InGaN light-emitting diodes: Efficiency-limiting processes at high injection

Abstract

The authors discuss a relatively comprehensive theoretical and experimental study aimed on unveiling the dominant efficiency loss mechanism at high injection levels in InGaN light-emitting diodes(LEDs), which still limits their application for general lighting despite the breathtaking performance demonstration. A large body of theoretical and experimental data ascribes the observed efficiency loss to overflow of hot electrons aggravated by nonuniform distribution of carriers in the active region as the primary origin of the efficiency droop-phenomenon, but Auger recombination has also been invoked as the genesis of the efficiency loss. The electron overflowand the associated efficiency loss can be reduced substantially by inserting, in the n-side of the InGaN active region, an InGaN stair-case electron injector (SEI) with a step-like increased indium composition to operate as an “electron cooler.” In contrast to electron-blocking layer usually employed to prevent the electron leakage from the active region, the SEI does not impede hole injection due to the absence of valence band offset with p-GaN. Moreover, SEI does not generate piezoelectric polarizationfield in addition to differential spontaneous polarization field that pulls down the conduction band at the AlGaN/GaN interface aggravating the electron rollover. In terms of the active region design, owing to their high three-dimensional density of states, it is argued that double heterostructures (DHs) are more attractive for general-lighting LEDs than necessarily quantum wells. The authors demonstrate that DH-based LED active regions,particularly wide ones and those composed of multiple DHs separated by thin (3 nm)In0.06Ga0.94N barriers of reduced barrier height, meant to allow efficient hole transport across the active regions, naturally act as an electron cooler, thus considerably reducing the electron overflow at high injection. However, a wide separation of electron and hole distribution functions in DHs wider than 6 nm substantially reduces the radiative recombination efficiency at injection current densities below∼200 A/cm2. Consequently, the LEDs with dual 6 nm and quad (4×) 3 nm DHs separated by 3-nm In0.06Ga0.94N barriers exhibit the highest external quantum efficiency with substantially reduced efficiency degradation at injection current densities of special interest for low-voltage general-lighting applications. The authors conclude that, for achieving the highest possible LED efficiency, it is imperative that optimum the SEI and the active region should be designed to operate in unison.

The efficiency challenge of nitride light‐emitting diodes for lighting

“…Carrier overshoot [11], i.e., non-capture of the electrons injected from the n-side, is avoided by the use of multiple quantum wells (QWs), where electrons overshooting one QW are captured by the next ones. Electron leakage into the p-GaN layer surrounding the lightemitting QWs is avoided by using wider bandgap (AlGaN) electron blocking layers.…”

On the internal quantum efficiency for InGaN/GaN light-emitting diodes grown on insulating substrates

“…More importantly, Li et al, by the electron holography, also measure and show a reduced electrostatic potential for the LED device with the InGaN insertion layer . However, a most recent physical model has been developed by Ni et al , Zhang et al , Li et al , Avrutin et al , Zhang et al , and Chang et al that a reduced electron leakage is caused by the phonon‐electron scattering taking place in the InGaN insertion layer. During the phonon‐scattering process, the electrons lose the energy of 92 meV, such that electrons become “cold” and the InGaN insertion layer functions as the “electron cooler (EC).” In addition, the study by Zhang et al further reveals the impact of the polarization‐induced electric field in the [0001]‐oriented InGaN EC layer on the electron energy .…”

Tailoring of Energy Band in Electron-Blocking Structure Enhancing the Efficiency of AlGaN-Based Deep Ultraviolet Light-Emitting Diodes

“…To counteract the polarization effect on EBL, several approaches have been suggested for blue LEDs, such as gradual or tapered EBL [4], [5], polarization-matched AlInGaN EBL [1], [6], and polarizationreversed electron-blocking structure [7], [8], etc. Besides, some strategies aimed to decelerate electron-transport or accelerate hole-transport in the active region, such as asymmetric multiquantum wells [9], and stair-case electron injector [10], [11], etc. Similar concepts could also be used for AlGaN DUV LEDs but require further study or optimization.…”