Formation of polyhedral voids at surface cusps during growth of epitaxial TiN/NbN superlattice and alloy films

Epitaxial TiN/NbN(001) superiattice thin films with periods A between 0 (alloy) and 9.6 nm have been grown by ultrahigh vacuum reactive magnetron sputter deposition on MgO(OOl) substrates in mixed Ar/N2 discharges. Cross-sectional transmission electron microscopy was used for characterizing layer and defect structure. Coherency strain relaxation close to the film/substrate interface resulted in nonplanar growth with surface cusps and the creation of apparent column boundaries at which TiN and NbN layers curved towards the substrate, but across which the basic lattice was ordered. Also, a fraction of the boundaries were associated with threading defects along the growth direction. The column boundaries and threading defects were decorated by voids that exhibited an orthorhombohedral shape, were elongated in the growth direction, and typically had flat surfaces on {100} planes. The origin of voids is suggested to be through self-shadowing at surface cusps in combination with limited adatom mobility. The voids subsequently attained equilibrium shape as the result of surface and bulk diffusion. The void density varied withλ and the void length in the growth direction was generally an integer number of periods, indicating that the void formation was influenced by the superiattice layers. The formation of limited voids is suggested to be characteristic of thin film growth close to the epitaxial temperature.