Showing papers on "Variable-range hopping published in 1984"
TL;DR: In this article, variable-range hopping in finite one-dimensional systems show large variations in the resistance as a function of chemical potential, which may explain recently observed structures in resistance versus gate voltage in narrow metaloxide-semiconductor field effect transistors.
Abstract: Numerical studies of variable-range hopping in finite one-dimensional systems show large variations in the resistance as a function of chemical potential. This may explain recently observed structures in resistance versus gate voltage in narrow metal-oxide-semiconductor field-effect transistors. Our explanation suggests the possibility of probing a particular pair of localized states.
121 citations
TL;DR: In this paper, a unified theory of the electrical current produced by electron hopping between localised states is presented, and the electron mobility in a multiquantum well structure is investigated.
Abstract: Presents a unifying theory of the electrical current produced by electron hopping between localised states. In this framework, the authors investigate the electron mobility in a multiquantum well structure. Finally, they compare the theoretical results with those experimentally obtained on a series of GaAs-GaxAl1-xAs superlattices.
79 citations
01 Jan 1984
TL;DR: In this article, the authors considered hopping conduction at temperatures which are so low that typical resistances between neighboring impurities become larger than those connecting some remote impurities whose energy levels happen to be very close to the Fermi level.
Abstract: This chapter deals with hopping conduction at temperatures which are so low that typical resistances between neighboring impurities become larger than those connecting some remote impurities whose energy levels happen to be very close to the Fermi level In this case the characteristic hopping length increases with lowering temperature (hence the name variable-range hopping, or VRH), and for a constant density of states one obtains the celebrated Mott’s law Derivation of this law is given in Sect 91 In that section it is also discussed how Mott’s law should be modified in the presence of a Coulomb gap Section 92 studies the effect of a magnetic field on hopping conduction in the VRH regime Section 93 describes a peculiar size effect which occurs in thin films of amorphous semiconductors with VRH conduction, and arises due to the finite volume accessible to current Finally, in Sect 94 we discuss theory of the pre-exponential factor in hopping conductivity, and compare results of different authors and their approaches to this problem
53 citations
TL;DR: In this paper, high resolution measurements of the Hall resistivity ρxy within the Hall plateaus show that the slope is finite at finite temperatures but decreases exponentially with inverse temperature.
31 citations
TL;DR: In this article, the conductivity and dielectric constant of quasi-one-dimensional orthorhombic TaS3 crystals are measured in the frequency range 3 to 105 Hz and temperature range 4.2 to 40 K.
Abstract: The conductivity and dielectric constant e′ of quasi-one-dimensional orthorhombic TaS3 crystals are measured in the frequency range 3 to 105 Hz and temperature range 4.2 to 40 K. Below 20 K the frequency-dependent component of conductivity is found to follow a σac ∼ ω8 law (s ≈ 0.8) and increase linearly with temperature; above 20 K no temperature dependence is found, while the frequency dependence is retained. The conductivity of samples with some disorder is shown to be due to variable range hopping near the chemical potential level. The value of E' is found to increase at lower frequencies and higher temperatures. High values of the dielectric constant (≈ 106) may be due to polarization of the pinned charge density wave.
[Russian Text Ignored].
19 citations
TL;DR: Amorphous Si1−xSnx alloys have been prepared by vapor deposition at a pressure of about 10−8 Torr on substrates maintained at 77 K in this article.
18 citations
TL;DR: In this article, an electrical conductivity and a thermopower of sintered Li 1.1 Ti 1.9 O 4 were measured and the Seebeck coefficient is proportional to temperature.
Abstract: An electrical conductivity and a thermopower of sintered Li 1.1 Ti 1.9 O 4 were measured. The Seebeck coefficient is proportional to temperature. The conductivity becomes larger with increasing temperature but it does not fit to the T 1/4 relation of a variable range hopping model nor to the relation for a small polaron hopping model at lower temperatures. To explain the transport, magnetic and thermal properties of Li 1+ x Ti 2- x O 4 , it is proposed a bipolaron state in a random field.
17 citations
01 Jan 1984
TL;DR: In this paper, the basic experimental facts related to hopping conduction, and the simplest models used in their interpretation are discussed, as well as the typical dependences of hopping conductivity on the temperature and impurity concentration.
Abstract: This chapter is concerned with the basic experimental facts related to hopping conduction, and the simplest models used in their interpretation. Section 4.1 describes the range of temperatures and degrees of compensation for which electrical conduction in semiconductors occurs by the hopping mechanism. It also shows the typical dependences of hopping conductivity on the temperature and impurity concentration. Section 4.2 shows how, following Miller and Abrahams, one can reduce the problem of calculating the hopping conductivity to that of calculating the conductivity of a random network of resistors connecting donor pairs. Naive approaches to that problem based on averaging either resistances or conductances are critically considered.
15 citations
TL;DR: In this paper, the effect of selenium-doping on the d.d.c. of germanium was investigated and it was shown that the addition of the element increased the amorphicity of the sample.
14 citations
TL;DR: In this paper, the authors calculated the ratio of the conductivity in the high-frequency regime (frequency greater than the maximum hopping frequency) and its dc value for doped trans-polyacetylene.
Abstract: Hopping mechanisms for conductivity are characterized by a broad distribution of hopping rates. We have calculated the ratio of the conductivity in the high-frequency regime (frequency greater than the maximum hopping frequency) and its dc value. The results for isoenergetic hopping and variable-range hopping are shown to differ, although both are found to be independent of the details of the hopping attempt frequency. These results are applied to the experimental data for doped trans-polyacetylene. Charge conduction in undoped and lightly doped trans-polyacetylene is in agreement with isoenergetic hopping consistent with intersoliton electron hopping. In contrast, the data for trans-polyacetylene doped to 0.01-0.05 ${\mathrm{I}}_{3}$ per carbon are in better agreement with variable-range hopping, consistent with the proposal of variable-range hopping among pinned solitons. These results support a change in the charge transport mechanism as polyacetylene is doped.
12 citations
TL;DR: The temperature dependence of dc conductivity σ has been measured in the temperature range T=100-500 K for a series of aSiHx:B thin-film specimens prepared by magnetron reactive sputtering.
Abstract: The temperature dependence of the dc conductivity σ has been measured in the temperature range T=100–500 K for a series of a‐SiHx: B thin‐film specimens prepared by magnetron reactive sputtering. Two groups of specimens were measured: (a) undoped specimens containing hydrogen concentration varying from zero to about 30 at. %, and (b) doped specimens containing 10 at. % hydrogen and boron concentration in the range 0.1–10 at. %. Activated band conduction (log σ∼T−1) was observed at high temperatures in undoped specimens containing about 5–10 at. % hydrogen. For all other specimens there is an extended temperature range of log σ∼T−1/4 consistent with charge transport by Mott’s variable range hopping mechanism. The density of states N(EF) at the Fermi level undergoes appreciable changes with increasing concentrations of hydrogen and boron. These changes are interpreted as evidence of substantial structural disorder in hydrogen‐rich and heavily doped a‐SiHx:B films.
TL;DR: In this article, the authors measured the electrical resistivity and thermoelectric power of evaporated amorphous antimony films from 85 K to their crystallization temperatures near ambient.
Abstract: We have measured the electrical resistivity and thermoelectric power of evaporated amorphous antimony films from 85 K to their crystallization temperatures near ambient. In addition to conduction by variable-range-hopping, as observed by others, we have identified a new contribution at higher temperatures due to thermally activated carriers. The thermoelectric power, which is here reported for the first time, appears also to be activated in the higher temperature range. Our analysis of the thermopower data in terms of bi-modal conduction implies an implausibly large contribution from variable range hopping which decreases with increasing temperature.
TL;DR: In this article, a coordinated study of the structural, optical, magnetic and transport properties of polyacetylene doped with iodine to these nearly metallic levels is conducted. And the measurements are in quantitative agreement with charge transport via variable range hopping among soliton-like levels.
TL;DR: In this paper, a study of d.c. and microwave conductivity and thermoelectric power of the organic conductor (TMTSF) 2 BrO 4 is presented.
01 Jan 1984
TL;DR: In this paper, it was shown that the density of states has two humps in its dependence on energy, and that the existence of correlation implies that the random resistor network model, which underlies the theory of hopping conduction, becomes strictly speaking inadequate.
Abstract: In this chapter it is shown that the density of states has two humps in its dependence on energy. Because of Coulombic correlations, the density of states vanishes at the Fermi level. This has an important effect on the temperature dependence of hopping conduction, especially in the variablerange hopping region. The existence of correlation implies that the random resistor network model, which underlies the theory of hopping conduction desribed in the preceding chapters, becomes strictly speaking inadequate. The model remains useful for physical problems, but its use requires special justification.
01 Jan 1984
TL;DR: In this paper, the effect of the magnetic field is taken into account with the help of a kinetic equation or an equation for the density matrix, which is a common tool in semiconductor physics.
Abstract: Measurement of magnetoresistance is a common tool in semiconductor physics. When the conduction is due to band carriers, the resistivity has a power-law dependence on the magnetic field. The theory of this phenomenon is well developed. The effect of the magnetic field is taken into account with the help of a kinetic equation or an equation for the density matrix.
TL;DR: In this paper, the eigenstate tunneling theory leads to some unexpected results for the temperature dependence of the conductance G(T), which may sometimes be metallic or even non-monotonic.
TL;DR: In this paper, the effect of thermal annealing on the optical and electrical properties of amorphous silicon (a•Si) was studied on the samples prepared by radio-frequency sputtering.
Abstract: The effect of thermal annealing on the optical and electrical properties of amorphous silicon (a‐Si) was studied on the samples prepared by radio‐frequency sputtering. Hydrogen evolution is likely responsible for the change in optical absorption, while oxygen incorporation during sputtering is probably responsible for the change in electrical conductivity after thermal annealing. Our data indicate the possibility of the variable range hopping in unhydrogenated a‐Si films.
TL;DR: In this paper, the preparation of amorphous films of WO3-x ·yH2O was described, and the electrical conductivity and thermopower were measured in the temperature range 100 to 500 K.
Abstract: This paper briefly describes the preparation of amorphous films of WO3-x ·yH2O. The films were coloured either photo- or thermo-chromically, and the electrical conductivity and thermopower were measured in the temperature range 100 to 500 K. The conductivity [sgrave] could be described by the relationship [sgrave] = [sgrave]0 exp [T/T 0]−¼, and the thermopower S was proportional to T ½. Both transport parameters are consistent with variable range hopping in states above the valence band edge.
TL;DR: In this paper, the electrical resistivity and magnetoresistance of 1T-TaS 2-x Se x (x = 0-0.4) have been measured from a few Kelvin to 30 mK.
Abstract: The electrical resistivity and magnetoresistance of 1T-TaS 2- x Se x ( x =0–0.4) have been measured from a few Kelvin to 30 mK. The resistivity follows a two dimensional variable range hopping process in the temperature range from a few Kelvin to 1 K and deviates upward at lower temperatures. On the other hand, the magnetoresistance is found to show positive and negative ones. The resistivity and the corresponding positive magnetoresistance become larger as the growth temperature of crystal becomes higher and/or as the doping amount of Se becomes more. Below about 0.1 K, only the negative magnetoresistance is observed. These results are qualitatively understood by taking into account the Zeeman effect of the electrons, suppression of hopping process by the magnetic field and also the magnetic order of the spins of the localized electrons.
01 Jan 1984
TL;DR: In this paper, the authors used percolation theory to calculate the activation energy for hopping conduction in the limit of K → 1, where the main contribution to the impurity level scatter results not from the Coulomb interaction but from fluctuations in the composition of the solution within the volume of an impurity state.
Abstract: In this chapter we shall be using the percolation method to calculate the activation energy є 3 for hopping conduction. The simplest and most accurate solution for this problem is available in the case of low compensation (Sect. 8.1). For this case we shall give a detailed comparison of the theory with experimental data. Behavior of the activation energies є 1 and є 3 in the limit of K → 1 is discussed in Sect. 8.2. As K → 1, both energies increase because of the lowering of the Fermi level into the forbidden gap (cf. Sect. 3.4). Section 8.3 develops a perturbation method for percolation theory and proves that in a lightly doped semiconductor the activation energy є 3 is independent of the temperature at any degree of compensation. The perturbation theory recipe is then used to calculate the activation energy for an isoelectronic solid solution of different semiconductors — when the main contribution to the impurity level scatter results not from the Coulomb interaction but from fluctuations in the composition of the solution within the volume of an impurity state.
TL;DR: In this article, a parallel conduction mechanism of thermal activation of charge carriers to high mobility extended band states was proposed for polyacetylene doped with iodine to nearly metallic levels.
Abstract: Polyacetylene doped in the range of one to five percent has been shown to have low Pauli susceptibility yet high electrical conductivity. Earlier studies of polyacetylene doped with iodine to these nearly metallic levels show that essentially all charges go into soliton-like states. The conductivity (σ) and thermopower (S) of many samples are in quantitative agreement with charge transport via variable range hopping among soliton-like levels. Other doped samples have a lover density of states at the Fermi level with differing σ(T) and S(T). The strong sensitivity of the density of states to disorder leads to the proposal of a parallel conduction mechanism of thermal activation of charge carriers to “high mobility” extended band states. This mechanism may dominate in less disordered samples.
TL;DR: In this article, the two-dimensional (2D) electron hopping energy integral integral was investigated in order to calculate the impurity density of states of doped semiconductors.
Abstract: The authors have investigated the two-dimensional (2D) electron hopping energy integral in order to calculate the impurity density of states of doped semiconductors. A cluster model is outlined for the two-dimensional disordered system. It is shown that the hopping matrix is very sensitive to a change in the dimensionality of the system, i.e. from 3D to 2D system. The impurity band is symmetric and has a considerable bandwidth for high concentration, while for low concentration it is drastically reduced by the cut-off of the long-range hopping energy. The results of other models are discussed.
01 Jan 1984
TL;DR: In this paper, the exponential factor for hopping resistivity ρ 3.6 is calculated for the case of large strain in n-and p-Ge, where the wave functions of impurities are associated with a single ellipsoid in the electron spectrum.
Abstract: The percolation method described in Sect. 5.6 is employed in this chapter to calculate the exponential factor for hopping resistivity ρ 3. The case of isotropic impurity-state wave functions is considered in Sect. 6.1. The obtained theoretical dependence of ρ 3 on the impurity concentration is compared with a large number of experimental data for different semiconductors. On the whole, a good agreement is found. In Sect. 6.2 the theory is applied to semiconductors with anisotropic impurity wave functions, of which a typical example is n-Ge. The effect of strain on hopping conduction is analyzed. The anisotropy of hopping conductivity is calculated for the case of large strain in n-and p-Ge, where the wave functions of impurities are associated with a single ellipsoid in the electron spectrum.
TL;DR: In this paper, an extension of effective medium theory for bond percolation problems is given to include effects of hopping to more than adjacent neighbours, and the phase boundary separating conduction and insulating phases is a Z-1-dimensional hyperplane in the space of occupation probabilities if Zth neighbours are included.
Abstract: An extension of effective-medium theory for bond percolation problems is given to include effects of hopping to more than adjacent neighbours. In this approximation the phase boundary separating the conduction and insulating phases is a Z-1-dimensional hyperplane in the space of occupation probabilities if Zth neighbours are included. Long-time and critical properties remain the same as for the nearest-neighbour model. A quasi-transition is found if the hopping rates for the nearest-neighbour model. A quasi-transition is found if the hopping rates for neighbours of different order differ grossly. The dynamical features are discussed numerically in two-dimensions.