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Energy@Rensselaer: Zeroing in on the Elusive Green LED
Researchers Discover New Method for Boosting the
Light Output of Green LEDs — A Critical Step Toward the
Development of LED Televisions and Displays
Researchers at Rensselaer Polytechnic Institute have
developed a new method for manufacturing green-colored LEDs
with greatly enhanced light output.
The research team, led by Christian Wetzel,
professor of physics and the Wellfleet Constellation Professor
of Future
Chips at Rensselaer, etched a nanoscale pattern at the
interface between the LED’s sapphire base and the layer of
gallium nitride (GaN) that gives the LED its green color.
Overall, the new technique results in green LEDs with
significant enhancements in light extraction, internal
efficiency, and light output.
The discovery brings Wetzel one step closer to his goal of
developing a high-performance, low-cost green LED.
“Green LEDs are proving much more challenging to create than
academia and industry ever imagined,” Wetzel said. “Every
computer monitor and television produces its picture by using
red, blue, and green. We already have powerful, inexpensive red
and blue LEDs. Once we develop a similar green LED, it should
lead to a new generation of high-performance, energy-efficient
display and illumination devices. This new research finding is
an important step in the right direction.”
Sapphire is among the least expensive and widely used
substrate materials for manufacturing LEDs, so Wetzel’s
discovery could hold important implications for the rapidly
growing, fast-changing LED industry. He said this new method
should also be able to increase the light output of red and
blue LEDs.
Results of the study, titled “Defect-reduced green GaInN/GaN
light-emitting diode on nanopatterned sapphire,” were published
last week in the journal Applied Physics Letters, and
are featured in today’s issue of the Virtual Journal of
Nanoscale Science & Technology, published by the
American Institute of Physics and the American Physical
Society. The paper may be viewed online at: http://dx.doi.org/10.1063/1.3579255
The research program is supported by the U.S. Department of
Energy National Energy Technology Laboratory (NETL) Solid-State
Lighting Contract of Directed Research, and the National
Science Foundation (NSF) Smart Lighting
Engineering Research Center (ERC), which is led by
Rensselaer.
LED lighting only requires a fraction of the energy required
by conventional light bulbs, and LEDs contain none of the toxic
heavy metals used in the newer compact fluorescent light bulbs.
In general, LEDs are very durable and long-lived.
First discovered in the 1920s, LEDs – light-emitting diodes
– are semiconductors that convert electricity into light. When
switched on, swarms of electrons pass through the semiconductor
material and fall from an area with surplus electrons into an
area with a shortage of electrons. As they fall, the electrons
jump to a lower orbital and release small amounts of energy.
This energy is realized as photons – the most basic unit of
light. Unlike conventional light bulbs, LEDs produce almost no
heat.
The color of light produced by LEDs depends on the type of
semiconductor material it contains. The first LEDs were
red, and not long thereafter researchers tweaked their formula
and developed some that produced orange light. Years later came
blue LEDs, which are frequently used today as blue light
sources in mobile phones, CD players, laptop computers, and
other electronic devices.
The holy grail of solid-state lighting, however, is a true
white LED, Wetzel said. The white LEDs commonly used in novelty
lighting applications, such as key chains, auto headlights, and
grocery freezers, are actually blue LEDs coated with yellow
phosphorus – which adds a step to the manufacturing process and
also results in a faux-white illumination with a noticeable
bluish tint.
The key to true white LEDs, Wetzel said, is all about green.
High-performance red LEDs and blue LEDs exist. Pairing them
with a comparable green LED should allow devices to produce
every color visible to the human eye – including true white,
Wetzel said. Today’s computer monitor and television produces
its picture by using red, blue, and green. This means
developing a high-performance green LED could therefore likely
lead to a new generation of high-performance, energy-efficient
display devices.
The problem, however, is that green LEDs are much more
difficult to create than anyone anticipated. Wetzel and his
research team and investigating how to “close the green gap,”
and develop green LEDs that are as powerful as their red or
blue counterparts.
For more information on the Wetzel’s research at Rensselaer,
visit:
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Published
April 25,
2011 |
Contact: Michael Mullaney
Phone: (518) 276-6161
E-mail: mullam@rpi.edu |
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