Variable Range Hopping Conduction Process Research Articles

Dynamics of charge carriers in La2CuO4 cuprates: Conductivity variation, scaling formalisms, and electric modulus study

In this work, we conducted a detailed investigation of the structural properties and the dynamics of the charges in the La2CuO4 Ruddlesden–Popper system that is prepared via the conventional solid-state process. Via the X-ray diffraction data and using the Rietveld refinement, we found that the compound exhibits a single crystallographic phase with a Bmab space group. The DC-conductivity investigation shows that the studied ceramic exhibits two electrical behaviors. Below 460 °C, the occurrence of a semiconductor behavior is attributed to the effect of the Variable range hopping conduction process. At high temperatures, the electrical nature of the material is linked to the thermally activated small polaron hopping conduction process via an activation energy Ea = 1.40 eV. Over the explored temperature range, conductivity spectra of the Ruddlesden–Popper are analyzed using the universal Double Jonscher power law. The dispersion region can be attributed to the coexistence of both hopping and tunneling processes. The conductivity spectra of the La2CuO4 Ruddlesden–Popper nicely follow the time-temperature superposition (TTSP) principle. This confirms that similar sources are responsible for relaxation and conduction processes. Using the Summerfield scaling approach, the superposition of the conductivity spectra into a single master curve confirms that the relaxation process is the microstructure's response independent. From the electrical modulus investigation, we found that the studied compound exhibits a non-Debye relaxation nature, over the explored temperature interval. Contribution of grain and grain boundary were resolved by complex modulus spectroscopy analysis. In addition, we found the absence of the electrode contribution at low frequencies. The correlation between both Z″ and the M″ peaks indicates that the conduction and relaxation processes are not related to the same origins.

Understanding the charge carriers dynamics in the La0.55Ca0.45Mn0.8Nb0.2O3 perovskite: scaling of electrical conductivity spectra.

The present work proposes the best realistic theoretical approaches to examine the experimental conductivity data taken for La0.55Ca0.45Mn0.8Nb0.2O3. For this purpose, we comprehensively discussed the structural, microstructural, and electrical properties of the La0.55Ca0.45Mn0.8Nb0.2O3 perovskite. Both X-ray diffraction and Rietveld analysis show the orthorhombic structure of the ceramic. Scanning electron microscope showed the existence of well-defined irregularly shaped particles with a grain-size distribution of 0.843 μm. The X-ray photoemission spectroscopy reveals the existence of Mn3+ and Mn4+ states. The complicated behavior of the lanthanum states is demonstrated using the La3d line. AC-conductivity responses are related to the correlated barrier hopping contribution. At high temperatures, the compound's semiconductor behavior is attributed to the activation of the polaronic transport. At low temperatures, the occurrence of semiconductor behavior in the La0.55Ca0.45Mn0.8Nb0.2O3 ceramic is attributed to the effect of the variable range hopping conduction process. The application of the time-temperature-superposition-principle and the Summerfield scaling formalisms leads to the superposition of the isotherms. Using the Ghosh formalism, the superposition of the spectra confirms that the number density and the hopping distance are temperature-dependent. The superposition of the spectra suggested the temperature-independent relaxation and polaronic processes. In addition, it confirms that the relaxation mechanism is independent of the microstructure response.

Polaron hopping conduction mechanism and magnetic properties of Pb‐doped LaMnO3

AbstractIn the present work, we have synthesized La1‐xPbxMno3 (0 ≤ x ≤ 0.20) perovskites through a conventional solid‐state reaction route. The powder x‐ray diffraction data analyzed through the Rietveld refinement program confirm monophase rhombohedral distorted structure (R‐3c space group). Fourier transform infrared spectra exhibit strong and well‐defined absorption bands of a typical perovskite structure. The x‐ray photoelectron spectroscopy studies confirm the elemental composition and oxidation state of various elements in the samples. Low temperature resistivity measurements do not reveal metal–semiconductor transition (MST) in the pristine sample while Pb‐doped samples show MST. The metallic region of the temperature‐dependent resistivity data is well fitted with ρ(T) = ρο + ρ2.5T2.5, which reveals the significance of electron–magnon interaction. Moreover, the semiconducting behavior of all the samples follows the Mott's variable range hopping conduction process. Additionally, the semiconducting part is also fitted with the small‐polaron hopping (SPH) model that establishes the non‐adiabatic SPH mechanism obeyed for the lower doping, whereas adiabatic mechanism for the higher Pb concentration. Magnetic studies clearly indicate a paramagnetic to weak ferromagnetic transition associated with the Pb doping that may be due to the double‐exchange mechanism between the manganese ions via oxygen orbitals. The outcomes are elucidated on the basis of substitution effect induced by Pb2+ ions.

VRH and NSPT conduction mechanisms in CuIn0.7Ga0.3Se2 near the liquid nitrogen temperature

Variable range hopping conduction process of Shklovskii-Efros type is identified, for the first time, in CuIn0.7Ga0.3Se2 bulk material in the temperature range [-190 °C, −10 °C]. Two different methods are considered to confirm this mechanism. The corresponding characteristic temperature and the localization length of mobile carriers are estimated. The AC electrical conductivity analysis suggests the predominance of Non-overlapping Small Polaron Tunneling mechanism. In the frequency range [20 Hz, 1 MHz], the equivalent circuit model of this material is established and the microscopic contributions to the total electrical conduction are discussed.

Effect of Ammonia Based Deprotonation on the variable range hopping conduction in Polypyrrole Nanotubes

This paper reports the facial synthesis of polypyrrole (PPy) nanotubes by an anionic azo dye {sodium 4-[4-(dimethyl-amino)-phenyldiazo] phenylsulfonate} assisted chemical oxidation method and their deprotonation by liquid ammonia treatment. Scanning and transmission electron microscopy studies reveal the synthesis of PPy nanotubes of diameters ∼155 nm and their lengths are found to be about 2–3 μm. The deprotonation of PPy nanotubes by liquid as well as gaseous ammonia has been experimentally confirmed by Raman and FTIR studies. These spectroscopic techniques suggest the proton transfer based mechanism for deprotonation of the PPy nanotubes. The deprotonation of PPy nanotubes has also been confirmed through the electrical measurements carried out in the presence of ammonia gas at different ppm levels. Furthermore, the effect of deprotonation on the charge transport properties of PPy nanotubes has also been investigated. The temperature dependent conductivity measurements are performed on both the pristine as well as deprotonated (by liquid ammonia) PPy nanotubes in a wide range of temperature (77–400 K) and analyzed for different conduction mechanisms. However, both the samples are found to follow Mott's variable range hopping conduction process in three dimensions (3D-VRH) for the charge transport. The conduction properties of both the samples are further investigated in terms of Mott's 3D-VRH variables such as Mott's characteristic temperature, hopping distance, hopping energy, and density of states, etc.

Effect of thermal annealing on structural, optical and electrical properties of transparent Nb2O5 thin films

We report on the structural, optical and electrical properties of RF sputtered Niobium pentoxide (Nb2O5) thin films. The structural studies have been carried out using XRD, and AFM techniques. Optical constants such as optical band gap energy, absorption coefficient, refractive index, complex dielectric constant and optical conductivity have been estimated for as-deposited and annealed Nb2O5 thin films. The estimated direct optical band gap energy values were found to be Egd=3.62 eV for the as deposited which decreases to Egd=3.07 eV for the annealed films at 973K. The dispersion curves of the refractive index of Nb2O5 thin films in the optical transparency region are explained by using single oscillator and Drude models. The correlations between optical parameters and the annealing temperature of the Nb2O5 thin films are discussed. The DC activation energy has been estimated by using two probe technique.Mott’s variable range hopping conduction process (VRH) and small-polaron hopping have been used to understand DC electrical conductivity.

Topological insulator thin films starting from the amorphous phase-Bi2Se3 as example

We present a new method to obtain topological insulator Bi2Se3 thin films with a centimeter large lateral length. To produce amorphous Bi2Se3 thin films, we have used a sequential flash-evaporation method at room temperature. Transmission electron microscopy has been used to verify that the prepared samples are in a pure amorphous state. During annealing, the samples transform into the rhombohedral Bi2Se3 crystalline structure which was confirmed using X-ray diffraction and Raman spectroscopy. Resistance measurements of the amorphous films show the expected Mott variable range hopping conduction process with a high specific resistance compared to the one obtained in the crystalline phase (metallic behavior). We have measured the magnetoresistance and the Hall effect at different temperatures between 2 K and 275 K. At temperatures T ≲ 50 K and fields B ≲ 1 T, we observe weak anti-localization in the MR; the Hall measurements confirm the n-type character of the samples. All experimental results of our films are in quantitative agreement with results from samples prepared using more sophisticated methods.

Effect of SnO2 concentration on the tuning of optical and electrical properties of ZnO-SnO2 composite thin films

ZnO-SnO2 composite thin films have been deposited at 400 °C on glass substrates using targets of different SnO2 content (1 to 40 wt. %) by pulsed laser deposition technique. The structural, optical, and electrical properties of the composite films have been studied as a function of SnO2 content. It is revealed from X-ray diffraction analysis that films are crystalline in nature and the crystallite size decreases from 20–23 nm to 5–7 nm with increase of SnO2 content. X-ray photoelectron spectroscopy analysis indicates that Sn is predominantly doped into the ZnO lattice upto a SnO2 content of 15 wt. % in the composite. For higher concentration, a separate SnO2 phase is segregated in the composite. The band gap energy as well as the electrical conductivity can be tuned by varying the SnO2 content in the composite. Low temperature electrical conductivity measurements show three dominant conduction mechanisms in the temperature range of 20–300 K. At high temperature range of 200–300 K, thermal activation conduction process is dominant. Nearest neighbor hopping conduction mechanism, which occurs in the shallow impurity bands, is dominant in the temperature range of 90–200 K. In the low temperature range of 20–90 K, the electronic transport occurs through Mott's variable range hopping conduction process.

Charge transport mechanism in high conductivity undoped tin oxide thin films deposited by reactive sputtering

This paper reports the charge transport mechanism at low temperatures in (110) oriented polycrystalline tin oxide (SnO2) films of less than 100nm thickness prepared by reactive sputtering in the substrate temperature (TS) range of 350–450°C. Undoped tin oxide films with high electrical conductivity of 11Ω–1cm–1 have been achieved at 350°C. The Hall mobility increases from 6.7 to 13.9cm2/V·s and carrier concentration decreases from 11×1018 to 0.9×1018cm–3 as TS is increased. In the 300–100K temperature range we found two types of conduction mechanisms: thermally activated conduction till 160K and nearest-neighbour-hopping conduction below 160K. At temperatures below 90K, the Mott variable-range-hopping (VRH) conduction governs the charge transport. In all the three regimes the activation energies of conduction increase with an increase in TS, being 15–50, 0.2–9, and 2–6meV respectively, consistent with a decrease in oxygen vacancies at higher TS. The analysis of the data in Mott VRH conduction regime suggests a systematic localization of the oxygen vacancy states with an increase in growth temperature. Even at 70nm thickness the films behave as three dimensional with regard to the Mott VRH conduction process.

Variable-range-hopping conduction processes in oxygen deficient polycrystalline ZnO films

We have fabricated oxygen deficient polycrystalline ZnO films by the rf sputtering deposition method. To systematically investigate the charge transport mechanisms in these samples, the electrical resistivities have been measured over a wide range of temperature from 300 K down to liquid-helium temperatures. We found that below about 100 K, the variable-range-hopping (VRH) conduction processes govern the charge transport properties. In particular, the Mott VRH conduction process dominates at higher temperatures, while crossing over to the Efros–Shklovskii (ES) VRH conduction process at lower temperatures. The crossover occurred at temperatures as high as a few tens degrees kelvin. Moreover, the temperature behavior of resistivity over the entire VRH conduction regime from the Mott-type to the ES-type process can be well described by a universal scaling law.

Structural characterization and electrical properties of quaternary CdGaInSe 4 thin films

Stoichiometric bulk ingot material of the quaternary CdGaInSe 4 was prepared by direct fusion of the constituent elements in vacuum-sealed silica tubes. Nearly stoichiometric films could be deposited by thermal evaporation of the ingot material in 10 −3 Pa vacuum at a deposition rate 1.5 nm/s. Crystal structure investigation was carried out using X-ray diffractometry and transmission electron diffraction. Elemental composition was determined by means of energy-dispersive X-ray spectrometry. CdGaInSe 4 possesses a tetragonal defective chalcopyrite structure (space group I ¯ 4 ) with lattice parameters a=0.5665 nm and c=1.1221 nm. All the films exhibited n-type conduction and ohmic behaviour with metallic films of Au, Cd, In, Ag and Sb. However, in the case of Al a nonlinear behaviour occurs. Analysis of the temperature dependence of the dark conductivity in the range 130–470 K has revealed three operating conduction mechanisms; a variable range hopping conduction process dominating at low temperatures below 270 K, followed by a transport of the charge carriers across intercrystalline barriers and grain boundaries in the temperature range 270–353 K, and finally an extrinsic conduction above 353 K.

Transport and optical properties of SiO 2–polypyrrole nanocomposites

Conducting polypyrrole-wide band gap semiconducting silica (SiO 2) nanocomposites are prepared by polymerizing pyrrole in the presence of colloidal silicon oxide sol. The nanocomposites are characterized by Fourier transform infrared spectrometer and scanning electron microscope. Three dimensional variable range hopping conduction process is involved in electric conduction. The current density ( J ) satisfies the power law, J ∝ V m , with 1 < m < 2 at voltage V > 0.5 V . Optical absorption spectra reveal that the π – π ∗ transition of polypyrrole shifts from 3.9 eV to 4.58 eV with increase of silica concentration.

ELECTRICAL PROPERTIES OF IN DOPED CDS THIN FILMS

Indium doped polycrystalline cadmium sulphide 〈CdS:In〉 thin films have been prepared by the spray pyrolysis technique on glass substrates in an enclosed dome. The different scattering mechanisms such as lattice, impurity and grain boundary scattering for CdS:In films are observed at low temperature, in the range of 303 to 120 K. The experimentally determined mobilities due to these scatterings are well interpreted with those of theoretically calculated mobilities. The d.c. conductivity for CdS:In films has also been studied in the same temperature region. The Mott variable range hopping conduction process followed below the temperature of 150 K. The Mott parameters such as N(EF), R, W and α are found to be 1.26 × 1019 eV−1cm−3, 9.8 × 10−-7cm, 0.02 eV−1 and 2.38 × 106cm−1, respectively from the conductivity data.

Variable-range-hopping-type transport near the superconductor-insulator transition observed in Nd2−x,CexCuO4thin films

Abstract Electrical transport properties of high-T c Nd2−xCexCuO4 single-crystal thin films were investigated to reveal the conduction mechanism near the superconductor-insulator transition. We find that two different types of localized state exist in the insulator side: one shows typical behaviour or weak localization and the other is the variable-range-hopping (VRH) conduction type. We claim this to be a new type of VRH conduction process with interlayer hopping by Cooper pairs.

Effect of laser irradiation on the electrical properties of amorphous germanium films

Amorphous films of germanium were grown using a vacuum evaporation technique, on glass substrates kept at room temperature. As-grown films were irradiated with Q-Switched Nd-YAG laser pulses (λ=1.06 μm, 20nsec, 10 to 50Jcm−2). The d.c. conductivity measurements were made in the temperature range 77 to 300 K. It was observed that the effect of laser irradiation was similar to the effect caused by the thermal annealing of the films. The d.c. conductivity data were analysed in the light of Mott's theory of a variable range hopping conduction process.