Atoms move through the crystal lattice of a solid by a process called diffusion. The atoms migrate by swapping places with ‘point defects’ such as vacancies – empty spaces in the lattice – and ‘interstitial’ atoms, which float between the regularly spaced atoms. In a semiconductor structure, a solid is doped with ‘foreign’ atoms that carry charge. Since these charge carriers move by a similar diffusion process, a better understanding of the phenomenon could enable scientists to develop more efficient semiconductors.

Hartmut Bracht and co-workers created a structure from gallium antimonide (GaSb) layers containing different concentrations of Ga and Sb isotopes. Using a technique called secondary ion mass spectrometry (SIMS), the team measured how far the isotopes travelled through the structure in 18 days. SIMS allows diffusion to be monitored for a longer period than was possible with earlier methods, which were limited by the half-lives of radioactive ‘tracer’ atoms they used. The SIMS results showed that Ga diffuses at a similar rate through both Sb-rich and Ga-rich regions. But the team was surprised to find that Sb moves through Sb-rich regions over a thousand times more slowly, and it hardly diffuses through the Ga-rich regions at all. “GaSb is a key ingredient in high-mobility transistors”, team member Eugene Haller told PhysicsWeb, “and our studies form the basis of the advanced understanding we need to make these devices”.

The discrepancy occurs near the melting point of GaSb, and cannot easily be explained by conventional theories of diffusion. Bracht’s team proposes a new mechanism for the effect, in which reactions between defects on the Ga and Sb sublattices remove the defects that are necessary for the migration of the Sb atoms. The group now plans more detailed experiments on GaSb.