Thermally Stimulated Current Spectra Research Articles

Effect of Etching Treatment on Thermally Stimulated Current in Spherulitic Polypropylene

Charging saturation and thermally stimulated currents (TSCs) were studied for positively corona-charged spherulitic and nonspherulitic polypropylene films which were etched with a mixture of H2SO4 and K2Cr2O7. From the experimental results of the etching treatment effect on the TSC spectra, it is concluded that the charging saturation is caused by a decrease in the number of traps located in the amorphous part of the polymer. For spherulitic polypropylene, estimated values of trap density which is effective at room temperature were 2.5-1.1×1014 [cm-3], depending on the extent of etching treatment. The densities of traps located in the crystalline parts of the nonspherulitic and spherulitic polypropylene were 0.93×1014 [cm-3] and 0.76×1014 [cm-3], respectively.

Study of charge carrier trapping in solution-grown alpha -HgI2 crystals by thermally stimulated currents

alpha -HgI2 crystals grown in solution, crystals irradiated by X- or gamma -rays, crystals aged in air, crystals aged after being polarized in a high DC field, and crystals stored in an iodine atmosphere were studied by thermally stimulated currents (TSC). The general TSC spectrum of the crystals shows two peaks: at 170 and 230 K. X-ray irradiation generates deep-donor-type traps and decreases the lifetime of the charge carriers. gamma -ray irradiation introduces a large number of deep traps closely spaced in energy. By aging the crystals in air deep donor traps are introduced into a narrow layer close to the surface of the crystals. Crystals aged in air and polarized in a high DC field show a depletion of the concentration of the deep donor traps in the active zone close to the surface induced by the field. The storage of the crystals in an iodine atmosphere prevents their degradation by aging. Apparently iodine diffusion into the crystals suppresses the slight Hg-rich deviation from the stoichiometry in crystals.

Effects of Contact Materials on the Thermally Stimulated Current Spectra of Mercuric Iodide

ABSTRACTMercuric iodide (HgI2) single crystals deposited with transparent indium-tin-oxide (ITO), and semitransparent gold and nickel contacts were investigated by thermally stimulated current spectroscopy (TSC). The differences in the TSC spectra from these samples indicate that the defect structure in HgI2 may be modified by the contact material. These defects act as carrier traps and have strong implications in the application of HgI2 nuclear detectors. A method of numerical analysis was developed to extract information such as carrier trap activation energy, capture cross-section, and trap concentration-lifetime product from the TSC measurements.

Thermally stimulated current and photorefractivity of bismuth silicon oxide and bismuth germanium oxide

Nominally undoped crystals of bismuth silicon oxide and bismuth germanium oxide have been grown by the Czochralski method and characterized by thermally stimulated current (TSC) spectroscopy and optical grating erasing techniques. TSC spectra from crystal samples of the same boule were found to be highly reproducible and varied only moderately among platelets from different boules. In contrast, the protorefractive parameters determined from grating erasing experiments varied drastically even among samples from the same boule.

Thermally stimulated current studies of sillénite-type oxide crystals: Retrapping kinetics and identity of trapped carriers

Thermally stimulated currents (TSC) of Bi12SiO20 and Bi12GeO20 crystals were measured after uv excitation at two different temperatures. This double excitation technique was demonstrated to be a simple but effective means of studying the detrapping and retrappping kinetics. In particular, it leads us to suggest that most, if not all, of the trap levels detected by TSC may be hole traps, and calls into question the general belief that these traps are electron traps. In addition, our previous contention regarding the stepwise trapping kinetics among the major traps observed in the TSC spectra was confirmed by this technique. It was found that some of the minor but deeper trap levels at higher temperatures are also related to shallower traps by such stepwise kinetics.

Charge Hopping Transport Studies in σ-Conjugated Polysilanes

Abstract Thermally Stimulated Current (TSC) and Time Of Flight (TOF) measurements were performed on polymethylphenylsilane (PMPS) to study the mechanism of interchain hole hopping transport in these materials. In TSC spectra a well defined current peak was observed between 100K-150K. The peak temperature was dependent on applied electric field and heating rate. The corresponding activation energy ranged between 0.20eV at 30V/μm to 0.33eV at 5V/μm, as obtained by both TSC and TOF spectroscopies. Photogeneration at 80K was studied as a function of illumination energy and electric field. For samples irradiated by more than ∼ ImJ, a secondary peak at ∼ 180K was observed in TSC. The secondary peak, with corresponding activation energy of 0.5eV, was associated with trapping states induced by the UV defect sites. The filling fraction of those states was dependent both on illumination energy and electric field.

TSC and PL spectroscopic studies of polyacetylene

The thermally stimulated current (TSC) and Photoluminescence (PL) spectra of conducting polymer polyacetylene are measured and analyzed. The new result on polarization of neutral polyacetylene at applied electrical field, which may be attributed to neutral polaron in it, are obtained. The isomerization from cispolyacetylene into trans-polyacetylene is observed at the TSC measurement of polyacetylene sample. A new method of computer fitting for TSC spectrum according to theoretical forms is suggested, to get activation energy.

Thermally Activated Charge Hopping Transport Studies in a Σ-Conjugated Polymer

ABSTRACTTime of flight (TOF) and thermally stimulated current (TSC) measurements were performed on poly(methylphenylsilane) (PMPS), poly(ethyl-p-methoxyphenylsilane) (PMeOPhEtS) and poly(ethylphenylsilane) (PEtPhS), to study the mechanism of interchain charge hopping transport. The value of the hole mobility at a given electric field and temperature varied with the substituent side groups. The activation energies, which were assigned to the hopping transport, ranged between 0.1-0.3eV and were dependent on the applied electric field. The nature of this dependence also varied with the substituent side groups. In the TSC spectra, a well defined current peak was observed in the temperature range between 100K-160K. The peak temperature was dependent both on the electric field and the heating rate. For a constant electric field and heating rate it was also dependent on the side groups, in a manner which was consistent with the results obtained by TOF. When the samples were exposed to high doses of UV radiation, a secondary peak at ∽180K was observed, which was attributed to surface defect sites produced by the UV exposure.

Thermally-Stimulated Current Caused by Trapped Charge Transfer Excitons in Tetracene Films

Thermally-stimulated currents (TSC) with sharp spectra were observed in tetracene films deposited at room temperature and cooled rapidly after deposition. The TSC spectra could not be explained by conventional TSC theoy. A theoretical TSC curve derived from the treatment as a two-carrier problem, that is, treatment where carriers react with trapped charge-transfer (CT) excitons, explained well the experimental results. The trap depth of 0.58 eV and dissociation energy of 0.69, 0.58, 0.49, and 0.38 eV of CT excitons were estimated from the observed sharp TSC.

Reorientable electric dipoles and cooperative phenomena in human tooth enamel.

A preliminary investigation of electric dipole reorientability in human tooth enamel (TE) in comparison to that in hydroxyapatite (OHAp) has been made with the fractional-polarization form of the thermally stimulated currents (TSC) method. The reorientable dipoles are the structural OH-ions. The OHAp exhibited compensation phenomena at 211.5 degrees C and at 356 degrees C which are associated here with the hexagonal form becoming quasi-statically stabilized and dynamically stabilized, respectively, against the monoclinic form. TE specimens were pretreated at various temperatures. All showed the onset of cooperative motions that could quasi-statically stabilize the hexagonal form at the same temperature, approximately 212 degrees C, as did OHAp, even though the TE was already statically stabilized in the hexagonal form. Parts of the TSC spectra that did not conform to the 212 degrees C compensation changed progressively with pretreatment temperature. Loss of incorporated H2O is identified as the most probable cause of most of these changes. This work shows considerable promise for TSC as a tool for further quantitative investigation of TE.

Thermally-Stimulated Current Measurements and Localized States in Polychlorophenylacetylene

Thermally-stimulated currents (TSC) in conjugated conducting polymer (polychlorophenylacetylene, PCPA) and their doping effects were studied. The TSC spectra in undoped and I2 doped PCPA are very complicated. The TSC is enhanced upon increasing the I2 concentration and a main peak is observed at around 30∼60°C. This peak was separated by the thermal-cleaning method. The trap depths evaluated from an Arrhenius plot (log i vs 1 / T) of the initial rising part of the separated peak were about 0.24 and 0.4 eV, respectively, regardless of the amount of dopant. It is considered that a localized center either comprises existing defects originally in the PCPA or defects produced by doping with I2.

Thermally stimulated currents from corona-charged polypropylene films: A thermal effect of vacuum deposition of metallic electrodes

It is demonstrated that the thermally stimulated currents (TSC) from positively or negatively corona-charged polypropylene strongly depend on the order of the following two processes: a process of vacuum deposition of Al electrodes on the sample polymer and a process of heat-treatment of the polymer. Observed results are explained by a thermal effect which is introduced during the vacuum deposition of metallic electrodes. This thermal effect of the vacuum deposition of metallic electrodes is the largest for Al and the smallest for Bi among Al, Au, Ag, and Bi. Observed TSC spectra have three peaks at about 68, 142, and above 147 °C for positively charged samples and four peaks at about 48, 90, 142, and above 147 °C for negatively charged samples, respectively. Origins of these TSC peaks are discussed in some detail.

THERMALLY STIMULATED CURRENT OF HEAT-TREATED POLYACRYLONITRILE

Thermally stimulated current (TSC) of polyacrylonitrile (PAN) films heat-treated at elevated temperatures in air and in vacuum has been investigated. In the TSC spectra of solvent cast PAN films, the current peak due to residual solvent was observed. The residual solvent could be removed by heat treatment in vacuum at 200°C for 60min. The TSC spectrum obtained for a solvent free PAN film was characterized by two current peaks observed at 110°C (αII peak) and 150°C (αI peak) due to the dipolar relaxations in the paracrystalline and amorphous phases, respectively. The amount of polarization calculated from TSC was in good agreement with theoretical value calculated from the dipolar orientation of nitril groups.The αII peak increased in the early stage of heat treatment at 230°C, while the αI peak decreased rapidly. The total polarization calculated from the both peaks decreased linearly with decreasing amount of nitril groups in a PAN film. It was found that a PAN film heat-treated at 230°C in air for longer times than about 200min did not show a significant TSC, although it was confirmed by infrared analysis that about 20% of nitril groups in PAN still remained after the heat treatment. This fact indicates that the pendant nitril groups of PAN heat-treated for prolonged time were immobilized by neighboring cyclized rigid segments and could not contribute to polarization.

Study of gap states density in A-Si: H using thermally stimulated current in a space charge zone

From a study of thermally stimulated current (TSC) performed on a-Si: H Schottky diodes, we exhibit the existence of several distinct families of levels. The exponential band tail states contribute partly to the TSC spectrum and a method of characterization is derived from the study of the associated peak. The other deepest levels are associated with ponctual defects such as impurities or structural defects and represent an other class of traps not commonly reported in the literature about a-Si: H.

THERMALLY STIMULATED CURRENT IN POLYACRYLONITRILE

Thermally stimulated current (TSC) spectra of polyacrylonitrile were measured for temperatures ranging from 10 to 170°C. The TSC spectrum obtained with the polarization in a low polarizing field is characterized by two peaks at 110°C (αI peak) and at 150°C (αII peak). In the case of the high field polarization, a current reversal phenomenum was observed near at the αII peak temperature. It is considered that the αI and αII peaks are due to the dipolar relaxations in the amorphous and paracrystalline phases, respectively. The activation energies for these two dipolar relaxations were determained by thermal sampling method. The values obtained were 36 kcal/mol for the αII relaxation and 43 kcal/mol for the αI relaxation. The degree of crystallinity, which was calculated as a percentage of the polarization charge responsible for the αII relaxation against the total polarization, was 32%.

Application of the Onsager theory to thermally stimulated currents

As it has been shown recently, the thermally stimulated current (TSC) technique may be successfully applied to investigate the effect of charged centers on electrical phenomena in organic solids. The Onsager theory of dissociation is applied to obtain a theoretical description of the field-dependent TSC spectrum, which appears as an effect of charged centers. Appearance of a TSC peak is considered in two cases: (a) due to ionization of donor or acceptor centers, and (b) due to thermal release of photogenerated and frozen at low temperature carrier pairs. It is shown that in the first case the maximum temperature of the TSC peak depends on the square root of the applied electric field. This dependence is linear in the intermediate range of fields (this is the case described by a simple Poole-Frenkel formalism), and tends to saturation for both low and high fields. In the case of photogenerated carrier pairs, for the range of trap parameters commonly accepted for amorphous solids, the TSC peak position is field-independent. The effect of charged centers on the activation energy found from the initial rise of TSC current is also discussed. The theoretical predictions are illustrated by the results of TSC experiments performed for poly/N-vinylcarbazole/ and its mixtures with polycarbonate.

-Search For a Correlation Between Stoichiometry,TSC Spectra and -NUclear Spectroscopic Capabilities of HCI2.

ABSTRACTMercuric iodide is, potentially, an interesting material for room temperature high efficiency gamma or X-ray spectroscopy. However, still several parameters limit the performance of these devices and even the crystal growth is not fully under control today, poor reproductibility being reached.An optimization of growth conditions seems difficult since no parameter of the material could be correlated with the spectrometer's performance. Therefore, the effective progress in recent years was rather limited.The goal of this paper is to investigate systematically if a correlation can be found between bulk stoichiometry for crystal grown under certain conditions, thermally stimulated current (TSC) measurements and nuclear performance. In this study, two families of crystals could be identified. Best results are obtained for nearly stoichiometric materials, exhibiting a specific TSC spectrum. In particular, the peak appearing at 173 K constitutes a good indication of the detection capabilities of the material, both for spectroscopic resolution and polarization.The growth conditions have been adjusted to produce in a fully reproducible way high performance devices.

Carrier Traps in Poly-p-Xylylene Thin Films

Carrier traps in poly (p-xylylene), PPX thin films (0.7 µm) polymerized from the vapour phase are discussed from an analysis of the X-ray-induced thermally-stimulated current (TSC). PPX polymerized at room temperature is a semicrystalline polymer with a monoclinic lattice (α-form). The TSC spectrum from α-form PPX shows five groups of TSC peaks in the range -180 to 200°C. Thermal treatment of α-form PPX above 230°C producel β-form PPX with a trigonal crystal lattice, causing a change in the TSC spectrum, that is, a decrease in the TSC peaks at -150∼-90°C, -40°C, 120°C and 160°C. Annealing the α-form PPX while retaining its α-form crystal lattice increased the TSC's from 120–160°C and produced a new peak at 50–90°C which was not changed by further annealing, or even by the crystalline phase transition. Considering these facts, these traps were considered to be molecular irregularities at the surface or in the surface layer of the crystallite, molecular defects in the crystallite, and irregular molecules.

Polarization studies by the TSC technique on a blend of cellulose acetate and polyvinyl acetate

AbstractThe thermally stimulated current (TSC) technique has been used to study solvent‐cast blends of a cellulose derivative with a vinyl polymer. TSC peaks are observed at 56, 80, and 120°C. Their origin is investigated because the TSC spectra of the blends differ from the spectra of the individual components. Data on blends with components in the weight ratios 25:75, 50:50, and 75:25 indicate that the 50:50 blend shows the greatest polarization. The enhancement of depolarization currents observed on blending is explained on the basis of a Maxwell–Wagner–Sillars polarization due to increased heterogeneity in the structure. Effects of forming conditions (time, temperature, field) on polarization have been investigated. Activation energies and relaxation times are calculated; there is good agreement between the values obtained from the initial‐rise and the full‐curve methods.

Thermal depolarisation current study of electron irradiated fluorinated ethylene propylene

Thermally stimulated current (TSC) studies have been reported in the co-polymer of tetrafluoro ethylene and hexafluoropropylene films. Depolarisation current peaks are obtained atα1,α2 andβ relaxation temperatures of the polymer and the detrapping process is explained on the basis of its molecular motion. A cross-over electron energy of 18 keV is observed where the nature of TSC spectra undergoes a remarkable change. This is explained in relation to the surface states in FEP. Five groups of trapping levels, 0.25±0.08, 0.57±0.10, 1.07±0.1, 1.3±0.25 and 2.3±0.4 eV are obtained.