Showing papers on "Variable-range hopping published in 1999"

High dielectric constant and tunability of epitaxial srtio3 thin film capacitors

Abstract: The dielectric and insulating properties of epitaxial SrTiO3(STO) thin film capacitors were studied. The films were grown by inverted cylindrical magnetron sputtering in the radio frequency mode on (100) STO substrates which were covered with a (001)-oriented YBa2Cu3O7−x (YBCO) layer as a ground electrode. As a top electrode we used YBCO or Au thin films. A high dielectric constant, e, of up to 5000 was observed at T=80 K. The capacitors revealed a large tunability, i.e., a nonlinear e(E) dependence, with respect to voltage biasing. By applying 3 V, e decreased to 1000 which was 20% of its maximum value. The frequency dependence of e, the temperature dependence of the dielectric loss factor, tan δ, and the direct currency conductivity reflected that variable range hopping via localized states was present and dominated the conduction process in the STO films at low temperatures. The field strength for the electrical breakdown amounted to 300 kV/cm even for rather thin films with a thickness of about 40 nm....

Variable range hopping conduction in bulk samples composed of single-walled carbon nanotubes

Abstract: An experimental study of the electrical transport phenomena in magnetic fields H up to 15 T in pressed samples of a porous system composed of single-walled carbon nanotubes in the temperature range 1.39 K

Multiphonon assisted hopping in strontium vanadate semiconducting glasses

Abstract: The temperature and compositional dependence of the electrical conductivity of semiconducting strontium vanadate glasses have been reported in the temperature range 80-500 K. It has been observed that the multiphonon assisted hopping model of small polarons in the nonadiabatic regime, which considers the strong interaction of electrons with both the optical and acoustical phonons, can interpret the temperature dependence of the conductivity data of these glasses over the entire temperature range of measurement. The parameters obtained from the fits of the experimental data to this model appear reasonable and are consistent with the glass composition. Mott's optical phonon assisted hopping model at high temperatures yields smaller values of the localization length. However, Mott's variable range hopping model can predict the low temperature data. Schnakenberg's model provided higher values of the hopping energy than the activation energy obtained at the highest temperature range.

Optical and electrical characterization of a conducting polypyrrole composite prepared by in situ electropolymerization

Abstract: A study of the optical and electrical properties of a conducting polypyrrole–polyoxyphenylene composite, PPy–POP, prepared by insitu electropolymerization is presented. Electropolymerization was performed potentiostatically in a solution of pH 9 which contained the monomers pyrrole, allylphenol and sodium 4-hydroxybenzenesulfonate (4HBS), by applying a potential of 1.25 V vs. SCE. The films obtained were characterized optically by UV/VIS and IR spectroscopy and electrically by measurements of the temperature dependence of the ac and dc conductivity. FTIR measurements indicated that the polymer blend obtained consists of PPy and the insulating polymer poly-2-allyloxyphenylene (POP), whereas the third monomer, 4HBS, is incorporated into the PPy–POP film as dopant for the conducting PPy. Furthermore, optical characterizations show a light degree of overoxidation of PPy in the PPy–POP composite. In the UV/VIS spectra, the formation of both polaron and bipolaron electronic states of the band structure of PPy can be seen, but the IR spectra demonstrate the transition of the PPy structure from a conducting quinoid to benzoid type with increasing polymerization potential. This is accompanied by the introduction of a carbonyl group into the PPy backbone and a reduction of the conjugation length of the polymer chain, which has a strong influence on the conductivity of the polymer composite. Despite this overoxidation process, the PPy–POP film retains a conductive character which allows the growth of thick films. The temperature dependence of the ac and dc conductivity of PPy–POP was investigated. The total ac conductivity, σtot(ω), in the frequency range 102–105 Hz, changes by approximately four orders of magnitude in the range from 77 to 300 K, showing a sub-linear dispersive behavior. The temperature dependence of the dc conductivity of such a polymer composite can be described by Mott's variable range hopping (VRH) model according to σ=σ0 exp[-(T0/T)γ], with γ=1/2

Semiconducting properties of magnesium vanadate glasses

Abstract: The electrical conductivity of semiconducting magnesium vanadate glasses has been reported for a wide composition range in the temperature range of 80–500 K. The experimental results have been analyzed in the framework of different hopping models. It has been observed that the multiphonon assisted hopping model of small polarons in the nonadiabatic regime, proposed by Emin, can interpret the temperature dependence of the conductivity data of these glasses over the entire temperature range of measurement. The parameters obtained from the fits of the experimental data to this model appear reasonable and are consistent with the glass composition. On the other hand, Mott’s optical phonon assisted hopping model at high temperatures provides smaller values of the localization length. However, Mott’s variable range hopping model is consistent with the low temperature data. Schnakenberg’s model yields higher values of the hopping and the disorder energies than the activation energy obtained at the highest and the lowest temperature ranges.

Ion implantation in TiO2: damage production and recovery, lattice site location and electrical conductivity

Abstract: Ions with different oxidation states were implanted in TiO 2 (rutile). The lattice disorder as well as the lattice site location of the implanted ions were determined using Rutherford Backscattering and Channeling (RBS-C) spectrometry. The production of disorder as a function of dose and temperature, and its recovery was studied in detail. Important results are the observation of dynamic recovery at 293 K and above, and one isothermal at 77 K and two thermal recovery stages between 170 and 210 K and between 260 and 293 K. The recovery at 77 K is proportional to ln t , indicating that the activation energy increases with decreasing disorder density. The results concerning the lattice site location of 14 ion species reveal that 13 ions occupy Ti lattice site. With increasing netcharge, the maximum soluble concentration decreases by the formation of impurity–defect complexes probably enforced by charge compensation. Directed displacements from the substitutional lattice site provide some hints on the structures of these complexes. The electrical conductivity of the implanted samples increased by about 12 orders of magnitude irrespective of the oxidation state of the implanted species. From the temperature and dose dependence of the electrical conductivity as well as from its similar behaviour for noble-gas ions and other species it is concluded, that the carrier transport occurs by single energy states excitation at low doses and by variable range hopping between localized states at high doses.

Negative magnetoresistance of γ-Fe2O3 observed in γ-Fe2O3/Ag granular nanocomposites

Abstract: The magnetoresistance of (γ-Fe2O3)xAg100−x (x=50–90), a granular system where insulating magnetic nanoparticles and nonmagnetic metal are intimately mixed, has been studied at room temperature. For high silver concentration (x⩽70), samples are metallic and exhibit ordinary positive magnetoresistance. Below the percolation threshold, which is 11.4 vol % of silver (x=71), samples are insulators. Our data suggest that direct tunneling between silver grains across γ-Fe2O3 barriers dominates at low temperature and variable range hopping becomes the main transport mechanism at high temperature. The variable range hopping within γ-Fe2O3 is believed to be associated with the presence of Fe2+ impurities which is determined by Mossbauer spectroscopy. Negative magnetoresistance up to −2% is found in (γ-Fe2O3)xAg100−x (x⩾72) in an applied field of 10 kOe at room temperature. It is proposed that the observed negative magnetoresistance is due to the field-dependent hopping rate of electrons from Fe2+ to Fe3+ which is enhanced due to the alignment of their moments by an applied magnetic field.

Low-temperature magnetoresistance in insulating a-Gd x Si 1-x alloys

Abstract: We report on low-temperature magnetotransport measurements on the amorphous semiconductor $a\ensuremath{-}{\mathrm{Gd}}_{x}{\mathrm{Si}}_{1\ensuremath{-}x}$ with $x\ensuremath{\sim}0.13,$ on the insulating side of the $T=0$ metal-insulator transition for this material. The samples exhibit a negative magnetoresistance of more than five orders of magnitude at 1 K, which grows exponentially larger at lower temperatures. The temperature dependence of the conductivity displays an activated form indicative of variable range hopping in the presence of a Coulomb gap in all magnetic fields from 0 to 9 T, while the characteristic temperature ${T}_{0}$ of the hopping conductivity decreases from over 300 K in 0 T to 6 K in 8.5 T. This enormous magnetoresistance must arise from an exchange interaction between the conduction electrons and the local Gd moments which are randomly oriented in zero field and become (partially) aligned in 9 T, and its consequent influence on the electron-electron interaction. However, it remains to be determined whether the effect of this magnetic interaction is a relative shift of the Fermi energy relative to the mobility edge and a modification to the Coulomb gap or a change in the bandwidth and its consequent effect on the electronic density of states.

Properties of CdSe polycrystalline thin films grown by chemical bath

Abstract: CdSe polycrystalline thin films were grown onto glass substrates by chemical bath deposition at 65°C. Structural, optical, and electrical properties of samples were characterized. The layers grew in the cubic phase as evidenced by X‐ray diffractograms. Using the first derivative of the optical absorption vs. phonon energy curves, two transitions were found, the fundamental at 1.8 eV and a second one at 2.2 eV, respectively. It is worthwhile to mention that the second transition has been reported only for the hexagonal CdSe modification. From the dark conductivity vs. 1/KT behavior on the 100–500 K range, it could be determined an activation energy of 0.3 eV at higher temperatures (>350 K) and a behavior following the variable range hopping model of Mott was satisfied at lower temperature. Electron dispersion spectroscopy measurements indicated stoichiometric CdSe material within the 1 % error bar. © 1999 The Electrochemical Society. All rights reserved.

Electrical properties of a periodic porous carbon replica of opal

Abstract: The electrical conductivity and magnetoconductance of pyrolized porous phenolic resin opal replicas have been studied as a function of their heat treatment temperature (HTT) up to 2380°C. Porous graphite can be formed by pyrolizing the phenolic resin opal replica. In the case of replicas with a low HTT, a crossover from Mott variable range hopping (VRH) to Efros and Shklovskii VRH upon decreasing the temperature has been experimentally observed. For those with a high HTT, the electrical properties are consistent with those of pyrolytic graphite. The occurrence of positive magnetoconductance indicates the existence of quantum effect at low temperatures.

Fabrication and low-temperature characterization of Si-implanted thermistors

Abstract: We have fabricated and characterized Si-implanted thermistors to be used as phonon sensors in very-low-temperature, single-quantum detectors. After a short review of the required thermistor properties for this type of application, a detailed description of the production process and of the characterization experimental techniques is reported. The data show that the resistivity-temperature behaviour of all of the devices follows the prediction of the variable range hopping conduction model in the investigated temperature range (4.2-0.03 K). Phonon-electron decoupling and excess low-frequency noise show up at low temperatures, reducing the thermistor sensitivity. These phenomena are discussed and conveniently parametrized in view of a complete detector optimization.

Variable-range-hopping conduction and metal-insulator transition in Cu-doped BaTiO3

Abstract: Cu-doped BaTiO3 polycrystals with the nominal composition of (x = 0.1 - 1) were synthesized. The samples are multiphase. The metallic conducting behaviour was seen for the sample with x = 0.5, while semiconducting behaviour appears for the other compositions. For x = 0.4, variable-range-hopping conduction was observed. The results are briefly discussed. It is proposed that the BaTiO3 phase modified by Cu doping might be responsible for the metallic behaviour observed.

Polaronic transport properties of some vanadate glasses: Effect of alkali-earth oxide modifiers

Abstract: Temperature and composition dependence of the electrical conductivity of alkali-earth vanadate glasses formed with ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$ as a unique glass network former have been investigated in the temperature range 80\char21{}500 K. The results for these glasses have been compared with those for the traditional vanadate glasses. The conductivity and the activation energy for the alkali-earth vanadate glasses having ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$ content g70 mol % depend slightly on the nature of the alkali-earth oxide. However, for the glasses with ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$ content \ensuremath{\leqslant}70 mol % the conductivity changes significantly with the nature and the content of alkali-earth oxides. The composition dependence of the electrical properties of the alkali-earth vanadate glasses has been observed stronger than those for the traditional vanadate glasses. The results for the alkali-earth vanadate glasses have been interpreted in terms of polaronic hopping models. The multiphonon assisted hopping model of small polarons in the nonadiabatic regime, which considers the strong interaction of electrons with both the optical and acoustical phonons, is the best to interpret the temperature dependence of the conductivity data of these glasses over the entire temperature range of measurement. The parameters obtained from the fits of the experimental data to this model appear reasonable and are consistent with the glass composition. On the other hand, Mott's optical phonon assisted hopping model at high temperature yields small values for the localization length, while Mott's variable range hopping model is consistent with the low-temperature data only. Schnakenberg's model provided higher values of the hopping and disorder energies than the activation energies obtained at the highest and the lowest temperature ranges, respectively. The addition of the alkali-earth oxide modifiers to the vanadate network alters the hopping mechanism from adiabatic to nonadiabatic.

Variable-range hopping : role of coulomb interactions

Abstract: The effect of Coulomb interactions on hopping conduction in the variable-range hopping regime is analyzed within a linear-response formalism. Here the conductivity and the dielectric function are related to the density-density response function for which a generalized master equation (GME) can be derived using the Mori-Zwanzig projector formalism. The GME can be thought of as a random resistor network with frequency-dependent internode conductances, whose values can be determined from a function related to the current-current correlator at the two nodes. We evaluate the internode conductances using a diagrammatic perturbation formalism. For a single electron hop with all the other charges frozen, we obtain hop rates correct to all orders in Coulomb interaction. This gives us a finite temperature generalization of existing results for the interacting system. We then incorporate relaxation effects that accompany electron hops, using a dynamical model of the Coulomb gap. We argue that the parameter that governs the local relaxation is related to the conductivity itself. These internode conductances are then used to calculate the dc conductivity of the network by effective- medium approximation. We show that a crossover from Efros-Shklovskii's ${T}^{1/2}$ behavior to Mott's ${T}^{1/4}$ behavior occurs due to the relaxation effects, as the temperature is increased. At low temperatures the relaxation is slow so that electrons hop in a frozen charge background and thereby sense the Coulomb gap. This gives the ${T}^{1/2}$ behavior. At higher temperatures the relaxation gets faster and the Coulomb gap is alleviated leading to Mott's behavior.

Variable-range hopping transport in modulation-doped n-channel Si/Si1-xGex quantum well structures

Abstract: We have measured the temperature- and field-dependent longitudinal conductivity, xx, in the Landau level tails of two modulation-doped Si/Si1-xGex two-dimensional electron gas samples at temperatures below 1 K. The temperature dependence of xx at the minima of the Shubnikov-de Haas oscillations obeyed a relation of the form xxmin(T)(1/T)exp[-(T0/T)1/2], in agreement with published models of variable-range hopping between localized states by Ono and by Polyakov and Shklovskii. However, the value and magnetic field dependence of the characteristic temperature, T0, cannot be explained quantitatively on Ono's model, which is based on Gaussian localization of the electron wavefunction on a scale given by the magnetic length. Polyakov and Shklovskii used exponential wavefunctions to derive an alternative expression for the characteristic temperature, and to model the conductivity in the vicinity of the peaks between adjacent quantum Hall plateaux. Our results have been analysed according to this theory, and show good agreement: the magnetic field dependence of the corresponding characteristic temperature, T1(), obeys the power law relation, T1(), as expected from theory, while the experimental value of , 0.90±0.07, agreed with that determined from a half-width analysis of the xx peaks; however, this value differs from the theoretically predicted figure of ~2.3.

Electrical properties of in doped cds thin films

Abstract: Indium doped polycrystalline cadmium sulphide 〈CdS:In〉 thin films have been prepared by the spray pyrolysis technique on glass substrates in an enclosed dome. The different scattering mechanisms such as lattice, impurity and grain boundary scattering for CdS:In films are observed at low temperature, in the range of 303 to 120 K. The experimentally determined mobilities due to these scatterings are well interpreted with those of theoretically calculated mobilities. The d.c. conductivity for CdS:In films has also been studied in the same temperature region. The Mott variable range hopping conduction process followed below the temperature of 150 K. The Mott parameters such as N(EF), R, W and α are found to be 1.26 × 1019 eV−1cm−3, 9.8 × 10−-7cm, 0.02 eV−1 and 2.38 × 106cm−1, respectively from the conductivity data.