Microwave-assisted nonaqueous sol-gel deposition of different spinel ferrites and barium titanate perovskite thin films

Rapid and selective heating of solvents by microwave irradiation coupled to nonaqueous sol-gel chemistry makes it possible to simultaneously synthesize metal oxide nanoparticles within minutes and deposit them on substrates. The simple immersion of substrates, such as glass slides, in the reaction solution results after microwave heating in the deposition of homogeneous porous thin films whose thickness can be adjusted through the precursor concentration. Here we use such a microwave-assisted nonaqueous sol-gel process for the formation of various spinel ferrite MFe2O4 (M = Fe, Co, Mn, Ni) and BaTiO3 nanoparticles and their deposition as thin films. The approach offers high flexibility with respect to controlling the crystal size by adjusting the reaction time and/or temperature. Based on the example of CoFe2O4 nanoparticles, we show how the crystal size can carefully be tuned from 4 to 8 nm, resulting in a continuous change of the magnetic properties.     

Aerosol deposition of dense lead zirconate titanate thin films at room temperature

Lead zirconate titanate (Pb(Zr,Ti)O₃, PZT) films were grown on silicon 1 0 0 substrate by aerosol deposition, using solid-state reacted powder containing donor oxide Nb₂O₅, while the substrate was maintained at room temperature. The PZT films were simultaneously sintered upon deposition on a highly densified ceramic layer. Crystalline phases of the deposited films have been determined by X-ray diffractometry (XRD), and microstructures analysed by transmission electron microscopy (TEM). The cross-section microstructure consisted of several thin layers, including the PZT film and the platinum electrode and titanium-buffered layers on the substrate. High-resolution images revealed that the PZT layer contained a mixture of randomly oriented grains of nanometre size, which were embedded in an amorphous matrix. In contrast to the conventional liquid-phase sintering mechanism, sintering of the PZT films involved amorphised phases generated by pressure-induced amorphisation (PIA) from plastic deformation when the initial powder particles collided amongst one another upon reaching the silicon substrate during aerosol deposition. An analogy may be drawn to the impact of extraterrestrial meteorites in which diaplectic glass, i.e., amorphised phase, was formed and retained metastably at room temperature. The individual PZT grains were joined with the amorphised phase(s) and sintered to become a dense, thin film on the silicon substrate.     

Influence of sputter deposition parameters on the properties of tunable barium strontium titanate thin films for microwave applications

Barium strontium titanate (BST) thin films are studied with respect to their application as tunable dielectric at microwave frequencies. BST thin films are deposited by means of radio-frequency magnetron sputtering on platinized Si substrates. The substrate to target distance during sputter deposition is varied and the effect on structure, topology, composition and electronic properties is monitored using X-ray diffraction, atomic force microscopy, Rutherford backscattering spectrometry and X-ray photoelectron spectroscopy. These findings are related to the dielectric measurements, which are carried out at 1 MHz and in the microwave range up to 8 GHz using metal-insulator-metal structures with Pt electrodes. For further device evaluation, leakage current measurements are carried out. Changing the process parameter strongly affects the composition of the films. The results emphasize the possibility for enhancing the microwave properties by fine-tuning of the chosen process parameter.     

Low-temperature preparation of hydrothermal lead zirconate titanate thin films

In preparing lead zirconate titanate thin films under hydrothermal conditions, we investigated the effects of concentrations of nutrient and mineralizer, reaction time and reaction temperature on crystallinity, grain size and shape. Experiments were performed in the ranges of 0.1-1.0M Pb(NO₃)₂, and 0-6.0M KOH with varying reaction time from 0 to 48 hours at 60-200 °C. In the experiment, lead zirconate titanate thin film of homogeneous crystalline grain was obtained through a 24 hour reaction with 0.4M Pb(NO₃)₂ and 5.0M KOH at 140 °C. The thickness of the film was 0.9-1.6 μm, and it exhibited a saturation polarization (Ps) of 18.3 μC/cm², remnant polarization (Pr) of 7.4 μC/ cm₂ and coercive field (Ec) of 0.41 kV/cm. The dielectric constant and loss (δ) measured at 1 kHz were approximately 1020 and 0.15, respectively.     

Investigation of polycrystalline lead titanate fibers and thin films by micro-Raman spectroscopy

Lead titanate thin films and fibers were prepared by sol-gel technique and the phase formation behavior, surface morphology and the effect of strain on the microstructural features of these samples was investigated. The x-ray and micro-Raman scattering results indicate the formation of the tetragonal perovskite phase in both films and fibers. The analysis of the Raman phonon modes reveals that the film is more stressed than the fibers. The variation of stress has been correlated to the microstructural features in these structures.     

Modified sol-gel process for preparation of lead zjrconate titanate thin films

A modified precursor preparation method is suggested for sol-gel processing of better quality PZT thin films. The PZT precursor, prepared using Pb acetate, Ti isopropoxide and Zr n-propoxide, was modified by changing the order of precursor mixing. It was found that there was a dramatic change in the shelf-life of the precursor and in the electrical properties of the PZT films. The processing parameters have been optimized and a sol-gel processing technique is suggested to obtain consistent and high quality PZT films. The films show improved electrical properties.     

Processing and electromechanical properties of lead zirconate titanate thick films by electrophoretic deposition

Electrophoretic deposition (EPD) was used for the fabrication of piezoelectric [lead zirconate titanate (PZT)] thick films on alumina substrates. The EPD was performed in constant current mode from an ethanol based suspension consisting of PZT and PbO particles. The influence of addition of ethyl cellulose (EC) and sintering temperature on the thickness, density, homogeneity and functional response of PZT thick films is studied. Results show that the highest electromechanical performance is obtained for the PZT thick films sintered at 900 or 950°C, with a thickness coupling factor k_t of 50%. The addition of EC influenced the thickness of the PZT thick films but had only minor effect on the porosity content for sintering temperatures over 900°C. Moreover, elastic constants of the thick films based on the suspension with EC were lower, which leads to lower acoustic impedance (15?MRa) while maintaining high (k_t) value. In this last case, a better acoustic matching can be expected with propagation media such as water or biological tissues for ultrasound medical imaging applications.     

Mechanical failure dependence on the electrical history of lead zirconate titanate thin films

Piezoelectric micromechanical systems (piezoMEMS) are often subjected to harsh mechanical and electrical loads during operation. This study evaluates the effects of the electrical history of a lead zirconate titanate (PZT) layer on the electro-mechanical response and structural limits of multilayer stacks. Electro-mechanical characterization was performed under biaxial bending employing the Ball-on-three Balls (B3B) test on virgin, poled, and DC biased (80 kV/cm) samples. No significant effect on the characteristic strength or Weibull modulus of the stack was observed. However, the crack initiation stress was highest for the virgin samples (σ₀ ～ 485 ± 30 MPa); this decreased for both poled samples (σ₀ ～ 410 ± 30 MPa), and samples measured under 80 kV/cm (σ₀ ～ 433 ± 30 MPa). in situ ε_r and loss tangent measurements suggested electromechanical loading conditions can destabilize the domain structure. Overall, the electrical history and electromechanical loading conditions can reduce the PZT film’s fracture resistance.     

Sol-gel processed barium titanate thin films

Ferroelectric barium titanate (BaTiO₃) thin films have been prepared by sol-gel technique from barium acetate [Ba(CH₃COO)₂] and titanium (IV) isopropoxied [Ti(CH₃)₂CHO)₄] precursors. The as-grown films were found to be amorphous which crystallized to tetragonal phase after annealing at 700°C for one hour in air. The room temperautre dielectric constant (ε) and loss tangent (tanδ) of the films were found to be 370 and 0.02 respectively. The values of the spontaneous polarization (P_s), remanent polarization (P_r) and coercive field (E_c) of the films determined from the polarization-field (P-E) hysteresis were found to be 14.0, 3.2μC-cm^-2 and 53KV-cm^-1 respectively. The coercive field of the film determined from the capacitance-voltage (C-V) characteristics is slightly lower than that determined from the P-E hysteresis loop.     

Epitaxial,electro-optically active barium titanate thin films on silicon by chemical solution deposition

Recent progress in the integration of BaTiO₃ thin films with silicon has shown great promise for the development of on-chip photonic devices. However, the highest performing thin films in the literature are deposited by costly and/or complex vacuum techniques. In this study, epitaxial BaTiO₃ thin films are deposited on thin SrTiO₃ template layers on Si(001) from an alkoxide-based chemical solution under atmospheric conditions and yield an effective Pockels coefficient of 27 ± 4 pm/V for an ~85 nm film. Film crystallinity, microstructure, and defect nature are examined by X-ray diffraction and high-resolution transmission and scanning electron microscopy techniques and discussed within the context of the growth method as well as the observed electro-optical response.     

Influence of tensile vs. compressive stress on fatigue of lead zirconate titanate thin films

Novel applications of ferroelectric films require a variety of different substrates, which exert different mechanical stress on the film. This raises the question of reliability of differently stressed films. This work compares the cycling-induced fatigue of the polarization hysteresis of PZT films in different stress states. A tensile stress of +270 MPa, for PZT on fused silica glass, causes gradual degradation, while degradation sets in abruptly under compressive stress of −100 MPa, for PZT on sapphire. The main fatigue mechanism is domain wall pinning on charged defects. Reversible and irreversible domain wall processes in the small- and large-signal permittivity reveal that the fatigue behavior results from a variation of the ferroelectric domain structure. Films under tensile stress contain more 90° domain walls, which get pinned continuously on isolated defects. Compressive stress creates more 180° domain walls, which require formation of defect agglomerates during a certain threshold cycle number for pinning.     

Thickness‐dependent domain wall reorientation in 70/30 lead magnesium niobate‐ lead titanate thin films

Continued reduction in length scales associated with many ferroelectric film‐based technologies is contingent on retaining the functional properties as the film thickness is reduced. Epitaxial and polycrystalline lead magnesium niobate‐lead titanate (70PMN‐30PT) thin films were studied over the thickness range of 100‐350 nm for the relative contributions to property thickness dependence from interfacial and grain‐boundary low permittivity layers. Epitaxial PMN‐PT films were grown on SrRuO₃/(001)SrTiO₃, while polycrystalline films with {001}‐Lotgering factors >0.96 were grown on Pt/TiO₂/SiO₂/Si substrates via chemical solution deposition. Both film types exhibited similar relative permittivities of ~300 at high fields at all measured thicknesses with highly crystalline electrode/dielectric interfaces. These results, with the DC‐biased and temperature‐dependent dielectric characterization, suggest irreversible domain wall mobility is the major contributor to the overall dielectric response and its thickness dependence. In epitaxial films, the irreversible Rayleigh coefficients reduced 85% upon decreasing thickness from 350 to 100 nm. The temperature at which a peak in the relative permittivity is observed was the only measured small signal quantity which was more thickness‐dependent in polycrystalline than epitaxial films. This is attributed to the relaxor nature present in the films, potentially stabilized by defect concentrations, and/or chemical inhomogeneity. Finally, the effective interfacial layers are found to contribute to the measured thickness dependence in the longitudinal piezoelectric coefficient.     

Stress-mediated solution deposition method to stabilize ferroelectric BiFe1-xCrxO3 perovskite thin films with narrow bandgaps

Ferroelectric oxides with low bandgaps are mainly based on the BiFeO₃ perovskite upon the partial substitution of iron with different cations. However, the structural stability of many of these perovskites is only possible by their processing at high pressures (HP, >1GPa) and high temperatures (HT, >700ºC). Preparation methods under these severe conditions are accessible to powders and bulk ceramics. However, transferring these conditions to the fabrication of thin films is a challenge, thus limiting their use in applications. Here, a chemical solution deposition method is devised, which overcomes many of these restrictions. It is based on the application of an external compressive-stress to the film sample during the thermal treatment required for the film crystallization, promoting the formation and stabilization of these HP perovskites. We demonstrate the concept on BiFe_1-xCr_xO₃ (BFCO) thin films deposited on SrTiO₃ (STO) substrates and with large chromium contents. The resulting BFCO perovskite films show narrow bandgaps (Eg～2.57 eV) and an excellent ferroelectric response (remnant polarization, P_R～ 40 μC cm^?2). The polarized thin films under illumination present a large out-put power of ～6.4 μW cm^?2, demonstrating their potential for using in self-powered multifunctional devices. This stress-mediated solution deposition method can be extended to other perovskite films which are unviable under conventional deposition methods.     

Processing of high performance lead lanthanum zirconate titanate thick films

The processing and ferroelectric properties of lead lanthanum zirconate titanate (PLZT) thick films on Pt-coated Al₂O₃ substrate were studied. The most critical processing issue is to prevent lead losses during heat treatment. Maintaining a high PbO vapour concentration during heat treatment of the layers maintains a liquid phase that enables densification also in constrained conditions, possibly because of relaxation of the shear stress originating from constrained sintering. Reaction of PbO-rich liquid phase at the platinum/alumina interface results in β-alumina crystals that deteriorate the contact between the platinum electrode and the substrate. Instead of platinum on Al₂O₃, a new Al₂O₃/PLZT/Pt structure was introduced taking advantage of the excellent mechanical stability of the Al₂O₃/PLZT and Pt/PLZT interfaces. PLZT thick films with ferroelectric properties equal to or better than those of hot pressed samples were obtained. Such a sintering and electrode strategy can be applied in processing of other lead-based thick films.     

Hydrothermal synthesis of lead titanate and lead zirconate titanate electroceramic particles

Hydrothermal synthesis of two lead-based perovskite powder systems, lead titanate and lead zirconate titanate, was investigated. Phase-pure perovskite lead titanate and lead zirconate titanate with various morphologies have been synthesized by hydrothermal methods at 150° and 175°C, respectively. Solution pH should be greater than 14 to obtain the phase-pure perovskite phases in a reasonable time. In addition, the KOH concentration and the stirring rate significantly influence particle morphology of the hydrothermally derived PT and PZT. Therefore, these parameters can be used to tailor particle size and morphology.     

Morphology of water-based chemical solution deposition (CSD) lead titanate films on different substrates: Towards island formation

This study aims at the deposition of PbTiO₃ (PT) islands prepared by a water-based chemical solution deposition (CSD). Two aqueous citrato-based PbTiO₃ precursor solutions, either with or without peroxide, are deposited by spin coating. The effect of different substrates on the formation of separated grains or islands is examined. It is observed that spin coating of a 0.6 M precursor solution on a Pt(1 1 1)/IrO₂/Ir/SiO₂/Si substrate gives the best results towards island formation. For this substrate, crystallizations are carried out between 600 °C and 900 °C. A final crystallization at 800 °C results in the highest degree of separation for the islands, while keeping the platinized substrate intact. The deposition of diluted precursors shows that it is possible to form islands from a precursor solution with a concentration down to 0.3 M. Solutions with a lower concentration result in irregularly shaped structures.     

Substrate temperature-dependent physical properties of nanocrystalline zirconium titanate thin films

Nanocrystalline zirconium titanate thin films were deposited by direct current magnetron reactive sputtering on to glass substrates at room temperature and at different substrate temperatures of 423, 473, 523, and 573 K under high vacuum conditions. The deposited films have been characterized to study the physical properties of the films as a function of substrate temperature. Though the film exhibited amorphous characteristics at room temperature the higher temperatures resulted in the evolution of crystallites in the films. The crystallinity increased with temperature from 423 K onwards and the film deposited at 523 K exhibited a high crystallite size of 22 nm. The SEM images of the films revealed the improvement in the crystallinity from 423 to 523 K with dense columnar structure normal to the substrate. Further higher treatment deteriorated the film properties. The films showed a good transmittance of above 80%. A high optical transmittance of 91% and a high packing density of 96% have been observed for the film deposited at 523 K. The thickness of the films remained consistent at ~230 nm (±6 nm). It is noticed that an increase in the substrate temperature enhanced the structural, optical, and electrical properties of the films up to 523 K.     

Dielectric relaxations of lead zirconate titanate films up to millimeter-wave frequencies

Modern high-frequency applications critically depend on the availability of data on the dielectric properties of functional materials in the microwave and millimeter-wave range. This paper investigates the dielectric response of polycrystalline lead zirconate titanate (PZT) thin films prepared by solution deposition at frequencies between 10 MHz and 70 GHz and temperatures of 305–395 K by measuring the S-parameters of coplanar waveguides (CPW). The real and imaginary parts of the permittivity of the PZT film are de-coupled from the electrical properties of both the fused silica substrate and the copper electrodes. Two dielectric relaxations are identified: one around 850 MHz, with room-temperature permittivity dropping from 1080 to 560, and one around 36 GHz, with permittivity dropping below 100, respectively. The low-frequency relaxation shows a shift to lower frequencies with increasing temperature; it is explained by the conventional Arlt model of mechanical shear mode resonance across the film thickness. The high-frequency relaxation, which is practically independent of temperature, is attributed to the response of ferroelectric domain walls.     

Structure and dielectric properties of barium titanate thin films for capacitor applications

BaTiO₃ is a typical ferroelectric material with high relative permittivity and has been used for various applications, such as multilayer ceramic capacitors (MLCCs). With the tendency of miniaturization of MLCCs, the thin films of BaTiO₃ have been required. In this work, BaTiO₃ thin films have been deposited on Pt-coated Si substrates by RF magnetron sputtering under different deposition conditions. The films deposited at the substrate temperature from 550 °C–750 °C show a pure tetragonal perovskite structure. The films deposited at 550 °C–625  °C exhibit (111) preferential orientation, and change to (110) preferential orientation when deposited above 650 °C. The film morphologies vary with working pressure and substrate temperature. The film deposited at 625 °C and 4.5 Pa has the relative permittivity of 630 and the loss tangent of 2% at 10 kHz.