A Study on Improvement in Luminescence Efficiency of InGaN/GaN Multiple Quantum Well based Light-Emitting Diodes

Abstract

The high brightness gallium nitride (GaN)-based light-emitting diodes (LEDs) have attracted much attention in recent years for various applications such as back light units for liquid crystal displays, traffic signals, automobiles, and solid-state lighting. However, in order to use for next-generation light source, the emission efficiency of GaN-based LEDs is needed to be further improved to replace the conventional solid-state light sources such as incandescent and fluorescent lamps. Although the internal quantum efficiency is close to 90%, the external quantum efficiency of conventional GaN-based LEDs is very still low because of the strict limitation of light extraction efficiency. The low extraction efficiency of LEDs is mainly caused by the large difference in refractive index between nitride epitaxial layer and surrounding air. Most of photons emitted from the active layer remain trapped within the high refractive index epitaxial layer by the total internal reflection at the interface between LED and air, which results in the low external quantum efficiency of GaN-based LEDs with a light escape probability of only 4%. Therefore, the improvement in the light extraction efficiency of LEDs is considered to be a critical issue. In this thesis, the several approaches to improve the extraction efficiency of GaN-based LEDs using a various nanostructures and surface texturing are presented. These methods of forming the nanostructures and surface texturing on top surface of LEDs effectively reduce the internal reflection and increase the probability of light escape, leading to the enhancement of light extraction efficiency of LEDs. First, the improvement in light extraction of GaN-based LEDs was achieved by using a surface coating of polystyrene (PS)/silica (SiO2) core-shell nanospheres. The SiO2 nanospheres and PS/SiO2 core-shell nanospheres synthesized by a Stöber procedures were coated on the top surface of LEDs using a simple spin-coating technique, respectively. The theoretically investigated results using the full three-dimensional (3D) finite-difference time-domain (FDTD) simulations showed the calculated light extraction efficiency of LEDs coated with PS/SiO2 core-shell nanospheres was much higher than that of the LEDs coated with SiO2 nanospheres. The relative electroluminescence (EL) intensity of LEDs coated with PS/SiO2 core-shell nanospheres was experimentally increased by 69% and 98% compared with that of LEDs coated with only SiO2 nanospheres and conventional LEDs without any nanospheres. The improvement in light extraction of LEDs using PS/SiO2 core-shell nanospheres is attributed to the multiple scattering within the core-shell spheres as well as increased probability of light escape by the reduced internal and Fresnel reflection. Second, the optical properties of GaN-based green LEDs were improved by using the surface chemical etching method. Using the phosphoric acid (H3PO4) solution as an etchant, the hexagonal-shaped pits were formed on the p-GaN surface of LEDs. In order to control the density and size of etch pits, the etching time was varied from 0 min to 20 min. When the etching time was increased, both the etch pit size and density were gradually increased. The relative intensity of photoluminescence (PL) of LEDs etched for 20 min is much higher than that of non-etched conventional LEDs and LEDs etched for 5 min and 10min. The increase of density and size of etch pits formed on top surface of LEDs enables the improvement in extraction efficiency because of the enlarged escape angle of photon emitted from the active layer.