Full-color monolithic hybrid quantum dot nanoring micro light-emitting diodes with improved efficiency using atomic layer deposition and nonradiative resonant energy transfer
Three subpixels of a green μLED, a blue NR-μLED, and a red QD-NR-μLED, with the deposition of a transparent conducting oxide layer and the pn electrodes.
Micro light-emitting diodes (μLEDs) are promising candidates for the next generation display technology. However, the efficiency of μLEDs decreases as the size shrinks. Additionally, in order to realize monolithic full-color micro-display, the cost of the RGB mass transfer is enormous, and the color conversion efficiency of the quantum dot (QD) color conversion technology still needs to be improved.
In order to enable μLEDs to be mass-produced as soon as possible. A research group, led by Prof. Hao-Chung Kuo from Institute of Electro-Optical Engineering in Taiwan Chiao Tung University and Dr. Tingzhu Wu from Department of Electronic Science in Xiamen University, carried out the study of full-color monolithic hybrid quantum dot nanoring (NR) μLEDs with improved efficiency. The research results are published in Photonics Research, Volume 7, Issue 4, 2019 (S. Huang Chen, et al., Full-color monolithic hybrid quantum dot nanoring micro light-emitting diodes with improved efficiency using atomic layer deposition and nonradiative resonant energy transfer).
This study reports a novel structure which is the hybrid quantum dot NR micro-LED (QD-NR-μLED). NR structures are fabricated on a green LED epitaxial wafer; the color of NR-μLEDs is tuned from green to blue through strain relaxation. Using the technology of atomic layer deposition (ALD), a 1-nm Al2O3 layer is deposited on the sidewall of blue NR-μLEDs, which improves the photoluminescence intensity of blue NR-μLEDs by 143.7%. Coupling with the exposed multiple quantum wells through nonradiative resonant energy transfer (NRET), red QDs are printed to blue NR-μLEDs for a full-color display. To further improve the color purity of the red light, a distributed Bragg reflector is developed to reuse the excitation light.
In this work, the NR structure provides a structural basis for monolithic full-color micro-display. The ALD sidewall passivation technology improves the luminous efficiency of μLEDs. The NRET mechanism improves the color conversion efficiency of red quantum dots. Therefore, Prof. Kuo and Dr. Wu believes that this novel QD-NR-μLEDs can provide a new idea and a new method for the application of full color micro-displays.
Further work will focus on the optimized thickness of ALD layer as well as how to combine QD-NR-μLEDs with a flexible substrate to achieve flexible transparent micro-displays.
基于纳米环/量子点结构的高效率全彩微型发光二极管
沉积了透明导电氧化层和pn电极的绿色μLED、蓝色NR-μLED和红色QD-NR-μLED的三个子像素。
微发光二极管（μLEDs）是下一代显示技术最有希望的候选者。然而，μLEDs的效率会随着尺寸的减小而降低。另外，为了实现单片全彩色微显示，RGB的巨量转移成本很高，量子点（QD）颜色的转换效率还有待提高。
为了使μLED尽快量产。由台湾交通大学电光工程研究所郭浩中教授和厦门大学电子科学系吴挺竹博士领导的研究小组，对如何提高全色单片混合量子点纳米环（NR）μLED的效率进行了研究。研究结果发表在Photonics Research 2019年第7卷第4期 (S. Huang Chen, et al., Full-color monolithic hybrid quantum dot nanoring micro light-emitting diodes with improved efficiency using atomic layer deposition and nonradiative resonant energy transfer)。
该课题组报道了一种新的量子点-纳米环混合μLED（QD-NR-μLED）结构。在绿色LED外延片上制备了纳米环结构，利用应变弛豫效应将NR-μLED由绿色变为蓝色。采用原子层沉积（ALD）技术，在蓝光NR-μLED的侧壁上沉积了1 nm的Al2O3层，使蓝光NR-μLED的光致发光强度提高了143.7%。接着在蓝光NR-μLED上喷涂红色量子点，利用非辐射共振能量转移（NRET）机制，通过量子点和暴露的多量子阱之间的耦合来提升量子点色转换效率并实现全彩显示。为了进一步提高红光的纯度，课题组研制了一种分布式布拉格反射镜，以重复利用激发光。
在这项工作中，纳米环结构为单片全彩微显示提供了结构基础，ALD侧壁钝化技术提高了μLEDs的发光效率，NRET机制增强了红色量子点的色转换效率。因此，郭教授和吴博士认为，"这种新型的QD-NR-μLED能够为全彩微显示的应用提供新的思路和方法。"
下一步的工作将集中在优化ALD的厚度以及如何将QD-NR-μLED与柔性基板结合，以实现柔性透明微显示。