Low-field Conductivity Research Articles

Impact of Top (Pt, Au, and Al) Electrodes on HfAlOx-Based MIM Capacitors

The electrical and dielectric properties of films deposited by radio frequency magnetron cosputtering of and targets with Au as the bottom electrode and Pt, Au, and Al as the top electrodes forming metal–insulator–metal (MIM) structures have been studied. layers are characterized by X-ray photoelectron spectroscopy to examine the chemical composition. Atomic force microscopy morphology study shows that root mean square value of surface roughness of the film decreases after annealing. From grazing incidence X-ray diffraction study, it is observed that the deposited films remain amorphous up to annealing temperature. For electrical characterization, current–voltage and capacitance–voltage characteristics were studied in the frequency range of . MIM capacitors with Pt top electrode were found to have the lowest leakage current of (at ) and low energy dissipation with improved voltage coefficients of capacitance values . Low field conduction mechanism was found to be dominated by Schottky emission, while at high field () it is dominated by Poole–Frenkel mechanism. Change in the quadratic voltage coefficient of capacitance under different stress voltages was found to be the lowest with Pt top electrode.

Nonlinear DC and Dispersive Conductivity of Ion Conducting Glasses and Glass Ceramics

Abstract Frequency-dependent third-order conductivity spectra σ 3 ´(ν) of various ion conducting glasses and glass ceramics were obtained by applying sinusoidal electric fields with high amplitudes and by analysing the resulting higher-harmonic currents. In the DC conductivity regime, the third-order conductivity σ 3, dc was found to be positive for all materials and at all temperatures. From the ratio of the third-order conductivity to the low-field conductivity, σ 3, dc/σ 1, dc, apparent jump distances were calculated. These apparent jump distances are much larger than jump distances between neighbouring sites in the glasses and decrease with increasing temperature. In (Li2O)1-x ·(Na2O) x ·Al2O3·(SiO2)4 glasses, the mixed alkali effect leads to a minimum in the apparent jump distance, while partial crystallisation of Li2O·Al2O3·(SiO2)2 glasses leads to an increase of the apparent jump distance. In the dispersive regime, the third-order conductivity σ 3 ´(ν) of all glasses and glass ceramics is negative and exhibits an approximate power-law dependence, however with a larger exponent than the dispersive low-field conductivity σ 1 ´(ν). For a given material, the third-order conductivity spectra σ 3 ´(ν) obey the time–temperature superposition principle and can be superimposed by using the Summerfield scaling. Remarkably, the shift between the σ 3 ´(ν) master curves of different materials is much stronger than the shift between the σ 1 ´(ν) master curves. In order to rationalize this effect, we calculate the nonlinear dispersive hopping conductivity in a double minimum potential approximation.

Nonlinear conductivity spectra of ionically conducting glasses and glass ceramics: Analysis of spectral shape and scaling properties

We present higher order conductivity spectra ${\ensuremath{\sigma}}_{3}^{\ensuremath{'}}(\ensuremath{\nu})$ of different ion conducting glasses and glass ceramics, which were taken over broad frequency ranges and at different temperatures. The ${\ensuremath{\sigma}}_{3}^{\ensuremath{'}}(\ensuremath{\nu})$ spectra are characterized by a change in sign, namely from positive values in the dc regime to negative values in the dispersive regime. In the dispersive regime, ${\ensuremath{\sigma}}_{3}^{\ensuremath{'}}(\ensuremath{\nu})$ exhibits an approximate power-law-type frequency dependence, albeit with a significantly larger exponent than the low-field conductivity ${\ensuremath{\sigma}}_{1}^{\ensuremath{'}}(\ensuremath{\nu})$. The ${\ensuremath{\sigma}}_{3}^{\ensuremath{'}}(\ensuremath{\nu})$ isotherms of an individual glass or glass ceramic can be superimposed by using the Summerfield scaling. The resulting ${\ensuremath{\sigma}}_{3}^{\ensuremath{'}}(\ensuremath{\nu})$ master curves of different materials show strong shifts on the scaled frequency axis with respect to each other. This implies strong differences between the materials regarding the nonlinearity of the dispersive conductivity. In order to rationalize this effect, we calculate the nonlinear dispersive hopping conductivity in a double-well potential approximation.

The influence of temperature and electric field history on the conductivity of the charge density wavesystem o-TaS3

We have studied the dependence of the low electric field conductivity of the charge density wave (CDW)system o-TaS3 on the temperature and applied electric field history. Without the application of highelectric fields, the conductivity is higher on cooling than on heating in a wide temperaturerange below the CDW transition. With the application of increasing field at a giventemperature the conductivity evolves towards a stable, history-independent value situatedbetween the cooling and heating values. The evolution is gradual but most pronouncedaround the threshold field for nonlinear conductivity. The relative change of conductivityafter the application of high field is asymmetric in respect of the temperaturehistory. We discuss the results within the model of quantized changes of the CDWwavelength induced by the variation of the temperature and the electric field.

Characterization of Y 2O 3 gate dielectric on n-GaAs substrates

Physical and electrical properties of sputtered deposited Y 2O 3 films on NH 4OH treated n-GaAs substrate are investigated. The as-deposited films and interfacial layer formation have been analyzed by using X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS). It is found that directly deposited Y 2O 3 on n-GaAs exhibits excellent electrical properties with low frequency dispersion (<5%), hysteresis voltage (0.24 V), and interface trap density (3 × 10 12 eV −1 cm −2). The results show that the deposition of Y 2O 3 on n-GaAs can be an effective way to improve the interface quality by the suppression on native oxides formation, especially arsenic oxide which causes Fermi level pinning at high- k/GaAs interface. The Al/Y 2O 3/n-GaAs stack with an equivalent oxide thickness (EOT) of 2.1 nm shows a leakage current density of 3.6 × 10 −6 A cm −2 at a V FB of 1 V. While the low-field leakage current conduction mechanism has been found to be dominated by the Schottky emission, Poole–Frenkel emission takes over at high electric fields. The energy band alignment of Y 2O 3 films on n-GaAs substrate is extracted from detailed XPS measurements. The valence and conduction band offsets at Y 2O 3/n-GaAs interfaces are found to be 2.14 and 2.21 eV, respectively.

Current–voltage characteristics of SnO2–Co3O4–Cr2O3–Sb2O5 ceramics

The effect of mechanical treatment in a planetary mill on the microstructure and electrical properties of tin dioxide based varistor ceramics in the system SnO2–Co3O4–Cr2O3–Sb2O5 sintered in the range 1150–1450 °C was studied. The mechanical treatment leads to an increase in shrinkage, decrease in porosity, decrease in sample diameter, change in colour of the sintered samples from grey to black and enhancement of nonlinearity. For the sample sintered at 1350 °C the mechanical treatment enhances the nonlinearity coefficient from 11 to 31 and decreases the electric field E1 (at 10−3 A cm−2) from 3500 to 2800 V cm−1. The observed changes in physical properties are explained in terms of an additional size reduction of oxide particles and a better mixing of oxide powder followed by the formation of potential barriers at the grain boundaries throughout the whole sample. In spite of the low porosity, the low-field electrical conductivity of mechanically treated ceramics is significantly increased with the growth of relative humidity. A higher humidity sensitivity is found for mechanically treated ceramics with higher barrier height and higher nonlinearity coefficient.

Cobalt Silicide Nanostructures: Synthesis, Electron Transport, and Field Emission Properties

Cobalt silicide nanostructures have been synthesized by a spontaneous chemical vapor transport and reaction method. The temperature and the vapor flow rate were shown to critically influence the growth of nanostructures. The effects of two main parameters on the growth of nanostructures were discussed. The phases formed were determined by the Gibbs free energy changes in the reactions. Various phases (CoSi, Co2Si) and morphologies, such as single-stem nanowires, three-dimensional (3D) nanowire networks, and aloelike nanostructures, have been synthesized. Very low turn-on field (1.42 V/μm) and good conductance obtained from field-emission and electrical property measurements, respectively, indicate that CoSi nanowires are potentially useful for electronic devices.

Tunnelling mafnetoresistance of (1 – x)La0.7Ca0.3MnO3 +xAg composites

Magnetization and transport of (1 – x)La0.7Ca0.3MnO3 +xAg (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5) composites has been reported in this paper. Low Field Magnetoresistance (LFMR), conductivity (σ) and metal – insulator transition temperature (Tp) increases with increasing content of Ag. Due to the presence of nomagnetic Ag in the ferromagnetic La0.7Ca0.3MnO3 material, dc magnetization decreases but the corresponding Tp increases. Based on the spin polarized transport of conduction electrons at the grains boundaries, we find that our experimental data (MR – H) agree quite well with the dependence of temperature on tunneling magnetoresistance (TMR) described by an expression of Hwang et al.

Electrical properties of SnO2 based varistor ceramics with CuO addition

Current–voltage characteristics, temperature dependences of the low-field dc conductivity and the relative dielectric permittivity of tin dioxide based SnO2–Co3O4–Nb2O5–Cr2O3 ceramics with CuO addition in the range 0–8 mol% are studied. These materials exhibit non-Ohmic conduction (the varistor effect). If 0.5 mol% CuO is added, the nonlinearity coefficient is increased from 54 to 75 and the electric field at fixed current density is decreased from 628 to 390 kV m−1. Higher amounts of CuO substantially diminish non-Ohmic behaviour. Thickness dependence of the nonlinearity coefficient and the electric field at fixed current density in SnO2 varistor ceramics is observed. Electrical properties are discussed in the frames of grain-boundary controlled conduction mechanism.

Electrical Transport in a Semimetal–Semiconductor Nanocomposite

Measurements are presented on the low-field electrical conductivity and moderate-field current-voltage characteristics in a nanocomposite structure of ErAs particles in an In0.53Ga0.47As host with Be compensation. The electrical conductivity displays strong temperature dependence with two types of transport mechanisms. At ~ 205 K and above, the low-field conductivity appears to be dominated by free electrons in In0.53Ga0.47As. Between 55 and 205 K, the conductivity is well explained by variable-range hopping, sigma = A exp(-B/T1/4), via Mott's law. The transport displays a soft breakdown effect at moderate bias fields that grows in threshold field with decreasing temperature. This is attributed to impact ionization of the Be dopants.

A physics-based model of electrical conduction decrease with time in amorphous Ge2Sb2Te5

The time-stability of the electrical characteristics of chalcogenide materials is one of the most important issues for their use in nonvolatile solid state memory applications. In particular the electrical conduction of the glassy phase evolves with time due to two different physical phenomena: the crystallization and the so-called low conductivity drift. Despite the physics of crystallization having been extensively studied in literature, the latter is mainly described by phenomenological relationships, and its physical comprehension is still under discussion. In this paper we study the amorphous phase low-field conductivity drift and its dependence on the temperature experienced by the device. We developed an experimental procedure able to separate the reversible change in the electrical conductivity with temperature due to the material semiconductorlike behavior from the nonreversible one related to the drift mechanism. A drift model explaining such nonreversible conductivity change as a band diagram modification is also provided and calibrated on experimental data. The present work finally introduces alternative metrics for drift quantification that can be useful in order to compare different materials.

Distinctive plasma density features of the topside ionosphere and their electrodynamics investigated during southern winter

This study utilizes a novel technique to map the Defense Meteorological Satellite Program (DMSP) data across the two hemispheres to learn about the morphology and plasma composition of the topside ionosphere, and the underlying ionospheric dynamics. In the southern winter hemisphere, the regional maps tracked a heavy‐ion (Ni‐O+) trough, aurora zone, polar hole, and large plasma density depletion. The latter appeared in the region of the South Atlantic Magnetic Anomaly (SAMA). The electron temperature (Te) map detected the thermal characteristics of these features, while the plasma drifts and flux maps tracked their dynamics. Results show that there were special electrodynamic effects in the SAMA region due to the low magnetic field and high conductivity. These increased the vertical downward (VZ) and the westward (VY) drifts. Independently, the VZ and VY maps registered the affected area that was depleted in heavy ions and rich in light ions. Some field‐aligned profiles tracked the impact of these SAMA effects on the heavy‐ion trough, which was a stagnation trough and appeared markedly differently at different longitudes. At trough latitudes ((56 ± 4)°S (geomagnetic) when Dstav = 0 nT), the elevated electron temperatures forming a Te peak indicated subauroral heating effects. A statistical study modeled the magnetic activity dependence of the Te peak's magnitude and location and revealed their linear correlation with the activity level. Statistically, the Te peak increased [10.226 ± 1.355]°K and moved equatorward [0.051 ± 0.009]° (geomagnetic) per 1 nT decrease in the averaged Dst index. Per 1 nT increase in the averaged AE index, its magnitude increased [1.315 ± 0.444]°K and the equatorward movement was [0.014 ± 0.003]°.

Zero Field Spin Splitting in GaN/AlGaN Heterostructures Probed by the Weak Antilocalization

We present the magnetoconductivity measurements of a high mobility two-dimensional electron gas confined at GaN/AlGaN interface. The sensitive measurements of low field conductivity revealed both quantum corrections, the weak localization and antilocalization effects. It indicates the importance of the spin–orbit coupling in this wide band gap material. The analysis of the data provided the information about the temperature dependence of the dephasing time and total spin–orbit relaxation time. The conduction band spin splitting energy amounts to 0.23 meV and 0.35 meV at electron densities 2.2× 10 cm−2 and 5.7× 10 cm−2, respectively.

Varistor and humidity-sensitive properties of SnO 2–Co 3O 4–Nb 2O 5–Cr 2O 3 ceramics with V 2O 5 addition

It is shown that tin dioxide based SnO 2–Co 3O 4–Nb 2O 5–Cr 2O 3 ceramic material exhibits not only varistor properties discussed in the literature but also humidity-sensitive properties. At low electric fields conductivity of SnO 2–Co 3O 4–Nb 2O 5–Cr 2O 3 ceramics is increased with relative humidity of air, however, highly nonlinear current–voltage characteristic is observed even at relative humidity 93%. Conduction mechanism in this material is grain-boundary controlled and decrease of the barrier height on voltage and on relative humidity is responsible, respectively, for varistor and sensor effects. Slow decrease of the low-field conductivity with time during several months after the sintering or high-temperature heat treatment is observed. Such ageing effect is accompanied by the decrease of the lowest value of the current density at highly nonlinear part of current–voltage characteristic. Addition of V 2O 5 to SnO 2–Co 3O 4–Nb 2O 5–Cr 2O 3 causes strong decrease of the nonlinearity coefficient, increase of the low-field conductivity, decrease of the activation energy of electrical conduction and decrease of the humidity sensitivity. At quite small V 2O 5 addition (about 1 mol.%) the transition to a percolation along the V 2O 5 phase is assumed. Secondary conductive V 2O 5 phase acts as a shunt to the grain boundaries.

Structural, magnetic, and ferroelectric properties of multiferroic BiFeO3 film fabricated by chemical solution deposition

Polycrystalline BiFeO3 film has been fabricated by a chemical solution deposition on Pt∕Ti∕SiO2∕Si(100) substrates. A ferroelectric hysteresis loop showed a high remanent polarization of 47μC∕cm2 at room temperature. Leakage current density was on the order of 10−1A∕cm2 at 100kV∕cm, indicating the high leakage current density in the present BiFeO3 film. The leakage current mechanism could be considered as follows: Ohmic conduction at low electric field and Poole-Frenkel trap-assisted conduction appeared as the electric field increased, and space-charge-limited current started at a high electric field. Weak ferromagnetism was observed at room temperature, and magnetic coercivity increased to 0.5kOe with small remanent magnetization of 2emu∕cm3 at 10K. In order to investigate the magnetoelectric effect of the BiFeO3 film, the ferroelectric hysteresis loop was measured under the magnetic field of 5kG at room temperature.

Hopping conduction in disordered carbon nanotubes

We report electrical transport measurements on individual disordered multiwalled carbon nanotubes, grown catalytically in a nanoporous anodic aluminum oxide template. In both as-grown and annealed types of nanotubes, the low-field conductance shows an exp [ − ( T 0 / T ) 1 / 2 ] dependence on temperature T , suggesting that hopping conduction is the dominant transport mechanism, albeit with different disorder-related coefficients T 0 . The electric field dependence of low-temperature conductance behaves as exp [ − ( ξ 0 / ξ ) 1 / 2 ] at high electric field ξ at sufficiently low T . Finally, both annealed and unannealed nanotubes exhibit weak positive magnetoresistance at T = 1.7 K . Comparison with theory indicates that our data are best explained by Coulomb-gap variable-range hopping conduction and permits the extraction of disorder-dependent localization length and dielectric constant.

Large Bychkov-Rashba spin-orbit coupling in high-mobilityGaN∕AlxGa1−xNheterostructures

We present low temperature magnetoconductivity measurements of a density-tunable and high mobility two-dimensional electron gas confined in the wide bandgap GaN/AlGaN system. We observed pronounced anti-localization minima in the low-field conductivity, indicating the presence of strong spin-orbit coupling. Density dependent measurements of magnetoconductivity indicate that the coupling is mainly due to the Bychkov-Rashba mechanism. In addition, we have derived a closed-form expression for the magnetoconductivity, allowing us to extract reliable transport parameters for our devices. The Rashba spin-orbit coupling constant is $\alpha_{so}$ $\sim$ 6$\times$ 10$^{-13}$eVm, while the conduction band spin-orbit splitting energy amounts to $\Delta_{so}$ $\sim$ 0.3meV at n$_e$=1$\times10^{16}$m$^{-2}$.

Memory Effects in Manganese Perovskites. Experiment and Theory

ABSTRACTElectrically induced quasi-permanent changes in low-field conductivity have been observed in single crystals of LaGa1-xMnxO3 in the broad range of Mn ion concentrations. The memory effects can last for a long time at room temperature and can be easily erased by heating up to Tc ∼300 C. The temperature dependence of the resistivity has a sharp drop around the phase transformation temperature, pointing to the role of phase transformation processes. The switching between high resistance and low resistance states is demonstrated. We explain our experimental data in terms of thermo induced local phase transition with the oxidation of Mn ions. The results of the ab initio calculations of the electron energy structure in Jahn-Teller-distorted and non-distorted cells confirm the mechanism suggested.

Application of nonlinear conductivity spectroscopy to ion transport in solid electrolytes

The field-dependent ion transport in thin samples of different glasses is characterized by means of nonlinear conductivity spectroscopy. Ac electric fields with strengths up to 77 kV/cm are applied to the samples, and the Fourier components of the current spectra are analyzed. In the dc conductivity regime and in the transition region to the dispersive conductivity, higher harmonics in the current spectra are detected, which provide information about higher-order conductivity coefficients. Our method ensures that these higher-order conductivity coefficients are exclusively governed by field-dependent ion transport and are not influenced by Joule heating effects. We use the low-field dc conductivity σ 1,dc and the higher-order dc conductivity coefficient σ 3,dc to calculate apparent jump distances for the mobile ions, a app. Over a temperature range from 283 K to 353 K, we obtain values for a app between 39 Å and 55 Å. For all glasses, we find a weak decrease of a app with increasing temperature. Remarkably, the apparent jump distances calculated from our data are considerably larger than typical values published in the literature for various ion conducting glasses. These values were obtained by applying dc electric fields. Our results provide clear evidence that the equation used in the literature to calculate the apparent jump distances does not provide an adequate physical description of field-dependent ion transport.

Incoherent charge hopping and conduction in DNA and long molecular chains

We establish some relations between the kinetics of incoherent, hopping charge transport in bridged large scale chemical systems or in a single-component duplex DNA, and the electrical properties (electric current ( j) and conductance ( g)) of these systems connected by two electrodes. We considered two distinct voltage distributions across the equienergetic chain (with N bridge elements, and an intersite hopping rate k), which involve the voltage being biased only across the edge bridge elements (case (i)), and the voltage being equally distributed across the bridge (case (ii)). For sufficiently long chains in the low voltage ( Φ) domain, we find that j = ( ek/ N) G( κ 1, κ −1)( eΦ/ k B T), where G() is a function of charge injection rates κ 1( κ −1) to (from) the electrode. The low field (constant) conductance is g = 1.6 × 10 −19( k/N) G Ω −1. At high voltages we established the existence of a maximal, constant, Φ independent current ( j max), where g → 0. For case (i) j max = ek/ N, being determined by the intersite hopping rate and by N −1, as appropriate for diffusional charge transport. For case (ii) j max = eκ 1, being independent of the chain length, and determined by the rate of charge injection from the electrode. Finally, we applied our kinetic model for the description of incoherent charge transport in and the electronic properties of a donor–acceptor pair connected by two electrodes.