Change In Conduction Mechanism Research Articles

Current Transport and 1/f Noise Characteristics in Ferromagnetic Permalloy/n-type Ge Schottky Contacts

The current transport mechanism in permalloy/n-type Ge Schottky diodes was studied over the temperature range from 200 to 400 K. At temperatures above 250 K, the forward current-voltage (I-V) characteristics of the diode were ideal and obeyed the thermionic emission theory. Below 250 K, however, the recombination process was found to contribute to current transport. Similarly, in reverse bias, the thermionic emission mechanism appeared to dominate current transport at temperatures above 250 K, and the carrier generation mechanism dominated the reverse current below 250 K. A temperature-driven change in the current conduction mechanism from conduction dominated by low-barrier-height patches to conduction dominated by high-barrier-height regions suggests inhomogeneity in the Schottky barrier height. The barrier height inhomogeneity led to deviations in the Richardson constant from its theoretical value at lower temperatures. The room-temperature low-frequency noise measurements taken at different forward biases for the permalloy/n-type Ge Schottky diodes showed a 1/fγ dependence with a tight variation of γ between 1.20 and 1.31. The current dependence of the noise power spectral density exhibited a 1/f noise behavior, indicating the operation of the permalloy/n-type Ge Schottky diodes in the thermionic emission mode.

Study of structural, electrical and magnetic properties of 1−x(Ba0.96Ca0.04TiO3)−x(BiFeO3) ceramics composites

Lead free 1−x(Ba0.96Ca0.04TiO3)−x(BiFeO3) (abbreviated as BCT–BF) ceramics were prepared by solid state reaction method. The structural studies were carried out by using XRD technique, which confirms that the samples have a tetragonal, with the cubic spinel ferrite phase at room temperature. Dielectric and impedance properties of BCT–BF ceramics have been characterized at different temperature and in 1 kHz–1 MHz frequency range. The complex impedance plot exhibited one semi-circle at higher temperatures. Centers of the semicircles lie below the real axis, which confirms that the material is the non-Debye type. Single semicircle indicating that bulk grain effect are dominated in system. Bulk resistance decreased with the increase of temperature, which indicating the negative temperature coefficient of resistance behavior in our samples. The activation energy determined from an Arrhenius type plot of conductivity (at 1 kHz) was also found to increase from 0.22 eV (x = 0.10) to 1.41 eV (x = 0.30), which is close to half of the optical band gap, suggesting a change in conduction mechanism due to extrinsic defects.

Investigation of charge transport mechanism in semiconducting La0.5Ca0.5Mn0.5Fe0.5O3 manganite prepared by sol–gel method

Here in, we report the charge transport mechanism in semiconducting La0.5Ca0.5Mn0.5Fe0.5O3 (LCMFO) polycrystalline material synthesized via sol–gel auto combustion route. X-ray diffraction (XRD) analysis confirmed the orthorhombic phase of the prepared material. Temperature dependent resistivity and impedance spectroscopy measurements have been carried out to probe the dielectric and electrical conduction mechanism which revealed a change of Mott variable range to the small polaronic hopping conduction mechanism around 303 K. The complex impedance and modulus spectra undoubtedly showed the contribution of both grain and grain boundary effect on the conduction properties of LCMFO. An equivalent circuit [(RgbQgb) (RgQg)] model has been used to address the electrical parameters associated with the different phases (grains and grain boundaries) having different relaxation times. The values of resistances of two phases obtained after fitting the equivalent circuit in the nyquist plot have been analyzed which confirmed the change of conduction mechanism around 303 K. The resultant change in conduction mechanism is also supported by the conductivity plots.

Electric field effect on variable-range hopping conductivity in Bi1.9Lu0.1Te3

Electric field effect on low-temperature electrical resistivity of n-type Bi1.9Lu0.1Te3 was examined and analyzed. It was found that with decreasing temperature from 35 down to 2.5 K, the conductivity mechanism changes from “metal” type to “semiconductor” one resulting in appearance of minimum in the specific electrical resistivity, ρ. The temperature of this minimum, Tm, depends on electric field strength, E. At low electric field of 0.15 Vˑm−1 this temperature is equal to ∼11 K and decreases as E increases up to 4.65 Vˑm−1. Variable-range hopping (VRH) conductivity mechanism was applied to explain the ρ(T, E) behavior below Tm. Average hopping distance estimated from the experimental ρ(T, E) dependences decreases as temperature or electric field strength increase, that is the VRH conductivity can be activated both temperature and electric field. It was also found that density of states at the Fermi level rapidly enhances as electric field increases. This enhancement could be attributed to high and sharp density of states within an impurity band formed from the electronic f-levels of Lu. The forming of this impurity band is believed to be responsible for enhancement of the thermoelectric figure-of-merit of Bi2Te3.

Effect of SiO2/B2O3 Ratio on the Crystallization Behavior and Dielectric Properties of Barium Strontium Titanate Glass–Ceramics Prepared by Sol–Gel Process

Ferroelectric glass–ceramics, with a basic composition 90 wt.% (Ba0.65Sr0.35)TiO3−10 wt.% (B2O3−nSiO2) (n = 0.5, 1, 3, 5) were synthesized by the sol–gel method and their phase development and dielectric properties were investigated by differential thermal analysis, x-ray diffraction, field emission scanning electron microscopy, dielectric temperature curves and impedance spectroscopy. From the differential thermal analysis, glass transition and crystallization behavior can be observed. From the x-ray diffraction study, two crystalline phases (Ba,Sr)TiO3 and Ba2TiSi2O8 were formed over the entire composition range of the glass–ceramics. In addition, the main crystal phase has undergone a transformation from (Ba,Sr)TiO3 to Ba2TiSi2O8 with the increase of n. A typical structure in which the crystal phase was surrounded by a glassy matrix has been observed in the scanning electron microscope images. As a result of temperature dependent dielectric property measurements, the dielectric constant increased obviously with the increase of n from 0.5 to 1. Further increasing n led to a reduction of the dielectric constant, which is in coincidence with the variation of the intensity of (Ba,Sr)TiO3 phase with n. According to the impedance spectroscopy analysis and the activation energy calculation, the relaxation peak in both Z″ and M″ data should be attributed to the crystal–glass interface, and the change of conduction mechanism with the increase of SiO2/B2O3 ratio may be attributed to the corresponding transition of the main crystal phase.

Structural and electrical properties of Cr-doped SrTiO3 porous materials

Series of single-phase materials with assumed formula SrTi1−xCrxO3 (where x = 0, 1, 4, 6 mol.%) were obtained by sol-gel method. The structure and microstructure of materials were characterised by X-ray diffraction and scanning electron microscopy methods. Moreover, the study of electrical properties and evaluation of chemical stability in CO2/H2O atmosphere was performed by electrochemical impedance spectroscopy and thermogravimery methods, respectively. The possibility of participation of Cr-doped strontium titanate in oxidation–reduction processes was analysed by temperature-programed reduction (TPR) and temperature-programed oxidation (TPOx) measurements. The changes of lattice parameters together with XPS analysis, the Seebeck coefficient measurements results and TPR profiles obtained for SrTi1−xCrxO3 materials prove the presence of chromium on +3 and +6 oxidation stages. Thus, chromium can be treated as both acceptor- and donor-type dopant in the SrTiO3 structure. The Cr3+/Cr6+ ratio strongly affects the electrical properties, as the change of conduction mechanism was observed. The results of performed stability test clearly indicate that incorporation of chromium into SrTiO3 structure results with decrease of chemical stability in CO2 atmosphere.

Effect of doping of alkaline metal ions on structural and electrical properties of Bi0.8M0.2FeO3-modified Na0.5Bi0.5TiO3 ceramics (M=Ca, Sr, and Ba)

In the present work the electrical properties of polycrystalline 0.9(Na0.5Bi0.5TiO3)–0.1(BiFeO3) and Bi0.8M0.2FeO3-modified Na0.5Bi0.5TiO3, (where M = Ca, Sr, and Ba) lead-free ceramics were investigated through impedance spectroscopy. X-ray diffraction analyses indicate the incorporation of alkaline earth metal ions at A-site of perovskite type ceramics revealing the conservation of crystal structure (rhombohedral structure with R3c space group) in the synthesized compositions. The substitution of alkaline earth metal ions at the A-site in perovskite-structured ceramics led to an enlargement of unit cell. The appearance of depressed semicircle arcs in Nyquist plots suggests non-Debye nature of relaxation occurring in studied samples. An equivalent circuit model comprising of parallel combination of resistance and constant phase element was used for the simulation of Nyquist plots. The mismatching of peaks in the combined spectroscopic plots of Z″ and M″ versus frequency also confirms the occurrence of non-Debye relaxation in BBFO sample. The ac conductivity studies show that the conduction mechanism changes from correlated barrier hopping model (CBH) to overlapping large polaron tunneling, when alkaline metal ions are substituted in place of Bi3+ ions. Activation energy values calculated from conductivity data suggest the presence of oxygen vacancies. The dielectric, impedance and conductivity behaviors are significantly influenced by the distortion of Ti/FeO6 octahedron.

Variable-range hopping conductivity in Lu-doped Bi2Te3

The Seebeck coefficient enhancement due to an increase of density-of-states effective mass of electron has been found in n-type Bi1.9Lu0.1Te3. This enhancement is assumed to be related to forming the narrow and non-parabolic impurity (Lu) band with local maximum of electronic density of states lying near the Fermi level. Minimum in the specific electrical resistivity, ρ, originated from change of conductivity mechanism was observed at temperature Tm ≈ 11 K. Above Tm, the ρ change is due to decrease of electron mobility via acoustic phonon scattering. Below Tm, the variable-range hopping conductivity takes place. The electron hops between the localized states of the impurity energy band occur via tunneling process. Using the temperature and magnetic field dependences of ρ, the localization radius of electron was estimated as ≈6 nm. Two parts in the magnetic field dependence of the electrical resistivity were found at temperature of 2 K. At weak magnetic fields, the ρ change is in agreement with the variable-range hopping conductivity mechanism. At high magnetic fields, the positive and almost linear transverse and longitudinal magnetoresistances were observed at low temperatures. Both variable-range hopping conductivity and positive linear magnetoresistance are characteristics of disordered and inhomogeneous semiconductors.

Bipolar resistive switching with coexistence of mem-elements in the spray deposited CoFe2O4 thin film

In the present investigation, we have experimentally demonstrated the bipolar resistive switching with the coexistence of three fundamental memelements in the Ag/CoFe2O4/FTO thin film metal-insulator-metal (MIM) device. The device shows the analog resistive switching behavior and charge transport follows the Ohmic and space charge limited conduction (SCLC) mechanisms. The device transforms from asymmetric to symmetric resistive switching when the SCLC conduction mechanism change to the Ohmic conduction mechanism at higher voltage sweep rates. It was observed that the I–V crossing location of MIM device shifted towards the higher voltage range with increasing voltage sweep rates for both bias regions due to the nanobattery effect. The significant tunneling gap between immature conductive filament(s) and percolation channels was responsible for the coexistence of memelements and nanobattery effect in the Ag/CoFe2O4/FTO thin film MIM device.

Study of the structural, impedance spectroscopy and dielectric properties of Na and Si co-doped NiO ceramics

Sodium and silicon co-doped NiO (NSNO) of the formula NaxSiyNi1 – x−yO (x = 0.05, 0.1 and y = 0.02, 0.05) were prepared by solid state reaction. Structural, electrical conduction and dielectric properties of the NSNO ceramics were investigated. The current NSNO ceramics exhibited giant dielectric constant (ε′ ~ 104 at 1.1 KHz and 300 K) and three dielectric relaxation peaks were found on the curve of the frequency dependence of the imaginary part of electric modulus M″. The complex impedance diagrams showed three electrical responses, which have been attributed to the domain, domain-boundaries and grain-boundaries in the NSNO ceramics. Analysis of the temperature dependence of the conductivity for the current ceramics indicated that the electrical conduction mechanism changes from variable range hoping at low temperatures (T < 220 K) to nearest neighbor hopping mechanism at higher temperatures. The electrically heterogeneous structure, revealed by the complex impedance analysis, combined with considerable polarization due to defect dipoles are thought to be the cause for the giant dielectric response of NSNO ceramics.

Surface sensitivity of four-probe STM resistivity measurements of bulk ZnO correlated to XPS

Multi-probe instruments based on scanning tunnelling microscopy (STM) are becoming increasingly common for their ability to perform nano- to atomic-scale investigations of nanostructures, surfaces and in situ reactions. A common configuration is the four-probe STM often coupled with in situ scanning electron microscopy (SEM) that allows precise positioning of the probes onto surfaces and nanostructures enabling electrical and scanning experiments to be performed on highly localised regions of the sample. In this paper, we assess the sensitivity of four-probe STM for in-line resistivity measurements of the bulk ZnO surface. The measurements allow comparisons to established models that are used to relate light plasma treatments (O and H) of the surfaces to the resistivity measurements. The results are correlated to x-ray photoelectron spectroscopy (XPS) and show that four-probe STM can detect changes in surface and bulk conduction mechanisms that are beyond conventional monochromatic XPS.

Conduction mechanism change with transport oxide layer thickness in oxide hetero-interface diode

An effective and facile strategy is proposed to demonstrate an engineered oxide hetero-interface of a thin film diode with a high current density and low operating voltage. The electrical characteristics of an oxide hetero-interface thin film diode are governed by two theoretical models: the space charge-limited current model and the Fowler-Nordheim (F-N) tunneling model. Interestingly, the dominant mechanism strongly depends on the insulator thickness, and the mechanism change occurs at a critical thickness. This paper shows that conduction mechanisms of oxide hetero-interface thin film diodes depend on thicknesses of transport oxide layers and that current densities of these can be exponentially increased through quantum tunneling in the diodes with the thicknesses less than 10 nm. These oxide hetero-interface diodes have great potential for low-powered transparent nanoscale applications.

Structure and Electrical-Transport Relations in Ba(Zr,Pr)O3-δ Perovskites.

Members of the perovskite solid solution BaZr1-xPrxO3-δ (0.2 ≤ x ≤ 0.8) with potential high-temperature electrochemical applications were synthesized via mechanical activation and high-temperature annealing at 1250 °C. Structural properties were examined by Rietveld analysis of neutron powder diffraction and Raman spectroscopy at room temperature, indicating rhombohedral symmetry (space group R3̅c) for members x = 0.2 and 0.4 and orthorhombic symmetry (Imma) for x = 0.6 and 0.8. The sequence of phase transitions for the complete solid solution from BaZrO3 to BaPrO3 is Pm3̅m → R3̅c → Imma → Pnma. The structural data indicate that Pr principally exists as Pr4+ on the B site and that oxygen content increases with higher Pr content. Electrical-conductivity measurements in the temperature range of 250-900 °C in dry and humidified (pH2O ≈ 0.03 atm) N2 and O2 atmospheres revealed an increase of total conductivity by over 2 orders of magnitude in dry conditions from x = 0.2 to x = 0.8 (σ ≈ 0.08 S cm-1 at 920 °C in dry O2 for x = 0.8). The conductivity for Pr contents x > 0.2 is attributable to positively charged electronic carriers, whereas for x = 0.2 transport in dry conditions is n-type. The change in conduction mechanism with composition is proposed to arise from the compensation regime for minor amounts of BaO loss changing from predominantly partitioning of Pr on the A site to vacancy formation with increasing Pr content. Conductivity is lower in wet conditions for x > 0.2 indicating that the positive defects are, to a large extent, charge compensated by less mobile protonic species. In contrast, the transport mechanism of the Zr-rich composition (x = 0.2), with much lower electronic conductivity, is essentially independent of moisture content.

Crystalline-like temperature dependence of the electrical characteristics in amorphous Indium-Gallium-Zinc-Oxide thin film transistors

A crystalline-like temperature dependence of the electrical characteristics of amorphous Indium-Gallium-Zinc-Oxide (a-IGZO) thin film transistors (TFTs) is reported, in which the drain current reduces as the temperature is increased. This behavior appears for values of drain and gate voltages above which a change in the predominant conduction mechanism occurs. After studying the possible conduction mechanisms, it was determined that, for gate and drain voltages below these values, hopping is the predominant mechanism with the current increasing with temperature, while for values above, the predominant conduction mechanism becomes percolation in the conduction band or band conduction and IDS reduces as the temperature increases. It was determined that this behavior appears, when the effect of trapping is reduced, either by varying the density of states, their characteristic energy or both. Simulations were used to further confirm the causes of the observed behavior.

Effects of temperature on conduction mechanism, ac electrical and dielectric properties of NdFe0.9Ni0.1O3 by employing impedance spectroscopy

In the present work, the effects of temperature and frequency on the ac electrical and dielectric properties of polycrystalline NdFe0.9Ni0.1O3, prepared by solid-state reaction method, are investigated. From the impedance spectroscopic measurements, three relaxation processes are observed, which are related to grains, grain boundaries and electrode-semiconductor contacts in the measured temperature and frequency range. For NdFe0.9Ni0.1O3 sample, the applicable equivalent circuit model is (R g Q g)(R gb Q gb)(R c Q c) in the measured temperature range. Decrease in resistances and relaxation times of the grains and grain boundaries with temperature confirm the involvement of thermally activated conduction mechanisms. With the increase in temperature, the relaxation frequencies increase for all three processes and the conduction mechanism changes from Mott variable range hole hopping to adiabatic small polaronic hole hopping in this material. The permittivity of grains is from 8 to 10, while the high permittivity observed at higher temperatures is extrinsic (due to Maxwell-Wagner type polarizations), which is primarily due to the formation of different Schottky barriers. The grain boundaries, ceramic surfaces and electrode-semiconductor contacts mainly contribute in the colossal value of permittivity of NdFe0.9Ni0.1O3. The ac conductivity plots show the positive values of the ‘dσ/dT’ which indicate that the charge carriers are localized. The results of conductivity signify that the delocalization of charge carriers augment with increase in the temperature in this system.

Impedance spectroscopy study of Bi 0.5 (Na 0.74 K 0.16 Li 0.10 ) 0.5 TiO 3 -Ba(Zr 0.05 Ti 0.95 )O 3 ceramics prepared via combustion technique

Abstract Lead free (1- x )Bi 0.5 (Na 0.74 K 0.16 Li 0.10 ) 0.5 TiO 3 - x Ba(Zr 0.05 Ti 0.95 )O 3 or BNKLT-100 x BZT, 0.025 ≤ x ≤ 0.125, ceramics with single-phase perovskite structure were prepared via a combustion technique. Impedance spectroscopy was conducted over a wide temperature and frequency range in order to investigate the influences of BZT on electrical properties and conduction mechanisms. Analysis using impedance complex plane plots combined with spectroscopic plots of the imaginary component of the impedance and electric modulus revealed an electrically homogeneous microstructure with increasing BZT content. With the increase in BZT content, the resistivity increased from ~10 6 Ω-cm at x = 0.025 to ~10 7 Ω-cm at x = 0.125. The activation energy determined from an Arrhenius-type plot of conductivity was also found to increase from 0.97 eV ( x = 0.025) to 1.62 eV ( x = 0.125), which is close to half of the optical bandgap, suggesting a change in conduction mechanism towards an intrinsic electronic conduction mechanism. IS measurement under different oxygen partial pressure was also performed on selected sample. The possible defect chemistry conditions responsible for the observed electrical properties will also be discussed.

Dielectric and electric relaxations induced by the complex defect clusters in (Yb+Nb) co‐doped rutile TiO2 ceramics

AbstractThe effect of the Yb+Nb substitution for Ti on the microstructure, crystal structures, and dielectric properties of (Yb1/2Nb1/2)xTi1−xO2 (0.01≤x≤0.1) ceramics is investigated in this study. The results reveal that the solid solubility limit of the (Yb1/2Nb1/2)xTi1−xO2 ceramics is x=0.07, and the average grain sizes considerably decrease from 12 μm to 6 μm with x increasing from 0.01 to 0.1. Three types of dielectric relaxations are observed at temperature ranges of 10‐30 K, 80‐180 K, and 260‐300 K, caused by the electron‐pinned defect dipoles, polaron hopping, and interfacial polarizations, respectively. The conduction mechanism changes from nearest‐neighbor‐hopping to polaron hopping mechanism, which is confirmed by ac conductivity measurements. The present work indentifies the correlation between the colossal permittivity and polaron hopping process in the titled compound.

Influence of Ammonia on Amorphous Carbon Resistive Random Access Memory

This letter investigates the influence of ammonia on amorphous carbon resistance random access memory by sputtering the carbon target with argon and ammonia mixed gas. The device fabricated with ammonia (C (NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> )-RRAM) showed remarkable improvement in memory window and high resistance state as compared to the device deposited without ammonia. Material analysis confirmed the absorption signal of amine. Current fitting indicated the conduction mechanism changes from hopping conduction to Schottky conduction with the addition of ammonia. Finally, we propose a model to explain the influence of ammonia on the resistive switching behaviors of the amorphous carbon RRAM.

Pressure dependence of electrical conductivity in forsterite

AbstractElectrical conductivity of dry forsterite has been measured in muli‐anvil apparatus to investigate the pressure dependence of ionic conduction in forsterite. The starting materials for the conductivity experiments were a synthetic forsterite single crystal and a sintered forsterite aggregate synthesized from oxide mixture. Electrical conductivities were measured at 3.5, 6.7, 9.6, 12.1, and 14.9 GPa between 1300 and 2100 K. In the measured temperature range, the conductivity of single crystal forsterite decreases in the order of [001], [010], and [100]. In all cases, the conductivity decreases with increasing pressure and then becomes nearly constant for [100] and [001] and slightly increases above 7 GPa for [010] orientations and a polycrystalline forsterite sample. Pressure dependence of forsterite conductivity was considered as a change of the dominant conduction mechanism composed of migration of both magnesium and oxygen vacancies in forsterite. The activation energy (ΔE) and activation volume (ΔV) for ionic conduction due to migration of Mg vacancy were 1.8–2.7 eV and 5–19 cm3/mol, respectively, and for that due to O vacancy were 2.2–3.1 eV and −1.1 to 0.3 cm3/mol, respectively. The olivine conductivity model combined with small polaron conduction suggests that the most part of the upper mantle is controlled by ionic conduction rather than small polaron conduction. The previously observed negative pressure dependence of the conductivity of olivine with low iron content (Fo90) can be explained by ionic conduction due to migration of Mg vacancies, which has a large positive activation volume.

Advancing the Understanding of Reverse Breakdown in Cu(In,Ga)Se2 Solar Cells

Reverse breakdown is investigated in multiple Cu(In,Ga)Se2 solar cells with varying buffer layer thicknesses. A method to extract transition voltage, which marks the change of conduction mechanism that leads to electrical breakdown, is described as an alternative to the often less-meaningful breakdown voltage. Transition voltages for samples with CdS and Zn x Sn1- x O y buffers are extracted from breakdown measurements performed in darkness and under illumination. The electric field is calculated for ZTO-based samples measured in darkness, and its implications for the energy band structure are examined. Fowler–Nordheim tunneling and Poole–Frenkel conduction are considered as candidates for the main breakdown mechanism in darkness. A model combining the two conduction mechanisms is proposed, and fits for experimental data are presented and discussed. Involvement of defects is debated, and defect-and-breakdown-related phenomena are showcased.