Variable Hopping Length Research Articles

Ionic conductivity of the Ag8GeSe6 compound

The temperature dependences of the conductivity and permittivity of the Ag8GeSe6 compound have been studied in direct and alternating electric fields. This phenomenon is associated with the process of charge accumulation at the boundary of the sample with ionic conductivity and the blocking electrode, and the formation of a double electric layer. The observed dielectric relaxation is associated with the transition of the crystal to the superionic state. It has been established that hopping conduction with a variable hopping length over localized states near the Fermi level takes place in an Ag8GeSe6 crystal.

The Effect of X‐Ray Irradiation on Conductivity of C and 2C Polytype TlInS2 Ferroelectrics

The influence of X‐ray irradiation on DC conductivity of TlInS2 C and 2C polytypes is studied in the temperature range of 100–300 K. The X‐ray irradiation (absorbed doses are 0.9 and 2.7 kGy) leads to an increasing conductivity of both C‐ and 2C‐samples in proportion to the absorbed dose; the conductivity decreases on cooling in the temperature region of 100–300 K. An anomalous increase in conductivity is observed in C‐polytype samples on cooling in the temperature range of the incommensurate phase (197–214 K). The X‐ray irradiation causes the “blur” of that anomaly. It is established that the mechanism of hopping conductivity with variable hopping length takes place for both polytypes in the temperature range of 125–175 K, and the densities of localized states near the Fermi level (NF) are determined. Under irradiation, the density NF increases from 3.7 × 1018 eV−1 cm−3 (C) to 6.9 × 1018 eV−1 cm−3 (2C) and exceeds the initial value more than three times at D = 2.7 kGy. Using the 1D Kroning–Penney model, the role of incommensurate periodic potential in modification of electron band structure at creation of the incommensurate phases in crystals is discussed.

Structure and Electrical Properties of (ZnO/SiO2)25 Thin Films

(ZnO/SiO2)25 thin-film multilayers consisting of nanocrystalline ZnO layers and amorphous SiO2 spacers with a bilayer thickness from 6 to 10 nm are synthesized in a single deposition process. An analysis of the temperature dependences of the electrical resistivity of (ZnO/SiO2)25 thin films shows that, in the temperature range of 77–300 K, the dominant conductivity mechanism successively changes from hopping conductivity with a variable hopping length in a narrow energy band near the Fermi level at temperatures of 77–250 K to thermally activated impurity conductivity around room temperature. Using the obtained temperature dependences of the electrical resistivity, the effective density of localized states at the Fermi level and the activation energy of impurity levels are estimated. The effect of heat treatment on the structure and electrical properties of the synthesized films is examined. It is established that in (ZnO/SiO2)25 thin-film systems at temperatures of 580–600°C, the ZnO and SiO2 layers chemically interact, which is accompanied by destruction of the multilayer structure and formation of the Zn2SiO4 compound with a tetragonal structure (sp. gr. I-42d).

Competition between band and hopping carrier transport in Ge : Mn thin films

Regularities of hole transport and its correlation with percolation magnetism caused by localized carriers simultaneously involved in the formation of the magnetic and electrical properties of Ge: Mn thin films are investigated. It is established that at temperatures of T > 22 K the activationless band carrier transport occurs in the Ge: Mn samples (2 at % Mn). At low temperatures, the hopping mechanism with a variable hopping length works.

Low-temperature conductivity of silicon doped with antimony

A detailed analysis of the experimental temperature dependences of the resistivity of silicon doped with arsenic with a concentration of 1018 cm−3 is performed for the region 1.8 K < T < 25 K. It is shown that, as a result of cooling to a temperature lower than 4.5 K, a transition from the Mott mode with variable hopping length to the mode of hopping conduction via nearest neighbors is observed, while, at T < 2.5 K, a transition to the Shklovskii-Efros mechanism is possible. A model for such a temperature crossover is suggested; the model is based on simplified solution of the percolation problem with the use of an interpolation expression for the density of states. Performed estimates show that the model is in satisfactory agreement with experimental data when the minimum number of adjustable parameters are used.

Electrical Properties of Composite Films with Silicon Nanocrystals in the Insulating Matrix

The electrical properties of nanocomposite SiO2(Si) films containing Si nanoclusters have been investigated. The films were formed by oxide assisted growth that included ion plasma sputtering (IPS) of Si target and following high temperature annealing. It was determined that electrical conductivity of the films correspond to the mechanism of hopping conductivity with variable hopping length through the traps near the Fermi level (Mott mechanism) due to the large number of silicon dangling bonds in the dielectric matrix. The peculiarities of charge capture in nanocomposite SiO2(Si) films for their application as the medium for charge storage in memory cells have been investigated by C-V method. The good charge storage possibility of SiO2(Si) films formed by IPS deposition with followed temperature annealing has been observed. The negative differential capacitance has been revealed in conditions of semiconductor surface accumulation. The physical model for explanation of the negative differential capacitance of MIS structures with nanocomposite SiO2(Si) films as the dielectric has been proposed. The model is based on the parallel conjunction of the oxide capacitance and nanocrystals capacitance.

Charge transfer along localized states in InSe and InSe⟨Sn⟩ single crystals

It is established that at temperatures T&amp;lt;200K hopping conductivity with variable hopping length exists across natural layers of undoped and doped (0.2 and 0.4at% Sn) InSe single crystals in a constant electric field. The density of states near the Fermi level (NF=5.36⋅1018–1.72⋅1019eV−1⋅cm−3), the energy spread of the states (ΔE=0.028–0.040eV), the localization radius (a=58Å), and the average hopping length (Rav=99.5–130Å) in the temperature interval 100–200K are evaluated.

Low-temperature thermopower in quasiamorphous carbons

Results are presented from a study of the thermopower of quasiamorphous carbons in the temperature interval from 20 to 200 K. It is shown that the thermopower of quasiamorphous carbons is described satisfactorily in a model based on the series connection of regions with metallic conductivity, hopping conductivity with a variable hopping length, and hopping conductivity with a constant hopping length. It is found that when considering the thermopower of the regions with the metallic conductivity, it is necessary to take into account the temperature dependence of the main mechanisms of carrier scattering. The parameters of the electronic structure of quasiamorphous carbon are calculated using experimental data on the temperature dependence of the electrical resistivity and thermopower.

Nonlinear percolation conductivity and negative differential resistivity in microcrystalline PbTe layers with an adjustable potential well

The effect of electric fields on the electrical conductivity of PbTe films with block sizes smaller than the Debye screening length is studied. As the temperature is varied, a readjustment of the potential well is observed due to thermal spread of barriers with height ϕkT and the expansion of higher barriers. Spatial ensembles, which consist of several blocks that increase rapidly with temperature, are established for each T. This process leads to an increase in the height of the potential barriers as the linear size of these ensembles increases. This determines the potential well in these films and their nonlinear properties, which originate in the nonlinear percolation conductivity of a microscopic crystalline system with intergranular barriers. A comparison with the experimental data of Shklovskii shows that the scale length of the spatial inhomogeneity a=3.7×10−6 cm at T=4.2 K corresponds to the average block size. The value of a increases with temperature, reaching 5×10−4 cm at T=240 K. This mechanism for electrical conductivity is compared with the hopping conductivity with a variable hopping length. The negative differential resistance in the structures examined here is found to be electrothermal in nature.

Anisotropy of the hopping conductivity in TlGaSe2 single crystals

The temperature dependences of the conductivities parallel and perpendicular to the layers in layered TlGaSe2 single crystals are investigated in the temperature range from 10 K to 293 K. It is shown that hopping conduction with a variable hopping length among localized states near the Fermi level takes place in TlGaSe2 single crystals in the low-temperature range, both along and across the layers. Hopping conduction along the layers begins to prevail over conduction in an allowed band only at very low temperatures (10–30 K), whereas hopping conduction across the layers is observed at fairly high temperatures (T⩽210 K) and spans a broader temperature range. The density of states near the Fermi level is determined, NF=1.3×1019eV·cm3)−1, along with the energy scatter of these states J=0.011 eV and the hopping lengths at various temperatures. The hopping length R along the layers of TlGaSe2 single crystals increases from 130 A to 170 A as the temperature is lowered from 30 K to 10 K. The temperature dependence of the degree of anisotropy of the conductivity of TlGaSe2 single crystals is investigated.