Tiny electronics via silicon with ‘afterburners’

RICE (US)—Scientists have found a way to attach molecules to semiconducting silicon. The discovery may help electronics manufacturers overcome a major roadblock to developing smaller and more powerful microprocessors.

Moore’s Law, suggested by Intel cofounder Gordon Moore in 1965, proposes the number of transistors that can be placed on an integrated circuit doubles about every two years. But even Moore has said the law cannot be sustained indefinitely.

The challenge is to get past the limits of doping, a process that has been essential to creating the silicon substrate that is at the heart of all modern integrated circuits, says James Tour, Chao Professor of Chemistry and professor of mechanical engineering and materials science and of computer science at Rice University.

To date, doping has been effective even in concentrations as small as one atom of boron, arsenic or phosphorus per 100 million of silicon. But as manufacturers pack more transistors onto integrated circuits while making the circuits ever smaller, doping gets problematic.

“When silicon gets really small, down to the nanoscale, you get structures that essentially have very little volume,” Tour explains.


“Attaching molecules to semiconducting silicon affects the threshold voltage, or gate voltage, required to create a conductive path between the source and drain electrodes (blue) and turn the device on. The molecules influence the amount of charge carriers available within the device layer (red). (Courtesy: Rice University)

Manufacturers who put billions of devices on a single chip need them all to work the same way, but that becomes more difficult with the size of a state-of-the-art circuit at only 45 nanometers wide, with smaller ones on the way. For comparison, a human hair is about 100,000 nanometers wide.

The researchers argue that attaching molecules to the surface of the silicone rather than mixing them, serves the same function as doping, but works better at the nanometer scale.

“We call it silicon with afterburners,” Tour says. “We’re putting an even layer of molecules on the surface. These are not doping in the same way traditional dopants do, but they’re effectively doing the same thing.”

Tour says the process will complement, rather than replace silicon. “It’s hard to compete with something that has trillions of dollars and millions of person-years invested into it.”

He anticipates wide industry interest in the process, in which carbon molecules could be bonded with silicon either through a chemical bath or evaporation. “This is a nice entry point for molecules into the silicon industry. We can go to a manufacturer and say, ‘Let us make your fabrication line work for you longer. Let us complement what you have.’

“This gives the Intels and the Microns and the Samsungs of the world another tool to try, and I guarantee you they’ll be trying this.”

The findings are published this month by the Journal of the American Chemical Society.

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