This year the Nobel prize in physics goes to Isamu Akasaki, Meijo University and Nagoya University, Hiroshi Amano, Nagoya University, and Shuji Nakamura, University of California, Santa Barbara, for inventing the blue light emitting diode (blue LED) 20 years ago.
After the announcement, when asked how he felt on being awarded the Nobel Prize, Akasaki said “It’s unbelievable.”
“Their inventions were revolutionary. Incandescent bulbs lit the 20th century; the 21st century will be lit by LED lamps,” notes a statement by the Royal Swedish Academy of Sciences, which awards the Nobel Prizes.
This is a prize that would be after Alfred Nobel’s own heart, because he had intended that the prizes should go to those who have “conferred the greatest benefit to mankind.”
The blue LED forms the long-awaited third in the set (red, green were already produced) of coloured LEDs that can together produce white light, in a way that is environment-friendly and energy-efficient. The blue LED can also be made to excite a phosphor into emitting red and green lights, with the mixture yielding white light.
LEDs basically consist of a junction of p-type (electron deficient or hole rich) and n-type (electron rich) semiconductors. When a voltage is applied across this junction, the holes and electrons flow across the junction and recombine, in the process, releasing light.
They do not use mercury or any such gas as is used in the fluorescent light. This makes them environment friendly. They do not require a filament to get heated and glow to shed light unlike the case of the tungsten light bulb.
In contrast to the incandescent bulbs and fluorescent lamps, the LEDS directly convert electricity to light particles. As a result, there is greater efficiency; in the other two cases, a great part of the electricity gets converted to heat.
The colour of the light emitted by the LED when voltage is applied may range from infrared to ultraviolet. Red and green LEDs have been around since the late 1950s, and these have been used extensively in digital displays and the like.
Junctions that emitted weak blue light when excited by an electron beam were made by Akasaki’s group in the late 1980s. Yet, the extraordinary difficulty in making LEDs that give off blue light of significant strength delayed the fabrication of the blue LED to the early 1990s, and this made it a prizewinning effort.
In the 1950s, the material that was commonly used to produce LEDs was Gallium Phosphide (GaP) with dopants (added impurities) like Zn-O or N. These gave out red and green light.
This led to the commercial manufacture of red and green LEDs in the 1960s. However, blue light still remained a challenge and a quest. After some research, it came to be believed that Gallium Nitride (GaN) was the material that would enable development of blue LEDs. But GaN crystals were notoriously difficult to grow in the lab.
The quest for fabricating the blue LED starting from GaN took shape in the 1950s itself. Researchers at the Philips Research Laboratories had produced light of a wide range of wavelengths from GaN.
However, the material was in a powder form and could not be grown into crystals to create p-n junctions So many researchers were giving up GaN and moving back to further research compounds such as GaP. Even as late as 1973, fabricating single GaN crystals and providing adequate p-doping remained the two great obstacles in the path of making the blue LED.
GaN crystals
It was into this scenario that Isamu Akasaki entered, in 1974. Working first at Matsushita Research Institute in Tokyo and later as a professor, with Hiroshi Amano and coworkers, in Nagoya University, he continued his research. In 1986, they succeeded in growing high-quality single crystals with good optical properties on a base of sapphire, for the first time. Shuji Nakamura, the other Nobel Prize awardee, who was working at Nichia Chemical Corporation, developed a similar method and published these results in a 1991 paper.
Second challenge
Still, the second major challenge remained, which was that making an LED requires a p-n junction. While it was easy to form the electron rich n-layer out of crystalline GaN, producing the electron deficient p-layer remained a problem.
By the late 1980s, Amano, Akasaki and coworkers seemed to have cracked this problem, but almost accidentally. They observed that when Zn-doped GaN was viewed under a scanning electron microscope, it seemed to emit more light. This implied that the p-doping had been improved and that a p-n junction had formed. Similarly, shining an electron beam on Mg-doped GaN showed better p-doping properties.
The duo did not however understand why this was happening and were therefore unable to exploit it. This was explained a few years later by Nakamura and coworkers: Acceptor impurities such as Magnesium or Zinc, which normally give rise to p-type conductivity, are trapped by hydrogen during the manufacturing process.
They therefore cannot perform their roles as providers of holes. When the acceptors get excited by electrons, they get activated and the holes are released.
Nakamura used a different approach to produce the p-layer. He found a simple heat treatment (annealing) would activate the acceptors, thus making the p-layer active.
In order to make the LED more efficient, both Akasaki’s and Nakamura’s teams in the 1990s moved on from simple p-n junctions to fabricating more complex, layered structures known as double heterojunctions.
In such multi-layered structures, the recombination of holes and electrons occurs more efficiently and with minimal losses. Having succeeded in the fabricating the basic structures, the groups then started work on to improving the efficiency of the heterojunctions.
In 1994, Nakamura and coworkers fabricated an efficient double heterojunction consisting of a combination of Indium- and Aluminium-doped Gallium nitride (InGaN/AlGaN).
This directly led to the development of efficient blue-LEDs. The teams did not stop there, they went on to develop more applications, such as blue laser emissions based on GaN, which was observed in 1995-96.
This application has advanced the technology for storing music, pictures and movies.
Today, LED lights are used in smart phones and lamps. White light from LEDs is more power-efficient than from other sources: If the amount of light flux produced per unit of power supplied is 16 for a tungsten bulb, and 70 for a fluorescent bulb, it is 300 for a LED supplied source. This would drastically lower our power consumption if LED lights are used more.
Solar-powered LED lights are also taking the world by storm. From providing illumination to possible future applications such as generating UV light for treating bacteria-infested water, the blue LED has come to stay.
