Internal stress and the deformation phase transition in nanoscale barium-strontium titanate films (original) (raw)
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Residual Stress in Very Thin Barium Strontium Titanate Films
Integrated Ferroelectrics, 2008
Residual stress in thin film barium strontium titanate is thought to be preferentially distributed within the thickness of the film, such that a high degree of residual stress is manifested at the film-substrate interface, and the level of residual stress gradually decreases towards the free surface. To investigate this phenomena, a series of very thin films of barium strontium titanate with both cubic (30-70 Ba-Sr ratio) and tetragonal (80-20 Ba-Sr ratio) lattice structures were fabricated on platinized silicon and magnesium oxide. The thickness of these films was varied from 4-25 nm and the level of residual stress was investigated. The results highlighted within this paper, indicate that the cubic and tetragonal films manifest stress via different mechanisms, and that the substrate affected the magnitude of the residual stress but not the mechanism of stress relief.
Physics of the Solid State, 2007
Heteroepitaxial thin films of Ba 0.7 Sr 0.3 TiO 3 (BST-0.3) solid solutions were grown on single-crystal (001) MgO substrates by high-frequency cathode sputtering of a stoichiometric ceramic target. The parameters of the tetragonal unit cell of a film were determined by x-ray diffraction methods, and the temperature dependence of the parameter c was studied depending on the synthesis conditions in the temperature range 293-520 K. An E (TO) soft mode was observed in the Raman spectra, the frequency of which correlates with two-dimensional stresses arising in films. It is shown that the two-dimensional stresses in a film are controlled not only by the film-substrate lattice constant mismatch and the difference of their thermal expansion coefficients but also are significantly dependent on the heteroepitaxial growth mechanism. It is shown that the phase transition to the tetragonal paraelectric phase during film heating occurs irrespective of the growth mechanism.
Applied Physics Letters, 2001
A series of heteroepitaxial BaxSr1-xTiO3 thin films with composition x=0.50 were deposited on (001) MgO substrates by pulsed-laser deposition. The thickness of the films was varied from 14 to 500 nm to produce a systematically decreasing level of in-plane tensile stresses. The microstructural and crystallographic features of the films were determined via transmission electron microscopy and x-ray diffraction. A theoretical treatment of the in-plane misfit strain as a function of film thickness is in agreement with the measured out-of-plane lattice parameters. Electrical measurements indicate a drop in the dielectric constant from 2350 for highly stressed thin films to 1700 for relaxed thicker films. The variation in the dielectric constant with the misfit strain is in accordance with a thermodynamic model developed. The relationship between the dielectric constant and electric field is also described by extending the thermodynamic model and taking the effect of electric field into account. A new definition of tunability is adopted to study the effect of strain on tunability.
Residual Stress Effects in Doped Barium Strontium Titanate Thin Films
Journal of The Electrochemical Society, 2005
Barium strontium titanate (Ba x Sr 1Ϫx TiO 3 ; BST͒ thin films are the principal materials of interest for microwave antenna design, primarily because of their low loss, high dielectric constant, and large tunability. The effects of acceptor ͑Mg͒ doping, added to further tune film properties, on the BST thin films were investigated in parallel to the annealing temperature. Films were deposited on MgO single-crystal substrates using metallorganic solution deposition. Residual stress, known to have a drastic effect on the mechanical, electrical, and dielectric properties, becomes of particular importance in thin-film materials, where the residual stress can be several orders of magnitude higher. The residual stress in the films was measured in three ways ͑reflective curvature, nanoindentation, and glancing angle X-ray diffraction͒. Measured stresses ͑as high as 2 Gpa͒ were tunable by varying the dopant concentration and annealing temperature. The results from all three stress measurement techniques showed that the films had a highly compressive stress at the interface that transitioned to a significantly lower, but still nonzero, stress at the top surface (ϳ200 nm) as theory would suggest. In this paper, we evaluate and report the effects of residual stress on acceptor-doped BST as a function of postdeposition annealing conditions.
Strain-dependence of χ(2) in thin film barium strontium titanate
AIP Advances
Optical second-harmonic generation (SHG) in reflection characterized variations in the second-order nonlinear optical susceptibility χ(2) of Ba1−xSrxTiO3 (BSTO) alloy films grown epitaxially on SrTiO3(001) (STO) as Sr molar ratio x changed from 0 to 1 and thickness from 10 to 50 nm. X-ray diffraction showed that BSTO films with x ≳ 0.1 were highly crystalline and compressively strained with out-of-plane (in-plane) lattice constant 4.09 > c > 3.97 (3.903 < a < 3.907 Å, pinned to substrate) for 0 < x < 1. Spectroscopic ellipsometry showed that the BSTO dielectric function was nearly x-invariant in the range of fundamental (780 nm) and SH (390 nm) wavelengths. SHG intensity peaked at x ∼ 0.3 for a given film thickness, suggesting that composition control can optimize ferroelectric and electro-optic properties of BSTO films grown on STO buffer layers.
Thickness dependence of the properties of epitaxial barium strontium titanate thin films
Physics of the Solid State, 2015
Heteroepitaxial Ba x Sr 1-x TiO 3 thin films of different thicknesses have been studied in the paraelectric phase at a temperature of 600°C. The lattice parameters of the film have nonlinear dependence on the thickness. The misfit strain and unit cell volume increase with decreasing thickness. The behavior of the misfit strain with variations in the thickness is well described in the model of the double electric layer formed during the synthesis of the film at the interface with the substrate.
The effect of stress on the dielectric properties of barium strontium titanate thin films
1999
Abstract Barium strontium titanate thin films are being developed as capacitors in dynamic random access memories. These films, grown on silicon substrates, are under tensile residual stress. By a converse electrostrictive effect, the in-plane tensile stress reduces the capacitance in the thickness direction of the film. We measured the substrate curvature change upon the removal of the film, and found the magnitude of the residual stress to be 610 MPa.
Structural phase transitions in nanosized ferroelectric barium strontium titanate films
Physics of the Solid State, 2008
The lattice parameters of epitaxial barium strontium titanate films with various thicknesses (from 6 to 960 nm) were measured as a function of temperature in the normal and tangential directions with respect to the film plane using x-ray diffraction. The films were grown through the layer-by-layer mechanism by rf cathode sputtering under elevated oxygen pressure. A critical film thickness (~50 nm) was found to exist, below and above which the films are subjected to compressive and tensile stresses, respectively. As the temperature varies from 780 to 100 K, the films undergo two diffuse structural phase transitions of the second order over the entire thickness range. The transitions in the films under tensile stresses are likely to be transformations from the paraelectric tetragonal to aa phase and then to r phase, whereas the transitions under compressive stresses are transformations from the tetragonal paraelectric to ferroelectric c phase and then, with further decreasing temperature, to r phase.
Geometrical effects in nanodimensional epitaxial films of barium strontium titanate
Technical Physics, 2007
The effect of the thickness of 6-to 950-nm-thick Ba 0.8 Sr 0.2 TiO 3 films epitaxially grown on (001)MgO on their ferroelectric properties is investigated. Raman scattering and X-ray diffraction data indicate a structural phase transition taking place at a film thickness of ≈ 70 nm, which changes drastically the lattice parameters of the film. Raman spectra taken of the films confirm that they are in the ferroelectric state and that their symmetry changes in going over a critical thickness, as revealed by a sharp displacement of the peaks corresponding to the A 1 (TO) and E (TO) components of the soft mode. At film thicknesses of <100 nm, the permittivity versus thickness dependence exhibits two peaks at thicknesses of ~18 and ~36 nm. Near the first peak, the dielectric nonlinearity is considerably higher than near the second one.
Applied Physics Letters, 2009
Ba x Sr 1−x TiO 3 is ideally suited as a tunable medium for radio frequency passive component. In this context we have studied the effect of biaxial strain on the dielectric and ferroelectric properties of Ba 0.5 Sr 0.5 TiO 3 thin films grown epitaxially on SrTiO 3 ͑001͒ substrates. The lattice parameters of the films determined by high-resolution x-ray diffraction with the thickness varying from 160 to 1000 nm indicated large biaxial compressive strain which decreased from 2.54% to 1.14% with increasing film thickness. Temperature-dependent measurements of the dielectric constant in our strained Ba 0.5 Sr 0.5 TiO 3 thin films revealed a significant increase in the Curie temperature as the film thickness is below 500 nm. Enhanced ferroelectric behavior was observed for highly strained films with a remanent polarization of 15 C / cm 2 in the 160-nm-thick layer. However, the thick films ͑Ն500 nm͒ exhibited weak temperature dependence of the dielectric constant without any pronounced peak corresponding to the Curie temperature, which may suggest inhomogeneous strain distribution in the thick films.