Special features of hopping conduction in p-Hg0.78Cd0.22Te crystals under conditions of dual doping

At T=4.2–125 K, the electrical conductivity and Hall effect were studied in p-Hg_0.78Cd_0.22Te crystals that contained 3×10¹⁶ cm^?3 Cu atoms and 1.83×10¹⁶ cm^?3 of Hg vacancies (either simultaneously or independently of each other). In such crystals, the ?₁ conductivity over the valence band is dominant at temperatures above 10–12 K, whereas the hopping conduction is prevalent at temperatures below 8–10 K. In the samples containing copper atoms and mercury vacancies simultaneously, conductivity with variable-range hopping is observed. It is found that the ?₁ conductivity of the copper-doped crystals is independent of the presence of mercury vacancies, whereas the hopping conductivity increases appreciably if these vacancies are introduced into the undoped crystal. This phenomenon is attributed to attachment of holes to the neutral mercury vacancies. The energy of this attachment is calculated, and it is found that this energy is equal to 3.7 meV for the ground state. The fluctuation-related broadening of the impurity band in the solid solutions gives rise to the overlap of the impurity bands formed by the copper acceptor levels and by the levels of holes attached to vacancies.     

Electrical Conductivity of V2O5–TeO2–Sb Glasses at Low Temperatures

Semiconducting glasses of the type 40TeO₂–(60 ? x) V₂O₅–xSb were prepared by rapid melt quenching and their dc electrical conductivity was measured in the temperature range 180–296 K. For these glassy samples, the dc electrical conductivity ranged from 2.26 × 10^?7  S cm^?1 to 1.11 × 10^?5 S cm^?1 at 296 K, indicating the conductivity is enhanced by increasing the V₂O₅ content. These experimental results could be explained on the basis of different mechanisms (based on polaron-hopping theory) in the different temperature regions. At temperatures above Θ _D/2 (where Θ _D is the Debye temperature), the non-adiabatic small polaron hopping (NASPH) model is consistent with the data, whereas at temperatures below Θ _D/2, a T ^?1/4 dependence of the conductivity indicative of the variable range hopping (VRH) mechanism is dominant. For all these glasses crossover from SPH to VRH conduction was observed at a characteristic temperature T _R ≤ Θ _D/2. In this study, the hopping carrier density and carrier mobility were determined at different temperatures. N (E _F), the density of states at (or near) the Fermi level, was also determined from the Mott variables; the results were dependent on V₂O₅ content.     

The concentration dependence of acceptor-state radii in p-Hg0.78Cd0.22Te crystals

Hopping conduction in undoped p-Hg_0.78Cd_0.22Te crystals containing native double-charged acceptors (Hg vacancies) with concentrations of 10¹⁶–10¹⁸cm^?3 was studied. Electrical conduction with a variable hopping range is dominant in the entire concentration range at temperatures below 6–16 K. The measured parameters of this conduction were used to calculate the acceptor-state radius as a function of vacancy concentration N _A . It is shown that, for N _A <4×10¹⁷ cm^?3, the low-temperature conduction occurs via the vacancy states whose radius is independent of N _A . For N _A >5×10¹⁷ cm^?3, the hopping conduction is governed by the states of uncontrolled shallow-level impurity acceptors. The radius of the state for these defects increases with increasing N _A owing to an increase in the effective permittivity of the medium.     

On the polarization caused by bulk charges and the ionic conductivity in TlInSe2 crystals

TlInSe₂ crystals are investigated in dc and ac electric fields in the temperature range of 100–400 K. A decrease in the electrical conductivity σ with time in a dc field is revealed. The complex-impedance spectra Z*(f) are measured in the frequency range of 10–10⁶ Hz. Diagrams in the (Z″-Z′) complex plane are analyzed using the method of equivalent circuits. It is shown that the electrical properties of TlInSe₂ crystals in the investigated ranges of temperatures and frequencies are determined by hopping conductivity and the accumulation of charge carriers near blocking platinum electrodes.     

Optical and electrical properties of thin wafers fabricated from nanocrystalline silicon powder

The infrared transmittance spectra and dark conductivity of wafers fabricated from nanocrystalline silicon powder (nc-Si) have been studied. The initial nc-Si powder is first synthesized by laser-induced silane dissociation, with the temperature of the surrounding buffer gas varied from 20 to 250°C, and then compacted at pressures from 10⁸ to 10⁹ Pa. It is found that compaction of the nc-Si powder results in formation of Si-H, Si-CH_x, and O_y-Si-H_x structures (x, y=1–3). The formed structures break down under annealing, with the Si-H and Si-CH_x complexes disintegrating at the lowest annealing temperature (t = 160°C). The dark conductivityof the nc-Si wafers is shown to increase along with the buffer gas temperature at which the starting powder was prepared. Two temperature regions are found in which the dark conductivity behaves in radically different ways. At wafer temperatures T ≥ 270 K, conductivity is mediated by free carriers, whereas, at lower temperatures, electron transport is governed by hopping conduction over localized states in the band gap.     

A small-molecule organic semiconductor

The electrical conductivity and optical properties of bis-diethylaminokumarin have been investigated. The electrical conductivity of the compound exhibited three-dimensional hopping conduction in the temperature range 295–321 K. The compound shows a typical semiconductor property, and its semiconducting property results from delocalization of the π-electrons in the structure. To determine the optical band gap of the compound, an optical absorption study was made in the wavelength range 250–600 nm. The optical study revealed that the optical transition is the allowed indirect one. The electronic parameters, such as the electrical conductivity at room temperature σ₂₅, activation energy E, and optical band gap E_g of the compound are 1.46×10^?5 S/cm, 0.42 eV, and 2.46 eV, respectively. These values are in agreement with electronic parameters of organic semiconductors.     

Hopping transport in doped (Pb0.78Sn0.22)1−x InxTe solid solutions

Transport coefficients in (Pb_0.78Sn_0.22)_1?x In_xTe solid solutions with indium content x=0.03 and 0.05 additionally doped with acceptors (Tl) or donors (Cl) have been measured and analyzed. The Seebeck coefficient S is positive for x=0.05 in the low temperature range 77–200 K; its sign changes to negative when the Tl acceptor is added. This unusual behavior of the thermoelectric power can be attributed to hopping conduction at a nonmonotonic energy dependence of the density of localized states. The density-of-states function has been determined at x=0.03 and 0.05 from experimental data on the thermopower. Theoretical estimates of the Nernst-Ettingshausen coefficient are made for x=0.03 additionally doped with Cl. The estimates are based on taking into account, along with the hopping conduction, the contribution from electrons with energies above the mobility edge and on using the critical electrical conductivity exponent obtained in the percolation theory. The activation energies characterizing the temperature dependences of conductivity and Hall and Nernst-Ettingshausen coefficients are discussed and compared.     

The role of trapping levels of nonequilibrium electrons during the formation of pinning centers for domain walls in the magnetic semiconductor CdCr<Subscript>2</Subscript>Se<Subscript>4</Subscript>

A complex study of electrical conductivity and photoconductivity in constant and variable electric fields, thermally stimulated conductivity, and the photoferromagnetic effect are carried out in the temperature range 10–300 K for a CdCr₂Se₄ magnetic semiconductor with various concentrations of Ga impurity and Se vacancies (V_Se). The phenomenon of hopping photoinduced conduction is observed for the first time. It is shown that shallow donor levels that exchange photoelectrons trapped by them with Cr³⁺ magnetic ions may be responsible for the photoferromagnetic effect. As a result of this exchange, the Cr³⁺ ions acquire valence and spin instabilities, which lead to nonequilibrium pinning of the domain wall by these ions.     

The mechanism of charge transfer in Bi2(Te0.9Se0.1)3 solid solution thin films

The conductivity, Hall effect, and magnetoresistance of Bi₂(Te_0.9Se_0.1)₃ solid solution thin films are studied in a wide temperature range from 2.5 to 300 K and in high magnetic fields of up to 8 T. It is found that the conductivity of Bi₂(Te_0.9Se_0.1)₃ solid solution thin films is of the insulator type, whereas the conductivity of the corresponding bulk single crystals is of metallic type. It is inferred that, at high temperatures (100–300 K), the conductivity is controlled mainly by thermally activated charge-carrier transport over extended states in the conduction band, with an activation energy of about 15 meV. At lower temperatures (2.5–70 K), conductivity controlled by charge-carrier hopping between localized states in a narrow energy region close to the Fermi level is dominant. From the magnetoresistance and conductivity data, the localization radius, the density of localized states, and the average charge-carrier hopping length are estimated.     

Dielectric Properties of Nanocrystalline Tungsten Oxide in the Temperature Range of 223–293 K

The dielectric properties of nanocrystalline tungsten oxide are studied in the temperature range of 223–293 K and in the frequency range ν = 10^–2–10⁶ Hz. Powders of WO₃ with particle sizes of 110, 150, and 200 nm are prepared by the heat treatment of ammonium paratungstate at various temperatures. It is established that the frequency dependences of the conductivity for all samples increase with an increase in frequency, while the polarization characteristics ε'(ν) and ε"(ν) decrease. It is found that the frequency dependences of the conductivity are described by a function of the form ν ^s with an index in the range of (0.83–0.90) ± 0.01, which is characteristic of the “hopping” mechanism of charged-particle motion (complexes) over localized states confined by potential barriers and structural defects.     

Chemical synthesis and charge transport mechanism in solution processed flexible polypyrrole films

Polypyrrole (PPy) has been synthesized by a chemical oxidation method using ammonium persulfate to obtain a solution processable PPy powder. The resultant PPy powder was then solution processed to deposit flexible thin films of PPy over flexible substrates. PPy film samples were then characterized using UV–vis spectroscopy, FTIR spectroscopy and X-ray diffraction. It was found that conductivity of PPy (σ=2.4×10⁻² S/cm) reduces by an order of magnitude after solution processing in the form of films. The temperature dependent conductivity of PPy pellet and flexible films of PPy were measured in the temperature range of 80–300 K. It was observed that PPy films show stronger temperature dependence than pelletized samples. Charge transport in PPy samples has been investigated using Kivelson׳s and Mott׳s variable range hopping models. Mott׳s parameters such as density of states at the Fermi level N(E_F), average hopping distance (R) and average hopping energy (W) have been estimated for PPy samples. The results showed that at room temperature average hopping distance for PPy film was about 22.3 Å and average hopping energy was 128.6 meV.     

Specific features of the low-temperature conductivity and photoconductivity of CuInSe2-ZnIn2Se4 alloys

n-type CuInSe₂-ZnIn₂Se₄ alloy single crystals are grown by the horizontal variant of the Bridgman method. The slight temperature dependence of the conductivity, high electron concentration, and the low photoconductivity of single crystals containing a low (5–10 mol %) fraction of ZnIn₂Se₄ are indicative of the nearly degenerate state of the crystals. It is established that, in the CuInSe₂-ZnIn₂Se₄ single crystals containing 15 and 20 mol % of ZnIn₂Se₄, the hopping mechanism of conductivity is dominant at temperatures of T ～ 27–110 K. At T ≥ 110 K, hopping conductivity gives way to activated conductivity. It is found that the specific feature of the low-temperature (27–77 K) photoconductivity spectrum of single crystals with ～15 and 20 mol % of ZnIn₂Se₄ is a single narrow peak at a wavelength of λ_max = 1190–1160 nm.     

Thermoelectric Properties and Conduction Mechanism of CaCu3Ti4O12 Ceramics at High Temperatures

The Seebeck coefficient and electrical conductivity of CaCu₃Ti₄O₁₂ (CCTO) ceramics were measured and analyzed in the high temperature range of 300°C to 800°C, and then the electrical conduction mechanism was investigated by using a combination of experimental data fitting and first-principles calculations. The Seebeck coefficient of the CCTO ceramic sintered at 1050°C is negative with largest absolute value of ～650 μV/K at 300°C, and the electrical conductivity is 2–3 orders greater than the value reported previously by other researchers. With increasing sintering temperature, the Seebeck coefficient decreases while the electrical conductivity increases. The temperature dependence of the electrical conductivity follows the rule of adiabatic hopping conduction of small polarons. The calculated density of states of CCTO indicates that the conduction band is mainly contributed by the antibonding states of Cu 3d electrons, therefore small-polaron hopping between CuO₄ square planar clusters was proposed. Possible ways to further improve the thermoelectric properties of CCTO are also discussed.     

Light-induced metal-insulator transition in n-GaAs/AlGaAs heterostructure: Acoustic methods of study

An n-GaAs/AlCaAs heterostructure “underdoped” with Si, with the dark conductance of the 2D channel at T = 4.2 K lower than 10^?8 Ω^?1, is studied. By successive illumination with a LED, the conductance of the structure could be raised by five orders of magnitude, up to ～10^?3 Ω^?1, which allowed studies of the metal-insulator transition (MIT) in the same sample at the same temperature. A new method of MIT studies using acoustoelectric effects is proposed. These effects have been measured at T = 4.2 K under successive illumination of the sample without a magnetic field and in a field up to 6 T. The real, σ₁, and imaginary, σ₂, parts of the high-frequency (HF) conductance σ^hf = σ₁ ? iσ₂ and their ratio σ₁/σ₂ was determined. The percolation mechanism of MIT has been established. It is found that, up to σ₁ ≈ 10^?7 Ω^?1, the system is in the insulating state, and electrons are localized at the minima of the random potential. In this situation, the HF hopping conductivity mechanism dominates and is characterized by the relation σ₂ ≥ σ₁. As the electron concentration increases, electron droplets become larger, and HF conductivity arises within these droplets. The conduction mechanism becomes mixed: the conduction by delocalized electrons within metallic droplets appears in parallel with hopping. As the conductance further increases, above 10^?5 Ω^?1, metallic droplets fill the entire conducting surface, and a metallic state with σ₂ = 0 appears. A curve demonstrating the dependence of the relative part of the surface area occupied by metallic droplets on the conductance of the 2D channel is plotted.     

Role of Si-doped Al<Subscript>0.3</Subscript>Ga<Subscript>0.7</Subscript>As layers in the high-frequency conductivity of GaAs/Al<Subscript>0.3</Subscript>Ga<Subscript>0.7</Subscript>As heterostructures under conditions of the quantum hall effect

The complex high-frequency conductivity of GaAs/Al_0.3Ga_0.7As heterostructures that are δ-doped and modulation-doped with silicon was investigated by acoustic methods under conditions of the integer quantum Hall effect. Both the real (σ₁) and imaginary (σ₂) parts of the complex conductivity σ(ω, H)=σ_i?iσ₂ were determined from the dependences of the absorption and velocity of surface acoustic waves on magnetic field. It is shown that, in the heterostructures with electron density n_s=(1.3–7)×10¹¹ cm^?2 and mobility μ=(1–2)×10⁵ cm²/(V s), the high-frequency conductivity near the centers of the Hall plateau is due to electron hopping between localized states. It is established that, with filling numbers 2 and 4, the conductivity of the Al_0.3Ga_0.7As:Si layer efficiently shunts the high-frequency hopping conductivity of the two-dimensional interface layer. A method of separating the contributions of the interface and Al_0.3Ga_0.7As:Si layers to the hopping conductivity σ(ω, H) is developed. The localization length of electrons in the interface layer is determined on the basis of the nearest neighbor hopping model. It is shown that, near the centers of the Hall plateau, both σ(ω, H) and n_s depend on the cooling rate of a GaAs/Al_0.3Ga_0.7As sample. As a result, the sample “remembers” the cooling conditions. Infrared light and static strain also change both σ(ω, H) and n_s. We attribute this behavior to the presence of two-electron defects (so-called DX^? centers) in the Al_0.3Ga_0.7As:Si layer.     

Conductivity and structure of Er-doped amorphous hydrogenated silicon films

A columnar structure of a-Si:H(Er) film serving as a working layer in electroluminescent structures has been demonstrated. The diameter of columns is in the range of 60–100 Å. In a structure of this kind, the conductivity depends on the direction of current. In the planar configuration, room-temperature transport occurs through hopping via localized states near the conduction band edge, within the band tail. In the sandwich configuration, the conduction occurs along the column boundaries, where the conductivity is higher, via hopping conduction at the Fermi level.     

Physical properties of FeGa2Se4 under an applied alternating current

The temperature and frequency dependences of the permittivity and conductivity of FeGa₂Se₄ crystals under an applied alternating current are investigated. The values of the permittivity are determined. It is assumed that an increase in ?′ is related to an increase in the defect concentration with temperature. It is established that the regularity σ ∝ f ^S (0.1 ≤ S ≤ 1.0) is fulfilled for conductivity in the temperature range of 294–374 K at frequencies of 10⁴-2 × 10⁵ Hz. In the FeGa₂Se₄ crystal, the variation in the frequency dependence of the conductivity can be explained as follows: there are the clusters in crystals containing localized states with almost identical energy, and the electron hopping occurs between them. In the FeGa₂Se₄ compound, the conductivity is characterized by band-hopping mechanisms.     

electrical properties of the proton-irradiated semi-insulating GaAs:Cr

The n-p conversion of the conduction type and a decrease in resistivity to 10² Ω cm at 300 K were revealed upon proton irradiation (5 MeV, 300 K, D≈2×10¹⁷ cm^?2) of semi-insulating GaAs:Cr (ρ≈(3–4)×10⁸ Ω cm). Temperature dependences of ρ for heavily irradiated samples indicate a hopping conduction in the temperature range of 400–120 K, with the transition to the conduction with variable-range hopping at T≤120 K. The effects of electronic switching were found in low-resistivity proton-irradiated GaAs:Cr at about 20 K. The isochronous annealing of radiation defects in the temperature range of 20–750°C was investigated.     

Effect of Nano-Ce-Doped TiO<Subscript>2</Subscript> on AC Conductivity and DC Conductivity Modeling Studies of Poly (<Emphasis Type="Italic">n</Emphasis>-Butyl Methacrylate)

Among the unique properties of polymer nanocomposites, electrical conductivity deserves a prominent place due to their wide applications in conducting adhesive, electromagnetic shielding and sensors. The present work focuses on the effect of cerium-doped titanium dioxide (Ce-TiO₂) nanoparticles on the conductivity studies of poly (n-butyl methacrylate), or PBMA, nanocomposites at different temperatures. The frequency-dependent alternating current (AC) electrical conductivity of PBMA/Ce-TiO₂ nanocomposites has been found to increase with increase in temperature and the concentration of Ce-TiO₂ nanoparticles. The activation energy calculated from the AC electrical conductivity has been found to decrease with frequency and increasing temperatures. The frequency exponent factor also showed a decrease with frequency, indicating the hopping conduction in the nanocomposites. The maximum AC conductivity has been observed for the composites with 7 wt.% sample. The direct current (DC) conductivity of PBMA/Ce-TiO₂ composites was also enhanced with the addition of Ce-TiO₂ nanoparticles. Experimental and theoretical investigations based on Scarisbrick, Bueche, McCullough and Mamunya modeling were undertaken to understand the observed DC conductivity differences induced by the addition of Ce-doped TiO₂ nanoparticles to PBMA matrix. The experimental conductivity showed good agreement with the theoretical conductivity observed using the Mamunya model.