July 23, 2009

Monolayer Nanotechnology Will Enable Silicon to Maintain Conductance for Smaller Devices and Sustain Moore's Law Progress

Scientists at Rice University and North Carolina State University have found a method of attaching molecules to semiconducting silicon that may help manufacturers reach beyond the current limits of Moore's Law as they make microprocessors both smaller and more powerful.

The electronic properties of silicon, such as the conductivity, are largely dependent on the density of the mobile charge carriers, which can be tuned by gating and impurity doping. When the device size scales down to the nanoscale, routine doping becomes problematic due to inhomogeneities. Here we report that a molecular monolayer, covalently grafted atop a silicon channel, can play a role similar to gating and impurity doping. Charge transfer occurs between the silicon and the molecules upon grafting, which can influence the surface band bending, and makes the molecules act as donors or acceptors. The partly charged end-groups of the grafted molecular layer may act as a top gate. The doping- and gating-like effects together lead to the observed controllable modulation of conductivity in pseudometal−oxide−semiconductor field-effect transistors (pseudo-MOSFETs). The molecular effects can even penetrate through a 4.92-μm thick silicon layer. Our results offer a paradigm for controlling electronic characteristics in nanodevices at the future diminutive technology nodes.

The paper suggests that monolayer molecular grafting -- basically, attaching molecules to the surface of the silicon rather than mixing them in -- essentially serves the same function as doping, but works better at the nanometer scale. "We call it silicon with afterburners," Tour said. "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 said years of research into molecular computing with an eye toward replacing silicon has yielded little fruit. "It's hard to compete with something that has trillions of dollars and millions of person-years invested into it. So we decided it would be good to complement silicon, rather than try to supplant 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."

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