Efficiency Droop In LEDs Research Articles

Increase in the Shockley–Read–Hall recombination rate in InGaN/GaN QWs as the main mechanism of the efficiency droop in LEDs at high injection levels

It is shown that the efficiency droop observed as the current through a GaN-based light-emitting diode increases is due to a decrease in the Shockley–Read–Hall nonradiative lifetime. The lifetime decreases with increasing current because a steadily growing number of traps in the density-of-states tails of InGaN/GaN quantum wells become nonradiative recombination centers upon the approach of quasi-Fermi levels to the band edges. This follows from the correlation between the efficiency droop and the appearance of negative differential capacitance, observed in the study. The correlation appears due to slow trap recharging via the trap-assisted tunneling of electrons through the n-type barrier of the quantum well and to the inductive nature of the diode-current variation with forward bias.

Direct Observation of the Biaxial Stress Effect on Efficiency Droop in GaN-based Light-emitting Diode under Electrical Injection.

Light-emitting diode (LED) efficiency has attracted considerable interest because of the extended use of solid-state lighting. Owing to lack of direct measurement, identification of the reasons for efficiency droop has been restricted. A direct measurement technique is developed in this work for characterization of biaxial stress in GaN-based blue LEDs under electrical injection. The Raman shift of the GaN E2 mode evidently decreases by 4.4 cm−1 as the driving current on GaN-based LEDs increases to 700 mA. Biaxial compressive stress is released initially and biaxial tensile stress builds up as the current increases with respect to the value of stress-free GaN. First-principles calculations reveal that electron accumulation is responsible for the stress variation in InxGa1−xN/GaN quantum wells, and then reduces the transition probability among quantum levels. This behavior is consistent with the measured current-dependent external quantum efficiency. The rule of biaxial stress-dependent efficiency is further validated by controlling the biaxial stress of GaN-based LEDs with different sapphire substrate thicknesses. This work provides a method for direct observation of the biaxial stress effect on efficiency droop in LEDs under electrical injection.

Hopping transport in the space-charge region of p-n structures with InGaN/GaN QWs as a source of excess 1/f noise and efficiency droop in LEDs

It is shown that the emission efficiency and the 1/f noise level in light-emitting diodes with InGaN/GaN quantum wells correlate with how the differential resistance of a diode varies with increasing current. Analysis of the results shows that hopping transport via defect states across the n-type part of the space-charge region results in limitation of the current by the tunneling resistance at intermediate currents and shunting of the n-type barrier at high currents. The increase in the average number of tunneling electrons suppresses the 1/f current noise at intermediate currents. The strong growth in the density of current noise at high currents, SJ ∝ J3, is attributed to a decrease in the average number of tunneling electrons as the n-type barrier decreases in height and width with increasing forward bias. The tunneling-recombination leakage current along extended defects grows faster than the tunneling injection current, which leads to emission efficiency droop.

基于混合型量子阱的GaN基垂直结构发光二极管性能

In order to solve the problem that Ga N-based vertical structure LEDs( VS-LEDs) suffer from efficiency droop under high current injection level,hybrid quantum wells( HQWs) with coupled quantum wells( CQWs) and normal quantum wells( NQWs) were employed. Compared to VSLEDs with NQWs,VS-LEDs with HQWs had better properties,wherein forward voltage reduced by0. 68 V and light-output power increased by 53. 0% at a forward current of 350 m A. Meanwhile,the relative external quantum efficiencies of VS-LEDs with NQWs and VS-LEDs with HQWs reduced to37. 7% and 67. 5% of their maximum,respectively. The results reveal that the efficiency droop of LEDs can become less severe by exploiting HQWs.

Auger effect identified as main cause of efficiency droop in LEDs

Auger effect identified as main cause of efficiency droop in LEDs

Mechanisms behind efficiency droop and degradation in InGaN/GaN LEDs

AbstractThe results of low frequency noise studies in power blue InGaN/GaN LEDs with external quantum efficiency 30‐45% classified by leakage current (LC) values before and after degradation are reported. The LC values at V< 2V integrally characterize electrical properties of extended defect system (EDS) that is typical for InGaN/GaN LEDs. The smaller concentration of extended defects and length of dilatation, dislocation boundaries, the lower LC value is. It has been demonstrated that non‐radiative recombination processes (NRRP) localized in extended defect system (EDS) contribute in efficiency droop of LEDs at j ≥ 10 A/cm2. The main characteristics of degradation processes such as: (1) increase in tunneling current values related with EDS, at U ≤ 2 V, (2) strong deterioration of p‐n junction rectification properties, (3) formation of conductive paths along EDS; (4) increase in non‐uniformity of current distribution, and (5) the formation of local overheating channels at j > 10 A/cm2 along EDS had been revealed. The NRRP are localized along EDS and can be considered as a general mechanism of efficiency droop and the ambiguous degradation process in power InGaN/GaN LEDs (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)