【作者】 岳庆炀；

【导师】 李康；

【作者基本信息】 山东大学 ， 无线电物理， 2014， 博士

【摘要】 21世纪以来,能源的消耗量急剧增加,导致能源的严重紧缺。为了满足人们对能源的紧急需求,能源的不合理开发和利用变得越来越严重,从而造成环境的污染。如何处理好能源利用和环境保护问题之间的关系是实现可持续发展的重要战略,关系到人类的生存和发展。除了开发新的清洁能源之外,节能减排成为了解决环境问题的一个重要手段。据不完全统计,全世界大约20%的电能被用来照明,然而传统的照明设备的转换效率很低,导致了大量电能的浪费。发光二极管(light-emitting diodes, LEDs)作为一种新型的电光转化器件,其效率已经超过了2001m/W,白光有机发光二极管(organic lightemitting diode, OLED)的发光效率也已经超过了1001m/W。与传统光源相比较,除了具有高的光电转换效率之外,还具有如体积小、寿命长、高色彩品质、方向性好等优点,成为了下一代照明器件的理想选择。LED在短期内占据照明市场不是很容易的,主要是因为LED的发光效率还不是很高。LED的发光效率主要由其内量子效率和光提取效率两部分决定。随着半导体加工工艺的发展,半导体器件的性能获得了突破性的进展,LED的内量子效率接近了理论极限。由于LED内存在全内反射和菲涅尔反射现象,从而会使LED的光提取效率很低。因此提高LED的外量子效率成了国内外科研单位和企业研究的热点,这也是本文研究的重点内容。目前国内外增强LED光提取效率的主要方法可以为以下几类：芯片变形技术(chip shaping)、背部反射技术(placing a back-surface mirror)和表面改造技术(surface modification)。表面改造技术包括表面粗化和光子晶体技术,它是现在提高LED出光效率的一种非常有效的主要手段。从目前的研究现状来看,实验是LED光提取效率研究的主要手段,但是实验具有一定的盲目性,因此理论研究显得尤其重要。目前,通过相关微纳结构技术确实增强LED的出光效率,而加入结构后LED内部的能量分布又会如何?这些结构的几何参数又会对光提取效率造成怎样的影响?这一系列的问题尚没有明确的理论指导。除此之外,LED结构参数的优化也缺少行之有效的手段,时域有限差分(Finite difference time domain method, FDTD)法作为当前流行的一种电磁计算方法,确实能在一定程度上为LED的设计提供帮助。由于LED的数值仿真属于电大尺寸问题,传统的数值计算方法FDTD遇到了巨大的计算量和冗长的计算时间两大难题,又加上LED结构的各个参数是相互关联的,这就更加大了其计算难度。由于图形处理器加速技术(如Nvidia CUDA)的出现,这个难题可以得到一定的解决。本论文选择纳米尺度的介质结构作为提高LED的出光效率的研究对象,以电磁场数值分析和电磁场理论分析为手段,对LED的光提取效率等科学问题进行了详细地研究。具体的研究内容主要包括以下几个方面：一、对LED的发光机理以及结构特性进行了分析,从原理上解释了LED有源层可以用FDTD中的电偶极子源来等效的方法,并完成了FDTD数值仿真模型的相关设置,并对GPU加速技术对我们的计算平台进行了改造,为下一步的仿真提供理论基础。二、利用三层平板模型解释了正装LED内的能量分布情况,解释了光子晶体增强发光效率的原理。在最佳光子晶体结构的基础上,研究了光子晶体中的缺陷和位置微扰对光提取效率的影响。三、研究了周期性缺陷光子晶体对LED光提取效率的影响,对其介电常数图进行了傅立叶变换分析,发现它比完美光子晶体具有更好的方向性,进一步对这个模型进行了优化。四、利用多层平板波导完成了对OLED内的能量分布分析,分别利用嵌入式光子晶体和表面光子晶体技术来提取其低阶模和高阶模。优化了它们的参数,提出了一种高效率的双层光子晶体倒置LED模型。五、完成了非晶态光子晶体的建模工作,继而用FDTD方法分析了非晶态光子晶体的安德森局现象,分析了其增强LED光提取效率的原因,并对这种LED结构进行了优化。六、利用经典电磁场理论分析了纳米线中电偶极子辐射场的分布情况,证明了纳米线中的导模在提高纳米线LED的光提取效率起着重要的作用。利用FDTD法分析了的纳米线的半径、间距以及高度等各个参数对纳米线LED光提取效率的影响。本论文研究的主要内容就是设计高发光效率的纳米介质结构LED模型,在此基础上利用相关的电磁场数值方法进行分析和优化,深入探索其增强LED发光效率的物理机制,为高性能LED的制造提供了重要的理论依据。在以下四个方面做到了创新。一、设计了一种高效率的周期性缺陷光子晶体LED模型,提出了一种利用介电常数的傅立叶变换图谱(空间频谱)设计光子晶体结构的方法。首先在LED平面波导模型的基础上讨论了LED中的能量在各种模式中的分布情况,从而给出了光子晶体提取LED内部导模的原理。接下来,利用FDTD算分析了光子晶体中的缺陷对LED的光提取效率的影响,仿真结果表明,在光子晶体中引入合适的周期性缺陷可以增强LED的光提取效率。通过对缺陷光子晶体的介电常数图的傅立叶变换分析可知,其比完美光子晶体具有更好的方向性,设计了一种缺陷光子晶体,它的光提取效率是完美光子晶体的1.6倍。最后证明了光子晶体的微小位置偏差不会影响LED的光提取效率。二、基于提取OLED (倒装结构LED)内的不同模式,设计了一种高效率的双层光子晶体有机发光二极管,并对其物理机制给出了详细的说明。首先利用多层平板模型分析了OLED内部的能量分布情况,提出了分别利用嵌入式光子晶体和表面光子晶体两种不同方法分别来提取OLED内部的低阶模和高阶模所携带的能量。接下来FDTD方法分析了这两种光子晶体的参数对OLED光提取效率的影响,并利用模式理论对其物理机制进行了分析。最后,给出了一个双层光子晶体OLED的最佳模型,其光提取效率提高了大约1.9倍。三、利用FDTD算法和傅立叶变换技术分析了光在非晶态光子晶体的安德森局域现象,证明了非晶态光子晶体中的强局域模可以提高LED的光提取效率,并优化了非晶态LED的结构参数。首先利用分子动力学的方法模拟出了二维非晶态光子晶体,随后利用FDTD研究了非晶态光子晶体的的各个参数对光局域效果的影响。通过模式理论分析,非晶态光子晶体的局域模可以使限制在横向的一个有限区域内,从而沿着纵向传播,来达到提高LED的发光效率的目的。在参数的优化的基础上,得到了一个可以增强光提取效率4.8倍的LED模型,最后并对这个模型的远场进行了分先比价.四、利用电磁理论结合FDTD算法分析了有源层位置、纳米线的半径、间距以及高度等各个参数对纳米线LED光提取效率的影响,提出了一种利用纳米线导模提高LED光提取效率的方法。首先利用经典电磁场理论分析了纳米线中电偶极子辐射场的分布情况,发现偶极子的位置对其辐射场的空间分布有着很大的影响,且纳米线中的导模在提高纳米线LED的光提取效率起着重要的作用。随后,利用FDTD法分析了的纳米线的半径、间距以及高度等各个参数对纳米线LED光提取效率的影响,最后经过优化给出了两种不同量子阱结构的最佳纳米线LED模型,并且这种分析方法具有普适性。本论文的选题来源于国家重点基础研究发展计划项目：金属／介质纳米异质结构中的局域耦合效应及其在光电转换器件中的应用。通过论文的研究,明确了不同纳米介质结构提LED光提取效率的物理机制,进一步完善了我们的GPU计算平台,设计了高出光效率的LED模型,为了LED的生产和制造提供理论支持。更多还原

【Abstract】 Since the21st century, the consumption of energy has increased dramatically, the energy shortage is getting more and more serious. In order to provide sufficient energy, irrational energy development and utilization has become more and more serious, which leads to environmental pollution. Dealing with the relationship between energy and environment is the key to ensure human survival and development. Except for developing new clean energy addition, energy conservation has become an important means to solve environmental problems. Among the energy, nearly20%is used for light. However, the efficiency of the conventional lighting is very low, resulting in a large waste of energy. As a novel electro-optical conversion device, the efficiency of light-emitting diodes (LED) has exceeded2001m/W, the efficiency of organic light-emitting diodes has also exceeded1001m/W. Compared with conventional light sources, LED has much advantages, such as energy saving,,long life, high color quality, good direction, etc.,and the LED will become the next generation light source.However, the applications of commercial LED production will need a long time. And the most important reason is that the efficiency of the LED be very low. As we know, the efficiencies of the LED include internal quantum efficiency and the light extraction efficiency. With the breakthrough of semiconductor technology, internal quantum efficiency of LED get closer to the theoretical limit. Owning to the high refractive index contrast between semiconductor material and air, the total internal reflection and Fresnel reflection phenomenon occurs at the semiconductor/air interface, resulting in low light extraction efficiency of the LED. Therefore, improving the external quantum efficiency of LED has become a research highlights, which is the focus of this dissertation.There are a number of approaches aimed to the increasing of light extraction efficiency from the LED chip:chip-shaped LED design, placing a back-surface mirror and surface modification. Surface modification technique, including surface roughening and photonic crystal technology, is an very effective method to improve light efficiency of LED. Now, the main means of enhancing the light extraction efficiency of LED experiment, but it has a certain blindness. Therefore, theoretical research on the issue is an urgent task. Nowadays, the fabricaton of the micro/nano structure on LED can enhance the light efficiency of LED. However, how different geometric parameters change the energy distribution within the LED structure and how to affect the extraction efficiency of LED are still not clear. Moreover, the optimization of LED structure parameters is lack of an efficient method. As a popular electromagnetic analysis method, FDTD method can provide help for LED design to some extent. The traditional FDTD method meets two problems, a large amount of calculation and lengthy computation time. In addition, all the parameters of LED structure are interrelated, the task of calculation becomes more difficult. The graphics processor units (GPU) can solve this problem to some extent.In this dissertation, the nano-scale dielectric structures were chosen to improving the light-emitting efficiency of LED. We did some study on these problems mentined above using electromagnetic fields numerical calculation methods and electromagnetic field theory analysis methods. The main contents of this dissertation include:1. This dissertation analyzed the luminescence mechanism and structure characteristics LED. This paper also illustrated the QWs of LED could be equivalent to a dipole source and established the revelent model in FDTD simulation. GPU is employed to accelerate the calculation. This provide a theoretical basis for the next simulation.2. The LED can be modeled by a slab waveguid and the energy distribution of LED is illustrated. The physical mechanism that the photonic crystals can improve the light extraction efficiency is given. On the basis of optimal photonic crystal structure, we study the Influence of position perturbation on the light extraction efficiency.3. The influence of the periodic defect in the photonic crystal LED is analyzed. We take Fourier transform for the dielectric constant distribution map, and found that it has a better direction than perfect photonic crystals.4. The typical OLED is multi-layer planer structure, the embedded PhCs and surface PhCs are used to extract low order modes and high order modes. The optimized OLED structure with double PhCs is proposed.5. The liquid-like amorphous PhCs is simulated by the use of the Metropolis Monte Carlo method with the purely repulsive short-range linear spring potential. The2D FDTD method is employed to investigate the frequency dependence of light propagation characteristics in the amorphous PhCs. The mechanisms by which amorphous PhCs can effectively improve LEE were explored, and optimized amorphous PhCs LED structure is proposed.6. Analyzing the emitted power distribution of the dipole in the semiconductor nanowire, we found that the guided mode has an important role in enhancing the LEE of LED. The parameter such as radius of nanowires and the distance between nanowires were discussed.The work of this dissertation is to design high light-emitting efficiency LED model with nano-scale dielectric structures, analyze and optimize the parameters using electromagnetic numerical methods, and provide important theoretical guidance for the designing and fabricating of high performance LED.The main innovations of the dissertation are as follows:(Ⅰ) A high efficiency LED model with periodic defect photonic crystal is designed, a method to design photonic crystal structure by using a Fourier transform of the dielectric constant pattern is presented. First, the LED planar waveguide model is given to discuss the energy distribution of the LED in various modes, the principle that PhCs can improve the light extraction efficiency of LED is also given. The simulation shows that a small amount of defects in photonic crystal LED does not reduce the LED light extraction efficiency. Instead, the light extraction efficiency of LED will be increased by adding some defects in the photonic crystals. We designed a kind of photonic crystals with defects, and its’light extraction efficiency achieves1.6times of the perfect photonic crystals.(Ⅱ) An OLED structure model with double PhCs is proposed, which can extract low order modes and high order modes from OLED, the mechanism is also discussed. A structure model of the real OLED for analyzing the mechanisms that affect the light out-coupling characteristics is provided, and the embedded PhCs and surface PhCs are used to extract these guide modes. The influence of the PhCs parameter on enhancing the efficiency of the OLED is analyzed by FDTD method. The enhancement of the extraction efficiency in excess of290%is observed for the optimized double photonic crystal OLED.(Ⅲ) FDTD method and Fourier transform technology are empolyed to investigate Anderson localization phenomena in amorphous photonic crystals, and the amorphous strong local mode is demonstrated to improve light extraction efficiency of LED. The liquid-like amorphous PhCs is simulated by the use of the Metropolis Monte Carlo method, and2D FDTD method is employed to investigate the frequency dependence of light propagation characteristics. These strong localization modes confirm that the corresponding modes are sufficiently well located inside a small area for a long time, and the light that propagates in the horizontal direction is redirected and escapes from the LEDs to increase the light extraction.(IV) FDTD method and Electromagnetic theory are used to analyze the influence of parameters of nanowire LED, such as the position of the active layer, the radius of the nanowire, distance between nanowires, on the light extraction efficiency. The guided mode is proved to improve LED light extraction efficiency. First, we analyze the emitted power distribution of the dipole in the semiconductor nanowire. we found that the LEE of LEDs agrees with guided power portion of the total power, that is respect with the radius of the nanowire and the distance between the nanowires. With the geometrical parameters optimized, a radial QWs structure and axial QWs structure are proposed.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, The mechanisms by which amorphous PhCs can effectively improve LEE were clearified; Our GPU computing platform is further improved; high performance LED model was proposed; all the conclusions drawn in this dissertation provided very important theriotical reference value in the designment and fabrication of high efficient white LED.更多还原