Enhanced light extraction efficiency in flip-chip GaN light ... - gist.ac.kr

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Dong-Min Jeon,2 Je Won Kim,2 and Yong Chun Kim2. 1Department of Materials Science and Engineering, Gwangju Institute of Science and Technology,.
APPLIED PHYSICS LETTERS 93, 021121 共2008兲

Enhanced light extraction efficiency in flip-chip GaN light-emitting diodes with diffuse Ag reflector on nanotextured indium-tin oxide Ja-Yeon Kim,1 Min-Ki Kwon,1 Il-Kyu Park,1 Chu-Young Cho,1 Seong-Ju Park,1,a兲 Dong-Min Jeon,2 Je Won Kim,2 and Yong Chun Kim2 1

Department of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea 2 Lighting Module Business Team, Samsung Electro-Mechanics, Suwon 443-743, Republic of Korea

共Received 13 December 2007; accepted 11 June 2008; published online 17 July 2008兲 We investigated a flip-chip light emitting diode 共FCLED兲 with a diffuse reflector fabricated by depositing a Ag film on a nanotextured indium-tin oxide 共ITO兲 layer. The FCLED with a diffuse Ag reflector showed remarkably good adhesion and high reflectance than that with a specular Ag reflector deposited on the planar ITO layer. The optical output power of FCLED with the diffuse Ag reflector was enhanced by 161.3% at 300 mA compared to that with the specular Ag reflector. © 2008 American Institute of Physics. 关DOI: 10.1063/1.2953174兴 Gallium nitride 共GaN兲 has attracted special attention with the recent development of high-brightness green, blue, and white light-emitting diodes 共LEDs兲 for solid state lighting and displays.1 An increase in light extraction efficiency is considered to be crucial to improve the efficiencies of GaNbased LEDs. Several methods have been proposed for enhancing the light extraction of LEDs, including chip shaping,2,3 surface texturing,4–7 photonic crystals,8–10 and flip-chip packaging.11 The flip-chip LED 共FCLED兲 configuration is particularly effective in enhancing the light extraction efficiency, and has been extensively used in the fabrication of high power and high efficiency LEDs.11 Highperformance FCLEDs require p-Ohmic reflector layers that have high reflectivity, low contact resistance, and good thermal stability. For this purpose, silver 共Ag兲 has been widely used for FCLED reflectors because of its high reflectance in visible light and reasonable Ohmic behavior. However, a Ag shows very poor adhesion and thermal characteristics.12–15 To solve these problems, transparent layers inserted between the Ag reflector and p-GaN,12,13 Ag-based alloy layers such as AgAl alloy layer,14 and AgCu alloy layer15 have been introduced. However, these studies were based mainly on a Ag reflector layers deposited on the planar surface of transparent conducting layers or p-GaN. In the reflector deposited on planar surface, the light reflected from the reflector with planar interface 共specular reflector兲 is circulated and trapped within the FCLED due to the large difference between the refractive index of GaN 共nGaN ⬃ 2.5兲 and air 共nair ⬃ 1兲. On the other hand, the light from FCLED with a reflector having a textured interface 共diffuse reflector兲 has a higher escape

probability and the light extraction efficiency of FCLED can be enhanced since the light in an FCLED with a diffuse reflector experiences multiple refraction, reflection, and scattering events inside the escape cone.16,17 In this work, we studied a Ag diffuse reflector formed on the nanotextured indium-tin oxide 共ITO兲 layer. The optical output power of FCLED with a Ag reflector deposited on the nanotextured ITO layer was significantly enhanced, compared to a FCLED using a Ag reflector deposited on a planar ITO layer. To investigate the effect of a Ag diffuse reflector, GaN FCLEDs with a 450 nm emission wavelength were grown by metalorganic chemical vapor deposition on a c-plane sapphire substrate. The GaN LED consisted of the following layers: a 2 ␮m thick Si-doped n-GaN layer; an multiple quantum well active layer consisting of five periods of 2 nm thick undoped InGaN wells and 8 nm thick undoped GaN barriers; a Mg-doped p-GaN with a thickness of 0.15 ␮m. For electrode fabrication, mesa patterns were formed by an inductively coupled plasma etching process with Cl2 / CH4 / H2 / Ar gases. An electrically conductive and highly transparent ITO film 共500 nm兲 was deposited on the p-GaN by electron-beam evaporation. To form the diffuse reflector, the ITO layer was textured by dipping in a 0.1% buffered-oxide-etch solution for 5, 7, and 10 s without using any etch masks. The Ag layer was deposited on the nanotextured ITO layer as depicted in Fig. 1共a兲. For comparison, a Ag specular reflector was formed by deposition on unetched, planar ITO, as shown in Fig. 1共b兲. Cr/ Au layer 共50/ 350 nm兲 was then deposited on the n-GaN as a n-pad electrode. The electrical and optical properties of the FCLEDs were mea-

FIG. 1. 共Color online兲 Schematic cross-sectional views of FCLEDs with 共a兲 Ag reflector on nanotextured ITO layer, and 共b兲 Ag reflector on planar ITO layer.

a兲

Author to whom correspondence should be addressed. Electronic mail: [email protected].

0003-6951/2008/93共2兲/021121/3/$23.00 93, 021121-1 © 2008 American Institute of Physics Downloaded 21 Jul 2008 to 203.237.48.41. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

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FIG. 2. 共Color online兲 AFM images of nanotextured ITO layer etched for 共a兲 0, 共b兲 5, 共c兲 7, and 共d兲 10 s, respectively.

sured using a parameter analyzer 共HP 4155A兲 and a calibrated Si photodiode connected to an optical power meter. The surface morphology of the ITO layer following wet etching was examined using atomic force microscopy 共AFM兲, as shown in Fig. 2. The root-mean-square 共rms兲 surface roughness values of ITO layers etched for 0, 5, 7, and 10 s were 5.7 关Fig. 2共a兲兴, 32.5 关Fig. 2共b兲兴, 41.6 关Fig. 2共c兲兴, and 52.6 nm 关Fig. 2共d兲兴, respectively, demonstrating that the surface roughness increased with increasing etching time. For more reliable and good control of nanotexture formation on ITO, it is needed to further optimize the etching condition such as etching time. Figure 3 shows scanning electron microscopy 共SEM兲 images of Ag reflector layers on p-GaN 关Figs. 3共a兲 and 3共b兲兴, on planar ITO 关Figs. 3共c兲 and 3共d兲兴, and on a nanotextured ITO layer etched for 7 s 关Figs. 3共e兲 and 3共f兲兴 before and after annealing process at 500 ° C for 1 min in air ambient. Figures 3共a兲 and 3共b兲 indicate that the surface of the Ag layer was severely roughened after annealing, resulting in a hole formation and agglomeration, while Figs. 3共c兲 and 3共d兲 demonstrate that the agglomeration of the Ag layer disappeared on the planar ITO layer. Figure 3共e兲 show that the surface morphology of a Ag reflector on nanotextured ITO etched for 7 s shows sphere-shaped protrusions before annealing process. After annealing, however, the surface of Ag reflector on the nanotextured ITO becomes smooth, as shown in Fig. 3共f兲. The AFM measurements of a Ag reflector before and after annealing revealed that the rms of Ag reflector on planar ITO layer was decreased from 5.7 to 3.1 nm and that of Ag reflector on nanotextured ITO layer was decreased from 41.6 to 9.2 nm. The smooth surface of Ag reflector deposited on planar ITO and nanotextured ITO is a result of their good adhesion of Ag reflector on the ITO layer compared to that of Ag reflector on the p-GaN layer. The tape-peel adhesion tests showed that 95% of Ag layers on p-GaN and 80% of Ag layers on planar ITO were removed, but Ag layers deposited on nanotextured ITO remained intact. Fig. 3共g兲 shows crosssection transmission electron microscopy 共TEM兲 image of Ag on the roughened ITO after a post annealing at 500 ° C. As shown in TEM images, the upper part of ITO is roughened by a wet etching process and the Ag layer and the nanotextured ITO are strongly interlocked without forming

any nanovoids at the interface of Ag and nanotextured ITO. It is believed that the roughened ITO surface can prevent the agglomeration of Ag layer by interrupting the Ag migration at the interface. In addition, the adhesion of Ag layer is also improved by interlocked structure and the increased interface area, as shown in TEM image. To study the diffuse reflection effect of a Ag reflector on the nanotextured ITO, the reflectance was measured by collecting the scattered light in all angles by using an integrating sphere. Figure 4 shows that the total reflectance of a Ag on nanotextured ITO layer 共etched for 7 s兲 is higher than that of a Ag reflector on the planar ITO layer over the wavelength range of 380– 580 nm. An enhancement of 2.7 times was observed at 450 nm. This result is attributed to an increased probability of escape for photons undergoing diffuse reflection and scattering at the diffuse interface between Ag reflector and nanotextured ITO layer. Figure 5共a兲 shows the I-V characteristics of blueFCLEDs fabricated using Ag reflectors on planar ITO layer and on nanotextured ITO layers. A FCLED with a Ag reflector on the planar ITO layer exhibited a forward voltage of 3.38 V at 20 mA and a series resistance of 11.9 ⍀. However, the FCLEDs with a Ag reflector on the nanotextured ITO layer etched for 5, 7, and 10 s showed a higher forwardvoltage of 3.45, 3.50, and 3.60 V at 20 mA and larger series resistance of 13.6, 13.5, and 16 ⍀, respectively. The increase in forward voltages and series resistances were small for FCLEDs with a Ag reflector on nanotextured ITO layer etched for 5 and 7 s but those of nanotextured ITO layers etched for the 10 s resulted in a marked increase in both parameters compared to those of FCLEDs with Ag reflector on planer ITO layer. These results are attributed to the increased sheet resistance of nanotextured ITO layers. The sheet resistances of nanotextured ITO layers etched for 0, 5, 7, and 10 s were 11, 15, 18, and 45 ⍀ / sq, respectively, as shown in the inset of Fig. 5共a兲. The large increase in sheet resistance of nanotextured ITO layer etched for 10 s can be attributed to the suppression of current spreading by the severely roughened surface, as shown in Fig. 2共d兲. Figure 5共b兲 shows the optical output power 共L-I兲 as a function of current for FCLEDs using nanotextured reflec-

FIG. 3. SEM images of 关共a兲 and 共b兲兴 Ag reflector on p-GaN, 关共c兲 and 共d兲兴 Ag reflector on the planar ITO layer, and 关共e兲 and 共f兲兴 Ag reflector on nanotextured ITO layer before and after annealing process at 500 ° C for 1 min in air, respectively. 共g兲 TEM image of Ag reflector on nanotextured ITO layer after annealing process at 500 ° C for 1 min in air.

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FIG. 4. 共Color online兲 Relative total reflectance of a Ag reflector on nanotextured ITO layer and on a planar ITO layer integrated over all angles, respectively.

tors. The light output power of the FCLEDs with the ITO etched for 5, 7, and 10 s were enhanced by 22.2%, 153.9%, and 120.6% at 20 mA and 22.2%, 161.3%, and 132.0% at 300 mA, respectively, compared to that of the FCLED with a Ag reflector on a planar ITO layer. The FCLED with a Ag

reflector on an ITO layer etched for 7 s showed the highest output power, indicating that the nanostructure was optimized for the maximum light extraction. Despite a large enhancement of 2.7 times of reflectance of Ag on nanotextured ITO, the output power was enhanced by 161.3%. The discrepancy can be attributed to the slight degradation in electrical properties of Ag reflector on nanotextured ITO compared to that of Ag on planar ITO as shown in Fig. 5共a兲. These results showed that the light output power of FCLEDs can be greatly increased by forming diffuse Ag reflectors on the nanotextured ITO layers and the enhanced properties are attributed to the good adhesion and the increased diffuse reflectance of Ag reflector. In summary, we investigated the properties of diffuse Ag reflectors formed on nanotextured ITO layers fabricated by a maskless wet etching process. The light extraction of FCLEDs employing diffuse reflectors was enhanced by 161.3% at 300 mA compared to an FCLED with a specular Ag reflector on a planar ITO layer. This result can be attributed to the good adhesion and high diffuse reflectance of diffuse Ag reflector. The use of Ag reflectors formed on nanotextured ITO layers is a promising technique for the production of high efficiency and high power GaN FCLEDs. This work was supported by the BK 21 program, the Korea Science and Engineering Foundation 共KOSEF兲 grant funded by the Korea government 共MOST兲 共No. R17-2007078-01000-0兲, and Samsung Electromechanics Co., Ltd., in Korea. E. F. Schubert and J. K. Kim, Science 308, 1274 共2005兲. M. R. Krames, M. Ochiai-Holcomb, G. E. Hofler, C. Carter-Coman, E. I. Chen, I.-H. Tan, P. Grillot, N. F. Gardner, H. C. Chui, J.-W. Huang, S. A. Stockman, F. A. Kish, M. G. Craford, T. S. Tan, C. P. Kocot, M. Hueschen, J. Posselt, B. Loh, G. Sasser, and D. Collins, Appl. Phys. Lett. 75, 2365 共1999兲. 3 J.-Y. Kim, M.-K. Kwon, J.-P. Kim, and S.-J. Park, IEEE Photonics Technol. Lett. 19, 1863 共2007兲. 4 S.-I. Na, G.-Y. Ha, D.-S. Han, S.-S. Kim, J.-Y. Kim, J.-H. Lim, D.-J. Kim, K.-I. Min, and S.-J. Park, IEEE Photonics Technol. Lett. 18, 1512 共2006兲. 5 D.-S. Han, J.-Y. Kim, S.-I. Na, S.-H. Kim, K.-D. Lee, B. Kim, and S.-J. Park, IEEE Photonics Technol. Lett. 18, 1406 共2006兲. 6 R. H. Horng, C. C. Yang, J. Y. Wu, S. H. Huang, C. E. Lee, and D. S. Wuu, Appl. Phys. Lett. 86, 221101 共2005兲. 7 D. S. Leem, T. Lee, and T. Y. Seong, Solid-State Electron. 51, 793 共2007兲. 8 J. J. Wierer, M. R. Krames, J. E. Epler, N. F. Gardner, M. G. Craford, J. R. Wendt, J. A. Simmons, and M. M. Sigalas, Appl. Phys. Lett. 84, 3885 共2004兲. 9 S. H. Kim, K.-D. Lee, J.-Y. Kim, M.-K. Kwon, and S.-J. Park, Nanotechnology 18, 055306 共2007兲. 10 J.-Y. Kim, M.-K. Kwon, K.-S. Lee, S.-J. Park, S. H. Kim, and K.-D. Lee, Appl. Phys. Lett. 91, 181109 共2007兲. 11 J. J. Wierer, D. A. Steigerwald, M. R. Krames, J. J. O’Shea, M. J. Ludowise, G. Christenson, Y.-C. Shen, C. Lowery, P. S. Martin, S. Subramanya, W. Gotz, N. F. Gardner, R. S. Kern, and S. A. Stockman, Appl. Phys. Lett. 78, 3379 共2001兲. 12 J.-O. Song, J. S. Kwak, Y. Park, and T.-Y. Seong, Appl. Phys. Lett. 86, 062104 共2005兲. 13 D. L. Hibbard, S. P. Jung, C. Wang, D. Ullery, Y. S. Zhao, W. So, H. Liu, and H. P. Lee, Appl. Phys. Lett. 83, 311 共2003兲. 14 J.-Y. Kim, S.-I. Na, G.-Y. Ha, M.-K. Kwon, I.-K. Park, J.-H. Lim, and S.-J. Park, Appl. Phys. Lett. 88, 043507 共2006兲. 15 H. Kim, K. H. Baik, J. Cho, J. W. Lee, S. Yoon, H. Kim, S.-N. Lee, C. Sone, Y. Park, and T.-Y. Seong, IEEE Photonics Technol. Lett. 19, 336 共2007兲. 16 J. K. Kim, H. Luo, Y. Xi, J. M. Shah, J. Gessmann, and E. F. Schbert, Electrochem. Solid-State Lett. 153, G105 共2006兲. 17 Y. J. Lee, H. C. Kuo, T. C. Lu, and S. C. Wang, IEEE J. Quantum Electron. 42, 1196 共2007兲. 1 2

FIG. 5. 共a兲 Current-voltage characteristic 共I-V兲 of FCLEDs using Ag reflector on nanotextured ITO etched for 0, 5, 7, and 10 s. The inset shows the sheet resistance of the nanotextured ITO layer after wet etching for 0, 5, 7, and 10 s. 共b兲 Light output power 共L-I兲 of FCLEDs using Ag reflector on nanotextured ITO etched for 0, 5, 7, and 10 s, respectively.

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