A blue light that might save the world
The humble LED may seem an unlikely successor to the Higgs boson for the Nobel Prize for physics, but as Cathal O'Connell reports, the discovery is the result of a grand and long-standing quest.
Yes, you heard right. The Nobel Prize this year goes to Isamu Akasaki, Hiroshi Amano and Shuji Nakamura for the invention of … a blue light? Sounds like a world apart from last year’s award for the Higgs boson, the long-sought particle that explains mass. But this year’s discovery is also the culmination of a grand and long-standing quest.
As Olle Inganäs, a member of this year’s physics Nobel Committee reminded us, the physics prize “is awarded for a discovery or invention of the greatest benefit to mankind”. There’s no doubt that since its blue rays first beamed from a lab-bench in Japan, the blue LED has had a profound impact.
In the lab, the best light-emitting diodes (LEDs) turn electricity into light using a third of the energy used by fluorescent lights. For this reason LEDs have long been poised to revolutionise the way we light the planet. But producing an LED light for practical use was not straightforward. For more than 33 years physicists used LED technology to produce lights for each colour of the spectrum – except blue. And without blue there could be no white (the reason is explained below). Only white light can usefully illuminate the interiors of our homes and workplaces.
The blue LED light was finally created in the early 1990s by the three Nobel-winning Japanese physicists who took the painstaking route of growing gallium nitride crystals to do the job.
Since then LEDs have been associated with state-of-the art devices like Blu-ray DVDs, smart-phone screens and TV screens. The intensity of tiny red, green and blue LED pixels can be adjusted to produce sharp images of any colour. But the real revolution is still to come, from the simple lighting of households, offices and streets.
According to the International Energy Agency 19% of all global electricity is used for lighting. The incandescent bulbs developed by Edison in 1879 run hot, wasting most of the electricity pumped into them. The fluorescent tubes that criss-cross the ceilings of today’s offices are about five times more efficient than incandescents. Today’s commercially available white LEDs have the same efficiency as fluorescents, but, while fluorescent technology has plateaued, LEDs just keep getting better and better. They’re expected to both double in efficiency and halve in price over the next 15 years. Added to this they have a useful lifetime of over 50,000 hours, meaning they need to be changed less than once a decade. Internationally, LED lighting sales are growing at 150% year-on-year, and that’s great news for the planet too. Currently about 6% of global CO2 emissions (more than two billion tonnes) come from lighting, and widespread LED use is expected to cut this in half.
The colour of light the LED emits is the result of the energy gap
between the rowdy and the sedate semiconductor materials.
The story of LEDs began in Russia in the 1920s when Oleg Losev, a brilliant young scientist, was discovering it was not a good time to be born into a noble family. Barred from a science career because of his family connections following the Bolshevik revolution, Losev was forced into a menial technical job in a Soviet radio laboratory. One day he took a radio receiver apart and noticed that the semiconductor diodes (the materials that control the flow of electrons) tended to glow when a current was passed through them. Unknown to Losev this strange effect had been noticed as far back as 1907, though it had never been explained. Researching in his spare time he discovered the light was “cold”, and that he could switch the diodes on and off at up to 78,500 times a second by hooking them up to an AC current.
He used quantum theory to explain how the LEDs worked and derived the basic formula that explains the emission, which is still used today. According to some reports he even wrote to Einstein to ask for help, but did not receive a reply. Sadly, after a short but brilliant career, Losev starved to death during the siege of Leningrad in 1942 at the age of 39.
The basic structure of an LED has not changed since. At its heart, it’s still the radio diode Losev studied – a sandwich composed of an n-type (negative) semiconductor on one side, and a p-type (positive) on the other. The n-type material is like a raging party for electrons – crowded and with all the attendees buzzing with energy. The p-type material is a more sedate soirée, with many empty chairs. Applying a voltage pushes the rowdy n-type electrons into the p-type material where, suddenly finding themselves in a quieter environment, they settle down. As each electron relaxes it gives off its excess energy in the form of a photon. The light we see from an LED is like the sigh of relaxing electrons. LEDs are so efficient because this relaxation step is a perfect conversion – all of the excess energy is turned into light and none into heat. As Losev realised after reading the work of Planck and Einstein, this cold emission could only be explained using quantum mechanics – the light comes from electrons falling from a high energy level to a lower one.
The colour of light the LED emits is the result of the energy gap between the rowdy and the sedate semiconductor materials. Red light has the lowest energy of all visible light and is released when the energy gap is smallest. This was easiest to achieve and so the first red LEDs blinked on in the 1960s. Green LEDs followed about a decade later.
Physicists have known the colour rules since Newton defined them 300 years ago. White light is made by mixing red and green with blue. If LEDs were going to take their place in the history books and revolutionise the illumination of the 21st century, as incandescents and fluorescents did in their time, they would have to make white light. And that meant first making blue.
But blue, the highest-energy visible light, was a formidable challenge.
None of the traditional semiconductors used in computers, such as silicon or germanium, could reach energy states high enough to emit blue light. Finding such a material became a quest for the world’s physicists.
“There was intense competition to see who could be first,” says Martin Green, a photovoltaics researcher at the University of New South Wales. Most groups believed zinc selenide was the answer as it had a wide natural energy gap quite close to that needed for blue light. Going against the grain, Hiroshi Amano and his mentor-professor Isamu Akasaki at Nagoya University focused on gallium nitride, a material dismissed by others because of the difficulty of controlling its crystal growth. But as Green puts it, “they blazed their own trail”. Beavering away through the 1980s the Nagoya pair found a way to grow the material on a sapphire coated with aluminium nitride, and worked on ways to tune the blue colour by adding indium as an alloy. They unveiled their functioning bright blue LED in 1992.
Meanwhile Shuji Nakamura, working at a small Japanese firm called Nichia Corporation, had dived into the same problem “after-hours”. Also working with gallium nitride, Nakamura overcame the crystallisation problem by carefully controlling the temperature. He patented his own efficient blue LED in 1993, but as a “salaryman” engineer the patent belonged to his company and Nakamura was given a bonus of 20,000 yen (about $200). He later sued Nichia and was awarded 844 million yen (more than $8.5 million) in one of the most significant intellectual property rulings in Japanese court history.
The blue LED did not only make it possible to produce white light by combining it with red and green. The real significance, says Green, is that the blue light emitted is powerful enough “to excite ‘phosphors’ that emit white light and thereby form white LEDs”.
Phosphors, like the white tint on fluorescent tubes, are man-made metallic compounds that capture high-energy light and re-emit it at lower wavelengths. In the case of fluorescent light they convert ultraviolet into white. In the case of LEDs a coating of phosphors can transform the humble blue LED into a highly efficient white lamp.
The increased efficiency of these new white LEDs is bringing the lighting revolution to the world’s poorest places. According to the UN, less than 30% of the population in sub-Saharan Africa have access to electricity, and the connections that do exist are prone to failure. Past efforts to deploy solar power have been limited by its poor efficiency and the need for large-scale panels and batteries. But efficient blue LED lights using phosphor coating can run on small portable solar-powered units.
According to figures released this month by Lighting Africa, a project of the International Financial Corp, more than 28 million Africans without access to electricity are now using solar-powered LED lights. Five years ago, there were fewer than six million.
As Per Delsing, chairman of the Nobel academy’s physics committee said during the award announcement, the invention truly upheld their founder’s tradition of working to benefit mankind. “I really think that Alfred Nobel would have been happy about this prize.”