In-plane Dielectric Properties Research Articles

On the Evolution of Stress and Microstructure in Radio Frequency-Sputtered Lead-Free (Ba,Ca)(Zr,Ti)O3 Thin Films

Thin-film piezoelectrics are widely investigated for actuators and energy harvesters, but there are few alternatives to toxic lead zirconate titanate. Biocompatible Ca- and Zr-modified BaTiO3 (BCZT) is one of the most promising lead-free alternatives due to its high piezoelectric response. However, the dielectric/piezoelectric properties and structural integrity of BCZT films, which are crucial for their applications, are strongly influenced by the substrate upon which the film is grown and the related processing methods. Here, the in-plane stress, microstructure, dielectric, and piezoelectric properties of 100–500 nm thick high-temperature RF-sputtered BCZT films on industrially relevant Si-based substrates were investigated. Obtaining polycrystalline piezoelectric films required deposition temperatures ≥ 700 °C, but this induced tensile stresses of over 1500 MPa, which caused cracking in all films thicker than 200 nm. This degraded the dielectric, piezoelectric, and ferroelectric properties of films with larger electrode areas for applications. Films on SrTiO3, on the other hand, had a compressive residual stress, with fewer defects and no cracks. The grain size and surface roughness increased with increasing deposition temperature. These findings highlight the challenges in processing BCZT films and their crucial role in advancing lead-free piezoelectric technologies for actual device applications.

Impact of Aluminum Modification on the Dielectric Properties of Graphene Oxide Membrane: Different Al Source

The high dielectric constant and low dielectric loss of Graphene oxide (GO) membranes made them good dielectric separator membranes for both dielectric capacitors and planar micro-supercapacitors. A GO membrane, which is GO sheets laminated in the form of “brick-and-mortar” stacking configuration, is an orthotropic material. This means that GO will have different dielectric properties parallel to the GO sheets (in-plane) and perpendicular to the GO sheets (through-plane). Tailoring strategies applied to the membrane will impact its in-plane and through-plane dielectric property differently. Better understanding and ability to tailor the dielectric properties of the GO membrane is crucial for its dielectric applications. The Al3+ has the potential to interact with OFGs of GO sheets on the edge (carboxyl groups), which mainly impacts in-plane properties, or on the basal plane (hydroxyl, carbonyl, and carboxyl groups), which mainly impacts through-plane properties. The uneven interaction of Al3+ with the functional group on the edge and basal plane of GO sheets could lead to different modification effects of the in-plane and through-plane dielectric properties of the GO membrane. It was reported before that Al3+ from different sources will bond with the GO sheet differently, thus it has the potential to impact the through-plane and in-plane dielectric properties of the GO membrane differently. Here, the Al3+-modification to the GO membrane from three different Al source, Al metal foil, chloride salt, and alumina, were applied to tailor the through-plane and in-plane dielectric properties of as-fabricated GO membranes in the frequency range from 0.01 Hz to 105 Hz. Plane-to-plane and edge-to-edge coordination of Al3+ with the functional group on the basal plane or edge of the GO sheets were proposed to understand the different impact of Al3+ on the through-plane and in-plane dielectric properties of the GO membranes. This study provides a facile route to impact the dielectric property of GO membrane through interaction with the oxygenated functional group on the edge and basal plane of the GO sheet, and demonstrate the possibility of designing dielectric GO membrane with controllable dielectric performance.

In-plane and through-plane dielectric properties of graphene oxide membrane: Effect of Al3+ modification

A graphene oxide (GO) membrane can be used in structural dielectric capacitors and planar micro-supercapacitors as a dielectric separator material due to its excellent through-plane and in-plane dielectric properties. The ability to tailor the dielectric properties of the GO membrane is crucial for dielectric applications. We believe this to be the first demonstration describing how Al3+ ions impact the dielectric properties of the GO membrane through cation to oxygenated functional groups interactions. In this study, the Al3+-modification to the GO membrane was realized through a facile pre-coordination method followed by layer-by-layer assembling. The influence of Al3+ on the GO membranes’ through-plane and in-plane dielectric properties (dielectric constant and dielectric loss) was investigated in the frequency range from 0.01 Hz to 105 Hz. Plane-to-plane and edge-to-edge Al3+coordination with the functional group on the basal plane or edge of the GO sheets was proposed to understand the metal ion's impact on the dielectric properties. The results demonstrated that the Al3+ modification was an effective strategy to tailoring the GO membranes’ dielectric properties in a low frequency regime and further suggested the possibility of designing dielectric GO membranes with controllable dielectric performance. At a low frequency range (0.01–1 Hz), the addition of Al3+ increased the through-plane dielectric constant and decreased the in-plane dielectric constant. In terms of dielectric loss, unmodified GO membranes exhibited a higher in-plane dielectric loss than the through-plane dielectric loss. Al3+-modified GO membranes exhibited increased through-plane dielectric loss and decreased in-plane dielectric loss compared to the unmodified GO membranes.

Abnormal in-plane permittivity and ferroelectricity of confined water: From sub-nanometer channels to bulk

Dielectric properties of nano-confined water are important in several areas of science, i.e., it is relevant in the dielectric double layer that exists in practically all heterogeneous fluid-based systems. Molecular dynamics simulations are used to predict the in-plane dielectric properties of confined water in planar channels of width ranging from sub-nanometer to bulk. Because of suppressed rotational degrees of freedom near the confining walls, the dipole of the water molecules tends to be aligned parallel to the walls, which results in a strongly enhanced in-plane dielectric constant (ε∥) reaching values of about 120 for channels with height 8 Å < h < 10 Å. With the increase in the width of the channel, we predict that ε∥ decreases nonlinearly and reaches the bulk value for h > 70 Å. A stratified continuum model is proposed that reproduces the h > 10 Å dependence of ε∥. For sub-nanometer height channels, abnormal behavior of ε∥ is found with two orders of magnitude reduction of ε∥ around h ∼ 7.5 Å, which is attributed to the formation of a particular ice phase that exhibits long-time (∼μs) stable ferroelectricity. This is of particular importance for the understanding of the influence of confined water on the functioning of biological systems.

In-plane characterization of PZT thin films for the creation of a general impedance model

The in-plane dielectric and ferroelectric properties of preferentially oriented lead zirconate titanate (PZT) thin films are characterized using interdigitated transducers (IDTs). By combining finite element method simulations and capacitance measurements, values of the dielectric constant of films with thicknesses between 150 nm and 800 nm are obtained. A modified Sawyer–Tower circuit is used to investigate the polarization loops measured in-plane using IDT electrodes. A well-defined hysteresis loop is obtained demonstrating the switching of the polarization of the ferroelectric domains. Leakage current measurements reveal high resistivity and are an indication of the high quality of the PZT film. The obtained characteristics are used to determine the total impedance of the IDT-PZT structure. Here, the structure is represented by an equivalent ladder circuit using the inductance and resistance of the IDT electrodes and the capacitance and conductance of the PZT film. The obtained total impedance matches low frequency measurements.

Anisotropic in-plane dielectric and ferroelectric properties of tensile-strained BaTiO3 films with three different crystallographic orientations

Ferroelectric properties of films can be tailored by strain engineering, but a wider space for property engineering can be opened by including crystal anisotropy. Here, we demonstrate a huge anisotropy in the dielectric and ferroelectric properties of BaTiO3 films. Epitaxial BaTiO3 films deposited on (100), (110), and (111) SrTiO3 substrates were fabricated by chemical solution deposition. The films were tensile-strained due to thermal strain confirmed by the enhanced Curie temperature. A massive anisotropy in the dielectric constant, dielectric tunability, and ferroelectric hysteresis loops was observed depending on the in-plane direction probed and the orientation of the films. The anisotropy was low for (111) BaTiO3, while the anisotropy was particularly strong for (110) BaTiO3, reflecting the low in-plane rotational symmetry. The anisotropy also manifested at the level of the ferroelectric domain patterns in the films, providing a microscopic explanation for the macroscopic response. This study demonstrates that the properties of ferroelectric films can be tailored not only by strain but also by crystal orientation. This is particularly interesting for multilayer stacks where the strain state is defined by the boundary conditions. We propose that other materials can be engineered in a similar manner by utilizing crystal anisotropy.

Epitaxial growth and in-plane dielectric properties of DyMnO3 by pulsed-laser deposition

Perovskite manganites DyMnO3 thin film was grown epitaxially on LaAlO3 (110) substrate by using pulsed-laser deposition (PLD) technique. The film showed perfectly orthorhombic crystallization and was well-aligned with the substrates. The in-plane dielectric properties of DyMnO3 film were measured as functions of temperature (80–300 K) and frequency (120–100 kHz) by using parallel-plate interdigital electrodes (IDEs). Distinct transitions can be observed in the complex permittivity and loss tangent and the respective peaks shifted to higher temperatures as the measuring frequency increased. By the Arrhenius plots, the determined activation energy of the thermally activated dielectric relaxations was about 0.20 eV. According to the relationship of the real and imaginary parts of the complex permittivity, the in-plane dielectric properties of epitaxial DyMnO3 thin film can be described as the universal dielectric response (UDR) behavior.

Structure, lattice dynamics, properties of bismuth titanate heterostructures

We investigated the epitaxial Bi4Ti3O12 (BTO) thin films with different thicknesses on a (0 0 1) MgO substrate with a Ba0.4Sr0.6TiO3 buffer layer (4 nm). The crystallographic axis [1 0 0] of Bi4Ti3O12 film is rotated on degrees relative to [1 0 0] MgO axis. When the thickness of Bi4Ti3O12 film is less than 40 nm, the unit cell of the film is compressed in the normal to the interface plane direction. In cases of large film thicknesses, the sign of strain changes and correspondingly the unit cell is stretched. Switched spontaneous polarization in the interface plane with 180° domain structure occurs at 10 nm film thickness and increases with a thickness up to 55 C cm−2. Changes in the Raman spectra of the Bi4Ti3O12 films indicates to increase of monoclinic distortion of the film unit cell in comparison with the bulk unit cell. The anisotropy of in-plane dielectric properties and the effect of internal strain on dielectric properties for the Bi4Ti3O12 films are confirmed by the study of the dielectric characteristics of the films.

Enhanced in-plane ferroelectricity in BaTiO3 thin films fabricated by aqueous chemical solution deposition

Ferroelectric BaTiO3 is widely used in capacitors, but the low Curie temperature limits a further use of BaTiO3. In this work we present an aqueous chemical solution deposition (CSD) route for BaTiO3 thin films, demonstrating that organic solvents are not required for CSD. Textured BaTiO3 thin films were deposited on SrTiO3 substrates. The in-plane dielectric properties were investigated using interdigitated electrodes and ferroelectric switching was observed up to 160±5 °C. The increased Curie temperature is proposed to result from thermal strain due to a mismatch in thermal expansion coefficient between the film and the substrate, and is in good agreement with the theory of strain engineering in BaTiO3. Finally, the decomposition and crystallization of BaTiO3 during thermal treatment were determined by the combination of thermal analysis, IR spectroscopy and X-ray diffraction of powder prepared from the solution.

Polar phonons in polycrystalline SrTiO3 thin films: The role of the substrate

Recently we have demonstrated that out-of-plane dielectric properties of polycrystalline SrTiO3 (ST) films depend on the strain/stress induced by the substrates due to thermal expansion misfit. Here, we present a complementary infra-red (IR) spectroscopy investigation of the role of Al2O3 and MgO substrates on the lattice dynamics and in-plane dielectric properties of ST thin films also prepared by sol-gel. We verify that ST films deposited on Al2O3 substrates, being under a tensile stress, present the soft mode behaviour similar to that of ST single crystals, deviating just below 150 K. In contrast, ST films deposited on MgO substrates, being under compressive stress, have higher soft mode frequency from 10 to 300 K. It is thus proved that for polycrystalline ST films the intrinsic in-plane relative permittivity may be tuned through an induced stress state.

Influence of the vicinal surface on the anisotropic dielectric properties of highly epitaxial Ba0.7Sr0.3TiO3 thin films.

Epitaxial thin films of Ba0.7Sr0.3TiO3 (BST) were grown on the designed vicinal single-crystal LaAlO3 (001) substrates to systematically investigate the evolution of microstructures and in-plane dielectric properties of the as-grown films under the strains induced by surface step terraces. Anisotropic dielectric properties were observed, which can be attributed to different tetragonalities induced by vicinal LaAlO3 substrates with miscut orientations along the [100] and [110] directions with different miscut angles of 1.0°, 2.75° and 5.0°. A terrace geometric model with both compressive and tensile strained domains in the BST film was established, which is in good agreement with the experimental results. Our experimental studies not only shed new light on the heteroepitaxial growth mechanism, but also provide a promising platform for the design and integration of high performance device applications.

Epitaxial growth and in-plane dielectric properties of orthorhombic HoMnO3 films

Orthorhombic HoMnO3 (HMO) thin films were grown epitaxially on LaAlO3 (001) substrates by using pulsed laser deposition technique. The films showed perfect orthorhombic crystallization and were well-aligned with the substrates. The in-plane dielectric constant and loss of HMO films were measured as functions of temperature (80–300 K) and frequency (120 Hz–100 kHz) by using coplanar interdigital electrodes. Two thermally activated dielectric relaxations were found, and the respective peaks shifted to higher temperatures as the measuring frequency increased. The in-plane dielectric properties of epitaxial orthorhombic HMO films were considered as universal dielectric response behavior, and the dipolar effects and the hopping conductivity induced by the charge carriers were used to explain the results.

In-plane dielectric properties of epitaxial orthorhombic HoMnO3 films grown on LaAlO3 substrates

Orthorhombic HoMnO3 thin films were grown epitaxially on LaAlO3 (001) substrates by using a pulsed laser deposition technique. The films showed perfectly orthorhombic crystallization and were well-aligned with the substrates. The in-plane dielectric constant and the dielectric loss of HoMnO3 films were measured as functions of temperature (80 ∼ 300 K) and frequency (120 ∼ 100 kHz) by using interdigital surface electrodes. Two thermally-activated dielectric relaxations were found, and the respective peaks shifted to higher temperatures as the measuring frequency was increased. The in-plane dielectric properties of epitaxial orthorhombic HoMnO3 films were considered to have a universal dielectric response behavior, and the dipoar effects and the hopping conductivity induced by the charge carriers were used to explain the results. The dielectric properties at low temperatures, which showed the multiferroicity, of the orthorhombic HoMnO3 films are discussed.

In-plane dielectric properties of epitaxial Ba0.7Sr0.3TiO3 thin films grown on GaAs for tunable device application

We have epitaxially deposited ferroelectric Ba0.7Sr0.3TiO3 (BST) thin films grown on GaAs substrate via SrTiO3 buffer layer by laser molecular beam epitaxy. Structural characteristics of the heterostructure were measured by various techniques. The in-plane dielectric properties of the heteroepitaxial structure under different applying frequency were investigated from −190 to 90 °C, indicating Curie temperature of the BST film to be around 52 °C. At room temperature, the dielectric constant of the heterostructure under moderate dc bias field can be tuned by more than 30% and K factor used for frequency agile materials is found to be close to 8. Our results offer the possibility to combine frequency agile electronics of ferroelectric titanate with the high-performance microwave capabilities of GaAs for room temperature tunable device application.

Barium Strontium Zirconate Titanate (Ba,Sr)(Zr,Ti)O3 Thin Films for Tunable Microwave Applications

(Ba0.55Sr0.45)(Zr0.1Ti0.9)O3 (BSZT) thin films were deposited by pulsed laser deposition on LSAT(001) substrates. X-ray diffraction characterization reveals a good quality of crystallization and epitaxial nature of the films. The in-plane dielectric properties of the films were characterized over a wide frequency range from 50 MHz to 20 GHz by using ring resonator-structured samples. Based on the S-parameter measurements and electromagnetic simulation it was found that microwave dielectric constant of the BSZT thin film is ∼350. Large dielectric tunability was also observed in this material.

Thickness dependence of strain and in-plane dielectric properties of highly (001) oriented (Ba,Sr)TiO 3 thin films

Highly (001) oriented (Ba,Sr)TiO 3 thin films, grown on (001) LaAlO 3 substrates by pulsed laser deposition, exhibit strong variation of strain over the thickness range of 20–800 nm. The tensile elastic residual strain reaches a minimum value at a thickness of 250 nm, while the inhomogeneous strain decreases gradually with increasing film thickness. The 250-nm-thick film has the largest in-plane dielectric constant due to a smallest tensile elastic strain in the film and the largest in-plane tunability of 40% is achieved in the thickest film.

In-plane homogeneity and lipid dynamics in tethered bilayer lipid membranes (tBLMs).

Tethered bilayer lipid membranes (tBLMs) were prepared by the self-assembly of thiolated lipidic anchor molecules on gold, followed by phospholipid precipitation via rapid solvent exchange. They were characterized by their in-plane structure, dynamics and dielectric properties. We find that the in-plane homogeneity and resistivity of the tBLMs depend critically on a well-controlled sample environment during the rapid solvent-exchange procedure. The in-plane dynamics of the systems, assessed by fluorescence correlation spectroscopy (FCS) as the diffusivity of free, labeled phospholipid dissolved in the membrane, depend on the density of the lipidic anchors in the bilayer leaflet proximal to the substrate as well as on details of the molecular structure of the anchor lipid. In DOPC tBLMs in which tethers are laterally dilute (sparsely tethered bilayer lipid membranes, stBLMs), measured diffusivities, D ≈ 4 μm(2) s(-1), are only slightly greater than those reported in physisorbed bilayers (M. Przybylo, J. Sykora, J. Humpolíckova, A. Benda, A. Zan and M. Hof, Langmuir, 2006, 22, 9096-9099). However, when we distinguish label diffusion in the proximal and in the distal bilayer leaflets, we observe distinct diffusivities, D ≈ 2 μm(2) s(-1) and 7 μm(2) s(-1), respectively. The value observed in the distal leaflet is identical to that in free membranes. stBLMs completed with phytanoyl lipids (DPhyPC) show consistently lower label diffusivity than those completed with unsaturated chains (DOPC). As the length of the tether chain increases, a reduction in the apparent diffusivity is observed, which we interpret as an increased propensity of the proximal bilayer leaflet to host free lipid. We also investigated preparation conditions that control whether the tBLMs are laterally homogeneous, as assessed by optical microscopy. In laterally heterogeneous bilayers, the label diffusivity varies only by a factor of ~2 to 4, indicating that the regions in the bilayers with different label solubilities do not correspond to distinct phases, such as a fluid phase coexisting with a gel phase.

Lipid Diffusion in Tethered Bilayer Lipid Membranes (tBLMs)

The complexity of cells makes molecular-scale characterizations of structure and interactions of biomembranes in vivo extremely difficult, thus driving the development of synthetic membrane models. tBLMs are resilient biomimetic systems stabilized by the proximity of an inorganic interface.1,2 We characterize their in-plane structure, dynamics and dielectric properties using fluorescence microscopy, fluorescence correlation spectroscopy (FCS) and electrochemical impedance spectroscopy (EIS). The in-plane dynamics of tBLMs depend on structural details of the anchor lipid and its lateral density in the bilayer leaflet proximal to the substrate.3 In tBLMs with homogeneous lateral label distributions, the fluidity of the distal leaflet is comparable to that in vesicle membranes (2D diffusion constant, D ∼ 7 μm2/s) while that in the proximal leaflet is moderately reduced (D ∼ 2-3 μm2/s). tBLMs completed with phytanoyl lipids (DPhyPC) show lower label diffusivity than those completed with unsaturated chains (DOPC). In laterally heterogeneous bilayers, the label diffusivity varies only slightly, indicating that distinct regions in the bilayers do not correspond to distinct phases. Concurrently, we investigate the effect of charged lipids (DOPS) and cholesterol on lipid diffusivity. This aims at a characterization of changes in membrane dynamics as amyloid-β (Aβ) oligomers associate with tBLMs where they have been shown to affect the resistance of the bilayer to ion conduction.4,5Supported by the NIH (1P01AG032131) and the AHAF (A2008-307).1McGillivray, D.J., et al. 2007. Biointerphases 2:21-33.2Vockenroth, I.K., et al. 2008. Biointerphases 3:FA68-73.3Shenoy, S., et al., 2009. Soft Matter, submitted.4Arispe, N., et al., 1993. Proc. Natl. Acad. Sci. U.S.A. 90:567-571.5Valincius, G., et al., 2008. Biophys. J. 95:4845-4861.

Compositional Dependence of Structure and Dielectric Properties in Ba(ZrxTi1 - x)O3 Thin Films Grown by Pulsed Laser Deposition

Ba(Zr x Ti 1− x )O 3 (BZT) thin films with various compositions (x = 0.20, 0.25, 0.30, 0.35) were epitaxially grown on (LaAlO 3 ) 0.3 (Sr 2 AlTaO 6 ) 0.35 [LSAT] (001) single crystal substrates using pulsed laser deposition. By x-ray diffraction measurements it was found that all the films exhibited a tetragonally distorted lattice structure, which is different from that of the constituent ceramics. Consequently, an upward shifted Curie temperature was observed in each of the films. The in-plane dielectric properties were determined. At room temperature, a large dielectric tunabilities of 50% (at 500 MHz) was achieved at x = 0.30, showing its potential for use in tunable microwave devices.

The In-Plane Dielectric and Microwave Properties of Barium Stannate Titanate Thin Films

The microstructure and in-plane dielectric and microwave properties of barium stannate titanate Ba(Sn 0.15 Ti 0.85 )O 3 (BTS) thin films grown on c-plane Al 2 O 3 substrates through sol-gel process are investigated. X-ray diffraction and field emission scanning electron microscopy are used to characterize phase structure and microstructure of the thin films, respectively. Using interdigital capacitors (IDC) with Au electrodes on thin films, the dielectric constant and loss tangent as a function of applied voltage (0–40 V) are measured at low frequencies up to 1 MHz. Microwave properties of the films are measured at 10 GHz. This work clearly reveals the highly promising potential of BTS films for application in tunable microwave devices.