CRYSTALLIZATION OF CO-SPUTTERED AMORPHOUS COBALT-CARBON ALLOYS - MORPHOLOGY AND KINETICS OF SPHERULITIC GROWTH

We investigated the crystallization behavior of co-sputtered amorphous Co75C25 films by in situ transmission electron microscopy (TEM). It was found that, at 250-280-degrees-C, the amorphous film transforms into crystalline phase (a mixture of c.p.h. cobalt and metastable delta'-carbide) by nucleation and growth of approximately circular or egg-shaped ''grains'', which we call ''spherulites''. The circular spherulites and, to some extent, the egg-shaped spherulites as well, possess distinct six-fold symmetry. It was found that, at the center of the spherulites, the c-axis of c.p.h. cobalt and metastable delta'-carbide points out of the plane of the films and, as the spherulites grow radially, the c-axis becomes inclined towards the growing direction. Moreover, the growth rate of the spherulite is high when the c-axis is parallel to the growing direction, while it is slow when the c-axis is perpendicular to the growing direction. The average activation energy for growth is 2.1 +/- 0.2 eV, which is higher than the activation energy for carbon diffusion in cobalt, 1.6 eV, but lower than that for self-diffusion of cobalt, 2.8 eV. This discrepancy between the activation energy of the growth and those for diffusion, as well as the dependence of the growth rates on the orientation of the crystalline phase, suggest that the growth of the spherulites is controlled by a process that does not involve the long-range diffusion of carbon or cobalt.