| GaN based Light-Emitting Diode (LED) has attracted much attention because of its advantages such as long lifetime, high efficiency, energy conservation, and environment protection. With the developments of epitaxial technology, chip processing, and packaging technology, the application areas of LED have expanded from indicator and backlight resource to the full color display and general illumination. However, due to the limitation of the current technology, the quantum efficiency of GaN based LED is not high enough and it is still necessary to improve its internal and external quantum efficiency, as well as the light output power. The limitation in internal quantum efficiency is related to the poor quality of materials, large strain and poor electron capturing ability. The low external quantum efficiency relates to the poor extraction efficiency because of the reflection of light at the interface. This thesis first designed and prepared the LED using AlInGaN/InGaN as the electron emitter layer to improve the internal quantum efficiency. Then in order to enhance the external quantum efficiency, some key technologies such as patterned sapphire substrate (PSS), distributed Bragg reflection (DBR) layer, and stealth dicing, have been studied in detail. The results show that these methods can effectively improve the performance and reliability of the chip. The major innovations of this thesis are summarized as follow.(1) Based on the theory of optoelectronic devices, the key factors affecting the internal quantum efficiency of LED have been studied by Matlab and APSYS. It is found that the internal quantum efficiency closely relates to the Auger recombination and leakage of carriers. When the Auger recombination and the leakage of carriers increased, the internal quantum efficiency drops quickly. In this thesis, AlInGaN/InGaN multi-quantum wells were used as the electron emitter layer to reduce the electron leakage. After the simulation and optimization, the LED using AlInGaN/InGaN multi-quantum as the electron emitter layer with high performance has been achieved. The experiment results show that the quantum efficiency of the new structure is improved significantly, which can be ascribed to two reasons. On one hand, compared to GaN barrier, AlInGaN has a higher band gap to reduce the electrons mobility in the vertical direction and enhance the lateral movement effectively, which can improve the efficiency of electron capture and the scalability of the current. On the other hand, the crystal quality has been improved with a reduced defect density to1.87×108/cm2after adopting the AlInGAN/InGaN multi-quantum wells electron emitter layer. In addition, some other properties of this new structure are better than those of the conventional one. The leakage current of this new structure is3.24μA at30V reverse bias, the luminous efficiency has been improved by more than26%, and the pass yield remains70%in the reverse5000V ESD (human model).(2) Based on the Monte Carlo method and non-sequential ray tracing, the influences of the geometric parameters of patterned sapphire substrate (PSS), such as distance between adjacent pyramid, radius of pyramid, height of pyramid and inclined angle between the edge and bottom of the pyramid, on the extraction efficiency have been investigated with Tracepro, and the optimized parameter ranges have been achieved. Through optimization of the photolithography and ICP etching process, the optimized pattern has been obtained with a height of1.5μm, an incline angle of50degrees, and an interval distance of0.5μm between each pyramid. After epitaxial growth with MOCVD, the quality of the epitaxial layer with PSS has been improved. The results of X-ray diffraction show that the full width at half maximum has been reduced from285arcsec to264arcsec. Compared with the performance of GaN based LED with planar substrate, the luminous flux of those with the optimized PSS has been increased by27%. This can be attributed to the improved extraction efficiency by PSS, which can change the light path and make the light originally totally reflected at the surface be extracted out.(3) The effects of the dielectric distributed Bragg reflection (DBR) layer and "DBR+metal film" on the improvement of light extraction efficiency of GaN based LED have been studied by TFCalc. It is found that a high reflectivity of97%of the forbidden band can be obtained through optimizing the DBR structure. Furthermore, the reflectivity could still be the same after bath in water at85℃, which means that the DBR stability is superior. In addition, the luminous flux of chip with dielectric DBR has been increased by about5.64%, compared with the one with only "DBR+metal film" after packaging. The retention rate of luminous flux has been improved by27%for the chip with dielectric DBR in the accelerated aging test with50℃for168h, compared with conventional one without DBR layers.(4) Since the burning caused by the laser surface dicing technology can destroy the crystal structure of sapphire, the light can be absorbed at the burning site, which can negatively affect the light extraction efficiency of GaN based LED. Stealth dicing is an advanced cutting technologies, which uses the picosecond laser with high photon density equal to the multi-photon absorption threshold of sapphire to damage valence bond of sapphire when the laser energy is absorbed. By comparing these two cutting technology, it is found that the optical power of chip with the laser stealth dicing before package is increased by11%and6%after package. The O/Al atomic ratio of the stealth dicing section is studied by EDS, which shows that only very small atomic ratio mismatched regions exist around the cavity while other regions remain the same as sapphire crystal structure. This reduces light absorption. In addition, by optimizing processing parameters such as the laser cutting power, focus depth, dot density, and Q-frequency, the abnormal chip leakage current can be under control. |
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