Variable Range Hopping Model Research Articles

Local Ir dedimerization followed by ligand-hole hopping in X-ray irradiated thiospinel

The extended X-ray-absorption fine structure of a spinel CuIr2(S0.95Se0.05)4 powder sample, which has the charge-ordered temperature TCO=190 K, was determined at the Ir L3 edge for a wide temperature range. All the local structure parameters on the Ir–Ir bonds signified the second structural change at T*~70 K. Remarkably, the number of Ir dimers, ND(T), decreased by ~10% below T* [ND(0)/ND(T*)~0.9], which indicates partial dedimerization by X-ray irradiation. We also reanalyzed the resistance data of CuIr2S4 [Furubayashi et al., Solid State Comm. 126, 617 (2003)] with the variable-range hopping model by focusing on the density of states at EF, g. The X-ray-induced decrease in resistance was quantitatively explained as a result of the increase in g, which was attributed to the release of holes from the 10%-dedimerized Ir ions. We suppose that the released holes hop over the ligand chalcogen site and discuss the origin of X-ray-induced structural modulations [Kiryukhin et al., Phys. Rev. Lett. 97, 225503 (2006)] in terms of segregation of broken dimers.

Exploring weak ferromagnetism and conduction mechanism in the layered oxide BiPbSr2MnO6

Layered oxide BiPbSr2MnO6 (BPSMO), isostructural to 10 K Bi-based cuprates, is prepared by employing solid-state route in nitrogen atmosphere. Phase purity of the compound is determined from the room temperature Rietveld refined X-ray diffraction (XRD) data. BPSMO crystallizes in an orthorhombic crystal structure, thereby confirming its space group as ‘A2aa’. Presence of weak ferromagnetism in BPSMO below magnetic transition temperature (TN) is validated from isothermal magnetization data accompanied by the Arrott plot at 2.8 K, in contrast to its antiferromagnetic behavior at 290 K. The coexistence of Mn2+ and Mn3+ is confirmed from X-ray photoelectron spectroscopy study. The conduction mechanism in BPSMO is studied following two different hopping models, i.e., Arrhenius model for T > 250 K and Mott's variable range hopping (MVRH) model for T < 250 K. The relaxation in BPSMO is found to be polaronic in nature and may have originated due to the mixed valence structure of the sample. Lastly, the frequency domain analysis of dielectric spectra reveals the presence of two distinct relaxations. Moreover, it is determined that the low-frequency and high-frequency relaxation have activation energy 0.35 eV and 0.48 eV, respectively.

Electrical Double Percolation of Polybutadiene/Polyethylene Glycol Blends Loaded with Conducting Polymer Nanofibers.

The critical phenomena of double percolation on polybutadiene (PB)/polyethylene glycol (PEG) blends loaded with poly-3-hexylthiophene (P3HT) nanofibers is investigated. P3HT nanofibers are selectively localized in the PB phase of the PB/PEG blend, as observed by scanning force microscopy (SFM). Moreover, double percolation is observed, i.e., the percolation of the PB phase in PB/PEG blends and that of the P3HT nanofibers in the PB phase. The percolation threshold (φcI) and critical exponent (tI) of the percolation of the PB phase in PB/PEG blends are estimated to be 0.57 and 1.3, respectively, indicating that the percolation exhibits two-dimensional properties. For the percolation of P3HT nanofibers in the PB phase, the percolation threshold (φcII) and critical exponent (tII) are estimated to be 0.02 and 1.7, respectively. In this case, the percolation exhibits properties in between two and three dimensions. In addition, we investigated the dimensionality with respect to the carrier transport in the P3HT nanofiber network. From the temperature dependence of the field-effect mobility estimated by field-effect transistor (FET) measurements, the carrier transport was explained by a three-dimensional variable range hopping (VRH) model.

Conduction Mechanism and Dielectric Properties of Polycrystalline La0.53Ca0.47Mn0.5Cr0.5O3

The electric conductivity and dielectric properties of polycrystalline La0.53Ca0.47Mn0.5Cr0.5O3 are studied with impedance spectroscopy techniques over a wide range of frequencies and temperatures from 100 to 1 MHz and 100 and to 400 K, respectively. The material exhibits a semiconductor behavior in the whole temperature range, characterized by a change in the slope at a specific temperature with a saturation at ~ 360 K. At low temperature, the conduction mechanism is governed by distributed trap localized charge carrier states described by a variable-range hopping model. At high temperature, the conduction mechanism is thermally activated by a small polaron hopping process. At low frequencies, the high dielectric constants of the material decrease rapidly with frequency and increase with temperature due to the free charges accumulated at the electrode-sample interface (interfacial Maxwell–Wagner polarization). These high dielectric constants can be interpreted by the existence of grain boundaries. Additionally, the Nyquist plots are presented by two depressed not centered semicircles on the real axis, indicating non-Debye behavior for the material. The resistance of the grain boundaries is larger than the resistance of the grains, confirming that the grain boundaries govern the conductivity in La0.53Ca0.47Mn0.5Cr0.5O3. Finally, the modulus analysis indicates a transition of carrier mobility from the long range in the low-frequency region to the short range in the high-frequency region.

Mg-substitution effect on microstructure, dielectric relaxation and conduction phenomenon of Fe based perovskite nanomaterials

In this work, we have investigated the electrical and dielectric properties of the Mg doped perovskite (La0.8Ca0.1Pb0.1Fe1-xMgxO3 (x = 0.0, x = 0.1 and x = 0.2)) using the electrical impedance spectroscopy in the temperature range of (200–320 K). The crystal structure evaluated by X-Ray diffraction proved an orthorhombic structure of all the studied compounds with a Pbnm space group. Impedance fitting based on an equivalent circuit and the M″ fitting confirmed the existence of two relaxation phenomena attributed to grains and grain boundaries contributions. The study of the dc electrical conductivity proved that all compounds followed the variable range hopping (VRH) model at low temperature and the Arrhenius model at high temperature range. From conductivity analysis and the temperature dependence of the Jonscher’s power low exponent, we noted an alternation of correlated barrier hopping (CBH) and non-overlap small polaron tunneling model (NSPT) conduction processes at the studied temperature range. The relaxation phenomenon for both contributions and the deduced activation energies were found to be sensitive to the Mg composition; for the grain contribution, a minimum activation energy value was recorded for x = 0.1 compound and increased for suplementary Mg content. While, for grain contribution the activation decreases with the increase in Mg ion concentration. Concerning the evolution of the magnetization versus temperature, it was measured to investigate the effect of the Mg amount on the magnetic behavior of the prepared compounds under the studied temperature range.

In-Situ Chemical Synthesis, Microstructural, Morphological and Charge Transport Studies of Polypyrrole-CuS Hybrid Nanocomposites

Hybrid nanocomposites (HNCs) of polypyrrole (PPy) and CuS were synthesized by an in situ chemical polymerization method. The HNCs were prepared by varying the CuS nanoparticles weight percentage (10–40%) in PPy matrix. The XRD and FESEM characterization indicated the uniform distribution of CuS nanoparticles in PPy matrix. The XRD pattern revealed the presence of hexagonal CuS peaks overlapped with amorphous PPy pattern. Williamson Hall method was employed to estimate intrinsic strain in HNCs. FTIR spectrum revealed the shifting of 1025 cm−1 peak toward higher wavenumber, indicating insertion of CuS nanoparticles in PPy. The room temperature electrical conductivity of PPy is found to be increased from 1.15 × 10−1 to 3.70 S/cm as the content of CuS nanoparticles increases up to 40 wt% in HNCs. To explore the charge transport mechanism in HNCs,the conductivity was measured in the temperature range of 300–15 K. The measured conductivity data was analyzed with the help of Arrhenius model and 3d Mott’s variable range hopping (VRH) model in the temperature range of 35–300 K. The approximate values of Mott’s parameters at 300 K such as the density of states at Fermi level, average hopping distance and average hopping energy of HNCs were estimated as ~ 4×1024 cm−3eV−1, ~ 1.7 A and ~ 10 meV respectively.

Three-Dimensional Variable Range Hopping and Thermally Activated Conduction Mechanism of Polypyrrole/Zinc Cobalt Oxide Nanocomposites

Zinc cobalt oxide (ZCO) nanoparticles and zinc cobalt oxide/polypyrrole (ZCO/PPy) nanocomposites were synthesized by hydrothermal and in situ chemical polymerization routes, respectively. X-ray diffraction (XRD) patterns of PPy and ZCO/PPy nanocomposites show the amorphous nature of PPy, while there is the presence of dominant crystalline peaks of ZCO in the PPy matrix. Field-emission scanning electron microscopy (FESEM) depicts a spherical geometry and shows a decrease in the particle size with an increase in the ZCO weight percentage, while the transmission electron microscopy (TEM) images depict the core–shell-type formation of particles with aggregated ZCO nanoparticles as a core embedded in the PPy shell. The Fourier transform infrared (FTIR) spectra of ZCO/PPy nanocomposites show all the asymmetrical stretching vibrations of PPy and ZCO along with an increase in intensities of the respective metal oxides peaks and slight shifts in a few bands. Further, the transport studies, i.e., direct current (DC) conductivities of the ZCO/PPy nanocomposites, were studied as a function of temperature (300–453 K) and reveal semiconducting transport behavior. The analysis of DC conduction in PPy and ZCO/PPy suggests that the conduction of charge carriers follows the three-dimensional Mott variable range hopping (3D-VRH) model at a lower temperature, while it follows a thermally activated conduction model at high temperature.

Growth mechanisms and related thermoelectric properties of innovative hybrid networks fabricated by direct deposition of Bi2Se3 and Sb2Te3 on multiwalled carbon nanotubes

Flexible thermoelectric generators are an emerging trend in the field of waste heat conversion, as well as wearable and autonomous devices. However, the energy conversion efficiency of the state-of-the-art flexible thermoelectric devices is too low for their wide application and commercialization. In this work, n- and p-type multiwalled carbon nanotube (MWCNT)-thermoelectric material hybrid networks that may become a promising building block for the fabrication of flexible thermoelectric devices are presented. The hybrid networks were fabricated by direct deposition of thermoelectric material (Bi2Se3, Sb2Te3) on the MWCNT networks using physical vapor deposition technique. Growth mechanisms of Bi2Se3 and Sb2Te3 on MWCNTs were investigated. The Seebeck coefficient and charge transport properties of MWCNT-Bi2Se3 and MWCNT-Sb2Te3 hybrid networks were studied as function of MWCNT wt% in the networks. Variable-range hopping models were applied for the interpretation of conductance mechanisms in the hybrid networks. The Seebeck coefficients of the MWCNT-Sb2Te3 and MWCNT-Bi2Se3 hybrid networks with low MWCNT wt% were found to be comparable with the Seebeck coefficients of pure inorganic Sb2Te3 and Bi2Se3 thin films. At the same time, flexibility tests of the MWCNT-Sb2Te3 and MWCNT-Bi2Se3 hybrid networks with MWCNT 50% showed no significant increase in the resistance when they were bent to a radius of 5 mm. This makes hybrid networks, presented in this work, the perspective for applications in flexible thermoelectrics as thermoelectric coatings for flexible substrates and fillers for polymers-based composites.

Effect of the biphase TiO2 nanoparticles on the dielectric and polaronic transport properties of PVA nanocomposite: Structure analysis and conduction mechanism

Polymeric composite films with tunable electrical and dielectric performance have several uses in electronics based on organic materials. This paper reports the fabrication and the role of TiO2 NPS (nanoparticles) on the dielectric and nanocomposites' conduction of pristine polyvinyl alcohol (PVA). The nanopowder of biphase TiO2 and the nanocomposites of different NPS weight percentages have been fabricated via the sol-gel and casting methods. Structure of the synthesized NPS and nanocomposites were analyzed via PXRD (Powder X-ray diffraction), HR-TEM (High-resolution microscopy of transmission electron), and FTIR (Fourier transform-IR). The average size of the biphase TiO2NPS crystallite is around 52 nm, as calculated from Scherer law. Frequency and temperature dependence of dielectric properties have been studied in a wide range. The dielectric constant of the host matrix has been enhanced by adding TiO2NPS. AC conductivity increases with the further addition of TiO2NPS contents, and its frequency dependence follows the universal power law. The nanocomposite with 6.6 wt% of TiO2NPS shows higher conductivity as compared to other concentrations. The exponent power (s) has values in the range of 0.2–0.9, indicating that the conduction mechanism is correlated with barrier hopping (CBH). The dependence of the DC conductivity on absolute temperature has been studied and obeys Arrhenius relation. Three discrete sections were recognized from the AC conductivity spectra. In light of polaronic hopping conduction models, the electrical conductivity data were analyzed. The analysis shows that the high-temperature conductivity is well explained by the polaronic model, while at an intermediate temperature, the Greaves VRH (variable-range hopping) model is appropriate. Also, the non-adiabatic conduction was established in the nanocomposite system via the application of different suggestions. The values of the decay constant and the density of states confirmed the presence of the localized states. Finally, a small polaron coupling constant (γp) is more significant than four, which indicates a strong electron-phonon interaction.

Composites (1 − x)La0.7Ca0.3MnO3/xLa0.7Sr0.2Ca0.1MnO3: Electrical transport properties and enhancing of low-field-magnetoresistance and colossal magnetoresistance

Composites of (1 − x)La0.7Ca0.3MnO3/xLa0.7Sr0.2Ca0.1MnO3 were prepared via the sol–gel method combined with the energy milling method. The electrical transport and low-field magnetoresistance properties of the composites were systematically studied. As the La0.7Sr0.2Ca0.1MnO3 level increased, the metal-insulator transition temperature of the composites shifted to a higher temperature. The transport mechanisms in the insulating region of the composites were comprehensively analyzed via small polaron hopping (SPH) and variable range hopping (VRH) models. The combination of electron–electron interaction and spin wave scattering was considered to estimate the conduction mechanism in the metallic region of the composites. Both intrinsic magnetoresistance and extrinsic low-field magnetoresistance were enhanced from 30 K to the Curie temperature for the system. The results were explained on the basis of the competition between spin-dependent scattering and spin-polarized tunneling at La0.7Ca0.3MnO3 grain boundaries induced by interactions between La0.7Ca0.3MnO3 and La0.7Sr0.2Ca0.1MnO3 manganites.

Structural, electrical and radiation shielding properties of polyvinyl alcohol doped with different nanoparticles

Both pure and doped polyvinyl alcohol (PVA) with 1 wt% of Ni, NiO and Fe2O3 nanoparticles (NPs) were prepared using the casting method. The structural and electrical properties of the composite were extensively studied. Moreover, the radiation shielding of these composites was studied. XRD results prove the semi-crystalline structure of pure PVA because of crystalline and amorphous phases are appeared. The electrical conductivity (σdc) and current–voltage (I–V) characterization for all PVA samples have been an investigation. The separation (R) and energy of hopping (W) were calculated. Also, the temperature dependence of the conductivity for all pure samples was investigated using Mott’s variable-range hopping model. The obtained results suggest strongly the applicability of these samples in the electronic applications. Furthermore, radiation shielding parameters have been calculated. Results show that PVA with 1% Ni sample has the highest values of mass attenuation coefficient, effective atomic number, fast neutrons removal cross-sections and absorption neutron scattering cross-section among other samples.

A-site K-doping to enhance room-temperature TCR of polycrystalline La0.8Sr0.2-xKxMnO3 ceramics

Herein, polycrystalline La0.8Sr0.2-xKxMnO3 (LSKMO, 0 ≤ x ≤ 0.2) ceramics with varying K concentration are successfully synthesized via a facile sol-gel method. The electrical transport properties and microstructural of the prepared samples are characterized and analyzed in detail. The different characterization results show that K+ ion substituted La3+ and Sr2+ ions significantly influenced the double exchange (DE) mechanism and Jahn-Teller (JT) effects. The electrical resistivity studies suggest that as the K doping content rose, the resistivity increased and peak resistance temperature decreased. Electrical transport properties of samples are analyzed via the combination of DE mechanism, JT effects, variable-range hopping and small-polaron hopping model. The temperature coefficient of resistance (TCR) measurement demonstrates the enhanced temperature sensitivity for the K-doped polycrystalline ceramics samples as compared to the pure La0.8Sr0.2MnO3 samples. The optimized x content for the polycrystalline LSKMO ceramics is determined to be x = 0.08. At this x value, the peak TCR of the resistivity measured 9.11% K−1 at 292.44 K. These results confirm that high room-temperature TCR values can be achieved by optimizing A-site K doping and demonstrate the potential of polycrystalline La0.8Sr0.12K0.08MnO3 ceramic in room-temperature infrared bolometers.

Charge transport mechanism and percolation model in La0.75Ca0.25−xNaxMnO3 (0 ≤ x ≤ 0.10) manganites

In this communication, La0.75Ca0.25−xNaxMnO3 (0 ≤ x ≤ 0.10) samples were synthesized by the Flux method and their structural and electrical properties were systematically undertaken. X-ray diffraction of the samples showed an orthorhombic structure with a Pbnm space group. Furthermore, to get a better understanding of the electrical properties, the resistivity ρ(T) as a function of temperature was measured by a standard four-probe method. The maximum resistivity values decreased and the insulator–metal transition temperature (TM–SC) increased with increasing magnetic field. At low-temperature region (T TM–SC) region, the transport mechanism was explained using the adiabatic small polaron hopping and the variable-range hopping models. Then, to get insights into the change in the resistivity plots, in entire temperature, ρ(T) was fitted by the percolation model. Interestingly, it is important to note that the activation energy (Ea) decreased with increasing Na+ content, which indicates the enhancement of the double-exchange (DE) interaction. Moreover the magnetoresistance (MR %) effect was studied.

Analysis and comparison of total resistance models for accurate static modeling of long- and short-channel OTFTs produced with various organic materials

In the present study, long- and short-channel organic thin film transistors (OTFTs) produced with various organic semiconductor materials as active layers were fabricated, characterized, and modeled. Two charge transport models, the Meyer-Neldel rule grain boundary-trapping (MNR-GBT) and variable range-hopping models (VRH), were applied to investigate the total device resistance of different types of OTFTs fabricated with various organic semiconductor materials. A generalized method of assessing the current-voltage characteristics of both short- and long-channel OTFTs fabricated with different materials is also presented. All of the devices are described theoretically in terms of their current-voltage characteristics by considering the total resistance's dependence on the gate voltage, which was calculated by substituting the total resistance into the drain current equation. The calculated and measured total resistance and output current-voltage characteristics of each device are presented. Using the best modeled values for the total resistance, the experimental data of output current-voltage characteristics ID(VD) of different kinds of OTFTs are in agreement with those calculated for four different types of OTFTs with long and short channels. The developed model including the total device resistance (according to the Meyer-Neldel rule grain boundary-trapping model) can be applied to long and short-channel devices produced with different materials generalizing all types of OTFTs.

Nanocomposites Based on Dendrimers and Layered Molybdenum Disulfide

Nanocomposites consisting of molybdenum disulfide (MoS2) and an organometallic dendrimer were synthesized by taking advantage of the exfoliating/restacking properties of lithiated molybdenum disulfide (LiMoS2). Based on the molecular weight of the dendrimer, nine different mole ratios of MoS2 to dendrimer were utilized in order to investigate the upper and lower limit of the dendrimer loading into the restacked MoS2. The developed nanocomposites were characterized by powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and variable-temperature electrical conductivity measurements. Our studies revealed that the ideal condition for preparing reproducible monophasic systems is at low dendrimer concentration, no greater than 1:0.125 mole ratio of MoS2 to dendrimer, and no lower than 1:0.002 mole ratio of MoS2 to dendrimer. Biphasic systems can be produced at mole ratio of MoS2 to dendrimer higher than 1:0.125. Additionally, the average crystallite size decreased as the mole ratio of the MoS2 to dendrimer was increased. Specifically, a change from 1:0.002 to 1:0.5 resulted in a noticeable decrease from 365 to 155 A. The electrical conductivity of the nanocomposites drops as the amount of dendrimer increases. Variable-temperature conductivity data are consistent with a variable-range hopping model.

Structural and transport properties of composite of La0.8Sr0.2MnO3 and BaTiO3 materials

We studied the effect of BaTiO3 phase on the structural, vibrational, and electrical properties of La0.8Sr0.2MnO3. The parent samples of La0.8Sr0.2MnO3, BaTiO3 and their composite (0.85) La0.8Sr0.2MnO3 + (0.15) BaTiO3 were synthesized by solid-state reaction route. Structural study using X-ray diffraction (XRD) for La0.8Sr0.2MnO3 phase revealed a rhombohedrally distorted (space group R3c) structure, while BaTiO3 compound was found to have crystallized in tetragonal structure (space group P4mm). The XRD results were verified using Rietveld refinement. The composite sample (0.85) La0.8Sr0.2MnO3 + (0.15) BaTiO3 display the presence of both of La0.8Sr0.2MnO3 and BaTiO3 phases. The addition of insulating BaTiO3 phase in the ferromagnetic metallic phase of La0.8Sr0.2MnO3 which exhibits well-characterized metal-to-insulator transition in the vicinity of 350 K enhances the resistivity and shifts the metal–insulator transition temperature TMI towards lower temperature. Further, with the application of magnetic field, resistivity of the composite sample is found to decrease and TMI in composite sample is observed to shift towards higher temperature. The magnetoresistance (MR) of composite sample (0.85) La0.8Sr0.2MnO3 + (0.15) BaTiO3 decreases monotonously in 200–300 K regime under magnetic field of 8 T. The analysis of electrical resistivity data in the metallic region (T TMI) is well described using Small polaron hopping (SPH) and Mott's variable range hopping (VRH) models.

Investigation of Structural, Optical and Electrical Transport Properties of Yttrium Doped La0.7Ca0.3MnO3 Perovskites

In the present study, bulk samples of yttrium doped La0.7−xYxCa0.3MnO3 (x = 0.3 and 0.4) are prepared through solid-state reaction route. The structural, optical and electrical transport properties have been explored through various analytical techniques. The x-ray diffraction (XRD) patterns affirm single phase and polycrystalline nature of the samples. Rietveld refinement analysis of the XRD data is used to determine the various structural parameters. It reveals that both the samples belong to orthorhombic crystal system with Pnma space group. Using Scherrer’s equation, a lower value of crystallite size is found in the higher concentration of yttrium doped sample. The band gap as determined from the UV–visible DRS data by employing the Tauc’s relation is found to enhance with the increase of yttrium concentration in La0.7Ca0.3MnO3. The low temperature resistivity measurements establish semiconducting nature of the samples over the temperature range of ~ 110–300 K. Moreover, the electrical resistivity data are interpreted within the variable range hopping (VRH) model to estimate the density of states at the Fermi level N(EF), mean hopping distance Rh(T) and hopping energy Eh(T). It is observed that the electrical quantities vary gradually with the change of Y content. The conduction mechanism in this system is governed in the framework of thermally activated small polaron hopping (SPH) model and accordingly various parameters namely Debye temperature (θD), activation energy (Ep) and optical phonon frequency (νph) are evaluated. The obtained results support a strong connection between structure and electrical behavior in our samples and can be elucidated on the basis of localization of charge carriers induced by the substituting ions.

Comparison of temperature dependences of electrical conductivity of composite rGO-SWNT film with rGO and SWNT films

Temperature dependences of electrical conductivity of reduced graphene oxide composite with single-walled nanotubes (rGO-SWNTs) and rGO films has been studied in the temperature range of 25–290 K. Both films were obtained by vacuum filtration of aqueous suspensions. Temperature dependences of conductivity of films were found similar to the conductivity observed in disordered semiconducting systems. It was demonstrated that the behavior of the conductivity temperature dependence of pure rGO, SWNTs and composite film is different. The temperature dependences of the resistance R(T) of the films were analyzed within the framework of the variable range hopping (VRH) transport in which electron motion is due to the thermo-activated quantum tunneling between localized states. Two-dimensional Mott VRH (Mott 2D VRH) and Efros-Shklovskii VRH (ES VRH) models were applied for analysis. Mott 2D VRH was observed for rGO-SWNT film in the interval of 25–200 K. At higher temperatures R(T) of rGO-SWNT was fitted with Arrhenius-like equation describing electron activation from localized states to delocalized ones. R(T) of the rGO film followed the Mott 2D VRH model from 165 to 290 K, however, at lower temperatures ES VRH model was exploited. From approximation of R(T) by these models the parameters of the electron transport in rGO-SWNT and rGO films were estimated. It was suggested that nanotubes in rGO-SWNT composite serve as conductive bridges among rGO sheets enhancing the conductivity as compared to rGO. The conductivity of rGO also influences on the composite properties because 2D character of electron motion is kept in sharp contrast to SWNT film which demonstrated three-dimensional Mott VRH electron transport in the same temperature range.

Observation of 2D Conduction in Ultrathin Germanium Arsenide Field-Effect Transistors.

We report the fabrication and electrical characterization of germanium arsenide (GeAs) field-effect transistors with ultrathin channels. The electrical transport is investigated in the 20–280 K temperature range, revealing that the p-type electrical conductivity and the field-effect mobility are growing functions of temperature. An unexpected peak is observed in the temperature dependence of the carrier density per area at ∼75 K. Such a feature is explained considering that the increased carrier concentration at higher temperatures and the vertical band bending combined with the gate field lead to the formation of a two-dimensional (2D) conducting channel, limited to few interfacial GeAs layers, which dominates the channel conductance. The conductivity follows the variable-range hopping model at low temperatures and becomes the band-type at higher temperatures when the 2D channel is formed. The formation of the 2D channel is validated through a numerical simulation that shows excellent agreement with the experimental data.

Structural, electrical and magnetic properties of Sr1-xBixTiO3-δ ceramics

ABSTRACTThe structural, electrical and magnetic properties of Sr1-xBixTiO3-δ, (x = 0.10, 0.20, and 0.30) ceramics have been investigated. The X-ray powder diffraction (XRD) patterns at room temperature indicate that the structure of the samples is cubic with Pm3 m space symmetry group. The conduction of all investigated samples follows the three-dimensional variable-range hopping (VRH) model within the temperature range 100 K ≤ T ≤ 300 K. It was noticed that the paramagnetic behaviour depends on the content of Bi ions. The effective paramagnetic magnetic moment increased as the Bi content increased.