Mott Variable-range Hopping Conduction Research Articles

Electrical Conduction Mechanism of β‐MnTe Thin Film with Wurtzite‐Type Structure Using Radiofrequency Magnetron Sputtering

Manganese telluride (MnTe) compound is known to be a polymorphic chalcogenide. Recently, it has been reported that the MnTe shows nonvolatile memory properties with a significant change in resistance via a polymorphic transition between NiAs‐type (NC) structure (low resistance) and wurtzite‐type (WZ) structure (high resistance). This crystalline polymorphic MnTe is expected to realize a phase‐change memory with fast operation speed and ultralow operation energy. While the NC‐MnTe, generally designated as α‐MnTe, is intensively studied, WZ‐MnTe is still poorly understood. Herein this study, electrical conduction mechanism of a β‐MnTe film with a WZ‐type structure is studied. A resistivity, Hall mobility, and Seebeck coefficient of the WZ‐MnTe film are measured at various temperatures. The temperature dependence of resistivity in the temperature range 120–300 K clearly indicates that the WZ‐MnTe film shows a variable‐range hopping (VRH) conduction. In this temperature region, with decreasing temperature, the conduction mechanism changes from Mott–VRH conduction to Efros–Shklovskii VRH conduction at about 210 K. Furthermore, the low thermally activated Hall mobility, occurrence of Hall‐effect sign anomaly, and relatively low activation energy for thermopower, which are the observed results, suggest that the small polaron hopping conduction is dominant above 310 K.

Charge transport mechanisms in inkjet-printed thin-film transistors based on two-dimensional materials

Printed electronics using inks based on graphene and other two-dimensional materials can be used to create large-scale, flexible and wearable devices. However, the complexity of ink formulations and the polycrystalline nature of the resulting thin films have made it difficult to examine charge transport in such devices. Here we report the charge transport mechanisms of surfactant- and solvent-free inkjet-printed thin-film devices based on few-layer graphene (semimetal), molybdenum disulfide (MoS2, semiconductor) and titanium carbide MXene (Ti3C2, metal) by investigating the temperature, gate and magnetic-field dependencies of their electrical conductivity. We find that charge transport in printed few-layer MXene and MoS2 devices is dominated by the intrinsic transport mechanism of the constituent flakes: MXene exhibits a weakly localized 2D metallic behaviour at any temperature, whereas MoS2 behaves as an insulator with a crossover from 3D Mott variable-range hopping to nearest-neighbour hopping around 200 K. Charge transport in printed few-layer graphene devices is dominated by the transport mechanism between different flakes, which exhibit 3D Mott variable-range hopping conduction at any temperature.

LaMn3Rh4O12: An Antiferromagnetic Quadruple Perovskite Synthesized at High Pressure.

A quadruple perovskite LaMn3Rh4O12 with A' = Mn and B = 4d transition metal was synthesized at high pressure (8 GPa) and temperature (1423 K) for the first time. Room temperature powder X-ray diffraction indicates that LaMn3Rh4O12 forms in cubic symmetry (Im3̅, a = 7.4997(1) Å). X-ray absorption near-edge spectroscopy shows predominantly Mn3+ and Rh3+ oxidation states. An antiferromagnetic transition at TN ∼ 41 K is corroborated by specific heat measurements. The resistivity measurements indicate a three-dimensional Mott variable-range hopping conduction mechanism between 300 and 160 K.

Magnetotransport and conductivity mechanisms in Cu2ZnSnxGe1\u2212xS4 single crystals

Resistivity, ρ(T), and magnetoresistance (MR) are investigated in the Cu2ZnSnxGe1−xS4 single crystals, obtained by the chemical vapor transport method, between x = 0–0.70, in the temperature range of T ~ 50–300 K in pulsed magnetic field of B up to 20 T. The Mott variable-range hopping (VRH) conductivity is observed within broad temperature intervals, lying inside that of T ~ 80–180 K for different x. The nearest-neighbor hopping conductivity and the charge transfer, connected to activation of holes into the delocalized states of the acceptor band, are identified above and below the Mott VRH conduction domain, respectively. The microscopic electronic parameters, including width of the acceptor band, the localization radius and the density of the localized states at the Fermi level, as well as the acceptor concentration and the critical concentration of the metal-insulator transition, are obtained with the analysis of the ρ(T) and MR data. All the parameters above exhibit extremums near x = 0.13, which are attributable mainly to the transition from the stannite crystal structure at x = 0 to the kesterite-like structure near x = 0.13. The detailed analysis of the activation energy in the low-temperature interval permitted estimations of contributions from different crystal phases of the border compounds into the alloy structure at different compositions.

Mechanisms of charge transfer and electronic properties of Cu2ZnGeS4 from investigations of the high-field magnetotransport

Recent development of the thin film solar cells, based on quaternary compounds, has been focused on the Ge contain compounds and their solid solutions. However, for effective utilization of Cu2ZnGeS4, deeper investigations of its transport properties are required. In the present manuscript, we investigate resistivity, ρ (T), magnetoresistance and Hall effect in p-type Cu2ZnGeS4 single crystals in pulsed magnetic fields up to 20 T. The dependence of ρ (T) in zero magnetic field is described by the Mott type of the variable-range hopping (VRH) charge transfer mechanism within a broad temperature interval of ~100–200 K. Magnetoresistance contains the positive and negative components, which are interpreted by the common reasons of doped semiconductors. On the other hand, a joint analysis of the resistivity and magnetoresistance data has yielded series of important electronic parameters and permitted specification of the Cu2ZnGeS4 conductivity mechanisms outside the temperature intervals of the Mott VRH conduction. The Hall coefficient is negative, exhibiting an exponential dependence on temperature, which is quite close to that of ρ(T). This is typical of the Hall effect in the domain of the VRH charge transfer.

Temperature-Dependent Electrical Characterization of Amorphous Indium Zinc Oxide Thin-Film Transistors

In this study, N-type amorphous indium zinc oxide thin-film transistors are fabricated and temperature-dependent electrical characteristics in the range of 170–295 K are analyzed through experimental measurements and using an equivalent-circuit model. In this model, thermionic field emission for reverse bias and a thermionic emission mechanism for forward bias are applied. The barrier height coefficient of a contact region between the channel and the Ti/Au electrode is 1.26 meV/K, and the resistance of the channel material decreases at a rate of $- 0.39 \Omega \cdot {\mathrm{ K}}^{-1}$ at various temperatures. The obtained energy level is experimentally confirmed through a Kelvin probe measurement. In addition, the simulation results of the channel resistance successfully describe the Arrhenius behavior of the drain current and the Mott variable-range hopping conduction mechanism in a low-temperature regime below 230 K.

Magnetotransport of Cu2ZnSnS4 single crystals in two regimes of variable–range hopping conduction

The resistivity, ρ(T), and the magnetoresistance (MR) of Cu2ZnSnS4 (CZTS) single crystals are investigated at temperatures T = 2–300 K in pulsed magnetic fields of B up to 20 T. The Mott variable–range hopping (VRH) conductivity over localized states of the defect acceptor band is observed between T ~ 50–150 K. The Shklovskii–Efros (SE) VRH conduction over the states of the Coulomb gap is found below T ~ 3–4 K. The positive MR is observed at all temperatures and magnetic fields, its value decreasing with T. In the Mott VRH conduction region, MR follows the law ln ρ(B) ∝ B 2 up to the highest applied fields. The joint analysis of the resistivity and MR data in this region has yielded values of the localization radius as well as a set of important microscopic parameters, including the mobility threshold in the acceptor band, the values of the density of localized states near the Fermi level and the critical concentration of the metal–insulator transition. In the SE region, the MR law above is observed only in much smaller fields, transformed into those of lnρ(B) ∝ B 2/3 or ∝ B 3/4 when B increases. Such transformation, accompanied by a strong increase of the localization radius, give evidence for an important role of scattering and interference phenomena in the VRH conduction at low temperatures.

Transition between Efros–Shklovskii and Mott variable-range hopping conduction in polycrystalline germanium thin films

We report on the electrical transport properties of polycrystalline germanium thin films which are grown by the DC magnetron sputtering method. The temperature dependent resistance of seven devices are measured from 290 K down to 10 K. The thermal excitation model dominating the transport properties at the high temperature regime (above ∼60 K) is demonstrated and the low temperature electron transport is governed by the variable-range hopping (VRH) mechanism. Moreover, we observed a transition from Efros–Shklovskii to Mott VRH at ∼25 K over the entire VRH conduction regime, which is well described by a universal scaling law.

Structural, Optical and Electrical Conductivity Properties of Stannite Cu2ZnSnS4

A precursor powder was obtained from drying the solutions of a mixture of different ratios of Cu, Zn and Sn chloride and thiourea. The Cu2ZnSnS4 (CZTS) samples were prepared from thermal decomposition of the corresponding precursors in vacuum, and were then characterized using scanning emission microscopy, energy dispersive x-ray analysis, x-ray powder diffraction and Raman scattering. According to x-ray diffraction analysis, all the synthesized samples had a tetragonal structure of space group \(I\overline{4} 2m\). The electrical properties of the CZTS samples were investigated in the temperature range of 10–325 K. The charge carrier concentration was measured to be about p = 1 × 1016 cm−3. A crossover from a nearest-neighbor hopping conduction mechanism at high temperatures (T > 150 K) to a Mott variable-range hopping conduction mechanism at low temperatures (T < 150 K) was observed. The activation energies of the CZTS samples were calculated for the low and high ranges of temperature. A thin film on quartz substrates was obtained using magnetron sputtering, for which a band gap of Eg = 1.3 eV was determined from transmittance measurements.

High positive magnetoresistance in Ge films at low temperatures

High positive magnetoresistance is found and investigated in p-Ge films on GaAs at low temperatures (1.4–4.2 K) and in magnetic fields up to 14 T. The film resistance grows exponentially with magnetic field. At a temperature of 1.4 K and in a magnetic field of 14 T, the film resistance is increased by a factor of 3.8. The magnetoresistance also grows exponentially as temperature is reduced. The Mott variable-range hopping conduction is observed in the films studied in the 1.4–21 K temperature range. It is shown that experimentally observed magnetoresistance agrees well with the theory of compression of wave functions of the localized states in the plane normal to magnetic field. This results in reduction of their overlapping, decrease of electron tunneling probability, and consequently, considerable growth of film resistance. Based on experimental resistance dependences on temperature and magnetic field, the parameters of hopping resistivity in Ge films on GaAs are determined. A possibility of transition to interference effects in conduction leading to localization of charge carriers and negative magnetoresistance at temperature decrease is also discussed.

Variable range hopping conduction in n-CdSe samples at very low temperature

We reanalyzed experimental data already published in Friedman J R, Zhang Y, Dai P, et al. Phys Rev B, 1996, 53(15): 9528. Variable range hopping (VRH) conduction in the insulating three-dimensional n-CdSe samples has been studied over the entire temperature range from 0.03 to 1 K. In the absence of a magnetic field, the low temperature conductivity σ of the three samples (A, B and C) obeys the Mott VRH conduction with an appropriate temperature dependence in the prefactor (σ = σ0 exp[–(T0/T)]p with p ≈ 0.25). This behavior can be explained by a VRH model where the transport occurs by hopping between localized states in the vicinity of the Fermi level, EF, without creation of the Coulomb gap (CG). On the contrary, no Efros-Shklovskii VRH is observed, suggesting that the density is constant in the vicinity of the EF.

Variable-range hopping conductivity in Cu2ZnGeSe4 single crystals

Resistivity, ρ(T), of the Cu2ZnGeSe4 single crystals is investigated between T~10 and 300K. The Mott variable-range hopping (VRH) conductivity is observed in the temperature interval of ~80–230K. Analysis of the Mott VRH conductivity yields the values of the semi-width of the acceptor band, W≈13–15meV, the relative acceptor concentration, NA/Nc≈0.86–0.89, at the critical concentration of the metal–insulator transition, Nc≈6.3×1018cm−3, the values of the localization radius, a/aB≈7–9 with the Bohr radius aB≈13.5Å, the mean acceptor energy E0≈82meV and the mean density of the localized states, g≈(1.7–2.1)×1017meV−1cm−3.

Disorder and variable-range hopping conductivity in Cu2ZnSnS4 thin films prepared by flash evaporation and post-thermal treatment

Resistivity, ρ(T), of as-grown and annealed Cu2ZnSnS4 films, obtained by flash evaporation, is investigated between T∼10 and 300K. A correlation between the transport properties and the growth conditions of the thin films is also explored. The behavior of ρ(T) in the as-grown films exhibits a close proximity to the metal–insulator transition (MIT), whereas annealing shifts the material from the MIT towards an insulating side. This is attributable to an increased microscopic lattice disorder, which is substantiated by the analysis of the Mott variable-range hopping conductivity observed up to T∼220–280K (120–180K) in the as-grown (annealed) films. An increased width of the acceptor band, a decreased relative acceptor concentration, N/Nc and lower values of the mean density of the localized states, g, are obtained after annealing.

Simultaneous γ(Co60)-quanta irradiation and isothermal annealing-induced insulator–metal transition in a-As4Se4Te2 chalcogenide composition

Amorphous chalcogenide composition As4Se4Te2 is prepared by the conveniential quenching technique. The separate isothermal annealing or γ irradiation not effected by the physical properties of the prepared sample. The prepared samples are subjected to simultaneous isothermal annealing at temperature 150°C and γ-quanta irradiation. The dark dc conductivity decreases with increasing time of γ-irradiation exposure. At irradiation dose 1.1 M rad, the dc conductivity starts to have metallic-like conductivity character. Also a crossover from the Efros–Shklovskii percolation mechanism of conduction to the Mott variable-range hopping conduction mechanism is detected and discussed. This crossover depends on both temperature and the composition characteristics.

Features of the acceptor band and properties of localized carriers from studies of the variable-range hopping conduction in single crystals of p-Cu2ZnSnS4

Resistivity, ρ(T), in single crystals of p-Cu2ZnSnS4 is investigated in the temperature interval of T∼300−10K. Below ∼200K, ρ(T) exhibits an activated character obeying between ∼130−150K and ∼30−70K the Mott variable-range hopping conduction law. Analysis of the experimental data yields typical values of the relative acceptor concentration, N/Nc∼0.72−0.84, where Nc ≈8×1018cm−3 is the critical concentration of the metal–insulator transition (MIT), and those of a/aB∼3.5−6.4, where a is the localization radius and aB ≈13Å is the Bohr radius, respectively. Width of the acceptor band, W∼11−22meV, centered at the energy EB∼50−60meV above the top of the valence band, is estimated implying a high degree of the microscopic lattice disorder in the investigated p-Cu2ZnSnS4 samples. Typical values of the density of the localized states, g∼(1−3)×1017cm−3meV−1, are estimated, as well.

Charge transport mechanism in high conductivity undoped tin oxide thin films deposited by reactive sputtering

This paper reports the charge transport mechanism at low temperatures in (110) oriented polycrystalline tin oxide (SnO2) films of less than 100nm thickness prepared by reactive sputtering in the substrate temperature (TS) range of 350–450°C. Undoped tin oxide films with high electrical conductivity of 11Ω–1cm–1 have been achieved at 350°C. The Hall mobility increases from 6.7 to 13.9cm2/V·s and carrier concentration decreases from 11×1018 to 0.9×1018cm–3 as TS is increased. In the 300–100K temperature range we found two types of conduction mechanisms: thermally activated conduction till 160K and nearest-neighbour-hopping conduction below 160K. At temperatures below 90K, the Mott variable-range-hopping (VRH) conduction governs the charge transport. In all the three regimes the activation energies of conduction increase with an increase in TS, being 15–50, 0.2–9, and 2–6meV respectively, consistent with a decrease in oxygen vacancies at higher TS. The analysis of the data in Mott VRH conduction regime suggests a systematic localization of the oxygen vacancy states with an increase in growth temperature. Even at 70nm thickness the films behave as three dimensional with regard to the Mott VRH conduction process.

Transport behavior and negative magnetoresistance in chemically reduced graphene oxidenanofilms

The electron transport behavior in chemically reduced graphene oxide (rGO) sheets with differentthicknesses of 2, 3, and 5 nm was investigated. The four-probe method for the sheet resistance (RS) measurement on the intensively reduced graphene oxide samples indicatesan Arrhenius characteristic of the electron transport at zero magnetic fieldB = 0, consistent with previous experimental results on well-reduced GO samples.The anticipated variable range hopping (VRH) transport of electrons in atwo-dimensional electron system at low temperatures was not observed. The measuredRS of the rGOsamples are below 52 kΩ/square at room temperature. With the application of a magnetic field up to 4 T, negativemagnetoresistance in the Mott VRH regime was observed. The magnetotransport featuressupport a model based on the spin-coupling effect from the vacancy-induced midgap statesthat facilitate the Mott VRH conduction in the presence of an external magnetic field.

Hopping conductivity of La1−xSrxMN1−yFeyO3

The resistivity, ρ, is investigated in ceramic La1−xSrxMn1−yFeyO3 samples with x = 0.3 and y = 0.03 – 0.25 at temperatures T ∼ 5 – 310 K in magnetic fields B up to 8 T. The metallic conductivity at y = 0.03 and the variable-range hopping (VRH) conductivity between y = 0.15 – 0.25 are observed. The Mott VRH conductivity takes place in all investigated samples with y ≥ 0.15 in different temperature intervals. In addition, at y = 0.25 the interval of the Shklovskii-Efros VRH conductivity, characteristic of the Coulomb gap in the density of the localized states (DOS), is observed. Analysis of the VRH conductivity yields the values of the microscopic parameters, including the DOS features and the localization radii of charge carriers.

Variable-range hopping conductivity ofLa1 − xSrxMn1 − yFeyO3

The temperature dependence of the resistivity,ρ, of ceramicLa1 − xSrxMn1 − yFeyO3 (LSMFO)samples with x = 0.3 and y = 0.03, 0.15, 0.20 and 0.25 (or simply #03, #15, #20 and #25, respectively) is investigated between temperaturesT ∼ 5 and 310 K inmagnetic fields B up to 8 T. Metallic conductivity in #03 is changed eventually to activated in #25. In #15 and #20 the behaviorof ρ(T) is more complicated, comprising of two extremes, divided byan interval of metallic behavior in #15, and two inflections ofρ(T) in #20 withinsimilar intervals ΔT below ∼ 100 K.Mott variable-range hopping (VRH) conductivity is observed in #15 above the ferromagnetic Curie temperature,TC. In #20 the Mott VRH conductivity takes place in three different temperature intervals atT > TC,T closeto TC and T < TC. In #25, the Mott VRH conductivity is observed in two different intervals, above and belowTC, divided by an intermediate interval of the Shklovskii–Efros VRH conduction regime.Analysis of the VRH conductivity yielded the values of the localization radius,α, and thedependence of α and of the density of the localized states,g, near the Fermilevel, on B. AboveTC the localization radiusin all samples at B = 0 hassimilar values, α≈1.0–1.2 Å,which is enhanced to α≈3.3 Å (#20) and 2.0 Å (#25) below TC. The sensitivity of α and g toB dependon y andT. Thecomplicated behavior of the mechanisms of the hopping charge transfer, as well as of the microscopicparameters α and g, is attributable to different electronic and magnetic phases of LSMFO varying withtemperature and Fe doping.

Variable-range-hopping conduction processes in oxygen deficient polycrystalline ZnO films

We have fabricated oxygen deficient polycrystalline ZnO films by the rf sputtering deposition method. To systematically investigate the charge transport mechanisms in these samples, the electrical resistivities have been measured over a wide range of temperature from 300 K down to liquid-helium temperatures. We found that below about 100 K, the variable-range-hopping (VRH) conduction processes govern the charge transport properties. In particular, the Mott VRH conduction process dominates at higher temperatures, while crossing over to the Efros–Shklovskii (ES) VRH conduction process at lower temperatures. The crossover occurred at temperatures as high as a few tens degrees kelvin. Moreover, the temperature behavior of resistivity over the entire VRH conduction regime from the Mott-type to the ES-type process can be well described by a universal scaling law.