The Michigan Engineer News Center

Xianhe Liu receives Best Poster Award at ICNS 2019

The research impacts development of high-efficiency, micro LEDs, used in a variety of applications. | Short Read

Xianhe Liu, visiting scholar working with Prof. Zetian Mi, received a Best Poster Award at the 13th International Conference on Nitride Semiconductors (ICNS) for research in colored, high-efficiency, micro LEDs.

The poster, Multi-Color InGaN Nanocrystal Micro LEDs Grown by Molecular Beam Epitaxy, was co-authored by Xianhe Liu, Kishwar Mashooq (ECE PHD student), Ayush Pandey (ECE PHD student), Walter J. Shin (ECE PHD student), and the team’s advisor, Prof. Zetian Mi.

The team is developing micro-LEDs based on GaN, one of the most important and promising routes for micro-LED displays and other similar applications.

According to Liu:

“High efficiency, micro-light emitting diodes (LEDs) that can operate efficiently in the blue, green and red spectra are in demand for a broad range of applications including mobile displays, virtual/mixed/augmented reality, bio-photonics, and communications.

However, GaN-based micro-LEDs exhibit very low efficiency, particularly in the green and red wavelengths, which has been limited by the presence of large defect densities, surface recombination and poor current injection.

We have investigated the design, epitaxy, and optical properties of dislocation-free InGaN nanocrystals, and have demonstrated small size (~2-10 µm) LEDs that can operate in the blue, green, yellow, and orange wavelengths.

Self-organized InGaN quantum dots are incorporated in the nanocrystals to serve as the active medium. InGaN nanocrystals are used as the building block in a carefully designed hexagonal lattice to form photonic crystal structures. Such photonic crystal structures allow for the efficient coupling of guided modes to vertical surface emission without any conventional surface roughening and encapsulation which would involve complicated processing.

Our InGaN nanocrystal micro-LEDs exhibit excellent current-voltage characteristic, with a turn on voltage ~3.5 V. Strong emission in the blue, green, yellow and orange wavelengths were measured. Negligible efficiency droop was observed at a high injection current of ~ 2 kA/cm2. Furthermore, a green surface emitting laser based on InGaN nanocrystals is also demonstrated using the photonic crystal structure.

Combining the dislocation-free crystalline quality and the freedom for engineering the light emission property with photonic crystal structure, our approach will be ideally suited for high efficiency multi-color micro-LED application and the monolithic integration of multi-color micro-LEDs on a single substrate.”

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Catharine June
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