Stoichiometry as key to ferroelectricity in compressively strained SrTiO3films (original) (raw)
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Stoichiometry as key to ferroelectricity in compressively strained SrTiO3films
Applied Physics Letters, 2016
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 SrTiO 3 films on (001) LSAT substrates were grown by hybrid molecular beam epitaxy where the Ti cation was supplied using a metal-organic titanium tetraisopropoxide (TTIP) 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 SrTiO 3 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.
Ferroelectricity in Strain-Free SrTiO3 Thin Films
Physical Review Letters, 2010
Biaxial strain is known to induce ferroelectricity in thin films of nominally nonferroelectric materials such as SrTiO 3 . By a direct comparison of the strained and strain-free SrTiO 3 films using dielectric, ferroelectric, Raman, nonlinear optical and nanoscale piezoelectric property measurements, we conclude that all SrTiO 3 films and bulk crystals are relaxor ferroelectrics, and the role of strain is to stabilize longerrange correlation of preexisting nanopolar regions, likely originating from minute amounts of unintentional Sr deficiency in nominally stoichiometric samples. These findings highlight the sensitive role of stoichiometry when exploring strain and epitaxy-induced electronic phenomena in oxide films, heterostructures, and interfaces.
ab initio Study of Strain-Induced Ferroelectricity in SrTiO 3
Japanese Journal of Applied Physics, 2005
Valley lines on total-energy surfaces for the zone-center distortions of freestanding and in-plane strained SrTiO 3 are investigated with a newly developed first-principles structure optimization technique [Jpn. J. Appl. Phys. 43 , 6785]. The results of numerical calculations confirmed that the ferroelectricity is induced, and the Curie temperature is increased, by applying biaxial compressive or tensile strains. Along the distortion, strong nonlinear coupling between the soft-and hard-modes is demonstrated.
Scientific reports, 2017
Due to the remarkable possibilities of epitaxially growing strontium titanate (SrTiO3 or STO) on silicon, this oxide is widely used as a buffer layer for integrating other perovskite oxides which allows for the development of various functional electronic devices on silicon. Moreover, STO is known to be an incipient ferroelectric in bulk but may become ferroelectric when in the form of strained ultrathin films. Given the importance of the potential applications for electronics if this property is demonstrated, we performed a spectroscopic study of STO on Si(001) templates coupling experimental and ab initio investigations. We selected six samples of ultrathin films: three strained samples (of thickness 4, 9 and 48 nm) and three relaxed samples (of equivalent thickness). Their infrared spectra show that both the mechanical stress and the thickness play major roles: higher energy modes evolve as soft modes in thinner strained films. In order to support these observations, the dynamica...
Applied Physics Letters, 2007
We investigated the ferroelectric properties of strontium titanate (STO) thin films deposited on SrTiO 3 (001) substrate with SrRuO 3 electrodes. The STO layer was grown coherently on the SrTiO 3 substrate without in-plane lattice relaxation, but its out-of-plane lattice constant increased with a decrease in the oxygen pressure during deposition. Using piezoresponse force microscopy and P-V measurements, we showed that our tetragonal STO films possess room-temperature ferroelectricity. We discuss the possible origins of the observed ferroelectricity.
Physical Review B, 2006
X-ray diffraction has been used to study the epitaxy and lattice expansion of SrTiO 3 thin films grown coherently on Si͑001͒ by kinetically controlled sequential deposition. The coherent growth of SrTiO 3 on Si produces films with greater in-plane compressive strain than previously attained, −1.66%. The measured expansion of the out-of-plane lattice constant exceeds the prediction of the bulk elastic constants of SrTiO 3 by nearly 100%. This expansion agrees with recent theoretical predictions and experimental findings of roomtemperature ferroelectricity in SrTiO 3 films under epitaxial mismatch strain. Our first principles density functional calculations determine an energetically favorable interfacial-defect and surface-charge structure that allows the ferroelectric polarization in these ultrathin films.
Systematic ab initio study of the phase diagram of epitaxially strained SrTiO[sub 3]
Journal of Applied Physics, 2006
We use density-functional theory with the local-density approximation to study the structural and ferroelectric properties of SrTiO3 under misfit strains. Both the antiferrodistortive (AFD) and ferroelectric (FE) instabilities are considered by calculating all the phases predicted by Pertsev et al. [Phys. Rev. B 61, R 825 (2000)] based on the phenomenological Landau theory. The rotation of the oxygen octahedra and the movement of the atoms are fully relaxed within the constraint of a fixed in-plane lattice constant. We find a rich misfit strain-induced phase transition sequence which is in overall agreement with the prediction by Pertsev et al. and is obtained only when the AFD distortion is taken into account. Nevertheless, the calculated locations of the phase boundaries are different from the prediction by Pertsev et al. We also find that compressive misfit strains induce ferroelectricity in the tetragonal low temperature phase only whilst tensile strains induce ferroelectricity in the orthorhombic phases only. The calculated FE polarization for both the tetragonal and orthorhombic phases increases monotonically with the magnitude of the strains. The AFD rotation angle of the oxygen octahedra in the tetragonal phase increases dramatically as the misfit strain goes from the tensile to compressive strain region whilst it decreases slightly in the orthorhombic (FO4) phase. This reveals why the polarization in the epitaxially strained SrTiO3 would be larger when the tensile strain is applied, since the AFD distortion is found to reduce the FE instability and even to completely suppress it in the small strain region. Finally, our analysis of the average polar distortion and the charge density distribution suggests that both the Ti-O and Sr-O layers contribute significantly to the FE polarization.
Strain effects on the electronic structure of SrTiO_{3}: Toward high electron mobilities
Physical Review B, 2011
Using the screened hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE) we explore the effects of strain on the energetic ordering and effective mass of the lowest conduction-band states in SrTiO 3. We predict that biaxial stress in the (001) or (110) planes results in the lowest-energy conduction-band state having significantly smaller electron mass in the in-plane directions compared to the unstrained SrTiO 3 , thus suggesting that pseudomorphic growth is a promising route to increasing the electron mobility in epitaxial films. We propose possible substrates that may lead to SrTiO 3 films with enhanced electron mobilities, and report deformation potentials that allow accurate prediction of conduction-band splittings for arbitrary strain configurations.
Applied Physics Letters, 2013
The effect of stoichiometry on the dielectric properties and soft mode behavior of strained epitaxial Sr 1þx TiO 3þd films grown on DyScO 3 substrates is reported. Direct comparisons between nominally stoichiometric and non-stoichiometric films have been performed through measurements of lattice parameters, temperature-dependent permittivities, second harmonic generation, and terahertz dielectric spectra. The nominally stoichiometric film shows dispersion-free low-frequency permittivity with a sharp maximum and pronounced soft mode behavior. Our results suggest that strained perfectly stoichiometric SrTiO 3 films should not show relaxor behavior and that relaxor behavior emerges from defect dipoles that arise from non-stoichiometry in the highly polarizable strained SrTiO 3 matrix.