High Dielectric Materials Research Articles

Fabrication and structural characterization of (Ba,Sr)TiO 3 thin films produced by Electrostatic Spray Assisted Vapour Deposition

Thin films of perovskite (Ba,Sr)TiO 3 (BST) system are promising candidates for microelectronic devices that can be integrated into semiconductor technology. The lead-free solid solution Ba x Sr 1− x TiO 3 is a high permittivity dielectric material with low loss and suitable for use in dynamic random access memory cells, tunable microwave devices, by-pass capacitors and infrared detectors. A versatile and cost-effective Electrostatic Spray Assisted Vapour Deposition (ESAVD) process has been used to fabricate nanostructured BST film. The crystal structure, surface morphology and homogeneity of composition of BST thin films have been investigated using a combination of XRD, SEM, AFM, Raman spectroscopy and XPS techniques.

Dielectric slab Rotman lens for microwave/millimeter-wave applications

A new form of a Rotman lens is proposed for microwave/millimeter-wave applications such as a collision-avoidance radar. The proposed lens can be described as a dielectric slab fed by slot lines. The new form is expected to show lower loss and lower mutual coupling than the conventional Rotman lenses fabricated with conducting plates at millimeter-wave frequency. Taking the field distribution inside the dielectric slab into account, the TE/sub 0/ mode was chosen to excite the dielectric slab lens. The dielectric Rotman lens consists of a dielectric slab, tapered slot structure, and the transitions between the antipodal slots and microstrip lines for subminiature A connectors. The conventional design equations have been modified for use in designing the dielectric slab Rotman lens with a high dielectric material. A prototype was implemented with nine beam ports and nine array ports. Measurements from 10 to 20 GHz show that mutual coupling can be lowered at higher frequency. The obtained efficiency of the dielectric slab lens system is approximately 30%. The efficiency of the lens is comparable to that of the conducting plate lenses even though there is a spillover loss from the dielectric slab.

Effect of mode-stirrer configurations on dielectric heating performance in multimode microwave applicators

In this paper, several mode-stirrer configurations are compared in order to establish their influence on the electric-field uniformity within an irradiated dielectric sample inserted in a microwave-heating applicator. Two different scenarios are evaluated with metallic sheets moving inside the multimode applicator. The different stirrer configurations are tested and compared for low-, medium-, and high-loss dielectric sample materials. Additionally, a straightforward procedure based on a generalized plane-wave approach is proposed and evaluated as a computationally efficient alternative for calculating the electric-field distribution inside materials processed in these microwave applicators with mode stirrers. Although very different electric patterns are achieved depending on stirrer geometry and sample permittivity, the plane-wave approach has been shown to provide a very good approximation for medium and high lossy dielectric materials.

Role of ceramic supports on microwave heating of materials

A detailed analysis has been carried out to study efficient heating due to microwaves for one-dimensional samples placed on ceramic supports (Al2O3, SiC). The greater effects on microwave heating of samples have been illustrated via average power within a sample versus sample thickness diagram for various cases. The maxima in power, also termed as “resonances,” is observed for specific sample thicknesses and the two consecutive maxima in average power are termed as R1 and R2 modes. The greater heating effects leading to hot spots would occur in water samples during both-sides incidence when the sample is kept on Al2O3 support. SiC support may be recommended for water samples due to uniform heating throughout the sample. In contrast, SiC support could cause local hot spots or thermal runaway for oil samples. The localized hot spots are more pronounced for the samples exposed to microwaves on both faces. The choice of support may not be trivial due to the complex dielectric response of sample-support assembly. Current analysis has been illustrated for low- and high-dielectric materials (water and oil) and a representative case study has also been shown for beef samples. Based on such observations, a generalized heating strategy for materials due to uniform plane waves has been derived.

Synthesis of nanotabular barium titanate via a hydrothermal route

As layer thickness of multilayer ceramic capacitors decreases, nanoparticles of high dielectric materials, especially BaTiO3,are needed. Tabular metal nanoparticles produce thin metal layers with low surface roughness via electrophoretic deposition. To achieve similar results in dielectric layers requires the synthesis and dispersion of tabular BaTiO3nanoparticles. In the current study, the synthesis of BaTiO3was investigated using a hydrothermal route. Transmission electron microscopy and atomic force microscpy analyses show that the synthesized particles are single crystal with a 〈111〉 zone axis and a median thickness of 5.8 nm and face diameter of 27.1 nm. Particle growth is likely controlled by the formation of {111} twins and the synthesis pH, which stabilizes the {111} face during growth. With limited growth in the 〈111〉 direction, the particles develop a platelike morphology. Physical property characterization shows the powder is of high purity with low hydrothermal defect concentrations and controlled stoichiometry.

Novel Dielectric Materials for Organic Electronics

AbstractWe present our latest results on the design and fabrication of very high capacitance dielectric materials for organic field-effect transistors. We will show that utilization of appropriate self-assembling siloxane building blocks and polymer matrices allows solution-processed, pinhole-free organic dielectrics. Electrical (MIS, TFT) data demonstrate that these insulators can be efficiently integrated into large TFT structures. These devices function for both p- and n-channel semiconductors, the molecular components of which exhibit greatly different core structures and substituent functionalities. Substantial TFT response is achieved at very low operational biases (<1V), without serious leakage currents (<10-8 A/cm at 1V).

Mechanically Tunable Nanophotonic Devices

AbstractWe report a novel tunable nanophotonic device concept based on Mechanically Controlled Photonic Crystal (MCPC), which is comprised of a periodic array of high index dielectric material and a low index polymer. Tunability is achieved by applying mechanical force with nano-/micro-electron-mechanical system actuators. The mechanical stress induces changes in the periodicity of the photonic crystal, to which the photonic band structure is extremely sensitive. This consequently produces tunability much greater than that achievable by electro-optic materials such as liquid crystal. Our theoretical investigations revealed that we could achieve dynamic beam steering over a wide range of angles up to 75° with only 10% mechanical stretching. We also predicted tunable sub-wavelength imaging in which we could tune the frequency response and focal length of negative index PC lens. For experimental demonstration, we fabricated the PC structures on Si-on-insulator substrates. Optical characterizations clearly showed the anticipated negative refraction in which the incident beam was refracted back to the side it was incident. The experimental demonstration of negative refraction at optical frequencies in a Si-based photonic crystal structure is a significant step toward the next-generation nanophotonics.

Experimental Verification of Mode Coupling Phenomenon in High Permittivity NRD Guide with Small Remaining Asymmetrically Air Gap

It is known that a high permittivity NRD guide suffers from irregular transmission phenomena. In this study, we clarified that this problem is caused by a mode coupling phenomenon between the operating and parasitic modes due to a small remaining asymmetrically air gap between the metal plates and high permittivity dielectric materials.

A compact wide-band EBG structure utilizing embedded resonant circuits

This letter presents a wide-band electromagnetic bandgap (EBG) structure designed utilizing embedded resonant circuits (ERCs). The EBG structure is a periodic material whose unit cell is composed of a number of small resonant LC circuits. For EBG composed of single resonant LC circuits above the resonant frequency, the structure behaves as a negative permeability material that provides the bandgap behavior. To design a compact wide-band EBG, a structure constructed of three layers of ERC, each having different resonant frequencies separated by layers of impedance inverters, is proposed. The impedance inverter layer is designed using I-shaped metallic inclusions printed in the host medium to achieve a high dielectric material so as to physically reduce the thickness of the quarter wavelength inverters. The design and performance characteristic of a complete EBG structure with isotropic bandgap behavior almost independent of incident angle and polarization state is also demonstrated.

Analysis of Microwave Propagation for Multilayered Material Processing: Lambert's Law versus Exact Solution

Microwave heating of a multilayered system has wide applications in thawing, drying, hyperthermia treatment, and many more. A typical multilayered system can be viewed as a combination of low and high dielectric materials. The propagation of microwaves within a multilayered system can be represented either by exponential Lambert's law or by an exact solution of Maxwell's equation. The exact solution for microwave propagation is based on both the transmitted and reflected waves, whereas Lambert's law is based purely on transmission. Lambert's law is a good approximation for a semi-infinite sample, and the validity of Lambert's law for a multilayered assembly is limited to orientation of various dielectrics. Two specific cases for a multilayered sample are considered, and the detailed scattering effects during microwave propagation have been analyzed via Lambert's law and the exact solution.

Antenna Miniaturization and Bandwidth Enhancement Using a Reactive Impedance Substrate

The concept of a novel reactive impedance surface (RIS) as a substrate for planar antennas, that can miniaturize the size and significantly enhance both the bandwidth and the radiation characteristics of an antenna is introduced. Using the exact image formulation for the fields of elementary sources above impedance surfaces, it is shown that a purely reactive impedance plane with a specific surface reactance can minimize the interaction between the elementary source and its image in the RIS substrate. An RIS can be tuned anywhere between perfectly electric and magnetic conductor (PEC and PMC) surfaces offering a property to achieve the optimal bandwidth and miniaturization factor. It is demonstrated that RIS can provide performance superior to PMC when used as substrate for antennas. The RIS substrate is designed utilizing two-dimensional periodic printed metallic patches on a metal-backed high dielectric material. A simplified circuit model describing the physical phenomenon of the periodic surface is developed for simple analysis and design of the RIS substrate. Also a finite-difference time-domain (FDTD) full-wave analysis in conjunction with periodic boundary conditions and perfectly matched layer walls is applied to provide comprehensive study and analysis of complex antennas on such substrates. Examples of different planar antennas including dipole and patch antennas on RIS are considered, and their characteristics are compared with those obtained from the same antennas over PEC and PMC. The simulations compare very well with measured results obtained from a prototype /spl lambda//10 miniaturized patch antenna fabricated on an RIS substrate. This antenna shows measured relative bandwidth, gain, and radiation efficiency of BW=6.7, G=4.5 dBi, and e/sub r/=90, respectively, which constitutes the highest bandwidth, gain, and efficiency for such a small size thin planar antenna.

Structural and electrical properties of (1− x)(Na 1/2Bi 1/2)TiO 3– xPb(Mg 1/3Nb 2/3)O 3 solid solution

Structural, dielectric and piezoelectric properties of (1− x)(Na 1/2Bi 1/2)TiO 3– xPb(Mg 1/3Nb 2/3)O 3 (NBT– xPMN) solid solution have been investigated. An addition of PMN into NBT transformed the structure of sintered samples from rhombohedral to pseudocubic phase where x is larger than 0.1. In calcined powders, however, the intermediate structure were observed between rhombohedral and cubic phases near x=0.1. The formation of solid solution between NBT and PMN modified the dielectric and piezoelectric properties of NBT to be suitable for high temperature dielectric and piezoelectric material. With increasing the content of PMN, the temperature-stability of ε r( T) increased and the high temperature dielectric loss decreased. In addition, the piezoelectric property of NBT– xPMN was enhanced, for the decrease of coercive field and conductivity promoted the domain reversal under the high electric field of the poling process.

VUV spectroscopic ellipsometry applied to the characterization of high-k dielectrics

In this study, we use vacuum UV spectroscopic ellipsometry (VUVSE) to characterize new high dielectric materials. All the candidates for high-k dielectrics become strongly absorbent when the wavelength is reduced down to 140nm. Due to the high dispersion of the refractive index in the VUV range, it is easier to determine the thickness and the refractive index independently than in the visible range and much more precise structural information can be deduced. HfO2, Al2O3 and mixed HfAlOx layers have been studied with and without thin SiO2 oxide at the interface. X-ray reflectometry (XRR) has been used to measure precisely the layer thickness and roughness. The two techniques are included in the same automated metrology system dedicated to 300mm technology which is also presented. We show in particular that VUVSE can detect the crystalline character of the layers and their composition can be measured in addition to the layer thickness. Results are compared to those obtained by transmission electron microscopy (TEM), X-ray fluorescence analysis (XRF) and X-ray photoemission (XPS).

High dielectric insulation coating for time domain reflectometry soil moisture sensor

Insulation coating is often applied to time domain reflectometry (TDR) soil moisture sensors to reduce the conduction loss of energy. However, because of low dielectric constants of common plastic insulation materials, sensitivity is reduced, and the measurements are strongly dependent on coating thickness. Analytical studies clearly showed that these unwanted features could be mitigated if a high dielectric material is used for coating. An epoxy‐ceramic nanocomposite was selected as a high dielectric insulation material because of its high dielectric constant and high adhesion. Experimental work using this composite indicated that the material has the potential to be used as a high dielectric coating for TDR soil moisture probes. On the basis of the results of these analytical and experimental studies a framework for designing an improved epoxy‐ceramic composite for coating TDR soil moisture sensors is suggested.

Preparation of BaTiO 3-based X7R ceramics with high dielectric constant by nanometer oxides doping method

High performance X7R dielectric materials were prepared by doping Nb 2O 5–Co 3O 4 nanometer composite oxide to BaTiO 3 powders, with a dielectric constant as high as 5200 and dielectric loss lower than 1.0%, when they were sintered at 1300 for 2 h. The effect of Nb 2O 5–Co 3O 4 nanometer composite oxide was discussed. It was concluded that Nb 5+, Co 3+ were easier to incorporate into BaTiO 3 lattice by using Nb 2O 5–Co 3O 4 nanometer composite oxide as dopants than by doping Nb 2O 5, Co 3O 4 separately. It was favorable by nanometer oxides doping to make the system hold high dielectric constant and still satisfy the requirement of EIA X7R specification, attributed to the uniform distribution of nanometer dopants in BaTiO 3 grain boundaries and the formation of abundant “core–shell” structure with relatively thinner grain shell.

Fundamental Investigation in Two Flashover-Based Trigger Methods for Low-Pressure Gas Discharge Switches

Modern switches for pulse-power technology have special requirements such as long lifetime, reliability in a wide pressure and voltage range, as well as small delay time. In order to meet these requirements, two trigger methods were developed and examined. These two different trigger methods based on a flashover were tested for the emission behavior by variation of different parameters. The first configuration is a semiconductor surface flashover trigger, where electron emission is based on a surface flashover between the contact area of a copper spring and a carbide cylinder. The second trigger concept is the high-dielectric trigger, where electrons are released by the field emission effect at the transition between metal-vacuum and dielectric. For this system, high dielectric materials with dielectric constants in the order of 2000 are available. The electrical and optical measurements of both trigger systems were done in a modular structured vacuum chamber. For lower pressure, the high-dielectric trigger shows better performances and higher emitted charge of the electron emission within all adjusted parameters like gas pressure, applied voltage, and different wirings. In addition to the higher emitted charge, the emitted electrons from the high-dielectric material have higher energies. For the lifetime characteristic, the high-dielectric trigger shows lifetimes much higher than 100 million discharges.

Complex, 3D Photonic Crystals Fabricated by Atomic Layer Deposition

ABSTRACTRecently we have demonstrated the potential of Atomic Layer Deposition (ALD) for the fabrication of advanced luminescent photonic crystal (PC) structures based on the inverse opal architecture.[1–3] PC's offer efficiency enhancement, decreased threshold, and other enhancements that improve phosphor performance. 3D PC structures are being extensively modeled, revealing that changes in the structures such as shifting the distribution of dielectric material can significantly improve photonic band gap (PBG) properties. For example, in the inverted “shell” structure, the width of the PBG can be increased from 4.25% to 8.6%.[4] Similarly, the PBG width can also be increased to 9.6% by formation of a non-close-packed structure.[5] Using the FDTD method, we have found that the PBG in a TiO2 non-closed packed structure can be as high as 5%. The performance of these structures depends critically on precisely and accurately placed high dielectric material. Using ALD, we have demonstrated infiltration of TiO2 films with extremely smooth surfaces (0.2–0.4 nm RMS roughness) while maintaining a high level of control over the infiltration coating thickness, enabling formation of composite infiltrated and inverse opals with nano-scale precision.Here we report progress in fabrication of multi-layered and non-close packed PCs using ALD. Two and three-layer inverse opals were formed by the deposition of thin layers of ZnS:Mn and TiO2 in stacked configuration, each exhibiting luminescence when excited by UV light. Evidence for modification of the emission characteristics by high order PBGs (gaps other than between the 2nd and 3rd bands) has been observed. In addition, non-close packed inverse opals have been formed by infiltrating heavily sintered silica opals with TiO2, etching the spheres with hydrofluoric acid, and backfilling the resulting inverse opal. Resulting structures were characterized using specular reflectance and transmission, photoluminescence, and SEM. This work demonstrates the enormous potential that ALD offers for the realization of high performance photonic crystal structures.

The connection between in vitro water uptake and in vivo skin moisturization.

Humectancy or hygroscopy is the water absorption tendency of a substance from the surroundings. Our interest, from the clinical point of view, consists of correlating this tendency in vitro and its effect in vivo for the development of drugs and formulations for the treatment of dry skin syndrome or diseases accompanied by dry skin. In vitro, water absorption was measured using the comparative isopiestic method. This method is based on bringing the vapor of the water to isothermal equilibrium between a reference system and the material to be studied. The in vivo model on guinea-pigs for the dry skin syndrome tested the therapeutic ability of mono-, di- and tri-glycerols to provide moisture to dry skin leading to healing. The moisture content in the stratum corneum was measured with a Corneometer CM 825 PC that measures merely the presence of high dielectric material (humectant or water), whereas the Mexameter MX 16 measures a pathological parameter - the erythema. Adding hydroxyl groups to a consecutive set of polyhydroxyalkanes increases the humectancy of the polyols in vitro. This elevation was found to be linear at low relative humidities (Relative humidity=31.9% and 37 degrees C). In vivo, moisture was returned to normal within a week in all three groups. However, only glycerol managed to abolish the erythema within 7 days. A rise in water absorption ability in vitro, at a rate of about 0.25 mol water per hydroxyl group was revealed in a consecutive set of glycerols (mono-, di- and tri-glycerols). One would expect that the better humectant is a material, i.e. in which the higher its physical ability to hold water in vitro the more effective it will be in recovering skin dryness. We have found, however, that glycerol, which has the lowest humectant activity in vitro, from the set of glycerols, di- and tri-glycerol, has been proven to be the best for eliminating the signs of skin dryness. Accordingly, we propose to distinguish between the in vitro humectancy (i.e. the water uptake of a material), and its in vivo moisturizing effect, i.e. its ability to cure skin dryness and erythema. This finding supports our conclusion that the connection between in vitro humectancy and in vivo moisturization is not a simple correlation.

Photonic band gaps in materials with triply periodic surfaces and related tubular structures

We calculate the photonic band gap of triply periodic bicontinuous cubic structures and of tubular structures constructed from the skeletal graphs of triply periodic minimal surfaces. The effect of the symmetry and topology of the periodic dielectric structures on the existence and the characteristics of the gaps is discussed. We find that the $\mathrm{C}({\mathrm{I}}_{2}\ensuremath{-}{\mathrm{Y}}^{**})$ structure with $\mathrm{Ia}3\ifmmode\bar\else\textasciimacron\fi{}d$ symmetry, a symmetry which is often seen in experimentally realized bicontinuous structures, has a photonic band gap with interesting characteristics. For a dielectric contrast of 11.9 the largest gap is approximately 20% for a volume fraction of the high dielectric material of 25%. The midgap frequency is a factor of 1.5 higher than the one for the (tubular) D and G structures. For a volume fraction of 25% the smallest dielectric contrast required to open a gap for the $\mathrm{C}({\mathrm{I}}_{2}\ensuremath{-}{\mathrm{Y}}^{**})$ structure is 4.5. A gap of width larger than 10% is obtained with dielectric contrasts of 7 and higher.

High-dielectric-constant all-polymer percolative composites

We report here an all-polymer high-dielectric (dielectric constant K>1000 at 1 kHz) percolative composite material, fabricated by a combination of conductive polyaniline particles (K>105) within a poly(vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene) terpolymer matrix (K>50). These high-K polymer hybrid materials also exhibit high electromechanical responses. For example, 1.5% strain, which is proportional to the square of the field applied, can be induced by a field of 9.5 MV/m, an eightfold reduction in field applied compared with that in a fluoroterpolymer matrix.