While last year’s Nobel Prize in Physics was for an esoteric advance in knowledge — theoretical work on sub-atomic particles, notably predicting the existence of the Higgs boson – the prize awarded during this past week was for a far more practical achievements, by three Japanese-born men: Isamu Akasaki, Hiroshi Amano and Shuji Nakamura. They received the award for playing seminal roles in the invention of blue light-emitting diodes.
“That’s all very well,” you might say. “But what’s that got to do with the price of tea in China?” And while you might be right that such diodes could seem unimpressive in isolation, note that they proved the key to creating white LEDs, and to making a full spectrum of colours on LED displays – including smartphone screens, along with modern televisions and computer monitors. Plus, blue laser diodes led to Blu-ray discs.
Light-emitting diodes – LEDs – produce light directly from electricity, rather than heating filaments as in light bulbs of the kind first commercialised by Thomas Edison in the late 19th century. Light emission from a diode was first reported in 1907, when electrical engineer Henry J. Round noted that some silicone carbide crystals gave out a yellowish light as he applied a voltage to them.
Russian technician Oleg V. Losev independently discovered this phenomenon, and went on to conduct extensive studies that qualified him as the discoverer of the LED. Despite lacking a formal education, he linked their properties to the emerging field of quantum mechanics. In a patent, Losev anticipated potential uses including for fast communications, and transmission of images. Yet partly thanks to political upheavals in Russia, his work was little recognised, and a 2007 commentary in Nature remarked Losev’s “arguably well-deserved reputation as one of the titans of twentieth century physics and engineering was never established.”
Though more obscure than many who have worked on LEDs, Losev is not alone in missing out on recognition. This week, news reports have included 85-year old Professor Nick Holonyak remarking on being overlooked for a Nobel Prize for his seminal work on inventing the first visible light-emitting diodes, in the early 1960s. These diodes emitted red light, and in tandem with a red laser diode that Holonyak helped invent they made possible technologies such as fibre-optics networks and DVDs.
The red LEDs came soon after the advent of LEDs emitting infrared, which became commonplace in applications such as television remote controls. Green LEDs also appeared in the 1960s. There were also blue LEDs, made from silicone carbide, but they were feeble.
The need for efficient blue LEDs was recognised by people such as James Tietjen, director of the Materials Research Division of the Radio Corporation of America. In 1968, he realised the potential for developing a flat panel television that could be hung on the wall like a painting. The red and green LEDs existed, albeit the green LEDs were a tad yellowish; all that was needed was to combine them with a bright blue LED.
Tietjen launched a blue LED project, with Herbert Maruska as the main researcher. They aimed to grow gallium nitride crystals to form the diodes. Eventually, Maruska created an LED that emitted light strong enough to be seen in a lit room. But by 1974, RCA was collapsing as revenues plunged, and the blue LED project was cancelled.
However, the quest for a blue LED continued, becoming something of a holy grail of the electronics world. Most work focused on using zinc selenide, partly as gallium nitride was considered too hard to work with, as it was tough to make crystals and they were prone to defects. There were successes, such as green laser diodes demonstrated in 1991, but these barely lasted 100 hours.
Isamu Akasaki was among the few researchers to concentrate on gallium nitride. He began growing crystals at Matsushita Research Institute during the 1960s, and continued this after moving to Natoya University. Here, he and graduate student Hiroshi Amano discovered that the addition of tiny amounts of magnesium enhanced the crystal properties, and in 1990 they achieved stimulated blue emission from a diode. "It was a shining blue, and I was so excited that my hands trembled," Akasaki later recalled of the experiment.
The third of the Nobel prize winners, Shuji Nakamura, was perhaps akin to a kindred spirit of Oleg Losev. He was born, raised and educated on Shinkoku, smallest and least populous of Japan’s four main islands. After obtaining a master’s degree in electrical engineering in 1979, he became a researcher at Nichia, a local company selling phosphors and aiming to enter the LED market. The R&D department comprised just Nakamura, his boss, and one other employee.
After the company sales manager decided Nichia should manufacture gallium phosphide crystals for use in LEDs, Nakamura had to make a furnace that would be fuelled by hydrogen and oxygen, melting the materials at 1500°C. Occasionally, the quartz shields cracked, oxygen entered, and there were explosions. Yet Nakamura persisted, eventually making high quality crystals, though these were not a commercial success.
Guided by the sales manager, Nakamura then worked with gallium arsenide, wearing full body covering and a respirator to guard against the arsenic. He even developed red and infrared LEDs, though several other companies already sold them.
Deciding a niche company should make niche products, Nakamura won his CEO’s support to investigate blue light LEDs. Eschewing the widely favoured zinc selenide, he chose to pursue gallium nitride.
Timing helped – he began as Akasaki and Amano were reporting on their key breakthrough in making thin films of magnesium doped gallium nitride. Nakamura solved a mystery of why these films could have surprisingly high electrical resistance: hydrogen could form complexes with the magnesium impurities. He switched from making the crystals within ammonia gas, to using a nitrogen atmosphere, greatly improving conductivity.
With his ingenuity and unique expertise in making furnaces, Nakamura also improved crystal deposition by blowing the reactant gas towards the surface where material would deposit, and blowing an inert gas across the surface. In 1995, Nakamura demonstrated a blue/green LED of comparable efficiency to the best red LEDs. Full-colour displays and white lamps were now possible.
Also that year, Nakamura achieved the blue laser emission, and with the shorter wavelengths of blue light this meant more information could be stored on discs.
Today, developments based on LEDs continue apace, spanning lighting, communications and even water purification. As noted by an article on the Noebel Prize website, “The invention of the blue LED is just twenty years old, but it has already contributed to creating white light in an entirely new manner to the benefit of us all.”