Strongly Localized Excitons for Efficient Micro-Light-Emitting-Diodes under Low Injection Levels

Recovery and isolation of damages on the sidewall are two main routes to improve the external quantum efficiency (EQE) of micro light-emitting-diodes (μLEDs) under low injection levels. In this work, strongly localized excitons in μLEDs were found to play a role in carrier isolation from damages. An efficient 5 μm-diameter InGaN blue μLED array with a high EQE of 11.77% at 0.2 A/cm2 was fabricated. The longitudinal optical-phonon sidebands (LO-PSBs) were observed near the zero-phonon line (ZPL) in electroluminescence (EL) and confocal photoluminescence (PL) spectra at different temperatures and excitation densities. The LO-PSBs originated from the coupling of the strongly localized excitons to the LO-phonons. According to the Huang–Rhys factor, about 55% of the total injected carriers became strongly localized excitons. Compared to the reference μLEDs, a higher fraction of strongly localized excitons corresponded to more suppression of nonradiative recombination in the sidewall region. By confocal PL and Raman mappings, the distribution of the strongly localized excitons was visible on the mesa. The exciton fraction was calculated using the Saha equation. The binding energy of the excitons in the efficient μLEDs at a current density of 0.05 A/cm2 was estimated to be above 35 meV. This work proposed a way to eliminate the edge effects of μLEDs.