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The Nobel Prize in Physics 2014 (Oct 7, 2014)
Discoveries
When Akasaki, Amano and Nakamura arrive in Stockholm in early December to attend the Nobel Prize ceremony, they will hardly fail to notice the light from their invention glowing in virtually all the windows of the city. The white LED lamps are energy-efficient, long-lasting and emit a bright white light. Moreover, and unlike fluorescent lamps, they do not contain mercury.
Red and green light-emitting diodes have been with us for almost half a century, but blue light was needed to really revolutionize lighting technology. Only the triad of red, green and blue can produce the white light that illuminates the world for us. Despite the high stakes and great efforts undertaken in the research community as well as in industry, blue light remained a challenge for three decades.
Akasaki worked with Amano at Nagoya University while Nakamura was employed at Nichia Chemicals, a small company located in Tokushima on the island of Shikoku. When they obtained bright blue light beams from their semiconductors, the gates opened up for a fundamental transformation of illumination technology. Incandescent light bulbs had lit the 20th century; the 21st century will be lit by LED lamps.
Saving energy and resources
A light-emitting diode consists of a number of layered semiconductor materials. In the LED, electricityis directly converted into light particles, photons, leading to efficiency gains compared to other lightsources where most of the electricity is converted to heat and only a small amount into light. In incan-descent bulbs, as well as in halogen lamps, electric current is used to heat a wire filament, making it glow.In fluorescent lamps (previously referred to as low-energy lamps, but with the advent of LED lamps thatlabel has lost its meaning) a gas discharge is produced creating both heat and light.
Thus, the new LEDs require less energy in order to emit light compared to older light sources. Moreover,they are constantly improved, getting more efficient with higher luminous flux (measured in lumen)per unit electrical input power (measured in watt). The most recent record is just over 300 lumen/watt,which can be compared to 16 for regular light bulbs and close to 70 for fluorescent lamps. As about onefourth of world electricity consumption is used for lighting purposes, the highly energy-efficient LEDscontribute to saving the Earth’s resources.
LEDs are also more long-lasting than other lamps. Incandescent bulbs tend to last 1,000 hours, asheat destroys the filament, while fluorescent lamps usually last around 10,000 hours. LEDs can last for100,000 hours, thus greatly reducing materials consumption.
Creating light in a semiconductor
LED technology originates in the same art of engineering that gave us mobile phones, computers andall modern electronics equipment based on quantum phenomena. A light-emitting diode consists ofseveral layers: an n-type layer with a surplus of negative electrons, and a p-type layer with an insufficientamount of electrons, also referred to as a layer with a surplus of positive holes.
Between them is an active layer, to which the negative electrons and the positive holes are driven whenan electric voltage is applied to the semiconductor. When electrons and holes meet they recombine andlight is created. The light’s wavelength depends entirely on the semiconductor; blue light appears at theshort-wave end of the rainbow and can only be produced in some materials.
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 theSoviet Union, Oleg V. Losev undertook closer studies of light emission. However, Round and Losevlacked the knowledge to truly understand the phenomenon. It would take a few decades before the pre-requisites for a theoretical description of this so-called electroluminescence were created.
The red light-emitting diode was invented in the end of the 1950s. They were used, for instance, in digi-tal watches and calculators, or as indicators of on/off-status in various appliances. At an early stage it wasevident that a light-emitting diode with short wavelength, consisting of highly energetic photons – a bluediode – was needed to create white light. Many laboratories tried, but without success.
Challenging convention
The Laureates challenged established truths; they worked hard and took considerable risks. They builttheir equipment themselves, learnt the technology, and carried out thousands of experiments. Most ofthe time they failed, but they did not despair; this was laboratory artistry at the highest level.
Gallium nitride was the material of choice for both Akasaki and Amano as well as for Nakamura, andthey eventually succeeded in their efforts, even though others had failed before them. Early on, thematerial was considered appropriate for producing blue light, but practical difficulties had proved enor-mous. No one was able to grow gallium nitride crystals of high enough quality, since it was seen as ahopeless endeavour to try to produce a fitting surface to grow the gallium nitride 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 Ph.D.-student at Nagoya University. Nakamura atNichia also chose gallium nitride before the alternative, zinc selenide, which others considered to bea more promising material.
Fiat lux – let there be light
In 1986, Akasaki and Amano were the first to succeed in creating a high-quality gallium nitride crystalby placing a layer of aluminium nitride on a sapphire substrate and then growing the high quality gal-lium nitride on top of it. A few years later, at the end of the 1980s, they made a breakthrough in creatinga p-type layer. By coincidence Akasaki and Amano discovered that their material was glowing moreintensely when it was studied in a scanning electron microscope. This suggested that the electronic beamfrom the microscope was making the p-type layer more efficient. In 1992 they were able to present theirfirst diode emitting a bright blue light.
Nakamura began developing his blue LED in 1988. Two years later, he too, succeeded in creating high-quality gallium nitride. He found his own clever way of creating the crystal by first growing a thin layerof gallium nitride 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 electronbeam removed the hydrogen that was preventing the p-type layer to form. For his part, Nakamura replacedthe 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 themmore efficient. They created different gallium nitride alloys using aluminium or indium, and the LED’sstructure 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 bluelaser emits a cutting-sharp beam. Since blue light has a very short wavelength, it can be packed muchtighter; 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 playbacktimes, as well as better laser printers.
Many home appliances are also equipped with LEDs. They shine their light on LCD-screens in televi-sion sets, computers and mobile phones, for which they also provide a lamp and a flash for the camera.
A bright revolution
The Laureates’ inventions revolutionized the field of illumination technology. New, more efficient,cheaper and smarter lamps are developed all the time. White LED lamps can be created in two differentways. One way is to use blue light to excite a phosphor so that it shines in red and green. When all col-ours 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 do the work of combining the three colours into white.LED lamps are thus flexible light sources, already with several applications in the field of illumination– millions of different colours can be produced; the colours and intensity can be varied as needed. Colour-ful light panels, several hundred square metres in size, blink, change colours and patterns. And everythingcan be controlled by computers. The possibility to control the colour of light also implies that LED lampscan reproduce the alternations of natural light and follow our biological clock. Greenhouse-cultivationusing artificial light is already a reality.
The LED lamp also holds great promise when it comes to thepossibility of increasing the quality of life for the more than 1.5billion people who currently lack access to electricity grids, asthe low power requirements imply that the lamp can be pow-ered by cheap local solar power. Moreover, polluted water canbe sterilised using ultraviolet LEDs, a subsequent elaborationof the blue LED.
The invention of the blue LED is just twenty years old, but ithas already contributed to creating white light in an entirelynew manner to the benefit of us all.
Story Source
The above story is based on materials provided by Nobel Foundation. Note: Materials may be edited for content and length.
Biography
For this and other work Isamu Akasaki was awarded the Kyoto Prize in Advanced Technology in 2009 and the IEEE Edison Medal in 2011. He was also awarded the 2014 Nobel prize in Physics, together with Hiroshi Amano and Shuji Nakamura.
Born in Kagoshima Prefecture, Akasaki graduated from Kyoto University in 1952, and received Dr. Eng. in Electronics from Nagoya University in 1964. He started working on GaN-based blue LEDs in the late 1960s. Step by step, he improved the quality of GaN crystals and device structures at Matsushita Research Institute Tokyo,Inc.(MRIT),where he decided to adopt metalorganic vapor phase epitaxy (MOVPE) as the preferred growth method for GaN.
In 1981 he started afresh growth of GaN by MOVPE at Nagoya University,and in 1985 he and his group succeeded in growing high-quality GaN on sapphire substrate by pioneering the low-temperature(LT) buffr layer technology.
This high-quality GaN enabled them to discover p-type GaN by doping with magnesium (Mg) and subsequent activation by electron irradiation (1989), to produce the first GaN p-n junction blue/UV LED(1989), and to achieve conductivity control of n-type GaN (1990) and related alloys(1991) by doping with silicon(Si),enabling the use of heterostructures and multiple quantum wells in the design of more efficient p-n junction light emitting structures.
They achieved stimulated emission from the GaN firstly at room temperature in 1990, and developed in 1995 the stimulated emission at 388 nm with pulsed current injection from high-quality AlGaN/GaN/GaInN quantum well device. They verified quantum size effect (1991) and quantum confined Stark effect(1997) in nitride system, and in 2000 showed theoretically the orientation dependence of piezoelectric field and the existence of non-/semi-polar GaN crystals, which have triggered today’s world-wide efforts to grow those crystals for application to more efficient light emitters.
Professor Akasaki's patents were produced from these inventions, and the patents have been rewarded as royalties. Nagoya University Akasaki Institute opened on October 20, 2006 .The cost of construction of the institute was covered with the patent royalty income to the university, which was also used for a wide range of activities in Nagoya University. The institute consists of an LED gallery to display the history of blue LED research/developments and applications, an office for research collaboration, laboratories for innovative research, and Professor Akasaki's office on the top sixth floor. The institute is situated in the center of the collaboration research zone in Nagoya University Higashiyama campus.
He joined Professor Isamu Akasaki's group in 1982 as an undergraduate student. Since then, he have been doing research on the growth, characterization and device applications of group III nitride semiconductors, which are well known as materials used in blue light-emitting diodes. In 1985, he developed low-temperature deposited buffer layers for the growth of group III nitride semiconductor films on a sapphire substrate, which led to the realization of group-III-nitride semiconductor based light-emitting diodes and laser diodes. In 1989, he succeeded in growing p-type GaN and fabricating a p-n-junction-type GaN-based UV/blue light-emitting diode for the first time in the world.
Nakamura graduated from the University of Tokushima in 1977 with a degree in electronic engineering, and obtained a master's degree in the same subject two years later, after which he joined the Nichia Corporation, also based in Tokushima. It was while working for Nichia that Nakamura invented the first high brightness GaN LED whose brilliant blue light, when partially converted to yellow by a phosphor coating, is the key to white LED lighting, which went into production in 1993.
Previously, J.I. Pankove and co-workers at RCA put in considerable effort, but did not manage to make a marketable GaN LED in the 1960s. The principal problem was the difficulty of making strongly p-type GaN. Nakamura was somewhat luckier than other workers in that another Japanese group led by Professor Isamu Akasaki published their method to make strongly p-type GaN by electron-beam irradiation of magnesium-doped GaN. However, this method was not suitable for mass production and its physics was not well understood. Nakamura managed to develop a thermal annealing method which was much more suitable for mass production. In addition, he and his co-workers worked out the physics and pointed out the culprit was hydrogen, which passivated acceptors in GaN.
At the time, many considered creating a GaN LED too difficult to produce, therefore Nakamura was fortunate that the founder of Nichia, Nobuo Ogawa (1912–2002) was initially willing to support his GaN project. However the company eventually ordered him to suspend work on GaN, claiming it was consuming too much time and money. Nakamura continued to develop the blue LED on his own and in 1993 succeeded in making the device.
He was awarded a Doctor of Engineering degree from the University of Tokushima in 1994. He left Nichia Corporation in 1999 and took a position as a professor of engineering at the University of California, Santa Barbara.
In 2001, Nakamura sued his former employer Nichia over his bonus for the discovery, which was originally ¥20,000 (~US$180). Although Nakamura originally won an appeal for ¥20 billion (~US$180 million), Nichia appealed the award and the parties settled in 2005 for ¥840 million (~US$9 million), at the time the largest bonus ever paid by a Japanese company.
Nakamura has also worked on green LEDs, and is responsible for creating the white LED and blue laser diodes, which are used in Blu-ray Discs and HD DVDs.
Nakamura is currently a professor of Materials at the University of California, Santa Barbara, and holds over 100 patents. In 2008, Nakamura, along with fellow UCSB professors Dr. Steven DenBaars and Dr. James Speck, founded Soraa, a leading developer of solid-state lighting technology built on pure gallium nitride substrates.
ResearchBib News Oct 7, 2014