Microstructure and functional properties of Sr-doped K0.5Na0.5NbO3 thin films

Abstract

The legislation against the use of lead-based piezoelectric materials in electronics has stimulated an increased research in the field of the lead-free piezoelectric ceramics and thin films after 2003. Among lead-free materials, special attention was paid to some compositions of potassium sodium niobate solid solution, K0.5Na0.5NbO3 (KNN) obtained by a partial replacement of A- and B- site atoms from the perovskite KNN crystal lattice with dopants. Excellent piezoelectric and ferroelectric properties of chemically modified KNN ceramics indicate that they can be used as the efficient lead-free counterparts to lead-based piezoelectrics. Although A-site doping of KNN ceramics with low amounts of alkaline earths (Ca2+, Sr2+, Ba2+), particularly with Sr2+ was reported as an useful way in improvement of their density and electrical properties [1, 2], there is no literature data concerning the microstructure and functional properties of KNN thin films influenced by the same chemical modification. In this contribution, liquid precursors of (K0.5Na0.5)1-ySryNbO3 (KNN-ySr) thin-films, where the Sr- dopant content was set at y = 0, 0.005, 0.01, were prepared from potassium and sodium acetates and niobium ethoxide in 2-methoxyethanol solvent with 5 mole % of potassium acetate excess, and an appropriate amount of strontium acetate dissolved in acetic acid. The approximately 250 nm thick KNN-ySr thin films on Pt/TiOx/SiO2/Si substrates were obtained through repeated spin-coating and pyrolysis steps at 300 oC for 2 min, followed by the rapid thermal annealing at 650 oC in air flow for 5 min with a heating rate of 12 K/s. According to X-ray diffraction analysis, all of the synthesized KNN thin films crystallize in pure perovskite phase with (100) preferential orientation. The surface and cross-section microstructure analysis, performed by the field emission scanning electron microscopy, reveals that the KNN-ySr films consist of equiaxed grains, the average size of which gradually decreases from about 90 nm to a few tens of nm by increasing the Sr-dopant content. Dielectric properties versus frequency, polarisation – electric field dependence and leakage current were followed in order to get information on how the Sr-dopant content influences the functional properties of the as-prepared films. In addition, the topography and the local piezoelectric response of the KNN-ySr films were analysed by atomic force microscopy coupled with a PFM mode. References: [1] B. Malič, J. Bernard, J. Holc, D. Jenko, M. Kosec, Alkaline-earth doping in (K,Na)NbO3 based piezoceramics, J. Eur. Ceram. Soc. 25 (2005) 2707-2711. [2] M. Demartin Maeder, D. Damjanović, N. Setter, Lead free piezoelectric materials, J. Electroceram. 13 (2004) 385-392.