ZnO 및 Ag 나노 구조를 이용한 LED의 광 특성 향상

Abstract

GaN-based light-emitting diodes have attracted extensive interest for solid-state lighting due to high brightness, energy saving, long lifetime, and environmentally properties. However, the luminous efficiency of LED is still low for application. One of the current major issues for conventional planar GaN-based LEDs is low light extraction efficiency due to total internal reflections within a limited escape cone due to large refractive index difference from GaN to air. Several methods have been proposed to enhance the light extraction, including surface texturing, patterned sapphire substrate, chip shaping, and photonic crystal. On the other hand, the proposed surface texturing methods involve expensive and energy consuming processes, long fabrication times. In addition, conventional GaN-based LED is the use of non-conductive sapphire substrate because it causes the current crowding near the p-contact by lateral carrier injection. To solve these problems, indium tin oxide (ITO) has been widely used as the transparent conductive electrode in GaN-based LEDs with advantage in high electrical conductivity and high optical transparency, but ITO is costly, and shows the poor transparency in the blue and ultraviolet range and instability in the presence of acids or base. Therefore, there is a significant need for a novel electrode material that can replace ITO. Recently, ZnO nanostructures grown by the hydrothermal method have been investigated widely to enhance the light extraction efficiency of the GaN based LEDs, because this has advantage in cost effective and low-temperature process. However most research based on ZnO nanostructures for LEDs have focused on enhancing the extraction efficiency of the LEDs. In this study, we will experimentally and numerically investigate the effect of the alignment of ZnO nano rod arrays (NRA) on the light extraction efficiency. To investigate the effect of the alignment of ZnO nano rod array on the light extraction behavior, we grow ZnO nano rod array with almost same parameters, like density, diameter and crystal quality, at the same level. The ZnO NRA with two different inclination angles, termed vertical and flower, were fabricated by the two-step hydrothermal method: forming seed and main ZnO NRA. The shape of the ZnO NRAs was controlled using the seed layers. To grow vertical ZnO NRA, the seed layer form by dipping the substrate into 0.005 M Zinc Acetate dihydrate dissolved in Ethanol at room temperature for 5 min while to grow flower shaped ZnO NRA, the seed layer form by dipping the substrate into 0.005 M zinc acetate dissolved in DI waver at 90°C for 5 min. After forming the ZnO seed layers, the main growth for the ZnO NRA was carried out in solutions of 0.05 M zinc nitrate hexahydrate and hexamethylenetetramine at 85°C for 60 min. The numerical analysis based on two-dimensional (2D) finite difference of time domain (FDTD) method shows that the extraction efficiency of LED with bare (without ZnO NRA), vertical ZnO NRA, flower shaped ZnO NRA is 37%, 60%, 49%, respectively. Current-Voltage characteristics for all of LEDs are same; however, the optical output power of LED with vertical ZnO NRA and flower ZnO NRA is improved by 50% and 30%, compared to that of LED with bare, respectively at an input current of 100 mA. This enhancement is agreed well with numerical analysis. Silver (Ag) nanowire based transparent conducting electrodes have also attracted significant attention for their advantage in high transparency and conductivity and low cost fabrication. However, there is no report on characteristic of LED with Ag nanowire transparent conductive electrode. In this study, we demonstrate the characteristic of LED with Ag nanowire transparent conductive electrode deposited by spin coating method. With optimization of deposition condition, silver nanowire TCO has a sheet resistance of 30~40 Ω/sq and an optical transmittance over 90%, which is similar with characteristics of ITO. The electrical and optical characteristics of LED with Ag nanowire transparent conductive electrode are comparable of those of LED with ITO.