Plastic ICs: Another Step Closer

The day when you can roll up your computer and put it in your pocket just got a little closer. Researchers at IBM (Yorktown Heights, N.Y.) have been able to produce thin films of an organic "plastic" material called pentacene with crystal grains 20 to 100× larger than previously observed and with semiconductor characteristics good enough to build electronic devices. While still far from a commercial product, this discovery in organic chemistry could make it easier to build computing devices by "printing" or "spraying" a thin film of semiconductors onto plastic and other materials.

The advancement is also a significant milestone in the world of molecular electronics (which some call "moletronics") where, instead of making a transistor from p-n junctions, you can make the equivalent of a transistor by using organic molecules. "One can imagine designing molecules where you might even have three electrical connections to a single molecule, and you would have something that would be functionally like a transistor but it would now be a thousand times smaller, and potentially a thousand times more dense," said Bob Hamers of the chemistry department at the University of Wisconsin-Madison. Hamers' work in how organic materials are deposited was instrumental in helping IBM researchers achieve their goal.

Large pentacene crystals grown by IBM on silicon substrates. (Source: IBM)

"For years and years, there have been these theories about how materials grow," Hamers said. "People have used those fairly extensively to guide what they do with inorganic materials. But, because organic materials are just really different, in particular because these organic molecules have complicated shapes, it's never really been clear to what extent these classical theories of nucleation and growth might actually be used to understand what happens with organic materials."

Hamers said an important aspect of the IBM work is that they made this connection between what happens in organic materials, and how it connects back to what has been used for many years, to understand inorganic materials. "By making that connection, they have paved the way to doing an even better job of understanding and manipulating and growing organic-based materials."

Much of the original work in making organic-based transistors used films with electrical properties that were relatively poor. "Over about the last five years, people at Bell Labs, in particular, have found some ways of growing single-crystal materials that are very large, and they've been able to make individual devices on them. They've been able to show, in fact, if you have a single crystal, the electrical properties are extremely good," Hamers explained. "But the problem is that the single crystals were grown in a way that is completely incompatible with standard microelectronics chemical vapor deposition processes."

The IBM group used a more manufacturing-friendly technique in which it grew large crystals on silicon, on the order of a tenth of a millimeter. "That's still pretty small, but it is big enough that one could actually make not only individual transistors, but small integrated circuits," Hamers said.

Using an electron microscopy technique called photoelectron emission microscopy (PEEM), the IBM team shot high-resolution videos that captured pentacene molecules in the early stages of crystal growth. The researchers found that, when pentacene is grown on a silicon substrate, it tends to stick to surface impurities, resulting in erratic growth and grain sizes too small for building high-quality devices. To grow larger grains, the IBM team first applied a "molecular buffer" — a single layer of cyclohexene molecules — on top of the silicon substrate. This layer covers the "sticky sites" on the silicon, resulting in a surface that is clean and allows the pentacene to grow very large grains.

The IBM work appears in the Aug. 2 issue of Nature. The authors of the report "Growth Dynamics of Pentacene Thin Films" are Frank-J. Meyer zu Heringdorf, Mark C. Reuter and Rudolf M. Tromp. The same issue contains an excellent analysis of the work by Hamers.

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