Let There Be Light

And let it come from cool, clear semiconductors rather than fragile, white-hot filaments

10 min read
Let There Be Light
Photo: Fioravanti

Photo: Fioravanti
The Fioravanti Yak concept car, unveiled last March at the Geneva Motor Show, is one of the first to use light-emitting diodes in front—as headlamps, fog lights, and turn signals.

graphic link to LED illustration

Illustrations: Bryan Christie/Photo: Lumileds Lighting LLC
Click on the image for the full illustration view.

This is part of IEEE Spectrum's special R&D report: They Might Be Giants: Seeds of a Tech Turnaround.

Kevin Dowling points the light at me and suddenly, in a head-splitting blaze of white, I'm ready to confess. It's true, I'm ready to blubber; I had been thinking that light-emitting diodes (LEDs) were basically just dim and dowdy indicator lights. I had no idea that they had become bright enough to make an iguana squint.

The light Dowling is wielding consists of 56 white and 7 amber-colored LEDs. The total package occupies an area not much bigger than a letter-sized sheet of paper and it throws about 1900 lumens—more than both headlights on the average automobile. And this little demonstrator is not even state of the art. Outfitted with the brightest of the white LEDs on the market, it would throw an astounding 7500 lm—almost as much as a typical sodium-vapor street light.

Dowling, who is vice president for strategy and technology at the start-up company Color Kinetics Inc. (Boston), is one of the hundreds of researchers harboring some very big dreams for some specks of gallium nitride roughly a millimeter square. With impressive longevity and potentially mind-boggling efficiency, these chips pour out photons in almost any color and with dazzling intensity. The idea that these tiny semiconductors might replace the venerable incandescent light bulb—a technology so successful that it has become the very symbol of innovative insight—has gone from preposterous to plausible indeed.

The key development in that transformation was the invention of the gallium nitride (GaN) LED, which made it possible to get white light from a semiconductor. It has been more than six years since a then-little-known researcher named Shuji Nakamura, at Nichia Corp. (Anan, Japan), stunned semiconductor researchers by coaxing beautiful blues, greens, and purples out of GaN—and doing so reliably. Nakamura has since decamped to the University of California, Santa Barbara, but the revolution he launched is just starting to get interesting.

GaN LEDs are already quietly transforming specialized illumination, including architectural and stage lighting, indoor and outdoor accent lighting, traffic and railway signaling, commercial and retail signs and displays, and outdoor illumination on bridges, walkways, gardens, and fountains. Every night, the almost surreally saturated hues of LEDs bathe a suspension bridge in Philadelphia, a nightclub in Miami, a movie palace in New York, hotels in Japan, the United States, and Europe, casinos everywhere, and too many other buildings to count. All told, the market for LEDs that throw at least several lumens of light was US $1.2 billion last year, according to Robert Steele, an analyst at the market research firm Strategies Unlimited (Mountain View, Calif.).

But the brightest is yet to come, researchers say. They have set their sights on the $12 billion-a-year market for sources of white light, including light bulbs and fluorescent tubes. In fact, you can already light your home with them, if you managed to escape the collapse of the tech bubble with vats and vats of cash.

You can now buy, from Lumileds Lighting LLC (San Jose, Calif.), a GaN-based white-light LED that throws about 120 lm. Six of them will use just 30 W but give you almost as much light as a 60-W incandescent bulb. Unfortunately, those six LEDs could cost you about US $120. A Lucerne, Switzerland-based bank lights its boardroom with white LEDs. And Osram-Sylvania (Danvers, Mass.) used 11 300 white LEDs and 5700 amber ones to light up part of the Jefferson Memorial (Washington, D.C.). White LEDs are also lighting the dashboards of a few luxury cars, and prototype vehicles with LED headlights have been unveiled [see photo]. Coming generations of cell phones are expected to use white LEDs for backlights.

Of course, semiconductor specialists have plenty of work to do before they can turn this expensive novelty into a mass-market source of illumination. Lumen for lumen, white LEDs cost roughly 100 times as much as an incandescent bulb. Not to worry, say researchers. Not only will they get the cost down, they are going to dazzle us with devices that will be 10 times as efficient as an incandescent and will last 100 times as long. By mixing light from LEDs of different colors, the devices will provide 1000 shades of white—or any hue under the sun—at the twist of a dial.

"The ability to change both intensity and color—that is going to bring on a new way of thinking about illumination," says Ihor Lys, an IEEE member and cofounder of Color Kinetics, which uses LEDs in sophisticated color lighting systems. What transforms the idea of a future lit by chips into more than just a vivid pipe dream is the fact that every one of the world's top lighting manufacturers, including General Electric [ranked (43) among the Top 100 R&D Spenders], Osram-Sylvania, and Philips (24), has sizable LED-lighting research efforts and joint ventures under way.

photo recent installations

Photos: Osram Opto Semiconductors; Bottom right: Zumtobel Staff
Recent installations include one with 14 000 LEDs designed by the Bartenbach Lighting Laboratory [top left], Aldrans, Austria. LEDs are also used in the lights over the boardroom table at Canton Bank in Lucerne, Switzerland [top right] and in small accent lights in the wall of the Ratskellar restaurant in Lemgo, Germany [above right]. A quote of Thomas Jefferson's in his memorial in Washington, D.C., is now lit with LEDs [above left]. All use LEDs from Osram Opto Semiconductors (Regensburg, Germany). Click on the photo for a larger view.

Wasted watts

An incandescent light bulb is just a big resistor. It gets hot enough to glow white, converting about 5 percent of electrical energy to light and wasting the rest. "It's a heater that happens to give off a little visible light by accident," jokes M. George Craford, an IEEE Fellow and chief technical officer of Lumileds.

Sad but true: all over the world, at any given instant, tens of gigawatts of electricity are doing nothing but producing unwanted heat. Worse, generating all those wasted gigawatts spews hundreds of millions of tons of carbon dioxide into the atmosphere every year.

An LED takes a different tack. It produces light by forcing together positive and negative electric charge carriers in an infinitesimal region where two different types of semiconductor material meet. LEDs are, after all, diodes: they join an n-type material, with an excess of electrons, to a p-type one, with an excess of positively charged electron deficiencies, called holes. Voltage drives the electrons and holes to an active layer at the boundary between the n- and p-type materials. When an electron anda hole meet, they release energy in the form of a photon.

In modern devices this active layer is actually one or more so-called quantum wells. These are regions so flat as to be almost two-dimensional. Electrons and holes confined in one of these thin layers behave quantum mechanically: their energy levels become constricted to certain values, or quantized. Limited to these levels, the electrons and holes become more likely to combine and emit photons.

In theory, at least, photons stream from an LED without any accompanying heat at all. But LEDs are not perfect, of course. Like any diode, they have some resistance, and although it is a minute fraction of what you get with an incandescent bulb, it is still enough to produce some heat.

The much bigger problem, though, is internal quantum efficiency, which at least for the moment is too low. For reasons researchers do not completely understand, when some electrons and holes meet, their combination creates not photons but rather heat-producing vibrations in the semiconductor crystal's lattice. Even when they do shoot off photons, not all of them escape from the device. Impurities in the semiconductors and the metal contacts to the chip, as well as flaws, called dislocations, in the semiconductors' crystalline structure, absorb photons and convert them to heat. Overall, plenty of photons are lost or never created in the first place—a whopping 90 percent for a green LED, 80 percent for blue, and 60 percent for red.

On the other hand, those numbers also indicate why researchers are so confident that the tiny chips will eventually defeat the mighty light bulb. The best LEDs are now roughly twice as efficient, in lumens per watt, as incandescent bulbs. But compared to incandescents, which have more than a century of development behind them, GaN LEDs were practically born yesterday. Better device designs and fewer imperfections should push the quantum efficiencies close to 100 percent.

When that happens, researchers say, LEDs will produce at least 10 times as many lumens as incandescents of the same wattage. "It's just a question of time," says Craford.

There are several ways to get white light out of an LED. The most common puts a blue LED chip beneath a film of yttrium-aluminum garnet (YAG) phosphor. The phosphor gives off yellow light when struck by the blue light; the mix of blue and yellow rays appears white.

To get a device to emit more of those rays, researchers must improve both the chips and their packaging. The latter boils down to finding ways to siphon off heat from the tiny semiconductors, a tricky business at which Lumileds excels. Its high-power devices have a copper heat sink mounted to a piece of silicon that in turn attaches to the LED chip and protects it from electrostatic discharges [see illustration, "Best and Brightest"].

photo of colored LEDs

Photos: Color Kinetics Inc.
Colored LEDs are brightening interior spaces, such as the Riley Hospital for Children in Indianapolis, Ind. The lighting scheme uses technology from Color Kinetics Inc. (Boston).

Machine of the moment

The challenge of building better chips is greater, and this is where the fundamental struggle is being waged. The weapon of choice in this battle is the MOCVD machine, an acronym that stands for metal-organic chemical vapor deposition. At the heart of the machine is a vacuum chamber, about the size of the inside of a small microwave oven. To grow the LEDs, technicians place a semiconductor substrate in the chamber and heat it to 900­1100 C. Then they introduce gases that flow over the substrate and react with one another to produce the layers that make up the finished device.

The reactants must be gaseous or at least have a high vapor pressure at room temperature, so that they can be finely controlled while being fed into the vacuum chamber. They must also break down as they flow over the hot substrate, leaving only the desired element—gallium, for example—without any contaminating atoms or molecules. Organic molecules incorporating the metal, such as trimethylgallium, work well (hence the term metal-organic).

By varying the substrate temperature and the pressures, flow rates, and compositions of the reactant gases, technicians fabricate devices layer by layer, in batches of several hundred on a single substrate wafer. An LED might have 40 layers, including a stack of perhaps five or 10 quantum wells, where electrons and holes combine to produce light. These quantum wells, just 3­4 nm thick, are separated by barrier layers and sandwiched in between the n-type and p-type regions.

Besides Lumileds, top competitors include Nichia, which in 2001 had close to 40 percent of the market for blue, green, and white LEDs, according to Strategies Unlimited's Steele. Cree Inc., a compound-semiconductor powerhouse, produces more than two million GaN LED chips per day at its huge fab facilities in Durham, N.C., but does not sell finished, packaged devices. One of Cree's big customers is GELcore LLC (Valley View, Ohio), a joint venture between General Electric Co. and Emcore Corp. (Somerset, N.J.), a maker of MOCVD equipment. Osram Opto Semiconductors (Regensburg, Germany) produces a line of GaN LEDs, as does Japanese chip maker Rohm Co. Toshiba Corp. (31) is allied with LED specialist Toyoda Gosei Co., and Mitsubishi Chemical Corp. (97) is funding research on LED lighting at the University of California, Santa Barbara.

From here to hundreds of lumens

The basic problem with GaN is that no one has yet figured out how to grow the semiconductor in bulk form. That is why devices made of the compound semiconductor must be fabricated today on substrates of sapphire or silicon carbide—flawless bulk GaN, which would work perfectly, simply isn't available.

Because materials other than GaN are used as substrates, there is inevitably some mismatch where the crystalline lattices of the substrate and the semiconductor meet. This mismatch produces imperfections called dislocations, which are a prime culprit in the conversion of photons to heat in the devices.

Thus a great deal of research on GaN LEDs is devoted to reducing the densities of these dislocations, which can exceed 10 000 million per cubic centimeter. Although researchers invariably decline to elaborate on their strategies, many of these involve variants of a technique known as lateral overgrowth. Using MOCVD, they deposit material in an elaborate pattern of layers and shapes designed to block faults from the mismatched lattices from intruding into the chip's quantum wells.

Sandia National Laboratories (Albuquerque, N.M.) is less inhibited than its private industry counterparts. Its researchers will talk specifics about their extensive research effort on GaN LEDs. Their variation on lateral overgrowth is called cantilever epitaxy. First, a sapphire wafer is etched with grooves that cover about 90 percent of its surface. The grooves are 5­10 µm wide, separated by long 2-µm-wide strips they call pedestals. Using MOCVD, the researchers grow GaN vertically on top of and in between the pedestal strips. Then, by changing the temperature of the substrate, they change the mode of growth, so that the GaN materializes horizontally. It grows outward from the tops of the pedestals, like tiny diving boards.

The tips of the diving boards from adjacent pedestals meet above a groove at ever so slight an elevation above the GaN material in the bottom of that groove. In the regions extending from the pedestals to where the diving boards meet, in so-called coalescence regions, the density of defects is about 10 million per square centimeter, or less than one-hundredth what you'd get by simply putting GaN on top of sapphire. And when Sandia's researchers fabricate an LED chip on top of that structure—the device sprawls across dozens or hundreds of pedestals—"we get a 1500 percent increase in brightness over the earlier LEDs we were growing," reports Jerry A. Simmons, manager of the semiconductor physics department at Sandia.

Through this and countless other advances, LED researchers promise that in the coming decade the diodes will follow a performance curve that will look like a rocket's trajectory. From about 25 lm/W today to 50 lm/W in 2005, to 75 lm/W in 2007, and on to 150 lm/W by 2012. Finally, they hope to reach 200 lm/W in 2020. Even then there will be room for improvement—the white LED's theoretical maximum is between 300 and 400 lm/W, depending on how much green you like in your white light.

At the same time, the wattage per device will continue climbing, from 5 W today to 10 W and 500 lm in 2005—more than a 40-W incandescent light bulb. In fact, one U.S. executive says she has already seen a 5-A, 500-lm LED in a laboratory in Asia. Although the device was not a white-light LED, it proves that astounding current levels are possible, she notes. By 2012, researchers pledge that the devices will reach 7 W and 1000 lm. Think about that for a moment: the device would be brighter than a 60-W bulb and yet draw an amount of current that could be provided by four D-size batteries.

Around then, Edison's "gas and glass," the scornful term the solid-state people apply to light bulbs, should be on its way out. "In 10 or 20 years, all lighting will be solid state," asserts Masayuki Ishikawa at Toshiba Corp.'s corporate R&D center in Kawasaki, Japan. The fat lady may not be singing, but the iguana sure is squinting.

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