Doping generally involves adding impurities to change the electronic properties of semiconducting materials by increasing the number of charge carriers, which can be electrons or "holes". Although the dopant atoms are randomly dispersed throughout the material, their distribution is assumed to be fairly uniform. However, as the size of semiconductor devices continues to shrink, this uniformity can no longer be taken for granted. Some regions will contain significantly more charge carriers than others, and this will adversely affect the performance of the device.

Shinada and co-workers use a single-ion implantation technique to overcome this problem. A small aperture is used to extract ions from a focussed ion beam and they are directed one by one into a nano-sized region of semiconductor until the required number of dopant atoms have been implanted. The team count the number of ions implanted by detecting secondary electrons.

A wide variety of ions - including beryllium, boron, phosphorus, iron and cobalt - can be implanted with a precision of 60 nm. When the Japanese team implanted phosphorus ions at 30 kilovolts into a 100 nm-wide channel in a transistor, they found that the threshold voltage decreased from -0.4 volts, the value for a conventional device with random doping, to just -0.2 volts. The group says that the improvement is a result of the electrostatic potential in the channel being more uniform due to the ordered distribution of dopant atoms (see figure).

Although the technique is too slow for high-volume chip manufacturing, the team plans to modify the ion beam so that single ions can be implanted with an accuracy of better than 10 nm, which should allow it to make single-atom devices.

"Ordered dopant arrays may enhance the prospects of making single-atoms devices whose properties are governed by individual dopant atoms, such as silicon-based solid-state computers," Shinada told PhysicsWeb. The team also plans to apply the techniques to biomedical materials.