Brilliant! chronicles Nobel winner Nakamura's LED innovations (MAGAZINE)
In his 2007 book Brilliant! Shuji Nakamura and the Revolution in Lighting Technology, author BOB JOHNSTONE (shown) predicted that Shuji Nakamura would win the Nobel Prize. Now that his prediction has come true, Prometheus Books has published a new and revised edition of Brilliant! documenting Shuji's award and bringing his story up to date, as this excerpt illustrates.
Like any good engineer Shuji Nakamura sees himself primarily as a problem solver. The biggest problem in gallium nitride work remained the lack of what is known in the jargon as a "native" substrate. All other semiconductor materials are grown on like material. For instance, silicon devices are fabricated on silicon wafers. With gallium nitride, however, because it was extremely difficult to produce salami-like boules of bulk crystal, researchers were forced to fall back on "foreign" substrates like sapphire or silicon carbide. Devices grown on such substrates were riddled with defects. This in turn meant that, to avoid catastrophic failure, light emitters could not be run at high power.
Even at lower currents, researchers had encountered a puzzling issue which they dubbed "droop." Crank up the current and the efficiency - how much electricity is converted into light - of the LED would sag. This was a big deal, because researchers are forever striving to improve the efficiency of their devices. Today's commercial LEDs are typically only fifty percent efficient and, with the addition of a phosphor, efficiency drops further. When I spoke to him a few days after the Nobel announcement, Shuji told me his ultimate goal is to push efficiency "as close to one hundred percent as possible."
But though bulk gallium nitride was hard to make, it was not impossible. Giant Japanese corporations like Sumitomo Electric, Hitachi Cable, and Mitsubishi Chemical had deep enough pockets to invest in production facilities. For such firms the reason for pouring resources into manufacturing this material was the lure of a lucrative niche market. Blu-ray digital video disk players and game machines like Sony's popular Playstation used violet-blue lasers, descendants of Shuji's work at Nichia. Unable to tolerate the defect densities of gallium nitride on sapphire, lasers had to be grown on GaN.
Shuji had long dreamed of growing gallium nitride devices on native substrates, "GaN-on-GaN" as it would later come to be known. Now, in 2006, with the availability of the new Blu-ray substrates, even though a single two-inch wafer of gallium nitride might cost several thousand dollars, it was finally possible for him to put his ideas into practice. At the time the prevailing notion was that GaN-on-GaN devices, even if practicable, would be prohibitively expensive. With sapphire wafers selling for a few dollars a pop, it was crazy even to think about making such things. But as ever, once Nakamura had determined that there was a better way to do things, it was impossible to dissuade him. Here again, his leadership would inspire others. "Shuji's always thinking of the newest thing to try," Steve DenBaars [Shuji's colleague at UC Santa Barbara] said. "GaN-on-GaN is something that people wouldn't have tried unless Shuji had pushed Jim [Speck], another UCSB colleague and me, and pushed our students, too."
In January 2007, Nakamura and his colleagues announced via a press release that they had used the new material to make "a major breakthrough," a new low-power blue-violet laser. Among the most interested readers of the press release was the venture capitalist Vinod Khosla. A Silicon Valley legend, Khosla has a track record of funding successful startups. His company Khosla Ventures has focused on investing in the cleantech sector. In other words, on technologies like LEDs that as well as providing commercial benefits also address environmental concerns such as climate change. Khosla had originally approached Shuji back in 2001, just after his move to Santa Barbara. He proposed that Nakamura, DenBaars, and Speck should start an LED company. "Shuji looked straight at this guy, who's a multi-billionaire," DenBaars recalled. "And he said No, the time's not right yet - we need to come up with our own new technology."
Six years on, Khosla flew down to Santa Barbara to meet with the three professors. "He said, OK, now you have a new technology, are you ready to start a company? We thought about it for about a week, and then we said, Yeah."
They named their startup Soraa, after the Japanese word for "sky." The trio had no business plan, they had not put together a cash-flow analysis or any of the other stuff that is supposed to be sine qua non for a startup. What they did have was impressive results. "That's what Shuji's so good at, he produces the results that can let you [go ahead]," DenBaars said. "It was just, spin-out the technology and good things will happen."
An oversimplification, as it turned out. Defying the conventional wisdom that GaN-on-GaN technology was ridiculously expensive, hence commercially unviable, made it impossible for the fledgling firm to convince other investors. In the company's early days employees faced month-to-month payroll cuts. Many times the firm came close to going under. Substantial personal loans were needed to keep Soraa afloat. In a sense, Vinod Khosla was like a latter-day Nobuo Ogawa [Nichia founder president], a patron who was prepared to back Shuji's vision all the way. By 2014, Soraa had managed to raise over $100 million, most of it from Khosla Ventures.
The allure of Nakamura's name enabled the startup to attract top talent. One of Soraa's first recruits was Mike Krames, who joined as chief technology officer. Krames had led the Advanced Laboratories at Lumileds, a subsidiary of Philips. This group had developed the LEDs used in Apple's iPhone. It had also identified the mechanism that caused the droop phenomenon, as "Auger recombination" (don't ask). In 2009 Krames signed on as Soraa's chief technology officer. The company moved its base up from Goleta, near Santa Barbara, to Fremont, Silicon
Valley's cleantech hub.
Though lasers had stimulated Soraa's founding, it soon became clear that GaN-on-GaN was ideal for better LEDs, too. The company's focus switched to making what it would term "LED 2.0," a second generation of light emitting diodes. In February 2012, after four years beavering away in stealth mode, Soraa startled the lighting industry with the announcement of its first product. It was a replacement for the 50-Watt MR16 halogen lamp. This was a product whose small size and intense, high-quality brightness LED makers had thus failed to match.
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With its much smaller chips, GaN-on-GaN had a natural advantage. Fewer wafers would be needed to produce the same number of lumens. Since gallium nitride wafers were still more than a hundred times more expensive than sapphire ones, reducing the cost of the substrate was imperative. The incumbent technology for bulk GaN production - hydride vapor phase epitaxy - left much to be desired in terms of material quality. But there was also another method, known as ammono-thermal growth, that promised to reduce both cost and defect density. This technology had been tried and tested in other areas. For example, it was used to produce thousands of tons of quartz annually, for applications that included crystal oscillators in watches and clocks.
As ever, Nakamura had long been eager to exploit this alternative method. "Shuji's been very big on ammono-thermal growth from day one," said Krames. In 1999, while still at Nichia, Nakamura had flown to Poland, to visit an outfit called Ammono which was attempting to develop ammono-thermal growth of gallium nitride. Just before he left Japan, Shuji had arranged for Nichia to fund development at Ammono. At UC Santa Barbara he continued to pursue the technology. The Poles had run into trouble adapting conventional autoclaves. Among other issues, growth rates in these reactors were painfully slow. Externally-heated autoclaves could not reach high enough temperatures and pressures. It was necessary to completely redesign the reactors and build new, internally-heated autoclaves that were customized for growing bulk gallium nitride. Better equipment would speed up growth rates. Substrate costs would be reduced by an order of magnitude. Bulk growth has become a hot topic. At recent nitride conferences, papers on growth methods outnumber those on any other subject.
Soraa licensed ammono-thermal patents from UCSB. With funding from the Department of Energy the company's researchers spent five years improving the technology. "It's a lot of heavy lifting for a startup to do this," Krames said. By late 2014 Soraa was able to grow boules of very high-purity gallium nitride that were two inches in diameter. For commercial production, however, four-inch boules were needed. Krames estimated that to reach the point where the company could supply its own needs in-house would take another two years.
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With the first generation of his LEDs already ubiquitous and a second generation on the way, was there anything left in solid-state lighting, I asked Shuji, for him to achieve? It turned out there was: laser lighting. For one thing lasers, unlike LEDs, do not suffer from droop. "So the efficiency is very high," Nakamura explained. "You can make very, very bright lights using tiny chips." UCSB was mounting a big effort on laser lighting. "That's where most of our PhD students work," DenBaars said. The first fruits of this new emphasis were already emerging in commercial applications. They included automobile headlights and cinema projectors.
For laser lighting to become ubiquitous, there were, Shuji acknowledged, still some challenges remaining. Safety was one: out in the real world it would not be possible, as it was in the lab, to wear safety glasses to prevent being blinded by laser light. And, as always, there was cost. "Everybody thinks lasers are expensive," he said, "so we have to reduce the cost." Would laser light be the next big thing? Industry observers were skeptical. But during the course of his brilliant career Shuji had confounded the skeptics many times before. The odds were good that, yet again, he would prove them wrong.
Editor's note: All links within this text are provided by LEDs Magazine and do not appear in the Prometheus Books publication of Brilliant!.
Republished from February 3, 2015 edition of Brilliant! Shuji Nakamura and the Revolution in Lighting Technology (Updated Edition) by Bob Johnstone (shown above). Copyright February 2015 by Prometheus Books, a division of Penguin Random House, Inc. (available at: http://bit.ly/1M1QIZq).
