【作者】 高晖；

【导师】 李康；

【作者基本信息】 山东大学 ， 无线电物理， 2012， 博士

【摘要】 当今世界,人类经济社会得到了飞速发展。然而,世界的能源形势却愈发严峻。在这日益紧缺的能源中,约占21%的电能被用在了照明领域。倘若在这一领域节省尽可能多的资源,势必会对人类社会的经济发展做出巨大贡献。这其中,半导体发光二极管(Light Emitting Diode, LED)由于其寿命长、发热小以及强大的节能潜力博得了国内外的研究者的广泛关注,并逐渐引领了人类第四代照明技术的革命。目前LED的研究及大规模的商业化还尚待实现。这其中一个重要原因,便是器件中产生的光子不能被有效地提取,进而转化为可用的光功率,从而导致整个器件的出光效率很低。如何合理地设计LED的结构,使其具有更高的出光效率,便成为了节能减排的大势所趋,也是本论文研究的重要意义所在。我们知道,LED的出光效率具体可分为内量子效率及外量子效率,其中外量子效率的提升主要可体现为光提取效率的提升。对于LED内量子效率的提升,可以采取光子态密度工程的手段。通过引入金属纳米粒子改变LED有源层附近的能量分布,可以一定程度上提升LED的内量子效率。研究和掌握这种能量的耦合规律和调控LED内量子效率增强的机制,可以为制备高性能LED提供重要的参考价值。此外,通过表面粗化、表面刻蚀光子晶体、掩埋低折射率限制层以及嵌入式光子晶体的方式均在一定程度上改变了LED结构内部能量分布,从而提升了LED的光提取效率然而,目前国内外的研究对于LED结构优化尚缺乏明确的理论指导,尤其在光提取效率提升方面,各种不同结构的几何参数如何改变LED结构内的能量分布进而如何最终影响LED的光提取效率,其机理尚不明确。迫切地需要我们通过研究去阐明。此外,当前对于LED的电磁建模计算也缺乏一种高效的手段。传统的时域有限差分方式在可见光频率受到网格大小的限制,对每一个模型的计算都将耗费冗长的计算时间和庞大的计算空间,从而难以满足人们对结构快速扫描优化的需要。因此,LED模型的快速参数优化,需要我们提供一个更为高效的研究方法,并做到在节省计算量的同时能够较为清晰地阐明LED结构参数对于其光提取效率的控制机制,从而为LED结构的优化提供重要的理论指导。本论文选取纳米尺度的金属/介质结构,以提高LED的出光效率为最终目的,通过电磁场数值分析并结合模式分析手段,对上述科学问题进行了细致地研究。论文的主要研究内容包括：1)对GaN基蓝光LED结构的发光机理和结构特性进行了分析,详细说明了GaN基蓝光LED有源层发光可通过偶极子源等效的原理,并完成相关的计算机数值仿真。2)对金属纳米粒子结构形成的局域化表面等离激元(Localized suface plasmon, LSP)对偶极子源自发辐射,也即LED有源层自发辐射的影响进行了详细地分析并优化了金属纳米粒子的结构。3)研究了金属纳米粒子结构形成的LSP对偶极子源辐射方向性的影响,建立了LSP模式近场和偶极子源自发辐射远场二者间的联系。4)研究了金属薄层平板结构形成的表面等离激元(Surface plasmon ploaritons, SPPs)对改变LED内量子效率的影响。优化了金属薄层平板的结构尺寸。5)探索了同时增强LED内量子效率和光提取效率的机制。提出了补偿SPPs动量损失的结构,并在获得LED内量子效率提升的同时也获得了较为可观的LED光提取效率的提升。6)通过等效折射率近似的方式,将光子晶体LED结构模型进行了简化。从而建立并完善了LED结构模型的模式分析理论,并通过相应的数值计算验证,为LED结构的快速优化提供了一个切实可行的方法。7)详细分析了二维光子晶体结构(包括表层深度刻蚀光子晶体、嵌入式光子晶体以及双层光子晶体结构)以及AlxGa1-xN低折射率限制层结构对GaN基蓝光LED光提取效率的优化准则。并对相应的结构参数进行了优化。本论文的研究工作主要围绕纳米尺度人工光学结构增强LED出光效率的电磁仿真进行。研究紧密结合当今国民经济和社会发展过程中与节能减排、环境保护相关的关键技术,通过相关的电磁场数值分析和优化,深入探讨了金属/介质纳米结构对GaN基蓝光LED光提取效率改变的机制,为高性能GaN基蓝光LED的分析、设计和制造提供了重要的理论参考价值。论文的主要创新点如下：(Ⅰ)论文阐明了LED中量子阱有源层的电磁模型可以用电偶极子源替代的原因。并详细分析了LSP、SPPs模式对LED内量子效率提升的机制。通过相关结构优化发现,当金属纳米粒子长短轴比接近4：1时,可使得相应系统的偶极子源自发辐射获得近5000倍的增强效果；并同时发现了偶极子源自发辐射谐振波长随纳米粒子减小的蓝移现象,使得在不同波长下获得偶极子源自发辐射谐振变为可行。(Ⅱ)论文发现了LSP的高阶模式会使得偶极子源的原偶极辐射图样消失,主瓣方向性发生改变的现象。并首次建立了LSP高阶模式的近场分布和偶极子源自发辐射远场方向性二者的联系。(Ⅲ)论文探讨了在增强LED内量子效率同时增强LED外量子效率的机制。通过文章提出的在p-GaN层中掩埋金属纳米粒子阵列结构的方式,在获得3.6倍内量子效率增强的基础上,通过引入倒晶格矢量抵消SPPs动量的方式,使得整个结构的光提取效率获得了接近2倍的增强。从而为同时增强LED内外量子效率的机制提供了一定的参考价值。(Ⅳ)论文对传统的时域有限差分方法分析LED的形式进行了创新。通过等效折射率近似,首次提出将LED模型看作多层介质波导进行模式分析的理论。并通过相关的数值仿真验证,完善了LED中的模式分析方法,从而为LED的建模仿真及相应结构参数的优化提供了更为便利的手段,免除了传统的数值分析算法所要面临的巨大的存储空间和冗长的计算时间的问题。同时,论文提出的方法更加直观,能够在一定程度上阐明LED各层结构中的结构参数对LED光提取效率影响的物理机制。(Ⅴ)论文分析指出,LED光提取效率的提升受到低阶导模影响最大,降低低阶导模从有源层中获取能量所占的比例是能够显著提升该效率的最为有效的手段。论文的这一结论为GaN基蓝光LED的结构优化指明了基本理论依据,在一定程度上消除了后续仿真优化工作的盲目性。(Ⅵ)论文详细优化了光子晶体LED结构,含低折射率限制层的LED结构的几何尺寸。首次提出了在双层光子晶体结构基础上引入薄层低折射率限制层,在不明显增加晶格位错和降低低阶导模携带能量的同时调制布洛赫模式分布,最终使得整个GaN基蓝光LED的光提取效率获得了八倍增强。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,阐明了金属/介质纳米异质结构在LED中的局域耦合效应的物理机制,为GaN基蓝光LED的结构优化探寻了高效的电磁分析方法,并且论文优化的模型为高性能GaN基蓝光LED的设计制造提供了重要的理论参考。更多还原

【Abstract】 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:(Ⅰ) 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.(Ⅱ) 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.(Ⅲ) 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.(Ⅳ)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. (Ⅵ)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.更多还原