Observation of two different temperature-dependent behaviors of the defects in III-nitride micro-LEDs

As micro-LED pixel sizes shrink, the volume-to-surface ratio increases, so defect-assisted non-radiative recombination becomes more important for LED pixel efficiency degradation. The donor-type defects on the pixel sidewalls are induced by inductively coupled plasma etching, and the carrier leakage through the sidewalls is determined by the density of surface traps. The defects in quantum wells provide places for the non-radiative recombination of electron-hole pairs, and the recombination rate is related to the thermal velocity of carriers and the trapping cross section of defects. The experimental results indicate that the significant improvement of quantum efficiency from sidewall passivation happens at current densities higher than 400 A/cm(2) at an environmental temperature of 300 K. When the temperature decreases to 150 K, the sample with sidewall passivation has 10% higher quantum efficiency at all current ranges. Numerical modeling is applied to evaluate the behaviors of two types of defects at different temperatures. Our observation from modeling reveals that the efficiency drop caused by surface defects is hardly affected by temperature changes, whereas results show that the Shockley-Read-Hall nonradiative recombination rate decreases rapidly at low temperature due to longer carrier lifetime and increased difficulty of electron and hole recombination in traps. Therefore, the significant increase in the efficiency at 150 K, especially in the low current density range, is due to the defects within the quantum well.