Science Interviews


Thu, 30th May 2013

Bringing LED lights into the main

Prof Colin Humphries, Cambridge University

Listen Now    Download as mp3 from the show Shedding light on LEDs

Now thereís long been a search for energy efficient lighting which could replace Conventional bulbthe wasteful incandescent bulbs which governments around the world have started programmes to phase out.

However, the current most commonly available energy efficient bulbs, Compact Fluorescents, take time after being switched on to warm up and are widely seen as producing a harsh blue colour of light.

Now though breakthroughs in LED technology are offering a cheap alternative. Weíre joined in the studio by Colin Humphries Director of Research at the Department of Materials Science and Metallurgy at the University of Cambridge whoís been working on bringing LED technology to the wider market

Dave - What actually is an LED?

Colin - An LED is a solid which when you pass a tiny electric current through it, it emits brilliant light and I brought along a childrenís toy here and this is a pig. And with one finger, I could just press this lever on this pig and you get bright light emitted from this pig, and if I stop pressing, I've actually put enough energy into this that bright light continues to shine now. So, they're extremely energy efficient.

Dave - So, far better than the conventional light bulb where you're just heating up a piece of wire and I guess wasting a lot of energy.

Colin - Far better than conventional lighting. If I may add just a slight correction, a conventional light bulb is actually only 5% efficient, 95% comes out of heat, right? A compact fluorescent lamp, these so-called low energy light bulbs, they're about 20% efficient. So, they're 80% inefficient, even these so-called low energy light bulbs.

Dave - So, what sort of efficiencies can you get out of LEDs?

Colin - LEDs at the moment, you can buy now a 30% efficient, so there are already something like 6 times efficient as an old fashioned light bulb. But in the laboratory, we have them at 60% efficient and theyíll be coming on to the market in the next few years.

Dave - So, that really is an improvement.

Colin - Theyíll be the most efficient source of lighting available, yes.

Dave - But one problem I guess with LEDs is if you want to actually go and buy them they're quite expensive. What's the problem with that and why are they so expensive?

Colin - So, they're really expensive because in order to get a sufficient light out, you have to have about 8 or 10 LEDs in a sort of light bulb shape. So, this is a Philips bulb which I got in John Lewis and it costs 13 pounds and itís a 48-watt equivalent bulb. Not many people will spend 13 pounds even though over its lifetime, you'll save lots of money in electricity, so you'll save over all. But the initial outlay is too expensive for people.

These are expensive because they're something like 8 or 10 LEDs. Each LED costs about 1 or 2 pounds. This is because they're grown on sapphire or silicon carbide wafers, and a wafer is Ė think of an ice cream wafer or a cheese wafer. So, a sapphire wafer, you actually grow a big piece of sapphire artifically, you slice it up into wafers. Sapphire is really expensive and silicon carbide is expensive. Silicon itself is really cheap. And so, you save a lot of money by growing on silicon.

Dave - What you're actually growing on the silicon or the silicon carbide?

Colin - Weíre depositing a material called gallium nitride which does not exist in nature. So, itís a man-made material and then the material which actually emits the light is something called indium gallium nitride. So, itís three different elements put together Ė indium, gallium and nitrogen and those elements are in very thin layers called quantum wells and it's those layers which emit the brilliant light.

Dave -  How do you get the colour of the LED? Are they naturally white?

Colin - A really good question. So, the light is emitted by the indium gallium nitride and if we have 15% indium in this indium gallium nitride, you get blue light. If you have 25% indium, you get green light. If you have 80% of indium, you get red light. Itís like cookery in a sense. You just change the composition, change the mixture and you get different lights. And in fact, I've got here some LEDs I brought along and if you push these buttons, you'll see all sorts of different colours.

And then to get white light, we take blue light and this is another demonstration I brought along. This is in fact, one of the first gallium nitride on silicon LEDs we made, itís not incredibly efficient in this demo. Weíve improved efficiency but itís still pretty bright. And so, this is blue light that you take.

Dave - So, thatís quite a deep blue light.

Colin - It is, thatís right and if you place what's called a phosphor on top, that converts the blue light to white light and a phosphor is a material where you put in high energy photons which are blue and you get out low energy photons which in this case are yellow. And the phosphor is very thin so the blue light shines through this yellow phosphor to give white light.

Dave - Is this an efficient process converting blue light into the other colours.

Colin - Well, thatís a really good question because you lose energy in the phosphor and also, in going from a high energy photon to a low energy photon, you use energy. So, the next generation after this, you'll get rid of phosphor altogether and you'll make white light by having red, green and blue LEDs individual ones. And you'll combine them together to make white light and you might have a little control like the control on a television set which actually can control the colour rendering of this white light. So, in the morning when you wake up, you might have bluish white light, you might have a romantic dinner with reddish white light. But you'd be able to control the colour of the light you get.

Dave - This might sound like a stupid question, but why aren't you doing that already?

Colin - We are not doing that already because for reasons we donít understand, this is where the research comes in, the efficiency of green LEDs is less than blue or red. We donít know why, so thatís a science problem to solve, but if you can solve that problem then weíll have very efficient white lighting from red, green and blue.

Dave - So at the moment when you buy an LED, itís based on a sapphire wafer which is an expensive thing, and you're replacing that with silicon, why isnít that easy?

Colin - All the commercial LEDs you can buy at the moment. They're grown on sapphire wafers or silicon carbide wafers, and they're both very expensive. So, we had the idea of growing on silicon wafers and thatís much more difficult to do scientifically because when you heat up silicon, it expands at a very different rate from when you heat up gallium nitride. And we grow these LEDs at 1,000 degrees centigrade. Itís very, very hot. So, when you cool down, the LEDs just crack because of this. And so, what we do, we deliberately had to introduce layers which introduce compression into the system and it matches the tension you'll get when it cools, so it doesnít crack. And the other thing is you get lots of defects when you grow on silicon, about 10 times more than when you grow on sapphire. And so, we introduce special clever techniques to actually minimise and reduce the defect density.

Dave - So, how much is growing it on silicon going to reduce the price of an LED?

Colin -  Well, silicon is really cheap. So, a 6-inch diameter silicon wafer costs about 20 pounds. A 6-inch diameter sapphire wafer will cost about 500 pounds. So, you know, itís much more than 10 times cheaper and then if you use silicon, you can go through a 6-inch processing line and all the fabrication, high yield and everything we associate with sillicon chips, you can get that on these LEDs.


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