Researchers at IBM have overcome an important obstacle to building computers based on carbon nanotubes, by developing a way to selectively arrange transistors that were made using the carbon molecules. The achievement, described in the current issue of Nano Letters, could help make large-scale integrated circuits built out of carbon nanotubes possible, leading to ultrafast, low-power processors.
For decades, the size of silicon-based transistors has decreased steadily while their performance has improved. As the devices approach their physical limits, though, researchers have started looking to less conventional structures and materials. Single-walled carbon nanotubes are one prominent candidate – already researchers have built carbon nanotube transistors that show promising performance (see The Nanotube Computer). According to estimates, carbon nanotubes have the potential to produce transistors that run 10 times faster than even anticipated future generations of silicon-based devices, while at the same time using less power.
But so far research in the field has hit a roadblock: not being able to control the placement of nanotube transistors, making it impossible to build complex integrated circuits. “The way most [nanotubes transistors] are made now, nanotubes are randomly dispersed on a surface in solution, then source and drain contacts are randomly printed using lithography, and then you search around until you find by chance a tube that goes between a source and a drain,” says James Hannon, one of the researchers involved with the work at IBM’s T.J. Watson Research Center in Yorktown Heights, NY.
To gain control over the arrangement of transistors, the IBM researchers coated the nanotubes with molecules that bind only to patterns of metal oxide lines on a surface, and not to the areas in-between.
To make working transistors, the researchers laid down lines of aluminum using a lithography technique. These wires serve as the gates that turn the transistors on and off. They then oxidized the aluminum to form a thin aluminum oxide layer on top of the wires, which acts as both a dielectric and the material to which the nanotubes will bind. After applying carbon nanotubes in solution and allowing them to bind to the aluminum oxide, the researchers deposited palladium leads perpendicular to the aluminum/aluminum oxide wires. These leads crossed over the nanotubes, becoming the source and drain of the transistor.
While developing this method of organizing nanotube transistors is an important step, much work remains to be done before commercial processors will be available. For one thing, exploiting the full potential of nanotube transistors will require improving the leads, possibly by using nanotubes in place of the palladium wires.
But perhaps a more pressing problem is finding reliable and inexpensive ways to isolate different types of carbon nanotubes. Current fabrication techniques produce a mix of nanotubes with different sizes and electronic properties, not all of which will work well in integrated circuits.
Because of these challenges, the first applications of carbon nanotube transistors will probably not be as high-performance processors, Hannon says, but highly sensitive sensors that work even with a mix of different nanotubes.
Meanwhile, others are developing devices that don’t rely on nanotubes’ high-end electrical properties, but rather on features such as their strength and flexibility. This skirts the need both to sort and to individually arrange the nanotubes. The Woburn, MA-based company Nantero, for example, takes advantage of nanotubes’ strength and flexibility to make memory devices. “We use [nanotubes] as electromechanical devices, so we just bend them up and down to represent zeros and ones,” says Nantero CEO Greg Schmergel. In this application, clusters of nanotubes rather than single tubes can be used, so they can be patterned using lithography.
Eventually, Schmergel says, nanotubes could replace every part of semiconductor devices by using all of the tubes’ features. “Nanotubes have quite a number of unique properties all combined in one material. They can replace memory, logic, the interconnect, ultimately they can replace everything in the chip, so it definitely makes sense to pursue all of those angles,” he says.