Structural and piezo-ferroelectric properties of K0.5Na0.5NbO3 thin films grown by pulsed laser deposition and tested as sensors

We have grown K0.5Na0.5NbO3 (KNN) films on Pt/TiO2/SiO2/Si substrates by using pulsed laser deposition. Thin films with pure KNN crystal phase and homogenous morphology were successfully prepared by optimizing the deposition conditions, mainly the oxygen partial pressure and deposition temperature. For oxygen pressure as low as 8 Pa a tungsten-bronze phase is formed, while for higher values i.e. 16 and 32 Pa, the desired KNN perovskite structure is obtained. To prepare pure KNN thin films, a deposition temperature as low as 675 °C is required, however, increasing the temperature to 725 °C promotes better homogeneity and densification of the films. The piezoelectric and ferroelectric behavior, confirmed by piezoresponse force microscopy, revealed a local piezoelectric coefficient of d33 = 37 pm/V and a coercive voltage of 3.5 V. Finally, we measured the frequency response of this material prepared as a sensor, using the photoacoustic method. This measurement shows that the resonant frequency of a KNN thin film of 230 nm thickness is centered at 3 MHz.