Chemical Processing and Characterization of Ferroelectric (K,Na)NbO₃ Thin Films

Abstract

Lead-free ferroelectric K 0:5 Na 0:5 NbO 3 (KNN) thin films have been synthesized by the chemical solution deposition. The optimization of excess amounts of K and Na in KNN precursor solutions was found to be effective for achieving perovskite KNN single-phase thin films with improved leakage current properties. It was revealed from Raman spectroscopic analysis data that a change in scattering mode was observed for the KNN thin films fabricated under various processing conditions. This change was due to the chemical composition fluctuation of K and Na in the KNN thin films after heat treatment. The leakage current and ferroelectric properties of the perovskite KNN thin films were strongly affected by the excess amounts of K and Na as well as the heating conditions of the precursor films. Optimized KNN thin films with 10 mol % excess K and Na exhibited a ferroelectric polarization-electric field (P-E) hysteresis loop with 2P r and 2E c values of 14 mC/cm 2 and 140 kV/ cm, respectively.

Microstructure-dependent leakage-current properties of solution-derived (K0.5Na0.5)NbO3 thin films

“…In addition to studies of bulk piezoelectric and ferroelectric ceramics, the research on thin films has been promoted by miniaturization trends in electronics and micro-electro-mechanical systems [2]. In both the vapour-and solution-based deposition routes for KNN films, volatilization of the alkali compounds has been identified as the major processing problem [3][4][5][6]. The loss of alkalis may hinder the control over the composition and promote the formation of undesirable, alkalideficient, secondary phases [4,5], as well as the formation of alkali and oxygen vacancies, which are thought to be the main reason for the large leakage currents observed in KNN films [5][6][7].…”

“…In both the vapour-and solution-based deposition routes for KNN films, volatilization of the alkali compounds has been identified as the major processing problem [3][4][5][6]. The loss of alkalis may hinder the control over the composition and promote the formation of undesirable, alkalideficient, secondary phases [4,5], as well as the formation of alkali and oxygen vacancies, which are thought to be the main reason for the large leakage currents observed in KNN films [5][6][7]. The non-ferroelectric response in KNN thin films was also related to chemical heterogeneities in the films evidenced by the presence of secondary phases [8].…”

Lead-Free KNN-Based Piezoelectric Materials

“…Due to the volatility of sodium and potassium, adding an excess amount of these elements in precursors can compensate for the loss of A-site elements and result in films with improved properties [77][78][79][80][81][82]86]. Excess sources of Na and K largely suppress the formation of cation and anion vacancies in the films, eliminate the formation of secondary phases such as K 4 Nb 6 O 17 , and improve the surface roughness.…”

“…The optimization of CSD-derived KNN films by the addition of proper amounts of K and Na sources to the precursor solutions improves the ferroelectric and leakage current behavior of the films by eliminating secondary phases (K 4 NbO 17 ) and decreasing the concentration of charge carriers [82]. The volatilization of A-site elements triggers the formation of oxygen vacancies to maintain charge neutrality in the film.…”

“…The addition of 10 mol% excess A-site elements (5% for each) compensates for the loss of Na and K during the high-temperature processing of chemically deposited thin films. Exposing the films to the excessively high annealing temperature or time facilitates the condition for further loss of alkaline elements and hence, deteriorates the electrical properties [82].…”