Synthesis of Highly Oriented Lead Zirconate–Lead Titanate Film Using Metallo-organics

Crack-free Pb(Zr,Ti)O₃ (PZT) thin films with preferred orientation were prepared successfully on MgO (100), SrTiO₃ (100), and Pt/Ti/SiO₂/Si substrates from metal alkoxide solutions. Calcination of precursor films in a H₂O─-O₂ gas mixture was found to be effective not only for low-temperature crystallization of perovskite PZT, but also for obtaining the preferred orientation of PZT films. Single-phase PZT films with high preferred orientation were synthesized on MgO (100) and Pt/Ti/SiO₂/Si substrates at 550° and 600°C for 2 h, respectively. The PZT film on the Pt/Ti/SiO₂/Si substrate showed a permittivity of 520, tan δ of 0.03, a remanent polarization of 24 μC/cm², and a coercive field of 54 kV/cm.     

Synthesis of Highly Oriented K(Ta,Nb)O3 (Ta:Nb = 65:35) Film Using Metal Alkoxides

Highly oriented K(Ta,Nb)O₃ (Ta:Nb = 65:35) (KTN) thin films of perovskite structure were synthesized successfully on Pt(100)/MgO(100) substrates from a metal alkoxide solution through reaction control. Homogeneous KTN coating solutions prepared from KOC₂H₅, Ta(OC₂H₅)₅, and Nb(OC₂H₅)₅ in ethanol were analyzed by ¹H, ¹³C, and ⁹³Nb NMR spectroscopy. The KTN precursor included a molecular-level mixture of K[M(OC₂H₅)₆] (M = Ta, Nb) units interacting in ethanol solution. X-ray pole figure measurement showed that perovskite KTN films crystallized on Pt(100)/MgO(100) substrates had not only a (100) orientation but also a three-dimensional regularity of grains. The remanent polarization and coercive field of the KTN film (thickness, 1.0 μm) crystallized at 700°C were 1.5 μC/cm² and 8.7 kV/cm, respectively, at 225 K.     

Preparation of Ba2YCu3O7-δ Thin Films with Preferred Orientation through an Organometallic Route

Superconducting Ba₂YCu₃O_7-δ thin films were prepared through an organometallic route. Single-phase Ba₂YCu₃O_7-δ thin films with preferred orientation were successfully prepared on SrTiO₃ (100) single-crystal substrates at 800°C by a dip coating method using partially hydrolyzed Ba-Y-Cu organometallic solutions. Preferentially oriented Ba₂YCu₃-O_7-δ thin films were also prepared on MgO (100) substrates. By controlling the partial hydrolysis conditions, a coating solution for precursor thin films was kept accurately at the stoichiometric composition. The use of ozone gas during the pyrolysis of the precursor thin films was found to suppress the formation of BaCO₃. Ba₂YCu₃O_7-δ thin films with c -axis orientation perpendicular to a SrTiO₃ (100) substrate, which were heat-treated at 900°C for 15 min, exhibited a superconductivity transition with an onset of 90 K and an end of 75 K.     

Synthesis of Oriented Ba2NaNb5O15 (BNN) Thin Films from an Alkoxy-derived Precursor

Ba₂NaNb₅O₁₅ (BNN) thin films with a tungsten bronze structure were fabricated using a precursor solution that was synthesized from barium, sodium, and niobium alkoxides. Highly (002)-oriented BNN films were prepared successfully on Pt(100)/MgO(100) substrates at 700°C, using a BNN underlayer. X-ray pole-figure measurement showed that the BNN films that crystallized on Pt(100)/MgO(100) substrates had two in-plane orientations. The remanent polarization and coercive field of the BNN film (thickness of 1.0 µm) that was crystallized at 700°C were 12.3 µC/cm² and 101 kV/cm, respectively, at –150°C (123 K). BNN films on fused silica substrates exhibited second harmonic generation upon irradiation with 1064 nm light.     

Synthesis of Ba2NaNb5O15 Powders and Thin Films Using Metal Alkoxides

Transparent and highly oriented Ba₂NaNb₅O₁₅ (BNN) thin films have been prepared by using metal alkoxides. A homogeneous precursor solution was prepared by the controlled reaction of NaOC₂H₅, Nb(OC₂H₅)₅, and barium metal. The BNN precursor included a molecular-level mixture of NaNb(OC₂H₅)₆ and Ba[Nb(OC₂H₅)₆]₂ in ethanol. The alkoxy-derived powder crystallized to a low-temperature phase, and then transformed to orthorhombic BNN (tungsten bronze) at 600°C. BNN precursor films on substrates crystallized to orthorhombic BNN at 800°C via the low-temperature phase. Highly (002) oriented BNN films of tungsten bronze structure were successfully prepared on MgO (100) substrates at 700°C by using BNN underlayer.     

Preparation of Potassium Tantalate Niobate by Sol–Gel Method

Perovskite potassium tantalate niobate (KTN) powders and thin films were synthesized from a metal alkoxide solution. Homogeneous KTN coating solutions were prepared from KOC₂H₅, Ta(OC₂H₅)₅, and Nb(OC₂H₅)₅ in absolute ethanol. The precursor crystallized to pyrochlore at ∼ 650°C, and then to perovskite at ∼ 750°C, depending upon Ta:Nb ratio. H₂O vapor during calcination was found to play a prominent role in the direct and predominant crystallization of perovskite films with preferred orientation. Highly oriented KTN films of perovskite structure were successfully prepared on MgO (100) substrates at 675°C.     

Sol-Gel Preparation of Pb(Zr0.50Ti0.50)O3 Ferroelectric Thin Films Using Zirconium Oxynitrate as the Zirconium Source

Lead zirconium titanate (Pb(Zr_0.5Ti_0.5)O₃, PZT) ferroelectric thin films were successfully deposited on platinum-coated silicon substrates and platinum-coated silicon substrates with a PbTiO₃ interlayer by using a modified sol–gel spin-coating process, using zirconium oxynitrate dihydrate as the zirconium source. The precursor solution for spin coating was prepared from lead acetate trihydrate, zirconium oxynitrate dihydrate, and tetrabutyl titanate. The use of zirconium oxynitrate instead of the widely used zirconium alkoxide provided more stability to the PZT precursor solution and a well-crystallized structure of PZT film at a relatively low processing temperature. PZT films that were annealed at a temperature of 700°C for 2 min via a rapid thermal annealing process formed a well-crystallized perovskite phase of PZT films and also had nanoscale uniformity. The microstructure and morphology of the prepared PZT thin films were investigated via X-ray diffractometry, transmission electron microscopy, and atomic force microscopy techniques. The values for the remnant polarization ( P ) and coercive electric field ( E ) of the PZT films that were obtained from the P–E loop measurements were 3.67 μC/cm² and 54.5 kV/cm, respectively.     

Ultraviolet Patterning of Alkoxy-Derived Lithium Niobate Film

Patterned lithium niobate (LN) films were synthesized successfully from a precursor film containing 1-phenyl-1,3-butanedione (PBD) by ultraviolet (UV) irradiation. A LN precursor solution composed of lithium ethoxide and niobium ethoxide was modified with PBD in ethanol to form LiNb(OEt)_{6- n} (PBD) _n , which was designed to have a specific UV absorption at 330 nm. The precursor film began to crystallize at 400°C on sapphire C substrates. The UV absorption band at 330 nm in the precursor film decreased in intensity with increased exposure time to UV light. The patterned LN films crystallized at 550°C on sapphire C substrates showed a (006) preferred orientation.     

Preparation of Strontium Barium Niobate by Sol–Gel Method

Crack-free SBN (Sr_xBa_1-xNb₂O₆) thin films have been prepared by a sol–gel method with metal alkoxides. A homogeneous and stable precursor solution was obtained from Sr and Ba metal and Nb(OEt)₅ in ethanol with a key additive of ethoxyethanol. SBN (where x = 0.5) powder crystallized to orthorhombic phase at 700°C, and then transformed completely to tetragonal phase at 1200°C. The formation of tetragonal SBN was observed on sapphire (R) substrates at 700°C, whereas the tetragonal phase began to appear in the powders at 1000°C. SBN films with highly preferred orientation were successfully synthesized on MgO (100) substrates at 670°C.     

Preparation of Strontium Titanate Thin Film on Titanium Metal Substrate by Hydrothermal-Electrochemical Method

Polycrystalline SrTiO₃ thin films have been prepared on Timetal substrates by the hydrothermal-electrochemical method. The films were prepared at temperatures ranging from 100° to 200°C and under saturated vapor pressures in electrolytic solutions of 1 N to 5 N Sr(OH)₂. A galvanostat was used to apply a direct current of 5 to 25 mA/cm². The films had smooth and homogeneous surfaces without any visible pores or defects. The dielectric constant of the films was approximately 129 assuming 25 nm for film thickness.     

Phase Equilibria in the System BaTiO3–SrTiO3

Phase equilibria in the binary system BaTiO₃–SrTiO₃ were studied above 1200°C. This system is characterized by a complete series of solid solutions with a minimum at 2.5 mole % SrTiO₃ and 1585°C. The hexagonal BaTiO₃ phase is suppressed to a region extending no farther than 0.5% SrTiO₃ at 1600°C. No immiscibility gap was found.     

Preparation of 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 Thin Films Via a Polyvinylpyrrolidone-Assisted Aqueous Sol–Gel Process and Dielectric Properties

Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ (PMN–PT) thin films were prepared by spin coating using aqueous solutions of metal salts containing polyvinylpyrrolidone, where niobium oxide layers and lead—magnesium–titanium oxide layers were laminated on Pt(111)/TiO _x /SiO₂/Si(100) substrates and fired at 750° or 800°C. 250 ± 20 nm thick 0.7PMN–0.3PT thin films of a single-phase perovskite could be prepared, and the film fired at 750°C had dielectric constants and dielectric loss of 1900 ± 350 and 0.13 ± 0.03, respectively, exhibiting polarization-electric field hysteresis with a remanent polarization of 5.1 μC/cm² and a coercive field of 21 kV/cm.     

Conversion Process of Strontium–Titanium Bimetallic Methoxyethoxide Precursor into SrTiO3 via Hydrolysis/Calcination

The conversion of a Sr–Ti bimetallic methoxyethoxide precursor into SrTiO₃ via hydrolysis and/or calcination was investigated. Hydrolysis with various water/metal molar ratios ( r _H; r _H= 0.5, 2, 4, 6, 8, 10) in tetrahydrofuran at reflux resulted in a decrease in the amount of the methoxyethoxyl groups, and the hydrolyzed products were soluble with r _H≤ 2. At r _H≥ 8, SrTiO₃ was crystallized without calcination. Both the hydrolyzed and unhydrolyzed precursors ( r _H= 0, 0.5, 2) were calcined in dry air at 550°–800°C. SrTiO₃ was crystallized on calcination at ≥550°C from amorphous materials with a considerable loss of carbon, which was present as both chars and carbonate ions.     

Sol–Gel Route to Porous Lanthanum Cobaltite (LaCoO3) Thin Films

Sol–gel-derived LaCoO₃ thin films were deposited on yttria-stabilized zirconia (YSZ) substrates from a lanthanum isopropoxide–cobalt acetate (with 2-methoxyethanol) precursor solution. A chelating agent (2-ethylacetoacetate) and polyethylene glycol (PEG) were used to modify the above-mentioned precursor solution. The La-Co precursor solution was sufficiently viscous, and transparent LaCoO₃ gel films were prepared successfully using a spin-coating technique. Crystallization behavior and microstructure evolution were investigated using X-ray diffractometry (XRD) and scanning electron microscopy (SEM). A single-phase perovskite thin film with the grain size of ∼50 nm was obtained by heat-treating the spin-coated gel film at a temperature of 600°C. SEM observations revealed that the microstructure of LaCoO₃ thin films that were prepared from the precursor solution with PEG was porous, and the LaCoO₃ thin film maintained its porous microstructure to a temperature of 800°C.     

Preparation of Strontium Titanate Thin Films by the Hydrothermal-Electrochemical Method in a Solution Flow System

SrTiO₃ thin films have been prepared on titanium substrates using strontium acetate solutions in newly constructed flow-system equipment by the hydrothermal-electrochemical method. The synthesis parameters (temperature of 120°-200°C and flow rate of 1-40 mL/min) allow fabrication of dense, single-phase films with grain sizes in the range of 80-340 nm by controlling nucleation and/or growth rates. The flow can be closed, enabling easy recycling of the solutions used. This processing route may serve as an inexpensive and environmentally friendly way of fabrication of single-phase SrTiO₃ thin films as well as functionally graded ferroelectric materials.     

Metalorganic Chemical Vapor Deposition of Ferroelectric Pb(Zr,Ti)O3 Thin Films

Ferroelectric Pb(Zr_xT_1–x)O₃, films were successfully and reproducibly deposited by both hot–wall metalorganic chemical vapor deposition (MOCVD) and cold-wall MOCVD. One of the important problems associated with the MOCVD techniques is the selection of ideal precursors. After an intensive investigation for the most suitable precursors for MOCVD PZT films, the safe and stable precursors, namely lead tetramethylheptadione [Pb(thd)₂], zirconium tetramethylheptadione [Zr(thd)₄], and titanium ethoxide [Ti(OEt)₄], were chosen. The films were deposited at temperatures as low as 550°C and were single-phase perovskite in the as-deposited state. Also, the films were smooth, specular, crack-free, and uniform, and adhered well to the substrates. The stoichiometry of the films can be easily controlled by varying the individual precursor temperature and/or the flow rate of the carrier gas. Auger electron spectroscopic (AES) depth profile showed good compositional uniformity through the thickness of the films. The AES spectra also showed no carbon contamination in the bulk of the films. As-deposited films were dense and showed uniform and fine grains (≅0.1 μm).The optical properties of the films on the sapphire disks showed high refractive index ( n = 2.413) and low extinction coeflicient ( k = 0.0008) at a wavelength of 632.8 nm. The PZT (82/18) film annealed at 600°C showed a spontaneous polarization of 23.3 μC/cm² and a coercive field of 64.5 kV/cm.     

Oxygen Tracer Diffusion in Lanthanum-Doped Single-Crystal Strontium Titanate

Strontium titanate (SrTiO₃) is known as a good high-temperature resistive oxygen sensor material; its response time depends on oxygen bulk diffusion and surface exchange processes. In the present work, ¹⁸O diffusion has been investigated in lanthanum-doped SrTiO₃, single crystals in the temperature range 700° to 900°C by secondary ion mass spectrometry (SIMS). Oxygen tracer diffusivities between 2 × 10⁻¹⁵ and 1 × 10⁻¹³ cm²/s have been calculated from the SIMS results. Low surface enrichment of ¹⁸O compared to the ¹⁸O concentration in the gas atmosphere gives clear evidence for a surface exchange reaction.     

Suppression of Liquid Film Migration and Improvement of Dielectric Properties in Niobium-Doped Strontium Titanate

In an attempt to improve the dielectric properties of SrTiO₃-based boundary-layer capacitors (BLCs), the effects of infiltrant composition on the liquid film migration and dielectric properties in 0.2-mol%-Nb₂O₅-doped SrTiO₃ were investigated. Powder compacts were sintered at 1480°C for 5 h in 5H₂·95N₂ and infiltrated with 80Bi₂O₃·20( x CaO–(1 – x )BaO) at 1300°C for various times in air. When the value of x was 0, 0.2, 0.7, and 1.0, liquid film migration occurred, which formed a new solid-solution layer containing solute species. On the other hand, when x = 0.5, no liquid film migration was observed. The effective dielectric constant was the highest in the sample with x = 0.5 (no liquid film migration), and it decreased as the migration distance increased. In addition, the loss tangent was the lowest, <1%, in the sample with x = 0.5. Agreement between the estimated effective dielectric constants and the measured values showed that the suppression of liquid film migration improved the dielectric constant. The agreement further indicated that the prediction of the dielectric constant in SrTiO₃-based BLCs was possible using a two-layer model with a liquid-infiltrated layer and a SrTiO₃-based oxidized layer.     

Effect of Bottom Electrode Materials and Annealing Treatments on the Electrical Characteristics of Ba0.47Sr0.53TiO3 Film Capacitors

The dielectric constant and leakage current density of Ba_0.47Sr_0.53TiO₃ (BST) thin films deposited by radio-frequency magnetron sputtering on various bottom electrode materials (Pt, Ir, IrO₂/Ir, Ru, RuO₂/Ru) before and after annealing in O₂ and N₂ ambient were investigated. Improvement in crystallinity of BST films deposited on various bottom electrodes was observed with annealing. The refractive index, dielectric constant, loss tangent, and leakage current of the films were also strongly dependent on annealing conditions. A BST thin film deposited on an Ir bottom electrode at 500°C, after 700°C annealing in O₂ for 20 min, had a dielectric constant of 593 ± 5%, a loss tangent of 0.019 ± 10% at 100 kHz, a leakage current of (2.1 ± 13%) 10⁻⁸ A/cm² at an electric field of 100 kV/cm with a delay time of 30 s, and a charge storage density of 53 ± 5% fC/μm² at an applied field of 150 kV/cm. Based on the dielectric constant, leakage current, and reliability, the optimum material for the bottom electrode with annealing was Ir. Interdiffusion of Ru and Ti at the interface between the BST film and Ru electrode was observed in 500°-700°C annealed samples. The 10 year lifetime of time-dependent dielectric breakdown (TDDB) studies indicated that BST on Pt, Ir, IrO₂/Ir, Ru, and RuO₂/Ru had long lifetime over 10 years of operation at a voltage bias of 1 V.     

Hydrothermal Strontium Titanate Films on Titanium: An XPS and AES Depth-Profiling Study

SrTiO₃ films in the 100-nm thickness range were grown on Ti foils by a 2–4 h hydrothermal treatment at 225°C in an aqueous solution of Sr(OH)₂. X-ray diffraction showed the principal reflections of cubic SrTiO₃. Photoelectron analysis revealed the presence of carbon contamination and suggested the presence of hydroxyl groups on the surface, which disappeared in the film bulk. The Auger depth profiles showed three distinct zones. The outermost layer, besides the common carbon contamination, had a large number of OH groups. The second zone was a clean, almost carbon-free SrTiO₃ film. The third zone was a diffuse interface region where the Sr and O concentrations slowly decreased toward the substrate.