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微纳米结构增强蓝光LED发光效率的研究
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摘要
在世界范围处于能源危机的当今社会,LED作为一种新型的绿色光源,具有环保节能、可靠性高、使用寿命长等优点,在照明领域将引领未来发展的趋势,有望成为继白炽灯、荧光灯、高强度气体放电灯之后的“第四代光源”。世界各国政府已开始重视LED的研究和开发,争相提出各自的发展计划或出台一些扶持措施。
     自LED问世以来,固体白光照明光源是人们一直努力追寻的目标。目前蓝光LED是制造白光LED的基础,蓝光LED主要是由GaN材料制成的,所以研究GaN基蓝光LED对发展白光LED有重要意义。LED的发光效率是衡量LED性能的重要指标,目前LED发光效率还存在很大的提升空间,特别是GaN基LED。由于GaN材料和空气之间大的折射率差异,使得大部分光在分界面处产生全内反射和菲涅尔反射而陷落在材料中。目前围绕如何提高GaN基LED光提取效率的问题开展了大量工作。多种方法已被提出来提高GaN基LED的光提取效率,如表面粗化、倒装芯片结构、光子晶体、图形化衬底技术、微纳米复合结构等。在制作微纳米结构增加LED发光效率的电磁场特性的研究中,LED样品的生长和微纳米结构的制作都是一个非常耗时、耗力和耗财的过程。为了节约光电器件制作成本,可以借助计算机技术来加快设计过程并且降低开发成本。
     本论文围绕提高蓝光LED发光效率为目的,采用微纳米GaN图形结构,通过计算机模拟和理论分析相结合的方法研究了微纳米图形结构对蓝光LED光提取效率的影响。基本思路是采用数值仿真技术通过对蓝光LED结构建模,研究了微纳米图形结构的结构参数对蓝光LED光提取效率的影响,通过对结构参数进行优化,并结合理论分析,最终获得具有高提取效率的LED优化结构,为实现高效蓝光LED提供理论指导。
     本论文开展的主要研究工作有以下几点:
     (1)分析并研究了FDTD算法和蒙特卡洛射线追踪法在进行LED光提取效率计算时的适用性。FDTD算法以将麦克斯韦旋度方程直接在时域-空域进行差分离散,通过逐步推进的方法求解空间电磁场。针对具有纳米图形结构的LED数值仿真是非常有效的。蒙特卡洛射线追踪法是依据光在均匀媒质中的直线传播定律以及光在两种不同媒质界面上的反射和折射定律为基础,把光看作为光能量传播方向上的几何线。运用此方法很容易模拟LED有源层电子-空穴对的自发辐射过程,并且适用于任何形状和尺度的结构。这种方法对具有微米量级图型结构的LED仿真是可行并可靠的。
     (2)通过数值建模,运用FDTD算法计算了光子晶体LED,特别是嵌入式光子晶体对LED的光提取效率的影响。重点研究了嵌入式光子晶体结构参数(如填充因子、位置、厚度)对蓝光LED光提取效率的影响,同时与表面光子晶体对LED光提取效率的影响进行了比较。结果表明嵌入式光子晶体的填充因子和位置对光提取效率影响较大,而厚度对其影响较小。光子晶体厚度对LED光提取效率的影响呈现一种震荡状态。在表面光子晶体深度刻蚀(深度250nm)时可使光提取效率有5倍以上的增加。表面光子晶体和嵌入式光子晶体在对LED进行光提取上起的作用不同。顶端光子晶体主要是通过对导模的衍射作用增加光提取效率,而嵌入式光子晶体主要贡献是改变LED中导模的分布。通过对嵌入式光子晶体结构参数的优化,更进一步通过双光晶体增加LED光提取效率提供了理论指导。
     (3)在光子晶体的制作过程中,不可避免的会出现结构上的缺陷和无序,而这些缺陷或无序会严重影响光子晶体的性能。本文通过程序设计产生了具有一定容差的光子晶体,即无序光子晶体。结合FDTD方法通过数值建模得到具有无序光子晶体结构的LED模型。同时研究了无序光子晶体LED有源层位置对光提取的影响。研究发现,无序结构的出现并未改变有源层位置随光提取效率的影响,与平板结构相同。
     (4)为了进一步得到光子晶体结构参数的无序变化对蓝光LED光提取效率的影响,分别选取了具有优化结构参数的光子晶体和非优化结构参数的光子晶体进行研究对比。主要研究了光子晶体空气孔位置、半径和刻蚀深度的无序变化对蓝光LED光提取效率的影响。研究表明,空气孔位置的无序变化一般呈现弱无序状态,对光提取效率的影响不显著;空气孔半径的无序变化可以显著的影响蓝光LED的光提取效率,最大变化幅度可达54%;空气孔在深度刻蚀情况下,刻蚀深度在20%的容差变化范围内,对光提取效率的影响可以忽略。对于优化的光子晶体结构,任何的无序变化都会使光提取效率下降,而对于非优化的光子晶体结构,结构参数的无序变化反而会使光提取效率增加。这部分研究结果说明不同结构参数的光子晶体LED受结构参数无序变化的影响不同,但每个光子晶体LED会存在一组优化参数,使光提取效率达到最大,其它光子晶体结构参数的无序变化,都会使光提取效率减小。
     (5)在LED表面制作微表面结构,一是可以增加光的散射,二是会起到一定抗反射的作用。利用蒙特卡洛射线追踪法研究了各种不同的周期图形阵列对蓝光倒装LED光提取效率的影响。并结合表面抗反射理论和等效折射率理论对数值研究结果给出了详尽的理论分析。结果表明,在四种不同的周期阵列中,微锥阵列可以提供最好的光提取效率,锥阵列结构要优于柱阵列结构,凸起结构要优于孔状结构。为了找到利于光提取效率增加的优化微结构,比较了微锥阵列结构和微金字塔阵列结构对光提取效率的影响。结果表明,在具有相同的图形底面积和倾斜角的情况下,微锥阵列优于微金字塔阵列。考虑到实际的图形结构制作工艺,严格的锥结构很难获得,研究了截断的锥或锥孔阵列顶半径对光提取效率的影响。研究表明具有一定顶半径的截断的锥或锥孔阵列反而更有利于光提取效率的增加。
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.
引文
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