Study Of Improving Light-emitting Efficiency Of Thin-Film Flip-Chip LED By Nano-scale Structures

As a novel solid state lighting devices, light-emitting diode (LED) has higher efficiency of electro-optic conversion. The GaN-based blue LED is the basis for manufacturing white LED, so it has more important significance to study the GaN-based blue LED for the high-efficiency white LED. Compared with the traditional incandescent and fluorescent lamps, the electro-optic conversion efficiency of current LED is increased about 15 times and 4 times respectively. If the total electricity consumption of lighting is about 300k million degrees each year in China, the electricity saving is about 200k million degrees by all replacement of LED lighting, similar to reducing about 2 tons of carbon missions. In addition, LED has much more advantages, such as energy saving, long life, high color quality, good direction, small volume, light weight, collision resistance etc.However, the low luminous efficiency of LED is the first key to limit the schedule of wide application by replacing traditional lighting sources, in addition to higher price factor. Therefore, many countries have put forward plans and programs for increasing the uminous efficiency of LED n the world, such as National research program on semiconductor lighting in America, Rainbow project brings color to LEDs in European Union and semiconductor lighting industry technology development in China. As one of the decisive factors in the LED luminous efficiency, internal quantum efficiency is close to the theoretical limit of 100%. So how to enhance the light extraction efficiency becomes a major factor to break the bottle neck of the LED’s luminous efficiency. Because of large refractive index difference between the semiconductor material and the air, the total reflection and Fresnel reflection limits most of the radiation into the air. Therefore, improving the light extraction efficiency has become a hot and difficult point in the research of high luminous efficiency LED, including the focus of this dissertation.There are a number of approaches and techniques to increase light extraction efficiency from the LED chip, including two main ideas. The first kind of method is that the maximum part of radiative light can be reflected to top emitting surface from the bottom and around sides. The other way is to gather the greatest degree of radiation light into extraction cone. In this dissertation, the study of light extraction efficiency from thin film flip chip LED combines the two extraction characteristics, using the micro-cavity effect of thin film flip chip LED and omnidirectional extraction of photonic crystal to enhance light extraction efficiency.In this dissertation, the nano-scale dielectric structures are chosen to improve the light-emitting efficiency of LED. We do some study on these problems mentained above using electromagnetic fields numerical calculation methods and electromagnetic field theory analysis methods. The main contents of this dissertation include:(1) This study analyzes the luminescence mechanism and structure characteristics about LED, and also illustrates that the QWs of LED can be equivalent to a dipole source embedded in model of FDTD simulation. The finite difference time domain method (FDTD) is selected to analysis LED structure characteristics and light extraction efficiency as the simulation method, and the numerical model with relevant setting is established.(2) Through the analysis of light extraction mechanism from Fabry-Perot cavity and photonic crystal, the effect of structure parameters on light extraction efficiency is discussed theoretically, such as microcavity thickness and the active layer relative position.(3) According to the extraction mechanism of photonic crystal, it is satisfied that the guided modes outside the extraction cone can be transfer into leaky modes by the diffraction conditions. This is the basis for structure parameters selection and optimization of the photonic crystal.(4) This study systematically investigates the light-extraction efficiency of GaN-based thin-film flip-chip light-emitting diodes (TFFC LEDs) with deeply etched photonic crystals (PhC). The optimal structure can be obtained by scanning the structural parameters using a three-dimensional finite-difference time-domain simulation.(5) The vertical light-extraction efficiency (LEE) of the thin-film flip-chip light-emitting diodes (TFFC LED) with the embedded photonic crystal (PhC) is investigated using the finite-difference time-domain method (FDTD). This study systematically analyzes the dependence of the vertical LEE on the vertical structure and the embedded PhC parameters.In this dissertation, the study focuses on the effect of vertical LED structure and nano-scale photonic crystal structure on light extraction efficiency by the computational simulation and theoretical analysis method. Based on the simulation results, the optimized structure is obtained by discussing the influence of structure parameters and physical mechanism. The study can be used as practical design guidelines for fabricating promising candidates of high-efficiency and high-brightness LED. The main innovations of the dissertation are as follows:(1) The two techniques of p-GaN deep-hole and embedded photonic crystal in conventional top-emitting LED is used to enhance the light extraction efficiency of thin-film flip-chip LED. By the comparison of light extraction between nano-strucutre and planar thin-film flip-chip LED, the calculation results show that the micro-cavity effect is not significantly affected, and the dependence of the light extraction efficiency on the relative position is obviously decreased.(2) The model of GaN-based thin-film flip-chip LED with deeply etched photonic crystals is designed, and this study systematically investigates the dependence of the light-extraction efficiency on the structure. The optimal structure can be obtained by scanning the structural parameters using a three-dimensional finite-difference time-domain simulation. The concentration of the internal emission pattern within a limited extraction cone is significantly influenced by several structural parameters. The light-collection enhancement within an extraction cone explains the peak light-extraction efficiency caused by optimized structural parameters such as the p-GaN thickness, the air-hole depth, the lattice constant and the filling factor. The maximum extraction efficiency can reach 65%.(3) The model of GaN-based thin-film flip-chip LED with double embedded photonic crystals is designed, this study systematically analyzes the dependence of the vertical LEE on the vertical structure and the embedded PhC parameters. A-1.8 folds enhancement in the vertical LEE is obtained for the optimized structure by scanning the double embedded PhCs parameters and the physical mechanisms for the enhancement of the vertical LEE are discussed.Through the research, the mechanisms by which deeply etched photonic crystals and embedded photonic crystal can effectively improve LEE of thin film chip flip LED are clarified. All the conclusions drawn in this dissertation can be used as practical...