| In recent years, a number of major technological breakthroughs have been made in the crystal growth and device fabrication of InGaN/GaN LED. Hence, it has been widely used in areas such as traffic display, large screen display, and street lamps. However, for entering the huge markers of indoor lighting commercially, great efforts still have to be taken to improve power efficiency, enhance device reliability and reduce costs. To meet above requirements, the vertical structure LED based on Si substrate is a promising device for both the advantages of vertical structure and silicon substrate. Firstly, the metal reflector and surface roughening which are quite effective on enhancing light extraction efficiency can be done on VLED. Moreover, VLEDs have better current uniformity and better thermal conductivity than traditional LEDs. In addition, it is known that the original substrate has to be removed to fabricate thin-film LED such as VLED and the removing of Si substrate, with lower cost, can be done easier than sapphire substrate. Si is also cheap and easy to obtain large size substrate. However, GaN LED based on Si substrate is just a comparatively new world. There is still lots of work needs to do both on its crystal growth and on its device fabrication.In this dissertation, effects of p-GaN thickness on light extraction efficiency of VLED and the stress evolution during devices fabrication have been studied systematically. Some significant and innovative results are achieved as follows.1. A large F-P cavity model was firstly applied to vertical LED without surface roughening, which was formed by the GaN-epoxy interface and the metal mirror, and a method of calculating light extraction efficiency was derived based on this model. This model is of great significance on the optimization of LED design.2. It is proved that p-GaN thickness has a significant influence on extraction efficiency of vertical LED without surface roughening. Therefore, the light extraction efficiency can be enhanced by the optimization of p-GaN thickness. In addition, the optimal p-GaN thickness (d) and the value of the maximum in extraction efficiency are affected by the type of the metal reflector. For InGaN/GaN VLED employing Ag, Al, and Pt reflectors, the optimal d are～0.69λn,～0.75λn, and～0.73λn respectively. Hence, it is necessary to make sure the metal reflector type when optimizing d.3. It is also worth knowing that the optimal p-GaN thickness via light power is different from the optimal value via light intensity in vertical direction. Hence, it is inappropriate to judge the optimal p-GaN thickness only by light intensity measured by LED Chip-Prober.4. The variation of stress in GaN LED thin film is large during the fabrication of VLEDs. At first, the LED film is in tension grown in LED film epitaxy. In order to relax the grown tensile stress, the silicon substrate was patterned into independent chips. As the chip becomes larger, the tensile stress becomes greater. Then, there is a remarkable relaxation of the tensile growth stress during transferring LED film to a new substrate. As a result, the LED film is in compression after the LED-film transferring. After that, the compressive stress is relaxed during surface roughening. It is worth noting that, if the LED film is in tension, the tensile stress will also be relaxed during surface roughness. Then, the tensile growth stress is relaxed when removing the chip fringe, and the chip large, the relaxation great.Above results have been used in the study on related National High Technology Research and Development Program 863 of our institute and in related industrialization. And good effects have been achieved. |
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