by Judy Purdy
The shiny, chrome-plated bumpers and hubcaps that graced vintage cars may ultimately pave the way for a new generation of microscopic semiconductors.
A new method of electrodeposition -- the process used to make chrome bumpers for classic Chevrolet Bel-Aires, Ford Fairlanes and port-hole Thunderbirds -- may one day yield microcircuits so small they can be measured by layers of atoms and so precise they can be used to track exhaust from MiG jets.
"There has been an elegant method for quite awhile to electrodeposit compounds, but it tends to result in disordered and non-ideal material," said UGA electrochemist John Stickney. "It doesn't have nearly the amount of controllability needed to form an electronic-grade material."
So Stickney developed a technique that may make it possible to electrodeposit materials for a wide variety of electronic devices, including semiconductors of sufficient quality to produce optical electronic devices. Such devices could be used in lasers for compact disk players and fiber optic systems or detectors for infrared instruments, heat seeking missiles and night vision goggles.
"People were depositing materials electrochemically, but the structures were very polycrystalline -- composed of many small crystals. There was little control over the resulting structure. That's where our effort is -- not in making the devices but in trying to understand fundamentally how to electrochemically form these materials," he said.
What's different about Stickney's recently patented technique is that everything is done on a very controlled and extremely fine scale. "It all comes down to atoms," he said.
For the past five years, the associate professor of chemistry has experimented with ways to alternately electrodeposit one-atom-thick layers of two or more elements, such as cadmium and tellurium, to form a compound. His technique, Electrochemical Atomic Layer Epitaxy (ECALE), results in compounds that might be useful for many kinds of semiconductors, such as those found in computers.
"We do this by depositing two different elements in a one- to-one ratio," he said.
To deposit one-atom-thick layers, Stickney combines Atomic Layer Epitaxy -- a chemical deposition process -- with a surface- limited reaction that stops when the electrode surface is covered. But he and his graduate students encountered stumbling blocks along the way. For instance, the first cadmium layer would "peel off" when they tried to deposit a second telluride layer on top of it. A different tellurium precursor solved their quandry.
One application of cadmium telluride is in the formation of photovoltaics. "Cadmium telluride is perfectly matched to adsorb the light produced by the sun, and thus has potential to produce very efficient photovoltaic devices for conversion of solar power to electrical power," he said.
ECALE also might have applications in making phosphor materials, which are essential to developing new high-definition display systems, such as television screens, for the planned information superhighway.
"The United States is trying to advance its research status in this area and that involves fundamental science, new technology and a lot of applied work," Stickney said. "We have a synthesis technique that may be able to form some phosphors better than other methods and would probably form some phosphors worse than other methods. We need to find out what it can and can't do and where it might be able to contribute."
Stickney's process also is more environmentally friendly. Others "produce a lot of waste gases that are very toxic and hard to dispose of," he said.
His process uses very small amounts of dilute solutions that are easily recycled.
"We only need the number of atoms we want to deposit," he said. "Theoretically, ECALE will cost less since it uses much smaller amounts of chemicals and the hardware is very simple."
Unlike other deposition processes, ECALE takes place at room temperatures, which results in more pronounced boundaries between different compound layers and, consequently, higher quality compounds.
But Stickney isn't content to stop there. He continues to experiment with various chemicals and surfaces to form thin films of other new compounds. To date, he has successfully deposited about 10 compounds. He also wants to find ways to deposit multilayers and to make thicker films.
"It's fun stuff, but to make these devices, you have to improve the quality," he said. "To build a laser or an optical electronic device, you need very highly structured materials."