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, N _F=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 Å to 170 Å 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.     

Optical and electrical properties of amorphous arsenic

Amorphous arsenic prepared by plasma decomposition of arsine has been characterized using field-effect conductance, thermopower, optical absorption, and photoconductivity measurements. It is found that the Fermi level is located in a density of states ～ 10¹⁷ cm^?3 eV^?1 approximately in the center of the forbidden gap, that conduction occurs via electrons in extended states in the conduction band, and that the optical and photoelectrical properties are very similar to those of bulk a-As. It is concluded that a model involving a negative correlation energy for localized states is inappropriate for this material.     

Thermopower in the region of hopping conduction in TlNiS<Subscript>2</Subscript>

Samples of the composition TlNiS₂ in the hexagonal system with the unit cell parameters a=12.28 Å, c=19.32 Å, and ρ=6.90 g/cm³ are synthesized. The results of the investigation into the electrical and thermoelectrical properties of TlNiS₂ samples in the temperature range 80–300 K indicate that TlNiS₂ is a p-type semiconductor. It is found that, at temperatures ranging from 110 to 240 K, TlNiS₂ samples in a dc electric field possess variable-range-hopping conduction at the states localized in the vicinity of the Fermi level. The density of localized states near the Fermi level is determined to be N_F=9×10²⁰ eV^?1 cm^?3, and the scatter of the states is estimated as J≈2×10^?2 eV. In the temperature range 80–110 K, TlNiS₂ exhibits activationless hopping conduction. At low temperatures (80–240 K), the thermopower of TlNiS₂ is adequately described by the relationship α(T)=A+BT, which is characteristic of the hopping mechanism of charge transfer. In the case when the temperature increases to the temperature of the onset of intrinsic conduction with the activation energy ΔE=1.0 eV, there arise majority intrinsic charge carriers of both signs. This leads to an increase in the electrical conductivity σ and, at the same time, to a drastic decrease in the thermopower α; in this case, the thermopower is virtually independent of the temperature.     

DC and AC conductivities of (As2S3)100−x(AsSe0.5Te0.5I)x chalcogenide glasses

The DC and AC conductivities of samples from the system (As₂S₃)_100−x(AsSe_0.5Te_0.5I)_x, where x=0, 5, 10, 15, 20, 25, 30, 35, 50, 70 and 90 mol%, were measured as a function of temperature. Besides, the AC conductivities of the samples with x=10 and 30 were measured as a function of frequency from room temperature to the glass transition temperature. The DC conductivity dependence on temperature is of the Arrhenius type, whereas the value of the pre-exponential factor suggests the electrical conduction by localized states in the band tails and by localized states near the Fermi level. The small values of the conduction activation energy (10⁻²-10⁻¹ eV) obtained at higher frequencies suggest that the conduction in these materials is due to hopping of charge carriers between close defect states near the Fermi level.     

Conduction through localized states in a single crystal of the TlGa<Subscript>0.5</Subscript>Fe<Subscript>0.5</Subscript>Se<Subscript>2</Subscript> alloy

Layered single crystals of the TlGa_0.5Fe_0.5Se₂ alloy in a dc electric field at temperatures ranging from 128 to 178 K are found to possess variable-range-hopping conduction along natural crystal layers through states localized in the vicinity of the Fermi level. The parameters characterizing the electrical conduction in the TlGa_0.5Fe_0.5Se₂ crystals are estimated as follows: the density of states near the Fermi level N_F = 2.8 × 10¹⁷ eV^?1 cm^?3, the spread in energy of these states ΔE = 0.13 eV, the average hopping length R_av = 233 Å, and the concentration of deep-lying traps N _t = 3.6 × 10¹⁶ cm^?3.     

Conductivity anisotropy and localization of charge carriers in TlInTe<Subscript>2</Subscript> single crystals

The electrical resistivity of TlInTe₂ chain-structure semiconductors in directions parallel and perpendicular to the chains is analyzed as a function of temperature. It is demonstrated that, in both cases, the temperature dependences of the electrical resistivity in the temperature range under investigation are characterized by two portions associated with different mechanisms of electrical conduction. In the high-temperature range, the electrical conduction is predominantly provided by thermally excited impurity charge carriers in the allowed band. In the low-temperature range, the conduction occurs through charge carrier hopping between localized states lying in a narrow energy band near the Fermi level. The activation energy for impurity conduction is determined. The localization lengths and the density of localized states near the Fermi level, the spread in energies of these states, and the average carrier-hopping distances are estimated for different temperatures.     

Effects of photoexcitation on the current transport mechanism in amorphous indium selenide thin films

The effect of photoexcitation on the current transport mechanism in amorphous indium selenide thin films was studied by means of dark and illuminated conductivity measurements as a function of temperature. Analysis of the dark electrical conductivity in the temperature range 110–320 K reveals behaviour characteristic of carriers excited to the conduction band and thermally assisted variable-range hopping (VRH) at the Fermi level above 280 K and below 220 K, respectively. In the temperature range 220–280 K, a mixed conduction mechanism was observed. A conductivity activation energy of ～300 meV (above 280 K), a density of localised states (evaluated assuming a localisation length of 5 Å) of 1.08 × 10²¹ cm^?3 eV^?1, an average hopping distance of 20.03 Å (at 120 K) and an average hopping energy of 27.64 meV have been determined from the dark electrical measurements. When the sample was exposed to illumination at a specific excitation flux and energy, the values of the conductivity activation energy, the average hopping energy and the average hopping range were significantly decreased. On the other hand, the density of localised states near the Fermi level increased when the light flux was increased. Such behaviour was attributed to a reversible Fermi level shift on photoexcitation.     

Generation of dangling bonds by high temperature annealing and hopping conduction in amorphous silicon films

By annealing evaporated a-Si films between 450°C and 620°C, it is found that new dangling bonds are generated. These dangling bonds can act as hopping centres. Quantitative analysis of experimental results on electrical resistivity and ESR measurements favours a conduction mechanism of hopping to nearest neighbours between room temperature and ?100°C. Films annealed just before crystallisation temperature have a density of states near Fermi level of about 3 × 10¹⁸ cm^?3 eV^?1.     

Conductivity anisotropy and localization of charge carriers in CuFeTe2 single crystals with a layered structure

The temperature dependences of the electrical resistivity of CuFeTe₂ semiconductor single crystals with a layered structure are investigated parallel and perpendicular to the plane of the crystal layers in the temperature range 5–300 K. It is demonstrated that, in both cases, the temperature dependences of the electrical resistivity in the temperature range studied are characterized by two portions associated with different mechanisms of electrical conduction. In the high-temperature range, the electrical conduction is predominantly provided by thermally excited impurity charge carriers in the allowed energy band. In the low-temperature range, the electrical conduction occurs through charge carrier hopping between localized states lying in a narrow energy band near the Fermi level. The activation energy for impurity charge carriers is determined. The density of localized states near the Fermi level, the spread in energies of these states, and the average carrier-hopping distances are estimated for different temperatures     

Investigation of dc hopping conduction in TlGaS2 and TlInS2 single crystals

It is established that variable-range hopping conduction takes place between states localized near the Fermi level in layered TlGaS₂ and TlInS₂ single crystals both along and across their natural layers in a constant electric field at T⩽200 K. The densities of states near the Fermi level and the hopping distances at different temperature are estimated. The occurrence of activationless hopping conduction is established in TlGaS₂ and TlInS₂ single crystals in the temperature range 110–150 K. Fiz. Tverd. Tela (St. Petersburg) 40, 612–615 (April 1998)     

Charge transfer in TlFeS2 and TlFeSe2

The temperature dependences of the conductivity and the thermoelectric coefficient in TlFeS₂ and TlFeSe₂ samples have been investigated in the temperature range 85–400 K. The variable-range hopping conduction has been established. It is found that the density of localized states N _F near the Fermi level is 1.7×10¹⁸ and 3.3×10¹⁸ eV^?1 cm^?3, and the average hopping length R is 109 and 104 Å for TlFeS₂ and TlFeSe₂, respectively. The non-Arrhenius (activationless) behavior of the hopping conductivity is established in the temperature region T<200 K for TlFeS₂ and T<250 K for TlFeSe₂.     

电致发光加速层二氧化硅的电子高场迁移率

根据非晶态半导体的能带理论，讨论了分层优化薄膜电致发光方案中非晶二氧化硅加速层中的电子在高电场中的输运行为.研究结果表明：在高电场下，由于电场的存在降低了陷阱之间的平均势垒高度.在费密能级附近处的杂质及缺陷定域态和导带尾定域态中，电子的输运主要表现为电场增强的热辅助式跳跃传导；而在导带扩展态中，电子的输运仍像晶态半导体那样表现为共有化运动.此外，以实验数据为基础，计算出了非晶二氧化硅中电子的迁移率、最小金属电导率、导带迁移率边界状态密度及费密能级处的状态密度. 关键词：     

Electrical conduction mechanism in nanocrystalline CdTe (nc-CdTe) thin films

The present paper reports the electrical characterization of nc-CdTe thin films in different temperature ranges. Thin films of nc-CdTe are deposited on the glass substrates by Physical Vapor Deposition (PVD) using the Inert Gas Condensation (IGC) method. The Transmission Electron Microscopy (TEM) studies are made on the CdTe nanocrystals. The surface morphology and structure of the thin films are studied by the Scanning Electron Microscope (SEM) and X-Ray Diffraction (XRD) measurements. Dark conductivity measurements are made on the nc-CdTe thin films in the temperature range 110–370 K in order to identify the conduction mechanism in this temperature range. The obtained results reveal three distinct regions at high, low, and sufficiently low temperature regions with decreasing activation energies. The analysis of the high temperature conductivity data is based on the Seto’s model of thermionic emission. At very low temperatures, dc conductivity (σ _d) obeys the law: lnσT ^1/2∝T ^?1/4, indicating variable-range hopping in localized states near the Fermi level. The density of the localized states N(E _F) and various other Mott’s parameters like the degree of disorder (T _O), hopping distance (R), and hopping energy (W) near the Fermi level are calculated using dc conductivity measurements at low temperatures. Carrier type, carrier concentration, and mobility are determined from the Hall measurements. The transient photoconductivity decay measurements are performed on the nc-CdTe thin films at different intensities in order to know the nature of the decay process.     

NMR and magnetic susceptibility of CeCu6

NMR and magnetic susceptibility of CeCu₆ intermetallic compound were investigated. The most important contributions to the magnetic susceptibility are the Curie-Weiss term, expressing the paramagnetism of the localized f-electrons, and a temperature independent term for which all the contributions were estimated. The phenomenological exchange constantJ _sf between 4f — electron spins and conduction — electron spins is derived to be ?0·012 eV. The Knight shift and Curie temperature are discussed in terms of the Ruderman-Kittel-Kasuya-Yosida theory and in the Rocher's virtual bound states model. The s — f exchange integralΓ and the Fermi wavevectork _F are derived to be — 0·8 eV and 1·32 Å^?1, respectively.     

Electrical properties of amorphous (Co45Fe45Zr10)x(Al2O3)1?x nanocomposites

The electrical properties of (Co₄₅Fe₄₅Zr₁₀)_x(Al₂O₃)_1−x granular nanocomposites have been studied. The concentration dependences of electrical resistivity are S-shaped (in accordance with the percolation theory of conduction) with a threshold at a metallic component concentration of ∼41 at. %. An analysis of the temperature behavior carried out in the range 300–973 K revealed that structural relaxation and crystallization of the amorphous phase are accompanied by a decrease in the electrical resistivity of the composites above the percolation threshold and by its increase below the percolation threshold. For metallic phase concentrations x<41 at. %, variable range hopping conduction over localized states near the Fermi level was found to be dominant at low temperatures (77–180 K). A further increase in temperature brings about a crossover of the conduction mechanism from Mott’s law ln(σ) ∝ (1/T)^1/4 to ln(σ) ∝ (1/T)^1/2. A model of inelastic resonance tunneling over a chain of localized states of the dielectric matrix was used to find the average number of localized states involved in the charge transport between metallic grains. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 11, 2004, pp. 2076–2082. Original Russian Text Copyright ? 2004 by Kalinin, Remizov, Sitnikov.     

Gap states in hydrogenated amorphous silicon: A comparison of photoemission and photoconductivity results

We report photoemission results from which we directly determined the density of states g(E) in the gap of a-Si:H between the top of the valence band E_v and the Fermi level. At 0.4 eV above E_v, g(E) was found to be ≈1×10²⁰ cm^-3 eV^-1 in the undoped film; P-doping increased g(E) in this region whereas annealing reduced it. The photoconductivity-derived optical absorption spectrum matched the shape of the photoemission spectrum, and thus supports the explanation that the photoconductivity shoulder at photon energies in the region of 1.3 eV is due to transitions from localized states above the valence band to the conduction band.     

Electrical conduction mechanism in amorphous Se80In20−xPbx films

DC conductivity measurements on thin films of a-Se₈₀In_20−xPb_x (where x=0, 2, 6 and 10) are reported in the temperature range 200–400 K. The density of states near the Fermi level is calculated using the DC conductivity (Mott parameters). The conduction in the low-temperature region is found to be due to variable range hopping while that in the high-temperature region is due to thermally assisted tunneling of the carriers in the localized states near the band edge.     

On the fermi velocity and static conductivity of epitaxial graphene

The models of the energy density of states of a metallic or semiconductor substrate, which does not further lead to divergences, have been proposed to calculate the characteristics of epitaxial graphene. The Fermi velocity of epitaxial graphene formed on a metal has been shown to be greater than that in free-standing graphene irrespective of the position of the Fermi level. On the contrary, the Fermi velocity of graphene formed on a semiconductor is lower so that the lower is the Fermi velocity, the closer is the Fermi level to the center of the band gap of the semiconductor. The zero-temperature static conductivity σ of epitaxial graphene has been calculated according to the Kubo-Greenwood formula. The quantity σ_m of undoped graphene on metal has been shown to decrease with an increase in the deviation of the Dirac point ?_D (which coincides with the Fermi level of the system) from the center of the conduction band of the substrate. In the case of the semiconductor substrate, the static conductivity σ_sc turns out to be nonzero and amounts to σ_sc = 2e ²/π?-only under the condition ?_F =?′_D, where ?′_D is the Dirac-point energy renormalized by the interaction with the substrate.     

Electronic states near fermi level in superconductors La2 − x Sr x CuO4 by means of x-ray absorption spectra near Cu-K and La-L edges

The electronic states of La_{2? x} Sr _x CuO₄ with 0.00 ≤ x ≤ 0.20 are studied by means of X-ray absorption spectroscopy (XANES, EXAFS) near the K-edge of Cu²⁺ ion and the L-edges of La³⁺ ion. It is found that characteristic white lines occurring near L _II and L _III edges of La³⁺ ion show a slight energy shift depending on substituted Sr²⁺ ions, x and temperature. The white lines suggest that unoccupied high-density 5dπ and 5dδ bands of La³⁺ ion just above a Fermi level transform to a hybridized single band of 5dπδ at 78?K in the superconductors with x = 0.10, 0.16 and 0.20. On the other hand, the XANES spectra near the Cu-K edge including a pre-edge region do not depend on x and temperature in the region of 0.00 ≤ x ≤ 0.20. It is considered that there is no reconstruction of electronic states at the Fermi level in a Mott–Hubbard band gap between an O 2p valence band and a Cu 3d conduction band. The electronic states at the Fermi level are probably consisted of the unoccupied 5dπδ band and an empty charge-transfer 3d?⁹ L band at low temperature, bands of which occur in a band gap between a filled O 2p valence band and an unoccupied O 2p conduction band. The insulator–superconductor–metal transitions in La_{2? x} Sr _x CuO₄ are related to the 5dπδ and 3d?⁹ L bands and holes, which site at a top region of the O 2p valence band near the Fermi level produced by a substitution of La³⁺ with Sr²⁺ ions.     

Nuclear Quadrupole spin-lattice relaxation in Rhombohedral Arsenic

The temperature dependence of the spin-lattice relaxation time T₁ in rhombohedral arsenic has been measured by nuclear quadrupole resonance. The relaxation time is inversely proportional to the temperature and of a magnitude which indicates that the relaxation results from the Fermi contact interaction of the conduction electrons and holes and the arsenic nuclei. The density of electrons and holes at the site of the nucleus, averaged over the Fermi surface is approximately 2.6 × 10²¹ carriers cm^?3.