Experimental and modeling study of charge carriers release from traps by interaction with molecular vibrations in silicon organic polymers

Abstract

The dependence of the charge carrier’s release from traps on the polymer ordering and the widths of the vibration bands are investigated in five silicon organic polymers using thermoluminescence in the 5–200?K temperature range and the Raman spectra. The influence of the vibrations on the thermoluminescence is caused by the processes of the charge carriers’ release from traps via the absorption of vibration quanta. The effect manifests itself in both the appearance of discrete levels of the carriers’ activation energy which coincides with the energy of the vibration quanta and the formation of a structure on the thermoluminescence curve. It is shown, that the magnitude of effects depends on the widths of optical vibration bands and the ordering of the polymers. The mechanisms of interactions of trapped charges with molecular vibrations are analyzed and the values of microscopical parameters of such interactions are determined. It is shown that the contribution of molecular vibrations in the charge release from the trap is great.     

Relaxation of polaronic charge carriers in lithium manganese spinels

Lithium manganese spinels Li_1+xMn_2−xO₄, 0 ⩽ x ⩽ 0.33, were prepared by wet chemistry technique followed by heat-treatment at 750 °C or 800 °C. Differential scanning calorimetry was used to reveal phase transitions. Electrical properties were studied by impedance spectroscopy. LiMn₂O₄ exhibited phase transition below room temperature. The transition, seen as an exothermic event in DSC and a steep decrease of conductivity upon cooling, was sharp in sample sintered at 800 °C and broadened over a range of temperature in sample sintered at 750 °C. In the low temperature phase of LiMn₂O₄, two relaxations of similar strength were observed in the frequency dependent permittivity. The low frequency process was identified as relaxation of charge carriers since the relaxation frequency followed the same temperature dependence as the dc conductivity. The high frequency process exhibited milder temperature dependence and was attributed to dipolar relaxation in the charge-ordered structure. The dipolar relaxation was barely visible in Li substituted samples, x ⩾ 0.05, which did not undergo structural phases transition. Measurements extended to liquid nitrogen temperature showed gradual lowering of the activation energy of conductivity and relaxation frequencies, behavior typical for phonon-assisted hopping of small polarons.     

Current Density‐Voltage and Capacitance‐Voltage Characteristics of Local p‐n Junction Structures in CuInSe2 Single Crystals

Local p‐n junction were obtained under room temperature conditions in CuInSe2 by applying strong electric field through small indium and copper contacts. The current density voltage (J‐V) and the capacitance‐voltage (C‐V) of three different samples were measured at room temperature. The J‐V method shows that the current is dominated by the drift component of the injected carriers. The C‐V method gave a barrier height of 1.04 eV for all three samples which agrees with the reported energy gap of this material. Analysis of these results indicate that the p‐n junction structures formed by strong electric fields are hihgly compensated and the current transport is dominated by the space charge limited current effect.     

Use of a small-voltage electrical response to measure sodium motion in rf sputtered SiO2 films

Low voltage transient and ac responses of rf sputtered SiO₂ films are studied in the temperature range 473–593 K. Experimental results can be explained in terms of a model with trapped carriers and with only one type of recombining mobile charge carriers. Two distinct phenomena clearly appear in transient current curves; the rapid one is attributed to the space charge effect, the other to generation recombination. Theoretical and experimental curves are in good agreement. The concentrations and mobilities are deducted of carriers identified as Na⁺ ions. Concentrations values are compared (for glass and silica substrates) with ion microprobe analysis results.     

Non-Langevin bimolecular recombination in low-mobility materials

We have investigated the bimolecular recombination coefficient (B) of charge carriers in low-mobility materials, in which the Langevin recombination is reduced: inorganic a-Si:H, μc-Si:H and the organic regioregular poly(3-hexylthiophene)/1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene (RR-PHT/PCBM) blend. We have been using a multitude of experimental techniques, namely space-charge-limited current (SCLC), photogenerated charge carrier extraction by linearly increasing voltage (photo-CELIV) and double injection (DoI) current transient techniques for investigation of temperature and electric field dependencies of B. For RR-PHT/PCBM blends, we observed a weak dependence of B on electric field and the most significant reduction of Langevin recombination.     

Investigation of charge carrier lifetime in high-resistivity semiconductor layers by the method of small charge photocurrent

A method of determination of charge carrier lifetime from experimental data of small charge current transient under constant electric field strength is discussed. On the basis of experimental results on transient of small charge photocurrent of holes and electrons which were obtained at different electric field strengths, an analysis of charge carrier lifetimes in a-Se layer has been performed. For interpretation of experimental results the approximation, according to which photogenerated electrons or holes can be trapped in levels of three types, has been used. We found that the process of localized charge carrier release is influenced by the electric field strength and that in a-Se layer, in the vicinity of the substrate, there is a narrow region where the density of localized states exceeds its mean in a layer.     

Relaxation and charge transport phenomena in polyaniline nanofibers: Swift heavy ion irradiation effects

Relaxation and charge transport mechanisms in hydrochloric acid (HCl) doped polyaniline (PAni) nanofibers synthesized using interfacial polymerization technique and the swift heavy ion irradiation effects have been investigated by dielectric relaxation spectroscopy. Non-Debye type relaxation with a distribution of relaxation times observed in PAni nanofibers is dominated by hopping of trapped charges. Swift heavy ion irradiation induces a benzenoid to quinoid transition in PAni nanofibers. Dynamic processes occurring at different frequencies in PAni nanofibers exhibit the same activation energy which reveals an enhanced coupling among the localized short range dipolar motions. The increase in conductivity relaxation time upon irradiation indicates an increase in the carrier hopping length thereby impeding charge transport. The ac conductivity of the pristine and irradiated PAni nanofibers has been found to obey the universal power law of frequency. Although charge transport in both the pristine and irradiated PAni nanofibers follows the correlated barrier hopping (CBH) model with polarons as major charge carriers, significant decrease in conductivity has been observed upon irradiation.     

An improved time domain analysis of the charge pumping current

A new time domain analysis of the charge pumping phenomenon in metal-oxide-semiconductor field-effect transistors is presented. With this theoretical approach we model the traps filling and the charge pumping current with any gate signal waveform and any energy distribution of the interface traps in the semiconductor bandgap. A method is also presented for the determination of the energy levels reached at the end of the non-steady-state emission of electrons and holes. The theoretical charge pumping response of a single trap is also investigated for uniform and non-uniform substrate doping profiles.     

Comparison of the carrier mobilities of annealed P3HT films by using charge carrier extraction by linearly increasing voltage and space-charge limited current

Vertical mobilities of poly-3-hexythiophene (P3HT) thin films annealed with various temperatures were estimated by using charge carrier extraction by linearly increasing voltage (CELIV) method, and were compared with those estimated from a conventional space-charge limited current (SCLC) method. Both methods revealed that the mobilities were improved with increasing annealing temperature because of enhanced crystallinity of the P3HT film. In high annealing temperature more than 150°C, SCLC method could not be applicable due to deviation from theoretical analysis; in contrast, CELIV method was valid for all annealing conditions.     

Evaluation of charge trapping properties of microcrystalline germanium thin films by Kelvin force microscopy

We have prepared highly-crystallized germanium (Ge) films on quartz and evaluated their local charge trapping and electrical conduction properties from topographic and surface potential images simultaneously taken by a conductive atomic force microscopy (AFM) during and after current application to Ge films. By applying a bias of 10 V at which the current of ~ 8 mA flows between the co-planer electrodes on Ge films, the surface potential image which was uniform before bias application shows in-plane inhomogeneity within ~ 1.0 mV commensurate with the surface morphology. Such potential images remained inhomogeneous at zero bias for more than two hours after bias application. The inhomogeneous potential images can be interpreted in terms of the difference in electron concentration in highly-crystallized Ge films presumably caused by electron charging in the grain boundaries, indicating direct detection of electrically separated grain structures and resultant percolation current pass.     

Conduction mechanism for sputtered a-C:H based structures

Metal–insulator–metal (MIM) and metal–insulator–semiconductor (MIS) structures fabricated by magnetron sputtering deposition of insulating hydrogenated amorphous carbon are analyzed by measuring their current–voltage characteristics in order to find the conduction mechanism. For MIM structures, the linearity of the logarithmic dependencies between current density and electric field intensity (log J–log E) for higher fields indicate a space charge limited current (SCLC) conduction mechanism. The calculated values of the effective mobility are in agreement with other literature results. For MIS structures, the power-law dependence between current and voltage also indicate a space–charge limited currents based conduction mechanism.     

A study of the effect of depletion charge on the stable crystals structures of heavy alkalis

In the framework of second-order perturbation theory and model potential formalism we investigate the effect of variation of depletion charge on the prediction of stable crystal structures of K, Rb, and Cs. In the formal calculation the HCP structure is found to have the lowest energy for all three metals, even though the phonon contributions were added in case of K. Nevertheless, we were able to predict correctly the observed BCC structure by changing the sign of the depletion hole charge.     

Electrical and optical properties of binary glasses from the arsenictelluriumselenium system

An investigation of binary glasses derived from the arsenicseleniumtellurium system is described in which measurements were made of d.c. conductivity over a range of temperatures, space charge limited current, density and absorption in the near infrared region. For the higher temperature region the electrical results were consistent with conduction being due to carriers excited into extended states. While conduction at lower temperatures may in some cases be due to carriers hopping in localized states at the band edges, an alternative approach in which the localized states were considered to act as donors and acceptors gave satisfactory agreement with the experimentally determined conductivity versus temperature and activation energy versus temperature relationships and with the value of the pre-exponental constant in the conductivity expression. An anomalous field dependence of conductivity was found for annealed specimens of two glasses and it is tentatively suggested that this is associated with the creation of an impurity band. Measurements of space charge limited current showed that gold gave ohmic contacts to the glass, whereas silver and copper did not. The measurements also provided evidence for the localized states having an exponential distribution. Changes of electrical properties and density as a function of glass composition are discussed in the light of possible structural changes taking place in the glasses. The absorption edges of the glasses in the infrared region were found to be exponential in form and had slopes that did not very greatly for the range of compositions studied.     

On the light intensity dependence of short-circuit current of bilayer organic photovoltaic cells

The work presents a model of light intensity dependence of short-circuit current for a bilayer organic system. The model is based on the assumption of a uniform electric field in both organic layers and it concerns a bimolecular and monomolecular recombination of charge carriers at a heterojunction interface. The comparison between theoretical calculations and experimental results carried out on the system formed from copper phthalocyanine or bormophosphorus phthaocyanine and perylene dye is presented in the work. A good correlation between theoretical and experimental results has been achieved.     

Soliton excitations in liquid crystal polyacetylene

Abstract

A disubstituted polyacetylene with lyotropic liquid crystallinity showing SmA was synthesized. The number-average-molecular weight of the polymer was 5.2?×?10⁵ g/mol. Vapor phase iodine doping for the disubstituted polyacetylene in the form of liquid crystal order allows the generation of charge carriers. The ESR measurements show the charge carriers exhibit Pauli paramagnetism, indicating that these carriers are polarons. Mott-type variable range hopping (VRH) conduction of the polymer was observed. The combination of SmA molecular architecture and electrical conduction by polyene as a molecular wire in the polyacetylene derivative results in liquid crystal electrical conduction via electro-soliton movement.     

Space charge analysis of electrothermally aged XLPE cable insulation

Cross-linked polyethylene (XLPE) is currently widely used as an insulating material for power cables due to its good physical properties, however when in use it undergoes an electrical ageing process. Its ability to trap electric charge can give rise to space charge accumulation in the bulk of the polymer and produce localised electric stresses that can lead to cable failure, since the electric field will be increased above the design stress in some regions favouring the initiation of degradation there. In this work the PEA (pulsed electroacoustic) method was used to compare the charge dynamics in three samples (XLPE cable peelings) aged in different ways (electrothermally in the laboratory, field aged in service and thermally aged in the laboratory). Very different transient behavior was found depending upon the ageing history. This is related to differences in the migration of chemical species in the insulation layer, which are known to act as charge traps. All materials showed heterocharge peaks when the space charge reached stability, the magnitude of which seems to be related to the severity of the ageing.     

Dielectric Properties of Tin Oxide Thin Film Capacitors

Thin films of several thicknesses in the form of MIM structures are prepared from the powders of tin oxide (SnO₂) by thermal evaporation technique in a vacuum of 10⁻⁵ Torr. The dielectric properties of tin oxide film capacitors have been studied with temperatures varying from 77 to 400 K and also with frequency. At 77 K the values of capacitance and loss tangent are small for these films. With increasing temperature the values of capacitance and loss tangent increase. The capacitance-temperature plots show peak values of capacitance at 345 K for 1 kHz, 350 K for 2 kHz, 360 K for 5 kHz and 368 K for 10 kHz. These peak values of capacitance can be eliminated by repeated heating and cooling of the structures. The activation energy for the migration of charge carriers is calculated for tin oxide films and is found to be 0.13 eV. The results obtained on the dielectric properties of tin oxide thin films are presented and discussed.     

Modifications of epitaxy in evaporated films by electric charge effects

Modifications of epitaxy in Ge, Si, CdS, β-Sn, and α-Fe films, 100–600 Å in thickness, evaporated onto air-cleaved and vacuum-cleaved (001) NaCl substrates under electron bombardment (∽10¹⁵ electrons/cm² sec, at 200–400 V) or an electric field (dc 100–300 V/cm) applied to the substrate surface were investigated by reflection electron diffraction, transmission electron microscopy and selected-area diffraction. Results indicated that epitaxial temperatures of Ge and Si films were considerably lowered, and a monocrystalline β-Sn film was produced by the application of electron bombardment. The application of electric field resulted in the formation of an epitaxial cubic CdS phase, and a single orientation in an α-Fe film. These results are discussed in terms of current speculation concerning the electric charge effects on alkali halide substrates.     

Evidence of hopping of charge carriers in the clustered state of manganites

The electrical resistivity and magnetization measurements as a function of both temperature and magnetic field provide clear evidence for three temperature intervals in which the magnetic and transport properties are altered in the La_0.55Y_0.15Ca_0.3MnO₃ compound. The intermediated-T regime, called clustered state, involves the presence of both ferromagnetic and paramagnetic phases and is shown to be closely related to the colossal magnetoresistivity effect. An extra hopping conducting process is proposed to exist in this clustered state. We use both the Abeles’ model and microstructural parameters to estimate the hopping activation energy between ferromagnetic clusters. The size (density) temperature dependence and the shape of the ferromagnetic metallic clusters are revealed to play a major role in the clustered state of this manganite.