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Energy Express

  • Editor: Christian Seassal
  • Vol. 22, Iss. S4 — Jun. 30, 2014
  • pp: A1071–A1078
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Enhanced light extraction from GaN based light-emitting diodes using a hemispherical NiCoO lens

Do-Hyun Kim, Dong Su Shin, and Jinsub Park  »View Author Affiliations


Optics Express, Vol. 22, Issue S4, pp. A1071-A1078 (2014)
http://dx.doi.org/10.1364/OE.22.0A1071


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Abstract

We report on the improvement of light extraction efficiency from GaN-based light-emitting diodes (LEDs) using Ni1-xCoxO nanoparticles (NPs) formed on the p-GaN layer. After formation of Ni1-xCoxO hemispherical lens arrays on the blue LEDs by drop-casting colloidal NPs, electroluminescence (EL) and photoluminescence (PL) measurements are conducted to investigate the electrical and optical properties. The PL and EL intensities from the blue LEDs with the Ni1-xCoxO NPs are 1.74 and 1.61 times greater, respectively, than a conventional LED. Finally, a hybrid approach using ZnO nanorod arrays grown on the NiCoO hemispherical lens shows further increase of light extraction by 3.8 and 6.2 times compared to LEDs with bare NiCoO NPs and without any NPs, respectively. Finite-difference time-domain (FDTD) simulation results also agree well with the experimental results. The enhancement of light extraction from LEDs with ZnO nanorods and NiCoO NPs can be attributed to an enlarged escape angle cone and increased probability of light scattering.

© 2014 Optical Society of America

1. Introduction

2. Experimental details

A solution-based synthesis was used to obtain NiCoO NPs; the raw materials were Ni(II) acetate tetrahydrate(98.0%), Co(II) acetate tetrahydrate(98.0%), and KOH potassium hydroxide(85.0%). The initial stock solutions were made with 150 mL deionized (DI) water, dissolving the nickel acetate- and cobalt acetate-based salts in a 70:30 weight ratio. The metallic precursor homogeneity was achieved by simply stirring the stock solutions continuously for 48 hrs prior to the precipitation reactions. The stock solutions were then treated with drops of 10 M KOH until precipitation ceased. The details for the NiCoO NP synthesis can be found in other reference [10

10. G. M. Kumar, D. H. Kim, and J. Park, “Colloidal Ni1-xCoxO nanostructures as anodic buffer layers in hybrid polypyrrole/Zn1-yInyO p–n junctions,” CrystEngComm 15(40), 8195–8201 (2013). [CrossRef]

]. After synthesis of NiCoO, the NPs are drop-casted onto the surface of GaN-based LED epi structures grown by metal organic chemical vapor deposition (MOCVD) [11

11. K. Song and J. Park, “Effects of the growth pressure of a-plane InGaN/GaN multi-quantum wells on the optical performance of light-emitting diodes,” Semicond. Sci. Technol. 28(1), 015010 (2013). [CrossRef]

]. Finally, ZnO nanorod arrays are formed on top of the NiCoO NPs using the hydrothermal method [6

6. T. Son, K. Y. Jung, and J. Park, “Enhancement of the light extraction of GaN-based green light emitting diodes via nanohybrid structures,” Curr. Appl. Phys. 13(6), 1042–1045 (2013). [CrossRef]

]. Photoluminescence (PL) and electroluminescence (EL) measurements were conducted to determine the effects of the NiCoO NPs and the ZnO nanorod arrays on the light extraction efficiency in LEDs.

3. Results and discussion

Fig. 1 Schematic diagram of fabricated blue LED structures with ZnO nanorod arrays on Ni1-xCoxO hemisphere lenses.
Figure 1 shows a schematic diagram of a blue LED structure with the NiCoO NP hemispherical lenses and ZnO nanorod arrays formed on the top-most layer. The layer structure of the blue LEDs with 450 nm emission was grown by MOCVD on c-plane Al2O3 substrates. After coating the NiCoO NPs on p-GaN layer, ZnO nanorod arrays are formed on top of the NiCoO hemispherical lens. Before the formation of p-electrode on p-GaN, the NiCoO NPs are partially removed.

To evaluate the optical properties of the NiCoO NPs, UV-Vis absorption measurements are conducted. The UV-Vis spectrum obtained from the NiCoO NPs reveals an absorption band edge with an intensity maximum centered around 300 nm, as shown in Fig. 3.
Fig. 3 UV-Vis absorbance spectrum of the colloidal Ni1-xCoxO dispersions in solvent.
Although there is a shift in absorption band edge by ligand–metal charge transfer processes [12

12. C. F. Windisch Jr, G. J. Exarhos, K. F. Ferris, M. H. Engelhard, and D. C. Stewart, “Infrared transparent spinel films with p-type conductivity,” Thin Solid Films 398-399, 45–52 (2001). [CrossRef]

], absorption bands are observed at 320 and 280 nm for the NiO and Co3O4 phases, respectively [13

13. X. Li, X. Zhang, Z. Li, and Y. Qian, “Synthesis and characteristics of NiO nanoparticles by thermal decomposition of nickel dimethylglyoximate rods,” Solid State Commun. 137(11), 581–584 (2006). [CrossRef]

, 14

14. L. H. Ai and J. Jiang, “Rapid synthesis of nanocrystalline Co3O4 by a microwave-assisted combustion method,” Powder Technol. 195(1), 11–14 (2009). [CrossRef]

]. Furthermore, the optical band gapof the synthesized oxide nanomaterial was extrapolated to be approximately 2.8 eV (442 nm) which indicates that the synthesized NiCoO NPs can be used as a transparent oxide layer in the visible range.

Fig. 4 (a) SEM image of Ni1-xCoxO hemisphere lens coated on top of p-GaN. (b) The histogram for the diameter distribution of hemispherical NiCoO NPs.
In order to investigate the morphology of the NiCoO NPs and ZnO nanorod arrays formed on p-GaN layer, scanning electron microscope (SEM) observations are performed. Figures 4(a) and 4(b) show SEM image of the NiCoO NP hemispheres formed on the p-GaN layer and a distribution of the nanoparticle sizes, respectively. A drop-casting method was used to form the NiCoO hemispherical lens. The SEM image shows that the hemisphere-type NiCoO NPs are uniformly distributed on the surface. From the SEM images, the average surface coverage and diameter of NPs on LED surface were measured as 50~60% and 73 nm, respectively. After formation of the NiCoO hemispherical lenses on the LED top surface, the ZnO nanorod arrays are formed on NiCoO NP coated LEDs by the hydrothermal synthesis method [6

6. T. Son, K. Y. Jung, and J. Park, “Enhancement of the light extraction of GaN-based green light emitting diodes via nanohybrid structures,” Curr. Appl. Phys. 13(6), 1042–1045 (2013). [CrossRef]

].

Fig. 5 SEM images obtained from the ZnO nanorod arrays on(5(a) and 5 (c)) a conventional LED and (5(b) and 5(d)) a NiCoO NP/LED. (c) and (d) show magnified SEM images for 5(a) and 5(b), respectively.
Figures 5(a) and 5(b) show ZnO nanorod arrays formed on a conventional blue LED and on a NiCoO NP coated LED, respectively. ZnO nanorods grown using a general hydrothermal growth condition (6 h at 95°C) show an average diameter and length of 60 nm and 1 µm, respectively. Figures 5(c) and (d) show magnified images of Figs. 5(a) and 5(b), respectively. Although the some deviation of direction occurred in the ZnO nanorod arrays grown on top of the NiCoO NPs, all samples show vertically aligned nanorod morphology along the growth direction of the GaN LED epi structures. The formation of NiCoO NPs effects on the growth of ZnO nanorods due to its spherical shape.

Fig. 6 The PL spectra of the LEDs with (red line) and without (black line) NiCoO hemisphere lens formed on top of p-GaN .
To investigate the effects of NiCoO hemispherical lens coatings on the surface of LEDs on the light extraction properties from blue LEDs, PL measurements are conducted using a 325 nm He-Cd excitation source at room temperature. Figure 6 shows the PL spectra of the uncoated and NiCoO NP-coated LED samples, whose emission peak wavelength was approximately 450 nm in all samples. The integrated PL luminescence intensity for LEDs with the hemispherical NiCoO lens arrays showed a 1.8-fold enhancement over that of the uncoated sample. Generally, in the case of GaN-based LEDs, the large refractive index difference between GaN (n = 2.5) and air (n = 1) makes it quite difficult for photons to escape from the nitride to air.

Specifically, the escape cone angle is very narrow, which means that due to total internal reflection, most of the light generated by the active regions is trapped in the higher refractive index semiconductor material [15

15. T. S. Kim, S. M. Kim, Y. H. Jang, and G. Y. Jung, “Increase of light extraction from GaN based light emitting diodes incorporating patterned structure by colloidal lithography,” Appl. Phys. Lett. 91(17), 171114 (2007). [CrossRef]

]. The improvement of light extraction in LED with NiCoO hemisphere array can be attributed to the effects of the graded refractive index. The refractive index of NiCoO is 1.7, intermediate value between those of GaN and air, which results in an enlargement of the escape cone and a corresponding increase of light scattering probability in the NPs [16

16. C. F. Windisch, K. F. Ferris, and G. J. Exarhos, “Synthesis and characterization of transparent conducting oxide cobalt–nickel spinel films,” J. Vac. Sci. Technol. 19(4), 1647–1651 (2001). [CrossRef]

, 5

5. S. H. Yeon and J. Park, “Enhancement of photoluminescence intensity of GaN-based light-emitting diodes with coated polystyrene/silica core-shell nanostructures,” J. Nanosci. Nanotechnol. 13(11), 7653–7657 (2013). [CrossRef] [PubMed]

].

EL measurements are conducted at various injection current levels to show the effects of NiCoO NPs as well as that of the combined ZnO nanorod and NiCoO NP structures on the light extraction in blue LED with turn-on status. The EL spectra for the various surface nano structured blue LEDs (bare ZnO nanorods, bare NiCoO NPs, ZnO nanorods/NiCoO NPs) under an injection current fixed at 50 mA are shown in Fig. 7.
Fig. 7 EL spectra obtained from blue LEDs with different surface nanostructures at an injection current of 50mA.
The EL intensity of LEDs with ZnO nanorod/NiCoO NPs is the highest. The integrated EL intensity of the samples covered with hemispherical NiCoO lenses and ZnO nanorod/NiCoO hemispherical arrays exhibited enhancements of 1.6 and 6.1 times, respectively, over that of the uncoated sample. EL intensity measurements were also conducted on LEDs with bare ZnO nanorods, which are generally used to make nanohybrid structures. Although the ZnO nanorods/LED structure shows a higher EL intensity than the NiCoO NPs/LED structure did, the combined ZnO nanorods/NiCoO NPs/LED structure showed a 20% further increase in EL intensity over LEDs with bare ZnO nanorods. The increase of light extraction efficiency in blue LEDs with the ZnO nanorod/NiCoO NP structures can be explained by a similar mechanism to that used for the PL intensity enhancement displayed in Fig. 6.

The enhancement in light extraction from LEDs can be attributed to the enlarged effective photon escape cone by the NiCoO hemisphere lens arrays, and additional reduced Fresnel reflections at the surface due to a gradual change of refractive indices at, respective, the air/ZnO nanorod/NiCoO NP/GaN interfaces [17

17. J. K. Kim, A. N. Noemaun, F. W. Mont, D. Meyaard, E. F. Schubert, D. J. Poxson, H. S. Kim, C. Sone, and Y. J. Park, “Elimination of total internal reflection in GaInN light-emitting diodes by graded refractive-index micropillars,” Appl. Phys. Lett. 93(22), 221111 (2008). [CrossRef]

,18

18. M. K. Lee, C. L. Ho, and P. C. Chen, “Light extraction efficiency enhancement of GaN blue LED by liquid-phase-deposited ZnO Rods,” IEEE Photon. Technol. Lett. 20(4), 252–254 (2008). [CrossRef]

]. In addition, the use of ZnO nanorods on NiCoO hemispheres further enhances the light interference and diffusing properties which may act on the diffusion center resulting in improvement of the light extraction [19

19. Z. Yin, X. Liu, H. Wang, Y. Wu, X. Hao, Z. Ji, and X. Xu, “Light transmission enhancement from hybrid ZnO micro-mesh and nanorod arrays with application to GaN-based light-emitting diodes,” Opt. Express 21(23), 28531–28542 (2013). [CrossRef] [PubMed]

, 20

20. Y. H. Ko and J. S. Yu, “Urchin-aggregation inspired closely-packed hierarchical ZnO nanostructures for efficient light scattering,” Opt. Express 19(27), 25935–25943 (2011). [CrossRef] [PubMed]

].

Finally, for the theoretical consideration of the relationship between various nanostructure formation and light extraction from LEDs, FDTD simulations are conducted using a Lumerical simulation method. A single polarized dipole source is located at the center of the InGaN quantum well active layer, and both the detection plane and line are placed over the top surface of the LEDs. The LED structures and the sizes of NiCoO hemisphere and ZnO nanorods used in the FDTD simulation are approximately similar to those used in our experiments. The light emission output was measured in the top-view direction of the LED structures. The simulated results for the electric field intensity, as an indication of light extraction are shown in Fig. 8.
Fig. 8 FDTD simulation results for light extraction characteristics as a function of various different nanostructures.
Figures 8(a)-8(d) indicate the spatial distribution of extracted light intensity for LEDs with the different nanostructures, as denoted in the Fig. 8. The integrated intensity values are indicated by the round dots in figure with respect to sample name. As in the experimental results, the ZnO nanorod/NiCoO NP/LED structure shows highest light extraction efficiency compared to the other LED structures. FDTD simulation results show that the improvement of light extraction was achieved by using the NiCoO hemispherical lenses and the combination of ZnO nanorod arrays on NiCoO NP. The blue LEDs with ZnO nanorod arrays on NiCoO NPs show a 10.9-fold light extraction improvement over LEDs without any nanostructures. For the bare NiCoO NPs formed on the LED surface, the light extraction intensity increased by 1.37 times over the reference LED. The simulation results were in good agreement with the experimental results.

4. Conclusions

In summary, we report on the improvement of light extraction from blue LEDs using NiCoO hemispherical lens arrays both alone and in a hybrid structure with ZnO nanorods. EL intensity of the LED with the hemispherical NiCoO lens was 1.61 times greater than the reference LEDs. In addition, the combined hybrid structure with ZnO nanorod arrays formed on top of the hemispherical NiCoO lenses show a further improvement in light extraction, approximately 6.5 times greater than the LED without nanomaterials. The experimentally measured results are entirely consistent with FDTD simulations, and the improvements in photon extraction can be attributed to the enlarged escape angle and reduction of Fresnel reflection loss due to the use of NiCoO and ZnO materials whose refractive indices are located between those of GaN and air.

Acknowledgments

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF-2012R1A1A1041930) and the BK21 PLUS(Brain Korea 21 Program for Leading Universities & Students), funded by the Ministry of Education, Science and Technology.

References and links

1.

S. Nakamura, T. Mukai, and M. Senoh, “Candela-class high-brightness InGaN/AlGaN double heterostructure blue light emitting diodes,” Appl. Phys. Lett. 64(13), 1687–1689 (1994). [CrossRef]

2.

S. N. Mohammad, A. A. Salvador, and H. Morkoç, “Emerging gallium nitride based devices,” Proc. IEEE 83(10), 1306–1355 (1995). [CrossRef]

3.

Q. Zhang, K. H. Li, and H. W. Choi, “InGaN light-emitting diodes with indium-tin-oxide sub-micron lenses patterned by nanosphere lithography,” Appl. Phys. Lett. 100(6), 061120 (2012). [CrossRef]

4.

Y. K. Ee, P. Kumnorkaew, R. A. Arif, H. Tong, H. Zhao, J. F. Gilchrist, and N. Tansu, “Optimization of light extraction efficiency of III-Nitride LEDs with self-assembled colloidal-based microlenses,” J. Select. Topics Quantum Electron. 15(4), 1218–1225 (2009). [CrossRef]

5.

S. H. Yeon and J. Park, “Enhancement of photoluminescence intensity of GaN-based light-emitting diodes with coated polystyrene/silica core-shell nanostructures,” J. Nanosci. Nanotechnol. 13(11), 7653–7657 (2013). [CrossRef] [PubMed]

6.

T. Son, K. Y. Jung, and J. Park, “Enhancement of the light extraction of GaN-based green light emitting diodes via nanohybrid structures,” Curr. Appl. Phys. 13(6), 1042–1045 (2013). [CrossRef]

7.

M. Khizar, Z. Y. Fan, K. H. Kim, J. Y. Lin, and H. X. Jiang, “Nitride deep-ultraviolet light-emitting diodes with microlens array,” Appl. Phys. Lett. 86(17), 173504 (2005). [CrossRef]

8.

L. E. Greene, M. Law, D. H. Tan, M. Montano, J. Goldberger, G. Somorjai, and P. Yang, “General route to vertical ZnO nanowire arrays using textured ZnO seeds,” Nano Lett. 5(7), 1231–1236 (2005). [CrossRef] [PubMed]

9.

F. Iacomi, G. Calin, C. Scarlat, M. Irimia, C. Doroftei, M. Dobromir, G. G. Rusu, N. Iftimie, and A. V. Sandu, “Functional properties of nickel cobalt oxide thin films,” Thin Solid Films 520(1), 651–655 (2011). [CrossRef]

10.

G. M. Kumar, D. H. Kim, and J. Park, “Colloidal Ni1-xCoxO nanostructures as anodic buffer layers in hybrid polypyrrole/Zn1-yInyO p–n junctions,” CrystEngComm 15(40), 8195–8201 (2013). [CrossRef]

11.

K. Song and J. Park, “Effects of the growth pressure of a-plane InGaN/GaN multi-quantum wells on the optical performance of light-emitting diodes,” Semicond. Sci. Technol. 28(1), 015010 (2013). [CrossRef]

12.

C. F. Windisch Jr, G. J. Exarhos, K. F. Ferris, M. H. Engelhard, and D. C. Stewart, “Infrared transparent spinel films with p-type conductivity,” Thin Solid Films 398-399, 45–52 (2001). [CrossRef]

13.

X. Li, X. Zhang, Z. Li, and Y. Qian, “Synthesis and characteristics of NiO nanoparticles by thermal decomposition of nickel dimethylglyoximate rods,” Solid State Commun. 137(11), 581–584 (2006). [CrossRef]

14.

L. H. Ai and J. Jiang, “Rapid synthesis of nanocrystalline Co3O4 by a microwave-assisted combustion method,” Powder Technol. 195(1), 11–14 (2009). [CrossRef]

15.

T. S. Kim, S. M. Kim, Y. H. Jang, and G. Y. Jung, “Increase of light extraction from GaN based light emitting diodes incorporating patterned structure by colloidal lithography,” Appl. Phys. Lett. 91(17), 171114 (2007). [CrossRef]

16.

C. F. Windisch, K. F. Ferris, and G. J. Exarhos, “Synthesis and characterization of transparent conducting oxide cobalt–nickel spinel films,” J. Vac. Sci. Technol. 19(4), 1647–1651 (2001). [CrossRef]

17.

J. K. Kim, A. N. Noemaun, F. W. Mont, D. Meyaard, E. F. Schubert, D. J. Poxson, H. S. Kim, C. Sone, and Y. J. Park, “Elimination of total internal reflection in GaInN light-emitting diodes by graded refractive-index micropillars,” Appl. Phys. Lett. 93(22), 221111 (2008). [CrossRef]

18.

M. K. Lee, C. L. Ho, and P. C. Chen, “Light extraction efficiency enhancement of GaN blue LED by liquid-phase-deposited ZnO Rods,” IEEE Photon. Technol. Lett. 20(4), 252–254 (2008). [CrossRef]

19.

Z. Yin, X. Liu, H. Wang, Y. Wu, X. Hao, Z. Ji, and X. Xu, “Light transmission enhancement from hybrid ZnO micro-mesh and nanorod arrays with application to GaN-based light-emitting diodes,” Opt. Express 21(23), 28531–28542 (2013). [CrossRef] [PubMed]

20.

Y. H. Ko and J. S. Yu, “Urchin-aggregation inspired closely-packed hierarchical ZnO nanostructures for efficient light scattering,” Opt. Express 19(27), 25935–25943 (2011). [CrossRef] [PubMed]

21.

Z. Yin, X. Liu, H. Wang, Y. Wu, X. Hao, Z. Ji, and X. Xu, “Light transmission enhancement from hybrid ZnO micro-mesh and nanorod arrays with application to GaN-based light-emitting diodes,” Opt. Express 21(23), 28531–28542 (2013). [CrossRef] [PubMed]

OCIS Codes
(160.6000) Materials : Semiconductor materials
(230.3670) Optical devices : Light-emitting diodes
(220.4241) Optical design and fabrication : Nanostructure fabrication

ToC Category:
Light-Emitting Diodes

History
Original Manuscript: February 24, 2014
Revised Manuscript: April 10, 2014
Manuscript Accepted: April 23, 2014
Published: May 28, 2014

Citation
Do-Hyun Kim, Dong Su Shin, and Jinsub Park, "Enhanced light extraction from GaN based light-emitting diodes using a hemispherical NiCoO lens," Opt. Express 22, A1071-A1078 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-S4-A1071


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References

  1. S. Nakamura, T. Mukai, and M. Senoh, “Candela-class high-brightness InGaN/AlGaN double heterostructure blue light emitting diodes,” Appl. Phys. Lett.64(13), 1687–1689 (1994). [CrossRef]
  2. S. N. Mohammad, A. A. Salvador, and H. Morkoç, “Emerging gallium nitride based devices,” Proc. IEEE83(10), 1306–1355 (1995). [CrossRef]
  3. Q. Zhang, K. H. Li, and H. W. Choi, “InGaN light-emitting diodes with indium-tin-oxide sub-micron lenses patterned by nanosphere lithography,” Appl. Phys. Lett.100(6), 061120 (2012). [CrossRef]
  4. Y. K. Ee, P. Kumnorkaew, R. A. Arif, H. Tong, H. Zhao, J. F. Gilchrist, and N. Tansu, “Optimization of light extraction efficiency of III-Nitride LEDs with self-assembled colloidal-based microlenses,” J. Select. Topics Quantum Electron.15(4), 1218–1225 (2009). [CrossRef]
  5. S. H. Yeon and J. Park, “Enhancement of photoluminescence intensity of GaN-based light-emitting diodes with coated polystyrene/silica core-shell nanostructures,” J. Nanosci. Nanotechnol.13(11), 7653–7657 (2013). [CrossRef] [PubMed]
  6. T. Son, K. Y. Jung, and J. Park, “Enhancement of the light extraction of GaN-based green light emitting diodes via nanohybrid structures,” Curr. Appl. Phys.13(6), 1042–1045 (2013). [CrossRef]
  7. M. Khizar, Z. Y. Fan, K. H. Kim, J. Y. Lin, and H. X. Jiang, “Nitride deep-ultraviolet light-emitting diodes with microlens array,” Appl. Phys. Lett.86(17), 173504 (2005). [CrossRef]
  8. L. E. Greene, M. Law, D. H. Tan, M. Montano, J. Goldberger, G. Somorjai, and P. Yang, “General route to vertical ZnO nanowire arrays using textured ZnO seeds,” Nano Lett.5(7), 1231–1236 (2005). [CrossRef] [PubMed]
  9. F. Iacomi, G. Calin, C. Scarlat, M. Irimia, C. Doroftei, M. Dobromir, G. G. Rusu, N. Iftimie, and A. V. Sandu, “Functional properties of nickel cobalt oxide thin films,” Thin Solid Films520(1), 651–655 (2011). [CrossRef]
  10. G. M. Kumar, D. H. Kim, and J. Park, “Colloidal Ni1-xCoxO nanostructures as anodic buffer layers in hybrid polypyrrole/Zn1-yInyO p–n junctions,” CrystEngComm15(40), 8195–8201 (2013). [CrossRef]
  11. K. Song and J. Park, “Effects of the growth pressure of a-plane InGaN/GaN multi-quantum wells on the optical performance of light-emitting diodes,” Semicond. Sci. Technol.28(1), 015010 (2013). [CrossRef]
  12. C. F. Windisch, G. J. Exarhos, K. F. Ferris, M. H. Engelhard, and D. C. Stewart, “Infrared transparent spinel films with p-type conductivity,” Thin Solid Films398-399, 45–52 (2001). [CrossRef]
  13. X. Li, X. Zhang, Z. Li, and Y. Qian, “Synthesis and characteristics of NiO nanoparticles by thermal decomposition of nickel dimethylglyoximate rods,” Solid State Commun.137(11), 581–584 (2006). [CrossRef]
  14. L. H. Ai and J. Jiang, “Rapid synthesis of nanocrystalline Co3O4 by a microwave-assisted combustion method,” Powder Technol.195(1), 11–14 (2009). [CrossRef]
  15. T. S. Kim, S. M. Kim, Y. H. Jang, and G. Y. Jung, “Increase of light extraction from GaN based light emitting diodes incorporating patterned structure by colloidal lithography,” Appl. Phys. Lett.91(17), 171114 (2007). [CrossRef]
  16. C. F. Windisch, K. F. Ferris, and G. J. Exarhos, “Synthesis and characterization of transparent conducting oxide cobalt–nickel spinel films,” J. Vac. Sci. Technol.19(4), 1647–1651 (2001). [CrossRef]
  17. J. K. Kim, A. N. Noemaun, F. W. Mont, D. Meyaard, E. F. Schubert, D. J. Poxson, H. S. Kim, C. Sone, and Y. J. Park, “Elimination of total internal reflection in GaInN light-emitting diodes by graded refractive-index micropillars,” Appl. Phys. Lett.93(22), 221111 (2008). [CrossRef]
  18. M. K. Lee, C. L. Ho, and P. C. Chen, “Light extraction efficiency enhancement of GaN blue LED by liquid-phase-deposited ZnO Rods,” IEEE Photon. Technol. Lett.20(4), 252–254 (2008). [CrossRef]
  19. Z. Yin, X. Liu, H. Wang, Y. Wu, X. Hao, Z. Ji, and X. Xu, “Light transmission enhancement from hybrid ZnO micro-mesh and nanorod arrays with application to GaN-based light-emitting diodes,” Opt. Express21(23), 28531–28542 (2013). [CrossRef] [PubMed]
  20. Y. H. Ko and J. S. Yu, “Urchin-aggregation inspired closely-packed hierarchical ZnO nanostructures for efficient light scattering,” Opt. Express19(27), 25935–25943 (2011). [CrossRef] [PubMed]
  21. Z. Yin, X. Liu, H. Wang, Y. Wu, X. Hao, Z. Ji, and X. Xu, “Light transmission enhancement from hybrid ZnO micro-mesh and nanorod arrays with application to GaN-based light-emitting diodes,” Opt. Express21(23), 28531–28542 (2013). [CrossRef] [PubMed]

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