hn-classics/_stories/2009/8421518.md

---
created_at: '2014-10-07T14:59:34.000Z'
title: The LED's Dark Secret (2009)
url: http://spectrum.ieee.org/semiconductors/optoelectronics/the-leds-dark-secret
author: spectruman
points: 79
story_text: ''
comment_text: 
num_comments: 45
story_id: 
story_title: 
story_url: 
parent_id: 
created_at_i: 1412693974
_tags:
- story
- author_spectruman
- story_8421518
objectID: '8421518'
year: 2009

---
![](/img/520659-1367523519626.jpg) Illustration: Bryan Christie Design

**The blue light-emitting diode,** arguably the greatest optoelectronic
advance of the past 25 years, harbors a dark secret: Crank up the
current and its efficiencies will plummet. The problem is known as
droop, and it’s not only puzzling the brightest minds in the field, it’s
also threatening the future of the electric lighting industry.

Tech visionaries have promised us a bright new world where cool and
efficient white LEDs, based on blue ones, will replace the wasteful
little heaters known as incandescent lightbulbs. More than a dozen
countries have already enacted legislation that bans, or will soon ban,
incandescent bulbs. But it’s hard to imagine LEDs dislodging
incandescents and coming to dominate the world electric lighting
industry, unless we can defeat droop.

In flashlights, in backlights for screens in cellphones and now
televisions, and in a bunch of other applications, white LEDs already
constitute a multibillion-dollar market. But that’s just a US $5 billion
niche compared to the overall lighting industry, whose sales next year
should reach $100 billion, according to the market research firm Global
Industry Analysts. The trick will be to make LEDs turn electricity into
light efficiently enough to offset their relatively high cost—roughly 16
cents per lumen, at lightbulb-type brightness, as opposed to about 0.1
cents or less for incandescents.

Look at the competition and you’d think the job was easy. Today’s
garden-variety incandescent bulbs aren’t much different from the ones
Thomas Edison sold more than a century ago. They still waste 90 percent
of their power, delivering roughly 16 lumens per watt. Fluorescent tubes
do a lot better, at more than 100 lm/W, but even they pale next to the
best LEDs. The current state-of-the-art white LED pumps out around 250
lm/W, and there’s no reason why that figure won’t reach 300 lm/W.

Unfortunately, these LEDs perform at their best only at low power—the
few milliamps it takes to backlight the little screen on your mobile
phone, for instance. At the current levels needed for general lighting,
droop kicks in, and down you go, below 100 lm/W.

[![LED
Architecture](/img/520290-620px-1412782576788.jpg)](/img/520290-1412782534145.jpg)

 

Illustration: Bryan Christie Design **LED Architecture:** At the heart
of every white LED is a semiconductor chip made from nitride-based
materials. The chip is traditionally positioned on top of the cathode
lead. Applying several volts across this device makes the chip emit blue
light. Passing the light through a yellow phosphor yields white light.
Modern, high-power LEDs are variants of this architecture, featuring
more complex packages for superior thermal management.

**The first-ever report of** light emission from a semiconductor was by
the British radio engineer Henry Joseph Round, who noted a yellowish
glow emanating from silicon carbide in 1907. However, the first devices
at all similar to today’s LEDs arrived only in the 1950s, at Signal
Corps Engineering Laboratories, at Fort Monmouth, in New Jersey.
Researchers there fabricated orange-emitting devices; green, red, and
yellow equivalents followed in the ’60s and ’70s, all of them quite
inefficient.

The great leap toward general lighting came in the mid-1990s, when Shuji
Nakamura, then at Nichia Corp., in Tokushima, Japan, developed the first
practical bright-blue LED using nitride-based compound semiconductors.
(Nakamura’s achievement won him the 2006 Millennium Technology Prize,
the approximate equivalent in engineering of a Nobel Prize.) Once you’ve
got blue light, you can get white by passing the blue rays through a
yellow phosphor. The phosphor absorbs some of the blue and reradiates it
as yellow; the combination of blue and yellow makes white.

All LEDs are fabricated as aggregated sections, or regions, of different
semiconductor materials. Each of these regions plays a specific role.
One region serves as a source of electrons; it consists of a crystal of
a compound semiconductor into which tiny amounts of an impurity, such as
silicon, have been introduced. Each such atom of impurity, or dopant,
has four electrons in its outer shell, compared with the three in an
atom of gallium, aluminum, or indium. When a dopant takes a place that
one of these other atoms would normally occupy, it adds an electron to
the crystalline lattice. The extra electron moves easily though the
crystal, acting as a carrier of negative charge. With this surfeit of
negative charges, such a material is called n-type.

At the opposite end of the LED is a region of p-type material, so called
because it has excess positive-charge carriers, created by doping with
an element such as zinc or magnesium. These metals are made up of atoms
with only two electrons in their outer shell. When such an atom sits in
place of an atom of aluminum, gallium, or a chemically similar element
(from group III in the periodic table), the lattice ends up an electron
short. That vacancy behaves as a positive charge, moving throughout the
crystal like the missing tile in a sort-the-number puzzle. That mobile
vacancy is called a hole.

In the middle of the sandwich are several extraordinarily thin layers.
These constitute the active region, where light is produced. Some layers
made of one semiconducting material surround a central layer made of
another, creating a “well” just a few atoms thick—a trench so confined
that the laws of quantum mechanics rule supreme. When you inject
electrons and holes into the well by applying a voltage to the n - and
p-type regions, the two kinds of charge carriers will be trapped,
maximizing the likelihood that they will recombine. When they do, a
photon pops out.

To make an LED, you must grow a series of highly defined semiconductor
layers on a thin wafer of a crystalline material, called a substrate.
The substrate for red, orange, and yellow LEDs is gallium arsenide,
which works wonderfully because its atoms are spaced out identically to
those of the layers built on top of it. Hardly any mechanical strain
develops in the semiconductor’s crystalline lattice during fabrication,
so there are very few defects, which would quench light generation.

Unfortunately, blue and green LEDs lack such a good platform. They’re
called nitride LEDs because their fundamental semiconductor is gallium
nitride. The n-type gallium nitride is doped with silicon, the p-type
with magnesium. The quantum wells in between are gallium indium nitride.
To alter the light color emitted from green to violet, researchers vary
the gallium-to-indium ratio in the quantum wells. A little indium
produces a violet LED; a little more of it produces green.

Such LEDs would ideally be manufactured on gallium nitride substrates.
But it has proved impossible to grow the large, perfect crystals of
gallium nitride that would be necessary to make such wafers. Unipress,
of Warsaw, the world leader in this field, cannot make crystals bigger
than a few centimeters, and then only by keeping the growth chamber at a
temperature of 2200 C and a pressure of almost 20 000 atmospheres.

So the makers of blue LEDs instead typically build their devices on
wafers of sapphire, whose crystalline structure does not quite match
that of the nitrides. And that discrepancy gives rise to many
defects—billions of them per square centimeter.

[![combatting droop
illustration](/img/520305-620-1412783091916.jpg)](/img/520305-620-1412782905853.jpg)

 

Illustration: Bryan Christie Design **Combatting Droop** Droop—the loss
of efficiency at high power—afflicts conventional nitride LED
structures. These feature an active region with gallium indium nitride
quantum wells and GaN barriers, and an electron-blocking layer to keep
electrons in this region. Researchers at Rensselaer Polytechnic
Institute have reduced droop with new active regions, made first by
combining GaInN wells and aluminum gallium indium nitride barriers and,
more recently, by pairing GaInN wells with GaInN barriers. Meanwhile,
Philips Lumileds has also developed a structure that is less prone to
droop, thanks to a far thicker quantum well.

It is amazing that such LEDs work at all. Any arsenide-based red,
orange, or yellow LED that contained as many defects would emit
absolutely no light. To this day, researchers, including Nakamura
himself—who moved to the University of California, Santa Barbara (UCSB)
in 1999—can’t agree on the cause of the phenomenon. Perhaps the solution
to this problem may also explain droop.

**The explanation won’t come easily.** When researchers set out to find
the cause of droop in nitride LEDs, one of their first suspects was
heat, which they knew could cause droop in arsenide LEDs. There, heat
imparts so much energy to the electrons and holes that the quantum well
can no longer trap them. Instead of recombining, some of them escape,
only to be swept away by the electric fields in the device. But
researchers dismissed this possibility after noting that nitride LEDs
suffered from droop even when driven by short, pulsed voltages spaced
far enough apart to let the devices cool down.

Another theory was proposed as far back as 1996 by Nakamura. He argued
that everything could be explained by the structure of the quantum well.
Nakamura and his colleagues looked at LEDs with a transmission electron
microscope and were surprised to find light and dark areas within the
quantum well, suggesting that the material there was not uniform. They
then investigated the crystalline structure more closely, using X-ray
diffraction, and found that the quantum well had indium-rich clusters
(bright) next to indium-poor areas (dark).

Nakamura conjectured that because the indium clusters were free from
defects, the electrons and holes would be trapped in them, making bright
emission possible, at least at low currents. Continuing with this line
of reasoning, Nakamura’s team argued that LEDs’ high efficiency at low
currents stemmed from a very high proportion of electron-hole
recombination in defect-free clusters. At higher currents, however,
these clusters would become saturated, and any additional charge
carriers would spill over into regions having defects dense enough to
kill light emission. The saturation at high current, they suggested,
accounted for the observed droop.

This theory has fallen out of favor in recent years. “To start with, we
saw indium-rich clusters in InGaN quantum wells, just as the rest of the
world did,” explains Colin Humphreys, the head of the Cambridge Centre
for Gallium Nitride at the University of Cambridge, in England. But then
he and his team began to suspect that their electron microscope was
causing the very thing it was detecting. So the group carried out
low-dose electron microscopy. “We looked at the first few frames—a very
low exposure—and saw no indium clustering at all. But as we exposed the
material to the beam, these clusters developed,” he says. They concluded
that the clustering was merely an artifact of measurement.

In 2003, Humphreys presented that jaw-dropping finding at the Fifth
International Conference on Nitride Semiconductors, in Nara, Japan. It
wasn’t well received. Many delegates contended that something must have
gone wrong with the Cambridge samples. So Humphreys’s group went back
and studied a wider variety of specimens, including LEDs supplied by
Nichia. Their work only reinforced their view that the clusters were
formed by electron-beam damage.

In 2007, Humphreys’s Cambridge team, together with researchers at the
University of Oxford, described how they had attacked the problem with
what’s known as a three-dimensional atom probe. This device applies a
high voltage that evaporates atoms on a surface, then sends them
individually through a mass spectroscope, which identifies each one by
its charge-to-mass ratio. By evaporating one layer after the other and
putting all the data together, you can render a 3-D image of the surface
with atomic precision.

The resulting images confirmed, again, what the electron microscope had
shown: There is no clustering. Discrediting the cluster theory was an
important step, even though it left the research community without an
alternative explanation for droop.

Then, on 13 February 2007, the California-based LED manufacturing giant
Philips Lumileds Lighting Co. made the stunning claim that it had
“fundamentally solved” the problem of droop. It even said that it
would soon include its droop-abating technology in samples of its
flagship Luxeon LEDs.

Lumileds kept the cause of droop under wraps for several months. Then,
at the meeting of the International Conference of Nitride
Semiconductors, held September 2007 in Las Vegas, it presented a paper
putting the blame on Auger recombination—a process, named after the
20th-century French physicist Pierre-Victor Auger, that involves the
interaction of an electron and a hole with another carrier, all without
the emission of light.

The idea was pretty radical, and it has had a mixed reception. Applied
Physics Letters published Lumileds’ paper only after repeated rejections
and revisions. “In my experience, it was one of the most difficult
papers to get out there,” says Mike Krames, director of the company’s
Advanced Laboratories.

**Krames’s team used a laser** to probe a layer of gallium indium
nitride, the semiconductor used for quantum wells in a nitride LED. They
tuned the laser to a wavelength that only the gallium indium nitride
layer would absorb, so that each zap created pairs of electrons and
holes that then recombined to produce photons. When the researchers
graphed the resulting photoluminescence against different intensities
impinging on the sample, they produced curves that closely fit an
equation that described the effects of Auger recombination.

The bad news is that you can’t eliminate this kind of recombination,
which is proportional to the cube of the density of carriers. So in a
nutshell, if you’ve got carriers—which of course you need to generate
light—you’ve also got Auger recombination. The good news, though, is
that Lumileds has shown that you can push the peak of your efficiency to
far higher currents by cutting carrier density—that is, by spreading the
carriers over more material. The company does so with what’s known as a
double heterostructure (DH), essentially a quantum well that’s 13
nanometers wide, rather than the usual 3 or 4 nm. It still shows quantum
effects, although they are not so pronounced, and the design is less
efficient than the standard one at low currents. Still, it excels at
higher currents. The Lumileds team has created a test version that
delivers a peak efficiency slightly higher than that of a conventional
LED.

Promising though this new crystalline structure may be, it is difficult
to grow. Perhaps this is why Lumileds has yet to incorporate the design
into its Luxeon LEDs. “There are multiple paths to dealing with droop,
and we’ve investigated most of these paths,” says Krames. “We have new
structures in the pipeline, DH as well as non-DH, and we will move
forward with the best structure.”

**Not everyone is convinced** that Auger recombination is the cause of
droop. One such skeptic is Jörg Hader, a University of Arizona theorist,
who works with former colleagues in Germany at Philipps-Universität
Marburg and at one of the world’s biggest LED manufacturers, Osram Opto
Semiconductors, in Regensburg.

“All \[Lumileds\] showed was that they can fit the results with a
dependence that is like Auger,” claims Hader. “It’s a fairly weak
argument to see a fit that fits, and see what might correspond to that
fitting.” In his view, there’s a good chance that the Lumileds data
could also be fitted with other density dependencies, as well as the
cubed dependence that is classically associated with Auger
recombination.

Hader has calculated the magnitude of direct Auger recombination for a
typical blue LED. The equations that describe this interaction of an
electron and a hole with a third carrier date back to the 1950s, but
that doesn’t mean that they are easy to solve. Hader says he took no
shortcuts. Instead, he accounted for all physical interactions in a
program tens of thousands of lines long, a program that in its initial
form would have taken several years to run. However, Hader says he’s
learned what he can omit safely in order to get the running time down to
just 1 minute. He says the model shows that Auger losses are too small
to account for LED droop, although he does allow that droop might be
caused by other processes related to Auger recombination. These
processors are more complicated because they also involve defects in the
material or thermal vibrations (phonons, in quantum terms) of the
semiconductor crystal.

Krames criticizes Hader’s calculations for leaving out the possibility
that electrons might occupy higher energy levels, known as higher
conduction bands. But Hader believes that including these bands would
hardly affect his conclusions.

This May, computer scientists at UCSB brought new evidence to bear on
this debate. Chris Van de Walle’s team included a second conduction band
in their calculations of Auger recombination in nitrides and concluded
that Auger contributes strongly to droop. However, they modeled only the
bulk materials, not realistic quantum wells, for which Van de Walle
admits his methods cannot handle the calculations, at least not on
today’s computers.

Hader does not doubt the general shape of the UCSB results. However, he
points out that the value Van de Walle’s team has taken for the second
conduction band substantially differs from that given in certain
academic papers. Using these published values would have profound
effects on any estimate of the magnitude of Auger recombination. The
conclusions of Hader and Van de Walle highlight the lack of consensus
among theorists over the cause of droop.

![Less Leakage: POLARIZATION FIELDS may cause LED droop](/image/520319)
Illustration: Bryan Christie Design **Less Leakage:** Polarization
fields may cause LED droop. Such fields are claimed to drive electrons
out of the active region and into the p-type layer, where some recombine
without emitting light \[top\]. A “polarization matched” structure
\[bottom\] has a far weaker internal field and therefore suffers less
electron leakage, leaving more electrons to recombine with holes.

Meanwhile, a group headed by E. Fred Schubert at the Rensselaer
Polytechnic Institute, in Troy, N.Y., has proposed yet another theory.
His team, in collaboration with Samsung, blames droop on the leakage of
too many electrons from the quantum well.

Interestingly, Schubert’s team, like the researchers at Lumileds, drew
its conclusions by pumping light into the nitride structures and
observing the light that those structures emitted in response. But
Schubert and company investigated full LED structures, and they compared
the results they’d obtained from optical pumping with light output
generated when a voltage was applied, as it is in normal operation. As
expected, droop kicked in when the device was pumped electrically. But
the researchers saw no sign of droop in the photoluminescence data.

They then brought in Joachim Piprek, a theorist from the NUSOD
Institute, a device simulation consultancy in Newark, Del. He used a
computer model to simulate the behavior of a blue LED and found that the
strong internal fields characteristic of nitrides must be causing
electrons to leak out of the wells.

Now Schubert and his colleagues have produced direct evidence to back up
their argument for leakage. They took an LED unconnected to any circuit
and hit it with light at a wavelength of 405 nm, which is absorbed only
in the quantum wells. The researchers detected a voltage across the
diode, implying that carriers must leave the wells, contradicting
Lumileds’ theory.

Schubert’s team has tried to control electron leakage by redesigning the
LED. By carefully selecting the materials for the active
region—switching from the conventional gallium nitride barrier to an
aluminum gallium indium nitride version—they have been able to eliminate
the charges that tend to form wherever distinct crystalline layers meet.
They say such “polarization matching” consistently cuts droop, raising
power output by 25 percent at high currents.

Schubert believes that the electrons that leak out of the wells
recombine with holes in the p -type region. If he could detect this
recombination, it would certainly add weight to his explanation. “We’ve
looked for that luminescence,” says Schubert, “but we have not seen it.”
He’s not surprised, though, because p -type gallium nitride is a very
inefficient light emitter, and the LED’s surface is nearby, so surface
recombination at the top contact is also likely.

However, it is possible to detect electrons in the p -type region by
modifying the standard LED structure, and researchers at UCSB have done
just this. This team, led by Steven DenBaars and Nakamura, did the job
of fitting the p -type region with an additional quantum well, one that
emits light of a color different from that of the main LED. At a
workshop in Montreux, Switzerland, in the fall of 2008, the group
reported that they had found just this sort of emission.

Although this experiment proved that electrons do flow into the p -type
region, it can’t tell us where they came from. And while Schubert’s
theory of electron leakage could explain the results, there may well be
other things that can also account for them. We can’t even rule out
Auger recombination as the dominant mechanism, because the proportion of
electrons flowing into the p -type region is still to be quantified.

**Each theory has its champions.** Theoreticians at Philipps-Universität
Marburg support Auger recombination, mainly the phonon-assisted form, as
the main cause of droop. So does Semiconductor Technology Research, a
device-modeling company based in Richmond, Va. Meanwhile, Hadis Morkoç’s
group at Virginia Commonwealth University seconds Schubert’s support of
electron leakage, which they attribute to the poor efficiency with which
holes are injected into the quantum well.

Confused? Join the club—and realize that this controversy is precisely
what you’d expect to find in a field that has suddenly begun to make
great progress. Even if we don’t have a universally agreed-upon theory
to account for droop, we do have a growing arsenal of proven weapons to
fight it—Schubert’s polarization-matched devices, Lumileds’ wide quantum
well structures, as well as designs that improve hole injection, among
others. Too bad that we still can’t agree on how they work.

The industry will move forward. LEDs are just starting to supplant
fluorescent as well as incandescent lighting. Someday, in our lifetimes,
incandescent filaments will finally stop turning tens of gigawatts into
unwanted heat. Smokestacks will spew less carbon into the global
greenhouse. And we won’t have to get up on stepladders to change
burned-out bulbs nearly so often as we do today.

And around that time, when you’re reading this magazine by the light of
an LED, perhaps the theorists will have watertight explanations for the
experimentalists, and we’ll know the answer to the burning question that
remains: What causes droop?

## About the Author

Richard Stevenson, author of “The LED’s Dark Secret” \[p. 22\], got a
Ph.D. at the University of Cambridge, where he studied compound
semiconductors. Then he went into industry and made the things. Now, as
a freelance journalist based in Wales, he writes about them. Between
assignments, he builds traditional class A hi-fi amplifiers, as opposed
to the class D type favored by IEEE Spectrum’s Glenn Zorpette. “If we
were to share an office,” Stevenson says, “many hours would be lost to
discussions of the path to hi-fi nirvana.”

## To Probe Further

The Philips Lumileds papers are “Auger Recombination in InGaN Measured
by Photoluminescence,” by Y. C. Shen, G. O. Mueller, S. Watanabe, N. F.
Gardner, A. Munkholm, and M. R. Krames, Applied Physics Letters 91 ****
141101, 1 October 2007, and “Blue-Emitting InGaN–GaN
Double-Heterostructure Light-Emitting Diodes Reaching Maximum Quantum
Efficiency Above 200 A/cm2,” by N. F. Gardner, G. O. Müller, Y. C. Shen,
G. Chen, S. Watanabe, W. Götz, and M. R. Krames, APL 91 **** 243506, 12
December 2007.

The papers from Rensselaer Polytechnic Institute are “Origin of
Efficiency Droop in GaN-Based Light-Emitting Diodes,” by M.-H. Kim, M.
F. Schubert, Q. Dai, J. K. Kim, and E. Fred Schubert, J. Piprek, APL 91
**** 183507, 30 October 2007; “Effect of Dislocation Density on
Efficiency Droop in GaInN/GaN Light-Emitting Diodes,” by M. F. Schubert,
S. Chhajed, J. K. Kim, and E. Fred Schubert, D. D. Koleske, M. H.
Crawford, S. R. Lee, A. J. Fischer, G. Thaler, and M. A. Banas, APL 91
**** 231114, 7 December 2007; and “Polarization-Matched GaInN/AlGaInN
Multi-Quantum-Well Light-Emitting Diodes With Reduced Efficiency Droop,”
by M. F. Schubert, J. Xu, J. K. Kim, E. F. Schubert, M.-H. Kim, S. Yoon,
S. M. Lee, C. Sone, T. Sakong, and Y. Park, APL 93 **** 041102, 28 July
2008.

The paper from Jorg Hader, et al., is “On the Importance of Radiative
and Auger Losses in GaN-Based Quantum Wells, APL 92 **** 261103, 1 July
2008.

The paper from Virginia Commonwealth University is “On the Efficiency
Droop in InGaN Multiple-Quantum-Well Blue-Light-Emitting Diodes and Its
Reduction with p-Doped Quantum-Well Barriers,” by J. Xie, X. Ni, Q. Fan,
R. Shimada, Ü. Özgür, and H. Morkoç, APL 93 **** 121107, 23 September
2008.