In situ measurements of cobalt thin-film surface roughening upon annealing

Atomic force and scanning tunneling microscopies were employed to investigate the surface roughness of 5.0- and 8.0-nm-thick Co films as a function of annealing temperature. Films were epitaxially grown on (110) Al2O3 at 315°C via molecular-beam epitaxy. The as-grown films had a surface roughness average of approximately 0.7nm. Images were acquired in situ at temperatures ranging from room temperature to 600°C. For the 5.0nm films, surface roughening abruptly starts at a critical temperature T0=459°C and increases depending on the final annealing temperature and annealing time. After waiting long periods of time, so that the film is in quasiequilibrium, the roughness increases with increasing temperature T with a power-law dependence (T∕T0−1)0.5. This sudden roughening process is irreversible and possibly related to the hcp-fcc martensitic phase transition in cobalt, which occurs at 415°C in bulk, or due to bulk defect diffusion. The autocorrelation function of the images above T0 shows that the in-plane island size during roughening is larger in a direction perpendicular to the sapphire terraces. The 8.0-nm-thick film sample revealed the formation of irregularly shaped pits with a lateral size of ∼350nm at temperatures greater than ∼550°C.