Study Of Enhancing Light-emitting Efficiency Of Blue LED By Micro-or Nano-Structures

In order to handle the energy crisis around the world, LED as a new green light source has the advantages, such an environmental protection and energy saving, high reliability, long-time life, which will lead the future trend of development in the lighting field. It is expected to become the fourth generation light source after incandescent lamp, fluorescent lamp, high intensity discharge lamp. Many governments have begun to attach importance to the research and development of LED and proposed development plan or some measures to support it.Since the LED was inverted, solid state lighting is a pursued goal. The blue LED is the basis for manufacturing white LED and is mainly made of GaN material, so it has important significance to study the GaN-based blue LED for the development of white LED. The luminous efficiency of the LED is an important parameter to decide the performance of the LED. There is still much room for improving the luminous efficiency, especially the GaN-based LED. Because the refractive index difference is large between the GaN material and air, most of light are collapsed into GaN materials by the total internal reflection and the Fresnel reflection. At present, a lot of work has been carried out about how to improve the light extraction efficiency of GaN-based LED. Many research teams has employed a variety of methods to improve the light extraction efficiency of GaN-based LED, such as surface roughening,, flip-chip structure, photonic crystals, patterned sapphire substrate technology, micro/nano hybrid structures etal. The growth of LED and the fabrication of the micro/nano structures are a very time-consuming, labor-consuming and cost-consuming process. In order to save the fabrication cost of the optoelectronic device, computer technology can be used to speed up the design process and reduce the cost of development.In this paper, we focused on enhancing the blue LED light emitting efficiency as the goal and used the micro/nano patterned structure as research way through the combination of computer simulation and theoretical analysis. The GaN-based LED models were established by computer simulation technology and the effects of micro/nano patterned structural parameters on the light extraction efficiency were studied and the optimized structure parameters were obtained by optimizing the structure, thus the results can provide a theoretical basis for high efficient GaN-based LED.In this paper, the main works are as follows:(1) The FDTD algorithm and Monte Carlo ray-tracing method were used to calculate the light extraction efficiency for different patterned structure. The FDTD algorithm is that the Maxwell equations are directly separated in time-space through thestep-by-step method. The method is very effective for the numerical simulation of LED with nano-patterned structure. Monte Carlo ray-tracing method is based on geometric optics. Propagation of light obeys the law of reflection and refraction on the interface of two different media.It is easy for this method to simulate spontaneous emission process of electron-hole pairs in active layer, it is applicable to the structures of any shape and size. The results are feasible and reliable for the LED with the micro-patterned structure(2) Through numerical simulation, the extraction efficiency of photonic crystal LED was calculated by using FDTD algorithm. The work focused on the effects of the embedded photonic crystal structure parameters (such as the fill factor, location, thickness) on light extraction efficiency of blue LED LED, compared with the effects of surface photonic crystals.The results showed that the filling factor and the position of embedded photonic crystals had larger effects on the light extraction efficiency, and the thickness had little influence. Effects of the thickness of photonic crystal on the light extraction efficiency appeared up and down. The etched depth (depth250nm) of surface photonic crystal can make the increment of the light extraction efficiency more than5times. Surface Photonic crystal and embedded photonic crystal surface played different role for light extraction. The surface photonic crystal mainly through the diffraction effect of guided mode to increase the light extraction efficiency, and the embedded photonic crystal is to change the distribution of guided modes in LED. Through the optimization of the structure parameters of embedded photonic crystal, further by double crystal LED it can provide theoretical guidance for increasing the light extraction efficiency by double photonic crystals.(3)During the fabrication of photonic crystals, there will inevitably be structural defect and disorder, and the defect or disorder will affect the performance of photonic crystals. In this paper, the photonic crystals with a certain tolerance were produced by computer programming, namely the disordered photonic crystals. The LED with disordered photonic crystals is obtained by combining with the FDTD method. The effect of the position of active layer in the LED with disordered photonic crystal LED was studied on the light extraction efficiency. The results showed that disordered structure didn’t change the effect, compared with conventional LED.(4) In the fabrication process of the photonic crystals, the disorder and the defect can affect seriously the properties of photonic crystals. In order to further get the effects of the disordered photonic crystal structural parameters on the light extraction efficiency of blue LED, photonic crystals with optimized structural parameters and with non-optimized structural parameters were selected. The effect of disordered change of photonic crystals is studied on the blue LED light extraction efficiency, such as air hole, the radius, and etched depth. The results showed that the disordered change of hole position was a weak disordered state and had no significant effect on the light extraction efficiency; disordered change of the radius of air hole can significantly influence the extraction efficiency of blue LED and the largest54%increment of the extraction efficiency could be obtained; For the deep-etched air holes, the etched depth change in the tolerance range of20%change can be ignored on the effect of light extraction efficiency. For the optimized photonic crystals, any disordered change will make the light extraction efficiency decreased. However, for non-optimized photonic crystals, disordered change of structural parameters can make the light extraction efficiency increased. These results showed that photonic crystal LEDs with the different structural parameters can lead to different effects due to the disordered change of structural parameters. But for each photonic crystal LED, there will be a set of optimization parameters to make the light extraction efficiency maximized, and disordered change of other photonic crystal parameters will cause the light extraction efficiency decreased.(5) Surface micro-structures used in LED can play an important role for light scattering and anti-reflection. The effects of periodic arrays on the extraction efficiency of the flip-chip LED were studied by Monte Carlo ray tracing method. The detailed theoretical analysis was given by the surface anti-reflection theory and the effective refractive index theory. The results showed that micro-cone arrays could provide the best light extraction efficiency in the four different periodic arrays. The cone arrays were better than pillar arrays and convex structure was better than well structure for light extraction. Furthermore, micro-cone arrays and the micro-pyramid structures were compared. The results showed that micro-cone arrays were better than micro-pyramid arrays at the same base area and the inclined angle. Considering the fabrication processes of actual cone arrays, the effects of the top radius were studied on light extraction efficiency of the truncated cone or cone well arrays. The results showed that the LEDs with cone or cone-hole arrays of certain top radius were preferable for light extraction.