Small-polaron mobility in nonstoichiometric cerium dioxide

The high temperature drift mobility (μ_d) of charge carriers in nonstoichiometric cerium dioxide (CeO_2?x) has been calculated by combining the electrical conductivity and nonstoichiometry data on the basis of the oxygen vacancy model with correct ionization state. The electrical conductivity was measured by a four-probe d.c. technique and the nonstoichiometry by thermogravimetric analysis. The dilute solution model of the point defects is valid up to x = 0.03. From the magnitude of μ_d and its temperature dependence, the charge carriers in CeO_2?x, are proposed to be small-polarons formed by localization of electrons at cerium sites and the charge transport process is proposed to occur by a hopping mechanism. The observed temperature dependence of μ_d is in accord with that derived by Holstein and Friedman for small-polaron transport by the hopping mechanism. The activation energy of mobility is found to increase with increasing x as expected for the hopping model.     

Anisotropic properties and conduction mechanism of TlInSe2 chain semiconductor

TlInSe₂ chain crystals were prepared using the modification of the Bridgman technique. The grown crystals were identified by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDX), and X-ray diffraction (XRD). We investigate the anisotropy of transport properties for the first time for TlInSe₂ crystals. Temperature dependence of the dc electrical conductivity, Hall coefficient, Hall mobility, and charge carrier concentration were investigated in the temperature range 184–455 K. The conduction mechanism of TlInSe₂ crystals was studied, and measurements revealed that the dc behavior of the grown crystals can be described by Mott’s variable range hopping (VRH) model in the low temperature range, while it is due to thermoionic emission of charge carriers over the chain boundaries above 369 K. The Mott temperature, the density of states at the Fermi level, and the average hopping distance are estimated in the two crystallographic directions. The temperature dependence of the ac conductivity and the frequency exponent, s, is reasonably well interpreted in terms of the correlated barrier-hopping CBH model.     

Charge carrier transport and d.c. conductivity in thin polycrystalline p-terphenyl films2

The electrical conductivity and charge carrier mobility in thin polycrystalline p-terphenyl layers have been measured. The transport and conductivity were interpreted in terms of a hopping mechanism. The activation energies of mobility and conductivity were obtained, and the density of states in the vicinity of the Fermi level 10²⁰ cm^?3 eV^?1 was estimated. The height of the potential barrier around the impurity levels changed according to the Poole-Frenkel effect.     

Mobility of charge carriers in porous silicon layers

The (conduction) mobility of majority charge carriers in porous silicon layers of the n and p types is estimated by joint measurements of electrical conductivity and free charge carrier concentration, which is determined from IR absorption spectra. Adsorption of donor and acceptor molecules leading to a change in local electric fields in the structure is used to identify the processes controlling the mobility in porous silicon. It is found that adsorption of acceptor and donor molecules at porous silicon of the p and n types, respectively, leads to a strong increase in electrical conductivity, which is associated with an increase in the concentration of free carrier as well as in their mobility. The increase in the mobility of charge carriers as a result of adsorption indicates the key role of potential barriers at the boundaries of silicon nanocrystals and may be due to a decrease in the barrier height as a result of adsorption.     

Electrical conduction of metallized BOPP films based on revised Poole–Frenkel effect

In high energy density capacitors, the electrical conduction in the dielectric may result in the leakage and the reduction of the energy efficiency. This paper investigates the electrical conduction mechanism of metallized biaxially oriented polypropylene (BOPP) film based on a revised Poole–Frenkel (PF) effect and field-enhanced carrier mobility. The field-enhanced carrier mobility which is not taken into account in traditional PF effect is investigated in the revised PF effect based on the carrier hopping mechanism. Moreover, the coefficient of the PF effect (β_PF) which stands for the variation of the barrier height is discussed. Experiments are performed to measure the electrical conductivity of the BOPP film under different electric fields. Influencing factors such as metallized electrodes, the interlayer pressure in the winding, and the crystallinity may decrease the conductivity. With a fitted jump distance of 0.6 nm, the calculated conductivities based on the revised PF effect do match well with experimental results.     

Interpretation of anomalous normal state optical conductivity of La-Sr-CuO cuprates

The observed frequency dependent optical response of hole doped cuprate La_1.85Sr_0.15CuO₄ superconductors, has been theoretically analysed. Starting from an effective two-dimensional (2D) interaction potential for superlattice of hole doped cuprates treated as a layered electron gas, the spectral function is formulated. Calculations of the optical conductivity σ(ω) have been made within the two-component scheme: one is the coherent Drude carriers responsible for superconductivity and the other is incoherent motion of carriers from one site to other leads to a pairing between Drude carriers. The approach accounts for the anomalies reported (frequency dependence of optical conductivity) in the optical measurements for the normal state. Estimating the effective mass from specific heat measurement and ε_∞ from band structure calculations for the low-energy charge density waves, the model has only one free parameter, the relaxation rate. The frequency dependent relaxation rates are expressed in terms of memory functions, and the coherent Drude carriers from the effective interaction potential lead to a sharp peak at zero frequency and a long tail at higher frequencies, i.e. in the infrared (IR) region. However, the hopping of carriers from one site to other (incoherent motion of doped carriers) yields a peak value in the optical conductivity centred at mid-IR (MIR) region. We find that both the Drude and hopping carriers in the superlattice of cuprates will contribute to the optical process of conduction in the CuO₂ planes and shows similar results on optical conductivity in the MIR as well as IR frequency regions as those revealed from experiments.     

Charge transport in PbMoO<Subscript>4</Subscript> crystals

Dc and ac electrical conductivity of lead molybdate crystals is studied in the temperature range 300–550 K. The electrical conductivity was shown to have electronic (hole) impurity character. The I–V characteristics are typical of a space charge-limited current. The carrier mobility was estimated to be 10^?5 cm² V^?1 sat T=300 K. The results of the study suggest the hopping mechanism of conduction in PbMoO₄ crystals.     

Small polaron electron transport in reduced CeO2 single crystals

An investigation of the electrical properties of reduced ceria, CeO_2?x, carried out on single crystals, shows that CeO_2?x provides one of the clearest examples of hopping conduction and the small polaron mechanism. Included are conductivity and Seebeck coefficient measurements at constant x, obtained by sealing off the specimen chamber after reduction. The Seebeck coefficient is independent of temperature, suggesting that the number of carriers is constant. On the other hand, the mobility is activated, with activation energy E_h = 0.40 eV at small x and increasing to 0.52 eV at x = 0.25. The results for the mobility preexponential are consistent with the adiabatic theory of small polaron behavior. A puzzling feature of the Seebeck data as a function of x is that, for low x, the data fit the well-known Heikes formula, without a degeneracy factor of 2 for spin. Nevertheless, these data are interpreted to show that the proportion of mobile carriers decreases as x increases, presumably because of the presence of short-range ordered configurations which immobilize some carriers.     

DC conductivity and magnetic properties of piezoelectric–piezomagnetic composite system

A series of composites (1−x) (Ni_0.8Zn_0.2Fe₂O₄)+x (BaTiO₃), where x=0%, 20%, 40%, 60%, 80% and 100% BT content, have been prepared by the standard ceramic technique, then sintered at 1200 °C for 8 h. X-ray diffraction analysis shows that the prepared composites consist of two phases, ferrimagnetic and ferroelectric. DC electrical resistivity, thermoelectric power, charge carriers concentration and charge carrier mobility have been studied at different temperatures. It was found that the DC electrical conductivity increases with increasing BT content. The values of the thermoelectric power were positive and negative for the composites indicating that there are two conduction mechanisms, hopping and band conduction, respectively. Using the values of DC electrical conductivity and thermoelectric power, the values of charge carrier mobility and the charge carrier concentration were calculated. Magnetic measurements (hysteresis loop and magnetic permeability) show that the magnetization decreases by increasing BT content. M–H loop of pure Ni_0.6 Zn_0.4 Fe₂O₄ composite indicates that it is paramagnetic at room temperature and that the magnetization is diluted by increasing the BT content in the composite system. The value of magnetoelectric coefficient for the composites decreases by increasing BT content for all the compositions except for 40% BT content, which may be due to the low resistivity of magnetic phase compared with the BT phase that causes a leakage of induced charges on the piezoelectric phase. Since both ferroelectric and magnetic phases preserve their basic properties in the bulk composite, the present BT–NZF composite are potential candidates for applications as pollution sensors and electromagnetic waves.     

Magnetic semiconductors

As far as the electrical conductivity is concerned, solids are usually classified as metals, semiconductors, or insulators. In metals the concentration of the charge carriers responsible for the electrical conductivity is large, whereas in semiconductors and insulators the carrier concentration is much smaller. The distinction between semiconductors and insulators is based on a difference in the nature of the conductivity. For semiconductors the charge carriers (electrons or holes) occupy the states of energy bands; these states are not Iocalized on particular atoms, but spread throughout the entire crystal. In such a situation the mobility of the carriers can be quite high and would, in fact, be infinite in a rigid periodic lattice; in this model the thermal motion of the atoms induces a scattering of the carriers and thus limits the conductivity to finite values. The classical examples of semiconductors are the elements Si and Ge and compounds such as GaAs, InSb, CdTe, ZnS, etc.     

Poole-Frenkel effect in chalcogenide semiconductors with various crystalline structures

The results of investigation of electrical conductivity of a large group of chalcogenide semiconductors having the layered, cubic, and orthorhombic structures in strong electric fields up to 10⁵ V/cm are presented. The revealed increase in electrical conductivity σ in strong electric fields has been explained by the Frenkel thermionic ionization. This has made it possible, along with other parameters (for example, activation energy and trap concentration, charge-carrier mean free path, permittivity), to evaluate the concentration and mobility of charge carriers in semiconductors under study. It has been shown that in strong electric fields in semiconductors, when the thermionic ionization of the traps occurs, their permittivity ?, which is caused by the electron polarization, is determined by the simple formula ? = n ², where n is the refractive index of light.     

螺旋位错对螺旋型硒化铋纳米片电学特性的影响

利用场效应晶体管和导电原子力显微术,系统研究了螺旋型和平面型硒化铋纳米片的电学特性.结果显示,两种纳米片均体现出高的电导率及类金属导电特性.与平面型样品相比,有更高的载流子浓度和更低的迁移率.导电原子力显微术表征表明,螺旋型纳米片中的螺旋位错边缘相比平台有更高的电导,反映出螺旋位错可以提供更多的载流子.补偿了样品的低迁移率特性,提升了样品的电导率.     

On the existence of free and self-trapped carriers in insulators: an abrupt temperature-dependent conductivity transition

A variational calculation of the eigenstates of the three-dimensional analogue of Holstein's Molecular Crystal Model is utilized as a basis for determining the conditions under which carrier self-trapping does or does not occur in this system. It is found that below a temperature-dependent critical value of the electron-lattice coupling strength self-trapping does not occur; the eigenstates then correspond to an excess electron being only weakly coupled to the vibratory motion. Above a larger temperature-dependent critical value of the electron-lattice coupling strength only self-trapped (small-polaron) eigenstates exist. Between these two (temperature-dependent) critical values of the electron-lattice coupling strength both types of solutions are found.

The condition for the existence of the weakly coupled situation, as well as that for the self-trapped circumstance, is shown to be derivable from arguments which are independent of the detailed variational calculation. These ancillary derivations provide a physical basis for understanding the two existence conditions. In addition, the results of the variational calculation are shown to agree with results obtained by non-variational means in the appropriate limits.

The temperature-dependent appearance or disappearance of states from the energy spectrum of the coupled system manifests itself through an abrupt change in the electrical transport properties of the material. The conductivity, Hall mobility, and thermoelectric power on both sides of such a transition are calculated by an ad hoc application of previously obtained results. The essential feature of the occurrence of this transition is that the carriers on the low-conductivity side of the transition are self-trapped; they possess the low thermally activated mobility that is associated with small-polaron hopping motion. On the high-conductivity side of the transition the mobility of the carriers is considerably higher, it being that which is generally associated with electronic motion in rigid-lattice bands. The possible relevance of the present theory to the so-called insulator-to-metal and insulator-to-insulator transitions is discussed.     

Charge carrier mobility in crystals of the Pb<Subscript>0.67</Subscript>Cd<Subscript>0.33</Subscript>F<Subscript>2</Subscript> superionic conductor

The frequency (ν = 10^?1–10⁷ Hz) dependences σ(ν) of the conductivity of single crystals of the Pb_0.67Cd_0.33F₂ superionic conductor with the fluorite-type structure (CaF₂) in the temperature range of 132–395 K have been studied. The dependences σ(ν) have been discussed in the framework of the hopping relaxation of ionic carriers, which are mobile anions F^?. From experimental curves σ(ν), the direct-current (dc) conductivity σ_dc and the average charge carrier hopping frequency ν_h have been determined. This has made it possible to calculate the charge carrier mobility μ_mob and charge carrier concentration n _mob in these crystals. At room temperature (293 K), the electrical parameters are σ_dc = 1.6 × 10^?4 S/cm, ν_h = 2.7 × 10⁷ Hz, μ_mob = 2.0 × 10^?7 cm²/(s V), and n _mob = 5.1 × 10²¹ cm^?3.     

Electron transport in Ti1−xNbxO2 solid solutions with x < 4%

By means of conductivity and thermopower measurements in niobium doped rutile TiO₂, we have analysed the electron transport. A previous study made in TiO₂ pointed out the existence of a large polaron transport. Nevertheless, some uncertainties remain as the precise dependence of the mobility with the temperature. The Nb doped TiU₂ studied in the present paper brings some new information: the mobility could follow an optical phonon scattering law. The FHIP model seems to be the more appropriate model to describe the mobility but the lack of theoretical calculation of transport energy does not allow definitive conclusions about the physical processes which occur in the intermediate range of temperature. However, we can say that the ionization activation energy of Nb is found to be close to 14 eV.Moreover, it is found that the localized electrons become mobile by hopping with increasing niobium content. This yields a conductivity composed by electrons which move in the 3d Ti band and localized electrons which hop from site to site. This hopping component and the number of localized electrons increase with niobium content. This leads to the disappearance of the band motion in the conductivity.     

The influence of Zn ions substitution on the transport properties of Mg-ferrite

The thermoelectric power and electrical conductivity measurements of Zn-substituted Mg-ferrites having the general formula Mg_1−xZn_xFe₂O₄ (where x=0, 0.2, 0.4 and 0.6) were carried out from room temperature to 773 K. The Seebeck coefficient is positive for all the compositions showing that these ferrites behave as p-type semiconductors and the majority charge carriers are holes. The temperature variation of the Seebeck coefficient is also discussed. The Fermi energy (E_F); the density of charge carriers (n) and the carriers mobility (μ) were determined for the studied system. The variation of log σ with reciprocal of temperature shows a discontinuity at Curie temperature. The DC electrical conductivity increases with increasing temperature ensuring the semiconducting nature of the samples. The Curie temperature determined from DC electrical conductivity was found in satisfactory agreement with that determined from initial magnetic permeability measurements. This transition temperature is found to decrease with increasing Zn concentration. The activation energy in the paramagnetic region is found to be lower than that in ferrimagnetic region. The variation of room temperature conductivity with composition indicates that conductivity increases with increasing Zn content. The dependence of the electrical conductivity of Mg-Zn ferrite on Zn content is explained on the basis of the cation distribution.     

Conductivity in a disordered medium and carrier localization in weakly doped lanthanum manganites

The dc and 9.2-GHz electrical resistivities and magnetoresistance observed in La_1?xA_xMnO₃ crystals (A=Sr, Ce, x≤0.1) in the temperature interval 77–300 K are accounted for by the contributions due to carriers, both nonlocalized and localized in the valence-band tail and near the Fermi level. The localized-state tail extends to a depth of 0.15–0.25 eV inside the band gap, and the hopping activation energy varies from 0.06 to 0.15 eV, depending on the sample composition. Within the temperature region where magnetic ordering sets in, the variations of the electrical resistivity and magnetoresistance with temperature and magnetic field are caused by variations in the carrier mobility and concentration.     

Transport properties of photo-excited carriers in slightly compensated Hg0.785Cd0.215Te

Photo-Hall measurements are presented for slightly compensated n-type Hg_0.785Cd_0.215Te at 4 K where the electron mobility is limited by charged scattering centers. Using a 10.6 μm laser as the optical source, we observe that the electron mobility passes through a maximum as a function of photo-excited carrier density. Mobility expressions based on the Kane-band model are modified to incorporate neutralization of charged scattering centers by photo-excited carriers. Calculated mobility values are found to be in satisfactory agreement with the data. The observed decrease in mobility for high carrier densities is attributed to electron-hole scattering. The mobility enhancement at lower carrier densities is explained in terms of neutralization of charged acceptors.     

Electrical conduction in ordered defect compounds

A comparative study of the temperature dependence of electrical resistivity, carrier concentration and carrier mobility of the Ordered Defect Compounds (ODCs) CuIn₃Se₅, CuIn₃Te₅, and CuIn₅Te₈ with their corresponding normal 1:1:2 phase is reported. Relatively lower carrier concentration and higher activation energy observed in ODCs is explained on the basis that shallow acceptor or donor levels observed in 1:1:2 phase are partially annihilated in these compounds due to attractive interaction between V_Cu⁻¹ and In_Cu⁺² defect pair. In the activation regime, the mobility is explained by taking into account a scattering mechanism of the charge carriers with donor–acceptor defect pairs. The electrical data at lower temperatures is explained with the existing theoretical expression for the nearest neighbor hopping conduction mechanism.     

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.