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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 6 — Mar. 25, 2013
  • pp: 7125–7130
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An improvement of light extraction efficiency for GaN-based light emitting diodes by selective etched nanorods in periodic microholes

Seung Hwan Kim, Hyun Ho Park, Young Ho Song, Hyung Jo Park, Jae Beom Kim, Seong Ran Jeon, Hyun Jeong, Mun Seok Jeong, and Gye Mo Yang  »View Author Affiliations


Optics Express, Vol. 21, Issue 6, pp. 7125-7130 (2013)
http://dx.doi.org/10.1364/OE.21.007125


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Abstract

We have demonstrated the enhancement of a GaN-based light emitting diode (LED) by means of a selective etching technique. A conventional LED structure was periodically etched, to form periodic microholes. It showed an improvement of the light extraction efficiency (LEE) of approximately 15%, compared to that of a conventional LED. Furthermore, nano-sized rods inside the microholes were randomly formed by using a powder mask, resulting in an LEE of 43%. From the result of confocal scanning electroluminescence measurement, the light emission arises mainly from the vicinity of the nanorods in the periodic microholes. Therefore, we found that nanorods randomly distributed in periodic microholes in a LED structure play a significant role in the reduction of total internal reflection, by acting as photon wave-guides and scattering centers. This method would be valuable for the fabrication of high efficiency GaN-based LED, in terms of technical simplification and cost.

© 2013 OSA

1. Introduction

GaN-based light emitting diodes (LEDs) have been attracting a great deal of interest, due to their powerful illumination, and use as back light units in liquid crystal displays, with high efficiency and reliable life-time for optoelectronic applications [1

1. I. Akssaki, “Key inventions in the history of nitride-based blue LED and LD,” J. Cryst. Growth 300(1), 2–10 (2007). [CrossRef]

,2

2. H. Jia, L. Guo, W. Wang, and H. Chen, “Recent progress in GaN-based light-emitting diodes,” Adv. Mater. 21(45), 4641–4646 (2009). [CrossRef]

]. However, light extraction of the LEDs strongly influences their external efficiency, which is typically limited due to the total internal reflection (TIR) of photons [3

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

]. It is well known that photons, which arise from GaN-based multiple quantum wells (MQW), only escape from holes in GaN layers to air within a critical angle of 23°, because of the large difference of refractive index between GaN (n = 2.5) and air (n = 1) [4

4. I. Schnitzer, E. Yablonovitch, C. Caneau, T. J. Gmitter, and A. Scherer, “30% external quantum efficiency from surface textured, thin‐film light‐emitting diodes,” Appl. Phys. Lett. 63(16), 2174–2176 (1993). [CrossRef]

]. The photons emitted over the critical angle can be reflected from the interface and then reabsorbed internally, resulting in the reduction of external efficiency for GaN-based LEDs. Thus, how the reflected photons can be effectively extracted from a GaN layer to air has been regarded as a critical issue. In addition, the probability of reabsorption inevitably increases, because the LED structure acts as a lateral waveguide. In order to enhance light extraction efficiency of the LEDs, several methods have so far been attempted, such as patterned sapphire substrate [5

5. H. Gao, F. Yan, Y. Zhang, J. Li, Y. Zeng, and G. Wang, “Improvement of the performance of GaN based LEDs grown on sapphire substrates patterned by wet and ICP etching,” Solid-State Electron. 52(6), 962–967 (2008). [CrossRef]

], and photonic crystal [6

6. J. Y. Kim, M. K. Kwon, K. S. Lee, S. J. Park, S. H. Kim, and K. D. Lee, “Enhanced light extraction from GaN-based green light-emitting diode with photonic crystal,” Appl. Phys. Lett. 91(18), 181109 (2007). [CrossRef]

]. These approaches showed improvement of the light extraction efficiency for GaN-based LEDs, but provided more complicated device processes, with higher cost in commercial lines. Others have suggested various texturing schemes inside the LED structure [7

7. C. Huh, K. S. Lee, E. J. Kang, and S. J. Park, “Improved light-output and electrical performance of InGaN-based light-emitting diode by microroughening of the p-GaN surface,” Appl. Phys. Lett. 93, 9383–9385 (2003).

10

10. 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]

] (i.e. coated transparent balls with a high refractive index on an n-GaN surface, and SiO2/polystyrene microlens arrays on a p-GaN surface [11

11. Y.-K. Ee, R. A. Arif, N. Tansu, P. Kumnorkaew, and J. F. Gilchrist, “Enhancement of light extraction efficiency of InGaN quantum wells light emitting diodes using SiO2/polystyrene microlens arrays,” Appl. Phys. Lett. 91(22), 221107 (2007). [CrossRef]

]). In these cases, textured surfaces of the layers can only be prepared by etching technique, again inducing economic drawback when applied to large areas. Specifically, complicated processing is required for the photonic crystal, a laser lift-off for removing a sapphire substrate for the coated transparent balls, and a rapid convective deposition is required for coating using nanospheres as microlenses [12

12. 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]

]. Many groups have reported high-performance GaN-based microdisk and microring LEDs [13

13. S. X. Jin, J. Li, J. Y. Lin, and H. X. Jiang, “InGaN/GaN quantum well interconnected microdisk light emitting diodes,” Appl. Phys. Lett. 77(20), 3236–3238 (2000). [CrossRef]

17

17. T. H. Hsueh, J. K. Sheu, H. W. Huang, J. Y. Chu, C. C. Kao, H. C. Kuo, and S. C. Wang, “Enhancement in light output of InGaN-based microhole array light-emitting diodes,” IEEE Photon. Technol. Lett. 17(6), 1163–1165 (2005). [CrossRef]

].

In this work, we investigated an enhancement in light output power for GaN-based LED, by using a selective etching technique with Alumina powder as an etching mask, to form nanorods in periodic microholes in a conventional LED structure. Field-emission scanning electron microscopy (FE-SEM) was performed to characterize the surface and microstructural properties of the etched nanorods in the microholes. In order to clarify photon extraction from the etched LED structure, a confocal scanning electroluminescence microscopy (CSEM) was carried out.

2. Experiment

The GaN-based LED structures were grown on Al2O3 (0001) substrate by metal organic chemical vapor deposition (MOCVD). The LED structure subsequently consists of a 30-nm-thick GaN buffer layer, a 2-μm-thick n-type GaN, a five-period InGaN/GaN MQW as an active region, and a 200-nm-thick p-type GaN layer. During the MOCVD growth, trimethylgallium, trimethylindium, and NH3 were used as precursors for the Ga, In, and N, respectively. Cp2Mg and SiH4 were employed as the p- and n-type dopant precursors, respectively. H2 was used as a carrier gas throughout the growth, except for the InGaN growth, where N2 was used. The reactor pressure was maintained at 200 Torr.

After the growth of the LED structure, the LED mesa structure was obtained by standard lithography patterning, followed by inductively coupled plasma (ICP) etching using Cl2 and BCl3 to expose the n-GaN layer. Then, a 200-nm-thick ITO transparent layer was evaporated on a p-GaN layer, to form a p-side contact layer and a current spreading layer. The ITO layer was patterned into a periodic circle pattern of 10 μm diameter by diluted HCl solution. The spacing between adjoining circle patterns was 14 μm. For the crystallization of ITO grains, these ITO films were annealed in a N2 and O2 mixed ambient at 600 °C for 60s in a rapid thermal annealing chamber. Then, to form the periodic microhole, the entire sample was covered with a photoresist, except for the periodic circle pattern of 10 μm diameter through a standard lithography patterning. Subsequently, for the fabrication of LED with periodic microholes (named as PM LED), the dry etching process was performed by ICP with a gas mixture of Cl2 and BCl3, at an inductive power of 1000 W and radio frequency power of 350 W. Then, for the fabrication of LED with nanorods in microholes (named as PMN LED), the Alumina was coated on the periodic circles pattern of 10 μm diameter by spin coating method at a speed of 4000 rpm, as an etching mask. Details of the alumina coating have been described elsewhere [18

18. S. H. Kim, T. K. Kim, S. S. Yang, J. K. Son, K. H. Lee, Y. G. Hong, S. J. Bae, K. H. Shim, J. W. Yang, and G. M. Yang, “Al2O3 powder coating and surface texturing for high efficiency GaN-based light emitting diodes,” Jpn. J. Appl. Phys. 48(9), 092101 (2009). [CrossRef]

]. The two samples were simultaneously etched by ICP etching at the same condition, and etched down to the n-GaN layer with an approximate depth of 820 nm. Finally, the p- and n-side electrodes composed of Cr and Au were evaporated onto the ITO transparent layer and the n-GaN layer, by an e-beam evaporator. Figure 1
Fig. 1 Schematic diagrams of (a) projective view of fabricated LED with nanorods in periodic microholes, and (b) cross-sectional views of LED with and without nanorods .
shows schematics of (a) a projective view of the fabricated LED with nanorods in periodic microholes, and (b) cross-sectional views of the PM LED and PMN LED. The surface morphologies were examined by FE-SEM at an acceleration voltage of 15 kV (Hitachi S-4300SE). Current-voltage (I-V) and light output-current (L-I) measurements were carried out, using a probe station system. A novel fiber-optic-based CSEM was employed for the investigation of the spatially resolved luminescence properties of the fabricated LED. In the CSEM system, a static current was applied to the sample during the scanning, using a Keithley 2400s Source Meter. Light collected from the focal planes was delivered to a monochromator through a multimode optical fiber, and detected by a cooled charge-coupled device (CCD) detector [19

19. H. Jeong, Y. H. Kim, T. H. Seo, H. S. Lee, J. S. Kim, E.-K. Suh, and M. S. Jeong, “Enhancement of light output power in GaN-based light-emitting diodes using hydrothermally grown ZnO micro-walls,” Opt. Express 20(10), 10597–10604 (2012). [CrossRef] [PubMed]

].

3. Results and discussion

Figure 2(a)
Fig. 2 SEM images of (a) top view of fabricated LED with nanorods in periodic microholes, and (b) cross-sectional view of formation of nanorods in a microhole
shows the SEM image of the top view of the fabricated PMN LED and an enlarged microhole consisting of nanorods [the inset of Fig. 2(a)]. From the SEM image, it was determined that the LED chip and diameter of microhole are 540 μm x 540 μm and 10 μm, respectively. Nanorods of approximately 830 nm height and 300 nm diameter were formed in the microhole, as shown in Fig. 2(b). And the occupation ratio of the microholes is about 25% in total area of unit chip. The randomly distributed nanorods in the microholes in the LED structure were prepared by using Alumina powder as an etching mask. The sizes of Alumina powder on the top of the nanorods were slightly reduced after the dry etching process.

Figures 3(a)
Fig. 3 Schematic diagrams of the photon escape traces in (a) the PM LED, and (b) PMN LED. Confocal scanning electroluminescence (EL) microscopy images of (c) the PM LED, and (d) PMN LED, operated at 1 mA.
and 3(b) show schematics of the photon escape paths in PM LED and PMN LED, respectively. Since the escape of photons from the LED is limited, due to the large difference in refractive index at the interface between GaN layer and air, we suggest that the photon wave-guiding effect and multiple scattering in microholes with nanorods leads to an increase in the light extracted from the LED chip. This is because photons emitted from the LED have difficulty escaping from the LED chip to air. At the PM LED, the reflected photons inward LED chip by TIR have a probability of escaping from inside the LED chip through the side walls of the microholes with 25% occupation ratio. That is, the area of side wall, which photons can escape, is increased as 25% occupation ratio of microholes than the reference LED. However, the photons were re-reflected by TIR at the bottom of microholes. In the case of the PMN LED, the generated photons that escape from inside the LED are greater than that of the PM LED, because the photons can extract from not only the side walls of microholes, but also the nanorods, each of which functions as a photon wave-guide and scattering center. To investigate the luminescence properties of the nanorods, we employed a CSEM system, which is known to be an effective experimental tool for measuring optical characteristics such as light propagation and local light output. Figures 3(c) and 3(d) show CSEM images of PM LED and PMN LED, respectively, operated at 1 mA; in Fig. 3(c), the emitted light is strongly extracted around the microhole. On the other hand, the emitted light from the PMN LED is enhanced in the side walls of microholes and the nanorods regions, as shown in Fig. 3(d). As mentioned above, the nanorods behave as photon wave-guides and scattering centers.

Figure 4
Fig. 4 Light far-field distributions of reference LED, PM LED, and PMN LED at 80 mA (measured by unit chip)
shows the light far-ðeld distributions from three kinds of LEDs at an injection current of 80 mA. The LEDs with microholes presented brighter emission intensity than the reference LED without microholes. As shown in Figs. 3(c) and 3(d), the emitted light is strongly propagated in the vertical direction. Therefore, the light far-field distributions of the PM LED and PMN LED are enhanced at from −40° to 40°. Compared with the PM LED and PMN LED, it can be seen clearly that the emission intensity of the LED with nanorods in microholes is higher in the vertical direction, because the probability of photons escaping from the LED via the nanorods in microholes is better than for the LED without the nanorods.

Figure 5
Fig. 5 Light output power-current-voltage (L-I-V) characteristics of reference LED, PM LED, and PMN LED
shows the current-voltage (I-V) characteristics of the GaN-based LED with and without nanorods in microholes, as well as that of a reference LED. The forward voltages of the reference LED, the PM LED, and PMN LED are similar at about 3.6 V at 80 mA. Figure 5 also shows the light output power versus current curves (L-I) of the reference LED, PM LED, and PMN LED. The light output power was measured from the top of the LED, using a calibrated Si photodiode connected to an optical power meter. As shown, light output powers of PM LED and PMN LED are higher than that of the reference LED. At an injection current of 80 mA, enhancements of about 15% and 43% are obtained in the light output power for the PM LED and PMN LED, respectively. The improvement of light output power for LED with and without microholes could be attributed to the side walls of microholes and nanorods acting as photon wave-guides and scattering centers.

4. Conclusion

In summary, we have demonstrated an enhancement in the light output power for the GaN-based LED with and without nanorods in microholes. The forward voltages of these LEDs were similar, at about 3.6 V at 80 mA. The light output powers of the GaN-based LED with and without nanorods in microholes are enhanced by ~43% and ~15%, respectively, compared to that of the reference LED at 80 mA. The enhanced light output power is attributed to the photon wave-guiding effect and scattering through the nanorods in microholes, by the decrease in total internal reflection to improve extraction efficiency. This study proposed a simple method for enhancing the light output power of GaN-based LED via incorporating microholes with nanorods, using Alumina by the spin coating method. Furthermore, through the optimization of the coating condition and ICP etching condition, we could further enhance the light output power.

Acknowledgment

This work was partly supported by the IT R&D program MKE/KEIT (No. 10039151) and Energy Resource R&D program (No. 2010201010020) of MKE.

References and links

1.

I. Akssaki, “Key inventions in the history of nitride-based blue LED and LD,” J. Cryst. Growth 300(1), 2–10 (2007). [CrossRef]

2.

H. Jia, L. Guo, W. Wang, and H. Chen, “Recent progress in GaN-based light-emitting diodes,” Adv. Mater. 21(45), 4641–4646 (2009). [CrossRef]

3.

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

4.

I. Schnitzer, E. Yablonovitch, C. Caneau, T. J. Gmitter, and A. Scherer, “30% external quantum efficiency from surface textured, thin‐film light‐emitting diodes,” Appl. Phys. Lett. 63(16), 2174–2176 (1993). [CrossRef]

5.

H. Gao, F. Yan, Y. Zhang, J. Li, Y. Zeng, and G. Wang, “Improvement of the performance of GaN based LEDs grown on sapphire substrates patterned by wet and ICP etching,” Solid-State Electron. 52(6), 962–967 (2008). [CrossRef]

6.

J. Y. Kim, M. K. Kwon, K. S. Lee, S. J. Park, S. H. Kim, and K. D. Lee, “Enhanced light extraction from GaN-based green light-emitting diode with photonic crystal,” Appl. Phys. Lett. 91(18), 181109 (2007). [CrossRef]

7.

C. Huh, K. S. Lee, E. J. Kang, and S. J. Park, “Improved light-output and electrical performance of InGaN-based light-emitting diode by microroughening of the p-GaN surface,” Appl. Phys. Lett. 93, 9383–9385 (2003).

8.

C. C. Yang, R. H. Horng, C. E. Lee, W. Y. Lin, K. F. Pan, Y. Y. Su, and D. S. Wuu, “Improvement in extraction efficiency of GaN-based light-emitting diodes with textured surface layer by natural lithography,” Jpn. J. Appl. Phys. 44(4B), 2525–2527 (2005). [CrossRef]

9.

C. L. Lee, S. C. Lee, and W. I. Lee, “Nonlithographic random masking and regrowth of GaN micro-hillocks to improve light-emitting diode efficiency,” Jpn. J. Appl. Phys. 45(1), L4–L7 (2006). [CrossRef]

10.

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]

11.

Y.-K. Ee, R. A. Arif, N. Tansu, P. Kumnorkaew, and J. F. Gilchrist, “Enhancement of light extraction efficiency of InGaN quantum wells light emitting diodes using SiO2/polystyrene microlens arrays,” Appl. Phys. Lett. 91(22), 221107 (2007). [CrossRef]

12.

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]

13.

S. X. Jin, J. Li, J. Y. Lin, and H. X. Jiang, “InGaN/GaN quantum well interconnected microdisk light emitting diodes,” Appl. Phys. Lett. 77(20), 3236–3238 (2000). [CrossRef]

14.

H. W. Choi, C. W. Jeon, and M. D. Dawson, “InGaN microring light-emitting diodes,” IEEE Photon. Technol. Lett. 16(1), 33–35 (2004). [CrossRef]

15.

R. A. Mair, K. C. Zeng, J. Y. Lin, H. X. Jiang, B. Zhang, L. Dai, H. Tang, A. Botchkarev, W. Kim, and H. Morkoc, “Optical properties of GaN/AlGaN multiple quantum well microdisks,” Appl. Phys. Lett. 71(20), 2898–2900 (1997). [CrossRef]

16.

K. C. Zeng, L. Dai, J. Y. Lin, and H. X. Jiang, “Optical resonance modes in InGaN/GaN multiple-quantum-well microring cavities,” Appl. Phys. Lett. 75(17), 2563–2565 (1999). [CrossRef]

17.

T. H. Hsueh, J. K. Sheu, H. W. Huang, J. Y. Chu, C. C. Kao, H. C. Kuo, and S. C. Wang, “Enhancement in light output of InGaN-based microhole array light-emitting diodes,” IEEE Photon. Technol. Lett. 17(6), 1163–1165 (2005). [CrossRef]

18.

S. H. Kim, T. K. Kim, S. S. Yang, J. K. Son, K. H. Lee, Y. G. Hong, S. J. Bae, K. H. Shim, J. W. Yang, and G. M. Yang, “Al2O3 powder coating and surface texturing for high efficiency GaN-based light emitting diodes,” Jpn. J. Appl. Phys. 48(9), 092101 (2009). [CrossRef]

19.

H. Jeong, Y. H. Kim, T. H. Seo, H. S. Lee, J. S. Kim, E.-K. Suh, and M. S. Jeong, “Enhancement of light output power in GaN-based light-emitting diodes using hydrothermally grown ZnO micro-walls,” Opt. Express 20(10), 10597–10604 (2012). [CrossRef] [PubMed]

OCIS Codes
(160.6000) Materials : Semiconductor materials
(180.1790) Microscopy : Confocal microscopy
(230.3670) Optical devices : Light-emitting diodes
(220.4241) Optical design and fabrication : Nanostructure fabrication

ToC Category:
Optical Devices

History
Original Manuscript: December 20, 2012
Revised Manuscript: February 20, 2013
Manuscript Accepted: March 1, 2013
Published: March 13, 2013

Citation
Seung Hwan Kim, Hyun Ho Park, Young Ho Song, Hyung Jo Park, Jae Beom Kim, Seong Ran Jeon, Hyun Jeong, Mun Seok Jeong, and Gye Mo Yang, "An improvement of light extraction efficiency for GaN-based light emitting diodes by selective etched nanorods in periodic microholes," Opt. Express 21, 7125-7130 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-6-7125


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References

  1. I. Akssaki, “Key inventions in the history of nitride-based blue LED and LD,” J. Cryst. Growth 300(1), 2–10 (2007). [CrossRef]
  2. H. Jia, L. Guo, W. Wang, and H. Chen, “Recent progress in GaN-based light-emitting diodes,” Adv. Mater. 21(45), 4641–4646 (2009). [CrossRef]
  3. J. K. Kim, A. N. Noemaun, F. W. Mont, D. Meyaard, E. F. Schubert, D. J. Poxson, H. Kim, C. Sone, and Y. Park, “Elimination of total internal reflection in GaInN light-emitting diodes by graded-refractive-index micropillars,” Appl. Phys. Lett. 93(22), 221111 (2008). [CrossRef]
  4. I. Schnitzer, E. Yablonovitch, C. Caneau, T. J. Gmitter, and A. Scherer, “30% external quantum efficiency from surface textured, thin?film light?emitting diodes,” Appl. Phys. Lett. 63(16), 2174–2176 (1993). [CrossRef]
  5. H. Gao, F. Yan, Y. Zhang, J. Li, Y. Zeng, and G. Wang, “Improvement of the performance of GaN based LEDs grown on sapphire substrates patterned by wet and ICP etching,” Solid-State Electron. 52(6), 962–967 (2008). [CrossRef]
  6. J. Y. Kim, M. K. Kwon, K. S. Lee, S. J. Park, S. H. Kim, and K. D. Lee, “Enhanced light extraction from GaN-based green light-emitting diode with photonic crystal,” Appl. Phys. Lett. 91(18), 181109 (2007). [CrossRef]
  7. C. Huh, K. S. Lee, E. J. Kang, and S. J. Park, “Improved light-output and electrical performance of InGaN-based light-emitting diode by microroughening of the p-GaN surface,” Appl. Phys. Lett. 93, 9383–9385 (2003).
  8. C. C. Yang, R. H. Horng, C. E. Lee, W. Y. Lin, K. F. Pan, Y. Y. Su, and D. S. Wuu, “Improvement in extraction efficiency of GaN-based light-emitting diodes with textured surface layer by natural lithography,” Jpn. J. Appl. Phys. 44(4B), 2525–2527 (2005). [CrossRef]
  9. C. L. Lee, S. C. Lee, and W. I. Lee, “Nonlithographic random masking and regrowth of GaN micro-hillocks to improve light-emitting diode efficiency,” Jpn. J. Appl. Phys. 45(1), L4–L7 (2006). [CrossRef]
  10. 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]
  11. Y.-K. Ee, R. A. Arif, N. Tansu, P. Kumnorkaew, and J. F. Gilchrist, “Enhancement of light extraction efficiency of InGaN quantum wells light emitting diodes using SiO2/polystyrene microlens arrays,” Appl. Phys. Lett. 91(22), 221107 (2007). [CrossRef]
  12. 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]
  13. S. X. Jin, J. Li, J. Y. Lin, and H. X. Jiang, “InGaN/GaN quantum well interconnected microdisk light emitting diodes,” Appl. Phys. Lett. 77(20), 3236–3238 (2000). [CrossRef]
  14. H. W. Choi, C. W. Jeon, and M. D. Dawson, “InGaN microring light-emitting diodes,” IEEE Photon. Technol. Lett. 16(1), 33–35 (2004). [CrossRef]
  15. R. A. Mair, K. C. Zeng, J. Y. Lin, H. X. Jiang, B. Zhang, L. Dai, H. Tang, A. Botchkarev, W. Kim, and H. Morkoc, “Optical properties of GaN/AlGaN multiple quantum well microdisks,” Appl. Phys. Lett. 71(20), 2898–2900 (1997). [CrossRef]
  16. K. C. Zeng, L. Dai, J. Y. Lin, and H. X. Jiang, “Optical resonance modes in InGaN/GaN multiple-quantum-well microring cavities,” Appl. Phys. Lett. 75(17), 2563–2565 (1999). [CrossRef]
  17. T. H. Hsueh, J. K. Sheu, H. W. Huang, J. Y. Chu, C. C. Kao, H. C. Kuo, and S. C. Wang, “Enhancement in light output of InGaN-based microhole array light-emitting diodes,” IEEE Photon. Technol. Lett. 17(6), 1163–1165 (2005). [CrossRef]
  18. S. H. Kim, T. K. Kim, S. S. Yang, J. K. Son, K. H. Lee, Y. G. Hong, S. J. Bae, K. H. Shim, J. W. Yang, and G. M. Yang, “Al2O3 powder coating and surface texturing for high efficiency GaN-based light emitting diodes,” Jpn. J. Appl. Phys. 48(9), 092101 (2009). [CrossRef]
  19. H. Jeong, Y. H. Kim, T. H. Seo, H. S. Lee, J. S. Kim, E.-K. Suh, and M. S. Jeong, “Enhancement of light output power in GaN-based light-emitting diodes using hydrothermally grown ZnO micro-walls,” Opt. Express 20(10), 10597–10604 (2012). [CrossRef] [PubMed]

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