微纳金字塔垂直结构LED量子效率研究

Despite the achievement of nitride based light emitting diodes(LEDs) that led to the realization of commercial productions, there are still severe limitations such as poor quantum efficiency and dramatic efficiency droop, for which the polarization field, dislocations, electron leakage and Auger recombination are considered as the major reasons. Progresses in low-dimensional micro- or nano-structure LEDs for overcoming the limitations have been steadily developed. However, a large number of threading dislocations (TDs) were still presented in micro-size LEDs, and material damages caused by dry etching processes are not negligible. On the other hand, nitride-based vertical-injection LEDs (VLEDs), of which the insulated sapphire substrates are removed to transfer the epilayer to a new thermal and electrical conductive substrates, have been considered as a promising candidate for high power applications. VLEDs provide many advantages, such as better current injection, excellent heat dissipation and enhanced reliability. In this study, hexagonal pyramids array micro vertical light emitting diodes (HP VLEDs) based on laser lift-off and N-polar wet etching were proposed. And graphene was first adopted in HP VLEDs as transparent conductive layer. The eliminating of TDs in hexagonal pyramids was realized by wet etching process. The improvement of internal quantum efficiency (IQE) was achieved by the reduction of TDs in HP VLEDs.Separating the active regions by N-polar wet etching process is an effective approach to fabricate the micro- to nano- size VLEDs array without detrimental ICP process and artificial lithography mask. Meanwhile, it provides a simple but effective way to further improve the crystal quality and light extraction of GaN based LEDs. Theoretically HP LVEDS can improve quantum efficiency and efficiency droop brilliantly due to the better crystal quality and higher extraction efficiency as the inherent advantage of hexagonal pyramid shape. And a perfect device model based on HP VLEDs was provided for carrier transport machanism research of nitrides LEDs. Key points in this study : mechanisms in electrochemical etching of N-polar GaN and approaches towards quasi-perfect micro-scale hexagonal pyramid VLEDs; contact characters of Graphene and n-GaN; fabrication processes of HP VLEDs; mechanisms of carrier transport, recombination and photon extraction; technologies for flexible HP VLEDs.

如何进一步提升高功率密度下的量子效率是制约GaN基LED发展及应用的重要技术瓶颈，揭示制约量子效率的关键物理机制并探索实现高量子效率发光器件是本项目主要研究目标。本项目以激光剥离垂直结构LED器件为基础，通过湿法腐蚀的方法制备微纳金字塔发光二极管，采用石墨烯作为电极互连材料，实现器件的电注入。该方案利用化学腐蚀的选择性，理论上实现了一个无位错、高提取效率的理想发光器件，通过对该器件的载流子输运复合机制、光子提取机制进行研究，分析限制氮化物LED量子效率的关键因素，并探索提升量子效率的技术手段。主要研究内容包括：N面氮化镓腐蚀机理； 微纳金字塔垂直结构LED制备技术及电极互连技术；微纳金字塔垂直结构LED中载流子输运与复合机制、光子提取机制；柔性有序化微纳金字塔垂直结构LED技术探索。

如何进一步提升高功率密度下的量子效率是制约GaN基LED发展及应用的重要技术瓶颈，揭示制约量子效率的关键物理机制并探索实现高量子效率发光器件是本项目主要研究目标。项目以激光剥离的垂直结构LED器件为基础，采用湿法腐蚀的方法制备微纳金字塔发光结构（HP VLED），利用湿法腐蚀对位错的选择性实现无位错纳米结构，同时避免了干法刻蚀造成的器件损伤，并自然形成了金字塔形貌，实现了一种兼具高内量子效率与高提取效率的理想器件结构。项目突破了N面氮化镓选择性刻蚀技术、微纳金子塔垂直结构LED电注入技术、石墨烯与N面n型氮化镓欧姆接触技术等关键技术，取得的主要成果包括：掌握了氮面氮化镓 (N-GaN) 蚀刻过程和机理，建立GaN 腐蚀的Electrochemical能带模型，为微纳金字塔垂直结构LED器件的研制奠定了理论与技术基础；研究了金属/蚀刻N-GaN面接触性质，突破了微纳金字塔垂直结构LED关键技术，对于降低LED工作电压，提高器件热稳定性具有重要意义；制备出微纳尺寸金字塔垂直结构LED器件，并研究了其量子效率和光电性能，实现了内量子效率的提升和Droop效应的改善；初步实现了柔性有序化微纳金字塔垂直结构LED，为探索新型柔性显示技术、可植入生物光源技术等提供了新的解决方案；项目实施过程中发表学术论文10篇，申请发明专利6项，培养研发和技术人员3人，研究生3人。本项目首次提出了基于垂直结构的微纳发光器件，并引入石墨烯作为互联电极实现电注入发光，对于氮化物LED新型器件结构、新型透明电极材料等前沿技术探索具有深远意义。柔性LED技术的突破也为超越照明创新应用提供了新的技术路径。