Towards quantification of the crucial Impact of auger recombination for the Efficiency droop in (AlInGa)N Quantum well structures

Recent experimental investigations on the reduction of internal quantum efficiency with increasing current density in (AlInGa)N quantum well structures show that Auger recombination is a significant contributor to the so-called "droop" phenomenon. Using photoluminescence (PL) test structures, we find Auger processes are responsible for at least 15 % of the measured efficiency droop. Furthermore, we confirm that electron-electron-hole (nnp) is stronger than electron-hole-hole (npp) Auger recombination in standard LEDs. The ratio of respective Auger coefficients is determined to be in the range 1 < C-nnp = C-npp <= 12. This asymmetry is shown to limit the detection efficiency of Auger processes in our PL-based approach. (C) 2016 Optical Society of Americ

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.TU Berlin, Open-Access-Mittel – 2020BMBF, 03ZZ0134A, Zwanzig20 - Advanced UV for Life - Verbundvorhaben: UV Power; TP1: Entwicklung von UVC Hochleistungsleuchtdioden um 280 nmDFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement

Ageing of InGaN-based LEDs: Effects on internal quantum efficiency and role of defects

This paper describes the degradation of InGaN-based LEDs submitted to constant current stress; based on combined electroluminescence, photoluminescence and deep-level transient spectroscopy we show that: (i) when submitted to constant current stress, LEDs can show a measurable decrease in the optical power, which is more prominent in the low current regime; (ii) the decrease in optical power is strongly correlated to the increase in the Shockley-Read-Hall recombination coefficient A, as estimated by differential lifetime measurements; (iii) stress induces the increase in the concentration of a trap level, with activation energy between 0.6 and 0.7 eV, which is supposed to be located next to/within the active region. The results suggest that the optical degradation can be ascribed to the increase in non-radiative recombination, rather than to a decrease in carrier injection efficiency. \ua9 2015 Elsevier Ltd

Influence of the Growth Substrate on the Internal Quantum Efficiency of Algan/Aln Multiple Quantum Wells Governed by Carrier Localization

International audienceThe influence of the growth substrate on the internal quantum efficiency (IQE) of deep ultraviolet (UV) light emitting diodes is studied. Two nominally identical Al-rich AlGaN/AlN multi-quantum-well (MQW) structures grown by metalorganic vapour phase epitaxy (MOVPE) on different substrates were investigated. The first MQW structure was grown on a native AlN substrate, while the second one was deposited on an AlN template on sapphire. By the combination of atomic force microscopy (AFM), photoluminescence (PL) and cathodoluminescence (CL) spectroscopy, we demonstrate that the dislocation-mediated spiral growth of MQWs on sapphire results in the more efficient localization of carriers. This effect helps to prevent non-radiative carrier recombination at point defects, improving the IQE of the structure. I

Point defect-induced UV-C absorption in aluminum nitride epitaxial layers grown on sapphire substrates by metal-organic chemical vapor deposition

Herein, the optical properties of aluminum nitride (AlN) epitaxial layers grown on sapphire substrates by metal-organic chemical vapor deposition (MOCVD) are reported. The structures investigated in this study are grown at highly different degrees of supersaturation in the MOCVD process. In addition, both pulsed and continuous growth conditions are employed and AlN is deposited on nucleation layers favoring different polarities. The samples are investigated by photoluminescence (PL), photoluminescence excitation (PLE), and absorption spectroscopy and are found to vary significantly in absorption and emission characteristics. Two distinct absorption bands in the UV-C spectral range are observed and examined in greater detail, with either giving rise to a significant absorption coefficient of around 1000 cm−1. The corresponding defect transitions are identified by PL spectroscopy. Combined with secondary-ion mass spectrometry (SIMS) measurements, these absorption bands are allocated to the incorporation of carbon and oxygen impurities, depending on the applied growth conditions. Furthermore, similarities with other epitaxial growth techniques serving as basis for UV-C applications are highlighted. These results are highly relevant for a better understanding of absorption issues in AlN templates grown by various deposition techniques. In addition, consequences for the growth of efficient UV-C devices by MOCVD on sapphire substrates are outlined.BMWi, 16IPCEI623, Errichtung und Ausstattung einer Produktionsumgebung für neuartige optoelektronische BauelementeBMBF, 03ZZ0134A, Zwanzig20 - Advanced UV for Life - Verbundvorhaben: UV Power; TP1: Entwicklung von UVC Hochleistungsleuchtdioden um 280 nmDFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, BauelementeDFG, 390837967, EXC 2123: QuantumFrontiers - Licht und Materie an der Quantengrenz

Adiponectin Lowers Glucose Production by Increasing SOGA

Adiponectin is a hormone that lowers glucose production by increasing liver insulin sensitivity. Insulin blocks the generation of biochemical intermediates for glucose production by inhibiting autophagy. However, autophagy is stimulated by an essential mediator of adiponectin action, AMPK. This deadlock led to our hypothesis that adiponectin inhibits autophagy through a novel mediator. Mass spectrometry revealed a novel protein that we call suppressor of glucose by autophagy (SOGA) in adiponectin-treated hepatoma cells. Adiponectin increased SOGA in hepatocytes, and siRNA knockdown of SOGA blocked adiponectin inhibition of glucose production. Furthermore, knockdown of SOGA increased late autophagosome and lysosome staining and the secretion of valine, an amino acid that cannot be synthesized or metabolized by liver cells, suggesting that SOGA inhibits autophagy. SOGA decreased in response to AICAR, an activator of AMPK, and LY294002, an inhibitor of the insulin signaling intermediate, PI3K. AICAR reduction of SOGA was blocked by adiponectin; however, adiponectin did not increase SOGA during PI3K inhibition, suggesting that adiponectin increases SOGA through the insulin signaling pathway. SOGA contains an internal signal peptide that enables the secretion of a circulating fragment of SOGA, providing a surrogate marker for intracellular SOGA levels. Circulating SOGA increased in parallel with adiponectin and insulin activity in both humans and mice. These results suggest that adiponectin-mediated increases in SOGA contribute to the inhibition of glucose production