Jun 5, 2008
Close-packed array improves LED output
Air holes between hexagonal-close-packed microspheres make an affordable template for etching light-extracting photonic crystal arrays, claim researchers. Scientists from the University of Hong Kong observed a three-fold enhancement in LED light emission intensity when they incorporated the nanopattern onto the surface of their test device.
"It takes just 5 ml of a 1% silica microsphere suspension, which costs only tens of dollars, to coat a 2 inch wafer," Anthony H W Choi of the university's department of electrical and electronic engineering told nanotechweb.org. "Compared with the huge investment of an e-beam writer, high maintenance costs and expensive electron resists, microsphere lithography is a cheap and practical nanotechnology."
To pattern the LED wafer, the researchers begin by spin coating a GaN substrate with a dispersion of silica microspheres (mean diameter of 500 nm) in deionized water. The particles spontaneously self-assemble into a single layered hexagonal-close-packed array, which doubles as an effective etching mask.
Choi and his colleagues use a dry etching process to transfer the pattern of air holes onto the LED wafer. The silica particles are subsequently removed from the substrate by sonification to reveal an array of triangular-shaped voids spaced at intervals of 290 nm along the semiconductor surface.
As the researchers explain, the introduction of a photonic crystal structure on the surface of an LED can improve light extraction by diffracting waveguided modes out of the semiconductor. The team observed that LEDs made without the array displayed interference effects caused by light bouncing to and fro within the device.
The researchers presented their work in Nanotechnology.
About the author
James Tyrrell is editor of nanotechweb.org.