The 2014 Nobel Prize in Physics has been awarded to Isamu Akasaki, Hiroshi Amano and Shuji Nakamura "for the invention of efficient blue light-emitting diodes which has enabled bright and energy-saving white light sources."
For those old enough to remember, in the 1970s and 1980s the world of light-emitting diodes (LEDs) had only a limited range of colours at its disposal. There were green, yellow and red LEDs fabricated from widely studied semiconductor compounds such as gallium arsenide, which, following pioneering work by Nick Holonyak Jr at General Electric as well as others, has been used to make red LEDs since the early 1960s.
Although these LEDs decorated many consumer electronic devices, one colour was missing: blue. The higher energy of blue light meant that new semiconductor materials with greater electronic transition energies had to be used. One of the most favourable candidates for this purpose was gallium nitride.
Yet, producing working LEDs from gallium nitride proved to be exceptionally difficult. All early fabricated materials were full of imperfections and defects that made light-emission very inefficient. A first breakthrough came in 1986, when Isamu Akasaki and Hiroshi Amano from Nagoya University in Japan developed a suitable growth method and device structure based on a chemical vapour deposition technique.
However, one issue remained. To fabricate an LED, additional dopants need to be incorporated into the semiconductor to deliver the positive and negative electrical charges to the active region, where they combine and emit light. For gallium nitride, growing the p-type layer was problematic. The dopants added into the material, usually zinc or magnesium, were neutralized, hampering the efficiency of the LEDs.
While Akasaki and Amano were working on improving their p-type layers, Shuji Nakamura from the Nichia Corporation worked independently on the problem. Noting that the Nagoya researchers observed an improvement in the brightness of their LEDs when they were irradiated with electrons in a scanning electron microscope, he worked on more practical techniques to improve the efficiency, and in 1993 developed a thermal annealing technique to remove the detrimental hydrogen that deactivated the p-type dopants. A first high-efficiency blue LED was presented in 1994.
Since then, we have experienced a technological revolution, especially when the blue LEDs are combined with fluorescent materials to realize white light. These white LEDs have become so efficient that they are used in applications as diverse as smartphones, light bulbs, car headlights and many more.
This year's prize in physics is therefore certainly in the spirit of Alfred Nobel's will, rewarding an invention that has been of great benefit to mankind. It follows recent awards for the development of efficient semiconductor lasers in 2000 and for optical fibres and CCD cameras in 2009. Together with last year's prize, awarded to Englert and Higgs for their work on the Higgs boson, it is encouraging to see the physics prize covering the full, rich spectrum of physics and related research, from the very fundamentals of our universe to the technologies that brighten up our daily life.
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