Silicon economy

LEDs could soon be manufactured for less expense using a novel technique for growing gallium nitride (GaN) semiconductors.

The technique, being developed by the Cambridge University-based Centre for Gallium Nitride, grows GaN on silicon wafers. Since the 1990s GaN, used to make LEDs, has been grown in labs on expensive sapphire wafers.

LEDs are an extremely attractive lighting choice because they emit bright light but use very little electricity. Until now, however, high production costs have made GaN lighting too expensive for widespread use in homes and offices.

Colin Humphreys, lead scientist on the project, said it was previously difficult to grow GaN on silicon because the lattices of the two elements are completely mismatched and their thermal expansion coefficients differ by as much as 50 per cent.

‘Gallium nitride is grown on silicon at a temperature of about 1,000C, and when cooling down from this growth temperature the difference of thermal expansion coefficient puts the GaN into tension and it cracks,’ he said. ‘The wafer also bows substantially.’

Humphreys’s team overcame this technical hurdle by deliberately introducing compressive layers made of aluminium gallium nitride into the GaN growth procedure. These compressive layers counter the tension in the GaN when the wafer is cooled.

‘So at room temperature the 6in wafer is totally free of cracks and also very flat,’ he said.

Before that could be accomplished, however, the Cambridge researchers had to overcome another challenge presented by the lattice mismatch. GaN grown on silicon is prone to a large number of atomic defects, which is measured in terms of dislocation density, and the researchers needed to straighten the microstructure out. Humphreys said that after adding compressive layers into the GaN growth procedure, the team introduced special dislocation reduction layers made of silicon nitride. With this, he said, they were able to reduce the dislocation density by a factor of 100.

GaN has been grown on silicon before, for electronic devices such as high electron mobility transistors, used in a range of equipment such as mobile phones and electronic warfare systems such as radar.

Yet it has never had successful application for LEDs. ‘Electronic devices are much less sensitive to high dislocation density than are optoelectronic devices, and no one previously has produced efficient LEDs from gallium nitride grown on 6in silicon wafers,’ said Humphreys.

The Cambridge research team hope their less-expensive method for producing LEDs will increase their use and reduce the amount of electricity consumption nationwide. The group estimates 20 per cent of electricity in the UK is used for lighting, and GaN LED lights used in every home and office would cut that number to 5 per cent. This means, their estimates indicate, the UK could close or not need to replace eight power stations.

It would be a welcome change for most consumers. It is claimed a GaN LED can burn for approximately 100,000 hours and, on average, only need replacing every 60 years.

GaN LEDs do not contain mercury as with current energy-saving light bulbs, so their disposal would have less of an impact of the environment. Also, GaN LEDs can be turned on instantly and dimmed.

The Cambridge researchers initially developed their process with funding from the Engineering and Physical Sciences Research Council; it is now being transferred to UK technology group Qinetiq. There the technique will be fully developed into a process that is reproducible run after run.

The production technology will then be passed on to semiconductor manufacturer RFMD UK in County Durham for full-scale commercial use. Commercial products should be available in a few years, it is claimed.

While most news has been pretty grim for the UK manufacturing industry, Humphreys said his group’s discovery shows innovation is still the way forward.

‘We know that around the world people have been trying to do this and they’ve failed,’ he said. ‘This gives us a good lead on technological know-how, which is important to keep the UK ahead for a bit.’