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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 22 — Nov. 4, 2013
  • pp: 26846–26853
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Room-temperature larger-scale highly ordered nanorod imprints of ZnO film

Zabu Kyaw, Wang Jianxiong, Kapil Dev, Swee Tiam Tan, Zhengang Ju, Zi-Hui Zhang, Yun Ji, Namig Hasanov, Wei Liu, Xiao Wei Sun, and Hilmi Volkan Demir  »View Author Affiliations


Optics Express, Vol. 21, Issue 22, pp. 26846-26853 (2013)
http://dx.doi.org/10.1364/OE.21.026846


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Abstract

Room-temperature large-scale highly ordered nanorod-patterned ZnO films directly integrated on III-nitride light-emitting diodes (LEDs) are proposed and demonstrated via low-cost modified nanoimprinting, avoiding a high-temperature process. with a 600 nm pitch on top of a critical 200 nm thick Imprinting ZnO nanorods of 200 nm in diameter and 200 nm in height continuous ZnO wetting layer, the light output power of the resulting integrated ZnO-nanorod-film/semi-transparent metal/GaN/InGaN LED shows a two-fold enhancement (100% light extraction efficiency improvement) at the injection current of 150 mA, in comparison with the conventional LED without the imprint film. The increased optical output is well explained by the enhanced light scattering and outcoupling of the ZnO-rod structures along with the wetting film, as verified by the numerical simulations. The wetting layer is found to be essential for better impedance matching. The current-voltage characteristics and electroluminescence measurements confirm that there is no noticeable change in the electrical or spectral properties of the final LEDs after ZnO-nanorod film integration. These results suggest that the low-cost high-quality large-scale ZnO-nanorod imprints hold great promise for superior LED light extraction.

© 2013 Optical Society of America

1. Introduction

2. Experiments

The studied GaN LEDs grown by our MOCVD system consist of a buffer layer, a Si-doped GaN layer and InGaN-GaN multiple quantum wells (MQWs), followed by Mg-doped GaN [13

13. Z. G. Ju, S. T. Tan, Z.-H. Zhang, Y. Ji, Z. Kyaw, Y. Dikme, X. W. Sun, and H. V. Demir, “On the origin of the redshift in the emission wavelength of InGaN/GaN blue light emitting diodes grown with a higher temperature interlayer,” Appl. Phys. Lett. 100(12), 123503 (2012). [CrossRef]

, 14

14. Z.-H. Zhang, S. T. Tan, Z. G. Ju, W. Lui, Y. Ji, Z. Kyaw, Y. Dikme, X. W. Sun, and H. V. Demir, “On the Effect of Step-Doped Quantum Barriers in InGaN/GaN Light Emitting Diodes,” J. Display Technology 9(4), 226–233 (2013). [CrossRef]

]. The active region contains 5-pair quantum well stack (In0.18Ga0.82N/GaN with 3 nm thick wells and 12 nm thick barriers). The devices were fabricated using standard fabrication process. The LED mesa was patterned through reactive ion etching with a size of 350 µm × 350 µm. A Ni/Au (5 nm/5 nm) film was deposited as the semi-transparent current spreading layer on the defined mesa, and then the thermal annealing was performed in the mixture of N2 and O2 for 5 min at 525 °C. Finally, Ti/Au (30 nm/150 nm) was deposited on the n-GaN layer and the current spreading layer as the n and p electrodes.

The ZnO nanorod pattern fabrication process is briefly shown in Fig. 1(a).
Fig. 1 (a) Modified imprinting process of ZnO-nanorod arrays: (I)-(III) illustrate the processes of transferring the pattern from the master plate to the PDMS mold, and (IV)-(VI) illustrate the processes of transferring the pattern from the PDMS mold to a GaN LED. (b) Schematic drawing of the nanorod-imprinted ZnO film directly integrated on a GaN LED.
A polydimethylsiloxane (PDMS) mold was first prepared using the patterned silicon master substrate. The ~2 µm thick photoresist was patterned with lithography process by exposing the top area where ZnO-nanorod arrays are to be imprinted. The excessive photoresist was thinned down to the same level of top mesa edge by O2 plasma. Subsequently, the large-scale ordered ZnO-nanorod films were directly imprinted using the precursor solution mixture, different than the recently reported pattern replication approach of non-wetting templates (PRINT) [15

15. K. Ishihara, M. Fujita, I. Matsubara, T. Asano, and S. Noda, “Direct Fabrication of Photonic Crystal on Glass Substrate by Nanoimprint Lithography,” Jpn. J. Appl. Phys. 45, 201–212 (2011).

], in which the sol-gel ZnO nanoparticles are first produced. Figure 1(a) briefly illustrates the proposed modified imprinting technique used here to fabricate ZnO-nanorod arrays. A polydimethylsiloxane (PDMS) mold was first prepared using the patterned silicon master substrate. The PDMS mold was pressed on the GaN LED that was coated with ZnO precursor solution using drop-casting. The ZnO precursor solution mixture was composed of 0.1 M Zn(Ac)2 solution and 0.1 M diethanolamine (DEA). The substrate was then heated for solvent evaporation. Subsequently, the mold was peeled off, leaving behind the periodical pillar arrays. Finally, the as-prepared sample was annealed at 250 ̊C for 30 min. The photoresist was then removed by acetone, leaving behind only the large-scale ordered ZnO-nanorods at top area and exposing the N-pad region. It should be noted that a ZnO wetting layer was formed underneath the ZnO nanorod arrays during the nanoimprinting process. A ZnO wetting layer thickness can be varied by controlling the coating thickness of the ZnO precursor solution, the concentration of Zn(Ac)2 solution in ZnO precursor solution, and the pressure applied on the PDMS mold. Figure 1(b) shows the ZnO-nanorod arrays on the mesa area of a single LED die.

3. Results and discussion

Fig. 2 (a). Optical image of the ZnO-nanorod arrays imprinted on the GaN LED. (b) Top view micrograph image of the ZnO-nanorod arrays, along with their enlarged (c) top view and (d) tilt view SEM images.
Figure 2(a) shows the optical image of the ZnO-nanorod arrays imprinted on the GaN LED where obvious blue reflection from the ZnO-nanorod arrays (resulting from the preferential scattering of the incoming light in blue off the nanorod arrays) is observed. Figure 2(b) presents the imprinted ZnO-nanorod arrays on the semi-transparent metal surface. The enlarged plane-view and tilt-view field-emission scanning electron microscopy (FE-SEM) images are given in Figs. 2(c) and 2(d), respectively. As seen from Figs. 2(a)-2(d), the imprinted ZnO-nanorod arrays exhibit uniform size distribution on the semi-transparent metal electrode, in an architecture designed for a 200 nm diameter and height with a 600 nm pitch.

The output optical power of the LEDs was measured using an integrating optical sphere.
Fig. 3 Light output power as a function of injection current for the NR-LED (with the ZnO wetting layer plus ZnO nanorods) and the C-LED (without ZnO film). The inset shows the I-V characteristics of LED samples, which are similar with and without ZnO nanorods.
Figure 3 shows the light output power (LOP) characteristics of the two fabricated LEDs as a function of the injection current. The LOP of the nanorod-film integrated LED (NR-LED) is clearly higher than that of the conventional LED (C-LED) without the wetting layer or nanorod array at the same injection current. The LOP of the NR-LEDs is twofold of that of the C-LED at 150 mA, as shown in Fig. 3.

It is well known that the efficiency of an LED is increased when the LED produces a higher level of optical output power while keeping the input electrical power the same. Since the forward voltage (Vf) directly influences the input electrical power, Vf should be kept constant after incorporating the ZnO nanostructures compared to the initial Vf before incorporating them. To address this problem, here we utilize nanoimprinting process to form ZnO nanorods at room-temperature and thus induce limited or no damage to the p-electrode. The inset of Fig. 3 shows the I-V curves of the NR-LED and C-LED. It can be seen that both the I-V curves are similar and no obvious increase of the forward voltage is observed. This suggests that the incorporation of these room-temperature imprinted ZnO nanorods does not degrade the electrical properties of LEDs.

Fig. 4 Electroluminescence (EL) spectra of (a) the NR-LED with wetting layer plus ZnO nanorods and (b) the C-LED without wetting layer and ZnO nanorods.
Figures 4(a) and 4(b) show the electroluminescence (EL) spectra of the fabricated NR-LED and C-LED at the injection current levels of 20, 50, and 150 mA, respectively. There are no significant differences in the EL peak positions (at 440 nm) of the two LEDs. In addition, the two EL spectra have essentially identical normalized spectral shape and linewidth. However, EL intensities obtained from the NR-LED were two times of those achieved from the C-LED at these injection currents of 20, 50, and 150 mA, which is consistent with the results of optical power output measurement shown in Fig. 3. Given that the NR-LED and the C-LED are from the same run of MOCVD growth and their EL spectral shape and linewidth are identical, we deduce that the 100% improvement in LOP is mainly attributed to the improvement of LEE while the internal efficiency of two LEDs remains about the same.

The much larger improvement in LEE in our case compared to those in [11

11. S.-H. Lee, K.-J. Byeon, H. Park, J.-Y. Cho, K.-Y. Yang, and H. Lee, “Enhancement of light extraction efficiency of GaN-based lightemitting diode using ZnO sol-gel direct imprinting,” Microelectron. Eng. 88(11), 3278–3281 (2011). [CrossRef]

, 12

12. S. Kim, S.-M. Kim, H.-H. Park, D.-G. Choi, J.-W. Jung, J. H. Jeong, and J.-R. Jeong, “Conformally direct imprinted inorganic surface corrugation for light extraction enhancement of light emitting diodes,” Opt. Express 20(S5Suppl 5), A713–A721 (2012). [CrossRef] [PubMed]

] is due to the structure differences of ZnO nanorod structure. In our work, the size of nanorods is much larger (diameter 200nm and height 200nm) than that in [12

12. S. Kim, S.-M. Kim, H.-H. Park, D.-G. Choi, J.-W. Jung, J. H. Jeong, and J.-R. Jeong, “Conformally direct imprinted inorganic surface corrugation for light extraction enhancement of light emitting diodes,” Opt. Express 20(S5Suppl 5), A713–A721 (2012). [CrossRef] [PubMed]

] (diameter 100nm and height 108nm), and is closer to the emitting wavelength which can generate multiple light scatterings at the LED surface and makes it more likely for photons to escape from the device. Furthermore, the generated light is also extracted through the larger vertical sidewalls of the ZnO nanorods. Hence, the total range of angles through which light is coupled out from the LED into the air is increased. The larger size of the ZnO nanorods also increases the effective refractive index leading to a better match to ZnO refractive index (nZnO = 2.1) of the wetting layer and more light can be coupled out because of it. Moreover, compared to the lack of ZnO wetting layer in [11

11. S.-H. Lee, K.-J. Byeon, H. Park, J.-Y. Cho, K.-Y. Yang, and H. Lee, “Enhancement of light extraction efficiency of GaN-based lightemitting diode using ZnO sol-gel direct imprinting,” Microelectron. Eng. 88(11), 3278–3281 (2011). [CrossRef]

] and thinner ZnO wetting layer of 35nm in [12

12. S. Kim, S.-M. Kim, H.-H. Park, D.-G. Choi, J.-W. Jung, J. H. Jeong, and J.-R. Jeong, “Conformally direct imprinted inorganic surface corrugation for light extraction enhancement of light emitting diodes,” Opt. Express 20(S5Suppl 5), A713–A721 (2012). [CrossRef] [PubMed]

] the presence of 200nm ZnO wetting layer in our work is critical in further enhancing LEE, because it provides smoother change of refractive index to buffer the refractive index mismatch between the air and GaN layer and leads to a more dispersed angular distribution of photons generated in MQWs, resulting in a larger escape cone for photons in the NR-LED than in the LEDs without ZnO wetting layer or with thinner ZnO wetting layer.

The qualitatively physical explanation above is further confirmed by a two-dimensional finite difference time domain (FDTD) simulation using a commercially available solver from Lumerical Inc. A simulation area of 30 µm × 15 µm is constructed to include multiple period of two dimensional ZnO nano-patterning. The non-uniform mesh with minimum mesh size of 0.25nm is used to mesh the simulated structure. Dipole light source is placed in an emission layer to replicate the incoherent light radiation generated due to electron-hole pair. The incoherent light radiation from the coherent dipole source is generated by rotating the dipole source in x, y and z directions and recording transmitted intensity respectively by frequency-domain field monitor placed outside the simulation structure but within FDTD simulation area. These transmitted intensities are then added incoherently to yield incoherent light radiation. Physically matched layers are used as boundary condition to surround the simulation area in order to absorb any light radiation impinging on it. The numerical simulation is repeated for three particular cases; (1) LED with ZnO wetting layer only, (2) LED with ZnO nanorods only and (3) LED with both ZnO wetting layer and nanorods (NR-LED). The LEE is measured at 440nm wavelength in the far-field integrating all extracted light radiation in 1° solid angle. Considering the Purcell effect, we simulated the effect of ZnO wetting layer and ZnO nanorods arrays on the internal quantum efficiency (IQE). In our FDTD simulations we have not found any discernible change in the IQE due to the presence of ZnO wetting layer and nanorods array. This can be interpreted because of the reason that the active layer in the GaN LED is distant from the ZnO wetting layer and nanorods on the top. Thus, the presence of the wetting layer and nanorods does not noticeably affect the dipole radiates.

The LEE measured from the FDTD simulation with respect to change in wavelength for three different above mentioned cases is shown in Fig. 5.
Fig. 5 Numerical FDTD simulation results of light extraction enhancements as a function of wavelength for the.LEDs with wetting layer plus ZnO nanorods, with only ZnO nanorods and with only wetting layer, respectively.
It can be clearly seen from this figure that the LEE for NR-LED with 200nm ZnO wetting layer is 1.9 times (90% improvement); whereas, the LEE for NR-LED without 200nm wetting layer is decreased to 1.7 times to the C-LED. Thus, ZnO wetting layer plays an important role to increase LEE. Based on the experimental results, the thickness of ZnO wetting layer also affects the LEE improvement and 200 nm ZnO wetting layer thickness is the optimal thickness which can provide the largest improvement of LEE given the dimensions of the nanorods arrays adopted here. The LEE for LED with 200nm ZnO wetting layer and no nanorods is also shown in Fig. 5 and compared to that of C-LED there is little improvement in LEE. Therefore, it is the combination of optimized ZnO nanorods dimensions and ZnO wetting layer with matched thickness that can provide the largest improvement in LEE. The simulation result of 90% improvement in LEE here is consistent with the experimental result of 100% LOP improvement as shown before, which confirms that LOP enhancement is mainly due to the light extraction enhancement.

Fig. 6 2D light intensity pattern from numerical FDTD simulation in case of (a) C-LED, (b) C-LED with ZnO wetting layer, (c) C-LED with ZnO nanorod array and (d) C-LED with ZnO nanorod array on top of ZnO wetting layer.
Figure 6 shows the simulated 2D intensity pattern of light exiting from LED in four different cases: (a) C-LED, (b) C-LED with ZnO wetting layer, (c) C-LED with ZnO nanorods and (d) C-LED with ZnO nanorod array on top of wetting layer. As shown by Fig. 6(a), most of the light that is generated in the active region of the LED, suffers the total internal reflection and is trapped within the LED structure. With the help of 200 nm thick ZnO wetting layer more amount of light can escape out the LED structure since ZnO wetting layer allows increase in the critical angle at air-ZnO boundary. In Fig. 6(c), light emission from the LED structure is studied with only ZnO nanorods. In this case, no ZnO wetting layer is present which helps to out-couple more light from the LED structure as shown in Fig. 6(b). However, with only ZnO nanorods present on the top of GaN LED surface, LEE of 1.7 times can be achieved due to multiple scattering from the side walls within ZnO nanostructure and emission from top surface [16

16. J. Strutt, “On the scattering of light by small particles,” Philos. Mag. 41, 447–454 (1871).

] [17

17. S. Nunomura, A. Minowa, H. Sai, and M. Kondo, “Mie scattering enhanced near-infrared light response of thin-film silicon solar cells,” Appl. Phys. Lett. 97(6), 063507 (2010). [CrossRef]

]. In the last case, light scattered from ZnO nanorod array on the top of the ZnO wetting layer is shown by Fig. 6(d). In this case, both wetting layer and nanorod array assist in to escape more light from the GaN LED that is trapped inside achieving the LEE of 1.9. Thus, the ZnO wetting layer out-couples more light into the air whereas, ZnO nanorods allows light enhancement due to multiple scattering from the sidewalls and top surface.

4. Conclusion

Acknowledgments

This work is supported by the National Research Foundation of Singapore under Grant No. NRF-CRP-6-2010-2 and NRF-RF-2009-09 and the Singapore Agency for Science, Technology and Research (A*STAR) SERC under Grant No. 112 120 2009.

References and links

1.

D. Zhu, C. McAleese, K. K. McLaughlin, M. Häberlen, C. O. Salcianu, E. J. Thrush, M. J. Kappers, W. A. Phillips, P. Lane, D. J. Wallis, T. Martin, M. Astles, S. Thomas, A. Pakes, M. Heuken, and C. J. Humphreys, “GaN-based LEDs grown on 6-inch diameter Si (111) substrates by MOVPE,” Proc. SPIE 7231, 723118, 723118-11 (2009). [CrossRef]

2.

S. T. Tan, X. W. Sun, H. V. Demir, and S. P. DenBaars, “Advances in the LED Materials and Architectures for Energy-Saving Solid-State Lighting Toward “Lighting Revolution”,” IEEE Photon. J. 4(2), 613–619 (2012). [CrossRef]

3.

M. Boroditsky and E. Yablonovitch, “Light extraction efficiency from light-emitting diodes”, Proc. SPI 3003, 119–122 (1997). [CrossRef]

4.

T. Fujii, Y. Gao, R. Sharma, E. L. Hu, S. P. DenBaars, and S. Nakamura, “Increase in the extraction efficiency of GaN-based light-emitting diodes via surface roughening,” Appl. Phys. Lett. 84(6), 855–857 (2004). [CrossRef]

5.

T. N. Oder, K. H. Kim, J. Y. Lin, and H. X. Jiang, “III-nitride blue and ultraviolet photonic crystal light emitting diodes,” Appl. Phys. Lett. 84(4), 466–468 (2004). [CrossRef]

6.

X. Guo, Y. L. Li, and E. F. Schubert, “Efficiency of GaNInGaN light-emitting diodes with interdigitated mesa geometry,” Appl. Phys. Lett. 79(13), 1936–1938 (2001). [CrossRef]

7.

H. Y. Lee, X. Y. Huang, and C. T. Lee, “Light output enhancement of GaN-based roughened LEDs using bias-assisted photoelectrochemical etching method,” J. Electrochem. Soc. 155(10), H707–H709 (2008). [CrossRef]

8.

S. J. An, J. H. Chae, G. C. Yi, and G. H. Park, “Enhanced light output of GaN-based light-emitting diodes with ZnO nanorod arrays,” Appl. Phys. Lett. 92(12), 121108 (2008). [CrossRef]

9.

S. Dalui, C. C. Lin, H. Y. Lee, C. H. Chao, and C. T. Lee, “Light output enhancement of gan-based light-emitting diodes using ZnO nanorod arrays produced by aqueous solution growth technique,” IEEE Photon. Technol. Lett. 22(16), 1220–1222 (2010). [CrossRef]

10.

J. Zhong, H. Chen, G. Saraf, Y. Lu, C. K. Choi, J. J. Song, D. M. Mackie, and H. Shen, “Integrated ZnO nanotips on GaN light emitting diodes for enhanced emission efficiency,” Appl. Phys. Lett. 90(20), 203515 (2007). [CrossRef]

11.

S.-H. Lee, K.-J. Byeon, H. Park, J.-Y. Cho, K.-Y. Yang, and H. Lee, “Enhancement of light extraction efficiency of GaN-based lightemitting diode using ZnO sol-gel direct imprinting,” Microelectron. Eng. 88(11), 3278–3281 (2011). [CrossRef]

12.

S. Kim, S.-M. Kim, H.-H. Park, D.-G. Choi, J.-W. Jung, J. H. Jeong, and J.-R. Jeong, “Conformally direct imprinted inorganic surface corrugation for light extraction enhancement of light emitting diodes,” Opt. Express 20(S5Suppl 5), A713–A721 (2012). [CrossRef] [PubMed]

13.

Z. G. Ju, S. T. Tan, Z.-H. Zhang, Y. Ji, Z. Kyaw, Y. Dikme, X. W. Sun, and H. V. Demir, “On the origin of the redshift in the emission wavelength of InGaN/GaN blue light emitting diodes grown with a higher temperature interlayer,” Appl. Phys. Lett. 100(12), 123503 (2012). [CrossRef]

14.

Z.-H. Zhang, S. T. Tan, Z. G. Ju, W. Lui, Y. Ji, Z. Kyaw, Y. Dikme, X. W. Sun, and H. V. Demir, “On the Effect of Step-Doped Quantum Barriers in InGaN/GaN Light Emitting Diodes,” J. Display Technology 9(4), 226–233 (2013). [CrossRef]

15.

K. Ishihara, M. Fujita, I. Matsubara, T. Asano, and S. Noda, “Direct Fabrication of Photonic Crystal on Glass Substrate by Nanoimprint Lithography,” Jpn. J. Appl. Phys. 45, 201–212 (2011).

16.

J. Strutt, “On the scattering of light by small particles,” Philos. Mag. 41, 447–454 (1871).

17.

S. Nunomura, A. Minowa, H. Sai, and M. Kondo, “Mie scattering enhanced near-infrared light response of thin-film silicon solar cells,” Appl. Phys. Lett. 97(6), 063507 (2010). [CrossRef]

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

ToC Category:
Optical Devices

History
Original Manuscript: August 8, 2013
Revised Manuscript: September 23, 2013
Manuscript Accepted: October 9, 2013
Published: October 30, 2013

Citation
Zabu Kyaw, Wang Jianxiong, Kapil Dev, Swee Tiam Tan, Zhengang Ju, Zi-Hui Zhang, Yun Ji, Namig Hasanov, Wei Liu, Xiao Wei Sun, and Hilmi Volkan Demir, "Room-temperature larger-scale highly ordered nanorod imprints of ZnO film," Opt. Express 21, 26846-26853 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-22-26846


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References

  1. D. Zhu, C. McAleese, K. K. McLaughlin, M. Häberlen, C. O. Salcianu, E. J. Thrush, M. J. Kappers, W. A. Phillips, P. Lane, D. J. Wallis, T. Martin, M. Astles, S. Thomas, A. Pakes, M. Heuken, and C. J. Humphreys, “GaN-based LEDs grown on 6-inch diameter Si (111) substrates by MOVPE,” Proc. SPIE7231, 723118, 723118-11 (2009). [CrossRef]
  2. S. T. Tan, X. W. Sun, H. V. Demir, and S. P. DenBaars, “Advances in the LED Materials and Architectures for Energy-Saving Solid-State Lighting Toward “Lighting Revolution”,” IEEE Photon. J.4(2), 613–619 (2012). [CrossRef]
  3. M. Boroditsky and E. Yablonovitch, “Light extraction efficiency from light-emitting diodes”, Proc. SPI 3003, 119–122 (1997). [CrossRef]
  4. T. Fujii, Y. Gao, R. Sharma, E. L. Hu, S. P. DenBaars, and S. Nakamura, “Increase in the extraction efficiency of GaN-based light-emitting diodes via surface roughening,” Appl. Phys. Lett.84(6), 855–857 (2004). [CrossRef]
  5. T. N. Oder, K. H. Kim, J. Y. Lin, and H. X. Jiang, “III-nitride blue and ultraviolet photonic crystal light emitting diodes,” Appl. Phys. Lett.84(4), 466–468 (2004). [CrossRef]
  6. X. Guo, Y. L. Li, and E. F. Schubert, “Efficiency of GaNInGaN light-emitting diodes with interdigitated mesa geometry,” Appl. Phys. Lett.79(13), 1936–1938 (2001). [CrossRef]
  7. H. Y. Lee, X. Y. Huang, and C. T. Lee, “Light output enhancement of GaN-based roughened LEDs using bias-assisted photoelectrochemical etching method,” J. Electrochem. Soc.155(10), H707–H709 (2008). [CrossRef]
  8. S. J. An, J. H. Chae, G. C. Yi, and G. H. Park, “Enhanced light output of GaN-based light-emitting diodes with ZnO nanorod arrays,” Appl. Phys. Lett.92(12), 121108 (2008). [CrossRef]
  9. S. Dalui, C. C. Lin, H. Y. Lee, C. H. Chao, and C. T. Lee, “Light output enhancement of gan-based light-emitting diodes using ZnO nanorod arrays produced by aqueous solution growth technique,” IEEE Photon. Technol. Lett.22(16), 1220–1222 (2010). [CrossRef]
  10. J. Zhong, H. Chen, G. Saraf, Y. Lu, C. K. Choi, J. J. Song, D. M. Mackie, and H. Shen, “Integrated ZnO nanotips on GaN light emitting diodes for enhanced emission efficiency,” Appl. Phys. Lett.90(20), 203515 (2007). [CrossRef]
  11. S.-H. Lee, K.-J. Byeon, H. Park, J.-Y. Cho, K.-Y. Yang, and H. Lee, “Enhancement of light extraction efficiency of GaN-based lightemitting diode using ZnO sol-gel direct imprinting,” Microelectron. Eng.88(11), 3278–3281 (2011). [CrossRef]
  12. S. Kim, S.-M. Kim, H.-H. Park, D.-G. Choi, J.-W. Jung, J. H. Jeong, and J.-R. Jeong, “Conformally direct imprinted inorganic surface corrugation for light extraction enhancement of light emitting diodes,” Opt. Express20(S5Suppl 5), A713–A721 (2012). [CrossRef] [PubMed]
  13. Z. G. Ju, S. T. Tan, Z.-H. Zhang, Y. Ji, Z. Kyaw, Y. Dikme, X. W. Sun, and H. V. Demir, “On the origin of the redshift in the emission wavelength of InGaN/GaN blue light emitting diodes grown with a higher temperature interlayer,” Appl. Phys. Lett.100(12), 123503 (2012). [CrossRef]
  14. Z.-H. Zhang, S. T. Tan, Z. G. Ju, W. Lui, Y. Ji, Z. Kyaw, Y. Dikme, X. W. Sun, and H. V. Demir, “On the Effect of Step-Doped Quantum Barriers in InGaN/GaN Light Emitting Diodes,” J. Display Technology9(4), 226–233 (2013). [CrossRef]
  15. K. Ishihara, M. Fujita, I. Matsubara, T. Asano, and S. Noda, “Direct Fabrication of Photonic Crystal on Glass Substrate by Nanoimprint Lithography,” Jpn. J. Appl. Phys.45, 201–212 (2011).
  16. J. Strutt, “On the scattering of light by small particles,” Philos. Mag.41, 447–454 (1871).
  17. S. Nunomura, A. Minowa, H. Sai, and M. Kondo, “Mie scattering enhanced near-infrared light response of thin-film silicon solar cells,” Appl. Phys. Lett.97(6), 063507 (2010). [CrossRef]

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