Tony SpringThorpe holds his thumb and forefinger about a centimetre apart, then offers this revealing quip about his long and distinguished career, “For more than 40 years I’ve been doing epitaxial growth and in that time I’ve grown this much material.”
This is how SpringThorpe sums up a lifetime of laying down thousands upon thousands of atom-thin crystal layers, a record that few people in the world can match. The lanky Brit, now a principal researcher with the Canadian Photonics Fabrication Centre in Ottawa, recalls how he first encountered the field in 1962. “I was a student attending an Institute of Physics exhibition in London and they had a gallium phosphide red light emitting diode,” he says. “I had a sense that there was a future to it.”
Tony SpringThorpe, a principal researcher with the Canadian Photonics Fabrication Centre in Ottawa, loading an MOCVD reactor with single crystal InP wafers on which will be deposited epitaxial layers of indium phosphide and gallium arsenide-based alloys to form laser structures. Photo credit: Tim Lougheed
That future is with us now, as the latest incarnation of those red light emitting diodes, or LEDs, can be found in billboards, traffic signals and automobile taillights on roads all over the world. Our homes and workplaces are likewise rapidly being populated by an even wider assortment of LEDs, from television monitors that can be hung on the wall like paintings to hardy replacements for the venerable incandescent light bulb.
Such innovations are only the most obvious indication of the progress that has taken place in the technique of epitaxy since it was introduced to SpringThorpe more than five decades ago. Semiconductor materials, which have become a defining feature of modern civilization, depend on an unprecedented ability to create films so pure and so thin that their electrical properties can be defined at the atomic level. Yet this ability, like the films themselves, is all too easily overlooked and taken for granted.
SpringThorpe, for his part, continues to push the boundaries of that work, as he has been doing since the 1960s. At that time the concept of epitaxy — from a pair of Greek root words meaning “arranging upon” — was no more than a decade old. He found himself cast as one of the pioneers in this pursuit with Britain’s Royal Radar Establishment, which had an interest in ternary analogs of gallium arsenide, such as zinc silicon diphosphide and zinc germanium arsenide. Thin films made up of these compounds served as semiconductor diodes, giving off Gunn effect microwaves that could be used for a variety of detection purposes.
By 1969, SpringThorpe found himself in Canada working with an ambitious company known as Northern Electric. The firm had a keen interest in ramping up production of high efficiency LEDs, which prompted him to look for better ways of growing gallium phosphide layers. The result was his take on liquid phase epitaxy, whereby substrates on wafers were inserted into a saturated solution of gallium phosphide dissolved in metallic gallium. This hot bath was steadily cooled, so that crystals formed on their surface. He eventually patented a means of manually dunking up to 20 wafers at a time on an elaborate “cake stand.” The result laid the foundation for some of the first handheld calculator displays and other products marking the advent of consumer electronics.
By the 1970s, says SpringThorpe, it was clear that such devices could make it possible to use light rather than electricity to package data for communications. “In order to have an effective optical fibre transmission system, you need a light source with a long life, emitting at the right wavelength,” he explains, noting that these qualifications could be met by LEDs operating as lasers. In contrast to the boxy, desktop-size lasers that represented the contemporary state of the art, epitaxy was pointing the way to laser diodes that could do the same job on a microscopic scale.
The first generation of such lasers was made using Organometallic Vapour Phase epitaxy, whereby gaseous versions of the various constituents were injected into a reactor at just the right combination of temperature and pressure so that they would combine in solid form on a surface. SpringThorpe’s team subsequently refined this method and renamed it Metalorganic Chemical Vapour Deposition (MOCVD). However, he concedes that the bevy of volatile inputs like arsenic and phosphorus led him to suggest that MOCVD could also stand for “Many
An increasing number of vehicle headlights — as well as traffic signals, billboards and television monitors, among other consumer products — are constructed of light emitting diodes, a once-exotic technology that is on its way to replacing conventional light bulbs as a mainstay of modern living. Photo credit: Wikimedia Commons
Opportunities to Come to a Violent Death.” Happily, he can think of no death — violent or otherwise — that could be put down to epitaxy.
Happily, too, these developments were contributing to his employer’s prime position at the very forefront of optical electronics. Northern Electric had become Nortel, the cornerstone of an Ottawa-based cluster of enterprises that was dubbed Silicon Valley North. The research efforts driven by these firms reached a fever pitch in the 1990s, when Nortel eventually had an operation that housed eight MOCVD reactors that could simultaneously process eight 7½ centimetre-wide wafers. From the 1980s he was also exploring another avenue for crystal growth, molecular beam epitaxy, which employs evaporated beams of the component elements in an ultra-high vacuum environment.
As the 21st century opened, the significance of epitaxy had prompted the federal government to invest in a dedicated facility that would provide a private clientele with everything from basic design services to epitaxial growth of finished components. The Canadian Photonics Fabrication Centre (CPFC), located on the National Research Council’s main campus in Ottawa’s east end, represented an investment of some $40 million. The funding would have allowed CPFC to outfit a modest array of laboratories but nothing that could have competed with the epitaxial output at Nortel’s campus on the other side of the city. As it turned out, Nortel was in the midst of an agonizing administrative meltdown that would reduce it to a corporate husk.
SpringThorpe and his colleagues had seen this crisis coming and were eager to preserve the sophisticated, highly specialized facilities they had spent decades assembling. Nortel’s beleaguered bureaucrats were in no state to negotiate and he estimates that CPFC acquired some $100 million of the world’s best epitaxial infrastructure, such as sealed walls surrounding dust-free clean rooms, for a fraction of that amount.
By the time such components had migrated to CPFC, the site had 1,000 square metres of clean rooms where epitaxial materials could be assembled and prepared. Even more importantly, the people who had built up Nortel carried on as CPFC employees, building a new epitaxial foundry for domestic and international corporate clients.
Although the list of those clients is privileged, it is easy to imagine companies that are making items such as high efficiency solar cells or fibre optic network hardware. SpringThorpe says one customer receives some 150 wafers each month, each of which could be used to make some 10,000 laser diodes. Such output ranks with any other facility in North America, although it is dwarfed by the Taiwanese or Japanese foundries that dominate the world’s epitaxial activity.
While much of Nortel’s former corporate memory has gone into this repository of knowledge, some of it is home grown. Bruno Riel was a graduate student 15 years ago when he started learning the nuts and bolts of epitaxy from SpringThorpe at CPFC. Riel then joined Cyrium, an Ottawa start-up, to work on a novel form of solar cell. These were produced with elements from groups III and V on the periodic table, as opposed to the more common silicon. These crystals served as the substrate for solar cells with an efficiency of 41 percent, far higher than other typical silicon-based products on the market. Cyrium did not withstand the aftereffects of the global economic crisis that began in 2008, although the intellectual property for the company’s technology has been retained. Riel is now with Fibics, an Ottawa firm that dates back to the original telecom boom. Here he is working on ion and electron microscopy as well as lithography, characterization and fabrication technologies that played an essential supportive role in the development of epitaxy.
Meanwhile, the ripples of Nortel’s downfall have expanded well beyond the Ottawa area. Richard Arès had been commissioning the company’s brand new chemical beam epitaxy (CBE) operation with a $6 million budget when management told him to shut everything down because they were selling the division. “I asked them to give me all the equipment in my lab, to donate it to a university where I could get hired and keep going,” he says. It ended up at Université de Sherbrooke in Quebec, where Arès founded a brand new laboratory for advanced epitaxy. Arès’ new position has given him a chance to go even further and spin off a company that could market machines suitable for what he anticipates will be a flourishing market. “Epitaxy is going through a revolution that started with the end of the telecom era, which was the golden age for epitaxy,” he says.
The high cost of epitaxial products was part of the downfall for Nortel and other companies that had hyped the expectations for telecommunications in the 1990s, Arès says. As long as the hardware remained so expensive, these products would never become part of delivering fibre optics to a mass market. That market is only now starting to emerge, he says, thanks to lower-cost Asian manufacturing of goods such as LED lighting and solar panels.
Meanwhile Arès continues to refine CBE, a further variation on the high vacuum approach to laying down crystals of exceptional purity. He makes it sound simple, “You supply the precursors, provide the right temperature and pressure conditions and we let nature take its course.” Nature, on the other hand, is much more demanding. Any impurity, even a countable number of molecules, could be enough to completely alter the makeup of a thin film and its properties. “One part per billion is extremely significant for us,” Arès says. “If I have a blotch of anything on my surface then it’s a shield and my crystal won’t grow.”
Arès insists that for all the mechanical precision and scientific rigour required by epitaxy, there remain many subjective aspects to its procedures that can only be learned first-hand. “You can do some calculations on temperatures or pressures that are most likely to be favourable but very often it’s a multi-variable process and you can’t take everything into account,” he says. “You’re stacking atoms on top of each other, one at a time. It takes a while to have a decent amount of material. You have to be patient.”
Arès agrees with SpringThorpe that epitaxy gets little or no glory in the high-tech world, where designers get the credit for a device’s success and crystal growers will only be consulted if the device fails. Nevertheless, Arès sees a growing appreciation of the role that these materials play in our everyday lives. “When it was very high-end military lasers and detectors or some optical components hidden inside fibre optic systems, nobody knew what epitaxy was,” he says. “But now it’s in cell phones, it’s in lighting, it’s in solar cells — it’s in things that people can see.”