Epitaxial Barium Strontium Titanate Research Articles

Investigation of the effects of misfit strain on barium strontium titanate thin films deposited on base metal substrates by a modified phenomenological model

The Landau-Devonshire phenomenological model, which has been utilized to investigate epitaxial barium strontium titanate (BST) thin films, was modified to investigate the effects of misfit strain on the dielectric properties of polycrystalline BST thin films deposited on base metal substrates. The modification considers the relaxation of lattice misfit stress resulting from the formation of in-plane misfit dislocations. The modified lattice misfit strain was calculated by referring to the ferroelectric critical grain size. Moreover, the misfit strain and dielectric properties of BST thin films with different structures and substrates were investigated by the models. It was found that the measured dielectric constant and tunability of BST thin films on different metal substrates overall agreed with the computed data. In addition, the good agreement was also observed for sandwich-like structural BST thin films deposited on LNO buffered stainless steel plates. Our results indicated that the modified L-D models might be utilized to predict dielectric properties of polycrystalline BST thin films for varied substrates and multilayer structures.

Strain engineering of electrical conductivity in epitaxial thin Ba0.7Sr0.3TiO3 film heterostructures

Thin epitaxial films have a great potential to be used in real life applications, such as oxide-on-silicon. However, they often contain a large amount of defects, leading to an enhanced electrical conductivity. This could be desirable in some applications (i. e. memristors), but the mechanism is not fully understood. Here we report on the investigation of epitaxial barium strontium titanate thin films deposited on strontium titanate single crystal substrates (Ba0.7Sr0.3TiO3/SrRuO3//SrTiO3 heterostructures) with a controlled epitaxial strain. The impedance analysis allowed us to propose a model, which explains changes in the temperature dependence of the conductivity based on the strain-dependent anisotropic change of electron/hole mobility.

DIELECTRIC PROPERTIES OF EPITAXIAL Ba1-xSrxTiO3 FILMS ON MgO SUBSTRATES

We have systematically studied the dielectric properties of epitaxial barium strontium titanate, Ba1-xSrxTiO3 (x = 0.0, 0.1, 0.2, …, 1.0), films on MgO substrates grown by pulsed laser deposition. The dielectric properties of Ba1-xSrxTiO3 films at a frequency of 1 MHz and room temperature are a strong function of Ba/Sr ratio. The degree of electric field dependent dielectric constant is more pronounced with decrease of x values. The largest dielectric tunability and the figure of merit (or tunability/loss) are obtained at x = 0.3 and at x = 0.4, respectively.

Structure and lattice dynamics of heterostructures based on bismuth ferrite and barium strontium titanate on magnesium oxide substrates

Bismuth ferrite films doped with neodymium on MgO single-crystal substrates with an epitaxial barium strontium titanate thin (1–2 nm) sublayer have been prepared by rf sputtering of ceramic targets at an elevated oxygen partial pressure and at temperatures below the ferroelectric and magnetic transition temperatures. It has been revealed using X-ray diffraction and Raman scattering spectroscopy that, in these bismuth ferrite films, a new phase (not observed in bulk samples) is formed. The symmetry of this phase is monoclinic, the unit cell contains two formula units, and the spontaneous polarization vector deviates from the [111]cub direction and can have different components along the x, y, and z axes.

Dielectric tunability and electro-optic effect of Ba0.5Sr0.5TiO3 thin films

We elucidated the relationship between the dielectric tunability and the electro-optic (EO) effect. A new measurement system for EO effect and dielectric permittivity (εr) was developed to investigate the accurate correlation between EO and dielectric properties of epitaxial barium strontium titanate, Ba0.5Sr0.5TiO3 (BST) thin films grown on SrTiO3 (STO) substrates. The BST films had a strained lattice with a large lattice parameter along the thickness direction. Small birefringence was induced in the as-deposited film by the alignment of the c-axis in the film plane. The tunability of dielectric permittivity calculated from the complex voltage and current with planer electrodes reached to 53.1%, and the tunability of birefringence by EO effect was 0.6%. The birefringence change from EO effect was much lower than the tunability of dielectric permittivity. Therefore, there is no strong correlation between the dielectric tunability and EO effect, and the materials with high tunability do not always exhibit high EO effect. Finally, we discussed how to obtain the materials with high EO effect considering these results.

Structural phase transitions in nanosized ferroelectric barium strontium titanate films

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.

Preparation and Characterization of Compositionally Graded Epitaxial Barium Strontium Titanate Thin Films via Scanning Probe Microscopy

Epitaxially graded barium strontium titanate (BaxSr1-x)TiO3 (x = 0.75, 0.8, 0.9, 1.0, abbreviated as BST75, BST80, BST90 and BTO respectively) thin films were fabricated by pulsed laser deposition method on the (La0.7Sr0.3)MnO3 (LSMO)/LaAlO3 (LAO) single crystal substrate. Scanning probe microscopy with a contact mode was used to characterize the temperature dependence of polarization from room temperature to 140°C. Results indicated that the piezo-response signal of the BST graded films had an obvious change with temperature, and that the graded structures had a flatter temperaturedependence of permittivity. Furthermore, the contrasts of the SPM images were lower for the ferroelectric – paraelectric (F-P) phase transition temperatures of BST 75, BST 80, and BST90, but higher for the F-P transition temperature of BTO.

IN-PLANE DIELECTRIC PROPERTIES OF BST THIN FILMS GROWN ON MgO SUBSTRATES USING THE THIN PEROVSKITE LAYERS FOR MICROWAVE TUNABLE ELECTRONICS

ABSTRACT Barium strontium titanate (BST) thin films have been intensively studied as a frequency and phase agile materials for the microwave tunable devices such as tunable filter, phase shifter and phased array antennas. In this work, we report on the epitaxial growth and the in-plane dielectric properties of (Ba, Sr)TiO3 (BST) thin films for high-performance microwave tunable device applications. Epitaxial BST thin films were deposited on MgO substrates by the insertion of thin perovskite ABO3 layer such as Ba(Zr, Ti)O3[BZT], Ba(Sn,Ti)O3 [BTS] and SrZrO3 [SZO], with an intermediate lattice constant between the BST films and the MgO substrate using a pulsed laser deposition. In-plane nonlinear dielectric properties of BST thin films were carried out using planar interdigitated (IDT) capacitor. The dielectric tunability and quality K-factor of BST thin film using thin BZT layer exhibited 78.8% and 150 at a frequency of 100 kHz and applied voltage of 40 V, respectively. These results indicate that the thin perovskite layer with an intermediate lattice constant between those of the BST film and the MgO substrate can be used as an excellent seed layer for improving the crystallization and dielectric properties of BST films, and that it is a promising candidate for high-performance microwave tunable devices at room temperature.

ENHANCED DIELECTRIC PROPERTIES OF (Ba, Sr)TiO3 THIN FILMS FOR HIGH-PERFORMANCE MICROWAVE PHASE SHIFTERS†

ABSTRACT Barium strontium titanate (BST) thin films have been intensively studied as a frequency and phase agile materials for the tunable microwave devices such as tunable filter, phase shifter and phased array antennas. The dielectric properties of BST thin films, including the dielectric constant, dielectric loss and tunability of capacitance are affected by many factors, such as Ba/Sr ratio, growth temperature, crystallographic orientation, grain size, defect chemistry, oxygen vacancies, strain and stress, and dopants. In this work, we report on the enhanced dielectric properties of (Ba0.6, Sr0.4)TiO3 (BST) thin films for high-performance microwave tunable device applications. Epitaxial BST thin films were deposited on MgO substrates by the insertion of thin BaTiO3 [BT] seed layers with intermediate lattice mismatch between the BST films and the MgO substrate using a pulsed laser ablation. Dielectric measurements were carried out using planar interdigitated (IDT) capacitor in ranges of RF/microwave frequency. IDT capacitor based on epitaxial BST film exhibited the large capacitance tunability of 77% at a frequency of 100 kHz and applied electric field of 80 kV/cm. CPW phase shifter showed the large phase tuning of 153° at 10 GHz and 40 V. These results indicate that it is a promising candidate for high-performance microwave phase shifters at room temperature. †Originally presented at 2004 ISIF, Kyung Ju, Korea, April 5-8, 2004.

High Dielectric Tunability of (Ba, Sr)TiO3 Thin Films and Their Coplanar Waveguide Phase Shifter Applications

In the present work, we report the epitaxial growth and dielectric properties of barium strontium titanate (BST) thin film which can be used to make frequency and phase agile tunable microwave (MW) components with a high performance, such as tunable filters, tunable oscillators, and phase shifters for application in phased array antennas. Epitaxial BST thin films were deposited on a MgO substrate by the insertion of a thin BaTiO3 (BT) seed layer between the BST films and the MgO substrate using pulsed laser ablation. The crystalline structure, dielectric tunability and microwave properties of the epitaxial BST thin films depended strongly on the thickness of the BT seed layer. The dielectric tunability and microwave phase tuning increased with increasing thickness of the BT seed layer. An interdigitated (IDT) capacitor based on the epitaxial BST/BT(12 nm) film on the MgO substrate exhibited a large capacitance tunability of 78% at a frequency of 100 kHz and an applied electric field of 133 kV/cm. Also, a coplanar waveguide (CPW)-type phase shifter fabricated on the BST/BT(12 nm) film showed a phase shift of 153° at 10 GHz and 40 V. These results indicate that the thin BT layer with an intermediate lattice constant between those of the BST film and MgO substrate can be used as an excellent seed layer for improving the crystallization and dielectric properties of BST films, and the high microwave performance of ferroelectric MW devices.

High tunability in compositionally graded epitaxial barium strontium titanate thin films by pulsed-laser deposition

Compositionally graded barium strontium titanate [(BaxSr1−x)TiO3—BST, x=0.75, 0.8, 0.9, and 1.0] thin films are fabricated by pulsed-laser deposition on a LaAlO3 substrate with (La0.7Sr0.3)MnO3 as the bottom electrode. A high dielectric permittivity and temperature characteristic without Curie–Weiss law are obtained. A tunability of over 70% is obtained at frequency of 1 MHz, which is higher than that of single BST layer with the same compositions. All the results indicate that the graded thin films have better electrical properties than a single-layer film.

Optimization of the tunability of barium strontium titanate films via epitaxial stresses

The tunability of epitaxial barium strontium titanate films is analyzed theoretically using a phenomenological model. The relative dielectric constant of Ba0.5Sr0.5TiO3 (BST 50/50) films as a function of the applied external electric field is calculated and an electric field–misfit strain phase diagram is developed to assist in the interpretation of the behavior. On the basis of these results, the tunability of BST 50/50 films as a function of the misfit strain is provided and compared with the experimental data in the literature. Analysis shows that a high tunability can be achieved by adjusting the misfit strain especially in the vicinity of a structural phase transformation. The misfit strain in epitaxial films can be controlled with the selection of a substrate material or variations in the film thickness. The film thickness dependence is due to misfit dislocation formation at the film growth temperature. A critical thickness to attain the maximum tunability can be defined for BST 50/50 films on MgO (∼90 nm) and LaAlO3 (∼120 nm) substrates. It is suggested that the selection of the substrate and/or the film thickness can be chosen as design parameters to manipulate the strain state in the film to achieve optimum tunability.

Compositionally graded epitaxial barium strontium titanate thin films derived from pulsed laser deposition

Epitaxial graded barium strontium titanate (Ba x Sr 1− x )TiO 3 ( x=0.75, 0.8, 0.9, 1.0) thin films were prepared through pulsed laser deposition method on the (La 0.7Sr 0.3)MnO 3 (LSMO)/LaAlO 3 (LAO) substrate. XRD analysis indicated the graded film had a broadened 2 θ peak at 45° for the (2 0 0) peak. The energy dispersive analysis of X-ray (EDAX) showed a gradient change of Ba/Sr composition. P–E loop measurement gave a remnant polarization about 2 μC cm −2, and without big up offset observed. A big room temperature dielectric constant as high as 1600 was obtained and has an approximately linear relationship with temperature from room temperature to 130 °C. All these results can be ascribed to the epitaxial and graded structure of BST thin films.

Phase diagrams and dielectric response of epitaxial barium strontium titanate films: A theoretical analysis

We develop phase diagrams for single-domain epitaxial barium strontium titanate films on cubic substrates as a function of the misfit strain based on a Landau–Devonshire phenomenological model similar to the one developed by Pertsev et al. [Phys. Rev. Lett. 80, 1988 (1998)]. The biaxial epitaxy-induced internal stresses enable phase transformations to unusual ferroelectric phases that are not possible in single crystals and bulk ceramics. The dielectric response of the films is calculated as a function of the misfit strain by taking into account the formation of misfit dislocations that relieve epitaxial stresses during deposition. It is shown that by adjusting the misfit strain via substrate selection and film thickness, a high dielectric response can be obtained, especially in the vicinity of structural instabilities. Theoretical estimation of the dielectric constant of (001) Ba0.7Sr0.3TiO3 and Ba0.6Sr0.4TiO3 films on (001) Si, MgO, LaAlO3, and SrTiO3 substrates as a function of misfit strain and film thickness is provided. An order-of-magnitude increase in the dielectric constant with increasing film thickness is expected for films on LaAlO3 and SrTiO3 substrates. A structural instability around 40 nm is predicted in films on MgO substrates accompanied by a substantial increase in the dielectric constant. For films on MgO substrates thicker than 40 nm, the analysis shows that the dielectric constant decreases significantly. We show that the theoretical approach not only predicts general trends but is also in good quantitative agreement with the experimental data reported in literature.

Misfit Strain Driven Phase Transformations in Epitaxial Barium Strontium Titanate Films

AbstractWe develop phase diagrams for single domain epitaxial barium strontium titanate films on cubic substrates as a function of the misfit strain based on a Landau-Devonshire phenomenological model similar to the one developed by Pertsev et al. [Phys. Rev. Lett. 80, 1988 (1998)]. Unusual ferroelectric phases that are not possible in single crystals and bulk ceramics are demonstrated in epitaxially constrained BST films. The misfit strain is correlated with the film thickness quantitatively by taking into account the formation of misfit dislocations that relieve epitaxial stresses during deposition. Theoretical estimation of the dielectric constant of (001) Ba0.7Sr0.3TiO3 and Ba0.6Sr0.4TiO3 films grown on Si, MgO, LaAlO3, and SrTiO3 substrates as a function of film thickness is provided. It is shown that the selection of the substrate and the film thickness can be chosen as design parameters to manipulate the internal stress level in the film to achieve enhanced dielectric response.

Tuning the Tunability in Epitaxial Barium Strontium Titanate Film via Internal Stresses

ABSTRACTThe tunability of epitaxial barium strontium titanate films is analyzed theoretically for the first time using a modified phenomenological Landau-Devonshire approach taking into account the formation of unusual ferroelectric phases that cannot form in bulk and single-crystal ceramics. It is shown that enhanced tunability can be achieved by adjusting the misfit strain especially in the vicinity of a structural phase transformation. The internal stresses in epitaxial films as characterized by the misfit strain can be controlled by changing the substrate material and/or varying the film thickness. The latter is due to the possibility of stress relaxation by formation of misfit dislocations that relieve epitaxial stresses at film growth temperature. Based on the thermodynamic model, we provide quantitative estimations of tunability of (001) Ba0.5Sr0.5TiO3 films on (001) LaAlO3 (LAO) and SrTiO3 (STO) as a function of film thickness. The analysis indicates that films on STO substrates should be as thick as possible in order to achieve optimum tunability. To obtain maximum tunability on LAO substrates, the thickness of films should be as close as possible to a critical film thickness (∼120 nm).

Thin films of CCVD Ba0.5Sr0.5TiO3 and SrTiO3 for electrically tunable coupled microstripline phase shifters

Epitaxial barium strontium titanate and strontium titanate thin films were deposited on (100) MgO single-crystal substrates, utilizing Combustion Chemical Vapor Deposition. The as-deposited films were patterned with gold electrodes to form interdigitated structure to function as electrically tunable devices, namely, coupled microstripline phase shifters (CMPS). Microwave dielectric properties were characterized at 11–20 GHz with an applied electric field up to ±35 V/μm. A maximum figure of merit of 53°/dB at 20 GHz and 23°C was measured. The demonstrated high degree of tunability and relatively low loss showed great potential of these films for the targeted frequency agile device applications. Characterization, using SEM, EDS, and XRD, is presented in addition to microwave performance as a function of dc bias and frequency.

Thin Film Dielectrics for Electronics Using Combustion Chemical Vapor Deposition

AbstractThis paper reviewed work to date on multicomponent oxides deposited, utilizing openatmosphere Combustion Chemical Vapor Deposition for electronic applications. Epitaxial barium strontium titanate and strontium titanate thin films were deposited on (100) MgO single crystal substrates. They were patterned to form interdigitated structures for electrically tunable devices, namely, coupled microstripline phase shifters (CMPS). The undoped, as-deposited perovskite dielectrics exhibited a figure of merit of 53°/dB at 20 GHz and 23°C, indicating high degree of tunability and fairly low loss. High-permittivity (ε=263), polycrystalline BST and SrTiO3 were studied for dynamic random access memory, and leakage current density of 10−7 A/cm2 was measured. Intended for non-volatile ferroelectric memory, lead zirconium titanate was deposited onto a seed layer of perovskite structure to prevent the growth of the unwanted pyrochlore phase. To function as buffer layers for superconductor applications, epitaxial CeO2, YSZ, SrTiO3, LaAlO3, Y2O3, and Yb2O3 coatings on single crystal and textured nickel substrate were investigated. Electronic analyses and characterization, using SEM, EDS, XRD, and X-ray pole figures, were presented.