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Register a new userStrain-coupled ferroelectric polarization in BaTiO3-CaTiO3 superlattices
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We report on growth and ferroelectric (FE) properties of superlattices (SLs) composed of the FE BaTiO3 and the paraelectric (PE) CaTiO3. Previous theories have predicted that the polarization in (BaTiO3)n/(CaTiO3)n SLs increases as the sublayer thickness (n) increases when the same strain state is maintained. However, our BaTiO3/CaTiO3 SLs show a varying lattice-strain state and systematic reduction in polarization with increasing n while coherently-strained SLs with n=1, 2 show a FE polarization of ca. 8.5 uC/cm^2. We suggest that the strain coupling plays more important role in FE properties than the electrostatic interlayer coupling based on constant dielectric permittivities.
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Ferroelectric BaTiO3 films with large polarization have been integrated with Si(001) by pulsed laser deposition. High quality c-oriented epitaxial films are obtained in a substrate temperature range of about 300 deg C wide. The deposition temperature critically affects the growth kinetics and thermodynamics balance, resulting on a high impact in the strain of the BaTiO3 polar axis, which can exceed 2% in films thicker than 100 nm. The ferroelectric polarization scales with the strain and therefore deposition temperature can be used as an efficient tool to tailor ferroelectric polarization. The developed strategy overcomes the main limitations of the conventional strain engineering methodologies based on substrate selection: it can be applied to films on specific substrates including Si(001) and perovskites, and it is not restricted to ultrathin films.
We report on quantification and elastic strain mapping in two artificial BaZrO3/BaTiO3 (BZ/BT) superlattices having periods of 6.6 nm and 11 nm respectively, grown on (001) SrTiO3 single crystal substrate by pulsed laser deposition technique. The methodology consists of a combination of high-resolution scanning transmission electron microscopy and nanobeam electron diffraction associated with dedicated algorithm for diffraction patterns processing originally developed for semiconductors to record the strains at atomic scale. Both in-plane and out-of-plane elastic strains were then determined at 2 nm spatial resolution and their average values were used to map the strains along and transverse to the epitaxial growth direction of both samples to determine its variation along several BZ/BT interfaces. In addition, the variation of the width of the inter-diffusion BT/BZ interfaces and intermixing between different layers are estimated. The obtained width average value measured in these inter-diffusion interfaces vary from 8 to 12% and from 9 to 11% for both superlattices having period of 6.6 nm and 11 nm respectively. These inter-diffusion interfaces and the inherent elastic strains due to the confined layers of the superlattices are known to be the most important parameters, responsible of the change in their functional properties.
Recent works suggest that the surface chemistry, in particular, the presence of oxygen vacancies can affect the polarization in a ferroelectric material. This should, in turn, influence the domain ordering driven by the need to screen the depolarizing field. Here we show using density functional theory that the presence of oxygen vacancies at the surface of BaTiO3 (001) preferentially stabilizes an inward pointing, P-, polarization. Mirror electron microscopy measurements of the domain ordering confirm the theoretical results.
First-principles density functional calculations are performed to investigate the interplay between inplane strains and interface effects in 1by1 PbTiO3/SrTiO3 and BaTiO3/SrTiO3 superlattices of tetragonal symmetry. One particular emphasis of this study is to conduct side-by-side comparisons on various ferroelectric properties in short-period superlattices and in constituent bulk materials, which turns out to be rather useful in terms of obtaining valuable insight into the different physics when ferroelectric bulks form superlattices. The various properties that are studied in this work include the equilibrium structure, strain dependence of mixing energy, microscopic ferroelectric off-center displacements, macroscopic polarization, piezoelectric coeffcients, effective charges, and the recently formulated k-dependent polarization dispersion structure. The details of our findings are rather lengthy, and are summarized in Sec. IV.
The integration of complex oxides on silicon presents opportunities to extend and enhance silicon technology with novel electronic, magnetic, and photonic properties. Among these materials, barium titanate (BaTiO3) is a particularly strong ferroelectric perovskite oxide with attractive dielectric and electro-optic properties. Here we demonstrate nanophotonic circuits incorporating ferroelectric BaTiO3 thin films on the ubiquitous silicon-on-insulator (SOI) platform. We grow epitaxial, single-crystalline BaTiO3 directly on SOI and engineer integrated waveguide structures that simultaneously confine light and an RF electric field in the BaTiO3 layer. Using on-chip photonic interferometers, we extract a large effective Pockels coefficient of 213 plus minus 49 pm/V, a value more than six times larger than found in commercial optical modulators based on lithium niobate. The monolithically integrated BaTiO3 optical modulators show modulation bandwidth in the gigahertz regime, which is promising for broadband applications.
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