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phys. stat. sol. (c) 6, No. 1, 323–326 (2009) / DOI 10.1002/pssc.200879896

© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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caphys. stat. sol. (c) 6, No. 1, 323–326 (2009) / DOI 10.1002/pssc.200879896

Time-resolved absorption and luminescence following electron-hole pair creation in ZnO

R. Andrew Wall1, Kyle C. Lipke1, K. B. Ucer1, R. T. Williams*, 1, D. Millers2, K. Smits2, and L. Grigorjeva2

1 Department of Physics, Wake Forest University, Winston-Salem, NC 27109, USA 2 Institute of Solid State Physics, University of Latvia, 8 Kengaraga St., 1063 Riga, Latvia

Received 15 June 2008, revised 26 June 2008, accepted 26 June 2008

Published online 21 October 2008

PACS 71.35.Cc, 78.47.jc, 82.53.Hn

* Corresponding author: e-mail williams@wfu.edu, Phone: +1 336 758 5132, Fax: +1 336 758 6142

© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Ucer et al. [1] have reported optical absorption on the 2–100 ps time scale induced by band-gap excitation of undoped, single crystal ZnO at T = 17 K. A significant part of the absorption signal could be fit with a decay time of 50 ps, which corresponds to the radiative de-cay time of the neutral donor bound exciton (DoX) previ-ously measured in the same ZnO sample material in our laboratory [2]. Given the dominance of DoX luminescence in the low-temperature spectrum of pure ZnO, the 50-ps component of absorption at low temperature was attributed to the population of neutral donor bound excitons. This report deals primarily with room temperature measurements of transient absorption induced in undoped, single crystal ZnO by two complementary experimental methods: The first is the sub-ps pulsed laser method used in Ref. [1], covering 2–300 picoseconds. The second measures lamp transmission following electron pulse irra-diation of the sample, covering the time range 8–300 nano-

seconds. Based on the spectrum observed in both experi-ments, we suggest that the absorption signal is proportional to the population of shallow trapped carriers produced by band-gap excitation of ZnO. In a highly defect-free mate-rial, pulsed band-gap excitation would populate a reservoir of free-carriers and excitons from which radiative recom-bination luminescence would be observed. In a realistic material, shallow trapped carriers also contribute to the reservoir of excitations, often as the main contributor. From the point of view of scintillator and excitonic light emission applications of ZnO, this “reservoir” of possibly interconverting forms of stored electronic energy repre-sents the maximum amount of energy one could possibly extract in a scintillation pulse or as illumination. In fast scintillator applications of ZnO, one hopes to extract as much excitonic recombination as possible in as short a time as possible. ZnO is attractive for this because its room-temperature excitonic luminescence (actually a pho-

We report transient absorption induced by electron-hole exci-

tation in undoped ZnO. A laser pump/continuum probe

method covers 2–300 ps, and an electron pulse with lamp

transmission covers 8–300 ns. The broad absorption spectrum

increases monotonically with wavelength from 900 to 1600

nm. Following a reasonable hypothesis that the free-carrier-

like induced infrared absorption is proportional to the total

number of free carriers, excitons, and shallow-trapped carri-

ers in the sample, these data allow setting an upper limit on

the quantum efficiency of a specified lifetime component of

luminescence. For the undoped commercial ZnO studied in

this report, the quantum efficiency of room temperature exci-

tonic luminescence is less than 5%. This means that there is

significant room for improvement in applications aiming to

use room-temperature excitonic blue luminescence of ZnO

for fast scintillators and light sources. Direct observation that

a large majority of excitations are tied up for more than 100

nanoseconds in shallow traps confirms the premise for studies

undertaken at Oak Ridge National Laboratory to improve the

excitonic luminescence yield and decay rate of ZnO by donor

doping. The preliminary results presented here on undoped

ZnO suggest that induced absorption measurements should be

a useful diagnostic of quantum efficiency while studying such

dopant effects.

324 R. A. Wall et al.: Time-resolved absorption and luminescence following e–h pair creation in ZnO

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non replica of the free exciton) has a decay time constant on the order of a few hundred picoseconds [3]. Measure-ments of relative or absolute quantum yield of this particu-lar spectral and lifetime component are of obvious interest in trying to optimize ZnO for fast scintillator applications [4]. The present measurements as discussed below provide a way of measuring quantum yield of a specific lifetime component. In our opinion, the results drive home in a vivid fashion the conclusion that very little of the excita-tion energy imparted to commercial undoped ZnO comes out in the desired excitonic component. Studies have indi-cated that it may be possible to increase the yield and de-cay rate of excitonic fluorescence by doping crystals with donor impurities [4]. A systematic investigation of this strategy has been undertaken by Neal et al. [5] at Oak Ridge National Laboratory, including picosecond time-resolved luminescence measured with the experimental set-up used in this report. 2 Experimental details The 300 fs pulsed-laser method of measuring transient absorption on the 2–300 ps time scale has been described in Ref. [1]. Repeating only a brief summary, optical absorption induced by 2-photon ex-citation across the band gap of ZnO is probed after a vari-able time delay by a continuum infrared pulse covering the range 900 nm to 1600 nm. To access the longer time range from 8 to 300 nanoseconds, companion measurements of electron pulse induced transient absorption were conducted at the Institute of Solid State Physics, University of Latvia. Details of the set-up have been described previously by Grigorjeva, Millers et al. [6], and will be summarized only briefly here. A pulse of 270 keV electrons, ~ 8 ns duration, penetrates one flat face of the sample to a small depth. To increase optical path length through the irradiated layer, the probe light is focused into one polished edge of the sample, and is incident on the irradiated flat face at a low angle. The light exits the opposite polished edge of the sample and is imaged onto the entrance slit of the spec-trometer. The light source is a xenon flash lamp. Data are acquired with a photomultiplier and digital storage oscillo-scope. The undoped ZnO sample used in both experiments was a c-axis oriented epitaxy-substrate wafer from Eagle Picher Co. Its thickness was 0.5 mm.

3 Results and discussion Figure 1 plots the

change in transmitted signal following the excitation pulse

divided by the transmitted signal immediately preceding

the pulse for a succession of probe wavelengths as labeled

on each trace. All data in Fig. 1 were measured at 293 K

(room temperature). Since the change in transmittance due

to creation of absorbing states is negative, the data in Fig. 1

start from ΔT/T0 = 0 in the upper left just before the laser

pulse and decrease toward -1, which would correspond to

induced opacity. Because of the desire to show data for a

large number of wavelengths by stacking the offset traces,

the level of maximum opacity for each trace is not shown.

Under the reasonable assumption that the 2-photon excita-

time (ps)

0 50 100 150 200 250

ΔT

/T0 (

no

rm.)

time (ps)

0 50 100 150 200 250

time (ps)

0 50 100 150 200 250

900 nm

1050 nm

1000 nm

950 nm

1150 nm

1100 nm

1250 nm

1200 nm

1300 nm

1350 nm

1600 nm

1500 nm

1450 nm

1400 nm

1550 nm

Figure 1 Change in transmittance induced by a 300-fs band-gap

excitation pulse in undoped ZnO at room temperature. Each trace

shows transmittance at a different wavelength as labelled in the

range 900-1600 nm. The traces have been offset vertically for

display.

tion density is approximately constant throughout the d = 0.5 mm thickness of the sample, the induced absorption coefficient, α, may be expressed in terms of

0

1ln 1

T

d T

Δα

⎛ ⎞= − +⎜ ⎟

⎝ ⎠

The common features of the transmission traces are that there is a spike of decreased probe transmission in the first 2 ps, and thereafter the absorption remains approxi-mately unchanged over the 300 ps time range that can be measured with our delay line. The initial spike of probe at-tenuation has been discussed in Ref. [1], where it was at-tributed to the phenomenon of 2-photon absorption across the band gap by one pump photon plus one probe photon, which is not of interest. The persistent absorption thereaf-ter is not completely flat, sometimes rising a bit versus time, and sometimes falling a bit. After repetitions of the measurement, we concluded that this variability was due to long-term drifts in the laser energy and/or pulse width over the 30-minute runs required to scan 300 ps. On average, the runs showed that the absorption induced at room tem-perature changes little between 5 ps and 300 ps at each of the wavelengths sampled in Fig. 1. The sample does not become permanently darkened, so we must presume that the transmission recovers on some time scale. That time scale was found to be in the few hundred nanosecond range using electron pulse experiments described below. From the data in Fig. 1, absorption spectra correspond-ing to ~ 10 ps after excitation are plotted in Fig. 2. The dominant trend of the spectra is increasing absorption ver-sus wavelength over the whole measured infrared range.

phys. stat. sol. (c) 6, No. 1 (2009) 325

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wavelength (nm)

1000 1200 1400 1600

Δα

(c

m-1

)

0

2

4

6

8

10

Figure 2 Spectrum of absorption induced 10 ps after band-gap

excitation of undoped ZnO by a 300-fs laser pulse at T = 293 K.

From decay curves at these and other wavelengths, the spectrum of induced absorption has been constructed. The spectrum of absorption measured 10 ns after excitation is plotted in Fig. 4 versus photon energy. Just as in Fig. 2, the absorption rises monotonically with decreasing photon en-ergy (longer wavelength). Figure 3 shows the production and decay of optical ab-sorption at 1.1 eV (1130 nm) and 1.6 eV (775 nm) follow-

time (ns)

-100 0 100 200 300

O.D

.

0.0

0.2

0.4

0.6

0.8

1.01.1 eV

1.6 eV

Figure 3 Decay of absorption at 1.1 eV (1130 nm) and 1.6 eV

(775 nm) following 8 ns electron pulse excitation at T = 293 K.

energy (eV)

0.8 0.9 1.0 1.1 1.2 1.3 1.4

Δα

(cm

-1

)

0

2

4

6

8

10

12

14

16

18

Figure 4 Transient absorption spectrum 10 ns after band-gap ex-

citation by a pulse of high-energy electrons at room temperature.

ing an electron pulse. The time resolution is approximately 8 ns and the time scale extends out to 300 ns. The decay is approximately exponential with the time constant of 120 ns. In Ref. [1] the excitation-induced IR absorption in the same ZnO at 17 K was seen to have nearly the same broad band shape increasing monotonically with wavelength over the photon energy range 1.3 to 0.8 eV. It was shown on the basis of lifetime analysis that the neutral donor bound exci-ton (DoX) accounts for about 38% of the IR absorption at T = 17 K. In other words, the IR absorption spectrum shown to be attributable to donor bound excitons looks approxi-mately like the band shape of free carrier absorption. It was argued in Ref. [1] that such a conclusion is reasonable when the optical transition energy so greatly exceeds the binding energy of the exciton. In such a circumstance, the spectral band shape is primarily determined by the density of final free-carrier states that are the photo-ionization break-up products of the highly-excited exciton (or by ex-tension, shallow-trapped carrier). Participation of a phonon greatly expands the number of momentum- and energy-conserving final states and so should control the spectral band shape more or less as in free-carrier absorption. Thus we hypothesize that the induced IR absorption we observe is due to the collection of all excitons, shallow-trapped carriers, and free carriers induced by the excitation pulse. Within this hypothesis, the transient absorption gives us a measure of the time history of the whole collec-tion of shallow trapped carriers, not just those that result in luminescence. An extension of this interpretation is that the fractional change in IR absorption attributable to a given decay time component gives a measure of the absolute quantum yield for luminescence with the corresponding decay time, assuming the luminescence decay is primarily radiative and that the number of deep traps filled is small. If not, the yield deduced represents an upper limit to the quantum efficiency. Combining the results of both the laser and electron-pulse experiments, we have a vision of the room-temperature decay of excitation-induced absorption ex-tending more than 5 decades from 2 ps to 300 ns, except for a gap from 0.3 to 8 ns that falls between presently ac-cessible methods for the two experimental methods. As-suming that there is no big decay component in the missing decade, we conclude that at room temperature in undoped ZnO, the free and shallow-trapped carrier population un-dergoes negligible depletion until approximately 120 ns af-ter excitation. We know that there must be a depletion of some of the carriers associated in excitons because the phonon-replica of free exciton luminescence at 3.26 eV decays at room temperature with the shortest two time con-stants being of order 0.44 ns and 5 ns. Because the inten-sity of a luminescence transition is proportional to the rate of radiative decay of the upper level population, the short-est time constant is typically dominant in terms of instan-taneous intensity. Many of the fast scintillator and light emitting applications considered for ZnO are attractive be-cause of the sub-nanosecond or few-nanosecond time con-

326 R. A. Wall et al.: Time-resolved absorption and luminescence following e–h pair creation in ZnO

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stant of its blue excitonic transitions. They are indeed at-tractively fast. But because the population contributing ra-diatively to the transition is proportional to the time inte-gral of the luminescence, a longer-lifetime, weaker com-ponent can often represent the larger excited-state popula-tion as seen in absorption measurements. The comparison of luminescence and induced absorption in the present study indicates that the excited-state depletion associated with the observed room-temperature blue luminescence occurring on a sub-nanosecond time scale is only a small fraction of the total available excited-state population. Most of the excited-state population monitored by the “free-carrier-like” IR absorption, likely due to shallow trapped carriers resembling free carrier spectra in the sam-pled range, decays in 120 ns. This is too slow for the corre-sponding luminescence to be interesting for many fast emitter applications. This luminescence of the long-trapped carriers could be in the visible defect bands, or recombina-tion could be non-radiative. The conclusion we draw from our hypothesis that the broad infrared absorption in ZnO represents a monitor of the population of all free and shallow-trapped carriers [1] is that the absolute quantum efficiency of the subnanosec-ond blue excitonic luminescence at room temperature is very low in undoped single-crystal ZnO typified by the Eagle-Picher material. An estimate of the maximum quan-tum efficiency in this hypothesis would correspond to the maximum fraction of induced absorption that can be as-signed a decay time corresponding to the 440 ps short time constant of phonon-assisted free exciton decay measured in the same crystal at room temperature. Fitting our data to a sum of exponentials with decay times of 8 ps, 440 ps and 120 ns, we find that the 440 ps component is 5% of the overall signal. According to our model this would repre-sent the upper limit on the quantum efficiency of the blue transition in this Eagle-Picher material. The first implica-tion is that there is a lot of room for improvement of mate-rial response. One may try to remedy this by going to no-ticeably more perfect ZnO or conversely by optimally dop-ing, e.g. with donor defects [see below]. The conclusion based on our hypothesis about the origin of the IR absorp-tion should ideally be checked by directly measuring the absolute quantum efficiency of the fast blue luminescence. This is in fact a difficult task to do accurately. In the mean-time the transient absorption may give a more trustworthy measure of an upper limit on the quantum efficiency of fast excitonic transitions with an identifiable lifetime. As an example, Ref. [1] succeeded by this method in assigning a quantum efficiency of 38% to the neutral donor bound ex-citon luminescence transition at T = 17 K. Such a value is plausible because the DoX luminescence is typically very strong and fast in undoped ZnO at low temperature. The low quantum efficiency of the blue excitonic room-temperature luminescence in undoped ZnO implied by the results of this work relates directly to the ongoing efforts of some of the present authors and others [5] to op-timize simultaneously the quantum efficiency and short

time response of room-temperature excitonic luminescence in ZnO by introducing dopants. Due to space limitations we must refer the reader to Ref. [5] for discussion of the application and detailed influence of doping on the yield and lifetime. However in the context of the present discus-sion, it is worth pointing out that the conclusion of this pa-per implying poor quantum yield of excitonic lumines-cence in ordinary undoped ZnO supports a viewpoint on scintillator design described to us by Derenzo [7]. The general concept is to focus on one carrier, e.g. holes, pro-duced in the initial excitation of e-h pairs. By doping, one hopes to make sure that an electron can find each hole in a short time to undergo recombination, rather than allow the holes (and scant supply of electrons) time to encounter traps and non-radiative recombination sites. Even if elec-trons and holes finally recombine radiatively after resi-dence on traps, it is likely to be beyond the time window of useful response, particularly for applications like modern fast scintillators where the time window could be sub-nanosecond. Getting rid of the traps is one fruitful way to get more prompt excitonic response. Another productive approach investigated by Derenzo et al. [4] and now Neal et al. [5] is to dope the material rather heavily with donors to sustain a constant supply of conduction electrons. In principle, one of the numerous conduction electrons should dive in almost immediately on each hole produced in the excitation event. Ref. [5] has investigated the effect on relative yield and on luminescence lifetime, as well as find-ing a limitation to the trend at high doping levels, which we attribute to onset of Auger decay. We are keen on including excited-state absorption measurements on the same doped sample series, to see how the excess free carriers (and the donors as traps, by the way) influence the evolution of free and shallow-trapped excitons and carriers, controlling the yield and lifetime of fast luminescence response.

Acknowledgements We would like to thank John S. Neal

and Lynn A. Boatner for helpful discussions.

References

[1] K. Burak Ucer, R. Andrew Wall, Kyle. C. Lipke, and R. T.

Williams, to be published in: Proceedings of the International

Conference on Excitonic Processes in Condensed Matter,

Kyoto, Japan, 2008.

[2] J. Wilkinson, K. B. Ucer, and R. T. Williams, Radiat. Meas.

38, 501 (2004).

[3] J. Wilkinson, K. B. Ucer, and R. T. Williams, Nucl. Instrum.

Methods A 537, 66 (2005).

[4] S. E. Derenzo, M. J. Weber, and M. K. Klintenberg, Nucl. In-

strum. Methods Phys. Res. A 486, 214 (2002).

[5] J. S. Neal, N. C. Giles, X. Yang, R. Andrew Wall, K. B. Ucer,

R. T. Williams, D. J. Wisniewski, L. A. Boatner, V. Rengara-

jan, J. Nause, and Bill Nemeth, IEEE Trans. Nucl. Sci. 55,

1397 (2008).

[6] L. Grigorjeva, D. Millers, E. A. Kotomin, and E. S. Polzik,

Solid State Commun. 104, 327 (1997).

[7] S. E. Derenzo, private communication.