Surface smoothing and crystalline reorientation in thin cobalt films

Epitaxial cobalt films in the thickness range of 3.9 to 8.6 nm were deposited on Al₂O₃[1120] substrates by dc magnetron sputtering at a substrate temperature of 315 ^∘{}C. In situ annealing was performed in a vacuum after which the samples were rapidly quenched to room temperature in order to preserve the high temperature structure. Ex situ atomic-force microscopy revealed that surface roughening takes place during annealing and reaches a maximum when the annealing temperature TA is equal to a critical temperature TC{~}500 ^∘{}C. We discovered that if TA>TC the surface becomes smooth again, although large rectangular pits that go down to the substrate also appear. X-ray-diffraction data show that unannealed samples are oriented along the hcp[0001] direction. Upon annealing samples transform to a preferentially fcc[111] orientation for TA<TC, and subsequently to a fcc[001] orientation for TA>TC. We show that surface or interface oxidation cannot be the sole cause of this effect. We speculate that an increasing interface strain at higher temperatures or a surface reconstruction of the substrate are possible mechanisms.