Thickness-dependent mechanical properties of epitaxial PMN-PT thin films studied by Nano-indentation

This study examines the thickness-dependent structural and mechanical properties of epitaxial [Pb(Mg1/3Nb2/3)O3]0.67-(PbTiO3)0.33 thin films, a high-performance piezoelectric material, deposited on (001)-oriented SrTiO3 substrates with a 100 nm SrRuO3 bottom electrode layer and a 25 nm Pb(Zr0.5Ti0.5)O3 interfacial layer. Structural characterization confirms phase-pure, (001)-oriented PMN-PT films with very slight in-plane tensile strain. The out-of-plane lattice constant evolves with thickness in the range of 300–1000 nm, shifting from values below the bulk value in thinner films to the bulk value for the thickest film. This indicates strain relaxation and defect accumulation. The XRD analysis indicates a transition from a tetragonal to a more bulk-like, rhombohedral lattice symmetry for the thickest PMN-PT layer, which is ascribed to the strain relaxation. Nanoindentation results show that the hardness and elastic modulus exceed those of bulk values, with the thickest film (1000 nm) standing out with the highest modulus (217 ± 17.7 GPa) and hardness (12.04 ± 0.84 GPa) values. This is attributed to the rhombohedral lattice symmetry for this sample. The observed correlation between deeper pop-in events and PMN-PT film thickness signify the strong effect of strain relaxation and structural defects on mechanical failure mechanisms. These findings highlight the potential to optimize mechanical response of PMN-PT thin films through thickness control, paving the way for advanced applications in piezoelectric devices.