Calcium Titanate Thin Films Research Articles

Role of Ca doping on oxygen vacancy production in modulating dielectric, ferroelectric and magnetic polarization in BaTiO3 thin films

Chemical bath deposition, being an application oriented electroless technique, is used for the deposition of calcium doped barium titanate (Ba1-xCaxTiO3) thin films using glass substrate. Ca concentration is varied as x = 0.0, 0.02, 0.04, 0.06, 0.08, 0.10 and 0.12. As-deposited thin films show amorphous behavior, whereas, annealed thin films show crystalline nature. X-ray diffraction (XRD) analysis of annealed thin films shows a pure tetragonal phase of Perovskite barium calcium titanate thin films up to x = 0.08 dopant concentration, while XRD peak corresponding to CaO is observed at x = 0.10 and 0.12. Maximum optical transmission of 65% is observed under undoped and as-deposited conditions. Annealed thin films show 83% transmission under undoped condition. While decrease in transmission is observed with the increase of dopant concentration due to the presence of sub bands. High value of dielectric constant (i.e. ∼3800 at log f = 1.3) and low value of tangent loss (i.e.∼0.15 at log f = 1.3) is observed for x = 0.08 dopant concentration. Cole–Cole plots reveal the presence of two electrically active grains and grain boundary regions. Maximum value of spontaneous polarization Pmax (18.46 μC/cm2) is observed for x = 0.08 dopant concentration. Mixed para-ferromagnetic response is observed under undoped condition. S-shaped magnetic response is observed for x = 0.02–0.04 whereas, ferromagnetic curves are observed for x = 0.06–0.12. It is important to mention here that tetragonal Perovskite calcium doped barium titanate thin films with U-shaped dielectric constant and good ferroelectric properties are hard to find in literature.

Tuning bipolar resistive switching by forming defect dipoles in A-site-deficient perovskite calcium titanate thin films

If we intentionally make the A-site in an ABO3 perovskite structure deficient, it is possible to tune the resistive switching effect by forming defect dipoles. In this study, an A-site-deficient calcium titanate (Ca0.95TiO3, CTO) thin film was fabricated on a Pt/Ti/SiO2/Si substrate as an active layer for resistive random-access memory. The Pt/CTO/Pt device exhibited stable bipolar resistive switching performance with good endurance and long retention. The resistive switching may be attributable to the formation and rupture of the conduction filaments due to the O vacancies and defect dipoles resulting from the interaction between the Ca and O vacancies.

The Investigation of E-beam Deposited Titanium Dioxide and Calcium Titanate Thin Films

Thin titanium dioxide and calcium titanate films were deposited using electron beam evaporation technique. The substrate temperature during the deposition was changed from room temperature to 600 °C to test its influence on TiO2 film formation and optical properties. The properties of CaTiO3 were investigated also. For the evaluation of the structural properties the formed thin ceramic films were studied by X-ray diffraction (XRD), energy dispersive spectrometry (EDS), scanning electron microscopy (SEM) and atomic force microscopy (AFM). Optical properties of thin TiO2 ceramics were investigated using optical spectroscope and the experimental data were collected in the ultraviolet-visible and near-infrared ranges with a step width of 1 nm. Electrical properties were investigated by impedance spectroscopy.It was found that substrate temperature has influence on the formed thin films density. The density increased when the substrate temperature increased. Substrate temperature had influence on the crystallographic, structural and optical properties also. DOI: http://dx.doi.org/10.5755/j01.ms.19.3.1805

Characterization of calcium titanate thin films deposited on titanium with reactive sputtering and pulsed laser depositions

In the present study, characteristics of calcium titanate thin films deposited on titanium by reactive sputtering and pulsed laser deposition techniques were investigated. In both techniques, a calcium titanate target was used as a deposition source, and the titanium substrate was heated at 873 K during the deposition. The oxygen flow for the reactive sputtering was in the range of 1 to 10 sccm, and the oxygen pressure for the pulsed laser deposition was in the range of 0.13 to 13 Pa. The deposited films were crystallized into perovskite-type calcium titanate; furthermore, a titanium-dioxide layer formed in the interface between the film and substrate. In the film deposited by reactive sputtering with low oxygen flow, titanium-to-calcium ratio ([Ti]/[Ca]) is lower than that of stoichiometric calcium titanate due to the formation of calcium hydroxide. The ratio increases with an increase of oxygen flow, and the ratio of the film deposited with a 10-sccm oxygen flow was almost in accordance with that of stoichiometric calcium titanate. On the other hand, in the pulsed laser deposition, [Ti]/[Ca] ratios of the deposited film were almost in accordance with that of stoichiometric calcium titanate at the oxygen pressure under the present experimental condition. In both deposition techniques, the thickness of the titanium-oxide layer increased with an increase of the amount of oxygen gases. The results indicate that the pulsed laser deposition has an advantage for the preparation of the stoichiometric calcium titanate film without formation of a thick titanium-oxide layer.

Leakage current and relaxation characteristics of highly (111)-oriented lead calcium titanate thin films

Highly (111)-oriented (Pb0.76Ca0.24)TiO3 (PCT) thin films were grown on Pt/Ti/SiO2/Si substrates by a sol–gel process. The Au/PCT/Pt metal–insulator–metal film capacitor showed well-saturated hysteresis loops at an applied field of 800 kV/cm with remanent polarization (Pr) and coercive electric field (Ec) values of 18.2 μC/cm2 and 210 kV/cm, respectively. The leakage current depended on the voltage polarity. At low electrical field and with Pt electrode biased negatively, the Pt/PCT interface exhibits a Schottky emission characteristics. The Au/PCT interface forms an ohmic contact. The conduction current when the Au electrode is biased negatively shows a space-charge-limited behavior. The dielectric relaxation current behavior of Au/PCT/Pt capacitor obeys the well-known Curie–von Schweidler law at low electric field. At higher fields, the currents have contributions to both dielectric relaxation current and leakage current.

Electrical properties of compositionally graded lead calcium titanate thin films

The compositional gradient (Pb 1− x Ca x )TiO 3 (PCT) thin films of x=0 and 24 mol% were fabricated on Pt/Ti/SiO 2/Si substrates by using a modified sol–gel process. The final structure consists of two layers, up-graded films starting from PbTiO 3 on the Pt electrode to the top PCT(24) layer, whereas films with opposite gradient are called down-graded films. Highly (100)-oriented graded PCT thin films have been obtained when the graded PCT thin films were annealed at 600 °C. At 100 kHz, the dielectric constants were 79 and 128, respectively, for the up-graded and down-graded PCT films. The pyroelectric coefficient ( p) of the graded films was measured by a dynamic technique. At room temperature, the values of p and figure-of-merits ( F D) of the up-graded and down-graded PCT films are 95 μC/m 2 K and 9.1×10 −6 Pa −0.5, 112 μC/m 2 K and 8.6×10 −6 Pa −0.5, respectively.

Oxygen Metastable Defects in Calcium Titanate Thin Films

Metastable defects of oxygen in calcium titanate thin films with perovskite structure were studied. Thin films of calcium titanate were deposited by a magnetron sputtering method. The oxygen defects were evaluated, using the results of oxygen diffusion experiments. The thin film process had tendencies of creation of a large amount of oxygen point defects and the wide range of non equilibrium solid solution at room temperature. The crystal distortion was increasing with deviation from the stoichiometric composition. Although the metastable defects were decreasing due to the annealing, annealed samples had more oxygen defects in a few magnitude orders than single crystals.

Seeding acceleration of perovskite crystallization in lead calcium titanate thin films

Seeding to enhance the kinetics of phase transformation was demonstrated with a special seeding process using crystallized ultrafine lead titanate powder to lower the temperature of perovskite formation of the modified lead calcium titanate thin films. X-ray diffraction and scanning electron microscopy show that the pyrochlore-to-perovskite phase transformation temperature of the seeded lead calcium titanate film can be lowered by as much as 50°C when compared with the unseeded lead calcium titanate film. This is most likely related to the decrease of activation energies of nucleation which, in turn, increases the kinetics of the perovskite formation, since numerous nucleation sites are provided. The similarity, or small lattice mismatch of crystal structure between the lead titanate and perovskite of the modified lead calcium titanate film supports these explanations.