Plastic Fantastic

Silicon, Tom Jackson readily acknowledges, is a remarkable material. The well-ordered atomic structure of single-crystal silicon, in particular, makes it an excellent semiconductor, essential to such marvels as the Pentium processor. But single-crystal silicon is too costly for use in cheap information-storing devices like smart cards or inventory control tags.

plastic

One solution is amorphous silicon. Inexpensive to make, this variety lacks the long-range structural order needed to carry a signal; by itself, Jackson asserts, "Its electronic properties are garbage." Fortified with a bit of hydrogen, however, amorphous silicon will straighten up and pass a current well enough for a smart card—or for the multitude of simple transistors needed to create the display screen of a laptop computer.

Like the single-crystal variety, however, a thin film of amorphous silicon requires a glass substrate to "grow" on, which is not exactly ideal for a product you're going to keep in your wallet. That's one of the reasons why interest has re-kindled in making transistors out of plastic.

"Back in the 40s, there was a good bit of interest in organic materials," says Jackson, Penn State professor of electrical engineering. "It petered out when silicon took off." Not that people didn't recognize plastic's real advantages: In addition to being lighter and more rugged, plastic semiconductors would be cheaper than those grown on glass. The problem has always been the speed—or rather the lack of it—with which plastic can shuttle a pulse; the property engineers call charge mobility.

About 20 years ago, Jackson says, during the first revival of interest in organic conductors, "people started trying to make simple electronic devices out of poly materials. They found you could make a transistor similar in structure to amorphous silicon." But the mobility of these prototypes was too low for any practical application. Nothing changed until about 10 years ago, when a French physicist named Francis Garnier started looking at an organic compound called sexithienyl.

This stuff was different; its molecules were small and tended toward a crystalline structure, unlike the large disorderly molecules that make up poly materials. And it could move current: Not nearly as fast as amorphous silicon, but far faster than poly materials can. Which brings the story round to Jackson. In 1992, after 12 years at IBM's T.J. Watson Research Center, where he had worked on computer displays and other semiconductor technologies, he came to Penn State with a yen to explore the possibilities of organic materials. Jackson, author of 19 patents, started by looking at sexithienyl, as many other labs are now doing. "We got similar mobilities to what everyone else has," he reports. Not satisfied, he decided to try something different.

"So I sat down with the Aldrich Structural Index [the "bible" of molecular structures] and a laundry list of criteria," he says. "I spent part of one Christmas holiday just looking for any material that seemed interesting." One that fit the bill was the plastic pentacene.

A simple molecule, pentacene is made up of five benzene rings fused in a line. It's short, well-ordered, and can be deposited at relatively low temperatures (75 degrees C), which means it is amenable to a plastic substrate. (A thin film is created by heating a solid under controlled conditions until it turns to vapor, which is then allowed to cool and settle, layer by layer, onto the desired substrate.)

"We bought some—it's commercially available—and deposited it," Jackson says. "The very first time, we got mobilities high enough to be interesting." Once they had gone back and removed impurities from their samples, mobilities jumped even higher, to a level comparable to that of amorphous silicon.

In order to be useful as a semiconductor, especially in something like a display screen, pentacene needs qualities beyond mobility, however. "It has to work well as a switch," Jackson says. The image on a high-resolution laptop display "refreshes" itself 50 to 100 times a second; any slower and the screen's visible flickering would boggle your eyeballs. To keep such a pace, the transistors that support each individual pixel must be able to turn on and off very, very quickly. Nor can they leak current when they are supposed to be "off," a particular concern for a portable, battery-powered device. In tests so far, Jackson and his graduate students Yen-Yi Lin and Dave Gundlach have found that pentacene scores acceptably on both these counts.

There remains a drawback, however. The plastic requires a higher than acceptable voltage to switch it on and off, about 80 volts, compared to five to 10 volts for amorphous silicon. "You'd need a generator," Jackson says. He and his students are now working on bringing that voltage down. One of the ways they hope to do so is by improving the thin-film deposition process.

"The thing about organic materials," Jackson says, "is that they're very sensitive to processing." When deposition is done slowly, he reports, at low enough temperatures, pentacene grows in large tree-shaped grains, with few defects. "But when it's blasted on by flash evaporation [a common, high-temperature technique], it doesn't have time to form crystals, and you get rotten characteristics.

"It all depends on how the material is grown," Jackson says, "and that's something we're just beginning to get a handle on."

Thomas N. Jackson, Ph.D., is professor of electrical engineering, 216 Electrical Engineering West Bldg., University Park, PA 16802; 814-863-8570; tnj1@psu.edu. Yen-Yi Lin and David Gundlach are Ph.D. students in electrical engineering. The research reported above is sponsored by the Defense Advanced Research Project Agency and the National Science Foundation. For additional information, visit Jackson's lab on the World Wide Web at jerg.ee.psu.edu

Last Updated May 01, 1998