Thermally Stimulated Current Research Articles

Surface flashover performance of phenolphthalein modified LDPE in vacuum

Since surface flashover of insulators in vacuum restricts the operation of the power equipment, improvement of surface flashover performance is of considerable importance in various fields. Previous studies show that surface flashover performance is closely related to the trap parameters of materials. In this paper, the crystallization behavior of low density polyethylene (LDPE) is modified by phenolphthalein. It results in the change of the trap parameters, leading to the enhancements in surface flashover performance. Results of differential scanning calorimeter (DSC) and scanning electron microscope (SEM) show that the crystallization behavior of LDPE is greatly changed by phenolphthalein. The crystallinity increases initially and then decreases with the increase of phenolphthalein concentration, and its maximum value occurs at 0.4 wt%. The spherulite size of LDPE composites decreases obviously with the increasing phenolphthalein concentration. The results of thermally stimulated current (TSC) show that both the shallow trap (γ peak) and the deep trap (α peak) become deeper with the increase of phenolphthalein concentration. Furthermore, the impulse and dc surface flashover voltages present the optimum values at 1 wt% phenolphthalein concentration, which can be enhanced by 30.8% and 48.4%, respectively. It is concluded that LDPE with the smaller spherulite size indicates the deeper trap and the better surface flashover performance. This paper reveals the relationship between surface flashover performance and crystallization behavior of LDPE, and proposes a new method to enhance the surface flashover performance of semi-crystalline polymers in vacuum.

Effects of IR irradiation on the transport properties through deep-level defects in CdZnTe crystals studied by thermally stimulated current

The effects of infrared (IR) irradiation on the transport properties in CdZnTe crystals were investigated by using thermally stimulated current (TSC) measurements. The temperature increasing process in TSC measurements could be described by the standard Shockley-Read-Hall (SRH) model. When applying IR irradiation to this process, the relative balance from the mixing effect of the six thermal and optical transitions could be described by the modified SRH model. Under steady-state condition at room temperature, the increase of the ionization probability of the deep donor (T4) from ~0.71 in the dark to ~0.93 with IR irradiation, theoretically predicted by the SRH model was identified, which is consistent with the increase of the effective density of T4 from (5.28±0.26)×1013cm−3 in the dark to (7.25±0.36)×1013cm−3 with IR irradiation, experimentally measured from TSC results. Therefore, the effects of IR irradiation on the transport properties, by suppressing trapping and encouraging de-trapping through the defect traps, would further contribute to improving the charge collection processes and the consequent spectroscopic characteristics of CZT detectors.

Correlated analysis of 2 MeV proton-induced radiation damage in CdZnTe crystals using photoluminescence and thermally stimulated current techniques

Radiation damage induced by 2MeV protons in CdZnTe crystals has been studied by means of photoluminescence (PL) and thermally stimulated current (TSC) techniques. A notable quenching of PL intensity is observed in the regions irradiated with a fluence of 6×1013p/cm2, suggesting the increase of non-radiative recombination centers. Moreover, the intensity of emission peak Dcomplex centered at 1.48eV dominates in the PL spectrum obtained from irradiated regions, ascribed to the increase of interstitial dislocation loops and A centers. The intensity of TSC spectra in irradiated regions decreases compared to the virgin regions, resulting from the charge collection inefficiency caused by proton-induced recombination centers. By comparing the intensity of identified traps obtained from numerical fitting using simultaneous multiple peak analysis (SIMPA) method, it suggests that proton irradiation under such dose can introduce high density of dislocation and A-centers in CdZnTe crystals, consistent with PL results.

(Invited) Interface Architecture Between TiO2/Perovskite, Perovskite/Hole Transport Layer, and Perovskite Grain Boundary for MAPbI3, MASnPbI3 and CsPbI3 Solar Cells

The relationship between photovoltaic performances and interface structures on MAPbI3, CsPbI3, and MAPbI3 are reported. In addition, role of Cl anions are at the interfaces are discussed. In order to examine the interface structure, quartz crystal microbalance sensor (QCM) was used. The amount of PbI2 adsorption on TiO2 surface increased as the adsorption time and leveled off at a certain time. PbI2 still remained even after the solvent only (DMF) was passed on the TiO2 layer on QCM (namely rinsing with DMF), suggesting that the PbI2 was tightly bonded on the TiO2 surface. The structure was found to be Ti-O-Pb linkage by XPS analysis. We concluded that the Ti-OH on the surface of TiO2 reacts with I-Pb-I to form Ti-O-Pb-I. The surface trap density was measured by thermally stimulated current (TSC) method. Before the PbI2 passivation, the trap density of TiO2 was 1019 cm3. The trap density decreased to 1016/cm3 after the PbI2 passivation, suggesting that the TiO2 surface trap was passivated with I-Pb-I. It was found that Jsc increased with an increase in the Ti-O-Pb density. We concluded that the interface between TiO2 and perovskite layer has passivation structure consisting of Ti-O-Pb-I which decreases the trap density of the interfaces and suppresses charge recombination. The effect of Cl anion on high efficiency is still controversial when perovskite layer is prepared by one step method from the mixture of MAI and PbCl2. It was found that adsorption density of PbCl2 on TiO2 surface was much higher than that of PbI2 from the experiment using QCM sensor. After the surface was washed with DMF, Cl and Pb were detected. These results suggest that the TiO2 surface was much more passivated by PbCl2 than by PbI2. This may explain partially the high efficiency when the perovskite layer was fabricated by one step process consisting of MAI and PbCl2 solution. We also observed that the crystal size increased with an increase in the amount of Cl anion which of course one of the explanation of the high efficiency. The interface of hole transport layer/perovskite layer, and between perovskite layer /perovskite layer (grain boundary) was passivated with organic amines in the same way. The passivation was also effective for increasing Voc and Jsc (Efficiency: 17.5%). This was explained by the results of transient absorption spectroscopy that the charge recombination time between hole transport payer/perovskite layer increased from 0.3 to 60 μsec. It was found that the interface between TiO2 and perovskite has the structure of Ti-O-Pb-I. The bonding becomes the anchoring moieties for perovskite layers on TiO2 and passivates the surface traps of TiO2, which suppresses the charge recombination at the interface. It was proved that the perovskite solar cell efficiency increased with an increase in the Ti-O-Pb-I density. Cl ions which have been still controversial also increased the density of Ti-O-Pb-Cl groups between TiO2 and perovskite lauers, which help enhance the efficiency. Passivation of perovskite/hole transport layer was also tried and it was proved to be effective for increasing the efficiency (17.5%).

Doping Effect of Graphene Nanoplatelets on Electrical Insulation Properties of Polyethylene: From Macroscopic to Molecular Scale.

The doping effect of graphene nanoplatelets (GNPs) on electrical insulation properties of polyethylene (PE) was studied by combining experimental and theoretical methods. The electric conduction properties and trap characteristics were tested for pure PE and PE/GNPs composites by using a direct measurement method and a thermal stimulated current (TSC) method. It was found that doping smaller GNPs is more beneficial to decrease the conductivity of PE/GNPs. The PE/GNPs composite with smaller size GNPs mainly introduces deep energy traps, while with increasing GNPs size, besides deep energy traps, shallow energy traps are also introduced. These results were also confirmed by density functional theory (DFT) and the non-equilibrium Green’s function (NEGF) method calculations. Therefore, doping small size GNPs is favorable for trapping charge carriers and enhancing insulation ability, which is suggested as an effective strategy in exploring powerful insulation materials.

Near-infrared photoluminescence and thermally stimulated current in Cu3Ga5Se9 layered crystals: A comparative study

Near-infrared photoluminescence (PL) and thermally stimulated current (TSC) spectra of Cu3Ga5Se9 layered crystals grown by Bridgman method have been studied in the photon energy region of 1.35–1.46eV and the temperature range of 15–115K (PL) and 10–170K (TSC). An infrared PL band centered at 1.42eV was revealed at T=15K. Radiative transitions from shallow donor level placed at 20meV to moderately deep acceptor level at 310meV were suggested to be the reason of the observed PL band. TSC curve of Cu3Ga5Se9 crystal exhibited one broad peak at nearly 88K. The thermal activation energy of traps was found to be 22meV. An energy level diagram demonstrating the transitions in the crystal band gap was plotted taking account of results of PL and TSC experiments conducted below room temperature.

Dielectric Study of Electron Capture Centers in Sillenite Crystals Bi<sub>12</sub>SiO<sub>20</sub>

The quantitative analysis of experimental results of research thermostimulated currents in crystals Bi12SiO20 (BSO) is lead carried out. Parameters of the capture centers in crystals BSO are determined.

Evaluation of Thermal Stability of Organic Electro-Optic Device by Using Thermally Stimulated Current.

Thermally stimulated current (TSC) measurement was employed to study the thermal stability of electro-optic (EO) polymers, i.e., guest/host polymer DR1/PMMA and side-chain polymer PMMA-co-DR1. Here the isothermal relaxation test showed that the relaxation time τ (85 °C) of side-chain polymer PMMA-co-DR1 is longer than that of guest/host polymer DR1/PMMA. TSC peaks appeared symmetrically in proportion to the poling electric field Ep, indicating that DR1 molecules make a dominant contribution to dipolar depolarization. Thermal sampling (TS) method showed that the activation energy of the DR1/PMMA is around 1 eV, while that of the PMMA-co-DR1 is distributed >1 eV. Results suggested that side-chain polymer is preferable to the guest/host polymer in the thermal stability. TSC measurement is helpful as a conventional method for studying the life time of EO polymers in terms of dipolar motion.

Influence of water, NaCl solution, and HNO<inf>3</inf> solution on high-temperature vulcanized silicone rubber

The absorption characteristics of high-temperature vulcanized (HTV) silicone rubber (SR) in deionized water, NaCl solution, and HNO 3 solution, as well as the influence of immersion in these liquids on HTV SR, were investigated. The effects of temperature, concentration, and AC electric field on the absorption characteristics of HTV SR in the three liquids were investigated. AC electric field exerts no significant effect on the NaCl and HNO 3 solution absorption of HTV SR. The effects of the three liquids on the dielectric constant, loss tangent, volume resistivity, AC breakdown strength, polarization current, polarization charge, thermally stimulated current (TSC), and physicochemical properties of HTV SR were also analyzed. The water absorbed by HTV SR increases the polarization current and polarization charge, and the immersion in high-temperature deionized water, NaCl solution, and HNO 3 solution lowers the activation energy of HTV SR.

A metal-oxide-semiconductor radiation dosimeter with a thick and defect-rich oxide layer

Enhancing the density of defects in the oxide layer is the main factor in improving the sensitivity of a metal-oxide-semiconductor (MOS) radiation dosimeter. This paper reports a novel MOS dosimeter with a very thick and defect-rich oxide layer fabricated by MEMS technology. The category of defects in SiO2 and their possible effect on the radiation dose sensing was analyzed. Then, we proposed combining deep-reactive-ion etching, thermal oxidation and low pressure chemical vapor deposition to realize an oxide layer containing multiple and large interfaces which can increase defects significantly. The trench-and-beam structure of silicon was considered in detail. The fabrication process was developed for obtaining a thick and compact MEMS-made SiO2. Our devices were irradiated by γ-rays of 60Co at 2 Gy per minute for 2 h and a thermally stimulated current (TSC) method was used to determine the readout of the dosimeters. Results show that there is a peak current of about 450 nA, indicating a total TSC charge of 158 μC and sensitivity of 1.1 μC mm−3·Gy, which is 40 times the sensitivity of previous MOS dosimeters.

Combined low-temperature photoluminescence and thermally stimulated current studies in Cu3In5S9 layered single crystals

Photoluminescence (PL) and thermally stimulated current (TSC) spectra of Cu3In5S9 layered crystals grown by Bridgman method have been studied in the photon energy region of 1.20–1.65eV and the temperature range of 15–70K (PL) and in the temperature region of 10–130K (TSC). A broad PL band centered at 1.484eV was observed at T=15K. Radiative transitions from shallow donor level located at 16meV to moderately deep acceptor level located at 120meV were suggested to be responsible for the observed PL band. TSC curve of Cu3In5S9 crystal exhibited one broad peak at about 60K. The activation energy of trap level was found to be 17meV. An energy level diagram showing transitions in the band gap of the crystal was plotted taking into account the results of PL and TSC measurements carried out below room temperature. Moreover, the absorption edge of Cu3In5S9 crystals have been studied through transmission and reflection measurements in the photon energy range 1.13–1.85eV. The band gap energy was determined as 1.61eV.

MeV electron irradiation of Si-SiO2 structures with magnetron sputtered oxide

MeV electrons influence on the characteristics of Si-SiO2 structure with magnetron sputtered oxide was studied by ellipsometry and the thermally stimulated current (TSC) method. The MOS structures used in this study were fabricated on <100> oriented p-Si wafers of 12.75-17,25 Ω.cm resistivity. Magnetron sputtered oxides with different thicknesses of 20 and 100 nm were deposited on p-Si substrates. Both groups of samples were irradiated by 23 MeV electrons. The oxide thicknesses and TSC characteristics of the MOS samples were measured before and after MeV electron irradiation with doses of 4.8×1015 and 4.8×1016 el.cm-2. The oxide thicknesses of both groups of samples increased after irradiation. The main defects generated by the MeV electrons were evaluated. It was shown that the trap concentration increases with the electron irradiation dose. The main peak in the TSC characteristics gives information about the main radiation defects at the Si-SiO2 interface of the MOS structures. These defects can be related to the vacancy-boron complexes which are associated with the main impurities in the p-Si substrate. These results correspond to our results reported earlier for MeV electron irradiated Si-SiO2 structures with thermally grown oxide. But (in this case) the effects observed are more pronounced for the magnetron sputtered oxide. A possible reason is the higher defect concentration generated in the magnetron sputtered oxide during its deposition on Si-substrates.

Interface traps in Al/HfO2/SiO2/4H-SiC metal-insulator-semiconductor (MIS) structures studied by the thermally-stimulated current (TSC) technique

Thermally-stimulated current (TSC) is presented as a powerful technique for investigation of shallow interface traps in metal-insulator-semiconductor (MIS) structures fabricated on wide bandgap semiconductors. This work highlighted often made mistakes during the characterization of MIS structures with this technique which was originally used for the characterization of the structures on silicon. Accuracy of the results can be significantly improved by the implementation of the proposed methods of analysis.

Electric characterization of LDPE films with TSC and dielectric spectroscopy

This contribution discusses thermally stimulated current (TSC) measurements on low density polyethylene (LDPE) in the form of thin films and virtually without additives. The TSC responses are divided into different parts attributed to capacitive currents, currents due to release of trapped carriers, and normal conduction currents. The three contributions dominate at low, intermediate, and high temperatures, respectively. Dielectric spectroscopy measurements at different temperatures indicate the relevance of the capacitive currents for the measurement of thermally stimulated polarization currents (TSPC). The separation of the conduction current, by subtracting reference curves from the total current, allowed the investigation of trap and conduction properties for different experimental parameters (e.g., excitation levels, fields, heating rates).

Understanding the conduction and breakdown properties of polyethylene nanodielectrics: effect of deep traps

Due to the variation of charge transport characteristics by introduction of nanostructured filler, the conduction and dielectric breakdown properties of nanodielectrics are poorly understood. This work studies on the effect of deep trap on the dc conduction and breakdown properties of nanodielectrics. X-ray diffraction technique is conducted to study the crystallization behavior of LDPE/Al2O3 nanocomposites. Thermally stimulated current (TSC) is applied to measure the trap parameters of specimens. Breakdown strength and volume resistivity are also measured. The results indicate that small amount of nanoalumina enhances the crystallinity, volume resistivity and breakdown strength, and decreases the crystallite size. The TSC results show that the deep trap level and density both increase at low nanoparticle loading samples (<1 wt%). It is concluded that the independent interfacial region brought by small amount of nanoparticles generates a new potential barrier ϕ 2. It interacts with the original trap sites in LDPE matrix, resulting in the increase of deep traps in nanocomposites. Nanoparticle may act as nucleating agents to modify the morphology and change the deep traps, leading to the reduction of electrical conduction and the improvement of breakdown properties. Both deep trap level and density are benefit to enhance the volume resistivity and breakdown strength. The reduction in mobility of charge carriers, the enhanced height of barrier, the decrease of low density region and the formation of homocharges caused by deep traps are of importance to the reduced conduction and high breakdown performance.

Defect Levels in Nuclear Detector Grade Cd0.9Zn0.1Te Crystals

Nuclear detector grade semi-insulating Cd0.9Zn0.1Te (CZT) crystals were grown by a low temperature solution method from in-house zone refined precursor materials. The extended defects due to secondary phases of tellurium inclusions/precipitates in the grown crystals were characterized by a non-destructive electron beam induced current (EBIC) imaging method. The EBIC results were correlated with the infrared (IR) transmittance mapping, which clearly shows the variation of contrast is due to non-uniform distribution of tellurium inclusions/precipitates in CZT crystals. Electrical characteristics of defect levels in the fabricated detectors were further investigated by thermally stimulated current (TSC) measurements. Pulse height spectra (PHS) measurements were carried out using nuclear radiation sources of 241Am (59.6 keV) and 137Cs (662 keV) and energy resolutions of 6.2% and 1.6% respectively were achieved.

Molecular mobility of hydroxyethyl cellulose (HEC) films characterised by thermally stimulated currents (TSC) spectroscopy

Molecular mobility has long been established to relate to textural properties and stability of polymer films and is therefore an important property to characterise to better understand pharmaceutical film formulations. The molecular mobility of solvent cast hydroxyethyl cellulose (HEC) films has been investigated by means of thermally stimulated current (TSC) below the temperature at which the film was formed. Preliminary physical characterisation of the films was performed using XRPD, TGA, DSC and texture analysis (tensile properties). XRPD results showed the films to be completely amorphous with Tg determined by DSC to be 127±1°C. TGA analysis showed the films to contain 8±1% water and film was dried to only 0.06±0.01% water content when heated to 160°C. Application of TSC detected molecular mobility in HEC films at sub-zero temperatures. Two motional transitions with average relaxation time of 50±3s were identified; a β-relaxation at −57±2°C, attributed to localised non-cooperative orientation of HEC polymer chain ends and the hydroxyethyl side groups and an α-relaxation, originating from cooperative segmental mobility, at −20±2°C. The tensile properties i.e., elongation, tensile strength and elastic modulus of the HEC film have been related to the molecular relaxation processes detected by TSC.

Comparative study of low-temperature photoluminescence and thermally stimulated current in quinary Tl4Ga3InSe6S2 layered crystals

Photoluminescence (PL) and thermally stimulated current (TSC) spectra of Tl4Ga3InSe6S2 layered crystals grown by Bridgman method have been studied in the wavelength region of 560–690nm and the temperature range of 15–45K (PL) and in the temperature region of 10–90K (TSC). A broad PL band centered at 620nm (2.00eV) was observed at T=15K. Variations of emission band has been studied as a function of excitation laser intensity in the 0.7–36.5mWcm−2 range. Radiative transitions from donor level located at 150meV below the bottom of conduction band to shallow acceptor level located at 10meV above the top of the valence band were suggested to be responsible for the observed PL band. TSC curve of Tl4Ga3InSe6S2 crystal exhibited one broad peak at about 41K. The activation energy of trap level was found to be 11meV. An energy level diagram showing transitions in the band gap of the crystal was plotted taking into account the results of PL and TSC measurements carried out below room temperature.

"Thermal Stabilization Effect" of Al2O3 nano-dopants improves the high-temperature dielectric performance of polyimide.

Insulation performance of the dielectrics under extreme conditions always attracts widespread attention in electrical and electronic field. How to improve the high-temperature dielectric properties of insulation materials is one of the key issues in insulation system design of electrical devices. This paper studies the temperature-dependent corona resistance of polyimide (PI)/Al2O3 nanocomposite films under high-frequency square-wave pulse conditions. Extended corona resistant lifetime under high-temperature conditions is experimentally observed in the 2 wt% nanocomposite samples. The “thermal stabilization effect” is proposed to explain this phenomenon which attributes to a new kind of trap band caused by nanoparticles. This effect brings about superior space charge characteristics and corona resistance under high temperature with certain nano-doping concentration. The proposed theory is experimentally demonstrated by space charge analysis and thermally stimulated current (TSC) tests. This discovered effect is of profound significance on improving high-temperature dielectric properties of nanocomposites towards various applications.

Investigation of defects in Gd doped GaN using thermally stimulated current spectroscopy

Abstract Defects in Gd-doped GaN layers are investigated using thermally stimulated current spectroscopy (TSC). The line-shape function, which commonly considers a delta function like density of state for the trap levels, is modified to take into account broad trap distributions. This function is used to fit the spectra which show two broad TSC features in all Gd-doped samples. The feature associated with the trap distribution peaking at about 45 meV from the band edge (TSC-1) can be attributed to unintentional oxygen donors, while the second feature for which the distribution peaks at ≈ 130 meV (TSC-2) from the band edge is assigned to the defects resulting due to Gd incorporation as the intensity of this feature increases with the Gd concentration. However, only a portion of the TSC-2 band, which is estimated to have an activation energy of 240 meV, is substantially reduced upon annealing at 800 °C. The annealing also results in a complete suppression of the magnetization. This implies that the observed magnetization is likely to be resulting from the defects associated with this portion of TSC-2 band. Furthermore, the energetic position of this portion of TSC-2 band (240 meV) matches very well with the activation energy ( ≈ 200 meV ) for the low temperature PL peak at 3.05 eV, which has already been attributed to the defects responsible for the observed magnetic behavior. This suggests that both the TSC and the PL features are resulting from the same defect type and therefore, could provide a vital clue in the search for the exact nature of the defect responsible.