Shuji Nakamura, Professor of Materials and Research Director of the Solid State Lighting and Energy Electronics Center (SSLEEC) at UCSB, recently received the 2014 Nobel Prize in Physics from the Royal Swedish Academy of Sciences for the invention of efficient blue light emitting diodes (LED). He shares this prize with Isamu Akasaki of Meijo University and Nagoya University, Japan and Hiroshi Amono of Nagoya University, Japan.
LED is a two-lead semiconductor light source; it functions when a photon is emitted as the electrons drop from the conduction band to the valence band. The color of light emitted depends on the band gap between the valence and conduction bands. Blue light needs a larger band gap than red light. Red and green LEDs had already been invented using aluminum indium gallium phosphide but the material had a limited band gap which could not be extended to the necessary band gap to make blue light.
Scientists began researching gallium nitride (GaN) as a potential material, but after many years they almost gave up on GaN systems for the production of blue light. Nakamura, however, succeeded in perfecting the method of doping gallium with magnesium in order to achieve P-type gallium for the emission of blue light. P-type material has increased valence electron deficiencies, or “holes,” so that electrons can be easily excited from the valence band into these extra energy levels.
Christopher Pynn, a third year graduate researcher in the Materials Department and SSLEEC, explained Nakamura’s method of doping gallium with magnesium.
“The hydrogen was forming complex magnesium-hydrogen complexes in the crystal lattice; it was neutralizing the magnesium and no longer making it an electron acceptor. So Nakamura figured out that if he placed the material into an oven with ambient nitrogen all the hydrogen would leave and when the hydrogen leaves, all that is going to be left is uncompensated magnesium. This is called activation and by incorporating magnesium, we get p-type material,” Pynn said.
In addition to gallium doping, Nakamura invented a Metal Organic Chemical Vapor Phase Deposition (MOCVD) reactor which then enabled him to create high quality gallium.
Stacy Kowsz, fifth year graduate student of Shuji Nakamura in the Materials Department, explained Nakamura’s invention.
“He figured out what kind of reactor to design to make good gallium nitride that is of high crystal quality that can emit light, that has low defects. He designed a Two Flow Reactor. In the standard reactors you either have vertical or horizontal flow reactors where you are flowing gas from the top of the your surface or horizontally. His reactor reactor has gas flowing in vertically and horizontally. This makes for a superior material,” Kowsz said.
Today, creating blue LED seems simple and is sometimes taken for granted, but it was remarkable that Nakamura was able to accomplish this. The invention of efficient blue LED is incredibly important because the combination of red, green and blue LED allows for efficient lighting, make full color displays on computers, TV screen, and mobile phones, has the potential to sterilize polluted water, and enabled the creation of white light.
Kowsz said the LED could replace the standard light bulb in the future due to its high efficiency and low heat production.