Temperature-Dependent Efficiency Droop in GaN-Based Blue Micro Light-Emitting Diodes

This work investigates the size-dependent decrease in external quantum efficiency (EQE) of various InGaN/GaN multiple-quantum-well flip-chip blue micro light-emitting diodes ( μ -LEDs) of sizes from 10 × 10 μ m 2 to 250 × 250 μ m 2 and proposes that the temperature-dependent efficiency droop is the main mechanism for decrease in EQE with reducing dimensions for well-passivated μ -LEDs. Experimental results show that the EQE increases with reducing μ -LED sizes to 50 × 50 μ m 2 . However, the EQE decreases as the μ -LED size is further reduced to 10 × 10 μ m 2 . The measured current-voltage characteristics, the minimum ideality factor, the light-emission patterns by the photon-emission microscope, and the transmission-electron-microscopy images consistently reveal that the decreased EQE of the smallest sized μ -LED is not due to the sidewall leakage: the decreased EQE is rather caused by the temperature-dependent efficiency droop (T-droop), which is systematically found by investigating the blueshift in peak emission wavelength and calculating the thermal resistance (R th ) that increases with the reduced mesa area. The decrease in peak EQE at 440 K compared to 300 K is also presented, which demonstrates that the reduction in peak EQE increases with reducing μ -LED sizes. It is pointed out that the small-sized μ -LEDs suffer from higher junction temperature due to lower heat dissipation caused by higher R th compared to large-sized μ -LEDs.