Nucleation and growth of Ti2AlN thin films deposited by reactive magnetron sputtering onto MgO(111)

The nucleation and growth of Ti2AlN thin films on MgO(111) substrates during dual DC reactive magnetron cosputtering from Ti and Al targets in an Ar/N2 atmosphere at a substrate temperature of 690°C have been investigated . Time and thickness dependent in situ specular x-ray reflectivity and x-ray diffraction (XRD) in coplanar scattering geometry in combination with cross-sectional transmission electron microscopy and Rutherford backscattering spectroscopy revealed the formation of competing phases. It is found that initially an ~380-Åthick epitaxial cubic (Ti1-xAlx)Ny layer of substoichiometric composition forms with a layer-by-layer growth mode. Nitrogen-vacancy driven diffusion of Ti and Al leads to spinodal decomposition of this metastable solid solution into nano-sized cubic TiNy and AlNy domains as well as to a solid-state reaction where Mg2(Al:Ti)O4 spinel forming below the interface to the MgO(111). Spatial and temporal fluctuations of the nitrogen activity on the (Ti1-xAlx)Ny surface at higher thicknesses yield thermodynamically favored Ti2AlN nucleation and polycrystalline growth. Concurrent Ti2AlN inward grain growth by diffusion of weakly bonded Al atoms along the Ti2AlN basal planes reduces the thickness of the phase-separated (Ti1-xAlx)Ny layer to ~60 Å. Individual Ti2AlN grains retain local epitaxy to the substrate with vertical grain sizes in the range of the final film thickness.