'Perfect' transistor built from a single atom

Posted by Emma Woollacott

Physicists have created a working transistor using only a single phosphorus atom placed precisely in a silicon crystal.

The development could mark a big step towards quantum computing. While single-atom transistors have been produced before, it's been by chance, with researchers either searching through many devices or tuning multi-atom devices to isolate one that works.

"But this device is perfect", says Professor Michelle Simmons, group leader and director of the ARC Centre for Quantum Computation and Communication at the University of New South Wales.

"This is the first time anyone has shown control of a single atom in a substrate with this level of precise accuracy."

The device even has tiny visible markers etched onto its surface so researchers can connect metal contacts and apply a voltage.       
"Our group has proved that it is really possible to position one phosphorus atom in a silicon environment - exactly as we need it - with near-atomic precision, and at the same time register gates," says UNSW research fellow Dr Martin Fuechsle.

The team used a scanning tunnelling microscope (STM) to see and manipulate atoms at the surface of the crystal inside an ultra-high vacuum chamber. Using a lithographic process, they patterned phosphorus atoms into functional devices on the crystal then covered them with a non-reactive layer of hydrogen.

Hydrogen atoms were removed selectively with the metal tip of the STM, and a controlled chemical reaction then incorporated phosphorus atoms into the silicon surface.
                           
Finally, the structure was encapsulated with a silicon layer and the metallic connections aligned using the etchedmarkers.

The electronic properties of the device conformed beautifully with theoretical predictions for a single phosphorus atom transistor.

Moore's Law predicts that commercial single-atom transistors will be achieved by about 2020 - and this development, says Simmons, could see it happen well ahead of schedule.