Study On Manufacturing Technology And Leakage Current Mechanisms For High Efficiency And High Power InGaN/GaN Light-Emitting Diodes

LEDs have been proven to be highly efficient and widely used in general lighting due to their advantages of high luminous efficiency,long operation lifetime,and low energy consumption.At present,white LEDs are realized through the excitation of yellow phosphor by blue LED chips mainly.Therefore,the performance enhancement of blue LED chips is crucial for improving lighting efficiency and energy saving.Thanks to the efforts of scientists and engineers,the performance of blue LEDs has been improved greatly in the past ten years.The luminous efficiency of high power blue LED chips has reached as high as 140?180 lm/W.However,the blue LED chips still face some urgent problems like low light extracting efficiency,efficiency droop at high input current,large leakage current,and so on.Furthermore,the key technology for high power LED chips is controlled by multi-national companies.The quality of high power LED chips made by domestic companies is still inferior to that of multi-national companies.Focused on the high power InGaN/GaN blue LEDs,theoretic analysis,simulation and experiments were used to study the methods to improve the optical and electrical performances of LED chips in this project.The physical mechanisms of leakage current and the effects of leakage current on the LED reliability were also analyzed.The detailed contents of this thesis contain:First,we analyzed the characteristics of the photon propagation in LED structure and discussed the fundamental schemes to improve light extraction efficiency of LEDs.Based on Monte Carlo ray tracing methods,the optical model of LED was established to analyze the effects of chip structure parameters and material properties on the light extraction efficiency and study the effect of patterned Indium tin oxide(ITO),reflector,patterned sidewall on the light extraction efficiency.The design and optimization of distributed Bragg reflectors(DBR)/omni-directional reflector(ODR)with high reflectivity were also discussed by comprehensive consideration the requirement of LED chip and packaging to improve the light extraction efficiency of blue light and yellow light.Second,in the light of current crowding phenomenon of LED chips under high input current,a number of current spreading models were analyzed with respect to the model simplification principle to find the fundamental method to improve the current spreading ability of LED chips.Based on the one dimensional drift-diffusion equation and carrier continuity equation,3D electrical-thermal coupling model was established by finite element method to simulate the current spreading characteristic,focusing on the relationshios between the geometric topology of electrodes and the current spreading ability of LEDs.The concept and fabrication method of 3D Ni/Ag and ITO/DBR electrodes with two-level metallization were presented.The characteristics and current spreading abilities of LEDs with fingerlike electrode,distributed CBL fingerlike electrode,discrete fingerlike electrode,3D electrode and two-level metallization 3D electrode were compared and analyzed.Third,two types of high performance high power high power blue LED chips were fabricated through chip fabrication technologies like epitaxy growth,photolithography,etching and thin film deposition.The top-emitting LED chip integrated distributed CBL structure,3D patterned ITO structure and patterned sidewall structure.Experimental results showed that distributed CBL structure,patterned ITO structure and patterned sidewall structure increased the light extraction efficiency of LEDs by 10.5%,13.9%and 7.4%,respectively.Through the fabrication and analysis of the flip-chip LED with two-level metallization 3D electrodes,great advantages of flip-chip structure and double metal layer 3D electrodes in light extraction efficiency,heat dissipation and current spreading were proved,which provided the design thinking and technological references for the fabrication of super high power and high current density LEDs.Fourth,in the light of the leakage current of LED,a number of carrier transport mechanisms of leakage current were systematically studied.According to the data obtained in the experiments,the forward carrier transport mechanism of LEDs was presented and the transport mechanism of reverse leakage current was identified.The transport mechanism of forward leakage current was discribed:electrons from n-GaN and holes from p-GaN could tunnel diagonally and recombine nonradiatively in the deep center of central forbidden band,which subsequently formed the "mixed diagonal tunneling".The transport mechanisms of reverse leakage current was discribed:variable-range-hopping(VRH)conduction dominates the leakage current when T<200 K;thermally assisted multi-step tunneling dominates the leakage current when T>250 K,which is enhanced by Poole-Frenkel effect.Through the accelerated aging experiments of LEDs with different leakage current,the relationship between leakage current and LED reliability and the effects of accelerated aging experiments on the leakage current were also studied.