SrBi2Nb2O9 Ferroelectric Powders and Thin Films Prepared by Sol-Gel

Pure SrBi2Nb2O9 powders and thin films were obtained using sol-gel synthesis from mixtures of niobium ethoxide, bismuth and strontium 2-ethylhexanoates. Powders crystallized for 2 hours at 700°C had grain sizes of about 100–150 nm. SrBi2Nb2O9 thin films were prepared by spin-coating a stable precursor solution onto Si/SiO2/TiO2/Pt substrates. Crystallization of pure SrBi2Nb2O9 phase occurred at about 500–550°C. Randomly oriented 0.3 m-thick crack-free films were obtained after 10 successive depositions and heating at 700°C for 2 hours. P-E hysteresis loops have confirmed the ferroelectric behavior of the films: they show a remanent polarization of 2.5 C/cm2 (5 V, 8 ms). No fatigue was observed up to 109 full switchings.     

Synthesis of Bi2O3 and Bi4(SiO4)3 Thin Films by the Sol-Gel Method

Bi2O3 thin films were prepared by dipping silica slides in ethanolic solutions of tris(2,2-6,6-tetramethylheptane-3, 5-dionato)bismuth(III) [Bi(dpm)3] [1] and heating in air at temperatures 500°C. Bi4(SiO4)3 homogeneous thin films were obtained from the reaction of the bismuth oxide coating with the silica glass substrate at temperatures higher than 700°C. For heat treatments at temperatures between 600°C and 700°C, Bi2SiO5 coatings were obtained. The composition and microstructure evolution of the films were determined by Secondary Ion-Mass Spectrometry (SIMS), X-Ray Photoelectron Spectroscopy (XPS) and Glancing Angle X-Ray Diffraction (GA-XRD). The synthesis procedure was reproducible and allowed the control of the Bi2O3 phase composition. Moreover, the thin film annealing parameters were correlated with the formation of bismuth silicates, among which Bi4(SiO4)3 (BSO) is very appealing for the production of fast light-output scintillators [2].     

Crystallization and Microstructure of (Sm, Pb)TiO3 Thin Films Prepared from Sol-Gel Synthesized Solutions

A sol-gel route is developed for the synthesis of samarium modified lead titanate precursor solutions. The solutions are used for the deposition of thin films. After thermal treatment of the films, two crystalline phases are observed by X-ray diffraction analysis: an undesirable pyrochlore phase and a ferroelectric perovskite. These two phases are clearly distinguished in the film microstructure, showing a fined grained fraction of pyrochlore anda-axis oriented rosette grains of perovskite. The development of these phases as well as the evolution of the perovskite/pyrochlore ratio in the films is related to the chemistry of the synthesized solutions and the thermal treatment used for crystallization.     

Effect of Niobium Modifications to PZT (53/47) Thin Films Made by a Sol-Gel Route

Niobium-modified lead zirconate titanate thin films (PNZT) with nominal compositions, Pb_(1–0.5x) (Zr_0.53 Ti_0.47)_1–x Nb _x O₃:x = 0.02–0.07, have been prepared using a diol based sol-gel route. Single-layer (0.5 m) films were fabricated on platinised silicon substrates by spin-coating. The effect of niobium additions with regard to phase development, microstructure, and ferroelectric and dielectric properties were investigated for different annealing temperatures. For comparison, unmodified PZT films were also prepared. Niobium substitution increased the crystallisation temperatures for perovskite PNZT phase formation. The values of remanent polarisation P _r and dielectric constant _r were found to decrease with the introduction of Nb. For example, in films heated at 700°C for 15 min, the P _r value of an unmodified PZT film was 31 C cm^–2, compared to 17 C cm^–2 for an x = 0.05 PNZT film, whilst respective relative permittivity values fell from 1190 to 600. The highest Nb concentration film, x = 0.07, did not display any switchable polarisation characteristics, which is consistent with high levels of intermediate pyrochlore phase.     

Atomic Layer Deposited Ti2O3 Thin Films

Ti₂O₃ thin films have been prepared through atomic layer deposition and subjected to electrical resistivity measurements as a function of temperature. The as-prepared films were stable for up to three weeks. In Ti₂O₃ thin films, the insulator-metal transition is observed at ∼80 K, with nearly 3–4 orders of magnitude change in resistivity. The anomalous increase in electrical resistivity in the films is in accordance with the two-band model. However, the energy interval between the bands depending on the crystallographic c/a ratio leads to a change in electrical resistivity as a function of temperature.     

Sol-Gel Derived Barium-Strontium Titanate Films

Sol-gel techniques for the preparation of barium-strontium titanate (BST) films are discussed. The evolution of film microstructure during heat treatment, and the dielectric properties of BST films prepared from alkoxide solutions and from alkoxide solutions modified by 2-ethylhexanoic acid were studied. It is shown that the extent of the modification of the precursors by 2-ethylhexanoic acid changes the precursor molecular complexity governing the microstructure of the films and their electrical properties.     

Sol-Gel Thin Films of BaFe12O19 from Chemically Modified Alkoxide Precursors

Homogeneous BaFe₁₂O₁₉ (BaM) thin films have been prepared by the dip-coating technique on microscope slides and SiO₂ wafers using barium and iron alkoxides as precursors, and diethanolamine as a modifier. In order to optimise the coating conditions, the influence of the alkoxide, water and modifier on the sol viscosity, thickness and appearance of the Fe₂O₃ films has been investigated. For comparison of the crystallisation and magnetic properties, powders of the same composition as the films have been also prepared by the sol-gel method. The BaM phase appears after annealing at 650°C for 6 hours. The BaM film and powder, heated at 850°C for 6 hours, give coercive force of about 3000 and 5500 Oe, respectively.     

Textured PLCT Thin Films Prepared on Pt/Ti/SiO2/Si Substrate by Metal-Organic Decomposition

Textured calcium modified (Pb,La)TiO₃ (PLCT) films were deposited on Pt/Ti/SiO₂/Si substrates by using a metal-organic decomposition (MOD) process. The PLCT films exhibit good ferroelectric properties, a very low leakage current and a sharp PLCT/Pt interface. The (100) texture of the PLCT film is growth-controlled; the (100) oriented grains grow preferentially so as to minimize the surface energy. Particularly, the (100) preferred orientation is easy to form in the PLCT film with a layered structure for which the substrate almost does not affect the nucleation and growth of the film.     

Photoconductivity in Mesostructured Thin Films

Mesostructured silica films are widely studied due to their different structures, properties and variety of possible applications. Sodium dodecyl sulfate (SDS)-templated sol-gel silica films possess highly ordered lamellar phase structure. It is expected that molecules and polymer chains line up with these layered structures when incorporated into the films. Mesostructured thin films were doped with Dispersed Red 1 (DR1) and carbazole ((C₆H₄)₂NH)). The films were poled by corona discharge at 120 C. Absorption spectra were recorded as function of the polarization time. Dependence of the absorption coefficient with polarization time was fitted with a Langevin-Debye equation. It shows a saturation level after 60 minutes of polarization. We compare the efficiency of mesostructured thin films with that of amorphous films. The photoconductivity technique was used to determine the charge transport mechanism of these films. From current density versus electrostatic applied field, the parameters for the photovoltaic effect and photoconductivity were determined. Results of the mesostructured thin films are also compared to those of KNbO₃ crystals.     

Chemistry-Crystallization-Microstructure Relations of Sol-Gel Derived Lanthanum Modified Lead Titanate Thin Films

Lanthanum modified lead titanate thin films have been obtained by the deposition of sol-gel solutions onto platinized (100) silicon substrates. Crystallization of perovskite films was achieved by thermal treatments at 650°C with slow or rapid heatings. Lead oxide excesses were used in the precursor solutions to counterbalance the lead losses produced during the thermal treatment. Rapid heatings and large excesses of lead produce a preferred orientation of the films. These films have more homogeneous and denser microstructures than slow heated films without lead excess.     

Ferroelectric Thin Films of Bismuth Strontium Tantalate Prepared by Alkoxide Route

Study of the interactions in Bi(OR)3-Ta(OR)5-ROH (R = Me, Et, iPr) at 20°C show that metal ethoxides are the best of the investigated precursors for the production of bismuth tantalates via alkoxides. Polycrystalline SrBi2Ta2O9 (SBTO) and BiTaO4 films on Si-SiO2-Ti-Pt substrates with thicknesses of 0.4–0.5 m and 0.4 m, respectively, have been formed by sol-gel processing. The most stable solutions for film application were obtained by using mixed-metal Bi-Ta ethoxides and solutions of Sr carboxylate (2-ethylhexanoic acid derivative) in 2-ethylhexanoic acid. Films annealed at 700–750°C for 30 min were single-phased. SBTO films demonstrated good ferroelectric properties: remanent polarization ranged from 3.5 to 4.0 C/cm2 and coercive voltage 1.5–2.0 V, whereas BiTaO4 films showed dielectric behavior.     

Preparation and Characterization of Sol-Gel Derived Y2O3 Thin Films

Sol-gel derived Y2O3 thin films have been prepared on platinum coated silicon wafers and fired to temperatures ranging from 400°C to 750°C. Multiple coats were used to obtain films up to 0.5 m thick with an intermediate firing of 400°C between coatings. Top Pt electrodes were sputtered to form monolithic capacitors. These films exhibited a dielectric constant of 18 and a leakage current of 10–11–10–7 A/cm2, making them attractive candidates for high dielectric constant dielectric films in high density DRAMs.     

Inorganic Thin Films Prepared from Soluble Powders and Their Applications

Sol-gel processing has proved to be a useful method for the preparation of a wide range of inorganic thin films. However, gelation or precipitation of coating solutions and thus limited shelf-life have been drawbacks for the industrial application of this technology.Soluble powders of various compositions can be prepared from modified metal alkoxides commonly employed in sol-gel processing. Even though these powders contain a considerable amount of organic moieties they can be stored indefinitely under ambient conditions, thus diminishing shelf life problems. Redissolution in polar organic solvents, solvent mixtures or even water yields ready-to-use coating solutions. Coating solutions for the preparation of titania (TiO₂), zirconia (ZrO₂) and lead zirconate titanate (PbTi _x Zr_1–x O₃, PZT) films have been synthesized by this method. Thickness, microstructure and properties of the resulting films can be varied by modification of the solvent, the coating procedure and thermal treatment. Inorganic thin films for various applications have been prepared.     

Sol-Gel Processing of PbZrO3 Thin Films

PbZrO₃ (PZ) thin films have been prepared by 2-methoxyethanol route from lead oxide or lead acetate and zirconium n-butoxide. The use of lead oxide as lead source and the seeding layer of TiO₂ on Pt/TiO₂/SiO₂/Si substrate facilitate the formation of the perovskite phase.     

Lead Zirconate-Titanate Films Prepared from Soluble Powders

Precursor powders for Pb(Zr1 – xTix) O3 (PZT) thin films were produced by the reaction of zirconium-and titanium-n-propoxides with acetylacetone and lead acetate trihydrate. The subsequent complete removal of volatile components yielded powders that can be handled in air. The powders are indefinitely stable under ambient conditions.High molarity (>2m) coating sols were prepared by dissolution of the precursor powders in mixtures of 1,3-propanediol, triethanolamine (TEA) and water. Excess amounts of lead to compensate lead loss during firing were easily introduced into these solutions.The deposition of these sols on steel substrates and firing at 600°C yielded PZT films. Many physical parameters like film thickness, morphology and electrical performance could be influenced by choice of the solvent mixture composition and oxide content of the sols. Depending on the preparation dielectric permittivities, r of up to 840 were measured at 1 kHz. By hysteresis measurements at 50 Hz and a field amplitude of 50 V/m a remanent polarization of about 40 C/cm2 and coercivity of about 8 V/m was obtained. The films were stable against dielectric breakdown up to 70V/m.     

Elaboration of ZnO Thin Films with Preferential Orientation by a Soft Chemistry Route

Microcrystalline ZnO films presenting well-defined and tunable orientation were obtained by spin coating of alcoholic sols by two different approaches, based on controlled hydrolysis-condensation of Zn-ethanolamine complexes. As-deposited films are formed by amorphous zinc oxide-acetate submicronic particles, which are transformed into oriented ZnO after thermal treatment. The orientation of ZnO depends on the synthesis method, and the solvent. While in ethanol and [Zn] = 0.05 mol·L^–1, films consist of rectangular platelets oriented with the (100) planes parallel to the substrate (a//n), the orientation of the particles changes to (c//n) for systems in 2-methoxyethanol (2-ME) and [Zn] = 0.75 mol·L^–1. A study of chemical factors that influence the orientation (precursor, solvent, MEA/Zn ratio, concentration, coating parameters, heat treatment) is presented.     

Ferroelectric Films

The preparation and properties of ferroelectric films are reviewed. Specific attention is directed to ferroelectric films prepared by wet chemical methods. Emphasis is placed on the microstructure and properties of SBT films for memory applications, and their dependence on chemistry and processing, as well as on the effects of chemistry and processing on the properties of piezoelectric and pyroelectric films. Emphasis is placed on recent results obtained in our laboratories, including modeling of the ferroelectric behavior of films and experimental results in each of these areas. Also considered is the use of ferroelectric films in memory, piezoelectric and pyroelectric devices of various types, with specific note made of the recently-announced use of pyroelectric sensors in automobiles.     

Strain Coupling of Conversion-type Fe3O4 Thin Films for Lithium Ion Batteries

Lithiation/delithiation induces significant stresses and strains into the electrodes for lithium ion batteries, which can severely degrade their cycling performance. Moreover, this electrochemically induced strain can interact with the local strain existing at solid–solid interfaces. It is not clear how this interaction affects the lithiation mechanism. The effect of this coupling on the lithiation kinetics in epitaxial Fe₃O₄ thin film on a Nb-doped SrTiO₃ substrate is investigated. In situ and ex situ transmission electron microscopy (TEM) results show that the lithiation is suppressed by the compressive interfacial strain. At the interface between the film and substrate, the existence of Li_xFe₃O₄ rock-salt phase during lithiation consequently restrains the film from delamination. 2D phase-field simulation verifies the effect of strain. This work provides critical insights of understanding the solid–solid interfaces of conversion-type electrodes.     

New Sol-Gel Route for Processing of PMN Thin Films

Pyrochlore free Pb(Mg_1/3Nb_2/3)O₃ (PMN) thin films were prepared from mixed-metal precursors solutions using the sol-gel process. Lead acetate [Pb(CH₃COO)₂], magnesium acetate [Mg(CH₃COO)₂] and niobium ethoxide [Nb(C₂H₅O)₅] were used as starting materials, while 2-isopropoxy-ethanol was chosen as solvent. The reactivity of the precursors was investigated in order to understand and control the process and thus to prevent the contamination of the PMN with the pyrochlore phase. The solution was spin-coated on TiO₂/Pt/TiO₂/SiO₂/Si substrate. The thin films were characterized by SEM and XRD while dielectric measurements were performed on the bulk ceramic.