Study On Enhancing The Light Extraction Efficiency Of GaN-based Light-emitting Diodes By Photonic Crystal And One-dimensional Grating Structure

LED (Light-emitting diodes), is a kind of semiconductor light-emitting device based on the principles of the p-n junction electroluminescent. LEDs has many advantages compared with traditional light sources, such as high light efficiency, long lifetime, energy saving and environmental protection, low power consumption, small size and light weight, high reliability, etc. High power light-emitting diodes (LEDs), with increasing luminous efficiency and cost performance in recent years, have more and more applications in general lighting and special lighting, such as LED road lighting, backlighting for LED TV, LED headlamp of automotive, and so on. GaN-based light-emitting diodes with high light extraction efficiency and excellent heat dissipation are essential for high-power and high-brightness applications. However, the realization of high light extraction LED is acritical issue, this is mainly due to the total internal reflection originating from the large refractive index differene between the semiconductor and air. The main research work of this thesis is around the design%modeling and simulation of high-light-extraction-efficiency LEDs. Photonic crystal and one-dimensional grating structure was employed to enhance the light extraction efficiency of LEDs. Finite-difference time-domain method was used to calculate the light extraction efficiency of LEDs. The following results are obtained:(1) Plane wave expansion method was employed to study the complete photonic band gap (PBG) of two-dimensional air annular photonic crystals (PCs). Comparison of the complete PBG of the air annular photonic crystal and the commonly used PCs (square-and triangular-lattice dielectric rods in air and square-and triangular-lattice air holes in dielectric background) reveals that a larger complete PBG can be obtained for the air annular PCs. Furthermore, the complete PBG is observed for the air annular PCs even if the dielectric contrast is low, while it is difficult to achieve a complete PBG for the commonly used PCs.(2) First, we investigated the effects of structural parameters (i.e., radius and slab thickness) on the first TE-like band gap for the square-and triangular-lattice two-dimensional GaN photonic-crystal slab with air holes. Based on the simulation results, we can optimize the structural parameters of the GaN photonic-crystal slab to obtain a larger photonic band gap. Then we calculated the light extraction efficiency of the GaN photonic-crystal slab LEDs using the3D finite-difference time-domain method. Simulation results demonstrated that when the emission wavelength is within the photonic band gap, the spontaneous light emission coupled to the guided mode is inhibited and the spontaneous emission is mostly coupled to the slab modes, so that the light extraction efficiency was significantly enhanced. The study provides a reference for the design of the GaN photonic-crystal slab LED with high light extraction efficiency.(3) One-dimensionl grating structure was employed to enhance the light extraction efficiency (LEE) of the thin-film flip-chip LEDs (TFFC-LEDs) and weaken the dependence of LEE on the p-GaN layer thickness. Finite-difference time-domain method was used to calculate the LEE of LEDs. First, dependence of LEE on the structural parameters (i.e., p-GaN layer thickness and n-GaN layer thickness) for the conventional LED and the TFFC-LED with flat surface was investigated. Then, we investigated the effects of the grating structural parameters and the p-GaN layer thickness on LEE for the TFFC-LED with triangular grating and semicircular grating structure. Simulation results show that LEE for the TFFC-LED with flat surface periodically oscillates with large amplitude as a function of the p-GaN layer thickness; this means that the p-GaN layer thickness for the TFFC-LED should be precisely controlled during epitaxial growth steps to obtain the maximum LEE. However, by adopting the surface one-dimensionl grating structure, not only the LEE was significantly enhanced, but also greatly weaken the dependence of LEE on the p-GaN layer thickness.