Design Of Structure And Fabrication Of High Luminous Efficiency GaN-Based Flip-Chip LEDs

Light emitting diode?LED?is the rapid rise of semiconductor solid-state lighting components in recent years.Compared with the traditional incandescent lamp,fluorescent lamp,it has the advantage of small size,compact construction,good vibration resistance,less heat,long life,high brightness,the luminous response speed,low working voltage and so on.GaN-based materials are wide bandgap direct bandgap semiconductor materials,bonding energy is very large,with good chemical stability and thermal stability,making it in the field of blue and ultraviolet optoelectronics technology occupies an important position.GaN-based LED chip has a formal,vertical and flip three structures,flip-chip structure is considered to achieve a high power chip in one direction.Compared with the traditional formal chip,the light luminous efficiency is higher,and the heat dissipation is better.Compared with the vertical structure,the production process is relatively simple and the yield is high.The structure is also a good solution to the problem of high current crowding effect and thermal resistance,and it can achieve a high current density and current uniformity.Currently,the flip-chip LED is a hot research area,especially in the flip-chip LED light efficiency,heat dissipation and other issues.There are many problems need to be solved.In this paper,we study the structure,optical and electrical aspects of the chip,and design and fabricate a high efficiency GaN-based flip chip structure.On the chip structure,the distributed Bragg reflector is used instead of the traditional metal mirror as the flip-chip reflector,and according to DBR structural design principles designed to prepare high reflectivity DBR structure.To overcome the traditional flip-chip LED due to the use of metal mirrors caused by the incident angle and wavelength of the insensitivity and reflectivity is not high problem.The forward voltage and the optical output power of the flip chip LED with high reflectivity DBR structure are compared with those of the purchased sample.It is found that the 457?m*760?m chip has the lowest forward voltage and the optical power at 120mA,350mA and 500mA respectively increased by 12.8%,21.91%and 27.39%respectively.In the aspect of optics,FDTD is used to simulate the effect of planar substrate and patterned sapphire substrate on light extraction efficiency.The simulation results show that the efficiency of light extraction of planar substrate LED,cylindrical substrate LED and conical substrate LED is 12.5%,25%and 32.5%respectively.The extraction efficiency of cylindrical and conical relative planar structures is increased 12%,20%.It can be seen that the extraction efficiency of the substrate is better than that of the planar substrate structure,and the conical substrate is the best for the light extraction efficiency.Based on the results of optical simulation,we fabricated the conical substrate LED chip by SiO₂ nanosphere mask and ICP etching.On the photoluminescence test,the photoluminescence spectroscopy wave crest intensity was two times higher than the LED chip of the planar substrate.In the aspect of electrical engineering,three-dimensional finite element electrical model was established by COMSOL finite element simulation method.The influences of different electrode structures on the current density of flip-chip LED was analyzed by COMSOL finite element simulation method.And the near-field optical test of the chip with different electrode structure is carried out to obtain the optimal electrode structure.In this thesis,the GaN-based flip-chip LED to improve the current diffusion and effectively to improve the LED chip extraction efficiency.The experimental results show that the semiconductor device fabrication is stable and the overall yield is improved.