半极性准同质外延绿光LED及量子效率提升技术研究

Light-emitting diodes (LEDs) have attracted considerable interest as candidates for next-generation lighting because they promise to reduce energy consumption enormously. Semiconductor-based white-light generation by combining the red, green, and blue light emitting diodes illustrates many merits as ideal technology route, such as the broad spectrum, high color rendering index and regulation of color temperature etc.. However, the peak internal quantum efficiency (IQE) of green LEDs is significantly lower than that of InGaN-based blue and AlGaInP-based red LEDs, which is characteristically called the "green gap". In this project, we plan to homoepitaxy the green LED along the semipolar orientation on the high quality GaN template obtained by hydride vapor phase epitaxy (HVPE) technology. By virtue of the obvious decrease of the polarization effect at high indium content, the quantum efficiency of green LED can be improved. The growth mechanism of quasi-homoepitaxy is investigated near the interface along the semipolar orientation. Using InAlGaN nuclear layer and multi-layer buffer for strain regulation, the residual stress in the quasi-homoepitaxial growth is obviously reduced. The annihilation mechanism of the threading dislocation (TDs) and basal-plane stacking faults (BSFs) is revealed by TEM and CL. With the combination of theory simulation and experiment results, the InGaN/GaN multiple quantum well (MQWs) structure design is optimized under the reduced polarization and the carrier transport and recombination physical course are understood in the semipolar MQWs. Additionally, the critical reasons with respect to the efficiency droop of green LED at high current are also comprehended. Furthermore, with the localized surface plasmon (LSP) methods and new-type vertical package technology, the light extraction efficiency of green LED is greatly increased. The concerned physical mechanism of LSP methods for green light enhancement is presented using self-organized metal nano-arrays. Finally, the semipolar green LED devices with high quantum efficiency are successfully fabricated.

红绿蓝三基色实现白光具有光谱范围宽、显色性高、色温可调等优点，是实现白光LED的理想技术路线，但目前绿光LED发光效率低下成为制约三基色白光LED发展和应用的瓶颈。本项目拟在HVPE技术制备的高质量半极性GaN复合衬底基础上，沿半极性方向准同质外延绿光LED实现降低极化电场和droop效应，进而提高绿光LED的效率。通过InAlGaN形核层和多层应力调制层技术研究准同质外延生长规律，揭示穿透位错、堆垛层错等缺陷的湮灭和控制机制；采用理论计算和实验相结合，优化有源区结构设计，掌握半极性InGaN多量子阱输运和复合物理过程，揭示半极性材料中辐射复合中心和非辐射复合中心角色的微观区域结构、和生长条件的关系及其形成机理，进而获取解决绿光大电流效率下降的关键因素；结合表面等离激元耦合和新型垂直封装技术，探索光出射增强物理机制，实现高光提取效率，最终研制出高效率InGaN基半极性绿光LED。

红绿蓝三基色实现白光具有光谱范围宽、显色性高、色温可调等优点，是实现白光LED 的理想技术路线，但目前绿光LED发光效率低下成为制约三基色白光LED 发展和应用的瓶颈。本课题围绕半极性准同质外延InGaN绿光LED的外延生长及器件研制中的基础科学问题，采用半极性GaN应力调制层生长技术，结合自组装SiO2纳米球和碳纳米管图形衬底，纳米外延来控制应力的释放和阻隔位错的延伸，从而获得高质量半极性GaN外延层。同时研究半极性GaN材料p型掺杂的物理本质、杂质补偿和激活机理，采用低激活能的半极性p-InGaN代替p-GaN作为空穴注入层，通过周期性梯度掺杂和In-Mg共掺杂技术提高p-GaN的空穴浓度。也系统研究了绿光LED量子阱载流子输运和复合的物理过程，探索绿光LED载流子泄漏、电子溢流以及载流子有效注入等详细过程和微观机理，揭示绿光LED器件Droop效应的本质原因，并采用表面等离激元和光子晶体等技术，实现外量子效率超过50%的提高。同时，我们还采用纳米选区外延生长，利用纳米球镜光刻实现了高In组分纳米柱绿光LED，金字塔纳米柱绿光LED位错密度和应力明显降低，量子限制斯塔克效应得到有效抑制，借助石墨烯作为连通电极，实现了器件电致发光，FDTD模拟也证明光提取效率大幅度提高，为绿光LED和无荧光粉白光LED效率进一步提升，提供了新的途径。