Showing papers on "Variable-range hopping published in 1985"
Book•
01 Jan 1985
TL;DR: In this article, the authors present a diagram technique for strong electron-phonon coupling in the presence of a magnetic field and the Potts model for percolation problems.
Abstract: Basic Elements of the Theory of Hopping Transport Small Polarons Hopping Conduction in Disordered Systems Classical Hopping Transport Recent Developments Appendix 1. Diagram Technique for Strong Electron-Phonon Coupling Appendix 2. Rate Equation in the Presence of a Magnetic Field Appendix 3. Percolation Problems and the Potts Model References.
576 citations
TL;DR: Using the CTRW approximation, the authors constructed the simplest possible nontrivial model of ac hopping conductivity in disordered solids, which predicts universality of the frequency-dependent conductivity, independent of temperature, chemical composition and conductivity mechanism, in resonably good agreement with experiments.
121 citations
TL;DR: In this paper, the current and energy decay in amorphous semiconductors is studied within a new theoretical approach based on nonequilibrium hopping transport between localized states in a band tail.
Abstract: Within a new theoretical approach the current and energy decay in amorphous semiconductors is studied. The relaxation of photoexcited carriers observed by transient photoconductivity experiments in amorphous silicon and chalcogenides can well be described by an algebraic power-law decay. The theoretical explanation is based on nonequilibrium hopping transport between localized states in a band tail. The theory reproduces the typical temperature dependence of the decay param- eter commonly explained within a multiple-trapping (MT) model. The observed deviations from the simple MT relation at low temperatures can be explained by the theory. Additionally, direct calculation of the energy relaxation reveals further insight into the dissipation process.
50 citations
TL;DR: In this article, the effect of electron-electron interactions on the thermoelectric power for hopping transport in two and three dimensions is calculated in the low-temperature limit.
Abstract: The effect of electron-electron interactions on the thermoelectric power for hopping transport in two and three dimensions is calculated in the low-temperature limit. While the conductivity requires a very precise measurement to extract a temperature exponent with enough precision to determine the role of the interactions, it is shown that the thermopower behaviour will reflect the presence of interactions in an unambiguous manner. In contrast to a Mott variable-range hopping model where the thermopower decreases to zero as the temperature decreases (S=0 at T=0), the thermopower in an Efros interaction mode, should approach a non-zero constant as T to 0.
41 citations
TL;DR: The metal-insulator transition in a-Si1−xAux system was studied through electrical conductivity measurements and tunnelling experiments at temperatures down to 10 mK and in magnetic fields up to 5 T as discussed by the authors.
Abstract: The metal-insulator transition in a-Si1−xAux system was studied through electrical conductivity measurements and tunnelling experiments at temperatures down to 10 mK and in magnetic fields up to 5 T. This system makes a metal-insulator transition at xc ∼ 0.14 at low temperatures. As the variable range hopping conduction has been observed in an insulating side, this transition has a character of Anderson transition. On the other hand, it has also a character of Mott transition, because the tunnelling experiments showed that a correlation gap begins to open at xc when x approaches xc from the metallic side. The existence of a strong spin-orbit interaction was revealed by the studies of temperature and magnetic field dependences of electrical conductivity σ(T, B) of metallic a-Si : Au thin films. The zero-temperature limit conductivity σ0 continuously goes to zero as the content of gold approaches xc from the metallic side. This behaviour is expressed by σ0α |x/xc − 1|s with xc ∼ 0.14 and s ∼ 1.0 both in 0 T and 5 T. The value of 1.0 is consistent with the one obtained by Altshuler and Aronov for interacting electrons in disordered electron system with a strong spin-orbit interaction.
33 citations
TL;DR: In this paper, the authors apply percolation theory to the small polaron hopping regime and evaluate the thermoelectric power at low and high temperatures for a situation in which we have a symmetrical band of localized states with the Fermi level in the middle, and for an asymmetrical case in which there are multiple localized states above the level of fermi.
Abstract: Percolation-theoretic considerations have been used by various workers to evaluate the dc hopping conductivity and the thermoelectric power in disordered systems. We apply percolation theory to the small polaron hopping regime. Expressions for the thermoelectric power at low and high temperatures are obtained for a situation in which we have a symmetrical band of localized states with the Fermi level in the middle, and for an asymmetrical case in which we have a band of localized states above the Fermi level, and the conductivity for the low temperature case is evaluated. The correlation between bonds due to the energy of the common site is included in the present treatment.
32 citations
TL;DR: The results of an extensive study of the transport properties of this regime coordinated with magnetic, optical and structural experiments, using ClO4 − and l3 − doped (CH)x are presented in this paper.
Abstract: Nearly metallic polyacetylene doped to the level, y, of -0.01 to 0.05 acceptors or donors per carbon has a high, “nearly metallic” conductivity yet susceptibility very much less than the metallic value. We present here the results of an extensive study of the transport properties of this regime coordinated with magnetic, optical and structural experiments, using ClO4 − and l3 − doped (CH)x. We conclude that for all samples studied, the charge conduction is via three dimensional variable range hopping among states near the Fermi level, cF. The variation of the density of states, N(ϵ), near ϵF, is measured using susceptibility, and shown to determine the detailed transport behavior. Our results are in agreement with the transition to an incommensurate Peierls semiconductor for higher doping levels, with disorder leading to a gapless condition.
27 citations
TL;DR: In this article, the authors present magnetotransport data on the magnetic semiconductor Gd−xvxS4 (v = vacancy) and show that at low temperatures (T ∼ 10 mK) the conductivity σ(0) ∝ (H - Hc) is a critical field below which the material is an insulator.
Abstract: This note reviews magnetotransport data on the magnetic semiconductor Gd−xvxS4 (v = vacancy). New data on the metallic side of the transition are presented. The results for two samples may be summarized as follows: 1) At low temperatures (T ∼ 10 mK) the conductivity, σ(0) ∝ (H - Hc), where Hc is a critical field below which the material is an insulator; 2) for the sample with the larger critical field, the data are not consistent with variable range hopping: 3) for H/Hc ≳ 1, the temperature dependence deviates only little from a √T law.
17 citations
TL;DR: In this article, a simple theory of phonon assisted hopping 1 f noise is presented for hydrogenated amorphous silicon and the noise shows a Lorentzian, indicative of generation-recombination noise.
Abstract: Excess noise measurements have been carried out in nonhydrogenated amorphous silicon material at room temperature. The temperature dependence of the conductivity has shown that, at room temperature, noise measurements deal with transport mechanisms dominated by hopping to nearest neighbors. At lower temperature there is variable range hopping from which the number of states near the Fermi level was determined. The ohmic behavior of structures such as molybdenum-amorphous silicon-molybdenum, and the V2 dependence of the noise spectral density indicate that 1 f noise is characteristic for this material. A simple theory of phonon assisted hopping 1 f noise is presented. From this the fluctuations. Some preliminary data are shown for hydrogenated amorphous silicon. The noise shows a Lorentzian, indicative of generation-recombination noise.
16 citations
TL;DR: In this article, the Hall effect observed in the region of hopping conduction is discussed on a percolative picture of hop conduction, and it is suggested that the results can be interpreted in terms of variable-range hopping if one assumes structure around the Fermi level in the hopping band.
Abstract: Electrical resistivity and Hall effect have been measured on phosphorus-doped silicon with Compensation ratio K ranging from 0 to 0·98. The experimental results are analysed with a view to clarifying the mechanism of hopping conduction. By comparing the plots of resistivity versus temperature for these samples, it is suggested that the results can be interpreted in terms of variable-range hopping if one assumes structure around the Fermi level in the hopping band. The Hall effect observed in the region of hopping conduction is discussed on a percolative picture of hopping conduction.
16 citations
TL;DR: In this paper, the combination of the tunneling recombination and the variable range hopping of carriers in the band tail was investigated for photoluminescence decay in amorphous As2S3.
TL;DR: In this article, the correlation effects of spinless fermion model on the electrical SP hopping conductivity were investigated, and the particle transfer was treated in lowest non-vanishing order of perturbation theory.
Abstract: On the basis of a spinless fermion model, which takes into account the intersite Coulomb interaction and the formation of small polarons (SP) due to the interaction with phonons, the correlation effects (due to the SP-SP interaction) on the electrical SP hopping conductivity are investigated. The particle transfer is treated in lowest nonvanishing order of perturbation theory, and the case of non-adiabatic SP hopping is considered. The polaronic short-range order (SRO) results in an additional activation of the dc conductivity due to hopping processes without and with changes of the Coulombic configurational energy. For sufficiently low SRO the dc conductivity is enhanced relative to its magnitude in the case of non-interacting SP. The SRO leads to an additional optical absorption and a positive shift of the absorption spectrum. The theory maybe relevant for the description of inhomogeneous mixed-valence compounds and superionic conductors.
TL;DR: In this article, the results of coordinated measurements of DC and microwave conductivity of polyacetylene doped with iodine to moderate levels are presented and discussed within a hopping model using the extended pair approximation.
Abstract: Results of coordinated measurements of DC and microwave conductivity of polyacetylene doped with iodine to moderate levels are presented and discussed within a hopping model using the extended pair approximation. At low temperatures the DC data follow Mott'sT−1/4 law indicating similarities between polyacetylene and amorphous semiconductors. A detailed analysis gives a qualitative description of the evolution of the density of states near the Fermi energy upon doping.
TL;DR: In this article, the conductivity sigma for electron transport via strongly localised electron states in a one-dimensional MOSFET has been determined by the direct solution of the Miller-Abrahams equivalent network equations.
Abstract: The conductivity sigma for electron transport via strongly localised electron states in a one-dimensional MOSFET has been determined by the direct solution of the Miller-Abrahams equivalent network equations (1960). Random fluctuations of log sigma proportional to T-1/2 are observed as the chemical potential mu moves through an energy band of width W. The fluctuations are smaller than those determined recently by Lee (1984) using a percolation argument and the proportionality constant is 60% higher. As the temperature decreases the fluctuations increase from approximately one order of magnitude when kT/W=0.008 to approximately nine orders of magnitude when kT/W=0.001. The conductivity decreases to an asymptotic limit as the length of the system increases.
TL;DR: In this paper, low-temperature data for variable-range-hopping conductivity in lightly doped, compensated n-InP was used to show that the corresponding Mott law for this conductivity mechanism is also an adequate description for three-dimensional systems.
Abstract: The authors low-temperature data for variable-range-hopping conductivity in lightly doped, compensated n-InP gives: sigma = sigma 0 exp(-(T0/T)s) with 0.21
TL;DR: In this paper, magnetoresistance measurements on uncompensated Si have been made in the temperature range 0.050 K < T < 4.2 K and in magnetic fields up to 23 Tesla.
Abstract: Magnetoresistance measurements on uncompensated Si:As samples in the concentration range (6.6 × 1018 to 10.2 × 1018/cc) spanning the MI transition have been made in the temperature range 0.050 K < T < 4.2 K and in magnetic fields up to 23 Tesla. The zero-field temperature dependence of σd.c for barely insulating samples exhibits Mott variable range hopping behavior. Insulating samples show a strong magnetoresistance characteristic of the percolation model, but with a much strong density dependence and a weakening temperature dependence as N → Nc. Metallic samples show a crossover from weakly metallic magnetoresistance behavior characteristic of Coulomb interactions to percolative behavior at a density-dependent crossover field suggesting the samples have been driven to a different type of conduction mechanism.
TL;DR: In this paper, a model based on orbital shrinking of the localized wave function (positive MR) and on a Zeeman shift of the energy levels (negative MR) was used to fit the data and yields the Bohr radius and the binding energy of the local states as well as the variation of the density of states at the Fermi level.
Abstract: has been measured between 4.2 K and 0.3 K in fields up to 3.4 Tesla. The data can be described by variable-range hopping (VRH) in a magnetic field and by postulating a second transport mechanism independent of the magnetic field. A model based on orbital shrinking of the localized wavefunction (positive MR) and on a Zeeman shift of the energy levels (negative MR) can be fit to the data and yields the Bohr radius and the binding energy of the localized states as well as the variation of the density of states at the Fermi level, which in most samples is found to be quadratic.
TL;DR: In this article, the authors show that the conduction occurs due to variable range hopping in the localized states near the Fermi level in the GexSe1−x sample.
Abstract: Bulk amorphous samples of GexSe1−x (0.5 ⩽ x ⩽ 0.7) were prepared by quenching. Dc conductivity measurements were carried out in the temperature range 77–300 K. In the low temperature region, the conduction occurs due to variable range hopping in the localized states near the Fermi level. The results are explained by Mott, Pollak and Butcher's models. Butcher's model which is based on the equivalent conduction network is compatible with the results.
TL;DR: In this article, the asymptotic behavior of impurity wave functions on distances much larger than mean interimpurity separation is determined by the behavior of an impurity electron by the other impurities situated near its tunneling path.
Abstract: Variable-range hopping conduction in semiconductors is determined by the asymptotic behavior of impurity wave functions on distances much larger than mean interimpurity separation. Scattering of an impurity electron by the other impurities situated near its tunneling path is shown to result in a correction Δa to electron localization lengtha. This correction depends on the impurity scattering length and impurity concentrationN and may be of the order ofa(Na3) ora(Na3)1/2.
TL;DR: The results of the temperature dependence of the DC electrical conductivity σ in hydrogenated amorphous silicon doped with metal impurities are reported in this article, where it is pointed out that this result is related with the transition from the hopping conduction associated with dangling bond electrons to that associated with the metal impurity.
Abstract: The results of the temperature dependence of the DC electrical conductivity σ in hydrogenated amorphous silicon doped with metal impurities are reported. The temperature dependence of log10σ as predicted by a simple variable range hopping model has not been observed with increasing impurity content. It is pointed out that this result is related with the transition from the hopping conduction associated with dangling bond electrons to that associated with the metal impurity. In addition, Meyer-Neldel rule is found in a-Si:H films doped with impurities.
TL;DR: In this paper, the wave function decay length α −1, the rate prefactor R 0, and the density of states ϱ f were derived for samples of R.F sputtered a-Ge:H, and some progress was made with the prefactor and parallel shift problems.
Abstract: The ac conductivity for samples of R.F sputtered a-Ge:H is qualitatively well explained by hopping in a constant density of states. The wavefunction decay length α −1 , the rate prefactor R 0 and the density of states ϱ f can be derived. However, their behaviour with hydrogenation is problematic so we generalize to a quadratic density of states. Some progress is made with the prefactor and parallel shift problems.
TL;DR: In this article, the authors performed ESR measurements on lapped and etched silicon samples and observed three kinds of defect centers: damaged silicon center, variable range hopping at low temperatures, and surface contamination or a bulk defect.
Abstract: Low-temperature transports in doped silicon near the metal-nonmetal transition depend strongly on the surface treatment processes; lapping, etching and thermal annealing. We have performed ESR measurements on lapped and etched silicon samples and have observed three kinds of defect centers. One, previously known as damaged silicon center, is related to variable range hopping at low temperatures. The other two probably come from a surface contamination or a bulk defect.
TL;DR: In this article, the intersoliton hopping rate for intramolecular and acoustic-phonon-mediated hopping was calculated, and used this to find electron mobilities, and the case of (N-methylphenazinium),54 (phenazine) 46 tetracyanoquino-dimethanide [(NMP) 54 (Phen)46., TCNQ] gave a maximum mobility at 100K between.06 and 1.0 cm 2/V-sec, due to intramoleslon-assisted hopping.
Abstract: Phonon-assisted electron hopping between solitons may contribute substantially to transport in the quarter-filled-band, quasi 1-dimensional (Id) compounds that exhibit large Coulomb repulsion between two electrons on the same site. We have calculated the intersoliton hopping rate for intramolecular- and acoustic-phonon-mediated hopping, and used this to find electron mobilities. Evaluation for the case of (N-methylphenazinium),54 (phenazine) 46 tetracyanoquino-dimethanide [(NMP) 54 (Phen)46., TCNQ] gives a maximum mobility at 100K between .06 and 1.0 cm 2/V-sec, due to intramolecular-phonon-assisted hopping.
01 Jan 1985
TL;DR: In this paper, the properties of systems whose transport is dominated by a single quenched fluctuation are discussed, and the Lee oscillations in the Matt hopping are specifically considered.
Abstract: This paper discusses the properties of systems whose transport is dominated by a single quenched fluctuation. The Lee oscillations in the Matt hopping are specifically considered.
TL;DR: The hydrazine intercalation complexes formed with the layered compound T-TaS2 exhibit very strong Fermi surface driven structural distortions as discussed by the authors, and they are non-metallic below room temperature due to Anderson localization of states near the FermI level which lies in the Peierls pseudo-gap.
Abstract: The hydrazine intercalation complexes formed with the layered compound T-TaS2 exhibit very strong Fermi surface driven structural distortions. They are non-metallic below room temperature due to Anderson localization of states near the Fermi level which lies in the Peierls pseudo-gap. Resistivity, ρ, and thermopower, S, measurements indicate that transport is by variable range hopping, and a proportionality is demonstrated between In ρ and /(S/T) over a wide temperature range. The conduction mechanism in this intercalation complex may be compared with that in lightly-doped polyacetylene, in which the Fermi level also falls in the Peierls gap.
Journal Article•
TL;DR: The van der Waals equation of state for a conduction-electron gas at concentrations near the Mott metal-to-insulator transition was examined in this paper, where it was shown that the MOTT concentration is universal for conductionelectron condensation.
Abstract: The van der Waals equation of state for a conduction-electron gas at concentrations near the Mott metal-to-insulator transition is examined. The Mott concentration is found to be universal for conduction-electron gas condensation.
TL;DR: In this article, it is argued that in those cases where the temperature dependence T of the d.c. electrical conductivity σ (T) as predicted by variable range hopping models is observed, the values obtained for α −1 (extend of the localised wave function) and N S (the density of localised carriers responsible for the transport) are in sharp disagreement with electron paramagnetic resonance.
Abstract: Various aspects of variable range hopping conduction in amorphous semiconductors are discussed. It is argued that in those cases where the temperature dependence T of the d.c. electrical conductivity σ (T) as predicted by variable range hopping models is observed, the values obtained for α −1 (extend of the localised wavefunction) and N S (the density of localised carriers responsible for the transport) are in sharp disagreement with electron paramagnetic resonance. To reconcile this disagreement it is proposed that the cases with “good” behaviour (in accordance with variable range hopping models) might be in fact reflecting the inhomogeneous nature of the studied systems.
01 Jan 1985
TL;DR: The first measurement of the magnetoresistance (MR) for amorphous semiconductors was made by Mell and Stuke (1970) and Mell (1974) as discussed by the authors, and the typical field dependence of MR for their evaporated a-Si sample is as follows: below about 1kg the MR increases approximately proportional to the absolute value of the field, then goes through a maximum and finally decreases down to negative values at high fields.
Abstract: The first measurement of the magnetoresistance (MR) for amorphous semiconductors was made by Mell and Stuke (1970) and Mell (1974) for amorphous Ge, Si, InSb and GeTe in the variable range hopping (VRH) conduction regime. The typical field dependence of MR for their evaporated a-Si sample is as follows: Below about 1kg the MR increases approximately proportional to the absolute value of the field, then goes through a maximum and finally decreases down to negative values at high fields. The positive contribution to MR increases with decreasing temperature and the absolute value of the negative contribution to MR also increases with decreasing temperature which is clearer in the case of a- Ge. Note that qualitative features of these field- and temperature- dependences are similar to those observed in the Anderson localized regime (Kobayashi and Muto,1979; Onuki et al., 1980).
TL;DR: In this paper, a trap-controled hopping in 1D systems is studied by means of the Berezinsky technique, where exact time and field dependences are given of the relaxation current produced by injected carriers.
Abstract: Trap-controled hopping in 1D systems is studied by means of the Berezinsky technique. In the case of a discrete trap niveau exact time and field dependences are given of the relaxation current produced by injected carriers. In an initial time interval any kind of traps acts as deep species and one obtains the same current kinetics as formerly found for the randomly broken bond model, i.e., some exponential dependences. At large times the traps acting as shallow species give rise to a diminished stationary current. The effect of spatial disorder in the hopping sites is discussed. Taking into account a small transverse hopping rate the 1D results are generalized to quasi-1D systems.
[Russian Text Ignored].