Advance brings low-cost, bright LED lighting closer to reality
WEST LAFAYETTE, Ind. -
Operating a "reactor"
 |
Researchers at Purdue University have overcome a major obstacle in
reducing the cost of "solid state lighting," a technology that could
cut electricity consumption by 10 percent if widely adopted.
The technology, called light-emitting diodes,
or LEDs, is about four times more efficient than conventional
incandescent lights and more environmentally friendly than compact
fluorescent bulbs. The LEDs also are expected to be far longer lasting
than conventional lighting, lasting perhaps as long as 15 years before
burning out.
"The 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.
The LED lights are about as efficient as compact fluorescent lights, which contain harmful mercury.
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.
Findings are detailed in a research paper
appearing this month in the journal Applied Physics Letters, published
by the American Institute of Physics.
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.
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.
"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.
Clean
For
many good reasons, an integral part of the new mix of energy
technologies that will be needed to solve these problems is Nuclear.
Wind, solar, geothermal — all available technologies are important and
will have their place in the ultimate solution to our global energy
problem. But the workhorse is going to be nuclear. (see 
