ARTICLE pubs.acs.org/JPCA
Formation Dynamics of Excitons and Temporal Behaviors of Fano Resonance Due to the ExcitonImpurityPhonon Configuration Interaction in ZnO C. C. Zheng,† S. J. Xu,*,† J. Q. Ning,† Y. N. Chen,†,|| B. K. Li,‡ J. N. Wang,‡ and C. M. Che§ †
Department of Physics and HKU-CAS Joint Laboratory on New Materials and §Department of Chemistry and Institute of Molecular Functional Materials (Areas of Excellence Scheme, University Grants Committee (Hong Kong)), The University of Hong Kong, Pokfulam Road, Hong Kong, China ‡ Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, China ABSTRACT: Formation dynamics of free and neutral donor bound excitons (FX and D0X) in a high quality ZnO single crystal are studied by means of time-resolved photoluminescence (TRPL) at various temperatures. At low-temperatures, FX and D0X formation times are determined to be ∼5 and ∼10 ps, respectively, by fitting the rise process with the Boltzmann sigmoidal function. Temporal information of FX- and D0Xlongitudinal optical (LO) phonon coupling is also acquired by measuring TRPL spectra of the first-order LO phonon-assisted FX and D0X transitions. In particular, interesting time evolution of luminescence intensity in the Fano resonance region due to the configuration interaction of excitonimpurityphonon is explored.
’ INTRODUCTION ZnO, a direct wide bandgap semiconductor with large exciton binding energy (∼60 meV), has reattracted intensive interest from scientific community for its potential application in efficient ultraviolet lasers, optoelectronics, and spintronics.1,2 Realization of these expected applications of ZnO demands a better and deeper understanding of fundamental optical physics properties of ZnO, especially the dynamics of photoexcited exciton-related emissions. Most previous reports concentrated on the recombination dynamics of excitons in ZnO materials.35 Few studies of the detailed exciton formation dynamics have been done.6 The exciton formation time was derived to be within 10100 ps after photoexcitation by Hendry et al.6 On the other hand, luminescence spectral observation and identification of Fano resonance induced by the excitonimpurityphonon configuration interaction in ZnO have been recently conducted by Xu et al.7,8 However, we have known nothing about temporal behaviors of this three-body Fano interference. In this study, we determined the exciton formation time by directly fitting the rise processes of free and neutral donor bound exciton (FX and D0X) emissions at different temperatures with the Boltzmann sigmoidal function. The FX formation time is found to be ∼5 ps when the D0X formation time is about 2 times longer. Temporal behaviors of the first-order longitudinal optical (LO) phonon sidebands of FX and D0X (FX-1LO and D0X-1LO) in ZnO are also studied. Different coupling strengths between FX-LO phonon interaction and D0X-LO interaction were revealed. In last part of this study, the temporal behaviors of the Fano interference due to the threebody configuration interaction are investigated. r 2011 American Chemical Society
’ EXPERIMENTAL DETAILS The ZnO sample used in this study is a high quality single crystal grown by melt method and terminated at Zn (Cermet, Inc.). Low-temperature photoluminescence (PL) tests of this sample show nearly no visible emission which indicates the low concentration of impurity/defect in the crystal (results no shown here). Time-resolved photoluminescence (TRPL) measurements of the sample were performed at different temperatures by utilizing a streak camera (C4742-95-12ER, Hamamatsu) working with a charge-coupled device (CCD, ER-150, Hamamatsu) under excitation of a femtosecond laser. The wavelength tunable femtosecond pulses with a repletion rate of 76 MHz were generated from a Ti:sapphire oscillator (Coherent), which was pumped by a solid state laser with a power of 5 W. Triple frequency generation of one optimized laser line (800 nm) from the oscillator was employed to get the 266.7 nm pulses (width ∼100 fs) for exciting the sample by one-photon absorption. A beam splitter was adopted to guide a portion of the excitation laser beam to a PIN photodiode, which was used to generate the synchrony signal that would be picked up by the trigger unit for the streak camera. The system has a time window of 2.2 ns and a temporal resolution less than 5 ps. The sample was mounted on the coldfinger of a cryostat whose temperature can be controlled between 25 and 300 K. Received: October 14, 2011 Revised: December 3, 2011 Published: December 09, 2011 381
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Figure 2. PL decay of the D0X, FX, FX-1LO, and D0X-1LO transitions (right column in semilogarithmic scale, from top down order, in different shapes of empty scatters) read directly from the TRPL image of the ZnO single crystal at 25 K. The corresponding fitting results by an exponential or a biexponential function, depending on which one gives a better fitting outcome, are shown in solid lines. The rise curves of corresponding peaks and their fitting results by the Boltzmann sigmoidal function are shown in the left column in linear scale by the same scatter shape as the right one and solid lines, respectively. The results are shifted vertically for clarity.
Figure 1. TRPL image of a ZnO single crystal at 25 K under onephoton excitation (c) and typical decay curves in semilogarithmic scale of FX and D0X (b), whose spectral positions are indicated by two vertical dashed lines in the time integrated spectrum (a) of the TRPL image (c). The temporal profile of the laser pulse is depicted by solid diamond dots +line in (b), which is shifted horizontally for clarity. The biexponential fitting results are shown in solid lines for FX and D0X transitions in (b), respectively. Three vertical arrows in (a) indicate the spectral positions of FX-1LO, the dip caused by Fano interference, and D0X-1LO.
where B1 is the initial value, B2 is the final value, tS indicates the time center, and τR represents the time constant for the rising process. Rise time constants (τR) of FX and D0X unveiled by Boltzmann sigmoidal fitting are 5.46 ( 0.24 and 10.53 ( 0.20 ps, respectively (results shown in Figure 2). Figure 2 shows the rise and decay curves of FX, D0X, FX-1LO, and D0X-1LO transitions together with their fitting results at 25 K. The rising processes were all fitted by eq 2, whereas the decay processes of FX-1LO and D0X-1LO were fitting by a single exponential function
’ RESULTS AND DISCUSSION 1. Exciton Formation Dynamics. Because the excitation photon energy (4.65 eV) is well above the band gap (3.37 eV at room temperature) of ZnO, excited electron and hole first undergo a fast thermalization process and then form an exciton, which will further relax to a K = 0 state before it radiates.3 Typical rise and decay curves of FX (3.3764 eV) and D0X (3.3620 eV) transitions4,7,9 are shown in Figure 1b in semilogarithmic scale, which are extracted from the TRPL image in Figure 1c. The timeintegrated spectrum of the TRPL image is depicted in Figure 1a. The spectral positions of FX and D0X are marked by two vertical dashed lines, respectively, and a vertical arrow indicates the spectral position cause by the Fano interference which will be discussed later. The temporal profile of the laser pulse is also shown in Figure 1b. Both decay curves of FX and D0X transitions were fitted by a biexponential function
IðtÞ ¼ A1 et=τ1 þ A2 et=τ2 þ Ib
IðtÞ ¼ Aet=τ0 þ Ib
As reflected by the fitting results, luminescence lifetimes of FX1LO and D0X-1LO are 451.8 ( 13.6 and 708.8 ( 30.8 ps, respectively, which are significantly longer than the corresponding zero-phonon lines (FX and D0X). This is most likely caused by the additional time needed for LO phonon collision and generation. Interestingly, the rise time of FX-1LO is 4.51 ( 0.79 ps which is quite close to the rise time of FX, whereas it is 14.17 ( 2.03 ps for D0X-1LO, which is a little bit longer than that of D0X. To further investigate the temporal properties of exciton formation and excitonLO phonon interactions, evolution of rise time constants of FX, D0X, FX-1LO, and D0X-1LO with temperature is acquired and analyzed. The PL rise intensities of FX and D0X transitions at various temperatures are shown by different shapes of empty scatters in Figure 3 together with the fitting results by Boltzmann sigmoidal function in solid lines for the corresponding experimental curves. Similar types of data for FX-1LO and D0X-1LO transitions are presented in Figure 4 except that the D0X-1LO transition was not resolved after 65 K due to the ionization of bound excitons and broadening of FX-1LO transition at higher temperatures. Together with the data presented in Figure 2, we are able to trace the rise times of relevant transitions in a temperature range of 25140 K. The temperature evolution of the rise time constants
ð1Þ
where Ib is the background noise. The fitting results are depicted by solid lines in Figure 1b. The fitting parameters are τ1 = 40.7 ( 0.4 ps and τ2 = 391 ( 10.6 ps for the FX transition and τ1 = 241.5 ( 5 ps and τ2 = 1280.9 ( 473 ps for the D0X transition. It is worth mentioning that the FX recombination lifetime was determined to be 245322 ps in ZnO at 2 K by Reynolds et al. using a single exponential decay function.10 And similar lifetimes (i.e., τ1 = 230 ps and τ2 = 570 ps) of the D0X transition in ZnO at 2 K were recently reported by Wagner et al.11 with a biexponential decay function. The rise time is determined by fitting the rising process with the Boltzmann sigmoidal function IðtÞ ¼ B2 þ
B1 B2 1 þ eðt tS Þ=τR
ð3Þ
ð2Þ 382
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Figure 5. Temperature evolutions of the rise time constants revealed by the fitting results for FX, D0X, FX-1LO, and D0X-1LO transitions shown by different shapes of empty scatters+lines, respectively. The errors existed in the fitting results are shown in vertical error bars for each result.
Figure 3. Rise processes of FX and D0X transitions at different temperatures displayed in linear scale in alternative rows by empty triangle scatters and empty rectangle scatters, respectively. The figure is divided into two sections (upper part and lower part) and in each section the data acquired at the same temperature are shown in one column. The fitting results of the rise processes by Boltzmann sigmoidal function are depicted in solid lines for the corresponding experimental results.
Figure 6. Nine decay curves around Fano resonance (in different shapes of empty scatters) at 25 K. Each decay curve possesses a wavelength interval of 0.278 nm to get a clear signal. The wavelength centers of the nine lines are 374.620, 374.898, 375.176, 375.454, 375.732, 376.011, 376.288, 376.567, and 376.845 nm, respectively. The exponential decay fitting curves of these lines are also presented by the solid lines. The spectral position of the Dip is indicated by a vertical arrow.
It is clear that the rise time of FX transition keeps ∼5 ps against temperature variation, reflecting the large binding energy and temperature robust of free excitons in ZnO. Also, it is consistently shorter than that of D0X transition by ∼5 ps at low temperatures up to 65 K. This time difference characterizes the capture time of FX by shallow impurities. Above 65 K, the ionization of the bound excitons into free excitons becomes very efficient. This results in a significant reduction of D0X transition. At the same time, FX transition quickly increases, accompanied by thermal broadening as the temperature goes up. As a result, the rise time of the D0X transition trends to be the same as that of the FX transition for temperature above 65 K. As for the firstorder LO phonon sidebands of FX and D0X, the rise time of FX1LO is almost the same as for the FX transition in the interested temperature range, as shown in Figure 5. However, the rise time of the D0X-1LO transition is significantly longer (by ∼10 ps)
Figure 4. Rise processes of FX-1LO and D0X-1LO transitions at different temperatures displayed in linear scale by empty diamond scatters and solid triangle scatters, respectively. Data acquired at each temperature are shown in one subsection. The fitting results of the rise processes by a Boltzmann sigmoidal function are depicted in solid lines for the corresponding experimental results. The D0X-1LO transition is not resolved after 65 K because of the ionization of BX, and the fast emerging and broadening of the FX-1LO transition at higher temperature.
for FX, D0X, FX-1LO, and D0X-1LO transitions revealed by the fitting results is shown in Figure 5. The fitting error bars are also given in the figure for the corresponding results. 383
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than that of the FX-1LO transition and also longer (by ∼5 ps) than that of D0X transition within the temperature range in which the D0X-1LO transition was clearly resolved. This indicates that the scattering characterization time between FX and the LO phonon is much shorter than that of D0X and the LO phonon, suggesting a stronger coupling strength between FX and the LO phonon in ZnO. Furthermore, the rise time of the D0X1LO transition reaches a peak value at a temperature around 45 K. In fact, the thermal ionization of the donor bound excitons (D0X) becomes noticeable as evidenced by the reduction of D0X related peaks at ∼45 K. As a consequence, the capturing process of free excitons by impurities to form the bound excitons becomes difficult. On the other hand, the coupling between the bound exciton and phonon tends to be stronger. Both mechanisms could jointly make the rise time constant of D0X1LO be a peak value at this temperature. 2. Temporal Behaviors of Fano Resonance. It is interesting to study the temporal properties around the characteristic asymmetric line shape of the Fano resonance to obtain a deeper understanding of the excitonimpurityphonon Fano interference process in ZnO.7,8 Nine decay curves were selected from the TRPL image at 25 K and are shown in Figure 6 with empty scatters. Each decay curve possesses a wavelength interval of 0.278 nm to get a clear time-evolution trace. The center wavelengths of the nine lines are 374.620, 374.898, 375.176, 375.454, 375.732, 376.011, 376.288, 376.567, and 376.845 nm, respectively. The exponential fitting results of these lines are also plotted in Figure 6 by solid lines. Again, the Boltzmann sigmoidal function was employed to fit the rise processes of the nine lines (results not shown in the figure). The decay and rise time constants vs center wavelength of the nine lines are depicted in Figure 7a,b, respectively, together with the time-integrated PL intensity within this wavelength range in Figure 7c. The data at 35 and 45 K were also shown in Figure 7 for a comparison because the observed Fano interference is sensitive to temperature.7 The spectral positions of FX-1LO, the Fano interference Dip, and D0X-1LO transitions are marked by vertical dashed lines in the figure.
As mentioned earlier, the FX-1LO transition has a faster decay time and a shorter rise time compared with those of the D0X1LO transition, as also seen by the data at the corresponding spectral locations of FX-1LO and D0X-1LO transitions in Figure 7a,b. Within the Fano resonance range, it is found that the rise time monotonically increases while the decay time first drops by some amount, reaches a lowest value near the Dip, and then increases again as the center wavelength scans from FX-1LO to D0X-1LO for the investigated three temperatures. The nonmonotonic variation of the decay time against wavelength (energy) indeed reflects energy dependence of the Fano interference strength. The shorter decay times near the spectral Dip indicate stronger destructive interference of the two alternative pathways involved in the luminescence process. Another finding is that the rise time remains almost the same at three different temperatures for the transitions while the decay time shows a tendency to increase when the temperature goes up, indicating that the rise process in the temporal profile mainly concerns the nature of the transition while the decay process is connected to the lattice vibration/scattering strength.
’ SUMMARY AND CONCLUSIONS In summary, the formation and recombination dynamics of FX and D0X transitions in ZnO at various temperatures have been studied by means of PL and TRPL. The rise times of FX and D0X transitions vs temperature were determined. The FX formation time is found to be ∼5 ps when the D0X formation time is about 2-fold longer. The coupling strength between FX and the LO phonon is found to be stronger than that of the D0X and the LO phonon, as evidenced by the shorter and similar rise times of both FX and FX-1LO transitions, respectively. The temporal behaviors of the transitions within the Fano resonance region are also studied. It is found that the rise time monotonically increases while the decay time first decreases, then reaches a lowest value, and increases again as the wavelength increases, showing an additional evidence of the Fano-type interference between the two alternative pathways. The findings and data not only enrich the current understanding of formation and recombination dynamics of free and bound excitons in semiconductors but also bring new insight on the excitonimpurityphonon many-body interactions in solids. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Present Addresses
)
Figure 7. Decay (a) and rise (b) time constants with error bars of the emission intensities within the Fano resonance region, and timeintegrated PL intensities (c) at 25, 35, and 45 K. The spectral positions of FX-1LO, Dip, and D0X-1LO are indicated by vertical dashed lines.
ARTICLE
Laboratoire “Materiaux et Phenomenes Quantiques”, Universite Paris Diderot-Paris 7, CNRS-UMR 7162, 75013 Paris, France.
’ ACKNOWLEDGMENT The work was supported by the Joint Research Fund for Overseas Chinese, Hong Kong and Macau Scientists of NSFC (Grant No. 61028012) and the HK RGC-GRF Grant (Grant No. HKU 7056/06P), and partially supported by a grant from the University Grants Committee Areas of Excellence Scheme of the Hong Kong Special Administrative Region, China (Project No. [AoE/P-03/08]). 384
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’ REFERENCES
€ ur, U.; € Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; (1) Ozg€ Dogan, S.; Avrutin, V.; Cho, S. J.; Morkoc-, H. J. Appl. Phys. 2005, 98, 041301. (2) Klingshirn, C.; Fallert, J.; Zhou, H.; Sartor, J.; Thiele, C.; MaierFlaig, F.; Schneider, D.; Kalt, H. Phys. Status Solidi B 2010, 247, 1424–1447. (3) Jung, S. W.; Park, W. I.; Cheong, H. D.; Yi, G. C.; Jang, H. M.; Hong, S.; Joo, T. Appl. Phys. Lett. 2002, 80, 1924–1926. (4) Teke, A.; Ozgur, U.; Dogan, S.; Gu, X.; Morkoc, H.; Nemeth, B.; Nause, J.; Everitt, H. O. Phys. Rev. B 2004, 70, 195207. (5) Lagarde, D.; Balocchi, A.; Renucci, P.; Carrere, H.; Zhao, F.; Amand, T.; Marie, X.; Mei, Z. X.; Du, X. L.; Xue, Q. K. Phys. Rev. B 2008, 78, 033203. (6) Hendry, E.; Koeberg, M.; Bonn, M. Phys, Rev, B 2007, 76, 045214. (7) Xu, S. J.; Xiong, S. J.; Shi, S. L. J. Chem. Phys. 2005, 123, 221105. (8) Jin, K. J.; Xu, S. J. Appl. Phys. Lett. 2007, 90, 032107. (9) Reynolds, D. C.; Look, D. C.; Jogai, B.; Litton, C. W.; Collins, T. C.; Harsch, W.; Cantwell, G. Phys. Rev. B 1998, 57, 12151–12155. (10) Reynolds, D. C.; Look, D. C.; Jogai, B.; Hoelscher, J. E.; Sherriff, R. E.; Harris, M. T.; Callahan, M. J. J. Appl. Phys. 2000, 88, 2152–2153. (11) Wagner, M. R.; Callsen, G.; Reparaz, J. S.; Schulze, J. H.; Kirste, R.; Cobet, M.; Ostapenko, I. A.; Rodt, S.; Nenstiel, C.; Kaiser, M.; Hoffmann, A.; Rodina, A. V.; Phillips, M. R.; Lautenschlager, S.; Eisermann, S.; Meyer, B. K. Phys. Rev. B 2011, 84, 035313.
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