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Three dimensional dielectric systems are simulated to obtain the relaxational polarization responses. The dielectric system consists of randomly distributed permanent dipoles that fluctuate due to thermal activation in double well potentials between two plane-parallel electrodes. The barrier height between the wells is assumed to be proportional to the dipole length. The electrodes' effect is considered with the method of images. The relaxational phenomena of two dielectric systems, one with fixed and the other with distributed dipole lengths having single and distributed relaxation times, respectively, are calculated with a Monte-Carlo simulation method. The local field at every dipole and consequently the transition probabilities are computed to determine the transient polarization response iteratively. Both dielectric systems are simulated over a wide range of temperatures for interacting and non-interacting cases. The mean relaxation times exhibit a Vogel-Fulcher law for interacting systems and an Arrhenius law for non-interacting systems. [ABSTRACT FROM AUTHOR]

Ultrathin films of poly(vinylidene fluoride-trifluoroethylene) copolymers were prepared by the Langmuir–Blodgett technique. These films show a hysteresis loop of the polarization versus the applied electrical field in a dry atmosphere which can be attributed to a ferroelectric phase. By increasing the relative humidity in the surrounding atmosphere, water molecules are absorbed by the samples yielding quasi-free charge carriers. These charges can fluctuate between the electrodes of the samples and build a space charge polarization superimposed to the ferroelectric polarization. Therefore, the hysteresis loops at low frequencies, where the quasi-free charges can contribute to the total polarization, are broadened and the measured remanent polarization is increased. Also, the real and the imaginary parts of the dielectric permittivity of the samples in the low frequency range are increased. Additionally, the transient decay of the retained charge is more pronounced for higher relative humidities. Furthermore, the transient desorption of the water from the polymer is measured by detection of the rapidly changed relative humidity. However, ellipsometric measurements show only a small increase in the sample thickness and no significant change in the refractive index due to the absorption of water. [ABSTRACT FROM AUTHOR]

Thin films of ion-conducting polyethylene oxide (PEO) are prepared as a model system for solid polymer electrolytes. Plane-parallel capacitance structures of the type Al-PEO-Al (Metal-Insulator-Metal, MIM) with blocking electrodes are produced. In the short time range t<=1 s after the application of an external field, an ionic space-charge polarization with an exponential time dependence prevails. In the long time range the samples exhibit a Kohlrausch behavior of the polarization current, i.e., j∝t-α. This time dependence can be explained by image charges in the electrodes which attract the mobile ions. This attraction of ions is responsible for the static and the dynamic behavior of the system. The remanent polarization appearing in hysteresis measurements is an affirmation of the presence of these image charges. Computer simulations of a three-dimensional hopping model yield qualitatively the same results. Here the attraction of ions at the electrodes due to the image charges can be observed, as well as the experimentally determined t-α currents and the hysteresis of the polarization. This behavior cannot be simulated with a standard continuum model. [ABSTRACT FROM AUTHOR]

The dielectric permittivity of amorphous SiO thin films could be increased by the evaporation of palladium electrodes. The evaporation of gold electrodes does not show a comparable effect. Gold electrodes reproduce the literature value for the dielectric permittivity of SiO. Because the preparation processes of both types of electrodes have been carried out in the same manner, the detected increase of the dielectric permittivity must be caused by the physical properties of the used evaporated material. A significant difference between gold and palladium is the ability of palladium to absorb large amounts of hydrogen. Therefore we assume in the case of evaporated Pd electrodes a diffusion process of hydrogen atoms into the insulator, which cause additional relaxation centers. The contribution of these relaxation centers to the polarization is measured as an enhanced dielectric permittivity of the SiO thin film. Measurements of the small signal response have been carried out in the frequency domain, the time domain, where a Kohlrausch law is found, and in dependence of the temperature. The large signal response was studied using isochronal time domain measurements. A double well potential model is discussed to interpret the experimental results quantitatively. It is shown that a distribution of relaxation times is caused by a distribution of distances between neighbored atoms in the amorphous state. c 2001 American Institute of Physics. [DOI: 10.1063/1.1384484]. [ABSTRACT FROM AUTHOR]

The non-exponential polarization and depolarization current density after a voltage step is investigated in the ion conducting polymer polyethylene oxide yielding a Kohlrausch or Curie-von Schweidler behavior in the long and in the short time range with different exponents respectively. To explain this behavior simulations with a three-dimensional hopping model are performed. Interactions between all charges in the system are considered. A distribution of sites is introduced to construct an amorphous structure. The intrinsic barrier heights between two neighboring sites in a multi-well energy structure depend here on the distance between the sites and the interaction between the charges yielding a distribution of relaxation times. Simulations with this model show a Kohlrausch behavior in the long time range as well as in the short time range. For short times the distribution of relaxation times is responsible for the Kohlrausch behavior. In the long time range a distribution of distances of the sites to the electrodes resulting in a distribution of image forces causes the Kohlrausch current. [ABSTRACT FROM AUTHOR]

The polarization and the depolarization current after application of a voltage step are investigated in the solid electrolyte polyethylene oxide. For low applied fields the current density follows a Kohlrausch behavior, i.e. j α f?, in the long time range during the polarization and the depolarization process respectively. By using sufficiently high applied fields charge injection sets in and the current during the polarization process is nearly constant at long times. It turns out that the transient depolarization current in this case shows a pronounced unforeseen maximum. The time to the maximum as well as the height of the current peak depend on the field applied before, the polarization time, the temperature, the LiClO4 doping, the electrode material, and the sample length. Simulations carried out using a discrete three-dimensional hopping model reveal qualitatively comparable results. It is concluded that the injection and later extraction of excess electrons accordi [ABSTRACT FROM AUTHOR]

Abstract: The scanning Kelvin probe is used for the contactless determination of the surface potential of the ion-conducting polymer polyethylene oxide (PEO). The potential distribution in the samples after application of an external voltage between two metal electrodes is strongly non-linear in the steady state. This is attributed to a space charge polarization which prevails in the PEO. Furthermore, the time dependent evolution of the surface potential at several fixed positions after a voltage step is measured. Here, the behavior is influenced by the movement of the mobile internal charges and also by injected charges from the electrodes. A comparison with simulations performed with a three-dimensional hopping model shows qualitatively the same results and yields negative mobile charges in the system. Additionally, it is observed that the dc current through the samples is independent of the sample length. This is caused by a depletion zone of mobile charges near the negative electrode which determines the resistance of the samples. [Copyright &y& Elsevier]

A novel method to prepare a passive integrated low-pass filter is described. The filter device is based on the layer sequence of a metal–insulator–semiconductor (MIS) structure. In contrast to the standard MIS system, the metal electrode of the filter device is structured to gain a local variation of its lateral resistance. The lateral resistance forms, together with the capacitance of the insulating film, a ladder filter RC network with low-pass characteristic. Devices with filter characteristics of fourth order and maximal attenuations of −110 dB have been realized until now. The cut-off frequencies have been varied between about 25 μHz and 1 kHz. To prepare MIS filters with cut-off frequencies in the sub-millihertz region, it is necessary to deposit metal electrodes with very high sheet resistances (up to 1015 Ω/□). These sheet resistances could be achieved by evaporation of very thin metal films with a granular, disordered morphology. Because of their granular structure, the thin films reveal a thermally activated conductivity. [Copyright &y& Elsevier]

The space charge distribution and therefore the distribution of the internal potential in ion-conducting polymers can strongly be influenced by electrode effects. Due to image forces the electrodes attract mobile ions to the electrolyte/electrode interface. Simulations performed with a three-dimensional hopping model yield a charge accumulation of mobile ions at the electrodes as well as a depletion zone near this accumulation layer even without external applied field. The resulting potential shows a maximum in the middle of the sample. Measurements of the surface potential performed with a scanning Kelvin probe using polyethylene oxide confirm the model. [ABSTRACT FROM AUTHOR]

The transient current density through ion conducting polyethylene oxide samples of different lengths is measured after application of voltage steps with different amplitudes. In the experiments, it is found that at a given voltage step the long-term current density is independent of the sample length. Qualitatively, the same behavior is attained from computer simulations using a three-dimensional hopping model which takes into account internal ionic charge movements and extrinsic electronic charge injection from the electrodes. It turns out that the width of a depletion zone of mobile ions at the electrode, which only depends on the applied voltage, is responsible for the independence of the current from the sample length. [ABSTRACT FROM AUTHOR]