Strong Frequency Dispersion Research Articles

Capacitance properties of a system of coupled nanowires

For a system of parallel quantum wires provided with two bulk metal contacts, the charge distribution induced by the applied voltage is found and the capacitance of the system C is calculated. The screening effects, very weak for a single wire, in the wire array are connected with the electrostatic interaction between wires, which makes the screening length equal to the interwire distance R. As a result, in the static regime C;R. At the signal frequencies v*D/R ~D is the diffusion coefficient in wires!, a strong frequency dispersion begins and C drops steeply down to the values determined by the distance between contacts. @S0163-1829~99!05604-0#

Electronic Properties of Semimetallic Heterojunctions

The properties of InAs–GaSb heterojunctions containing a semimetallic electron–hole double layer at the heterointerface have been considered. By self-consistent calculations of interface accumulation and inversion layers, band diagrams, capacitance–voltage and current–voltage characteristics were analyzed for all possible combinations of the semiconductor doping types, namely, for n–p, n–n, p–p and p–n-heterojunctions. The n-InAs–p-GaSb junction containing accumulation layers in both semiconductors, has the most unusual properties including the capacitance growth with the increase of contact potential and metallic conductivity caused by activationless generation–recombination processes at the interface. A phenomenological theory of these processes is developed, which can also describe the conductivity of long-period InAs–GaSb superlattices. One more remarkable effect is the strong frequency dispersion of capacitance in p-InAs–n-GaSb junctions caused by the re-charging of a single or double inversion layer at the interface.

Dielectric properties of BaTiO3/SrTiO3 multilayered thin films prepared by pulsed laser deposition

BaTiO 3 /SrTiO 3 multilayered thin films were deposited on Si and Pt/Si substrates. X-ray diffraction clearly shows the formation of the superstructures. Phase transition properties were studied via dielectric measurements. Glassy behavior, characterized by a strong frequency dispersion of dielectric properties, was found in samples with a total thickness of 400 nm, while in samples with a total thickness of 800 nm, normal ferroelectric phase transitions with two dielectric peaks were observed. A preliminary interpretation assumes that size effects which frustrate long range ferroelectric ordering may lead to the relaxational behavior in BaTiO3/SrTiO3 superstructures.

Dielectric study of KNbO3 ceramics over a large range of frequency (102-109Hz) and temperature (300-800 K)

KNbO3 ceramics were sintered at 1263K thanks to a LiF additive. The ferroelectric-paraelectric transition is of first order and occurs at T c∼670K. A dielectric study in a large frequency range (102-109Hz) shows: – a strong frequency dispersion of the permittivities at low frequency for T > 500 K which comes mainly from conductivity phenomena – a dispersion at high frequency in both ferroelectric and paraelectric regions which comes from a network relaxation of correlation chains of Nb5 + cations. The latter effect was revealed thanks to a new high frequency-high temperature device usable up to 109Hz and 800 K simultaneously.

Spectroelectrochemical and electrical characterization of low bandgap polymers

In situ FT-IR spectroscopy, and d.c. and a.c. electrical measurements were used to characterize an electronically conducting polymer prepared from the electropolymerization of a bridged dithienyl monomer. Electronic absorption bands were observed during both n- and p- doping processes. The solid state data showed that the metal|polymer|metal diodes displayed no rectification although capacitance and loss data showed a strong frequency dispersion characteristic of a Schottky diode.

Polar state in a SrTiO3-KTaO3 solid solution

Dielectric properties of ceramic samples of a 0.85SrTiO3-0.15KTaO3 solid solution have been studied. The temperature dependence of the dielectric permittivity exhibits maxima with a strong frequency dispersion, which is described by the Vogel-Fulcher law with a characteristic temperature Tg=26 K. These features are apparently connected with transition to a polar glasslike state.

A-site cation ordering and dielectric behavior of lead-calcium titanate

The microstructure and dielectric behavior of lead-calcium thanate (Pb0.5Ca0.5)TiO3 ceramics were investigated Three types of ordering—1/2{111}, 1/2{110} and 1/2{100} are found to be coexisted in (Pb0.5Ca0.5)TiO3 and the ordering degree can be intensified by long time annealing. The effects of ordering degree on the dielectric behavior were studied, which showed a less strong frequency dispersion (ΔTo), but a more diffused FE → PE phase transition (5) with the ordering degree increased.

Ferroelectricity and ionic conductivity in Ba5Li2Ti2Nb8O30 ceramics

Tetragonal-tungsten-bronze-type Ba5Li2Ti2Nb8O30 ceramics have been densified. Such materials are both ferroelectric and ionic conductor. A maximum of ε'r appears at Tc ≈580K. Another small peak has been found at about 305K, it probably results from a crystallographic transition. In addition, a very strong frequency dispersion, related to the ionic conduction, occurs at T≥Tc.

Optical studies of electronic states and coupled vibrations in substituted polyacetylene

The electronic states and coupled vibrations in films of substituted polyacetylene (poly( o-trimethylsilylphenylacetylene); PTMSiPA) have been studied using the techniques of resonant Raman scattering (RRS) and electroabsorption (EA) spectroscopies. Strong RRS frequency dispersion has been observed for the C--C and C=C stretching vibrations with excitation photon energy from 1.55 to 3.42 eV, which is similar to that of other nonluminescent conducting polymers such as trans-polyacetylene, polydiacetylene and polydiethynylsilane. The RRS dispersion has been analyzed by means of the amplitude mode formalism placing the energy of the first allowed odd-parity exciton E(1Bu) of the longest chains at 1.85 eV. The rather weak EA spectrum is mainly composed of Stark shift of an energy distribution of 1Bu exciton and a positive contribution centered at 2.6 eV due to an even-parity exciton (mAg).

Dispersion of resonant Raman scattering in luminescent and non-luminescent conjugated polymers

We found that luminescent and non-luminescent π-conjugated polymers can be distinguished by their remarkably different behavior of the phonon frequency dispersion measured by resonant Raman scattering (RRS). Non-luminescent conjugated polymers, like trans-polyacetylene, show strong phonon frequency dispersion, whereas strongly luminescent conjugated polymers, like PPV, show a weak RRS dispersion. We show that the RRS dispersion is not directly due to the commonly assumed variation of the optical gap with the conjugation length, but is mainly due to that of the first optically forbidden state. In luminescent conjugated polymers, the optically allowed state is the lowest lying excited state which varies with the conjugation length (as observed by the broad absorption spectrum), yet the RRS shows only a weak dispersion. In non-luminescent polymers, on the other hand, the lowest excited state is optically forbidden, and they show strong RRS dispersion

Influence of postdeposition annealing on the structural and optical properties of sputtered zinc oxide film

The influence of postdeposition annealing on the structural and optical properties of rf sputtered insulating zinc oxide films has been investigated. The as-grown films deposited on quartz substrates were highly c-axis oriented and in a state of stress. These films become almost stress free after a postdeposition annealing treatment at 673 K for 1 h in air. Above 673 K, a process of coalescence was observed which causes major grain growth resulting in microcrack formation and surface roughness. The refractive index shows a strong frequency dispersion below the interband absorption edge. The optical dispersion data have been fitted to (1) a single oscillator model and (2) the Pikhtin–Yas’kov model. The origin of optical dispersion at different annealing temperatures has been discussed in the light of these models. A packing density of more than 99% is estimated in the film annealed at 673 K, indicating that these films are almost void free.

Effects of lanthanum modification on rhombohedral Pb(Zr1−xTix)O3 ceramics: Part I. Transformation from normal to relaxor ferroelectric behaviors

The interruption of long-range polar order in rhombohedral ferroelectricPb(Zr1−xTix)O3 (PZT) ceramics has been systematically studied by incorporating La onto the A-site of the perovskite (ABO3) structure for Zr/Ti ratios of 65/35 and 80/20 and various La contents. Studies have been performed by hot-stage transmission electron microscopy, dielectric spectroscopy, and Sawyer–Tower polarization (P-E) techniques. The evolution of a polar nanodomain state from a normal micron-sized domain state with increasing La content was observed. The emergence of this polar cluster state was characterized by the onset of strong frequency dispersion in the dielectric response, indicative of relaxor behavior. The La content that drives the structure into the relaxor state was found to be related to the lattice distortion of the undoped base composition.

Dielectric dispersion in pure and doped lithium rubidium sulphate

Abstract The frequency (102 – 105 Hz) dependence of the dielectric properties of lithium rubidium sulphate (LRS) are reported in the vicinity of the transition temperature Tc = 477 K. The a.c. conductivity σ(ω) shows a strong temperature dependence and weak frequency response. The dielectric constant in this region shows a strong frequency dispersion. A Cole-Cole diagram was used to determine the distribution parameter and the molecular relaxation time. The effect of doping with Dy+3, Sm+3 and V+3, was also studied. It was found that doping gives rise to localized states which produce a disorder in the structure of LiRbSO4.

Influence of Iodine on Low Frequency Dielectric Properties of Anthrone

Low frequency dielectric properties of polycrystalline anthrone and anthrone/iodine charge-transfer complex are presented. Polycrystalline anthrone proves to be a typical low-loss material. Iodization of anthrone pellets leads to a remarkable change in dielectric properties. Dielectric response of anthrone/iodine CT complex shows comparatively strong low frequency dispersion. The change in dielectric properties proves to be reversible to great extend as removing of iodine by heating at 373 K for a few hours almost completely restores the dielectric values of a virgin anthrone sample.

Temperature and Frequency Dependent Characteristics of Amorphous Silicon thin film Transistors

ABSTRACTOn the basis of our experimental studies of the temperature dependence of amorphous silicon thin film transistor current-voltage and capacitance-voltage characteristics, we have developed an analytical device model suitable for implementation in circuit simulators. This model describes the above-threshold (on) current and the subthreshold (off) current [1]. In addition, the model is able to incorporate changes in the distribution of localized states which arise from thermal and/or bias stress. In this paper, we identify the temperature-dependent parameters, which describe the temperature dependence of both the on and off currents, and we model the leakage current at large negative gate biases. The modeling results are in good agreement with our experimental data. We also discuss capacitance-voltage characteristics of amorphous silicon thin film transistors for varying gate lengths, temperatures, and frequencies. The measured capacitance-voltage characteristics show strong frequency dispersion, which is related to the trap-limited transport of carriers in the channel. The characteristic time constant, which determines when the channel capacitance becomes dependent on frequency, is on the order of the transit time calculated with the field-effect mobility and the electric field. The field-effect mobility takes into account carrier trapping by the localized states and is a function of gate voltage and temperature.

Polarization relaxation in disordered ferroelectrics

Mixed or disordered ferroelectrics such as K 1−χLi χTaO 3 are shown to display a complex relaxation behavior. Their dielectric constant exhibits a strong frequency dispersion and hysteresis loops are observed in their polarization even above the ferroelectric transition. Both manifestations are related and the complex relaxation behavior can be attributed to the presence of randomly distributed but interacting dipoles in a highly polarizable crystal.

Hopping conduction in insulating rf-sputtered zinc oxide films.

The dielectric constant and ac conductivity of rf-sputtered insulating zinc oxide films has been measured in the metal-insulator-metal configuration over a wide temperature (90--500 K) and frequency (0.1--100 KHz) range. The films sputtered from oxide targets in argon were c-axis oriented having a resistivity of \ensuremath{\simeq}${10}^{12}$ \ensuremath{\Omega} cm. The dielectric constant shows a strong frequency dispersion at temperatures above 330 K which is the intrinsic behavior of the films. The dielectric dispersion data has been fitted to (i) Debye-type relaxation (with a distribution of relaxation times), (ii) a random-free-energy-barrier model (which assumes that for hopping conduction, the dielectric dispersion is correlated to the dc conductivity), and (iii) the dielectric modulus (which assumes that microscopic inhomogeneities and the distribution of conductivity relaxation times is responsible for dielectric dispersion). The origin of dielectric dispersion has been discussed in the light of these models. It has been found that hopping conduction dominates in ZnO films up to 450 K.

Electric field induced effect on the optical, dielectric and ferroelectric properties of Pb(Fe2/3W1/3)O3single crystals

Abstract Single crystals of the complex perovskite Pb(Fe2/3 W1/3)O3 [PFW], synthesized from high temperature solution growth, have been studied by optical and dielectric characterizations at zero field and under an electric bias field. It was found that at zero electric field the PFW crystals remain optically isotropic down to 10 K, and that the diffuse maximum of the permittivity near 178 K results from a typical dielectric relaxation with strong frequency dispersion. Application of an electric field can induce a macroscopically polar phase, which remains metastable when removing E at low temperatures. Reversing the polarity of the applied field leads to a reversal of the induced polarization P ind , giving rise to the dielectric hysteresis loop of ferroelectric character. Thus the ferroelectricity in PFW results in reality from the induction of the macropolar phase. The coercive field for the establishment of the induced polarization increases sharply with decreasing temperature. The switching of the in...

Characterization of conduction in PZT thin films produced by laser ablation deposition

Abstract Both direct current (d.c.) and alternating current (a.c.) conductivity measurements were undertaken on lead zirconate titanate (PZT) films synthesized by laser ablation deposition. Direct current (I) displayed an initial time dependence of the form I ∝ t−γ (γ ∝ 0.5–1.0). The possible reasons for this time dependence are discussed. At lower temperatures, the a.c. electrical conductivity shows a frequency dependence of the form σ ∝ ω′ which is explained as electrical charge hopping. At higher temperatures, the d.c. component of electrical conductivity becomes dominant, and is accompanied by a strong low frequency dispersion of the dielectric constant. The results are compared to published data on conductivity in SrTiO3, and discussed in terms of the latest theories for dielectric response of materials.

Low-frequency ac conduction and dielectric relaxation in vinyl chloride:vinyl acetate copolymers

ac conductivity σ(ω), dielectric constant ε′, and loss ε″ of vinyl chloride:vinyl acetate (VC:VAc) copolymers having 3%, 10%, and 17% VAc content (by weight) have been measured in the temperature range 77–410 K and in the frequency range 50 Hz–100 kHz. At low temperatures up to 250 K, the ac conductivity can be expressed by σ(ω)=Aωs, where the slope s is close to unity and its value decreases with the increase in temperature. The dielectric constant in this temperature region shows a very weak frequency and temperature dependence. At temperatures above 300 K, the ac conductivity shows a strong temperature dependence; however, in this temperature region the dielectric constant shows a strong frequency dispersion. The measured dielectric loss as a function of temperature reveals the β1, the β2, and the α relaxations. The β1 relaxation is associated with the movement of the more flexible side group dipoles and the β2 relaxation is due to the movement of rigidly attached side group dipoles, whereas the α relaxation is attributed to the segmental motion of the main chain of the copolymer. Both the latter relaxations seem to be long range in nature with distribution of relaxation times associated with the cooperative motion. The mechanism of conduction in the low- and high-temperature regions have been discussed in light of existing theoretical models.