[Nmcaver] New LED Breakthrough

[Mark Minton]

	The National Institute of Standards has developed a new way of 
making LEDs which makes them vastly more 
efficient: 
<http://www.nist.gov/public_affairs/techbeat/tb2006_0720.htm#novel>.

Mark Minton


Novel Nano-Etched Cavity Makes LEDs 7 Times Brighter

Researchers at the National Institute of Standards and Technology 
(NIST) have made semiconductor light-emitting diodes (LEDs) more than 
seven times brighter by etching nanoscale grooves in a surrounding 
cavity to guide scattered light in one direction. The novel 
nanostructure, which may have applications in areas such as in 
biomedical imaging where LED brightness is crucial, is described in 
the July 17 issue of Applied Physics Letters.*

Semiconductor LEDs are used increasingly in displays and many other 
applications, in part because they can efficiently produce light 
across a broad spectrum, from near-infrared to the ultraviolet. 
However, they typically emit only about two percent of the light in 
the desired direction: perpendicular to the diode surface. Far more 
light skims uselessly below the surface of the LED, because of the 
extreme mismatch in refraction between air and the semiconductor. The 
NIST nanostructured cavity boosts useful LED emission to about 41 
percent and may be cheaper and more effective for some applications 
than conventional post-processing LED shaping and packaging methods 
that attempt to redirect light.

The NIST team fabricated their own infrared LEDs consisting of 
gallium arsenide packed with "quantum dots" of assorted sizes made of 
indium gallium arsenide. Quantum dots are nanoscale semiconductor 
particles that efficiently emit light at a color determined by the 
exact size of the particle. The LEDs were backed with an alumina 
mirror to reflect the light emitted backwards. The periphery of each 
LED was turned into a cavity etched with circular grooves, in which 
the light reflects and interferes with itself in an optimal geometry.

The researchers experimented with different numbers and dimensions of 
grooves. The brightest output was attained with 10 grooves, each 
about 240 nanometers (nm) wide and 150 nm deep, and spaced 40 nm 
apart. The team spent several years developing the design principles 
and perfecting the manufacturing technique. The principles of the 
method are transferable to other LED materials and emission 
wavelengths, as well as other processing techniques, such as 
commercial photolithography, according to lead author Mark Su.

* M.Y. Su and R.P. Mirin. Enhanced light extraction from circular 
Bragg grating coupled microcavities. Applied Physics Letters. July 17