Electron Hopping Conduction Research Articles

A comparative study of the electrical properties of reduced and unreduced LiTaO3 crystals

We have carried out a comparative study of the electrical properties of lithium tantalate (LiTaO3) crystals in a wide temperature range (300–900 K) before and after reductive treatment in H2O vapor and subsequent oxidative annealing. The results demonstrate that, in the temperature range of Li+ ion conduction (550–900 K), the activation enthalpy for ionic conduction in the reduced lithium tantalate crystal is H a = 1.37 eV, which slightly exceeds that in the initial state of the crystal (1.34 eV). In the temperature range 390–450 K, the σ(T) data for the unannealed crystal are well represented by the Arrhenius law in the presence of two carrier types, with activation energies E 1 = 1.03 eV and E 2 = 0.29 eV, characteristic of proton and electron hopping conduction, respectively. After reductive annealing, the activation energy for conduction is ~0.65 eV, characteristic of the activation energy for bipolaron conduction. After subsequent oxidative annealing of the reduced crystals in dry air, the activation energy is ~1.2 eV. It seems likely that the presence of oxygen vacancies in the reduced LiTaO3 crystal stimulates hydrogen release from the crystal during oxidative annealing.

Reactive Solid-Phase Epitaxy and Electrical Conductivity of Layered Sodium Manganese Oxide Films

Layered alkali ion-containing metal oxides (LAMOs) are promising as candidate material for energy storage and conversion applications. Although high-quality epitaxial films are useful to clarify their physical properties, it is very difficult to fabricate LAMO films by a conventional vapor phase epitaxy method because of the high vapor pressure of alkali metals at elevated temperatures. Here we report reactive solid-phase epitaxy (R-SPE) [Cryst. Growth Des. 2005 5, 25] of layered sodium manganese oxide, which is a candidate as cathode active material for Na-ion batteries. In the R-SPE method, manganese oxide epitaxial film with spinel structure was first grown on sapphire substrate, and then the film was changed into Na≈2/3MnO2 epitaxial film by heating the film at 700 °C with Na2CO3 powder. Hydrated Na≈0.61MnO2· ≈ 0.42H2O film was also prepared by immersing a Na≈2/3MnO2 film into water at room temperature. The electron hopping conductivity of Na≈2/3MnO2 and Na≈0.61MnO2 ≈ 0.42H2O films was clarified as ∼1...

Characterization of new negative temperature coefficient thermistors based on Zn–Ni–O system

Y2O3-doped Zn1-xNi x O (x = 0, 0.3, 0.4, 0.5, 0.6, 0.7, and 0.9) powders were prepared by a wet chemical synthesis method, and the related ceramics were obtained by the traditional ceramic sintering technology. The phases and related electrical properties of the ceramics were investigated. The analysis of X-ray diffraction (XRD) indicates that the prepared ceramics with Ni substitution have a cubic crystalline structure. The resistance–temperature feature indicates that all the ceramics show a typical effect of negative temperature coefficient (NTC) of resistivity with the thermal constants between 3998 and 5464 K, and have high cyclical stability in a temperature range from 25 to 300 °C. The impedance analysis reveals that both grain effect and grain boundary effect contribute collectively to the NTC effect. The electron hopping and band conduction models are proposed for the grain (bulk) conduction, and the thermally activated charge carrier transport overcoming the energy barrier is suggested for the grain boundary conduction.

Characterization of a new system of NTC temperature-sensitive ceramics based on Al/F modified NiO simple oxides

Al/F modified NiO, Ni1−xAlxO1−yFy (x ≤ 0.05, y ≤ 0.05), ceramics were prepared by using wet-chemical synthesis methods followed by a traditional ceramic sintering technology. The phase component and related electrical properties of the ceramics were investigated. The results show that all the prepared ceramics have the effect of negative temperature coefficient (NTC) of resistivity. The room temperature resistivities (ρ25) and material constants (B25/85) of the Ni1−xAlxO1−yFy NTC ceramics can be adjusted by changing the concentrations of Al and F. The B25/85 values are from 1705 to 5884 K for the Al-content changes from 0 to 0.05 in Ni1−xAlxO0.96F0.04. The investigations by analyzing the electrochemical impedance spectra at various temperatures show that both grain effect and grain-boundary effect contribute to the NTC feature of the ceramics. The conduction mechanisms combining the electron-hopping model and band conduction are proposed for the NTC effect in the Ni1−xAlxO1−yFy ceramics.

Spin excitations in systems with hopping electron transport and strong position disorder in a large magnetic field

We discuss the spin excitations in systems with hopping electron conduction and strong position disorder. We focus on the problem in a strong magnetic field when the spin Hamiltonian can be reduced to the effective single-particle Hamiltonian and treated with conventional numerical technics. It is shown that in a 3D system with Heisenberg exchange interaction the spin excitations have a delocalized part of the spectrum even in the limit of strong disorder, thus leading to the possibility of the coherent spin transport. The spin transport provided by the delocalized excitations can be described by a diffusion coefficient. Non-homogenous magnetic fields lead to the Anderson localization of spin excitations while anisotropy of the exchange interaction results in the Lifshitz localization of excitations. We discuss the possible effect of the additional exchange-driven spin diffusion on the organic spin-valve devices.

Electrical properties and temperature sensitivity of Li/Fe-modified NiO-based ceramics as NTC thermistors

To develop a new system of negative temperature coefficient (NTC) thermistors, Li/Fe-modified NiO-based ceramics, Ni1−x−y Li x Fe y O (x: 0.00–0.08; y: 0.00–0.06), were prepared through a wet chemical process. The phase component and microstructure of the ceramics were analyzed respectively by using X-ray diffraction and electron microscopy, and the related electrical properties were investigated by resistance–temperature measurement and complex impedance analysis. The results show that all the ceramics have the cubic-type NiO crystalline structure and show typical NTC characteristic. The NTC material constant B 25/85 ranges from 335 to 5169 K and room-temperature resistivity lnρ 25 ranges from 3.97 Ω cm to 1.99 MΩ cm, when the ceramics were modified by various contents of Li- and Fe-ions. The conduction mechanisms combining the electron-hopping model and band conduction are supposed for the NTC effect in the Li/Fe-modified NiO ceramics.

Colossal dielectric constant of NaNbO3doped BaTiO3ceramics

AbstractBaTiO3ceramics doped with 0.40 mol% NaNbO3were prepared using a traditional approach by sintering at temperature of 1250 °C to 1290 °C. The prepared ceramics was characterized by very good dielectric properties, such as high dielectric constant (1.5 × 105), low dielectric loss (0.1), and good dielectric temperature stability in the −40 °C to 100 °C range for the sample sintered below 1270 °C. The dielectric characteristics obtained with XPS confirmed that Ti4+ions remain in the state without any change. The huge increase in dielectric constant in NaNbO3doped BaTiO3samples occurs when large amount of Ba2+ions are excited to a high energy bound state of Ba2+− e or Ba+to create electron hopping conduction. For samples with the content of NaNbO3higher than 0.40 mol%, or sintering temperature higher than 1280 °C, compensation effect is dominated by cation vacancies with sharply decreasing dielectric constant and increased dielectric loss. The polaron effect is used to explain the relevant mechanism of giant dielectric constant appearing in the ferroelectric phase.

Computational Study of Excess Electron Mobility in High-Pressure Liquid Benzene

In recent years, excess electron transfer in organic liquids has attracted increasing interest owing to the emerging class of liquid organic semiconductors. In this study, to achieve a comprehensive understanding of electron conduction in liquids, we investigate hopping electron conduction in liquids from an atomistic viewpoint. High-pressure liquid benzene is chosen as a simple model system. Hopping electron mobility is computed using a combination of molecular dynamics simulations, quantum chemical calculations, and kinetic Monte Carlo methods. The computed electron mobility is in good agreement with the experimental values. Because the amplitude of the intermolecular vibration observed in liquids is larger compared to that in solids, the effect of dynamic disorder on electron mobility is investigated. The time scale of the change in electronic couplings due to the rotation of molecules is comparable to that of the electron residence time at each benzene molecule at the absence of change in the arrangement of a benzene dimer. Thus, the effect of dynamic disorder is evaluated by a Monte Carlo-based analysis. It is shown that the effect of dynamic disorder on electron mobility is small even though electronic couplings between molecules fluctuate by more than an order of magnitude.

Study of the magnetite to maghemite transition using microwave permittivity and permeability measurements

The microwave cavity perturbation (MCP) technique is used to identify the transition from magnetite (Fe3O4) to the meta-stable form of maghemite (γ-Fe2O3). In this study Fe3O4 was annealed at temperatures from 60 to 300 °C to vary the oxidation. Subsequent to annealing, the complex permittivity and magnetic permeability of the iron oxide powders were measured. The transition to γ-Fe2O3 was corroborated with x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and vibrating sample magnetometry (VSM). XRD, XPS and VSM implied that the starting powder was consistent with Fe3O4 and the powders annealed at more than 200 °C were transitioning to γ-Fe2O3. The MCP measurements gave large differences in both complex permittivity and magnetic permeability of the two phases in the frequency range of 2.5–10.2 GHz. Magnetic permeability decreased with annealing temperature, though magnetic losses showed frequency dependent behaviour. Complex permittivity measurements showed a large decrease in both dielectric constant and losses at all measurement frequencies, as well as a prominent loss peak centred around the phase transition temperatures. We interpret the loss peak as being a consequence of field effects due to an intermediate multi-phase mixture. Additionally, almost no frequency dependence was observed. The reduction in complex permittivity implies that the cations in the lattice provide a significant contribution to polarization at microwave frequencies and the effects of are nominal in comparison. The change in loss can be explained as a combination of the differences in the effective conductivity of the two phases (i.e. Fe3O4 exhibits electron-hopping conduction whereas the presence of vacancies in γ-Fe2O3 nullifies this). This shows that the non-invasive MCP measurements serve as a highly sensitive and versatile method for looking at this phase transition in iron and potentially the effects of oxidation states on the polarization in other iron oxides.

Zr-substituted SnO2-based NTC thermistors with wide application temperature range and high property stability

Zr-substituted SnO2-based ceramics (Sn0.95−x Sb0.05Zr x O2, x ≤ 0.1) were prepared by using a wet-chemical synthesis method. The results show that the prepared ceramics have a pure tetragonal phase as that of SnO2 and have a typical characteristic of negative temperature coefficient (NTC) of resistivity over a wide temperature range from −50 to 300 °C. The room-temperature resistivity ρ 25 and material constant B 25/85 of the NTC thermistors increase from 4.77 Ω cm to 4.89 × 107 Ω cm and from 641 to 5085 K, respectively, when the content of Zr, x, changes from 0 to 0.1. The NTC thermistors also show high cyclic stability and ageing resistance. The analysis of impedance spectra reveals that the NTC effect mainly resulted from the grain-boundary contribution, and the conduction mechanisms of the Sn0.95−x Sb0.05Zr x O2 thermistors combine with the electron-hopping conduction and band conduction.

Efficiency droop in GaN LEDs at high current densities: Tunneling leakage currents and incomplete lateral carrier localization in InGaN/GaN quantum wells

The phenomenon of the emission efficiency droop of InGaN/GaN quantum wells (QWs) in light-emitting diode p-n structures is studied. The influence exerted by two basic processes on the emission efficiency is considered: tunnel injection into a QW and incomplete lateral carrier localization in compositional fluctuations of the band-gap width in InGaN. The sharp efficiency peak at low currents and the rapid efficiency droop with increasing current are due to tunneling leakage currents along extended defects, which appear as a result of a local increase in the electron hopping conductivity via the depletion n region and a corresponding local decrease in the height of the injection p barrier. A less sharp efficiency peak and a weak, nearly linear, decrease in efficiency with increasing current are caused by incomplete lateral carrier localization in the QW due to slowing-down of the carrier energy-relaxation rate and to the nonradiative recombination of mobile carriers.

Dielectric and Ac Conductivity Studies of Mn-Doped Na1.86Li0.10K0.04Ti3O7Ceramics

Dielectric-spectroscopic and ac conductivity studies on 0.01 and 1.0 molar percentage manganese doped layered Na1.86Li0.10K0.04Ti3O7 ceramics have been reported. The dependence of loss tangent (tan δ) and relative permittivity (er) on temperature in the range 350 775 K and on frequency in the range 10 kHz 1 MHz have been undertaken. The losses are the characteristics of dipole mechanism, electrical conduction and space charge polarization. The obtained conductivity plots between log(σacT ) versus 1000/T have been divided into four regions namely region I, II, III, and IV. The mechanism of conduction in region I is acknowledged to electronic hopping conduction. The less frequency and more temperature dependent region II is ascribed as a mixed mechanism associated interlayer ionic conduction, electron hopping, and alkali ion hopping conduction . The unassociated interlayer ionic conduction along with alkali ion hopping conduction mechanisms are contributing to the transport process in the mid temperature region III. The mechanism of conduction in the highest temperature region IV may be recognized as the modi ed interlayer ionic conduction. The conductivity versus frequency curves lead to conclude that the electronic hopping conduction diminishes with the rise of temperature.

A Physics-Based Compact Model of Metal-Oxide-Based RRAM DC and AC Operations

A physics-based compact model of metal-oxide-based resistive-switching random access memory (RRAM) cell under dc and ac operation modes is presented. In this model, the conductive filament evolution corresponding to the resistive switching process is modeled by considering the transport behaviors of oxygen vacancies and oxygen ions together with the temperature effect. Both the metallic-like and electron hopping conduction transports are considered to model the conduction of RRAM. The model can reproduce both the typical I-V characteristics of RRAM in high-/low-resistance state (LRS) and the nonlinear characteristics in LRS. Moreover, to accurately model ac operation mode, the effects of parasitic capacitance and resistance are included in our model. The developed compact model is verified and calibrated by measured data in different HfOx-based RRAM devices under dc and ac operation modes. The excellent agreement between the model predictions and experimental results shows a promising prospect of the future implementation of this compact model in large-scale circuit simulation to optimize the design of RRAM.

FTIR and dielectric studies of nickel doped potassium hexa-titanate (K2Ti6O13) fine ceramics

The dielectric and electrical properties such as relative permittivity \((\varepsilon^{\prime } )\), loss tangent (tanδ), and ac conductivity (σac) have been studied in the temperature range 373–773 K at three different frequencies 100, 200, 400 kHz for doped potassium hexa-titanate (K2Ti6O13) samples. It was observed that dielectric constant and loss tangent decreases while ac conductivity increases with the increase in frequency. The temperature dependent relative permittivity showed a phase transition for all samples. Dielectric loss mechanism was observed to include space charge polarization and dipole orientation. Moreover, electron-hopping conduction was observed to be dominant in the low temperature region, whereas intratunnel ionic conduction prevailed at higher temperatures. Fourier transform infrared spectroscopy analysis was also carried out to identify the chemical bonds present in the specimens.

High-Performance Flexible ${\rm Ni}/{\rm Sm}_{2}{\rm O}_{3}/{\rm ITO}$ ReRAM Device for Low-Power Nonvolatile Memory Applications

A high-performance very low-power flexible Ni/Sm2O3/indium tin oxide (ITO) resistive device for nonvolatile memories is demonstrated. The device exhibits a good memory margin of >103 ON/OFF current ratio with very low switching power of 105 at 85°C and switching endurance of 104 program-read-erase-read cycles is achieved in the highly flexible Ni/Sm2O3/ITO device. The low-power switching operation is believed to be because of the low-energy electron hopping conduction via oxide defects.

EFFECTS OF COPPER DOPING ON DIELECTRIC AND A.C. CONDUCTIVITY IN LAYERED SODIUM TRI-TITANATE CERAMIC

Electron paramagnetic resonance (EPR) spectra of 0.01, 0.1 and 1.0 molar percentage (mp) of CuO doped derivatives of layered Na 2 Ti 3 O 7 ceramic have been reported. The results show that copper substitutes as Cu 2+ at Ti 4+ octahedral sites. From the dependence of loss tangent ( tan δ) and the relative permittivity (ε′) on temperature and frequency, it is concluded that all the derivatives are of polar nature. The relaxation peaks at lower temperatures have been attributed to the presence of different types of dipoles, whereas peaks in the higher temperature region indicate possible ferroelectric phase transition. The dependence of conductivity on temperature show that electron hopping (polaron) conduction exists in a wide span of temperature range. However, the associated interlayer ionic conduction exists in a small temperature range. Interlayer alkali ion hopping mechanism of conduction has been proposed toward higher temperatures. The conductivity versus frequency plots reveal that the polaron conduction plays a prominent role toward the lower temperature side that diminishes with the rise in temperature. The most probable relaxation times for 0.01 and 0.1 mp CuO doped derivatives are almost same but it records an increased value for 1.0 mp doped material. This again attributes to the possible change in the symmetry of copper environment.

A.C. Conductivity Investigations on Layered Na2-x-yLixKyTi3O7 Ceramics

Frequency and temperature dependence of a.c. electrical conductivity of layered mixed ionic alkali trititanates, Na1.89Li0.10K0.01Ti3O7, Na1.88Li0.10K0.02Ti3O7, Na1.86Li0.10K0.04Ti3O7, and Na1.85Li0.10K0.05Ti3O7, have been investigated over a wide temperature 350 K ≤T≥ 725 K and frequency 10 kHz to 1 MHz range. For this, Arrhenius plots are used for a.c. electrical conductivity of these compounds. The obtained conductivity plots have been divided into four distinct regions and discussed the relevant theory. According to slop variation, the conduction mechanisms occurring are different in different temperature regions. At lower temperatures, the hopping electron disorders the surroundings by moving to its neighboring Ti atoms from their equilibrium positions, causing structural defect in the polycrystalline network named small polaron. At higher temperatures, associated/unassociated interlayer ionic conduction occurs along with the alkali ions hopping through the interlayer space and electron hopping (small polaron) conduction through Ti–Ti chains in these layered polar alkali titanates.

NTC characteristic of SnSb0.05O2–BaTi0.8Fe0.2O3−δ composite materials

SnSb x O2 (x = 0.003, 0.01, 0.02, 0.05 and 0.07) ceramics and SnSb0.05O2–BaTi0.8Fe0.2O3-δ composite ceramics were prepared by using wet-chemical synthesis methods. The phases and related electrical properties of the ceramics were investigated. The results show that all the prepared ceramics have the effect of negative temperature coefficient (NTC) of resistivity over a wide temperature range. The room-temperature resistivities (ρ 25) and material constants (B 25/85) of the SnSb x O2 NTC ceramics increase with the Sb concentration increases. The B 25/85 of the ceramics can be enhanced obviously when a certain content of BaTi0.8Fe0.2O3-δ was added in SnSb x O2. The analysis of impedance spectra reveals that both grain and grain boundary contribute to the NTC effect of the ceramics. The conduction mechanisms combining the electron-hopping model and band conduction are proposed for the NTC effect in the SnSb0.05O2–BaTi0.8Fe0.2O3-δ composite ceramics.

Electrical and thermoelectric attributes of Ba2Co2Fe12−2x(Ti–Mn)xO22 and their catalytic activity for complete N2O decomposition

A series of Ti–Mn co-doped Ba2Co2Fe12−2x(Ti–Mn)xO22 (x = 0.2–1.0) Y type hexaferrites has been synthesized by sol–gel method. Single phase hexagonal crystal structure was confirmed by X-ray diffraction analysis and Rietveld refinement with a crystallite size of 30–54 nm of the particles. Ti–Mn doping was uniform in the grains and there was no effect on the microstructure. Alternating current conductivity and dielectric properties were studied over a range of frequencies (20 Hz–3 MHz) for all the compositions. The resistivity increased with the dopant content while the dielectric constant decreased due to valence alteration of Fe3+ ions from the octahedral site by the dopants. Thermoelectric studies along with direct current resistivity results established the electron hopping conduction mechanism in the doped ferrites. The doped hexaferrites were used for the first time as the catalysts for complete N2O decomposition at a low temperature of 873 K. The electronic properties along with surface vacancies were found to be responsible for their efficient catalytic activity. Stability of the prepared material for several hours at the reaction temperature added to its catalytic application.

Probing radiation damage by alternated current conductivity as a method to characterize electron hopping conduction in DNA molecules

Analysis of AC electrical conductivity of deoxyribonucleic acid (DNA) thin films, irradiated with ultraviolet (UV) light, revealed that electrical conduction arises from DNA chain electron hopping between base-pairs and phosphate groups. The hopping distance calculated from correlated barrier hopping model equals the distance between DNA base-pairs, which is consistent with the loss of conductivity with irradiation time arising from a decrease in phosphates groups. In the high frequency regime, at a given frequency, real part of conductivity strongly depends on irradiation time particularly for low dose levels suggesting the use of DNA based films for UV radiation sensors.