Portland, Ore. -- Researchers at the University of Wisconsin in Madison have discovered a surface treatment that enables nanoscale membranes of silicon to conduct better as they get thinner, thereby extending Moore's Law all the way to atomic dimensions.
"If you make silicon half as thick, you would expect it to conduct half as well," said materials science and engineering professor Paul Evans. "But it turns out that silicon can conduct even better at the nanoscale if the surface is well-prepared." Evans performed the work with professors Mark Eriksson, Irena Knezevic and Max Lagally, research scientist Donald Savage and doctoral candidates Pengpeng Zhang, Emma Tevaarwerk and Byoung-Nam Park.
The team discovered that as silicon layers dip below 200 nanometers in thickness--what the team calls "nano- membranes"--the normal determinants of conductivity, such as dopants, become irrelevant. Instead, it is the atomic accuracy of the surface that determines conductivity. In fact, the team found that when atomically accurate silicon nanomembranes were cleaned of oxide contaminations, they became 10 times more conductive. And as nanomembrane thickness shrunk toward 10 nm, a careful surface preparation could increase conductivity as much as 1 million times.
"What this tells us is that if you're building nanostructures, the surface is what's really important," said Evans.
The team made the discovery by accident: It was not setting out to increase the conductivity of nanomembranes, but merely attempting to clean their surfaces for more accurate imaging with a scanning-tunneling microscope. Working with a silicon-on-insulator substrate, the team was trying to clean off unintended oxidation atop its silicon nanomembranes that result whenever chips are exposed to air.
The usual method of removing top-layer oxidation, high-temperature annealing, could not be used, because temperatures above 1,200°C cause nanomembranes to ball up. Instead, the team put its test chip in an ultrahigh vacuum and slowly deposited a few monolayers of silicon and germanium to displace the unwanted oxidation and replace it with an atomically precise cap.
The resultant scanning-tunneling microscopy revealed that such carefully prepared nanomembranes had greatly enhanced conductivity. To explain the surprising results, the team characterized nanomembranes ranging from 200 down to 15 nm and found that careful surface preparations increased conductivity as the membranes got thinner--the opposite of what happens to unprepared silicon films.
The mechanism by which conductivity increases as nano-membrane thickness decreases is not yet well-understood, but the researchers speculated that the monolayer surface preparation has holes in its lattice that attract electrons from inside the membrane. As electrons flow upward to fill in the surface holes, they leave behind holes inside the nano-membrane that facilitate increased conduction. The thinner the nanomembrane becomes, the bigger the proportion of holes opened up inside it when electrons are attracted to the surface, thereby enhancing conductivity more and more as nanomembranes get thinner and thinner.
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