Capacitance Relaxation Research Articles

Enhanced Crystallinity of SrTiO3 Films on a Silicon Carbide Substrate: Structural and Microwave Characterization

Thin films of strontium titanate, which reveal high structure quality and tunable properties prospective for microwave applications at room temperature, were grown on a semi-insulating silicon carbide substrate using magnetron sputtering for the first time. The films’ growth mechanisms were studied using medium-energy ion scattering, and the films’ structures were investigated using X-ray diffraction. The electrical characteristics of planar capacitors based on strontium titanate films were measured at a frequency of 2 GHz using a high-precision resonance technique. It is shown that the tendency to improve the crystalline structure of strontium titanate film with an increase in the substrate temperature is most pronounced for films deposited at elevated working gas pressure under low supersaturation conditions. Planar capacitors formed on the basis of oriented SrTiO3 films on silicon carbide showed tunability n = 36%, with a loss tangent of 0.008–0.009 at a level of slow relaxation of capacitance, which is significantly lower than the data published currently regarding planar tunable ferroelectric elements. This is the first successful attempt to realize a planar SrTiO3 capacitor on a silicon carbide substrate, which exhibits a commutation quality factor more than 2500 at microwaves.

Dirac Electrons with Molecular Relaxation Time at Electrochemical Interface between Graphene and Water.

The time dynamics of charge accumulation at the electrochemical interface between graphene and water is important for supercapacitors, batteries, and chemical and biological sensors. By using impedance spectroscopy, we have found that measured capacitance (Cm) at this interface with the gate voltage Vgate ≈ 0.1 V follows approximate laws Cm~T1.2 and Cm~T0.11 (T is Vgate period) in frequency ranges (1000-50,000) Hz and (0.02-300) Hz, respectively. In the first range, this dependence demonstrates that the interfacial capacitance (Cint) is only partially charged during the charging period. The observed weaker frequency dependence of the measured capacitance (Cm) at frequencies below 300 Hz is primarily determined by the molecular relaxation of the double-layer capacitance (Cdl) and by the graphene quantum capacitance (Cq), and it also implies that Cint is mostly charged. We have also found a voltage dependence of Cm below 10 Hz, which is likely related to the voltage dependence of Cq. The observation of this effect only at low frequencies indicates that Cq relaxation time is much longer than is typical for electron processes, probably due to Dirac cone reconstruction from graphene electrons with increased effective mass as a result of their quasichemical bonding with interfacial molecular charges.

On how Doping with Atoms of Gadolinium and Scandium affects the Surface Structure of Silicon

The experimental team has developed a technology of step-by-step low-temperature diffusion doping of gadolinium and scandium into silicon that allows the clusters of impurity atoms to be uniformly distributed throughout the entire bulk of the silicon material. It was shown that, unlike the samples obtained under the high-temperature diffusion doping technology, in the samples obtained under the novel technology the team had detected any surface erosion or formation of alloys and silicide in the near-surface region. The authors have revealed increased thermostability and radiation resistance of silicon samples dotted with clusters of impurity atoms of gadolinium and scandium. The authors have conducted comprehensive studies by using the techniques of tagged atoms, autoradiography, measurement of conductivity and Hall effect, isothermal relaxation of capacitance and current of diffusion, solubility, and electrophysical properties of scandium in silicon under various doping conditions and for a wide temperature range (1100 ÷ 1250 0С). Diffusion parameters, solubility and acceptor nature of scandium in silicon as well as thermal stability of silicon doped with gadolinium and scandium impurity atoms have been established.

Insight into the Charging and Relaxation Dynamics of Diffusive Memristors in Integration-and-fire Neuron Applications

Artificial neural networks (ANNs) have been studied to mimic biological neurons because of the limitations of conventional computing. Among various ANNs, the spike neural network (SNN) is advantageous owing to its energy efficiency. To demonstrate the effectiveness of the SNN, circuits of integrate-and-fire (IF), leaky IF (LIF), and Hodgkin-Huxley model have been studied using various methods. These circuits contain an external capacitor to mimic membrane behavior. In this study, it is expected that the LIF circuits can be simplified by adopting a diffusive memristor made of Pt/Ag-doped HfOSUBx/SUB/Pt. Volatile threshold switching was observed and modeled by performing electrical measurements. Their capacitive properties and relaxation behavior were interpreted by the internal capacitor and dissolution of the conducting filament. Pulse trains were adjusted to confirm the possibility of implementing an LIF neuron without an external capacitor.

Detection of near-interface traps in NO annealed 4H-SiC metal oxide semiconductor capacitors combining different electrical characterization methods

Fast near-interface (NI) traps have recently been suggested to be the main cause for poor inversion channel mobility in nitrided SiC metal-oxide-semiconductor-field-effect-transistors. Combining capacitance, conductance, and thermal dielectric relaxation current (TDRC) analysis at low temperatures of nitrided SiC MOS capacitors, we observe two categories of fast and slow near-interface traps at the SiO2/4H-SiC interface. TDRC reveals a suppression of slow near-interface traps after nitridation. Capacitance and conductance analysis reveals a high density of fast NI traps close to the SiC conduction band edge that are enhanced by nitridation. The very fast response of NI traps prevents them from detection using TDRC or deep level transient spectroscopy.

Sequential and simultaneous ion transfer into carbon nanopores during charge–discharge cycles in electrical double layer capacitors

Electrical double-layer capacitance was reduced not only by degradation of ions, but also by the presence of idle ions. Meanwhile, the fast relaxation of capacitance and slow ion dynamics facilitate charge–discharge efficiency in wide pores.

Laplace DLTS studies of the 0.25 eV electron trap properties in n-GaN

The thermal emission rate of electrons from a 0.25 eV trap, widely observed in epitaxial GaN by conventional deep-level transient spectroscopy (DLTS), is investigated by the Laplace DLTS (LDLTS) technique. It is demonstrated that the capacitance relaxation waveforms, from which the properties of this trap have been determined until now by the correlation procedure, contain two exponential components induced by the thermal emission of electrons from two traps with the activation energies of 173 meV and 232 meV. In lightly silicon-doped and carbon-doped epitaxial GaN grown on sapphire substrates, the concentrations of these traps were found to be (5.5–7.2) × 1013 and (1.5–3.2) × 1013 cm−3, respectively. Based on formerly reported experimental results and recently published results of theoretical calculations performed with the implementation of the density functional theory with hybrid functionals, the point defects being a potential source of these traps is proposed. It is shown, that a high resolution of LDLTS gives a new insight into the properties of energy levels related to point defects in epitaxial GaN.

Peculiarities of electron emission from high-density deep levels of nanodefects in oxygen-implanted silicon

The peculiarities of electron emission from electronic states of nanodefects formed at the early stages of oxygen precipitation in oxygen-implanted silicon annealed at 700°C were investigated with a combination of capacitance and current transient spectroscopy of the space charge region (SCR) in semiconductors. It was established that the particular properties of acceptor-like states are due to their high density and their localization at the back side of the implanted region. A model is suggested that explains an apparent emission rate slowdown and the appearance of an unexpected sign of capacitance relaxation signal as a result of the non-monotonic shape of the potential of the Schottky-diode.

Discharging behavior of confined bipolar electrodes: Coupled electrokinetic and electrochemical dynamics

In this study we numerically investigate the intimately coupled transient electrokinetic and electrochemical dynamics of a nanoconfined bipolar electrode system, using experimental results to guide our analysis. Our recently developed 2D numerical model implements the Poisson-Nernst-Planck-Stokes system of equations to describe nanoscale chemical species transport by advection, migration, and diffusion, as well as the presence of both homogenous and heterogeneous reactions. By eschewing the assumption of electroneutrality and resolving diffuse-charge screening effects, our model uniquely accounts for a wide range of nonlinear transient effects including bipolar electrode (BPE) surface polarization, Faradaic charge accumulation, induced-charge electroosmotic flow, and ion concentration polarization. Using this model, we demonstrate that upon the removal of a polarizing electric field, excess accumulated charge at a BPE surface electrochemically discharges following the capacitive relaxation of diffuse space charge in the electrical double layers (EDLs) surrounding the BPE extremities. These EDLs continue to polarize the BPE as they relax by a process of drift-diffusion, wherein the counter-ionic space charge at each pole promotes a large influx of oppositely charged ions to restore electroneutrality. We numerically reproduced this electrokinetic enhancement effect that was first observed in a recently reported experimental system in which charged fluorophores were used as tracer molecules. Our results also support experimental evidence that, following capacitive EDL relaxation, nanoconfined BPEs can exhibit pseudocapacitance-like discharging behavior that is localized to a single end of the electrode; we experimentally linked this localization to surface oxidation of the anodic BPE pole under the applied field. In addition to providing important insight into the interplay between nanoscale electrokinetic and electrochemical phenomena that govern transient electrode processes, our model and the results presented in this work reinforce the notion that the domain of bipolar electrochemistry constitutes a promising frontier for developing “wirelessly” tunable charge storage and visual detection approaches which exploit both electrokinetic and Faradaic mechanisms.

Theory for impedance response of grain and grain boundary in solid state electrolyte

We develop a generic phenomenological theory for the electrochemical impedance response of solid state electrolytes (SSEs). The theory describes dynamics of various physical processes in grain (g) and grain boundary (gb). Leaking capacitor model for the dynamics of ions in grain accounts compositional heterogeneity, ion reorganization and transfer across the surface of the grain. Model for ion relaxation in the grain boundary is determined through generalized electric double layer (EDL) theory over heterogeneous surface (Singh and Kant, 2013). The theory accounts for ion transfer and reorganization in grain boundary has the following components: (i) ions distribution at energetically heterogeneous surface of grain; (ii) space charge layer (SCL) in the grain boundary. Predicted impedance response of SSE evinces three frequency regimes, viz. (i) high frequency regime, attributed to relaxation of ions in grain and grain boundary (SCL), (ii) intermediate frequency regime, related to ion reorganization and transfer across heterogeneous grain and grain boundary (iii) low frequency regime, ascribed to the SSE/metal interface. The model emphasizes that ions encounter energetic heterogeneity in the grain causing distribution of relaxation time with a characteristic exponent γg. Doped grain with aliovalent cations further enhances the structural/energetic heterogeneity and concentration of mobile ions therefore decreases the ion transfer resistance from grain to grain boundary. Doping also enhances the surface concentration of mobile ions at the grain boundary region resulting in enhanced capacitance and prolonged relaxation time of compact layer.

Binding of Potassium Ions Inside the Access Channel at the Cytoplasmic Side of Na+,K+-ATPase

Binding of potassium ions through an access channel from the cytoplasmic side of Na+,K+-ATPase and the effect of pH and magnesium ions on this process have been studied. The studies were carried out by a previously developed method of measuring small increments of the admittance (capacitance and conductivity) of a compound membrane consisting of a bilayer lipid membrane with adsorbed membranes fragments containing Na+,K+-ATPase. The capacitance change of the membrane with the Na+,K+-ATPase was induced abruptly by release of protons from a bound form (caged H+) upon a UV-light flash in the absence of magnesium ions. The change of admittance consisted of an initial fast jump and a slow relaxation to a stationary value within a time of about 1–2 s. The kinetics of the capacitance relaxation depended on pH and the concentration of magnesium and potassium ions. The dependence of the rapid capacitance jump on the potassium concentration corresponded to the predictions of the model developed earlier that describes binding of sodium or potassium ions in competition with protons. The effect of magnesium ions can be explained by the assuming that they bind to the Na+,K+-ATPase and affect binding of potassium ions because of either changes in protein conformation or the creation of an electrostatic field in the access channel on the cytoplasmic side.

The Impedance Response of a Passive Film Revisited by a Double Modulation Technique

AbstractThe interfacial capacitance measured in electrochemical experiments is usually considered to be independent of the faradaic processes involved at the electrode surface. In this work, we report on the use of a double modulation technique, which allows the simultaneous measurement of electrochemical impedance spectroscopy (EIS) and modulation of the interfacial capacitance transfer function (MICTF). We show that the capacitance ascribed to the response of passive films formed on pure iron electrodes in phosphoric acid solution exhibits two different time‐constants in low frequency domains, namely the response of intrinsic properties of the film linked to charge carriers and a relaxation ascribed to the thickness relaxation of the oxide film. Interestingly, the low frequency limit of MICTF is in good agreement with the capacitance obtained from EIS measurements. A detailed data analysis allows determination of both, the capacitance of the oxide film and the double layer capacitance to the overall capacitance to be measured. This technique thus provides a unique way of analysing the different contributions ascribed to the relaxation of the interfacial capacitance.

Sequential entrapping of Li and S in a conductivity cage of N-doped reduced graphene oxide supercapacitor derived from silk cocoon: a hybrid Li–S-silk supercapacitor

Li and S compounds are currently exploited for their applications in battery industry. Here, we discovered that Li–S compounds exhibit supercapacitor like properties in a context-dependent manner viz., when Li and S atoms are entrapped in a conductivity cage of N-doped reduced graphene oxide (ND–RGO) supercapacitor derived from silk cocoon, it resulted in the formation of a superior hybrid Li–S-silk (ND–RGO–Li–S) supercapacitor. Interestingly, ND–RGO–Li–S proves to be a better supercapacitor than ND–RGO alone. Electrochemical properties of ND–RGO versus ND–RGO–Li–S indicated that the later has higher capacitance (~ 10.72%), lower resistance (~ 2.98%), and higher time constant or relaxation time (~ 7.52%). Thus, in one of the first attempts, caging Li and S in ND–RGO supercapacitor matrix offers a new role for Li–S, as an improved supercapacitor, unlike its current application as a battery.

On the Understanding of Cathode Related Trapping Effects in GaN-on-Si Schottky Diodes

Cathode related current collapse effect in GaN on Si Schottky barrier diodes is investigated in this paper. Capacitance and current relaxation measurements on diodes and gated-Van Der Pauw are associated with temperature dependent dynamic ${R_{\mathrm{ ON}}}$ transients analysis to identify the parasitic trapping locations in the devices. We show here that the main part of the current collapse at the cathode comes from a combination of electron trapping in the passivation layer and in a carbon related trap in the GaN buffer layers ( ${E_{A}} = \mathrm {E_{T}} - {E_{V}} \simeq 0.9 \,\mathrm {eV}$ ) that can be studied independently by using the appropriate stress configurations. These two parasitic effects can lead to long recovery time (>1 ks) after reverse bias stress.

High frequency electroporation efficiency is under control of membrane capacitive charging and voltage potential relaxation

The study presents the proof of concept for a possibility to achieve a better electroporation in the MHz pulse repetition frequency (PRF) region compared to the conventional low frequency protocols. The 200ns×10 pulses bursts of 10–14kV/cm have been used to permeabilize Chinese hamster ovary (CHO) cells in a wide range (1Hz–1MHz) of PRF. The permeabilization efficiency was evaluated using fluorescent dye assay (propidium iodide) and flow cytometry. It was determined that a threshold PRF exists when the relaxation of the cell transmembrane potential is longer than the delay between the consequent pulses, which results in accumulation of the charge on the membrane. For the CHO cells and 0.1S/m electroporation medium, this phenomenon is detectable in the 0.5–1MHz range. It was shown that the PRF is an important parameter that could be used for flexible control of electroporation efficiency in the high frequency range.

Exploring the Impedance Signatures of Electrochemical/Mechanical Transport Coupling in Concentrated Liquid Electrolytes

The Warburg impedance response [1] in electrochemical impedance spectroscopy data is commonly recognized as a signature of molecular diffusion processes. One can go beyond a qualitative perspective based on equivalent circuits and employ impedance data to quantify material properties relevant to transport phenomena. The Warburg impedance in particular provides useful data for electrolyte characterization. More often than not, however, properties such as transference numbers and diffusivities have been extracted by applying the Poisson-Nernst-Planck (PNP) model [2] to impedance data; the PNP model neglects Darken factors, and therefore does not bear accurate results when applied to concentrated solutions. This work builds on the detailed impedance formulation discussed by Pollard and Comte [3], who rely on Onsager-Stefan-Maxwell theory instead of PNP theory. As well as accounting for thermodynamic factors and using thermodynamically meaningful potentials [4], the Pollard-Comte model involves parameters that quantify solute-volume effects, namely, the excluded-volume effect and Faradaic convection [5]. Faradaic convection, which refers to bulk electrolyte flow induced by interfacial electrochemical reactions, is one instance of the electrochemical/mechanical coupling this presentation will explore. The Pollard-Comte analysis is extended to an asymmetric cell, in which different half-reactions occur at the working and counter electrodes. Faradaic convection is an essentially kinematic phenomenon. A far more substantial augmentation of the Pollard-Comte model is needed to account for dynamical effects, such as those arising from local pressure variations. Liu and Monroe showed that in multiple spatial dimensions, Newman’s concentrated-solution theory requires a local momentum balance in addition to the volume-explicit equation of state [5]. Appending the momentum balance, and its associated state-variable definitions and constitutive laws, is a non-trivial exercise, because the pressure dependence of partial molar volumes makes the governing system of transport equations highly coupled – even in a one-dimensional case. Mechanical responses whose characteristic times differ substantially from those associated with molecular diffusion or capacitive relaxation arise from both liquid inertia and viscosity. Impedance analysis is therefore a good strategy for deconvoluting dynamical effects and validating the augmented transport model. It also appears that mechanical effects may be associated with new impedance elements that have not been considered to date.

Compressive capacitance relaxation of carbon black filled silicone rubber composite

The changes in the capacitance of carbon black filled silicone rubber composite during the compressive stress relaxation, which is defined as the “compressive capacitance relaxation,” is researched. During the compressive stress relaxation, the capacitance of the composite increases gradually and tends to be stable over time. Based on the viscoelastic theory, the mathematical model for the compressive capacitance relaxation is established. The capacitance relaxation times and the stable normalized capacitance decrease with the increase of the conductive phase content. The capacitance relaxation times hold constant, and the stable normalized capacitance increases with the increase of the initial stress. POLYM. COMPOS., 39:3446–3451, 2018. © 2017 Society of Plastics Engineers

Mapping the ionic fingerprints of molecular monolayers.

We have previously proposed, and experimentally resolved, an ionic charge relaxation model for redox inactive self-assembled monolayers (SAMs) on metallic electrodes in contact with a liquid electrolyte. Here we analyse, by capacitance spectroscopy, the resistance and capacitance terms presented by a range of thiolated molecular films. Molecular dynamics simulations support a SAM-specific energy barrier to solution-phase ions. Once surmounted, the entrapped ions support a film embedded ionic capacitance and non-faradaic relaxation, which can be assigned as a particular case of general electrochemical capacitance.

Structural and frequency-dependent dielectric properties of PVP-SiO2-TMSPM hybrid thin films

In the present study, thin films of PVP-SiO2-TMSPM (polyvinylpyrrolidone-silicon dioxide- 3-trimethoxysilyl propyl metacrylate) were deposited on p-type Si (111) substrates using spin coating technique. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and energy dispersive X-ray spectrometry (EDS) were applied to investigate the chemical bonds and structural properties of the samples. Morphology of the hybrid thin films was studied using atomic force microscopy (AFM) and scanning electron microscopy (SEM) techniques. The frequency dependence of dielectric properties such as dielectric constant (ε′), dielectric loss (ε″), loss tangent (tan δ) as well as the real component of electric modulus (M′), imaginary component of electric modulus (M″), and AC electrical conductivity (σAC) was studied in Al/PVP-SiO2-TMSPM/PSi used as a metal-polymer-semiconductor (MPS) device. Analysis of dielectric relaxation behavior was performed in the frequency range of 0.1 KHz to 1 MHz. In the frequency range of 1 KHz to 1 MHz, the σAC data were varied from 6.35 × 10−6 to 9.02 × 10−6 for the sample with 0.15 wt ratios of TMSPM and equivalent values of both PVP and SiO2. The dielectric, modulus, and AC conductivity analyses were considered the useful factors to detect the effects of the capacitance, ionic conductivity, and dielectric relaxation process.

Interface passivation of liquid‐phase crystallized silicon on glass studied with high‐frequency capacitance–voltage measurements

The passivation quality at the interface between dielectric interlayer (IL) stacks and liquid‐phase crystallized silicon (LPC‐Si) was investigated by means of high‐frequency capacitance–voltage (C–V) measurements. The developed device structure was based on a molybdenum layer sandwiched between the glass substrate and the IL/LPC‐Si stack. C–V curves were discussed in terms of hysteresis formation and capacitance relaxation. We varied the nitrogen and hydrogen content in the a‐SiOxNy:H layer adjacent to the LPC‐Si and studied the effects on the defect state density at the IL/LPC‐Si interface (Dit) as well as on the effective charge density in the IL (QIL,eff). Both parameters are crucial for the analysis of chemical and field‐effect passivation. Furthermore, the effect of an additional hydrogen plasma treatment (HPT) on Dit and QIL,eff was investigated. A Gaussian‐like defect distribution at around 0.1 eV above the mid gap energy is significantly reduced by the additional HPT. With additional HPT, the lowest Dit and highest QIL,eff at mid gap, i.e., Dit = (3.5 ± 0.7) × 1011 eV−1 cm−2 and QIL,eff = (1.6 ± 0.3) × 1012 cm−2, correspond to the passivation by an a‐SiOxNy:H layer with a low nitrogen and high hydrogen content.