Transmission Electron Microscopy Characterisation of Lead-Free KNN Thin Films

Abstract

Lead-free piezoelectric ceramics have in recent years received an increased interest. Potassium sodium niobate (KNN) has shown promising results, exhibiting properties close to todays dominating piezoelectric material lead zirconate titanate (PZT). It is essential for the KNN thin films to be highly textured in order to obtain comparable electrical properties.

Five differently heat treated (Na0.5K0.5)NbO3 thin films were studied in a transmission electron microscope (TEM). The films were synthesised using non-toxic materials and an aqueous solution for chemical solution deposition on (100) oriented strontium titanate (STO) substrates. Focused ion beam (FIB) and precision ion polishing system (PIPS) were used for preparation of cross-section TEM specimens. The specimens were studied using scanning precession electron diffraction (SPED), bright field imaging and high resolution TEM. The sets of SPED data were analysed using automated crystal orientation mapping.

Crystallisation during pyrolysis was found to be in direct correlation with the final crystallographic structure of the films after sintering at 700 degrees celsius. The microstructure of the films is dominated by columnar growth in which the crystal structure of the initial layer has been approximately templated through the entire film thickness. The effect of pyrolysis on the microstructure was proven to be consistent with what is given in literature, with lower temperatures producing more epitaxial growth. The crystal texture in the growth direction shows indications of a weak [100] texture attributed to the small amounts of epitaxy in the films. Sintering at 700 degrees celsius proved necessary for good bonding between substrate and thin film.