Showing papers on "Variable-range hopping published in 1996"

Anneal‐tunable Curie temperature and transport of La0.67Ca0.33MnO3

Abstract: Resistivity measurements on a La0.67Ca0.33MnO3 film are reported for a series of argon anneals at successively higher temperatures. Tc, the ferromagnetic ordering temperature, increases uniformly with increasing annealing temperature and annealing time. Hence, Tc can be tuned by appropriate annealing. In order to fully anneal these samples, i.e., achieve bulk properties, it proves sufficient to anneal them in argon. Further annealing in oxygen produces only minor changes in the resistivity. Data from Tc up to 1200 K show activated conduction with ρ=BTeEa/kT, the temperature dependence predicted by the Emin–Holstein theory of adiabatic polaron hopping. Their model fits both data from the partially annealed and fully annealed samples better than the variable range hopping or semiconductor models which have been used by previous workers. The activation energy Ea and resistivity coefficient B decrease with increasing maximum anneal temperature. These changes, together with the increase in Tc, are consistent w...

Mechanism of charge transport in polypyrrole, poly(N‐methyl pyrrole) and their copolymers

Abstract: The temperature dependence of the dc conductivity of electrochemically polymerized films of polypyrrole, poly(N‐methyl pyrrole) and their copolymers, poly(N‐methyl pyrrole‐pyrrole), having different percentage of BF−4 ions has been investigated in the temperature range 77–350 K. The observed behavior could be explained in terms of Mott’s variable range hopping model involving a single phonon process. The estimated values of polaron radius yield the realistic values of density of states at the Fermi level which are in good agreement with the values reported earlier for other polyconjugated systems.

Universal Crossover between Efros-Shklovskii and Mott Variable-Range-Hopping Regimes.

Abstract: A universal scaling function, describing the crossover between the Mott and the Efros-Shklovskii hopping regimes, is derived, using the percolation picture of transport in strongly localized systems. This function is agrees very well with experimental data. Quantitative comparison with experiment allows for the possible determination of the role played by polarons in the transport.

Hopping conduction in SiO2 films containing C, Si, and Ge clusters

Abstract: Temperature dependence of electrical characteristics of SiO2 films doped with C, Si, and Ge were studied. The conductivity σ was found to vary with the temperature as lnσ∝T−1/4 over a wide temperature range, indicative of the conduction by the variable range hopping (VRH) mechanism. Since the previous optical studies for the same films indicate the existence of clusters in the films, it is very likely that the VRH through localized states associated with clusters dominates the conduction process, irrespective of the kind of group IV elements.

Carrier transport in amorphous silicon-based thin-film transistors studied by spin-dependent transport

Abstract: Carrier transport processes in hydrogenated amorphous silicon-based thin-film transistors (a-Si:H TFT's) are investigated by spin-dependent transport (SDT). Spin-dependent photoconductivity (SDPC) signals arising from less than ${10}^{6}$ spins in a small transistor are detected with an adequate signal-to-noise ratio. SDPC measurements reveal two different limiting steps for the light-induced leakage current in TFT's depending on the gate voltage: bulk recombination in undoped a-Si:H and recombination near the source junction. Also, the leakage current mechanism under high source-drain fields is identified by SDT measurements in the dark as electron hopping via defect states located at the interface between undoped a-Si:H and the passivation silicon nitride layer. Both silicon dangling bonds and nitrogen dangling bonds seem to be involved in the electron hopping process. At temperatures below 100 K, spin-dependent hopping of electrons in conduction-band tail states is observed. The change of the dominant transport path from extended state conduction to variable range hopping conduction with decreasing temperature is confirmed by SDT measurements. \textcopyright{} 1996 The American Physical Society.

Effective one-dimensionality of universal ac hopping conduction in the extreme disorder limit

Abstract: A phenomenological picture of ac hopping in the symmetric hopping model ~regular lattice, equal site energies, random energy barriers! is proposed according to which conduction in the extreme disorder limit is dominated by essentially one-dimensional ‘‘percolation paths.’’ Modeling a percolation path as strictly one dimensional with a sharp jump rate cutoff leads to an expression for the universal ac conductivity that fits computer simulations in two and three dimensions better than the effective medium approximation. @S0163-1829~96!06745-8#

Electrical conductivity of V2O5SrOB2O3 glasses

Abstract: Glass samples from the V2O5SrOB2O3 ternary system were prepared by the rapid quenching method The glass formation region determined by X-ray diffraction analysis was in the range of V2O5 30 to 85 mol%, SrO 15 to 50 mol% and B2O3 0 to 40 mol% The electrical conduction mechanism in these glasses is discussed in terms of the small polaron hopping theory by Mott At temperatures higher than room temperature 1n σ versus T−1 is approximately linear and exhibits small polaron hopping conduction However at temperatures below room temperature, the conductivity (σ) was proportional to T − 1 4 This fact shows that the conduction may be due to variable range hopping (VRH)

Einstein's relationship for hopping electrons

Abstract: Photoconductivity in amorphous semiconductors at very low temperatures is temperature-independent, being determined by the energy-loss hopping of carriers through localized band-tail states. In such a hopping relaxation, neither diffusion coefficient, D , nor mobility of carriers, μ, depend on temperature and the conventional form of the Einstein's relationship μ = eD kT is not valid. The relationship between μ and D for the hopping relaxation of electrons in the exponential band tail is calculated and it is shown that it has the form μ ∝ eD ϵ 0 , where ϵ 0 is the energy scale of the exponential band tail.

Low-temperature relaxations of charge carriers in disordered hopping systems

Abstract: Energetic relaxation, drift, diffusion and recombination of charge carriers are considered at low temperatures in disordered hopping systems. It is shown that, due to its scaling form, an exponential energetic distribution of the density of localized states (DOS) generates very specific transport characteristics which cannot be obtained for other types of DOS functions. In contrast with other distributions, only an exponential DOS function allows (i) the introduction of a low-temperature analogue of the Einstein relation between carrier mobility and diffusivity for some initial time domain of relaxation and (ii) the description of carrier drift in strong electric fields in terms of a field-dependent effective temperature.

Crossover from Mott to Efros-Shklovskii variable range-hopping in Si:P

Abstract: The low-temperature conductivity of uncompensated insulating Si:P with P concentration just below the metal-insulator (MI) transition shows with decreasingN a crossover from Mott variable range hopping (VRH) to Efros-Shklovskii VRH. From the concentration dependence of the Mott temperatureT M a correlation-length exponent ν=1.1 is obtained which is compatible with the conductivity exponent μ=1.3 for metallic samples.

Transport mechanism in nonconventional bismuth cuprate glass

Abstract: Temperature and compositional dependence of the electrical transport properties of the nonconventional binary bismuth cuprate glasses are reported for the first time. It has been observed that the phonon assisted small polaron hopping model in the nonadiabatic regime is consistent with the data only at high temperatures. The variable range hopping model can fit the low temperature data qualitatively. The polaron hopping models proposed by Schnakenberg [Phys. Status Solidi 28, 623 (1968)] and Emin [Phys. Rev. Lett. 32, 303 (1974)] can predict quantitatively the conductivity data in the entire temperature range of measurement for all glass compositions. The physical parameters obtained from the best fits of these models are found reasonable and consistent with the glass compositions.

Universal spin-induced magnetoresistance in the variable-range hopping regime

Abstract: The magnetoresistance in the variable-range hopping regime due to Zeeman spin-splitting and intra-impurity interactions is calculated analytically and shown to be a universal function of μH/kT log R. Good agreement with numerical calculations in one and two dimensions is observed. With the inclusion of quantum interference effects, excellent agreement with recent experiments is obtained.

Absence of the Coulomb gap at the Fermi level in the variable-range hopping regime of zinc stannate polycrystalline systems

Abstract: Measurements of the direct current resistivity, , on ceramic zinc stannate polycrystalline samples that had been annealed in a reducing atmosphere at temperatures of C, C, C and C were made as a function of temperature T from room temperature down to the liquid nitrogen temperature. In this temperature range, the data showed that the main contribution to the conductivity comes from carriers that hop directly between localized states executing variable-range hopping processes. Analysis of the data showed that the resistivity - temperature characteristics were well described by the Mott law, confirming the absence of a Coulomb gap at the Fermi level and suggesting that the density of states at the Fermi level is constant. The variation in the characteristic hopping temperature with the annealing temperature showed that was thermally activated with an activation temperature of .

Charge transport in silicon carbide: Atomic and microscopic effects

Abstract: It is shown that charge transport in SiC ceramics includes atomic mechanisms as well as phenomena which depend on the microstructure of the material. Both aspects are revealed by the analysis of temperature‐dependent dc and ac measurements. The complex dielectric function (DF) of boron‐doped SiC ceramics with various additives has been measured at frequencies from 5 Hz to 2 GHz and at temperatures between 100 and 330 K. In addition, the dc conductivity was measured between 40 and 220 K. A transport mechanism on an atomic scale determines the temperature dependence of the dc conductivity. At low temperatures 3D variable range hopping between boron impurity states or point defects takes place whereas at higher temperatures Arrhenius‐like carrier activation becomes dominant. The ac behavior depends on the dc conductivity, but it reflects phenomena on a larger microscopic scale as well. The real part of the DF has huge values of up to 104. Two polarization processes have been identified. The low‐frequency pro...

On the description of hopping-energy relaxation and transport in disordered systems

Abstract: Some recent theoretical approaches to the hopping of electrons through localized band-tail states are discussed. Particular attention is given to the approximation in which energy relaxation of non-equilibrium electrons is treated as diffusion in energy space and to the approximation based on the averaging of hopping rates. It is shown that both approaches lead to results different from those based on Mott's semiquantitative technique with averaging hopping distances, not the rates. Detailed analysis shows that both diffusion approximation and averaging of hopping rates are inapplicable to hopping relaxation and transport of carriers in band tails.

Electrical properties of semiconducting (1−x)(90V2O5–10P2O5)+xBaTiO3 glass and glass–ceramics

Abstract: Homogeneous (1−x)(90V2O5–10P2O5)+x(BaTiO3) glasses (abbreviated as VPBT) are formed with lower concentrations (x) of BaTiO3 (x 0.3 microcrystallization of BaTiO3 appears during glass formation (corresponding samples are termed as glass–ceramics). All of these oxide glasses show semiconducting behavior. Electrical conductivity data measured between 80 and 450 K follow the adiabatic small polaron hopping conduction mechanism for glasses with lower concentrations (x 0.2) the said conduction mechanism is changed to the nonadiabatic regime. Low temperature (T≤150 K) conductivity data of these glass samples can be well fitted with the variable range hopping model. At higher temperatures, Mott’s model of phonon assisted small polaron hopping between nearest neighbors is consistent with the conductivity data. The polaron hopping model of Schnakenberg can predict the conductivity data in the entire temperature range of measurement. However, the percolati...

Conductivity and Structural Studies on Disordered Amorphous Conducting Carbon Films

Abstract: Amorphous carbon films are prepared by the pyrolysis of Tetra Chloro Phthalic Anhydride (TCPA) at different temperatures (700 degrees C to 900 degrees C). DC Conductivity measurements are done on the films in the temperature range 300K to 4.2K. It shows an activated temperature dependence with a small activation energy (0.02eV to 0.003eV). Variable range hopping is observed at low temperatures. The films are characterised by XRD, SEM, TEM, AFM and microRaman. The electronic structure of the film is used to explain the electrical behaviour.

Metal - insulator transition in Sb-doped short-period Si/SiGe superlattices

Abstract: Symmetrically strained Si/SiGe superlattices (25 A wells/14 A barriers) with homogeneous Sb doping concentrations of 4.5 and show low-temperature magnetoconductivity effects which can be explained by single-electron backscattering and disorder-induced electron - electron interaction for an anisotropic three-dimensional case. The insulating phase of two lower doped samples (2.0 and ) is documented by the observation of variable range hopping conductivity. In addition, the extrapolated zero-temperature conductivities for in different magnetic fields show a magnetic-field-induced metal-to-insulator transition, demonstrating a three-dimensional behaviour of this superlattice with respect to localization.

Non-adiabatic small-polaron hopping conduction in SrTiO3-doped 90V2O5-10Bi2O3 glassy semiconductors

Abstract: Dc electrical conductivities [sgrave]dc of 90V2O5−10Bi2O3 (VB) glasses doped with different SrTiO3 concentrations (0–40 wt%) are reported between 80 and 450 K. At higher temperatures, the Mott model of phonon-assisted small-polaron hopping between nearest neighbours is consistent with the conductivity data while, at low temperatures, the variable-range hopping model is found to be appropriate. The Holstein model indicates that, in the high-temperature regime, hopping occurs by a non-adiabatic process in contrast with the vanadate (VB) glasses. The percolation model of Triberis and Friedman applied to the small-polaron hopping regime is found to be consistent with the conductivity data. However, the consistency of the low-temperature conductivity data with the prediction of different theoretical models cannot be easily distinguished for these glasses. The estimated model parameters such as density of states, localization length and carrier mobility are found to be consistent with the formation of ...

Variable range hopping conduction in LaC2, CeC2, or GdC2 crystals encapsulated carbon nanocages

Abstract: An experimental study of electrical transport phenomena in magnetic fields H up to 15 T in pressed samples of three porous systems composed of carbon nanocages in which LaC2, CeC2, or GdC2 crystals are encapsulated is reported. The exp[(T0/T)1/3] temperature dependence in the zero‐field dc electrical resistance R(0) is found at low temperatures. Below 50, 50, and 35 K for the LaC2, CeC2, and GdC2 system, respectively, transverse magnetoresistance (MR), [R(H)–R(0)]/R(0), is negative. In the weak field region (H<0.15 T), the negative MR is quadratic in H, and becomes linear in the field range 0.15–0.5 T. The linear and negative MR shows T−1.04±0.10 temperature dependence. The temperature dependence of R(0) is described in terms of Mott’s law for two‐dimensional variable range hopping (2D VRH) conduction. The low field magnetotransport features support a predicted quantum interference effect on 2D VRH conduction.