Showing papers on "Variable-range hopping published in 2020"

Observation of 2D Conduction in Ultrathin Germanium Arsenide Field-Effect Transistors

Abstract: We report the fabrication and electrical characterization of germanium arsenide (GeAs) field-effect transistors with ultrathin channels. The electrical transport is investigated in the 20–280 K tem...

Synthesis, Transfer, and Properties of Layered FeTe2 Nanocrystals.

Abstract: Different from layered two-dimensional (2D) transition metal dichalcogenides (TMDs), iron dichalcogenides crystallize in the most common three-dimensional pyrite or marcasite structures. Layered iron dichalcogenides are rarely reported and little is known about their structures and properties. Here, layered hexagonal phase iron ditelluride FeTe2 (h-FeTe2) nanocrystals are grown on mica by atmospheric pressure chemical vapor deposition (APCVD) method and are fully characterized by various methods. Like other 2D layered TMD materials, the FeTe2 nanoflakes exhibit regular hexagon, half hexagon, or triangle shapes with a controllable thickness of 6-95 nm and lateral length from a few to tens of micrometers. A simple and effective method is used to transfer the FeTe2 nanoflakes from the mica substrate onto any other substrates without quality deterioration by using polystyrene (PS) as a support polymer, which can also be operated in ethanol or ethylene glycol in a glovebox to avoid contact with water and air. Temperature-dependent electrical transport demonstrates that the FeTe2 nanoflake is a semiconductor with a variable range hopping (VRH) conduction, and its nonsaturated linear magnetoresistance (MR) reaches up to 10.4% under magnetic field of 9 T at 2 K, both probably due to its structure disorders. No signature of magnetic ordering is observed down to 2 K. The CVD growth of this layered FeTe2 represents an addition to the extensive library of 2D materials, particularly iron chalcogenides or alloys. Synthesis, properties, and even doping of phase pure h-FeTe2 call for further study in the future.

Transport phenomena of Cu–S–Te chalcogenide nanocomposites: frequency response and AC conductivity

Abstract: In this work, the development and electrical characterization of several chalcogenide nanocomposites have been reported. X-ray diffraction (XRD) has been used to reveal their microstructures. Mott's variable range hopping model has been used to interpret the DC conductivity data of the nanocomposites at lower temperatures. The DC conductivity data at higher temperatures has been explained well using Greave's model. To explain the AC conductivity data, the Meyer–Neldel (MN) conduction rule has been employed. The AC conductivity spectra at different temperatures have been analyzed using Almond–West formalism. Different conduction models, namely, correlated barrier hopping (CBH) and modified non-overlapping small polaron tunneling (NSPT), have been used to interpret the conduction mechanism of the nanocomposites. Scaling of the AC conductivity spectra reveals that the electrical relaxation process is independent of temperature, but depends on the nanocomposite composition. The conductivity mechanism is explained using a schematic structural model.

Structuraland Thermoelectric Properties of BlackArsenic–Phosphorus

Abstract: Here, we report the structural and temperature-dependent transport properties of AsxP1–x (x = 0, 0.2, 0.5, 0.83, and 1) alloys. It is observed that black phosphorous-related phonon modes in the all...

Usefulness of theoretical approaches and experiential conductivity measurements for understanding manganite-transport mechanisms

Abstract: Experimental measurements and numerous theoretical models have been employed to comprehend the dynamics of charge carriers in manganite system. The present work highlights the electrical transport of La0.5Ca0.4Ag0.1MnO3 compound in the temperature range [80 K–700 K]. It is found that the conductivity spectra of the studied sample present a double Jonscher variation, universal Jonscher evolution and Drude variation respectively for low, intermediate and high temperature ranges. The temperature dependence of the frequency exponent confirms the efficiency of the quantum mechanical tunneling, the non-overlapping small polaron tunneling and the correlated barrier hopping mechanisms at separated dispersive regions. DC conductivity analysis confirms the semiconductor behavior of the studied sample. Then, the transport properties have been explained in term of a thermally activated small polaron hopping and Mott-variable range hopping processes respectively at high and low temperature sides. Also, the disorder energy is predicted to decrease with rising frequency which could be due to variation in the separation path between the hop centers. Moreover, Greaves variable range hopping model is the suitable one that describes the transport properties in the intermediate temperature range. The scaling approach indicates that the time temperature superposition principle is valid in the temperature side [180 K–360 K].

Charge carrier transport and electrochemical stability of Li2O doped glassy ceramics

Abstract: We study electrical conductivity of new Li2O doped glassy ceramics in wide frequency and temperature regime not only for their applicability in various fields like lithium ion conductors but also for academic interest. Here, we employ “Jonscher’s power law model and Almond-West formalism” to interpret mixed conduction process in the present system. We use “Mott’s variable range hopping” model to analyse low temperature DC conductivity data. It also points that the ratio of power law pre-factor to the exponent (–log10 A/S) indicates temperature independency and strong composition dependency of present conductors. Higher lithium content in the composition may disturb “electrochemical stability” of the present system. We do believe that these glassy ceramics are suitable candidates for lithium ion battery application with lower lithium content.

SrTiO3/Epoxy Nanodielectrics as Bulk Energy Storage and Harvesting Systems: The Role of Conductivity

Abstract: Composite nanodielectric materials with strontium titanate nanoparticles embedded within an epoxy resin matrix were prepared and studied, varying the filler content. Broadband dielectric spectroscopy was employed for determining the dielectric response of the prepared specimens. Dielectric results reveal the presence of three relaxations processes, which are attributed to (a) glass to rubber transition of the polymer matrix (α-mode), (b) rearrangement of polar side groups (β-mode), and (c) interfacial polarization between systems’ constituents. The stored and harvested energy of the examined nanodielectrics was also evaluated under DC conditions in the applied voltage range of 10–240 V. The coefficient of energy efficiency (neff) was determined for all filler’s content, varying the applied temperature and field. The effect of temperature appears to be pronounced, causing a sigmoidal dependence of neff. Furthermore, filler content enhances neff, reaching the highest value of 69.41% for the 10 phr SrTiO3 nanocomposite at 50 V. Finally, both AC and DC conductivities were evaluated as a function of filler and temperature, and the corresponding activation energies were calculated. Hopping conductivity appears to be the predominant conduction mechanism in all nanocomposite systems, since experimental data are in accordance with variable range hopping model.

Low Field Vertical Charge Transport in the Channel and Buffer Layers of GaN-on-Si High Electron Mobility Transistors

Abstract: Substrate ramps and stepped stress transient measurements are applied to study vertical charge transport mechanisms in GaN-on-Si power HEMTs. By choosing appropriate bias points for substrate stress it is possible to single out the dominant charge transport mechanism: at low negative biases transport through carbon-doped GaN manifests itself in negative (decreasing) current transients with apparent activation energy ( $\text{E}_{\text {A}}) = {0.29}$ eV, while at larger negative voltages transport through unintentionally doped GaN is characterized by positive (increasing) current transients ( $\text{E}_{\text {A}} = {0.38}$ eV). We present experimental evidence for 3D variable range hopping taking place in C-doped GaN and 1D hopping along the dislocations in unintentionally doped GaN. By investigating transients obtained from bidirectional voltage steps of 10 V potential difference in the range 0 to −140 V, we observe that hopping transport through dislocations shows non-Ohmic behavior at low substrate biases, which manifests itself in a time constant $\tau $ strongly dependent on bias. We propose that this can be explained by the existence of a diode junction between the dislocation core and the 2D electron gas (2DEG).

Three-Dimensional Variable Range Hopping and Thermally Activated Conduction Mechanism of Polypyrrole/Zinc Cobalt Oxide Nanocomposites

Abstract: Zinc cobalt oxide (ZCO) nanoparticles and zinc cobalt oxide/polypyrrole (ZCO/PPy) nanocomposites were synthesized by hydrothermal and in situ chemical polymerization routes, respectively. X-ray dif...

Electrical characterization of GaN Schottky barrier diode at cryogenic temperatures

Abstract: In this report, electrical characteristics of the Ni/GaN Schottky barrier diode grown on sapphire have been investigated in the range of 20 K–300 K, using current–voltage, capacitance–voltage, and deep level transient spectroscopy (DLTS). A unified forward current model, namely a modified thermionic emission diffusion model, has been developed to explain the forward characteristics, especially in the regime with a large ideality factor. Three leakage current mechanisms and their applicability boundaries have been identified for various bias conditions and temperature ranges: Frenkel–Poole emission for temperatures above 110 K; variable range hopping (VRH) for 20 K–110 K, but with a reverse bias less than 20 V; high-field VRH, in a similar form of Fowler–Nordheim tunneling, for cryogenic temperatures below 110 K, and relatively large bias (>25 V). Four trap levels with their energy separations from the conduction band edge of 0.100 ± 0.030 eV, 0.300 eV, 0.311 eV, and 0.362 eV have been tagged together with their capture cross sections and trap concentrations. The significantly reduced DLTS signal at 100 K suggested that traps practically became inactive at cryogenic temperatures, thus greatly suppressing the trap-assisted carrier hopping effects.

Paramagnetism, hopping conduction, and weak localization in highly disordered pure and Dy-doped Bi2Se3 nanoplates

Abstract: Breaking the topological protection of surface states of topological insulators is an essential prerequisite for exploring their applications This is achievable by magnetic doping, in reduced dimensions, and predictably by introducing disorder beyond a critical level In certain cases, the former is also known to induce a transition from weak anti-localization (WAL) to weak localization (WL) Here, we report the occurrence of paramagnetism, hopping conduction, and WL in chemically prepared unannealed Dy xBi 2 − xSe 3 ( x = 0, 01, and 03) nanoplates primarily via dc magnetization, resistivity, and magnetoconductance measurements The paramagnetism in the magnetic-atom-free Bi 2Se 3 nanoplates is ascribed, using density functional theory calculations, to the acquisition of magnetic moments by defects The defect density in pure Bi 2Se 3 is estimated to be high ( ∼ 10 19 defects/cm 3) Successive Dy doping brings in further incremental disorder, apart from the Dy atomic moments The nanoplates are shown to sequentially exhibit thermally activated band conduction, nearest neighbor hopping, Mott variable range hopping (VRH), and Efros–Shklovskii VRH with decreasing temperature WL is evident from the observed positive magnetoconductance Annealing converts the WL behavior to WAL, arguably by setting in the topological protection on a substantial reduction of the disorder

Non-Ohmic Variable-Range Hopping and Resistive Switching in SrTiO 3 Domain Walls

Abstract: We report observation of electric field driven conductivity with negative differential conductance and resistive switching in insulating ${\mathrm{SrTiO}}_{3}$ samples over a wide range of applied voltages at low temperatures. The observed current follows $I={I}_{0}\mathrm{exp}[\ensuremath{-}{({E}^{*}/E)}^{1/2}]$ at large applied electric field, corresponding to variable range hopping conduction with a Coulomb gap in domain walls. Our data are sufficient to discriminate unambiguously between Shklovskii and Mott hopping via their different electric field exponent. Under some conditions space-charge-limited currents are observed, and the charge mobility limit is determined to be in the range of 17 and $210\text{ }\text{ }{\mathrm{cm}}^{2}/\mathrm{Vs}$.

Investigation of Structural, Optical and Electrical Transport Properties of Yttrium Doped La0.7Ca0.3MnO3 Perovskites

Abstract: In the present study, bulk samples of yttrium doped La0.7−xYxCa0.3MnO3 (x = 0.3 and 0.4) are prepared through solid-state reaction route. The structural, optical and electrical transport properties have been explored through various analytical techniques. The x-ray diffraction (XRD) patterns affirm single phase and polycrystalline nature of the samples. Rietveld refinement analysis of the XRD data is used to determine the various structural parameters. It reveals that both the samples belong to orthorhombic crystal system with Pnma space group. Using Scherrer’s equation, a lower value of crystallite size is found in the higher concentration of yttrium doped sample. The band gap as determined from the UV–visible DRS data by employing the Tauc’s relation is found to enhance with the increase of yttrium concentration in La0.7Ca0.3MnO3. The low temperature resistivity measurements establish semiconducting nature of the samples over the temperature range of ~ 110–300 K. Moreover, the electrical resistivity data are interpreted within the variable range hopping (VRH) model to estimate the density of states at the Fermi level N(EF), mean hopping distance Rh(T) and hopping energy Eh(T). It is observed that the electrical quantities vary gradually with the change of Y content. The conduction mechanism in this system is governed in the framework of thermally activated small polaron hopping (SPH) model and accordingly various parameters namely Debye temperature (θD), activation energy (Ep) and optical phonon frequency (νph) are evaluated. The obtained results support a strong connection between structure and electrical behavior in our samples and can be elucidated on the basis of localization of charge carriers induced by the substituting ions.

Miller-Abrahams random resistor network, Mott random walk and 2-scale homogenization

Abstract: The Miller-Abrahams (MA) random resistor network is given by a complete graph on a marked simple point process with edge conductivities depending on the marks and decaying exponentially in the edge length. As Mott random walk, it is an effective model to study Mott variable range hopping in amorphous solids as doped semiconductors. By using 2-scale homogenization we prove that a.s. the infinite volume conductivity of the MA resistor network is given by an effective homogenized matrix $D$. Moreover $D$ admits a variational characterization and equals the limiting diffusion matrix of Mott random walk. This result clarifies the relation between the two models and it also allows to extend to the MA resistor network the existing bounds on $D$ in agreement with the physical Mott law [12,14]. The latter concerns the low temperature stretched exponential decay of conductivity in amorphous solids. The techniques developed here can be applied to other models, as e.g. the random conductance model [11], without ellipticity assumptions.

Fabrication and electrical properties of reduced graphene oxide doped nanocrystalline vanadium pentoxide films

Abstract: In this work, the fabrication, structural and electrical properties of reduced graphene oxide (rGO) doped nanocrystalline vanadium pentoxide V2O5 films were studied. rGO was synthesized using modified Hummer method and confirmed by HRTEM imaging. XRD analysis revealed the structure of the prepared films as nanocrystals in highly oriented form toward c-axis normal to the surface of the substrate of the film. The XRD analysis also revealed that the (0 0 2) line is noticeable in the pure film but weakened by the addition of rGO, which indicates a layer of intercalation between the vanadium layers. The average particle size decreased with increasing rGO content from 4.86 nm to 3.17 nm. The activation energy was calculated and found to be increasing from 0.157 eV to 0.181 eV as rGO increases from 0 to 0.35 mol%. The vanadium ions concentration N were calculated and found to be increasing from 1.11 × 1022 cm−3 to 1.38 × 1022 cm−3, while the spacing of vanadium ions R decreased from 0.448 nm to 0.417 nm. The hopping carrier mobility (μ) and carrier density (Ne) of the prepared samples were also calculated at fixed temperature. Greaves model for variable range hopping was studied for the prepared samples.

Electrical conductivity and magnetoresistance in twisted graphene electrochemically decorated with Co particles

Abstract: Application of magnetic metal/graphene hybrid structures in magnetosensorics requires the formation of high-quality low-ohmic (barrier-free) contacts and understanding of mechanisms of electric charge transfer near and through the metal/graphene contact area. In present paper we fabricate samples of twisted graphene electrochemically decorated with Co particles (Co- G/SiO2) which demonstrate perfect ohmic electric contact between Co and graphene sheets. Temperature and magnetic field dependencies of surface resistance for pure twisted graphene (G/SiO2) and Co-G/SiO2 samples are considered within the models of 3D Mott variable range hopping and 2D weak-localization quantum corrections to the Drude conductivity . Phenomenological model is proposed explaining the experimentally observed transition from predominantly negative magnetoresistive effect in weak magnetic fields B (below 1–2 T) to positive magnetoresistance (PMR) at B beyond 5 T assuming the growth of PMR due to the distortion of current-conducting routes under the influence of Lorentz force which originates from the enhancement of large-scale potential relief in Co- G/SiO2 sample. This work considers the new approach to the application of G/SiO2 decoration with Co particles for creation both metallic (distributed, defragmented) shunts and high-quality ohmic electrodes in magnetic sensing.

Dielectric and impedance spectroscopy of Nd2CoIrO6 double perovskite.

Abstract: Polycrystalline Nd2CoIrO6 double perovskite crystallizes in monoclinic crystal structure with P21/n space group. The average grain size of powder sample is 400-500 nm. The dielectric, impedance and ac conductivity of the sample were studied in the temperature range 5-300 K and in the frequency range 20 Hz-2 MHz. Dielectric constant reveals a step like increase from low temperature value of ∼5 to colossal value of ∼104 at high temperature. High value of dielectric constant is associated with Maxwell-Wagner polarization due to large grain boundary capacitance. Cations (Co2+ and Ir4+) disorder leads to variable range hopping conduction of electrons in grain and grain boundary regions. Distribution of grain size induces distribution of relaxation time as confirmed from depressed semicircles in Nyquist plots. Frequency dependent conductivity follows universal power law behavior.

Large mobility modulation in ultrathin amorphous titanium oxide transistors

Abstract: Recently, ultrathin metal-oxide thin film transistors (TFTs) have shown very high on-off ratio and ultra-sharp subthreshold swing, making them promising candidates for applications beyond conventional large-area electronics. While the on-off operation in typical TFTs results primarily from the modulation of charge carrier density by gate voltage, the high on-off ratio in ultrathin oxide TFTs can be associated with a large carrier mobility modulation, whose origin remains unknown. We investigate 3.5 nm-thick TiOx-based ultrathin TFTs exhibiting on-off ratio of ~106, predominantly driven by ~6-decade gate-induced mobility modulation. The power law behavior of the mobility features two regimes, with a very high exponent at low gate voltages, unprecedented for oxide TFTs. We find that this phenomenon is well explained by the presence of high-density tail states near the conduction band edge, which supports carrier transport via variable range hopping. The observed two-exponent regimes reflect the bi-exponential distribution of the density of band-tail states. This improved understanding would be significant in fabricating high-performance ultrathin oxide devices. The origin of large mobility modulation in ultrathin oxide transistors, promising for their high on-off ratio, remains mostly unknown. Here, a 106 gate-induced mobility modulation in 3.5 nm-thick TiOx transistors is explained by a high density of tail states, mediating variable range hopping of carriers.