[tt] Cheap, super-efficient LED lights on the horizon - tech - 29 January 2009 - New Scientist

[Brian Atkins]

http://www.newscientist.com/article/dn16496-cheap-superefficient-led-lights-on-the-horizon.html?DCMP=OTC-rss&nsref=online-news

Incandescent tungsten-filament light bulbs face a global switch-off as 
governments push for energy efficient fluorescent lamps to become the standard. 
But the light could soon go out on those lamps too, now that UK materials 
scientists have discovered a cheaper way to produce LED bulbs, which are three 
times as efficient as fluorescent lamps.

Although the ultimate dominance of LED lights has long been predicted, the 
expense of the super-efficient technology has made the timescale uncertain. The 
researchers now say LED bulbs based on their new process could be commercially 
available within five years.

Gallium nitride (GaN) LEDs have many advantages over compact fluorescent lamps 
(CFLs) and incandescent bulbs. They switch on instantly, with no gradual 
warm-up, and can burn for an average of 100,000 hours before they need replacing 
- 10 times as long as fluorescent lamps and some 130 times as long as an 
incandescent bulb. CFLs also contain small levels of mercury, which makes 
environmentally-friendly disposal of spent bulbs difficult.
Cracking up

The cost of production has kept the LEDs far from the commercial marketplace, 
however. Gallium nitride cannot be grown on silicon like other solid-state 
electronic components because it shrinks at half the rate of silicon as it 
cools. Crystals of GaN must be grown at 1000°C, so by the time a new LED made on 
silicon has cooled, it has already cracked[ok?], rendering the devices unusable.

One solution is to grow the LEDs on sapphire, which shrinks and cools at the 
same rate as GaN. But the expense is too great to be commercially competitive.

Now Colin Humphreys's team at the University of Cambridge has discovered a 
simple solution to the shrinkage problem.

They included layers of aluminium gallium nitride in their LED design. These 
layers shrink at a much faster rate during cooling and help to counteract the 
slow-shrinkage of pure gallium nitride. These LEDs can be grown on silicon as so 
many other electronics components are. "They still work well as LEDs even with 
those extra layers inside," says Humphreys.
Early switch-over

A 15-centimetre silicon wafer costs just $15 and can accommodate 150,000 LEDs 
making the cost per unit tiny. That levels the playing field with CFLs, which 
many people only ever saw as a stopgap solution to the lighting problem.

Humphreys reckons that the UK government encouraged consumers to drop tungsten 
bulbs too soon. "We should have stayed with tungsten for another five years and 
then switched to LEDs," he says.

Humphreys team is being funded by the UK government's Technology Strategy Board 
to turn the new technology into a commercial process.

-- 
Brian Atkins
Singularity Institute for Artificial Intelligence
http://www.singinst.org/