Achieving high-efficiency, long-wavelength, and high-uniformity InGaN red micro-LEDs through polarization effect and stress engineering of the AlGaN capping layer

InGaN red light-emitting diode (LED) has attracted increasing interest in recent years due to its important role in full-color micro-LED displays. Covering an AlGaN capping layer on top of a high-indium-composition InGaN quantum well can improve the performance of an InGaN red LED, which is considered to compensate for stress and suppress the decomposition of InN. However, the AlGaN capping layer can also cause changes in the polarization electric field, which have been almost overlooked in previous studies. In this work, theoretical simulations reveal that the polarization effect of the AlGaN capping layer necessitates a trade-off between the long wavelength and high luminous intensity of InGaN red LEDs, thereby yielding an optimal Al composition of 0.4. Meanwhile, experimental results demonstrate that the micro-LED with an Al composition of 0.4 in the capping layer exhibits the most uniform luminescence. The underlying reason for this optimal luminous uniformity is elucidated by stress engineering via time-of-flight secondary ion mass spectrometry characterization, which verifies the in-plane uniformity of the indium composition within the quantum wells—a feature not addressed in previous research. The 30 × 30 μm2 micro-scale light-emitting diode achieves the longest emission wavelength of ∼650 nm and the highest on-wafer external quantum efficiency of 1.8%, which further corroborates the theoretical simulation results.