Electrode-dielectric Interface Research Articles

Ultrafast piezocapacitive soft pressure sensors with over 10 kHz bandwidth via bonded microstructured interfaces

Flexible pressure sensors can convert mechanical stimuli to electrical signals to interact with the surroundings, mimicking the functionality of the human skins. Piezocapacitive pressure sensors, a class of most widely used devices for artificial skins, however, often suffer from slow response-relaxation speed (tens of milliseconds) and thus fail to detect dynamic stimuli or high-frequency vibrations. Here, we show that the contact-separation behavior of the electrode-dielectric interface is an energy dissipation process that substantially determines the response-relaxation time of the sensors. We thus reduce the response and relaxation time to ~0.04 ms using a bonded microstructured interface that effectively diminishes interfacial friction and energy dissipation. The high response-relaxation speed allows the sensor to detect vibrations over 10 kHz, which enables not only dynamic force detection, but also acoustic applications. This sensor also shows negligible hysteresis to precisely track dynamic stimuli. Our work opens a path that can substantially promote the response-relaxation speed of piezocapacitive pressure sensors into submillisecond range and extend their applications in acoustic range.

HfO2-Based Switching Devices with Nitridation of the Bottom Electrode

HfO2 -based Resistive Switching devices are a promising candidate for in-memory computing due to their lowpower consumption, good endurance, reliability, and compatibility with CMOS technology [1]. Controlling theoxygen vacancy (Vo) distribution near the top and bottom electrodes by selecting proper metal-dielectric-metalstack materials has impact on the performance of the device [2-3]. Significant improvement in switching poweris achieved when Vo distribution is higher near the top electrode (TE) and lower near the bottom electrode [4].Introducing nitridation treatment to the TiN bottom electrode shows a significant reduction in switching powerand superior conductance quantization with nanosecond pulse width of programing pulsed operation [5].In this work we studied different TiN bottom electrode (BE) films in the TiN/HZO/TiN MIM toinvestigate its impact of BE TiN on the switching characteristics. Fig. 1 shows schematic for the two differentswitching devices with identical top electrodes (10 nm PVD TiN) and same switching dielectric (10 nm HZO) butwith different N2 levels of 30 nm TiN at the BE. Device-B has higher level of N2 compared to device-A. Formingprocess was performed by applying DC voltage sweep with 1nA compliance current. The device-A has the lowestforming power (0.86nW) as shown in Fig. 2(a). An increase in the forming power consumption was observed fordevice-B (2.45nW) as shown in Fig. 2(b). We carried out the carrier transport characterization at the highresistance state (HRS) prior to forming process. Fig. 2(c)&(d) shows the conduction process in both devices. Indevice-A, the current conduction mechanism followed the hopping conduction in the dielectric as shown in Fig.2(c). The hopping conduction is a bulk-limited conduction mechanism depends on the electrical properties of thedielectric itself such as the trap energy level [6]. The conduction mechanism in device-B (high N2) (Fig. 2(d)) andwas dominated by the Schottky conduction in the HZO layer, which is an electrode-limited conduction thatdepends on the electrical properties at the electrode-dielectric interface [6]. When the active nitrogen atoms areintroduced to the BE, it diffuses to the dielectric due to increased concentration, then connect to the hafnium iondangling bonds as their lone-pair electrons, since the bonding energy of the Hf–N bond is very low (535eV)compared with that of the Hf–O bond (801eV) [7]. This allows a migration of Vo towards TE and reducing the Voconcentration near the BE for device-A leading to carrier hopping. On the other hand, Vo concentration near theBE is completely purged because of high nitrogen concentration (device-B) leading to Schottky conduction. Thisexplains the reduction of forming power in device-A. Fig. 3 shows the resistance distribution after applying 30DC sweeps with reading voltage of 0.1V after each sweep. The device-B (high N2) shows the largest windowbetween HRS and LRS compared to the other devices. That further confirms the forming process in these devices.In summary, low level of N2 reduces the forming power while higher level of N2 enhances the resistance windowbetween LRS and HRS because of Vo concentration variation near BE.

Investigation on AC and DC Breakdown Mechanism of Surface-Ozone-Treated LDPE Films under Varied Thicknesses.

Electrical breakdown is an important physical phenomenon in power equipment and electronic devices. Recently, the mechanism of AC and DC breakdown has been preliminarily revealed as electrode-dielectric interface breakdown and bulk breakdown, respectively, based on space charge dynamics through numerical calculations. However, the AC breakdown mechanism still lacks enough direct experimental support, which restricts further understanding and the design and development of electrical structures. Here, in this study, LDPE films with various thicknesses ranging from 33 μm to 230 μm were surface modified with ozone for different durations to experimentally investigate DC and AC breakdown mechanism. The results indicate that carbonyl groups (C=O) were introduced onto the film surface, forming shallow surface traps and leading to a decreased average trap depth and an increased trap density. Such a surface oxidation modulated trap distribution led to enhanced space charge injection and bulk electrical field distortion, which decreased DC breakdown strength as the oxidation duration went longer, in all film thicknesses. However, such decreases in breakdown strength occurred only in films below 55 μm under AC stresses, as the enhanced electrical field distortion at the electrode-dielectric interface was more obvious and dominating in thin films. These experimental results further confirm the proposed electrode-dielectric interface breakdown of dielectric films and provide new understandings of space charge modulated electrical breakdown, which fulfills dielectric breakdown theory and benefits the miniaturization of power equipment and electronic devices.

Polymer Dielectrics with Outstanding Dielectric Characteristics via Passivation with Oxygen Atoms through C-F Vacancy Carbonylation.

The development of advanced electrical equipment necessitates polymer dielectrics with a higher electric strength. Unfortunately, this bottleneck problem has yet to be solved because current material modification methods do not allow direct control of deep traps. Here, we propose a method for directly passivating deep traps. Measurements of nanoscale microregion charge characteristics and trap parameters reveal a significant reduction in the number of deep traps. The resulting polymer dielectric has an impressively high electrical strength, less surface charge accumulation, and a significantly increased flashover voltage and breakdown strength. In addition, the energy storage density is increased without sacrificing the charge-discharge efficiency. This reveals a new approach to increasing the energy storage density by reducing the trap energy levels at the electrode-dielectric interface. We further calculated and analyzed the microscopic physical mechanism of deep trap passivation based on density functional theory and characterized the contributions of orbital composition and orbital hybridization.

All-Organic PTFE Coated PVDF Composite Film Exhibiting Low Conduction Loss and High Breakdown Strength for Energy Storage Applications

Plastic film capacitors are widely used in pulse and energy storage applications because of their high breakdown strength, high power density, long lifetime, and excellent self-healing properties. Nowadays, the energy storage density of commercial biaxially oriented polypropylene (BOPP) is limited by its low dielectric constant (~2.2). Poly(vinylidene fluoride) (PVDF) exhibits a relatively high dielectric constant and breakdown strength, making it a candidate material for electrostatic capacitors. However, PVDF presents significant losses, generating a lot of waste heat. In this paper, under the guidance of the leakage mechanism, a high-insulation polytetrafluoroethylene (PTFE) coating is sprayed on the surface of a PVDF film. The potential barrier at the electrode-dielectric interface is raised by simply spraying PTFE and reducing the leakage current, and then the energy storage density is increased. After introducing the PTFE insulation coating, the high-field leakage current in the PVDF film shows an order of magnitude reduction. Moreover, the composite film presents a 30.8% improvement in breakdown strength, and a 70% enhancement in energy storage density is simultaneously achieved. The all-organic structure design provides a new idea for the application of PVDF in electrostatic capacitors.

Novel high voltage polymer insulators using computational and data-driven techniques.

One of the key bottlenecks in the development of high voltage electrical systems is the identification of suitable insulating materials capable of supporting high voltages. Under high voltage scenarios, conventional polymer based insulators, which are one of the popular choices of insulators, suffer from the drawback of space charge accumulation, which leads to degradation in desirable electronic properties and facilitates dielectric breakdown. In this work, we aid the development of novel polymers for high voltage insulation applications by enabling the rapid prediction of properties that are correlated with dielectric breakdown, i.e.,the bandgap (Egap) of the polymer and electron injection barrier (Φe) at the electrode-insulator interface. To accomplish this, density functional theory based methods are used to develop large, chemically diverse datasets of Φe and Egap. The deviation of the computed properties from experimental observations is addressed using a statistical technique called Bayesian calibration. Furthermore, to enable rapid estimation of these properties for a large set of polymers, machine learning models are developed using the created dataset. These models are further used to predict Egap and Φe for a set of 13k previously known polymers. Polymers with high values of these properties are selected as potential high voltage insulators and are recommended for synthesis. Finally, the models developed here are deployed at www.polymergenome.org to enable the community use.

Initial electrical properties of tantalum oxide resistive memories influenced by oxygen defect concentrations

The initial electrical properties of tantalum oxide resistive memory were investigated using four metal electrodes (TiN, Ti, Ta, and Al) and two kinds of tantalum oxide with different amounts of intrinsic oxygen vacancies. The initial resistance depended on the electrode material. This indicated that oxygen scavenging by the electrodes contributed to a reduction in the resistance. However, the resistance change depended on the intrinsic oxygen vacancy concentration introduced during the tantalum oxide deposition. The forming voltage also depended on the electrode metal. For the device with an Al electrode, a clear aluminum oxide layer was identified at the electrode–insulator interface, which was hypothesized to be the origin of the high forming voltage. All factors concerning the oxygen vacancies, i.e. intrinsic vacancies introduced via film deposition and extrinsic vacancies caused by the electrode scavenging effect, influenced the initial state of tantalum oxide, and thus, its switching performance as a resistive memory.

All-organic dielectric polymer films exhibiting superior electric breakdown strength and discharged energy density by adjusting the electrode–dielectric interface with an organic nano-interlayer

The surface morphology of dielectric films has a great effect on the insulation performance. Remarkably improved capacitive performance is realized by adjusting the electrode–dielectric interface.

Enhanced surface insulating performance for polystyrene by atmospheric pressure plasma jet deposition

Surface flashover at the solid dielectric/vacuum interface is a fundamental physical phenomenon, which restricts the development of pulsed power system and high voltage electrical equipment. The initiation and developing processes of surface flashover at the solid dielectric are associated with electrode-dielectric interface, dielectric surface and other external factors. Especially, the surface charges transport properties that related with surface flashover developing process are determined by the physicochemical of the dielectric surface. Therefore, the surface insulating performance can be enhanced by deposited with appropriate functional films. TiN film has a low secondary electron yield (SEY), which can help to suppress the secondary electron multipactor effect and suppress the surface flashover. In this paper, a preparation method of film contained TiCxOyNz using atmospheric pressure plasma jet (APPJ) is proposed. The experimental devices include AC power supply, plasma jet chamber and gas/precursor system. The AC power supply has a power of 1000 VA and frequency of 20 kHz. The precursor tetraethyl titanate (TTEO) is pumped into plasma jet chamber by argon gas and the working gas is nitrogen. The experimental results show that the maximum SEY of sample with TiCxOyNz film is approximately decreased to 1.5. The flashover voltage of sample with TiCxOyNz film is increased obviously. The morphology of the dielectric surface indicates that the rough surface is introduced after modification. Also, the physical processes of film deposition are investigated by theemissionspectroscopicanalysis.

Analysis of large bandgap dielectrics by dual plasmon-photon excitation

This work demonstrates a non-destructive approach that uses low-energy radiation in conjunction with low-voltage electric bias to analyze deep defect states in large bandgap dielectric materials. The method is based on a double electronic excitation process, where visible light is employed to energize electrons to overcome, an otherwise too large, band-gap barrier. It is demonstrated over a nanometer-sized, semiconductor-insulator-metal (transparent top electrode, large bandgap dielectric, metallic bottom electrode) test structure. During the measurements, hot-electrons are produced at the transparent electrode by irradiating a plasmonic probe (i.e. Au). These energetic electrons are then transported to the top electrode-dielectric interface, where the same radiation excites them over the energy barrier. This two-step process results in a photocurrent that allows charge to occupy dielectric defect states via relaxation. Furthermore, introducing a non-overlapping time shift between photonic and voltage pulse stimulations is shown to produce a ‘load and shoot’ effect. In this manner, some defect states are preset prior to illumination thus affecting the photocurrent pattern. An analysis of defect density is then performed by spectral decomposition of this spatiotemporal pattern.

Significantly enhanced energy storage density of epitaxial Ba0.53Sr0.47TiO3 thin films by optimizing bottom electrode material

Electrode materials of dielectric thin-film capacitors have significant effect on their energy storage properties. In this work, Ba0.53Sr0.47TiO3 thin films were successfully deposited on LaNiO3 or La0.7Sr0.3MnO3 buffered (001) SrTiO3 substrates by pulsed laser deposition method (abbreviated as BST/LNO/STO and BST/LSMO/STO, respectively). The effect of LNO or LSMO bottom electrode on energy storage properties of BST films was systematically studied. It was found that the recoverable energy density of BST/LSMO/STO films is 36.1 J/cm3 at 3400 kV/cm, which is 59.1% higher than that of BST/LNO/STO thin films, i.e., 22.7 J/cm3 at 2765 kV/cm. Besides, the breakdown strength of the BST/LSMO/STO thin films is 22.9% higher than that of the BST/LNO/STO films. Both films show the good thermal stability from 20 °C to 160 °C as well as excellent fatigue resistance after up to 109 electrical cycles. The results show that the lattice mismatch strain of the film, surface roughness of the electrode and Schottky barrier height at the electrode-dielectric interface greatly affect the energy storage properties of BST thin-film capacitors. Our work offers a practical methodology to enhance the energy storage performances of thin-film capacitors by optimizing bottom electrode materials.

Distribution of Strain and Crystal Domain on Electrode-dielectric Interface in the Practical Ferroelectric Materials Studied by Micro X-ray Diffraction

Distribution of Strain and Crystal Domain on Electrode-dielectric Interface in the Practical Ferroelectric Materials Studied by Micro X-ray Diffraction

Relevance between a Structure of Electrode-Dielectric Interface and a Dielectric Property in the Practical Ferroelectric Materials using Micro X-ray Diffraction

Relevance between a Structure of Electrode-Dielectric Interface and a Dielectric Property in the Practical Ferroelectric Materials using Micro X-ray Diffraction

Quantum‐Tunneling Metal‐Insulator‐Metal Diodes Made by Rapid Atmospheric Pressure Chemical Vapor Deposition

AbstractA quantum‐tunneling metal‐insulator‐metal (MIM) diode is fabricated by atmospheric pressure chemical vapor deposition (AP‐CVD) for the first time. This scalable method is used to produce MIM diodes with high‐quality, pinhole‐free Al2O3 films more rapidly than by conventional vacuum‐based approaches. This work demonstrates that clean room fabrication is not a prerequisite for quantum‐enabled devices. In fact, the MIM diodes fabricated by AP‐CVD show a lower effective barrier height (2.20 eV) at the electrode–insulator interface than those fabricated by conventional plasma‐enhanced atomic layer deposition (2.80 eV), resulting in a lower turn on voltage of 1.4 V, lower zero‐bias resistance, and better asymmetry of 107.

Pulsed flashover across a solid dielectric in vacuum

Flashover across a solid surface in vacuum greatly restricts the insulation capability of pulsed power devices and impedes its overall performance. For more than half a century of painstaking research, there are still some ill-defined aspects that need to be studied. This paper, organized in chronological order, briefly reviews the milestones of research progress regarding surface flashover in vacuum. The main influential factors on flashover are analyzed, which are generalized into three aspects, i.e., electrode-dielectric interface, dielectric surface and external factors. The mainstream theories on flashover are discussed. In recent decades, the approach to improve surface insulation performance are summarized, including surface modification techniques, new insulating materials and new insulation structures. It is hoped that this paper will provide a guide for future related scientific research, engineering design and application.

Experimental investigation of surface charge accumulation behaviors on PTFE insulator under DC and impulse voltage in vacuum

Surface charge accumulated on insulators plays an important role in flashover process in vacuum, which has not yet been fully understood. In this paper, charging behaviors on insulator surface were experimentally examined under negative DC and impulse voltage. For the case of DC voltage, two charging stages were observed, i.e., the area near cathode was at first negatively charged under lower voltage (stage I) and then followed by a positively charged cathode-anode surface (stage II). Surface charging at stage I is considered to be caused by Schottky injection of homo-charges from electrode into sample surface layer. While at stage II secondary electron emission (SEE) process is responsible to the positively charged anode-cathode surface. However, charging at stage I is quite sensitive to pre-stress process, as well as the electrode-insulator interface conditions. The ongoing injection of electrons from cathode can lead to gradually decreasing in surface charge accumulation as the voltage duration increases. For the case of impulse voltage, Schottky injection of homo-charges has little influence on surface charging as the time is too short for charge injection and migration in surface layer. The SEE process will govern surface charging due to its extremely fast establishment process. A good linear relationship is obtained between saturated surface charge density and applied voltage, which is generally consistent with the numerical calculation result based on secondary electron emission avalanche theory. At last, it is confirmed that the ionization of desorbed gas from insulator surface and subsequent neutralization of electrons with positive surface charge is the main cause to eliminate surface charge in flashover.

Oxide composition studies of electrochemically grown tantalum oxide on sintered tantalum using XPS depth-profiling and co-relation with leakage properties

Chemical structure and leakage properties of electrochemically grown tantalum oxide films on high-surface area tantalum were studied. Tantalum oxide films with different thicknesses were grown on sintered tantalum under potentio-dynamic conditions by modifying the anodization dwell-voltages. X-ray photoelectron spectroscopy depth-profile studies were performed to investigate the surface chemical composition and oxide species of the grown tantalum oxide. Furthermore, the strength of the dielectric film was probed by analyzing the leakage properties of the tantalum oxide of different thicknesses. Different leakage current models were used to identify the plausible defect mechanisms in anodized tantalum oxide films. A mix of the stoichiometric pentavalent oxide and non-stoichiometric sub-oxides were observed in the tantalum oxide film with a distribution gradient along the oxide growth direction. A gradual drop was observed in the content of stoichiometric phase from the oxide surface to the oxide-metal interface irrespective of the oxide thickness. The non-stoichiometric content of oxide gave rise to crystalline nature in the oxide which acted as a catalyst for leakage conduction in oxide dielectrics. The electrode-dielectric interface gave rise to various defect mechanisms which also contributed to leakage in addition to crystalline content in the oxide. To the best of the authors’ knowledge, this is the first scientific reporting on composition and structure of anodically-grown tantalum oxide films on high-surface area tantalum along with identification of mechanisms responsible for their leakage properties.

Redesigning Multilayer Ceramic Capacitors by Preservation of Electrode Conductivity and Localized Doping.

Both Li2CO3-coated nickel particles and fast firing technique were utilized in the manufacturing of MLCCs. They preserved the conductivity of Ni electrodes and provided the possibility of sintering the devices in oxidizing atmospheres. By using our method, the partial pressure of oxygen increased from 10-10 atm in conventional methods to 10-4 atm. The oxidizing atmosphere reduced the oxygen vacancy concentration as illustrated by the color change of the samples, and the results of EELS (electron energy loss spectroscopy). A systematic test with variable parameters, Li2CO3 coating, the sintering schedule, and the oxygen flow during sintering were executed, and the dissipation factor and the capacitance of the MLCCs were documented. Three type of MLCCs were studied: Conventional (fired with the conventional technique), Uncoated (fast fired with uncoated Ni particles), and Coated (fast fired with the coated Ni particles). The maximum oxygen activity during sintering (i.e., pO2 = 1.2 × 10-4 atm) was obtained for coated samples, and due to the minimum VO•• concentration, their dissipation factor decreased up to 60% relative to the Conventional ones. In addition, the impedance spectroscopy, together with the map of Li ion distribution, suggested that Li ions accumulated around the electrode-dielectric interface and amplified the activation energy at these interfaces. This eventually caused the coated MLCCs to show higher capacitance than their uncoated counterparts. As a conclusion, it is shown that the manufacturing process described in this paper can provide a better MLCC with higher capacitance, and lower dissipation factor and leakage current.

Peculiarities of the dielectric response of the silver-modified-zeolite porous microstructure

The aim of this study was to characterize electrical conductivity and dielectrical properties of the silver-exchanged zeolite - natural clinoptilolite from Western part of Turkey and Azerbaijan in the range of frequencies from 200 Hz to 1 MHz and at room temperature. For a better understanding the effect of concentration and content of silver in the nanoporous zeolite volume on the conductivity, a study of the dielectric properties of an un-modified and silver-modified zeolite plates with different amounts of Ag ions and Ag nanoparticles is performed. Un-modified and three different types of the silver ion-exchanged modified clinoptilolite plates were prepared. It was found, that with increasing silver concentration, resistance of zeolite plate monotonically decreases at the same time a capacitance is increases. It is suggested an explanation of the observed frequency dependence of the capacitance and resistance of zeolite plates on the silver concentrations may be explain on the basis of an electrode-dielectric interface gap model. At the same time, the observed phenomenon can be explained by considering the fact that with increasing content of silver the conductivity increases. These results show that Ag nanoparticles play significant role for performance improvement in plasma electronic devices with zeolite cathode.

Calibration of pulsed electroacoustic method considering electrode–dielectric interface status and porosity

Pulsed electroacoustic (PEA) method is widely used, but measurement conditions not always agree with the underlying PEA assumptions. This necessitates space charge recovery; however, existing research only addresses the attenuation and dispersion in lossy media. The effects of electrode–dielectric interface contact status and porosity on the accuracy of space charge distribution are discussed in the present article. It is shown that the presence of nonlinear interface and porosity can introduce severe error. However, because the properties of acoustic propagation of waves (which are generated from charge on the electrode and in the bulk) are different, the conventional recovery algorithm is no longer suitable for calibrating the charge density. To obtain accurate space charge profiles, it is necessary to eliminate these effects. A method has been proposed which is based on the original measurement process. The validity of the proposed method was tested by reasonable post-recovery electric field distributions.