Thickness-dependent structural and electromechanical properties of (Na0.85K0.15)0.5Bi0.5TiO3 multilayer thin film-based heterostructures

Publication date: 5 July 2018
Source:Materials & Design, Volume 149
Author(s): Yunyi Wu, Siu Wing Or
(Na0.85K0.15)0.5Bi0.5TiO3 (NKBT) multilayer thin films with different thicknesses of 100–700nm, corresponding to 2–14 layers with each layer of ~50nm thickness, are synthesized on Pt(111)/Ti/SiO2/Si substrates to form Pt/NKBT/Pt/Ti/SiO2/Si heterostructures using different spin-coating and annealing conditions in a modified aqueous sol-gel process. The multilayer thin films spin-coated by two steps (step 1/2) at 600/4000rpm for 6/30s and annealed at 700°C for 5min with a heating rate of 30°C/s show a dense, uniform, and continuous morphology as well as a pure perovskite structure with a rhombohedral–tetragonal phase transition at ~140°C and no preferential orientation in the heterostructures. Their structural and electromechanical properties exhibit consistent improvement trends with increasing thickness from 100 to 550nm (i.e., 2–11 layers). The 550nm-thick, 11-layer films demonstrate the best ferroelectric, dielectric, piezoelectric, and electric performance in terms of the highest remnant polarization, saturation polarization, dielectric constant, and effective piezoelectric constant of 18.3μC/cm², 53.6μC/cm², 463, and 64pm/V, as well as the lowest coercive field, dielectric loss tangent, and leakage current density of 116kV/cm, 0.057, and 27μA/cm², respectively. The observed thickness-dependent improvement is explained by an interfacial passive layer effect where the motion of both 180° and non-180° domain walls is enhanced in the thicker multilayer thin films by weakening the influence of domain pinning in the interfacial passive layers between the multilayer thin films and the substrates.

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