15.6.2006
MILLENNIUM TECHNOLOGY PRIZE WINNER PROFESSOR SHUJI NAKAMURA AND HIS WORK
Professor Shuji Nakamura is one of the most significant inventors of our time. In 1993, he stunned the optoelectronic community with the announcement of very-bright blue GaN-based light emitting diodes (LEDs). In rapid succession, he then announced a green GaN-based LED, a blue laser diode, and a white LED. All these developments were things that other researchers in the semiconductor field had spent decades trying to do.
Professor Nakamura’s story is unique. Born in 1954 in Japan on the island called Shikoku, he received his master’s degree in 1979 at the University of Tokushima. He started his scientific and technological career outside mainstream Japanese technology, working as an engineer at Nichia Chemical, a small phosphor company in the countryside.
At Nichia Chemical’s laboratory, with only a limited budget and modest support from company management, Nakamura developed a highly-original two-flow growth system which led to the successful epitaxial growth of gallium nitride (GaN) in 1989. Three years later, he managed to produce p-type GaN, a fundamental breakthrough in III-V nitride research. Since the beginning of research into GaN almost three decades earlier, no-one had been able to create this particular compound.
In 1993, to universal surprise, Nakamura demonstrated bright-blue LEDs. Two years later he announced a green GaN-based LED, a blue laser diode, and a white LED. Professor Nakamura patented his innovations.
Innovative MOCVD technique
Nakamura's road to his innovations began with his development of a new technique for Metal-Organic Chemical Vapour Deposition (MOCVD). In the conventional MOCVD technique, semiconductors are manufactured by passing reactant gases over a substrate. Nakamura pioneered a method whereby the gases flow in two directions instead of just one, improving material quality. This novel MOCVD technique first enabled him to make a bright-blue LED, which led to a white LED and then to a blue laser.
Semiconductors are crystalline materials in which electrons moving from higher to lower energy levels in the structure emit photons of light whose frequency (i.e. colour) is determined by the size of the gap between the energy levels. Optoelectronic engineers call this difference in energy levels the "band gap." As the gap that an electron has to traverse to emit a blue photon is greater than that for any other visible colour, the electron must have a higher initial level of energy to be able to give off the higher frequency blue photon.
Blue LEDs – a breakthrough in semiconductor research
The usual semiconductor picture is rather too simplified for an understanding of Nakamura's breakthrough in making a blue LED. Blue LEDs actually consist of a two-sided crystal in which the "sides" represent an n-type and a p-type semiconductor. The n-type conducts electrons, and the p-type conducts holes, which are an absence of electrons. The electrons flow in one direction, the holes flow in the opposite direction. The location in the crystal where electrons and holes fall into, or are injected into, is called the junction, and that is where the photons - particles of light - are emitted.
Nakamura discovered how to grow semiconductor crystals so that they have the structure required to create "quantum wells" for electrons at the junction. One of the key techniques for creating these wells was the addition of indium to the GaN crystal. Without the indium, GaN produces a higher frequency of ultraviolet light which is not in the visible spectrum. Adding indium results in a lowering of the frequency of the emitted photons to visible blue, but the indium also creates the required quantum well effect, so that electrons that fall into passing holes first fall into the well and gain additional mass before being injected into the holes. This adding of mass in the well creates a more vigorous injection - and therefore more light.
Green LED – additional spectrum colours
With the addition of a fraction more indium, a blue LED can be turned into a green LED. Before Nakamura’s innovation, the green in full-colour displays was a phosphorescent yellow. His blue LED technology makes the greens even in large full-panel LED displays really green.
White LED – revolutionizing illumination
Professor Nakamura's next step was to add a novel phosphor to his blue chip to obtain white light.
Domestic 60-watt light bulbs emit a lot of electromagnetic energy in the infrared section of the spectrum. While this radiation cannot be seen, it can be felt as heat. The essence of this innovation is to eventually replace the world’s inefficient incandescent light bulbs with white LEDs to reduce the amount of energy required to produce light. An additional benefit will then be accomplished through a significant reduction in air-conditioning costs.
Not only do white LEDs produce light energy more efficiently, they have a working life of orders of magnitude longer than conventional light bulbs.
Blue lasers – multiplying information storage capacity
In the mid-1990s when Nakamura was using his blue LEDs to make white LEDs, he was also adapting his blue LED technology to make a blue laser.
In blue LEDs, the photons emitted fall in a range of similar frequencies – resulting in the blue colour. In lasers, the frequency of the photons must all be the same. To amplify a single light frequency in a crystal, Nakamura worked out how to etch a highly-polished mirror on each side of the crystal so that the light bouncing back and forth between the two mirrors resonated at the required frequency. His breakthrough was not only to form the mirrors on each side of the crystal, but also to make it possible for the crystal to accept the strong electrical currents necessary to create high-frequency blue laser light.
Blue lasers are a substitute for the infrared lasers used in compact-disc (CD) players. Using them means that the information storage capacity of a CD is increased five times. Blue lasers mean not only more data on CDs, but also on DVDs. Next-generation high-definition DVDs employing blue lasers are about to reach the market.
Professor Nakamura’s work continues
All of Nakamura's impressive innovations depend on the use of GaN semiconductors. Current research developments based on this material appear to herald a revolution in which gallium nitride will replace gallium arsenide as the semiconductor material of choice. Although gallium is common to both materials, it is the move from its combination with arsenic to a combination with nitrogen that is key. Unlike the former, the latter is an environmentally-friendly element.
In 1994, Nakamura received his doctorate in engineering at the University of Tokushima. Five years later he left Japan and joining the faculty of the University of California, Santa Barbara (UCSB). At UCSB he has built up a significant research programme in new areas of nitride research.
Professor Nakamura’s current research interests are the growing of optoelectronic materials and the fabrication of novel semiconductor devices. In more specific terms, he is working on new devices including full-colour LEDs, an efficient white-LED light bulb, laser diodes and high-power, microwave communication devices.
Nakamura’s inventions in both GaN materials and associated devices are having an extensive impact in many areas that improve human quality of life and promote sustainable development. Applications that have already been developed by using Nakamura’s technology can reduce energy consumption, bring reading lights to the outermost areas of developing countries, sterilise water in a more efficient and cheaper way, and store data in much smaller spaces. New applications for the technology and ways of using it to improve human quality of life are being developed all the time.
Information for the media
Millennium Prize Foundation
Dr. Tech. Tapio Alvesalo
Secretary General
Phone: +358 400 341 497
tapio.alvesalo@millenniumprize.fi
