave that is incident at an angle on the semiconductor surface. Here, we assume that the EMW undergoes a sequence of transmission and reflection events at the sample surfaces, using the Fresnel equations and classical optical ray tracing. The signal detected by the receiving antenna is amplified by about 40 dB with the two amplifier stages. The received signal is a function of the conductivity of the sample and changes with the photoconductivity. Thus, the transmission of EMWs from the transmitter to the receiver is modulated by the conductivity of the sample. With a short pulse light source incident on the sample, the receiver signal varies with the transient photoconductivity. Under pulsed optical excitation, the amplified signal is received and directed to one input of a mixer in order to extract the dc transient component. A portion of the signal from the oscillator is passed through a phase shifter and input to the second terminal of the mixer. When the two inputs are in phase, the mixer output produces a signal of 2f and a dc-transient component that decays with the injected carrier population. Digitizing, analyzing, and storing the transient photoconductive response contains the carrier lifetime data. The stored data allows the extraction of the carrier lifetime information. The lifetime is quantified by fitting the slope of the semi log plot of the rectified receiver output voltage versus time. Here, we are assuming that the mobility is independent of excess carrier density. When that is not the case, the data can be processed with an algorithm that corrects for the mobility variation.
2.2. Theory of TMPCD operation Fig. 1 shows the schematic representation of our current breadboard apparatus that has been used to make the measurements shown here. The sample bridges the transmitting and receiving coils directing the electromagnetic wave (EMW) from the transmitter to the receiver. A portion of the transmitted wave penetrates the sample and is ‘‘trapped’’ by total internal reflection. As such, the wave travels along the lateral direction of the sample, and a portion of that wave radiates from the bottom surface to the receiving antenna.
0.4 10-3
10-2
10-1 σ (ohm-cm)-1
100
Fig. 3. The calculated transmission coefficient of a 600 mm thick silicon wafer at 500 MHz at normal incidence.
In this analysis, we have ignored interference effects on the transmission of the EMW through the sample. The diffraction effects occur because the EM wavelength is much longer ( 60 cm) than the mechanical dimensions of the sample and measurement system. A more detailed analysis will occur in the near future and will consider these effects. The absorption coefficient of an EMW in a semiconducting medium has been calculated [11]. The absorption coefficient of the EMW is given by vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi !ffi u ume s2 1þ 2 2 1 a ¼ ot 2 o e where a is the attenuation coefficient of the electric vector in the medium and s the conductivity E ¼ E0 expðax=2Þ exp½iðotbxÞ When the conductivity, s, is increased by photon absorption, the E-vector component of the EMW is attenuated. At the frequency used here ( 500 MHz), the attenuation factor is fairly small, but the larger path produced by wave trapping increases the interaction length. This longer path length provides the larger signals that we observe in our breadboard apparatus. Fig. 3 shows a calculation of the 500 MHz-EMW transmission of a sample as a function of sample conductivity. The calculation is simply derived using Maxwell’s equations. The planar 500 MHz wave is assumed to be normally incident to the semiconductor surface. The optical path length is assumed to be 600 mm and the dielectric constant of silicon was used in the calculation. The transmission coefficient drops steeply when the bulk conductivity becomes larger than about 0.01 (O-cm) 1. In future work, we will extend this simple theory to include diffraction and interference effects. This extension will more accurately describe the interaction of the transient conductivity with the EMW. 2.3. Carrier lifetime measured by TMPCD The response time of TMPCD is considerably faster than that of RCPCD. We are currently limited in our ability to measure the exact risetime as we do not have a laser in the system that has a sufficiently short pulse width. The risetime difference is shown in Fig. 4 where the initial photoconductive response of a bare silicon wafer is measured by both systems with the same incident flux
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R.K. Ahrenkiel, D.J. Dunlavy / Solar Energy Materials & Solar Cells 95 (2011) 1985–1989
103
100
TMPCD RCPCD
RCPCD
1.03 μs
10-1 0.69 μs V (mV)
V (normalized signal)
TMPCD
10-2 1.68 μs 102 1.78 μs
10-3 0
0.05
0.1
0.15
t (µs) 0 Fig. 4. The rise time response data of TMPCD and RCPCD using the same pulsed optical light source and pulse intensity. The TMPCD amplitude is somewhat larger but both signals are normalized to unity. The excitation pulse width is about 6 ns full-width half maximum. The sample is a bare silicon wafer.
103 117 μs TMPCD RCPCD
V (mV)
119 μs
74.1μs 66.2 μs 102 0
50
100
150
200
250
t (μs) Fig. 5. The transient photoconductive decay of a Si3N4-passivated float zone wafer measured by both TMPCD and RCPCD. The photoresponse was produced by a YAG laser operating at 532 nm. The incident pulse intensity was set at the same value for both measurements.
intensity. The signal here has been normalized to unity. The TMPCD pulse rise is very close to the integrated time dependence of the excitation pulse, which has a full width half-maximum of about 6 ns. The RCPCD pulse rise time is much longer and is in the range of 30 or 40 ns. Fig. 5 shows data that were measured by both RCPCD and TMPCD using the same pulsed light source (a mini-YAG laser) that is operated in the frequency-doubled (532 nm) mode. As stated earlier, the pulse width of the laser pulse is about 6 ns full width half maximum. The excitation intensity is adjusted to be about 5 mJ/cm2 and was identical for each measurement. The sample is a float zone grown silicon wafer that has been passivated with Si3N4. The two-component decay, seen in both curves, is typical of a silicon sample pulsed into high injection. At high injection, the deep impurity levels, that dominate recombination, become partially filled with minority carriers. Therefore, the lifetime at
0.5
1
1.5
2 t (µs)
2.5
3
3.5
4
Fig. 6. Comparison of TMPCD and RCPCD data on a 4 mm-thick InGaAs film grown on an InP substrate. The InGaAs has a passivating epitaxial layer at each surface. The excitation source is a YAG laser operating at 532 nm.
high injection is larger than at low injection. Here the values are about 117 and 66 ms, for the high- and low-injection lifetimes, respectively. The data shows a slightly steeper curve for the TMPCD curve at t ¼0. The high-injection RCPCD lifetime is 119 ms whereas the low-injection lifetime is 74 ms. A reason for the small difference in low-injection lifetime is not known until this time. The TMPCD data shows a steeper drop in the initial (t ¼0) decay time than does the RCPCD data. This steep feature at t ¼0 is indicative of surface recombination and has been analyzed in the recent publication [12]. The high sensitivity of RCPCD is related to the very high quality factor (Q) of the coupling circuit, whereas, the TMPCD does not require a high Q. Circuit theory shows that as the Q increases, the system response time increases. As the detection system physics is different here, we are able to measure much shorter lifetimes by means of TMPCD. The ability to extend the measurement to thin films of silicon, compound semiconductors, and nanostructures may provide a significant advantage over other techniques. The technique has been successfully applied to thin films grown on substrates. Fig. 6 shows TMPCD data measured on a thin ( 4 mm) epitaxial film of InGaAs grown on a semi-insulating InP substrate. The active layer was grown between confinement/ passivating quaternary III–V epi-layers. The TMPCD data show a shorter lifetime at high injection than does the RCPCD data. At low injection, the lifetimes agree quite closely and within experimental error (+ / 5%). This difference will be explored by further experiments and the development of the measurement theory. These data again show that TMPCD has sensitivity equal to or exceeding RCPCD and in addition TMPCD has a much faster response time. The risetime differences are clear from the pulse appearance at t ¼0. The data of Fig. 7 shows TMPCD measurements of two thin films (samples A and B) of nano-Si/a-Si that was grown by CVD. The ratio of the nano-Si/a-Si components is 50:50. As the carrier mobility of a-Si is much less than unity, one suspects that nano-Si dominates the photoconductive transport shown in these data. We are limited in measuring the carrier lifetime by the finite pulse width of our excitation laser. The carrier lifetime is very comparable to the laser pulse width, and can be analyzed by the deconvolution process as being in the 10 ns range.
R.K. Ahrenkiel, D.J. Dunlavy / Solar Energy Materials & Solar Cells 95 (2011) 1985–1989
102 Sample A Sample B
TMPCD
V (mV)
3 ns
12 ns
101
100 0
0.01
0.02
0.03 t (μs)
0.04
0.05
Fig. 7. TMPCD data measured on two films (sample A and B) of nano-Si/a-Si using an excitation wavelength of 532 nm. The laser pulse has a full-width halfmaximum of about 6 ns. The longer decay time of the response is a convolution of the finite laser pulse width and the sample recombination lifetime. Both events are in the nanosecond range.
1989
alloy ZnxCd1 xSySe1 y [14] nanowires with composition (bandgap) continuously graded from ZnS (3.5 eV) to CdSe (1.74 eV). The light emission covers the entire visible spectrum by the bandgap emission on a single substrate that is 2 cm long. We measured one of these nanowire composites that was imbedded in a polymer and coated on a glass substrate. These data are shown in Fig. 8, and show a complex decay process. The latter is likely the result of the percolation transport that occurs in these films. The TMPCD technique was shown to measure carrier transport and recombination in these polymer imbedded structures and demonstrated high sensitivity for such measurements. Our technology will expedite materials research in this field to progress more rapidly as contacting is not required to produce the lifetime data. In summary, these results indicate that TMPCD has highly promising characteristics that will be useful for future research in a variety of materials systems. The developed technique could provide a significant advantage to many applications in that the measurement time is several seconds. Also, the increased sensitivity and faster response time extends the TMPCD technique to thin film and low-dimensional materials with intrinsically shorter lifetimes. The measurement is very rapid and requires very minimal tuning and adjustment. Quality measurements can be made in several seconds in most cases, depending on the repetition rate of the laser. The latter is significant in future materials research.
Acknowledgements 9.6 ns
The authors would like to thank Prof. C.Z. Ning for providing the II–VI nanowires used in these studies. 101 V (mV)
References 14.7 ns
36.3 ns
100 0
0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09
0.1
t (μs) Fig. 8. The TMPCD response of a ZnxCd1 xSySe1 y nanowire array that was encapsulated in a polymer on a glass substrate. The data were obtained using 532 nm pulsed excitation.
Measurements of the same film, with the same laser excitation source, produced null results using our RCPCD setup. With our microwave reflection system operating at 20 GHz, we could see a signal of several mv amplitude, but the signal was quite weak. For nanocrystalline silicon, we concluded that the sensitivity of TMPCD is superior to the other techniques used in our laboratory. The measurement of carrier lifetime in nanotubes and nanowires is critical for the evaluation of future generation PV materials. We have used TMPCD to measure the carrier lifetime in II–VI compound nanowires that contain a large bandgap gradient. The group of Pan and coworkers developed an alloy nanowire grading technique [13]. They demonstrated the first quaternary
[1] M. Kunst, G. Beck, The study of charge carrier kinetics in semiconductors by microwave conductivity measurements, J. Appl. Phys. 60 (1986) 3558–3566. [2] G. Beck, M. Kunst, Contactless scanner for photoactive materials using laserinduced microwave absorption, Rev. Sci. Instrum. 57 (1988) 197–201. [3] R.K. Ahrenkiel, S.W. Johnston, An advanced technique for measuring minority-carrier parameters and defect properties of semiconductors, Mater. Sci. Eng. B102 (2003) 161–172. [4] R.A. Sinton, A. Cuevas, Contactless determination of current–voltage characteristics and minority-carrier lifetimes in semiconductors from quasisteady-state photoconductance data, Appl. Phys. Lett. 69 (1996) 2510–2512. [5] W.K. Metzger, D. Albin, D. Levi, P. Sheldon, X. Li, B.M. Keyes, R.K. Ahrenkiel, Time-resolved photoluminescence studies of CdTe solar cells, J. Appl. Phys. 94 (2003) 3549–3555. [6] R.K. Ahrenkiel, in Minority Carriers in III–V Semiconductors: Physics and Applications, SEMICONDUCTORS AND SEMIMETALS-V39, Willardson Beer series; Academic Press, 1993, pp. 39–150. [7] U.S. Patent be 5,929,652, Apparatus for measuring minority carrier lifetimes in semiconductor materials, R.K. Ahrenkiel, 1999. [8] U.S. Patent 6,275,060 Richard Ahrenkiel and Steven Johnston, Apparatus and method for measuring minority carrier lifetimes in semiconductor materials, August 14, 2001. [9] U.S. Patent 6,369,603, Steven Johnston and Richard Ahrenkiel, April 9, 2002. [10] R.K. Ahrenkiel, S.W. Johnston, Lifetime analysis of silicon solar cells by microwave reflection, Sol. Energy Mater. Sol. Cells 92 (2008) 830–835. [11] R.K. Ahrenkiel, S.W. Johnston, The interaction of microwaves with photoelectrons in semiconductors, J. Vac. Sci. Technol. B 26 (2008) 1508–1515. [12] R.K. Ahrenkiel, S.W. Johnston, An optical technique for measuring surface recombination velocity, Sol. Energy Mater. Sol. Cells 93 (2009) 645–649. [13] A. Pan, W. Zhou, E. Leong, A. Chin, R. Liu, B. Zou, C.Z. Ning, Continuous alloycomposition spatial grading and superbroad wavelength-tunable nanowire lasers on a single chip, Nano Letters 9 (2009) 784–788. [14] A.L. Pan, R.B. Liu, M.H. Sun, C.Z. Ning, Quaternary alloy semiconductor nanobelts with bandgap spanning the entire visible spectrum, J. Am. Chem. Soc. (Communication) 131 (2009) 9502–9503, doi:10.1021/ja904137m.
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
A new lifetime diagnostic system for photovoltaic materials Richard K. Ahrenkiel n, Donald J. Dunlavy Department of Metallurgical and Materials Engineering and Renewable Energy Materials Research Science and Engineering Center, 1500 Illinois Street, Colorado School of Mines, Golden, Colorado, 80401, USA
a r t i c l e in fo
abstract
Article history: Received 9 September 2009 Received in revised form 1 February 2010 Accepted 13 February 2010 Available online 19 March 2010
We have developed a new minority-carrier lifetime measurement apparatus for measuring the recombination lifetime in semiconductors. We describe the technique as transmission modulated photoconductive decay (TMPCD). This is a contactless, non-invasive technique that produces transient photoconductive lifetime data. The measurement procedure is very sensitive to small signals and has a superior time response for measurement of short carrier lifetimes. This technology has several advantages over resonant coupled photoconductive decay (RCPCD) and transient microwave reflection photoconductive decay (mPCD). The response time advantage provides a capability to measure very short lifetimes in thin film materials, such as nanocrystalline silicon films and nanowire composites. This is accomplished while maintaining a sensitivity that is at least comparable to RCPCD. The new technique has been successfully applied to silicon wafers, compound semiconductor thin films, nano-crystalline silicon films, and II–VI nanowires. & 2010 Elsevier B.V. All rights reserved.
Keywords: Carrier lifetime Recombination lifetime Semiconductors Photovoltiacs Thin film Photoconductive decay
1. Introduction
2. Current lifetime measurement techniques
For short minority-carrier lifetime applications, microwave reflection (mPCD) is currently used for measuring photoconductive transients [1,2] The time resolution is determined by the microwave detector response time and the laser pulse width. The mPCD sensitivity is somewhat limited in low-mobility, shortlifetime films, and does not always provide the desired data. The RCPCD technique is more sensitive than mPCD but has limitations in the ability to measure short lifetimes. The minimum lifetime that can be measured by RCPCD is about 40 ns, and depends on the quality factor (Q) of the given RCPCD measurement system. With the new technique, we have been able to measure carrier lifetimes in some non-conventional films that could not be measured with either of the other techniques. The measured response time of the TMPCD is appreciably faster than that of RCPCD. Recently, we have been able to measure transient photoconductivity in nanocrystalline silicon thin films. These films produced no signal in our RCPCD system, showing the improved performance of TMPCD. We have also used the technique to measure carrier lifetime in II–VI nanowires. This technology provides a promising new diagnostic that measures lifetime data for a wide variety of semiconductor materials.
There are a number of techniques that are currently in use for contactless measurement of the carrier lifetime in semiconducting and photovoltaic materials. A contactless, non-invasive technique is the only viable option for rapid and non-contaminating sample evaluation of carrier recombination.
n
Corresponding author: Tel.: + 1 303 273 3543; fax: + 1 393 988 8172. E-mail addresses: [email protected], [email protected] (R.K. Ahrenkiel).
0927-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.02.037
Transient microwave reflection measurements [1,2] have become an industry standard. This technique is especially powerful when combined with a lifetime mapping apparatus. Resonance coupled photoconductive decay (RCPCD), [3], was invented at NREL and is the primary lifetime tool used by the Measurements and Characterization group for silicon and other indirect bandgap PV materials. Quasi-steady state photoconductivity (QSSPC) [4] is the standard technique used in the wafer silicon community. Time-resolved photoluminescence (TRPL) [5,6] is a common technique used to measure the strongly, light-emitting semiconductors such as GaAs and related compounds. Many semiconducting materials can be characterized by at least one of the above techniques. All of the photoconductive decay techniques measure the mobility-excess carrier density product. Consequently, mobility variations with excess carrier density may corrupt the recombination lifetime determination if not corrected in the data analysis. The QSSPC technique (3) is designed for silicon but does not function for other PV materials. Microwave reflection is applicable to most materials, but works best with
1986
R.K. Ahrenkiel, D.J. Dunlavy / Solar Energy Materials & Solar Cells 95 (2011) 1985–1989
planar structures. Microwave mapping provides very useful lifetime maps of multi-crystalline silicon. Technique 2, (RCPCD), is used for measurements of most of the popular materials and is very sensitive to low-level signals. However, the lower limit for lifetime measurement is about 40 ns with our current apparatus. The RCPCD technology has become the primary method for measuring recombination lifetime at the National Renewable Energy Laboratory. Technique 4 (TRPL) is only applicable for direct bandgap materials, but is not applicable to silicon, germanium, and other indirect bandgap materials. This technique is applicable to very low mobility materials, in contrast to the above photoconductive technique. TRPL also measures the excess carrier decay directly, and is not influenced by mobility variations. All of these techniques are commercially available except for RCPCD. The RCPCD technique was developed at NREL [7–9] and has been shown to be very useful for materials with lifetimes larger than about 40–50 ns. However, it cannot measure lifetimes in the shorter lifetime range that are commonly found in compound semiconductor materials and thin film silicon. The RCPCD technique has a large dynamic range and allows measurement over several orders of injection level. By contrast,
mPCD is able to measure lifetimes in the ns range, but has a very small dynamic range and lower signal sensitivity [10]. None of the photoconductive techniques function well when a metal back contact is a component of the sample. This limits measurements on the thin films that are grown on metal substrates such as copper indium gallium disulfide (CIGS). Our objective in this work was to find a contactless technique with comparable sensitivity to RCPCD but with a much faster response to short lifetime materials. We have developed a technique that is described as transmission modulated photoconductive decay (TMPCD), and TMPCD appears to meet the above criteria. 2.1. Transmission modulated photoconductive decay (TMPCD) A schematic representation of the current basic TMPCD setup is shown in Fig. 1. Transmitting and receiving coil antennas are embedded in two adjacent metal enclosures and are isolated from each other by metal shielding. The transmitting antenna is driven by an oscillator at a frequency (f) that is adjustable. We have adjusted this frequency for the maximum signal that is observed with a silicon wafer bridging the two coils. The operating
LIGHT PULSE
SAMPLE RECEIVING COIL
TRANSMITTING COIL
METAL SHIELD
OSCILLATOR 500 MHZ
AMPLIFIER
PHASE SHIFTER
AMPLIFIER
AMPLIFIER
MIXER
V (mV)
TRANSIENT DIGITIZER
SPLITTER
t
IN
Fig. 1. Schematic diagram of the TMPCD apparatus.
R.K. Ahrenkiel, D.J. Dunlavy / Solar Energy Materials & Solar Cells 95 (2011) 1985–1989
1987
1
12°
SAMPLE
0.9
45° Transmission
0.8
0.7
0.6
0.5
TRANSMITTER
SHIELD
RECEIVER
Fig. 2. Ray tracing analysis of TMPCD operation. This neglects interference and diffraction effects.
frequency in these experiments was set at about 500 MHz. The peak response of the passive, receiving circuit is tuned to the transmitter frequency by means of a series capacitor. There is virtually no signal in the receiving circuit unless a sample bridges the two regions. The sample acts as a transmission line or waveguide to conduct the electromagnetic wave (EMW) to the receiving antenna. Fig. 2 shows a simple schematic of a classical EMW wave that is incident at an angle on the semiconductor surface. Here, we assume that the EMW undergoes a sequence of transmission and reflection events at the sample surfaces, using the Fresnel equations and classical optical ray tracing. The signal detected by the receiving antenna is amplified by about 40 dB with the two amplifier stages. The received signal is a function of the conductivity of the sample and changes with the photoconductivity. Thus, the transmission of EMWs from the transmitter to the receiver is modulated by the conductivity of the sample. With a short pulse light source incident on the sample, the receiver signal varies with the transient photoconductivity. Under pulsed optical excitation, the amplified signal is received and directed to one input of a mixer in order to extract the dc transient component. A portion of the signal from the oscillator is passed through a phase shifter and input to the second terminal of the mixer. When the two inputs are in phase, the mixer output produces a signal of 2f and a dc-transient component that decays with the injected carrier population. Digitizing, analyzing, and storing the transient photoconductive response contains the carrier lifetime data. The stored data allows the extraction of the carrier lifetime information. The lifetime is quantified by fitting the slope of the semi log plot of the rectified receiver output voltage versus time. Here, we are assuming that the mobility is independent of excess carrier density. When that is not the case, the data can be processed with an algorithm that corrects for the mobility variation.
2.2. Theory of TMPCD operation Fig. 1 shows the schematic representation of our current breadboard apparatus that has been used to make the measurements shown here. The sample bridges the transmitting and receiving coils directing the electromagnetic wave (EMW) from the transmitter to the receiver. A portion of the transmitted wave penetrates the sample and is ‘‘trapped’’ by total internal reflection. As such, the wave travels along the lateral direction of the sample, and a portion of that wave radiates from the bottom surface to the receiving antenna.
0.4 10-3
10-2
10-1 σ (ohm-cm)-1
100
Fig. 3. The calculated transmission coefficient of a 600 mm thick silicon wafer at 500 MHz at normal incidence.
In this analysis, we have ignored interference effects on the transmission of the EMW through the sample. The diffraction effects occur because the EM wavelength is much longer ( 60 cm) than the mechanical dimensions of the sample and measurement system. A more detailed analysis will occur in the near future and will consider these effects. The absorption coefficient of an EMW in a semiconducting medium has been calculated [11]. The absorption coefficient of the EMW is given by vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi !ffi u ume s2 1þ 2 2 1 a ¼ ot 2 o e where a is the attenuation coefficient of the electric vector in the medium and s the conductivity E ¼ E0 expðax=2Þ exp½iðotbxÞ When the conductivity, s, is increased by photon absorption, the E-vector component of the EMW is attenuated. At the frequency used here ( 500 MHz), the attenuation factor is fairly small, but the larger path produced by wave trapping increases the interaction length. This longer path length provides the larger signals that we observe in our breadboard apparatus. Fig. 3 shows a calculation of the 500 MHz-EMW transmission of a sample as a function of sample conductivity. The calculation is simply derived using Maxwell’s equations. The planar 500 MHz wave is assumed to be normally incident to the semiconductor surface. The optical path length is assumed to be 600 mm and the dielectric constant of silicon was used in the calculation. The transmission coefficient drops steeply when the bulk conductivity becomes larger than about 0.01 (O-cm) 1. In future work, we will extend this simple theory to include diffraction and interference effects. This extension will more accurately describe the interaction of the transient conductivity with the EMW. 2.3. Carrier lifetime measured by TMPCD The response time of TMPCD is considerably faster than that of RCPCD. We are currently limited in our ability to measure the exact risetime as we do not have a laser in the system that has a sufficiently short pulse width. The risetime difference is shown in Fig. 4 where the initial photoconductive response of a bare silicon wafer is measured by both systems with the same incident flux
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t (µs) 0 Fig. 4. The rise time response data of TMPCD and RCPCD using the same pulsed optical light source and pulse intensity. The TMPCD amplitude is somewhat larger but both signals are normalized to unity. The excitation pulse width is about 6 ns full-width half maximum. The sample is a bare silicon wafer.
103 117 μs TMPCD RCPCD
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t (μs) Fig. 5. The transient photoconductive decay of a Si3N4-passivated float zone wafer measured by both TMPCD and RCPCD. The photoresponse was produced by a YAG laser operating at 532 nm. The incident pulse intensity was set at the same value for both measurements.
intensity. The signal here has been normalized to unity. The TMPCD pulse rise is very close to the integrated time dependence of the excitation pulse, which has a full width half-maximum of about 6 ns. The RCPCD pulse rise time is much longer and is in the range of 30 or 40 ns. Fig. 5 shows data that were measured by both RCPCD and TMPCD using the same pulsed light source (a mini-YAG laser) that is operated in the frequency-doubled (532 nm) mode. As stated earlier, the pulse width of the laser pulse is about 6 ns full width half maximum. The excitation intensity is adjusted to be about 5 mJ/cm2 and was identical for each measurement. The sample is a float zone grown silicon wafer that has been passivated with Si3N4. The two-component decay, seen in both curves, is typical of a silicon sample pulsed into high injection. At high injection, the deep impurity levels, that dominate recombination, become partially filled with minority carriers. Therefore, the lifetime at
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Fig. 6. Comparison of TMPCD and RCPCD data on a 4 mm-thick InGaAs film grown on an InP substrate. The InGaAs has a passivating epitaxial layer at each surface. The excitation source is a YAG laser operating at 532 nm.
high injection is larger than at low injection. Here the values are about 117 and 66 ms, for the high- and low-injection lifetimes, respectively. The data shows a slightly steeper curve for the TMPCD curve at t ¼0. The high-injection RCPCD lifetime is 119 ms whereas the low-injection lifetime is 74 ms. A reason for the small difference in low-injection lifetime is not known until this time. The TMPCD data shows a steeper drop in the initial (t ¼0) decay time than does the RCPCD data. This steep feature at t ¼0 is indicative of surface recombination and has been analyzed in the recent publication [12]. The high sensitivity of RCPCD is related to the very high quality factor (Q) of the coupling circuit, whereas, the TMPCD does not require a high Q. Circuit theory shows that as the Q increases, the system response time increases. As the detection system physics is different here, we are able to measure much shorter lifetimes by means of TMPCD. The ability to extend the measurement to thin films of silicon, compound semiconductors, and nanostructures may provide a significant advantage over other techniques. The technique has been successfully applied to thin films grown on substrates. Fig. 6 shows TMPCD data measured on a thin ( 4 mm) epitaxial film of InGaAs grown on a semi-insulating InP substrate. The active layer was grown between confinement/ passivating quaternary III–V epi-layers. The TMPCD data show a shorter lifetime at high injection than does the RCPCD data. At low injection, the lifetimes agree quite closely and within experimental error (+ / 5%). This difference will be explored by further experiments and the development of the measurement theory. These data again show that TMPCD has sensitivity equal to or exceeding RCPCD and in addition TMPCD has a much faster response time. The risetime differences are clear from the pulse appearance at t ¼0. The data of Fig. 7 shows TMPCD measurements of two thin films (samples A and B) of nano-Si/a-Si that was grown by CVD. The ratio of the nano-Si/a-Si components is 50:50. As the carrier mobility of a-Si is much less than unity, one suspects that nano-Si dominates the photoconductive transport shown in these data. We are limited in measuring the carrier lifetime by the finite pulse width of our excitation laser. The carrier lifetime is very comparable to the laser pulse width, and can be analyzed by the deconvolution process as being in the 10 ns range.
R.K. Ahrenkiel, D.J. Dunlavy / Solar Energy Materials & Solar Cells 95 (2011) 1985–1989
102 Sample A Sample B
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Fig. 7. TMPCD data measured on two films (sample A and B) of nano-Si/a-Si using an excitation wavelength of 532 nm. The laser pulse has a full-width halfmaximum of about 6 ns. The longer decay time of the response is a convolution of the finite laser pulse width and the sample recombination lifetime. Both events are in the nanosecond range.
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alloy ZnxCd1 xSySe1 y [14] nanowires with composition (bandgap) continuously graded from ZnS (3.5 eV) to CdSe (1.74 eV). The light emission covers the entire visible spectrum by the bandgap emission on a single substrate that is 2 cm long. We measured one of these nanowire composites that was imbedded in a polymer and coated on a glass substrate. These data are shown in Fig. 8, and show a complex decay process. The latter is likely the result of the percolation transport that occurs in these films. The TMPCD technique was shown to measure carrier transport and recombination in these polymer imbedded structures and demonstrated high sensitivity for such measurements. Our technology will expedite materials research in this field to progress more rapidly as contacting is not required to produce the lifetime data. In summary, these results indicate that TMPCD has highly promising characteristics that will be useful for future research in a variety of materials systems. The developed technique could provide a significant advantage to many applications in that the measurement time is several seconds. Also, the increased sensitivity and faster response time extends the TMPCD technique to thin film and low-dimensional materials with intrinsically shorter lifetimes. The measurement is very rapid and requires very minimal tuning and adjustment. Quality measurements can be made in several seconds in most cases, depending on the repetition rate of the laser. The latter is significant in future materials research.
Acknowledgements 9.6 ns
The authors would like to thank Prof. C.Z. Ning for providing the II–VI nanowires used in these studies. 101 V (mV)
References 14.7 ns
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t (μs) Fig. 8. The TMPCD response of a ZnxCd1 xSySe1 y nanowire array that was encapsulated in a polymer on a glass substrate. The data were obtained using 532 nm pulsed excitation.
Measurements of the same film, with the same laser excitation source, produced null results using our RCPCD setup. With our microwave reflection system operating at 20 GHz, we could see a signal of several mv amplitude, but the signal was quite weak. For nanocrystalline silicon, we concluded that the sensitivity of TMPCD is superior to the other techniques used in our laboratory. The measurement of carrier lifetime in nanotubes and nanowires is critical for the evaluation of future generation PV materials. We have used TMPCD to measure the carrier lifetime in II–VI compound nanowires that contain a large bandgap gradient. The group of Pan and coworkers developed an alloy nanowire grading technique [13]. They demonstrated the first quaternary
[1] M. Kunst, G. Beck, The study of charge carrier kinetics in semiconductors by microwave conductivity measurements, J. Appl. Phys. 60 (1986) 3558–3566. [2] G. Beck, M. Kunst, Contactless scanner for photoactive materials using laserinduced microwave absorption, Rev. Sci. Instrum. 57 (1988) 197–201. [3] R.K. Ahrenkiel, S.W. Johnston, An advanced technique for measuring minority-carrier parameters and defect properties of semiconductors, Mater. Sci. Eng. B102 (2003) 161–172. [4] R.A. Sinton, A. Cuevas, Contactless determination of current–voltage characteristics and minority-carrier lifetimes in semiconductors from quasisteady-state photoconductance data, Appl. Phys. Lett. 69 (1996) 2510–2512. [5] W.K. Metzger, D. Albin, D. Levi, P. Sheldon, X. Li, B.M. Keyes, R.K. Ahrenkiel, Time-resolved photoluminescence studies of CdTe solar cells, J. Appl. Phys. 94 (2003) 3549–3555. [6] R.K. Ahrenkiel, in Minority Carriers in III–V Semiconductors: Physics and Applications, SEMICONDUCTORS AND SEMIMETALS-V39, Willardson Beer series; Academic Press, 1993, pp. 39–150. [7] U.S. Patent be 5,929,652, Apparatus for measuring minority carrier lifetimes in semiconductor materials, R.K. Ahrenkiel, 1999. [8] U.S. Patent 6,275,060 Richard Ahrenkiel and Steven Johnston, Apparatus and method for measuring minority carrier lifetimes in semiconductor materials, August 14, 2001. [9] U.S. Patent 6,369,603, Steven Johnston and Richard Ahrenkiel, April 9, 2002. [10] R.K. Ahrenkiel, S.W. Johnston, Lifetime analysis of silicon solar cells by microwave reflection, Sol. Energy Mater. Sol. Cells 92 (2008) 830–835. [11] R.K. Ahrenkiel, S.W. Johnston, The interaction of microwaves with photoelectrons in semiconductors, J. Vac. Sci. Technol. B 26 (2008) 1508–1515. [12] R.K. Ahrenkiel, S.W. Johnston, An optical technique for measuring surface recombination velocity, Sol. Energy Mater. Sol. Cells 93 (2009) 645–649. [13] A. Pan, W. Zhou, E. Leong, A. Chin, R. Liu, B. Zou, C.Z. Ning, Continuous alloycomposition spatial grading and superbroad wavelength-tunable nanowire lasers on a single chip, Nano Letters 9 (2009) 784–788. [14] A.L. Pan, R.B. Liu, M.H. Sun, C.Z. Ning, Quaternary alloy semiconductor nanobelts with bandgap spanning the entire visible spectrum, J. Am. Chem. Soc. (Communication) 131 (2009) 9502–9503, doi:10.1021/ja904137m.