Onset Of Efficiency Droop Research Articles

Recombination efficiency in c-plane (In,Ga)N/GaN quantum wells: saturation of localisation sites versus Auger–Meitner recombination

Abstract Light emitting diodes based on c-plane (In,Ga)N/GaN quantum wells (QWs) can have >90% emission efficiency at modest current densities but this drops significantly at higher excitation, an effect known as efficiency droop that limits device efficacy at high brightness. Several explanations for this have been proposed including the saturation of carrier localisation sites at high excitation densities, resulting in a greater exposure of carriers to defects and hence a significant increase in the associated non-radiative recombination processes. Here, power- and temperature-dependent photoluminescence spectroscopy of c-plane (In,Ga)N/GaN QWs is used to investigate the relationship between the saturation of localised states and emission efficiency. For the samples studied, we find that the saturation of localised sites broadly coincides with the onset of efficiency droop. However, it is also found that as the localised states saturate with increasing excitation, the relative contribution of defect-associated non-radiative processes to overall recombination decreases rather than increases. Based on these observations and on modelling of recombination processes in the QW, it is concluded that the saturation of localised states does not significantly contribute to the reduction in emission efficiency at high excitation. Our studies rather suggest that defect-related non-radiative recombination is out-competed by radiative and Auger–Meitner recombination at the carrier densities required for saturation.

Efficiency droop in zincblende InGaN/GaN quantum wells.

The decrease in emission efficiency with increasing drive current density, known as 'droop', of c-plane wurtzite InGaN/GaN quantum wells presently limits the use of light-emitting diodes based on them for high brightness lighting applications. InGaN/GaN quantum wells grown in the alternative zincblende phase are free of the strong polarisation fields that exacerbate droop and so were investigated by excitation-dependent photoluminescence and photoreflectance studies. Polarisation-resolved measurements revealed that for all excitation densities studied the emission from such samples largely originates from similar microstructures or combinations of microstructures that form within the quantum well layers. Emission efficiency varies significantly with excitation at 10 K showing that non-radiative recombination processes are important even at low temperature. The onset of efficiency droop, as determined by photomodulated reflection measurements, occurred at a carrier density of around 1.2 × 1020 cm-3 - an order of magnitude greater than the value reported for a reference wurtzite quantum well sample using the same method. The high carrier density droop onset combined with the much shorter carrier lifetime within zincblende InGaN/GaN quantum wells indicate they have the potential to effectively delay efficiency droop when used in GaN based light-emitting diodes. However, the material quality of the quantum well layers need to be improved by preventing the formation of microstructures within these layers, and the importance of the role played by non-radiative centres in the QW layer needs to be elucidated, to fully realise the material's potential.

Effect of Micron-scale Photoluminescence Variation on Droop Measurements in InGaN/GaN Quantum Wells

Micro-photoluminescence maps reveal micron-scale spatial variation in intensity, peak emission energy and bandwidth across InGaN/GaN quantum wells. To investigate the effect of this spatial variation on measurements of the dependence of emission efficiency on carrier density, excitation power-dependent emission was collected from a bright and dark region on each of blue-and green emitting samples. The onset of efficiency droop was found to occur at a greater carrier density in the dark regions than in the bright, by factors of 1.2 and 1.8 in the blue and green-emitting samples, respectively. By spatially integrating the emission from progressively larger areas, it is also shown that collection areas greater than ∼50 μm in diameter are required to reduce the intensity variation to less than 10%.

Carrier Dynamics in Al‐Rich AlGaN/AlN Quantum Well Structures Governed by Carrier Localization

The carrier dynamics of Al‐rich AlGaN/AlN quantum well (QW) structures in the presence of strong carrier localization is reported. Excitation density‐dependent photoluminescence (PL) measurements at low temperatures reveal a clear correlation between the onset of efficiency droop and the broadening of the time‐integrated PL spectra. While the droop onset is heavily impacted by the localization strength, the PL emission broadening is observed almost exclusively on the high energy side of the emission spectrum. Spectrally resolved PL decay transient measurements reveal a strong dependency of the carrier lifetimes on the emission photon energy across the spectrum, consistent with a distribution of localized states, as well as on the temperature, depending on the localization strength of the investigated structure. The characteristic “S”‐shaped temperature dependence of the PL emission energy is shown to be directly correlated to the thermal redistribution of carriers between localized states. Based on these findings, the role of carrier localization in the recombination processes in AlGaN QW structures is underlined and its implications for efficiency droop are discussed.

Impact of carrier localization and diffusion on photoluminescence in highly excited cyan and green InGaN LED structures

Localization of charge carriers is of crucial importance in InGaN light emitting devices since it governs carrier transport and ensures high radiative efficiency. In this work, we observe the dynamics of carrier localization from an untypical redshift of photoluminescence spectra. We investigate two light emitting diode structures grown by MOCVD on c-plane sapphire and emitting at 500nm (cyan) or 530nm (green). For the study, we employ photoluminescence, differential transmission, and light induced transient gratings techniques. We observe non-monotonous dynamics of photoluminescence peak position: a blueshift at short delay times (<10ns) is later replaced by a redshift, which increases both with time and excitation. The redshift values as large as 30meV in the cyan and 20meV in the green structure were observed at 40ns delay. We attribute this redshift to density-dependent and diffusion-driven carrier redistribution between the shallower and deeper localized states. The carrier delocalization with increasing density is confirmed by growing diffusivity of carriers with excitation. We also demonstrate a correlation between the growth of diffusion coefficient and the onset of efficiency droop.

Investigating efficiency droop in InGaN/GaN quantum well structures using ultrafast time‐resolved terahertz and photoluminescence spectroscopy

AbstractThe mechanisms governing efficiency droop in an In0.18Ga0.82N/GaN multiple quantum well structure were investigated using a combination of ultrafast time‐resolved terahertz and photoluminescence spectroscopy. From excitation fluence dependent studies, a reduction in the room temperature photoluminescence efficiency to 3% of its maximum value was observed for an excitation fluence of 0.96 mJcm‐2. A correlation was found between the onset of efficiency droop and the emergence of a peak on the high‐energy side of the quantum well emission with a 1/e decay time of 19.6 ps. These characteristics were attributed to the saturation of localised states and the population of higher energy delocalised states. Time‐resolved studies revealed different scaling behaviours between the terahertz and photoluminescence decay dynamics, suggesting that the saturation of localised hole states may be playing a part in the onset of efficiency droop. (© 2016 The Authors. Phys. Status Solidi C published by WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

High excitation density recombination dynamics in InGaN/GaN quantum well structures in the droop regime

AbstractWe report on the low temperature time‐integrated and time‐resolved photoluminescence properties of an (0001) InGaN/GaN multiple quantum well structure. From excitation power dependent studies, we observe a clear correlation between the onset of efficiency droop and the broadening of the time‐integrated luminescence spectra. Time‐resolved spectroscopy reveals that this spectral broadening is associated with a large and rapid red‐shift of the spectrum with increasing time after the excitation pulse. This dynamic shift of the spectrum also modifies the form of the luminescence transients, yet the spectrally integrated luminescence dynamics have the same non‐exponential shape as those obtained under much lower excitation levels. This observation is consistent with the emission originating from the recombination of independently localised carriers, even under very high excitation levels. Nevertheless, at these high excitation levels we propose that there also is a population of weakly localised carriers, probably electrons. We suggest that these more mobile carriers are effective in screening the built‐in electric field across the quantum well as well as being more susceptible to non‐radiative loss. The collapse in this population, due to radiative and non‐radiative recombination, restores the built‐in polarisation induced field and results in the observed time dependent redshift of the luminescence spectrum. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Measurement of electron overflow in 450 nm InGaN light-emitting diode structures

Test structures were developed to experimentally measure the presence of electron overflow in light-emitting diodes (LEDs) under typical bias conditions. These test structures are comprised of a standard LED structure with an extra Mg-doped quantum well inserted on the p-type side of the electron blocking layer. Electrons escaping the active region recombine in the extra quantum well and the corresponding photon emission is observed. No electron overflow was observed at low current densities. At intermediate current densities where efficiency droop occurs, overflow was observed and increased with increasing current density. The onset of electron overflow occurred at slightly lower current densities than the onset of efficiency droop. Auger-assisted overflow, a by-product of the Auger process, is considered in addition to traditional overflow mechanisms.