SURFACE SMOOTHING AND CRYSTALLINE REORIENTATION IN THIN COBALT FILMS

Epitaxial cobalt films in the thickness range of 3.9 to 8.6 nm were de posited on Al2O3[11 (2) over bar 0] substrates by de magnetron sputter ing at a substrate temperature of 315 degrees C. In situ annealing was performed in a vacuum after which the samples were rapidly quenched t o room temperature in order to preserve the high temperature structure . Ex situ atomic-force microscopy revealed that surface roughening tak es place during annealing and reaches a maximum when the annealing tem perature TA is equal to a critical temperature T(C)similar to 500 degr ees C. We discovered that if T-A>T-C the surface becomes smooth again, although large rectangular pits that go down to the substrate also ap pear. 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 T-A<T-C, and subsequently to a fcc[001] orientation for T-A>T-C. We show that surface or interface o xidation cannot be the sole cause of this effect. We speculate that an increasing interface strain st higher temperatures or a surface recon struction of the substrate are possible mechanisms.