research review
Saturation of radiative recombination gets the blame for LED droop
As LEDs plunge to cryogenic temperatures,droop kicks in at lower drive currents due to diminished utilization of the active region.
A TEAM of Korean researchers is proposing an alternative explanation for variations in LED droop with temperature. The researchers account for droop, the decline in device efficiency as the current is cranked up, with a theory involving carrier overflow, reduced effective active volume and saturation of the radiative recombination rate.
The researchers from Hanyang University, Korea, started by measuring the internal quantum efficiency (IQE) of blue and green InGaN LEDs at drive currents ranging from less than 1 µA to just below 10 mA at temperatures from 50K to 300K.
At cryogenic temperatures LEDs suffered from severe droop, attributed to carrier overflow. It is claimed that the degree of overflow is governed by the extent of the reduction in effective active volume and the subsequent saturation of the radiative recombination rate.
Measurements were made on blue and green LEDs with indium compositions of 20 percent and 35 percent and chip sizes of 350 µm by 430 µm and 400 µm by 400 µm, respectively. These devices were placed in a closed-cycle cryostat and temperatures measured at an aluminium plate that mounts to the TO package of the LED.
“The thermal resistance from the aluminium plate to the LED chip is not considered high, and even if there is a temperature gradient from the aluminium plate to the active layer of the LED, it shouldn’t be high for the given current level that the efficiency droop starts to occur,” says corresponding author Jong-In Shim from Hanynag University.
Shim points out that for temperatures below 100 K, droop kicks in below 0.1 mA, so power dissipating at the chip will be very small. He believes, therefore, that any differences between the actual and recorded temperatures for the LED will not be big enough to have impact on the conclusions that have been drawn from these measurements.
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www.compoundsemiconductor.net June 2012
One of the findings of these measurements was that when the reduction in the temperature of blue and green LEDs was small, IQE increased and there was no significant change in the onset of droop. Shim and his co-workers believe that this occurs because decreases in temperature lead to a monotonic reduction in the number of defects in the quantum well that undergo non-radiative recombination.
When temperature falls further – below 200 K for blue LEDs and less than 250 K for their green siblings – device behaviour changes dramatically, with droop kicking in at a far lower drive current.
This led the team to draw two conclusions: The dominant droop mechanism undergoes a qualitative change at low temperatures; and Auger recombination is not a major cause of droop, because it cannot account for severe droop at low temperatures.
To shed more light on the cause of droop, the team measured electroluminescence spectra at various temperatures. In both LEDs, reductions in temperature led to the emergence of an emission peak around 400 nm (see Figure). This peak is claimed to originate from the overflow of electrons to the p-GaN cladding layer, where they undergo a recombination process involving a magnesium acceptor level.
Shim and his colleagues believe that electron overflow is the dominant cause of droop at low temperatures. And they argue that the degree of this overflow gets more severe as the temperature plummets because the radiative recombination rate saturates at lower currents, and the non- radiative recombination rate reduces.
Saturation of the radiative recombination rate is thought to occur at lower currents, due to a reduction in the effective active volume. This volume diminishes as temperature falls, due to a reduction in the utilization of the active region, which is associated with degradation in carrier transport. Specifically, the combination of inferior hole transport at lower temperatures and the activation of
Cooling unveils electron overflow
fewer holes leads to a reduction in utilization of the wells, trimming the effective active volume. The upshot is an earlier saturation of the radiative recombination rate, and a greater overflow of electrons into p-GaN.
Measurements by the researchers also suggest that electron overflow is more severe in green LEDs than blue ones. One reason for this is that the green variants have greater indium, leading to wells with greater inhomogeneity and more indium clustering.
Competing theories Shim and his co-workers are not alone in studying the influence of temperature on LED efficiency and using these measurements to draw conclusions on the origins of droop. For example, Fred Schubert’s group – which has measured the efficiency of blue LEDs at various currents and temperatures – claims that droop is caused by transport issues that stem from asymmetry in electron and hole concentrations, plus differences in carrier injection (see Appl. Phys. Lett. 99 25115 (2011)).
“It is true that transport is one of the factors that affects efficiency droop, but we
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