ABSTRACT
Jeong Wook Lee, Jaehee Cho, Sukho Yoon, Hyunsoo Kim, Youn Joon Sung, Hyungkun Kim, Cheolsoo Sone, Yongjo Park Photonics Program Team, Samsung Advanced Institute of Technology, P.O. Box 111, Suwon 440-600, Korea
Tae Geun Kim Department of Electronic Engineering, Korea University, l-5Ka, Anamdong, Seongbuk-ku, Seoul 136-701, Korea
Abstract. The external quantum efficiency of nitride light-emitting diodes (LEDs) grown on sapphire substrates is greatly influenced by the internal reflection of lights at the nitride-substrate interface. To improve the external quantum efficiency of nitride LEDs with an InGaN multi quantum well (MQW) structure, we introduced various patterned structures at the interface. The output power was increased by 1.7 times by incorporating an optimized patterned structure in nitride LEDs. In this study, the corrugated interface substrate (CIS) pattern was fabricated by standard photolithography and subsequent reactive ion etching (RIE) process. When the flip-chip type LED grown on CIS was operated at a forward bias current 20mA at room temperature, emission wavelength, output power, and external quantum efficiency were 400 nm, 21 mW and 34%, respectively. 1
1. Introduction In a general LED, an important issue for improving external quantum efficiency is how efficiently can the light, which is generated in an internal active layer, be extracted externally. To efficiently extract light propagated in the vertical direction of the sapphire substrate and nitride layers, many efforts have been attempted to form transparent electrodes or high reflective layers [1]. However, most of the emitted lights from the active layer propagate through the nitride layer because the refractive index of the IIInitrides is higher than that of the sapphire substrate. As a result, the photons emitted in the active region are mainly coupled into the high-index (GaN/Sapphire) guided mode. Moreover, as the reflectivity of p-electrode is not unity, this electrode absorbs some portion of the propagation light whenever it is reflected at the electrode [2-4]. Therefore, in case of flip-chip LEDs, the light extraction efficiency is at approximately 30% due to the difference in index of refraction between the GaN and the sapphire substrate and optical absorption at the electrode [4,5]. To prevent total internal reflection of light, several methods have been suggested such as forming the inclined sidewall of LEDs, i.e.
