Single atom transistor ahead of its time

An individual phosphorus atom is the active component that has been placed in a silicon crystal to create a single atom transistor.

This feat of microengineering was achieved by physicists at the University of NSW and is described in a published paper. The phosphorus, patterned between atomic scale electrodes and electrostatic control gates, may be the building block for a future quantum computer that would have unprecedented efficiency.

Although single atom transistors have been created before, they have usually been made by accident.

“This new device, however, is perfect,” says Prof Michelle Simmons, group leader and director of the ARC Centre for Quantum Computation and Communication Technology at the university.

It is the first time anyone has shown control of a single atom in a substrate with this level of accuracy. Etched into its surface are visible markers to which can be attached metal contacts allowing a voltage to be applied, according to Dr Martin Fuechsle of the university.

The researchers used a scanning, tunnelling microscope to see and manipulate atoms at the surface of the crystal inside an ultrahigh 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 then removed selectively in precisely defined regions with the superfine metal tip of the microscope. A controlled chemical reaction then incorporated the phosphorus into the silicon surface.

Finally, the structure was encapsulated in a silicon layer and the device contacted electrically using an intricate system of alignment markers on the silicon to align metallic connects. The electronic properties of the device were in excellent agreement with theoretical predictions for a single phosphorus atom transistor.

“This is remarkable,” says Dr Fuechsle, because its electronic characteristics exactly match theoretical predictions undertaken with Prof Gerhard Klimeck’s group at Purdue University in the US and Prof Hollenberg’s group at the University of Melbourne, who were joint authors on the paper along with Dr Oliver Warschkow of Sydney University’s School of Physics, who contributed to the understanding of the phosphorus positioning mechanism.

It is predicted that transistors will reach the single atom level by about 2020 to keep pace with Moore’s Law, which describes an ongoing trend that sees the number of chip components double every 18 months.

“This atom transistor has developed the technology to make this 2020 deadline well ahead of schedule and gives valuable insights to manufacturers into how devices will behave once they reach the atomic limit,” says Prof Simmons.