2018-02-23 18:58:03 +00:00
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---
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created_at: '2014-10-07T14:59:34.000Z'
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title: The LED's Dark Secret (2009)
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url: http://spectrum.ieee.org/semiconductors/optoelectronics/the-leds-dark-secret
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author: spectruman
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points: 79
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story_text: ''
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comment_text:
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num_comments: 45
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story_id:
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story_title:
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story_url:
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parent_id:
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created_at_i: 1412693974
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_tags:
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- story
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- author_spectruman
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- story_8421518
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objectID: '8421518'
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2018-06-08 12:05:27 +00:00
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year: 2009
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2018-02-23 18:58:03 +00:00
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---
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2018-03-03 09:35:28 +00:00
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![](/img/520659-1367523519626.jpg) Illustration: Bryan Christie Design
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2018-02-23 18:19:40 +00:00
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2018-03-03 09:35:28 +00:00
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**The blue light-emitting diode,** arguably the greatest optoelectronic
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advance of the past 25 years, harbors a dark secret: Crank up the
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current and its efficiencies will plummet. The problem is known as
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droop, and it’s not only puzzling the brightest minds in the field, it’s
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also threatening the future of the electric lighting industry.
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2018-02-23 18:19:40 +00:00
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2018-03-03 09:35:28 +00:00
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Tech visionaries have promised us a bright new world where cool and
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efficient white LEDs, based on blue ones, will replace the wasteful
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little heaters known as incandescent lightbulbs. More than a dozen
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countries have already enacted legislation that bans, or will soon ban,
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incandescent bulbs. But it’s hard to imagine LEDs dislodging
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incandescents and coming to dominate the world electric lighting
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industry, unless we can defeat droop.
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2018-02-23 18:19:40 +00:00
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2018-03-03 09:35:28 +00:00
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In flashlights, in backlights for screens in cellphones and now
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televisions, and in a bunch of other applications, white LEDs already
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constitute a multibillion-dollar market. But that’s just a US $5 billion
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niche compared to the overall lighting industry, whose sales next year
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should reach $100 billion, according to the market research firm Global
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Industry Analysts. The trick will be to make LEDs turn electricity into
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light efficiently enough to offset their relatively high cost—roughly 16
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cents per lumen, at lightbulb-type brightness, as opposed to about 0.1
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cents or less for incandescents.
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Look at the competition and you’d think the job was easy. Today’s
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garden-variety incandescent bulbs aren’t much different from the ones
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Thomas Edison sold more than a century ago. They still waste 90 percent
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of their power, delivering roughly 16 lumens per watt. Fluorescent tubes
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do a lot better, at more than 100 lm/W, but even they pale next to the
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best LEDs. The current state-of-the-art white LED pumps out around 250
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lm/W, and there’s no reason why that figure won’t reach 300 lm/W.
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Unfortunately, these LEDs perform at their best only at low power—the
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few milliamps it takes to backlight the little screen on your mobile
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phone, for instance. At the current levels needed for general lighting,
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droop kicks in, and down you go, below 100 lm/W.
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[![LED
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Architecture](/img/520290-620px-1412782576788.jpg)](/img/520290-1412782534145.jpg)
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Illustration: Bryan Christie Design **LED Architecture:** At the heart
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of every white LED is a semiconductor chip made from nitride-based
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materials. The chip is traditionally positioned on top of the cathode
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lead. Applying several volts across this device makes the chip emit blue
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light. Passing the light through a yellow phosphor yields white light.
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Modern, high-power LEDs are variants of this architecture, featuring
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more complex packages for superior thermal management.
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**The first-ever report of** light emission from a semiconductor was by
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the British radio engineer Henry Joseph Round, who noted a yellowish
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glow emanating from silicon carbide in 1907. However, the first devices
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at all similar to today’s LEDs arrived only in the 1950s, at Signal
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Corps Engineering Laboratories, at Fort Monmouth, in New Jersey.
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Researchers there fabricated orange-emitting devices; green, red, and
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yellow equivalents followed in the ’60s and ’70s, all of them quite
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inefficient.
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The great leap toward general lighting came in the mid-1990s, when Shuji
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Nakamura, then at Nichia Corp., in Tokushima, Japan, developed the first
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practical bright-blue LED using nitride-based compound semiconductors.
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(Nakamura’s achievement won him the 2006 Millennium Technology Prize,
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the approximate equivalent in engineering of a Nobel Prize.) Once you’ve
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got blue light, you can get white by passing the blue rays through a
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yellow phosphor. The phosphor absorbs some of the blue and reradiates it
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as yellow; the combination of blue and yellow makes white.
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All LEDs are fabricated as aggregated sections, or regions, of different
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semiconductor materials. Each of these regions plays a specific role.
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One region serves as a source of electrons; it consists of a crystal of
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a compound semiconductor into which tiny amounts of an impurity, such as
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silicon, have been introduced. Each such atom of impurity, or dopant,
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has four electrons in its outer shell, compared with the three in an
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atom of gallium, aluminum, or indium. When a dopant takes a place that
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one of these other atoms would normally occupy, it adds an electron to
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the crystalline lattice. The extra electron moves easily though the
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crystal, acting as a carrier of negative charge. With this surfeit of
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negative charges, such a material is called n-type.
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At the opposite end of the LED is a region of p-type material, so called
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because it has excess positive-charge carriers, created by doping with
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an element such as zinc or magnesium. These metals are made up of atoms
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with only two electrons in their outer shell. When such an atom sits in
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place of an atom of aluminum, gallium, or a chemically similar element
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(from group III in the periodic table), the lattice ends up an electron
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short. That vacancy behaves as a positive charge, moving throughout the
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crystal like the missing tile in a sort-the-number puzzle. That mobile
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vacancy is called a hole.
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In the middle of the sandwich are several extraordinarily thin layers.
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These constitute the active region, where light is produced. Some layers
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made of one semiconducting material surround a central layer made of
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another, creating a “well” just a few atoms thick—a trench so confined
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that the laws of quantum mechanics rule supreme. When you inject
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electrons and holes into the well by applying a voltage to the n - and
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p-type regions, the two kinds of charge carriers will be trapped,
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maximizing the likelihood that they will recombine. When they do, a
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photon pops out.
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To make an LED, you must grow a series of highly defined semiconductor
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layers on a thin wafer of a crystalline material, called a substrate.
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The substrate for red, orange, and yellow LEDs is gallium arsenide,
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which works wonderfully because its atoms are spaced out identically to
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those of the layers built on top of it. Hardly any mechanical strain
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develops in the semiconductor’s crystalline lattice during fabrication,
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so there are very few defects, which would quench light generation.
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Unfortunately, blue and green LEDs lack such a good platform. They’re
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called nitride LEDs because their fundamental semiconductor is gallium
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nitride. The n-type gallium nitride is doped with silicon, the p-type
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with magnesium. The quantum wells in between are gallium indium nitride.
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To alter the light color emitted from green to violet, researchers vary
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the gallium-to-indium ratio in the quantum wells. A little indium
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produces a violet LED; a little more of it produces green.
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Such LEDs would ideally be manufactured on gallium nitride substrates.
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But it has proved impossible to grow the large, perfect crystals of
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gallium nitride that would be necessary to make such wafers. Unipress,
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of Warsaw, the world leader in this field, cannot make crystals bigger
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than a few centimeters, and then only by keeping the growth chamber at a
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temperature of 2200 C and a pressure of almost 20 000 atmospheres.
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So the makers of blue LEDs instead typically build their devices on
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wafers of sapphire, whose crystalline structure does not quite match
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that of the nitrides. And that discrepancy gives rise to many
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defects—billions of them per square centimeter.
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[![combatting droop
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illustration](/img/520305-620-1412783091916.jpg)](/img/520305-620-1412782905853.jpg)
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Illustration: Bryan Christie Design **Combatting Droop** Droop—the loss
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of efficiency at high power—afflicts conventional nitride LED
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structures. These feature an active region with gallium indium nitride
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quantum wells and GaN barriers, and an electron-blocking layer to keep
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electrons in this region. Researchers at Rensselaer Polytechnic
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Institute have reduced droop with new active regions, made first by
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combining GaInN wells and aluminum gallium indium nitride barriers and,
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more recently, by pairing GaInN wells with GaInN barriers. Meanwhile,
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Philips Lumileds has also developed a structure that is less prone to
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droop, thanks to a far thicker quantum well.
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It is amazing that such LEDs work at all. Any arsenide-based red,
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orange, or yellow LED that contained as many defects would emit
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absolutely no light. To this day, researchers, including Nakamura
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himself—who moved to the University of California, Santa Barbara (UCSB)
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in 1999—can’t agree on the cause of the phenomenon. Perhaps the solution
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to this problem may also explain droop.
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**The explanation won’t come easily.** When researchers set out to find
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the cause of droop in nitride LEDs, one of their first suspects was
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heat, which they knew could cause droop in arsenide LEDs. There, heat
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imparts so much energy to the electrons and holes that the quantum well
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can no longer trap them. Instead of recombining, some of them escape,
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only to be swept away by the electric fields in the device. But
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researchers dismissed this possibility after noting that nitride LEDs
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suffered from droop even when driven by short, pulsed voltages spaced
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far enough apart to let the devices cool down.
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Another theory was proposed as far back as 1996 by Nakamura. He argued
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that everything could be explained by the structure of the quantum well.
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Nakamura and his colleagues looked at LEDs with a transmission electron
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microscope and were surprised to find light and dark areas within the
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quantum well, suggesting that the material there was not uniform. They
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then investigated the crystalline structure more closely, using X-ray
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diffraction, and found that the quantum well had indium-rich clusters
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(bright) next to indium-poor areas (dark).
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Nakamura conjectured that because the indium clusters were free from
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defects, the electrons and holes would be trapped in them, making bright
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emission possible, at least at low currents. Continuing with this line
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of reasoning, Nakamura’s team argued that LEDs’ high efficiency at low
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currents stemmed from a very high proportion of electron-hole
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recombination in defect-free clusters. At higher currents, however,
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these clusters would become saturated, and any additional charge
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carriers would spill over into regions having defects dense enough to
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kill light emission. The saturation at high current, they suggested,
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accounted for the observed droop.
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This theory has fallen out of favor in recent years. “To start with, we
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saw indium-rich clusters in InGaN quantum wells, just as the rest of the
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world did,” explains Colin Humphreys, the head of the Cambridge Centre
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for Gallium Nitride at the University of Cambridge, in England. But then
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he and his team began to suspect that their electron microscope was
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causing the very thing it was detecting. So the group carried out
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low-dose electron microscopy. “We looked at the first few frames—a very
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low exposure—and saw no indium clustering at all. But as we exposed the
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material to the beam, these clusters developed,” he says. They concluded
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that the clustering was merely an artifact of measurement.
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In 2003, Humphreys presented that jaw-dropping finding at the Fifth
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International Conference on Nitride Semiconductors, in Nara, Japan. It
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wasn’t well received. Many delegates contended that something must have
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gone wrong with the Cambridge samples. So Humphreys’s group went back
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and studied a wider variety of specimens, including LEDs supplied by
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Nichia. Their work only reinforced their view that the clusters were
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formed by electron-beam damage.
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In 2007, Humphreys’s Cambridge team, together with researchers at the
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University of Oxford, described how they had attacked the problem with
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what’s known as a three-dimensional atom probe. This device applies a
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high voltage that evaporates atoms on a surface, then sends them
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individually through a mass spectroscope, which identifies each one by
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its charge-to-mass ratio. By evaporating one layer after the other and
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putting all the data together, you can render a 3-D image of the surface
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with atomic precision.
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The resulting images confirmed, again, what the electron microscope had
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shown: There is no clustering. Discrediting the cluster theory was an
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important step, even though it left the research community without an
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alternative explanation for droop.
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Then, on 13 February 2007, the California-based LED manufacturing giant
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Philips Lumileds Lighting Co. made the stunning claim that it had
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“fundamentally solved” the problem of droop. It even said that it
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would soon include its droop-abating technology in samples of its
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flagship Luxeon LEDs.
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Lumileds kept the cause of droop under wraps for several months. Then,
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at the meeting of the International Conference of Nitride
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Semiconductors, held September 2007 in Las Vegas, it presented a paper
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putting the blame on Auger recombination—a process, named after the
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20th-century French physicist Pierre-Victor Auger, that involves the
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interaction of an electron and a hole with another carrier, all without
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the emission of light.
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The idea was pretty radical, and it has had a mixed reception. Applied
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Physics Letters published Lumileds’ paper only after repeated rejections
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and revisions. “In my experience, it was one of the most difficult
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papers to get out there,” says Mike Krames, director of the company’s
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Advanced Laboratories.
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**Krames’s team used a laser** to probe a layer of gallium indium
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nitride, the semiconductor used for quantum wells in a nitride LED. They
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tuned the laser to a wavelength that only the gallium indium nitride
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layer would absorb, so that each zap created pairs of electrons and
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holes that then recombined to produce photons. When the researchers
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graphed the resulting photoluminescence against different intensities
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impinging on the sample, they produced curves that closely fit an
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equation that described the effects of Auger recombination.
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The bad news is that you can’t eliminate this kind of recombination,
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which is proportional to the cube of the density of carriers. So in a
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nutshell, if you’ve got carriers—which of course you need to generate
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light—you’ve also got Auger recombination. The good news, though, is
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that Lumileds has shown that you can push the peak of your efficiency to
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far higher currents by cutting carrier density—that is, by spreading the
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carriers over more material. The company does so with what’s known as a
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double heterostructure (DH), essentially a quantum well that’s 13
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nanometers wide, rather than the usual 3 or 4 nm. It still shows quantum
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effects, although they are not so pronounced, and the design is less
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efficient than the standard one at low currents. Still, it excels at
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higher currents. The Lumileds team has created a test version that
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delivers a peak efficiency slightly higher than that of a conventional
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LED.
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Promising though this new crystalline structure may be, it is difficult
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to grow. Perhaps this is why Lumileds has yet to incorporate the design
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into its Luxeon LEDs. “There are multiple paths to dealing with droop,
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and we’ve investigated most of these paths,” says Krames. “We have new
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structures in the pipeline, DH as well as non-DH, and we will move
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forward with the best structure.”
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**Not everyone is convinced** that Auger recombination is the cause of
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droop. One such skeptic is Jörg Hader, a University of Arizona theorist,
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who works with former colleagues in Germany at Philipps-Universität
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Marburg and at one of the world’s biggest LED manufacturers, Osram Opto
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Semiconductors, in Regensburg.
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“All \[Lumileds\] showed was that they can fit the results with a
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dependence that is like Auger,” claims Hader. “It’s a fairly weak
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argument to see a fit that fits, and see what might correspond to that
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fitting.” In his view, there’s a good chance that the Lumileds data
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could also be fitted with other density dependencies, as well as the
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cubed dependence that is classically associated with Auger
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recombination.
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Hader has calculated the magnitude of direct Auger recombination for a
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typical blue LED. The equations that describe this interaction of an
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electron and a hole with a third carrier date back to the 1950s, but
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that doesn’t mean that they are easy to solve. Hader says he took no
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shortcuts. Instead, he accounted for all physical interactions in a
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program tens of thousands of lines long, a program that in its initial
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form would have taken several years to run. However, Hader says he’s
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learned what he can omit safely in order to get the running time down to
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just 1 minute. He says the model shows that Auger losses are too small
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to account for LED droop, although he does allow that droop might be
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caused by other processes related to Auger recombination. These
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processors are more complicated because they also involve defects in the
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material or thermal vibrations (phonons, in quantum terms) of the
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semiconductor crystal.
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Krames criticizes Hader’s calculations for leaving out the possibility
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that electrons might occupy higher energy levels, known as higher
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conduction bands. But Hader believes that including these bands would
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hardly affect his conclusions.
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This May, computer scientists at UCSB brought new evidence to bear on
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this debate. Chris Van de Walle’s team included a second conduction band
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in their calculations of Auger recombination in nitrides and concluded
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that Auger contributes strongly to droop. However, they modeled only the
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bulk materials, not realistic quantum wells, for which Van de Walle
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admits his methods cannot handle the calculations, at least not on
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today’s computers.
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Hader does not doubt the general shape of the UCSB results. However, he
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points out that the value Van de Walle’s team has taken for the second
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conduction band substantially differs from that given in certain
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academic papers. Using these published values would have profound
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effects on any estimate of the magnitude of Auger recombination. The
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conclusions of Hader and Van de Walle highlight the lack of consensus
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among theorists over the cause of droop.
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![Less Leakage: POLARIZATION FIELDS may cause LED droop](/image/520319)
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Illustration: Bryan Christie Design **Less Leakage:** Polarization
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fields may cause LED droop. Such fields are claimed to drive electrons
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out of the active region and into the p-type layer, where some recombine
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without emitting light \[top\]. A “polarization matched” structure
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\[bottom\] has a far weaker internal field and therefore suffers less
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electron leakage, leaving more electrons to recombine with holes.
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Meanwhile, a group headed by E. Fred Schubert at the Rensselaer
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Polytechnic Institute, in Troy, N.Y., has proposed yet another theory.
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His team, in collaboration with Samsung, blames droop on the leakage of
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too many electrons from the quantum well.
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Interestingly, Schubert’s team, like the researchers at Lumileds, drew
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its conclusions by pumping light into the nitride structures and
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observing the light that those structures emitted in response. But
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Schubert and company investigated full LED structures, and they compared
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the results they’d obtained from optical pumping with light output
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generated when a voltage was applied, as it is in normal operation. As
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expected, droop kicked in when the device was pumped electrically. But
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the researchers saw no sign of droop in the photoluminescence data.
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They then brought in Joachim Piprek, a theorist from the NUSOD
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Institute, a device simulation consultancy in Newark, Del. He used a
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computer model to simulate the behavior of a blue LED and found that the
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strong internal fields characteristic of nitrides must be causing
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electrons to leak out of the wells.
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Now Schubert and his colleagues have produced direct evidence to back up
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their argument for leakage. They took an LED unconnected to any circuit
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and hit it with light at a wavelength of 405 nm, which is absorbed only
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in the quantum wells. The researchers detected a voltage across the
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diode, implying that carriers must leave the wells, contradicting
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Lumileds’ theory.
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Schubert’s team has tried to control electron leakage by redesigning the
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LED. By carefully selecting the materials for the active
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region—switching from the conventional gallium nitride barrier to an
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aluminum gallium indium nitride version—they have been able to eliminate
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the charges that tend to form wherever distinct crystalline layers meet.
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They say such “polarization matching” consistently cuts droop, raising
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power output by 25 percent at high currents.
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Schubert believes that the electrons that leak out of the wells
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recombine with holes in the p -type region. If he could detect this
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recombination, it would certainly add weight to his explanation. “We’ve
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looked for that luminescence,” says Schubert, “but we have not seen it.”
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He’s not surprised, though, because p -type gallium nitride is a very
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inefficient light emitter, and the LED’s surface is nearby, so surface
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recombination at the top contact is also likely.
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However, it is possible to detect electrons in the p -type region by
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modifying the standard LED structure, and researchers at UCSB have done
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just this. This team, led by Steven DenBaars and Nakamura, did the job
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of fitting the p -type region with an additional quantum well, one that
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emits light of a color different from that of the main LED. At a
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workshop in Montreux, Switzerland, in the fall of 2008, the group
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reported that they had found just this sort of emission.
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Although this experiment proved that electrons do flow into the p -type
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region, it can’t tell us where they came from. And while Schubert’s
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theory of electron leakage could explain the results, there may well be
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other things that can also account for them. We can’t even rule out
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Auger recombination as the dominant mechanism, because the proportion of
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electrons flowing into the p -type region is still to be quantified.
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**Each theory has its champions.** Theoreticians at Philipps-Universität
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Marburg support Auger recombination, mainly the phonon-assisted form, as
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the main cause of droop. So does Semiconductor Technology Research, a
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device-modeling company based in Richmond, Va. Meanwhile, Hadis Morkoç’s
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group at Virginia Commonwealth University seconds Schubert’s support of
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electron leakage, which they attribute to the poor efficiency with which
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holes are injected into the quantum well.
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Confused? Join the club—and realize that this controversy is precisely
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what you’d expect to find in a field that has suddenly begun to make
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great progress. Even if we don’t have a universally agreed-upon theory
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to account for droop, we do have a growing arsenal of proven weapons to
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fight it—Schubert’s polarization-matched devices, Lumileds’ wide quantum
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well structures, as well as designs that improve hole injection, among
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others. Too bad that we still can’t agree on how they work.
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The industry will move forward. LEDs are just starting to supplant
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fluorescent as well as incandescent lighting. Someday, in our lifetimes,
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incandescent filaments will finally stop turning tens of gigawatts into
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|
unwanted heat. Smokestacks will spew less carbon into the global
|
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|
greenhouse. And we won’t have to get up on stepladders to change
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burned-out bulbs nearly so often as we do today.
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And around that time, when you’re reading this magazine by the light of
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an LED, perhaps the theorists will have watertight explanations for the
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experimentalists, and we’ll know the answer to the burning question that
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remains: What causes droop?
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## About the Author
|
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Richard Stevenson, author of “The LED’s Dark Secret” \[p. 22\], got a
|
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Ph.D. at the University of Cambridge, where he studied compound
|
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|
|
semiconductors. Then he went into industry and made the things. Now, as
|
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|
a freelance journalist based in Wales, he writes about them. Between
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|
assignments, he builds traditional class A hi-fi amplifiers, as opposed
|
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to the class D type favored by IEEE Spectrum’s Glenn Zorpette. “If we
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were to share an office,” Stevenson says, “many hours would be lost to
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discussions of the path to hi-fi nirvana.”
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## To Probe Further
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The Philips Lumileds papers are “Auger Recombination in InGaN Measured
|
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|
by Photoluminescence,” by Y. C. Shen, G. O. Mueller, S. Watanabe, N. F.
|
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Gardner, A. Munkholm, and M. R. Krames, Applied Physics Letters 91 ****
|
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|
141101, 1 October 2007, and “Blue-Emitting InGaN–GaN
|
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|
Double-Heterostructure Light-Emitting Diodes Reaching Maximum Quantum
|
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Efficiency Above 200 A/cm2,” by N. F. Gardner, G. O. Müller, Y. C. Shen,
|
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G. Chen, S. Watanabe, W. Götz, and M. R. Krames, APL 91 **** 243506, 12
|
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|
December 2007.
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The papers from Rensselaer Polytechnic Institute are “Origin of
|
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|
Efficiency Droop in GaN-Based Light-Emitting Diodes,” by M.-H. Kim, M.
|
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|
|
F. Schubert, Q. Dai, J. K. Kim, and E. Fred Schubert, J. Piprek, APL 91
|
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|
**** 183507, 30 October 2007; “Effect of Dislocation Density on
|
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|
Efficiency Droop in GaInN/GaN Light-Emitting Diodes,” by M. F. Schubert,
|
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|
S. Chhajed, J. K. Kim, and E. Fred Schubert, D. D. Koleske, M. H.
|
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|
Crawford, S. R. Lee, A. J. Fischer, G. Thaler, and M. A. Banas, APL 91
|
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|
**** 231114, 7 December 2007; and “Polarization-Matched GaInN/AlGaInN
|
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|
|
Multi-Quantum-Well Light-Emitting Diodes With Reduced Efficiency Droop,”
|
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|
|
by M. F. Schubert, J. Xu, J. K. Kim, E. F. Schubert, M.-H. Kim, S. Yoon,
|
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|
S. M. Lee, C. Sone, T. Sakong, and Y. Park, APL 93 **** 041102, 28 July
|
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|
2008.
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The paper from Jorg Hader, et al., is “On the Importance of Radiative
|
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|
and Auger Losses in GaN-Based Quantum Wells, APL 92 **** 261103, 1 July
|
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|
|
2008.
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The paper from Virginia Commonwealth University is “On the Efficiency
|
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|
Droop in InGaN Multiple-Quantum-Well Blue-Light-Emitting Diodes and Its
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|
Reduction with p-Doped Quantum-Well Barriers,” by J. Xie, X. Ni, Q. Fan,
|
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|
R. Shimada, Ü. Özgür, and H. Morkoç, APL 93 **** 121107, 23 September
|
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|
2008.
|