Synthesis of Potassium Niobiate (KNbO₃) Thin Films by Low-Temperature Hydrothermal Epitaxy

Abstract

Orthorhombic KNbO 3 thin films were deposited on SrTiO 3 (100) and LiTaO 3 (001) substrates by low-temperature hydrothermal epitaxy at 180-210°C using aqueous solutions containing Nb 2 O 5 and KOH. The films were thoroughly characterized by X-ray diffraction, Rutherford backscattering, and scanning electron microscopy. The KNbO 3 films were uniformly distributed on the entire substrate area, exhibited high stoichiometry, and did not show evidence of interdiffusion or chemical reaction at the film/substrate interface. Their microstructures were determined by the substrate used, the synthesis temperature, and the concentration of the reactants. The KNbO 3 films on SrTiO 3 (100) substrates with thickness of 45 nm to 1.28 µm were smooth and transparent, with microstructures strongly suggesting single crystal growth mechanism. They were pure orthorhombic phase with (011) orientation and probably single-domain. Conversely, the KNbO 3 films with higher thickness (>3.5 µm) exhibited very rough microstructure with textured columnar grains. Such films were also orthorombic but with mixed (100)/(011) orientation. This work demonstrated for the first time deposition of heteroepitaxial KNbO 3 films on single-crystal substrates using low-temperature hydrothermal epitaxy in a well-defined range of synthesis conditions. The low synthesis temperatures, which are below the 225°C phase transition temperature of KNbO 3 , resulted in formation of films that appear to exhibit improved features as compared to the films prepared by high-temperature processing. IntroductionPotassium niobiate (KNbO 3 ) is a very promising material for surface acoustic wave (SAW) devices and high-performance bulk-wave transducers. 1 The elctromechanical coupling coefficient (k 2 ) of the surface wave in a single-crystal KNbO 3 can be as high as 53%, which is one order of magnitude larger than that of LiNbO 3 . 2 The bandwidth is about 20% and insertion losses are 2-6 dB. 2 The acoustic wave velocities in KNbO 3 are 3500-7500 m/s 3 and the temperature coefficient of frequency (TCF) ranges between 0 and 170 ppm/K depending upon crystallographic orientation. 2 The KNbO 3 exhibits large piezoelectric nonlinearity. The 45°r otated Y-cut X-propagated KNbO 3 has 25 dB larger efficiency than Y-Z LiNbO 3 ; thus the KNbO 3 single crystal is considered as a promising material for SAW elastic convolvers with super high efficiency and process gain. 4 In addition to excellent piezoelectric properties, the KNbO 3 has a large electrooptic coefficient, high nonlinear optical coefficient, and excellent photorefractive characteristics 5 that make it a very promising candidate for optical applications such as optical waveguides, frequency doublers, 6 intensity modulators, 7

Hydrothermal epitaxy of KNbO3 thin films and nanostructures

“…In contrast, hydrothermally grown KNbO 3 films in this study (15 M KOH) and those grown in 5.5 M KOH and higher elsewhere [16] exhibit both the {1 1 0) o and (0 0 1) o orientations. Suchanek [16] ascribed the presence of both the {1 1 0) o and (0 0 1) o orientations to the relaxation of thermoelastic stresses by multiple domain formation.…”

“…[15] and less than 5.5 M in Ref. [16]), the film lattice parameters are close to the bulk values and the lattice mismatches at room temperature along the a pc and c pc directions are 3.4% and 1.8%, respectively. At the growth temperature of 200 1C, a pc remains relatively unchanged, but c pc increases significantly (a ¼ 7:11 Â 10 À5 K À1 ) [26], resulting in lattice mismatches of 3.3% and 2% (using a STO ¼ 11 Â 10 À6 K À1 ) [27].…”

“…Subsequently, Goh and Donthu [15] reported the growth of epitaxial NaNbO 3 films and showed that the use of a KTaO 3 buffer layer improved the alignment of the subsequent epitaxial KNbO 3 film. Suchanek [16] also grew KNbO 3 films by hydrothermal epitaxy, using KOH concentrations less than 10 M. In this report, a much higher KOH concentration of 15 M is utilized, whereupon instead of a smooth film, the growth gave rise to an assemblage of tower-like structures that were epitaxially related to the substrate and with tip diameters of about 50 nm. This study presents an initial effort to understand how the pseudo-cubic KNbO 3 system can form such highly anisotropic and epitaxial nanostructures under hydrothermal growth conditions.…”

“…Suchanek [16] ascribed the presence of both the {1 1 0) o and (0 0 1) o orientations to the relaxation of thermoelastic stresses by multiple domain formation. The implied assumption was that a KNbO 3 film with a single orientation would also be single domain, which following his argument meant that two orientations indicated the formation of multiple domains.…”

Preparation and photocatalytic activity of triangular pyramid NaNbO 3

“…Hydrothermal method is a good way to prepare the sample with special morphologies, and free substrate was usual in this method [22,23]. However, some single crystal substrate could be used as the substrate to grow the sample exposed certain facets [24,25]. Moreover, heterogeneous and homogeneous substrates will impact on the morphology of the sample due to the different atomic arrangement of interface.…”