Showing papers on "Dielectric published in 1997"

A new integral equation formalism for the polarizable continuum model: Theoretical background and applications to isotropic and anisotropic dielectrics

Abstract: We present a new integral equation formulation of the polarizable continuum model (PCM) which allows one to treat in a single approach dielectrics of different nature: standard isotropic liquids, intrinsically anisotropic medialike liquid crystals and solid matrices, or ionic solutions. The present work shows that integral equation methods may be used with success also for the latter cases, which are usually studied with three-dimensional methods, by far less competitive in terms of computational effort. We present the theoretical bases which underlie the method and some numerical tests which show both a complete equivalence with standard PCM versions for isotropic solvents, and a good efficiency for calculations with anisotropic dielectrics.

Spontaneous polarization and piezoelectric constants of III-V nitrides

Abstract: The spontaneous polarization, dynamical Born charges, and piezoelectric constants of the III-V nitrides AlN, GaN, and InN are studied ab initio using the Berry-phase approach to polarization in solids. The piezoelectric constants are found to be up to ten times larger than in conventional III-V and II-VI semiconductor compounds, and comparable to those of ZnO. Further properties at variance with those of conventional III-V compounds are the sign of the piezoelectric constants (positive as in II-VI compounds) and the very large spontaneous polarization.

Non-volatile semiconductor memory cell utilizing asymmetrical charge trapping

Abstract: A novel apparatus and method for programming and reading a programmable read only memory (EPROM) having a trapping dielectric layer (20) sandwiched between two silicon dioxide layers (18, 20) is disclosed that greatly reduces the programming time of conventional PROM devices. Examples of the trapping dielectric material are silicon oxide-silicon, nitride-silicon oxide (ONO) and silicon dioxide with buried polysilicon islands. A nonconducting dielectric layer functions as an electrical charge trapping medium. This charging trapping layer is sandwiched between two layers of silicon dioxide acting as an electrical insulator. A conducting gate layer (24) is placed over the upper silicon dioxide layer (22). The memory device (10) is programmed in the conventional manner. The device, however, is read in the opposite direction from which it was written, meaning voltages are applied to the gate (24) and the source (14) while the drain is grounded. For the same applied gate voltage, reading in the reverse direction greatly reduces the potential across the trapped charge region.

Plasmon Resonance Shifts of Au-Coated Au 2 S Nanoshells: Insight into Multicomponent Nanoparticle Growth

Abstract: In this Letter we present a coherent description of the optical properties of a unique type of nanoparticle, metal-coated dielectric nanoparticles. These structures occur naturally in the form of Au-coated ${\mathrm{Au}}_{2}\mathrm{S}$ nanoparticles. During the course of nanoparticle growth, the plasmon-related absorption peak undergoes very large shifts in wavelength, from $\ensuremath{\sim}650$ to $\ensuremath{\sim}900\mathrm{nm}$. We show that this plasmon peak shift is purely classical in origin and is determined solely by the relative thickness of the Au shell and the ${\mathrm{Au}}_{2}\mathrm{S}$ core diameter. This understanding of the optical properties of these nanoparticles is used to elucidate the nanoparticle growth kinetics.

Investigation of the temperature dependence of dielectric relaxation in liquid water by thz reflection spectroscopy and molecular dynamics simulation

Abstract: We report measurements of the real and imaginary part of the dielectric constant of liquid water in the far-infrared region from 0.1 to 2.0 THz in a temperature range from 271.1 to 366.7 K. The data have been obtained with the use of THz time domain reflection spectroscopy, utilizing ultrashort electromagnetic pulses generated from a photoconductive antenna driven by femtosecond laser pulses. A Debye model with an additional relaxation time is used to fit the frequency dependence of the complex dielectric constants. We obtain a fast (fs) and a Debye (ps) relaxation time for the macroscopic polarization. The corresponding time correlation functions have been calculated with molecular dynamics simulations and are compared with experimental relaxation times. The temperature dependence of the Debye relaxation time is analyzed using three models: Transition state theory, a Debye–Stoke–Einstein relation between the viscosity and the Debye time, and a model stating that its temperature dependence can be extrapolated from a singularity of liquid water at 228 K. We find an excellent agreement between experiment and the two latter models. The simulations, however, present results with too large statistical error for establishing a relation for the temperature dependence.

Dielectric Relaxation of Biological Water

Abstract: Dielectric relaxation and NMR spectrum of water in biological systems such as proteins, DNA, and reverse micelles can often be described by two widely different time constants, one of which is in the picosecond while the other is in the nanosecond regime. Although it is widely believed that the bimodal relaxation arises from water at the hydration shell, a quantitative understanding of this important phenomenon is lacking. In this article we present a theory of dielectric relaxation of biological water. The time dependent relaxation of biological water is described in terms of a dynamic equilibrium between the free and bound water molecules. It is assumed that only the free water molecules undergo orientational motion; the bound water contribution enters only through the rotation of the biomolecule, which is also considered. The dielectric relaxation is then determined by the equilibrium constant between the two species and the rate of conversion from bound to free state and vice versa. However, the diele...

Processing and Electromechanical Properties of (Bi0.5Na0.5)(1−1.5x)LaxTiO3 Ceramics

Abstract: Bismuth sodium titanate (Bi0.5Na0.5TiO3, BNT) with 0–6 at.% lanthanum was prepared by the conventional mixed oxide method. Each composition was calcined at 800–900°C for 2–5 h to form a pure perovskite phase. Green pellets were sintered at 1050–1150°C for 1–4 h to obtain dense ceramics with at least 95% of theoretical density. X–ray diffraction (XRD) showed phase distortion as lanthanum was added to this system. Meanwhile, a small amount of La was found to affect the grain size and had an influence on the poling conditions and electrical properties. The BNT–based composition with 1 at.% La doping provided a dielectric constant (K) of 560, a piezoelectric charge constant (d33) of 92 pC/N, and a hydrostatic piezoelectric coefficient (dh) of 72 pC/N.

Dynamics of glass-forming liquids. III. Comparing the dielectric α- and β-relaxation of 1-propanol and o-terphenyl

Abstract: We have measured the dielectric relaxation of the glass-former 1-propanol for temperatures between 65 and 350 K in the frequency range 10−2 to 2⋅1010 Hz and the photon correlation spectro-scopy decays near Tg. Attributing the strong Debye-type process of 1-propanol to distinct -OH group effects leaves two faster processes related to the structural relaxation which can be identified as α-relaxation and Johari–Goldstein type β-relaxation characteristic of nonhydrogen-bonding supercooled liquids. From the temperature dependent relaxation times τ(T) regarding the three distinct loss peaks, we can specify an α-β-bifurcation temperature Tβ, which coincides with characteristic qualitative changes in the τ(T) behavior, as also observed for ortho-terphenyl and other glass-forming liquids. This assignment is confirmed by the correla-tion times derived from incoherent quasielastic light-scattering data obtained from the simultaneously measured photon-correlation spectroscopy.

The dielectric response as a function of temperature and film thickness of fiber-textured (Ba,Sr)TiO3 thin films grown by chemical vapor deposition

Abstract: The temperature- and field-dependent permittivities of fiber-textured Ba0.7Sr0.3TiO3 thin films grown by liquid-source metalorganic chemical vapor deposition were investigated as a function of film thickness. These films display a nonlinear dielectric response under conditions representative of those encountered in dynamic random access memories or other integrated capacitor applications. This behavior has the exact form expected for a classical nonlinear, nonhysteretic dielectric, as described in terms of a power series expansion of the free energy in the polarization as in the Landau–Ginzburg–Devonshire approach. Curie–Weiss-like behavior is exhibited above the bulk Curie point (∼300 K), although the ferroelectric phase transition appears frustrated. Small-signal capacitance measurements of films with different thicknesses (24–160 nm) indicate that only the first term in the power series expansion varies significantly with film thickness or temperature. Possible origins for this thickness dependence are...

The analysis of piezoelectric/piezomagnetic composite materials containing ellipsoidal inclusions

Abstract: A unified method based on the inclusion formulation is proposed to determine the magnetic, electric, and elastic fields in a composite with piezoelectric and piezomagnetic phases. The composite reinforcements are treated as ellipsoidal inclusions that enable the reinforcement geometries ranging from thin flakes to continuous fibers. Utilizing the proposed method, the magneto-electro-elastic tensors analogous to Eshelby tensors for elastic ellipsoidal inclusions are obtained. With these tensors, the magnetic, electric, and elastic fields around the inclusion as well as concentration factors are determined. Furthermore, based upon the Mori–Tanaka mean-field theory [Acta Metall. 21, 571 (1973)] to account for the interaction between inclusions and matrix, the effective magneto-electro-elastic constants (elastic moduli, piezoelectric coefficients, dielectric constants, piezomagnetic coefficients, magnetoelectric, and magnetic permeability) of the composites are expressed explicitly in terms of phase propertie...

Microdischarge behaviour in the silent discharge of nitrogen - oxygen and water - air mixtures

Abstract: In this paper measurements of the behaviour of single microdischarges between metal and glass electrodes for both negative and positive polarities of the metal (designated and transitions, respectively), as well as the overall discharge behaviour between glass electrodes (GG), will be presented for different nitrogen - oxygen and water vapour - air mixtures. Increasing the oxygen concentration in nitrogen decreases the transferred charge per microdischarge for both polarities ( and ), while the total transferred charge per cycle is increased. With increasing water content in air, more charge is transferred per microdischarge for the polarity , but no significant change in the amount of transferred charge was found for a microdischarge. Less total charge is transferred per cycle with increasing water content. This is also true for double-dielectric barrier discharges (GG). The results suggest that water vapour coats the dielectric, reducing the surface resistance and increasing the effective dielectric capacity. This mechanism is supported by analysis of the voltage versus charge plot (Lissajous figure) for the involved capacitances of a double-barrier discharge. The reduction of the surface resistance of the dielectric and the resulting increase in the effective dielectric capacitance, are shown by photographs of spatially isolated microdischarges in a metal - pin dielectric arrangement.

Rotational Dynamics of Coumarin 153: Time-Dependent Friction, Dielectric Friction, and Other Nonhydrodynamic Effects

Abstract: Subpicosecond fluorescence anisotropy measurements are used to characterize the rotational dynamics of coumarin 153 (C153) in 35 common solvents and eight solvent mixtures at room temperature. The rotational anisotropy decays of C153 are generally nonexponential as a result of the non-Markovian nature of the friction on its rotational motion. Rotational correlation times are observed to be larger in polar solvents than in nonpolar solvents of the same viscosity. This difference is examined in the context of theories of dielectric friction, which relate the extra friction in polar solute/solvent systems to long-range dipole -dipole interactions. Since the latter interactions have been thoroughly characterized via dynamic Stokes shift measurements for the same solute/solvent combinations studied here, the present data provide a unique opportunity to test general concepts of dielectric friction. Contrary to expectations, the departures from simple hydrodynamic behavior cannot be modeled using only theories of rotational dielectric friction. More important than dielectric friction is the role that the relative solute/solvent size plays in determining the extent of solute -solvent coupling. Once this size dependence is approximately accounted for, the remaining departures from simple hydrodynamic behavior are relatively small in all solvents. In polar aprotic solvents, solvation data indicate that dielectric friction effects should be rather modest (10-20% of the total friction). In these solvents no clear correlation is found between dielectric friction predictions and the observed solute -solvent coupling. However, in normal alcohol solvents the effects of dielectric friction are predicted to be large and well beyond the scatter in the experimental data. No evidence for such an important dielectric friction contribution is observed in these solvents, in spite of the fact that long-time components of the solvation dynamics do appear to be present in the rotational friction.

Complex coordinate stretching as a generalized absorbing boundary condition

Abstract: By complex coordinate stretching and a change of variables, it is shown simply that PML is reflectionless for all frequencies and all angles. Also, Maxwell's equations for PML media reduce to ordinary Maxwell's equations with complex coordinate systems. Many closed-form solutions for Maxwell's equations map to corresponding closed-form solutions in complex coordinate systems. Numerical simulations with the closed-form solutions show that metallic boxes lined with PML media are highly absorptive. These closed-form solutions lend a better understanding to the absorptive properties of PML media. For instance, they explain why a PML medium is absorptive when a dielectric or metallic interface extends to the edge to a simulation region where PML media reside. More importantly, the complex coordinate stretching method can be generalized to non-Cartesian coordinate systems, providing absorbing boundary conditions in these coordinate systems. © 1997 John Wiley & Sons, Inc. Microwave Opt Technol Lett 15: 363–369, 1997.

Intrinsic dead layer effect and the performance of ferroelectric thin film capacitors

Abstract: We apply the Thomas theory of ferroelectricity to bulk and thin film perovskite ferroelectrics in the paraelectric regime above the transition temperature. From available data on bulk SrTiO3 we are able to fully determine the parameters in the Thomas theory for this material, with overall reasonable results, supporting its validity. In a new application of the Thomas theory to the surface of a thin ferroelectric film in the linear response regime, it is found that there is anticipated to be an intrinsic “dead layer effect” on the surface of a dielectric film which significantly reduces the effective dielectric constant observed in capacitor applications. Two predictions of the theory are verified from recent experimental data. An experiment is suggested to distinguish between linear and nonlinear surface effects.

Characterization of thick lead zirconate titanate films fabricated using a new sol gel based process

Abstract: Lead zirconate titanate (PZT) films 60 μm in thickness have been fabricated using a new sol gel based process. PZT powders are dispersed in a sol gel matrix to form a 0–3 ceramic/ceramic composite. The dielectric properties of these films have been studied as a function of powder concentration, frequency, and temperature. The characteristic Curie point is observed at 420 °C. The ferroelectric behavior measured in terms of the remanant polarization (Pr=35 μC/cm2) and coercive field (Ec=20 kV/cm) was an improvement over values quoted for thin PZT films but lower than that of bulk ceramic. The piezoelectric properties d33 (325 pC/N) and d31 (−80 pC/N) were comparable with those of the bulk ceramic.

Implantable medical device microstrip telemetry antenna

Abstract: A microstrip RF telemetry antenna is formed on or within the exterior surface of an implantable medical device housing that is formed either of a conductive metal or of a non-conductive dielectric material The microstrip antenna is formed of an electrically conductive radiator patch layer that is laminated upon an exterior facing side of a dielectric substrate layer of relatively constant thickness A conductive ground plane layer is formed on the opposite side of the dielectric substrate layer to extend parallel to and at least coextensively with the radiator patch layer The radiator patch layer is coupled to the transceiver circuitry within the implantable medical device housing by a feedthrough extending through the dielectric substrate layer, the ground plane layer and the implantable medical device housing side wall If the implantable medical device housing is conductive it may form the ground plane layer over which the dielectric substrate layer and the radiator patch layer are formed through deposition or other techniques If the implantable medical device housing is formed of a suitable non-conductive dielectric material, the ground plane layer is formed on an interior surface thereof and the radiator patch layer is formed on an exterior housing surface thereof, preferably by deposition techniques The ground plane layer may be recessed to form a cavity backed ground plane that receives the dielectric layer and radiator patch layer within the cavity The exterior surfaces of the radiator patch layer, the dielectric layer and any exposed surface of the ground plane layer may be electrically insulated by a radome layer

Optical properties of hexagonal GaN

Abstract: Single-crystalline hexagonal GaN (α-GaN) films have been grown on (0001) sapphire substrates by metalorganic chemical vapor deposition at 1040 °C. The complex dielectric functions, e(E)=e1(E)+ie2(E), of the epitaxial films have been measured by spectroscopic ellipsometry (SE) for E⊥c in the region between 1.5 and 5.0 eV at room temperature. Previously published ultraviolet SE spectra of α-GaN are examined by considering the effects of surface roughness using an analysis based on an effective medium model. Ex situ atomic force microscopy is used to assess independently surface flatness. By mathematically removing the effects of surface roughness, the most reliable e(E) values for α-GaN are presented in the 1.25–10 eV photon–energy range. Theoretical dispersion analysis suggests that the E0 structure could be characterized by a three-dimensional M0 critical point and the E1α (α=A,B,C) structures by two-dimensional M1 critical points. To facilitate design of various optoelectronic devices, dielectric-functio...

Charge transport of the mesoscopic metallic state in partially crystalline polyanilines

Abstract: Charge transport properties, including temperature-dependent dc conductivity, thermoelectric power, electron paramagnetic resonance, microwave frequency dielectric constant and conductivity, and electric-field-dependent conductance of partially crystalline (``physically'' cross-linked) HCl-doped polyaniline correlated with x-ray structure studies, demonstrate that charge delocalization in physically cross-linked polyaniline systems is structurally controlled. Further, we observe a positive dielectric constant at room temperature which increases (to values $g~{10}^{4})$ with increasing percent crystallinity, the size of crystalline regions, and polymer chain alignment in the disordered regions, supporting the establishment of mesoscopic metallic regions. We propose an inhomogeneous disorder model for this system in which ordered (crystalline) regions, described by three-dimensional metallic states, are connected through amorphous regions of polymer chains where one-dimensional disorder-induced localization is dominant. We utilize the metallic box, interrupted metallic strands, and Nakhmedov's phonon-induced delocalization models to account for the temperature dependence of charge transport properties of the various partially crystalline polyanilines. Analyses for the sample and temperature-dependent electron paramagnetic resonance linewidth and thermoelectric power are presented.

Method for forming high dielectric constant metal oxides

Abstract: A method for forming a metal gate (20) structure begins by providing a semiconductor substrate (12). The semiconductor substrate (12) is cleaned to reduce trap sites. A nitrided layer (14) having a thickness of less than approximately 20 Angstroms is formed over the substrate (12). This nitrided layer prevents the formation of an oxide at the substrate interface and has a dielectric constant greater than 3.9. After the formation of the nitrided layer(14), a metal oxide layer (16) having a permittivity value of greater than roughly 8.0 is formed over the nitrided layer (14). A metal gate (20) is formed over the nitrided layer whereby the remaining composite gate dielectric (14 and 16) has a larger physical thickness but a high-performance equivalent oxide thickness (EOT).

Broadband nonhomogeneous multi-segmented dielectric resonator antenna system

Abstract: A dielectric resonator antenna system is disclosed wherein a dielectric material having a high dielectric constant is placed between a dielectric resonator antenna (DRA) and the antenna feed. Preferably the dielectric material having a high dielectric constant is either in the form of an insert within a cavity of the DRA or alternatively is in the form of a thin layer between the feed and the DRA for enhancing coupling therebetween. It is preferred that the high dielectric constant material be at least twice the value of the dielectric resonator antenna.

Experimental investigation of scaling laws for electrospraying: Dielectric constant effect

Abstract: Experiments were performed to investigate the effect of liquid dielectric constant on existing scaling laws for the electrospraying process. The variations of the droplet size and the emitted current were measured as a function of the dielectric constant for the electrospray operating in the cone-jet mode. Eight different solvents with dielectric constants, κ, ranging from 12.5 to 182 were tested. The residue particle size distributions were measured using a TSI scanning mobility particle sizer, (SMPS). The produced liquid droplet sizes were then calculated from the known solution concentrations. The results show that: (1) For the produced droplet size, Dd , experimental data are in agreement with the scaling law (G(κ) = 1.66κ −1/6) proposed by Ganan-Calvo et al. (1994) for solvents with high dielectric constants. The derivation in low dielectric constant cases may be that the assumption of Ganan-Calvo (1994) on the characteristic length may not hold for the present system; (2) for the emitted cu...

Planar microwave integrated phase-shifter design with high purity ferroelectric material

Abstract: Ferroelectric materials (FEM's) are very attractive because their dielectric constant can be modulated under the effect of an externally applied electric field perpendicular to the direction of propagation of a microwave signal. FEM may be particularly useful for the development of a new family of planar phase shifters which operate up to X-band. The use of FEM in the microwave frequency range has been limited in the past due to the high losses of these materials; tan /spl delta/=0.3 at 3 GHz is typical for commercial BaTiO/sub 3/ (BTO) and due to the high electric field necessary to bias the structure in order to obtain substantial dielectric constant change. In this paper, a significant reduction in material losses is demonstrated. This is achieved by using a new sol-gel technique to produce barium modified strontium titanium oxide [Ba/sub 1-x/Sr/sub x/TiO/sub 3/ (BST)], which has ferroelectric properties at room temperature. Also demonstrated is how the use of thin ceramics reduces the required bias voltage below 250 V, with almost no power consumption required to induce a change in the dielectric constant. A phase shift of 165/spl deg/ was obtained at 2.4 GHz, with an insertion loss below 3 dB by using a bias voltage of 250 V. Due to the planar geometry and light weight of the device, it can be fully integrated in planar microwave structures.

Lead-free piezoelectric ceramics based on (Bi1/2Na1/2)TiO3-NaNbO3

Abstract: Dielectric, ferroelectric and piezoelectric pŕoperties of a BNT-based solid solution, (1-x)(Bi1/2Na1/2)TiO3-xNaNbO3, that is, Bi(1-x)/2Na(1+x)/2 Ti1-x Nb x O3 (BNTN-100x), are studied from the viewpoint of a new group of lead-free piezoelectric ceramics. The conventional ceramic fabricating technique was used to prepare BNTN ceramics. D-E hysteresis loops could be easily observed with a relatively large remanent polarization, P r, of about 25∼33 μC/cm2 and a high coercive field of about 50 kV/cm near the composition of BNT side. The electromechanical coupling factors, k p, k t, and k 33 were about 0.108, 0.424 and 0.433 for BNTN-3. BNTN ceramics seem to be a candidate for lead-free piezoelectric ceramics.

Dielectric Properties of AB2O6 Compounds at Microwave Frequencies (A=Ca, Mg, Mn, Co, Ni, Zn, and B=Nb, Ta)

Abstract: The microwave dielectric properties of AB2O6 compounds, where A=Ca, Mg, Mn, Co, Ni, or Zn, and B=Nb or Ta, were investigated. All samples were prepared using the mixed oxide method. In particular, AB2O6 compounds with Mg2+ and Zn2+ revealed very good dielectric properties with respect to dielectric resonator applications. However, their temperature coefficient of resonant frequencies should be modified. It was found that the electronegativity of AB2O6 compounds is relevant to their dielectric constant. The temperature coefficient of resonant frequency was discussed in terms of volume contraction.

Temperature dependence of elastic, dielectric, and piezoelectric properties of “single crystalline’’ films of vinylidene fluoride trifluoroethylene copolymer

Abstract: Elastic, dielectric, and piezoelectric constant matrix elements of a “single crystalline’’ (SC) film of vinylidene fluoride trifluoroethylene copolymer, P(VDF/TrFE), in which the orthorhombic [001] and [110] axes of fully extended chain crystals are preferentially oriented parallel to the stretching axis and normal to the surface, respectively, were measured at temperatures ranging from 10 K to the Curie point (402–404 K) by using a piezoelectric resonance method. All of the electromechanical coupling factors (k31, k32, k33, k24, and k15) are larger than those of conventional lamellar crystalline films. Some of the matrix elements for a P(VDF/TrFE) single crystal are derived from the measured values of constant matrix elements for the SC film. Some features characteristic of the SC film are revealed. The SC film has a large Young’s modulus for the stretching direction (1/s11) (121 GPa at 10 K). The properties related to the molecular motions along the chain axis, such as 1/s11, shear stiffness constant c55, shear piezoelectric constant e15, etc., exhibit strong relaxations around 250 K. The origin of these relaxations in the crystalline phase is discussed.

High spatial resolution quantitative microwave impedance microscopy by a scanning tip microwave near-field microscope

Abstract: A recently developed scanning tip microwave near-field microscope has been improved to achieve a spatial resolution of 100 nm (∼λ/106). Furthermore, explicit calculations of the field distribution using a simplified model allow quantitative microscopy of dielectric properties for dielectric materials. A detection sensitivity of δe/e∼6×10−4 has been achieved.