The Unsung Inventor Who Chased the LED Rainbow

As the space race heated up, Craford’s interest in space science waned, in spite of a summer job analyzing data returned from the first satellites. He had learned a bit about semiconductors, an emerging field, and Van Allen pointed him toward the solid-state physics program at the University of Illinois, where Craford studied first for a master’s degree, then a PhD.

The glowing Dewar

For his doctoral thesis, Craford began investigating tunneling effects in Josephson junctions. He had invested several years in that research when Holonyak, a pioneer in visible lasers and light-emitting diodes, left his position at General Electric Co. and joined the Illinois faculty. Craford met him at a seminar, where Holonyak was ex­plaining his work in LEDs. Recalled Craford: “He had a little LED—just a red speck—and he plunged it into a Dewar of liquid nitrogen, and it lit up the whole flask with a bright red light.”

Entranced, Craford immediately spoke to his thesis adviser about switching, a fairly unusual proposal, since it involved dropping years of work. “My thesis adviser was good about it; he had been spending less time around the lab lately, and Holonyak was building up a group, so he was willing to take me on.”

Craford believes he persuaded the laser pioneer to accept him, the senior man recalls things differently.

Craford’s adviser “was running for U.S. Congress,” Holonyak said, “and he told me, ‘I’ve got this good student, but I’m busy with politics, and everything we do someone publishes ahead of me. I can’t take good care of him. I’d like you to pick him up.”’

However it happened, Craford’s career path was finally set—and the lure of the glowing red Dewar never dimmed.

Holonyak was growing gallium arsenide phosphide and using it successfully to get bright LEDs and lasers. He assigned his new advisee the job of borrowing some high-pressure equipment for experiments with the material. After finding a professor with a pressure chamber he was willing to lend, Craford set up work in the basement of the materials research building. He would carry GaAsP samples from the lab to the materials research basement, cool them in liquid nitrogen, increase the pressure to study the variation of resistivity, and see unexpected effects.

“Just cooling some samples would cause the resistance to go up several times. But add pressure, and they would go up several orders of magnitude,” Craford said. “We couldn’t figure out why.”

Eventually, Craford and a co-worker, Greg Stillman, determined that variations in resistance were related not only to pressure but also to light shining on the samples. “When you cooled a sample and then shone the light on it, the resistance went down—way down—and stayed that way for hours or days as long as the sample was kept at low temperature, an effect called persistent photoconductivity.” Further research showed that it occurred in samples doped with sulfur but not tellurium. Craford and Stillman each had enough material for a thesis and for a paper published in the Physical Review.

The phenomenon seemed to have little practical use, and Craford put it out of his mind, until several years later when researchers at Bell Laboratories found it in gallium aluminum arsenide. “Bell Labs called it the DX Center, which was catchy, studied it intensively, and over time, many papers have been published on it by various groups,” Craford said. Holonyak’s group’s contribution was largely forgotten.

“He doesn’t promote himself,” Holonyak said of Craford, “and sometimes this troubles me about George; I’d like to get him to be more forward about the fact that he has done something.”

Move to Monsanto

After receiving his PhD, Craford had several job offers. The most interesting were from Bell Laboratories and the Monsanto Co. Both were working on LEDs, but Monsanto researchers were focusing on gallium arsenide phosphide, Bell researchers on gallium phosphide. Monsanto’s research operation was less well known than Bell Labs’ and taking the Monsanto job seemed to be a bit of a risk. But Craford, like his hero—adventurer Richard Burton, who spent years seeking the source of the Nile—has little resistance to choosing the less well-trodden path.

Besides, “Gallium phosphide just didn’t seem right,” said Craford, “but who knew?”

In his early days at Monsanto, Craford experimented with both lasers and LEDs. He focused on LEDs full time when it became clear that the defects he and his group were encountering in growing GaAsP on GaAs substrates would not permit fabrication of competitive lasers.

[He] didn’t toot his own horn. “When George [Craford] published the work, he put the names of the guys he had growing crystals and putting the things together ahead of his name.”
—Nick Holonyak

The breakthrough that allowed Craford and his team to go beyond Holonyak’s red LEDs to create very bright orange, yellow, and green LEDs was prompted, ironically, by Bell Labs. A Bell researcher who gave a seminar at Monsanto mentioned the use of nitrogen doping to make indirect semiconductors act more like direct ones. Direct semiconductors are usually better than indirect for LEDs, Craford explained, but the indirect type still has to be used because of band gaps wide enough to give off light in the green, yellow, and orange part of the spectrum. The Bell researcher indicated that the labs had had considerable success with Zn-O doping of gallium phosphide and some success with nitrogen doping of gallium phosphide. Bell Labs, however, had published early experimental work suggesting that nitrogen did not improve GaAsP LEDs.

Nonetheless, Craford believed in the promise of nitrogen doping rather than the published results. “We decided that we could grow better crystal and the experiment would work for us,” he said.

A small team of people at Monsanto did make it work. Today, some 25 years later, these nitrogen-doped GaAsP LEDs still form a significant proportion—some 5-10 billion—of the 20-30 billion LEDs sold annually in the world today.

“The initial reaction was, ‘Wow, that’s great, but our customers are very happy with red LEDs. Who needs other colors?’”

—George Craford

Again, Holonyak complains, Craford didn’t toot his own horn. “When George published the work, he put the names of the guys he had growing crystals and putting the things together ahead of his name.”

His peers, however, have recognized Craford as the creative force behind yellow LEDs, and he was recently made a member of the National Academy of Engineering to honor this work.

Craford recalls that the new palette of LED colors took some time to catch on. “The initial reaction,” he said, “was, ‘Wow, that’s great, but our customers are very happy with red LEDs. Who needs other colors?’”

Westward ho!

After the LED work was published, a Monsanto reorganization bumped Craford up from the lab bench to manager of advanced technology. One of his first tasks was to select researchers to be laid off. He recalls this as one of the toughest jobs of his life, but subsequently found that he liked management. “You have more variety; you have more things that you are semi-competent in, though you pay the price of becoming a lot less competent in any one thing,” he told
IEEE Spectrum.