Researchers at Xiamen University have demonstrated that adopting a hexagonal mesa structure significantly enhances the performance of indium gallium nitride (InGaN) micro green light-emitting diodes (LEDs). Published in Optics Express (Vol. 33, p. 42747, 2025), the study details how optimizing the geometric design of micro-LED mesas addresses current uniformity challenges and improves optoelectronic efficiency, which is crucial for next-generation display and communication technologies. Mesa refers to a raised surface on a micro-LED that forms the light emitting surface (LES).
The research compares circular, square, and hexagonal mesa geometries fabricated from InGaN/GaN multilayers grown on patterned sapphire substrates by metal-organic chemical vapor deposition (MOCVD). The hexagonal mesa, featuring six evenly spaced vertices, minimizes the maximum distance between the central p-electrode and the mesa edges. This geometry enhances current diffusion uniformity across the active region, alleviates current crowding at corners (a common issue in square mesas), and reduces low-current-density regions that degrade performance. With a balanced perimeter-to-electrode area (P/A) ratio, the hexagonal design optimizes carrier injection and mitigates parasitic recombination.
Green micro-LEDs, operating within the human eye’s peak sensitivity range, are essential for high-fidelity color displays, AR/VR systems, phototherapy, and visible light communication. The findings demonstrate that structural geometry optimization at the microscale can lead to measurable performance gains without altering material composition. The hexagonal mesa’s superior current spreading, reduced non-radiative loss, and enhanced EQE collectively position it as a promising architecture for energy-efficient microdisplay and communication LEDs, aligning with industry trends toward miniaturized, high-brightness, and longer-lifetime photonic components.
All three device geometries exhibited consistent turn-on voltages near 3.3 V. However, at higher bias levels (10 V), the hexagonal LEDs demonstrated substantially greater current density—285.8 A/cm² compared to 199.9 A/cm² for squared and 164.7 A/cm² for circular mesas. These results imply enhanced carrier injection efficiency due to optimized current spreading. The hexagonal LED also presented a noticeable 2.9 nm blue-shift in emission wavelength with increasing current, indicating reduced quantum confinement effects from better carrier distribution.
Optical measurements showed that the hexagonal micro-LED achieved an output power density of 4.94 W/cm² at 200 A/cm² current injection—surpassing both the circular (3.86 W/cm²) and square (3.14 W/cm²) variants. Its external quantum efficiency (EQE) peaked at 19.9% at 10.41 A/cm², outperforming the circular (16.9%) and square (17.6%) devices. Furthermore, the EQE droop—a measure of efficiency decline with increasing current—was minimized in the hexagonal design (48.2%), compared to 52.4% for circular and 56.1% for square geometries, suggesting improved thermal and electron-hole recombination balance.
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All images courtesy: https://opg.optica.org/oe/fulltext.cfm?uri=oe-33-20-42747
Image: (a) Optical output power density and (b) EQE of circular, square, and hexagonal green micro-LEDs as a function of injected current density.
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