Influence of thickness on the epitaxial stabilisation of SmNiO3 thin films

SmNiO3 thin films have been prepared by liquid injection Metal Organic Chemical Vapour Deposition on SrTiO3 (100) and LaAlO3 (100) single crystalline substrates. The influence of the film thickness on the epitaxial stabilisation has been studied for both substrates by X-ray diffraction and Atomic Force Microscopy (AFM). In the case of SmNiO3 on LaAlO3, the nickelate is obtained as a single phase up to a thickness of 200 nm; the surface remains smooth in agreement with the small lattice mismatch between film and substrate. In the case of the SrTiO3 substrate, SmNiO3 is never stabilised as single phase: NiO and Sm2O3 single oxides both appear beside the perovskite. We show that the strain state is driven by the choice of the substrate and the thickness of the film. Two different relaxation mechanisms of the film are evidenced depending on the substrate used. For LaAlO3, the good in-plane lattice match leads to a relaxation only in the growth direction. On the contrary, in the case of SrTiO3 which presents a strong lattice mismatch with the film, the out-of plane lattice parameter remains constant and constraints are relaxed through the chemical dissociation of SmNiO3 into single oxides.