The Enhancement of the Light Extraction Efficiency of GaN based Light-Emitting Diodes
by micro-semispherical and nano-cone composite Structure
Hongxin Yin1, Yujin Wang1, Chuanrui Zhu1, Yan Shen2, Haifang Yang1,*, Zhe Liu1, Changzhi Gu1,*, Baoli
Liu1, Xiangang Xu2
1
2
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of
Sciences, Beijing 100190, China
State Key Laboratory of Crystal Materials, Shandong University, 27 South Shanda Road, Jinan, Shandong
250100, China
[5] Lin C L, Chen P H, Chan C-H, Lee C C, Chen C-C, Chang J-Y and Liu C Y, Appl. Phys.Lett. 90 (2007)
242106
[6] Kim D, Lee H, Cho N, Sung Y and Yeom G, Jpn. J. Appl. Phys. 44 (2005) L18
[7] J. H. Lee, J. T. Oh, S. B. Choi, Y. C. Kim, H. I. Cho, and J. H. Lee, Ieee Photonics Technology Letters.
20 (2008) 345-347
[8]H. Kim, K. K. Choi, K. K. Kim, J. Cho, S. N. Lee, Y. Park, J. S. Kwak, and T. Y. Seong, Optics Letters.
33(2008) 1273-1275
[9] H.-X. Yin, C.-R. Zhu, Y. Shen, H.-F. Yang, Z. Liu, C.-Z. Gu, B.-L. Liu, X.-X. Gang, Appl. Phys. Lett.
104 (2014) 061113
e-mail: [email protected], [email protected]
Keywords: Light extraction efficiency, Light emitting diodes, micro/nano composite structure
[1] Ponce F A and Bour D P, Nature 386 (1997) 351
[2] E Fred Schubert and Jong Kyu Kim, Science. 308 (2005) 1274
[3] O B Shchekin, J E Epler, T A Trottier, T Margalith, D A Steigerwald, M O Holcomb, P S Martin and M
R Krames, Appl. Phys.Lett. 89 (2006) 071109
[4] Son J H, Kim J U, Song Y H, Kim B J, Ryu C J and Lee J-L, Adv. Mater. 24 (2012) 2259
Figure 1. Schematic diagram of the micro/nano
composite structure fabrication process.
Figure 2. The SEM image of the micro/nano
composite structure on V-LEDS surface. The scale
bar is 2 m.
0.12
160
micro/nano composite LED
micro structure LED
nano structure LED
reference LED
0.10
micro/nano composite LED
Reference LED
0.08
Light Output Power(mW)
Current(A)
Gallium nitride (GaN)-based light-emitting diodes (LEDs) have recently attracted significant attention
because of the demand for backlighting in liquid crystal displays, traffic signal lamps, vehicle lamps and
becoming a major contender in ecofriendly light sources [1-3]. However, the external quantum efficiency is
mainly limited by low light extraction efficiency. The primary reason for low light extraction efficiency is
total internal reflection at the interface due to the large difference in refractive index between the GaN film
(n=2.5) and air (n=1.0). Most of photons were trapped inside the GaN-based LED device and converted to
heat, which degrades the performance and the durability of the device. Roughening the top surface of the
LEDs is an effective and simple method to improve the light extraction. The micro structures and nano
structures were prepared on the surface of vertical-LEDs (V-LEDs) and has an obvious effect on the LED
light extraction enhancement such as conelike surface[4], honeycomb structure[5], microlens arrays[6] et
al. People found that the micro/nano composite structures[7-8] which prepared the nano structures on the
basis of the micro structures have a more obvious enhancement effects than that of the component of micro
structure or nano structure.
In this study, the composite of micro semispherical structure and nano-cone structure was fabricated on the
surface of vertical-LEDs (V-LEDs) and an obvious enhancement of the extraction efficiency had been
obtained. Fig.1 shows the schematic diagram of the fabrication process. The micro concave semispherical
structure was obtained by under exposure method based on conventional optical lithography system and
ICP dry-etching [9]. The nano-cone structure on the surface of V-LEDs was obtained through Ag
nanoparticles as mask by ICP etching. By optimizing the process parameters, the micro/nano composite
structure was fabricated on the surface of V-LED chip, as shown in Fig. 2. Fig.3 shows I-V characteristics
of V-LEDs for the flat surface sample and the micro/nano composite structure samples. The result reveals
that the patterning process has negligible influence on the electrical properties of the LED chips. The light
output power of V-LEDs for the flat surface and the micro/nano composite structure chips are measured
with the injection current from 50 to 380mA, as shown in Fig.4. The light output power of V-LEDs with
micro/nano composite structure has an obvious enhancement compared to fiat surface V-LEDs. The VLEDs with micro/nano composite structure showing about 220% increase in light output power compared
to the V-LEDs with flat surfaces when the injection current is 350mA.
0.06
0.04
0.02
120
80
40
0.00
-4
-2
0
2
4
Voltage(V)
Figure 3. The I-V characteristics of V-LEDs for the
flat surface sample and the sample with micro/nano
composite structure.
0
0
50 100 150 200 250 300 350 400 450
Injection Current(mA)
Figure 4. The light output power of V-LEDs for the
flat surface sample and the patterned samples as a
function of injection current (50 to 380mA).