Study Of Improving Light-emitting Efficiency Of GaN Based Blue LED By Nano-scale Metal/Dielectric Structures

Nowadays, with the rapid development of social economics, the energy shortage is getting more and more serious. Among the energy, nearly21%is used for light. Hence, saving as much energy as possible in this field can contribute a lot to the development of our social economics. With energy saving, long life, high reliability, small size and many other advantages, light emitting diodes (LED) are attracting many research interests of the scientists all over the world. And the LED will become the fourth generation light source, namely the semi-conductor lighting.However, the large-scale commercialized production of LED will not be widely available for some time. There are many reasons. And the most important one is the energy emitted by the sources can not be extracted efficiently. This make the efficiency of the LED be very low. So, desiging an LED model which can achieve high light extraction efficiency and save as much energy as possible is an unavoidable trend in the lighting industry. This is exactly the meaning of this dissertationAs we know, the efficiencies of the LED include the internal efficiency and the external one. In some ways, the external efficiency can be regarded as the light extraction efficiency (LEE). Controlling the photonic density can improve the internal efficiency of the LED. More definitely, using nano-metal particles can change the energy distribution of the LED active layer. Hence the internal efficiency can be improved. Besides, surface roughness, surface etched photonic crystals; embedded photonic crystals all can change the energy distribution in the LED structure. Hence the light extraction efficiency of the LED can be dramatically improved. Analyzing this physical mechanism can be a very promsing candidate for high performance LEDs.However, the present domestic and foreign researches for LED structure optimization are lack of a clear theoretical guidance. Especially in light extraction efficiency promotion, how different geometric parameters change the energy distribution within the LED structure, then how to affect the extraction efficiency of LED are still not clear. They are in urgent need of us to elucidate through our research.Moreover, the current electromagnetic analysis of LED is lack of an efficient method. Take the FDTD method for example, it is restricted by the mesh size in the visible band. To calculate a single model, people need very long time and very large memory space, let alone to optimize the LED structure. So, a more efficient and more convenient mothod is badly needed. Furthermore, we hope this mothod can show the influence of the LEE by these structural parameters more clear and can provide reliable thereotical guidance.In this thesis, the nano-scale metal/dielectric structures were chosen. In order to improve the light-emitting efficiency of LED, we did a detailed study on the problems mentined above using numerical calculation methods of electromagnetic fields and the mode analysis methods. The main contents of this paper include:1) This paper analyzed the luminescence mechanism and structure characteristics of GaN based blue light LED. This paper also illustrated the active layer of the blue LED could be equivalent to a dipole source and established the revelent simulation model.2) This paper provided a detailed research of the influence of the localized surface Plasmon (LSP), which is caused by nano-scale metal particles, on the spontaneous emission of the dipole source. And the geometrical parameters of these particles were also optimized.3) This paper analyzed the influence of the LSP on the far field directivity of the dipole source, and established the link between the near field of LSP and the far filed of dipole radiation.4) This paper analyzed the influence of the Surface Plasmon ploaritons (SPPs), which is caused by the thin nano-scale metal films, on the internal efficiency of LED. Similarly, the geometrical parameters of the metal films were also optimized.5) This paper explored a mechanism which can improve both the internal efficiency and the light extraction efficiency of LED. A new model, which can compensate the momentum loss of SPPs, was builded. This model finally improved the internal efficiency of GaN based blue LED by3.6times as well as improved the light extraction efficiency of the LED by nearly2times.6) Through the approximation of average refractive index, this paper simplified the classical model of the LED. The new proposed model could provide a more simple and convenient analysis method.7) This paper finally provided detailed optimization principles of various GaN based blue LEDs including LED with surface etched photonic crystals, LED with embedded photonic crystals and LED with confined low index layer. And the corresponding geometrical parameters were also optimized.The work of this paper is closely around improving the light-emitting efficiency of GaN based blue LED by nano-scale metal/dielectric optical structures. This work combines the key technologies in energy saving and emission reduction during the national economic and social development. Through the correlation analysis and optimization, this paper discusses the control mechanism of the nano-scale metal structures and the nano-scale dielectric optical structures on the improvement of light-emitting efficiency of LED. This discussion can provide important theoretical guidance for the designing and fabricating of high performance GaN based blue LED.The main innovations of the thesis are as follows:(â… ) This paper illustrated why the active layer of the LED can be replaced by an dipole source. This paper discussed in detail the influence of LSP and SPPs on the improvement of internal efficiency of the LED. Through the optimization, we found that the aspect ratio of the nano-scale metal particles being approximate to4:1can make the spontaneous emission of the dipole source improve by nearly5000times. Moreover, we also found that the resonant wavelength of the spontaneous emission decreases as the particle become small. This finding let us find the suitable particle size which can make the spontaneous emission enhance dramatically at different wavelength.(â…¡) This paper found that the dipole radiation patterns will dissaper if the high order modes of the LSP are excited. For the first time, the link between the near field of the LSP and the far filed of the dipole radiation was established.(â…¢) A mechanism which can improve both the internal efficiency and the light extraction efficiency of the GaN based blue LED was discussed in this paper. By embedding the array of the nano-scale metal particles in the layer of p-GaN, the momentum loss of the SPPs was compensated. The internal efficiency of the LED was enhanced by3.6times. Meanwhile, the light extraction efficiency of the LED was improved by nearly2times. This model provided a certain reference value of the designment of the LED strucuture which can both improve the internal and the external efficiencies.(â…£)Compared to the traditional FDTD calculation method, this paper provided a simpler and more convenient method. By using the approximation of the refactive index, the original LED structure with photonic crystals can be equalized to multilayer planar waveguide. This simplization could avoid the needs of long calculation time and large memory space of the FDTD method. Hence it could be very simple and convenient for the optimization of LED structure. Furthermore, this method could provide the control mechanism of the geometrical parameters on the light-emtting efficiency. Hence it also could reduce the blindness in the structural optimization.(V) This paper pointed out that, the light extraction efficiency of LED is significantly affected by the low order modes localized in the GaN buffer. Decreasing the energy of the low order modes obtained from the active layer or decreasing the percentage of the energy of low order modes in the total modes energy is the most effective method to improve the light extraction efficiency of the LED. To a great extent, this conclusion can reduce the blindness in the following structural optimization. (â…¥)The structure parameters of various kinds of LEDs were optimized in this paper in detail, which included the LED with surface etched photonic crystals, the LED with embedded photonic crystals, the LED with double layer photonic crystals, and the LED with confined low index layer. For the first time, this paper proposed a new model which composed of double layer photonic crystals and a thin confined low index layer. This model can excited cap layer mode and modified the Bloch modes localized in the photonic crystals without introducing high lattice mismatch. And by the structural optimization, this model finally gets more than eight enhancements of extraction efficiency of the GaN based blue LED.This work is supported by the project of National basic science research:The coupling effect in the structure of Metal/dielectric nanoscale heterogeneity and its application in photoelectric conversion device. Through our research, some mechanisms of the coupling effect in the Metal/dielectric nanoscale heterogeneity were clearified; a faster and more convenient method which can be used in the LED's structural optimization was proposed; all the conclusions drawn in this paper provided very important theriotical reference value in the designment and fabrication of high efficient GaN based blue LED.