Exciton distribution-induced efficiency droop in green microscale light-emitting diodes at cryogenic temperatures

Abstract

The anomalous droop in the external quantum efficiency (EQE) induced by the localization of excitons in GaN/InGaN green micro-light-emitting diodes (micro-LEDs) has been demonstrated at temperatures ranging from 25 to 100 K. At cryogenic temperatures, the random distribution of excitons among local potential energy minima limits the radiative recombination and reduces the EQE of green micro-LEDs. As the temperature increases from 25 to 100 K, the hopping of excitons from shallow potential energy minima to the potential energy valley contributes to the enhancement of radiative recombination. The distribution of excitons among local potential energy minima at cryogenic temperatures is also affected by the current density due to the influence of Coulomb screening of the polarization field and the band-filling effect.

Impact of Localization on the Optical Properties of InGaN-Based Red Light-Emitting Diodes Grown on a Silicon Substrate

“…Given that the sidewall effect has less impact on the overall efficiency reduction in large-scale LEDs, the intrinsic recombination mechanism at low temperatures should be associated with the localization centers in the inner region of the mesa. The effective active region volume is reduced with decreasing temperature, and the concentration of carriers involved in recombination via effective channels in the active region can be written as n = EQE × I η normalinj η normalext B q V QW β where η ext and η inj denote the extraction efficiency and current injection efficiency, respectively, B denotes the radiative recombination coefficient, and β denotes the ratio of the effective active region volume, which is fitted to be 1 above 180 K. q denotes the elementary charge, I denotes the injected current, and V QW is the active region volume in the QWs. The effective active region volume can be affected by carrier injection, carrier distribution, and the thermal hopping of carriers.…”

“…Instead of reaching the maximum intensity at the lowest measured temperature, the temperature-dependent EL possesses a maximum at 80 K before experiencing thermal quenching. Such a negative-quenching behavior can be attributed to the thermal hopping of carriers from the shallow potential minima into the potential energy valley . Herein, the thermal activation energy of nonradiative recombination and the coefficient of negative quenching can be extracted from the following relation: I EL = 1 + N 0 ⁡ exp ( − normalΔ E normalk normalB T ) 1 + A 0 ⁡ exp ( − normalΔ E normalA normalk normalB T ) where A 0 and N 0 are dimensionless coefficients, k B is the Boltzmann constant, and E A and Δ E are the activation energy of thermal quenching and hopping, respectively.…”

“…where η ext and η inj denote the extraction efficiency and current injection efficiency, respectively, B denotes the radiative recombination coefficient, and β denotes the ratio of the effective active region volume, 16 which is fitted to be 1 above 180 K. q denotes the elementary charge, I denotes the injected current, and V QW is the active region volume in the QWs. The effective active region volume can be affected by carrier injection, carrier distribution, and the thermal hopping of carriers.…”

“…Such a negative-quenching behavior can be attributed to the thermal hopping of carriers from the shallow potential minima into the potential energy valley. 16 Herein, the thermal activation energy of nonradiative recombination and the coefficient of negative quenching can be extracted from the following relation: 34…”

“…Several approaches have been proposed to realize high-performance InGaN-based red LEDs, , some of which have made progress on these issues to a certain extent, particularly, the silicon substrate has been employed to improve the crystal quality and hole injection efficiency of InGaN-based red LEDs . Moreover, the potential inhomogeneity in the InGaN wells could lead to the local band-filling effect, thermal delocalization of carriers in localized states, and electron leakage and overflow. At low injections, carriers would be localized in the potential minima of the QWs, which is much more significant in InGaN-based red LEDs due to the increase in indium-content and severe alloy fluctuation in the multiquantum wells (MQWs). , However, until now, the effects of carrier localization on the temperature-dependent electroluminescence (EL) of InGaN-based red LEDs have not been fully investigated, although the understanding of this issue is significant for improving EQE.…”