Enhanced Light Extraction Efficiency Of LEDs By Using Photonic Crystal And Omni-directional Reflector

Light-emitting diode (LED) is an active optical device which can change electrical energy into optical energy. It has many advantages, such as energy saving, environment friendly, long life span, high reliability, small volume and so on. So, LED can be widely used in many domains, for example mobile devices, traffic signal lights, flat panel display, traffic illumination, liquid crystal display, general illumination and so on. And that reveals the splendid application prospect. As related technologies developed rapidly, LEDs will replace incandescent light bulbs, fluorescent tubes and become the next generation solid state light source with great advantages. However, the output optical power of tradition LEDs is still very low. So it is a crucial problem to improve the quantum efficiency. Because of the high internal quantum efficiency of LEDs adopting some new technologies, which up to 80%, it becomes an attractive hotpot how to improve the extraction efficiency of LEDs.First, the LED and GaN-based LED developing courses are introduced in the paper. It is necessary to analyze the application prospect of LEDs. The LED is an electroluminescent device consisting of PN junction. As a result, the luminescence principle of LEDs is discussed from the point of semiconductor and energy gap theory. Then some characteristic parameters of LEDs are introduced. The parameter, extraction efficiency and the reason of low extraction efficiency are mainly stressed. The paper focuses on improving the extraction efficiency by adopting photonic crystal and an omni-directional reflector, so it also refers to photonic crystal concept, characteristics and research methods. Then it basically reaches to how the periodical structures to extract the light from planar waveguide. Two numerical methods, FDTD and RCWA, are mainly explored. The main methods to improve light extraction efficiency are summarized. We propose and design the LED structure with an optimized omni-directional reflector and photonic crystal, which is simulated and analyzed by FDTD and RCWA. The simulation results indicate the composite DBR can solve the problem of light absorption by substrate and electrodes, the photonic crystal can extract the inner lights by destroying the waveguide mode. The light extraction efficiency can hence be effectively enhanced, and the designed photonic crystal is fabricated more easily correspondingly. At last, the research work is concluded, and then because there are some deficiencies and defects and some new methods come up, the next research directions are looked forward to.In conclusion, the work is mainly devoted to designing photonic crystal and an omni-directional reflector to improve the extraction efficiency of LEDs because of their characteristics. The composite distributed Bragg reflector (DBR) is laid between the active layer and substrate layer, so it can reflect the downward light back to the upper surface. That will minish light loss induced by substrate, electrode and some other materials under the active layer. Photonic crystal is designed in p-GaN layer. By its bandgap and diffraction the guided mode in LEDs can be destroyed, and total internal reflection is also decreased. So lights in the LEDs can be extracted at utmost. The simulation results indicate that the structures designed can improve the extraction efficiency of LEDs, and the results are analyzed and discussed in detail.