Influence of K4Nb6O17 secondary phase on ferroelectric behavior of K0.5Na0.5NbO3 thin films

Abstract

K_xNa_1−xNbO₃ (KNN) perovskite thin films deposition by various methods poses significant challenges due to alkali cations losses, which result in vacancies or secondary phases. In this study, KNN thin films were obtained by pulsed laser deposition using a stoichiometric commercial target and a 60% potassium-enriched target on platinized silicon substrates. Potassium-enrichment target was used as an alternative to obtain a pure perovskite phase and enhance piezoelectric and ferroelectric behavior of the thin films. It was observed that the alkali losses during the deposition of KNN thin films mainly promote the formation of the secondary phase K₄Nb₆O₁₇ and its hydrated form. Importantly, K₄Nb₆O₁₇ adversely affects ferroelectric and piezoelectric properties owing to its high sensitivity to moisture. It was observed that these KNN films grown from a stoichiometric commercial target (denoted as KNNs) present a perovskite structure and K₄Nb₆O₁₇ secondary phase presence, while thin films deposited with a K-enriched target (denoted as KNNe) show a single perovskite structure. The topography and piezoresponse force microscopy amplitude exhibit topographic differences and a null-piezoelectric response in large and flat grains in KNNs, disrupting the distribution of small and rounded grains associated with piezoelectric signal. Meanwhile KNNe films present strong piezoelectric behavior. Null-piezoelectric response in KNNs agree with scanning electron microscopy and energy-dispersive X-ray spectroscopy analysis, where areas exhibiting such large and flat grains displayed a pronounced deficiency in alkali cations. In addition, characterization by using X-ray diffraction, Raman spectroscopy, scanning and transmission electron microscopy coupled to energy dispersive X-ray spectroscopy showed the presence of low amount of sodium incorporated into the K₄Nb₆O₁₇ structure during KNN films deposition, although only the K₄Nb₆O₁₇ was reported. On the other hand, thin films deposited from a K-enriched target presents a single perovskite phase devoid of any secondary phase. Their electrical measurements, performed on 0.25 × 0.25 mm² capacitors, show a d_33,f value ∼40 pm V⁻¹ and typical ferroelectric behavior with saturation polarization = 20 μC cm⁻², remnant polarization (2P_r) = 17.5 μC cm⁻² and coercive voltage (2V_c) = 3 V, comparable to earlier works reported in the literature, making KNNe thin films suitable candidates for further engineering applications through their optimization, contrary to KNNs thin films.