Effect of statistical fermi level shift on the Meyer-Neldel rule of a-Si:H conductivity

Effect of the statistical Fermi level shift on the Meyer-Neldel rule of a-Si:H conductivity was studied using the temperature dependence of MOSFET conductivity. A model is presented in which the temperature dependence of surface band bending in a MOSFET should compensate for the statistical Fermi level shift of bulk a-Si:H. This compensation is verified by the SiO₂ thickness dependence of the MOSFET conductivity. The pre-exponential factor obtained by analysis of the temperature dependence of MOSFET conductivity does not show the Meyer-Neldel rule. It is therefore concluded that the Meyer-Neldel rule observed in bulk a-Si:H conductivity is predominantly caused by the statistical Fermi level shift.     

Phase transitions of p-type (Pb,Sn,Ge)Te-based alloys for thermoelectric applications

p-type (Pb,Sn,Ge)Te-based alloys for thermoelectric applications were prepared using Bridgman technique. Second-order, rhombohedral to cubic phase transitions are involved, as evaluated from anomalies in the temperature dependence values of Seebeck coefficient, electrical conductivity, heat flow and the elongation, in the vicinity of the phase transition temperature, T_c. The correlation between these anomalies in both the electronic and thermodynamic properties was interpreted by means of the relationship of Fermi energy to the chemical potential (or to the molar Gibbs free energy).     

Polymorphic transformation in TlSe and the electrical properties of phases

Alloys in the TlSe-Se system on the side of the TlSe compound in the phase diagram have been investigated using differential thermal, X-ray powder diffraction, and microstructural analyses. The phase diagram of the system has been constructed, and the temperature dependences of the electrical conductivity of the phases obtained have been examined. An analysis of the thermograms of alloys in the (TlSe)_0.96-Se_0.04 system has revealed a structural phase transition at a temperature of 470 ± 1 K. Investigations into the temperature dependences of the electrical conductivity in the range 120–450 K have demonstrated that the temperature dependence of the electrical conductivity for the (TlSe)_0.96-Se_0.04 alloy exhibits metallic behavior.     

The field effect in amorphous chalcogenides: An investigation of localized states and electronic transport

The temperature dependence of the field effect response permits an unambiguous determination of the identity of those states responsible for electrostatic screening in the amorphous chalcogenides. We observe (1) in As₂Te₃, field effect screening by localized states at the Fermi level at low temperatures (～ 10¹⁹ cm^?3 eV^?1) and by mobile charge carriers (～ 10¹⁸ cm^?3 at 300 K) at high temperatures, and a transition from p-type to two-carrier (primarily n-type) conductivity as the temperature is raised above ～320 K; (2) in As₂SeTe₂, screening by mobile charge carriers (～ 10¹⁸ cm^?3 at 300 K) with strongly type conductivity; (3) in As₂Se₂Te, screening by localized states at the Fermi level (～ 10¹⁹ cm^?3 eV^?1) with strongly p-type conductivity; and (4) in Sb₂Te₃, a very high density of localized states at the Fermi level (～ 2 × 10²⁰ cm^?3 eV^?1) with both electron and hole contributions to the conductivity. Correlation with thermoelectric power results suggests that the p-type conductivity in As₂Te₃ is due to near-equal contributions from two processes: hopping in localized states plus extended state conduction. Aging and annealing behavior is described with the aid of a “chaotic potential model” that appears to be able to account for large changes in mobile carrier density that leave the conductivity unaltered.     

Conduction on localized states in Nb/Al2O3 semiconducting films

Electrical conduction in co-evaporated semiconducting Nb/Al₂O₃ films (1: 1 volume ratio) is determined by the high density of localized levels in the energy gap of the alumina. The tunnelling between the metal particles occurs via a hopping mechanism on these states. The transitions have an activation energy of the order of kT, due to the continuous distribution in energy of the localized levels. The conductivity dependence on temperature in the ohmic range leads to a density of states times mobility product increasing exponentially above the Fermi level. The current-field dependence in the high-field regime gives information about the density of states, which is inferred to increase linearly with energy above the Fermi level.     

Electrical properties of TiO2 thin films

The electrical properties of n-type titanium dioxide thin films deposited by magnetron-sputtering method have been investigated by temperature-dependent conductivity. We observed that the temperature dependence of the electrical conductivity of titanium dioxide films exhibits a crossover from T^?1/4 to T^?1/2 dependence in the temperature range between 80 and 110 K. Characteristic parameters describing conductivity, such as the characteristic temperature (T₀), hopping distance (R_hop), average hopping energy (Δ_hop), Coulomb gap (Δ_C), localization length (ξ) and density of states (N(E_F)), were determined, and their values were discussed within the models describing conductivity in TiO₂ thin films.     

Sm-substitution effects on structural and transport properties of PbSe

Polycrystalline single phase of Pb_1−xSm_xSe (x = 0.00–0.09) samples were prepared by the solid state reaction. XRD and SEM/EDAX methods were used for structural and chemical characterization of the obtained materials. The investigated samples were also characterized by the measurements of the samarium concentration dependence of the electrical conductivity and Seebeck coefficient. The power factor shows that these materials have the potential for high temperature applications with promising thermoelectric performance.     

Electrical properties of annealed a-SiC:H thin films

In the present work the dependence of electrical properties of a-SiC:H thin films on annealing temperature, T_a, has been extensively studied. From the measurements of dark dc electrical conductivity, σ_D, in the high temperature range (from 283 up to 493 K), was found that the conductivity activation energy, E_a, is invariant for T_a ⩽ 673 K and equal to 0.64 eV, whereas for T_a from 673 up to 873 K, E_a increases at about 0.2 eV reaching to a maximum value 0.85 eV at T_a = 873 K, suggesting the optimum material quality. This behavior of E_a as a function of T_a is mainly attributed to relaxation of the strain in the amorphous network, which is possibly combined with weak hydrogen emission for temperatures up to 873 K. For further increase of T_a (>873 K) the phenomenon of hydrogen emission, causes rapid decrease of E_a down to 0.24 eV at T_a = 998 K, deteriorating the material quality. These results are also supported by the measurements of dark dc electrical conductivity in the low temperature range (from 133 up to 283 K), where the dependence of the density of gap states at the Fermi level, N(E_F), on annealing temperature presents the minimum value at T_a = 873 K. The Meyer–Nelder rule was found to hold for the a-SiC:H thin films for annealing temperatures up to 873 K. Finally, the dependence of dark dc electrical conductivity at room temperature, σ_DRT, on T_a showed to reflect directly the dependence of E_a on T_a.     

Electrical and optical properties of chalcogenide amorphous semiconductors modified with Ni

The electrical and optical properties of the chalcogenide semiconductor (Se₃₂Te₃₂As₄Ge₃₂)_100?xNixitx have been studied. As the Ni concentration is increased the electrical dc conductivity is drastically increased and variable range hopping conduction becomes dominant even above room temperature. The optical energy gap decreases with the Ni concentration from 1.18–0.95 eV. Ni-atoms in the chalcogenide semiconductor donate free electrons which occupy the gap state. This occupation causes the shift of the Fermi level toward the conduction band. It is an effect of this shift that the thermal activation energy is decreased. The decrease in optical energy gap is independent of the shift of the Fermi level and is ascribable to the appearance of the additional level located at 0.95 eV above the top of the valence band. This level originates from the 3d-level of the Ni-atom.     

Electrical investigation of polythiophene–poly(vinyl acetate) composite films via VTF and impedance spectroscopy

Polythiophene (PTP) and poly (vinyl acetate) (PVAc) composite films were prepared by chemical oxidative polymerization method with FeCl₃ as an oxidant, in methanol at room temperature. Their dc conductivities as a function of temperature (313–358 K) were measured. An attempt has been made to investigate the effect of temperature and concentration of ferric chloride oxidant on the conductivity of polythiophene–poly(vinyl acetate) composite films. For fixed wt% of PVAc, the dc electrical conductivity of the film initially increases with the molar concentration of FeCl₃ and then decreases with the further increase in the concentration of FeCl₃. The temperature dependence of conductivity showed Arrhenius behavior. The conductivity isotherms show VTF behavior which is typical for ion conducting polymers. The Nyquist plot of Z′ vs. Z″ were plotted for frequency range 100 Hz–200 kHz and temperature range of 313–358 K. These plots consist of semicircles for higher temperatures. This suggests Debye type relaxation mechanism. The equivalent circuit consists of parallel combination of bulk resistance R_B and bulk capacitance C_B. The bulk resistance R_B of the samples decreased with increase in temperature.     

Electronic conduction in (Bi,Pb)–Sr–Ca–Cu–O granular superconductors

Granular superconductors are very interesting materials owing to their untypical electrical properties caused by the presence of Coulomb effects, electron tunnelling, Josephson coupling between granules and various aspects of disorder. The electrical properties of (Bi,Pb)–Sr–Ca–Cu–O granular superconductors obtained by the solid state crystallization method have been studied in this paper. The materials may be considered as a system of granules of a high-temperature suprconductor embedded in the insulating matrix. The main parameter which determines the properties of granular materials is the tunnelling conductivity between the neighboring grains. Composed of small weakly coupled granules they are characterized by hopping conductivity exponential dependence. Samples containing relatively large granules of the 2212 phase (above 30 nm), with larger inter-grain conductivity, above the transition temperature, exhibit a logarithmic temperature dependence of resistivity.     

Effect of Doping and Irradiation on Optical Parameters of Triglycine Sulphate Single Crystals

Crystals of triglycine sulphate (TGS) doped with orthonitroaniline (ONA) irradiated with different doses of gamma-radiation were used to investigate gamma-radiation effect on electrical and optical parameters. The absorption coefficient and the reflectance were measured and hence the extinction coefficient, the optical refractive index and the dielectric constants (e_r′, e_r″) of unirradiated and irradiated TGS crystals were calculated. Values of the allowed indirect optical energy gap E_g^opt. of TGS doped with ONA were calculated as a function of gamma-dose. Value of E_g^opt. decreases from 4.72 eV to 4.25 eV with increasing gamma-doses from 0 up to 3 Mrad. The values of the static dielectric constant e_r(0) and the effective electrical conductivity sigma₀ at room temperature and their dependence on gamma-dose were also calculated.     

The mixed alkali effect in lithium-sodium borate glasses

The electrical conductivity of a series of 0.35 (Li, Na)₂O·B₂O₃ glasses shows a minimum at the composition Na/(Na+Li)～0.6, which becomes stronger as the temperature is decreased; the activation enthalpy for electrical conductivity shows a maximum at this composition. In general, replacing 1% of the total oxygen concentration by chlorine or bromine (keeping the total alkali content fixed) in these glasses increases the conductivity; fluorine doping has an opposite effect. The mixed alkali effect, expressed in terms of the compositional dependence of the activation enthalpy for conductivity, is enhanced when borate glass is doped with fluorine, but is slightly diminished when doped with chlorine or bromine. The results are explained in terms of the structure of halogenated alkali-borate glasses, and discussed in relation to the origin of the mixed alkali effect.     

Optical and electrical properties of Tl-S glasses

The frequency and temperature dependence of ac conductivity and optical absorption have been measured for four Tl-S glasses, TlS, Tl₂S₃,TlS₂ and Tl₂S₅, prepared by a melt-quenching method. The ac conductivity has been measured over a frequency range 0.1 Hz to 1.8 GHz and a temperature range 190-273 K. The optical absorption was measured at room temperature over a wavelength range 200-2600 nm. We have determined the electrical and optical band gaps from the experimental results. For each glass, the electrical band gap is larger than the optical band gap and the difference increases with increasing sulfur concentration. The frequency dependence of ac conductivity varies with composition of the glasses. We suggest that these results are due to an increase of localized states in the band gap with increasing sulfur concentration.     

Electrical conductivity of Cs<Subscript>2</Subscript>CuCl<Subscript>4</Subscript> crystals

The electrical conductivity of Cs₂CuCl₄ single crystals, synthesized by crystallization from aqueous solutions in the CsCl–CuCl₂–H₂O system, has been investigated. The temperature dependence of the electrical conductivity of crystals in a temperature range of 338–584 K exhibits no anomalies. The electrical transfer activation enthalpy is ΔH_σ = 0.72 ± 0.05 eV and the conductivity is σ = 3 × 10^–4 S/cm at 584 K. The most likely carriers in Cs₂CuCl₄ are Cs⁺ cations, which transfer electric charge according to the vacancy mechanism.     

Electronic properties of vitreous and liquid AsSe alloys (I). Solid alloys

Measurements of the temperature dependence of the d.c. conductivity, therm?opower and optical absorption (near the fundamental edge) for five vitreous AsSe alloys containing from 30% to 50% As are presented. A conductivity maximum at the stoichiometric composition, As₂Se₃, corresponding to minima in the activation energies for conduction and thermopower, is accompanied by a minimum in the optical gap at this composition. These data suggest a minimum mobility gap at As₂Se₃, and a Fermi level independent of composition. Chemical bonding and bond edge state localization arguments are put forward to justify this proposal, and also to indicate why other amorphous semiconducting binary alloys behave differently at stoichiometric compositions.     

Thermal conductivity of FeS2 pyrite crystals in the temperature range 50–300 K

The thermal conductivity of two single-crystal samples of pyrite FeS₂ are investigated by the method of stationary longitudinal heat flux in the temperature range 50–300 K. The low electrical conductivity of the crystals with a small impurity content causes an identical value of experimental lattice thermal conductivity. The temperature dependence of the phonon mean free path is established.     

Influence of photonic excitations on the electrical parameters of TlInS2 crystals

The photo‐excitation effect on the current transport mechanism in TlInS₂ crystals has been studied by means of dark and illuminated conductivity measurements. The temperature‐dependent electrical conductivity analysis in the temperature region of 110‐340 K revealed the domination of the thermionic emission and the thermally assisted variable range hopping (VRH) of charge carriers above and below 160 K, respectively. Above 160 K, the conductivity activation energies in the dark are found to be 0.28 and 0.15 eV in the temperature regions of 340‐240 K and 230‐160 K, respectively. In the temperature region of 110‐150 K, the dark variable range hopping analysis revealed a density of localized states of 1.99×10²² cm^–3eV^–1, an average hopping distance of 0.53 nm and an average hopping energy of 79.65 meV. When the sample was photo‐excited, the values of the conductivity activation energies, the density of localized states near the Fermi level and the average hopping energy were observed to decrease sharply with increasing illumination intensity. On the other hand, the average hopping distance increased with rising illumination intensity. Such behaviours were attributed to the Fermi level shift and/or trap density reduction by electron‐hole recombination. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ac conductivity of Ag-doped bulk,glassy As2S3

Measurements of the electrical conductivity of Ag-doped bulk As₂S₃ glasses have been made as functions of temperature, pressure, frequency and Ag doping level. A Debye-like loss peak was observed near 10⁴ Hz. The frequency of the loss peak is dependent on temperature, pressure and doping level, but these dependences are different from those of the dc conductivity. The ac loss is attributed to the Maxwell-Wagner losses characteristic of inhomogeneous materials. The materials are presumed to be inhomogeneous mixtures of As₂S₃ and Ag₂S. We have also searched unsuccessfully for ac conductance in several bulk chalcogenide glasses.     

Electronic relaxation in zinc iron phosphate glasses

The electrical and dielectric properties of 10ZnO-30Fe₂O₃-60P₂O₅ (mol%) glasses, melted at different temperatures were measured by impedance spectroscopy in the frequency range from 0.01 Hz to 3 MHz and over the temperature range from 303 to 473 K. It was shown that the dc conductivity strongly depends on the Fe(II)/[Fe(II) + Fe(III)] ratio. With increasing Fe(II) ion content from 17% to 37% in these glasses, the dc conductivity increases. Procedure of scaling conductivity data measured at various temperatures into a single master curve is given. The conductivity of the present glasses is made of conduction and conduction-related polarization of the polaron hopping between Fe(II) and Fe(III), both governed by the same relaxation time, τ. The high frequency dispersion in electrical conductivity arises from the distribution in τ caused by the disordered glass structure. The evolution of the complex permittivity as a function of frequency and temperature was investigated. At low frequency the dispersion was investigated in terms of dielectric loss. The thermal activated relaxation mechanism dominates the observed relaxation behavior. The relationship between relaxation parameters and electrical conductivity indicates the electronic conductivity controlled by polaron hopping between iron ions.