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E1.9.1

Moth-Eye Light-Emitting Diodes

Hideki Kasugai1, Yasuto Miyake1, Akira Honshio1, Takeshi Kawashima1, Kazuyoshi Iida1, Motoaki Iwaya1, Satoshi Kamiyama1, Hiroshi Amano1, Isamu Akasaki1, Hiroyuki Kinoshita2 and Hiromu Shiomi2 1 Department of Materials Science and Engineering, 21st-Century COE Program “Nano Factory”, Meijo University, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya 468-8502, Japan. 2 SiXON Ltd., 47 Umezutakase-cho, Ukyo-ku, Kyoto 615-8686, Japan.

ABSTRUCT

Nitride-based blue-light-emitting diodes having a moth-eye structure were fabricated on the backside of a 6H-SiC substrate. The light extraction efficiency and the corresponding output power were increased by 3.8 times compared with those of an LED having the conventional structure. The results of theoretical analysis agree with these findings.

INTRODUCTION

Group III nitride-based light-emitting diodes (LEDs) in blue and green wavelength regions have been developed as a result of outstanding breakthroughs such as the improvement of crystalline quality by using a low-temperature-deposited buffer layer[1] and the realization of conductivity control of nitrides.[2-4] Applications of group III nitride-based LEDs, such as large-scale full-color displays, traffic signals, and backlight sources for cellular phones, are increasing. The realization of high-performance general lighting is the next target of nitride-based white LEDs. However, even though the performance of nitride-based LEDs is continuously improving, the efficiency of white LEDs is still insufficient, particularly for general lighting applications. Improvement of the efficiency is one of the most critical issues concerning the future expansion of LED applications. There are several factors that limit the efficiency of LEDs, such as nonradiative recombination, carrier confinement, and light extraction efficiency. Among such factors, light extraction efficiency is one of the most important point for LED efficiency. The use of a GaInN active layer can render the internal quantum efficiency satisfactorily high via the by optimization of the crystal growth conditions and structure. On the other hand, the external quantum

E1.9.2

efficiency strongly depends on the light extraction efficiency. Improvement of the light extraction efficiency of LEDs has been evaluated for many years. In the nitride-based LEDs, transparent substrates of sapphire and SiC are used. However, Fresnel reflection due to the refractive-index difference between the semiconductor chip and air causes a low light extraction efficiency. Surfaces with roughness on the scale of 2~4 µm to 400~600nm or less than 200nm with the small aspect ratio were introduced in order to improve the light extraction efficiency, because reflection is diffused at roughened surfaces.[5,6,7,8] The result is the suppression of the critical reflection and enhancement of light extraction. However, light extraction efficiency is only expected to improve by 50% at maximum due to the diffused reflection. In case of photonic crystals, it is necessary to us