When I wrote Materials: A Very Short Introduction (published later this month) I made a list of all the Nobel Prizes that had been awarded for work on materials. There are lots. The first was the 1905 Chemistry prize to Alfred von Baeyer for dyestuffs (think indigo and denim). Now we can add another, as the 2014 Physics prize has been awarded to the three Japanese scientists who discovered how to make blue light-emitting diodes. Blue LEDs are important because they make possible white LEDs. This is the big winner. White LED lighting is sweeping the world, and that’s something whose value we can all easily understand. (Well done to the Nobel Foundation, by the way: this year the Physics and Medicine prizes are both about things we can all get the hang of.)
Red and green LEDs have been around for a long time, but making a blue one was a nightmare, or at least a very long journey. It was the sustained target of industrial and academic research for more than twenty years. (Baeyer’s indigo by the way was a similar case. In the late nineteenth century, making an industrial indigo dye was everyone’s top priority, but the synthesis proved elusive.) What Akasaki, Amano, and Nakamura did was to work with a new semiconductor material, gallium nitride GaN, and find ways to build it into a tiny club sandwich. Layered heterostructures like this are at the heart of many semiconductor devices — there was a Nobel Prize for them in 2000. So it is not so much the concept of the blue LED that the new Nobel Prize recognizes as inventing methods to make efficient, reliable devices from GaN materials. In this Akasaki, Amano, and Nakamura succeeded where many others had failed.
The commercial blue LED is formed by two crystalline layers of GaN between which is sandwiched a layer of GaN mixed with closely related semiconductor indium nitride InN. The InGaN layer is only a few atoms thick: in the business it is called a quantum well. Finding how to grow these exquisitely precise layers (generally depositing atoms from a vapor on a smooth sapphire surface) took many years.
The quantum well is where the action occurs. When a current flows through the device, negative electrons and positive holes are briefly trapped in the quantum well. When they combine, there is a little pop of energy, which appears as a photon of blue light. The efficiency of the device depends on getting as many of the electron-hole pairs as possible to produce photons, and to prevent the electrical energy from leaking off into other processes and ending up as heat. The blue LED achieves conversion efficiencies of more than 50%, an extraordinary improvement on traditional lighting technology.
How does this help us to get white light? Well, one route is to combine the light from blue, red, and green LEDs, and with a nod to Isaac Newton the result is white light. But most commercial white LEDs don’t work that way. They contain only a blue LED, and are constructed so that the blue light shines through a thin coating of a material called a phosphor. The phosphor (commonly a yttrium garnet doped with cerium) converts some of the blue light to longer wavelength yellow light. The combination of yellow and blue light appears white.
Perhaps we should pay more attention to how amazing little devices such as these are made. And how they are packaged, and sold for next to nothing as components for everyday consumer products. Low cost and availability are important. It is easy to see that making a white-light LED which can produce say 200 lumens of light for every watt of electrical energy it uses is a big step in reducing energy consumption in lighting homes, offices, industries, in street lighting, in vehicles, and so on. They replace the old incandescent lamp which produced perhaps 15 lumens per watt. Since 20% of our electricity is used for lighting, a practical white LED lamp is transformative.
But the white LED has another benefit, in bringing useful light to communities all over the world that do not have a public electricity supply. One day, I took to pieces a little solar lamp, which sells for a few dollars. I wanted to see exactly what was in it, and in particular how many chemical elements I could find. When I totted them up I had found more than twenty, about a quarter of all the elements in the Periodic Table. This little lamp has a small solar panel, a lithium battery and at its heart a white LED. It brings white light to people who previously had only dangerous kerosene lamps, or perhaps nothing at all. And it provides a solar-powered charger for a phone too. Four of the more exotic elements in this lamp are in the LED light, indium and gallium in the LED heterostructure, and yttrium and cerium in the phosphor. Is this solar lamp really the simple product that it seems? Or is it, like thousands of other everyday articles, a miracle of material ingenuity?
Featured image: Blue light emitting diodes over a proto-board by Gussisaurio. CC-BY-SA-3.0 via Wikimedia Commons.