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.

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.