Sodium Niobate Thin Films Research Articles

Ultrahigh electromechanical response from competing ferroic orders

Materials with electromechanical coupling are essential for transducers and acoustic devices as reversible converters between mechanical and electrical energy1–6. High electromechanical responses are typically found in materials with strong structural instabilities, conventionally achieved by two strategies—morphotropic phase boundaries7 and nanoscale structural heterogeneity8. Here we demonstrate a different strategy to accomplish ultrahigh electromechanical response by inducing extreme structural instability from competing antiferroelectric and ferroelectric orders. Guided by the phase diagram and theoretical calculations, we designed the coexistence of antiferroelectric orthorhombic and ferroelectric rhombohedral phases in sodium niobate thin films. These films show effective piezoelectric coefficients above 5,000 pm V−1 because of electric-field-induced antiferroelectric–ferroelectric phase transitions. Our results provide a general approach to design and exploit antiferroelectric materials for electromechanical devices.

Composition Regulation of Potassium Sodium Niobate Thin Films through Post-Annealing under Alkali Element Atmospheres

Amorphous potassium sodium niobate (KNN) films were synthesized at 300 °C through the radio frequency magnetron sputtering method and subsequently crystallized by post-annealing at 700 °C in various alkali element atmospheres (Na and K). The as-deposited film is notably deficient in alkali metal elements, particularly K, whereas the loss of alkali elements in the films can be replenished through annealing in an alkali element atmosphere. By adjusting the molar ratio of Na and K in the annealing atmosphere, the ratio of Na/K in the resultant film varied, consequently suggesting the efficiency of this method on composition regulation of KNN films. Meanwhile, we also found that the physical characteristics of the films also underwent differences with the change of an annealing atmosphere. The films annealed in a high Na atmosphere exhibit large dielectric losses with limited piezoelectric vibration behavior, while annealing in a high K atmosphere reduces the dielectric losses and enhances the piezoelectric behavior. Furthermore, the results of vibration measurement demonstrated that the film annealed in a mixed powder of 25% Na2CO3 and 75% K2CO3 exhibits an optimal vibration displacement of ~400 pm under the sinusoidal excitation voltage of 8 V. This approach of altering the composition of KNN films through post-annealing may introduce the new concept of property design of KNN as well as other similar films.

Tailoring growth modes by excess alkali addition in magnetron sputtered potassium sodium niobate thin films

This work reports on the deposition of thin film potassium sodium niobate sputtered from equimolar and excess alkali targets, showing that the growth mode can be shifted towards recrystallized grain growth without substrate heating, while structural and electrical material properties are improved. In particular, we investigate the influence of abnormal grain growth and its attenuation in KNN thin films. This is accompanied by a comprehensive crystallographic analysis of the obtained crystal structures involving wide-range reciprocal space mapping, temperature-dependent X-ray diffraction and elemental analysis associated with the different growth modes. Moreover, we report on the phase segregation within the processed thin film discovered by scanning transmission electron microscopy, which does not show in conventional elemental analysis of the film surface. Verification of the ferroelectric properties of the films is given by piezoresponse force microscopy, which also enables a study of the ferroelectric domains.

Study of light-emitting defects induced by 100 MeV Ag ion irradiation in potassium sodium niobate thin films

Here, we report the effects of swift heavy ion (SHI) irradiation on the photoluminescence (PL) properties of (K,Na)NbO3 (KNN) films deposited using RF magnetron sputtering in mixed atmosphere of argon and oxygen gas. In order to comprehend the role of defects created by SHI irradiation on PL properties, films were irradiated using 100 MeV Ag ions by varying ion fluence. X-ray photoelectron spectroscopy (XPS) results show the enhancement in oxygen vacancies upon irradiation with ion fluence which is confirmed from the reduction in contribution of Nb5+ state as well as lattice oxygen. The enhanced hump towards lower binding energy of Nb 3d XPS spectra indicates the increase in sub-oxides after irradiation. PL results revealed the band-to-band or near-band-edge transition in near ultraviolet region along with blue emission associated with defects present in KNN. PL results show that the deep-traps between the valence band maximum and conduction band minimum are increased with ion fluence, and the transitions from defect states are enhanced due to increase in the density of defects at higher fluence. The decay lifetime is found to improve with ion fluence up to 5 × 1012 ions/cm2 and is decreased at higher fluence. In this report, SHI irradiation induced variation in the defect emission with decay lifetime of KNN films is explained.

Effects of strontium doping on microstructure and functional properties of solution-derived potassium sodium niobate thin films

The effects of strontium doping (0-2mol%) on structure, microstructure and functional properties of potassium sodium niobate (KNN) thin films deposited on Pt(111)/TiOy/SiO2/Si substrates were investigated. Incorporation of Sr up to 1mol% into the KNN crystal lattice hindered the grain growth, vertical roughness and contributed to the fine-grained and dense thin film microstructure with monoclinic crystal syngony. This effectively reduced leakage current and improved ferroelectric characteristics. Higher doping content (2mol%) led to the formation of secondary phases and complete deterioration of functional properties. Stabilization of 1mol% Sr-doped KNN solution with diethanolamine resulted in the film with dielectric constant and losses of 394 and 0.018 at 100 kHz, respectively, leakage current of 3.8 • 10−8 A/cm2 at 100 kV/cm and well saturated ferroelectric hysteresis with Pr of 6.8 μC/cm2 and low Ec of 85 kV/cm. Benefiting from improved leakage current characteristics at high electric fields and less defect structure, the film showed maximal local piezoelectric coefficient, d33 ∼ 110 pm/V determined by piezo-response force microscopy (PFM), ability to reach fully saturated local hysteresis under low switching DC voltage of 15 V, and good ferroelectric domain mobility proven by successful in-situ poling of chosen area using PFM lithography.

Irradiation induced modification of structural and optical properties of potassium sodium niobate thin films

In the present study, the effects of swift heavy ion induced modification on the structural, morphological and optical properties of potassium sodium niobate (KNN) thin films have been investigated. KNN thin films were deposited using RF magnetron sputtering onto Si and quartz substrates. Subsequently, as-deposited films were annealed at 700 °C in air ambience for crystallization. Eventually, these crystalline films were irradiated using 100 MeV Ag ions at various fluences ranging from 1 × 1012 to 1 × 1013 ions/cm2. The crystalline and irradiated films were characterized using various techniques such as X-ray diffraction (XRD), atomic force microscopy (AFM), Raman spectroscopy, and UV–Vis spectroscopy. XRD results reveal that the crystallinity of films decreases drastically upon irradiation and almost disappeared at 1 × 1013 ions/cm2. Raman spectra show the different vibration modes of NbO6 octahedra. Raman peaks intensity is decreased and the peaks get broadened due to irradiation which indicates the amorphous nature of films. Variation in surface morphology and roughness of films before and after irradiation is studied using AFM. The minimum value of roughness is observed at 5 × 1012 ions/cm2. Ion beam irradiation results in the variation of transmittance and optical band gap of the films. The optical band gap of crystalline KNN film is found to be 3.82 eV which decreased to 3.72 eV upon irradiation at 5 × 1012 ions/cm2. The monotonous decrease in the refractive index and packing density of films is also observed with ion fluence.

Effect of Solution Conditions on the Properties of Sol-Gel Derived Potassium Sodium Niobate Thin Films on Platinized Sapphire Substrates.

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.

The effect of calcium zirconate modifications on the microstructure and functional properties of sodium niobate thin films prepared by chemical solution deposition

Sodium niobate thin films doped with manganese (NN), and NN films modified with 5 or 10 mol % calcium zirconate (CZ) on platinized silicon substrates were prepared by chemical solution deposition. The 250-nm-thick films crystallize in a perovskite phase with fine, equiaxed grains. The NN films exhibit well-shaped ferroelectric loops with a remanent polarization and coercive field of ∼10 μC/cm2 and ∼100 kV/cm, respectively. The modification with CZ strongly influences the ferroelectric response of the films: the remanent polarization progressively decreases to around 2.5 μC/cm2. The absence of an anti-ferroelectric response, which has previously been confirmed in bulk NN-CZ ceramics, is attributed to the nanoscale microstructure and residual thermal stresses. All the studied films exhibit a piezoelectric response with the highest piezoelectric d33 coefficient of 35 pm/V at 300 kV/cm bias field for the NN modified films with 5 mol % CZ, making them candidates for lead-free piezoelectric thin-film applications.

Strain-Mediated Substrate Effect on the Dielectric and Ferroelectric Response of Potassium Sodium Niobate Thin Films

If piezoelectric thin films sensors based on K0.5Na0.5NbO3 (KNN) are to achieve commercialization, it is critical to optimize the film performance using low-cost scalable processing and substrates. Here, sol–gel derived KNN thin films are deposited using a solution with 5% of potassium excess on Pt/TiO2/SiO2/Si and Pt/SrTiO3 substrates, and rapid thermal annealed at 750 °C for 5 min. Despite an identical film morphology and thickness of ~335 nm, an in-plane stress/strain state is found to be tensile for KNN films on Pt/TiO2/SiO2/Si, and compressive for those on Pt/SrTiO3 substrates, being related to thermal expansion mismatch between the substrate and the film. Correspondingly, KNN films under in-plane compressive stress possess superior dielectric permittivity and polarization in the parallel-plate-capacitor geometry.

Highly accelerated lifetime testing of potassium sodium niobate thin films

Highly accelerated lifetime tests of 2 and 3 μm thick potassium sodium niobate [(K0.5, Na0.5)NbO3, KNN] films with different thicknesses were measured under electric fields ranging from 160 to 350 kV/cm and temperatures ranging from 90 to 210 °C. The medium time to failure (t50) was determined from a lognormal distribution plot of failure times of up to 22 electrodes per measurement condition. The activation energy (Ea) for failure was 0.74 ± 0.04 eV and 0.92 ± 0.05 eV for the 2 μm and 3 μm KNN films, respectively. The voltage acceleration factor was 3.5 ± 0.34 for the 3 μm film. But the electric field dependence of t50 for the 2 μm film showed two regions with similar N, 6.67 and 6.94 ± 0.23, respectively. Energy-dispersive X-ray spectroscopy was employed to investigate the Na+ and K+ ion distributions in KNN films.

Lead‐Free Ferroelectric Potassium Sodium Niobate Thin Films from Solution: Composition and Structure

Lead‐free potassium sodium niobate (K0.5Na0.5)NbO3 thin films on Pt(111)/TiO2/SiO2/Si substrates have been prepared from the acetate–alkoxide‐based precursor solutions with the stoichiometric composition and with 5 or 10 mol% excess of sodium or potassium acetate. Upon heating to 750°C, the films crystallize in pure perovskite phase. The increasing amount of alkali excess in solutions increases the degree of {100} orientation and it influences the nucleation and growth processes in the films. The microstructure of about 250 nm thick films prepared from the stoichiometric and 5 mol% excess solutions consists of equiaxed grains of about 50 nm across, whereas the grain size in the about 220 nm thick films prepared from the 10 mol% excess solutions is about 200 nm. The energy dispersive X‐ray spectroscopy revealed that the alkali excess in precursor solutions contributed to a higher level of chemical homogeneity of the films on the micrometer scale. The chemical composition of the films prepared from the 5% potassium‐excess solution was closest to the (K0.5Na0.5)NbO3 composition among all the samples as a result of more pronounced potassium losses upon heating. These films had the room temperature values of dielectric permittivity, dielectric losses, remnant polarization, and coercive field measured at 1 kHz 610, 0.015, 8 μC/cm2, and 80 kV/cm, respectively.

Precursor Homogeneity and Crystallization Effects in Chemical Solution Deposition‐Derived Alkaline Niobate Thin Films

Chemical solution deposition‐derived potassium niobate and sodium niobate thin films have been prepared under protective atmosphere using an alkoxide approach. A profound dependency of the quality and homogeneity of the resulting thin films on the preparation of the alkaline precursor has been found. In addition to this, an additional increase in microstructure homogeneity for the significantly more sensitive potassium niobate system is obtainable if the precursor solution is further stabilized by acetylacetone although ambient humidity is already strongly suppressed. The observed behavior is attributed to the stabilization of the heterometallic complex in the precursor solution until film deposition commenced. Further on, the crystallization pathway of the respective alkaline niobate thin films has been evaluated and indication of crystallization through a metastable phase has been provided which seems to play a key role in orientation control. Finally, the temperature dependency of the lattice parameter spacing for potassium niobate and sodium niobate thin films has been presented and evidence of a strong substrate‐induced lattice deformation has been given.

Comments on the processing of the niobium component for chemical solution derived niobium oxide-based thin-films

The impact of chemical precursor modification on the electronic properties of chemical solution deposition derived niobium oxide thin-films has been evaluated. It has been found that the application of certain chemical modifications is mandatory in order to obtain electrically insulating thin-films at low processing temperatures. It is emphasized that the devised optimal way of processing for the niobium component is widely contrary to the solution based processing of potassium sodium niobate films reported so far. Regarding the physical nature of the observed instabilities, the phase evolution of solution processed niobium oxide films has been studied. It has been detected that the organic fraction in the precursor solution is stable up to high temperatures and as a result, the low temperature crystalline TT-phase of niobium oxide is preserved up to unusually high processing temperatures. The inherent structural distortion of the unit cells may present a new defect mechanism that has to be further investigated regarding the inferior ferroelectric properties of chemical solution derived potassium sodium niobate thin-films, which are often observed.