news review
Cambridge and Plessey all set and ready to glow
A NEW technique for manufacturing gallium nitride LEDs on silicon is to be exploited in the UK, putting mass- produced, energy-efficient lighting within reach. Researchers say that the novel process is paving the way for manufacturing affordable LED lights and could have a dramatic impact on carbon emissions. Plessey, a semiconductor manufacturer based in Plymouth, will use the process, developed at the University of Cambridge. It involves growing GaN crystals on silicon and could drastically reduce the cost of making LEDs for lighting in offices and homes.
Researchers estimate that a worldwide switch to LEDs, which are far more energy- efficient, would enable the closure of 560 power plants and result in annual CO2 savings equivalent to the output of all the cars on the planet.
LEDs are also being designed to give off a more natural quality of light, which will be better for people’s health. They could potentially be used as “smart lighting” that can switch itself on or off when a person enters or leaves a room, or adjust its brightness according to the amount of daylight. Now a team led by Colin Humphreys at the Department of Materials Science and Metallurgy, University of Cambridge, says it has developed a new method which will be commercialised by a new arm of Plessey.
Called Plessey Lighting, the division will initially make LEDs on silicon for external manufacturers, but in time hopes to develop its own light bulbs in-house. “We’ve got higher efficiency for growing gallium nitride on silicon than anyone else we know,” Humphreys said. “Potentially this is a deal that puts Britain right at the forefront of LED research.”
“LED light bulbs currently cost as much as £40, but we expect to be able to reduce that cost by a factor of five by growing on silicon. Mass manufacturing may reduce the cost further. Eventually I think that we will see LED lighting being fitted throughout the world.”
Value for money is high as LEDs last as long as 100,000 hours. Since the average light bulb in Britain is on for four hours a
day, LED light bulbs would only have to be changed once in every person’s lifetime. LEDs, which require far less energy than conventional bulbs, are currently about 30% efficient and Humphreys hopes to raise that figure to 60% in time. In the UK alone, the researchers estimate that the nationwide use of LEDs would save 15% of the electricity generated by power stations, resulting in a similar reduction in CO2 emissions. There are also potential cost savings, as a (2010) US Department of Energy report estimates that savings of $20 billion per year would result if LED light bulbs became widespread in the US.
The technique developed by Humphreys resolves several problems with trying to grown GaN on silicon. In particular, it addresses difficulties to do with thermal expansion under the growth temperature of 1,0000 C. Gallium nitride expands at a very different rate to silicon and when the two substances cool down, the material tends to crack.
Humphreys’ team have been able to introduce layers to the process that put the GaN in a state of compression before it heats up. “This compression balances the tension when it cools down, which allows the material to relax,” he said. “As a result, we can grow LED structures which are totally crack-free.”
The new process will grow LEDs on 6-inch silicon wafers. Each costs about £20 ; cheaper than the 2-inch sapphire wafers used in the past, and ten times more LEDs can be grown on a 6-inch wafer than on a 2-inch. Plessey was chosen because it already has the equipment to manufacture silicon wafers of this size. What’s more, its work in electronics means that, in time, the company may be able to develop “smart lighting” from LED light bulbs as well.
“It’s very important to us that this research will be exploited here in the UK,” Humphreys added. “If we had stopped at the research stage, our work would probably have been picked up and commercialised overseas. This way, we can create more jobs in a low-employment part of the country and potentially turn Britain into a major centre for better, greener lighting.”
An LED emitting light on a 6-inch silicon wafer.(Credit: Colin Humphreys)
Minister for Universities and Science David Willetts said, “Professor Humphreys’ work shows the potential of science to drive growth and create the high-tech jobs of the future. Not only could his research result in a highly marketable, low carbon alternative to the everyday light bulb, but he has worked closely with industry to ensure that the commercialisation process also happens in the UK.”
There are a number of players developing GaN on silicon technology for LEDs. Osram Opto is marketing InGaN LED chips fabricated on 150 mm (6-inch) silicon wafers. Researchers at the company have succeeded in manufacturing high performance prototypes of blue and white LEDs, in which the light-emitting GaN layers are grown on 150 mm silicon wafers.
German based Azzurro Semiconductors also grows on 150 mm silicon and its patented technology allows the firm to grow GaN epitaxial layers up to 8 µm thick. The firm says the bow of the 150 mm wafers is below 20 µm and allows the use of standard CMOS-fabs for a massive cost breakthrough for LEDs.
Last October, Bridgelux closed an additional $15 million in financing raised specifically to further accelerate research, development, and scaling of the company’s position in GaN on silicon LED chip technologies ; it’s targeted for commercialisation in 2013.
Siltronic AG and Belgian nano-electronics research institute imec are setting their sights even higher and are developing a process to grow on 200 mm silicon wafers.
They are collaborating on the development of silicon wafers with a GaN layer as partner of imec’s GaN on silicon industrial affiliation program (IIAP). The venture aims to enable future production of LEDs and power devices.
March 2012
www.compoundsemiconductor.net 9
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