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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 13 — Jun. 18, 2012
  • pp: 14556–14563
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Enhancement of bandgap emission of Pt-capped MgZnO films: Important role of light extraction versus exciton-plasmon coupling

W. F. Yang, Y. N. Xie, R. Y. Liao, J. Sun, Z. Y. Wu, L. M. Wong, S. J. Wang, C. F. Wang, Alex Y. S. Lee, and H. Gong  »View Author Affiliations


Optics Express, Vol. 20, Issue 13, pp. 14556-14563 (2012)
http://dx.doi.org/10.1364/OE.20.014556


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Abstract

We present on a systematic study of the contribution of surface plasmon (SP) coupling and light extraction toward emission enhancement of Platinum (Pt) nano-patterns capped MgZnO films. Time resolved Photoluminescence (PL) results indicate that the Pt coating can greatly reduces the non-radiative recombination rate by passivation of surface states, making the decay slow down. Temperature dependence of the integrated photoluminescence intensity reveals that the Pt nano-patterns can offer a large amount of light transfer and scattering, which enormously increase the light extraction efficiency up to 3.8-fold. These results indicate that the increased light extraction efficiency caused by surface modification via Pt coating rather than SP coupling plays a dominant role in increasing bandgap emission of MgZnO film.

© 2012 OSA

1. Introduction

Surface plasmon (SP), excited by the interaction between light and metal surface [1

1. T. W. Ebbesen, H. J. Lezec, H. F. Ghaemi, T. Thio, and P. A. Wolff, “Extraordinary optical transmission through sub-wavelength hole arrays,” Nature 391(6668), 667–669 (1998). [CrossRef]

3

3. J. Li, H. Iu, D. Y. Lei, J. T. K. Wan, J. B. Xu, H. P. Ho, M. Y. Waye, and H. C. Ong, “Dependence of surface plasmom lifetimes on the hole size in two-dimensional metallic arrays,” Appl. Phys. Lett. 94(18), 183112 (2009). [CrossRef]

] is known as an effective means to improve the emission efficiency of optoelectronic devices [4

4. A. A. Toropov, T. V. Shubina, V. N. Jmerik, S. V. Ivanov, Y. Ogawa, and F. Minami, “Optically enhanced emission of localized excitons in InxGa1-xN films by coupling to plasmons in a gold nanoparticle,” Phys. Rev. Lett. 103(3), 037403 (2009). [CrossRef] [PubMed]

, 5

5. K. Okamoto, I. Niki, A. Shvartser, Y. Narukawa, T. Mukai, and A. Scherer, “Surface-plasmon-enhanced light emitters based on InGaN quantum wells,” Nat. Mater. 3(9), 601–605 (2004). [CrossRef] [PubMed]

]. Recently, numerous studies have been conducted to improve the band edge emission in ZnO based materials and different metallic materials (Ag, Au, Pt, graphene etc.) have been used as capping layers [6

6. Y. J. Wang, H. P. He, Y. L. Zhang, L. W. Sun, L. Hu, K. W. Wu, J. Y. Huang, and Z. Z. Ye, “Metal enhanced photoluminescence from Al-capped ZnMgO films: The roles of plasmonic coupling and non-radiative recombination,” Appl. Phys. Lett. 100(11), 112103 (2012). [CrossRef]

14

14. S. W. Hwang, D. H. Shin, C. O. Kim, S. H. Hong, M. C. Kim, J. Kim, K. Y. Lim, S. Kim, S. H. Choi, K. J. Ahn, G. Kim, S. H. Sim, and B. H. Hong, “Plasmon-enhanced ultraviolet photoluminescence from hybrid structures of Graphene/ZnO films,” Phys. Rev. Lett. 105(12), 127403 (2010). [CrossRef] [PubMed]

]. Enhancements in these types of ultraviolet (UV) emissions were mainly interpreted in terms of coupling between plasmons of the attached metallic materials and excitons in the ZnO material. However, Y. J. Fang et al. [15

15. Y. J. Fang, J. Sha, Z. L. Wang, Y. T. Wan, W. W. Xia, and Y. W. Wang, “Behind the change of the photoluminescence property of metal-coated ZnO nanowire arrays,” Appl. Phys. Lett. 98(3), 033103 (2011). [CrossRef]

] have shown that the large enhancement of the UV emissions of metal coated ZnO nanowires originates from the electron transfer process (from metal to ZnO) due to the Ohmic contact formed between them. J. Song et al. [16

16. J. Song, X. An, J. Zhou, Y. Liu, W. Wang, X. Li, W. Lan, and E. Xie, “Investigation of enhanced ultraviolet emission from different Ti-capped ZnO structures via surface passivation and surface plasmon coupling,” Appl. Phys. Lett. 97(12), 122103 (2010). [CrossRef]

] and A. Dev et al. [17

17. A. Dev, J. P. Richters, J. Sartor, H. Kalt, J. Gutowski, and T. Voss, “Enhancement of the near-band-edge photoluminescence of ZnO nanowires: Important role of hydrogen incorporation versus plasmon resonances,” Appl. Phys. Lett. 98(13), 131111 (2011). [CrossRef]

] have shown that metal-deposition on ZnO can lead to strong enhancement of the UV emissions by passivating some defect states. Actually, surface modification by metal coatings may impact light extraction efficiency at metal/ZnO, which plays an important role in the quantum efficiency of the emitters due to the SP and scattering properties of metals [18

18. M. C. Tam, H. Su, K. S. Wong, X. Zhu, and H. S. Kwok, “Surface-plasmon-enhanced photoluminescence from metal-capped Alq3 thin Films,” Appl. Phys. Lett. 95(5), 051503 (2009). [CrossRef]

]. However, there have been few reports to clearly explicate the contribution of light extraction caused by surface modification toward emission enhancement of SP-emitter, and SP coupling mechanism of excitons and light scattering mechanism are far from being fully understood. On the other hand, MgZnO, which can be achieved by alloying ZnO with MgO, has been regarded as a very promising material for short wavelength optoelectronic device applications, such as UV and deep UV light emitting and laser diodes [19

19. Z. P. Wei, B. Yao, Z. Z. Zhang, Y. M. Lu, D. Z. Shen, B. H. Li, X. H. Wang, J. Y. Zhang, D. X. Zhao, X. W. Fan, and Z. K. Tang, “Formation of p-type MgZnO by nitrogen doping,” Appl. Phys. Lett. 89(10), 102104 (2006). [CrossRef]

, 20

20. Y. Tian, X. Y. Ma, D. S. Li, and D. R. Yang, “Electrically pumped ultraviolet random lasing from heterostructures formed by bilayered MgZnO films on silicon,” Appl. Phys. Lett. 97(6), 061111 (2010). [CrossRef]

]. However, MgO has a rock-salt structure which is dissimilar to the wurtzite structure of ZnO. Thus, the incorporation of Mg in ZnO may deteriorate the crystalline quality and usually produces defects with the complication of lattice mismatch and Mg substitutional impurities, which limits the emission efficiency. It is therefore of high importance to study the influence of metals on the optical properties of MgZnO films in detail, which will be of great significance to their future applications in the field of MgZnO-based optoelectronics.

In this work, we have investigated the contribution of SP coupling and light extraction toward light enhancement of Pt nano-patterns (NPs) capped MgZnO films. Temperature dependence of the integrated photoluminescence (PL) intensity indicates that this improvement can be attributed to the surface modification and surface plasmonic coupling, while increased extraction efficiency caused by surface modification via Pt plays a dominant role as compared to SP coupling in increasing bandgap emission of MgZnO films. Here we give a detailed interpretation about the mechanism of this interesting phenomenon.

2. Experimental details

Mg0.06Zn0.94O films with thickness of ~200 nm were prepared on c-plane sapphire substrate by pulsed laser deposition in an ultrahigh vacuum chamber, and were then subsequently capped with Pt coatings. A monolayer of highly ordered polystyrene (PS) spheres (400 nm in diameter) was self-assembled on the surface of MgZnO sample. Pt was subsequently sputtered onto the monolayer PS/MgZnO sample. After sputtering, the PS spheres were removed by acetone dissolving for 2 min in an ultrasonic bath, then Pt NPs were formed on the top of MgZnO film. In addition, a Pt film was directly sputtered onto MgZnO film as the reference sample and the thickness of Pt (~8 nm) is the same as the maximum height of Pt NPs. All samples were cut from one MgZnO wafer and were identical. Atomic force microscopy (AFM) was utilized to characterize the morphology of the Pt NPs/MgZnO film. PL spectra were obtained with a 325 nm He–Cd laser excitation. The time resolved PL (TRPL) spectra were measured at room temperature to determine the decay dynamics by time-correlated single photon counting with a resolution of 10 ps. A 266 nm coherent femtosecond pulsed laser with a repetition rate of 76 MHz, a pulse width of 200 fs, and an excitation power of 10 mW was used as the excitation source.

3. Results and discussion

To get a clearer understanding of the giant PL intensity enhancement and the dominantly increased extraction efficiency from Pt/MgZnO system, we presented a hypothesis and illustrated it in Fig. 4
Fig. 4 Schematic showing the enhancement mechanism of the bandgap emission for: (a) MgZnO, (b) Pt film/MgZnO, and (c) Pt NPs/MgZnO. Krad: radiative recombination; knon: non-radiative recombination.
. Usually, exciton photo-generated in semiconductor materials decay through both radiative (krad) and non-radiative recombination (knon) processes (Fig. 4(a)). Only a small fraction of the lights generated inside MgZnO film can escape because of total internal reflection at the interface of MgZnO and the outer medium air. A rough estimation for the MgZnO/air single interface leads to an extraction efficiency of about ηext~1/4n2~6% [22

22. A. I. Zhmakin, “Enhancement of light extraction from light emitting diodes,” Phys. Rep. 498(4-5), 189–241 (2011). [CrossRef]

]. For bare MgZnO film, the surface defect states are rather high according to the observation of a strong defect emission. After blanketing with Pt coating, the surface states were passivated, which in turn decreases the non-radiative recombination rate and increases radiative recombination. On the other hand, the Pt layer can increase the light ηext since the photons can be converted to free space radiation via scattering by rough surface of Pt layer (Fig. 4(b) and Fig. 4(c)). This implies that the increase of ηextraction after Pt coating is attributed to the enhanced light extraction from Pt coating/MgZnO film by the surface modification. By proper engineering of the metal structures, light can be energy transferred from MgZnO film into a thin metal layer and scattered from the metal layer into free space, thereby enormously increase ηextractionof the light. While the distinction of ηextractionin the two metal patterns implies that, for Pt NPs, surface roughness of Pt film/MgZnO can enhance bandgap emission, but some of the energies can be thermally dissipated by non-radiative recombination (knon) (Fig. 4(b)). As for the Pt NPs capping, the demonstrated eight-fold enhancement of bandgap emission is attributed to the fact that the periodic Pt NPs layer not only increase the ηint but also offer a large amount of light transfer and scattering, which enormously increase the ηextraction up to 3.8-fold (Fig. 4(c)).

4. Conclusion

In summary, we present on a systematic study of the contribution of SP coupling and light extraction toward emission enhancement of Pt-capped MgZnO films. Temperature dependence of the integrated PL intensity indicates that this improvement can be attributed to the increased extraction efficiency as well as increased internal quantum efficiency due to the surface modification and surface plasmonic coupling, while increased light extraction efficiency caused by surface modification plays a dominant role in increasing bandgap emission of MgZnO films. The TRPL results indicate that the Pt coating can greatly reduce the non-radiative recombination rate by passivation of surface states, making the decay slow down.

Acknowledgments

Financial support from the Clean Energy Research Program (Grant Nos. NRF2008EWT-CERP002-041, NUS R284-000-081-592) under Singapore EDB and Du Pont Apollo is acknowledged.

References and links

1.

T. W. Ebbesen, H. J. Lezec, H. F. Ghaemi, T. Thio, and P. A. Wolff, “Extraordinary optical transmission through sub-wavelength hole arrays,” Nature 391(6668), 667–669 (1998). [CrossRef]

2.

A. Bouhelier, R. Bachelot, G. Lerondel, S. Kostcheev, P. Royer, and G. P. Wiederrecht, “Surface plasmon characteristics of tunable photoluminescence in single gold nanorods,” Phys. Rev. Lett. 95(26), 267405 (2005). [CrossRef] [PubMed]

3.

J. Li, H. Iu, D. Y. Lei, J. T. K. Wan, J. B. Xu, H. P. Ho, M. Y. Waye, and H. C. Ong, “Dependence of surface plasmom lifetimes on the hole size in two-dimensional metallic arrays,” Appl. Phys. Lett. 94(18), 183112 (2009). [CrossRef]

4.

A. A. Toropov, T. V. Shubina, V. N. Jmerik, S. V. Ivanov, Y. Ogawa, and F. Minami, “Optically enhanced emission of localized excitons in InxGa1-xN films by coupling to plasmons in a gold nanoparticle,” Phys. Rev. Lett. 103(3), 037403 (2009). [CrossRef] [PubMed]

5.

K. Okamoto, I. Niki, A. Shvartser, Y. Narukawa, T. Mukai, and A. Scherer, “Surface-plasmon-enhanced light emitters based on InGaN quantum wells,” Nat. Mater. 3(9), 601–605 (2004). [CrossRef] [PubMed]

6.

Y. J. Wang, H. P. He, Y. L. Zhang, L. W. Sun, L. Hu, K. W. Wu, J. Y. Huang, and Z. Z. Ye, “Metal enhanced photoluminescence from Al-capped ZnMgO films: The roles of plasmonic coupling and non-radiative recombination,” Appl. Phys. Lett. 100(11), 112103 (2012). [CrossRef]

7.

K. W. Liu, Y. D. Tang, C. X. Cong, T. C. Sum, A. C. H. Huan, Z. X. Shen, L. Wang, F. Y. Jiang, X. W. Sun, and H. D. Sun, “Giant enhancement of top emission from ZnO thin film by nanopatterned Pt,” Appl. Phys. Lett. 94(15), 151102 (2009). [CrossRef]

8.

D. Y. Lei and H. C. Ong, “Enhanced forward emission from ZnO via surface plasmons,” Appl. Phys. Lett. 91(21), 211107 (2007). [CrossRef]

9.

M. Liu, S. W. Qu, W. W. Yu, S. Y. Bao, C. Y. Ma, Q. Y. Zhang, J. He, J. C. Jiang, E. I. Meletis, and C. L. Chen, “Photoluminescence and extinction enhancement from ZnO films embedded with Ag nanoparticles,” Appl. Phys. Lett. 97(23), 231906 (2010). [CrossRef]

10.

W. F. Yang, R. Chen, B. Liu, G. G. Gurzadyan, L. M. Wong, S. J. Wang, and H. D. Sun, “Surface-plasmon enhancement of bandgap emission from ZnCdO thin films by gold particles,” Appl. Phys. Lett. 97(6), 061104 (2010). [CrossRef]

11.

C. W. Cheng, E. J. Sie, B. Liu, C. H. A. Huan, T. C. Sum, H. D. Sun, and H. J. Fan, “Surface plasmon enhanced bandedge luminescence of ZnO nanorods by capping Au nanoparticles,” Appl. Phys. Lett. 96(7), 071107 (2010). [CrossRef]

12.

C. W. Lai, J. An, and H. C. Ong, “Surface-plasmon-mediated emission from metal-capped ZnO thin films,” Appl. Phys. Lett. 86(25), 251105 (2005). [CrossRef]

13.

S. Kim, D. H. Shin, C. O. Kim, S. W. Hwang, S. H. Choi, S. Ji, and J. Y. Koo, “Enhanced ultraviolet emission from hybrid structures of single-walled carbon nanotubes/ZnO films,” Appl. Phys. Lett. 94(21), 213113 (2009). [CrossRef]

14.

S. W. Hwang, D. H. Shin, C. O. Kim, S. H. Hong, M. C. Kim, J. Kim, K. Y. Lim, S. Kim, S. H. Choi, K. J. Ahn, G. Kim, S. H. Sim, and B. H. Hong, “Plasmon-enhanced ultraviolet photoluminescence from hybrid structures of Graphene/ZnO films,” Phys. Rev. Lett. 105(12), 127403 (2010). [CrossRef] [PubMed]

15.

Y. J. Fang, J. Sha, Z. L. Wang, Y. T. Wan, W. W. Xia, and Y. W. Wang, “Behind the change of the photoluminescence property of metal-coated ZnO nanowire arrays,” Appl. Phys. Lett. 98(3), 033103 (2011). [CrossRef]

16.

J. Song, X. An, J. Zhou, Y. Liu, W. Wang, X. Li, W. Lan, and E. Xie, “Investigation of enhanced ultraviolet emission from different Ti-capped ZnO structures via surface passivation and surface plasmon coupling,” Appl. Phys. Lett. 97(12), 122103 (2010). [CrossRef]

17.

A. Dev, J. P. Richters, J. Sartor, H. Kalt, J. Gutowski, and T. Voss, “Enhancement of the near-band-edge photoluminescence of ZnO nanowires: Important role of hydrogen incorporation versus plasmon resonances,” Appl. Phys. Lett. 98(13), 131111 (2011). [CrossRef]

18.

M. C. Tam, H. Su, K. S. Wong, X. Zhu, and H. S. Kwok, “Surface-plasmon-enhanced photoluminescence from metal-capped Alq3 thin Films,” Appl. Phys. Lett. 95(5), 051503 (2009). [CrossRef]

19.

Z. P. Wei, B. Yao, Z. Z. Zhang, Y. M. Lu, D. Z. Shen, B. H. Li, X. H. Wang, J. Y. Zhang, D. X. Zhao, X. W. Fan, and Z. K. Tang, “Formation of p-type MgZnO by nitrogen doping,” Appl. Phys. Lett. 89(10), 102104 (2006). [CrossRef]

20.

Y. Tian, X. Y. Ma, D. S. Li, and D. R. Yang, “Electrically pumped ultraviolet random lasing from heterostructures formed by bilayered MgZnO films on silicon,” Appl. Phys. Lett. 97(6), 061111 (2010). [CrossRef]

21.

M. Trunk, V. Venkatachalapathy, A. Galeckas, and A. Yu. Kuznetsov, “Deep level related photoluminescence in ZnMgO,” Appl. Phys. Lett. 97(21), 211901 (2010). [CrossRef]

22.

A. I. Zhmakin, “Enhancement of light extraction from light emitting diodes,” Phys. Rep. 498(4-5), 189–241 (2011). [CrossRef]

OCIS Codes
(230.0250) Optical devices : Optoelectronics
(240.6680) Optics at surfaces : Surface plasmons
(250.5230) Optoelectronics : Photoluminescence
(220.4241) Optical design and fabrication : Nanostructure fabrication

ToC Category:
Optoelectronics

History
Original Manuscript: March 27, 2012
Revised Manuscript: April 27, 2012
Manuscript Accepted: May 5, 2012
Published: June 14, 2012

Citation
W. F. Yang, Y. N. Xie, R. Y. Liao, J. Sun, Z. Y. Wu, L. M. Wong, S. J. Wang, C. F. Wang, Alex Y. S. Lee, and H. Gong, "Enhancement of bandgap emission of Pt-capped MgZnO films: Important role of light extraction versus exciton-plasmon coupling," Opt. Express 20, 14556-14563 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-13-14556


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References

  1. T. W. Ebbesen, H. J. Lezec, H. F. Ghaemi, T. Thio, and P. A. Wolff, “Extraordinary optical transmission through sub-wavelength hole arrays,” Nature391(6668), 667–669 (1998). [CrossRef]
  2. A. Bouhelier, R. Bachelot, G. Lerondel, S. Kostcheev, P. Royer, and G. P. Wiederrecht, “Surface plasmon characteristics of tunable photoluminescence in single gold nanorods,” Phys. Rev. Lett.95(26), 267405 (2005). [CrossRef] [PubMed]
  3. J. Li, H. Iu, D. Y. Lei, J. T. K. Wan, J. B. Xu, H. P. Ho, M. Y. Waye, and H. C. Ong, “Dependence of surface plasmom lifetimes on the hole size in two-dimensional metallic arrays,” Appl. Phys. Lett.94(18), 183112 (2009). [CrossRef]
  4. A. A. Toropov, T. V. Shubina, V. N. Jmerik, S. V. Ivanov, Y. Ogawa, and F. Minami, “Optically enhanced emission of localized excitons in InxGa1-xN films by coupling to plasmons in a gold nanoparticle,” Phys. Rev. Lett.103(3), 037403 (2009). [CrossRef] [PubMed]
  5. K. Okamoto, I. Niki, A. Shvartser, Y. Narukawa, T. Mukai, and A. Scherer, “Surface-plasmon-enhanced light emitters based on InGaN quantum wells,” Nat. Mater.3(9), 601–605 (2004). [CrossRef] [PubMed]
  6. Y. J. Wang, H. P. He, Y. L. Zhang, L. W. Sun, L. Hu, K. W. Wu, J. Y. Huang, and Z. Z. Ye, “Metal enhanced photoluminescence from Al-capped ZnMgO films: The roles of plasmonic coupling and non-radiative recombination,” Appl. Phys. Lett.100(11), 112103 (2012). [CrossRef]
  7. K. W. Liu, Y. D. Tang, C. X. Cong, T. C. Sum, A. C. H. Huan, Z. X. Shen, L. Wang, F. Y. Jiang, X. W. Sun, and H. D. Sun, “Giant enhancement of top emission from ZnO thin film by nanopatterned Pt,” Appl. Phys. Lett.94(15), 151102 (2009). [CrossRef]
  8. D. Y. Lei and H. C. Ong, “Enhanced forward emission from ZnO via surface plasmons,” Appl. Phys. Lett.91(21), 211107 (2007). [CrossRef]
  9. M. Liu, S. W. Qu, W. W. Yu, S. Y. Bao, C. Y. Ma, Q. Y. Zhang, J. He, J. C. Jiang, E. I. Meletis, and C. L. Chen, “Photoluminescence and extinction enhancement from ZnO films embedded with Ag nanoparticles,” Appl. Phys. Lett.97(23), 231906 (2010). [CrossRef]
  10. W. F. Yang, R. Chen, B. Liu, G. G. Gurzadyan, L. M. Wong, S. J. Wang, and H. D. Sun, “Surface-plasmon enhancement of bandgap emission from ZnCdO thin films by gold particles,” Appl. Phys. Lett.97(6), 061104 (2010). [CrossRef]
  11. C. W. Cheng, E. J. Sie, B. Liu, C. H. A. Huan, T. C. Sum, H. D. Sun, and H. J. Fan, “Surface plasmon enhanced bandedge luminescence of ZnO nanorods by capping Au nanoparticles,” Appl. Phys. Lett.96(7), 071107 (2010). [CrossRef]
  12. C. W. Lai, J. An, and H. C. Ong, “Surface-plasmon-mediated emission from metal-capped ZnO thin films,” Appl. Phys. Lett.86(25), 251105 (2005). [CrossRef]
  13. S. Kim, D. H. Shin, C. O. Kim, S. W. Hwang, S. H. Choi, S. Ji, and J. Y. Koo, “Enhanced ultraviolet emission from hybrid structures of single-walled carbon nanotubes/ZnO films,” Appl. Phys. Lett.94(21), 213113 (2009). [CrossRef]
  14. S. W. Hwang, D. H. Shin, C. O. Kim, S. H. Hong, M. C. Kim, J. Kim, K. Y. Lim, S. Kim, S. H. Choi, K. J. Ahn, G. Kim, S. H. Sim, and B. H. Hong, “Plasmon-enhanced ultraviolet photoluminescence from hybrid structures of Graphene/ZnO films,” Phys. Rev. Lett.105(12), 127403 (2010). [CrossRef] [PubMed]
  15. Y. J. Fang, J. Sha, Z. L. Wang, Y. T. Wan, W. W. Xia, and Y. W. Wang, “Behind the change of the photoluminescence property of metal-coated ZnO nanowire arrays,” Appl. Phys. Lett.98(3), 033103 (2011). [CrossRef]
  16. J. Song, X. An, J. Zhou, Y. Liu, W. Wang, X. Li, W. Lan, and E. Xie, “Investigation of enhanced ultraviolet emission from different Ti-capped ZnO structures via surface passivation and surface plasmon coupling,” Appl. Phys. Lett.97(12), 122103 (2010). [CrossRef]
  17. A. Dev, J. P. Richters, J. Sartor, H. Kalt, J. Gutowski, and T. Voss, “Enhancement of the near-band-edge photoluminescence of ZnO nanowires: Important role of hydrogen incorporation versus plasmon resonances,” Appl. Phys. Lett.98(13), 131111 (2011). [CrossRef]
  18. M. C. Tam, H. Su, K. S. Wong, X. Zhu, and H. S. Kwok, “Surface-plasmon-enhanced photoluminescence from metal-capped Alq3 thin Films,” Appl. Phys. Lett.95(5), 051503 (2009). [CrossRef]
  19. Z. P. Wei, B. Yao, Z. Z. Zhang, Y. M. Lu, D. Z. Shen, B. H. Li, X. H. Wang, J. Y. Zhang, D. X. Zhao, X. W. Fan, and Z. K. Tang, “Formation of p-type MgZnO by nitrogen doping,” Appl. Phys. Lett.89(10), 102104 (2006). [CrossRef]
  20. Y. Tian, X. Y. Ma, D. S. Li, and D. R. Yang, “Electrically pumped ultraviolet random lasing from heterostructures formed by bilayered MgZnO films on silicon,” Appl. Phys. Lett.97(6), 061111 (2010). [CrossRef]
  21. M. Trunk, V. Venkatachalapathy, A. Galeckas, and A. Yu. Kuznetsov, “Deep level related photoluminescence in ZnMgO,” Appl. Phys. Lett.97(21), 211901 (2010). [CrossRef]
  22. A. I. Zhmakin, “Enhancement of light extraction from light emitting diodes,” Phys. Rep.498(4-5), 189–241 (2011). [CrossRef]

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