Temperature-dependent Electrical Transport Properties Research Articles

Crystallographic dependence of the electrical transport mechanism in La-Mn-O thermosensitive thin films

Bimetallic oxide materials with amorphous state usually exhibit singular peculiarities due to their periodic disorder structure, implying great potential applications as high-performance electronic devices. In this report, La-Mn-O thin films with different crystallographic features (amorphous and polycrystalline) were deposited on Si (100) substrates using magnetron sputtering method. The as-deposited thin films were successfully convert from the amorphous state to the polycrystalline state by tuning the post-annealing temperatures. X-ray diffraction (XRD) confirmed that La-Mn-O was amorphous at annealing temperatures of 500 °C, while the films were polycrystalline at higher annealing temperatures (600 °C and 700 °C). Scanning electron microscopy (SEM) and high-magnification transmission electron microscopy (HRTEM) distinguished the microscopic features of LaMnO3 thin films in polycrystalline and amorphous states. The effects of annealing temperatures and structural disorder on the temperature-dependent electrical transport properties of La-Mn-O thin films are investigated. The resistance-temperature relationship curves of both polycrystalline and amorphous films show negative temperature coefficient behavior, and the amorphous films follow more than one conduction mechanism. Results show that the electrical transport of the crystalline La-Mn-O films is mainly controlled by thermal activation mechanism, while the electron transport behavior of the amorphous film is simultaneously influenced by thermal activation and Mott VRH variable range hopping conduction.

X-ray photoelectron spectroscopy and temperature-dependent electrical transport properties of MnFe2O4-reduced graphene oxide nanocomposites

The reduced graphene oxide (rGO) sheets are assembled with MnFe2O4 (MFO) nanoparticles via an in-situ sol–gel autocombustion route to obtain MFO@rGO nanocomposites. The investigation of their structural and electrical properties is carried out through X-ray photoelectron spectroscopy (XPS) and impedance analyzer, respectively. XPS study discloses the presence of C, O, Fe, and Mn in the MFO@rGO composite, indicating the decoration of MnFe2O4 nanoparticles by rGO sheets. XPS study also reflects the mixed Fe3+/Fe2+ oxidation states of Fe and mixed Mn2+/Mn3+oxidation states of Mn in the composite. The highest value of ac conductivity (σac = 7.51 × 10−6 Ω−1 cm−1 at 1 MHz) at room temperature is obtained within the vicinity of the percolation threshold when rGO content is 20 wt%. The ac conductivity of composites increases with frequency up to 100 °C and then becomes independent of the frequency with a further increase in temperature. The impedance gradually decreases with temperature, revealing the occurrence of negative temperature coefficient of resistance effect in composites. The findings suggest that the MFO@rGO composites can be a potential nominee for the fabrication of electrodes for supercapacitors and flexible electronic devices of low resistance and high electrical conductivity for high-frequency applications.

Multiple carrier transport in high-quality α-Sn films grown on CdTe (001) by molecular beam epitaxy

The epitaxial growth of tin in an alpha phase (α-Sn) is desired for its topological properties. In this study, we have successfully grown a series of α-Sn films on CdTe (001) substrates by molecular beam epitaxy with different thicknesses. A (2 × 1) surface reconstruction of CdTe is obtained due to efficient cleaning by atomic hydrogen, which favors the α-Sn growth. The high quality of the α-Sn films has been confirmed by x-ray diffraction, atomic force microscopy, etc. Thickness and temperature-dependent electrical transport properties have been studied. All the samples show a p-type transport at room temperature, but transitions in transport type are observed at lower temperatures. These transport behaviors can be well explained by a three-band model, and a phase diagram illustrating the transport behaviors in α-Sn is presented.

Electrical transport properties of thermally stable n-ZnO/AlN/p-Si diode grown using RF sputtering

The temperature dependent electrical transport properties of n-ZnO/AlN/p-Si heterojunction diode fabricated by RF sputtering system have been investigated over a wide temperature range of 303 K–413 K. The AlN buffer layer in-between ZnO and Si lowers the mismatch in thermal expansion coefficient/lattice constant for improved electrical and structural characteristics. XRD pattern and FESEM confirm the crystalline nature and good quality of ZnO thin film with uniform grain size and crack free structure respectively. As measured from the temperature dependent I–V curve, the apparent barrier height reduces as Ideality factor rises which shows good thermal stability within the temperature range studied here. The mean value of barrier height as well as the Richardson constant have been evaluated. Additionally, by taking into account the Gaussian distribution of inhomogeneous barrier heights, the modified Richardson constant has been estimated.

Electrical transport properties of focused ion beam lithography Au contacts on PbTiO3 nanorods

PbTiO3 nanorods with tetragonal phase were synthesized by hydrothermal method and heat treatment, and temperature-dependent electrical transport properties of individual PbTiO3 nanorod were investigated. The results show that the conductivities of PbTiO3 nanorods are gradually enhanced with temperature increasing from 77.4 to 295[Formula: see text]K, and exhibit the typical nonlinear I–V characteristics. The barrier height between Au electrode and nanorod is reduced from 0.137 to 0.088[Formula: see text]eV with increasing bias from 0.2 to 1[Formula: see text]V. The corresponding values of thermal activation energies are 0.172 and 0.06[Formula: see text]eV below the conduction band for 180–295 and 77.4–180[Formula: see text]K, respectively. This semiconductor-like behavior may result from the larger number of surface defects or localized states in the amorphorized PbTiO3.

Impact of crystal stacking sequence on electrical transport and dielectric properties of the nanocrystalline BaCo0.9Mn0.1O3-δ

We investigate the electrical transport and dielectric properties of the nanocrystalline BaCo0.9Mn0.1O3-δ under 2H- and 12H-type hexagonal phases. Temperature dependent electrical transport properties under 2H- and 12H-type phases show semiconducting-like behavior above the magnetic transition. The numerical value of electrical resistivity of the 12H-type phase at low temperature is 10-fold higher than the 2H-type counterpart, following the trend of structural stacking sequence. The 12H-type phase exhibits rarely known characteristics of ferromagnetic-insulator. The conduction mechanism under both phases is studied using variable range and small polaron hopping conduction mechanism. The 12H-phase shows large magnetoresistance (∼60%) under an application of 5 T of magnetic field at 50 K. The relation between magnetization and electrical resistivity around the magnetic ordering temperature is studied. The relation has been used to correlate the magnetic entropy change derived by magnetization and electrical resistivity measurements. Temperature dependence and magnetic field induced dielectric permittivity of the 2H- and 12H-type hexagonal phases is studied around the magnetic ordering temperature. Both phases exhibit dielectric suppression by magnetic ordering. The field dependence dielectric behavior shows magnetocapacitance value of ∼4% for 2H-phase and ∼94% for 12H-phase at their magnetic ordering temperature under external magnetic field of 5 T. The existence of intrinsic magneto-dielectric coupling is studied under the framework of Ginzburg-Landau theory for ferroelectromagnet. The origin of extrinsic magneto-dielectric contribution is justified using combinational effect of large negative magnetoresistance and Maxwell-Wagner effect. The magnetic and electrical transport properties of the nanocrystalline BaCo0.9Mn0.1O3-δ compound are also studied using density of states. Nanocrystalline BaCo0.9Mn0.1O3-δ sample shows novel magnetic field-induced effect on the magnetic, electric and dielectric properties around the same temperature window which could be utilized for multi-functional applications.

Investigations on electrical and thermal transport properties of Cu2SnSe3 with unusual coexisting nanophases

The ternary diamond-like compound Cu2SnSe3 is a potential thermoelectric material. Its Cu-Se conducting network leaves Sn as a likely doping site to optimize the carrier concentration without much deterioration to the carrier mobility. Thus, the precise determination on the intricate phase structures of Cu2SnSe3 is critical. Here, we first use the atomically resolved scanning transmission electron microscopy and reveal an unusual phase coexistence (monoclinic and orthorhombic phases) in Ag-doped Cu2SnSe3. Owing to coexisting phases and the order–disorder transition in the orthorhombic phase, the Ag-doped Cu2SnSe3 shows an unusual three-stage behavior in its temperature-dependent electrical transport properties and achieves record high power factors in this system. The observed three-stage behavior due to the phase coexistence is supported by the first principle calculations, Hall measurements, and optical diffuse reflectance measurements. To fully understand the band structures and the coherent interface between these coexisting phases, a band contact model is proposed that could well explain the three-stage electrical transport behavior. Moreover, the phase coexistence is observed at the nanoscale regime, thus providing a high density of phase boundaries. Such coexisting nanophases play an important role in lowering the lattice thermal conductivity. As a result, the ZT value obtained in Cu2Sn0.93Ag0.07Se3 is double that of undoped Cu2SnSe3.

Temperature Dependent Electrical Transport Properties of High Carrier Mobility Reduced Graphene Oxide Thin Film Devices

We report temperature dependent electrical transport properties of high mobility reduced graphene oxide (RGO) thin films fabricated by pulse laser deposition. The temperature dependent (5K–350K) four terminal electrical transport property measurements confirm variable range hopping and thermally activated transport mechanism of the charge carriers at low (5K–210K) and high temperature (210K–350K) regions, respectively. The calculated localization length, the density of states near the Fermi level ( $E_{F}$ ), hopping energy, and Arrhenius energy gap provide useful information to explain the excellent electrical properties of the RGO films. Hall mobility measurement confirms ${p}$ -type characteristics of the thin films. The charge carrier Hall mobility can be engineered by tuning the growth parameters, and the measured maximum mobility was 1596 cm2v−1s−1. The optimization of the improved electrical property is well supported by structural properties such as the defect density, average size of sp2 clusters and degree of reduction, which were investigated by Raman spectroscopy and X-ray diffraction analysis.

Dielectric and ac Conductivity Studies on Nanocrystalline Mn Modified Cobalt Ferrite

The ac conductivity and dielectric properties on CoFe2−xMnxO4 for x = 0.00, 0.10, 0.15 and 0.20 have been studied in detail. All the samples were prepared in nanocrystalline size. These materials are found to be crystallized to Fd <mml:math display="block"> <mml:semantics> <mml:mover accent="true"> <mml:mi>3</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:semantics> </mml:math> m space group in cubic spinel structure. The dielectric constant and ac conductivity has been discussed as a function of frequency, temperature and composition. The relation between dielectric constant and ac conductivity has been analyzed and the results validate each other. The frequency response of ac conductivity (σac) obeys Johnschers power law and the parameters obtained, explain the sources of ac and dc electrical conductivity in the material. The frequency response of σac follows Maxwell–Wagner two-layer model. The influence of frequency as pumping force on activation energy has been determined. The temperature dependent ac conductivity shows the Arrhenius behavior. The σac observed to be enhanced with increase in frequency as well as temperature. The semiconducting behavior (NTCR) was also evident from temperature dependent electrical transport properties study. The low value of ac conductivity suggests a possible use of this material in dielectric applications.

Current Transport Properties of Monolayer Graphene/n-Si Schottky Diodes

The present work reports on the fabrication and the detailed macroscopic and nanoscale electrical characteristics of monolayer graphene/n-Si Schottky diodes. The temperature dependent electrical transport properties of monolayer graphene/n-Si Schottky diodes were investigated. Nanoscale electrical characterizations were carried out using Kelvin probe force microscopy and conducting atomic force microscopy. Most the values of ideality factor and barrier height are found to be in the range of 2.0–4.4 and 0.50–0.70 eV for monolayer graphene/n-Si nanoscale Schottky contacts. The tunneling of electrons is found to be responsible for the high value of ideality factor for nanoscale Schottky contacts.

Study of frequency- and temperature-dependent electrical transport in heavy fermion systems

This paper focuses on the frequency- and temperature-dependent electrical transport properties of heavy fermion (HF) systems. For this, Kondo lattice model (KLM) with Coulomb correlation between [Formula: see text]–[Formula: see text] electrons at the same site is considered. The Hamiltonian is treated in mean-field approximation (MFA) for the Kondo hybridization and Heisenberg-type interaction to get mean-field Hamiltonian and it is written after the Fourier transformation. The Hartree–Fock-type approximation is considered for the Coulomb repulsion between [Formula: see text]–[Formula: see text] electrons, the perturbed part of the Hamiltonian. The two Green’s functions for the conduction and [Formula: see text]-electrons are calculated to define the self-energy. Then the frequency- and temperature-dependent optical conductivity and resistivity are calculated by using the Kubo’s formula within the linear dynamical response approach. They are studied by varying the model parameters. The anomalies and results obtained are compared with experimental data.

Analysis of temperature-dependent electrical transport properties of nonvolatile organic field-effect transistor memories based on PMMA film as charge trapping layer

The temperature-dependent electrical transport properties of nonvolatile organic field-effect transistor (OFET) memories with Poly (methyl methacrylate) (PMMA) as a charge trapping layer were characterized at four typical temperatures (20 °C, 60 °C, 80 °C and −78.5 °C). It was found that the OFET memories showed strong temperature dependence. The performance degradations, including the memory windows and the retention characteristics, could be observed at both high and low temperatures. The degradations of the OFET memories at 60 °C, 80 °C and −78.5 °C were attributed to both the less electrons trapped by the PMMA film and the easier release of the trapped electrons from the PMMA film, which is caused by the lower crystallinity of pentacene film and the larger contact area between pentacene film and PMMA film, respectively.

Temperature-dependent electrical transport properties in graphene/Pb(Zr0.4Ti0.6)O3 field effect transistors

We report the temperature and gate voltage dependent electrical properties of PZT gated graphene field effect transistors (PZT-GFETs) in the vacuum atmosphere. The PZT-GFETs exhibit p-type characteristics which are attributed to the chemical doping induced the Fermi level shifting below the Dirac point. Meanwhile, it also shows a large memory window. The temperature dependencies of the source-drain current in the range of 20–300K indicate thermally activated hysteresis behaviors. The hysteresis in the transfer characteristics of PZT-GFETs shows a simultaneous enlargement with increasing temperature. The hysteresis appears to stem from the screening of charges that are transferred from graphene to traps at the interface of PZT and graphene. The magnitude of the charge neutrality point under opposite gate voltage sweep are enhanced with the increase of temperature and gate voltage can be ascribed to the common effects of the temperature and voltage magnitude dependent mechanisms such as interface charge trapping process and the polarization effects of PZT films.

Effect of Doping with Substituent Bi Atoms on the Electrical Transport Properties of a Bi0.4Sb1.6Te3 Film Fabricated by Molecular Beam Epitaxy

A highly crystalline Bi0.4Sb1.6Te3 film was fabricated on a Si substrate by molecular beam epitaxy (MBE) at a substrate temperature of 280°C. On the basis of study of x-ray diffraction patterns and high-resolution transmission electron microscopy lattice fringes it was inferred that Bi atoms were successfully incorporated into Sb lattice sites, forming substituent Bi impurities. Reduction of the carrier concentration was ascribed to the increased resistance to formation of antisite defects when Sb was substituted by Bi. The reduced mobility was a result of enhanced grain boundary scattering and attraction by substituent Bi atoms. Analysis of temperature-dependent electrical transport properties revealed that introduction of Bi atoms resulted in deeper energy level impurities in the Bi0.4Sb1.6Te3 film and higher activation energy (43.2 meV) than the normal value at room temperature, leading to semiconductor characteristics of the film in the temperature range −50°C to 150°C.

Heterogeneous integration of epitaxial Ge on Si using AlAs/GaAs buffer architecture: suitability for low-power fin field-effect transistors.

Germanium-based materials and device architectures have recently appeared as exciting material systems for future low-power nanoscale transistors and photonic devices. Heterogeneous integration of germanium (Ge)-based materials on silicon (Si) using large bandgap buffer architectures could enable the monolithic integration of electronics and photonics. In this paper, we report on the heterogeneous integration of device-quality epitaxial Ge on Si using composite AlAs/GaAs large bandgap buffer, grown by molecular beam epitaxy that is suitable for fabricating low-power fin field-effect transistors required for continuing transistor miniaturization. The superior structural quality of the integrated Ge on Si using AlAs/GaAs was demonstrated using high-resolution x-ray diffraction analysis. High-resolution transmission electron microscopy confirmed relaxed Ge with high crystalline quality and a sharp Ge/AlAs heterointerface. X-ray photoelectron spectroscopy demonstrated a large valence band offset at the Ge/AlAs interface, as compared to Ge/GaAs heterostructure, which is a prerequisite for superior carrier confinement. The temperature-dependent electrical transport properties of the n-type Ge layer demonstrated a Hall mobility of 370 cm2/Vs at 290 K and 457 cm2/Vs at 90 K, which suggests epitaxial Ge grown on Si using an AlAs/GaAs buffer architecture would be a promising candidate for next-generation high-performance and energy-efficient fin field-effect transistor applications.

Preparation and electrical transport properties of quasi free standing bilayer graphene on SiC (0001) substrate by H intercalation

We investigate the temperature dependent electrical transport properties of quasi-free standing bilayer graphene on 4H-SiC (0001) substrate. Three groups of monolayer epitaxial graphene and corresponding quasi-free standing bilayer graphene with different crystal quality and layer number homogeneity are prepared. Raman spectroscopy and atomic-force microscopy are used to obtain their morphologies and layer number, and verify the complete translation of buffer layer into graphene. The highest room temperature mobility reaches 3700 cm2/V·s for the quasi-free standing graphene. The scattering mechanism analysis shows that poor crystal quality and layer number inhomogeneity introduce stronger interacting of SiC substrate to the graphene layer and more impurities, which limit the carrier mobility of the quasi-free standing bilayer graphene samples.

Temperature dependent electrical transport of disordered reduced graphene oxide

We report on the simple route for the synthesis of chemically reduced graphene oxide (rGO) using ascorbic acid (a green chemical) as a reducing agent. Temperature-dependent electrical transport properties of rGO thin films have been studied in a wide range (50 K T 400 K) of temperature. Electrical conduction in rGO thin films was displayed in two different temperature regimes. At higher temperatures, Arrhenius-like temperature dependence of resistance was observed indicating a band gap dominating transport behavior. At lower temperatures, the rGO sample showed a conduction mechanism consistent with Mottʼs two-dimensional variable range hopping (2D-VRH). An unsaturated negative magnetoresistance (MR) was observed up to 3 T field. A decrease in negative MR at high temperatures is attributed to the phonon scattering of charge carriers.

Electronic transport and photovoltaic properties in Bi2Sr2Co2Oyepitaxial heterostructures

Epitaxial heterostructures constructed from the thermoelectric cobalt Bi2Sr2Co2Oy thin films and SrTiO3 as well as SrTi0.993Nb0.007O3 substrates were fabricated by pulsed-laser deposition. The scanning transmission electron microscopy results confirm that the heterostructures are epitaxial, with sharp and coherent interfaces. The temperature-dependent electrical transport properties and the Hall effects were systematically investigated. The Bi2Sr2Co2Oy/SrTi0.993Nb0.007O3 p-n heterostructure exhibits good rectifying current-voltage characteristics over a wide temperature range. A strong photovoltaic effect was observed in the Bi2Sr2Co2Oy/SrTi0.993Nb0.007O3 heterostructure, with the temperature-dependent photovoltage being systematically investigated. The present work shows a great potential of this new heterostructures as photoelectric devices.

Universal scaling of resistivity in bilayer graphene

We report the temperature dependent electrical transport properties of gated bilayer graphene devices. We see a clear evidence of insulating behavior due to electron-hole charge puddles. The electrical resistivity increases while the mobility decreases with decreasing temperature, a characteristic due to carrier inhomogeneity in graphene. The theoretical fittings using an empirical formula of single electron tunneling indicate that electrical resistivity follows a universal curve with a scaling parameter. The scaling parameter is determined to be a measure of the fluctuations in the electron-hole puddle distribution.

One dimensional ternary Cu–Bi–S based semiconductor nanowires: synthesis, optical and electrical properties

Ternary Cu–Bi–S based compounds have been thought to be alternative materials for well-known CuInS2 because of their abundance. Cu–Bi–S based nanomaterials have been less studied. We here report the synthesis, optical and electrical properties of single crystal Cu4Bi4S9 nanowires. High-quality Cu4Bi4S9 nanowires were synthesized through a modified solvothermal route by controlling the reaction sources and temperature. The optical bandgap for Cu4Bi4S9 nanowires were determined by using UV-vis-NIR and cyclic voltammetry techniques. Single nanowire devices were fabricated by using lithographic techniques. The devices exhibit photoconductive response with high external quantum efficiency (2.9 × 108%). Temperature-dependent electrical transport properties were also investigated. We observed that the transport properties of Cu4Bi4S9 nanowire show typical semiconductor behaviour in the temperature region 10–140 K and metal-like character in the temperature region of 150–300 K. The carrier transport in Cu4Bi4S9 nanowires can be described by the small polaron model in temperature region of 60–140 K and the variable range hopping mechanism in temperature region of 10–50 K. We further studied the properties of Cu4Bi4S9 nanowires in field-emission devices. The devices exhibit a relatively low turn-on field (6.9 V μm−1). The potential applications of Cu4Bi4S9 nanowires as field emitting materials and light absorbers in detectors are indicated.