Abstract
If piezoelectric thin films sensors based on K0.5Na0.5NbO3 (KNN) are to achieve commercialization, it is critical to optimize the film performance using low-cost scalable processing and substrates. Here, sol–gel derived KNN thin films are deposited using a solution with 5% of potassium excess on Pt/TiO2/SiO2/Si and Pt/SrTiO3 substrates, and rapid thermal annealed at 750 °C for 5 min. Despite an identical film morphology and thickness of ~335 nm, an in-plane stress/strain state is found to be tensile for KNN films on Pt/TiO2/SiO2/Si, and compressive for those on Pt/SrTiO3 substrates, being related to thermal expansion mismatch between the substrate and the film. Correspondingly, KNN films under in-plane compressive stress possess superior dielectric permittivity and polarization in the parallel-plate-capacitor geometry.
Highlights
Piezoelectric loops obtained for KNN films on Pt/SrTiO3 by piezoelectric force microscopy (PFM), and shown in Figure S2d, support the presence of out-of-plane polarization in these films, in contrast to KNN films on Pt/TiO2 /SiO2 /Si, as marked in Table 1 as well. Despite these loops are rather qualitative, and PFM is a microscopic technique, whereas the Pr values are obtained macroscopically, they confirm that an increase in Pr observed for KNN films on Pt/SrTiO3, in comparison to Pt/TiO2 /SiO2 /Si substrates, is, due to a stronger out-of-plane ferroelectric behavior, rather than to the higher losses at low frequencies
In this work, we have demonstrated that, besides the alkali excess amount and annealing conditions, the coefficient of thermal expansion of the substrate can contribute to the final electrical properties of polycrystalline KNN thin films
KNN films were produced from 0.2 M precursor solutions with 5% of potassium excess by rapid thermal annealing (RTA) at 750 ◦ C on platinized Si and SrTiO3 substrates
Summary
Alexander Tkach 1, * , André Santos 1 , Sebastian Zlotnik 1,2 , Ricardo Serrazina 1 , Olena Okhay 1,3 , Igor Bdikin 3 , Maria Elisabete Costa 1 and Paula M. Received: 28 September 2018; Accepted: 3 December 2018; Published: 6 December 2018
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