Variable Range Hopping Mechanism Research Articles

Investigation of phase coexistence and correlation in La1−xBixMnO3+δ (x = 0.4 and 0.6)

Polycrystalline samples of bismuth-doped (x = 0.4 and 0.6) have been synthesized using conventional solid state reaction methods. After characterizing the samples by x-ray diffraction, scanning electron microscope, and energy dispersive x-ray spectrometer, a systematic investigation of magnetic, electrical, and magneto-transport properties has been carried out. Our magnetic measurements indicate spin-glass magnetic behaviour accompanied by a ferromagnetic phase at low temperatures. Also, the evidence for phase coexistence, leading to overlapping of the nanoscale ferromagnetic phase in the paramagnetic host matrix is observed. The electrical resistivity measurements suggest the semiconducting nature of the samples in the measured temperature range. For both samples, the conductivity in the temperature range above the (for T > θD/2) is observed to be due to the adiabatic hopping of small polarons, whereas below the (for T < θD/2), the Shklovskii–Efros variable range hopping mechanism appears to be suitable. The dielectric permittivity as a function of temperatures and frequencies studied in the samples exhibits different relaxation processes, which are understood due to the Debye+Maxwell—Wagner relaxation mechanisms. The observed negative magneto-resistivity and the dielectric relaxation in the paramagnetic region can be attributed to the phase coexistence scenario.

Magnetic frustration and dielectric relaxation in insulating Nd2NiMnO6 double perovskites

We have investigated structural, dielectric, and magnetic properties of polycrystalline double perovskite Nd2NiMnO6 compound. The compound crystallizes in monoclinic P21∕n symmetry and is partially B-site disordered depending on the synthesis conditions. It undergoes second-order ferromagnetic transition at 192 K and shows glassy behaviour at low temperature. The glassy phase is due to anti-site disorder within the homogeneous sample. Temperature and frequency dependent dielectric measurements reveal colossal values of dielectric constant and is best interpreted using Maxwell-Wagner interfacial polarization model. Impedance spectroscopy has been used to analyse the intrinsic dielectric response. This enabled us to differentiate the conduction process at the grain and grain boundaries. Arrhenius behaviour is favoured at the grain boundary, while variable range hopping mechanism is considered most suitable within the grain region. dc conductivity measurements corroborate variable range hopping conduction.

Room temperature magnetism and metal to semiconducting transition in dilute Fe doped Sb1−xSex semiconducting alloy thin films

The magnetism was observed in very dilute Fe doped alloy thin film Fe0.008Sb1−xSex, for x = 0.01 to 0.10. These thin films were grown on silicon substrate using thermal evaporation technique. Structural, electrical, optical, charge carrier concentration measurement, surface morphology and magnetic properties were observed using glancing incidence x-ray diffraction (GIXRD), four probe resistivity, photoluminescence, Hall measurement, atomic force microscopy (AFM) and magnetic force microscopy (MFM) techniques, respectively. No peaks of iron were seen in GIXRD. The resistivity results show that activation energy increases with increase in selenium (Se) concentration. The Arrhenius plot reveals metallic behavior below room temperature. The low temperature conduction is explained by variable range-hopping mechanism, which fits very well in the temperature range 150–300 K. The decrease in density of states has been observed with increasing selenium concentration (x = 0.01 to 0.10). There is a metal-to-semiconductor phase transition observed above room temperature. This transition temperature is Se concentration dependent. The particle size distribution ∼47–61 nm is evaluated using AFM images. These thin films exhibit ferromagnetic interactions at room temperature.

Anomalous Fe diffusion in Si-ion-implanted β–Ga2O3 and its suppression in Ga2O3 transistor structures through highly resistive buffer layers

The thermal behavior of Fe as a compensating acceptor impurity in β-Ga2O3 (010) was studied in view of growing interests in semi-insulating Fe-doped Ga2O3 substrates for the realization of high-performance Ga2O3 field-effect transistors (FETs). An anomalous redistribution of Fe beyond the extent of intrinsic thermal diffusion was revealed by secondary ion mass spectroscopy in device-relevant structures where Ga2O3 grown homoepitaxially on Fe-doped substrates was doped by Si ion (Si+) implantation and annealed at high temperatures. The enhanced Fe diffusion was attributed to an athermal process involving intrinsic defects from the region of implantation damage. An undoped Ga2O3 buffer between the Si+-implanted layer and the Fe-doped substrate effectively suppressed Fe outdiffusion by protecting the substrate against unintentional ion damage or defects from a remote source, thereby preserving the electrical integrity of the Si-doped material. Temperature-dependent current-voltage measurements indicated that the undoped Ga2O3 buffer was highly resistive with inter-device leakage attributable to surface conduction via a variable-range hopping mechanism. The buffer scheme, together with dielectric passivation to eliminate surface leakage, was proposed to constitute an integral process module for future lateral Ga2O3 FET devices.

Transport characteristics of focused beam deposited nanostructures

AbstractWe review the transport properties of different nanostructures produced by ion- and electron-beam deposition, as prepared as well as after certain treatments. In general, the available literature indicates that the transport properties are determined by conduction processes typical for disordered metallic grains embedded in a carbon-rich matrix, including intergrain tunneling and variable range hopping mechanisms. Special emphasis is given to the superconducting behavior found in certain Tungsten-Carbide nanostructures that, in a certain field and temperature range, is compatible with that of granular superconductivity. This granular superconductivity leads to phenomena like magnetic field oscillations as well as anomalous hysteresis loops in the magnetoresistance.

Electronic excitations and structure of Li2IrO3 thin films grown on ZrO2:Y (001) substrates

Thin films are a prerequisite for application of the emergent exotic ground states in iridates that result from the interplay of strong spin-orbit coupling and electronic correlations. We report on pulsed laser deposition of Li2IrO3 films on ZrO2:Y (001) single crystalline substrates. X-ray diffraction confirms preferential (001) and (10-1) out-of-plane crystalline orientations with well defined in-plane orientation. Resistivity between 35 and 300 K is dominated by a three-dimensional variable range hopping mechanism. The dielectric function is determined by means of spectroscopic ellipsometry and, complemented by Fourier transform infrared transmission spectroscopy, reveals a small optical gap of ≈300 meV, a splitting of the 5d-t2g manifold, and several in-gap excitations attributed to phonons and possibly magnons.

1/f Noise in Mott Variable Range Hopping Conduction in p-type Amorphous Silicon

ABSTRACTWe report on electrical conductivity and noise measurements made on p-type hydrogenated amorphous silicon (a-Si:H) thin films prepared by Plasma Enhanced Chemical Vapor Deposition (PECVD). The temperature dependent electrical conductivity can be described by the Mott Variable Range Hopping mechanism. The noise at temperatures lower than ∼ 400K is dominated by a 1/f component which follows the Hooge model and correlates with the Mott conductivity. At high temperatures there is an appreciable G-R noise component.

Transport properties of La1−xSrxMnO3 ceramics above metal–insulator transition temperature

Single phase Sr-doped lanthanum manganite (La1−xSrxMnO3, 0≤x≤0.4) ceramics have been prepared to study the transport property and conduction mechanism in these materials above the metal–insulator transition temperature (Tp). The measurement of resistivity showed that the conductivity increased with the increasing of Sr concentration. The samples with 0.1≤x≤0.3 exhibited Tp between 258 and 281K, depending on x. We then fitted the resistivity data in the semiconducting region (T>Tp) to check the possible conduction mechanism through two models, i.e., small polaron hopping (SPH) and variable range hopping (VRH). It was found that the transport mechanism in the higher temperature region above θD/2 (θD is Debye temperature) was dominated by the SPH model. But for the lower temperature region (between Tp and θD/2), the conductivity data could be well fitted to the VRH model, where some fundamental parameters (hopping energy Eh and hopping distance Rh) estimated from Mott's relation also obeyed the VRH mechanism. The doping of Sr in the samples caused the reducing of the localization of charge carriers, which thereby led to a decreasing in Eh and Rh.

Temperature evolution of magnetic and transport behavior in 5d Mott insulator Sr2IrO4: significance of magneto-structural coupling

We have investigated the temperature evolution of magnetism and its interrelation with structural parameters in the perovskite-based layered compound Sr2IrO4, which is believed to be a Jeff = 1/2 Mott insulator. The structural distortion plays an important role in this material and induces a weak ferromagnetism in an otherwise antiferromagnetically ordered magnetic state with a transition temperature around 240 K. Interestingly, at low temperatures, below around 100 K, a change in the magnetic moment has been observed. Temperature dependent x-ray diffraction measurements show that sudden changes in structural parameters around 100 K are responsible for this. Resistivity measurements show insulating behavior throughout the temperature range across the magnetic phase transition. The electronic transport can be described with Mott's two-dimensional variable range hopping (VRH) mechanism, however, three different temperature ranges are found for VRH, which is a result of varying the localization length with temperature. A negative magnetoresistance (MR) has been observed at all temperatures in contrast to positive behavior generally observed in strongly spin-orbit coupled materials. The quadratic field dependence of MR implies the relevance of a quantum interference effect.

An insight to the low temperature conduction mechanism of c-axis grown Al-doped ZnO, a widely used transparent conducting oxide

Al-doped ZnO thin films were synthesized from oxygen reactive co-sputtering of Al and Zn targets. Explicit doping of Al in the highly c-axis oriented crystalline films of ZnO was manifested in terms of structural optical and electrical properties. Electrical conduction with different extent of Al doping into the crystal lattice of ZnO (AZnO) were characterized by frequency dependent (40 Hz–50 MHz) resistance. From the frequency dependent resistance, the ac conduction of them, and correlations of localized charge particles in the crystalline films were studied. The dc conduction at the low frequency region was found to increase from 8.623 µA to 1.14 mA for the samples AZnO1 (1 wt% Al) and AZnO2 (2 wt% Al), respectively. For the sample AZnO10 (10 wt% Al) low frequency dc conduction was not found due to the electrode polarization effect. The measure of the correlation length by inverse of threshold frequency (ω0) showed that on application of a dc electric field such length decreases and the decrease in correlation parameter(s) indicates that the correlation between potentials wells of charge particles decreases for the unidirectional nature of dc bias. The comparison between the correlation length and the extent of correlation in the doped ZnO could not be made due to the observation of several threshold frequencies at the extent of higher doping. Such threshold frequencies were explained by the population possibility of correlated charge carriers that responded at different frequencies. For AZnO2 (2% Al), the temperature dependent (from 4.5 to 288 K) resistance study showed that the variable range hopping mechanism was the most dominating conduction mechanism at higher temperature whereas at low temperature region it was influenced by the small polaronic hopping conduction mechanism. There was no significant influence found in these mechanisms on applications of 1, 2 and 3 V as biases.

Terahertz-infrared spectroscopy of overdoped manganites La1−xCaxMnO3

Terahertz and infrared spectra of dielectric permittivity and optical conductivity of overdoped manganites La1−xCaxMnO3 (0.5≤x≤1) have been measured at frequencies ν from 4cm−1 to 700cm−1 and at temperatures T from 5K to 300K. The samples were prepared in the form of bulk polycrystals (ceramics) and epitaxial films (free-standing and on MgO substrates). Strongly asymmetric absorption bands have been found in the compounds which are in charge-ordered state (0.5<x<0.85) and have commensurate calcium doping. The bands are located close to the position of lowest energy van Hove singularity of the folded Brillouin zone. The bands are associated with a boson peak, i.e., with the excess of lattice acoustical vibrational states over the standard Debye contribution (~ν2). In addition, Raman scattering measurements on ceramic samples have been conducted, which confirmed our assignment of the bands’ origin. We firmly identify contribution to the low energy spectra due to quasi-free charge carriers. In the paramagnetic phase all samples show a Drude-like response corresponding to the band type of charge transport. At low temperatures, for the range of dopings 0.5≤x<0.85, the conductivity spectra show signatures of variable-range hopping mechanisms of transport.

Detailed analysis on electrical conduction transition from 2D variable range hopping to phonon-assisted 3D VRH mechanism belonging to Bi-site La substituted Bi-2212 system

The main scope of this comprehensive study is to determine the critical dopant level of metal to insulator transition (MIT) for Bi-site La substituted Bi-2212 superconducting ceramics by means of bulk density, X-ray diffraction (XRD), scanning electron microscopy (SEM), electron dispersive X-ray (EDX), dc resistivity (ρ–T) and transport critical current density (Jc) experimental methods. The samples produced in this work are prepared by the conventional solid state reaction technique at the constant temperature of 840°C for the annealing duration of 60h. All the experimental evidences obtained indicate that the microstructural, electrical and superconducting properties suppress with the increment of the La dopant level in the Bi-2212 superconducting matrix up to x=0.20 beyond which the crucial characteristics diminish rapidly due to not only the deformation between the Bi-2212 layers and grain boundaries but also the disorder and hole localization (filling) problem in the crystal structure. As for the numerical values, onset and offset critical temperatures systematically decrease from 85.67K and 84.49K to 82.74K and 59.38K with the impurity content until x=0.15. However, the sample doped with the content level of x=0.20 exhibits further pseudo-critical transition (related to Bi-2201 phase) at about 23K as a consequence of the existence of inhomogeneities in the oxidation state of superconducting grains. This fact is even verified by the degree of the broadening. Moreover, residual resistivity (temperature-independent) value tends to increase rapidly because of the impurity scattering and defect interfaces such as grain boundaries, stacking faults, voids, planar and micro defects, being favored by the bulk density and porosity analyses. Similarly, critical current density is measured to be in the range of 771A/cm2 (for the pure sample)–78A/cm2 (for the compound doped with x=0.20). The critical current densities and temperatures are not measurable for the compounds substituted by the excess La concentration level (x>0.20) due to the beginning point of the metal to insulator transition (nonsuperconducting behavior). In other words, the disappearance of the superconductivity results from the destruction of the phase coherence (phase breaking). Additionally, surface morphology and interaction between the superconducting grains retrograde significantly with the La concentration. EDX measurement evidences also show that during the sample preparation procedure the elements each successfully enter into the Bi-2212 crystal structure and the observed peaks of Cu and especially Bi degrade regularly with the La inclusions, meaning that the foreign particles may substitute for the elements (especially Bi). Based on the XRD measurement results, a axis length is observed to elongate regularly due to both the induced electrons into anti-bonding orbital and ionic radius of La3+>that of Bi3+ whereas the cell parameter c is obtained to shrink considerably as a result of the required extra energy for the bond formation in the orbital hybrid process. Moreover, the electrical conduction transition from the 2D variable range hopping (VRH) to phonon-assisted 3D VRH mechanism is theoretically discussed with the aid localization length, probable jump distance, average hopping energy and inelastic diffusion length parameters.

Study on the Structural and Electrical Properties of Sequentially Deposited Ag–Ga–In–Te Thin Films

The structural properties and electrical conduction mechanisms of Ag–Ga–In–Te thin films deposited by a combination of e-beam and thermal evaporation methods were studied for various annealing temperatures. Structural analysis showed the existence of In $$_{4}$$ Te $$_{3}$$ and In $$_{2}$$ Te $$_{5}$$ binary phases at the early stage of crystallization and monophase of AgGa $$_{0.5}$$ In $$_{0.5}$$ Te $$_{2}$$ with the main orientation along (112) direction following the post-annealing at 400 $$^\circ $$ C. The effects of the structural changes on electrical properties and temperature dependence of the electrical conductivity of Ag–Ga–In–Te thin films were studied in the temperature range of 90–400 K. The analysis of electrical conductivity revealed the Efros–Shklovskii variable range hopping (VRH) mechanism in between 90 and 210 K and Mott VRH mechanisms for the temperature range of 250–400 K for all deposited films. The VRH parameters including average hopping distance, average hopping energy and characteristic temperature coefficient for Efros–Shklovskii and Mott VRH mechanisms were determined and discussed in detail.

Dielectric Constant and Transport Mechanism of Percolated Polyaniline Nanoclay Composites.

We report the dielectric constant and transport mechanism of intercalated nanoclay–polyaniline composite, an industrially ready to use novel nanocomposite, which is prepared by a simple mechanochemical method. The effects of clay concentration on structure and structure variations on properties were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, and impedance spectroscopy (20 Hz–1 MHz and temperature range from 300–380 K). The phenomenon of percolation was observed in these composites. The values of Mott’s temperature, density of states at the Fermi level, hopping distance, and barrier height for polyaniline–nanoclay (PANC) composites were calculated. By applying Mott’s theory, it is found that the PANC composites obey the one-dimensional variable range hopping mechanism. This type of percolated sample can be used as a nanocapacitor in many devices because of its enhanced transport properties.

Thermally stimulated discharge conductivity study of zinc oxide thermoelectrets

The present work deals with transmission electron microscopy (TEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC) and thermally stimulated discharge current (TSDC) study of inorganic metal oxide (ZnO) nanoparticles and its thermoelectrets. The thermoelectrets were prepared by applying different electric polarizing field (E P) at constant polarizing temperature (T P), for constant polarization time (t p). The TSDC study was carried out in the temperature region of 313–473 K. It was observed that the conductivity of ZnO samples increases with the increase in temperature and polarizing field. The dependence of TSDC data on polarizing agents, i.e. field and temperature shows Arrhenius type of behviour and is explained on the basis of variable range hopping mechanism.

Observation of negative magneto-resistance in SrFe1−xTixO3−δ (x = 0 to 0.3) systems

Resistance and magneto-resistance of nanocrystalline SrFe1−xTixO3−δ (x = 0 to 0.3) systems are investigated. Substitution of non magnetic Ti4+ stabilizes the single phase tetragonal crystal structure with I4/mmm space group and Ti introduces bond disorder as well as site disorder. With increase in Ti content, the disorder increase leading to localization and large change in resistivity is observed in all the systems. In undoped system, the increase in resistivity is associated with the magnetic transition. All the Ti doped samples show large change in resistivity much above the magnetic ordering temperature and three dimensional Variable Range Hopping mechanism seems to explain the transport and magneto-transport behavior in these systems.

Temperature- and magnetic field-dependent electrical transport studies of poly(-3,4 ethylenedioxythiophene) nanoparticles

Poly(3,4-ethylenedioxythiophene) nanoparticles with average diameter of 13 nm have been synthesized by oxidative polymerization method using dodecylbenzene sulfonic acid (DBSA) as surfactant. Thermogravimetric analysis shows the improvement of thermal stability with increasing surfactant concentration. The red shifting of the 1,381 cm−1 band with increasing DBSA concentration indicates the transition from benzenoid to quinoid structure. The temperature dependence of resistivity reveals that the charge transport takes place through variable range hopping (VRH) mechanism. The slopes of reduced activation versus temperature plot confirm that the charge transport takes place through 3D VRH mechanism. The negative slope of the reduced activation energy versus temperature plot indicates that our samples fall in the insulating regime. Magnetoresistance study shows a transition from positive to negative with increasing temperature. The negative value of magnetoresistance at higher temperature is due to the quantum interference effect where the charge carrier hopping distance in comparable to the localization length and the positive magnetoresistance at low temperature results due to the shrinkage of impurity orbitals under magnetic field. The higher value of positive magnetoresistance than that of the negative magnetoresistance indicates that wave function shrinkage effect is stronger compared to quantum interference effect.

Electrical Properties of Polypyrrole Conducting Polymer at Various Dopant Concentrations

Polypyrrole conducting polymer was prepared by chemical reaction method with various concentrations of iron (III) chloride (FeCl3) as dopant. The dc conductivity was obtained from current-voltage characteristic by using parallel-plate techniques in the temperature range of 100-300K. With the involvement of chloride, Cl- in the polymeric chain, the conductivity increased as temperature and the dopant concentration increased. To describe the electrical transport process, Mott’s 1-D, 2-D and 3-D variable range hopping (VRH) models have been considered. The result gives evidence of transport mechanism based on Mott’s 3-D VRH mechanism for all various dopant concentrations studied.

Magnetic, dielectric and transport characteristics of Ln2CoMnO6 (Ln=Nd and Sm) double perovskite ceramics

Magnetic and dielectric properties of Nd2CoMnO6 and Sm2CoMnO6 double perovskite ceramics have been investigated by comparing with those of La2CoMnO6 and Ln2NiMnO6 (Ln=La, Nd and Sm) ceramics. Differing from the single magnetic state of Ln2NiMnO6 (Ln=La, Nd and Sm) ceramics, both Nd2CoMnO6 and Sm2CoMnO6 are multiphase magnetic state with the monoclinic symmetry (space group P21/n). Ferromagnetic Curie point TC generally decreases with decreasing 〈Ni/Co–O–Mn〉 bond angles which depends on lanthanide ionic radius (RLn). This result indicates that the 〈Ni/Co–O–Mn〉 bond angle plays an important role in magnetic properties of Ln2MMnO6 (Ln=La, Nd and Sm, M=Ni and Co) ceramics. Meanwhile, only one relaxor-like dielectric peak with strong frequency dispersion is observed in Nd2CoMnO6 and Sm2CoMnO6 ceramics, which is similar with that of La2CoMnO6. The dielectric constant ε′ also decreases with decreasing RLn. Both magnetic and dielectric properties of Ln2CoMnO6 (Ln=La, Nd and Sm) ceramics have similar variation tendency with decreasing RLn, and have been correlated with cos2〈Co–O–Mn〉, demonstrating some coupling between magnetic and dielectric response. In addition, transport behaviors of the present ceramics conform to the three-dimensional variable-range hopping mechanism, which may be attributed to the phase separation nature.

Electrical Conductivity of V2O5–TeO2–Sb Glasses at Low Temperatures

Semiconducting glasses of the type 40TeO2–(60 − x) V2O5–xSb were prepared by rapid melt quenching and their dc electrical conductivity was measured in the temperature range 180–296 K. For these glassy samples, the dc electrical conductivity ranged from 2.26 × 10−7 S cm−1 to 1.11 × 10−5 S cm−1 at 296 K, indicating the conductivity is enhanced by increasing the V2O5 content. These experimental results could be explained on the basis of different mechanisms (based on polaron-hopping theory) in the different temperature regions. At temperatures above ΘD/2 (where ΘD is the Debye temperature), the non-adiabatic small polaron hopping (NASPH) model is consistent with the data, whereas at temperatures below ΘD/2, a T−1/4 dependence of the conductivity indicative of the variable range hopping (VRH) mechanism is dominant. For all these glasses crossover from SPH to VRH conduction was observed at a characteristic temperature TR ≤ ΘD/2. In this study, the hopping carrier density and carrier mobility were determined at different temperatures. N (EF), the density of states at (or near) the Fermi level, was also determined from the Mott variables; the results were dependent on V2O5 content.