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LED breakthrough may revolutionize lighting
http://eartheasy.com/article_led_breakthrough.html
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.
http://eartheasy.com/article_led_breakthrough.html
http://eartheasy.com/article_led_breakthrough.html
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.
http://eartheasy.com/article_led_breakthrough.html