Research On Influence Of Structure Optimization Of Photonic Crystals On Enhancement Of Extracting Efficiency Of GaN-based LED

With the development of LED-related technology, applications of LED are ranging from being indicator at first to the field of illumination, and at present the potential for LED to replace incandescent and fluorescent to become the most advantaged solid-state light source for next generation of illumination has been evident day after day. And GaN-based LED, as the core component in manufacturing white LED, has an extremely important position in solid-state lighting, multi-color and high brightness LED.The premise for LED going into the field of illumination is to overcome the low external quantum efficiency, and the key technology of that is to enhance the low light extraction efficiency resulted from the total internal reflection in the interface between air and LED. For GaN-based LED the same bottleneck also should be faced with.At present, the main technologies to enhance the light extraction efficiency of LED are photonic crystals, flip chip, surface roughening, substrate stripping, surface plasma and so on. And with the ability to control the radiation direction and the better enhancement efficiency, photonic crystal technology has attracted focus attention now. The research of photonic crystal LED now mainly focuses on experimental preparation, testing and theoretical analysis of photonic crystal, and in terms of the systematic analysis of variable parameters influence on the extraction efficiency, the researches are now relatively few.This research work focuses on utilizing photonic crystal technology to enhance the light extraction efficiency of GaN-based LED. And through the simulation and analysis of the structural parameters, the influence of the parameters to the light extraction efficiency has been studied. Research has made the following main results and conclusions:1. Researches on influence of lattice period, filling ratio and height on the position, width, and effect of the band-gap of photonic crystals. Based on the scan simulation of structural parameters of triangular and square lattice photonic crystals, discussions of influence of these parameters on band-gap have been made:(1) The lattice period of the photonic crystal has a maximum effect on the band-gap position, and when the photonic crystal has a certain height, the lattice period is inversely proportional to the center frequency and the width of the band-gap. (2) Increasing the height of the photonic crystal, the band-gap position will slightly down, while the width is basically the same. (3) Increasing the filling ratio of the photonic crystal, the low frequency of the band-gap will slightly go up, while the forbidden band width and the effect improve greatly. (4) For photonic crystals with different lattice shapes, the impact trend of these factors is consistent.2. Researches on collection effect in vertical direction for photonic crystals. Considering with the effect of band-gap and diffraction effects of periodic grids of photonic crystal, this article concludes collection effect in vertical direction for photonic crystals, and by comparing the radiation patterns of different photonic crystals, with the center frequency of GaN-based blue LED in or outside the band-gap, it is verified that for the former, the photonic crystal really presents a convergence phenomena in direction without band-gap. During the study process, it was also found that photonic crystal structure with triangular lattice has a better collection effect than that with square lattice in the same structural parameters condition.3. Researches on the influence of the optimized parameters of photonic crystal GaN-based LED on the light extraction efficiency. By scanning and simulating the height of each layer in photonic crystal LED, this article analyzes the impacts of the change of layer height to the power radiation and verifies that this impaction has a relationship with the wavelength in the medium. At the same time the antireflection effects of the photonic crystal layer in GaN-based LED structure have been simulated, and in which a method to calculate the equivalent refractive index of photonic crystals by using the non-reflection conditions in electromagnetic field has been proposed. The simulation results demonstrate that this method can calculate the equivalent refractive indices in different filling ratio more accurately, so that when the photonic crystal height is taken as the odd times of a quarter of wavelength, the output power reaches a peak.4. Analysis about influence of dual-photonic crystals structure on light extraction efficiency. Considering with photonic band-gap, diffraction effects of periodic grid and the collection effect in the vertical direction for photonic crystals, this paper sums up the convergence effect theory of dual-photonic crystals:in this structure the top photonic crystal will pull light upward, while the bottom one will push it downward, and for the bottom light, after being reflected by the reflective layer, it can radiate outside the upper surface in an improved angle, then the overall light extraction efficiency can be enhanced dramaticlly.The simulation results show that the radiation pattern of dual-photonic crystals structure presents clear vertical aggregation phenomenon compared to pure dielectric plate structure. Comparing with single photonic crystal structure and pure dielectric plate one, the dual-photonic crystal structure also has more output power at the entire light angle, in some angle the increase even up to about 2 times. And for the total radiation at all angles, the total top surface output power of the dual-photonic crystals structure can reach 1.3 times of that of the pure dielectric plate structure, so it can effectively improve the top surface output power of LED structure.The main innovations of this paper are:1. This paper summarizes the impact of the structural parameters of photonic crystal to the position, width and effect of the band-gap. It also summarizes and verifies that photonic crystal has the collection effect in vertical direction to the light wave. These conclusions will provide parameter setting and theory basis for the following analysis of the overall structure of photonic crystal LED.2. In this paper, a method to calculate the equivalent refractive index of photonic crystals of GaN-based LED structure by using the non-reflection conditions in electromagnetic field has been proposed, which can calculate the equivalent refractive indices in different filling ratio more accurately, and then provides parameter setting basis for the use of photonic crystals to improve the light extractive efficiency of LED structure in theory of antireflection effects.3. In this paper, the convergence effect theory of dual-photonic crystals of GaN-based LED structure has been proposed and demonstrated, which can achieve a better result in radiation angle and output power than single photonic crystal LED and pure media plate LED. So it can provide new ideas and methods for the use of photonic crystal technology to enhance the LED light extraction efficiency.