Low-temperature deposition of Li substituted (K,Na)NbO₃ films by a hydrothermal method and their structural and ferroelectric properties

Li substituted (K,Na)NbO₃ films with K-rich composition against polymorphic phase boundary were deposited at 240°C on (100)La:SrTiO₃ substrates by hydrothermal method. The amount of Li content in films was controlled by changing the nominal composition of raw materials, A = [LiOH]/([KOH] + [NaOH] + [LiOH]). X-ray diffraction measurement showed that {001}_c-oriented epitaxial (K,Na,Li)NbO₃ films were obtained for all nominal compositions (A = 00.1). However, the diffraction peaks originated from a secondary phase were also detected in the range of A = 0.050.1. Plan-view scanning electron microscopy observation revealed that (K,Na,Li)NbO₃ films possessed square-shape feature in accordance with the epitaxial relationship between film and substrate, while the secondary phase exhibited triangular- and pyramid-shaped features. The distribution of the elements across the thickness was investigated by time-of-flight secondary ion mass spectrometry. Transmission electron microscopy revealed the columnar structure of the film. In addition, nanoscale-pores have been found to exist in the boundary of each column by scanning transmission electron microscopy. (K,Na,Li)NbO₃ films, A up to 0.03, showed ferroelectricity, and the maximum remanent polarization was observed at A = 0.01.