Thickness independence of true phase transition temperatures in barium strontium titanate films

The functional properties of two types of barium strontium titanate (BST) thin film capacitor structures were studied: one set of structures was made using pulsed-laser deposition (PLD) and the other using chemical solution deposition. While initial observations on PLD films looking at the behavior of T-m (the temperature at which the maximum dielectric constant was observed) and T-c(*) (from Curie-Weiss analysis) suggested that the paraelectric-ferroelectric phase transition was progressively depressed in temperature as BST film thickness was reduced, further work suggested that this was not the case. Rather, it appears that the temperatures at which phase transitions occur in the thin films are independent of film thickness. Further, the fact that in many cases three transitions are observable, suggests that the sequence of symmetry transitions that occur in the thin films are the same as in bulk single crystals. This new observation could have implications for the validity of the theoretically produced thin film phase diagrams derived by Pertsev [Phys. Rev. Lett. 80, 1988 (1998)] and extended by Ban and Alpay [J. Appl. Phys. 91, 9288 (2002)]. In addition, the fact that T-m measured for virgin films does not correlate well with the inherent phase transition behavior, suggests that the use of T-m alone to infer information about the thermodynamics of thin film capacitor behavior, may not be sufficient. (C) 2004 American Institute of Physics.

Electrode field penetration: A new interpretation of tunneling currents in barium strontium titanate (BST) thin films

This paper shows that penetration of the applied electric field into the electrodes of a ferroelectric thin film capacitor produces both an interfacial capacitance and an effective mechanism for electron tunneling. The model predictions are compared with experimental results on Au-BST-SrRuO3 capacitors of varying thicknesses, and the agreement is excellent.

Multiferroic domain dynamics in strained strontium titanate

Multiferroicity can be induced in strontium titanate by applying biaxial strain. Using optical second harmonic generation, we report a transition from 4/mmm to the ferroelectric mm2 phase, followed by a transition to a ferroelastic-ferroelectric mm2 phase in a strontium titanate thin film. Piezoelectric force microscopy is used to study ferroelectric domain switching. Second harmonic generation, combined with phase-field modeling, is used to reveal the mechanism of coupled ferroelectric-ferroelastic domain wall motion. These studies have relevance to multiferroics with coupled polar and axial phenomena.

Surface effect ferromagnetism in pure and reduced strontium titanate

A room temperature ferromagnetic hysteresis is observed in single crystal strontium titanate substrates as purchased from several manufacturers. It was found that polishing all sides of the substrates removed this observed hysteresis, suggesting that the origin of the ferromagnetic behavior resides on the surface of the substrates. X-ray diffraction and energy dispersive x-ray spectra were measured however they were unable to detect any impurity phases. In similar semiconducting oxides it was previously suggested that ferromagnetism could originate in oxygen vacancies or from disorder within the single crystal. To this end substrates were annealed in both air and vacuum in a range of temperatures (600°C to 1100°G) to both create bulk oxygen vacancies and to heal surface damage. Annealing in vacuum was found to create a measureable number of oxygen vacancies however their creation could not be correlated to the ferromagnetic signal of the substrate. Annealing in air was found to effect the remnant moment of the substrate as well as the width of the x-ray diffraction peaks on the unpolished face, weakly suggesting a relation between surface based disorder and ferromagnetism. Argon ion bombardment was employed to create a layer of surface disorder in the polished crystal, however it was not found to induce ferromagnetism. It was found that acid etching was sufficient to remove the ferromagnetism from as purchased samples and similarly simulated handling with stainless steel tweezers was sufficient to re-create the ferromagnetism. It is suggested that the origin of this ferromagnetism in SrTi03 is surface contaminants (mainly iron).

Barium strontium titanate powders prepared by spray pyrolysis

Ultasonic spray pyrolysis (SP) has been investigated for the production of the barium strontium titanate (BST) powders from the polymeric precursors. The processing parameters, such as flux of aerosol and temperature profile inside the furnace, were optimized to obtain single phase BST. The powders were characterized by the methods of X-ray diffraction analysis, SEM, EDS and TEM. The obtained powders were submicronic, consisting of spherical, polycrystalline particles, with internal nanocrystalline structure. Crystallite size of 10 nut, calculated using Rietveld refinement, is in a good agreement with results of HRTEM. (c) 2005 Elsevier B.V. All rights reserved.

Electrical properties of lanthanum doped Bi4Ti3O12 thin films annealed in different atmospheres

Pure-and lanthanun doped Bi4Ti3O12 thin films were deposited on Pt/Ti/SiO2/Si substrate using a polymeric precursor solution. Annealing in static air and oxygen atmosphere was performed at 700 degrees C for 2 h. The obtained films were characterized by X-ray diffraction and atomic force microscopy. The dielectric constant and dissipation factor were measured in the frequency region from 1 kHz to 1 MHz. Electrical characterization of the films pointed to ferroelectricity via hysteresis loop. Films annealed in static air possess a dielectric constant higher than films annealed in oxygen atmosphere due to differences in the grain size, crystallinity and structural defects. A regularly shaped hystereses loop is observed after annealing in static air. The obtained results suggest that the annealing in oxygen atmosphere can increase the trapped charge and the relaxation phenomenon. (c) 2006 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Microwave-hydrothermal synthesis of barium strontium titanate nanoparticles

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Rietveld analysys and electrical properties of lanthanum doped BiFeO3 ceramics

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Lanthanum doped BiFeO3 powders: Syntheses and characterization

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

The influence of crystallization route on the properties of lanthanum-doped Bi4Ti3O12 thin films prepared from polymeric precursors

Pure and lanthanum-doped Bi4Ti3O12 thin films were deposited on Pt/Ti/SiO2/Si substrate using a polymeric precursor solution. The spin-coated films were specular and crack-free and crystalline after annealing at 700 degreesC for 2 h. Crystallinity and morphological evaluation were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Multilayered films obtained using the intermediate-crystalline layer route present a dense microstructure with spherical grains. Films obtained using the intermediate-amorphous layer, present elongated grains around 250 nm in size. The dielectric and ferroelectric properties of the lanthanum-doped Bi4Ti3O12 films are strongly affected by the crystallization route. The hysteresis loops are fully saturated with a remnant polarization and drive voltage of the films, heat-treated by the intermediate-crystalline (P-r = 20.2 muC/cm(2) and V = 1.35 V) and for the film heat-treated by amorphous route (P-r = 22.4 muC/cm(2) and V = 2.99 V). (C) 2004 Elsevier B.V. All rights reserved.

Size effect on structure and dielectric properties of Nb-doped barium titanate

It is known that the dielectric properties of BaTiO3 (BT) are strongly dependent on its grain size. Coarse-grained ceramics of pure BT showed lower dielectric constant at room temperature then fine grained. Many authors considered that when the grain size is lower than 700 nm, the lattice of BT changes from tetragonal to pseudocubic, and the dielectric constant value is very low. In the doped BT this effect is more complex, because it is necessary to consider also the influence of dopants. The grain size effect on the structure and dielectric properties of niobium-doped barium titanate was investigated. Niobium-doped barium titanate was prepared from powders obtained by doping of commercial barium titanate and from organometallic complex using citrates as precursors (Pechini procedure). The crystal and microstructure of sintered niobium-doped barium titanate were determined. Dielectric constant and dissipation factor were measured. The observation confirmed that the structure and properties are strongly dependent on grain size. (C) 2003 Elsevier B.V. B.V. All rights reserved.

Ferroelectric and dielectric properties of lanthanum-modified bismuth titanate thin films obtained by the polymeric precursor method

Lanthanum-modified bismuth titanate, Bi4-xLaxTi3O12 (BLT), thin films with a La concentration of 0.75 was grown on Pt/Ti/SiO2/Si substrates by using the polymeric precursor solution and spin-coating method. The scanning electron microscopy (SEM) showed rounded grains, which is not typical for these system. The BLT films showed well-saturated polarization-electric field curve which 2P(r) = 41.4 muC/cm(2) and V-c = 0.99 V. The capacitance dependence on the voltage is strongly nonlinear, confirming the ferroelectric properties of the film resulting from the domains switching. These properties make BLT a promising material for FERAM applications.

Electrical characterization of lanthanum-modified bismuth titanate thin films obtained by the polymeric precursor method

Lanthanum-modified bismuth titanate, Bi4-xLaxTi3O12 (BLT), thin films with a La concentration ranging from 0 to 0.75 were grown on Pt/Ti/SiO2/Si substrates using a polymeric precursor solution and spin-coating method. The scanning electron microscopy shows a change of morphology with increasing the lanthanum concentration. The BLT films show well-saturated polarization-electric field curves whit remnant polarizations of 14.7, 20.5, 21.5, and 20.4 muC/cm(2) for x = 0, 0.25, 0.50 and 0.75, respectively. The dielectric constant of BLT (x = 0.75 mol% La) is equal to 158 while dielectric loss remain low (tandelta = 0.0018).