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Luminescence enhancement of ZnO nanoparticles on metal surface
ARTICLE IN PRESS Journal of Luminescence 129 (2009) 1759–1761
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Luminescence enhancement of ZnO nanoparticles on metal surface Y. Harada a,, I. Tanahashi a, N. Ohno b a b
Nanomaterials Microdevices Research Center, Osaka Institute of Technology, Osaka 535-8585, Japan Academic Frontier Promotion Center, Osaka Electro-Communication University, Neyagawa, Osaka 572-8530, Japan
a r t i c l e in fo
abstract
Available online 6 May 2009
We have studied luminescence enhancement of zinc oxide (ZnO) nanoparticles with the average size of 30 nm on several metal surfaces at low temperatures. Bandedge luminescence originated from bound exciton (BE) annihilation is observed at 3.360 eV, and strongly depends on the kind and surface roughness of metal. The luminescence intensity is about 10 times larger for Ag surface than that for quartz surface. Furthermore, the luminescence increases remarkably when the roughness of Ag surface is almost the same as the particle size. The intensity ratio of the fast decay component to the slow one decreases for Ag surface compared with quartz. These results suggest that the luminescence enhancement is partially attributed to suppressing of the nonradiative recombination process in ZnO nanoparticles on metal surface. & 2009 Elsevier B.V. All rights reserved.
PACS: 78.55.Et 78.67.Bf 71.35.Gg Keywords: ZnO nanoparticle Metal surface Roughness Bound exciton Luminescence enhancement Decay lifetime
1. Introduction Zinc oxide (ZnO) has a wide bandgap of 3.37 eV at room temperature and a large exciton binding energy of 60 meV, which could be expected to be used for UV–blue light-emitting devices and laser diodes. Recently, ZnO nano-particle, -rod, and -wire have been extensively investigated [1–6], because the semiconductor nanocrystals could exhibit superior optical properties owing to the quantum-size and/or surface effects as compared with the bulk crystal. In ZnO films coated with various metals, it has been reported that the exciton luminescence at room temperature increases significantly compared with that of no metal coating [7,8]. The luminescence enhancement is proposed to be originated from the resonant coupling between excitons and surface plasmon of metal or from increasing of the local electric fields induced by surface plasmon. The luminescence enhancement of Ag-coated ZnO films has been investigated for various Ag island sizes [9]. The enhancement strongly depends on the island size. The roughness may affect the coupling between exciton of ZnO and surface states of metal, and vary the luminescence intensity as a result. However, the influence of roughness of metal surface on the enhancement mechanism has not been understood well. In the present work, we have investigated the enhancement of exciton luminescence of ZnO nanoparticles on several metal surfaces by photoluminescence (PL) and time-resolved PL mea Corresponding author.
E-mail address: [email protected] (Y. Harada). 0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2009.04.055
surements at low temperatures. The luminescence enhancement has been studied with respect to the influence of the roughness of metal surface. It is known that the suppression of nonradiative recombination process plays a crucial role for the luminescence enhancement. To make clear the influence of the suppression on the enhancement, we have examined decay dynamics of exciton annihilation.
2. Experimental procedure Commercial ZnO nanoparticles with the average size of 30 nm grown by a plasma vapour deposition method were used for optical measurements. A SEM image of ZnO nanoparticles is shown in Fig. 1. For the SEM measurements, quite a lot of particles purposely were crowded onto a carbon sheet. Each particle seems to have a polygonal shape and smooth surfaces. The particles were dispersed in ultrapure water, and mixed in an ultrasonic stirrer. A small amount of the liquid was spread uniformly on the Ag, Au, Al, and quartz surface by using a spincoater, and dried rapidly in air. For Ag, the surfaces with different roughness were prepared by sputtering evaporation or physical grinding. The root-mean-square (RMS) roughness of the metal surface was estimated from the data obtained by a laser scanning microscope with a blue laser. A He–Cd laser (3.81 eV, 50 mW) was used as an excitation source for PL measurements. The emitted light from the sample was detected by a photon-counting system with a photomultiplier through a 75 cm monochromator. A second harmonic light
ARTICLE IN PRESS 1760
Y. Harada et al. / Journal of Luminescence 129 (2009) 1759–1761
ZnO nanoparticles (30nm) Excited by He-Cd laser (3.81 eV) 9K
Intensity (arb. units)
100 nm
BE
Ag surface FE-1LO
Au surface
quartz surface
Fig. 1. A SEM image of ZnO nanoparticles with the average size of 30 nm.
FE-2LO
(3.54 eV) of a mode-locked Ti:Sapphire laser with a repetition rate of 2 MHz and a pulse width of about 2 ps was used for timeresolved PL measurements. The emitted light was detected by a photon-counting streak camera through a spectrometer. The time resolution of the system was about 20 ps.
3. Results and discussion Fig. 2 shows luminescence spectra of ZnO nanoparticles on Ag and Au surfaces with the RMS roughness (30 nm) measured at 9 K. Luminescence spectrum of the particles on quartz surface is also shown for comparison. Two sharp luminescence lines due to the annihilation of bound excitons (BE) are observed at 3.360 and 3.364 eV. Broad luminescence bands appearing at 3.31 and 3.24 eV are ascribed to one and two longitudinal-optical (LO) phonon replicas of free exciton luminescence. The BE luminescence (3.360 eV) intensity is about 10 times larger for Ag and about five times for Au than that for quartz. As can be seen clearly, the bandedge luminescence of ZnO nanoparticles on Ag and Au surfaces increases remarkably compared with that on quartz. It is to be noted that the BE luminescence on Al surface has almost the same intensity as that for Ag surface (not shown in Fig. 2). These facts indicate that the enhancement of exciton luminescence of ZnO nanoparticles strongly depends on the kind of metal. On the other hand, there is no apparent difference in green luminescence related to oxygen vacancies [10] for the samples investigated. We have examined the influence of the roughness of metal surface on the exciton luminescence in detail. Fig. 3(a) shows luminescence spectra of ZnO nanoparticles on Ag surfaces with various RMS roughnesses. One can see that the luminescence intensity varies significantly with the RMS roughness, although the spectral shape does not change. Fig. 3(b) shows the BE luminescence intensity observed at 3.360 eV as a function of RMS roughness. As decreasing RMS roughness from 150 nm, the intensity increases gradually, reaching a maximum at around 30 nm, which is the same value as the average size of particles. Below 30 nm, the luminescence intensity decreases abruptly. The results indicate that the roughness of metal surface plays an important role in the luminescence enhancement, and that the luminescence is enhanced remarkably when the RMS roughness agrees with the particle size. Fig. 4 shows decay profiles of the BE luminescence (3.360 eV) of ZnO nanoparticles on Ag and quartz surfaces at 12 K. The decay curves can be well-fitted with two exponential functions; the
0 3.20
3.25
3.30 Photon Energy (eV)
3.35
3.40
Fig. 2. Luminescence spectra of ZnO nanoparticles (30 nm) on Ag, Au, and quartz surfaces at 9 K.
decay lifetimes are estimated to be 90 and 270 ps for Ag surface, and 65 and 260 ps for quartz surface. The lifetime of the fast component for Ag surface is somewhat longer than that for quartz surface. It is noteworthy that the intensity ratio of the fast decay component to the slow one decreases apparently for Ag surface compared with quartz. The luminescence enhancement of exciton luminescence has already been reported in CdSe nanocrystals on Au surface [11] and InGaN quantum wells coated with Ag and Al [12]. In InGaN, the 14-fold enhancement of the exciton luminescence near the bandgap was observed for Ag-coated sample, but no clear enhancement was seen in Au-coated samples. The enhancement of exciton luminescence in metal-coated semiconductors has been explained by the resonant coupling between excitons and surface plasmon of metal [13–15] or by the increasing of local electric field induced by the surface plasmon [8,14]. The roughness dependence of metal surface on the exciton luminescence intensity has been examined in CdSe/ZnS nanocrystals on Au surface [15,16]. The bandedge luminescence for rough Au surface increases significantly as compared with that for the flat surface. The luminescence intensity was also found to be sensitive to the crystal size. Their simulations have shown that the strength of local electric field is enhanced largely on rough metal surface. As the electric field enhancement can vary the strength of coupling between excitons and surface plasmon on metal surface, it seems that the exciton luminescence depends on the roughness of metal surface. The present results show that the luminescence intensity increases when the RMS roughness of metal surface is corresponding to the particle size. The large luminescence enhancement in the accordance between the roughness and the particle size may suggest that the mechanism is attributed to the electric field enhancement. It has been reported that the fast decay component originates from nonradiative recombination process of BE annihilation near the surface [2]. The present results obtained from the decay lifetime measurements suggest that the nonradiative recombination process of BE annihilation is suppressed in these ZnO nanoparticles on metal surface as compared with that on quartz
ARTICLE IN PRESS Y. Harada et al. / Journal of Luminescence 129 (2009) 1759–1761
1761
0.0 20 40
1.2 ZnO nanoparticles (30nm) 9K
1.0 Intensity (arb. units)
0.5
Intensity (arb. units)
1.0
60
3.0 3.1 3.2 3.3 3.4 Photon Energy (eV)
0.6 0.4 0.2
80
160
0.8
ss hne nm) g 100 u ( o S r ace 120 RM g surf 140 A of
0.0 0
50 100 150 200 RMS roughness of Ag surface (nm)
Fig. 3. (a) Luminescence spectra and (b) BE luminescence (3.360 eV) intensity observed of ZnO nanoparticles (30 nm) on Ag surface with various RMS roughnesses.
Acknowledgement ZnO nanoparticles (30nm) 12 K
Normalized Intensity (arb. units)
100
The authors would like to thank Y. Danhara for helping with the time-resolved PL measurements. This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports, and Culture. References
10-1 Ag surface
10-2
quartz surface
10-3 0.0
0.2
0.4 0.6 Time (ns)
0.8
1.0
Fig. 4. Decay profiles of BE luminescence observed at 3.360 eV in ZnO nanoparticles (30 nm) on Ag and quartz surfaces at 12 K. Each profile is normalized with the maximum intensity.
surface. It is probable that the suppression of the nonradiative recombination process could not be disregarded for the luminescence enhancement.
[1] X.W. Du, Y.S. Fu, J. Sun, X. Han, J. Liu, Semicond. Sci. Technol. 21 (2006) 1202. [2] T. Hirai, Y. Harada, S. Hashimoto, M. Matsumura, N. Ohno, Jpn. J. Appl. Phys. 46 (2007) L522. [3] J.G. Lu, Z.Z. Ye, J.Y. Huang, L.P. Zhu, B.H. Zhao, Z.L. Wang, Sz. Fujita, Appl. Phys. Lett. 88 (2006) 063110. [4] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [5] C.X. Xu, X.W. Sun, Z.L. Dong, M.B. Yu, T.D. My, X.H. Zhang, S.J. Chua, T.J. White, Nanotechnology 15 (2004) 839. [6] T.W. Kim, T. Kawazoe, S. Yamazaki, M. Ohtsu, T. Sekiguchi, Appl. Phys. Lett. 84 (2004) 3358. [7] C.W. Lai, J. An, H.C. Ong, Appl. Phys. Lett. 86 (2005) 251105. [8] W.H. Ni, J. An, C.W. Lai, H.C. Ong, J.B. Xu, J. Appl. Phys. 100 (2006) 026103. [9] P. Cheng, D. Li, Z. Yuan, P. Chen, D. Yang, Appl. Phys. Lett. 92 (2008) 041119. [10] K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, J.A. Voigt, B.E. Gnade, J. Appl. Phys. 79 (1996) 7983. [11] O. Kulakovich, N. Strekal, A. Yaroshevich, S. Maskevich, S. Gaponenko, I. Nabiev, U. Woggon, M. Artemyev, Nano Lett. 2 (2002) 1449. [12] K. Okamoto, I. Niki, A. Shvartser, Y. Narukawa, T. Mukai, A. Scherer, Nat. Mater. 3 (2004) 601. [13] A. Neogi, C.W. Lee, H.O. Everitt, T. Kuroda, A. Tackeuchi, E. Yablonovitch, Phys. Rev. B 66 (2002) 153305. [14] H. Song, T. Atay, S. Shi, H. Urada, A.V. Nurmikko, Nano Lett. 5 (2005) 1557. [15] K. Matsuda, Y. Ito, Y. Kanemitsu, Appl. Phys. Lett. 92 (2008) 211911. [16] Y. Ito, K. Matsuda, Y. Kanemitsu, Phys. Rev. B 75 (2007) 033309.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Luminescence enhancement of ZnO nanoparticles on metal surface Y. Harada a,, I. Tanahashi a, N. Ohno b a b
Nanomaterials Microdevices Research Center, Osaka Institute of Technology, Osaka 535-8585, Japan Academic Frontier Promotion Center, Osaka Electro-Communication University, Neyagawa, Osaka 572-8530, Japan
a r t i c l e in fo
abstract
Available online 6 May 2009
We have studied luminescence enhancement of zinc oxide (ZnO) nanoparticles with the average size of 30 nm on several metal surfaces at low temperatures. Bandedge luminescence originated from bound exciton (BE) annihilation is observed at 3.360 eV, and strongly depends on the kind and surface roughness of metal. The luminescence intensity is about 10 times larger for Ag surface than that for quartz surface. Furthermore, the luminescence increases remarkably when the roughness of Ag surface is almost the same as the particle size. The intensity ratio of the fast decay component to the slow one decreases for Ag surface compared with quartz. These results suggest that the luminescence enhancement is partially attributed to suppressing of the nonradiative recombination process in ZnO nanoparticles on metal surface. & 2009 Elsevier B.V. All rights reserved.
PACS: 78.55.Et 78.67.Bf 71.35.Gg Keywords: ZnO nanoparticle Metal surface Roughness Bound exciton Luminescence enhancement Decay lifetime
1. Introduction Zinc oxide (ZnO) has a wide bandgap of 3.37 eV at room temperature and a large exciton binding energy of 60 meV, which could be expected to be used for UV–blue light-emitting devices and laser diodes. Recently, ZnO nano-particle, -rod, and -wire have been extensively investigated [1–6], because the semiconductor nanocrystals could exhibit superior optical properties owing to the quantum-size and/or surface effects as compared with the bulk crystal. In ZnO films coated with various metals, it has been reported that the exciton luminescence at room temperature increases significantly compared with that of no metal coating [7,8]. The luminescence enhancement is proposed to be originated from the resonant coupling between excitons and surface plasmon of metal or from increasing of the local electric fields induced by surface plasmon. The luminescence enhancement of Ag-coated ZnO films has been investigated for various Ag island sizes [9]. The enhancement strongly depends on the island size. The roughness may affect the coupling between exciton of ZnO and surface states of metal, and vary the luminescence intensity as a result. However, the influence of roughness of metal surface on the enhancement mechanism has not been understood well. In the present work, we have investigated the enhancement of exciton luminescence of ZnO nanoparticles on several metal surfaces by photoluminescence (PL) and time-resolved PL mea Corresponding author.
E-mail address: [email protected] (Y. Harada). 0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2009.04.055
surements at low temperatures. The luminescence enhancement has been studied with respect to the influence of the roughness of metal surface. It is known that the suppression of nonradiative recombination process plays a crucial role for the luminescence enhancement. To make clear the influence of the suppression on the enhancement, we have examined decay dynamics of exciton annihilation.
2. Experimental procedure Commercial ZnO nanoparticles with the average size of 30 nm grown by a plasma vapour deposition method were used for optical measurements. A SEM image of ZnO nanoparticles is shown in Fig. 1. For the SEM measurements, quite a lot of particles purposely were crowded onto a carbon sheet. Each particle seems to have a polygonal shape and smooth surfaces. The particles were dispersed in ultrapure water, and mixed in an ultrasonic stirrer. A small amount of the liquid was spread uniformly on the Ag, Au, Al, and quartz surface by using a spincoater, and dried rapidly in air. For Ag, the surfaces with different roughness were prepared by sputtering evaporation or physical grinding. The root-mean-square (RMS) roughness of the metal surface was estimated from the data obtained by a laser scanning microscope with a blue laser. A He–Cd laser (3.81 eV, 50 mW) was used as an excitation source for PL measurements. The emitted light from the sample was detected by a photon-counting system with a photomultiplier through a 75 cm monochromator. A second harmonic light
ARTICLE IN PRESS 1760
Y. Harada et al. / Journal of Luminescence 129 (2009) 1759–1761
ZnO nanoparticles (30nm) Excited by He-Cd laser (3.81 eV) 9K
Intensity (arb. units)
100 nm
BE
Ag surface FE-1LO
Au surface
quartz surface
Fig. 1. A SEM image of ZnO nanoparticles with the average size of 30 nm.
FE-2LO
(3.54 eV) of a mode-locked Ti:Sapphire laser with a repetition rate of 2 MHz and a pulse width of about 2 ps was used for timeresolved PL measurements. The emitted light was detected by a photon-counting streak camera through a spectrometer. The time resolution of the system was about 20 ps.
3. Results and discussion Fig. 2 shows luminescence spectra of ZnO nanoparticles on Ag and Au surfaces with the RMS roughness (30 nm) measured at 9 K. Luminescence spectrum of the particles on quartz surface is also shown for comparison. Two sharp luminescence lines due to the annihilation of bound excitons (BE) are observed at 3.360 and 3.364 eV. Broad luminescence bands appearing at 3.31 and 3.24 eV are ascribed to one and two longitudinal-optical (LO) phonon replicas of free exciton luminescence. The BE luminescence (3.360 eV) intensity is about 10 times larger for Ag and about five times for Au than that for quartz. As can be seen clearly, the bandedge luminescence of ZnO nanoparticles on Ag and Au surfaces increases remarkably compared with that on quartz. It is to be noted that the BE luminescence on Al surface has almost the same intensity as that for Ag surface (not shown in Fig. 2). These facts indicate that the enhancement of exciton luminescence of ZnO nanoparticles strongly depends on the kind of metal. On the other hand, there is no apparent difference in green luminescence related to oxygen vacancies [10] for the samples investigated. We have examined the influence of the roughness of metal surface on the exciton luminescence in detail. Fig. 3(a) shows luminescence spectra of ZnO nanoparticles on Ag surfaces with various RMS roughnesses. One can see that the luminescence intensity varies significantly with the RMS roughness, although the spectral shape does not change. Fig. 3(b) shows the BE luminescence intensity observed at 3.360 eV as a function of RMS roughness. As decreasing RMS roughness from 150 nm, the intensity increases gradually, reaching a maximum at around 30 nm, which is the same value as the average size of particles. Below 30 nm, the luminescence intensity decreases abruptly. The results indicate that the roughness of metal surface plays an important role in the luminescence enhancement, and that the luminescence is enhanced remarkably when the RMS roughness agrees with the particle size. Fig. 4 shows decay profiles of the BE luminescence (3.360 eV) of ZnO nanoparticles on Ag and quartz surfaces at 12 K. The decay curves can be well-fitted with two exponential functions; the
0 3.20
3.25
3.30 Photon Energy (eV)
3.35
3.40
Fig. 2. Luminescence spectra of ZnO nanoparticles (30 nm) on Ag, Au, and quartz surfaces at 9 K.
decay lifetimes are estimated to be 90 and 270 ps for Ag surface, and 65 and 260 ps for quartz surface. The lifetime of the fast component for Ag surface is somewhat longer than that for quartz surface. It is noteworthy that the intensity ratio of the fast decay component to the slow one decreases apparently for Ag surface compared with quartz. The luminescence enhancement of exciton luminescence has already been reported in CdSe nanocrystals on Au surface [11] and InGaN quantum wells coated with Ag and Al [12]. In InGaN, the 14-fold enhancement of the exciton luminescence near the bandgap was observed for Ag-coated sample, but no clear enhancement was seen in Au-coated samples. The enhancement of exciton luminescence in metal-coated semiconductors has been explained by the resonant coupling between excitons and surface plasmon of metal [13–15] or by the increasing of local electric field induced by the surface plasmon [8,14]. The roughness dependence of metal surface on the exciton luminescence intensity has been examined in CdSe/ZnS nanocrystals on Au surface [15,16]. The bandedge luminescence for rough Au surface increases significantly as compared with that for the flat surface. The luminescence intensity was also found to be sensitive to the crystal size. Their simulations have shown that the strength of local electric field is enhanced largely on rough metal surface. As the electric field enhancement can vary the strength of coupling between excitons and surface plasmon on metal surface, it seems that the exciton luminescence depends on the roughness of metal surface. The present results show that the luminescence intensity increases when the RMS roughness of metal surface is corresponding to the particle size. The large luminescence enhancement in the accordance between the roughness and the particle size may suggest that the mechanism is attributed to the electric field enhancement. It has been reported that the fast decay component originates from nonradiative recombination process of BE annihilation near the surface [2]. The present results obtained from the decay lifetime measurements suggest that the nonradiative recombination process of BE annihilation is suppressed in these ZnO nanoparticles on metal surface as compared with that on quartz
ARTICLE IN PRESS Y. Harada et al. / Journal of Luminescence 129 (2009) 1759–1761
1761
0.0 20 40
1.2 ZnO nanoparticles (30nm) 9K
1.0 Intensity (arb. units)
0.5
Intensity (arb. units)
1.0
60
3.0 3.1 3.2 3.3 3.4 Photon Energy (eV)
0.6 0.4 0.2
80
160
0.8
ss hne nm) g 100 u ( o S r ace 120 RM g surf 140 A of
0.0 0
50 100 150 200 RMS roughness of Ag surface (nm)
Fig. 3. (a) Luminescence spectra and (b) BE luminescence (3.360 eV) intensity observed of ZnO nanoparticles (30 nm) on Ag surface with various RMS roughnesses.
Acknowledgement ZnO nanoparticles (30nm) 12 K
Normalized Intensity (arb. units)
100
The authors would like to thank Y. Danhara for helping with the time-resolved PL measurements. This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports, and Culture. References
10-1 Ag surface
10-2
quartz surface
10-3 0.0
0.2
0.4 0.6 Time (ns)
0.8
1.0
Fig. 4. Decay profiles of BE luminescence observed at 3.360 eV in ZnO nanoparticles (30 nm) on Ag and quartz surfaces at 12 K. Each profile is normalized with the maximum intensity.
surface. It is probable that the suppression of the nonradiative recombination process could not be disregarded for the luminescence enhancement.
[1] X.W. Du, Y.S. Fu, J. Sun, X. Han, J. Liu, Semicond. Sci. Technol. 21 (2006) 1202. [2] T. Hirai, Y. Harada, S. Hashimoto, M. Matsumura, N. Ohno, Jpn. J. Appl. Phys. 46 (2007) L522. [3] J.G. Lu, Z.Z. Ye, J.Y. Huang, L.P. Zhu, B.H. Zhao, Z.L. Wang, Sz. Fujita, Appl. Phys. Lett. 88 (2006) 063110. [4] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [5] C.X. Xu, X.W. Sun, Z.L. Dong, M.B. Yu, T.D. My, X.H. Zhang, S.J. Chua, T.J. White, Nanotechnology 15 (2004) 839. [6] T.W. Kim, T. Kawazoe, S. Yamazaki, M. Ohtsu, T. Sekiguchi, Appl. Phys. Lett. 84 (2004) 3358. [7] C.W. Lai, J. An, H.C. Ong, Appl. Phys. Lett. 86 (2005) 251105. [8] W.H. Ni, J. An, C.W. Lai, H.C. Ong, J.B. Xu, J. Appl. Phys. 100 (2006) 026103. [9] P. Cheng, D. Li, Z. Yuan, P. Chen, D. Yang, Appl. Phys. Lett. 92 (2008) 041119. [10] K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, J.A. Voigt, B.E. Gnade, J. Appl. Phys. 79 (1996) 7983. [11] O. Kulakovich, N. Strekal, A. Yaroshevich, S. Maskevich, S. Gaponenko, I. Nabiev, U. Woggon, M. Artemyev, Nano Lett. 2 (2002) 1449. [12] K. Okamoto, I. Niki, A. Shvartser, Y. Narukawa, T. Mukai, A. Scherer, Nat. Mater. 3 (2004) 601. [13] A. Neogi, C.W. Lee, H.O. Everitt, T. Kuroda, A. Tackeuchi, E. Yablonovitch, Phys. Rev. B 66 (2002) 153305. [14] H. Song, T. Atay, S. Shi, H. Urada, A.V. Nurmikko, Nano Lett. 5 (2005) 1557. [15] K. Matsuda, Y. Ito, Y. Kanemitsu, Appl. Phys. Lett. 92 (2008) 211911. [16] Y. Ito, K. Matsuda, Y. Kanemitsu, Phys. Rev. B 75 (2007) 033309.