Green Light for Spectroscopy

Traffic lights, the sun, and semi-conductors. What’s the connection? Dr Roger Reeves at Canterbury University shines a light on the answer for Mike Bodnar…

If all the bulbs in Japan’s traffic lights were replaced with light-emitting diodes, generating green, amber and red light, the country could shelve some of its future nuclear power proposals.

That’s just one practical application of finding materials capable of emitting light of a specific wavelength, rather than using standard bulbs covered with coloured lenses.

There’s other interesting stuff coming out of the new MacDiarmid-funded laboratory facility at Canterbury University too, where a small team is working on some fundamental yet vital questions relating to advanced materials, their properties and their uses.

Headed by Dr Roger Reeves, the Laser Photoluminescence Laboratory is undertaking some ground-breaking work using spectroscopy.

Excitement in the lab

Principally a spectroscopist, Roger Reeves uses laser light sources to excite materials. Energy is absorbed in the form of photons which are usually affected by the material in some way. Spectroscopy is the science of analysing the colours of the emitted photons – light – which, says Roger, reveal a lot about the nature of the material.

“The colours that come out are a sort of fingerprint of what the atoms inside the material are doing.”

The lab employs tuneable lasers as a source of photons to bombard the materials. “They’re very flexible,” explains Roger. “We can change their wavelength, they’re high-powered, they’re directional so we can target them.”

It’s the tuning that enables the lab to undertake such precise work. The scientists need to resonantly excite atoms inside materials using exactly the right colours in the laser. If they’re not right, explains Roger, nothing happens. The material doesn’t bounce back anything to analyse.

“[If] it’s completely transparent, the light goes through, nothing goes on. So we need to be able to tune the laser to the right transition and get some exciting things to happen and get some sort of absorption. Once you’ve got absorption you’ve got the potential for light to come out.”

Roger says that spectroscopy is proving invaluable in the analysis of new materials, such as those made in collaboration with Steve Durbin from the Electrical Engineering Department at Canterbury. Steve’s a materialsgrower in molecular beam epitaxy – MBE – a huge ultra-high vacuum system where material is evaporated onto a substrate and very slowly a new material is built up layer by layer.

The problem with such new materials is finding out exactly what’s been made, so semi-conductor spectroscopy is applied.

“Once you make something you’ve got to figure out, ‘did we make what we wanted?’, ‘what does it look like?’” says Roger. “This is where spectroscopy comes in because I can take their samples, put them in my system and shine the laser on them and see what comes out.”

Band on the run

Band-gaps are another fascinating area of investigation related to spectroscopy. The ‘Group III Nitride semiconductors’ – gallium nitride, aluminium nitride, and indium nitride – form a system where the fundamental band-gap of the semi-conductors vary, which has Roger positively charged, as it relates to differing colours of emitted light.

“Aluminium nitride is six electron volts, so that’s a gap way up in the ultraviolet region; it’s completely transparent to any visible light whatsoever. Then we get gallium nitride in the middle – with a band-gap of about 3.5 electron volts, meaning it’s transparent to light up to about 340 nanometres and it’s very absorbing of light in the ultra violet and higher energy than 340 nanometres. And then we go down to the last member… indium nitride.”

Which is the fly in the ointment. Until about the 1980s indium nitride was thought to have a band-gap of about two electron volts, meaning it was transparent to light up to about 500 nanometres, which Roger says is “kind of green” and very absorbing for light blue light, ultraviolet light. But the semiconductor spectroscopy at Canterbury lab has revealed an apparent discrepancy in indium nitride’s bandgap.

“That [1980s two electron volts result] is turning out to be a bit controversial. We’ve been able to make materials in Steve’s machine, and… it looks like the band-gap is more like 0.7 electron volts. Very, very different.

“So it’s quite controversial, and there are a lot of people working on this around the world, trying to resolve this difference – this huge difference – going from 0.7 which is in the infra-red region, to two electron volts which is vastly different.”

As with any controversial results, multiple experiments have been necessary to ensure the readings are not just one-off anomalies. “Of the hundred or so samples, we’re really seeing quite a good correlation to say that it’s a low-energy band-gap.”

Efficient blue green light

So is it a given that the band-gap of indium nitride should show up as a constant and specific measurement? Roger: “It should be. In an ideal perfect crystal that only God can make it will be this number, with extremely accurate precision, but God’s not making these things, we mere mortals are making these, so every single one that’s made is slightly different, and understanding why they’re different is important.”

Which brings us back to our Japanese intersections and the traffic lights that control them, because nitride technology is, as Roger explains, extremely important for what’s called the blue-green light source industry and the creation of efficient light-emitting diodes – LEDs.

“Those LEDs are semiconductor systems, they’re quite cheap and they’re also extremely economical. I’ve seen figures for example that in Japan… if they convert all their traffic lights to LED systems – gallium nitride technology LED systems – it will mean they won’t have to build five new nuclear power stations in the next 50 years.”

Escaping the solar cell

Solar cells have been around for years, and for years have never been quite as efficient as we’d like them to be, mainly because the silicon used is relatively inefficient.

“It comes down to the fact that the band-gap width of silicon is about one electron volt, so if silicon absorbs a photon from the sun at four volts of energy – which is an ultraviolet photon – three volts is wasted.” This, he says, is a fundamental limit of silicon-based solar cell design.

However, there’s seemingly great potential in gallium nitride technology for solar cell development because as well as emitting light the semiconductors also absorb light. Now the talk is of a far more efficient “tapered” solar cell. The concept requires a range of solar cells in layers, electrically connected, with the band-gaps tailored to follow the solar spectrum. Roger says the issue is that they also have to be compatible with each other and to have an interaction between them.

“So you come back to Group III nitride system – aluminium nitride at six volts, gallium nitride at three and a half volts, and possibly indium nitride at 0.7 volts – to be determined.”

With these raw materials Roger believes it should be possible to build systems that result in different voltage combinations.

“So what actually indium nitride is, is a very important question in this sort of technology or idea.”

And spectroscopy, as the science that can help unravel the properties and characteristics of the Group III nitrides, is equally important.