Cho et al. Nanoscale Research Letters 2011, 6:578 http://www.nanoscalereslett.com/content/6/1/578 N A N O E X P R E S S Open Access Improvement of photon extraction efficiency of GaN-based LED using micro and nano complex polymer structures Joong-Yeon Cho1, Kyeong-Jae Byeon1, Hyoungwon Park1, Jinseung Kim1, Hyeong-Seok Kim2 and Heon Lee1*† Abstract A micro- and nanoscale complex structure made of a high refractive index polymer (n = 2.08) was formed on the ITO electrode layer of an edge-emitting type GaN blue light-emitting diode (LED), in order to improve the photon extraction efficiency by suppressing total internal reflection of photons. The nanoimprint lithography process was used to form the micro- and nanoscale complex structures, using a polymer resin with dispersed TiO2 nano- particles as an imprint resin. Plasma processing, such as reactive ion etching, was used to form the micro- and nano-scale complex structure; thus, plasma-induced damage to the LED device can be avoided. Due to the high refractive index polymeric micro- and nanostructure on the ITO layer, the electroluminescence emission was increased up to 20%, compared to an identical LED that was grown on a patterned sapphire substrate to improve photon extraction efficiency. Introduction High brightness GaN-based light-emitting diodes (LEDs) have been widely used for solid-state lighting sources due to their low power consumption, long lifetime, compact form factor, and eco-friendly nature [1-3]. The internal quantum efficiency (hint) of GaN-based LEDs has been drastically improved by the progress of GaN-based epitax- ial growth and device fabrication technologies [4,5]. Accordingly, many attempts have been made to maximize the external quantum efficiency (photon extraction effi- ciency) of LEDs. However, much room remains for improvement of the external quantum efficiency. One of the biggest issues surrounding the current high brightness LEDs is their low light extraction efficiency (hext) due to the total internal reflection (TIR) of light at the interface of the LED structure with ambient [6]. Various attempts, including surface roughening [7,8], the formation of photonic crystals [9,10], the use of patterned sapphire substrates (PSS) [11,12], and the use of an air- gap structure inside the LED [13], were made to suppress the TIR. * Correspondence: heonlee@korea.ac.kr † Contributed equally 1Department of Materials Science and Engineering, Korea University, Anam- dong 5-ga, Seongbuk-gu, Seoul 136-713, South Korea Full list of author information is available at the end of the article The TIR can be minimized by the scattering of light at the interface, which was enhanced by forming the photo- nic crystal structure or other micro- and nanoscale com- plex structures. In order to form those structures, plasma processing, such as reactive ion etching (RIE) or inductive coupled plasma etching, is inevitably used along with the lithography process and this can deteriorate the LED’s electrical performance [14-16]. Therefore, micro- or nanoscale complex structures need to be formed on the LED structure without plasma processing. In this study, micro- and nanoscale complex structures made of high refractive index polymer were formed to enhance the LED light extraction efficiency. The micro- and nanoscale structures were obtained from the photo- electro chemical (PEC)-etched surface of the N-faced GaN. The GaN epilayer of a vertical LED was detached from the sapphire substrate and placed over metallic heat sink; thus, the N-faced GaN surface was exposed. In order to improve the photon extraction efficiency of the vertical LED, the N-faced GaN surface was etched using the PEC process to form micro- and nanoscale structure [17]. Micro- and nanoscale patterns of N-faced GaN was repli- cated using a polydimethylsiloxane (PDMS) molding pro- cess and transferred to the ITO electrode surface of conventional edge emitting type GaN blue LED devices using nanoimprint lithography. Due to the micro- and © 2011 Cho et al; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.