Deep Trap Density Research Articles

Unraveling the mechanism of ultraviolet-induced optical gating in Zn1−xMgxO nanocrystal solid solution field effect transistors

We report ultraviolet (UV)-induced optical gating in a Zn1−xMgxO nanocrystal solid solution (NCSS) field effect transistor (FET) through a systematic study in which UV-induced charge transport properties are probed as a function of Mg composition. Change in the electrical properties of Zn1−xMgxO NCSS associated with electronic traps is investigated by field effect-modulated current–voltage characteristic curves in the dark and under illumination. Under UV illumination, significant threshold voltage shift to a more negative value in an n-channel Zn1−xMgxO NCSS FET is observed. Importantly, as the Mg composition increases, the effect of UV illumination on the threshold voltage shift is alleviated. We found that threshold voltage shift as a function of Mg composition in the dark and under illumination is due to difference in the deep trap density in the Zn1−xMgxO NCSS. This is supported by Mg composition dependent photoluminescence intensity in the visible range and reduced FET mobility with Mg addition. The presence of the deep traps and the corresponding trap energy levels in the Zn1−xMgxO NCSS are ensured by photoelectron spectroscopy in air.

Temperature dependent trap level characteristics of graphene/LDPE nanocomposites

The trap level characteristics are attracting much attentions in electrical and electronic engineering, which are closely associated with electrical properties of insulating materials. This paper try to modify the trap level characteristics of Low-density Polyethylene (LDPE), by incorporating graphene nanofillers into polymer with a filler content of 0.001, 0.003, 0.005, 0.007 and 0.01 wt%. The effects of filler content and ambient temperature on the trap level distributions are investigated, based on the isothermal surface potential decay (ISPD) method under 30, 50 and 70 °C. The experimental results show that the deep trap level and density increase with the filler content from 0 to 0.005 wt%, and then decrease with the further increase to 0.01 wt%. The increases of deep trap level and density affect the detrapping processes and suppress the electron injection into polymer bulks and cause a much slower downward trend in ISPD curves, especially at ambient temperature of 70 °C. Additionally, the apparent deep trap levels increase as temperature increases, since deeper and deeper traps are involved in the charge-release mechanism. It is concluded that the graphene nanofiller loading with a content of 0.005 wt% introduces large quantities of deep charge traps in the polymer-filler interfacial regions, which reduce the detrapping rate of charges and suppress the charge injection into polymer.

New Method for Shallow and Deep Trap Distribution Analysis in Oil Impregnated Insulation Paper Based on the Space Charge Detrapping

Space charge has close relation with the trap distribution in the insulation material. The phenomenon of charges trapping and detrapping has attracted significant attention in recent years. Space charge and trap parameters are effective parameters for assessing the ageing condition of the insulation material qualitatively. In this paper, a new method for calculating trap distribution based on the double exponential fitting analysis of charge decay process and its application on characterizing the trap distribution of oil impregnated insulation paper was investigated. When compared with the common first order exponential fitting analysis method, the improved dual-level trap method could obtain the energy level range and density of both shallow traps and deep traps, simultaneously. Space charge decay process analysis of the insulation paper immersed with new oil and aged oil shows that the improved trap distribution calculation method can distinguish the physical defects and chemical defects. The trap density shows an increasing trend with the oil ageing, especially for the deep traps mainly related to chemical defects. The greater the energy could be filled by the traps, the larger amount of charges could be trapped, especially under higher electric field strength. The deep trap energy level and trap density could be used to characterize ageing. When one evaluates the ageing condition of oil-paper insulation using trap distribution parameters, the influence of oil performance should not be ignored.

Isothermal relaxation current and microstructure changes of thermally aged polyester films impregnated by epoxy resin

In this study, to understand the effect of thermal aging on polymer films degradation, specimens of polyester films impregnated by epoxy resin with different thermal aging temperatures (80 and 130 °C) and aging times (500, 1600, 2400 and 3000 h) are prepared, then charge de-trapping properties of specimens are investigated via the isothermal relaxation current (IRC) measurement, the distributions of trap level and its corresponding density are obtained based on the modified IRC model. It is found that the deep trap density increases remarkably at the beginning of thermal aging (before 1600 h), but it decreases obviously as the aging degree increases. At elevated aging temperature and, in particular considering the presence of air gap between two-layer insulation, the peak densities of deep traps decrease more significant in the late period of aging. It can be concluded that it is the released energy from de-trapping process leads to the fast degradation of insulation. Moreover, after thermal aging, the microstructure changes of crystallinity and molecular structures are analyzed via the x-ray diffraction experiment and Fourier transform infrared spectrometer. The results indicate that the variation of the deep trap density is closely linked with the changes of microstructure, a larger interface of crystalline/amorphous phase, more defects and broken chains caused by thermal aging form higher deep trap density stored in the samples.

Trap distribution of electron beam irradiated epoxy resin under repetitive pulse voltage

Epoxy resin has been widely used as dielectric material in irradiation environment, such as nuclear power plants, aerospace equipment and superconducting power apparatus. This paper investigated the effects of different electron beam irradiation doses on the trap distribution of epoxy resin under repetitive pulse voltage. Samples were irradiated at room temperature (20 °C) in air by high energy electron beam of 7.5 MeV with beam density ranging from 0.35 to 0.45 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The total irradiation dose was 0, 100, 500 and 700 kGy, respectively. The corona charging was performed under ± 6 kV pulse voltage with different pulse frequencies in atmospheric air with relative humidity of 20%. The surface potential was collected through an electrostatic voltmeter to obtain the trap distribution at room temperature, and the isothermal discharge current of the samples processed with different irradiation dose was measured to calculate the trap distribution in liquid nitrogen (LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ). The experimental results indicate that both at room temperature and in LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , the trap level decreases first and then increases with the increase of irradiation dose. The trap level and the trap density have the positive correlation with pulse voltage frequency. At room temperature, the electron traps have higher energy level and deep trap density, but have lower shallow trap density than the positive charge traps.

The role of micro-structure changes on space charge distribution of XLPE during thermo-oxidative ageing

In order to investigate the differences of the space charge distribution of XLPE aged at temperatures below and above the melting point, thermo-oxidative ageing was conducted on 110 kV cross-linked polyethylene (XLPE) cable insulation at 100 °C and much elevated temperature of 160 °C respectively. The XLPE insulation showed moderate changes on micro-structure and space charge distribution until the moment when the Oxidation Induction Time (OIT) dropped to zero. It was deduced by the results of Fourier Transform Infrared (FTIR) Spectroscopy, X-Ray Diffraction (XRD), Differential Scanning Calorimetry (DSC) and Scanning Electron Microscope (SEM) tests that thermo-oxidative ageing could lead to a significant increase in carbonyl groups and a decrease in crystallinity. At 100 °C, the thermo-oxidative ageing mainly affected the amorphous regions because of the much denser structures of crystalline regions, which might make the oxygen diffusion more difficult. At 160 °C, the thermo-oxidative ageing affected the melted spherulites and leaded to obvious impact on lamellae, resulting in a significant decrease in crystallinity and the destruction of spherulites. The space charge distribution of aged XLPE samples was studied by Pulsed Electroacustic Method (PEA) test. Hetero-charges could be found in aged XLPE samples. These hetero-charges were derived from ionization of polar groups in XLPE generated during ageing. The surface trap parameters of XLPE samples were investigated by Isothermal Surface Potential Decay (ISPD) test. It was deduced that shallow traps with the energy level of about 0.95 eV and deep traps with the energy level of about 1.05 eV on the surface of XLPE were introduced by carbonyl groups and interfaces of crystal/amorphous respectively. Thermo-oxidative ageing at 160 °C dramatically decreased the deep trap density because of the destruction of spherulites, leading to enhanced electrode injection. As a result, compared with those of unaged samples and samples aged at 100 °C, increased amount of homo-charges were found in the XLPE samples aged at 160 °C.

Remarkably improved electrical insulating performances of lightweight polypropylene nanocomposites with fullerene

A polypropylene/fullerene nanocomposite was fabricated via solution processing with good dispersion of the fullerene nanoparticles. It was observed that a small amount of fullerene (~1 wt. ‰) can significantly improve the direct current (DC) electrical insulation performance (i.e. improved space charge distribution, greater DC dielectric breakdown strength, and larger DC volume resistivity) of polypropylene. Thermally stimulated depolarization current tests indicate that fullerene doping remarkably increases the deep trap density of pure polypropylene. The mechanisms by which the electrical properties of the polypropylene nanocomposites were improved are the introduction of deep traps and the high electron affinity of fullerene. The deep traps result from the interfaces between polypropylene and fullerene. This work provides a potential method to tailor lightweight nanodielectric materials with excellent electrical insulating performances.

A surface-potential-based drain current compact model for a-InGaZnO thin-film transistors in Non-Degenerate conduction regime

A physics-based drain current compact model for amorphous InGaZnO (a-InGaZnO) thin-film transistors (TFTs) is proposed. As a key feature, the surface potential model accounts for both exponential tail and deep trap densities of states, which are essential to describe a-InGaZnO TFT electrical characteristics. The surface potential is solved explicitly without the process of amendment and suitable for circuit simulations. Furthermore, based on the surface potential, an explicit closed-form expression of the drain current is developed. For the cases of the different operational voltages, surface potential and drain current are verified by numerical results and experimental data, respectively. As a result, our model can predict DC characteristics of a-InGaZnO TFTs.

気相合成法ZnS:Ag, Al蛍光体における深欠陥密度の低減化

気相合成法ZnS:Ag, Al蛍光体における深欠陥密度の低減化

Charge-Based Compact Model for DC Current in Organic TFT Including Non-Linear Injection Effects with a Close Link to Electrical Device Parameter

In this paper a closed-form current model for organic TFTs is presented which leads to a charge-based continuous current model with threshold voltage and slope as model parameters, while still keeping a single expression valid from below to above threshold operation. Most compact models for organic field-effect transistors (OFETs) are based on a two-piece description: a set of equations for above threshold operation is combined with a current model valid in subthreshold region by means of an analytical smoothing function [1]. Thereby the threshold voltage is introduced as fitting parameter without any relation to physical parameters. In [2] a charge-based current model with a continuous equation valid in all regions of operation has been presented. While solving Poisson’s equation the model preserves a close link to physical parameters like shallow and deep trap densities and avoids the introduction of a threshold voltage and subthreshold slope as model parameters. However, from a circuit designer’s perspective a model providing a close link to electrical device parameters is desirable because parameters as threshold voltage and slope can easily be extracted from I/V curves and are performance measures of a device. In this paper an improvement of the model from [2] is presented which leads to a charge-based C∞-continuous current model with threshold voltage and slope as model parameters, while still keeping a single expression valid from below to above threshold operation. To include the effect of hopping transport the empirical power-law mobility model [3] and a contact resistance [4] have been incorporated in the effective mobility. Channel length modulation effects have been included according to [5]. Furthermore, from physical parameters given by trap densities the threshold voltage and subthreshold slope of the device can be calculated. In the case of co-planar device structures the output characteristics in the linear operation regime show a superlinear behaviour due to non-linear injection effects. If the barrier height between source electrode and the channel material is not negligible the channel current is limited by the source injection current. The Schottky barrier height is modulated by the gate and drain potential. A strong electric field causes thinning of the barrier, allowing more carriers to enter channel. An analytical equation for the electric field at the injection barrier has been derived by applying the conformal mapping technique. The bias-dependent lowering of the Schottky barrier, and hence the maximum injection current can be calculated. An equivalent resistance of the Schottky barrier is calculated from the voltage drop Vds and the maximum possible injection current. To arrive at a closed-form current equation for the intrinsic device together with the injecting Schottky barrier this equivalent resistance is incorporated in the current equation by a first order approximation which is commonly used for the consideration of parasitic resistances. In this case the equivalent resistance is bias dependent and reduced with increasing Vds due to Schottky barrier lowering. The model has been validated vs. measurements on devices with small molecule organic semiconductors (Fig. 1, 2). The results show good agreement, even for the 1st and 2nd derivatives of the transfer characteristics. Acknowledgements: This project was funded by the German Federal Ministry of Education and Research contract No.03FH013PX2 (FhProfUnt). This project was also funded by the Horizon 2020 Programme of the European Union under contract RISE 2014 645760 ("DOMINO"). We acknowledge CEA-Liten (Grenoble) and Max Planck Institute for Solid State Research (Stuttgart) for providing measurement data. We thank AdMOS GmbH for support and Keysight Technologies for license donation of IC-CAP.

Influence of filler content on trap and space charge properties of epoxy resin nanocomposites

Epoxy resin nanocomposites have excellent properties such as the suppression of space charge accumulation, high resistivity, and high electrical breakdown strength, which play an important role in developing the direct current power equipment. However, the influencing mechanisms of filler content on trap, conductivity, and space charge of nanocomposites have not been clear to date. In the present paper, a method to calculate the densities of shallow traps and deep traps in interaction zones is proposed based on the multi-region structure model of interaction zones, and the dependence of shallow traps and deep traps on filler content is obtained. It is found that the shallow trap density increases with the increase of filler content, while the deep trap density first increases and then decreases with increasing the filler content, which is caused by the overlap of interaction zones. Then, the relation between the shallow trap controlled carrier mobility and the filler content is investigated. With the filler content increasing, the density of shallow traps increases and their mean distance decreases, leading to an increase in the shallow trap controlled carrier mobility. Considering the charge injection from cathode into dielectrics, carrier hopping in shallow traps, charge trapping into and detrapping from deep traps, a unipolar charge transport model is established to study the conductivity and distributions of space charges and electric field in epoxy resin nanocomposites. At relatively low filler content, the charge transport is dominated by deep traps in interaction zones and the conductivity decreases with the increase of filler content. However, the charge transport is determined by shallow traps at relatively high filler content and the conductivity increases.

Effects of crystal morphology on space charge transportation and dissipation of SiC/silicone rubber composites

The addition of silicon carbide (SiC) micro powder into silicone rubber (SiR) matrix has been found to introduce a marked nonlinear conductivity into the resulting composites, without a degradation of the insulating properties under low electric field. Owing to these properties, the SiR-based nonlinear conductive composites have a potential use as the field grading material (FGM) layer in high voltage direct current (HVDC) cable accessories, which is employed to uniform electric field and suppress charge accumulation at the interface between crosslinked polyethylene (XLPE) and accessory insulation. In this paper, the space charge transportation and dissipation behaviors of SiC/SiR composites were investigated by pulsed electro-acoustic (PEA) test, in view of the effects of SiC filler size and crystal morphology. The results indicated that with the enhancement of nonlinear conductivity, the charge transportation and dissipation processes were accelerated. Compared with α-SiC particles, the β-SiC whiskers could provide a larger nonlinear conductivity. Meanwhile, the trap level distributions of SiC/SiR composites showed that the β-SiC whiskers introduced a shallower trap level at 0.81 eV than the α-SiC particles (0.85 eV). In addition, for the α-SiC particles filled composites, with the increase of the filler size from 0.45 μm to 5.0 μm, the shallow trap density increased, and the deep trap density decreased, resulting in an enhancement of space charge dissipation.

Pyrene–Fullerene Interaction and Its Effect on the Behavior of Photovoltaic Blends

Improving the dispersion properties of unfunctionalized fullerenes in photovoltaic blends is a key factor for replacing the commonly employed butyric acid methyl ester derivatives (PBCM and PC71BM), with economical and light harvesting advantages. We consider here the effects of a pyrene derivative (PyBB) as dispersant of neat fullerenes (C60 or C70) in photovoltaic blends prepared with a low energy gap conjugated polymer (PTB7) as electron donor. The morphological and spectroscopic properties in the presence and absence of PyBB were evaluated by AFM, emission fluorescence, and light induced EPR, and the deep trap density of states was calculated in the two cases. The electrical properties of the devices prepared with or without PyBB were investigated and compared. The intimate interaction of fullerene/PyBB aromatic species revealed by the spectroscopic analysis is in agreement with the enhanced dispersion of the three-component blends. At the same time the deep trap density of states is varied, and the mobility of negative charge carriers is reduced in films prepared with PyBB dispersant, frustrating the beneficial effect of PyBB on the blend morphology.

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.

Deep-level transient spectroscopy at platinum/titanium-dioxide hydrogen sensors

Abstractauthoren Deep traps in platinum/mesoporous-titanium-dioxide hydrogen sensors are studied by deep-level transient spectroscopy (DLTS) before, during, and after the gas detection. This is possible for atmospheric hydrogen volume fractions in the low ppm regime where the Schottky barrier at the Pt/TiO2 interface is retained. In the absence of hydrogen, two deep levels with binding energies of 470 and 720 meV are found. Their occurrence increases as hydrogen is offered, but to a smaller extent than the hydrogen-induced doping density. An additional level develops during hydrogen exposure at a binding energy of ≈870meV. After the sensing sequence, the sample recovers only partially, but the deep trap densities can be reset by annealing in dry air. The data suggest that besides donor formation, exposure of mesoporous TiO2 to hydrogen also generates deep traps. The results furthermore show that despite the large interfacial disorder, which in the present case is essential for the functionality, DLTS can be used to obtain information about deep traps generated during utilization of the device as gas sensor.

Electrical Property Variations in Organic Bistable Devices Utilizing CdSe/CdS/ZnS Core-Shell-Shell Nanoparticle Layer Embedded in a Polystyrene Layer Due to an Inserted WO3 Layer.

Organic bistable devices (OBDs) based on CdSe/CdS/ZnS core-shell-shell nanoparticles embedded in a polystyrene (PS) layer with an inserted WO3 layer were fabricated by using the spin coating method and the thermal evaporation. The current density-voltage (J-V) curves for the Al/CdSe/CdS/ZnS core-shell-shell nanoparticles embedded in PS layer/WO3/ITO devices with different thicknesses of WO3 layers exhibited electrical bistabilities. Simulated J-V curves for the OBDs containing CdSe/CdS/ZnS core-shell-shell nanoparticles were in reasonable agreement with the experimental results. The simulated J-V characteristics for the OBDs at various trap densities utilizing the double Gaussian distribution showed that the current level decreased with increasing shallow trap density and that the transition voltage increased with increasing deep trap density.

Mechanism of charge recombination in meso-structured organic-inorganic hybrid perovskite solar cells: A macroscopic perspective

In the currently popular organic-inorganic hybrid perovskite solar cells, the slowness of the charge recombination processes is found to be a key factor for contributing to their high efficiencies and high open circuit voltages, but the underlying recombination mechanism remains unclear. In this work, we investigate the bimolecular recombination (BR) and the trap-assisted monomolecular recombination (MR) in meso-structured perovskite solar cells under steady state working condition, and try to reveal their roles on determining the device performance. Some interfacial effects such as the injection barriers at the selective contacts are examined as well. Based on the macroscopic device modeling, the recombination resistance-voltage (Rrec−V) and the current density-voltage (J–V) curves are calculated to characterize the recombination mechanism and describe the device performance, respectively. Through comparison with the impedance spectroscopy extracted Rrec data, it is found that under the typical BR reduction factor and deep trap densities observed in experiments, the MR dominates the charge recombination in the low voltage regime, while the BR dominates in the high voltage regime. The short circuit current and the fill factor could be reduced by the significant MR but the open circuit voltage is generally determined by the BR. The different electron injection barriers at the contact can change the BR rate and induce different patterns for the Rrec–V characteristics. For the perovskites of increased band gaps, the Rrec's are significantly enhanced, corresponding to the high open circuit voltages. Finally, it is revealed that the reduced effective charge mobility due to the transport in electron and hole transporting material makes the Rrec decrease slowly with the increasing voltage, which leads to increased open circuit voltage.

Improved GaN films with low background doping and low deep trap density grown by hydride vapor phase epitaxy

AbstractElectrical properties and deep electron and hole traps spectra are compared for undoped n‐GaN films grown by hydride vapor phase epitaxy (HVPE) on sapphire substrates at 850 °C (Group 1) and 950 °C (Group 2). Group 1 samples have very low residual donor concentration ((2‐5)×1014 cm‐3), very low density of electron and hole traps, low leakage current of Schottky diodes. Group 2 samples are not very different from the usual thick HVPE samples in terms of predominant electron and hole traps and MCL spectra. But the concentration of both electron and hole traps is quite high and could contribute to rather high reverse leakage current, low photocurrent and pronounced tunneling. (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Understanding temperature dependence of threshold voltage in pentacene thin film transistors

Threshold voltage in the pentacene-based organic thin film transistors is found to be linearly increased with decreasing measuring temperature, indicating that the threshold voltage is dominated by the deep hole trapping in an approximately energy-independent trap distribution. The slope of threshold voltage vs temperature is greatly reduced at 200–210 K when temperature is decreased, corresponding to a reduction in the deep trap density. H2O confined in pentacene is considered as one of the sources of the deep hole traps, and the slope change in the temperature dependence of threshold voltage is attributed to the phase transition of supercooled H2O.

Investigation on relaxation loss mechanism of CaCu3Ti4O12 ceramic

The dielectric loss of the CaCu3Ti4O12 ceramic is high, and the mechanism of the loss is not clear, which restricts its application. The CaCu3Ti4O12 ceramic samples are synthesised by solid state reaction method and coprecipitation method. The electronic relaxation of deep bulk traps at the depletion layer edge, carrier relaxation and the dielectric loss of CaCu3Ti4O12 ceramic are investigated. Both perfect double Schottky barrier and low impurity density can reduce the DC conductivity, thus reducing the low-frequency dielectric loss. High-frequency dielectric loss is controlled by deep bulk trap density, especially in the one whose activation energy is 0.12 eV. At room temperature, when the frequency is 1 kHz, the dielectric constant and loss of CaCu3Ti4O12 ceramic prepared by coprecipitation method are 1.4× 104 and 0.037, indicating a good improvement.