Monday, February 27, 2006

"CHIPS: Surface prep yields a super conductor"

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. 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. 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.