Microstructure and non-basal plane growth of epitaxial MAX phase Ti2AlN thin films

Thin films of the MAX phase Ti2AlN were epitaxially grown onto single crystal MgO(111) and MgO(100) substrates by DC reactive magnetron co sputtering from Ti and Al targets in an Ar/N2 gas mixture at a temperature of 690 °C. To promote the nucleation of the MAX phase, an fcc (Ti0.63Al0.37)N seed layer was deposited before changing to Ti2AlN growth parameters. The nucleation processes have been studied by real time in situ specular x ray reflectivity. Independent of substrate orientation, the seed layer shows no roughening until its final thickness of approximately 100 Å indicating pseudomorphic layer by layer growth within the descriptive step-flow growth model of Kodambaka et al. [1]. The MAX phase shows heteroepitaxial layer by layer growth on MgO(111), with increased surface roughening up to approximately 200 Å, whereas on MgO(100) the growth mode changes to Volmer Weber type already after three monolayers. X ray scattering in Bragg Brentano geometry of the final, approximately 1000 Å thick, Ti2AlN film reveals lattice parameters of c = 13.463 Å and a = 2.976 Å on the MgO(111) substrate and c = 13.740 Å and a = 2.224 Å on the MgO(100) substrate. From ex situ pole figure measurements the epitaxial relationship between film and substrate lattice was determined to be MgO{111} // Ti2AlN{101‾2}, regardless of the substrate orientation. This tilted, non basal plane growth leads to a threefold grain orientation of Ti2AlN along the MgO directions and a polycrystalline morphology, which is also confirmed by cross sectional transmission electron microscopy. The growth can be assumed to take place in a step-flow mode, i.e. emerging steps on the high surface free energy (0001) plane where adatoms come to rest after diffusion along A-facets, irrespective of epitaxial relationship.