The motion of electrons in a solid is determined by the periodic arrangement of its atoms. Any break in the uniformity of this lattice radically alters the electrons’ behavior. For example, a break in the symmetry of an atomic lattice at a surface creates new energy states for electrons to occupy. These states do not exist deeper in the material, and so the electrons become trapped at the surface, although they remain free to move in two dimensions.

As the energy of electron states at the surface is influenced by the strain, altering the strain in a material is one way to control surface electrons and put them to use in a device. This can be achieved by growing ultrathin metallic layers on top of a semiconductor, but the problem is that any imperfections in the metal film cause strain relaxation. Hiroyuki Hirayama and his team at the Tokyo Institute of Technology in Japan1 have now quantified the local effect that such dislocations have on the properties of surface electrons.

Fig. 1: A sharp tip can be used to investigate the electronic states of electrons at a surface and the way in which they are affected by dislocations.

Hirayama and his co-workers began looking at imperfections in a 20-atom-thick layer of silver deposited on a silicon substrate using a probe technique known as scanning tunneling spectroscopy. By applying a voltage to a sharp tip brought atomically close to the silver surface (Fig. 1), the researchers were able to obtain information about the surface-state energy from the current that flowed across the gap. They found that an individual imperfection, called a ‘dislocation’ in the atomic lattice, lowered the energy by as much as 40 meV compared to states near blemish-free parts of the film. They also showed that the effect could still be ‘felt’ by an electron 15 nm away from the dislocation.

When a very thin layer of metal is grown on a surface, the atoms align with the substrate’s atomic lattice. However, as the interatomic spacing in natural silver is shorter than that in the silicon substrate, the silver atoms are spaced further apart than they would like to be, which induces a strain force. At a dislocation, this strain relaxes, causing the observed change in electron energy.

Hirayama and his team hope to study the effect of dislocations on the electronic states in very small structures. “In the future, device sizes will be in the nanometer region, and the effect of misfit dislocations will become a decisive factor in performance.”