Variable-range Hopping Conduction Research Articles

Magnetotransport and conductivity mechanisms in Cu2ZnSnxGe1\u2212xS4 single crystals

Resistivity, ρ(T), and magnetoresistance (MR) are investigated in the Cu2ZnSnxGe1−xS4 single crystals, obtained by the chemical vapor transport method, between x = 0–0.70, in the temperature range of T ~ 50–300 K in pulsed magnetic field of B up to 20 T. The Mott variable-range hopping (VRH) conductivity is observed within broad temperature intervals, lying inside that of T ~ 80–180 K for different x. The nearest-neighbor hopping conductivity and the charge transfer, connected to activation of holes into the delocalized states of the acceptor band, are identified above and below the Mott VRH conduction domain, respectively. The microscopic electronic parameters, including width of the acceptor band, the localization radius and the density of the localized states at the Fermi level, as well as the acceptor concentration and the critical concentration of the metal-insulator transition, are obtained with the analysis of the ρ(T) and MR data. All the parameters above exhibit extremums near x = 0.13, which are attributable mainly to the transition from the stannite crystal structure at x = 0 to the kesterite-like structure near x = 0.13. The detailed analysis of the activation energy in the low-temperature interval permitted estimations of contributions from different crystal phases of the border compounds into the alloy structure at different compositions.

Magnetotransport in type-enriched single-wall carbon nanotube networks

Single-wall carbon nanotubes (SWCNTs) exhibit a wide range of physical phenomena depending on their chirality. Nanotube networks typically contain a broad mixture of chiralities, which prevents an in-depth understanding of SWCNT ensemble properties. In particular, electronic-type mixing (the simultaneous presence of semiconductor and metallic nanotubes) in SWCNT networks remains the single largest hurdle to developing a comprehensive view of ensemble nanotube electrical transport, a critical step toward their use in optoelectronics. Here, we systematically study temperature-dependent magnetoconductivity (MC) in networks of highly enriched semiconductor and metal SWCNT films. In the semiconductor-enriched network, we observe two-dimensional variable-range hopping conduction from 5 to 290 K. Low-temperature MC measurements reveal a large, negative MC from which we determine the wave-function localization length and Fermi energy density of states. In contrast, the metal-enriched film exhibits positive MC that increases with decreasing temperature, a behavior attributed to two-dimensional weak localization. Using this model, we determine the details of the carrier phase coherence and fit the temperature-dependent conductivity. These extensive measurements on type-enriched SWCNT networks provide insights that pave the way for the use of SWCNTs in solid-state devices.

Intersecting Aslamazov–Larkin and hopping conduction in superconducting RE-123

The coherence lengths of three superconductors have been extracted in the framework of Aslamazov–Larkin (AL) equation. A superconductor with an upturn in the normal state is found to have a very high coherence length in comparison to that of the two others with metallic normal states. We have suggested that an approximated Aslamazov–Larkin (AL) equation can be used to express the resistivity as a function of temperature in the paracoherent region. Our suggested form for ρAL (T) has been compared with the experimental resistivity ρ(Τ) to extract the coherence length in NBCO. An intersection of the suggested ρAL (T) and the variable range hopping (VRH) conduction ρVRH (T) has been observed to be sensitive to the coherence length. The ratio of the localization length and the coherence length is found to be very effective in shifting of crossing point towards the onset critical temperature in NBCO.

Temperature Dependence of Electrical Conductivity and Thermoelectric Power of Transparent SWCNT Films Obtained by Aerosol CVD Synthesis

The effect of pressing on temperature dependences of electrical conductivity σ(T) and thermoelectric power (Seebeck effect) S of single‐walled carbon nanotubes (SWCNTs) transparent films on polyester substrate has been studied. σ(T) has been described by the fluctuation‐induced tunneling model (FIT) conductance in the temperature range 20–298 K. At a temperature T < 20 K, three‐dimensional variable range hopping conductivity (VRHC) has been observed. Based on the estimation of current concentration n ≈1020 cm−3 from VRHC dependence, it has been found that metallic SWCNTs have made the main contribution to conductivity at T < 20 K. The value n obtained from S has been similar to the value obtained from VRHC estimation. The energy barrier between SWCNTs has been changed by pressing of films. Such pressing has led to increase in the electrical conductivity of the film by two orders of magnitude at room temperature. The measurement of conductivity in the process of synthesis has allowed us to control the repeatability of results and select samples with the necessary resistance values. Raman and optical spectroscopy, SEM and TEM have been used to determine the structural and geometric characteristics of the SWCNTs.

Magnetothermopower and giant magnetoresistance in the spin-glass CuCrTiS4 thiospinel

The transport properties of the thiospinel CuCrTiS4 have been investigated in detail, focusing on the possible interplay between charges and spins, and its consequence on transport, and more specifically on the thermoelectric properties. The chemical homogeneity of a CuCrTiS4 polycrystalline sample synthesized by a direct reaction of the constitutive elements has been carefully probed by X-ray diffraction and transmission electron microscopy techniques. This thiospinel exhibits below 75 K, in its paramagnetic regime, a variable range hopping (VRH) conduction, associated with a giant magnetoresistance (MR), which persists below the spin-glass transition temperature at 8 K, reaching -95% in 9 T at 5 K. All the MR data can be rescaled as a function of H/T and described using a Brillouin function with S = 3/2. This demonstrates that the paramagnetic fluctuations strongly impact the transport properties, and this is confirmed by the Seebeck coefficient which depends on the magnetic field at low temperature, with a large negative magnetothermopower (MTEP) effect of -26% at 20 K, i.e., the thermopower is reduced as the spin alignment reduces the spin entropy. This first report of MTEP in CuCrTiS4 shows that this thiospinel behaves rather similarly to strongly correlated oxides, with MR and MTEP enhanced in the case of VRH transport with paramagnetic spins, such as in cobalt misfits or colossal MR manganites.

Polyaniline inclusion into ordered mesoporous silica matrices: Synthesis, characterization and electrical transport mechanism

Abstract Composites of polyaniline (PANI) with MCM-41 and SBA-15 ordered mesoporous silicas were prepared by liquid and vapor monomer adsorption methods. Both methods resulted in polymer encapsulation inside the mesopores, but a higher amount was obtained using the vapor adsorption method. Composites presented a semiconductor behavior and the charge transport mechanism was consistent with a hopping process between localized states. Based on the results of one-dimensional variable range hopping (1D-VRH) conduction parameters, the SBA-15 with polyaniline incorporated by vapor adsorption was the more conductive and ordered material.

Polaron relaxation and charge dynamics near the mobility edge of a possible Wigner lattice of polarons

We performed impedance spectroscopy and optical investigations below the reported mobility edge of two models systems (${X}_{n}$/${X}_{1.95\ensuremath{-}\ensuremath{\delta}}{\mathrm{Al}}_{1.95}{\mathrm{Si}}_{2.05}{\mathrm{O}}_{8.00}$; $X=\mathrm{K}$ or Rb, $n$ is the doping density of guest atoms) used to explore the metal-insulator transition in a deformable lattice. The bulk ac conductivity and imaginary dielectric permittivity of insulating samples near the mobility edge in both systems show a power-law behavior and variable-range hopping (VRH) conduction that is indeed related to the proposed polaron relaxation and conduction. We show that polaron relaxation and VRH occur simultaneously above a particular density of an interacting gas of small (bi)polarons, which is commensurate with the electron-lattice coupling strength and the intersite electron transfer energy. Based on our experimental results, we propose a rich phase diagram for the charge dynamics in a deformable lattice from the perspective of a Wigner lattice of small (bi)polarons.

Structural, Magnetic, and Electrical Transport Properties of Nanocrystalline SrCo0.5Mn0.5O<inline-formula> <tex-math notation="LaTeX">$_{3-\delta}$ </tex-math> </inline-formula>

We report the structural, magnetic, and electrical transport response of nanocrystalline SrCo <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3-δ</sub> compound synthesized by the sol-gel method. X-ray diffraction pattern shows that the sample crystallizes in the hexagonal crystal structure with cubic stacking at room temperature. The compound retains its structural symmetry down to 20 K. The transmission electron microscopy image confirms the surface morphology of the nanocrystalline sample, and the average particle size was found to be ~90 nm. Temperature dependence magnetization data show bifurcation in the zero field cooled (ZFC) and field cooled (FC) magnetization measured under the applied magnetic field of 500 Oe with onset irreversible temperature Tirr ~ 242 K. At low temperature, the small peak has been observed in ZFC curve, suggesting the spin blocking below TB ~ 35 K. The peak has disappeared on the application of higher magnetic field, and the magnitude of bifurcation has also been reduced. The electrical transport data reveal semiconducting such as behavior and follow Mott's variable range hopping (VRH) conduction mechanism in the temperature range 260-300 K. The density of states in the vicinity of Fermi energy is found to be N(EF) ~ 5.702 χ 1018 eV <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> · cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> using the VRH fit. The density of state value is in good agreement with the other reported semiconducting oxides. It shows maximum positive magnetoresistance of ~7% at 200 K for the magnetic field change of 3 T.

Mott variable range hopping and bad-metal in lightly doped spin-orbit Mott insulator BaIrO3

The Mott transition and the vicinity of a Mott insulating phase have constantly been a fertile ground for exploring exotic quantum states, most notably the high-${T}_{c}$ cuprates. The layered iridate represents another intriguing example. The nature of the Mott insulator phase and transition mechanism to a metallic state is still under debate. Much of the challenge originates from a series of energy scales involved in the electronics phases. Here, we report synthetization, characterization, and transport measurements on doped and undoped $\mathrm{B}{\mathrm{a}}_{1\ensuremath{-}x}\mathrm{L}{\mathrm{a}}_{x}\mathrm{Ir}{\mathrm{O}}_{3}$ films grown on $\mathrm{SrTi}{\mathrm{O}}_{3}$. The films are fully strained up to 70 nm thick and have a tetragonal lattice structure. For a doping level of $x=0.1$, a bad-metal state with linear temperature dependence of the resistivity beyond the Mott-Ioffe-Regel limit emerges in a wide temperature range down to a critical temperature ${T}_{c}\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, below which the system shows Mott variable range hopping (Mott VRH) conduction behavior. The ground state is confirmed to be insulating by the Mobius criterion. A strong correlation between the bad-metal state and Mott VRH localized state is found as the slope of the linear resistivity is inversely proportional to film thickness and the size of the Mott VRH activation energy $\mathrm{\ensuremath{\Delta}}$ is linearly proportional to ${T}_{c}$. We further show that upon doping the spin-orbit Mott insulator, itinerant metallic regions coexist with localized regions within a nanoscale phase-separated ground state with a small activation energy. Our results shed light on the nature of the metallic state and a crossover to a bad-metal phase for doping the spin-orbit Mott insulator.

Electric field effect on variable-range hopping conductivity in Bi1.9Lu0.1Te3

Electric field effect on low-temperature electrical resistivity of n-type Bi1.9Lu0.1Te3 was examined and analyzed. It was found that with decreasing temperature from 35 down to 2.5 K, the conductivity mechanism changes from “metal” type to “semiconductor” one resulting in appearance of minimum in the specific electrical resistivity, ρ. The temperature of this minimum, Tm, depends on electric field strength, E. At low electric field of 0.15 Vˑm−1 this temperature is equal to ∼11 K and decreases as E increases up to 4.65 Vˑm−1. Variable-range hopping (VRH) conductivity mechanism was applied to explain the ρ(T, E) behavior below Tm. Average hopping distance estimated from the experimental ρ(T, E) dependences decreases as temperature or electric field strength increase, that is the VRH conductivity can be activated both temperature and electric field. It was also found that density of states at the Fermi level rapidly enhances as electric field increases. This enhancement could be attributed to high and sharp density of states within an impurity band formed from the electronic f-levels of Lu. The forming of this impurity band is believed to be responsible for enhancement of the thermoelectric figure-of-merit of Bi2Te3.

Variable-Range Hopping Conduction in the Kesterite and Wurtzstannite Cu2ZnGeS4 Single Crystals

Progress in the solar cell science and technology, based on quaternary compounds, recently has been focused on the materials with a partial or full cation substitution of Ge for Sn. In particular, this stimulated an interest to the Cu2ZnGeS4 (CZGeS) compound. However, for its effective utilization, deeper investigations of CZGeS transport properties are required. In the present manuscript, the resistivity, ρ(T), of the p-type CZGeS single crystals with the kesterite (KS) and wurtzstannite (WS) structures is investigated in the temperature range of ~50–300 K. The dependence of ρ(T) is described by the Mott type of the variable-range hopping (VRH) charge transfer mechanism within a broad temperature interval of ~60–150 K for the KS samples and of ~100–260 K for the WS samples. A quantitative analysis of the resistivity data in the VRH conduction regime permitted determination or estimation of a series of important microscopic parameters. These include the localization radius of acceptors, their concentration and density of the acceptor states, as well as positions of the mobility threshold and of the Fermi level in the acceptor band.

Reverse leakage current characteristics of InGaN/GaN multiple quantum well ultraviolet/blue/green light-emitting diodes

We investigated the reverse leakage current characteristics of InGaN/GaN multiple quantum well (MQW) near-ultraviolet (NUV)/blue/green light-emitting diodes (LEDs). Experimental results showed that the NUV LED has the smallest reverse leakage current whereas the green LED has the largest. The reason is that the number of defects increases with increasing nominal indium content in InGaN/GaN MQWs. The mechanism of the reverse leakage current was analyzed by temperature-dependent current–voltage measurement and capacitance–voltage measurement. The reverse leakage currents of NUV/blue/green LEDs show similar conduction mechanisms: at low temperatures, the reverse leakage current of these LEDs is attributed to variable-range hopping (VRH) conduction; at high temperatures, the reverse leakage current of these LEDs is attributed to nearest-neighbor hopping (NNH) conduction, which is enhanced by the Poole–Frenkel effect.

Electric and optical properties of some new functional lower-rim-substituted calixarene derivatives in thin films

Electric (d.c. conduction mechanism, the influence of environmental humidity, thermistor effect) and optical properties of new five substituted calixarene derivatives in thin films have been studied. The films behave as typical n-type polycrystalline semiconductors with activation energy of the electric conduction between 0.26 and 2.36 eV. In the higher temperature range, the electron transport was explained in the frame of the band gap representation model, while at lower temperatures, the Mott’s variable-range hopping conduction model was found as appropriate. In terms of electric conductivity, these compounds are practically insensitive to ambient humidity below 53% RH, their sensitivity increasing greatly with the humidity in the range of 53–98% RH. Study of optical absorption spectra in the photon energy range of 0.71–4.03 eV (UV–Vis–NIR) revealed direct optical band gaps between 2.64 and 3.83 eV. Actual compounds show promising potential for thermistor applications.

Characterization of SiO2/SiC interface states and channel mobility from MOSFET characteristics including variable-range hopping at cryogenic temperature

The characteristics of SiC MOSFETs (drain current vs. gate voltage) were measured at 0.14−350 K and analyzed considering variable-range hopping conduction through interface states. The total interface state density was determined to be 5.4×1012 cm−2 from the additional shift in the threshold gate voltage with a temperature change. The wave-function size of interface states was determined from the temperature dependence of the measured hopping current and was comparable to the theoretical value. The channel mobility was approximately 100 cm2V−1s−1 and was almost independent of temperature.

Effect of substitutions on dielectric behavior of La2NiMnO6

The substituted La2NiMnO6 (LNMO) double perovskite powder samples are prepared by hydroxide co-precipitation method. The electrical properties such as conductivity, dielectric constant and impedance analysis of the substituted sample, have been studied. The activation energy is determined by dc conductivity plot gives the fair idea of conduction mechanism. The dielectric and impedance analysis suggests the contribution of extrinsic and intrinsic effects in dielectric relaxation due to substitution at A and B site cations. The Nyquist plot reveals the role of grain and grain boundaries in charge conduction mechanism. The large grain and grain boundary resistance value and activation energy ∼100 meV of substituted LNMO samples shows the presence of variable range hopping (VRH) conduction mechanism.

Fully patterned p-channel SnO TFTs using transparent Al2O3 gate insulator and ITO as source and drain contacts

SnO p-type was used as active layer to fabricate thin film transistors (TFTs) through photolithography and dry etching processes. The SnO p-type thin films (25 nm) were deposited by DC reactive sputtering with variable oxygen (O2) flow rate to then be annealed in air at 250 ◦C. Al2O3 gate dielectric (15 nm) was deposited by atomic layer deposition. Hall measurements showed p-type carrier concentration (Nh) of around 1 × 1018 cm−3 and Hall mobilities (μHall) between 0.35 and 2.64 cm2 V−1 s−1, depending on the O2 flow rate during deposition. The hole transport was dominated by variable-range hopping conduction. A change in the preferred crystalline orientation in the SnO films from (101) to (110) was associated with the increase in μHall. In addition, Raman vibrational modes at 110 and 209 cm−1 of polycrystalline SnO films showed certain dependence with the grain orientation. The SnO-based TFTs showed p-type behavior with low threshold voltages (VT) and low sub threshold swing (SS) in the range from 1.76 to 3.50 V and 1.63 to 3.24 V/dec., respectively. The TFTs mobilities in the saturation regime (μsat) were in the range of 0.12 and 1.32 cm2 V−1 s−1. The current on/off ratio (ION/IOFF) was in the order of 102, approximately. The large values of the interface trap density (DIT) contributed to the high IOFF and the low ION/IOFF of the TFTs.

Effect of Nickel Distributions Embedded in Amorphous Carbon Films on Transport Properties

Electrical properties of C/Ni films are studied using four mosaic targets made of pure graphite and stripes of nickel with the surface areas of 1.78, 3.21, 3.92 and 4.64%. The conductivity data in the temperature range of 400–500 K shows the extended state conduction. The conductivity data in the temperature range of 150–300 K shows the multi-phonon hopping conduction. The Berthelot-type conduction dominates in the temperature range of 50–150 K. The conductivity of the films in the temperature range about T < 50 K is described in terms of variable-range hopping conduction. In low temperatures, the localized density of state around Fermi level N(EF) for the film deposition with 3.92% nickel has a maximum value of about 56.2 × 1017 cm−3eV−1 with the minimum average hopping distance of about 3.43 × 10−6 cm.

Variable-range hopping conductivity in Lu-doped Bi2Te3

The Seebeck coefficient enhancement due to an increase of density-of-states effective mass of electron has been found in n-type Bi1.9Lu0.1Te3. This enhancement is assumed to be related to forming the narrow and non-parabolic impurity (Lu) band with local maximum of electronic density of states lying near the Fermi level. Minimum in the specific electrical resistivity, ρ, originated from change of conductivity mechanism was observed at temperature Tm ≈ 11 K. Above Tm, the ρ change is due to decrease of electron mobility via acoustic phonon scattering. Below Tm, the variable-range hopping conductivity takes place. The electron hops between the localized states of the impurity energy band occur via tunneling process. Using the temperature and magnetic field dependences of ρ, the localization radius of electron was estimated as ≈6 nm. Two parts in the magnetic field dependence of the electrical resistivity were found at temperature of 2 K. At weak magnetic fields, the ρ change is in agreement with the variable-range hopping conductivity mechanism. At high magnetic fields, the positive and almost linear transverse and longitudinal magnetoresistances were observed at low temperatures. Both variable-range hopping conductivity and positive linear magnetoresistance are characteristics of disordered and inhomogeneous semiconductors.

Structural properties and electrical conduction mechanisms of Bi0.9Sm0.05Tb0.05FeO3 Thin Film

Bismuth ferrite BiFeO3 (BFO) is a fascinating multiferroic material, which exhibits strong magnetic and ferroelectric (FE) characteristics simultaneously at room temperature. The present work was aimed at the controlled growth of Bi0.9Sm0.05Tb0.05FeO3 (BSTFO) thin film prepared by hydrothermal method to enhance its magnetic moment as well as the electric properties. In this work, variation of electrical conduction with temperature is explained by employing few, relevant to perovskite oxide materials, conduction mechanisms. Variable-range hopping (VRH) and small-polaron hopping conduction mechanisms are used to analyze the conduction mechanism. Structure, ferromagnetic and dc electrical properties of Bi0.9Sm0.05Tb0.05FeO3thin films grown on SrTiO3 (100) substrates by hydrothermal method are reported. Saturation magnetization and room temperature resistivity values are found to be 160 emu/cm3 and 1.9×105Ωm respectively. Raman spectroscopy study indicates anomalous changes of vibration modes around the Neel’s temperature. The temperature dependent electrical resistivity shows two distinct regions suggesting two different conduction mechanisms which suggest that small polaron and variable-range-hopping conduction mechanisms are operative in 300–160 K and 160–90 K temperature regions respectively. The density of states, for our samples, at the Fermi level is found to be ∼1020eV−1cm−3.

Leakage current transport mechanism under reverse bias in Au/Ni/GaN Schottky barrier diode

The leakage current transport mechanism under reverse bias of Au/Ni/GaN Schottky diode is studied using temperature dependent current-voltage (I-V-T) and capacitance-voltage (C-V) characteristics. I-V measurement in this study is in the range of 140 K–420 K in steps of 10 K. A reduction in voltage dependent barrier height and a strong internal electric field in depletion region under reverse bias suggested electric field enhanced thermionic emission in carrier transport via defect states in Au/Ni/GaN SBD. A detailed analysis of reverse leakage current revealed two different predominant transport mechanisms namely variable-range hopping (VRH) and Poole-Frenkel (PF) emission conduction at low (<260 K) and high (>260 K) temperatures respectively. The estimated thermal activation energies (0.20–0.39 eV) from Arrhenius plot indicates a trap assisted tunneling of thermally activated electrons from a deep trap state into a continuum of states associated with each conductive threading dislocation.