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學生：黃順源老師：管 鴻 教授

Light Extraction Efficiency Enhancement of GaN Blue LED by Liquid-Phase-Deposited ZnO Rods copy

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outline

Abstract INTODUCTION EXPERIMENT RESULTS AND DISCUSSION CONCLUSION REFERENCES

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Abstract

We investigate the light extraction efficiency of a GaN light-emitting diode (LED) by using liquid-phase-deposited ZnO rods at near-room temperature.

Zinc nitrate and hexamethylenetetramine were used as the deposition precursors. Compared with the conventional GaN LED, the optical power output of the GaN LED with crystalline ZnO rods on its surface has about 1.6 times enhancement

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INTODUCTION

The internal quantum efficiency of GaN blue LEDs is close to 100%, however, most light is lost in the materials.

From the refractive indexes of GaN ( n=2.5 at 440 nm) and the air, the critical angle is 23.6 . Only about 4% emitted light radiates through the top and bottom [1].

ZnO film is a promising material for the optoelectronic devices as its wide bandgap (3.37 eV), large excitonic binding energy (60 meV) and high optical gain (300 cm^-1 ).

In this letter, a simple and nonexpensive liquid-phase-deposition (LPD) method was used to grow high crystalline and uniform size ZnO rods on GaN blue LED at near-room temperature to enhance the light extraction efficiency.

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EXPERIMENT

The GaN LED grown by metal–organic chemical vapor deposition consists of buffer and Si-doped layers, InGaN–GaN quantum wells (QWs), followed by Mg-doped GaN layers. The active region for blue LED was five-period In0.18Ga0.82N–GaN QWs(at 440 nm) and the chip area is 300×300 μm^2 . Ti–Au and Ni–Al metals were used as ohmic contacts on p- and n-GaN, respectively.

An HP4145B semiconductor-parameter analyzer was used for current–voltage characterization. The electroluminescence (EL) was measured at the forward current from 20 to 100 mA

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The aqueous solutions of zinc nitrate (Zn (NO3)2 , 0.1 M, 20 ml) and hexamethylenetetramine (C6H12N4 , (HMT), 0.1 M, 20 ml) were used to deposit ZnO rods [11]. The LED chip was immersed in the solution and the temperature is kept at 50 C.

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RESULTS AND DISCUSSION

(a)2.6×10^9㎝ ^-2(b)8.0×10^9㎝ ^-2(c)1.9×10^9㎝ ^-2(e)Height 240 nm and diameter 200 nm with ±10%

(FE-SEM)

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HR-TEM

The lattice spacing is approximately 2.6 A between two (002)crystal planes

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The refractive indexes of ZnO and GaN are 2.0 and 2.5 at the wavelength of 440 nm. The critical angle related to the air, ZnO and GaN is

calculated by Snell’s law 【 θC＝ sin^-1（ n2/n1）】 . From the aspect ratio (240 nm/200 nm) and rod density of 8.0×10^9cm^-2 in the above

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The incident angle of the light emission from the GaN active layer has an increase of 40% 【（ 53.2°-43.9°） /23.63°×100 ％】 through the side wall of each ZnO rod

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RESULTS AND DISCUSSION

By increasing the aspect ratio with a fixed density, the light escaping though the top surface decreases due to the increase of ZnO absorption loss from the additional total reflection and transmission length

By decreasing the aspect ratio, part of the original light escaping though the side wall will be totally reflected and then absorbed in GaN.

By increasing the rod density with a fixed diameter, the light coupling between rods increases and hence the absorption increases

By decreasing the rod density, the light enhancement decreases due to the decrease of ZnO rod coverage

Therefore, the light emission intensity depends on the rod shape and density

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EL spectra and I-V characteristics

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Optical power output

30° enhanced to1.65 times60° enhanced to1.73 times 90° enhanced to3.25 times

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CONCLUSION

ZnO rods have been directly grown on GaN LED by LPD. We have achieved in the LED an enhancement of optical power output to 1.6 times with the ZnO rod density of 8.0×10^9cm^-2 . It also shows a stronger and wider angular optical power distribution. The dependence of light emission efficiency on the rod density and size will be further studied.

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[1] M. Boroditsky and E. Yablonovitch, “Light extraction efficiency from light-emitting diodes,” Proc. SPIE, vol. 3002, pp. 119–122, Apr. 1997. [2] 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., vol. 63, no. 16, pp.2174–2176, 1993.[3] 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., vol. 90, no.20, p. 203515, 2007.[4] Y. Chen, D. M. Baghall, H. Koh, K. Park, K. Hiraga, Z. Zhu, and T. Yao, “Plasma assisted molecular beam epitaxy of ZnO on c-plane sapphire: Growth and characterization,” J. Appl. Phys., vol. 84, no. 19, pp.3912–3914, 1998.[5] A. Ohtomo, M. Kawasaki, Y. Sakurai, I. Ohkubo, R. Shiroki, Y. Yoshida, T. Yasuda, Y. Segawa, and H. Koinuma, “Fabrication of alloys and superlattices based on ZnO towards ultraviolet laser,” Mater. Sci. Eng. B, vol. 56, no. 2-3, pp. 263–266, 1998.[6] 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, vol. 292, no. 5523, pp. 1897–1899, Jun. 8, 2001.[7] Y. Li, G. W. Meng, L. D. Zhang, and F. Phillipp, “Ordered semiconductor ZnO nanowire arrays and their photoluminescence properties,” Appl. Phys. Lett., vol. 76, no. 15, pp. 2011–2013, 2000.[8] W. I. Park, D. H. Kim, S. W. Jung, and G. C. Yi, “Metalorganic vaporphase epitaxial growth of vertically well-aligned ZnO nanorods,” Appl.Phys. Lett., vol. 80, no. 22, pp. 4232–4234, Jun. 3, 2002.[9] H. Gao, F. Yan, J. Li, Y. Zeng, and J. Wang, “Synthesis and characterization of ZnO nanorods and nonoflowers grown on GaN-based led epiwafer using a solution deposition method,” J. Phys. D, vol. 40, no. 12, pp. 3654–3659, 2007.

REFERENCES

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REFERENCES[10] H. Q. Le, S. J. Chua, Y. W. Koh, K. P. Loh, Z. Chen, C. V. Thompson, and E. A. Fitzgerald, “Growth of single crystal zno nanorods on GaN using an aqueous solution method,” Appl. Phys. Lett., vol. 87, no. 10, pp. 101908–1101908, 2005.[11] L. Vayssieres, “Growth of arrayed nanorods and nanowires of ZnO from aqueous solutions,” Adv. Mater., vol. 15, no. 5, pp. 464–466, Mar.4, 2003.[12] H. Zhang, D. Yang, Y. Ji, X. Ma, J. Xu, and D. Que, “Low temperature synthesis of flowerlike zno nanostructures by cetyltrimethylammonium bromide-assisted hydrothermal process,” J. Phys. Chem. B, vol. 108, no. 13, pp. 3955–3958, 2004.[13] C. C. Lee and J. Park, “Temperature measurement of visible light-emitting diodes using nematic liquid crystal thermography with laser illumination,” IEEE Photon. Technol. Lett., vol. 16, no. 7, pp. 1706–1708, Jul. 2004.[14] R. D. Vispute, V. Talyansky, S. Choopun, R. P. Sharma, T. Venkatesan, M. He, X. Tang, J. B. Halpern, M. G. Spencer, Y. X. Li, L. G. Salamanca- Riba, A. A. Lliadis, and K. A. Jones, “Heteroepitaxy of ZnO on GaN and its implications for fabrication of hybrid optoelectronic devices,” Appl. Phys. Lett., vol. 73, no. 3, pp. 348–350, 1998.

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