Thermally Stimulated Current Research Articles

Space charge behavior in low density polyethylene at low temperatures

In this study, a pulsed electroacoustic (PEA) system, which can measure the space charge distribution of polymer films from low to high temperatures, is introduced. The space charge behavior of direct current (DC) stressed low density polyethylene (LDPE) film is investigated at temperatures from -40 to 70°C. The space charge results show that homocharges accumulate in the vicinity of electrodes at low temperatures (-40 to 0°C), whereas heterocharges that appear during short circuit at the ambient temperature (30°C), and negative charges gradually dominate the distribution with the increase of temperature. Moreover, the accumulated space charge density at -20°C reaches the maximum within the investigated temperature range, which may be related to the traps formed by the polyethylene molecular chain branches. This finding coincides with the dynamic mechanical analysis (DMA) result and the thermally stimulated current (TSC) of LDPE reported by other researchers. Besides, the apparent mobilities of carriers at various temperatures calculated based on space charge decaying data indicates that the mobility decreases first and then increases with the increase of temperature from -40 to 70°C. The variation of the apparent mobility can be explained by the hopping model. In addition, with the help of atomic force microscope (AFM) with Kelvin probe, the contact mode of aluminum/ LDPE/semiconductor was found to be the main reason that resulted in the slower decay of injected holes around the anode, as compared to the injected electrons around the cathode.

The suppression of space charge accumulation in CB/LDPE nanocomposites and its association with molecule relaxation

AbstractSpace charge accumulation within insulating material poses a threat to the reliability in the operation of DC power cables. To investigate the influence of carbon black (CB) on the space charge accumulation of low density polyethylene (LDPE), both conductive carbon black (C-CB) and insulating carbon black (I-CB) were employed as filler particles. The space charge distributions of LDPE and CB/LDPE nanocomposites were obtained by the pulsed electro-acoustic (PEA) method. Additionally, dynamic mechanical analysis (DMA) and thermally stimulated current (TSC) spectroscopy were applied to explore the mechanism of improving space charge performance. Both the C-CB/LDPE and I-CB/LDPE nanocomposites can effectively suppress space charge accumulation. It was concluded that the improvement in space charge characteristics of CB/LDPE nanocomposites was attributable to the interaction between the CB particles and the LDPE, which reduces the number of defects formed from molecules participating in α relaxation and decreases the density of traps within the LDPE.

Electronic defects in the halide antiperovskite semiconductor Hg3Se2I2

Halide perovskites have emerged as a potential photoconducting material for photovoltaics and hard radiation detection. We investigate the nature of charge transport in the semi-insulating chalcohalide ${\mathrm{Hg}}_{3}{\mathrm{Se}}_{2}{\mathrm{I}}_{2}$ compound using the temperature dependence of dark current, thermally stimulated current (TSC) spectroscopy, and photoconductivity measurements as well as first-principles density functional theory (DFT) calculations. Dark conductivity measurements and TSC spectroscopy indicate the presence of multiple shallow and deep level traps that have relatively low concentrations of the order of ${10}^{13}\ensuremath{-}{10}^{15}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$ and capture cross sections of $\ensuremath{\sim}{10}^{\ensuremath{-}16}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}$. A distinct persistent photoconductivity is observed at both low temperatures ($l170\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) and high temperatures ($g230$ K), with major implications for room-temperature compound semiconductor radiation detection. From preliminary DFT calculations, the origin of the traps is attributed to intrinsic vacancy defects (${V}_{\mathrm{Hg}}, {V}_{\mathrm{Se}}$, and ${V}_{\mathrm{I}}$) and interstitials (${\mathrm{Se}}_{\mathrm{int}}$) or other extrinsic impurities. The results point the way for future improvements in crystal quality and detector performance.

Study of Trapping Phenomena in SrTiO3 by Thermally Stimulated Techniques

Thermally stimulated current (TSC), thermally stimulated depolarization current (TSDC), and thermally stimulated luminescence (TSL) spectroscopies were combined to study trapping phenomena in undoped bulk SrTiO3 crystals. Electrical measurements were also performed and showed that the crystals are highly resistive in the dark but exhibited an unusually high photocurrent upon 400-nm illumination. Several traps were revealed in both TSC and TSDC spectra between 83 K and 450 K in such a broad temperature range and their activation energies were extrapolated from the trap positions (peaks). TSL spectra demonstrate similar characteristics comparable to TSC and TSDC spectra, though there are some differences because of different excitation and recombination mechanisms. This work reveals the presence of large number of traps in SrTiO3 single crystals, which are most likely the source of many of the interesting phenomena in SrTiO3 such as transient and persistent photoconductivity.

A modified thermally stimulated current analysis method for direct determination of trap energy distribution

A modified method is proposed to directly determine the trap energy distribution in dielectrics after high electric field charge injection based on thermally stimulated current (TSC) measurement. Firstly, through a series of equation derivation, it is found that obtaining trap energy distribution from measured TSC data becomes solving a nonnegative least squares problem. Then, an iteration algorithm is introduced to solve the problem in detail. Finally, the modified method is applied to analyze TSC experiment results of polyethylene terephthalate (PET) film, which are compared with other researchers' work. The experiment results show that there is a wide apparent peak in TSC curves, extending from ~325 K to ~400 K. After analyzing with the method, the peak is composed of two components formed by released charges previously trapped in the traps of two major depths of ~1.09 eV and ~1.18 eV in the range of 0.5-1.3 eV. Altogether, these results generally agree with other researchers' analysis of PET, which acquired by various measurements and methods. Moreover, in order to check the reliability of the algorithm, R-square values are calculated. The values are close to 1 which manifests that the numerical solutions are almost coincide with experiment data and the method is proved to be trustworthy, realistic and stable.

Phase behavior and chain dynamics of elastin-like peptides versus amino acid sequences

Elastin fibrillogenesis is conditioned by multiple self-assembly processes. Previous studies have evidenced the crucial influence of amino acid specificities on molecular organization of glycine-rich elastin-like peptides, but also the important role of environment on the self-assembly processes. For the first time, we combined a differential scanning calorimetry (DSC) study on aqueous solutions of three elastin-like peptides with thermally stimulated currents (TSC) experiments in the condensed state. We have studied three pentadecapeptides having the XGGZG motif threefold repeated with X and Z residues constituted of valine and leucine, known to form fiber structures. Valine and leucine moieties differ only by the presence of –CH2– spacer occupying in the pattern the first or the fourth position. Both of the residues are among the most abundant in elastin. Via DSC, we showed that the simple substitution of one amino acid strongly influences the surrounding hydration of the pentadecapeptides. During the self-assembly process, a slow exchange between bound water and bulk water is highlighted for (VGGLG)3, whereas a fast exchange of water molecules is found for(VGGVG)3 and (LGGVG)3. In the pre-fibrillar condensed state, TSC analysis reveals localized and delocalized motions and gives a fingerprint of the dynamics via activation parameters. At the localized level, a profound difference in the carbonyl environment is observed between(VGGLG)3 and the other peptides. The delocalized chain dynamics of the three peptides can be connected to the different conformations. The dominant unordered conformation of (VGGLG)3 leads to a softer system, while the large amount of b sheets and b turns in (VGGVG)3 and (LGGVG)3 leads to stiffer systems. Around the physiological temperature occurs a structural, isochronal phasetransition, sequence specific, suggested to be associated with the ferroelectricity of such elastin-like peptides.

Study of gain homogeneity and radiation effects of Low Gain Avalanche Pad Detectors

Silicon detectors with intrinsic charge amplification implementing a n++-p+-p structure are considered as a sensor technology for future tracking and timing applications in high energy physics experiments. The performance of the intrinsic gain in Low Gain Avalanche Detectors (LGAD) after irradiation is crucial for the characterization of radiation hardness and timing properties in this technology. LGAD devices irradiated with reactor neutrons or 800 MeV protons reaching fluences of 2.3×1016neq/cm2 were characterized using Transient Current Technique (TCT) measurements with red and infra-red laser pulses. Leakage current variations observed in different production lots and within wafers were investigated using Thermally Stimulated Current (TSC). Results showed that the intrinsic charge amplification is reduced with increasing fluence up to 1015neq/cm2 which is related to an effective acceptor removal. Further relevant issues were charge collection homogeneity across the detector surface and leakage current performance before and after irradiation.

Trap center study in hybrid organic-inorganic perovskite using thermally stimulated current (TSC) analysis

This paper presents results of a study that allowed identifying states of traps in thin films of hybrid organic-inorganic perovskite compounds based on methylammonium lead halide with different compositions CH3NH3PbX3 (X is Cl, Br or I) prepared by spin-coating, through Thermally Stimulated Current (TSC) measurements. Special emphasis was done in studying the influence of the composition and adsorption of oxygen on the depth of traps and on the density of states associated with trapping centers. Deconvolution of the TSC curves obtained from measurements made inside a vacuum chamber under different oxygen partial pressures, revealed the presence of traps centers whose activation energies are affected by the oxygen concentration. It was also found that TSC peaks do not appear in TSC measurements carried out at pressures less than 0.1 mbar, indicating that the possible nature of the identified traps centers is related to oxygen adsorbed superficially and/or located into the grain boundaries.

Thermally stimulated currents depolarization study of gamma ray irradiated PMMA

ABSTRACTWe will investigate the effect of γ-irradiation on the polymethyl methacrylate (PMMA) properties. The Fourier transform infrared spectroscopy technique in attenuated total reflexion mode (FTIR-ATR) results allowed us to show that the irradiated PMMA undergoes a scission in its lateral chain combined to an oxidation phenomenon. In fact, a decrease in the carbonyl index, versus the radiation dose, is observed. The differential scanning calorimetry and the thermostimulated depolarization currents revealed that the scission in the lateral chain provides a better flexibility to the PMMA chain and an increase in the free volume of the material. The consequences of these phenomena appear in the decrease of the activation energy of PMMA and the appearance of a local order in the material at a temperature Tc which decreases versus the radiation dose.

Dielectric and mechanical properties of various species of Madagascan woods

The low-temperature molecular mobility of three different wood species was analyzed, with the two major constituents—cellulose and lignin—as reference. Mechanical and dielectric dynamic techniques were used. In order to observe the fine structure of the broad relaxation modes of wood, a very low-frequency analysis was carried out by thermostimulated current technique. Low-temperature relaxations of rosewood were assigned to low-temperature relaxations of cellulose. There was no dielectric response of lignin in rosewood. Contrarily, both cellulose and lignin responses were distinguished in ebony and varongy. Thermostimulated currents analyses exhibit the specific behavior of lignin in the various wood species. Moreover, the relaxation mode of cellulose observed at lower temperature remains localized in rosewood, while it tends to delocalize in varongy and ebony. The nature and intensity of physical interactions that induce variation of phase miscibility might be responsible for the observed differences. Even at the scale of the c relaxation, physical interactions modify molecular mobility.

Role of Ag additives on light-induced metastable defects in a Se–In glassy system

The paper reports the role of Ag additives on light induced metastable defects in Se–In glassy alloys using a well-known thermally stimulated current (TSC) technique. Amorphous thin films of Se90In10−xAgx (x=0, 2, 4, 8) were prepared by thermal evaporation. Measurements were made in the given thin films prior to and after exposing amorphous thin films to white light for different exposure times (0–1.5hrs). The density of light induced metastable defects increases with exposure time, and the fractional increase in light induced metastable defect density decreases as Ag concentration increases.

The effects of deep-level defects on the electrical properties of Cd0.9Zn0.1Te crystals**Project supported by the National Natural Science Foundation of China (No. 51502234) and the Scientific Research Plan Projects of Shaanxi Provincial Department of Education of China (No. 15JS040).

The deep-level defects of CdZnTe (CZT) crystals grown by the modified vertical Bridgman (MVB) method act as trapping centers or recombination centers in the band gap, which have significant effects on its electrical properties. The resistivity and electron mobility–lifetime product of high resistivity Cd0.9Zn0.1Te wafer marked CZT1 and low resistivity Cd0.9Zn0.1Te wafer marked CZT2 were tested respectively. Their deep-level defects were identified by thermally stimulated current (TSC) spectroscopy and thermoelectric effect spectroscopy (TEES) respectively. Then the trap-related parameters were characterized by the simultaneous multiple peak analysis (SIMPA) method. The deep donor level ( dominating dark current was calculated by the relationship between dark current and temperature. The Fermi-level was characterized by current–voltage measurements of temperature dependence. The width of the band gap was characterized by ultraviolet-visible-infrared transmittance spectroscopy. The results show the traps concentration and capture cross section of CZT1 are lower than CZT2, so its electron mobility–lifetime product is greater than CZT2. The Fermi-level of CZT1 is closer to the middle gap than CZT2. The degree of Fermi-level pinned by of CZT1 is larger than CZT2. It can be concluded that the resistivity of CZT crystals increases as the degree of Fermi-level pinned near the middle gap by the deep donor level enlarges.

Direct Measurement of Pyroelectric and Electrocaloric Effects in Thin Films

Author(s): Pandya, S; Wilbur, JD; Bhatia, B; Damodaran, AR; Monachon, C; Dasgupta, A; King, WP; Dames, C; Martin, LW | Abstract: An understanding of polarization-heat interactions in pyroelectric and electrocaloric thin-film materials requires that the electrothermal response is reliably characterized. While most work, particularly in electrocalorics, has relied on indirect measurement protocols, here we report a direct technique for measuring both pyroelectric and electrocaloric effects in epitaxial ferroelectric thin films. We demonstrate an electrothermal test platform where localized high-frequency (approximately 1 kHz) periodic heating and highly sensitive thin-film resistance thermometry allow the direct measurement of pyrocurrents (l10 pA) and electrocaloric temperature changes (l2 mK) using the 2-omega and an adapted 3-omega technique, respectively. Frequency-domain, phase-sensitive detection permits the extraction of the pyrocurrent from the total current, which is often convoluted by thermally-stimulated currents. The wide-frequency-range measurements employed in this study further show the effect of secondary contributions to pyroelectricity due to the mechanical constraints of the substrate. Similarly, measurement of the electrocaloric effect on the same device in the frequency domain (at approximately 100 kHz) allows for the decoupling of Joule heating from the electrocaloric effect. Using one-dimensional, analytical heat-transport models, the transient temperature profile of the heterostructure is characterized to extract pyroelectric and electrocaloric coefficients.

Studies on the deep‐level defects in CdZnTe crystals grown by travelling heater method

The variation of deep level defects along the axis of CZT:In ingots grown by Travelling Heater Method was investigated by the means of thermally stimulated current (TSC) spectra. Models for the reaction among different defects In, Tei, and VCd were used to analyze the variation of deep level defects along the growth direction. It was found that the density of In dopant‐related defects is lower in the tip, but those of Te antisites and Te interstitials are higher in the tip. The density of cadmium vacancy exhibits an initial increase followed by a decrease from the tip to tail of the ingot. In PL spectra, the intensities of (D0, X), (DAP) and Dcomplex peaks obviously increase from the tip to the tail, due to the increase of the density of In dopant‐related defects (), Cd vacancies, and impurities. The low concentration of net free holes was found by Hall measurements, and high resistivity with p‐type conduction was demonstrated from I–V analysis. The mobility for electrons was found to increase significantly from 634 ± 26 cm2 V−1 s−1 in the tip to 860 ± 10 cm2 V−1 s−1 in the tail, due to the decrease of the deep level defect densities.

Experimental Study on Influence of Trap Parameters on Dielectric Characteristics of Nano-Modified Insulation Pressboard.

In order to study the influence of trap parameters on dielectric characteristics of nano-modified pressboards, pressboards were made using the nano doping method with different nanoparticle components. The dielectric characteristics of the modified pressboards were measured, and the trap parameters were investigated using the thermally stimulated current (TSC) method. The test results indicated that the conductivity initially declined and then rose with the increase of nano-Al2O3 content, whereas it solely rose with the increase of nano-SiC content. Moreover, the conductivity exhibited nonlinear characteristics with the enhancement of electric field stress at high nanoparticle content. The relative permittivity of modified pressboard declines initially and then rises with the increase of nanoparticle content. In addition, the breakdown strength of modified pressboards exhibited a pattern of incline followed by decline with the increase of nano-Al2O3 content, while it always declined with the increase of nano-SiC content. The analysis based on the energy band theory on trap parameters of the constructed multi-core model concludes that the nanoparticle components added in pressboard altered both the depth and density of traps. It is therefore concluded that trap parameters have significant influence on the dielectric characteristics of nano-modified insulation pressboard.

A time-resolved current method and TSC under vacuum conditions of SEM: Trapping and detrapping processes in thermal aged XLPE insulation cables

Thermal aging of cross-linked polyethylene (XLPE) can cause serious concerns in the safety operation in high voltage system. To get a more detailed picture on the effect of thermal aging on the trapping and detrapping process of XLPE in the melting temperature range, Thermal Stimulated Current (TSC) have been implemented in a Scanning Electron Microscope (SEM) with a specific arrangement. The XLPE specimens are molded and aged at two temperatures (120°C and 140°C) situated close to the melting temperature of the material. The use of SEM allows us to measure both leakage and displacement currents induced in samples under electron irradiation. The first represents the conduction process of XLPE and the second gives information on the trapping of charges in the bulk of the material. TSC associated to the SEM leads to show spectra of XLPE discharge under thermal stimulation using both currents measured after electron irradiation. It was found that leakage current in the charging process may be related to the physical defects resulting in crystallinity variation under thermal aging. However the trapped charge can be affected by the carbonyl groups resulting from the thermo-oxidation degradation and the disorder in the material. It is evidenced from the TSC spectra of unaged XLPE that there is no detrapping charge under heat stimulation. Whereas the presence of peaks in the TSC spectra of thermally aged samples indicates that there is some amount of trapped charge released by heating. The detrapping behavior of aged XLPE is supported by the supposition of the existence of two trap levels: shallow traps and deep traps. Overall, physico-chemical reactions under thermal aging at high temperatures leads to the enhancement of shallow traps density and changes in range of traps depth. These changes induce degradation of electrical properties of XLPE.

Development of system for automatic measurement of transient photocurrent and thermally stimulated current

This paper presents details of the design and implementation of a system for measuring of thermally stimulated current (TSC) and transient photocurrent (Iph), developed using the Virtual Instrumentation concept. For that we have used National Instrument hardware and the LabView® package as software. The system is controlled by a virtual instrument (VI) which includes facilities to perform measurements of photocurrent keeping the temperature of the sample and the pressure of the chamber of measurement controlled as well as real time display of the Iph vs t and TSC vs T curves. The system was tested by performing transient photocurrent and TSC measurements on CH3NH3PbI3 thin films that are generally used as absorbent layer of solar cells. This type of characterization is very useful to get information of the trapping and recombination processes that affect the transport properties of the devices.

The relationship between deep-level defects and high resistivity characteristic in CdZnTe crystals

The internal defects of CdZnTe crystals grown by the modified vertical Bridgman (MVB) method act as trapping centers or recombination centers in the band gap, which have effects on its resistivity. The relationship between deep-level defects and high resistivity characteristic in Cd0.9Zn0.1Te:In single crystal was studied. The deep-level defects were identified by thermally stimulated current (TSC) spectroscopy and thermoelectric effect spectroscopy (TEES) in the temperature range of 18–315 K. The trap-related parameters, e.g., activation energy, capture cross section, trap density were characterized by the simultaneous multiple peak analysis (SIMPA) method and Arrhenius fitting. The deep donor level (\({{E}_{\text{DD}}}\)) dominating dark current was about 0.664 eV near the middle gap by fitting the plots of the natural logarithm of dark current intensity ln(IDC) versus 1/(kT). The doubly ionized Te antisite (\(\text{Te}_{\text{Cd}}^{\text{2+}}\)) below the conduction band acting as a deep donor level was mainly the origin of \({{E}_{\text{DD}}}\) level. The energy value of Fermi-level was about 0.671 eV characterized by current–voltage (I–V) measurements of temperature dependence in the temperature range from 275 K to 315 K. The resistivity was about 8.17 × 109 Ω·cm measured by I–V measurement at room temperature. The high resistivity performance of Cd0.9Zn0.1Te:In crystal is mainly due to the Fermi-level pinned near the middle gap by the \({{E}_{\text{DD}}}\) level.

Characterization of defect states in Ga-rich gallium arsenide crystals by thermally stimulated current

The trap levels in Ga-rich GaAs crystals were studied in the temperature range of 10–300 K using thermally stimulated currents (TSC) technique. During the experiments we utilized a constant heating rate of 0.2 K/s. Experimental evidence is found for two electron trapping centers in the crystal with activation energies of 44 and 50 meV. The analysis of the experimental TSC curve suggests slow retrapping. The capture cross sections of the traps were determined as 8.8 × 10−25 and 1.0 × 10−25 cm2 with concentrations of 2.7 × 1011 and 1.3 × 1011 cm−3, respectively.

Thermal analysis of organically modified Ca2+-montmorillonite using DSC and TSC techniques

Thermal analysis on organically modified Ca2+-montmorillonite (OMON) and its source materials—octadecylamine (ODA) and Ca2+-montmorillonite (Ca2+-Mon)—was studied using thermally stimulated current (TSC) technique. The appearance of ρ MON peak with the T max = 75 °C shows the ability of the developed TSC system to demonstrate the relaxation effects of dehydration in Ca2+-Mon. It appeared within the temperature range of DSC endothermic peak (30–100 °C) where the T mMON = 58 °C. Segmental motions of ODA chains and structural disruptions in the modifier agent compound produced TSC α ODA, ρ ODA and ρ 1ODA peaks that are comparable to thermal transition and endothermic peaks in DSC profile (T gODA, T m1ODA and T m2ODA). The effect of localized motion in ODA chains as revealed by the TSC βOMON peak (T max = −23 °C), however, is absent in the DSC profile of OMON. It shows TSC technique has high sensitivity in detecting various relaxation behaviors at molecular level. More evidences are demonstrated by the ρ OMON (T max = 86 °C) and ρ 1OMON (T max = 105 °C) peak originated from the ODA chains structures. These peaks also confirm the intercalation of the modifier cations inside the Ca2+-Mon gallery.