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Tuesday, July 22, 2008

Purdue research have developed new production methods which will cut LED lighting costs

Purdue's Professor Sands and a graduate researcher man the reactor, which produces the first blue LEDs based on a silicon process. The new LEDs promise greater efficiency, longer lifetimes, and much lower costs.

Breakthrough Allows LED Lighting to be Processed on Silicon at Great Savings
The research was funded by the U.S. Department of Energy's solid-state lighting program, which the L Prize is based on. The project is part of a broader effort by Purdue to perfect white LED lighting, and perhaps take home the L Prize.
Purdue research have developed new production methods which will cut LED lighting costs to about a twentieth of current expenses

Purdue research have developed new production methods which will cut LED lighting costs to about a twentieth of current expenses

The U.S. Department of Energy recently announced a $20M USD "L Prize" for the first solid-state lighting meeting a strict standard of criteria. While the prize sounded very intriguing, the fact of the matter is that solid-state lighting on the market today falls short of the requirements by a sizable margin. Furthermore, it is far too expensive to see mass adoption.

However, a new breakthrough in processing from the Purdue University may change all of that. Researchers at Purdue have developed a technique to manufacture LED solid-state lighting at low cost using metal-coated silicon wafers.

Traditionally, the light-emitting layer of an LED light is a gallium nitride crystal. In sapphire based LEDs, used for green or blue lighting, mirror-like reflectors are need to reflect and resend emitted light, increasing the efficiency. Typically, this layer is extremely expensive to produce, part of the reason the current generation of LED lighting costs so much, costing at least 20 times more than conventional incandescent and fluorescent bulbs. Also, the LEDs are built on sapphire crystals, which provide the color, but are extremely expensive. The method uses aluminum nitride to provide the tint.

The new LEDs use a layer of zirconium nitride to provide the mirror effect. Normally, zirconium nitride reacts with silicon, making a silicon process difficult. However, by isolating the zirconium nitride with a protective layer to prevent reaction, scientists are able to fully deposit the need layers, including the gallium nitride necessary to build a full LED.

Timothy D. Sands, the Basil S. Turner Professor of Materials Engineering and Electrical and Computer Engineering states, "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. One of the main achievements in this work was placing a barrier on the silicon substrate to keep the zirconium nitride from reacting."

With the advance, for the first time the LEDs will be able to be produced on standard silicon wafers. The new wafers can be made using cheap existing processes. To deposit the colored layer, reactive sputter deposition is used. Aluminum is bombarded with positive Argon ions, which send it flying out into the air, reacting with nitrogen gas and being deposited on the silicon. For the zirconium reflective layer, an identical process is used with zirconium metal in place of aluminum. The final gallium layer is deposited using organometallic vapor phase epitaxy; a common deposition technique performed using high heat.

The new techniques yield a crystalline structure aligned to the crystalline silicon. This means that the LEDs are less prone to defects and will perform more efficiently. Further, by using common techniques costs are dramatically reduced from using more expensive alternative methods like crystal growth on glass using sapphire crystals.

Another advantage is that silicon dissipates heat more effectively than sapphires. This will reduce damage during operation and lead to longer lifetimes and more reliability.

The new device is extremely promising as it may allow lighting to finally do primarily what it was intended -- make light. Traditional incandescent bulbs are better heaters than lights, wasting 90 percent of energy as heat. LEDs currently on the market have efficiencies from 47 to 64 percent of energy converted into light, with the Purdue design expected to fall on the high-end of this range.

With one third of U.S. electricity going to lighting and tremendous lighting-related consumption worldwide, widespread adoption of LED lighting could cut world electric usage by 10 percent. Says Professor Sands, "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. That represents a 10 percent reduction of electricity consumption and a comparable reduction of related carbon emissions."

Professor Sands expects the process to be commercially adopted and operating within two years. A final hurdle for it to overcome is a problem with the gallium nitride layer cracking during cooling. He believes this problem will soon be solved, though, with a bit more research. He states, "These are engineering issues, not major show stoppers. 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 researchers' findings are reported in this month's edition of the journal Applied Physics Letters, published by the American Institute of Physics.

The other researchers contributing to the project led by Professor Sands were Jeremy L. Schroeder, David A. Ewoldt, Isaac H. Wildeson, Robert Colby, Patrick R. Cantwell and Vijay Rawat; Eric A. Stach, an associate professor of materials engineering.

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