Researchers at the National Institute of Standards and Technology (NIST) have made GaAs-based 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.*
LEDs typically emit only about two percent of the light in the
desired direction: perpendicular to the diode surface. Far more light is
internally reflected and stays within 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 (GaAs) packed with quantum dots of assorted sizes made of
indium gallium arsenide (InGaAs). 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 light
towards the top surface. 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, 2006
Etched nanostructured rings around an LED can make it more than
seven times brighter. The novel technique developed at NIST may have
applications in areas such as in biomedical imaging where LED brightness is
crucial. Credit: NIST
