Variable Range Hopping Mechanism Research Articles

Transport properties, charge conduction mechanism and magnetic behavior of La0.3Ca0.7MnO3:ZnO Nanocomposites

Nanocomposites consist of ZnO and La0.3Ca0.7MnO3 (LCMO) nanoparticles, prepared by cost effective sol–gel method, have been studied for their transport and magnetoresistance (MR) behaviors. Different ZnO nanoparticles contents (weight ratio) were mixed with LCMO nanostructured manganite to understand the possible interactions between ZnO and LCMO nanoparticles. X–ray diffraction (XRD) measurement was performed for all composites and structural quality was verified by performing Rietveld refinements. Variation in an average crystallite size for LCMO and ZnO nanoparticles in different composites has been discussed. Temperature dependent resistivity measurements under different applied magnetic fields suggest the semiconducting nature of all nanocomposites under all applied magnetic fields. Mott type variable range hopping (VRH) mechanism has been employed to understand the dependence of charge carrier localization in LCMO nanostructured manganite lattice on the applied magnetic field and ZnO nanoparticles content within the nanocomposites. Observed MR behavior has been understood under the influence of temperature and ZnO nanoparticles content in the context of spin polarized tunneling (SPT) and magnetic field induced improved conduction across the manganite lattice. Grain and grain boundary contributions to the MR behavior have been explained by using theoretical model fits to obtained MR data at different temperatures for all studied LCMO:ZnO nanocomposites. Magnetic nature of studied LCMO:ZnO nanocomposites has been understood by performing zero field cooled (ZFC) and field cooled (FC) measurement protocols based magnetization and magnetic isotherms measurements that confirm the coexisting magnetic phase in all studied nanocomposites. Magnetic nature has been understood on the basis of weakening of zener double exchange (ZDE) mechanism within the magnetic lattice of LCMO manganite and magnetic contribution from the free charge carriers within the ZnO clusters.

Optical and electrical studies on MgTiO3– Ba5Nb4O15 composite thin films for integrated microwave circuit applications

A correlative study on optical and electrical properties of MgTiO3 (MTO) -Ba5Nb4O15 (BNO) composite thin films (MTBN) with respect to film thickness (260–450 nm) is reported. The optical bandgap energy of MTBN composite thin films (3.76 eV–3.65 eV) are found to be intermediate to MTO (3.44 eV) and BNO (3.85 eV) monoliths. Further, the optical dispersion in MTBN composite thin films can be defined by Cody-Lorentz dispersion law. Higher photoluminescence (PL) emission intensity and shorter carrier lifetime (0.8376 ns) ascribed to local symmetry break is observed for 450 nm film. Moreover, the Mott's variable range hopping mechanism was dominant conduction process in the MTBN composite films. Further, density of states near the Fermi level were found to be around 7.85×1018 eV−1cm−3 - 3.478×1019 eV−1cm−3 for 260 nm–450 nm MTBN films. The obtained optical and electrical properties prospect a potential application of the MTBN composite thin films in integrated microwave circuits.

Intra‐Island Coulomb Correlations in PEDOT:PSS Thin Films; Saturation of Spin Polarization Magnetoresistance

AbstractMagnetic field and temperature dependence of the magneto‐conductance (MC) of thin films made of self‐p‐doped poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) having room temperature conductivity of ≈200 S cm−1 in the range of fields B = 0–14 T and temperatures T = 1.8–300 K are reported. At B = 0 and for current parallel to the film surface, the conductance follows a variable range hopping mechanism in two dimensions. It is shown that the finite Coulomb correlations, U, within the nano‐crystalline PEDOT islands dominates MC at low temperatures and high fields. At T = 1.8 K, the intra‐island Coulomb correlation spin polarization mechanism is already saturated for B > 10 T establishing a finite minimum of ≈2 meV for U. This small value of U is in line with the delocalized nature of the holes within the PEDOT islands and the relatively high conductivity that makes it essential in organic opto‐electronics.

Polaronic Relaxation and Variable-Range-Hopping Conductivity in Fe3O4 Nanoparticles

We report the polaronic relaxation and Variable Range Hopping (VRH) conductivity in 100nm average particle size cold pressed Fe3O4 nanoparticles. The crystal structure was studied using XRD showing spinel structure with (311) intense peak. The AC transport in the cold pressed powder was studied from 10 K to room temperature and from 100 Hz to 5.5 MHz. It was observed that the solid couples capacitively with the perturbing electric field initially and as the frequency increases it becomes more inductive. The impedance data was modelled using modified Havriliak–Negami relaxation model. It was observed that the cold pressing introduces an asymmetric broadening in the relaxation process. The change in impedance with temperature showed a drastic drop at around 120 K due to Verwey transition in real as well as imaginary part. The change in DC resistance with temperature indicated VRH mechanism of charge transport in the solid with a metal insulator transition at around 120 K. The relaxation times calculated indicated that the hopping process has an attempt frequency of the order of 30 microseconds. The activation energy change across the phase transition with relaxation time and resistance indicated that the charge transport is coherent. The activation energy of ∼0.05eV above Varway transition indicated that the transport is small polaronic in nature.

Suppression of multiple magnetic ordering induced by Nb and Ru substitution in SrCoO3-δ systems

Abstract In this paper, we report detailed study on magnetic behavior of perovskite SrCoO3-δ compound with partial substitution of non-magnetic Nb and magnetic Ru at Co site. All the samples show single-phase nature with tetragonal structure of P4/mmm space group. The semiconducting behavior is evident from temperature dependent resistivity (ρ) measurement. The resistivity exhibits Variable Range Hopping and Small Polaron Hopping mechanisms in different temperature regimes. The magnetoresistance increases with substitution and reaches up to ~35% at 10 K with 70 kOe magnetic field. The samples show weakly ferromagnetic-type transition below room temperature. The saturation magnetization and exchange bias (EB) increase with Nb concentration but the effect is less prominent for Ru substituted samples. Different magnetic properties are explained on the basis of valence states of Nb5+ and mixed valence of Ru4+/5+ replacing Co3+. The existence of ferromagnetic correlated region in antiferromagnetic matrix and irreversible glass state with competing exchange interactions impart the EB effect in these samples. The EB value HEB~7.45 kOe at 10 K is quite significant which might be useful for further technological implementation in electronics/spintronics devices.

Epitaxial strain tunable conductivity and charge transport of Ba0.6La0.4TiO3 thin films deposited by pulsed laser deposition

In this work, Ba0.6La0.4TiO3 (BLTO) thin films with a thickness of ∼30 nm were deposited by pulsed laser deposition on four different types of substrates, [MgO(100), LaAlO3(100), DyScO3(110), and GdScO3(110)], and their structures, charge transport behaviors, and related transport mechanisms were comprehensively explored. X-ray diffraction characterization revealed that all the BLTO films are epitaxially grown with strain ranging from ∼+0.65% to ∼−0.63%. All the films show a distinct semiconductor–metal transition behavior, and the transition temperature (TSM) decreases upon increasing the compressive strain. Their conductivities follow successively the variable-range hopping and small-polaron hopping mechanisms below TSM, and thermal excitation mechanism above TSM, while the corresponding temperature ranges are substrate dependent. Finally, Hall effect measurements confirmed that all the BLTO films possess semiconductor conduction behaviors, and the epitaxial strain can enhance carrier density. These results demonstrate that epitaxial strain can effectively tune the charge transport behaviors of BLTO thin films.

Electrical transport mechanism of aluminum substituted barium hexaferrite magnetic semiconductor

The polycrystalline M-type barium hexaferrite (BaFe12−xAlxO19) with x = 0.0, 1.0, 2.0, and 3.0 have been prepared by the sol–gel Method. The crystal structure of all the samples is found to be in hexagonal symmetry with P63/mmc space group. The impedance was studied over a range of frequencies (1 Hz–1 MHz) for all the compositions. Direct current (DC) electrical resistivity measurements of all the samples were carried out in the temperature range of 303–775 K. All the samples exhibit the semiconducting behavior. The resistivity increases with the increase in Al3+ substitution. The impedance along with DC resistivity results established the electron hopping conduction mechanism in the Al3+ substituted barium hexaferrites. The electrical conductivity has been well explained by the Mott variable-range hopping mechanism of localized polarons. The dielectric dipoles are frozen at low temperature and activated at high temperature as observed two transition temperatures in temperature versus impedance plot. A correlation between ac impedance and DC resistivity has been established in the M-type hexaferrite magnetic semiconductor.

Comments on the electronic transport mechanisms in the crystalline state of Ge—Sb—Te phase-change materials

It is known that phase-change materials, such as Ge–Sb—Te ternary system, are promising resistive non-volatile random-access memory applications with ultra-rapid reversible transformations between the crystalline and amorphous phases. This class of electronic transition is categorized to be the metal-insulator transition (MIT). The Anderson-type MIT has been discussed extensively in phase-change materials (PCMs) and isothermal annealing of amorphous PCMs (a-PCMs) which, above a certain temperature, leads to the metallic (crystalline) phase. In the insulator regime near the MI transition, Mott-type variable-range hopping (VRH) and/or Efros-Shkolvskii hopping (ESH) at low temperatures below 20 K (and down to 1 K) in Ge1Sb2Te4 (GST124) have been discussed extensively, however, we criticize the above argument through a detailed discussion of physical parameters that support the VRH mechanism. It is not clear whether or not the density-of-states (DOS) near the Fermi level is localized (like the Fermi glass) in the crystalline phase. It is also suggested that grain boundaries are expected to interfere with the electronic transport in the crystalline state. We should take into account the grain boundary effects on the electronic transport in the crystalline phase of Ge—Sb—Te system.

Charge localization crossover from Mott to Efros-Shklovskii type variable range hopping mechanism in In1−xPbxTe compounds

We have investigated the electrical transport properties of the In1−xPbxTe compounds. Small Pb-doping is not incorporated in the In-site but randomly distributed in the matrix, found from the X-ray diffraction and elemental mapping by energy-dispersive X-ray spectroscopy measurements. The random distribution of Pb elements induces robust charge localization at low temperatures causing the variable range hopping (VRH) transport. The electrical resistivity ρ(T) of pristine InTe exhibits semiconducting to metal transition near 175 K. From the analysis of temperature exponent, we found that the VRH transport is changed from Mott to Efros-Shklovskii (ES) type with decreasing temperature, where the crossover temperatures are found as 14.4 K (x=0.01) and 13.36 (x=0.02), respectively. The magnetoresistance (MR) of the InTe shows that the weak antilocalization at low temperature (T≤3 K) and low magnetic field (H≤1 T) competes with weak localization with increasing temperature (T≥5 K). It is noteworthy that small Pb-doping exhibits unconventional negative MR (NMR) behavior because it is not a magnetic or topological material. The unconventional NMR behavior of Pb-doped compounds is attributed to the quantum mechanical interference under the magnetic field in ES type VRH transport. The charge localization crossover from Mott to ES type VRH transport mechanism suggests the strong electron-electron Coulomb interaction in the compounds, leading to the significant change of density of states and inducing the Coulomb gap at low temperatures.

Frequency dependence of the hopping and disorder energies and conduction mechanisms in Cr-(Pr/Ca) MnO3

Transport properties of Pr0.7Ca0·3Mn0·85Cr0·15O3(Cr-PCMO) manganite have been studied in a large frequency and temperature ranges by mean of impedance spectroscopy. The conductance spectra are discussed in the outline of a double and single Jonscher laws for the temperature ranges [80 K–110 K] and [120–170 K] respectively. The correlated barrier hopping and the non small polaron tunneling (NSPT) conduction models are used to explain the dynamic of charge carriers in the ac regime. The lower temperature range is characterized by the coexistence of such two conduction processes. Below the frequency ν = 105 Hz, the compound exhibits a semiconductor behavior with no detectable electrical transition. A metal - semiconductor transition at around T = 190 K was observed at high frequency range (ν = 2.106 Hz). Beyond the Debye temperature, θD/2 = 130 K, the small polaron hopping (SPH) conduction process is thermally activated. Over the investigated frequency interval ν ≤ 105, the activation of the SPH conduction process needs the same activation energy Ea1 = 112 meV. At lower temperature range, the interaction between electron and acoustic phonon becomes important, the variable range hopping (VRH) mechanism was activated via a disorder energy WD. The temperature dependence of the local energy proves that type of the VRH process is sensitive to the frequency parameter and the investigated temperature range. The hopping and the disorder energies are strongly sensitive to frequency.

Critical behavior of the insulator-to-metal transition in Te-hyperdoped Si

Hyperdoping Si with chalcogens is a topic of great interest due to the strong sub-band-gap absorption exhibited by the resulting material, which can be exploited to develop broadband room-temperature infrared photodetectors using fully Si-compatible technology. Here, we report on the critical behavior of the impurity-driven insulator-to-metal transition in Te-hyperdoped Si layers fabricated via ion implantation followed by nanosecond pulsed-laser melting. Electrical transport measurements reveal an insulator-to-metal transition, which is also confirmed and understood by density functional theory calculations. We demonstrate that the metallic phase is governed by a power-law dependence of the conductivity at temperatures below 25 K, whereas the conductivity in the insulating phase is well described by a variable-range hopping mechanism with a Coulomb gap at temperatures in the range of 2--50 K. These results show that the electron wave function in the vicinity of the transition is strongly affected by the disorder and the electron-electron interaction.

Dielectric and electrical studies on iron oxide (α-Fe2O3) nanoparticles synthesized by modified solution combustion reaction for microwave applications

Hematite (α-Fe2O3) nanoparticles were synthesized by modified solution combustion method using a mixture of hexamethylenetetramine (HMTA) and glycine fuels at fuel to oxidant molar ratio (ϕ = 1) and fuel (HMTA) to fuel (glycine) weight ratio (1:3). DC-conductivity and dielectric studies of (α-Fe2O3) nanostructures were reported as a function of temperature. Above room temperature (RT), the conductivity showed an increasing trend with temperature which confirmed the semiconducting behaviour of the material. The conductivity behaviour followed Motts law which confirmed the three dimensional variable range hopping (3D VRH) mechanism in the synthesized system. Dielectric studies were carried out at room temperature with frequency range (1000 Hz to 1.5 MHz) as well as varying temperature (100 °C to 400 °C) at different frequencies (0.1 MHz, 0.3 MHz, 1 MHz, 1.3 MHz, and 1.5 MHz).The dielectric permittivity (є’)and tangent loss (tan δ) was found to increase with increase in temperature. In this synthesised material low value of dielectric loss was observed which depicted these materials as potential candidates for microwave applications. More than that studied material was characterised by Raman spectroscopy which revealed that seven active Raman modes were present with different intensities, namely two “A1g” modes (225 and 498 cm−1) and five “Eg” modes (247, 293, 299, 412 and 613 cm−1). The most representative band present in the synthesised iron oxide nanoparticles was found at 225 cm−1.

Optical, magnetic and magneto-transport properties of Nd 1-xAxMn0.5Fe0.5O3-δ (A=Ca, Sr, Ba; x=0, 0.25)

The effect of A-site doping by alkaline earth metals (A = Ca, Sr and Ba) on optical, magnetic, and electrical properties of Nd1-xAxMn0.5Fe0·5O3-δ (x = 0, 0.25) has been investigated. The UV–vis absorption spectra show that the A-site doping decreases the absorption. Two values of optical band-gap energy (Eg) can be estimated for each studied sample: the higher Eg value is associated with the charge transfer involving iron cations and the lower Eg value – with the charge transfer via manganese cations. Partial substitution of neodymium by alkaline earth metals decreases the Néel temperature (TN) and induces significant irreversibility between the zero-field cooled (ZFC) and field-cooled (FC) data below TN. The field-dependent magnetization at 3 K indicates antiferromagnetic ordering with a spin canting. Temperature dependencies of resistivity demonstrate the change from metallic to semiconductor-type conduction (with increasing temperature) at T = Tp < TN. The Tp temperature significantly decreases with doping. The introduction of alkaline earth metals in Nd1-xAxMn0.5Fe0·5O3-δ noticeably reduces the resistivity in the semiconducting region. The small polaron hopping (SPH) mechanism of conduction is suggested at T > TN. Within the range of Tp < T < TN, the resistivity data are interpreted by the variable range hopping (VRH) mechanism. A significant value of magnetoresistance (∼44%) is observed only for Nd0.75Sr0·25Mn0·5Fe0·5O3-δ at 120 K.

Crystal structures and electrical properties of cobalt manganese spinel oxides

Crystal structures and hopping motions of cobalt manganese spinel oxides across a wide composition range (CoxMn3−xO4 (CMO), 0.9 ≤ x ≤ 2.7) are investigated in order to clarify the corresponding mechanisms of electrical behaviors as negative temperature coefficient thermistors. Based on the diffraction measurements, a tetragonal to cubic spinel transition are observed mostly with increasing Co content. Hopping distance for a series of CMO are calculated by the variable range hopping mechanism, which is changed as a function of composition, resulting to the observed unique electrical properties. According to the electrical and structural analysis, the electrical properties of CMO compounds with different Co contents majorly determined by tetragonal to cubic phase transition, cation distribution, and hopping distance. In addition, the formation of secondary phase (i.e., CoO) and another CMO phase may also significantly affect the electrical properties.

Variable range hopping mechanism and modeling of isolation leakage current in GaN-based high-electron-mobility transistors

Off-state leakage current of AlGaN/GaN high-electron-mobility transistors (HEMTs) with implant and mesa isolation was studied. Comparison of isolated pad structures with HEMTs shows that isolation leakage flowing through contact pads is the major leakage source in the studied GaN-based HEMTs whose gate finger is connected to an Ohmic contact pad. Then, circular metal-oxide-semiconductor field-effect-transistor devices were used to identify this isolation leakage path as surface isolation leakage at the SiN/nitride interface rather than bulk leakage in the buffer layer or implanted region. The temperature-dependent measurement shows that the two-dimensional variable range hopping mechanism dominates both the implant and mesa isolation leakage current. Mesa isolation results in a larger hopping probability and isolation leakage current than implant isolation. The isolation leakage current through gate and drain contact pads results in a non-zero switch of gate current in rectangular devices with either a Schottky gate or a metal-oxide-semiconductor gate, which is not observed in circular devices. Gate voltage for the switch of gate leakage current is linearly correlated with drain bias voltage, and the slope of the linear model represents the influence of drain bias on gate leakage current. This empirical model is independent of the fabrication process, provided that the same layout is used. With an increase in the source–drain distance from 4 μm to 10 μm, the ratio of gate–drain isolation resistance to gate–source isolation resistance increases from 1.38 to 2.33, leading to a decrease in the slope of empirical lines from 0.42 to 0.30.

Influence of channel thickness on charge transport behavior of multi-layer indium selenide (InSe) field-effect transistors

Atomically thin Van der Waals solids that exhibit direct band gap in their few layer form can substantially impact the field of two dimensional (2D) materials based electronic devices and related applications. Here we report on electronic charge transport behavior of multi layer n-type InSe field-effect transistor (FET) devices fabricated on SiO2/Si substrate with seven different channel thicknesses, t > 20 nm, well within its direct band gap regime. Through gate dependent conductivity measurements over a wide range of temperatures (T; 20 K <T < 310 K), we show that, irrespective of the channel thickness, carrier transport in these materials occurs via Arrhenius-type activated process above a certain characteristic temperature (T*), whereas for temperatures T < T* conduction occurs via variable range hopping (VRH) mechanism consistent with Mott’s 2D VRH formalism, manifesting the presence of disorder induced localized states in these channels. Although the broad mechanisms of charge transport in all these devices are similar, the variation of correlation energy value (T1) of VRH with channel thickness indicates fewer density of localized states in thinner channels and hence higher electron mobility even without sufficient charge impurity screening from the substrate. Such intricate interplay between channel thickness and charge impurity screening and its influence on overall disorder landscape and hence on carrier localization in 2D layered systems underscores the need for strategic optimization of device parameters for their future prospect in electronic applications.

Electrical transport properties of polyaniline grafted radiation crosslinked chitosan

Present work reports about electrical properties of polyaniline (PANI) grafted with electron beam crosslinked chitosan. Measurement of temperature dependent dc & ac conductivity shows a significant improvement in the ac and dc conductivity after doping with 1 M HCl for different time intervals. The dc conductivity results were analyzed using Mott’s variable range hopping mechanism. The ac conductivity of grafted polymer obeys power law and behavior of frequency exponent‘s’ has been interpreted by the correlated barrier hopping (CBH) model. The variation of dielectric constant with temperature and frequency is consistent with the Debye type dispersion.

Probing charge carrier transport regimes in BiFeO3 nanoparticles by Raman spectroscopy

We report a finding of two transport regimes in multiferroic BiFeO3 nanoparticles, which correspond to variable range hopping (VRH) mechanisms. Switch between the two regimes coincides with the transition between antiferromagnetic (AF) and paramagnetic (PM) phase. At low temperatures, it was found that temperature dependence of indirectly extracted resistivity is consistent with Efros-Shklovskii mechanism. As with high temperatures, data fit favourably to Mott VRH theory. Estimated resistivity value of ~ 350mΩcm violates the Mott-Ioffe-Regel criterion, corroborating that the transport does not proceed through conduction bands. Conductivity was probed noninvasively via Raman contactless setup and studied as a function of temperature.

Magnetism and Charge Order in Nanocrystalline Orthorhombic SrFeO3-δ

Orthorhombic SrFeO3-δ is synthesized through solid-state reaction in nanocrystalline form with controlled oxygen flow. Neutron diffraction measurement with Rietveld refinement and selected area electron diffraction (SAED) confirms the crystal structure. Magnetization against temperature shows that orthorhombic SrFeO3-δ is an antiferromagnet with Neel temperature (TN) of 230 K. Field-cooled hysteresis (M-H) measurements show exchange bias present in this system. It undergoes charge ordering near the Neel temperature and large change in resistivity around TN is found. Behavior of transport and magnetotransport properties are understood using the three-dimensional variable range hopping (3DVRH) mechanism and hysteresis is observed around TN in transport data. Ferromagnetic resonance (FMR) study shows that the linewidth is minimum around TN. Charge order and magnetic phase transition are coupled to each other around TN in the orthorhombic SrFeO3-δ system.

Modified Solution Combustion Synthesis of Nickel-Doped Magnetite Nanoparticles and the Influence of Annealing on Their Optical, Electrical, and Magnetic Properties

In this work, nickel-doped magnetite nanoparticles (NPs) are synthesized by a modified solution combustion method and subsequently annealed at different temperatures. We report the structural, optical, dielectric, electrical, and magnetic properties of nickel-doped magnetite nanoparticles (NPs) annealed at 800°C, 1000°C, and 1200°C for 180 min. X-ray diffraction is utilized to explore the structural properties of the prepared magnetic nanoparticles. The surface morphology indicates that the particles annealed at 800°C and 1000°C are spherical, while particles annealed at 1200°C are polyhedral in shape and have uniform distribution. Energy-dispersive spectroscopy reveals the presence of the elements Ni, Fe, and O in the synthesized nanoparticles. The optical spectra obtained from UV–visible spectroscopy reveal that heat-treated synthesized samples have allowed a direct energy band gap. Also, the optical band gap shows a decreasing trend from 1.736 eV to 1.679 eV, while Urbach energy shows an increasing trend from 0.279 eV to 0.539 eV, with increasing annealing temperature. Photoluminescence spectra show a small redshift in band-edge emission from 433.77 nm to 440.12 nm with an increase in annealing temperature from 800°C to 1200°C. The dielectric studies are carried out at (1) varying frequency (20 Hz–2 MHz) at room temperature and (2) varying temperature (100–400 K) at constant frequency of 100 kHz. Both temperature- and frequency-dependent dielectric studies show an increase in the dielectric constant and dielectric loss with an increase in annealing temperature. Also, the conductivity increases with increased temperature in all studied samples, indicating semiconducting behavior, and increased conductivity is observed with increased annealing temperature. The conductivity behavior follows Mott's law, confirming the variable-range hopping mechanism in all samples. Magnetic measurements show high coercivity of 199.12 Oe and high remanent magnetization of 27.35 emu/g at lower annealing temperature. Thus, the magnetic properties of the synthesized ferrite nanoparticles can be tuned by adjusting particle size through annealing temperature.