Yu. A. Genenko

Self-consistent analytical solution of a problem of charge-carrier injection at a conductor/insulator interface

We present a closed description of the charge-carrier injection process from a conductor into an insulator. Common injection models are based on single electron descriptions, being problematic especially once the amount of charge-carriers injected is large. Accordingly, we developed a model, which incorporates space-charge effects in the description of the injection process. The challenge of this task is the problem of self-consistency. The amount of charge carriers injected per unit time strongly depends on the energy barrier emerging at the contact, while at the same time the electrostatic potential generated by the injected charge carriers modifies the height of this injection barrier itself. In our model, self-consistency is obtained by assuming continuity of the electric displacement and the electrochemical potential all over the conductor/insulator system. The conductor and the insulator are properly taken into account by means of their respective density of state distributions. The electric-field distributions are obtained in a closed analytical form and the resulting current-voltage characteristics show that the theory embraces injection-limited as well as bulk-limited charge-carrier transports. Analytical approximations of these limits are given, revealing physical mechanisms responsible for the particular current-voltage behavior. In addition, the model exhibits the crossover between the two limiting cases and determines the validity of respective approximations. The consequences resulting from our exactly solvable model are discussed on the basis of a simplified indium tin oxide/organic semiconductor system.

Self-consistent theory of unipolar charge-carrier injection in metal∕insulator∕metal systems

A consistent device model to describe current-voltage characteristics of metal∕insulator∕metal systems is developed. In this model the insulator and the metal electrodes are described within the same theoretical framework using density of states distributions. This approach leads to differential equations for the electric field which have to be solved in a self-consistent manner by considering the continuity of the electric displacement and the electrochemical potential in the complete system. The model is capable of describing the current-voltage characteristics of the metal∕insulator∕metal system in forward and reverse biases for arbitrary values of the metal∕insulator injection barriers. In the case of high injection barriers, approximations are provided offering a tool for comparison with experiments. Numerical calculations are performed exemplarily using a simplified model of an organic semiconductor.

Polarization switching dynamics by inhomogeneous field mechanism in ferroelectric polymers

The understanding of polarisation switching dynamics of ferroelectrics is of great importance for practical applications and is being steadily advanced for ferroelectric ceramics and polymers for more than half a century. The temporal behaviour of polarisation reversal in ferroelectric polymers like polyvinylidenedifluoride (PVDF) or its copolymer with trifluoroethylene P(VDF-TrFE) cannot be satisfactory explained by simple models such as the classical Kolmogorov-Avrami-Ishibashi nucleation and growth theory [1–3] or by models considering stretched exponential laws.

The effect of a superconducting surface layer on the optical properties of a dielectric photonic composite

The effect of a strongly anisotropic superconducting surface layer on the transmittance, reflectance and absorptance of a one-dimensional, layered dielectric composite with periodically alternating, isotropic constituents for linearly polarized, normally incident electromagnetic radiation is studied both analytically and numerically. The underlying model of the electric permittivity of the superconducting constituent permits photonic excitation at frequencies both below and above the superconductor pair breaking frequency as well as thermal and normal scattering right up to the superconductor critical temperature. The optical properties addressed reveal traits such as band-like patterns of the transmittance and reflectance, but also step-like or smeared-out patterns of the reflectance and absorptance, displaying a marked reference to the particular type of polarization by virtue of the anisotropy of the superconducting layer covering the dielectric composite. Thus, in switching from transverse electric to transverse magnetic polarization, the maximum optical selectivity can become gigantic, given an appropriate thickness of the superconducting layer, with a moderate dependence on temperature. This fact offers unique possibilities regarding practical applications of such a novel photonic composite as an efficient polarization filter for electromagnetic radiation tunable via the thickness of the covering layer and temperature.

The Einstein relation in systems with trap-controlled transport

The applicability of the Einstein relation considering diffusivity and mobility of charge carriers with respect to disordered systems has been a matter of serious discussion in recent years. Here the validity of the Einstein relation, using the generalized form, is checked for two specific density-of-states functions within a multiple trapping model and the mobility edge concept. It is shown that the classical Einstein relation is valid for almost all charge-carrier densities relevant for experimental purposes. It is demonstrated that this result is dependent on the charge-carrier mobility definition.

Superconductor strip near a magnetic wall of finite thickness

Transport current distributions in a flux-free superconductor strip located near a soft magnet wall of arbitrary thickness were studied numerically for different magnetic permeabilities using the method of surface magnetic charge. It was established that the reduction of the edge current peaks caused by magnetic shielding saturates rapidly with increasing thickness of the magnet. This makes possible the use of thin magnetic films for shielding of superconductor strips with the aim of improving their current-carrying capability so that a small thickness of shields may be compensated by a large magnetic permeability.

Bipolar charge-carrier injection in semiconductor/insulator/conductor heterostructures: Self-consistent consideration

A self-consistent model of bipolar charge-carrier injection and transport processes in a semiconductor/insulator/conductor system is developed, which incorporates space-charge effects in the description of the injection process. The amount of charge carriers injected is strongly determined by the energy barrier emerging at the contact, but at the same time the electrostatic potential generated by the injected charge carriers modifies the height of this injection barrier itself. In our model, self-consistency is obtained by assuming continuity of the electric displacement and of the electrochemical potential all over the system. The constituents of the system are properly taken into account by means of their respective density of state distributions. The consequences resulting from our model are discussed on the basis of an indium tin oxide/organic semiconductor/conductor structure. The distributions of the charge carriers and the electric field through the electrodes and the organic layer are calculated. ...

Self-consistent model of unipolar transport in organic semiconductor diodes: Accounting for a realistic density-of-states distribution

A self-consistent, mean-field model of charge-carrier injection and unipolar transport in an organic semiconductor diode is developed utilizing the effective transport energy concept and taking into account a realistic density-of-states distribution as well as the presence of trap states in an organic material. The consequences resulting from the model are exemplarily discussed on the basis of an indium tin oxide/organic semiconductor/metallic conductor structure. A comparison of the theory to experimental data of a unipolar indium tin oxide/poly-3-hexyl-thiophene/Al device is presented.

Hysteretic ac losses in a superconductor strip between flat magnetic shields

Hysteretic ac losses in a thin, current-carrying superconductor strip located between two flat magnetic shields of infinite permeability are calculated using Beans model of the critical state. Exact numerical calculations and approximate analytical forms delineating the penetration of magnetic flux and the energy dissipated during a cycle of the ac transport current, per unit length of the strip, are derived. For the shields oriented parallel to the plane of the strip, the penetration of the self-induced magnetic field is found to be enhanced, with the analytical current dependence of the ac loss conditionally resembling that of an isolated superconductor slab; for the shields oriented perpendicular to the plane of the strip, the penetration of the self-induced magnetic field is found to be impaired, with the analytical current dependence of the ac loss conditionally duplicating that of a regular set of curved superconducting tapes extending longitudinally around a surface of cylindrical shape. Thus, hysteretic ac losses can strongly augment or, respectively, wane when the shields approach the strip.

Fatigue effect on polarization switching dynamics in polycrystalline bulk ferroelectrics

Statistical distribution of switching times is a key information necessary to describe the dynamic response of a polycrystalline bulk ferroelectric to an applied electric field. The Inhomogeneous Field Mechanism (IFM) model offers a useful tool which allows extraction of this information from polarization switching measurements over a large time window. In this paper, the model was further developed to account for the presence of non-switchable regions in fatigued materials. Application of the IFM-analysis to bipolar electric cycling induced fatigue process of various lead-based and lead-free ferroelectric ceramics reveals different scenarios of property degradation. Insight is gained into different underlying fatigue mechanisms inherent to the investigated systems.