Variations in Decay Rate of Green Photoluminescence in ZnO under

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Variations in Decay Rate of Green Photoluminescence in ZnO under Above- and Below-Band-Gap Excitation Kanako Kodama, and Takashi Uchino J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp506953h • Publication Date (Web): 25 Sep 2014 Downloaded from http://pubs.acs.org on September 30, 2014

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The Journal of Physical Chemistry

Variations in Decay Rate of Green Photoluminescence in ZnO under Above- and Below-Band-Gap Excitation

Kanako Kodama and Takashi Uchino* Department of Chemistry, Graduate School of Science, Kobe University, Nada, Kobe 657-8501, Japan

Abstract The mechanism of the commonly observed green photoluminescence (PL) emission in ZnO is still controversial and is being actively discussed. Herein, through comprehensive time-resolved and excitation-energy-dependent PL measurements on variously annealed ZnO samples, we show that the PL decay profile, which is well fitted to a triple-exponential function, depends not only on temperature but also on excitation energy. Under excitation with photons near the band-gap energy, the decay profile is governed by three decay components with decay times of ~2, ~10, and ~50 µs. The contribution from the slower ~10− and ~50−µs decay components become dominant when the photoexcitation energy is ~100 meV below the band gap energy of bulk ZnO. It has also been revealed the highest green PL intensity is achieved under excitation of

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photons with energies ~100 meV below the band gap energy of bulk ZnO as well. These PL features are interpreted in terms of the formation of sub-band-gap states, band edge perturbation, and the related thermally activated trapping/detrapping processes, which can simultaneously increase the intensity and decay time of the green PL emission.

Keywords:, zinc oxides, thermal excitation, sub-band-gap states, time-resolved photoluminescence spectroscopy

1. INTRODUCTION Luminescence in the green region of spectrum in ZnO, which is a wide band-gap semiconductor with a band-gap energy of 3.37 eV at room temperature, has attracted considerable attention in the field of display devices, such as vacuum fluorescent displays and field emission displays.1 It has been well documented that the green photoluminescence (PL) emission with a peak around 2.5 eV is substantially enhanced when ZnO is annealed in a reducing atmosphere.2-6 Thus, the green PL emission is considered to result from a certain defect center, including oxygen vacancies, zinc interstitials and/or other related point defects.7-9 In spite of the extensive research that has been carried out during the past decades, however, the origin of the green luminescence center is not completely understood, and its emission mechanism is still a matter of debate.4-14 Difficulties for the understanding of the green emission in ZnO come principally


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from its complicated luminescence behaviors. For example, the green PL band does not show a simple thermal quenching behavior but often shows an intensity maximum at T ~ 200−~250 K or a negative thermal quenching behavior.15-18 A below-gand-gap photoluminescence excitation (PLE) response at ~3.3 eV is developed with increasing temperature.19-22 Furthermore, the entire PL decay profiles tend to become slower with an increase in temperature18,23 although the opposite tendency is normally observed because of the dominance of the thermally activated nonradiative decay process. Previous electron magnetic resonance (EPR) and optically detected magnetic resonance (ODMR) measurements have demonstrated that the shallow donor signal is likely to be transferred to green emission by a shunt process or a spin-dependent energy transfer process.24-26 All these results implies the existence of certain thermally activated emission routes, which complicate the entire emission process upon photoexcitation. To shed light on these complex PL behaviors we have recently carried out a systematic research on the temperature dependence of PL and PLE characteristics of the green emission band in ZnO.27 We have proposed in Ref. 27 that two competing emission processes coexist in ZnO: one is a direct (or normal) emission process (process I) and the other is a thermally assisted luminescence process (process II), which begins to dominate over the direct luminescence process especially under below-band-gap excitation and at temperatures above ~150 K. In process II, the below-band-gap excitation is followed by the thermally assisted process to promote the photoexcited electrons in the sub-band-gap states into the conduction band. The resulting electrons in



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the conduction band will eventually be relaxed to the emitting state, contributing to the green PL emission in ZnO. In this work, we will further explore the green PL emission especially via process II in ZnO in terms of the variations of the decay rates under excitation with above and below band-gap laser light. The temperature-dependent time-resolved PL measurements of variously annealed ZnO powders are carried out using excitation energies ranging from ~3.1 eV (below-band-gap light) to ~3.5 eV (above-band-gap light). We have found that the decay dynamics are highly influenced by excitation energy in a similar manner as the PL intensity. The observed excitation energy dependence on the decay profiles has found to be consistent with the proposed thermally activated emission model relevant to the sub-band-gap states.

2. EXPERIMENTAL SECTION We used high purity polycrystalline ZnO (Kojundo Chemical Laboratory, 99.999%, average particle size ~200 nm) as a starting material. Since the green PL emission of the as-received ZnO sample is very weak, we annealed the sample powder in a reducing atmosphere (5%H2-95%Ar mixed flowing gas) at temperatures ranging from 700 to 1100 °C for 1 h. Powder x-ray diffraction (XRD) patterns of differently annealed samples were obtained with a diffractometer (Rigaku, SmartLab) using Cu Ka radiation. Scanning electron microscopy (SEM) was conducted with a scanning electron microscope (JEOL,


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JSM-5610LVS). Ultraviolet-visible (UV-vis) diffuse reflectance spectra were collected on a UV-vis spectrophotometer (Hitachi, U3500) fitted with an integrating sphere. The resulting reflectance spectra (R) were transformed into apparent absorption spectra using the Kubelka–Munk function F(R):28

=

1 − 1 2

Steady-state PL and PLE measurements were performed using a monochromated Xe lamp (150 W) with a spectrofluorometer (JSSCO, FP-6600). Time-resolved PL measurements were carried out using a mode lock Ti:sapphire laser (Spectra Physics, Millennia-pumped Tsunami) with a 130 fs pulse duration operating at 80 MHz. The laser output was sent through a frequency doubler and pulse selector (Spectra Physics, Model 3980) to obtain excitations tunable between 350–400 nm (3.54–3.10 eV) at 10 kHz. PL emission is collected at a 90° geometry and detected with a monochromator with a 150 lines/mm grating and a synchroscan streak camera (Hamamatsu, C4334). During the steady-state and time-resolved PL measurements, the sample temperature was controlled in a closed-cycle N2 cryostat in the temperature region from 77 to 350 K.

3. RESULTS 3.1. XRD patterns and SEM images. Figure 1 shows the XRD patterns of the ZnO samples annealed at different temperatures in a reducing H2/Ar atmosphere along 5 ACS Paragon Plus Environment


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with that of the as-purchased sample. We do not see any changes in the XRD patterns after annealing, confirming that all the samples used in this work are characterized by a single phase wurtzite structure. As shown in Table SI in the Supporting Information, the refined lattice parameters of the respective ZnO samples are virtually unchanged irrespective of the annealing temperature. As for the SEM images of the samples, however, one sees a noticeable change in particle morphology depending on the annealing condition (see Figure 2). An appreciable increase in diameter of the particles due to sintering is observed with an increase in annealing temperature. For example, the particles sizes of the samples annealed at 700 and 1100 °C are ~0.5 µm and ~3-~5 µm, respectively. It is hence probable that the higher the annealing temperature, the less the surface-related properties become evident.

Figure 1. XRD patterns of the as-purchased and the differently annealed ZnO samples. Annealing was carried out in a reducing H2/Ar atmosphere at the designated temperature for 1 h.


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Figure 2. SEM images of (a) the as-purchased and (b)-(f) the differently annealed samples. Annealing was carried out at temperatures of (b) 700, (c) 800, (d) 900, (e) 1000, and (f) 1100 °C for 1 h in a reducing H2/Ar atmosphere.

3.2. Kubelka–Munk Spectra. Room temperature UV-vis Kubelka-Munk spectra of the as-purchased and differently annealed ZnO samples are shown in Figure 3. For all

Figure 3. Room temperature Kubelka-Munk spectra of the as-purchased and the differently annealed samples. Annealing was carried out in a reducing H2/Ar atmosphere at the designated temperature for 1 h. 7 ACS Paragon Plus Environment


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the samples investigated, one see a sudden decrease in the Kubelka-Munk function F(R) at 3.30−3.35 eV, corresponding to the band-gap energy in ZnO at room temperature (3.37 eV). As for the annealed samples, one also notices a substantial modification of the band edge by the formation of tail states in the sub-band-gap region. The density of the tail states becomes higher for the samples annealed at higher temperatures. Thus, it has been revealed that the present H2/Ar annealing process not only modifies the morphology and particle size of ZnO powders, but also changes their electronic structure by creating energy states in the sub-band-gap region. 3.3. Steady State Photoluminescence and Photoluminescence Excitation Spectra. Figure 4 shows room-temperature PL and PL excitation (PLE) spectra of the green band measured for the as-purchased sample and the samples annealed at differnet

Figure 4. Room-temperature PL and PLE spectra of the as-purchased and the differently annealed samples. Annealing was carried out in a reducing H2/Ar atmosphere at the designated temperature for 1 h. The inset shows the enlarged plot of the PLE spectra in the sub-band-gap region. 8 ACS Paragon Plus Environment

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temperatures under H2/Ar atmosphere. The PLE spectra were measured by monitoring the emission intensity at 2.5 eV, and the PL spectra were obtained under excitation by photons with the PLE peak energy of the corresponding samples. Although the green PL band of the as-purchased sample is hardly visible, the emission intensity shows a substantial increase with increasing annealing temperature. This indicates that the concentration of the emission center, possibly in the form of oxygen vacancies, increases with increasing annealing temperature. As for the PL spectra, the spectral shape and the peak position (~2.5 eV) of the differently annealed samples are almost unchanged irrespective of the annealing temperature. However, this is not the case for the PLE spectra. The onset energy of the PLE spectra shifts to lower energies with increasing annealing temperature (see also the inset of Fig. 4). In particular, a sharp peak is developed in the excitation energy Eex region around ~3.2 eV for the samples annealed at temperatures higher than 1000 °C. Although the PLE peak at ~3.2−3.3 eV has often been observed in previous studies on ZnO,19-22 such a systematic change in the peak intensity and position with annealing conditions as seen in Fig. 4 has not been reported, to our knowledge. It should also be worth mentioning that the development of the below-band-gap PLE response of the annealed samples corresponds well with the band-edge modification induced by H2/Ar annealing. It is hence reasonable to assume that the annealing induced sub-band-gap states will play a vital role in enhancing the green PL emission. Considering that the size of the ZnO powders increases with increasing annealing temperature, as mentioned in the previous subsection, we suggest



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that the development of the sub-band-gap states along with the related PLE response is not ascribed only to surface defect states, but also to certain defect states in the subsurface and/or in the bulk. In the PLE spectra, however, the optical response below ~3 eV (see the inset of Fig. 4) is not conspicuous as compared with that of the Kubelka-Munk function shown in Fig. 3. This indicates that the photoexcited electrons

Figure 5. Temperature dependent PL spectra of (a) S700 and (b) S1000 under excitation with photons of 3.42 and 3.54 eV, respectively. In the upper parts, the temperature dependence of the PL spectra is shown as a contour plot. The lower parts show a conventional 2D representation of the selected PL spectra measured at the designated temperatures. 10 ACS Paragon Plus Environment

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Figure 6. Temperature dependent PLE spectra of (a) S700 and (b) S1000 monitored at 2.51 eV. In the upper parts, the temperature dependence of the PLE spectra is shown as a contour plot. The solid black line indicates the temperature dependence of the band-gap energy in ZnO bulk crystal.30 The lower parts show a conventional 2D representation of the selected PLE spectra measured at the designated temperatures. created under excitation by photons below ~3 eV hardly contribute to the green emission. We next analyze the temperature dependence of the PL and PLE spectra of differently annealed ZnO samples in the temperature region from 77 to 350 K. In what follows, the samples annealed at T1 K under H2/Ar atmosphere will be referred to as ST1; for example, S1000 indicates the sample annealed at 1000 K.



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The temperature dependent PL and PLE spectra of all the annealed samples are given in Figures S1 and S2 in the Supporting Information. We here show some typical examples of the measurements observed for S700 and S1000. The temperature induced variations of the PL (PLE) spectra are illustrated as a contour plot in the upper parts of Figures 5 (Figure 6). In the lower parts of Figures 5 and 6, we also show a conventional 2D representation of the selected spectra measured at several representative temperatures. One sees from Figure 5 that the position of the green PL band (~2.5 eV) remains almost constant irrespective of temperature. On the other hand, the intensity of the green band becomes maximum at ~225 K in S700 and ~150 K in S1000. As will be shown below, the temperature at which the green PL intensity reaches a maximum depends strongly on the excitation energy for both the samples. In Figure 5, one can also recognize a band-edge UV luminescence at ~3.3 eV, which shows a substantial red-shift with increasing temperature. The intensity of the ~3.3-eV band of S700 is stronger than that of S1000 in the whole temperature region investigated. The UV PL band peaking at ~3.3 eV is often observed in ZnO and has been generally assigned to a free-to-bound (e, A0) transition of the electron from the conduction band to acceptor like states.27,29,30 The temperature dependence of the ~3.3-eV peak can hence be interpreted in terms of the temperature-induced narrowing of the bandgap.27,30 The temperature dependence of the PLE spectra shown in Figure 6 is also worth mentioning. The peak position of the PLE peak shows a substantial red shift with


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Figure 7. PL decay profiles for the green PL band of S1000 measured (a) in the 77-250 K and (b) in the 275-350 K temperature ranges. The excitation energy is 3.22 eV. The solid lines are the fit of the data with eq 2. increasing temperature, and its intensity reaches a maximum at ~150−~200 K depending on the excitation energy. The reported temperature dependence of the band gap energy in ZnO bulk crystal31 is superimposed on the contour plot, showing that the PLE peak of S700 and S1000 is located ~50 and ~100 meV below the band gap, respectively. The above results indicate that the temperature and excitation energy dependence of the PLE peak varies strongly depending on the samples. Thus, the expected thermally assisted emission process is highly influenced by the nature of the sub-band gap states, which can differ substantially from sample to sample. 3.4. Time-Resolved Photoluminescence Measurements. First, we will show the results of the time-resolved PL measurements observed for S1000 since this sample exhibits a rather intense PL emission in the green spectral region. Figure 7 shows a typical decay curves of the green PL emission of S1000 measured at different 13 ACS Paragon Plus Environment


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Figure 8. PL decay profiles for the green PL band of S1000 measured at 250 K under different excitation energies. (a) The higher excitation energy range from 3.26 to 3.54 eV. (b) The lower excitation energy range from 3.10 to 3.22 eV. The solid lines are the fit of the data with eq 2.

temperatures using a pulsed Ti:sapphire laser at a fixed photon energy of Eex=3.22 eV (385 nm). In basically agreement with the previous studies,18,23 the PL decay is characterized by a non-exponential behavior on the order of several tens of microseconds. The entire decay profiles become slower with increasing temperature up to ~250 K, as shown in Figure 7. This shows that under 3.22-eV excitation, the effect of the thermally assisted emission process on the decay dynamics is most prominent in the temperature region around 250 K. We next measured the PL decay profiles of S1000 by changing the excitation photon energy from 3.10 eV (400 nm) to 3.54 eV (350 nm). The measurements were carried out at 250 K, where the thermally assisted emission process occurs in a well effective manner. Figure 8 shows series of decay profiles recorded


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under different excitation energies. One sees from Figure 8 that the decay profile depends highly on the excitation energy. The contribution of the long decay component increases with decreasing excitation energy from 3.54 to 3.26 eV [see Figure 8(a)] and then decreases abruptly as the excitation energy goes down from ~3.2 eV to 3.10 eV [see Figure 8(b)]. The decay profiles of S1000 measured at 300 K and those of S700 measured at 250 K also exhibit a similar excitation energy dependence on the decay profiles, as shown in Figure S3 in the Supporting Information.

4. DISCUSSION Thus, we have demonstrated that the decay profiles of the green PL emission in ZnO are influenced not only by the measurement temperature T but also by the excitation energy Eex. To further discuss the effect of T and Eex on the PL decay dynamics in a quantitative manner, we performed the numerical fitting of the decay data. In agreement with a previous study,18 the decay profiles of the green PL emission shown in Figures 7 and 8 have been found to be fitted to a triple-exponential decay function,

= exp −

+ exp − + exp − 2



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where I(t) is the time dependent luminescence intensity, τ1, τ2, and τ3 are the decay times of the fast, middle and slow decay components, respectively, and Ai (i=1, 2, 3) is the

Figure 9. Fitted values of (a) τi and (b) CFi obtained from the fitting of the PL decay curves shown in Figure 7. Lines are guides for the eye.

Figure 10. Fitted values of (a) τi and (b) CFi obtained from the fitting of the PL decay curves shown in Figure 8. The values of CF1 are shown in the inset of (b). Lines are guides for the eye. The dotted curve shown in (b) is the PLE spectrum of S1000 measured at 250 K. 16 ACS Paragon Plus Environment

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fitting constant. From the fitted values of Ai, one can estimate the contribution factor of the ith decay component CFi as follows:

CFi = Ai/(A1 + A2 + A3)

(3)

The fitted decay parameters obtained for the data shown in Figures 7 and 8 are summarized in Figures 9 and 10, respectively. The fitted values of τ1, τ2, and τ3 are ~1−2, ~10, ~50 µs, respectively, which depend hardly on T [see Fig. 9(a)] and Eex [see Fig. 10(a)]. However, the contribution factor of each decay component varies strongly with T [see Fig. 9(b)] and Eex [see Fig. 10(b)]. Figure 9(b) shows that the values of CF1 and those of CF2 and CF3 show a minimum and a maximum, respectively, at T=250 K. This clearly demonstrates that the contribution of the middle and slow decay components increases with increasing temperature up to ~250 K. The excitation energy dependence of CF2 and CF3 should also be mentioned. As shown in Fig. 10(b), the values of CF2 and CF3 tend to increase as the excitation energy decreases from 3.54 to 3.26 eV. Upon further decrease in Eex, however, they exhibit a sudden decrease, resulting in a steep maximum at Eex ~3.26 eV. Accordingly, the contribution factor of the fast decay component CF1 shows an opposite tendency, showing a sharp minimum at Eex ~3.26 eV [see the inset of Figure 10(b)].



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It is interesting to point out that the excitation energy dependence of CF2 and CF3 is quite analogous to the PLE spectrum of the same sample measured at the same temperature [see the broken line spectrum in Figure 10(b)], also showing a peak at Eex ~3.26 eV. This close correspondence strongly suggests that the observed increase in the PL intensity under below-band-gap excitation of Eex~3.26 eV intrinsically accompanies the increase in the PL decay time. To corroborate this assumption, further curve fitting was performed for the decay data obtained for S1000 at 300 K and S700 at 250 K (see Figure S3 in the Supporting Information), and the resulting CFi values were compared with the corresponding PLE spectra (see Figure 11). Also in these cases, a good correspondence between the excitation energy dependence of CF2 and CF3 and the corresponding PLE spectrum can be recognized. It can hence be concluded that under appropriate below-band-gap excitation conditions there occurs a thermal excitation process that can simultaneously increase the intensity and the decay time during the

Figure 11. Fitted values of CFi obtained from the fitting of the PL decay curves measured for (a) S1000 at 300 K and (b) S700 at 250 K. The original decay curves are given in Figure S3 in the Supporting Information. The dotted curves indicate the PLE spectra of the corresponding samples measured at the same temperature. 18 ACS Paragon Plus Environment

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entire green PL emission process. As mentioned in the Introduction, we have recently propose a model that can account for the observed below-band-gap PLE response in ZnO.27 The model is based on the assumption that the green PL results from two competing processes, namely, the direct emission process (process I) and the thermally assisted indirect process (process II). Process I mainly governs the green PL emission occurring at low temperatures (T ~150 K) and/or below-band-gap excitation, however, process II tends to dominate the entire emission process. In process II, the photoexcited electrons in the sub-band-gap states can be transferred to the emitting state by being thermally excited to the conduction band (see Figure 12). Recent transient absorption spectroscopy on ZnO has also demonstrated that the green emission originates from the recombination between the electrons in the conduction band and/or shallow donor (sub-band-gap) levels and the holes trapped at the green emission centers.10 If the sub-band-gap states behave as shallow trap, the decay rate of the emission process will become slower since the electron in the sub-band-gap states cannot directly be transferred to the emission state but will experience a thermally activated trapping/detrapping process before finally relaxing to the emitting state.23 If the above assumption is valid, a slow PL component is expected to grow especially under below-band-gap excitation because of a prolonged transport period associated with the relevant trapping/detrapping process. In accordance with the expectation, we indeed



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Figure 12. A model of the below-band-gap photoexcitation and the subsequent thermal excitation, trapping/detrapping and relaxation processes that are responsible for the green PL emission (adapted from Ref. 27).

observed the development of the middle and slow decay components when the excitation energy lies ~100 meV below the band gap. To validate the above model, however, we have to answer the following question: why can electrons overcome such a large energy (~100 meV) using their thermal energy kT, e.g., ~20 meV eV at ~250 K? The room temperature optical absorption spectra shown in Figure 3 can provide a clue to answer the question. As shown in Figure 3, the H2/Ar annealing leads to the formation of tails of states at room temperature, extending the bands into the energy gap. It is hence most likely that the band edges are highly perturbed, resulting in the large potential fluctuation of the band edges. Accordingly, the energy separation between the localized sub-band-gap states and the extended states in the conduction band will be reduced, allowing the thermal excitation of the


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photoexcited electrons with less energy than expected. Thus, we believe that the proposed emission scheme based on the thermally assisted emission channel shown in Fig. 12 is not far from realistic. The observed middle (τ2) and slow (τ3) decay times will represent the carrier capture times of the relevant sub-band-gap (trap) states, which can indirectly contribute to the enhancement of the green PL emissions. In contrast to the model mentioned above, Foreman et al.32 have recently proposed a different model for the explanation of the temperature dependent behavior of the green PL emission. They32 proposed that thermal delocalization and photoionization of I3a donor-bound excitons (DBEs) lead to the I9 DBE emission, free exciton(X)-longitudinal optical (LO) emission, and/or green emission. Their model hence suggests a strong connection between green emission intensity and bound excitons. As far as the present H2/Ar annealed samples are concerned, however, the DBE-based model cannot fully account for the spectral features observed for the green PL emission. As shown in Figs. 4 and 6, the PLE peak position of the green emission measured at high temperatures (T > ~150 K) depends strongly on the sample preparation temperature. This implies that the energy states associated with process II depends strongly on the sample preparation conditions or on the type and/or concentration of defects related to the sub gap states and the green emission centers. These changes in the PLE spectral features may not be explained by the exciton-based model. Our samples are highly defective and yield substantial long tail sub gap states, as mentioned repeatedly in this paper. It hence follows that the excitons tend to dissociate easily in a region of perturbed band



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potentials because of the presence of local fields induced by the potential fluctuations of the band edges.33 It is hence most likely that the lifetime of an exciton, even if exists, will be very short in the present ZnO crystals, virtually resulting in a free electron and a free hole under below-band-gap excitation. Thus, we believe that the energy transfer from bound excitons to green-emitting defects is less likely to be realized in our samples and also that the thermally activated emission process (process II) can be reasonably interpreted in terms of the scheme shown in Figure 12. Finally, we will give a brief explanation as to why high temperature H2/Ar annealing induces the sub-band-gap states and enhances the green emission via process II. It should be reminded that the highly reducing condition is achieved by the H2/Ar annealing process, probably yielding sub-stoichiometric ZnOx (x < 1) oxides. It is hence most likely that the higher the annealing temperature, the more the oxygen vacancies and/or zinc interstitials are introduced into the crystalline lattice. In ZnO, it is generally believed that the green PL centers are attributed to oxygen vacancies,5−9 whereas zinc interstitials are expected to form shallow donor levels.7,8 As mentioned in our previous paper,27 the shallow donor electrons can be thermally ionized, forming free carriers in the conduction band and in turn creating empty donor levels. These empty donor levels are mostly likely responsible for the optical absorption below ~3.3 eV, or the sub-band-gap states, eventually contributing to the green PL emission via process II. Thus, we consider that the observed PL characteristics can be reasonably interpreted in terms of the introduction of the oxygen vacancies and zinc interstitials under reducing


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conditions. However, further work will be needed to clarify the origin of the sub-band-gap states created by the H2/Ar annealing process.

5. CONCLUSIONS From the time-resolved PL measurements on the green PL in the H2/Ar annealed ZnO samples, we have found that the PL decay profiles are dependent strongly on excitation energy Eex. The PL decay data were analyzed using a triple-exponential function, yielding the decay constant τi of each decay component i and the related contribution factor CFi. The fitted values of τ1, τ2, and τ3 are ~1−2, ~10, ~50 µs, respectively, showing almost no excitation energy dependence. On the other hand, the CF values of the middle (CF2) and slow (CF3) decay components show a maximum at Eex ~3.26 eV. The observed excitation energy dependence of CF2 and CF3 is compared well with the spectral shapes of the PLE spectra in the below-band-gap energy region. The observed correspondence leads us to conclude that under below-band-gap excitation there exists a thermal excitation process that can simultaneously increase the PL intensity and the PL decay time. These observations support the emission model that the photoexcited electrons in the sub-band-gap states can be transferred to the emitting state via thermally activated trapping/detrapping processes. The annealing induced perturbation of the band edges will lower the energy separation between the sub-band-gap states and the extended states in the conduction band. This will facilitate the photoexcited electrons to eventually transfer to the emitting state by thermal energy.



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ASSOCIATED CONTENT Supporting Information Refined lattice constants and temperature dependent PL and PLE spectra of all the annealed samples. The PL decay profiles of S1000 measured at 300 K and those of S700 measured at 250 K. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +81 78 803 5681. Fax: +81 78 803 5681 Notes The authors declare no competing financial interest.

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TOC graphics


Variations in Decay Rate of Green Photoluminescence in ZnO under

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