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Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 22, Iss. 15 — Jul. 28, 2014
  • pp: 17959–17967
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Temperature dependence of the radiative recombination time in ZnO nanorods under an external magnetic field of 6T

W. Lee, T. Kiba, A. Murayama, C. Sartel, V. Sallet, I. Kim, R. A. Taylor, Y. D. Jho, and K. Kyhm  »View Author Affiliations


Optics Express, Vol. 22, Issue 15, pp. 17959-17967 (2014)
http://dx.doi.org/10.1364/OE.22.017959


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Abstract

The Temperature dependence of the exciton radiative decay time in ZnO nanorods has been investigated, which is associated with the density of states for the intra-relaxation of thermally excited excitons. The photoluminescence decay time was calibrated by using the photoluminescence intensity in order to obtain the radiative decay time. In the absence of an external magnetic field, we have confirmed that the radiative decay time increased with temperature in a similar manner to that seen in bulk material (∼ T1.5). Under an external magnetic field of 6T parallel to the c-axis, we found that the power coefficient of the radiative decay time with temperature decreased (∼ T1.3) when compared to that in the absence of a magnetic field. This result can be attributed to an enhancement of the effective mass perpendicular to the magnetic field and a redshift of the center-of-mass exciton as a consequence of perturbation effects in the weak-field regime.

© 2014 Optical Society of America

1. Introduction

The wide band gap in ZnO is beneficial for UV-optoelectric devices, and ZnO nanorods are used for various applications in nanotechnology such as optical resonators, solar cells, piezo-electric energy generators, and in bio-medical applications [1

1. Zhong Lin Wang, “Zinc oxide nanostructures: growth, properties and applications,” J. Phys.: Condens. Matter 16, R829–R858 (2004).

]. Although nanorods are generally considered to be one-dimensional (1D) nanostructures, their 1D nature is determined by the degree of lateral confinement. As the exciton binding energy in ZnO is large (∼ 60meV), the lateral size of a ZnO nanorod needs to be comparable to the exciton diameter (∼ 3.6nm) to show this 1D nature. Otherwise, large-diameter nanorods are likely to produce bulk-like excitons or show weak confinement. When confinement dimensionality is determined by comparing the confinement size to the exciton diameter, it is difficult to distinguish the limits of weak and no confinement [2

2. Mark Fox, Optical Properties of Solids, 2nd Edition (OXFORD University Press, 2010).

]. Alternatively, quantum confinement can also be induced by applying a strong external magnetic field. For example, if a high magnetic field is applied perpendicular to the lateral plane of a quantum well, quantum dot-like three dimensional confinement can be achieved. Because these magnetically-induced quantum dots have very uniform size-homogeneity when compared to inhomogeneous self-assembled quantum dots, long-range spatial coherence can be achieved. This enables collective coherent emission such as super-fluorescence and super-radiance [3

3. S. Schimitt-Rink, J. B. Stark, W. H. Knox, D. S. Chemla, and W. Schäfer, “Optical properties of quasi-zero-dimensional magneto-excitons,” Appl. Phys. A 53, 491–502 (1991). [CrossRef]

, 4

4. Y. D. Jho, Xiaoming Wang, J. Kono, D. H. Reitze, X. Wei, A. A. Belyanin, V. V. Kocharovsky, Vl. V. Kocharovsky, and G. S. Solomon, “Cooperative Recombination of a Quantized High-Density Electron-Hole Plasma in Semiconductor Quantum Wells,” Phys. Rev. Lett. 96, 237401 (2006). [CrossRef] [PubMed]

]. Likewise, 1D nanostructures can be induced from bulk by applying a large magnetic field, taking the system into the strong-field regime, where the magnetic cyclotron energy (h̄ωc) is larger than the exciton binding energy ( Ry*). On the other hand, for a lower magnetic field, the so-called weak-field regime ( h¯ωcRy*), it is difficult to define the dimensionality of magnetically-induced confinement [5

5. Claus F. Klingshirn, Semiconductor Optics, 4th Edition (Springer, 2012). [CrossRef]

].

As an alternative method for evaluating the confinement dimensionality, the temperature dependence of the radiative decay time can be used. Because the intra-relaxation of the thermally excited exciton towards the bottleneck range of radiative decay for the center-of-mass exciton dispersion depends on the density of states, confinement dimensionality affects the temperature dependence of the radiative decay time. The validity of this method was confirmed in quantum wires (1D), quantum wells (2D), and bulk (3D) [6

6. G. W. ’t Hooft, W. A. J. A. van der Poel, L. W. Molenkamp, and C. T. Foxon, “Giant oscillator strength of free excitons in GaAs,” Phys. Rev. B 35, 8281–8284 (1987). [CrossRef]

, 7

7. H. Akiyama, S. Koshiba, T. Someya, K. Wada, H. Noge, Y. Nakamura, T. Inoshita, and A. Shimizu, “Thermalization Effect on Radiative Decay of Exciton in Quantum Wires,” Phys. Rev. Lett. 72, 924–927 (1994). [CrossRef] [PubMed]

]. Although this method is rarely used for magneto-photoluminescence, it is useful to evaluate magnetically-induced confinement. Under a sufficiently large magnetic field ( h¯ωcRy*), the induced one-dimensional nature from bulk can be confirmed in terms of the temperature dependence of radiative decay time similar to the case of quantum wires ( ~T) [7

7. H. Akiyama, S. Koshiba, T. Someya, K. Wada, H. Noge, Y. Nakamura, T. Inoshita, and A. Shimizu, “Thermalization Effect on Radiative Decay of Exciton in Quantum Wires,” Phys. Rev. Lett. 72, 924–927 (1994). [CrossRef] [PubMed]

]. However, because of the large exciton binding energy of ZnO, the intermediate-field regime ( h¯ωc~Ry*) requires ∼ 81T. In the case of the weak-field regime ( h¯ωcRy*), the Coulomb energy still dominates and the external magnetic field can be treated as a perturbation [5

5. Claus F. Klingshirn, Semiconductor Optics, 4th Edition (Springer, 2012). [CrossRef]

]. Recently, enhancement of the effective mass anisotropy was claimed as a consequence of magnetic perturbation [8

8. F. V. Kyrychenko, Y. D. Jho, J. Kono, S. A. Cooker, G. D. Sanders, D. H. Reitze, C. J. Stanton, X. Wei, C. Kadow, and A. C. Gossard, “Interband magnetoabsorption study of the shift of the Fermi energy of a 2DEG with an in-plane magnetic field,” Phisica E 22, 624–627 (2004). [CrossRef]

]. Although a 1D nature cannot be achieved by a weak-field, the increase of the effective mass perpendicular to the external magnetic field may affect the degree of the induced-confinement.

In this work, we have evaluated the confinement dimensionality of our ZnO nanorods in terms of the temperature dependence of the radiative decay time (τr) with and without an external magnetic field. For large diameter ZnO nanorods (250nm) in the absence of a magnetic field, a bulk nature was confirmed (τrT1.5). On the other hand, when 6T is applied (the weak-field regime), we conclude that the perturbation effect of the weak magnetic field results in a quasi-dimensionality between 2D and 3D according to the power coefficient of the radiative decay time with temperature (τrT1.3).

2. Experiment

The ZnO nanorods were grown by metalorganic chemical vapor deposition (MOCVD) on c-plane sapphire in a horizontal reactor operating at a reduced pressure of 50 Torr. Diethylzinc (DEZn) and nitrous oxide (N2O) were used as the zinc and oxygen sources with helium gas. The concentration of DEZn was 40 μmol/mn. Vertical ZnO nanorods with high aspect ratio and hexagonal section were grown at 875 °C under moderate oxygen/zinc molar ratio (RVI/II) of 1150 in order to favor 1D growth (high RVI/II above 14000 leads to two dimensional layer growth). The deposition time is 40 minutes [9

9. Wei Geng, Sergei Kostcheev, Corinne Sartel, Vincent Sallet, Michael Molinari, Olivier Simonetti, Gilles Lérondel, Louis Giraudet, and Christophe Couteau, “Ohmic contact on single ZnO nanowires grown by MOCVD,” Phys. Status Solidi C. 10, 1292–1296 (2013).

]. Top and side views of SEM images of the nanorods are shown in Fig. 1(a), where a single rod is of 5 μm length and hexagonal type and its diameter is approximately 250 nm. Third harmonic emission (266nm) generated from a Ti:sapphire laser (∼ 800nm) emitting pulses of 80fs duration operating at a repetition frequency of 80MHz was used for excitation. In order to avoid many-body effects, a weak excitation intensity (10pJ per pulse) was used. The beam was focused onto the sample mounted in a temperature-controlled cryostat housing a superconducting magnet, where an external magnetic flux density of 6T was applied parallel to the nanorod direction. Time-resolved photoluminescence (TR-PL) was measured by using a streak camera.

Fig. 1 SEM images of ZnO nanorods, where the average length is ∼ 5μm (left image) and the size of a hexagonal crosssection is ∼ 250nm (right image) (a). PL spectrum at 10 K, 20 K, and 30 K in absence/presence of magnetic field (b)/(c), where the individual PL spectrum of D0X (dotted) and FX (shadow) are extracted by Gaussian fitting, and the central emission energy and the linewidth in eV are shown in the parentheses

3. Results and discussion

Although several papers have already reported TR-PL in bulk ZnO and ZnO nanorods in the context of the temperature dependence of the radiative decay time, the physics underpinning this temperature dependence for the exciton was often misunderstood [10

10. X. H. Zhang, S. J. Chua, A. M. Yong, H. Y. Yang, S. P. Lau, S. F. Yu, X. W. Sun, Lei Miao, Masaki Tanemura, and Sakae Tanemura, “Exciton radiative lifetime in ZnO nanorods fabricated by vapor phase transport method,” Appl. Phys. Lett. 90, 013107 (2007). [CrossRef]

, 11

11. Fang-Yi Jen, Yen-Cheng Lu, Cheng-Yen Chen, Hsiang-Chen Wang, C. C. Yang, Bao-ping Zhang, and Yusaburo Segawa, “Temperature-dependent exciton dynamics in a ZnO thin film,” Appl. Phys. Lett. 87, 252117 (2005). [CrossRef]

], where the following aspects were overlooked. First of all, although the photoluminescence (PL) spectrum is dominated by donor bound excitons (D0Xs), free excitons (FXs) need to be resolved from the PL spectrum in order to evaluate the dimensional nature of the recombination. Secondly, the excitation intensity should be low enough to avoid many-body effects. As our excitation intensity (0.1μJcm−2) is lower by two-orders of magnitude compared to that of reported work (40μJcm−2), bandgap renormalization can be ruled out. Thirdly, the temperatures used should be low, otherwise the extraction of a pure radiative decay time becomes difficult as thermal non-radiative processes become significant, such as thermal dissociation, excitation, and quenching.

As shown in Figs. 1(b) and 1(c), the individual PL spectra of both the D0Xs and FXs with and without external magnetic field were resolved for various temperatures by double Gaussian fitting [12

12. Cheol Hyoun, Ahn Sanjay, Kumar Mohanta, Nae Eung Lee, and Hyung Koun Cho, “Enhanced exciton-phonon interactions in photoluminescence of ZnO nanopencils,” Appl. Phys. Lett. 94, 269104 (2009).

]. Although various peaks are seen at energies below that of the D0X, which are possibly other kinds of donor-bound excitons, acceptor-bound excitons, and exciton LO-phonon replicas, we include the D0Xs to improve the fitting accuracy of the FXs. Given the fitted linewidths of FXs (∼ 15meV), it was not possible to resolve the two peaks expected from the small Zeeman splitting (∼ 0.9meV) under an external magnetic flux density of 6T, which was estimated by using the g-factor of the electron (1.957) and the hole (2.45) in ZnO [13

13. Walter R. L. Lambrecht, Anna V. Rodina, Sukit Limpijumnong, B. Segall, and Bruno K. Meyer, “Valence-band ordering and magneto-optic exciton fine structure in ZnO,” Phys. Rev. B 65, 075207 (2002). [CrossRef]

]. Rather the overall magnetic effect causes a redshift of ∼ 5meV. Recently, a similar result was observed in a quantum well; when the effect of an external magnetic field applied along the in-plane is considered as a perturbation, the weak-field gives rise to a redshift in the absorption spectrum. They also found that the diamagnetic effect eventually gave rise to a blueshift beyond the threshold magnetic field (∼ 30T) [8

8. F. V. Kyrychenko, Y. D. Jho, J. Kono, S. A. Cooker, G. D. Sanders, D. H. Reitze, C. J. Stanton, X. Wei, C. Kadow, and A. C. Gossard, “Interband magnetoabsorption study of the shift of the Fermi energy of a 2DEG with an in-plane magnetic field,” Phisica E 22, 624–627 (2004). [CrossRef]

]. Therefore, the PL redshift under 6T in Fig. 1(c) can be attributed to a weak-field perturbation effect.

Each individual PL spectrum of the FX was obtained as a function of temperature by using Gaussian fitting as shown in Figs. 1(b) and 1(c), whereby the peak PL energy was estimated and a corresponding TR-PL trace was also measured up to a temperature of ∼ 100K within the spectral resolution of the TR system ∼ 10meV. As shown in Figs. 2(a) and 2(b), both the TR-PL with and without magnetic field show a monotonic decay on a log plot, where the PL decay is more rapid at low temperature. Although the increase in the PL decay time for increasing temperature contains the essential physics associated with the density of states, it is crucial to distinguish the radiative decay time from the PL decay time. As shown schematically in Fig. 3(b), FXs decay radiatively only in the small k-range where the center-of-mass FX wave vector is small when compared to that of the photon (k < k0) at finite temperature, where k0 is determined by the intersection of the parabolic center-of-mass FX dispersion (E = 2k2/2M) and the linear photon dispersion ( E=h¯ck/ε). Given the total FX mass (M = 0.69m0) and the dielectric constant ε0(ε) = 8.656(3.67), where m0 is the electron rest mass, the maximum kinetic energy for radiative decay of the FXs ( Δ=h¯2k02/2M~0.14 meV(0.059 meV)) could be determined [14

14. S. J. Pearton, D. P. Norton, K. Ip, Y. W. Heo, and T. Steiner, “Recent advances in processing of ZnO,” J. Vac. Sci. Technol. B 22, 932–948 (2004). [CrossRef]

, 15

15. Brendan Enright and Donald Fizmaurice, “Spectroscopic Determination of Electron and Hole Effective Masses in a Nanocrystalline Semiconductor Film,” J. Phys. Chem. 100, 1027–1035 (1996). [CrossRef]

]. On the other hand, for the FXs with large center-of-mass wave vector (k > k0) the decay is dominated by non-radiative processes during their intra-relaxation along the dispersion via various inelastic scattering processes to conserve their momentum. As a result, the PL decay time (τ) represents the population decay as a consequence of both radiative and non-radiative processes. Provided that high quality samples are measured at very low temperature (< 10K), the non-radiative decay can be negligible [6

6. G. W. ’t Hooft, W. A. J. A. van der Poel, L. W. Molenkamp, and C. T. Foxon, “Giant oscillator strength of free excitons in GaAs,” Phys. Rev. B 35, 8281–8284 (1987). [CrossRef]

]. Otherwise the non-radiative decay time (τnr) needs to be considered in the PL decay.

Fig. 2 TR-PL intensity of the FX for various temperatures in the absence/presence ((a)/(b)) of magnetic field. Temperature dependence of the PL decay time (τ(T)) (c) and ratio of the IQE with temperature to the IQE at 4K (ζ(T) = η(T)/η(4K)) (d) are shown with (open triangles) and without (filled triangles) external magnetic field up to ∼ 40K, beyond which non-radiative decay dominates, resulting in significant increase in τ(T) and a decrease in ζ(T) as shown in insets.
Fig. 3 (a) Temperature dependence of the radiative decay time with and without magnetic field. (b) Excitons in the small k-range (kex < k0) decay radiatively, whilst the population ratio with all excited excitons determines the temperature dependence of the radiative decay time (τr(T)). In the case of the weak-field regime, the perturbation energy of external magnetic field (B = 6T) gives rise to a redshift, and the effective mass perpendicular to the c-axis in a ZnO nanorod becomes heavier relatively ( M*>M).

For the monotonic decay of the time-resolved PL intensity, the population of FXs (N) after transient excitation (g) is given by
dNdt=gNτ,
(1)
whereby the population in the steady state N = can also be obtained. As long as the relation 1/τ = 1/τr + 1/τnr holds true, the time-integrated PL intensity at a given temperature can be described as I(T) ∼ N/τr(T) = (T), where η(T) = τ/τr is the quantum efficiency for that temperature [16

16. R. C. Miller, D. A. Kleinman, W. A. Nordland Jr., and A. C. Gossard, “Luminescence studies of optically pumped quantum wells in GaAs-AlxGa1−xAs multilayer structures,” Phys. Rev. B 22, 863–871 (1980). [CrossRef]

]. Consequently, the PL intensity ratio of I(T) to I(4K) represents the ratio of (T) to (4K) as
I(T)I(4K)=η(T)η(4K)=ζ(T),
(2)
where the calibration factor ζ(T) enables us to measure the temperature dependence of the radiative decay time in terms of τr(T)η(4K) as [17

17. D. Takamizu, Y. Nishimoto, S. Akasaka, H. Yuji, K. Tamura, K. Nakahara, T. Onuma, T. Tanabe, H. Takasu, M. Kawasaki, and S. F. Chichibu, “Direct correlation between the internal quantum efficiency and photoluminescence lifetime in undoped ZnO epilayers grown on Zn-polar ZnO substrates by plasma-assisted molecular beam epitaxy,” J. Appl. Phys. 103, 063502 (2008). [CrossRef]

]
τr(T)η(4K)=τ(T)ζ(T).
(3)
Even though the absolute internal quantum efficiency (IQE) at 4K (η(4K)) is unknown, the temperature dependence of the radiative decay time can be analyzed in terms of the power order (α) as τr(T) ∼ Tα, i.e., η(4K) contributes as a scaling factor, but does not change α.

Theoretically, τr(T) is given by
τr(T)=τ0r(T),
(4)
r(T)=0ΔD(ε)eε/kBTdε0D(ε)eε/kBTdε,
(5)
where τ0 is the intrinsic radiative decay time of the FX at k ∼ 0 and kB is the Boltzmann constant. The fraction of the small wave vector FXs (k < k0) with respect to the thermally excited ones (r(T)) is associated with the exciton density of states for a given energy (ε) as D(ε)∼ εβ. Because β depends on the confinement dimension, the dimensional nature of the nanorod can be determined in terms of α. As a result, the power order of the density of states (β = −0.5, 0, 0.5) for 1D, 2D, and 3D determines the power order of the temperature dependence (α = 0.5, 1, 1.5), respectively [6

6. G. W. ’t Hooft, W. A. J. A. van der Poel, L. W. Molenkamp, and C. T. Foxon, “Giant oscillator strength of free excitons in GaAs,” Phys. Rev. B 35, 8281–8284 (1987). [CrossRef]

, 7

7. H. Akiyama, S. Koshiba, T. Someya, K. Wada, H. Noge, Y. Nakamura, T. Inoshita, and A. Shimizu, “Thermalization Effect on Radiative Decay of Exciton in Quantum Wires,” Phys. Rev. Lett. 72, 924–927 (1994). [CrossRef] [PubMed]

].

Figure 2(c) shows τ(T) obtained from the monotonic decay of the TR-PL in the absence and presence of a magnetic field (Figs. 2(a) and 2(b)). The temperature dependence of the calibration factor ζ(T) was also obtained by normalizing I(T) to I(4K) as shown in Fig. 2(d). While τ(T) increases with temperature, a critical change in the temperature dependence is seen beyond 40K, where I(T) also becomes significantly suppressed (insets of Figs. 2(c) and 2(d)). These results indicate that the PL decay is dominated by non-radiative decay due to thermal effects when the temperature increases beyond 40K. It should also be noted that the accuracy of τ(T) deteriorates when the temperature is higher than 40K. Although Figs. 2(a) and 2(b) are normalized, the raw intensity weakens with a small signal-to-noise ratio as the temperature rises. As a result, the error bar for τ(T) becomes large. Since the dimensional nature of the nanorod is associated with radiative decay processes, α should be obtained by considering lifetime data below the critical temperature (< 40K) only. The enhancement of η(T) up to ∼ 40K can be attributed to suppression of FX trapping to donors [18

18. W. I. Park, Y. H. Jun, S. W. Jung, and Gyu-Chul Yi, “Excitonic emissions observed in ZnO single crystal nanorods,” Appl. Phys. Lett. 82, 964–966 (2003). [CrossRef]

]. Zhang et al. measured a quadratic dependence for τ(T), but calibration was overlooked [10

10. X. H. Zhang, S. J. Chua, A. M. Yong, H. Y. Yang, S. P. Lau, S. F. Yu, X. W. Sun, Lei Miao, Masaki Tanemura, and Sakae Tanemura, “Exciton radiative lifetime in ZnO nanorods fabricated by vapor phase transport method,” Appl. Phys. Lett. 90, 013107 (2007). [CrossRef]

]. Although Jen et al. considered the temperature dependence of the integrated PL intensity in order to calibrate τ(T), a linear temperature dependence of τr(T) was obtained in a ZnO film, which is in contrast to the 3D case (τr(T) ∼ T1.5). This disagreement possibly results from the inappropriate temperature range. Because the temperature dependence of the FXs was considered from 40K up to 180K [11

11. Fang-Yi Jen, Yen-Cheng Lu, Cheng-Yen Chen, Hsiang-Chen Wang, C. C. Yang, Bao-ping Zhang, and Yusaburo Segawa, “Temperature-dependent exciton dynamics in a ZnO thin film,” Appl. Phys. Lett. 87, 252117 (2005). [CrossRef]

], the linear temperature dependence is likely to arise from thermal linewidth broadening. On the other hand, as we have divided τ(T) by the calibration factor (ζ(T)) for the temperature dependence of the radiative decay time according to Eq. (3), the power-order of τr(T) decreases when compared to that of τ(T). As shown in Fig. 3(a), we have confirmed that the temperature dependence of τr(T) in our ZnO nanorods follows that expected for the bulk case (∼ T1.5). Given that the unknown η(4K) is less than 1, τr(T) seems somewhat long compared to the tens of picoseconds expected for the intrinsic radiative decay time in a semiconductor due to the long range spatial coherence of the small k. This can be explained by the large PL linewidth of the FXs in the nanorods; when the linewidth broadening is large, the uncertainty of the wave vector becomes comparable to k0. This limits the spatial coherence by localization, resulting in a relatively long radiative decay time.

When a strong magnetic field is applied, a one-dimensional structure can be induced from the bulk through the Landau levels, where the quantized levels in the plane perpendicular to an external magnetic field are analogous to the confined levels in quantum wires, and the 1D-excitons remain free to move along the direction of the quantum wires and external magnetic field, respectively [19

19. Richard L. Liboff, Introductory Quantum Mechanics, 4th Edition (Addison - Wesley, 2003).

, 20

20. Kerson Huang, Statistical Mechanics, 2nd Edition (John Wiley & Sons, 1987).

]. Alternatively, the reduction in confinement dimension can be understood in terms of the density of states. In the case of bulk, the states are equally spaced (Δk) in k-space for the various coordinate configurations (kx, ky, kz). On the other hand, an external magnetic field gives rise to quantized Landau levels in the magnetically-induced parabolic potential for the rotating radius. Schematically, the equally spaced (Δk) points in the k-plane (kx, ky), which represent the states in the absence of magnetic field, turn into the rings, which correspond to the Landau levels in the presence of an external magnetic field. The lateral distance between the rings in the k-plane (Δkm) decreases as the Landau levels are equidistant in energy. As a result, the density of states is reduced in the presence of an external magnetic field [19

19. Richard L. Liboff, Introductory Quantum Mechanics, 4th Edition (Addison - Wesley, 2003).

, 20

20. Kerson Huang, Statistical Mechanics, 2nd Edition (John Wiley & Sons, 1987).

].

As the level spacing versus the thermal energy can be a criterion for confinement instead of a comparison of the confinement size to the exciton diameter, the magnetically-induced confinement can be evaluated in terms of the Landau level spacing (h̄ωc) compared to the exciton binding energy ( Ry*), where the cyclotron resonance frequency (ωc = eB/μ) can be estimated by the reduced mass of the exciton (μ) and the applied magnetic flux density (Bẑ). Therefore, a 1D nature can be claimed in the so-called strong-field regime ( h¯ωcRy*). It would be challenging to measure the gradual change of this quasi-dimension towards the ideal 1D by increasing the magnetic field to this strong-field regime. However, at a flux density of 6T we are still in the weak-field regime ( h¯ωcRy*) as the Landau level spacing (h̄ωc ∼ 4.4meV) is still less than the exciton binding energy ( Ry*~60meV) [5

5. Claus F. Klingshirn, Semiconductor Optics, 4th Edition (Springer, 2012). [CrossRef]

, 21

21. P. Zu, Z. K. Tang, G. K. L. Wong, M. Kawasaki, A. Ohtomo, H. Koinuma, and Y. Segawa, “Ultraviolet spontaneous and stimulated emissions from ZnO microcrystallite thin films at room temperature,” Solid State Commun. 103, 459–463 (1997). [CrossRef]

]. This is somewhat similar to the center-of-mass exciton confinement when the exciton binding energy is larger than the confinement energy. For a diameter of 250nm in our ZnO nanorod, Δk = 2.5 × 105 cm−1 was estimated. Δkm = 7.0×105 cm−1 for 6T was also obtained by using the level spacing between the ground and the first excited state of the Landau levels. However, this value can be further reduced when Coulomb interactions are considered. In the case of the weak-field regime, the magnetically-induced confinement effect is likely to be suppressed due to thermal effects, but a reduction in the density of states might be expected as the arrayed points in k-space merge into the rings.

As shown in Fig. 3(a), we found that τr(T) in the presence of a flux density of 6T increases with temperature as τr(T) ∼ T1.3. Although the power order 1.3 may not represent the presence of an intermediate dimension, the power order of the temperature-dependent radiative decay time can be used to evaluate the nature of the effective confinement. On the other hand, when the magnetic length ( L=h¯eB) becomes comparable to the exciton Bohr radius (aB), it is also known that the oscillator strength of electron-hole pair becomes enhanced as the magnetic field increases due to shrinkage of the wavefunction of the relative motion of the electron and hole in the magnetic field. As a result, the radiative decay time decreases. We have estimated that the magnetic length (L = 10.4nm) at 6T is still larger than aB ∼ 1.8nm in ZnO, which confirms again the weak-field regime. Therefore, the relative decrease of the radiative decay time at 6T compared to that in the absence of a magnetic field may not be dominated by the wavefunction shrinkage effect [22

22. J. D. Berger, O. Lyngnes, H. M. Gibbs, G. Khitrova, T. R. Nelson, E. K. Lindmark, A. V. Kavokin, M. A. Kaliteevski, and V. V. Zapasskii, “Magnetic-field enhancement of the exciton-polariton splitting in a semiconductor quantum-well microcavity: The strong coupling threshold,” Phys. Revs. B 54, 1975–1981 (1996). [CrossRef]

].

As the exciton binding energy ( Ry*~60meV) is still larger than the Landau level spacing (h̄ωc ∼ 4.4meV), the diamagnetic perturbation energy of the center-of-mass exciton ( δε(B)=e2B2r28M) should be considered, where M is the total FX mass in the plane perpendicular to external field (Bẑ) corresponding to the vector potential A=12rBθ^ in polar coordinates (r, θ). The first-order perturbation of the diamagnetic effect is always of positive value (ε(1) > 0), but the second-order perturbation energy for the ground state can be negative (ε(2) < 0) in particular conditions. It is noticeable that the second-order perturbation for the excited states are also of positive value. In the case of a quantum well, the negative ε(2) occurs only when the state filling from the ground state is extended beyond a half of the energy between the ground state and the first excited state. Therefore, the redshift under 6T in Fig. 1(c) is possibly a consequence of the two competing terms (δε(B) ≃ ε(1) + ε(2) < 0), where ε(2) is more pronounced than ε(1). It was known that ZnO is intrinsically n-type without intentional doping due to zinc interstitials, oxygen vacancies, and hydrogen. As the doping density is high (1017 ∼ 1018cm−3), the negative ε(2) in our ZnO nanorods is possibly associated with state filling (or the Fermi level), which is similar to the case of a doped quantum well. However, further work is necessary to uncover the origin of the negative ε(2), which is not clear at the moment. Nevertheless, when ε(2) is negative, it is also known that the effective mass in the plane perpendicular to magnetic field becomes heavier ( M*>M) [8

8. F. V. Kyrychenko, Y. D. Jho, J. Kono, S. A. Cooker, G. D. Sanders, D. H. Reitze, C. J. Stanton, X. Wei, C. Kadow, and A. C. Gossard, “Interband magnetoabsorption study of the shift of the Fermi energy of a 2DEG with an in-plane magnetic field,” Phisica E 22, 624–627 (2004). [CrossRef]

]. As shown schematically in Fig. 3(b), suppose a weak magnetic field gives rise to a redshift and an enhancement of the effective mass in the plane perpendicular to the external magnetic field for the center-of-mass exciton dispersion, the range of radiative decay becomes smaller, i.e., both Δ and k0 decrease. These effects may explain why the power order of the temperature-dependent radiative decay time has decreased (τr(T) ∼ T1.3) in the presence of a weak external magnetic field (6T) when compared to the bulk nature in the absence of a magnetic field (τr(T) ∼ T1.5). Intuitively, the induced anisotropy of effective mass seems to change the confinement dimensionality effectively, i.e., the heavier effective mass in the plane perpendicular to the magnetic field affects transport, by which the lateral confinement effect is enhanced although the weak magnetic field is not enough to induce a one-dimensional nature.

4. Conclusion

The dimensional nature of confinement has been investigated in ZnO nanorods by using the temperature dependence of the radiative decay, where the temperature dependence of the integrated PL intensity was also used as a calibration factor. We confirmed that 250nm-diameter ZnO nanorods behave in a similar manner to bulk material (τrT1.5) at low temperatures (< 40K), whereby the alternative method of temperature-dependent decay time for determining dimensionality has been verified. In the weak magnetic field regime for ZnO (6T), a significant decrease in the power order of the temperature dependence of the radiative decay time (∼ T1.3) was attributed to an enhancement of the effective mass perpendicular to the magnetic field and a redshift of the center-of-mass exciton as a consequence of perturbation effects in the weak-field regime.

Acknowledgments

This work was supported by Basic Science Research Program through the NRF funded by Ministry of Education, Science and Technology ( BRL2011-0001566 and 2012R1A1A2006913) and Pioneer Research ( NRF-2013M3C1A3065522). T. Kiba and A. Murayama are supported from the Japan Science and Technology Agency and the Japan Society for the Promotion of Science (a Grant-in-Aid for Scientific Research (S) No. 22221007).

References and links

1.

Zhong Lin Wang, “Zinc oxide nanostructures: growth, properties and applications,” J. Phys.: Condens. Matter 16, R829–R858 (2004).

2.

Mark Fox, Optical Properties of Solids, 2nd Edition (OXFORD University Press, 2010).

3.

S. Schimitt-Rink, J. B. Stark, W. H. Knox, D. S. Chemla, and W. Schäfer, “Optical properties of quasi-zero-dimensional magneto-excitons,” Appl. Phys. A 53, 491–502 (1991). [CrossRef]

4.

Y. D. Jho, Xiaoming Wang, J. Kono, D. H. Reitze, X. Wei, A. A. Belyanin, V. V. Kocharovsky, Vl. V. Kocharovsky, and G. S. Solomon, “Cooperative Recombination of a Quantized High-Density Electron-Hole Plasma in Semiconductor Quantum Wells,” Phys. Rev. Lett. 96, 237401 (2006). [CrossRef] [PubMed]

5.

Claus F. Klingshirn, Semiconductor Optics, 4th Edition (Springer, 2012). [CrossRef]

6.

G. W. ’t Hooft, W. A. J. A. van der Poel, L. W. Molenkamp, and C. T. Foxon, “Giant oscillator strength of free excitons in GaAs,” Phys. Rev. B 35, 8281–8284 (1987). [CrossRef]

7.

H. Akiyama, S. Koshiba, T. Someya, K. Wada, H. Noge, Y. Nakamura, T. Inoshita, and A. Shimizu, “Thermalization Effect on Radiative Decay of Exciton in Quantum Wires,” Phys. Rev. Lett. 72, 924–927 (1994). [CrossRef] [PubMed]

8.

F. V. Kyrychenko, Y. D. Jho, J. Kono, S. A. Cooker, G. D. Sanders, D. H. Reitze, C. J. Stanton, X. Wei, C. Kadow, and A. C. Gossard, “Interband magnetoabsorption study of the shift of the Fermi energy of a 2DEG with an in-plane magnetic field,” Phisica E 22, 624–627 (2004). [CrossRef]

9.

Wei Geng, Sergei Kostcheev, Corinne Sartel, Vincent Sallet, Michael Molinari, Olivier Simonetti, Gilles Lérondel, Louis Giraudet, and Christophe Couteau, “Ohmic contact on single ZnO nanowires grown by MOCVD,” Phys. Status Solidi C. 10, 1292–1296 (2013).

10.

X. H. Zhang, S. J. Chua, A. M. Yong, H. Y. Yang, S. P. Lau, S. F. Yu, X. W. Sun, Lei Miao, Masaki Tanemura, and Sakae Tanemura, “Exciton radiative lifetime in ZnO nanorods fabricated by vapor phase transport method,” Appl. Phys. Lett. 90, 013107 (2007). [CrossRef]

11.

Fang-Yi Jen, Yen-Cheng Lu, Cheng-Yen Chen, Hsiang-Chen Wang, C. C. Yang, Bao-ping Zhang, and Yusaburo Segawa, “Temperature-dependent exciton dynamics in a ZnO thin film,” Appl. Phys. Lett. 87, 252117 (2005). [CrossRef]

12.

Cheol Hyoun, Ahn Sanjay, Kumar Mohanta, Nae Eung Lee, and Hyung Koun Cho, “Enhanced exciton-phonon interactions in photoluminescence of ZnO nanopencils,” Appl. Phys. Lett. 94, 269104 (2009).

13.

Walter R. L. Lambrecht, Anna V. Rodina, Sukit Limpijumnong, B. Segall, and Bruno K. Meyer, “Valence-band ordering and magneto-optic exciton fine structure in ZnO,” Phys. Rev. B 65, 075207 (2002). [CrossRef]

14.

S. J. Pearton, D. P. Norton, K. Ip, Y. W. Heo, and T. Steiner, “Recent advances in processing of ZnO,” J. Vac. Sci. Technol. B 22, 932–948 (2004). [CrossRef]

15.

Brendan Enright and Donald Fizmaurice, “Spectroscopic Determination of Electron and Hole Effective Masses in a Nanocrystalline Semiconductor Film,” J. Phys. Chem. 100, 1027–1035 (1996). [CrossRef]

16.

R. C. Miller, D. A. Kleinman, W. A. Nordland Jr., and A. C. Gossard, “Luminescence studies of optically pumped quantum wells in GaAs-AlxGa1−xAs multilayer structures,” Phys. Rev. B 22, 863–871 (1980). [CrossRef]

17.

D. Takamizu, Y. Nishimoto, S. Akasaka, H. Yuji, K. Tamura, K. Nakahara, T. Onuma, T. Tanabe, H. Takasu, M. Kawasaki, and S. F. Chichibu, “Direct correlation between the internal quantum efficiency and photoluminescence lifetime in undoped ZnO epilayers grown on Zn-polar ZnO substrates by plasma-assisted molecular beam epitaxy,” J. Appl. Phys. 103, 063502 (2008). [CrossRef]

18.

W. I. Park, Y. H. Jun, S. W. Jung, and Gyu-Chul Yi, “Excitonic emissions observed in ZnO single crystal nanorods,” Appl. Phys. Lett. 82, 964–966 (2003). [CrossRef]

19.

Richard L. Liboff, Introductory Quantum Mechanics, 4th Edition (Addison - Wesley, 2003).

20.

Kerson Huang, Statistical Mechanics, 2nd Edition (John Wiley & Sons, 1987).

21.

P. Zu, Z. K. Tang, G. K. L. Wong, M. Kawasaki, A. Ohtomo, H. Koinuma, and Y. Segawa, “Ultraviolet spontaneous and stimulated emissions from ZnO microcrystallite thin films at room temperature,” Solid State Commun. 103, 459–463 (1997). [CrossRef]

22.

J. D. Berger, O. Lyngnes, H. M. Gibbs, G. Khitrova, T. R. Nelson, E. K. Lindmark, A. V. Kavokin, M. A. Kaliteevski, and V. V. Zapasskii, “Magnetic-field enhancement of the exciton-polariton splitting in a semiconductor quantum-well microcavity: The strong coupling threshold,” Phys. Revs. B 54, 1975–1981 (1996). [CrossRef]

OCIS Codes
(160.3820) Materials : Magneto-optical materials
(160.6000) Materials : Semiconductor materials
(250.5230) Optoelectronics : Photoluminescence
(300.6500) Spectroscopy : Spectroscopy, time-resolved

ToC Category:
Materials

History
Original Manuscript: May 6, 2014
Revised Manuscript: July 9, 2014
Manuscript Accepted: July 9, 2014
Published: July 17, 2014

Citation
W. Lee, T. Kiba, A. Murayama, C. Sartel, V. Sallet, I. Kim, R. A. Taylor, Y. D. Jho, and K. Kyhm, "Temperature dependence of the radiative recombination time in ZnO nanorods under an external magnetic field of 6T," Opt. Express 22, 17959-17967 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-15-17959


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References

  1. Zhong Lin Wang, “Zinc oxide nanostructures: growth, properties and applications,” J. Phys.: Condens. Matter16, R829–R858 (2004).
  2. Mark Fox, Optical Properties of Solids, 2nd Edition (OXFORD University Press, 2010).
  3. S. Schimitt-Rink, J. B. Stark, W. H. Knox, D. S. Chemla, and W. Schäfer, “Optical properties of quasi-zero-dimensional magneto-excitons,” Appl. Phys. A53, 491–502 (1991). [CrossRef]
  4. Y. D. Jho, Xiaoming Wang, J. Kono, D. H. Reitze, X. Wei, A. A. Belyanin, V. V. Kocharovsky, Vl. V. Kocharovsky, and G. S. Solomon, “Cooperative Recombination of a Quantized High-Density Electron-Hole Plasma in Semiconductor Quantum Wells,” Phys. Rev. Lett.96, 237401 (2006). [CrossRef] [PubMed]
  5. Claus F. Klingshirn, Semiconductor Optics, 4th Edition (Springer, 2012). [CrossRef]
  6. G. W. ’t Hooft, W. A. J. A. van der Poel, L. W. Molenkamp, and C. T. Foxon, “Giant oscillator strength of free excitons in GaAs,” Phys. Rev. B35, 8281–8284 (1987). [CrossRef]
  7. H. Akiyama, S. Koshiba, T. Someya, K. Wada, H. Noge, Y. Nakamura, T. Inoshita, and A. Shimizu, “Thermalization Effect on Radiative Decay of Exciton in Quantum Wires,” Phys. Rev. Lett.72, 924–927 (1994). [CrossRef] [PubMed]
  8. F. V. Kyrychenko, Y. D. Jho, J. Kono, S. A. Cooker, G. D. Sanders, D. H. Reitze, C. J. Stanton, X. Wei, C. Kadow, and A. C. Gossard, “Interband magnetoabsorption study of the shift of the Fermi energy of a 2DEG with an in-plane magnetic field,” Phisica E22, 624–627 (2004). [CrossRef]
  9. Wei Geng, Sergei Kostcheev, Corinne Sartel, Vincent Sallet, Michael Molinari, Olivier Simonetti, Gilles Lérondel, Louis Giraudet, and Christophe Couteau, “Ohmic contact on single ZnO nanowires grown by MOCVD,” Phys. Status Solidi C.10, 1292–1296 (2013).
  10. X. H. Zhang, S. J. Chua, A. M. Yong, H. Y. Yang, S. P. Lau, S. F. Yu, X. W. Sun, Lei Miao, Masaki Tanemura, and Sakae Tanemura, “Exciton radiative lifetime in ZnO nanorods fabricated by vapor phase transport method,” Appl. Phys. Lett.90, 013107 (2007). [CrossRef]
  11. Fang-Yi Jen, Yen-Cheng Lu, Cheng-Yen Chen, Hsiang-Chen Wang, C. C. Yang, Bao-ping Zhang, and Yusaburo Segawa, “Temperature-dependent exciton dynamics in a ZnO thin film,” Appl. Phys. Lett.87, 252117 (2005). [CrossRef]
  12. Cheol Hyoun, Ahn Sanjay, Kumar Mohanta, Nae Eung Lee, and Hyung Koun Cho, “Enhanced exciton-phonon interactions in photoluminescence of ZnO nanopencils,” Appl. Phys. Lett.94, 269104 (2009).
  13. Walter R. L. Lambrecht, Anna V. Rodina, Sukit Limpijumnong, B. Segall, and Bruno K. Meyer, “Valence-band ordering and magneto-optic exciton fine structure in ZnO,” Phys. Rev. B65, 075207 (2002). [CrossRef]
  14. S. J. Pearton, D. P. Norton, K. Ip, Y. W. Heo, and T. Steiner, “Recent advances in processing of ZnO,” J. Vac. Sci. Technol. B22, 932–948 (2004). [CrossRef]
  15. Brendan Enright and Donald Fizmaurice, “Spectroscopic Determination of Electron and Hole Effective Masses in a Nanocrystalline Semiconductor Film,” J. Phys. Chem.100, 1027–1035 (1996). [CrossRef]
  16. R. C. Miller, D. A. Kleinman, W. A. Nordland, and A. C. Gossard, “Luminescence studies of optically pumped quantum wells in GaAs-AlxGa1−xAs multilayer structures,” Phys. Rev. B22, 863–871 (1980). [CrossRef]
  17. D. Takamizu, Y. Nishimoto, S. Akasaka, H. Yuji, K. Tamura, K. Nakahara, T. Onuma, T. Tanabe, H. Takasu, M. Kawasaki, and S. F. Chichibu, “Direct correlation between the internal quantum efficiency and photoluminescence lifetime in undoped ZnO epilayers grown on Zn-polar ZnO substrates by plasma-assisted molecular beam epitaxy,” J. Appl. Phys.103, 063502 (2008). [CrossRef]
  18. W. I. Park, Y. H. Jun, S. W. Jung, and Gyu-Chul Yi, “Excitonic emissions observed in ZnO single crystal nanorods,” Appl. Phys. Lett.82, 964–966 (2003). [CrossRef]
  19. Richard L. Liboff, Introductory Quantum Mechanics, 4th Edition (Addison - Wesley, 2003).
  20. Kerson Huang, Statistical Mechanics, 2nd Edition (John Wiley & Sons, 1987).
  21. P. Zu, Z. K. Tang, G. K. L. Wong, M. Kawasaki, A. Ohtomo, H. Koinuma, and Y. Segawa, “Ultraviolet spontaneous and stimulated emissions from ZnO microcrystallite thin films at room temperature,” Solid State Commun.103, 459–463 (1997). [CrossRef]
  22. J. D. Berger, O. Lyngnes, H. M. Gibbs, G. Khitrova, T. R. Nelson, E. K. Lindmark, A. V. Kavokin, M. A. Kaliteevski, and V. V. Zapasskii, “Magnetic-field enhancement of the exciton-polariton splitting in a semiconductor quantum-well microcavity: The strong coupling threshold,” Phys. Revs. B54, 1975–1981 (1996). [CrossRef]

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