| dc.description.abstract | The light extraction efficiency of light-emitting diodes (LEDs) is limited by many effects. For this reason, we present a quantitative analysis model for LED light extraction characteristics based on Monte Carlo ray tracing method, According to this model, the light extraction and loss mechanisms in GaN-based LEDs is studied. We furthermore verify the validity of the Monte Carlo simulation results by packaging experiments at low temperature.
In the thesis, there are five topics is discussed in detail. First at all, we analyze the issue of absorption in active layer, the simulation also take into account the effect of photon recycling. According to the simulation results, the light extraction efficiency of LED with light extraction structure is more sensitive to active layer absorption than that of typical LED. Secondly, the realistic geometry of chip scale is analyzed. According to the simulation results, the structure of chip shaping and patterned substrate is suitable for small chip, and the ThinGaN LED with Surface texture is suitable for large chip. Besides, the suggested thickness for sapphire substrate is 50~100um. Then we analyze the issue of contact and current spreading. The results show that the light extraction efficiency decreases due to the shadow effect of the p-contact while the current crowds near the p-contact. In order to overcome this effect, the light extraction efficiency can be greatly improved by integrating high reflectivity contact, current blocking layer and diffused bottom mirror. We also compare the light extraction efficiency of LED packaging with bare chip. From the optical point of view, the patterned substrate should be regarded as the most effective way in enhancing light extraction efficiency in an encapsulated flip-chip LED. Besides, the diameter of encapsulant lens must be larger 2.5 times than chip size, and to increase the encapsulant’s refractive index usually is more effective at improving light extraction efficiency than light extraction structure. Finally, in order to improve light extraction efficiency, the LED with micro-structure is analyzed and optimized. The simulation results show that the light extraction efficiency is improved by multiple reflections and reflected scattering in a LED chip, so that increasing reflectivity of the bottom mirror is an important key factor. | en_US |