Showing papers on "Variable-range hopping published in 2022"

Delocalization Enhances Conductivity at High Doping Concentrations

Abstract: Many applications of organic semiconductors require high electrical conductivities and hence high doping levels. Therefore, it is indispensable for effective material design to have an accurate understanding of the underlying transport mechanisms in this regime. In this study, own and literature experimental data that reveal a power‐law relation between the conductivity and charge density of strongly p‐doped conjugated polymers are combined. This behavior cannot consistently be described with conventional models for charge transport in energetically disordered materials. Here, it is shown that the observations can be explained in terms of a variable range hopping model with an energy‐dependent localization length. A tight‐binding model is used to quantitatively estimate of the energy‐dependent localization length, which is used in an analytical variable range hopping model. In the limit of low charge densities, the model reproduces the well‐known Mott variable range hopping behavior, while for high charge densities, the experimentally observed superlinear increase in conductivity with charge density is reproduced. The latter behavior occurs when the Fermi level reaches partially delocalized states. This insight can be anticipated to lead to new strategies to increase the conductivity of organic semiconductors.

Studies of low-temperature electrical measurements in some multicomponent selenium rich glassy alloys: Role of sliver modifier

Abstract: The aim of the present work is devoted towards the inspection of the incorporation of silver impurity on electric/dielectric properties in a novel third-generation glassy system at low temperatures. For electrical characterization, the temperature and frequency have been chosen in the range 170 K to 285 K and 800 Hz to 500 kHz, respectively. The investigation of the results indicates the relevance of modeling proposed by Austin-Mott for thermally governed a.c. conduction. However, the relevance of Mott's formalism of variable range hopping (VRH) is observed for d.c. conduction at temperatures below the room temperature. Various electrical parameters like the activation energies and pre-factors of electrical conductivity σ corresponding to d.c./a.c. conduction, the number of charged defects levels per unit volume N(EF), energy values of trapping levels, and the potential barrier energy have been evaluated and examined. The dependence of the calculated parameters upon glassy compositions has been explained by using constraint theory.

Evidence of surface delocalization in ultrathin films of topological insulator in presence of intersurface hybridization and disorder

Abstract: The study of surface transport in ultrathin films of few quintuple layers thick topological insulators (TIs) and its evolution with disorder is important for TI based device engineering. Here, we demonstrate the absence of Anderson localization in surface states of Bi2Se3 films on varying the disorder from the weak to moderate regime. On increasing the disorder, bulk of topological insulator transforms from diffusive to hopping transport while surface remains in quantum diffusive regime. The weak antilocalization (WAL) of surface states suppresses in thinner films, which could occur due to disorder or inter surface hybridization. Our analysis rules out the role of disorder in WAL suppression and shows that films crossover from decoupled surface states to a single coupled surface bulk channel and finally to hybridized surface states on reducing the film thickness. The dephasing mechanism of surface transport strongly depends on the nature of bulk transport. It is dominated by 2D electron–electron scattering for diffusive bulk transport while it is dominated by direct surface bulk charge puddle coupling and surface to hopping transport coupling for bulk in the variable range hopping regime. The surface to hopping transport coupling weakens with intersurface hybridization.

Grain-Size-Induced Collapse of Variable Range Hopping and Promotion of Ferromagnetism in Manganite La0.5Ca0.5MnO3

Abstract: Among transition metal oxides, manganites have attracted significant attention because of colossal magnetoresistance (CMR)—a magnetic field-induced metal–insulator transition close to the Curie temperature. CMR is closely related to the ferromagnetic (FM) metallic phase which strongly competes with the antiferromagnetic (AFM) charge ordered (CO) phase, where conducting electrons localize and create a long range order giving rise to insulator-like behavior. One of the major open questions in manganites is the exact origin of this insulating behavior. Here we report a dc resistivity and magnetization study on manganite La1−xCaxMnO3 ceramic samples with different grain size, at the very boundary between CO/AFM insulating and FM metallic phases x=0.5. Clear signatures of variable range hopping (VRH) are discerned in resistivity, implying the disorder-induced (Anderson) localization of conducting electrons. A significant increase of disorder associated with the reduction in grain size, however, pushes the system in the opposite direction from the Anderson localization scenario, resulting in a drastic decrease of resistivity, collapse of the VRH, suppression of the CO/AFM phase and growth of an FM contribution. These contradictory results are interpreted within the standard core-shell model and recent theories of Anderson localization of interacting particles.

Low‐Temperature Carrier Transport Mechanism of Wafer‐Scale Grown Polycrystalline Molybdenum Disulfide Thin‐Film Transistor Based on Radio Frequency Sputtering and Sulfurization

Abstract: Molybdenum disulfide (MoS2) synthesis methods have become diverse and enable wafer‐scale growth for high‐performance optoelectronic applications. However, there has been limited research on the carrier transports of wafer‐scale deposited MoS2 thin‐film transistors (TFTs). In this paper, the first demonstration of the electron transport mechanism in top‐gated polycrystalline crystalline MoS2 (poly‐MoS2) TFTs grown by a wafer‐scale deposition method is presented. The MoS2 is synthesized via radio frequency (RF) magnetron sputtering and gas flow chemical vapor sulfurization. A surface analysis is performed to determine the basic ingredients and grain size of the grown MoS2. Furthermore, the electrical properties and charge transport behaviors of the poly‐MoS2 TFTs are characterized using current–voltage measurement at low temperatures (93–273 K). The extracted parameters (e.g., field‐effect mobility, contact and channel resistance, activation energy, and hopping distance) and 2D Mott variable range hopping (VRH) of the poly‐MoS2 TFTs support the notion that the primary mechanism of carrier transport in the poly‐MoS2 TFTs involves thermally active hopping and grain effects. For advanced poly‐MoS2‐based devices, an increase of grain size will be the principal factor using the relationship between the grain size and electron hopping distance of poly‐MoS2.

Electrical Conduction Mechanism of β‐MnTe Thin Film with Wurtzite‐Type Structure Using Radiofrequency Magnetron Sputtering

Abstract: Manganese telluride (MnTe) compound is known to be a polymorphic chalcogenide. Recently, it has been reported that the MnTe shows nonvolatile memory properties with a significant change in resistance via a polymorphic transition between NiAs‐type (NC) structure (low resistance) and wurtzite‐type (WZ) structure (high resistance). This crystalline polymorphic MnTe is expected to realize a phase‐change memory with fast operation speed and ultralow operation energy. While the NC‐MnTe, generally designated as α‐MnTe, is intensively studied, WZ‐MnTe is still poorly understood. Herein this study, electrical conduction mechanism of a β‐MnTe film with a WZ‐type structure is studied. A resistivity, Hall mobility, and Seebeck coefficient of the WZ‐MnTe film are measured at various temperatures. The temperature dependence of resistivity in the temperature range 120–300 K clearly indicates that the WZ‐MnTe film shows a variable‐range hopping (VRH) conduction. In this temperature region, with decreasing temperature, the conduction mechanism changes from Mott–VRH conduction to Efros–Shklovskii VRH conduction at about 210 K. Furthermore, the low thermally activated Hall mobility, occurrence of Hall‐effect sign anomaly, and relatively low activation energy for thermopower, which are the observed results, suggest that the small polaron hopping conduction is dominant above 310 K.

Conductivity vs functionalization in single-walled carbon nanotube films

Abstract: Abstract Diazo functionalization is a chemical method that changes the conductance of metallic single-walled carbon nanotubes (SWCNTs) by disrupting the C–C double bonds. Its application to native mixtures of metallic and semiconducting SWCNTs is a promising way of large-scale production of semiconducting SWCNTs for use in electronics. This has been well studied on isolated SWCNTs, but the implications on the conductivity of SWCNT materials are still unclear. Here, we study the conductivity of such functionalized SWCNT films with a progressively decreased metallic/semiconducting ratio in a wide range of temperatures (4–300 K) to unravel the charge transport mechanisms of metallic and semiconducting SWCNT subnetworks to show how these components participate in the total conductivity of the films. At low functionalization degree (below 0.2 mol%), the conductivity is dominated by a subnetwork of metallic SWCNTs through two parallel mechanisms: a Luttinger liquid mechanism and a Variable Range Hopping process. Higher functionalization (over 0.4 mol%) destroys the Luttinger liquid mechanism, and a second parallel Variable Range Hopping process arises, attributed to the conduction through the semiconducting SWCNTs. At these high functionalization degrees, the SWCNT film behaves as a material with the desired semiconducting properties. Graphical abstract We studied the conductivity of chemically functionalized Single Walled Carbon Nanotube films with a progressively decreased metallic/semiconducting ratio in a wide range of temperatures (4–300 K) to unravel the charge transport mechanisms of metallic and semiconducting SWCNT subnetworks to show how these components participate in the total conductivity of the films.

The Effect of Bi2O3 and Fe2O3 Impurity Phases in BiFeO3 Perovskite Materials on Some Electrical Properties in the Low-Frequency Field

Abstract: Pure bismuth ferrite (BFO) and BFO with impurity phases (Bi2O3 or Fe2O3) were synthesized by the hydrothermal method. Complex dielectric permittivity (ε) and electrical conductivity (σ) were determined by complex impedance measurements at different frequencies (200 Hz–2 MHz) and temperatures (25–290) °C. The conductivity spectrum of samples, σ(f), complies with Jonscher’s universal law and the presence of impurity phases leads to a decrease in the static conductivity (σDC); this result is correlated with the increased thermal activation energy of the conduction in impure samples compared to the pure BFO sample. The conduction mechanism in BFO and the effect of impurity phases on σ and ε were analyzed considering the variable range hopping model (VRH). Based on the VRH model, the hopping length (Rh), hopping energy (Wh) and the density of states at the Fermi level (N(EF)) were determined for the first time, for these samples. In addition, from ε(T) dependence, a transition in the electronic structure of samples from a semiconductor-like to a conductor-like behavior was highlighted around 465–490 K for all samples. The results obtained are useful to explain the conduction mechanisms from samples of BFO type, offering the possibility to develop a great variety of electrical devices with novel functions.

Single polaron hopping in Fe doped glassy semiconductors: Structure–electrical transport relationship

Abstract: The development of glassy nanocomposites, xFe-(1−x) (0.5 [Formula: see text]–0.4 CdO–0.1 ZnO) is particularly important not only for exploring their microstructures using x-ray diffraction, FT-IR, and UV–Vis techniques but also for exploring their electrical conduction mechanism in terms of hopping of small polarons. The presence of various nanophases, such as ZnO, CdO, Cd9.5Zn0.5, ZnV, and Zn3V2O8, have been identified and the size of estimated nanocrystallites is found to decrease with more incorporation of the Fe content in the compositions. As the value of lattice strain increases with the increase of the Fe content in the compositions, the present system becomes more and more unstable, which may be favorable for better electrical transport phenomena via the polaron hopping process. Electrical conductivity of the system has been analyzed using modified correlated barrier hopping model, Almond–West formalism, and the alternating-current conductivity scaling. Experimental data reveal that both optical photon and acoustical phonon transitions are responsible for the entire electrical conduction process. Polaron hopping is expected to be of percolation type, which has been validated from an estimated range of frequency exponents. All experimental data have been used to frame a schematic model to explore the conduction mechanism inside the present glassy system.

Variable Range Hopping Model Based on Gaussian Disordered Organic Semiconductor for Seebeck Effect in Thermoelectric Device

Abstract: We investigate the carrier concentration dependent Seebeck coefficient in Gaussian disordered organic semiconductors (GD-OSs) for thermoelectric device applications. Based on the variable-range hopping (VRH) theory, a general model predicting the Seebeck effect is developed to reveal the thermoelectric properties in GD-OSs. The proposed model could interpret the experimental data on carrier concentration- and temperature-dependence of the Seebeck coefficient, including various kinds of conducting polymer film and small molecule based field-effect transistors (FETs). Compared with the conventional Mott's VRH and mobility edge model, our model has a much better description of the relationship between the Seebeck coefficient and conductivity. The model could deepen our insight into charge transport in organic semiconductors and provide instructions for the optimization of thermoelectric device performance in a disordered system.

Conductivity vs functionalization in single-walled carbon nanotube films

Abstract: Abstract Diazo functionalization is a chemical method that changes the conductance of metallic single-walled carbon nanotubes (SWCNTs) by disrupting the C–C double bonds. Its application to native mixtures of metallic and semiconducting SWCNTs is a promising way of large-scale production of semiconducting SWCNTs for use in electronics. This has been well studied on isolated SWCNTs, but the implications on the conductivity of SWCNT materials are still unclear. Here, we study the conductivity of such functionalized SWCNT films with a progressively decreased metallic/semiconducting ratio in a wide range of temperatures (4–300 K) to unravel the charge transport mechanisms of metallic and semiconducting SWCNT subnetworks to show how these components participate in the total conductivity of the films. At low functionalization degree (below 0.2 mol%), the conductivity is dominated by a subnetwork of metallic SWCNTs through two parallel mechanisms: a Luttinger liquid mechanism and a Variable Range Hopping process. Higher functionalization (over 0.4 mol%) destroys the Luttinger liquid mechanism, and a second parallel Variable Range Hopping process arises, attributed to the conduction through the semiconducting SWCNTs. At these high functionalization degrees, the SWCNT film behaves as a material with the desired semiconducting properties. Graphical abstract We studied the conductivity of chemically functionalized Single Walled Carbon Nanotube films with a progressively decreased metallic/semiconducting ratio in a wide range of temperatures (4–300 K) to unravel the charge transport mechanisms of metallic and semiconducting SWCNT subnetworks to show how these components participate in the total conductivity of the films.