Trapped Space Charge Research Articles

On the Importance of the Work Function and Electron Carrier Density of Oxide Electrodes for the Functional Properties of Ferroelectric Capacitors

It is important to understand the effect of the interfaces between the oxide electrode layers and the ferroelectric layer on the polarization response for optimizing the device performance of all‐oxide ferroelectric devices. Herein, the effects of the oxide La0.07Ba0.93SnO3 (LBSO) as an electrode material in an PbZr0.52Ti0.48O3 (PZT) ferroelectric capacitor are compared with those of the more commonly used SrRuO3 (SRO) electrode. SRO (top)/PZT/SRO (bottom), SRO/PZT/LBSO, and SRO/PZT/2 nm SRO/LBSO devices are fabricated. Only marginal differences in crystalline properties, determined by X‐ray diffraction and scanning transmission electron microscopy, are found. High‐quality polarization loops are obtained, but with a much larger coercive field for the SRO/PZT/LBSO device. In contrast to the SRO/PZT/SRO device, the polarization decreases strongly with increasing field cycling. This fatigue problem can be remedied by inserting a 2 nm SRO layer between PZT and LBSO. It is argued that strongly increased charge injection into the PZT occurs at the bottom interface, because of the low PZT/LBSO interfacial barrier and the much lower carrier density in LBSO, as compared with that in SRO, causing a low dielectric constant, depleted layer in LBSO. The charge injection creates a trapped space charge in the PZT, causing the difference in fatigue behavior.

Effect of thermal ageing on charge dynamics and material properties of 320 kV HVDC XLPE

This paper reports the effect of thermal ageing on the space charge characteristics, DC conductivities and trap energy distributions of 320 kV HVDC XLPE. Specimens were prepared by a heating press method in the laboratory. The specimens were placed in a thermal oven and thermally aged at 130 °C up to 30 days. Space charges were measured by a pulsed electro-acoustic setup. The results showed that the reference samples had limited hetero charge accumulations near both electrodes in the samples. The homo-charge accumulation was appeared with an increase of the aged time. The space charge accumulation in the samples was transferred from hetero-charge accumulation to homo-charge accumulation with an increase of the ageing time. The electric field distribution in the sample gradually increased with the appearance of the homo-charge accumulation, whereas the homo charges evidently distorted the electric field in the samples with the increasing ageing time. The DC conductivities of the aged samples consistently increased with the increase of the measured electric field. The obtained results of apparent mobility of the charge carriers, the activation energy and the trap energy density illustrated that thermal ageing could facilitate the homo-charge accumulation in the sample. The dominant effect of thermal ageing on the sample was elucidated by using various analytical techniques. It was found that the thermal ageing would affect the material properties and facilitate the increase of the trap energy density and the space charge accumulations in the samples.

Effect of gamma irradiation on space charge and charge trap characteristics of epoxy–MgO nanocomposites

Insulating materials used in power apparatus required to be space charge free. Epoxy–magnesium oxide (MgO) nanocomposites were prepared and gamma rays were irradiated for different dosages to understand the dielectric parameters variation, space charge characteristics, surface potential variations, and for getting charge trap characteristics of the material. It is realised that permittivity increases and then reduces when the irradiation level is increased. Tan (δ) marginally increased with increasing dosage of gamma irradiation. Space charge accumulation reduces with increase in wt% of MgO in epoxy resin. 3 and 5 wt% MgO added samples showed stable space charge accumulation and fast decay during depoling, irrespective of irradiation level. Surface potential decay time reduces with irradiated specimens. After irradiation, epoxy nanocomposites showed both shallow trap formation and deep trap formation and found that shallow trap density is high compared to the deep trap density. All the irradiated specimens showed a decrease in the trap energy level compared to the virgin specimens.

Electrical breakdown of sandwiched polymers — The effect of dielectric properties and trapped electrons

Nanocomposite (NC) polymers are commonly employed because of their high breakdown strength, but the underlying mechanism of this phenomenon is not well understood. In the present study, specimens of polyethylene terephthalate (PET) films coated with epoxy resin (EP) and filled with nano-SiO 2 and Si 3 N 4 at concentrations of 0, 0.5, 1, 2, 5 and 10 wt % were subjected to DC and impulse voltages to breakdown. The effects of the types of nano-fillers and concentrations on the dielectric properties, crystallinity, and space charge trap characteristics were investigated on the breakdown strength. The experimental results show that improved DC breakdown strength of a sandwich structure of PET/nano-EP film is obtained even at 10 wt%, which is attributed to the effect of increased trap density for deep traps. In addition, to experimentally establish the dominant factors during impulse breakdown, a systematic theoretical model was used to account for the variations of impulse breakdown performance. It is concluded that the frequency-dependent dielectric characteristics plays an important role in determining the absorption of activation energy for trapped electrons, which affects the instantaneous insulating performance of NC polymers.

Relationship between space charge behavior and trap energy density distribution: A simultaneous measurement

This paper gives a detailed introduction of a simultaneous measurement system that can obtain the space charge distribution and the thermally stimulated current (TSC) of the same sample over the temperature range 213–373 K. This measurement system adopts a 4-electrode structure and a time sharing strategy to avoid the mutual interference between the space charge measurement and the current measurement. In addition, a space charge recovery method is proposed to obtain the real space charge distribution from the original data under varying temperature situation. Furthermore, a simultaneous measurement is performed on a cross-linked polyethylene (XLPE) sample. The experimental results show that the positive charges accumulate and dominate in the bulk during polarization and the cooling stages. During the thermal stimulation stage, the accumulated charges decay after ∼330 K, and the thermally stimulated current increases significantly after ∼330 K. The analysis results imply that the release of trapped charges may be the main cause of the thermally stimulated current, and most of the charges are trapped in traps more than 1 eV.

Studies on the effects of moisture and ageing on charge de‐trapping properties of oil‐impregnated pressboard based on IRC measurement

The dielectric properties of oil-paper insulation degrade due to moisture ingression and ageing. This degradation significantly impacts the space charge accumulation and charge trapping behaviour in the insulation, which are vital parameters for insulation health under the high-voltage direct current environment. In this work, an improved model based on the isothermal relaxation current (IRC) has been developed to study the charge trapping behaviour of an oil pressboard under the influence of moisture and ageing. The conventional IRC model considers the total relaxation current because of charge de-trapping only. However, in the case of a composite dielectric like oil pressboard, dipolar relaxation also affects the relaxation current. In this work, a methodology has been proposed to investigate charge de-trapping behaviour of oil-pressboard insulation considering the dipole relaxation process from IRC measurements. For this purpose, frequency domain spectroscopy measurements and IRC measurements have been performed on oil-impregnated pressboard specimens carefully prepared in the laboratory having different ageing conditions and moisture contents. Results presented in this work depict that charge trapping parameters, i.e. the trap depth and trapped charge distribution are highly affected both by ageing and moisture. It was observed that ageing leads to the generation of deeper traps, while moisture mainly enhances the density of shallow traps.

Relationship between space charge behavior and trap energy density distribution: A simultaneous measurement

Relationship between space charge behavior and trap energy density distribution: A simultaneous measurement

Improvement of memristive properties in CuO films with a seed Cu layer

We have prepared CuO-based structures including Au/CuO/Pt and Au/CuO/Cu/Pt films and studied the memristive characteristics of the two samples. Current-voltage curves of the samples suggest that both CuO-based films are typical memristive devices, showing conventional pinched hysteresis loops. Furthermore, the presented Au/CuO/Cu/Pt structure with a seed Cu layer between CuO and bottom electrodes Pt demonstrates more typical memristive and better retention characteristics than Au/CuO/Pt. In order to explore the conducting mechanism for improved memristive properties, the electric transport characteristics of the device are analyzed using a space charge limited current and charge trap defect theory.

Erratum: Influence of nano layer interface on space charge behavior and trap levels using polyimide sandwich structure

Measurements of space charge density and trap levels are informative ways to predict failure conditions of insulation materials. The charges are injected from electrodes and penetrate from surface towards deep inside of the insulation material. In order to resist the charge injection, trapping, migration and increase charge recombination, de-trapping and extraction rate, layer (nanocomposite-PI-nanocomposite) sandwich structure samples are used. Two techniques Pulse Electro Acoustic (PEA) and Thermal Stimulated Current (TSC) are combined to characterize space charge behavior and trap levels. Lab-view based program is used to detect the charges density distribution and total charge decay, different peaks of current using TSC technique are used to get activation energy and trap level density. The effects of structural interface (single, double, three layers) and types of nano (Al 2 O 3 , SiO 2 ) fillers are analyzed to influence charge accumulation polarity and trap levels. Trap levels are calculated using TSC data as well as total charge decay data. The trap values calculated from both data match each other. Al 2 O 3 -PI-Al 2 O 3 sandwich structure showed remarkable improvements to resist accumulation of space charge for longer time. Mostly charges are shallow traps with 1.126eV trap level and recombination rate is higher. In double layer structure in which interface is made up of SiO 2 and Al 2 O 3 , deep trap level of 1.26 eV and higher trap density is observed. The charges decay is faster in three layer structure than double and single layer structure. It is concluded that the interface formed with different type of nano and structure of insulation, both are an important parameters for manufacturing industry to suppress space charges accumulation and traps.

Mechanism of bulk charging behavior of ethylene propylene rubber subjected to surface charge accumulation

To investigate the bulk charging behavior of ethylene propylene rubber subjected to surface charge accumulation, surface potential decay (SPD) measurement and electro-acoustic method were employed, after the sample was treated by a corona discharge for different time durations. The space charge behavior and trap distribution were obtained. The results show that the surface potential decay (SPD) property correlated with charging time and surface charge polarity. The decay rate of surface potential correlates with charging time, where SPD rate decreases with the increase of charging time. The accumulated surface charges can be injected into the bulk state, and the bulk charging property has a strong correlation with corona treatment time duration. The long-time corona treated sample has a lower initial SPD value. Surface trap distribution influences the accumulation and injection characteristics of the carriers. A plurality of charges will be injected into the material and then trapped as space charges in the interior material. As a result, the trapped space charge can affect surface potential significantly, especially for samples with longer corona treatment duration.

Electron transport phenomena at the interface of Al electrode and heavily doped degenerate ZnO nanoparticles in quantum dot light emitting diode

ZnO nanoparticles (NPs) of 4–5 nm, widely adopted as an electron transport layer (ETL) in quantum dot light emitting diodes (QD-LEDs), were synthesized using the solution-precipitation process. It is notable that synthesized ZnO NPs are highly degenerate intrinsic semiconductors and their donor concentration can be increased up to ND = 6.9 × 1021 cm–3 by annealing at 140 °C in air. An optical bandgap increase of as large as 0.16–0.33 eV by degeneracy is explained well by the Burstein–Moss shift. In order to investigate the influence of intrinsic defects of ZnO NP ETLs on the performance of QD-LED devices without a combined annealing temperature between ZnO NP ETLs and the emissive QD layer, pre-annealed ZnO NPs at 60 °C, 90 °C, 140 °C, and 180 °C were spin-coated on the annealed QD layer without further post-annealing. As the annealing temperature increases from 60 °C to 180 °C, the defect density related to oxygen vacancy (VO) in ZnO NPs is reduced from 34.4% to 17.8%, whereas the defect density of interstitial Zn (Zni) is increased. Increased Zni reduces the width (W) of the depletion region from 0.21 to 0.12 nm and lowers the Schottky barrier (ФB) between ZnO NPs and the Al electrode from 1.19 to 0.98 eV. We reveal for the first time that carrier conduction between ZnO NP ETLs and the Al electrode is largely affected by the concentration of Zni above the conduction band minimum, and effectively described by space charge limited current and trap charge limited current models.

Influence of TiO2 nanoparticle size on creeping flashover property of transformer oil-impregnated pressboards

Nanoparticles have exhibited great potential in improving insulating property of liquid and solid dielectrics. To explore their modification mechanism on transformer oil-impregnated pressboards, TiO2 nanoparticles with different diameters of 5nm, 10nm, and 15nm were synthesized and used to prepare transformer oil-based nanofluids. The creeping flashover characteristics of both pure oil-impregnated paperboards (OPs) and nanofluid-impregnated pressboards (NPs) were examined under lightning impulse and AC voltages. It was found that the creeping flashover property of NPs is significantly improved with the decreasing of nanoparticle size. Especially, it is enhanced by 2.44 times under positive lighting impulse as nanoparticle size reduced from 15 nm to 5 nm. The results of space charge decay and trap characteristics reveal the decrease of nanoparticle size can lower trap depth and greatly enhance the space charge dissipation rate, leading to more uniform electric field distribution at the interface between nanofluid and pressboards and higher creeping flashover property.

Investigating the Components of Signals Produced by the Pockels CCD Imaging Technique in CdZnTe

Several groups have demonstrated the potential of the Pockels effect in cadmium zinc telluride as a means to detect ionizing radiation. Migrating charge carriers are believed to generate the signal detected via the Pockels effect due to the distortions they create within the electric field; however, trapped space charge beneath the cathode has been regularly observed, which suggests field changes away from the charge generation area. Consequently, the Pockels signal amplitude contains a charge integrated, i.e., dose element. In this paper, the effects of electric field collapse at the location of charge carrier generation are demonstrated. This opens the potential to apply the technique for imaging dose rate distributions. Charge-coupled device images representing the changes in an electric field within the crystal were taken, and the response to illumination from a collimated 1550-nm, 4.5-mW IR laser and irradiation from 150-kVp X-rays was measured. The data demonstrate that the signal acquired is a combination of both the local change in the electric field at the location where the carriers are being released/generated and an element caused by carriers becoming trapped, leading to electric field enhancement due to space charge near the cathode and field reduction toward the anode.

Influence of nano layer structure of polyimide film on space charge behavior and trap levels

Measurements of space charge density and trap levels are informative ways to predict failure conditions of insulation materials. The charges are injected from electrodes and penetrate from surface towards deep inside of the insulation material. In order to resist the charge injection, trapping, migration and increase charge recombination, detrapping and extraction rate, layer (nanocomposite-PI-nanocomposite) sandwich structure samples are used. Two techniques Pulse Electro Acoustic (PEA) and Thermal Stimulated Current (TSC) are combined to characterize space charge behavior and trap levels. Lab-view based program is used to detect the charge density distribution and total charge decay, different peaks of current using TSC technique are used to get activation energy and trap level density. The effects of structural interface (single, double, triple layers) and types of nano (Al 2 O 3 , SiO 2 ) fillers are analyzed to influence charge accumulation polarity and trap levels. Trap levels are calculated using TSC data as well as total charge decay data. The trap values calculated from both data match each other. Al 2 O 3 -PI-Al 2 O 3 sandwich structure showed remarkable improvements to resist accumulation of space charge for longer time. Mostly charges are shallow traps with 1.126 eV trap level and recombination rate is higher. In double layer structure in which interface is made up of SiO 2 and Al 2 O 3 , deep trap level of 1.26 eV and higher trap density is observed. The charges decay is faster in three-layer structure than double and single layer structure. It is concluded that the interface formed with different type of nano and structure of insulation, both are an important parameter for manufacturing industry to suppress space charges accumulation and traps.

Failure Characteristics and Mechanism of Nano-Modified Oil-Impregnated Paper Subjected to Repeated Impulse Voltage.

Nano-modification is a prospective method for improving the electrical properties of transformer oil. In most situations, transformer oil combined with cellulose paper is used to construct an insulation system for power equipment, such as power transformers. However, the influence of nanoparticles on the electrical performance of oil-impregnated paper is still unclear. Therefore, in this paper, we identify the failure characteristics of both fresh and nano-modified oil/paper. Specifically, the accumulative failure characteristics of nano-oil-impregnated paper (NOIP) are experimentally determined. The space charge distribution and trap characteristics of fresh paper and NOIP were measured, and the effect of nanoparticles on the space charge behavior are then analyzed. Finally, we measure the microstructure of fresh paper and NOIP subjected to repeated impulses. The test results indicate that nano-titanium oxide (TiO2) particles have a limited effect on the breakdown voltage of NOIP. However, the particles can dramatically improve the resistant ability of NOIP against repeated impulses. For the NOIP with a nano-concentration of 0.25 g/L, the improvement reaches 62.5% compared with fresh paper. Under repeated applications of impulse voltages, the space charge density of NOIP is much lower than that of fresh paper. The deep trap density of NOIP is much higher than that of fresh OIP, whereas shallow trap density is relatively lower. Micropores are generated in paper insulation subjected to repeated impulses. The amount of the generated micropores in NOIP is lower than that in fresh paper. Nano-TiO2 particles suppress the accumulation of space charge in the oil paper insulation, which weakens the electric field distortion in the dielectric. However, nanoparticles reduce the accumulative damage caused by repeated impulses. The above two points are considered the main reasons to improve the resistant ability against repeated impulses.

Structural and electrical properties of pulsed laser deposited BaPbO3 conductive thin films and its effect on the ferroelectric properties of PbZr0.52Ti0.48O3 /BaPbO3 heterostructures

ABSTRACTBaPbO3 (BPO) thin films of different thickness were deposited by pulsed laser deposition on p type Si/SiO2 substrates. The influence of film thickness on the structural and electrical properties is studied. It is found that the BPO thin films of 300 nm has good crystallinity and grain size of 21nm. The carrier mobility increases with film thickness for films above 150 nm due to the ordered structure resulting in large grain size and small number of boundaries which enhances charge transport.The effect of BPO film thickness on the ferroelectric properties of Pb(Zr0.52Ti0.48)O3 (PZT) thin films grown over BPO electrode were also investigated. Stress analysis on the thin films was carried out to investigate the dependence of BPO thickness on the ferroelectric properties of PZT. The mean grain size of PZT obtained by the Williamson – Hall Plots are 33, 45, 57 and 39 nm with increasing BPO film thickness. It was found that the Pr values of PZT film deposited over BPO of thickness 225 nm is relatively large with low Ec. It is observed that stress and grain size effects have a competing effect. Larger grain size possess a decreased grain boundary density, which reduces the probability of space charge trapping at grain boundaries and facilitate effective domain mobility which result in the improvement of ferroelectric properties.

Analysis of electrical properties of heterojunction based on ZnIn2Se4

Heterojunction of n-ZnIn2Se4/p-Si was fabricated using thermal evaporation of ZnIn2Se4 thin films of thickness 473 nm onto p-Si substrate at room temperature. The characteristics of current–voltage (I-V) for n-ZnIn2Se4/p-Si heterojunction were investigated at different temperatures ranged from 308 K to 363 K. The junction parameters namely are; rectification ratio (RR), series resistance (Rs), shunt resistance (Rsh) and diode ideality factor (n) were calculated from the analysis of I-V curves. The forward current showed two conduction mechanisms operating, which were the thermionic emission and the single trap space charge limited current in low (0 ≤ V ≤ 0.5 V) and high (V ≥ 0.7 V) ranges of voltage, respectively. The reverse current was due to the generation through Si rather than the ZnIn2Se4 film. The built-in voltage and the width of the depletion region were determined from the capacitance-voltage (C-V) measurements. The photovoltaic characteristics of the junction were also studied through the (I-V) measurements under illumination of 40 mW/cm2. The cell parameters; the short-circuit current, the open-circuit voltage and the fill factor were estimated at room temperature.

Modeling of space charge dielectric constant

The space charge polarization/depolarization is considered, and to relate the geometrical arrangement of constitutive phases to the dielectric response, three space charge dielectric constants, viz., trapped space charge dielectric constant et, drift space charge dielectric constanted and dielectric constant e are modeled. It is observed that the order of et is same for AgI and 0.9[0.75AgI:0.25AgCl]:0.1SiO2 nanocomposite in β− and α−phases. In β− phase, ed and e are 10 times larger and 10 times smaller in the second system of respective β− and α−phases. In β− phase, the ionic concentration provides the main contribution to the dielectric response of both systems, and in α− phase, the ionic mobility has the main contribution to the dielectric nature.

Simulations of the effects of electric field and temperature on space charge traps in polymer

The simulations of the structure and behavior of the molecule in the simulation software are an effective way to analyze the microscopic mechanism associated with performance change of space charge trap in the polymer. To achieve this, in this paper we first present the polyethylene molecular model which is developed by using the simulation software Materials Studio (MS). Then, the microstructure and property of space charge trap are analyzed by the changes with the energy and the free volume in the polyethylene due to the chain segment motion under the universal force field (UFF), respectively. Some important findings are extracted from simulation results. First, in the process of the temperature gradually increasing from 298 K to 363 K, the phenomena of slippage and diffusion of the molecule due to the enhanced thermal motion of molecules are observed. These phenomena lead to the free volume increasing and the space charge trap energy level decreasing gradually, whose maximum value is 1542.073 and the minimum value is 0.66 eV when the temperature is 363 K. Second, when an electrostatic field of 0.0007 Hartree/Bohr is applied to the polymer, molecular chain segments are oriented by the Maxwell stress that is generated by the electric effect. Molecular chain segment orientations induce the van der Waals interaction energy to increase to -360.18 kcal/mol (1 kcal/mol = 4.18 kJ/mol), the free volume to decrease by 279.77 3, and the space charge trap energy level to decrease by 0.45 eV. Third, by comparing the cases of applying the temperature field and the electric field to the polyethylene, it is found that the electric field has stronger effect on charge trap. Specifically, the space charge trap energy level of the polyethylene associated with 0.0007 Hartree/Bohr electric field is reduced by 0.17 eV compared with that associated with the temperature of 363 K. Moreover, simulation results and measured results are compared with each other and they are well consistent. Finally, it is concluded that using electric effect and molecular thermodynamic movement is an very effective way to analyze the microscopic mechanism of changes with free volume and van der Waals interaction energy. This analysis confirms that molecular motion changes the microstructure of the polyethylene and generates charge traps. In addition, it confirms that the influence of the electric field on the polyethylene generates the lower level of space charge trap than the effect of the temperature field.

Fabrication, temperature dependent current-voltage characteristics and photoresponse properties of Au/α-PbO2/p-Si/Al heterojunction photodiode

Thin film of lead dioxide, α-PbO2, has been grown by thermal evaporation technique on the single crystal of p-Si substrate and heterojunction photodiode, Au/α-PbO2/p-Si/Al, was fabricated. The current-voltage characteristics of the diode have been studied in the temperature ranged from 303 to 373K and the voltage applied during measurements varied from −1 to 1.5V. It was found from the (I-V) characteristics of the diode that the conduction mechanisms in the forward bias direction are controlled by the thermionic emission at bias potential ≤0.7V followed by single trap space charge limited current (SCLC) conduction in the voltage range >0.7V. The capacitance-voltage characteristics of the device were studied at room temperature in dark condition and it has been shown that the diode is abrupt junction. The carrier concentration on both sides of the depletion layer has been determined. Energy band diagram for α-PbO2/p-Si device was constructed. The device under illumination with light of intensity 20W/m2 gives acceptable values of photoresponse parameters such as photosensitivity and photoconductivity. The presented photodiode parameters exhibit the typical photosensor applications with reproducibility phenomenon.