Formation and microstructural analysis of co-sputtered thin films consisting of cobalt nanograins embedded in carbon

Cobalt–carbon thin films were deposited with a carbon concentration ranging from 27 to 57 at.% at different substrate temperatures. The morphology and phases of as-deposited films were investigated by transmission electron microscopy and x-ray diffraction. The effect of annealing on the microstructure is reported. Under particular conditions of substrate temperature, carbon concentration, and subsequent annealing, a granular morphology consisting of nanocrystalline cobalt grains embedded in graphitelike carbon was obtained. The cobalt grains were uniform in size. The particle size could be controlled in the range from 4 to 7 nm by varying the carbon concentration. The cobalt phase was found to depend on the carbon concentration and substrate temperature. The hexagonal close-packed cobalt phase was observed only via the formation of the metastable carbide ${\delta }^{\prime }-{\mathrm{Co}}_{\mathrm{2}}\mathrm{C}$ and its subsequent decomposition upon annealing. Otherwise the cobalt phase has a heavily faulted close-packed structure or a random stacking structure.