Notes from the Editors

# Blue Was the Hardest Color

Physics 7, 103
The winners of the 2014 Physics Nobel Prize solved several major engineering problems over a period of decades before finally producing a useful blue LED.

The 2014 Nobel Prize in Physics went to three researchers, Isamu Akasaki of Meijo and Nagoya Universities in Japan; Hiroshi Amano of Nagoya University, Japan; and Shuji Nakamura of the University of California, Santa Barbara. They invented the blue light emitting diode (LED) in the 1990s, an achievement that led to energy-saving white LED lighting and advances in digital displays. Even though red and green LEDs were already commercialized by the late 1960s, developing the blue LED required the researchers to overcome a series of physics and materials challenges.

LEDs are made of two semiconductor materials—an $n$-type material, where electrons carry the current, and a $p$-type material, where holes are the charge carriers. At the LED’s $n$- $p$ interface, high-energy holes combine with low-energy electrons to emit photons. The semiconductor band gap is the difference in energy between an electron and a hole before they combine and is the largest energy an emitted photon can have. But if there are crystal defects or impurities, electrons and holes can merge in a process where some or all of the energy goes into heat instead of light.

Preventing the particles from combining in the wrong way is one of the difficulties in developing an LED, especially when the band gap is large, as in a blue LED, says semiconductor physicist Christian Wetzel of Rensselaer Polytechnic Institute in Troy, New York. “The taller the building, the more support it needs to stay up,” he says. Another challenge for wide-band-gap LEDs was the limited number of candidate materials.

The Nobel laureates chose gallium nitride (GaN), despite the difficulties that other researchers had encountered with it, and they ultimately solved three major problems. First, starting in the mid 1980s, Akasaki and Amano, and separately Nakamura, developed techniques for producing large, high-quality, GaN crystals, after decades of failures by others. Second, it was difficult to make GaN into a good $p$-type semiconductor because GaN is naturally $n$-type. Both teams came up with techniques (electron irradiation and temperature cycling) to solve this problem.

The final problem was to improve the efficiency. The researchers developed layered structures involving pure GaN and alloys of GaN that “corralled” the electrons and holes into a small volume, safely away from any defects that could sap their energy. The dramatically improved efficiency allowed blue LEDs to become a viable commercial technology.

Wetzel says the development of blue LEDs advanced the devices from small, barely visible indicators to bright, adjustable light sources tailored to human vision. And the use of highly efficient LED lighting “will help decrease the need to build new power plants,” he says.

–David Ehrenstein

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