Peculiarity of self-assembled cubic nanolamellae in the TiN/AlN system: Epitaxial self-stabilization by element deficiency/excess

Abstract

Synthesis of self-assembled thin films with multi-layered microstructures and outstanding functional properties represents a challenging task. In this work, detailed microstructural and chemical analyses of a self-assembled ∼3.8 μm thick cubic (c) (AlxTi1-x)yN1-y film grown by low pressure chemical vapour deposition on a Al2O3(0001) substrate is discussed. The film with an overall x fraction of ∼0.8 consists of alternating non-stoichiometric cubic Al-rich and Ti-rich nanolamellae with thicknesses of ∼11 and ∼1.5 nm. X-ray diffraction, electron microscopy and electron energy loss spectroscopy indicate that the nanolamellae coherency is primarily a result of an N deficiency in Ti-rich nanolamellae and an N excess in Al-rich nanolamellae, which induce a decrease and an increase in nanolamellae lattice parameters, compared to the lattice parameters of stoichiometric rock-salt c-TiN and c-AlN, respectively. Therefore the self-assembly allows a formation of c-(AlxTi1-x)yN1-y nanolamellae with Al atomic fraction x of 0.9–1.0, which are stabilized by neighbouring Ti-rich nanolamellae as a result of cube-on-cube epitaxy. The effect of the lattice parameter self-adjustment in the coherent nanolamellae by element deficiency and excess is verified by ab initio calculations. The compositional and morphological matches of the nanolamellae interfaces at the grain boundaries, the terraced growth with tetrahedral surface morphology and unzipped facets as well as the uniform nanolamellae thickness across the film depth indicate that the nanolamellae are formed as a result of kinetically-controlled oscillating reactions during the film growth. The understanding of this fascinating self-assembled nanolamellar microstructure containing a meta-stable c-AlNy, which does not exist in a bulk form at ambient conditions, represents a milestone in thin film technology.