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.     

In-Plane Polarized 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 Thin Films

Ferroelectric 0.7Pb(Mg_1/3Nb_2/3)O₃–0.3PbTiO₃ (PMN-PT) thin films were deposited on ZrO₂/SiO₂/silicon substrates using a chemical-solution-deposition method. Using a thin PZT film as a seed layer for the PMN-PT films, phase-pure perovskite PMN-PT could be obtained via rapid thermal annealing at 750°C for 60 s. The electrical properties of in-plane polarized thin films were characterized using interdigitated electrode arrays on the film surface. Ferroelectric hysteresis loops are observed with much larger remanent polarizations (∼24 μC/cm²) than for through-the-thickness polarized PMN-PT thin films (10–12 μC/cm²) deposited on Pt/Ti/Si substrates. For a finger spacing of 20 μm, the piezoelectric voltage sensitivity of in–plane polarized PMN-PT thin films was ∼20 times higher than that of through-the-thickness polarized PMN-PT thin films.     

Ferroelectric Thin Films of Bismuth-Containing Layered Perovskites: Part I, Bi4Ti3O12

Ferroelectric thin films of bismuth-containing layered perovskite Bi₄Ti₃O₁₂ have been fabricated by a metalorganic decomposition (MOD) method. Crack-free and crystalline films of ∼5000 Å thickness have been deposited on Pt/Ti/SiO₂/Si substrates. Different heat treatments have been studied to investigate the nucleation and growth of perovskite Bi₄Ti₃O₁₂ crystallites. If the same composition and final annealing temperature are used, films with different orientations are obtained by different heating schedules. These films show a large anisotropy in ferroelectric properties. Theoretical considerations are presented to suggest that nucleation control is responsible for texture and grain-size evolution. Moreover, the origin of the ferroelectric anisotropy is rooted in the two-dimensional nature of layered polarization.     

Ferroelectric and Dielectric Properties of Pb(Mg1/3Ta2/3)0.7Ti0.3O3 Thin Films Derived from RF Magnetron Sputtering

Pb(Mg_1/3Ta_2/3)_0.7Ti_0.3O₃ thin films of single perovskite phase were successfully synthesized by using the RF sputtering deposition technique, followed by post-thermal annealing. While the perovskite structure of Pb(Mg_1/3Ta_2/3)_0.7Ti_0.3O₃ is rather unstable, phase evolution in the thin films was manipulated by controlling both working pressure during the sputtering process and post-thermal annealing temperature. The desirable perovskite phase was promoted by increasing the working pressure in the range of 10–25 mTorr, followed by thermal annealing at 600°C. The ferroelectric, dielectric, and polarization behaviors of Pb(Mg_1/3Ta_2/3)_0.7Ti_0.3O₃ films were characterized over a wide range of frequencies. They are strongly affected by the film thickness, where the relative permittivity and remanent polarization increase, while the coercive field decreases with increasing film thickness in the range of 115–360 nm.     

Substrate Effect on the Magnetoelectric Behavior of Pb(Zr0.52Ti0.48)O3 Film-On-CoFe2O4 Bulk Ceramic Composites Prepared by Direct Solution Spin Coating

Magnetoelectric (ME) composite structures of Pb(Zr_0.52Ti_0.48)O₃ (PZT) and CoFe₂O₄ (CFO) were prepared by directly growing PZT films on highly dense CFO ceramics via a simple solution spin coating, rather than by conventional high-temperature cofiring. An obvious ME response, which had the same bias-dependent trend as the piezomagnetic coefficient of CFO ceramics, was observed in such film-on-bulk ceramic composites. It was found that the PZT films showed a good ferroelectric feature, and the ME response of the composites strongly depended on the resistivity of the CFO ceramics as both a substrate and a bottom electrode. The results suggest plenty of room for further enhancing the ME response of such films-on-ceramic substrate composites.     

Chemical Solution Deposition of Transparent and Metallic La0.5Sr0.5TiO3+x/2 Films Using Topotactic Reduction

Metallic and transparent La_0.5Sr_0.5TiO_{3+ x /2} films were prepared by the chemical solution deposition (CSD) method using topotactic reduction processing. The use of Si powder as the reducing agent was facile and allowed easy manipulation. It was observed that metallic (resistivity at 300 K ∼2.43 mΩ cm) and transparent (∼80% transmittance at visible light) La_0.5Sr_0.5TiO_{3+ x /2} films could be obtained with an annealing temperature of 900°C, which was significantly lower than the hydrogen reduction temperature (∼1400°C). The successful preparation of metallic and transparent La_0.5Sr_0.5TiO_{3+ x /2} films using CSD has provided a feasible route for depositing other perovskite-structured functional layers on La_0.5Sr_0.5TiO_{3+ x /2} films using this low-cost all CSD method.     

Pressure-Induced Phase Transformation of Controlled-Porosity Pb(Zr0.95Ti0.05)O3 Ceramics

Chemically prepared Pb(Zr_0.95Ti_0.05)O₃ (PZT 95/5) ceramics were fabricated with a range of different porosity levels, while grain size was held constant, by systematic additions of added organic pore former (Avicel). Use of Avicel in amounts ranging from 0 to 4.0 wt% resulted in fired ceramic densities that ranged from 97.3% to 82.3%. Hydrostatic-pressure-induced ferroelectric (FE) to antiferroelectric (AFE) phase transformations were substantially more diffuse and occurred at lower hydrostatic pressures with increasing porosity. An ∼12 MPa decrease in hydrostatic transformation pressure per volume percent added porosity was observed. The decrease in transformation pressure with decreasing density was quantitatively consistent with the calculated macroscopic stress required to achieve a specific volumetric macrostrain (0.40%). This strain was equivalent to experimentally measured macrostrain for FE-to-AFE transformation. The macroscopic stress levels were calculated using measured bulk modulus values that decreased from 84 to 46 GPa as density decreased from 97.3% to 82.3%. Good agreement between calculated and measured values of FE-to-AFE transformation stress was obtained for ceramics fired at 1275° and 1345°C.     

Long-Time Relaxation from Relaxor to Normal Ferroelectric States in Pb0.91 La0.06 (Zr0.65 Ti0.35 O3

Pb_0.91 La_0.06 (Zr_0.65 Ti_0.35)O₃ (PLZT 6/65/35) is a relaxor ferroelectric near and above the temperature of the dielectric maximum (∼180°C). The relaxor state can persist to room temperature upon fast cooling. However, this relaxor state gradually changes to a normal ferroelectric over a long time period at 25°C, characterized by an elimination of relaxor-like dielectric dispersion and a significant rhombo-hedral broadening and subsequent splitting of the (220) X-ray diffraction peak. A transmission electron microscopy (TEM) bright-field image of a long-time relaxed sample revealed normal micrometersized ferroelectric domain contrast with relaxor-like "tweed" structure on the submi-crometer scale. The gradually structural evolution is discussed in terms of development of correlations between relaxor polar clusters.     

Structure Development Study of Pb (Zr, Ti)O3 Thin Films by an Optical Method

Lead zirconate titanates (Pb (Zr_xTi_1–x)O₃ or PZT) are well-known materials with useful ferroelectric properties for nonvolatile memory applications. The device applications usually require processes which form the ferroelectric perovskite phase at low temperatures. Understanding the various aspects of structure development of PZT films is the key to develop low-temperature processes. An effective, versatile, and nondestructive optical method was developed for the study of the structure development in PZT films. Also, models for the structure development were proposed and were verified by this optical method. Using this method, the characteristic temperatures at which both pyrochlore phase and perovskite phase are initiated and completed were identified. The pyrochloreperovskite transformation was initiated at lower temperature and completed faster for the films on the titanium-rich side than those on the zirconium-rich side. In addition, the volume fractions of the perovskite phase were obtained as a function of annealing temperature by this optical method.     

Titanium Diffusion into (K0.5Na0.5)NbO3 Thin Films Deposited on Pt/Ti/SiO2/Si Substrates and Corresponding Effects

Lead-free (K_0.5Na_0.5)NbO₃ (KNN) thin films were prepared on Pt/Ti/SiO₂/Si substrates by a sol–gel processing method, and titanium diffusion from the substrates into the KNN films under different thermal treatment conditions were investigated by the secondary ion mass spectroscopy depth profile and X-ray photoelectron spectroscopy surface analysis. Titanium diffusion was evident in all the KNN thin films, which was further aggravated not only by increasing the annealing temperature, but also surprisingly by higher ramping rate attributed to the resulting larger grain boundaries. The pronounced effects of the titanium diffusion and the resulting substitution of Ti⁴⁺ for Nb⁵⁺ with different valence states on the composition, structure, and electrical properties of the KNN thin films are analyzed and discussed. The results showed that the Ti diffusion from the substrate played a crucial role in affecting the structure and electrical properties of the ferroelectric KNN thin films deposited on Pt/Ti/SiO₂/Si substrate.     

Effect of Carrier Gas Species on Ferroelectric Properties of PZT/Stainless-Steel Fabricated by CO2 Laser-Assisted Aerosol Deposition

Pb(Zr,Ti)O₃ (PZT) thick films were deposited onto stainless-steel substrates by aerosol deposition using different kinds of carrier gases and were irradiated by CO₂ laser both during and after deposition, for the recovery of ferroelectricity. The ferroelectric and dielectric properties of PZT films deposited using oxygen and nitrogen gases and irradiated by CO₂ laser were superior to those of films deposited using He gas and irradiated by the laser. Some kinds of defects within the film were relaxed by CO₂ laser irradiation during deposition, and grain growth in the film was promoted by post-annealing using laser irradiation.     

PbTiO3–PbO–SiO2 Glass-Ceramic Thin Film by a Sol–Gel Process

PbTiO₃(PT)-PbO-SiO₂ glass-ceramic thin films were pro-duced by a sol-gel process. The crystallization of PT oc-curred at ∼700°C and was higher than that in PT-PbO-B₂ O₃ sol-gel glass-ceramics. A pinhole-free thin film was obtained by a rapid thermal annealing process when the designed glass-forming phase content in the thin film was >24 vol%. The measured dielectric constants of the films fairly agreed with the predicted values, based on a parallel mixing model. The dielectric constant was 219 and the di-electric loss was 0.04 in the 0.6PT-0.4(PbO-SiO₂) film that was fired at 700°C.     

The Optical Properties and Band Gap Energy of Nanocrystalline La0.4Sr0.6TiO3 Thin Films

The effects of microstructure on the optical properties of La_0.4Sr_0.6TiO₃ thin films were investigated. Dense films with the thickness of ∼200 nm and grain size 14–30 nm were produced on monocrystalline sapphire substrates by using a polymeric precursor spin coating technique at annealing temperatures under 800°C. X-ray data showed the formation of a single-phase cubic perovskite-type structure similar to undoped SrTiO₃ for annealing temperatures >500°C. The results of optical measurements showed that the optical spectra varied with the change of the grain size. From these data, the absorption coefficients were calculated and the band gap energy determined. In agreement with the quantum confinement model, it was shown that the band gap energy increased as the grain size decreased.     

Dielectric Properties of Pb0.97La0.02(Zr0.87−xSnxTi0.13)O3 Thin Films with Compositions near the Morphotropic Phase Boundary

Pb_0.97La_0.02(Zr_{0.87− x} Sn _x Ti_0.13)O₃ (PLZST, x =0.27, 0.17, 0.07)) thin films with the compositions in ferroelectric rhombohedral (FE_R) region, near the morphotropic phase boundary (MPB), were deposited on the Pt-electroded silicon (PtSi) substrates by the sol–gel process. The phase structure and surface morphology of PLZST thin films were analyzed by XRD and SEM, respectively. The dc electric field and temperature-dependent dielectric properties of the PLZST thin films were investigated in detail. The results indicated that the dielectric constant, remnant polarization, and the Curie temperature ( T _c) of PLZST films were elevated with the decrease of Sn content. Hence, the larger dielectric tunability (τ) was obtained for PLZST thin films with x =0.07, and the maximum τ value was 78.1%.     

Effects of Residual Stress on the Electrical Properties of PZT Films

The effects of the residual stress (either compressive or tensile) induced during the heat-treatment process on the electrical properties of Pb(Zr_0.52Ti_0.48)O₃ (PZT) films were investigated. The PZT films were deposited on platinized silicon substrates by the rf-magnetron sputtering method using a single oxide target. After their deposition, the films were bent elastically by means of a specially designed fixture during the annealing process. Residual stress was induced in the film by removing the substrate from the fixture after annealing. The ferroelectric and piezoelectric properties of the films were markedly changed by the residual stresses; the remnant polarization ( P _r) and saturation polarization ( P _sat) increased when a compressive stress was induced. On the other hand, the piezoelectric properties increased when a tensile stress was induced in the film.     

Gas-Sensing Properties of Spinodally Decomposed (Ti,Sn)O2 Thin Films

The gas-sensing properties of spinodally decomposed (Ti,Sn)O₂ thin films on sapphire substrates were investigated for CO, C₃H₈, and C₂H₅OH, and hydrogen gases at a temperature of 500°C. The variation in the d -spacing of the (101) plane of (Ti_0.5Sn_0.5)O₂ films showed behavior that was typical of spinodal decomposition during annealing at a temperature of 900°C. Transmission electron micrographs of the spinodally decomposed (Ti,Sn)O₂ films on sapphire (0112) substrates revealed the characteristic modulated structure. The modulated lamella microstructure consisted of TiO₂- and SnO₂-rich regions at intervals of ∼10 nm. The films were very sensitive to hydrogen gas and revealed anisotropic electrical conduction that was influenced by the modulated microstructure, which is characteristic of spinodal decomposition.     

Structural Variation of the BaTi4O9 Thin Films Grown by RF Magnetron Sputtering

BaTi₄O₉ thin films were grown on a Pt/Ti/SiO₂/Si substrate using rf magnetron sputtering and the structure of the thin films were then investigated. For the films grown at low temperature (≤350°C), an amorphous phase was formed during the deposition, which then changed to the BaTi₅O₁₁ phase when the annealing was conducted below 950°C. However, when the annealing temperature was higher than 950°C, a BaTi₄O₉ phase was formed. On the contrary, for the films grown at high temperature (>450°C), small BaTi₄O₉ grains were formed during the deposition, which grew during the annealing. The homogeneous BaTi₄O₉ thin films were successfully grown on Pt/Ti/SiO₂/Si substrate when they were deposited at 550°C and subsequently rapid thermal annealed at 900°C for 3 min.     

BiScO3 Doped (Na0.5K0.5)NbO3 Lead-Free Piezoelectric Ceramics

Lead-free potassium sodium niobate-based piezoelectric ceramics (1− x )(Na_0.5K_0.5)NbO₃– x BiScO₃ (KNN–BS) ( x =0∼0.05) have been prepared by an ordinary sintering process. Single perovskite phase of KNN–BS exhibits an orthorhombic symmetry at x <0.015 and pseudocubic symmetry at x >0.02, separating by a MPB at 0.015≤ x ≤0.02. Piezoelectric and ferroelectric properties are significantly enhanced in the MPB, which are as follows: piezoelectric constant d ₃₃=203 pC/N, planar coupling coefficient k _p=0.36, remnant polarization P _r=24.4 μC/cm². These solid solution ceramics look promising as a potential lead-free candidate materials.     

Physical and Dielectric Properties of (1–x)PbZrO3·xBaTiO3 Thin Films Prepared by Chemical Solution Deposition

Physical and dielectric properties of (1– x )PbZrO₃· x BaTiO₃ thin films prepared by a chemical coating process have been investigated as a function of BaTiO₃ ( x ) content (0≤ x ≤0.2). Changing the molar ratio between propylene glycol and water prior to the deposition optimized the chemical precursors. (1– x )PbZrO₃- x BaTiO₃ thin films that contained a majority of perovskite phase, but also contained large amounts of other phases, were fabricated. These films could withstand fields of 250 kV/cm at 1 kHz. The microstructure of the thin films was found to depend on the BaTiO₃ content. The phase transition from antiferroelectric to ferroelectric was gradually induced as the BaTiO₃ content increased. A maximum dielectric constant of ∼809 was obtained at the composition of x = 0.1. A maximum dielectric constant of ∼809 was obtained at the composition of x = 0.1. A thin film at the low-field antiferroelectric-ferroelectric phase boundary with x = 0.05 exhibited the highest P _sat and P _r values. The maximum values of these were 45 and 31 μC/cm², respectively.