Interplay of point defects, extended defects, and carrier localization in the efficiency droop of InGaN quantum wells light-emitting diodes investigated using spatially resolved electroluminescence and photoluminescence

Abstract

We perform both spatially resolved electroluminescence (SREL) as a function of injection current and spatially resolved photoluminescence (SRPL) as a function of excitation power on InGaN quantum well blue light-emitting diodes to investigate the underlying physics for the phenomenon of the external quantum efficiency (EQE) droop. SREL allows us to study two most commonly observed but distinctly different droop behaviors on a single device, minimizing the ambiguity trying to compare independently fabricated devices. Two representative devices are studied: one with macroscopic scale material non-uniformity, the other being macroscopically uniform, but both with microscopic scale fluctuations. We suggest that the EQE–current curve reflects the interplay of three effects: nonradiative recombination through point defects, carrier localization due to either In composition or well width fluctuation, and nonradiative recombination of the extended defects, which is common to various optoelectronic devices. By comparing SREL and SRPL, two very different excitation/detection modes, we show that individual singular sites exhibiting either particularly strong or weak emission in SRPL do not usually play any significant and direct role in the EQE droop. We introduce a two-level model that can capture the basic physical processes that dictate the EQE–current dependence and describe the whole operating range of the device from 0.01 to 100 A/cm2.

Defects dynamics during ageing cycles of InGaN blue light-emitting diodes revealed by evolution of external quantum efficiency - current dependence

“…Defects growth, Ohmic contact degradation, and darkening in the encapsulation can quench the luminous intensity slowly. Several mechanisms have been considered to be responsible for the EQE droop, such as Auger recombination [3,4], electron leakages [5,6], insufficient hole injection [7,8], and defect-related mechanisms [9][10][11][12]. In our recent work, we associated the EQE droop with extended defects (EDs).…”

Three dimensional characterization of GaN-based light emitting diode grown on patterned sapphire substrate by confocal Raman and photoluminescence spectromicroscopy

“…The efficiency droop behavior in GaN-based LEDs differs with the level of effective band gap fluctuation. This band gap fluctuation not only enhances the localization effect, which increases the carrier wave function overlap and prevent carrier leakage through dislocations, but also improves the internal quantum efficiency (IQE)20. Therefore the choice of substrate is expected to play a significant role in determining many of the LEDs’ characteristics including dislocation, strain, and indium distribution in the MQWs.…”

“…Therefore the choice of substrate is expected to play a significant role in determining many of the LEDs’ characteristics including dislocation, strain, and indium distribution in the MQWs. The PL emission pattern for GaN-based LEDs has been reported to be quite non-uniform by several groups202122. It was generally believed that the non-uniformity influences the LED efficiency.…”

Temperature-dependent efficiency droop in AlGaN epitaxial layers and quantum wells

“…It is expected to be less pronounced in AlGaN due to the anticipated reduction of Auger coefficient, both direct and phonon-or impurity-assisted, with increasing band gap. 11,12 On the other hand, the connection between carrier localization and efficiency droop has been demonstrated in InGaN quantum well structures, 9,13,14 AlGaN epilayers, 15 and AlGaN quantum wells. 16 Recently, we proposed the ratio k B T/σ between the carrier thermal energy k B T and localization parameter σ (i.e.…”

“…Thus, the distance a carrier can move in real space during its lifetime is increased, and the nonradiative recombination at extended defects starts to dominate. 14,18 Moreover, at high carrier densities, the carrier mobility is additionally enhanced due to carrier degeneracy. 19 The carrier-density-enhanced recombination at the extended defects with distances between them larger than the average distance the carrier can travel during its lifetime at low temperature but comparable at room temperature is consistent with the supposed recombination at growth domains observed in AlGaN epilayers by scanning near-field optical microscopy.…”