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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 9 — May. 6, 2013
  • pp: 10483–10489
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Electrically pumped random lasing from FTO/porous insulator/n-ZnO/p+-Si devices

Yanjun Fang, Yewu Wang, Xi Ding, Ren Lu, Lin Gu, and Jian Sha  »View Author Affiliations


Optics Express, Vol. 21, Issue 9, pp. 10483-10489 (2013)
http://dx.doi.org/10.1364/OE.21.010483


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Abstract

Electrically pumped random lasing (RL) has been realized in FTO/porous insulator/n-ZnO/p+-Si devices. It is demonstrated that RL originates from the confining and recurrent scattering of light in the random cavities within the insulating layer, which are formed due to the glow discharge. The glow discharge also induces the observed negative differential resistance (NDR) effect following the normal I-V characteristics. The results present a new strategy to realize electrically pumped RL in ZnO-based metal-insulator-semiconductor device by simply modifying the morphology of the insulating layer.

© 2013 OSA

1. Introduction

Short-wavelength semiconductor lasers have raised considerable attention due to the potential applications in high capacity data storage, display, and lighting [1

1. S. Nakamura, “The roles of structural imperfections in InGaN-based blue light-emitting diodes and laser diodes,” Science 281(5379), 956–961 (1998). [CrossRef] [PubMed]

]. ZnO, as a wide band gap (Eg = 3.37 eV) semiconductor material with a high exciton-binding energy of 60 meV, shows promising prospects in realizing the low-threshold laser at room temperature [2

2. M. H. Huang, S. Mao, H. Feick, H. Q. Yan, Y. Y. Wu, H. Kind, E. Weber, R. Russo, and P. D. Yang, “Room-temperature ultraviolet nanowire nanolasers,” Science 292(5523), 1897–1899 (2001). [CrossRef] [PubMed]

]. The lasing mechanisms of ZnO-based nanostructures can be mainly classified into two catalogues, i.e. the cavity-lasing and the random-lasing [3

3. K. Okazaki, T. Shimogaki, K. Fusazaki, M. Higashihata, D. Nakamura, N. Koshizaki, and T. Okada, “Ultraviolet whispering-gallery-mode lasing in ZnO micro/nano sphere crystal,” Appl. Phys. Lett. 101(21), 211105 (2012). [CrossRef]

]. The random-lasing (RL) is particularly attractive compared to the cavity-lasing due to its board angular distribution that are suitable for lighting and display application, as well as its easy realization [4

4. H. Zhu, C. X. Shan, J. Y. Zhang, Z. Z. Zhang, B. H. Li, D. X. Zhao, B. Yao, D. Z. Shen, X. W. Fan, Z. K. Tang, X. Hou, and K. L. Choy, “Low-threshold electrically pumped random lasers,” Adv. Mater. 22(16), 1877–1881 (2010). [CrossRef] [PubMed]

]. Since the discovery of the optically pumped RL in zinc oxide polycrystalline films [5

5. H. Cao, Y. G. Zhao, H. C. Ong, S. T. Ho, J. Y. Dai, J. Y. Wu, and R. P. H. Chang, “Ultraviolet lasing in resonators formed by scattering in semiconductor polycrystalline films,” Appl. Phys. Lett. 73(25), 3656–3658 (1998). [CrossRef]

], much attention has been focused on the investigation of ZnO-based RL [6

6. S. F. Yu, C. Yuen, S. P. Lau, and H. W. Lee, “Zinc oxide thin-film random lasers on silicon substrate,” Appl. Phys. Lett. 84(17), 3244–3246 (2004). [CrossRef]

, 7

7. S. F. Yu, C. Yuen, S. P. Lau, W. I. Park, and G. C. Yi, “Random laser action in ZnO nanorod arrays embedded in ZnO epilayers,” Appl. Phys. Lett. 84(17), 3241–3243 (2004). [CrossRef]

]. Compared with the optically pumped RL, the electrically pumped lasing is more desirable. In 2006, Leong et al. [8

8. E. S. P. Leong and S. F. Yu, “UV random lasing action in p-SiC(4H)/i-ZnO–SiO2 nanocomposite/n-ZnO:Al heterojunction diodes,” Adv. Mater. 18(13), 1685–1688 (2006). [CrossRef]

] demonstrated the first electrically pumped RL in heterostructural p–i–n junctions using patterned ZnO–SiO2 nanocomposite film as the light-emitting layer. Later, Ma et al. presented a new strategy to realize electrically pumped RL through a Au/SiOx/ZnO metal-insulator-semiconductor (MIS) structure on Si substrate [9

9. X. Ma, P. Chen, D. Li, Y. Zhang, and D. Yang, “Electrically pumped ZnO film ultraviolet random lasers on silicon substrate,” Appl. Phys. Lett. 91(25), 251109 (2007). [CrossRef]

]. Regarding the formation mechanism of RL in MIS structure, it is generally believed that the recurrent scattering of light among the grain boundaries of the ZnO polycrystalline film accounts for it [4

4. H. Zhu, C. X. Shan, J. Y. Zhang, Z. Z. Zhang, B. H. Li, D. X. Zhao, B. Yao, D. Z. Shen, X. W. Fan, Z. K. Tang, X. Hou, and K. L. Choy, “Low-threshold electrically pumped random lasers,” Adv. Mater. 22(16), 1877–1881 (2010). [CrossRef] [PubMed]

, 9

9. X. Ma, P. Chen, D. Li, Y. Zhang, and D. Yang, “Electrically pumped ZnO film ultraviolet random lasers on silicon substrate,” Appl. Phys. Lett. 91(25), 251109 (2007). [CrossRef]

]. As a result, the light scattering capability of the ZnO layer, which is determined by its morphology, is crucial in realizing RL. For example, Tian et al. [10

10. Y. Tian, X. Ma, L. Jin, and D. Yang, “Electrically pumped ultraviolet random lasing from ZnO films: Compensation between optical gain and light scattering,” Appl. Phys. Lett. 97(25), 251115 (2010). [CrossRef]

] have demonstrated that the light scattering in sol-gel derived ZnO film is better than the sputtered one, which is favorable for the formation of electrically pumped RL. As a viable alternative to realize the RL action, however, the insulating layer may also be designed properly with certain morphology to function as the light confining and scattering cavities.

Herein, the electrically pumped RL has been realized from FTO/porous insulator/n-ZnO/p+-Si devices. The porous morphology of the insulating layer was proved to account for the RL, which presents a new strategy to realize the electrically pumped RL in MIS devices. In addition, our devices show a remarkable negative differential resistance (NDR) effect under sufficiently high forward bias following a normal I-V characteristic. Although the NDR effect was already observed in some cases [11

11. P. Chen, X. Ma, D. Li, Y. Zhang, and D. Yang, “Electrically pumped ultraviolet random lasing from ZnO-based metal-insulator-semiconductor devices: Dependence on carrier transport,” Opt. Express 17(6), 4712–4717 (2009). [CrossRef] [PubMed]

], the mechanism still remains an open issue. We think the glow discharge is responsible for the observed NDR effect in our devices.

2. Experiment

The ZnO films were fabricated on 1cm*1cm sized p+-Si (111) substrates via a simple hydrothermal method. Prior to the growth of ZnO films, thin ZnO seed layers with a thickness of around 30 nm were pre-deposited onto the substrates by magnetron sputtering. Then, the substrates were annealed at 550 °C in air for 1 h to improve the crystallinity of the seed layers. Afterwards, the substrates were put into a glass bottle filled with solution of zinc nitrate hexahydrate (Zn(NO3)2•6H2O) and methenamine (C6H12N4) in equal concentration of 0.05 M or 0.1 M. The reaction was kept at 95 °C for 2 h in an oven. After growth, the samples were rinsed with deionized water and dried. Finally, they were annealed at 400 °C in air for 1 h to promote the crystallization of the ZnO film.

The structure of the light-emitting devices is schematically illustrated in Fig. 1(a)
Fig. 1 Schematic of the FTO/insulator/n-ZnO/p+-Si device. (b) Room temperature PL spectrum of the as-grown ZnO film. The inset shows the top-view SEM image of the ZnO film grown with the precursor concentration of 0.05 M. The scale bar is 1 μm.
. Firstly, 150 nm thick Al electrode was deposited on the back of the Si substrates by magnetron sputtering as the bottom electrode, and the samples were rapid-annealed at 500 °C in N2 atmosphere for 1 min to obtain ohmic contact between Al and Si. Then, a layer of transparent insulating material (Poly(methyl methacrylate) (PMMA) or spin on glass (SOG)) with a thickness of around 50 nm was spin-coated on the surface of the ZnO film. Subsequently, the samples were either annealed at 200 °C for 30 min (in the case of PMMA) or at 500 °C for 1 h (in the case of SOG) in air to solidify the insulating layer. At last, a piece of FTO glass was directly contacted with the top surface of the sample as the top transparent electrode.

The morphology of the samples was characterized by scanning electron microscopy (SEM, KYKY-3200) and field-emission SEM (FESEM, Hitachi S-4800). The photoluminescence (PL) and electroluminescence (EL) measurements were carried out on a spectrometer (Edinburgh Instruments, FLS 920). For the PL measurement, a Xe lamp with the exciting wavelength of 300 nm was used as the excitation source. To acquire the EL spectra, the devices were applied with forward bias utilizing a Keithley 2400 source meter, with the positive voltage connected to the bottom Al electrode.

3. Results and discussions

The EL spectra of the ZnO film with PMMA as the insulating layer are shown in Fig. 2
Fig. 2 Room temperature EL spectra of the FTO/insulator/n-ZnO/p+-Si devices under different forward bias voltages with PMMA as the insulating layer.
. At low bias voltage, the spectrum is dominated by the yellow peak centered at round 575 nm, while the intensity of UV peak at 406 nm is relatively weak. We believe that this can be attributed to the spontaneous emission at the interface of the n-ZnO/p-Si heterojunction [15

15. S. W. Lee, H. D. Cho, G. Panin, and T. W. Kang, “Vertical ZnO nanorod/Si contact light-emitting diode,” Appl. Phys. Lett. 98(9), 093110 (2011). [CrossRef]

, 16

16. O. Lupan, T. Pauporté, and B. Viana, “Low-temperature growth of ZnO nanowire arrays on p-Silicon (111) for visible-light-emitting diode fabrication,” J. Phys. Chem. C 114(35), 14781–14785 (2010). [CrossRef]

]. It is noted that the EL spectra of our device are clearly different from its PL spectrum, which is due to the fact that EL is an interfacial process while PL probes the bulk property of the material [16

16. O. Lupan, T. Pauporté, and B. Viana, “Low-temperature growth of ZnO nanowire arrays on p-Silicon (111) for visible-light-emitting diode fabrication,” J. Phys. Chem. C 114(35), 14781–14785 (2010). [CrossRef]

]. With the further increase of the applied voltage, however, there is a sudden decrease of the current from above 200 mA to about 3 mA, nearly two orders of magnitude. At the same time, discrete sharp peaks with very narrow line-width (less than 0.1 nm) begin to emerge in the spectra. The spectra are very similar to RL, but considering that the spontaneous emission should also be observable in addition to the stimulated emission for a typical RL, they can hardly be regarded as RL. Moreover, the life-time of the emission is quite short. Its intensity will drop over an order of magnitude in a few seconds. Therefore, they are more like glow discharge spectra [17

17. S. Y. Moon, W. Choe, and B. K. Kang, “A uniform glow discharge plasma source at atmospheric pressure,” Appl. Phys. Lett. 84(2), 188–190 (2004). [CrossRef]

], and the magnitude of the current (several mA) is consistent with that of glow discharge. We have also changed the insulating layer from PMMA to SOG, and observed similar phenomenon. Hence, the glow discharge instead of the commonly observed RL occurred in our MIS devices.

During the EL measurement, we observed the formation of a layer of white substance on FTO at the same time of the current drop. When the FTO glass was detached from the sample, it could be clearly seen that the white substance firmly attached on both the FTO glass and the sample. The typical top-view SEM images of the FTO glass and SiOx-coated ZnO film before the EL measurement are shown in Figs. 3(a)
Fig. 3 Typical top-view SEM images of (a) FTO glass and (b) SiOx-coated ZnO film before the glow discharge, and of the SiOx patterns attached on (c) the FTO glass and (d) the surface of the ZnO film after the glow discharge. The inset of (c) is an enlarged version of the same sample.
and 3(b), respectively. Figures 3(c) and 3(d) are the top-view SEM images of the white substance on both the FTO glass and the ZnO film after the glow discharge when SOG was used as the insulating layer. It clearly shows that the white substance forms porous pattern on the FTO glass, and the feature size between walls is about several micrometers as shown in Fig. 3(c). In addition, Fig. 3(d) shows that the thin SiOx(x<2) layer we have coated on the ZnO film also has changed to porous and rough structure after the glow discharge. Hence, we can conclude that the white pattern on the FTO glass was formed during the EL measurement. Considering the glow discharge phenomenon we have observed, the SiOx porous pattern was most probably formed due to the sputtering of SiOx onto the FTO glass from the insulating layer during the discharge process. Interestingly, the obtained SiOx pattern was porous with numerous random cavities, which may formed due to the fact that the ZnO film we used was composed of discrete ZnO nanocolumns with high surface roughness, and the protrusion parts are more likely to be sputtered onto the FTO. Due to the unique morphology of the porous pattern, it is possible to use it as the random resonant cavities to realize RL.

The results point out that the RL can be realized in MIS structure by simply changing the morphology of the insulating layer. In fact, it is expected that the porous insulating pattern can be intentionally fabricated using the common photolithography method to replace the solid insulating film, which may become a viable manner to realize RL in the future.

4. Conclusion

In conclusion, electrically pumped RL has been realized in FTO/porous insulator/n-ZnO/p+-Si devices. It is believed that RL originates from the confining and recurrent scattering of light in the closed-loop random cavities within the insulating layer, which are formed due to the glow discharge. The glow discharge is also thought to be responsible for the observed NDR effect. The results here present a new strategy to realize the electrically pumped UV RL, and may provide more insights into its formation mechanism.

Acknowledgments

This work was supported by National Natural Science Foundation of China (Nos. 60976012 and 51272232), Program for New Century Excellent Talents in University, the Fundamental Research Funds for the Central Universities and the Science and Technology Innovative Research Team of Zhejiang Province (2009R50010).

References and links

1.

S. Nakamura, “The roles of structural imperfections in InGaN-based blue light-emitting diodes and laser diodes,” Science 281(5379), 956–961 (1998). [CrossRef] [PubMed]

2.

M. H. Huang, S. Mao, H. Feick, H. Q. Yan, Y. Y. Wu, H. Kind, E. Weber, R. Russo, and P. D. Yang, “Room-temperature ultraviolet nanowire nanolasers,” Science 292(5523), 1897–1899 (2001). [CrossRef] [PubMed]

3.

K. Okazaki, T. Shimogaki, K. Fusazaki, M. Higashihata, D. Nakamura, N. Koshizaki, and T. Okada, “Ultraviolet whispering-gallery-mode lasing in ZnO micro/nano sphere crystal,” Appl. Phys. Lett. 101(21), 211105 (2012). [CrossRef]

4.

H. Zhu, C. X. Shan, J. Y. Zhang, Z. Z. Zhang, B. H. Li, D. X. Zhao, B. Yao, D. Z. Shen, X. W. Fan, Z. K. Tang, X. Hou, and K. L. Choy, “Low-threshold electrically pumped random lasers,” Adv. Mater. 22(16), 1877–1881 (2010). [CrossRef] [PubMed]

5.

H. Cao, Y. G. Zhao, H. C. Ong, S. T. Ho, J. Y. Dai, J. Y. Wu, and R. P. H. Chang, “Ultraviolet lasing in resonators formed by scattering in semiconductor polycrystalline films,” Appl. Phys. Lett. 73(25), 3656–3658 (1998). [CrossRef]

6.

S. F. Yu, C. Yuen, S. P. Lau, and H. W. Lee, “Zinc oxide thin-film random lasers on silicon substrate,” Appl. Phys. Lett. 84(17), 3244–3246 (2004). [CrossRef]

7.

S. F. Yu, C. Yuen, S. P. Lau, W. I. Park, and G. C. Yi, “Random laser action in ZnO nanorod arrays embedded in ZnO epilayers,” Appl. Phys. Lett. 84(17), 3241–3243 (2004). [CrossRef]

8.

E. S. P. Leong and S. F. Yu, “UV random lasing action in p-SiC(4H)/i-ZnO–SiO2 nanocomposite/n-ZnO:Al heterojunction diodes,” Adv. Mater. 18(13), 1685–1688 (2006). [CrossRef]

9.

X. Ma, P. Chen, D. Li, Y. Zhang, and D. Yang, “Electrically pumped ZnO film ultraviolet random lasers on silicon substrate,” Appl. Phys. Lett. 91(25), 251109 (2007). [CrossRef]

10.

Y. Tian, X. Ma, L. Jin, and D. Yang, “Electrically pumped ultraviolet random lasing from ZnO films: Compensation between optical gain and light scattering,” Appl. Phys. Lett. 97(25), 251115 (2010). [CrossRef]

11.

P. Chen, X. Ma, D. Li, Y. Zhang, and D. Yang, “Electrically pumped ultraviolet random lasing from ZnO-based metal-insulator-semiconductor devices: Dependence on carrier transport,” Opt. Express 17(6), 4712–4717 (2009). [CrossRef] [PubMed]

12.

A. B. Djurisić and Y. H. Leung, “Optical properties of ZnO nanostructures,” Small 2(8-9), 944–961 (2006). [CrossRef] [PubMed]

13.

H. Wang, N. Koshizaki, L. Li, L. Jia, K. Kawaguchi, X. Li, A. Pyatenko, Z. Swiatkowska-Warkocka, Y. Bando, and D. Golberg, “Size-tailored ZnO submicrometer spheres: Bottom-up construction, size-related optical extinction, and selective aniline trapping,” Adv. Mater. 23(16), 1865–1870 (2011). [CrossRef] [PubMed]

14.

Y. J. Fang, Y. W. Wang, Y. T. Wan, Z. L. Wang, and J. A. Sha, “Detailed study on photoluminescence property and growth mechanism of ZnO nanowire arrays grown by thermal evaporation,” J. Phys. Chem. C 114(29), 12469–12476 (2010). [CrossRef]

15.

S. W. Lee, H. D. Cho, G. Panin, and T. W. Kang, “Vertical ZnO nanorod/Si contact light-emitting diode,” Appl. Phys. Lett. 98(9), 093110 (2011). [CrossRef]

16.

O. Lupan, T. Pauporté, and B. Viana, “Low-temperature growth of ZnO nanowire arrays on p-Silicon (111) for visible-light-emitting diode fabrication,” J. Phys. Chem. C 114(35), 14781–14785 (2010). [CrossRef]

17.

S. Y. Moon, W. Choe, and B. K. Kang, “A uniform glow discharge plasma source at atmospheric pressure,” Appl. Phys. Lett. 84(2), 188–190 (2004). [CrossRef]

18.

P. Chen, X. Ma, and D. Yang, “Fairly pure ultraviolet electroluminescence from ZnO-based light-emitting devices,” Appl. Phys. Lett. 89(11), 111112 (2006). [CrossRef]

OCIS Codes
(160.6000) Materials : Semiconductor materials
(230.2090) Optical devices : Electro-optical devices
(250.5960) Optoelectronics : Semiconductor lasers

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: March 13, 2013
Revised Manuscript: April 15, 2013
Manuscript Accepted: April 15, 2013
Published: April 22, 2013

Virtual Issues
Vol. 8, Iss. 6 Virtual Journal for Biomedical Optics

Citation
Yanjun Fang, Yewu Wang, Xi Ding, Ren Lu, Lin Gu, and Jian Sha, "Electrically pumped random lasing from FTO/porous insulator/n-ZnO/p+-Si devices," Opt. Express 21, 10483-10489 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-9-10483


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References

  1. S. Nakamura, “The roles of structural imperfections in InGaN-based blue light-emitting diodes and laser diodes,” Science281(5379), 956–961 (1998). [CrossRef] [PubMed]
  2. M. H. Huang, S. Mao, H. Feick, H. Q. Yan, Y. Y. Wu, H. Kind, E. Weber, R. Russo, and P. D. Yang, “Room-temperature ultraviolet nanowire nanolasers,” Science292(5523), 1897–1899 (2001). [CrossRef] [PubMed]
  3. K. Okazaki, T. Shimogaki, K. Fusazaki, M. Higashihata, D. Nakamura, N. Koshizaki, and T. Okada, “Ultraviolet whispering-gallery-mode lasing in ZnO micro/nano sphere crystal,” Appl. Phys. Lett.101(21), 211105 (2012). [CrossRef]
  4. H. Zhu, C. X. Shan, J. Y. Zhang, Z. Z. Zhang, B. H. Li, D. X. Zhao, B. Yao, D. Z. Shen, X. W. Fan, Z. K. Tang, X. Hou, and K. L. Choy, “Low-threshold electrically pumped random lasers,” Adv. Mater.22(16), 1877–1881 (2010). [CrossRef] [PubMed]
  5. H. Cao, Y. G. Zhao, H. C. Ong, S. T. Ho, J. Y. Dai, J. Y. Wu, and R. P. H. Chang, “Ultraviolet lasing in resonators formed by scattering in semiconductor polycrystalline films,” Appl. Phys. Lett.73(25), 3656–3658 (1998). [CrossRef]
  6. S. F. Yu, C. Yuen, S. P. Lau, and H. W. Lee, “Zinc oxide thin-film random lasers on silicon substrate,” Appl. Phys. Lett.84(17), 3244–3246 (2004). [CrossRef]
  7. S. F. Yu, C. Yuen, S. P. Lau, W. I. Park, and G. C. Yi, “Random laser action in ZnO nanorod arrays embedded in ZnO epilayers,” Appl. Phys. Lett.84(17), 3241–3243 (2004). [CrossRef]
  8. E. S. P. Leong and S. F. Yu, “UV random lasing action in p-SiC(4H)/i-ZnO–SiO2 nanocomposite/n-ZnO:Al heterojunction diodes,” Adv. Mater.18(13), 1685–1688 (2006). [CrossRef]
  9. X. Ma, P. Chen, D. Li, Y. Zhang, and D. Yang, “Electrically pumped ZnO film ultraviolet random lasers on silicon substrate,” Appl. Phys. Lett.91(25), 251109 (2007). [CrossRef]
  10. Y. Tian, X. Ma, L. Jin, and D. Yang, “Electrically pumped ultraviolet random lasing from ZnO films: Compensation between optical gain and light scattering,” Appl. Phys. Lett.97(25), 251115 (2010). [CrossRef]
  11. P. Chen, X. Ma, D. Li, Y. Zhang, and D. Yang, “Electrically pumped ultraviolet random lasing from ZnO-based metal-insulator-semiconductor devices: Dependence on carrier transport,” Opt. Express17(6), 4712–4717 (2009). [CrossRef] [PubMed]
  12. A. B. Djurisić and Y. H. Leung, “Optical properties of ZnO nanostructures,” Small2(8-9), 944–961 (2006). [CrossRef] [PubMed]
  13. H. Wang, N. Koshizaki, L. Li, L. Jia, K. Kawaguchi, X. Li, A. Pyatenko, Z. Swiatkowska-Warkocka, Y. Bando, and D. Golberg, “Size-tailored ZnO submicrometer spheres: Bottom-up construction, size-related optical extinction, and selective aniline trapping,” Adv. Mater.23(16), 1865–1870 (2011). [CrossRef] [PubMed]
  14. Y. J. Fang, Y. W. Wang, Y. T. Wan, Z. L. Wang, and J. A. Sha, “Detailed study on photoluminescence property and growth mechanism of ZnO nanowire arrays grown by thermal evaporation,” J. Phys. Chem. C114(29), 12469–12476 (2010). [CrossRef]
  15. S. W. Lee, H. D. Cho, G. Panin, and T. W. Kang, “Vertical ZnO nanorod/Si contact light-emitting diode,” Appl. Phys. Lett.98(9), 093110 (2011). [CrossRef]
  16. O. Lupan, T. Pauporté, and B. Viana, “Low-temperature growth of ZnO nanowire arrays on p-Silicon (111) for visible-light-emitting diode fabrication,” J. Phys. Chem. C114(35), 14781–14785 (2010). [CrossRef]
  17. S. Y. Moon, W. Choe, and B. K. Kang, “A uniform glow discharge plasma source at atmospheric pressure,” Appl. Phys. Lett.84(2), 188–190 (2004). [CrossRef]
  18. P. Chen, X. Ma, and D. Yang, “Fairly pure ultraviolet electroluminescence from ZnO-based light-emitting devices,” Appl. Phys. Lett.89(11), 111112 (2006). [CrossRef]

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