Thermally Stimulated Current Research Articles

Charging Phenomena at the Interface Between High-k Dielectrics and SiOx Interlayers

The transition regions of GdSiO/SiOx and HfO2/ SiOx interfaces have been studied with the high-k layers deposited on silicon substrates. The existence of transition regions was verified by medium energy ion scattering (MEIS) data and transmission electron microscopy (TEM). From measurements of thermally stimulated current (TSC), electron states were found in the transition region of the HfO2/SiOx structures, exhibiting instability attributed to the flexible structural molecular network expected to surround the trap volumes. The investigations were focused especially on whether the trap states belong to an agglomeration consisting of a single charge polarity or of a dipole constellation. We found that flat-band voltage shifts of MOS structures, that reach constant values for increasing oxide thickness, cannot be taken as unique evidence for the existence of dipole layers.

Preparation of glycine polymorphs crystallized in water and physicochemical characterizations

This study shows that it is possible to obtain pure α-glycine or a mixture of glycine polymorphs (α and γ) in aqueous solution, depending on the mode of preparation, either continuous or semi-continuous. The differences between the two samples of glycine in solid state and in aqueous solution have been studied, using the following techniques: X-ray analysis, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), thermal microscopy, calorimetric analysis, thermally stimulated current (TSC) analysis and electrochemical impedance analysis. These analyses show that the presence of γ-glycine significantly modifies the physico-chemical properties of these two samples.

Influence of poly (3,4-ethylenedioxythiophene)-poly (styrenesulfonate) in polyfluorene-based light-emitting diodes: Evidence of charge trapping at the organic interface

Although the improvement of the indium tin oxide (ITO) anode by PEDOT:PSS has often been reported in literature, the origin of it is not well known. In this paper, we investigate polyfluorene-based devices with a relatively high work function (Al) cathode by electroabsorption (EA) spectroscopy and thermally stimulated current (TSC) techniques. A 0.9 V increase of built-in voltage evidenced by EA in PFV-based devices was similar to that obtained previously in PFO-based devices. It was inconsistent with the work function difference between ITO (4.9 eV) and PEDOT:PSS (5.2 eV). The formation of new types of traps related to the presence of the PEDOT:PSS layer was detected by TSC in PF-N-Ph based devices, confirming that the PEDOT:PSS/PF contact layer acts as an electron trapping surface. We conclude that the TSC technique should allow a deeper characterization of charge traps created at the electrodes and a better modelling of charge injection in future studies.

Microscopic view of aging mechanism of polyimide film under pulse voltage in presence of partial discharge

Inverter-fed motors are damaged due to pulse voltage. The situation leads to early failure of insulation. Polyimide is an important insulating material in the inverter-fed motors. Its insulation aging process and failure mechanism are different from traditional ac and dc voltage. In this paper, structure change of polyimide film aging under pulse voltage above partial discharge inception voltage (PDIV) is investigated through scanning electron microscopy (SEM), thermally stimulated current (TSC) and lifetime test. It reveals that the degradation of structure in polyimide propagates from surface to interior with continuous aging, creating three layers in section. In the TSC spectrum, two main well-resolved peaks, namely α and β, exist in the 20-200°C range. The configuration corresponding to α peak in polyimide is damaged more severely than the configuration corresponding to β peak and its degradation is accelerated with increase in frequency.

Spatial Localization of Carrier Traps in 4H-SiC MOSFET Devices Using Thermally Stimulated Current

Carrier traps in 4H-SiC metal–oxide–semiconductor (MOS) capacitor and transistor devices were studied using the thermally stimulated current (TSC) method. TSC spectra from p-type MOS capacitors and n-channel MOS field-effect transistors (MOSFETs) indicated the presence of oxide traps with peak emission around 55 K. An additional peak near 80 K was observed due to acceptor activation and hole traps near the interface. The physical location of the traps in the devices was deduced using a localized electric field approach. The density of hole traps contributing to the 80-K peak was separated from the acceptor trap density using a gamma-ray irradiation method. As a result, hole trap density of N t,hole = 2.08 × 1015 cm−3 at 2 MV/cm gate field and N t,hole = 2.5 × 1016 cm−3 at 4.5 MV/cm gate field was extracted from the 80-K TSC spectra. Measurements of the source-body n +–p junction suggested the presence of implantation damage in the space-charge region, as well as defect states near the n + SiC substrate.

Characterization and parameter optimization of a microcellular polypropylene electret under an external inertial load

In this study, microcellular polypropylene (PP) electrets were fabricated under various charge and process conditions. This was followed by thermally stimulated current (TSC) experiments to examine in a qualitative manner the charge stability of the electrets and the origin of the surface and space charges. In addition, a series of experiments was performed to obtain the effective piezoelectric constant in the thickness direction using an equation that considers external inertial loading effects. An optimization process using the Taguchi method was conducted to find the optimal conditions for the charging process and the electret properties that maximize the piezoelectric effects. To compare the contributions of signal parameters, a pooled analysis of variance (pooled ANOVA) was used. From these results, the factors that most significantly influence the piezoelectric effects were identified. A response surface was constructed to predict the optimal values of these factors to obtain the best piezoelectric effects. As a consequence, the applied voltage and the modified film thickness were turned out to be the most influential factors, and it was predicted that the optimal conditions are a 30-kV corona discharge and use of 60-µm-thick film.

Trapping centers and their distribution in [formula omitted] layered single crystals

Thermally stimulated current (TSC) measurements have been carried out on Tl 2In 2Se 3S layered single crystals in the temperature range of 10–175 K. The TSC spectra reveal the presence of two peaks (A and B). The electronic traps’ distributions have been analyzed by different light illumination temperature techniques. It was revealed that the obtained traps’ distribution can be described as an exponential one. The variations of one order of magnitude in the traps’ density for every 30 meV (A peak) and 59 meV (B peak) were estimated. Moreover, the mean activation energy, attempt-to-escape frequency, capture cross section and concentration of the traps were determined.

Effect of magnetic field on electrical properties of nanocrystalline poly(vinylidene fluoride) samples

AbstractBACKGROUND: The electrical properties of nanocrystalline poly(vinylidene fluoride) (PVDF) samples of 20 µm in thickness were measured in terms of thermally stimulated current (TSC), conduction current and dielectric constant after application of a magnetic field.RESULTS: TSC shows the release of trapped charges inside the material that enhances the current with magnetic field. The reason for the polarity reversal of the current with reversal of the magnetic field polarity is due to the change in spin of electrons depending upon the direction of the magnetic field.CONCLUSION: The magnetic field causes trapping of charge carriers in different traps, as the reason for the increase of activation energy with increasing field. The flow of conduction current at constant temperature in magnetically polarized PVDF is governed by Poole–Frenkel and Schottky–Richardson mechanisms. The decrease in dielectric constant at a certain alternating current (AC) frequency and magnetic field with temperature is caused by magnetic polarization in addition to the AC field. Copyright © 2009 Society of Chemical Industry

Characterization of defects in undoped non c-plane and high resistance GaN layers dominated by stacking faults

Deep defects in undoped c- and a-planes GaN layers grown by MOVPE on sapphire substrates were investigated by photoluminescence (PL), photocurrent (PC) and thermally stimulated current spectroscopy (TSC). In a-plane GaN the PL spectra are dominated by basal stacking faults and donor–acceptor pair recombinations correlated signals, whereas the near band-edge emission is the dominating in c-plane GaN layers. In TSC, no evidence is found for a strong trap emission of a-plane GaN correlated with the basal stacking fault-related PL signal. But, the high concentration of stacking faults in a-plane GaN layers induces a high concentration of deep defects with activation energies of 220 and 320 meV causing the DAP transition in Pl, the broad peak B in PC and TSC trap emissions in the temperature range T2. The trap at 220 meV is well known from c-plane GaN and related to dislocations, which support the assumption that this deep trap is introduced by the stacking faults in a-plane GaN.

Study of trap states in polyspirobifluorene based devices: Influence of aging by electrical stress

The influence of aging by electrical stress on the formation of defects has been studied in two types of polymers: blue emitting spiro copolymer and white emitting spiro blend polymer. The white emitting spiro polymer was obtained by adding green and red chromophores into the host blue copolymer. The defect states were investigated by using two complementary techniques: charge based deep level transient spectroscopy and thermally stimulated current (TSC). Six trap levels were retrieved in blue emitting diodes with a depth of 0.17–0.85 eV, and the onset of an additional trap with a depth of 0.49 eV was observed in white emitting ones. The density of these traps is of the order of 1016–1017 cm−3. The fractional TSC experiments allowed us to determine the distribution of five trap types in white emitting diodes, which could be described by Gaussian functions. The aging of devices was performed by electrical stress and not by exposure of samples to air or oxygen, in contrast to previous investigations on other organic materials. Upon aging, no new traps have been created or suppressed in both doped and undoped polymers. Furthermore, an enhancement in trap density has been observed. The analysis of the trap characteristics has highlighted that the aging would not uniformly affect the defect levels in the polymer. The density of deeper traps (above 0.3 eV) is increased, and their distribution is also modified, while the shallow traps (below 0.3 eV) remain stable. Furthermore, the investigation of the doping influence on the degradation process has shown that the chromophore addition seems to stabilize the polymer, whose degradation rate is slower than that of the undoped material.

Annealing study of a bistable cluster defect

This work deals with the influence of neutron and proton induced cluster related defects on the properties of n-type silicon detectors. Defect concentrations were obtained by means of Deep Level Transient Spectroscopy (DLTS) and Thermally Stimulated Current (TSC) technique, while the full depletion voltage and the reverse current were extracted from capacitance–voltage ( C– V) and current–voltage ( I– V) characteristics. The annealing behaviour of the reverse current can be correlated with the annealing of the cluster related defect levels labeled E 4 a and E 4 b by making use of their bistability. This bistability was characterised by isochronal and isothermal annealing studies and it was found that the development with increasing annealing temperature is similar to that of divacancies. This supports the assumption that E 4 a and E 4 b are vacancy related defects. In addition we observe an influence of the disordered regions on the shape and height of the DLTS or TSC signals corresponding to point defects like the vacancy-oxygen complex.

Diamond-based off-line dosimeters for environmental control in space flights

Biological experiments in space and ongoing human space missions devoted to the solar system exploration require significant advancements in the radiation environment monitoring systems. Radiation hazard has to be continuously monitored and the evaluation of the biological damage suffered should be calculated within short time. In this paper we demonstrate the feasibility of using polycrystalline diamond films as dosimeters for space applications. The charge trapped into deep intra-gap defect levels during radiation exposure, and released during a high-temperature thermal scan to give thermally stimulated current (TSC), has been integrated as a function of time to evaluate the absorbed dose. The capability of diamond films to detect low doses has been demonstrated down to the mGys range. First application of these dosimeters in a real twelve-day, low Earth orbit, space mission has been carried out. TSC results have been proved to give correct evaluation of the dose absorbed during the space mission, assessing the capabilities of synthetic diamond and TSC read-out system as a proper dosimetry technique for space applications.

Thermally stimulated current observation of trapping centers and their distribution in as-grown TlGaSeS layered single crystals

Thermally stimulated current (TSC) measurements were carried out in as-grown TlGaSeS layered single crystals. The investigations were performed in temperatures ranging from 10 to 160 K with heating rate of 0.8 K s −1. The analysis of the data revealed three electron trap levels at 13, 20 and 50 meV. The activation energies of the traps have been determined using various methods of analysis, and they agree with each other. The calculation for these traps yielded 3.3 × 10 −24, 1.0 × 10 −24, and 1.1 × 10 −24 cm 2 for capture cross-sections and 1.9 × 10 12, 2.9 × 10 11 and 4.5 × 10 10 cm −3 for the concentrations, respectively. It was concluded that in these centers retrapping was negligible, as confirmed by the good agreement between the experimental results and the theoretical predictions of the model that assumes slow retrapping. An exponential distribution of electron traps was also revealed from the analysis of the TSC data obtained at different light excitation temperatures. This experimental technique provided values of 9 and 77 meV/decade for traps distribution of peaks A and C, respectively.

Changes in the physical structure and chain dynamics of elastin network in homocysteine‐cultured arteries

The thermal and dielectric properties of the elastin network were investigated in arteries cultured with physiological and pathological concentrations of homocysteine, an aminoacid responsible of histological impairments in human arteries. The physical structure of this amorphous protein was investigated by differential scanning calorimetry (DSC). To explore the molecular dynamics of the elastin network in the nanometer range, we used thermally stimulated currents (TSC), a dielectric technique running at low frequency, and measuring the dipolar reorientations in proteins subjected to a static electrical field. Combining DSC and TSC experiments reveals the molecular mobility of the proteins, both in the glassy state and in the liquid state. Significant differences are evidenced in the physical structure and relaxation behavior of elastin network in cultured arteries (physiological and pathological concentrations of homocysteine) and discussed.

Modelling thermally-stimulated currents in disordered semiconductors

We report a computer modelling study of thermally stimulated currents in thin film semiconductors. In a multi trapping context, we examine this method as a ‘spectroscopy’ of the energy distribution of localised states. We consider so-called ‘weak’ and ‘strong’ re-trapping conditions, the use of non-physical ‘effective’ attempt-to-escape frequencies and the effect of lifetime variation as distributions relax. We show that for strong re-trapping in distributed states, the energy scale can be better approximated by the quasi-Fermi level position and propose that the density of states may alternatively be computed using the easily determined derivative of the quasi-Fermi energy with respect to temperature.

Electric polarization memory effect in a magnetoelectric multiferroic CuFe1−xGaxO2

Magnetic oxide CuFeO2 is a magnetoelectric multiferroic with new type of spin–polarization coupling different from that in the spin-current mechanism, where magnetic field-induced or nonmagnetic impurity-induced proper helical magnetic ordering generates a spontaneous electric polarization parallel to the helical axis. Using a CuFe1−xGaxO2 sample with x=0.035, in which the single ferroelectric phase is realized below TN∼8 K in zero magnetic field unlike CuFe1−xAlxO2, we have performed pyroelectric current, thermally stimulated current (TSC) and polarized neutron diffraction measurements to study a memory effect that the electric polarization is partially preserved even for 2nd-cooling from above TN without poling electric field. It was found that the charge trapped during 1st-cooling with poling electric field, which is released as TSC on heating, plays an important role in the memory effect.

Trapping centers and their distribution in Tl 2Ga 2Se 3S layered single crystals

Thermally stimulated current (TSC) measurements with current flowing perpendicular to the layers were carried out on Tl 2Ga 2Se 3S layered single crystals in the temperature range of 10–260 K. The experimental data were analyzed by using different methods, such as curve fitting, initial rise and isothermal decay methods. The analysis revealed that there were three trapping centers with activation energies of 12, 76 and 177 meV. It was concluded that retrapping in these centers was negligible, which was confirmed by the good agreement between the experimental results and the theoretical predictions of the model that assumes slow retrapping. The capture cross section and the concentration of the traps have been also determined. An exponential distribution of electron traps was revealed from the analysis of the TSC data obtained at different light illumination temperatures. This experimental technique provided values of 10 and 88 meV/decade for the traps distribution related to two different trapping centers.

Influence of homocysteine on the physical structure and molecular mobility of elastin network in cultured arteries

The thermal and dielectric properties of the elastin network were investigated in arteries cultured with physiological and pathological concentrations of homocysteine, an aminoacid responsible of histological impairments in human arteries. The glass transition of this amorphous protein was investigated by Differential Scanning Calorimetry (DSC). To explore the molecular dynamics of the elastin network in the nanometer range, we used Thermally Stimulated Currents (TSC), a dielectric technique running at low frequency and measuring the dipolar reorientations in proteins subjected to a static electrical field. Combining TSC and DSC experiments with determination of the activation parameters of relaxation times reveals the molecular mobility of the proteins. The major differences in the relaxation behavior of elastin between arteries cultured with physiological and pathological concentrations of homocysteine are discussed.

Determination of trap depth and trap density in Se70Te30−xZnx thin films using thermally stimulated current measurements

Abstract In the present paper concentration of traps ( N t ) and trap depth ( E t ) have been calculated by thermally stimulated current (TSC) measurements in amorphous Se 70 Te 30− x Zn x ( x =2, 4) thin films. These measurements are carried out at three different heating rates. It is observed that the amount of thermally stimulated current gradually increases and the temperature ( T m ), at which maxima in TSC occurs, shifts to higher temperatures with increasing heating rates ( β ) as expected. The trap depth is found to be quite different for x =2 and x =4. The concentration of traps also increases slightly at higher concentration of Zn.

The mechanism of dielectric relaxation and proton conductivity in an α-LiIO3 nanostructure

Thermostimulated depolarization currents are studied and analyzed. As a result, a mechanism of dielectric relaxation is developed for α-LiIO3 crystals with the interatomic distance about 0.55 nm. This mechanism is due to formation of the Н3О+ and OН− defects resulting from the displacement of a proton along the valence bond in water molecules and its following migration through tunneling both inside the IO 3 − ions and between these ions in the neighboring layers. In addition, the НIO3 and НI3О8 impurities can serve as proton donors. The negative maximum of thermostimulated depolarization currents caused by false hysteresis loops due to a conductivity lag is explained.