LED breakthrough may revolutionize lighting

LED breakthrough may revolutionize lighting

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Purdue researchers achieve LED production breakthrough which clears the way for low-cost, high-efficiency lighting.

LED light bulbs are about four times more efficient than conventional incandescent lights and, because they contain no mercury, more environmentally friendly than compact fluorescent bulbs. LEDs are also longer lasting than conventional lighting, lasting perhaps as long as

15 years before burning out.

"LED technology has the potential of replacing all incandescent and compact fluorescent bulbs, which would have dramatic energy and environmental ramifications," said Timothy D. Sands, the Basil S. Turner Professor of Materials Engineering and Electrical and Computer Engineering at Purdue University.

But LED lights now on the market are prohibitively expensive, in part because they are created on a substrate, or first layer, of sapphire. The Purdue researchers have solved this problem by developing a technique to create LEDs on low-cost, metal-coated silicon wafers, said Mark H. Oliver, a graduate student in materials engineering who is working with Sands.

LEDs designed to emit white light are central to solid-state lighting, semiconducting devices made of layers of materials that emit light when electricity is applied. Conventional lighting generates light with hot metal filaments or glowing gasses inside glass tubes.

The LEDs have historically been limited primarily to applications such as indicator lamps in electronics and toys, but recent advances have made them as bright as incandescent bulbs.

The light-emitting ingredient in LEDs is a material called gallium nitride, which is used in the sapphire-based blue and green LEDs, including those in traffic signals. The material also is used in lasers in high-definition DVD players. The sapphire-based technology, however, is currently too expensive for widespread domestic-lighting use, costing at least 20 times more than conventional incandescent and compact fluorescent light bulbs.

One reason for the high cost is that the sapphire-based LEDs require a separate mirrorlike collector to reflect light that ordinarily would be lost. In the new silicon-based LED research, the Purdue engineers "metallized" the silicon substrate with a built-in reflective layer of zirconium nitride.

"When the LED emits light, some of it goes down and some goes up, and we want the light that goes down to bounce back up so we don't lose it," said Sands, the Mary Jo and Robert L. Kirk Director of the Birck Nanotechnology Center in Purdue's Discovery Park.

Ordinarily, zirconium nitride is unstable in the presence of silicon, meaning it undergoes a chemical reaction that changes its properties.

The Purdue researchers solved this problem by placing an insulating layer of aluminum nitride between the silicon substrate and the zirconium nitride.

"One of the main achievements in this work was placing a barrier on the silicon substrate to keep the zirconium nitride from reacting," Sands said.

Until the advance, engineers had been unable to produce an efficient LED created directly on a silicon substrate with a metallic reflective layer. Until the advance, engineers had been unable to produce an efficient LED created directly on a silicon substrate with a metallic reflective layer.

The Purdue team used a technique common in the electronics industry called reactive sputter deposition. Using the method, the researchers bombarded the metals zirconium and aluminum with positively charged ions of argon gas in a vacuum chamber. The argon ions caused metal atoms to be ejected, and a reaction with nitrogen in the chamber resulted in the deposition of aluminum nitride and zirconium nitride onto the silicon surface. The gallium nitride was then deposited by another common technique known as organometallic vapor phase epitaxy, performed in a chamber, called a reactor, at temperatures of about

1,000 degrees Celsius, or 1,800 degrees Fahrenheit.

As the zirconium nitride, aluminum nitride and gallium nitride are deposited on the silicon, they arrange themselves in a crystalline structure matching that of silicon.

"We call this epitaxial growth, or the ordered arrangement of atoms on top of the substrate," Sands said. "The atoms travel to the substrate, and they move around on the silicon until they find the right spot."

This crystalline formation is critical to enabling the LEDs to perform properly.

"It all starts with silicon, which is a single crystal, and you end up with gallium nitride that's oriented with respect to the silicon through these intermediate layers of zirconium nitride and aluminum nitride," Sands said. "If you just deposited gallium nitride on a glass slide, for example, you wouldn't get the ordered crystalline structure and the LED would not operate efficiently."

Using silicon will enable industry to "scale up" the process, or manufacture many devices on large wafers of silicon, which is not possible using sapphire. Producing many devices on a single wafer reduces the cost, Sands said.

Another advantage of silicon is that it dissipates heat better than sapphire, reducing damage caused by heating, which is likely to improve reliability and increase the lifetime of LED lighting, Oliver said.

The widespread adoption of solid-state lighting could have a dramatic impact on energy consumption and carbon emissions associated with electricity generation since about one-third of all electrical power consumed in the United States is from lighting.

The widespread adoption of solid-state lighting could have a dramatic impact on energy consumption and carbon emissions associated with electricity generation since about one-third of all electrical power consumed in the United States is from lighting.

"If you replaced existing lighting with solid-state lighting, following some reasonable estimates for the penetration of that technology based on economics and other factors, it could reduce the amount of energy we consume for lighting by about one-third," Sands said. "That represents a 10 percent reduction of electricity consumption and a comparable reduction of related carbon emissions."

Incandescent bulbs are about 10 percent efficient, meaning they convert 10 percent of electricity into light and 90 percent into heat.

"Its actually a better heater than a light emitter," Sands said.

By comparison, efficiencies ranging from 47 percent to 64 percent have been seen in some white LEDs, but the LED lights now on the market cost about $100.

"When the cost of a white LED lamp comes down to about $5, LEDs will be in widespread use for general illumination," Sands said. "LEDs are still improving in efficiency, so they will surpass fluorescents. Everything looks favorable for LEDs, except for that initial cost, a problem that is likely to be solved soon."

He expects affordable LED lights to be on the market within two years.

Two remaining hurdles are to learn how to reduce defects in the devices and prevent the gallium nitride layer from cracking as the silicon wafer cools down after manufacturing.

"The silicon wafer expands and contracts less than the gallium nitride," Sands said. "When you cool it down, the silicon does not contract as fast as the gallium nitride, and the gallium nitride tends to crack."

Sands said he expects both challenges to be met by industry.

"These are engineering issues, not major show stoppers," he said. "The major obstacle was coming up with a substrate based on silicon that also has a reflective surface underneath the epitaxial gallium nitride layer, and we have now solved this problem."

The research, based at the Birck Nanotechnology Center and funded by the U.S. Department of Energy through its solid-state lighting program, is part of a larger project at Purdue aimed at perfecting white LEDs for lighting.

References: Science Daily Adapted from materials provided by Purdue University. Purdue University (2008, July 21). Advance Brings Low-cost, Bright LED Lighting Closer To Reality. ScienceDaily.

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In , rpautrey2 wrote the following that caught me a bit as hype:

Same as CFLs - and that is for better than is achieved by most LED lighting products.

Although this is true when you have LED outperforming CFL (which is not common in lighting), keep in mind that CFL is better for the environment than incandescent - and close to a draw if even considering only mercury! Coal combustion is a huge source of mercury pollution, and CFLs achieving

6,000 hour life when replacing incandescents as low as 60 watts will actually not increase mercury pollution even if their recycling rate is zero! And to improve upon that -
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Also, Home Depot takes worn-out CFLs for proper disposal!

"Perhaps as long as"?

What about the widely-touted 100,000 hours?

What about better major brand white ones achieving 50,000 hours before fading by at least 30% "with good treatent" including heatsinking to extent of achieving temperature significantly cooler than "dataseet temperature limit"?

Also, rated efficiency tends to be achieved when either "heatsinkable surface" or the hottest internal point of the chip(s) of the LED is cooled to 25 degrees C!

It has been brought to my attention a few times already over the past many years how there were supposed to be in-the-works good-high-efficiency LED chips with silicon substrate!

Cree Inc. has been for a goodly few years already been achieving such LEDs with silicon carbide substrate - and even appears to me to have achieved transparent silicon carbide!

A mirror under the chip is dirt-cheap!

That good? I have yet to find a 120V 100W A19 incandescent even rated

1750 lumens of 7-+ efficiency at converting input to electromagnetic radiation of wavelengtths 400-700 nm!

Please tell me mfr, part number and supplier for any white LED that I can buy that achieves 47-64% efficiency, along with conditions for achieving such efficiency. That sounds to me like about 140-190 lumens/watt.

So surpassing of fluorescents has yet to be achieved? I thought that has been recently achieved already by small margin with lower wattages with great upfront-cost-per-whatever.

Read the posting that I quoted from if you need to see everything that I snipped. And buyer beware!

- Don Klipstein ( snipped-for-privacy@misty.com)

Reply to
Don Klipstein

Don't take press releases from such sites without a pinch of salt.

It'll be years before they become mainstream and many false claims have already been made along the route.

Incidentally, the mercury claim is essentially bogus and purely there to appeal to those who succumb to the 'fear factor'. Making semiconductors is pretty nasty too.


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If every stupid premature university lab "breakthrough" press release was posted here, there would be no way to sift out anything useful.

Let me know when these things are on sale at Safeway.

How much does 1e-8 square meters of sapphire substrate cost anyhow? Sapphire is just aluminum oxide.


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John Larkin


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John Fields

I would like to know where that one gets 47-64% efficiency for "some white LEDs".

I have one piece of hard data for number of lumens in 1 watt of "white LED light": 331. This figure will vary with spectral characteristics.

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That LED achieved 138 lumens/watt while achieving 41.7% conversion efficiency. (The LED makers like to call that "wallplug efficiency", even though that is milliwatts of light out per 100 milliwatts delivered to the LED, not per 100 milliwatts from the power supply.)

It appears to me that the 150 lumen/watt laboratory prototype developed afterwards achieves conversion efficiency somewhere around 45%.

That article on the 41.7% efficient white LED does mention 63.3% quantum efficiency for a blue LED chip. That means 633 photons emitted per 1,000 electrons pushed through. However, each photon has less energy than was expended to push each electron through - in the ballpark of 2.7 eV vs. 3.2 eV. Meanwhile, the phosphor that is added to make a blue LED into a white one has both quantum loss and Stokes loss. Because of the Stokes loss, the average energy per photon from a usual white LED is more like 2.3-2.35 eV.

And the Osram laboratory prototype that I mentioned in a different thread achieved 170 lumens/watt when very greatly underpowered. Also, that one's light is more yellowish/greenish than that of most other white LEDs, so its light probably has more than 331 lumens per radiated watt. So that looks to me like about 50% conversion efficiency at the fairly severe degree of underpowering that results in maximum efficiency.

Yes, I see a fair amount of hype in press releases, and also by LED lighting product manufacturers and sellers.

- Don Klipstein ( snipped-for-privacy@misty.com)

Reply to
Don Klipstein

Don, I've been using CFL's and other flourescent lighting for many years. My experience with the CFL's is some don't come on instantly and their reliability isn't what it's claimed to be. I'm looking forward to LED lighing. I'm pleased with the light and reliability of my LED flashlights and lanterns.







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Peter Hucker

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