LEDs find wide use in displays and other applications, partly because they efficiently emit light across a broad spectrum, from near-infrared to ultraviolet.
However, only about 2% of LED light goes in the desired direction, perpendicular to the diode surface. The rest skims uselessly away because of the extreme mismatch in refraction between air and the semiconductor.
Now, researchers at the National Institute of Standards and Technology (NIST) have built LEDs that are more than 7 X brighter by etching nanoscale grooves in a surrounding cavity. The grooves guide scattered light in one direction, boosting useful emission to about 41%. The approach may cost less and work better for some applications compared with conventional shaping and packaging methods that attempt to redirect light.
The NIST infrared LEDs consist of gallium arsenide packed with quantum dots of indium gallium arsenide. Quantum dots are nanoscale pieces of semiconductor that efficiently emit light at a color determined by their size. An alumina mirror reflects the emitted light backwards. The researchers tried different numbers and dimensions of grooves. It turns out the brightest output comes with 10 grooves, each about 240-nm wide, 150-nm deep, and spaced 40-nm apart. The method is transferable to other LED materials and wavelengths as well as other processing techniques such as commercial photolithography, says the group.