Formation of pyramid-like nanostructures during cobalt film growth by magnetron sputtering

Cobalt pyramid-like nanostructures with sharp tips were formed on the surfaces of cobalt thin films grown by magnetron sputtering only when a negative bias was applied. There are two types of pyramids, which grew on top of a columnar grain structure directly from Si(001) substrates. The formation of pyramid-like nanostructures is only selective to a ε-Co (hcp) thin film but not a α-Co (fcc) thin film, where the basal plane is 1010 with several well-defined low-energy faceted planes. When grown on Si(111) substrates, the shape of the basal plane changes to a pentagon or other polygon. The island nucleation seems to depend on adatom energy, sputtering rate and the interface stress between the Co thin film and the substrate, but growth is significantly determined by the minimum surface configuration of the structure. PACS 61.10.Nz; 68.55.Ac; 68.37.Lp; 66.30.Pa; 68.35.Md