It was recalled that the diffusion of substrate material to the surface of some epitaxial thin-film systems had been reported to occur very rapidly; even at homologous temperatures which were lower than 0.40, where bulk interdiffusion was expected to be negligible. The use of in situ scanning tunnelling microscopy revealed that Cu diffusion through ultra-thin Co films, grown under ultra-high vacuum onto Cu(100) substrates, occurred via a surface diffusion process rather than bulk interdiffusion. During annealing, pores nucleated at weak points of the films; for example, near to step bands in the substrate. As Cu diffused from the substrate, through the pores, and to the top of the film, pits which were up to several tens of nm in linear dimension were formed.

A.K.Schmid, D.Atlan, H.Itoh, B.Heinrich, T.Ichinokawa, J.Kirschner: Physical Review B, 1993, 48[4], 2855-8