  An integro-differential expression for the diffusion current of the
impurities diffusing by the mechanism of bound impurity-defect pairs has been
derived. The ensuing nonlocal diffusion equation generalizes the existing
theories of diffusion by the pair mechanism in unbounded systems with
homogeneous defect distributions on the systems with boundaries and variable
defect concentration. It has been established that the nonlocality manifests
itself only at short diffusion times while at large times, in particular, under
the stationary conditions, the predictions of the nonlocal theory would
coincide with existing approaches based on local diffusion currents.
Possibilities of experimental verification of the nonlocal theory have been
suggested. In particular, the explanation of the uphill diffusion in silicon by
the pair drag can be tested within slightly modified conventional experimental
setups. Also, it has been shown that in the nonlocal theory the impurity
segregation profile caused by the drag should differ at the initial stage of
evolution from the predictions of the local theories. Because the segregation
influences mechanical, corrosive, and electronic properties of materials, the
nonlocal character of the pair diffusion may have important implications for
nanostructured materials.
