Abstract
If piezoelectric micro-devices based on K0.5Na0.5NbO3 (KNN) thin films are to achieve commercialization, it is critical to optimize the films’ performance using low-cost scalable processing conditions. Here, sol–gel derived KNN thin films are deposited using 0.2 and 0.4 M precursor solutions with 5% solely potassium excess and 20% alkali (both potassium and sodium) excess on platinized sapphire substrates with reduced thermal expansion mismatch in relation to KNN. Being then rapid thermal annealed at 750 °C for 5 min, the films revealed an identical thickness of ~340 nm but different properties. An average grain size of ~100 nm and nearly stoichiometric KNN films are obtained when using 5% potassium excess solution, while 20% alkali excess solutions give the grain size of 500–600 nm and (Na + K)/Nb ratio of 1.07–1.08 in the prepared films. Moreover, the 5% potassium excess solution films have a perovskite structure without clear preferential orientation, whereas a (100) texture appears for 20% alkali excess solutions, being particularly strong for the 0.4 M solution concentration. As a result of the grain size and (100) texturing competition, the highest room-temperature dielectric permittivity and lowest dissipation factor measured in the parallel-plate-capacitor geometry were obtained for KNN films using 0.2 M precursor solutions with 20% alkali excess. These films were also shown to possess more quadratic-like and less coercive local piezoelectric loops, compared to those from 5% potassium excess solution. Furthermore, KNN films with large (100)-textured grains prepared from 0.4 M precursor solution with 20% alkali excess were found to possess superior local piezoresponse attributed to multiscale domain microstructures.
Highlights
Owing to the environmental issues with the state-of-the-art piezoelectric materials based on lead zirconate titanate and other lead containing compounds, potassium sodium niobate (K0.5Na0.5NbO3, KNN) has become an extensively investigated system [1] due to its elevated Currie temperature (TC), up to 420 ◦C, and high longitudinal piezoelectric coefficient (d33) reported by Saito et al on KNN-based ceramics in 2004 [2]
energy dispersive spectroscopy (EDS) and scanning electron microscopy (SEM) analysis has shown that nearly stoichiometric KNN films and ~100 nm average grain size were obtained using 5% potassium excess solution, while 20% alkali excess solutions gave (Na + K)/Nb ratio in the prepared films of 1.07–1.08 and the grain size of 500–600 nm
Using XRD analysis, KNN thin films were found to have a perovskite structure without clear preferential orientation in the case of 5% potassium excess solution, whereas a texture in (100) direction appears for 20% alkali excess solutions, being strong for the 0.4 M solution concentration
Summary
Owing to the environmental issues with the state-of-the-art piezoelectric materials based on lead zirconate titanate and other lead containing compounds, potassium sodium niobate (K0.5Na0.5NbO3, KNN) has become an extensively investigated system [1] due to its elevated Currie temperature (TC), up to 420 ◦C, and high longitudinal piezoelectric coefficient (d33) reported by Saito et al on KNN-based ceramics in 2004 [2]. Platinized Si, mainly reported as substrate for KNN [6,7,8] and other piezoelectric films [9,10], has been shown to induce large tensile residual stress in sol–gel derived undoped KNN films due to thermal expansion mismatch, diminishing out-of-plane dielectric, ferroelectric, and piezoelectric response of the films [11]. On the other hand, platinized SrTiO3 substrates, possessing much larger thermal expansion coefficient (TEC), have been shown to induce residual compressive stress in KNN films, significantly enhancing out-of-plane permittivity, polarization, and piezoresponse [11]. In this work and within this context, crack-free KNN thin films are prepared by chemical solution deposition from 0.2 and 0.4 M precursor solution with 5% solely potassium excess and 20% alkali (potassium and sodium) excess on platinized sapphire substrates, being annealed using RTA. Performing atomic force microscopy (AFM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analyses in combination with X-ray diffraction, dielectric spectroscopy and piezo-force microscopy (PFM) characterization, a correlation between the electrical properties in the parallel plate capacitor geometry, piezoresponse and the solution conditions for KNN films deposited on platinized Al2O3 was found
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