Stoichiometry as key to ferroelectricity in compressively strained SrTiO3 films

While strain is a powerful tuning parameter for inducing ferroelectricity in thin film oxides, the role of stoichiometry control is critical, but far less explored. A series of compressively strained SrTiO3 films on (001) (LaAlO3)0.3(Sr2AlTaO6)0.35 substrates were grown by hybrid molecular beam epit...

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Bibliographic Details
Published in:Applied physics letters 2016-07, Vol.109 (3)
Main Authors: Haislmaier, R. C., Engel-Herbert, R., Gopalan, V.
Format: Article
Language:English
Subjects:
Applied physics
Electron diffraction
Epitaxial growth
Ferroelectric materials
Ferroelectricity
Molecular beam epitaxy
Parameters
Polarimetry
Second harmonic generation
Stoichiometry
Strontium titanates
Substrates
Temperature dependence
Thin films
Titanium
X ray reflection
X-ray diffraction
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Summary:While strain is a powerful tuning parameter for inducing ferroelectricity in thin film oxides, the role of stoichiometry control is critical, but far less explored. A series of compressively strained SrTiO3 films on (001) (LaAlO3)0.3(Sr2AlTaO6)0.35 substrates were grown by hybrid molecular beam epitaxy where the Ti cation was supplied using a metal-organic titanium tetraisopropoxide molecule that helps systematically and precisely control Sr:Ti stoichiometry in the resulting films. A stoichiometric growth window is located through X-ray diffraction and in-situ reflection high-energy electron diffraction measurements, which show a minimum out-of-plane lattice parameter as well as constant growth rate within the stoichiometric growth window range. Using temperature dependent optical second harmonic generation (SHG) characterization, a ferroelectric-to-paraelectric transition at T ∼ 180 K is observed for a stoichiometric SrTiO3 film, as well as a higher temperature structural transition at T ∼ 385 K. Using SHG polarimetry modeling, the polar point group symmetry is determined to be tetragonal 4mm with the polarization pointing out-of-plane of the film. The SHG coefficients, d 31/d 15=3 and d 33/d 15=21, were determined at 298 K. The ferroelectric transition disappears in films grown outside the growth window, thus proving the critical role of stoichiometry control in realizing strain-induced ferroelectricity.
ISSN:0003-6951
1077-3118
DOI:10.1063/1.4959077