A. N. Lachinov

Modulation of optical absorption in poly(diphenylenephthalide) films near the switching threshold

Optical properties of poly(diphenylenephthalide) nonconjugated polymer films near the electronic switching threshold have been studied by modulation spectroscopy. The measured spectra exhibit specific features in the range of the band gap at ∼2.4 eV. The results obtained have been interpreted using quantum-chemical calculations of the electron energy characteristics of the polymer molecule. It has been established that the injection of charge carriers into the polymer film and interaction of this excess charge with the macromolecule can lead to the formation of deep trap states near the center of the band gap.

The conducting polymer/polymer interface

It is well-known that interfaces between various substances are widely used in modern electronics. Recently it has been found out that the interfaces of two dielectrics can possess unusual electronic properties. This work shows that high-conductivity interface can be obtained not only on the boundary of two crystals but also on the interface of amorphous insulator polymers with the carrier mobility extremely high for organic materials (0.4 cm2/Vs). Conductivity of the structure exceeds the bulk conductivity of the materials used by more than five orders of magnitude. Moreover, the polymer/polymer interface has a metallic conductivity.

Bipolar space charge formation and switching effect in thin polymer films

A theoretical model of bipolar charge formation in a polymer film is proposed. High density of metastable charge pairs within a thin trapping layer results from their delayed recombination. The conditions for the highest density of charge pairs are discussed. The formation of conductive channels by charge pairs starting from the trapping layer together with the high mobility of “Coulomb holes” in the channels can switch the structure to the highly conductive state.

Current-voltage characteristics of poly(diphenylenephthalide) thin films

Charge transfer near the threshold of polymer film transition (induced by a low uniaxial pressure) to the high-conductivity state is studied in an attempt to tackle the question of how the energy band structure of a wide-gap organic insulator varies near this threshold. The I-V characteristics of poly(diphenylenephthalide) films of various thickness versus uniaxial pressure are analyzed. The results obtained are treated within the model of space-charge-limited injection currents. The parameters of the injection model, such as the equilibrium concentration of electrons, electron mobility, the occupation of traps, etc., are estimated. It is concluded that deep traps due to an excess charge may appear in the energy gap of the polymer near the imref. This probably causes a narrow subband to arise, and charge transfer via this subband increases the charge carrier mobility and, hence, conductivity.

Charge transfer in thin polymer films of polyarylenephthalides

Transitions from an insulating state to a high-conductivity state are observed in thin polymer films of polyarylenephthalides. The temperature dependences of the conductivity for thin films of wide-band-gap polymer insulators are determined, the relaxation of excess charge is investigated, and the mechanisms of charge transfer in the temperature range preceding the transition to the high-conductivity state are revealed. It is shown that hopping transport over trap states in the band gap is the main mechanism of charge transfer in polyarylenephthalide film.

On the role of spin polarization of electrons in the effect of giant injection magnetoresistance in the Ni-polymer-Cu system

The effect of spin polarization of electrons injected from a ferromagnet on the giant injection magnetoresistance was investigated for a ferromagnet-polymer-nonmagnetic metal experimental structure. The degree of spin polarization was varied by introducing a depolarizing nonmagnetic metal (Cu) layer between the ferromagnet and the organic transport layer. It was established that the coefficient of giant injection magnetoresistance depends significantly on the thickness of the depolarizing layer. In particular, the effect was not observed at a thickness larger than 12 nm and decreased exponentially at a smaller thickness as the thickness increased. The conclusion was drawn that the spin polarization of electrons plays a decisive role in the effect of giant injection magnetoresistance.

On the nature of giant magnetoresistance in a ferromagnet-polymer-nonmagnetic metal system

The role of magnetostriction in manifestations of the giant magnetoresistance (GMR) in a nickel-poly(arylenephthalide)-copper system has been studied. It is established that the appearance (disappearance) of GMR at the nickel-polymer interface is not correlated with a significant change in magnetostrictive deformations, since the transverse and longitudinal magnetostriction in the substrate plane remains almost unchanged in the entire interval of magnetic fields inducing the transition. The role of the longitudinal magnetostriction perpendicular to the sample plane cannot be considered as determining, since the same sample exhibits a change in the sign of the conductivity switching depending on the initial conditions for the same sign of the longitudinal magnetostriction in the substrate. The possibility of controlling the GMR threshold field by changing the applied bias voltage is also not indicative of the influence of magnetostriction on this phenomenon.

Effect of excessive pressure on the drift mobility of charge carriers in poly(diphenylene phthalide) films

The electron-hole transport in poly(diphenylene phthalide) films has been investigated. The dependence of the drift mobility of charge carriers on the excessive mechanical pressure has been studied using the time-of-flight method. It has been revealed that, with an increase in the thickness of the polymer film, the dispersive transport of charge carries gives way to the quasi-dispersive transport. In thin films in the prethreshold range (i.e., before switching of the samples to the highly conductive state under excessive pressure), the electron mobility increases and exceeds the hole mobility. The experimental results have been discussed in the framework of the model describing the transport through the channels formed by metastable electron-hole pairs.

Anomalously high conductivity along the interface of two dielectric polymers

The possibility of creating a region of anomalously high conductivity at the interface between two dielectric polymers has been demonstrated. The charge carrier mobility and the specific conductance in this region have been estimated by several methods. An analysis of the results makes it possible to hypothesize that a layer with a quasi-two-dimensional electron gas is formed in the interface region between two dielectric polymer films.

Properties of the transport layer formed at the interface between two polymer films

Electronic transport properties of a 2D boundary layers formed by two films of an organic polymer material are investigated. It is established that the conductivity of such a layer is several orders of magnitude higher than the surface conductivity of individual films forming the experimental structure. Temperature measurements show that transport of charge carriers over this layer can be described using the models of Schottky thermionic emission and hopping transport over trap levels.