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Strain Effect on the Properties of K0.5Na0.5NbO3 Thin Films
Keywords: KNN, piezoelectric , strain/stress
One of the piezoelectric ceramics most widely used for electromechanical and energy harvesting applications is lead zirconate titanate (PZT). However, it contains more than 60 wt.% of lead that is toxic for humans and environment. In 2003, a directive from European Union has prohibited the use of potentially hazardous elements as lead. Due to the lack of competitive materials for PZT replacement an exception was created until a competitive alternative be found. Potassium sodium niobate (KNN) due to its high Curie temperature and promising piezoelectric properties is currently one of the most perspective lead-free materials for PZT substitution. Whereas most literature was focused on KNN-based bulk materials, recently, KNN based films have received more attention as one of the promising alternatives in various applications, such as sensors, actuators, energy harvesting systems and microelectromechanical systems (MEMS). However, two main issues still inhibit the fabrication of high- quality KNN-based films: stress/strain exerted between the KNN film and the substrate and the loss of alkali oxides during its preparation. Within this context, the influence of stress/strain applied to KNN films on their electrical properties is studied in this work. For this purpose, dense and crack free ~380 nm thick KNN films with 5% of potassium excess and 0.2 M concentration were deposited on Si/SiO2/TiO2/Pt, Al2O3/Pt and SrTiO3/Pt substrates. Using X-ray diffraction (XRD) analysis it was found that KNN thin films have a perovskite structure without secondary phases and any preferential orientation. A pseudo-cubic out-of- plane lattice parameter estimated from XRD profiles increases from ~3.957 0.001 for Si/SiO2/TiO2/Pt through ~4.000 0.009 for Al2O3/Pt to ~4.006 0.006 for SrTiO3/Pt substrates. Moreover, KNN thin films deposited on Si/SiO2/TiO2/Pt substrates are found to be under a tensile strain of 0.655 GPa, while the KNN films deposited on Al2O3/Pt and SrTiO3/Pt substrates are under compressive strains of 1.015 and 1.220 GPa, respectively. Taking into account that KNN thin film should be strained on cooling from growth temperature by the thermal expansion mismatch (αSrTiO3 > αAl2O3 > αKNN > αSi), a lattice distortion is expected allowing the Nb ionic motions dominantly out-of-plane for compressive in-plaine strain (Al2O3 and SrTiO3 substrates) and in-plain for tensile in-plain strain (Si substrate). As result, the dielectric properties of these films should be enhanced and suppressed, respectively, in the parallel plate capacitor geometry. Indeed, the ferroelectric measurements indicate that films with the lowest remanent polarization Pr values measured in in the parallel plate capacitor geometry are the KNN films under tensile strain deposited on Si/SiO2/TiO2/Pt substrates and the highest Pr values of 4.55 μC/cm2 (at 50 Hz) are found in the KNN films under highest compressive strain deposited on SrTiO3/Pt substrates. As a result, variation of the polarization can be achieved by controlling the stain/stress level on the films via the choice of the substrate. The piezo-force microscopy have shown that the KNN thin films deposited on SrTiO3/Pt and Al2O3/Pt substrates have well defined micrometre scale domains and local piezoelectric response.
Andr dos Santos, Strain Effect on the Properties of K0.5Na0.5NbO3 Thin Films
Universidade de Aveiro
Aveiro, Aveiro

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