Scientists have found a way to increase the light-output efficiency of LEDs through pioneering nanoelectronics.
Dr Faiz Rahman is putting millions of holes into light emitting diodes to make them brighter. He's not, as you might imagine, trying to increase their surface area. Instead, he's overcoming the physical properties that keep most light trapped inside. And when he's managed to scale up his hole-making process, it'll mean bright low-energy LED light bulbs for the home.
As a nanoelectronics researcher in the University of Glasgow, Rahman knows that LEDs are very energy-efficient. But there's a catch. Although 70% of the energy is converted to light, just 20% of that light escapes. Why? A high refractive index at the LED-air interface means the light is reflected straight back inside. Some LEDs are made from gallium nitride, perhaps the most optically-awkward semiconductor material of all.
Trapping the light
"It's transparent and has a high refractive index. It traps light very effectively," says Rahman. "But LEDs have the potential to be as bright as ordinary bulbs." To realise this potential, Rahman is working on a BERR (DTI)-funded technology project with the Institute of Photonics, University of Strathclyde, Mesophotonics Ltd, and Sharp Laboratories of Europe. "We know how to extract light out of LEDs and that's by making a series of holes on the surface of the LED," says Rahman, the project's lead researcher. He's already made several hole-patterned prototype LEDs, the precise details of which are secret.
These holes are extremely small indeed. At 200 nanometres (nm) in diameter, they are 400 times narrower than a human hair but only penetrate 100nm into the LED's surface. Spaced 300nm apart, 160 holes would fit across a hair's width. Although a single LED chip may be around one-third of a square millimetre (0.3mm by 0.3mm), that's enough space for hundreds of thousands of holes.
So how do these holes work? At such incredibly small scales, structuring the top of the LED semiconductor material with a periodic repeating pattern, or lattice, changes the refractive index (a measure of how light slows inside the medium). This change also reduces the internal reflections from the material's surface which would otherwise trap emitted light inside. Rahman refers to this as a ‘photonic crystal’ - an optical nanostructure that manipulates light. "In a photonic-lattice patterned LED, around 80% of the generated light can be extracted. More, probably, would be possible by future design improvements," he says.
Put aside any ideas of a nano-sized drill; Rahman first used a converted electron microscope — electron beam lithography — to pattern the holes. "It works fine, but it's a very slow process for production," he says. "As yet, LEDs have not been introduced as the standard lighting in homes because the process of making the holes is time-consuming and expensive." To create holes in LED chips using an electron beam, it takes Rahman about 30 minutes to process just a few LEDs. They're first covered with ‘resist’ material. The electron beam rapidly marks the position of the individual holes, which are later etched out using a dry gas mixture of hydrogen and methane, safer than handling liquid acids or solvents. These holes need barely penetrate the active surface of a typical half-millimetre thick LED chip to have the desired optical effect.
While the silicon chip industry has mastered expensive ways of making ultra-small features using photolithography and etching, it's the wrong approach for quickly patterning LEDs on a production line.
Many individual finished LEDs are necessary for a household light bulb, so high-volume manufacturing is essential.
Intricate patterns
He's now turned to nanoimprint lithography. Put simply, he first makes a stamp with nanoscale protrusions where the holes will be. When pressed down onto an LED chip, or more likely, an entire 6-inch wafer of LED devices in the future, these deform the imprint ‘resist’ on the LED surface, leaving a pattern with dry etching techniques later forming the holes as before.
Encouraged by several prototype LEDs, he hopes to tackle small-scale production trials in 18 to 24 months' time. But will he succeed? Aimin Song, professor of nanoelectronics at the University of Manchester, believes there doesn't seem to be a huge challenge in integrating such methods into LED production lines economically. "Nanoimprint (lithography) seems to be the most practically suitable technology for this. One could use e-beam to make an array of holes over a small area and then recombine into a millimetre-sized nanoimprint stamp which is a relatively easy process," says Song.
