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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

Open Access Open Access

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

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Figures (8)
Fig. 1
Fig. 1 Schematic diagram of fabricated blue LED structures with ZnO nanorod arrays on Ni1-xCoxO hemisphere lenses.

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Fig. 2
Fig. 2 (a) Powder XRD patterns and plane indices of the A(NiO) and B(Co3O4) phase in Ni1-xCoxO NPs. (b) TEM images of the Ni1-xCoxO NPs at low magnification (left) and high magnification (right)

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Fig. 3
Fig. 3 UV-Vis absorbance spectrum of the colloidal Ni1-xCoxO dispersions in solvent.

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Fig. 4
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.

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Fig. 5
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.

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Fig. 6
Fig. 6 The PL spectra of the LEDs with (red line) and without (black line) NiCoO hemisphere lens formed on top of p-GaN .

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Fig. 7
Fig. 7 EL spectra obtained from blue LEDs with different surface nanostructures at an injection current of 50mA.

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Fig. 8
Fig. 8 FDTD simulation results for light extraction characteristics as a function of various different nanostructures.

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