Critical thickness for transformation of epitaxially stabilized cubic AlN in superlattices

The epitaxial stabilization and transformation of cubic AlN layers in AlN/VN and AlN/TiN superlattices, grown by reactive sputtering on MgO (001), is described. In AlN/VN, the critical AlN thickness lC for transformation from cubic to hexagonal increased from ≈3.0 to >4 nm when the VN superlattice layer thickness was increased from 2.0 to 6.0 nm. The effect of lattice mismatch was observed by comparing AlN/VN (mismatch=1.46%) and AlN/TiN (mismatch=3.84%). The lC values were smaller, 2–2.5 nm, for the larger mismatch AlN/TiN system. The dependence of lC on the lattice mismatch and stabilizing layer thickness is discussed based on models of epitaxial stabilization.