Abstract
The effect of strongly-imbalanced carrier concentration and mobility on efficiency droop is studied by comparing the onset voltage of high injection, the onset current density of the droop, and the magnitude of the droop, as well as their temperature dependence, of GaInN-based blue and green light-emitting diodes (LEDs). An n-to-p asymmetry factor is defined as σn/σp, and was found to be 17.1 for blue LEDs and 50.1 for green LEDs. Green LEDs, when compared to blue LEDs, were shown to enter the high-injection regime at a lower voltage, which is attributed to their less favorable p-type transport characteristics. Green LEDs, with lower hole concentration and mobility, have a lower onset current density of the efficiency droop and a higher magnitude of the efficiency droop when compared to blue LEDs. The experimental results are in quantitative agreement with the imbalanced carrier transport causing the efficiency droop, thus providing guidance for alleviating the phenomenon of efficiency droop.
Highlights
Light-emitting diodes (LEDs) in a GaInN material system suffer from a reduction in efficiency at high injection currents, which is known as the “efficiency droop.” An understanding of the physical origin of the efficiency droop is critical for future progress in light-emitting diodes (LEDs), when operated at high current densities
In our samples, we found that the JOnset-of-droop in the green LED is lower than that of the blue LED, indicating that the effect of the asymmetric carrier concentration and mobility in the green LED on the JOnset-of-droop is stronger than the effect of SRH recombination
We found experimental evidence that the imbalance in carrier concentration and mobility is closely related to the efficiency droop
Summary
Light-emitting diodes (LEDs) in a GaInN material system suffer from a reduction in efficiency at high injection currents, which is known as the “efficiency droop.” An understanding of the physical origin of the efficiency droop is critical for future progress in LEDs, when operated at high current densities. There have been several reports on the lack of hole injection and electron leakage being caused by a much lower concentration and mobility of holes compared to those of electrons, leading the authors to the conclusion that electron leakage contributes to the efficiency droop [14,15,16,17]. We investigate the effect of imbalanced carrier concentration and mobility in GaInN-based pn-junction diodes on efficiency droop by comparing GaInN-based green and blue LEDs. Compared with blue LEDs, a higher indium (In) content is required in the MQW active region of green LEDs. It is well known that GaInN MQWs with a high In content become unstable at high epitaxial growth temperatures, because of the high volatility of In and the tendency to form InN clusters [18,19].
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