Epitaxial strain stabilization of ferroelectric phase in PbZrO3 thin films

PbZrO3/SrRuO3/SrTiO3 (100) epitaxial heterostructures with different thickness of the PbZrO3 (PZO) layer (d(PZO) similar to 5-160 nm) were fabricated by pulsed laser deposition. The ultrathin PZO films (d(PZO) <= 10 nm) were found to possess a rhombohedral structure. On increasing the PZO film thickness, a bulk like orthorhombic phase started forming in the film with d(PZO) similar to 22 nm and became abundant in the thicker films. Nanobeam electron diffraction and room-temperature micro-Raman measurements revealed that the stabilization of the rhombohedral phase of PZO could be attributed to the epitaxial strain accommodated by the heterostructures. Room-temperature polarization vs electric field measurements performed on different samples showed characteristic double hysteresis loops of antiferroelectric materials accompanied by a small remnant polarization for the thick PZO films (dPZO >= 50 nm). The remnant polarization increased by reducing the PZO layer thickness, and a ferroelectric like hysteresis loop was observed for the sample with d(PZO) similar to 22 nm. Local ferroelectric properties measured by piezoresponse force microscopy also exhibited a similar thickness-dependent antiferroelectric-ferroelectric transition. Room-temperature electrical properties observed in the PZO thin films in correlation to their structural characteristics suggested that a ferroelectric rhombohedral phase could be stabilized in thin epitaxial PZO films experiencing large interfacial compressive stress.

Increasing recoverable energy storage in electroceramic capacitors using “dead-layer” engineering

The manner in which ultrathin films of alumina, deposited at the dielectric-electrode interface, affect the recoverable energy density associated with (BiFeO3)0.6–(SrTiO3)0.4 (BFST) thin film capacitors has been characterised. Approximately 6 nm of alumina on 400 nm of BFST increases the maximum recoverable energy of the system by around 30% from 13 Jcc1 to 17 Jcc1.
Essentially, the alumina acts in the same way as a naturally present parasitic “dead-layer,” distorting the polarisation-field response such that the ultimate polarisation associated with the BFST is pushed to higher values of electric field. The work acts as a proof-of-principle to illustrate how the design of artificial interfacial dielectric “dead-layers” can increase energy densities in simple dielectric capacitors, allowing them to compete more generally with other energy storage technologies.

Adsorption-controlled molecular-beam epitaxial growth of BiFeO3

BiFeO3 thin films have been deposited on (111) SrTiO3 single crystal substrates by reactive molecular-beam epitaxy in an adsorption-controlled growth regime. This is achieved by supplying a bismuth overpressure and utilizing the differential vapor pressures between bismuth oxides and BiFeO3 to control stoichiometry. Four-circle x-ray diffraction reveals phase-pure, untwinned, epitaxial, (0001)-oriented films with rocking curve full width at half maximum values as narrow as 25 arc sec (0.007 degrees). Second harmonic generation polar plots combined with diffraction establish the crystallographic point group of these untwinned epitaxial films to be 3m at room temperature. (C) 2007 American Institute of Physics.

Ionically-Mediated Electromechanical Hysteresis in Transition Metal Oxides

Nanoscale electromechanical activity, remanent polarization states, and hysteresis loops in paraelectric TiO2 and SrTiO3 thin films are observed using scanning probe microscopy. The coupling between the ionic dynamics and incipient ferroelectricity in these materials is analyzed using extended Landau-Ginzburg-Devonshire (LGD) theory. The possible origins of electromechanical coupling including ionic dynamics, surface-charge induced electrostriction, and ionically induced ferroelectricity are identified. For the latter, the ionic contribution can change the sign of first order LGD expansion coefficient, rendering material effectively ferroelectric. The lifetime of these ionically induced ferroelectric states is then controlled by the transport time of the mobile ionic species and well above that of polarization switching. These studies provide possible explanation for ferroelectric-like behavior in centrosymmetric transition metal oxides.

Strain dependent microstructural modifications of BiCrO3 epitaxial thin films

Strain-dependent microstructural modifications were observed in epitaxial BiCrO3 (BCO) thin films fabricated on single crystalline substrates, utilizing pulsed laser deposition. The following conditions were employed to modify the epitaxial-strain: (i) in-plane tensile strain, BCOSTO [BCO grown on buffered SrTiO3 (001)] and in-plane compressive strain, BCONGO [BCO grown on buffered NdGaO3 (110)] and (ii) varying BCO film thickness. A combination of techniques like X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (TEM) was used to analyse the epitaxial growth quality and the microstructure of BCO. Our studies revealed that in the case of BCOSTO, a coherent interface with homogeneous orthorhombic phase is obtained only for BCO film with thicknesses, d < 50 nm. All the BCOSTO films with d = 50 nm were found to be strain-relaxed with an orthorhombic phase showing 1/2 <100> and 1/4 <101> satellite reflections, the latter oriented at 45° from orthorhombic diffraction spots. High angle annular dark field scanning TEM of these films strongly suggested that the satellite reflections, 1/2 <100> and 1/4 <101>, originate from the atomic stacking sequence changes (or “modulated structure”) as reported for polytypes, without altering the chemical composition. The unaltered stoichiometry was confirmed by estimating both valency of Bi and Cr cations by surface and in-depth XPS analysis as well as the stoichiometric ratio (1 Bi:1 Cr) using scanning TEM–energy dispersive X-ray analysis. In contrast, compressively strained BCONGO films exhibited monoclinic symmetry without any structural modulations or interfacial defects, up to d ~ 200 nm. Our results indicate that both the substrate-induced in-plane epitaxial strain and the BCO film thickness are the crucial parameters to stabilise a homogeneous BCO phase in an epitaxially grown film.

A Lead-Free and High-Energy Density Ceramic for Energy Storage Applications

In this work, we demonstrate a very high-energy density and high-temperature stability capacitor based on SrTiO3-substituted BiFeO3 thin films. An energy density of 18.6 J/cm3 at 972 kV/cm is reported. The temperature coefficient of capacitance (TCC) was below 11% from room temperature up to 200°C. These results are of practical importance, because it puts forward a promising novel and environmentally friendly, lead-free material, for high-temperature applications in power electronics up to 200°C. Applications include capacitors for low carbon vehicles, renewable energy technologies, integrated circuits, and for the high-temperature aerospace sector. © 2013 Crown copyright

High-resolution electron energy-loss spectroscopy of Ba Ti O 3∕ Sr Ti O 3 multilayers

The dielectric properties of BaTiO3 thin films and multilayers are different from bulk materials because of nanoscale dimensions, interfaces, and stress-strain conditions. In this study, BaTiO3/SrTiO3 multilayers deposited on SrTiO3 substrates by pulsed laser deposition have been investigated by high-energy-resolution electron energy-loss spectroscopy. The fine structures in the spectra are discussed in terms of crystal-field splitting and the internal strain. The crystal-field splitting of the BaTiO3 thin layer is found to be a little larger than that of bulk BaTiO3, which has been interpreted by the presence of the internal strain induced by the misfit at the interface. This finding is consistent with the lattice parameters of the BaTiO3 thin layer determined by the selected area diffraction pattern. The near-edge structure of the oxygen K edge in BaTiO3 thin layers and in bulk BaTiO3 are simulated by first-principle self-consistent full multiple-scattering calculations. The results of the simulations are in a good agreement with the experimental results. Moreover, the aggregation of oxygen vacancies at the rough BaTiO3/SrTiO3 interface is indicated by the increased [Ti]/[O] element ratio, which dominates the difference of dielectric properties between BaTiO3 layer and bulk materials.