KNbO3 Films Research Articles

Piezoelectrically active hydrothermal KNbO3 thin films

In this study, a post-growth treatment has been developed for epitaxial KNbO3 films grown hydrothermally at 200 °C so that they are ferroelectrically active. The treatment comprised of an O2 plasma followed by thermal annealing that removed lattice hydroxyls and also reduced the concentration of oxygen vacancies. This lowered the leakage current sufficiently such that films could be poled to saturation. This has led to the first demonstration of piezoelectrically active KNbO3 films grown hydrothermally.

Growth of Orientation-Controlled Epitaxial KNbO3 Thin Film by Hydrothermal Method

KNbO3 thin films were deposited at 240 oC on various kinds of single crystal substrates by hydrothermal method using aqueous solutions containing Nb2O5 and KOH. The deposition amount of the films was found not to be much dependent on the orientation and the surface roughness of the substrates, but on the electrical conductivity of the substrate. Epitaxial {100}, {110} and {111}-oriented KNbO3 films with ferroelectricity were grown on (100)c, (110)c, and (111)c -oriented SrRuO3//(100)SrTiO3 substrates, respectively.

Synthesis of KNbO3 Films from K2NbO3F Solution on Oxide Substrates

Method to synthesize KNbO3 crystals using K2NbO3F solutions is of interest. In this paper, films of KNbO3 crystals are synthesized on several oxide substrates from K2NbO3F solutions. The films have some orientation due to a reaction with the substrates. The reason is expected the solution after the synthesis is strongly basic.

Effect of Deposition Time on Film Thickness and Their Properties for Hydrothermally-Grown Epitaxial KNbO3 Thick Films

Deposition behavior and the crystal structure of the epitaxial KNbO3 films, grown at 240 °C on (100)cSrRrO3∥(100)SrTiO3 substrates using hydrothermal method, as well as their ferroelectric and piezoelectric properties were systematically investigated. Film deposition was detected to have begun after 2 h from the start, and the thickness increased with increasing the deposition time up to 4 h, but contrary decreased after 9 h due to the etching by KOH. X-ray diffraction (XRD) patterns of the films were basically the same regardless of the deposition time. However, the films deposited for 15 h introduces large amount of voids than those for 2 h. Ferroelectric and piezoelectric response of the films deposited for 2 and 15 h were measured to have similar film thickness, but the maximum applied electric field for the 2-h films was twice as that for the 15-h ones. The prototype ultrasonic transducer was fabricated using the KNbO3 films, which were deposited by hydrothermal method at 240 °C for 3 h, and proved to be able to transmit and receive ultrasonic signals above 90 MHz in degassed water.

Thermodynamics and ferroelectric properties of KNbO3

The Landau–Ginzburg–Devonshire phenomenological theory is employed to model and predict the ferroelectric phase transitions and properties of single-domain potassium niobate (KNbO3). Based on the LGD theory and the experimental data of KNbO3 single crystal, an eighth-order polynomial of free energy function is proposed. The fitted coefficients are validated by comparing to a set of experimental measured values including phase transition temperatures, spontaneous polarization, dielectric constants, and lattice constants. The effects of hydrostatic pressure and external electric field on phase transition temperatures and piezoelectric coefficients are investigated. The free energy function may be used to predict ferroelectric domain structures and properties of KNbO3 bulk and films by phase-field approach.

New Complexes of Alkali-Metals as Precursors for MOCVD of Ferroelectric (K, Na)NbO3 Thin Films

Three new mixed ligand complexes of Na and K were synthesized and characterized. The thermal behavior of synthesized compounds was investigated by TGA in vacuum and mass spectrometry from Knudsen cell. Obtained compounds demonstrate sufficient volatility in vacuum and were tested as precursor for MOCVD deposition of KNbO3 films. Single phase highly oriented films were obtained.

Growth of Epitaxial KNbO3 Thick Films by Hydrothermal Method and Their Characterization

KNbO3 films were grown at 240 °C by the hydrothermal method using KOH and Nb2O5 as source materials. Film thickness increased with reaction time up to 3 h, however decreased for longer reaction times. A 16-µm-thick epitaxially grown KNbO3 film with {100}pc orientation were successfully grown on (100)cSrRuO3∥SrTiO3 substrate for 3 h. The relative dielectric constant and dielectric loss at 100 kHz were 415 and 8%, respectively. Clear hysteresis loops originating from ferroelectricity were observed and the remanent polarization was 20 µC/cm2 at the maximum applied electric field of 220 kV/cm. The effective longitudinal piezoelectric constant, d33eff, obtained using a laser Doppler velocimeter, was 86 pm/V.

Epitaxially grown ferroelectric thin films for agile devices

Agile devices based on ferroelectric material thin films provide a promising approach to solve the problem of multi-standard communication systems. As an alternative to BaxSr1− x TiO3, the potentialities of two other materials, namely SrBi2Nb2O9 and KTa1− x Nb x O3 have been explored. This review gets an insight into the growth of thin films of these materials, including the control of stoichiometry and of their orientation on various substrates usable in the microwave range. Dielectric measurements show that the Curie temperature is close to the value reported on bulk materials and that the transition is of first order, while the Curie-Weiss temperature is frequency dependant in the example of KNbO3 films. Microwave test devices, patterned on such films, show promising agility levels, while it appears that further efforts have to be done to reduce the dielectric losses.

Low-Temperature Epitaxy of KTaO3 and KNbO3 Films

Epitaxial perovskite films were grown hydrothermally on (100) oriented single-crystal substrates in a solution at 170°C. Transmission electron microscopy, Rutherford backscattering spectroscopy, and optical measurements showed that despite the low growth temperature, the obtained films were highly crystalline. Also, determination of the residual strain due to grain coalescence, lattice mismatch, and thermal expansion mismatch indicated that differential contraction between the substrate and film during cooling generated sufficient compressive stress to cause film buckling at poorly bonded regions between the film and substrate.

Growth of KNbO3 Films by Solid-State Diffusion Technique

We attempted the growth of KNbO3 films by the solid-state diffusion technique. First, amorphous KNbO3 films were deposited on (100) MgO substrates at room temperature by pulsed laser deposition. The chemical composition of the amorphous films did not differ from that of the target. A (110)-oriented KNbO3 film was obtained by annealing the amorphous film at 750°C for 1 hour in air. Ruddlesden-Popper-type layered perovskite K2NbO3F films were also grown by the solid-state diffusion technique. The XRD pattern of the film after immersion in water indicated the formation of a KNbO3 film.

Self Assemble Synthesis of Potassium Niobate at Room Temperature

Nanoparticles and thin films of potassium niobate KNbO3 were prepared using a unique solution reaction at room temperature. Precursor solutions and as-prepared samples were characterized by Raman spectroscopy. The data demonstrated that the niobate nanosheets consist of distorted Nb-O polyhedra.

Ferroelectric properties in KNbO3 thin films probed by optical second harmonic generation

Ferroelectric phase transitions and the structure of KNbO3 nanometer-thick films deposited by Metal-organic chemical-vapor deposition technique on a MgO(100) substrate are studied. Thin KNbO3 films of the average thickness from 20to150nm possess a grainlike structure with a lateral grain size of about 250nm. Such films reveal ferroelectric properties similar to KNbO3 single crystal. Temperature dependencies of optical second harmonic generation intensity show that the Curie temperature TC of the nanometer-thick KNbO3 films is about 30°C lower than the TC of KNbO3 single crystal, which is attributed to size effects in KNbO3 nanograins.

Pulsed laser deposited KNbO3 thin films for applications in high frequency range

Potassium niobate thin films were grown by pulsed laser deposition on various substrates. Influence of deposition conditions on film characteristics was studied. Structural investigation evidenced that single phase polycrystalline randomly oriented films were grown on sintered alumina whereas epitaxial films were grown on (100)SrTiO3 and (100)MgO substrates. The microstructure was highly controlled by the structural characteristics. Interdigited capacitors built from KNbO3 films on two different substrates (alumina and MgO) showed the strong influence of the structural characteristics on the dielectric behavior. The variation of the equivalent capacitance measured on the interdigital capacitor on MgO was 6.4% at 2.5 GHz while it was 1.5% on alumina, in both cases for a moderate applied field of ∼15 kV cm−1. The results show the potentiality of these ferroelectric materials for use in frequency agile microwave electronics.

Interface Effect on Ferroelectricity at the Nanoscale

Interfaces play a critical role in nanoscale ferroelectricity. We perform a first-principles study of ultrathin KNbO(3) ferroelectric films placed between two metal electrodes, either SrRuO(3) or Pt. We show that bonding at the ferroelectric-metal interfaces imposes severe constraints on the displacement of atoms, destroying the bulk tetragonal soft mode. If the interface bonding is sufficiently strong, the ground-state represents a ferroelectric domain with an interface domain wall, driven by the intrinsic oppositely oriented dipole moments at the two interfaces. The critical thickness for the net polarization of the KNbO(3) film is predicted to be about 1 nm for Pt and 1.8 nm for SrRuO(3) electrodes.

LPE Growth of Textured KNbO3 Film Using SrTiO3 Substrate and V2O5 Flux

KNbO3 thin films were grown on (100) and (110) SrTiO3 substrates by liquid phase epitaxy (LPE) technique. The film orientation and surface morphology were characterized by XRD and AFM, respectively. The limited phase diagram of K2O-Nb2O5-V2O5 system was prepared by DTA measurement to investigate the effect of V2O5 flux on the LPE growth of KNbO3 film. The use of V2O5 flux enhanced a film growth rate at lower growth temperature.

Development of transparent single-crystalline KNbO3 thin film by LPE technique

Single-crystalline KNbO3 thin film has been successfully formed on SrTiO3 substrate from high-temperature K2CO3–Nb2O5 solution by the liquid phase epitaxy (LPE) technique. The growth morphology was strongly influenced by the melt composition and film growth temperature. The starting material for the film preparation was a powder mixture of K2CO3 and Nb2O5. The oxides were mixed in non-stoichiometric proportion with excess K2CO3 as flux. Under the optimized film growth conditions using melt compositions including K2CO3/Nb2O5Z52.5/47.5, 60.0/40.0 and 65.0/35.0, transparent single-crystalline KNbO3 thin films could be obtained. The synthesized KNbO3 thin film was subjected to precession X-ray photography in order to evaluate the crystallographic relationship with SrTiO3 substrate, and the result was compared with a simulated diffraction pattern. The precession X-ray photography clearly indicated that the [010]KNbO3 is not placed on the same diffraction line as [010]SrTiO3 but is slightly shifted with a difference in angle of approximately 38, while the [100] and [001] agree in direction for KNbO3 and SrTiO3. The observed lattice parameter c of KNbO3 film was calculated to be4.043 Å which was slightly (1.7%) larger than 3.974 Å reported for KNbO3 bulk crystal. In-plane rotation and elongation toward substrate normal for KNbO3 lattice on SrTiO3 substrate were discussed from the viewpoint of release of elastic energy accumulated by lattice mismatch on the substrate. © 2004 Elsevier Ltd. Allrights reserved.

Fabrication of KNbO3 Epitaxial Thin Films on Sapphire Substrates by Pulsed Laser Deposition

KNbO3 epitaxial thin films were fabricated on sapphire r-plane substrates with an MgO buffer layer by pulsed laser deposition. X-ray diffraction measurements revealed that both MgO and KNbO3 were epitaxially grown on the Al2O3 (1-102) substrate with an orientation relationship as KNbO3 (100)pc/MgO (100)/Al2O3 (1-102) (out-of-plane) and KNbO3 [010]pc//MgO [010]//Al2O3 [11-20] (in-plane), although both MgO and KNbO3 were tilted 6–9° with respect to the substrate surface. Raman scattering measurements revealed that the KNbO3 film was composed of the orthorhombic ferroelectric phase, and that the (010)ortho (Y) domain, in which the polarization axis was parallel to the substrate surface, was dominant. The present study enables the production of KNbO3 epitaxial thin-film wafers, which correspond to Y-cut KNbO3 single crystals with a large electromechanical coupling coefficient (k2=0.53), for surface acoustic wave propagation.

Single-Crystalline KNbO3Thin Film Grown by Liquid Phase Epitaxy

The effects of melt composition on the transmittance of single-crystalline KNbO3 films grown on SrTiO3 substrates by liquid phase epitaxy (LPE) technique were investigated. The growth morphology and transmittance of the KNbO3 films strongly depended on the extra K2CO3 self-flux content in K2CO3 (K2O)-Nb2O5 system. With increasing K2CO3 content close to the eutectic composition (≈66 mol%) in the phase diagram, the transmittance of KNbO3 film/SrTiO3 substrate increased up to 70% in the visible wavelength range. Atomic force microscopy (AFM) clearly showed that the parallel flat steps formed toward the dipping direction and their interval decreased with increasing the K2CO3 content.

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

AbstractFor Abstract see ChemInform Abstract in Full Text.

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

Orthorhombic KNbO3 thin films were deposited on SrTiO3 (100) and LiTaO3 (001) substrates by low-temperature hydrothermal epitaxy at 180−210 °C using aqueous solutions containing Nb2O5 and KOH. The films were thoroughly characterized by X-ray diffraction, Rutherford backscattering, and scanning electron microscopy. The KNbO3 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 KNbO3 films on SrTiO3 (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 KNbO3 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 KNbO3 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 KNbO3, resulted in formation of films that appear to exhibit improved features as compared to the films prepared by high-temperature processing.