Credit: © 2007 AAAS

During the construction of silicon nanodevices, other atoms may be added to act as electrical carriers. If these impurities are unevenly distributed in the silicon lattice, they may cause fluctuations in the electrical characteristics of the device.

Now, imaging technology capable of observing individual atoms is helping researchers such as Keith Thompson and his co-workers1 at the Imago and IBM corporations in the USA to understand how such fluctuations occur, and how they may be controlled. Arsenic atoms were implanted into a silicon wafer, which was then annealed at 600 °C. The sample was studied using a technique called atom probe tomography, which involves evaporating individual atoms from the sample onto a collection plate to build up a three-dimensional image. The images revealed spheroidal defects in the silicon lattice, caused by impacts from the heavier arsenic atoms.

After further annealing at 1,000 °C, the spheroidal defects transformed into stable loop-shaped lattice dislocations surrounded by a cloud of arsenic atoms. Such structures, known as Cottrell atmospheres, are generally around three nanometres in size — too small to cause problems in current devices. However, emerging technology may use components as small as 10 nanometres in size, in which the influence of Cottrell atmospheres could be a significant one.