Barium titanate

Summary

Barium titanate (BTO) is an inorganic compound with chemical formula BaTiO3. Barium titanate appears white as a powder and is transparent when prepared as large crystals. It is a ferroelectric, pyroelectric, and piezoelectric ceramic material that exhibits the photorefractive effect. It is used in capacitors, electromechanical transducers and nonlinear optics. Structure Perovskite (structure) The solid exists in one of four polymorphs depending on temperature. From high to low temperature, these crystal symmetries of the four polymorphs are cubic, tetragonal, orthorhombic and rhombohedral crystal structure. All of these phases exhibit the ferroelectric effect apart from the cubic phase. The high temperature cubic phase is easiest to describe, as it consists of regular corner-sharing octahedral TiO6 units that define a cube with O vertices and Ti-O-Ti edges. In the cubic phase, Ba2+ is located at the center of the cube, with a nominal coordination number of 12. Lower sym

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Significant progress has been made in integrating novel materials into silicon photonic structures in order to extend the functionality of photonic circuits. One of these promising optical materials is BaTiO3 or barium titanate (BTO) that exhibits a very large Pockels coefficient as required for high-speed light modulators. However, all previous demonstrations show a noticable reduction of the Pockels effect in BTO thin films deposited on silicon substrates compared to BTO bulk crystals. Here, we report on the strong dependence of the Pockels effect in BTO thin films on their microstructure, and provide guidelines on how to engineer thin films with strong electro-optic response. We employ several deposition methods such as molecular beam epitaxy and chemical vapor deposition to realize BTO thin films with different morphology and crystalline structure. While a linear electro-optic response is present even in porous, polycrystalline BTO thin films with an effective Pockels coefficient r(eff). = 6 pmV(-1), it is maximized for dense, tetragonal, epitaxial BTO films (r(eff) = 140 pm V-1). By identifying the key structural predictors of electro-optic response in BTO/Si, we provide a roadmap to fully exploit the linear electro-optic effect in novel hybrid oxide/semiconductor nanophotonic devices.

Institute of Physics2017

Cylindrical BaTiO3 nanorods embedded in < 100 >-oriented SrTiO3 epitaxial film in a brushlike configuration are investigated in the framework of the Ginzburg-Landau-Devonshire model. It is shown that strain compatibility at BaTiO3/SrTiO3 interfaces keeps BaTiO3 nanorods in the rhombohedral phase even at room temperature. Depolarization field at the BaTiO3/SrTiO3 interfaces is reduced by an emission of the 109 degrees or 71 degrees domain boundaries. In case of 10-80-nm diameter nanorods, the ferroelectric domains are found to form a quadruplet with a robust flux-closure arrangement of the in-plane components of the spontaneous polarization. The out-of-plane components of the polarization are either balanced or oriented up or down along the nanorod axis. Switching of the out-of-plane polarization with coercive field of about 5 x 10(6) V/m occurs as a collapse of a 71 degrees cylindrical domain boundary formed at the curved circumference surface of the nanorod. The remnant domain quadruplet configuration is chiral, with the C-4 macroscopic symmetry. More complex stable domain configurations with coexisting clockwise and anticlockwise quadruplets contain interesting arrangement of strongly curved 71 degrees boundaries.

Amer Physical Soc2014

Surface Kinetics of Titanium Isopropoxide in Chemical Vapor Deposition of Titanium Dioxide and Barium Titanate

Michael Oliver Reinke

All chemical vapor deposition (CVD) processes rely on the adsorption and decomposition of precursors on a substrate to deposit the desired material. The growth rate of the film is determined by the surface kinetics of the utilized precursor molecules and generally dependent on precursor concentration and substrate temperature. Investigation of surface kinetics is challenging in CVD techniques as precise quantification of the precursor concentration (or precursor flux) is difficult. Furthermore gas-phase reactions can influence the film growth. In this thesis we develop a method to characterize precursor surface kinetics during deposition processes using high vacuum chemical vapor deposition (HV-CVD). In a high vacuum environment precursor transport takes place in the molecular flow regime and gas phase reactions are efficiently suppressed. Furthermore precursor impinging rates can be calculated using a relatively simple mathematical model. Relating the number of impinging precursor molecules to the absolute amount of deposited material allows investigating the deposition kinetics in a very precise manner. We apply this method to characterize titanium isopropoxide and water in chemical vapor deposition conditions over large parameter range. Fitting a surface kinetic model to experimental data enabled us to derive activation energies for desorption, hydrolysis and pyrolysis. The surface kinetic model is then applied to characterize the TTIP based HV-CVD and ALD process of titanium dioxide in detail. Furthermore, we have studied the surface kinetics of TTIP when the substrate is additionally exposed to barium precursors aiming to deposit barium titanate. We demonstrate that HV-CVD is capable of growing epitaxial barium titanate films on magnesium oxide, strontium titanate and strontium titanate buffered silicon substrates at 400°C process temperature; this is the lowest reported substrate temperature for epitaxial growth of barium titanate by CVD. Finally, we investigate two experimental techniques to selectively grow titanium dioxide: a lift-off technique based on shading masks and a surface passivation technique, based on the local modification of surface reactions. We will demonstrate, that the surface kinetics of TTIP are not optimal for the local deposition of titanium dioxide in the first case, but that HV-CVD is capable of enhancing selectivity compared to previously reported values in the latter approach.

EPFL2016

Surface Kinetics of Titanium Isopropoxide in Chemical Vapor Deposition of Titanium Dioxide and Barium Titanate

Michael Oliver Reinke

EPFL2016