We present a detailed analysis of the ferroelectric domain structure of KNaNbO thin films on (110) TbScO grown by metal-organic chemical vapor deposition. Upon piezoresponse force microscopy and nanofocus x-ray diffraction measurements we derive a domain model revealing monoclinic M domains. The complex domain pattern is formed out of four co-existing in-plane orientations of the shearing direction of the monoclinic unit cell resulting in four types of superdomains each being composed of well-ordered stripe domains. Finally, we present surface acoustic wave (SAW) experiments that exhibit extraordinary signal intensities given the low thickness of the tested film. Moreover, the SAW propagation is found to occur selectively along the identified shearing directions.
In this work, we demonstrate the electronic tunability of surface acoustic waves (SAWs) in epitaxially strained relaxor-type ferroelectric thin films. Epitaxial K0.7Na0.3NbO3 thin films of typically 30 nm in thickness are grown via pulsed laser deposition on (110)-oriented TbScO3. A partial plastic lattice relaxation of the epitaxial strain in these samples leads to a relaxor-type ferroelectricity of these films, which strongly affects the SAW properties. Without electronic bias, only tiny SAW signals of ∼0.2 dB can be detected at room temperature, which can be boosted up to ∼4 dB by a static voltage bias added to the high frequency driving current of the SAW transducers. Upon field cooling below the freezing temperature of polar nanoregions (PNRs), this strong SAW signal can be preserved and is even enhanced due to a release of the electronically fixed PNRs if the bias is removed. In contrast, at elevated temperatures, a reversible switching of the SAW signal is possible. The switching shows relaxation dynamics that are typical for relaxor ferroelectrics. The relaxation time τ decreases exponentially from several hours at freezing temperature to a few seconds (<5 s) at room temperature.
Epitaxial K0.7Na0.3NbO3 thin films are grown via metal-organic chemical vapor deposition on (110)-oriented TbScO3. The films are strained due to the substrate–film lattice mismatch and therefore exhibit a strong and anisotropic modification of all its ferroelectric properties. The compressive in-plane strain leads to a reduction of the ferroelectric transition temperature from approximately 700 K for unstrained K0.7Na0.3NbO3 to 324 K and 330 K with maximum permittivities of 10 270 and 13 695 for the main crystallographic directions [001]TSO and [11¯0]TSO, respectively. Moreover, the quite thin films (approx. 30 nm thick) exhibit very large piezoelectric properties. For instance, surface acoustic waves with intensities of up to 4.7 dB are recorded for wave propagation along the [11¯0]TSO direction. The signal is smaller (up to 1.3 dB) along [001]TSO, whilst for the intermediate direction [11¯2]TSO, the signal seems to vanish (<0.1 dB). The results indicate that the choice of material, (K,Na)NbO3, in combination with strain-engineering via epitaxial growth onto lattice-mismatched substrates represents a promising way to optimize ferroelectric materials for piezoelectric thin-film applications.
In this work, we demonstrate that extremely thin strain-engineered K0.7Na0.3NbO3 (KNN) films are ideal candidates for highly sensitive and also potentially selective surface acoustic wave (SAW) sensor applications. The strength of the use of these films in SAW sensors is based on their piezoelectric properties and their thinness. The latter leads to a strong concentration of the SAW energy at the very surface of the sensor's delay line and the generation of higher harmonics with significant amplitudes. Thin epitaxial films of typically 30 nm in thickness are grown via liquid-delivery spin metal-organic vapor phase epitaxy on different (110)-oriented scandate substrates (TbScO3 and GdScO3). The epitaxial strain is induced by the lattice mismatch between a substrate and a film. The SAW signal of thin KNN films and the resulting sensitivity of an SAW thin KNN film sensor are compared with conventional bulk SAW sensors based on LiNbO3 (LN) using identical electrode designs for the generation and detection of the SAW for both systems. Compared to the conventional LN SAW sensor, our KNN-based sensor shows a sensitivity that is approximately 14 times higher. This was achieved using only the third and fifth harmonics. Using even higher harmonics, the improvement could potentially be boosted up to a factor > 40. Moreover, we showed that simultaneous sensor recording of mass loading at different harmonics is possible with the KNN sensor. Similar to other sensor concepts, the resulting multiple signals might provide a fingerprint of the detected material and, thus, lead to a selective detection of the mass load.
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