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An answer to LED droop?
A major obstacle to solid-state lighting is so-called "droop" - the way that
the efficiency falls at high drive currents. Its origins were unclear: until a
recent spate of papers appeared, that is...
The US Department of Energy's roadmap for white LEDs demands a rapid
improvement in LED efficacy per dollar. To stick to this roadmap,
manufacturing costs must fall whilst light output is increased – particularly at
the high drive currents required for general lighting applications.
But high efficacies at high drive currents are not something that we can
associate with today's GaN devices. That's because LED internal quantum
efficiency falls as current increases – a detrimental phenomenon known as
"droop".
Tackling droop starts with establishing a thorough understanding of its root
cause, but progress in this direction has been slow. Quite simply, until recently
nobody had provided a convincing explanation of the physics behind this
phenomenon.
However, this situation has now moved on, following the release of several
research papers last fall that offered different explanations. In these papers,
Philips Lumileds claimed that Auger recombination was the dominant cause for
droop; Fred Schubert and his team from Rensselaer Polytechnic Institute (RPI)
said carrier leakage was responsible; Yun-Li Charles Li and co-workers from
National Taiwan University stated that thin quantum wells were to blame; and
Bo Monemar from Linköping University pointed the finger at defects.
Lumileds backs thicker wells
Before Mike Krames and his colleagues from Lumileds identified Auger
recombination as the dominant cause for droop, they sought to rule out several
alternative explanations.
Temperature-related effects were discarded first. "If you take a diode and run
it in low-duty-factor pulsed operation you still observe this characteristic,"
explained Krames.
The team then considered whether droop could be caused by carrier leakage
from the active region, which would be driven by the electric field across the
device.
Early device results suggested that this might be the case, so Krames
attempted to verify the findings with resonant photo-excitation measurements
on a very simple structure – a thick InGaN layer (Appl. Phys. Lett. 91 141101).
This experiment was expected to show a linear increase of light output above
threshold.
"To our surprise we didn't see that – we saw the droop," remarked Krames.
"That, by itself, immediately rules out any sort of transport effect, any
interfacial effect, or any quantum-well effect that may be caused by the
presence of built-in fields."
According to Krames, these results actually showed that droop was a bulk
effect that occurs for epilayers grown in any orientation.
The Lumileds team went on to consider whether defects were causing the
droop by probing two samples with significantly different dislocation densities.
But high-dislocation and low-dislocation density samples had very similar
photoluminescence characteristics, ruling out the extended defect theory.
Krames says that this meant that the most likely cause of droop was a highorder, carrier-density dependent physical phenomenon, specifically Auger
recombination. This non-radiative process involves the transfer of energy from
electron-and-hole recombination to a third carrier.
Pushing out the efficiency peak
Armed with this knowledge, Krames' team went on to fabricate a range of LEDs
that are less susceptible to Auger recombination, which feature thicker wells to
reduce the carrier density in the active region. Specifically, 9 and 13 nm thick
wells with In0.14Ga0.86N and In0.16Ga0.84N compositions were used for the active
region (Appl. Phys. Lett. 91 243506, 2007 for details).
The researchers then compared the performance of these devices with
conventional designs featuring two or more 2.5 nm thick InGaN quantum wells.
Pulsed measurements with a 1% duty factor showed that the standard devices
produced a peak efficiency at 10 A/cm2, with droop kicking in as current
increased. In contrast, the LEDs with the thicker active region peaked at more
than 200 A/cm2, backing up the Auger theory.
The new designs emit at 430–445 nm and produce 20% more light at 1.8 A than
the company's state-of-the-art quantum well structures. They can also deliver
2.3 W at 2 A.
Figure 1
Krames' view that thicker wells can combat droop is backed up by research
from Yun-Li Charles Li and co-workers from National Taiwan University (see
Appl. Phys. Lett. 91 181113, 2007 for details).
These scientists have measured the efficiency droop in 460 nm InGaN LEDs and
have focused on quantum wells ranging from 0.6 to 1.5 nm thick. The droop is
virtually eliminated as the quantum well thickness increases to 1.5 nm, but
this improvement comes with the unwanted penalty of lower internal quantum
efficiency.
The elimination of droop is surprising in wells that are this thin, because
commercial LEDs suffer from droop and are typically 2–3 nm thick. Li is also
puzzled by this, but he says that droop could depend on the growth process for
the electron-blocking layer, or the overall growth rate.
Schubert goes for polarization-balancing
The RPI team, which has been working with Samsung Electro Mechanics
Corporation, Korea, has also been using optical measurements to search for the
origins of droop. However, they have arrived at a different conclusion – that
the effect results from carrier recombination outside the multi-quantum-well
active region under forward bias (Appl. Phys. Lett. 91 183507, 2007).
Figure 2
This team performed simulations that accurately matched the output of their
devices at a range of currents, before making additional calculations for LEDs
with polarization-matched electron-blocking layers and multiple quantum-wells
(figure 2).
"We're proposing a revolutionary design of the active region," explained
Schubert. According to the calculations, droop can be eliminated by replacing
the GaN/InGaN multiple quantum wells and the AlGaN electron blocking layers
with polarization-balancing quantum barriers and blocking layers made from
AlGaInN.
The scientists are now trying to produce this structure, but it's quite a
challenge, as the barriers and blocking layers require well-controlled aluminum
and indium content.
Schubert's group has also been working with Sandia National Labs on a study
that examines the relationship between defects and droop (Appl. Phys. Lett.
91 231114, 2007).
Measurements show that conventional 440 nm InGaN LEDs with high-defect
densities do not suffer from the droop that plagues lower-defect density
equivalents. However, the trade-off is a low overall efficiency. "Obviously, that
isn't the right way to solve the droop problem," said Schubert.
Meanwhile, Linköping's Bo Monemar is backing yet another explanation for
droop – carrier localization at defects (Appl. Phys. Lett. 91 181103, 2007). In
particular, he points a finger at phonon-assisted transport of holes via
tunneling at defect sites and threading dislocations that can induce a parasitic
tunneling current.
So who's right?
What's clear is that these research teams have differing views and they can't all
be right. So what do they make of each other's theories?
Krames remains confident, saying that none of the other droop explanations
have changed his view that Auger recombination is the dominant factor.
While Monemar argues that the improved droop observed in the best non-polar
LEDs is thanks to low epilayer defect densities (see Massive power boost for
non-polar GaN LEDs), Krames believes that it actually results from the thicker
quantum wells used. According to him, these thicker wells increase the
recombination volume, which reduces carrier density, and make three-body
interactions required for Auger recombination far less likely.
The Lumileds man suspects that the problem with the RPI result is that
insufficiently high excitation densities were used in those optical experiments.
"That's a trap that we also fell into initially," admitted Krames, "before we
invested a great deal of time and money in building a completely new optical
set-up with an [high-power] alexandrite laser."
Schubert, however, is concerned that recent Lumileds papers employ a new
quantity, "recombination thickness", which is used to determine the carrier
concentration in the quantum well. He believes that the way this quantity is
employed could lead to an overestimation of both carrier concentration and
Auger recombination.
These differences in opinion do not seem to be generating any ill-feeling,
however. "We have a wonderful situation at this time," said Schubert. "Many
different mechanisms have been proposed and I believe that the scientific
community will be smart enough to find out what are the dominant causes and
the minor effects."
Let's hope that he's right, for droop-free LEDs could be the key breakthrough to
unlocking the general lighting market.
About the author
Richard Stevenson is features editor of Compound Semiconductor magazine.