Aug 1, 2006
Nanoetching makes LEDs brighter
Researchers in the US claim to have made a resonant cavity LED (RCLED) seven times brighter over a 130 nm bandwidth by etching circular Bragg gratings into the device structure (Applied Physics Letters 89 033105). The technique could have applications in displays as well as medical imaging - in particular, say the researchers, optical coherence tomography (OCT).
"We have demonstrated a 7.5x enhancement of light extraction from a RCLED, which in turn has an enhancement factor of 2.7x over a conventional LED," Mark Su, lead author of the paper, told optics.org. "Whereas only around 2% of the light generated within a conventional LED can be collected, we estimate that we harvested 41% of the light generated within the LED."
A resonant cavity is a way to improve the light extraction of a regular LED by placing a mirror below the active layer. However, even in this configuration, most of the emitted light is waveguided below the semiconductor surface and lost. "We used a cavity defined by a circular Bragg grating to diffract the guided waves into waves which can be extracted - resulting in the improved light extraction," explained Su.
The team, based at the National Institute for Standards and Technology (NIST), made an infrared-emitting LED composed of gallium arsenide packed with quantum dots of indium gallium arsenide. The grating was etched by electron-beam lithography.
According to the researchers, the technique is immediately applicable to infrared LEDs and could have applications in areas such as biomedical imaging; in particular, OCT that requires bright sources with a wide bandwidth.
"Present-day broadband OCT sources have many undesirable properties, such as cost, bulkiness, and problems with the shape of the spectrum they output. So there is room for improvement," said Su.
The team believes that nanophotonic structures could also benefit high performance LEDs for applications such as projectors and displays. "For the visible part of the spectrum, bright LEDs incorporating design features such as circular Bragg gratings could enable energy-efficient projection TVs, or even a new generation of miniature portable projectors," said Su.
Su adds that the present-day visible sources that could be replaced by LED technology are devices like mercury arc lamps and halogen bulbs, which are power-hungry, hot, bulky, burn out and are costly to replace.
About the author
Darius Nikbin is technology reporter for optics.org and Optics & Laser Europe.