The University of Glasgow is to combine quantum dot phosphors with its LED-boosting photonic crystals, aiming at lighting.
"We have just begun to try to insert quantum dots into photonic crystal holes to tightly couple light to the quantum dots," researcher Dr Faiz Rahman told Electronics Weekly.
Late last year, Rahman revealed that he was making photonic crystals in the surface of LED to increase light extraction.
"The problem with all LEDs is they trap a lot of the light they generate," he said. "Only about 30 per cent of generated photons manage to get out of the surface."
Light leaving at right angles to the LED surface escapes, but anything trying to leave at an angle less than the critical angle is bounced back into the die by total internal reflection.
Manufactures know this and, according to Rahman, abrade the surface of LED wafers causing random marks that increase light extraction to 40 per cent: "Most LEDs these days use roughened wafers."
However, "we etch a diffraction grating into the surface to couple the light out. It changes the propagation of light from a trapped horizontal mode to an escaping vertical mode", he said. "We are probably getting something like 60 per cent."
The crystal takes the form of an array of holes typically 200nm diameter, spaced by 200nm, and 200nm deep - too shallow to damage the device junction.
Rahman says his group is not the only one working with hole-based photonic crystals to improve LED light extraction, but only Glasgow is using quasi-crystals.
"Everyone else is using regular 100 per cent periodic photonic crystals. We are using quasi crystals which look regular on a large scale but are quite irregular on short scales," he said, explaining: "Quasi-crystal patterns can control the direction of escaping light."
What sort of irregularity?
"It is quite complex, we use a powerful supercomputer in the University of Southampton to model them, said Rahman. "On the LED we repeat the quasi-crystal."
The holes are too small to define by conventional lithograph, and electron beam techniques have been used for research. Now Rahman is moving to nano-imprint lithography, where a physical stamp and pressure is used to emboss etch resist, to develop a production technique. "Everyone is of the opinion nano-imprint lithography will make it into the mainstream," he said.
Glasgow has been collaborating with Sharp Labs at Oxford to improve LED backlighting for LCD panels, but is broadening its research in response to huge interest in LEDs for lighting. "We are thinking very seriously of extending to room and space lighting," said Rahman, "and are trying various experiments with quantum dots."
Almost all 'white' LEDs are a blue LED with a broadband yellow phosphor on top. These appear white, but do not render colours as well as many other light sources.
Appearing in the wings is quantum dot phosphor technology.
When made into nano-sale particles, certain photo-active materials - cadmium telluride is an example, absorb incident light and re-emit it at a wavelength determined by the physical size of the particle. And there are cadmium-free alternatives, Nanoco of Manchester is working on one.
Photonic crystal holes etched into the surface of a GaN LED chip. Photo credit - Dr Ian Watson, Institute of Photonics, University of Strathclyde.
By carefully creating or selecting the particles, narrow band phosphors can be created that will convert blue LED light to red or green, potentially enabling white LEDs which render colours more accurately.
Putting quantum dots inside photonic crystal holes will give Rahman tight control of many parameters. "Maybe it will give us a useful boost in efficiency," he said.
See also: Electronics Weekly's roundup of content related to LEDs, with a special focus on both white LEDs and coloured LEDs:
LED technology - White LEDs
LED technology - Coloured LEDs
LED technology - LEDs general