Variable-range Hopping Conduction Research Articles

Quantum Hall effect and hopping conductivity in n-InGaAs/InAlAs nanoheterostructures

The longitudinal and Hall magnetoresistances are measured in the quantum Hall effect regime in the n-InGaAs/InAlAs heterostructures at temperatures of T = (1.8–30) K in magnetic fields up to B = 9 T. Temperature-induced transport in the region of the longitudinal resistance minima, corresponding to the plateau regions at Hall resistance, is investigated within the framework of the concept of hopping conductivity in a strongly localized electron system. The analysis of variable-range hopping conductivity in the region of the second, third, and fourth plateau of the quantum Hall effect provides the possibility of determining the localization length exponent.

Reverse Leakage Current Characteristics of GaN/InGaN Multiple Quantum-Wells Blue and Green Light-Emitting Diodes

The mechanisms of reverse leakage in InGaN/GaN multiple-quantum-wells (MQWs) blue and green light-emitting diodes (LEDs) were studied through analyzing the current–voltage ( I–V ) characteristics with the temperature ranging from 50 to 350 K. When the reverse bias V is below 12 V, the leakage current is dominated by the variable-range hopping conduction $(T\rm{ and Poole–Frenkel emission $(T\rm{ > 250\,K)}$ , respectively. As the reverse bias increases $(\vert V\vert \rm{ > 12\,V)}$ , the space-charge-limited conduction mechanism is responsible for the leakage current. It is found that the leakage current is highly associated with the defects density, as well as the trapping capacity of deep level centers; therefore, the lower leakage current of blue LED is mainly attributed to higher thermal activation energy $E_{a}$ due to less deep level centers within GaN/InGaN MQWs structures.

Towards p-Type Conductivity in SnO2 Nanocrystals through Li Doping and Its Applications in Porous Diodes for Room Temperature Chlorine Gas Detection

This work examined electrical transport properties and Li doping in SnO2 synthesized by the sol–gel method. Solid-state 7Li-NMR line shapes revealed that Li ions occupy two distinct sites with differing dynamic mobilities. The chemical exchange rate between the two sites is, however, too slow for detection on the NMR timescale. Compressed nanoparticulate films of this doped semiconductor exhibited a positive Seebeck coefficient implying a p-type conductivity. A variable-temperature direct current conductivity, over a 25–350 °C temperature range, followed an Efros–Shklovskii variable range hopping (ES-VRH) conduction mechanism (ln(ρ) versus T −1/2) at temperatures below 100 °C with a crossover to 2D Mott variable range hopping (M-VRH) (ln(ρ) versus T −1/3) conduction at temperatures above 250 °C. In a transition region between these two limiting behaviors, direct current resistivity exhibited an anomalous temperature-independent plateau. We suggest that its origin may lie in a carrier inversion phenomenon wherein the majority carriers switch from holes to electrons due to Li ion expulsion from the crystalline core and creation of oxygen vacancies generated by loss of oxygen at elevated temperatures. Using a compression technique, porous diodes consisting of n-type (antimony doped SnO2) and the p-type (lithium-doped) nano-particulate films were prepared. Typical current-voltage curves of such devices resembled the behavior of a typical diode but for one key difference; owing to the porosity of the film, areas near the p-n interface and the p- and n-regions are accessible to gaseous analytes. This access is not possible in conventional diodes fabricated through contemporary planar technology. The gas sensing properties of the diodes were investigated by using Cl2, Br2, HCl, NO, NO2, CHCl3, NH3 and H2. Of all the gases tested, the sensor had very good sensitivity to Cl2 and had a response of 80 at 400 ppb of Cl2. The response time of the sensor was ~30 s at room temperature but the recovery time was temperature dependent. At room temperature the recovery time was 80s after 400 ppb chlorine exposure. The sensors were also found to recover quickly (~ 30 s) when the junctions were biased. Reverse biasing the diode provides a controllable band-bending at a p-n junction that could influence the electron-transfer rates from the adsorbates which create intra-band localized states. As a result reverse biasing can provide a means of desorbing the analyzed species. This approach eliminates the requirement to remove adsorbates by elevating the sensor temperature and hence improves on the energy efficiency of these types of sensors.

N codoping induced room temperature ferromagnetic enhancement in (Fe, N)-codoped In2O3 films by experimental and computational insights

The (Fe, N)-codoped In2O3 films with cubic bixbyite structure have been prepared by magnetron sputtering methods. A combination of the XRD, XPS with X-ray absorption spectroscopy techniques reveals that the codoped Fe and N impurities were substitutionally dissolved at In and O sites of the In2O3 lattice, respectively, and form FeIn + 2VO complex in the first coordination shell of Fe. The Mott variable-range hopping (VRH) conduction mechanism was observed to dominate at the low temperature region, which is accompanied by an obvious room-temperature ferromagnetic behavior for both the films, while the N codoping remarkably enhances the saturated magnetization (Ms) of the (Fe, N)-codoped In2O3 film. Based the first principles calculations, it was found that N codoping in the (Fe, N)-codoped In2O3 film can change the magnetic coupling between two Fe ions from antiferromagnetic to ferromagnetic due to the strong N 2p-Fe 3d hybridization interaction. It can be concluded that the indirect interaction between bound magnetic polarons (BMP) based oxygen vacancies as well as the Fe-N-Fe ferromagnetic superexchange interaction may be responsible for the intrinsic ferromagnetic ordering in the (Fe, N)-codoped In2O3 film. These results may provide a new sight for understanding the magnetic mechanism of In2O3 based DMS systems.

Influence of scattering and interference effects on the low-temperature magnetotransport of Cu2ZnSnS4 single crystals

Activated resistivity, ρ(T), and positive magnetoresistance (MR) are observed in Cu2ZnSnS4 single crystals within the temperature interval between T ∼ 2 and 300 K in pulsed magnetic fields of B up to 20 T. Between T ∼ 50 and 150 K, the charge transfer is described by the Mott variable-range hopping (VRH) transport over localized states of the defect acceptor band with width W ∼ 12–25 meV. Up to the highest applied fields, MR follows the law ln ρ(B) ∝ B2 pertinent to the VRH conduction at λ ≫ a, where λ is the magnetic length and a is the localization radius. The joint analysis of the MR and ρ(T) data yielded a series of microscopic parameters, including the values of a ≈ 22–45 Å, depending on the proximity of a sample to the metal-insulator transition. However, below T ∼ 3–4 K the Shklovskii-Efros VRH conduction is observed. Here, the behavior of MR changes drastically, exhibiting a sharp contraction of the quadratic MR region and transformation of the MR law into those of ln ρ (B) ∝ B2/3 or ∝ B3/4 when B is increased. In addition, such behavior of MR is accompanied by a dramatic increase of the localization radius a ≈ 50–170 Å. Both anomalies above are explained quantitatively by the same reason, connected to the constructive interference of different paths arising from multiple scattering of hopping carriers at low temperatures. In particular, the low-temperature values of the localization radius, evaluated in frames of the theory of interference phenomena in the VRH conduction, exhibit a reasonable agreement with the experimental data.

Structural transition, subgap states, and carrier transport in anion-engineered zinc oxynitride nanocrystalline films

In this work, anion alloying is engineered in ZnON nanocrystalline films, and the resultant evolution of the structural transition, subgap states, and carrier transport is investigated. A broad distribution of sub-gap states above the valence band maximum is introduced by nitrogen due to the hybridization of N 2p and O 2p orbitals. The phase transition from partially amorphous states to full crystallinity occurs above a characteristic growth temperature of 100 °C, and the localized states are suppressed greatly due to the reduction of nitrogen composition. The electronic properties are dominated by grain boundary scattering and electron transport across boundary barriers through thermal activation at band edge states at high temperatures. The conductivity below 130 K exhibits a weak temperature dependence, which is a signature of variable-range hopping conduction between localized states introduced by nitrogen incorporation.

Effect of High Cobalt Concentration on Hopping Motion in Cobalt Manganese Spinel Oxide (CoxMn3–xO4, x ≥ 2.3)

Hopping motions in cobalt manganese spinel oxides with high cobalt concentration (CoxMn3–xO4, 2.3 ≤ x ≤ 2.7) are investigated in order to clarify the origin of unusual electrical behaviors as negative temperature coefficient (NTC) thermistors. Based on the resistance versus temperature (R–T) characteristics, hopping conduction mechanisms in MCO compounds (x = 2.3 and 2.5) are attributed to variable range hopping (VRH) motion with a parabolic distribution of the density of states (DOS) near the Fermi level. However, when Co content increases up to 2.7, transition in the hopping motion occurs from VRH to the nearest neighboring hopping (NNH) motion, which can be responsible for a huge increase of the resistance accompanied by decrease of the factor of thermal sensitivity (B value) in MCO compounds (x = 2.7). Also, hopping distance and activation energies for MCO (x = 2.3 and 2.5) compounds following VRH conduction are calculated as a function of temperature, indicating that higher B value observed in MCO (x...

Carbon films embedded by nickel nanoparticles: fluctuation in hopping rate and variable-range hopping with respect to annealing temperature

In this work, the electrical properties of carbon–nickel films annealed at different temperatures (573, 773, 1073 and 1273 K) in the temperature range 15–300 K were investigated. The films were grown by radio frequency magnetron co-sputtering on quartz substrates at room temperature. The multiphonon hopping conduction mechanism is found to dominate the electrical transport in the temperature range 150–300 K. It can be seen that the room-temperature hopping rate (ΓRT) at 773 K has maximum value of 56.8 × 105 s−1. Our results of conductivity measurements at high temperature are in good agreement with strong carrier–lattice coupling model; on the other hand, the conductivity in the range 15–50 K is well described in terms of variable-range hopping (VRH) conduction mechanism. The localized state density around Fermi level N(E F) and the average hopping energy W hop at low temperature for the films annealed at 773 K have maximum value of 2.23 × 1023 (cm−3 eV−1) and minimum value of 9.74 × 10−4 eV, respectively.

Growth and characterization of textured YBaCo2O5 + δ thin films grown on (001)-SrTiO3 via DC magnetron sputtering

Thin films of the layered cobaltite YBaCo2O5+δ were successfully grown on (001)-oriented SrTiO3 single-crystal substrates by means of DC magnetron sputtering. The 112 phase of the compound was stabilized by choosing an adequate Co reactant and through careful thermal treatment of the target. The results demonstrate the strong influence of these variables on the final phase of the compound. A substrate temperature of 1053K and an oxygen pressure of 300Pa seemed to be appropriate growing conditions for depositing (00ℓ)-textured YBaCo2O5+δ thin films onto the chosen substrate. In like fashion to the polycrystalline YBaCo2O5+δ, the films showed a clear sequence of antiferromagnetic–ferromagnetic–paramagnetic transitions within a narrow temperature range. Well-defined hysteresis loops were observed at temperatures as high as 270K, which supports the existence of a FM order in the films. In turn, the dependence of the resistivity on the temperature shows a semiconductor-like behavior, without any distinguishable structure, within the temperature range measured (50–350K). The analysis of the experimental data showed that the transport mechanism in the films is well described by using the Mott variable range hopping (VRH) conduction model.

Variable-Range Hopping Conductivity in Quantum Hall Regime for HgTe-Based Heterostructure

We have measured the longitudinal and Hall resistivities in the quantum Hall regime at magnetic fields B up to 9 T and temperatures \(T =(2.9\div 50)\) K for the HgCdTe/HgTe/HgCdTe heterostructure with a wide HgTe quantum well. The temperature-induced transport at the resistivity minima corresponding to the quantum Hall plateaus has been studied within the concept of hopping conduction in a strongly localized electron system. An analysis of the variable-range hopping conductivity in the regions of the first and second quantum Hall plateaus provided an opportunity to determine the value and the magnetic-field dependence of the localization length with the experimental estimation of the critical indices.

Multiphonon hopping conduction in carbon–nickel composite films at different deposition time

In this work, the electrical properties of carbon–nickel composite films deposited at different time (50–600 s) were investigated. The films were grown by radio frequency magnetron sputtering on glass substrates at room temperature. The electrical conductivity of the films was investigated in the temperature range of 15–500 K. The conductivity data in the temperature range of 400–500 K show the extended state conduction mechanism, while the multiphonon hopping (MPH) conduction is found to dominate the electrical transport in the temperature range of 150–300 K. The films deposited at 180 s have the maximum conductivity. The conductivity at T < 60 K could be described in terms of variable range hopping (VRH) conduction. The localized state density around Fermi level (N(EF)) at low temperature for the films deposited at 180 s has the minimum value of about 4.02 × 1021 cm−3·eV−1. The average hopping distance (Rhop) for the films deposited at 180 s has the maximum value of about 3.51 × 10−7 cm.

Study on temperature and electric field dependence of conductivity in polyimide

The deep dielectric charging (DDC) imposes a potential threat on spacecrafts. On the one hand, this kind of polymer insulator dielectric, represented by polyimide, is significantly dependent on temperature; on the other hand, during the charging process the high electric field (at the level of 107 V/m) will enhance the conductivity of the dielectric. Therefore, in order to make a precise assessment of DDC by computer simulation, the conductivity model should take into account the temperature and electric field dependences. In this field, two conductivity models are usually adopted for DDC simulation. One of them is proposed by Adamec. It puts emphasis on the enhanced conductivity due to high electric field, while its temperature dependence is based on the famous Arrhenius formula. Adamec model can make good performance versus electric field, but it is inappropriate in low temperatures. Another model combines the thermally assistant hopping conductivity and the variable-range hopping conductivity together, so it shows advantage in the temperature dependence, which is named as TAH VRH model. Although this model also can include the influence from electric fields, the effectiveness is not so good as that of Adamec model. In order to combine the advantages of these two models, i.e. the Adamec model and TAH VRH model, a new conductivity model is proposed with fewer parameters than those in TAH VRH. It is derived by replacing the Arrhenius formula in Adamec model with a simplified temperature model referred to as TAH VRH model. This formulation enables the new model to deal with a wider temperature range and keep the good performance versus high electric fields. The proposed model is verified partly by the measured data of a kind of polyimide. Satisfactory agreement is obtained in data fitting by using the new model, where the temperature dependence is better than that of Adamec model. In addition, to overcome the unreasonable increase in conductivity in low temperature and high electric field, a useful technique is proposed. By temperature mapping in the electric field correlated factors namely the carrier concentration and mobility enhancement factor, this technique can extend the feasible temperature range to a lower limit. This is done according to the assumption that the carrier concentration is small at low temperatures, and consequently the electric field influence should not be large. At high temperatures or in low electric fields, the temperature mapping is of little effect. Finally, analysis of the model's sensitivity versus several parameters is provided, demonstrating the advantage of applicability of the new model with fewer parameters.

Study of conduction mechanism in Fe2O3 doped Na2O·Bi2O3·B2O3 semiconducting glasses

Conduction mechanism in Fe2O3 doped Na2O·Bi2O3·B2O3 semiconducting glass system was studied in frequency range 10Hz to 1MHz and at temperatures between room temperature and 663K. The total conductivity spectrum follows universal power law with frequency exponent ‘s’ value less than unity and lies in the range 0.51≤s≤0.78. These ranges of ‘s’ values indicate that the carrier transport is predominately due to hopping electrons between charged defects and show temperature dependence as predicted by correlated barrier hopping (CBH) model. The change in activation energy of dc conductivity with temperature reveals the change in conduction mode from small polaron hopping (SPH) at high temperatures (T>θD/2) to variable range hopping (VRH) at low temperatures (T<θD/2). The range of density of states at Fermi level N (EF)=7.25×1021–1.32×1021eV−1cm−3 at temperatures below θD/2 corresponds to localized states near Fermi level. The large values of activation energy W2 (0.067–0.155eV) dominated the conduction may results in high range of temperature (T=503– 423K) for variable-range hopping conduction in these glasses.

Variable range hopping conduction in n-CdSe samples at very low temperature

We reanalyzed experimental data already published in Friedman J R, Zhang Y, Dai P, et al. Phys Rev B, 1996, 53(15): 9528. Variable range hopping (VRH) conduction in the insulating three-dimensional n-CdSe samples has been studied over the entire temperature range from 0.03 to 1 K. In the absence of a magnetic field, the low temperature conductivity σ of the three samples (A, B and C) obeys the Mott VRH conduction with an appropriate temperature dependence in the prefactor (σ = σ0 exp[–(T0/T)]p with p ≈ 0.25). This behavior can be explained by a VRH model where the transport occurs by hopping between localized states in the vicinity of the Fermi level, EF, without creation of the Coulomb gap (CG). On the contrary, no Efros-Shklovskii VRH is observed, suggesting that the density is constant in the vicinity of the EF.

Hopping conduction range of heavily Al-doped 4H-SiC thick epilayers grown by CVD

To outline the hopping conduction range, the electrical characteristics of CVD-grown heavily Al-doped 4H-SiC thick epilayers (2.0 × 1019–4.0 × 1020 cm−3) were investigated in a wide temperature regime (20–900 K). It is found that, below 100 K, hopping conduction dominates the carrier transport for all epilayers, and the corresponding hopping conduction activation energy shows a maximum of ∼30 meV at around 1.1 × 1020 cm−3. With increasing doping level, the temperature dependence of resistivity evolves and finally obeys the ∼1/T1/4 law in the entire temperature regime, which gives direct evidence of variable-range hopping conduction.

Transverse resistance of YBa2Cu3O7−δsingle crystals with different oxygen deficiency

The resistivity of YBa2Cu3O7−δ single crystals across the layers was investigated in the temperature range Tc–300 K and in the range of δ corresponding to the change in Tc from 93 to 33 K. The temperature dependence of the resistivity is adequately described by an empirical expression, which includes “semiconductor” behavior and fluctuation conductivity in the three-dimensional Aslamazov–Larkin model near Tc. For high Tc, the charge carriers are scattered mainly by phonons. For low Tc, scattering by defects dominates; the coherence length ξab(0) and the electron–phonon interaction constant λ reach the values characteristic of conventional (low-temperature) superconductors with strong coupling. Heterogeneity of the samples causes anisotropy of Tc and variable-range hopping conductivity between different phases.

Thermoelectric properties of thin-film Sb0.9Bi1.1Te2.9Se0.1–C composites

Thin-film nanocomposites of a Sb0.9Bi1.1Te2.9Se0.1 semiconductor solid solution in the carbon matrix have been synthesized. The low-temperature dependences of the electrical resistivity and thermoelectric power have been investigated in the temperature range of 77–300 K. It has been shown that there is a successive change in the dominant mechanisms of electrical conduction: variable-range hopping conduction over localized states lying in a narrow energy band near the Fermi level, hopping conduction over the nearest neighbors, and hopping electron transfer associated with charge carriers excited into localized states near the band edges.

Structural And Electrical Properties Of Self-standing Polyaniline Films Modified With Gold Nanoparticles

Self-standing polyaniline (Pani) films modified with gold nanoparticles (Au NP's), where Au NP's are added in different successive weight percents, have been synthesized by conventional chemical polymerization technique. An in-depth investigation of the structural and electrical characteristics of prepared films has been conducted using various characterizations. The X-ray diffraction (XRD) validates the presence of Au NP's in Pani and the results are supported well by energy dispersive X-ray analyzer (EDX). The field emission scanning electron microscopy (FESEM) clearly shows thorough dispersion of Au NP's in the amorphous host matrix with minor aggregation. The Fourier transform infrared red (FTIR) studies give the information of possible chemical interaction between the nanoparticles and polymer which is in good agreement with charge transfer mechanism proposed in the manuscript. The temperature dependent dc electrical conductivity has been observed to depend strongly on the nanoparticle loading and follows Mott's three-dimensional variable range hopping (3D VRH) conduction mechanism. Parameters obtained from Hall Effect measurements are of same order as is calculated by dc measurements which indicates a very good corroboration of results. Higher ac conductivity, dielectric constant and dielectric loss of nanocomposites have also been observed as compared to that of pure Pani. Copyright © 2015 VBRI Press.

Poole-Frenkel effect and variable-range hopping conduction in metal/YBCO resistive switching devices

Current-voltage (IV) characteristics and the temperature dependence of the contact resistance [R(T)] of Au/YBa2Cu3O7−δ (optimally doped YBCO) interfaces have been studied at different resistance states. These states were produced by resistive switching after accumulating cyclic electrical pulses of increasing number and voltage amplitude. The IV characteristics and the R(T) dependence of the different states are consistent with a Poole-Frenkel (P-F) emission mechanism with trapping-energy levels Et in the 0.06–0.11 eV range. Et remains constant up to a number-of-pulses-dependent critical voltage and increases linearly with a further increase in the voltage amplitude of the pulses. The observation of a P-F mechanism reveals the existence of an oxygen-depleted layer of YBCO near the interface. A simple electrical transport scenario is discussed, where the degree of disorder, the trap energy level, and the temperature range determine an electrical conduction dominated by non-linear effects, either in a P-F emission or in a variable-range hopping regime.

Temperature-dependent contact resistivity of radio frequency superimposed direct current sputtered indium tin oxide ohmic contact to p-type gallium nitride

The temperature-dependent contact resistivity of radio frequency (RF) superimposed direct current (DC) sputtered indium tin oxide (ITO) contacts to p-type gallium nitride (p-GaN) films as well as the temperature-dependent sheet resistivity of p-GaN films were investigated to understand the carrier transport mechanism of the sputtered ITO ohmic contacts to p-GaN. As the measurement temperature was decreased from 400 to 200K, the contact resistivity increased by three orders of magnitude. Furthermore, the sheet resistivity of the p-GaN increased linearly with exp (1/Temperature(T))1/4 from 200 to 340K, indicating variable-range hopping (VRH) conduction via the Mg-related deep level defect (DLD) band. Based on the VRH conduction model, the effective barrier height between the sputtered ITO and the DLD band were calculated to be 0.12eV, which is sufficiently low to explain the formation of the low contact resistivity of the RF superimposed DC sputtered ITO contacts to p-GaN (~10−2Ω-cm2).