NIST Industrial Impact Company: Eagle-Picher Research Laboratory, Environmental Science &
Technology Department, Miami, Oklahoma Physicist Jan Schetzina knows how to show-and-tell. In June 1994, at the end of a technical talk he was presenting at a meeting on semiconductor devices at the University of Colorado, he held up a little black box, his finger poised on a button. When he pressed it, a pencil-thin beam of bright green light from the box hit the back wall. He then swept the light like a beacon across the audience of several hundred. The next thing Schetzina remembers is that his ears were filled with a round of spontaneous applause. Those gathered at the meeting--the Institute of Electrical and Electronics Engineers' Device Research Conference--knew what that light signified. It meant that Schetzina of the North Carolina State University (NCSU) and his colleagues had achieved a milestone toward the goal of making highly controlled semiconductor crystals that emit green or blue wavelengths of light. Schetzina's show-and-tell glory rests partly on his team's expertise with a crystal growing technique called molecular beam epitaxy (MBE), which allows them to build and vary crystal structures atomic layer by atomic layer. The applause for Schetzina's demonstration was for more than skilled MBE work at NCSU. Also pivotal were a unique supply of pristine zinc selenide (ZnSe) crystals from Eagle-Picher Research Laboratory and what both Schetzina and Eagle-Picher representatives describe as a critical assist from the National Institute of Standards and Technology (NIST) in the form of an Advanced Technology Program grant. There actually is nothing new about light-emitting diodes (LEDs), which were the kind of devices Schetzina was talking about. Nice bright ones have been shining for years inside CD players, or from the panels of consumer electronic products. But no one yet has made a true green or blue LED that lasts more than a few seconds. Those LEDs now marketed as "green" lack intensity and are actually a disappointing hue of yellow green, Schetzina says. There is no shortage of technological carrots for green and blue LEDs and laser diodes, which emit a much narrower range of wavelengths than LEDs. For one, their availability would complete the basic spectrum of solid-state light-emitting devices and that could launch a new branch of the flat-panel display industry for TVs, computer monitors, control panels, and electronic games. Even more important, according to several key players in the field, is that their shorter wavelengths can pack more information than the redder ones now at the heart of optical communications. Moreover, those same shorter wavelengths could lead to data recording and storage systems with much greater capacities than today's best. Companies such as 3M, Xerox, and Sony in Japan, military planners, and academic researchers have been hot on the trail of this technology for years. There have been plenty of dead ends. Schetzina and many other research teams around the world had made green LEDs by depositing II-VI semiconductors (so-named because they are made by mixing and matching elements from the second and sixth columns of the Periodic Table) atop a crystal of gallium arsenide (GaAs, a III-V compound). Anyone who has ever tried to make GaAs-ZnSe combos though has run into a show-stopping problem--the devices last only a short time, usually a few seconds, before their crystal structures become riddled with light-quenching defects. The seeds for destruction probably are sown as soon as the first layers of ZnSe are deposited on the gallium arsenide since the respective spacing between the atoms in each layer are not quite matched. Like a plastic top forced onto a mismatched deli container, these differences elicit strains, which researchers think are the source of the structural defects that appear soon after any current courses into the crystals. Like other semiconductor researchers, Schetzina suspected that the only way to grow durable ZnSe-based LEDs would be on top of ZnSe crystals, in which case there would be no mismatch between atomic spacings. That is a scenario much easier to imagine than make real. No one had ever succeeded in growing ZnSe crystals large enough to offer commercial promise, Schetzina says. Until 1991, that is. That is when researchers at Eagle-Picher, led by Gene Cantwell and William Harsch, announced that they had come up with a patented method to grow the coveted ZnSe crystals. Called seeded physical vapor transport, the method involves a heated tube that contains purified zinc selenide powder whose atoms boil off and then migrate toward seed crystals on either end of the heated tube on which large pristine crystals then grow. Harsch was confident that he and colleagues would be able to come up with the ZnSe crystal that others had given up on, but he wanted to take the crystal to the next step of technology development.So in 1991, Harsch gave Schetzina a call. Their subsequent collaboration already has yielded prototype LEDs that emit a deep emerald green light, which is more than 50 times brighter and more efficient than commercial yellow-green devices. Recently, the NCSU/Eagle-Picher team also has demonstrated bright blue LEDs, says Schetzina. The timing of these successes was crucial. Eagle-Picher's original efforts to make ZnSe had been funded since the mid-1980s by a federal agency that shifted its emphasis entirely to gallium arsenide substrates in the early 1990s. The subsequent loss of government funds for ZnSe research led company management to seriously rethink the material's commercial potential. "I was told either to do something with the technology or to get out of the [ZnSe] business," Harsch recalls. Knowing "that the technology was too good to drop, though," the company applied for support from the Advanced Technology Program, Harsch says. In late 1992, Eagle-Picher was awarded a 3-year ATP award totaling $1,759,000. "Without the ATP, they would not have had the incentive or help that they needed at a very critical time when they were about to make internal decisions," Schetzina says. Adds Harsch: "It gave our program credibility in the minds of our corporate people." The award also enabled the company to greatly leverage its own investment for a high-risk project with potentially high payoffs, just what ATP awards are chartered to do. "This is turning into a textbook case of technology transfer," says Harsch. Since the award was granted, the Eagle-Picher group has been able to improve their success rate at growing usable ZnSe crystals from intermittent to nearly 100 percent, he says. By the fall of 1994, they expect to receive a production MBE system exactly like the one Schetzina's group uses at NCSU and the company plans to hire some graduates from Schetzina's lab. The hardware purchase was part of the ATP agreement. Says Harsch: "By the end of the year, we could be selling the world's brightest and most durable green LEDs." August 1994 |