Shallow Trap Density Research Articles

Increased Deep Trap Density in Interfacial Engineered Nanocomposite Revealed by Sequential Kelvin Probe Force Microscopy for High Dielectric Energy Storage.

Nanocomposites combining inorganic nanoparticles with high dielectric constant and polymers with high breakdown strength are promising for the high energy density storage of electricity, and carrier traps can significantly affect the dielectric breakdown process. Nevertheless, there still lacks direct experimental evidence on how nanoparticles affect the trap characteristics of nanocomposites, especially in a spatially resolved manner. Here, a technique is developed to image the trap distribution based on sequential Kelvin probe force microscopy (KPFM) in combination with the isothermal surface potential decay (ISPD) technique, wherein both shallow and deep trap densities and the corresponding energy levels can be mapped with nanoscale resolution. The technique is first validated using the widely-used commercial biaxially oriented polypropylene, yielding consistent results with macroscopic ISPD. The technique is then applied to investigate polyvinylidene fluoride-based nanocomposites filled with barium titanate nanoparticles, revealing higher deep trap density around surface-modified nanoparticles, which correlates well with its increased breakdown strength. This technique thus provides a powerful spatially resolved tool for understanding the microscopic mechanism of dielectric breakdown of nanocomposites.

Impact of hydrogen plasma treatment on fluorine-contained silicon nitride films

It has been suggested that the trap energy level in silicon nitride (SiN) films becomes shallow due to hydrogen and fluorine incorporation, which causes a charge migration in the MONOS memory cells and deteriorates the data retention characteristics. To solve these issues, we have proposed the hydrogen plasma treatment and demonstrated the reduction of the shallow trap density consequent to the removal of hydrogen from SiN films. In this study, the hydrogen plasma treatment is applied to the fluorine-contained SiN films. The results show that deepening of the trap energy level contributing to hole current, decreasing of the shallow trap density contributing to charging and the fluorine concentration near at the SiN surface. These results are regarded to be due to the removal of fluorine in the SiN film through the reaction with hydrogen radicals (H*) during the hydrogen plasma treatment.

Time-resolved photoluminescence: a precision tool for shallow trap density determination in perovskite films

Time-resolved photoluminescence: a precision tool for shallow trap density determination in perovskite films

Near-infrared afterglow enhancement of ZnGa2O4:Cr3+via regulating trap distribution guided by the VRBE diagram.

Lanthanide ions are commonly used as co-dopant ions for trap regulation in afterglow phosphors. However, rationally designing trap distribution to improve the afterglow performance remains challenging. Herein, the vacuum referred binding energy (VRBE) diagram was constructed to aid in the search for effective lanthanide ions to improve the near-infrared afterglow properties of ZnGa2O4:Cr3+. The constructed VRBE diagram indicates that Ln3+ (Ln = Sm, Yb, Tb) ions can create traps in ZnGa2O4, which is confirmed by the luminescence characterization. Results show that doping with Ln3+ (Ln = Sm, Yb, Tb) ions can significantly improve the afterglow intensity and duration of the phosphor due to the increased shallow trap density and trap depth. Among these samples, the Sm3+-doped sample exhibits the best afterglow properties. The afterglow enhancement mechanism by Ln3+ doping is discussed in detail. This work not only presents the lanthanide ions that can be used to regulate the trap distribution of ZnGa2O4:Cr3+ phosphors, but also provides new insights for the design of new afterglow phosphors with practical application value.

Suppression of dielectric surface flashover induced by strong electromagnetic field at multiple spatial scales based on above/sub-surface discharge development mechanisms

Spacecraft charging and electrostatic discharging (ESD) are prone to occur in harsh space environments. In particular, in the case of coupling strong electromagnetic field (EMF), ESD damages may occur at a low charging potential, posing a serious threat to on-orbit spacecraft missions. To investigate the mechanism and the pertinent suppression method for vacuum surface discharge induced by EMF, a specially-designed platform for EMF-induced surface discharge was set up. Surface structures with various spatial scales were created separately by using different surface engineering strategies, including direct fluorination, mechanical polishing, and 3D-printed grooving. The resulting surface physicochemical characteristics of the samples were examined. Furthermore, the surface discharge characteristics for different methods induced by strong EMF were systematically analyzed, considering the surface trap state distribution and secondary electron yield (SEY). The findings indicate that the proposed surface treatment methods demonstrate varying levels of improvement in mitigating EMF-induced discharge. Direct fluorination was found to produce lower SEY and to accelerate surface charge dissipation due to an elevated shallow trap density, making it favorable for suppressing the EMF-induced discharge. In addition, suitable surface roughness and groove size can effectively impede the development of the multipactor, thereby preventing EMF-induced discharge. This research is expected to provide valuable insights into the protection design of EMF-induced discharge on spacecraft.

Tailoring Organic/Inorganic Interface Trap States of Metal Oxide/Polyimide toward Improved Vacuum Surface Insulation.

High-voltage and high-power devices are indispensable in spacecraft for outer space explorations, whose operations require aerospace materials with adequate vacuum surface insulation performance. Despite persistent attempts to fabricate such materials, current efforts are restricted to trial-and-error methods and a universal design guideline is missing. The present work proposes to improve the vacuum surface insulation by tailoring the surface trap state density and energy level of the metal oxides with varied bandgaps, using coating on a polyimide (PI) substrate, aiming for a more systematical workflow for the insulation material design. First-principle calculations and trap diagnostics are employed to evaluate the material properties and reveal the interplay between trap states and the flashover threshold, supported by dedicated analyses of the flashover voltage, secondary electron emission (SEE) from insulators, and surface charging behaviors. Experimental results suggest that the coated PI (i.e., CuO@PI, SrO@PI, MgO@PI, and Al2O3@PI) can effectively increase the trap density and alter the trap energy levels. Elevated trap density is demonstrated to always yield lower SEE. In addition, increasing shallow trap density accelerates surface charge dissipation, which is favorable for improving surface insulation. CuO@PI exhibits the most remarkable increase in shallow trap density, and accordingly, the highest flashover voltage is 42.5% higher than that of pristine PI. This study reveals the critical role played by surface trap states in flashover mitigation and offers a novel strategy to optimize the surface insulation of materials.

Probing the Genuine Carrier Dynamics of Semiconducting Perovskites under Sunlight.

Understanding the nature of photogenerated carriers and their subsequent dynamics in semiconducting perovskites is important for the development of solar cell materials and devices. However, most ultrafast dynamic measurements on perovskite materials were conducted under high carrier densities, which likely obscures the genuine dynamics under low carrier densities in solar illumination conditions. In this study, we presented a detailed experimental study of the carrier density-dependent dynamics in hybrid lead iodide perovskites from femtosecond to microsecond using a highly sensitive transient absorption (TA) spectrometer. From the dynamic curves with low carrier density in the linear response range, we observed two fast trapping processes that occurred in less than 1 ps and tens of picoseconds, attributed to the shallow traps, and two slow decays with lifetimes of hundreds of nanoseconds and longer than 1 μs, related to the trap-assisted recombination and trapping at deep traps. Further TA measurements clearly show that PbCl2 passivation can effectively reduce both shallow and deep trap densities. These results provide insights into the intrinsic photophysics of semiconducting perovskites with direct implications for photovoltaic and optoelectronic applications under sunlight.

Effect of Trap Regulation on Vacuum DC Surface Flashover Characteristics of Nano-ZnO/PI Film.

The operating safety of spacecraft in space environments is closely related to the surface discharging phenomenon of dielectrics such as polyimide (PI) film in solar arrays; moreover, carrier traps in the dielectric can affect its insulation performance. Therefore, to improve the vacuum surface flashover characteristics of PI film by nano modification and reveal the effect of trap distribution on the flashover of PI composite film, first, the original PI and nano-ZnO/PI composite films with different additive amounts (0.5, 1, 2, and 3 wt.%) were prepared by in situ polymerization and their performance was evaluated by the physicochemical properties characterized by methods such as thermogravimetric analysis; second, the surface traps of the original and nanocomposite films were measured and calculated by surface potential decay method, and the carrier mobility was also obtained; finally, the vacuum direct current (DC) surface flashover characteristics and bulk resistivity of all the film samples were measured and analyzed. The experiment results showed that with the increase in the amount of nano-ZnO, both the shallow and deep trap density increased significantly, while the trap energy varied slightly, and the surface flashover voltage also increased obviously. Based on the multi-core model, the increases in the shallow and deep trap density after the introduction of nano-ZnO into the PI matrix was analyzed from the microscopic perspective of the interface. Based on the comparative analysis of the trap distribution and surface flashover voltage characteristics, a bilayer model of vacuum DC surface flashover development was proposed. In the bilayer model, deep traps and shallow traps play a dominant role in the vacuum–solid interface and the inner surface of the dielectric, respectively, and increasing the trap density could effectively inhibit secondary electron multiplication on the surface and accelerate charge dissipation inside the film. Consequently, nano-ZnO can purposefully control the trap distribution, and then improve the flashover characteristics of nano-ZnO/PI composite films, which provides a new approach for improving the spacecraft material safety.

Inhibition of electrical trees degradation of crosslinked polyethylene at high temperatures by electron‐buffering voltage stabilizers

AbstractThe effect of adding electron‐buffering voltage stabilizer (1 wt% m‐aminobenzoic acid) on the electrical tree‐induced degradation of cross‐linked polyethylene (XLPE) was studied at high temperatures. The electrical tree inception, growth, and partial discharge characteristics were simultaneously analyzed at 30, 50, and 70°C, respectively. The results showed that the growth rate of electrical tree and partial discharge activity as well as the discharge repetition rate, significantly increased with the temperature for pure XLPE and XLPE stabilizer blend. However, the XLPE stabilizer blend exhibited higher tree inception voltage, lower tree growth rate, and partial discharge activity than pure XLPE. The addition of the voltage stabilizer reduced the carbon layer thickness in the tree channel and offered higher resistance to the electrical tree growth. Surface potential decay demonstrated that the voltage stabilizer addition decreased the trap energy level, but increased the shallow trap density of the XLPE. Quantum chemical calculations showed that the voltage stabilizer enhanced the ability to buffer the high‐energy electrons in the XLPE. A high‐energy electron buffering mechanism was also proposed to explain the effect of voltage stabilizer on the electrical treeing and partial discharge activity in the XLPE at high temperatures.

Simulation of DC surface flashover of epoxy composites in compressed nitrogen

To date, numerical simulation techniques for surface flashover are still under development. In this work, a DC surface flashover numerical simulation model is constructed based on a gas–solid coupling surface flashover theoretical model with a multilayered structure at the gas–solid interface. Considering the effects of solid, gas, and gas–solid interaction on surface flashover, bipolar charge transport in the solid surface layer, collision ionization in the gas phase layer, secondary electron emission, and gas adsorption in the gas surface layer are combined to calculate the surface flashover voltage. By initializing model parameters, surface charge transport dependent dc surface flashover voltages of epoxy composites in compressed nitrogen are calculated. The results indicate that the surface flashover voltage increases with surface deep trap level, deep trap density, shallow trap density, and carrier mobility; however, surface flashover voltage decreases with surface shallow trap level and surface charge density. To further investigate the effects of surface trap on surface flashover, a “U-shaped” curve is constructed to describe the relationship between surface flashover voltage and surface trap level by the simulation method which shows good agreement with experimental results. The simulation indicates surface flashover voltage of epoxy composites is influenced by surface deep and shallow traps in the solid surface layer—shallow traps mainly influence surface charge dissipation, while deep traps mainly influence electron emission on the solid surface. The value of Ptr/Pde is crucial for the dominating surface trap in surface flashover.

Influence of thermally induced self‐assembly shish‐kebab crystal on charge transport behaviour in polypropylene/elastomer blends

This paper reports on the influence of thermally induced self-assembly shish-kebab crystal on charge transport behaviour in polypropylene (PP) and PP/propylene-based elastomer (PBE) blends. The film samples with the shish-kebab crystal were prepared by adding a β nucleating agent TMB-5 under thermally induced self-assembly. Polarised optical microscope （POM）, X-ray diffraction (XRD) and differential scanning calorimetry (DSC) measurements were performed to understand the crystallisation characteristics of samples. Carrier trap distribution was analysed by isothermal surface potential decay (ISPD) method, and DC breakdown strength measured as well. The decrease of shish-kebab crystal size, the reduction of trap level, the increase of shallow trap density and the decrease of deep trap density were obtained with the TMB-5 content. The hopping distance of charges decreased with the shallow trap density increasing. The DC breakdown strength for PP/PBE/TMB-5 was higher than that for PP/TMB-5. It is suggested that deep traps are formed on the clear shish-kebab crystal boundaries, while shallow traps are induced by the elastomer and the un-crystallisation nucleating agents. Both the carrier trap formation on the crystal boundary and the physical channel orientation of the shish-kebab crystals affect the charge transport behaviour.

The Effect of Shallow Trap Density on the Electrical Characteristics of an Organic Nonvolatile Memory Device Based on Eight-Hydroxyquinoline

An organic nonvolatile memory is proposed based on the synergistic effect of the (three-aminopropyl) triethoxysilane (APTES) self-assembled monolayer (SAM) and the polystyrene (PS):eight-hydroxyquinoline (8-HQ) blend layer, where 8-HQ is used to provide the trapping sites for the nonvolatile memory, and PS is used to form the matrix for 8-HQ in order to improve the memory property. In addition, the introduced APTES SAM aims to further increase the memory window. Two kinds of PS:8-HQ blend films are adopted, with lower 8-HQ concentration of 50 mg/mL and high spin-coating speed of 5000 rpm to form smooth thin film, while with high 8-HQ concentration of 100 mg/mL and low spin-coating speed of 1000 rpm to form rough thick film. The roughness and the surface modification property of the PS:8-HQ blend film greatly affect the morphology of the active layer, and thus the shallow trap density is also influenced, which further affects the electrical characteristics and memory properties of the nonvolatile memory device. The better characteristics of the memory device with thin 8-HQ layer indicate the potential applications in many domains.

Simple method to estimate the shallow interface trap density near the conduction band edge of MOSFETs using Hall effect measurements

A simple method to estimate the shallow interface trap density of states and the energy level of a MOSFET channel is proposed in this paper. This method estimates the trap density of states directly from the surface carrier density in the channel determined by Hall effect measurements, without any combination of the other multiple measurement methods. Fermi–Dirac statistics was applied to calculate the surface carrier density at a certain surface potential. Incomplete ionization of doped impurities in the equilibrium region was also considered for a wide bandgap semiconductor such as SiC. This method was demonstrated using n-channel 4H-SiC MOSFETs and was confirmed to be reasonable as the estimated trap density of states was equivalent to that obtained via a previous method combined with Hall effect and split capacitance–voltage measurements.

Bulk and interface defects analysis of n-CdS/p-Si heterojunction solar cell

Interface defects are detrimental for the performance of n-CdS/p-Si heterojunction solar cells. A model based on simulation studies has been developed in this work to correlate the defect density and the cell performance. Deep trap density, shallow trap density, interface defect density, and electron selective layer thickness are used as simulation parameters. Interface traps are found to be more dominant in deteriorating the device performance than the deep traps and the shallow traps. The effects of Cz-Si and FZ-Si are also studied in terms of trap density. The best efficiency determined through simulation in the present study for n-CdS/p-Si heterojunction solar cell is 18.72%. • Developed a self-consistent simulation model for n-CdS/p-Si heterojunction cell. • Simulation model for n-CdS/p-Si heterojunction cell produces 18.72% efficiency. • Interface defects are detrimental problem for n-CdS/p-Si heterojunction cell. • FZ-Si has better performance than Cz-Si in n-CdS/p-Si cell. • Thicker CdS layer is detrimental for the performance of n-CdS/p-Si cell.

Effect of micro and nano-size boron nitride and silicon carbide on thermal properties and partial discharge resistance of silicone elastomer composite

This study introduces silicone elastomers with micro and nano-sized boron nitride (BN) and silicon carbide (SiC) particles with various doping levels to improve thermal properties and partial discharge resistance. The effect of micro and nano-fillers on thermal conductivity, coefficient of thermal expansion (CTE), and thermal stability are investigated. Polarization and depolarization current and partial discharge are measured to investigate the non-linear conductivity, trap energy density distribution, and partial discharge resistance. Experimental results show higher thermal conductivity, lower CTE, and better thermal stability than the original silicone elastomer. Large size fillers dominate the thermal conductivity when the doping level is low, whereas the composite microstructure plays a significant role in the thermal conductivity when the doping level is high. The combination of different filler type and size has less effect on thermal stability and CTE compared to the effect of the doping level. According to the depolarization current, the composite's trap depth is generally shallower than that of the original silicone elastomer. With increased doping level, the shallow trap density increases, providing hopping sites for carriers, whereas the deep trap density decreases, reducing the number of trapped charges. The partial discharge inception voltage is higher than that of the original silicone elastomer, and it increases with the doping level, which may be because the electrostatic field generated by diffused charges reduces the local electric field near the high voltage electrode.

Characterization and electrical modeling of polycrystalline silicon vertical thin film transistors

Thin film transistors (TFTs) with lateral channels are limited in current density due to the design rule. For many applications with improved integration, the introduction of vertical channels reduces channel lengths while increasing current density per unit surface area. In previous works, vertical TFTs have been designed and manufactured using low-temperature polycrystalline silicon technology (T ≤ 600 °C), with a solid phase crystallization (SPC) based process. In this case, the introduction of an insulating layer between source and drain films has resulted in a significant improvement in the electrical characteristics, mainly in the On/Off state current (Ion/Ioff) ratio. However, the active layer is deposited on the sidewalls obtained by plasma etching, and the etching process results in morphological defects on the sidewalls that adversely affect the electrical characteristics. The purpose of this paper is to understand the origin and effects of these defects using different models. Thus, the transfer characteristics are analyzed in detail, with Suzuki method to calculate the density of states, while subthreshold slope method and Grünewald method are adopted to verify the Suzuki method for the deep and shallow trap densities, respectively. These methods provide an approach for DOS calculation independent of temperature-related measurement.

Trap Level Characteristics and DC Breakdown Performance of Isotactic/Syndiotactic/Atactic Polypropylene Blend Insulation

Polypropylene (PP) material as a DC cable insulation material has great prospects as a recyclable alternative to cross-linked polyethylene. This paper aims at investigate the effect of PP blends crystallinity morphology and trap energy level distribution on DC breakdown strength. Five different ratios iPP/sPP/aPP blends samples are prepared by melt-blending method. Polarizing optical microscope (POM) experiments are conducted to characterize the crystallinity morphology of five iPP/sPP/aPP blends and isothermal surface potential decay (ISPD) methods are employed to measure the trap levels distribution in composite dielectric. The DC breakdown strength is tested by ball-plate electrodes equipment, and the result of DC breakdown experiment indicates that sPP30 exhibits the highest breakdown field strength, which is 29.2% higher than sPP0. The addition of sPP can act as a nucleus to help the crystallization behavior of iPP, and the distribution of spherulites in sPP30 is the most uniform and dense among the five iPP/sPP/aPP blends samples, which can improve the trap characteristics of polymers. The ISPD results show that the decay rate of the sPP30 sample is the slowest at 50 °C, 70 °C and 90 °C, indicating that sPP30 is more conducive to suppressing the dissipation process of carriers during the depolarization process. Moreover, sPP30 has higher deep trap density and lower shallow trap density and exhibits superior breakdown performance. It is deduced that the addition of sPP can effectively regulate the crystallinity morphology of iPP, and then effectively improve the trap energy level distribution of iPP/sPP/aPP blends, and it shows the improvement of DC breakdown performance macroscopically.

Energy storage properties of polyimide/BaTiO3 nanocomposite films and their breakdown mechanism in a wide content range

Polyimide (PI) has excellent dielectric properties with superior thermal stability, and it is considered as a promising polymer dielectric for energy storage capacitors. Here, we studied the energy storage properties of PI composite films with BaTiO3 (BT) nanoparticles in a wide content range. Benefiting from the high breakdown strength (520 kV/mm), the nanocomposite filled with 0.05 wt. % of BT exhibits an increased energy density of 4.51 J/cm3 at room temperature and keeps a good thermal stability (3.22 J/cm3 at 100 °C). Additionally, trap properties of the nanocomposites and their effect on breakdown strength were studied by thermally stimulated depolarization current. It was found that the dominant trap parameter on breakdown strength variation, with the increase in the BT content, is correlated with deep traps, deep and shallow traps especially their depths, and shallow trap density. Then, a physical quantity of average trap depth was introduced, the value of which decreases from 0.81 eV to 0.44 eV as the BT content increases to 50 wt. %, and a clear correspondence was discovered between the average trap depth and breakdown strength.

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.

Temperature and trap distribution dependence of electrical tree growth characteristics in polypropylene/elastomer blends for recyclable cable insulation

Polypropylene (PP)/elastomer blends as a promising cable insulation material have to face the challenge of electrical treeing which possibly leads to unexpected insulation failure. In this work, two types of elastomers, i.e. polyolefin elastomer and propylene-based elastomer, have been blended into isotactic PP. Effects of the elastomer content and type on electrical treeing behaviours have been investigated under different ambient temperatures. Isothermal surface potential decay measurement has been performed to explore the influence of trap distribution on the treeing behaviour, scanning electron microscope and differential scanning calorimetry measurements were performed to characterise the structure of blends. It was found that with the increase of elastomer content, the trap energy level was reduced and the shallow trap density was increased, which resulted in the enhancement of the tree growth rate and the decrease in the breakdown time. With the temperature changing from 20 to 100°C, the growth rate tended to be increased except that an anomalous reduction in the rate occurred at 80°C. It is suggested that the compatibility between PP matrix and elastomer plays an important role in determining the morphology of blends, which in turn influences the trap distribution and the temperature dependent treeing behaviours.