Australasian Science: Australia's authority on science since 1938

A Seven Atom Transistor

By Stephen Luntz

Twenty years since 35 xenon atoms were manipulated into the shape of the IBM logo, the same technique has been used to form a transistor from just seven phosphorus atoms precisely placed in silicon. The achievement represents another step towards the creation of a quantum computer.

“The significance of this achievement is that we are not just moving atoms around or looking at them through a microscope,” says Prof Michelle Simmons of the University of NSW Centre for Quantum Computer Technology. “We are manipulating individual atoms and placing them with atomic precision in order to make a working electronic device.”

The Nature paper announcing the atomic IBM logo noted that scanning tunnelling microscopes might one day be used for the “ultimate in device miniaturization”. However, Simmons says that five more steps were needed to achieve this goal, which is now published in Nature Nanotechnology.

“Originally metal atoms were manipulated on a metal surface,” says Simmons. This was done by applying a voltage to the microscope’s tip to attract atoms and move them around.

“We are placing atoms in a semiconductor, which is much harder because the bonds are much stronger. We had to find another way to manipulate the atoms. We did this by creating a pattern on the surface, using a hydrogen mask and using the scanning tunnelling microscope tip to knock off hydrogen atoms in the right places. Then we had to find the right chemistry to make the phosphorus atoms bond to the exposed silicon surface.”

After this, the phosphorus atoms were encapsulated so that they stayed in place. Doing this presented one challenge; proving it afterwards by imaging them beneath the surface was another.

Finally the device was removed from the scanning tunnelling microscope environment and connected to electrodes. For this the team had to place markers that were visible to other forms of microscopes around the transistor and ensure that these markers survived the manufacturing process.

Connecting a few tens to hundreds of quantum transistors together should be enough to produce computers more powerful than conventional computers containing billions of transistors, but Simmons says there is still a long way to go to achieve this, particularly since the quantum spin state of atoms must be controlled for each transistor.

Recently a commercial single state atom transistor was measured, but Simmons says its creators had to measure hundreds of devices to find one statistically where a single atom was in the right place. Simmons counters that her team has succeeded in placing the atoms with atomic precision in all three dimensions, forming the smallest precision-built transistor.