On Optimization Design And Manufacturing Technology For High Efficiency And High Power GaN-based Light-Emitting Diodes

Recently, light-emitting diodes (LEDs) have attracted lots of attention for the wide application due to theirs high efficiency, environmental-protection, long lifetime, and high reliability, especially for the high power gallium nitride (GaN)-based LEDs, which is the base of white LED lighting. The research of GaN-based LEDs is involved in the materials, optics, electronics, and semiconductor physics, thus the complexity of the research leads to many problems with manufacturing high power GaN-based LEDs. Although significant progress has been made on GaN-based LEDs, they are still suffering from low luminous efficiency. The efficiency consists of radiation recombination, current injection efficiency, and light extraction efficiency, which are mainly decided by the structure parameter and manufacturing technology of epitaxial layer and chip. Some key technical issues relating to high power GaN-based LEDs have not been solved. In this thesis, we try to improve the radiation recombination, current injection efficiency, and light extraction efficiency through optimizing epitaxial structure and chip structure. GaN-based LEDs with output power of485.7mW at350mA are fabricated.Firstly, the three-dimensional electro-thermal model of GaN-based LEDs is built up to analyze the epitaxial structure and current spreading. The core of the model is based on carrier drift-diffusion theory. The generation, transport, and recombination of carrier are calculated through solving the Poisson’s equation, continuity equation for charge carriers, and so on. Effects of the Auger recombination, temperature, polarization, and carrier leakage on the internal quantum efficiency and performance of LEDs are investigated in detail based on this model. In order to improve the internal quantum efficiency and inhibit efficiency droop by improving the hole injection, the multi-quantum wells (MQWs) coupled graded-thickness quantum wells and barriers are designed.Secondly, in order to achieve uniform current spreading and decrease the forward voltage, the effects of structure parameter and material characteristics of LEDs on the electrical performance are researched in detail based on the electro-thermal model. The sheet resistance of ITO and n-GaN, temperature, MQWs and electrode pattern have significant effects on the current spreading. Uniform current spreading results from the equal sheet resistance of ITO and n-GaN, however, current still crowds around the pad and it worsens with the increase of the injection current and temperature. The simulation results are very useful to manufacturer GaN-based LEDs with uniform current spreading by optimization electrode material and electrode pattern.Thirdly, the effects of structure parameters and material properties of GaN-based LEDs on the light extraction efficiency are investigated based on Monte-Carlo ray-tracing method. On the one hand, the optical micro/nano-structure should be fabricated to increase the possibility of photon escaping into the air and then the patterns of sapphire substrate and ITO are optimized. On the other hand, light extraction efficiency is increased with the decrease of the absorption of semiconductor and electrode material. Thus, the distribution Bragg reflector (DBR) are simulated by comprehensive consideration the requirement of LED chip and packaging to improve the light extraction efficiency of blue light and yellow light.Finally, the high power GaN-based LEDs are manufactured based on optimization design of epitaxial layer and chip structure. The internal quantum efficiency is increased and the efficiency droop is inhibited by the LEDs with graded-thickness quantum wells and barriers. The effects of pattern of ITO and sapphire substrate on optical and electrical characteristic are investigated. The light extraction efficiency is decreased with the increase of current due to current crowding effect based on the electro-optical model. Thus, the reflective current blocking layer is designed to improve the light extraction efficiency. The effects of different DBR on the blue light, yellow light and junction temperature are compared.