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
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
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. 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.