Pulsed-Laser Deposition of Barium Titanate Thin Films

Barium titanate (BaTiO₃) thin films have been deposited on single-crystal magnesia (MgO) substrates by pulsed-laser ablation. Temperature dependence of capacitance measurements show a peak of 110°C, indicative that a ferroelectric phase transition has occurred at this temperature. This value is lower than that determined for single-crystal BaTiO₃, but consistent with that found for BaTiO₃ thin films produced by other methods. It has been demonstrated that the microstructure of the films can be varied by changes in the deposition parameters. Optimizing these variations permits the formation of thin films with controlled microstructures and properties.     

Formation of Monoclinic Hafnium Titanate Thin Films Via the Sol–Gel Method

Hafnium titanate films are generating increasing interest because of their potential application as high- k dielectrics materials for the semiconductor industry. We have investigated sol–gel processing as an alternative route to obtain hafnium titanate thin films. Hafnia-titania films of different compositions have been synthesized using HfCl₄ and TiCl₄ as precursors. The HfO₂–TiO₂ system composition with 50 mol% of TiO₂ and 50 mol% of HfO₂ has allowed the formation of a hafnium titanate film after annealing at 1000°C. The films exhibited a homogeneous nanocrystalline structure and a monoclinic hafnium titanate phase that has never been obtained before in thin films. The films resulted in the formation of homogeneously distributed nanocrystals with an average size of 50 nm. Different compositions, with higher or lower hafnia contents, produced anatase crystalline films after annealing at 1000°C.     

Raman Spectroscopy of Ferroelectric Thin Films and Superlattices

Recent results of Raman spectroscopy studies of lattice dynamics and phase transitions in ferroelectric thin films and superlattices are reviewed. Raman studies of SrTiO₃, BaTiO₃, and Ba _x Sr_{1− x} TiO₃ thin films in comparison with corresponding single crystals are presented; essential differences in the lattice dynamics behavior of thin films and single crystals are discussed. Application of ultraviolet Raman spectroscopy for studies of nanoscale ferroelectric heterostructures, such as BaTiO₃/SrTiO₃ superlattices, is demonstrated.     

Gold-Particle-Enhanced Crystallite Growth of Thin Films of Barium Titanate Prepared by the Sol-Gel Process

The effects of gold-particle incorporation on the structure of barium titanate (BaTiO₃) thin films was investigated. BaTiO₃ thin films (110 nm thick) with different gold concentrations were prepared via sol-gel processing and analyzed using X-ray diffractometry. The diffraction results showed that the crystal structure of the thin films changed from predominantly the cubic phase to the tetragonal phase, and the crystallite size increased as the gold concentration increased. Possible mechanisms for the gold-enhanced crystallization have been discussed.     

Fabrication and Characterization of Pyroelectric Ba0.8Sr0.2TiO3 Thin Films by a Sol-Gel Process

A sol-gel process was used to prepare pyroelectric Ba_0.8Sr_0.2TiO₃ thin films with large columnar grains (100–200 nm in diameter) on Pt/Ti/SiO₂/Si substrates, via using a 0.05 M solution precursor. The relationship between dielectric constant and temperature (ɛ_r- T ) showed two distinctive phase transitions in the Ba_0.8Sr_0.2TiO₃ thin films. Both the remnant polarization and the coercive field decreased as the temperature increased from −73°C to 40°C. Its low dissipation factor (tan δ= 2.6%) and high pyroelectric coefficient ( p = 4.6 × 10⁻⁴ C·(m²·K)⁻¹ at 33°C), together with its good insulating properties, made the prepared Ba_0.8Sr_0.2TiO₃ thin films promising for use in uncooled infrared detectors and thermal imaging applications.     

Nonepitaxial Orientation in Sol–Gel Bismuth Titanate Films

Control over crystallographic orientation in ceramic thin films is important for highly anisotropic structures. Layered perovskites, like Bi₄Ti₃O₁₂, have interesting properties associated with their ferroelectric nature, which may be fully exploited only when films are highly textured. Textured films of this titanate were fabricated via a sol–gel technique without using epitaxial growth. Orientation in the film is confirmed by XRD and SEM, and supported by refractive index and dielectric measurements. In an attempt to explain the orienting effect, light scattering experiments were conducted to yield information about the molecular size, shape, and conformation of macromolecules as the sol–gel solution ages and condensation reactions proceed. These experiments clearly show an increase in the size of molecular clusters with time. We believe that it is the organization of these large clusters during spin coating, and the relationship of the backbone chemistry to the crystal structure of Bi₄Ti₃O₁₂, that are responsible for the observed orientation.     

New Phase Transitions in Perovskite Oxides: BiFeO3, SrSnO3, and Pb(Fe2/3W1/3)1/2Ti1/2O3

This paper reports on two new aspects of oxide perovskites: the first part is devoted to new phase transitions, especially at high temperatures; multiferroics such as BiFeO₃ and Pb–Fe–W–titanate are emphasized but nonmagnetic materials such as SrSnO₃ are included. The work summarized on bismuth ferrite emphasizes its metal–insulator transition near 1200 K (atmospheric pressure) and 47 GPa (room temperature); that on SrSnO₃ emphasizes order–disorder phase transitions; and that on lead–iron tungstate–titanate exhibits a classic second-order ferroelectric phase transition, of which rather few are known in the literature. The second part of the paper presents a discussion of constant phase elements for oxide perovskite ceramics; this is a modern way of characterizing their dielectric relaxation, particularly near phase transition temperatures.     

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.     

Single-Crystal Ferrite Films Prepared by Cathodic Sputtering

Single-crystal thin films were prepared by sputtering in a glow discharge system from a ceramic cathode of nickel ferrite onto a magnesium oxide substrate cleaved parallel to the (100) plane. X-ray and electron diffraction patterns obtained from the films yield d values which are consistent with values for nickel ferrite spinel. Chemical analysis shows that the iron-to-nickel atom ratio is approximately 2:1 as in NiFe₂O₄. Magnetic measurements also indicated that the films were single-crystal but the values obtained differ from those of bulk nickel ferrite.     

Textured Magnetoplumbite Fiber-Matrix Interphase Derived from Sol-Gel Fiber Coatings

A calcium hexaluminate (CaAl₁₂O₁₉, magnetoplumbite structure) sol was used to coat alumina and yttrium-aluminum-garnet (YAG) single-crystal fibers and single-crystal alumina plates. When the coated substrates were either annealed or hot-pressed in polycrystalline alumina and YAG matrices, the calcium hexaluminate basal cleavage planes were aligned parallel with the fiber-matrix interface. A complex series of reactions and phase transformations contributed to texture formation on alumina substrates. The alumina fibers and plates seeded the phase transformation of sol-derived transition aluminas to α-Al₂O₃. CaAl₁₂O_l9 and CaAl₄O₇ formed between the seeded α-Al₂O₃, and CaAl₄O₇ later reacted with the seeded α-Al₂O₃ to form CaAl₁₂O₁₉, resulting in a single-phase coating. Several different mechanisms may be responsible for the texture. The microstructure, phase evolution, and possible mechanisms for texture formation of CaAl₁₂O₁₉ powders, sol-derived thin films, and coated plates and fibers, with and without hot-pressed matrices, were studied and are discussed. Deflection and propagation of cracks within the fiber-matrix interphase in thin foils suggests that such an inter-phase may protect fibers from matrix cracks.     

Low-Temperature Crystallization of Sol-Gel Processed Pb0.5Ba0.5TiO3: Powders and Oriented Thin Films

A novel sol-gel process suitable for depositing thin-film lead barium titanate has been developed. X-ray diffraction analysis showed perovskite phase crystallization to occur at a temperature as low as 400°C with single-phase Pb_0.5Ba_0.5TiO₃ (PBT) resulting at a temperature as low as 500°C. Small concentrations of barium carbonate were evident by X-ray diffraction at 400°C, and indications of minor, carbonate-containing phases were evident by FTIR at 600°C. Deposition of the sol by spin coating on single-crystal and thin-film MgO on silicon resulted in highly oriented PBT films after calcination at 600°C. Mixed (100)/(001) films were obtained on single-crystal MgO, whereas entirely (100) films were obtained on thin-film MgO.     

Sol—Gel Fabrication of Pb(Zr0.52 Ti0.48)O3 Thin Films Using Lead Acetylacetonate as the Lead Source

Lead zirconium titanate (PZT) thin films of the morphotropic phase boundary composition [Pb(Zr_0.52Ti_0.43)O₃] were deposited on platinum-coated silicon by a modified sol-gel process using lead acetylacetonate as the lead source. The precursor solution for spin coating was prepared from lead acetylacetonate, zirconium n -butoxide, and titanium isopropoxide. The use of lead acetylacetonate instead of the widely used lead acetate trihydrate provided more stability to the PZT precursor solution. Films annealed at 700°C for 12 min formed well-crystallized perovskite phase of Pb(Zr_0.52Ti_0.48)O₃. Microstructures of these films indicated the presence of submicrometer grains (0.1 to 0.2 μm). The dielectric constant and loss values of these films measured at 10 kHz were approximately 1200 and 0.04, respectively, while the remanent polarization and coercive field were ∼ 13 μC/cm² and ∼ 35 kV/cm. Aging of the solution had almost no effect on the dielectric and ferroelectric properties of these films.     

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.     

Characterization and Tribological Investigation of Sol–Gel Titania and Doped Titania Thin Films

Titania (TiO₂) and doped TiO₂ ceramic thin films were prepared on a glass substrate by a sol–gel and dip-coating process from specially formulated sols, followed by annealing at 460°C. The morphologies of the original and worn surfaces of the films were analyzed with atomic force microscopy (AFM) and scanning electron microscopy. The chemical compositions of the obtained films were characterized by means of X-ray photoelectron spectroscopy (XPS). The tribological properties of TiO₂ and doped TiO₂ thin films sliding against Si₃N₄ ball were evaluated on a one-way reciprocating friction and wear tester. The AFM analysis shows that the morphologies of the resulting films are very different in nanoscale, which partly accounts for their tribological properties. XPS analysis reveals that the doped elements exist in different states, such as oxide and silicate, and diffusion took place between the film and the glass substrate. TiO₂ films show an excellent ability to reduce friction and resist wear. A friction coefficient as low as 0.18 and a wear life of 2280 sliding passes at 3 N were recorded. Unfortunately, all the doped TiO₂ films are inferior to the TiO₂ films in friction reduction and wear resistance, primarily because of their differences in structures and chemical compositions caused by the doped elements. The wear of the glass is characteristic of brittle fracture and severe abrasion. The wear of the TiO₂ thin film is characteristic of plastic deformation with slight abrasive and fatigue wear. The doped TiO₂ thin films show lower plasticity than the TiO₂ thin film, which leads to large cracks. The propagation of the cracks caused serious fracture and failure of the films.     

Microstructure and Electrical Properties of MgAl2O4 Thin Films for Humidity Sensing

Active elements for humidity sensors based upon MgAl₂O₄ thin films or sintered pellets were investigated. Thin films were deposited on Si/SiO₂ substrates by radiofrequency (rf) sputtering. Sintered MgAl₂O₄ pellets were prepared by traditional ceramic processing. Scanning electron microscopy (SEM) analysis showed that the thin films were rather dense and homogeneous, made up of clustered particles of about 20–30 nm, while the pellets showed a wide pore-size distribution. X-ray photoelectron spectroscopy (XPS) demonstrated that the thin films have a stoichiometry close to that of MgAl₂O₄. Sintered MgAl₂O₄ is crystalline, while it is disordered in thin-film form. The presence of two different components of the Al 2 p peaks was correlated with the structural difference between pellets and thin films. The relationship between good film–substrate adhesive properties and the chemical composition at the interface was studied. The electrical properties of the sensing elements were studied at 40°C in environments at different relative humidity (RH) values between 2% and 95%, using ac impedance spectroscopy. MgAl₂O₄ thin films showed interesting characteristics in terms of their use in humidity-measurement devices. Resistance versus RH sensitivity values showed variations as high as 4 orders of magnitude in the RH range tested for thin films, and 5 orders of magnitude for pellets. The differences in the electrical behavior of MgAl₂O₄ pellets and thin films were correlated with their different microstructures.     

Preparation of a Monodispersed Suspension of Barium Titanate Nanoparticles and Electrophoretic Deposition of Thin Films

A transparent and stable monodispersed suspension of nanocrystalline barium titanate was prepared by dispersing a piece of BaTiO₃ gel into a mixed solvent of 2-methoxyethanol and acethylacetone. The results of high-resolution transmission electron microscopy (HR-TEM) and size analyzer confirmed that the BaTiO₃ nanoparticles in the suspension had an average size of ∼10 nm with a narrow size distribution. Crystal structure characterization via TEM and X-ray diffraction indicated BaTiO₃ nanocrystallites to be a perovskite cubic phase. BaTiO₃ thin films of controlled thickness from 100 nm to several micrometers were electrophoretic deposited compactly on Pt/Ti/SiO₂/Si substrates. The deposited thin film had uniform nanostructure with a very smooth surface.     

Field-Forced Antiferroelectric-to-Ferroelectric Switching in Modified Lead Zirconate Titanate Stannate Ceramics

Electric-field-forced antiferroelectric-to-ferroelectric phase transitions in several compositions of modified lead zirconate titanate stannate antiferroelectric ceramics are studied for ultra-high-field-induced strain actuator applications. A maximum field-induced longitudinal strain of 0.85% and volume expansion of 0.95% are observed in the ceramic composition Pb_0.97La_0.02(Zr_0.66Ti_0.09Sn_0.25)O₃ at room temperature. Switching from the antiferroelectric form to the ferroelectric form is controlled by the nucleation of the ferroelectric phase from the antiferroelectric phase. A switching time of <1 μs is observed under the applied field above 30 kV/cm. The polarization and strains associated with the field-forced phase transition decrease with increasing switching cycle, a so-called fatigue effect. Two types of fatigue effects are observed in these ceramic compositions. In one, the fatigue effects only proceed to a limited extent and the properties may be restored by annealing above the Curie temperature, while in the other, the fatigue effects proceed to a large extent and the properties cannot be restored completely by heat treatment. Hydrostatic pressure increases the transition field and the switching time. But when the applied electric field is larger than the transition field, the induced polarization and strain are not sensitive to increasing hydrostatic pressure until the transition field approaches the applied field.     

In–Situ Transmission Electron Microscopy Crystallization Studies of Sol–Gel-Derived Barium Titanate Thin Films

Barium titanate (BaTiO₃) thin films that were derived from methoxypropoxide precursors were deposited onto (100) Si, Pt/Ti/SiO₂/(100) Si, and molecular-beam-epitaxy-grown (MBE-grown) (100) BaTiO₃ on (100) Si substrates by spin coating. The crystallization behavior of the amorphous-gel films was characterized using in-situ transmission electron microscopy heating experiments, glancing-angle X-ray diffraction, and differential thermal analysis/thermogravimetric analysis. Amorphous-gel films crystallized at a temperature of ∼600°C to an intermediate nanoscale (5–10 nm) barium titanium carbonate phase, presumably BaTiO₂CO₃, that subsequently transformed to nanocrystalline (20–60 nm) BaTiO₃. Random nucleation in the bulk of the gel film was observed on all substrates. In addition, oriented growth of BaTiO₃ was concurrently observed on MBE-grown BaTiO₃ on (100) Si. High-temperature decomposition of the intermediate carbonate phase contributed to nanometer-scale residual porosity in the films. High concentrations of water of hydrolysis inhibited the formation of the intermediate carbonate phase; however, these sols precipitated and were not suitable for spin coating.     

Lead Zirconate Titanate Thin Films on Base-Metal Foils: An Approach for Embedded High-Permittivity Passive Components

An approach for embedding high-permittivity dielectric thin films into glass epoxy laminate packages has been developed. Lead lanthanum zirconate titanate (Pb_0.85La_0.15(Zr_0.52Ti_0.48)_0.96O₃, PLZT) thin films were prepared using chemical solution deposition on nickel-coated copper foils that were 50 μm thick. Sputter-deposited nickel top electrodes completed the all-base-metal capacitor stack. After high-temperature nitrogen-gas crystallization anneals, the PLZT composition showed no signs of reduction, whereas the base-metal foils remained flexible. The capacitance density was 300–400 nF/cm², and the loss tangent was 0.01–0.02 over a frequency range of 1–1000 kHz. These properties represent a potential improvement of 2–3 orders of magnitude over currently available embedded capacitor technologies for polymeric packages.     

The Role of TiN in the Intergranular Phase-Forming Process in TiN-Dispersed Si3N4 Nanocomposites

Internal interfaces in two ceramic systems, monolithic Si₃N₄ (SN) and TiN-dispersed Si₃N₄ nanocomposite (STN), were characterized by analytical transmission electron microscopy (TEM). In monolithic SN both MgO and Y₂O₃ dopants are preferentially hosted by the vitreous intergranular phase in pockets at triple grain junctions (TJ), whereas in STN composites the highest dopant concentrations were observed in grain and phase boundaries. The width of grain boundary films, as revealed by high-resolution TEM imaging, varied between ≈0.8 nm in monolithic SN and ≈1.0–1.2 nm in STN. Intergranular films with increased width ≈1.8 nm were detected in SN–TiN phase boundaries. Although no enrichment of Ti could be detected in the intergranular phase, it appears that the presence of TiN dispersants indirectly contributes to the intergranular phase formation. It is assumed that TiO₂ impurities sitting on TiN particle surfaces react with the matrix phase, resulting in a more oxidic nature of intergranular films due to increased SiO₂ supply in intergranular regions. Phase-specific Si-L_2,3 energy-loss near edge structure features, which could serve as fingerprints for phase identification, were observed in spatial-difference electron energy-loss spectra from grain boundary films and TJ pockets.