Blue LED Inventors Win The Nobel Prize For Physics
Isamu Akasaki, Hiroshi Amano and Shuji Nakamura have been rewarded the 2014 Nobel Prize for Physics for their invention of the blue LED. At the time, Akasaki was working with Amano at Nagoya University while Nakamura was employed at Nichia Chemicals, a small company located in Tokushima on the island of Shikoku. The prize awards an invention of greatest benefit to mankind; by using blue LEDs, white light can be created in a new way. With the advent of LED lamps we now have more long-lasting and more efficient alternatives to older light sources.
The most recent record for LED lamps is just over 300 lumen/watt, which can be compared to 16 for regular incadescent light bulbs and close to 70 for fluorescent lamps. LEDs can last for 100,000 hours, compared to 1,000 hours for iIncandescent bulbs and 10,000 hours for fluorescent lamps.
Creating light in a semiconductor
The first report of light being emitted from a semiconductor was authored in 1907 by Henry J. Round, a co-worker of Guglielmo Marconi, Nobel Prize Laureate 1909. Later on, in the 1920s and 1930s, in the Soviet Union, Oleg V. Losev undertook closer studies of light emission. But it took decades before the prerequisites for a theoretical description of electroluminescence were created.
Red LEDs were invented towards the end of 1950s and were used in the first digital watches and calculators, or as indicators of on/off-status in appliances. At an early stage it was evident that an LED with short wavelength, consisting of highly energetic photons - a blue diode - was needed to create white light. Many laboratories tried, but without success.
GaN was the material of choice for both Akasaki and Amano as well as for Nakamura. Early on, the material was considered appropriate for producing blue light, but practical difficulties had proved enormous. No one was able to grow GaN crystals of high enough quality, since it was seen as a hopeless endeavour to try to produce a fitting surface to grow the GaN crystal on. Moreover, it was virtually impossible to create p-type layers in this material.
Nonetheless, Akasaki was convinced by previous experience that the choice of material was correct, and continued working with Amano, who was a PhD-student at Nagoya University. Nakamura, who was employed at Nichia Chemicals, also chose GaN before the alternative, zinc selenide, which others considered to be a more promising material.
Growing hIgh quality GaN crystals
In 1986, Akasaki and Amano were the first to succeed in creating a high-quality GaN crystal by placing a layer of AlN on a sapphire substrate and then growing the high quality GaN on top of it. A few years later, at the end of the 1980s, they made a breakthrough in creating a p-type layer. By coincidence Akasaki and Amano discovered that their material was glowing more intensely when it was studied in a scanning electron microscope. This suggested that the electronic beam from the microscope was making the p-type layer more efficient. In 1992 they were able to present their first diode emitting a bright blue light.
Nakamura began developing his blue LED in 1988. Two years later, he too, succeeded in creating high- quality GaN. He found his own way of creating the crystal by first growing a thin layer of GaN at low temperature, and growing subsequent layers at a higher temperature.
Nakamura could also explain why Akasaki and Amano had succeeded with their p-type layer: the electron beam removed the hydrogen that was preventing the p-type layer to form. For his part, Nakamura replaced the electron beam with a simpler and cheaper method: by heating the material he managed to create a func- tional p-type layer in 1992. Hence, Nakamura's solutions were different from those of Akasaki and Amano.
During the 1990s, both research groups succeeded in further improving their blue LEDs, making them more efficient. They created different GaN alloys using aluminium or indium, and the LED's structure became increasingly complex.
Akasaki, together with Amano, as well as Nakamura, also invented a blue laser in which the blue LED, the size of a grain of sand, is a crucial component. Contrary to the dispersed light of the LED, a blue laser emits a cutting-sharp beam. Since blue light has a very short wavelength, it can be packed much tighter; with blue light the same area can store four times more information than with infrared light. This increase in storage capacity quickly led to the development of Blu-ray discs with longer playback times, as well as better laser printers.
A bright revolution
The Laureates' inventions revolutionised the field of illumination technology. New, more efficient, cheaper and smarter lamps are developed all the time. LEDs are also flexible light sources. For instance white LED lamps can be created in two different ways. One way is to use blue light to excite a phosphor so that it shines in red and green. When all colours come together, white light is produced. The other way is to construct the lamp out of three LEDs, red, green and blue, and let the eye combine the three colours into white The possibility to control the colour of light also implies that LED lamps can reproduce the alternations of natural light and follow our biological clock. Greenhouse-cultivation using artificial light is already a reality.
The LED lamp also holds great promise when it comes to the possibility of increasing the quality of life for the more than 1.5 billion people who currently lack access to electricity grids, as the low power requirements imply that the lamp can be pow-ered by cheap local solar power. Moreover, polluted water can be sterilised using ultraviolet LEDs, a subsequent elaboration of the blue LED.