Titanate de baryum

Résumé

Le titanate de baryum est un composé chimique de formule . Ce matériau céramique se présente sous la forme d'un solide blanc ferroélectrique à hystérésis prononcée ayant également un effet photoréfractif et un effet piézoélectrique. Il trouve des applications notamment dans les condensateurs, les transducteurs électromécaniques, les thermistances CTP et en optique non linéaire. Il existe également sous forme naturelle dans un minéral rare appelé baryopérovskite. Propriétés Il s'agit d'un oxyde mixte de titane et de baryum cristallisant dans une structure dont la symétrie dépend de la température : pérovskite cubique au-dessus de sa température de Curie de , elle est tétragonale à température ambiante, puis orthorhombique en dessous de et trigonale en dessous de . La forme tétragonale est caractérisée par des paramètres cristallins valant et . Dans cette structure, le cation de titane Ti des octaèdres est décalé par rapport aux anions oxyde O, ce qui génère un moment di

Source officielle

https://fr.wikipedia.org/wiki/Titanate_de_baryum

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Course dealing with the design and fabrication of fundamental components of smart phones front-end communication (filters, duplexers, quadplexers).

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Microstructure and ferroelectricity of BaTiO3 thin films on Si for integrated photonics

Patrik Hoffmann, Chiara Marchiori, Michael Oliver Reinke

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

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Closed-circuit domain quadruplets in BaTiO3 nanorods embedded in a SrTiO3 film

Nava Setter

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

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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

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EPFL2016