Effect of grain boundary on electrical properties of polycrystalline lanthanum nickel oxide thin films

In the present work, lanthanum nickel oxide (LNO) thin films were prepared by the sol-gel method. Microstructures of the films were tailored by changing sol concentration so as to investigate the effect of grain boundary on the transport properties of electrons in the polycrystalline LNO films. Based on the temperature dependence of the resistivity and the magnetic field dependence of the magnetoresistance (MR) at various temperatures, the factors that dominate the transport behavior in the polycrystalline LNO films were explored in terms of weak localization and strong localization. The results show that the grain boundary has a significant influence on the transport behavior of the electrons in LNO films at a low-temperature region, which can be captured by a variable-range hopping (VRH) model. The increase of metal–insulator (M–I) transition temperature is ascribed to Anderson localization in grain boundary. At a high-temperature region, electron–electron scattering and electron–phonon scattering predominates in the films. In this case, the existence of more grain boundary shows a minor effect on the transport behavior of the electrons but elevates the residual resistivity of the films.     

Direct current hopping conductance in one-dimensional diagonal disordered systems

Based on a tight-binding disordered model describing a single electron band, we establish a direct current (dc) electronic hopping transport conductance model of one-dimensional diagonal disordered systems, and also derive a dc conductance formula. By calculating the dc conductivity, the relationships between electric field and conductivity and between temperature and conductivity are analysed, and the role played by the degree of disorder in electronic transport is studied. The results indicate the conductivity of systems decreasing with the increase of the degree of disorder, characteristics of negative differential dependence of resistance on temperature at low temperatures in diagonal disordered systems, and the conductivity of systems decreasing with the increase of electric field, featuring the non-Ohm's law conductivity.     

Mechanism of dc conduction in polyaniline doped with sulfuric acid

A temperature variation of dc conductivity in the range 77–300 K has been carried out in order to explore the mechanism of charge transport in polyaniline (PAN) doped with sulfuric acid. The variable range hopping (VRH) exponent changes as the transition of the PAN lattice takes place in a narrow pH range thereby indicating that the charge transport is crucially composition dependent. A decrease in activation energy has been observed as the doping level is increased. Spin concentration of charge carriers determined by electron spin resonance spectroscopy has also been found to depend on the doping level of the specimen. Polarons and bipolarons formed during the doping process are the charge carriers in this system. The temperature dependence of dc conductivity and activation energy data are indicative of existence of both VRH and mixed conduction for various doping levels in these samples.     

Anisotropic properties and conduction mechanism of TlInSe2 chain semiconductor

TlInSe₂ chain crystals were prepared using the modification of the Bridgman technique. The grown crystals were identified by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDX), and X-ray diffraction (XRD). We investigate the anisotropy of transport properties for the first time for TlInSe₂ crystals. Temperature dependence of the dc electrical conductivity, Hall coefficient, Hall mobility, and charge carrier concentration were investigated in the temperature range 184–455 K. The conduction mechanism of TlInSe₂ crystals was studied, and measurements revealed that the dc behavior of the grown crystals can be described by Mott’s variable range hopping (VRH) model in the low temperature range, while it is due to thermoionic emission of charge carriers over the chain boundaries above 369 K. The Mott temperature, the density of states at the Fermi level, and the average hopping distance are estimated in the two crystallographic directions. The temperature dependence of the ac conductivity and the frequency exponent, s, is reasonably well interpreted in terms of the correlated barrier-hopping CBH model.     

Variable-range hopping in Fe70Pt30 catalyzed multi-walled carbon nanotubes film

Using low-pressure chemical vapour deposition (LPCVD), multi-walled carbon nanotubes (MWNTs) are grown on nanocrystalline Fe₇₀Pt₃₀ film. The Fe₇₀Pt₃₀ nanocrystalline film is deposited by vapour condensation technique. The size of the nanoparticles varies from 5–10 nm, as inferred from SEM micrographs of Fe₇₀Pt₃₀ film. SEM and TEM observations of as-grown CNTs film reveal that these are multi-walled and their diameter varies from 30–80 nm and length is of the order of several micrometers respectively. There is a structural change from ordinary geometry of CNTs to bamboo shaped as suggested by TEM image. Raman spectra shows sharp G and D bands with a higher intensity of G band showing the presence of graphitic nature of the nanotubes. An experimental study of the temperature dependence of electrical conductivity of MWNTs film is done over a wide temperature range from (293–4 K). The measured data gives a good fit to variable-range hopping (VRH) and the results are interpreted using Mott's (VRH) model. The conduction mechanism of the MWNTs film shows a crossover from the exp[ -(T_o/T)^1/4] law in the temperature range (293–110 K) to exp[ -(T_m/T)^1/3] in the low temperature range (110–4 K). This behaviour is attributed to temperature-induced transition from three-dimension (3D) to two-dimension (2D) VRH. Various Mott's parameters like characteristic temperature (T_m), density of states at Fermi level N(E_F), localization length (ξ), hopping distance (R), hopping energy (W) have also been calculated using above-mentioned model.     

Electrical and optical properties of thin film of amorphous silicon nanoparticles

Electrical and optical properties of thin film of amorphous silicon nanoparticles (a-Si) are studied. Thin film of silicon is synthesized on glass substrate under an ambient gas (Ar) atmosphere using physical vapour condensation system. We have employed Field Emission Scanning Electron Microscopy (FESEM), Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM) to study the morphology and microstructure of this film. It is observed that this silicon film contains almost spherical nanoparticles with size varying between 10 and 40 nm. The average surface roughness is about 140 nm as evident from the AFM image. X-ray diffraction analysis is also performed. The XRD spectrum does not show any significant peak which indicates the amorphous nature of the film. To understand the electrical transport phenomena, the temperature dependence of dc conductivity for this film is studied over a temperature range of (300-100 K). On the basis of temperature dependence of dc conductivity, it is suggested that the conduction takes place via variable range hopping (VRH). Three-dimensional Mott's variable range hopping (3D VRH) is applied to explain the conduction mechanism for the transport of charge carriers in this system. Various Mott's parameters such as density of states, degree of disorder, hopping distance, hopping energy are estimated. In optical properties, we have studied Fourier transform infra-red spectra and the photoluminescence of this amorphous silicon thin film. It is found that these amorphous silicon nanoparticles exhibits strong Si-O-Si stretching mode at 1060 cm⁻¹, which suggests that the large amount of oxygen is adsorbed on the surface of these a-Si nanoparticles. The photoluminescence observed from these amorphous silicon nanoparticles has been explained with the help of oxygen related surface state mechanism.     

Electron transport in a multichannel one-dimensional conductor: molybdenum selenide nanowires

We have measured electron transport in small bundles of identical conducting molybdenum selenide nanowires where the number of weakly interacting one-dimensional chains ranges from 1 to 300. The linear conductance and current in these nanowires exhibit a power-law dependence on temperature and bias voltage, respectively. The exponents governing these power laws decrease as the number of conducting channels increase. These exponents can be related to the electron-electron interaction parameter for transport in multichannel 1D systems with a few defects.     

Variable range hopping and/or phonon-assisted tunneling mechanism of electronic transport in polymers and carbon nanotubes

We review and compare two models recently used to describe electronic transport in polymer fibers/nanotubes and carbon nanotubes including graphene nanoribbons, namely, variable range hopping (VRH) in different versions and their modifications on the one hand and electric-field-induced phonon-assisted tunneling (PhAT) on the other hand. The VRH model is mainly approved on behalf of the results of temperature dependences. However, the field dependencies of the conductivity in the framework of this model remain practically unexplained. At the same time, the PhAT model describes properly not only temperature dependence of conductivity measured in a wide temperature range, but also conductivity/current dependences on field strength using the same set of parameters characterizing the materials     

Statistical properties of Lyapunov exponents and of quantum conductance of random systems in the regime of hopping transport

The statistics of the zero-temperature conductance and the Lyapunov exponents of one-, two- and three-dimensional disordered systems in the regime of strong localization is studied numerically. In one dimension, the origin of the universality of the moments of the conductance is explained. The relation between the most probable value of the conductance and its configurational average is discussed. The relative fluctuations of the conductance (and of the resistance) are shown to grow exponentially with the system length. In higher dimensions the conductance is almost entirely determined by the smallest of the Lyapunov exponents. The statistics of the conductance is therefore the same as in the one dimensional case. A model is proposed for the treatment of the fluctuations in hopping transport at finite temperatures. An exponential dependence of the relative fluctuations of the conductance/resistance on the temperature is predicted, log (δg/g) ∞ T^?a with α = 1/(d+1). It is concluded that the presently available experimental data on the temperature dependence of the conductance fluctuations in the hopping regime can be understood by replacing the system size in the zerotemperature result for the fluctuations of the conductance by the hopping length.     

Possible evidence of a spontaneous spin polarization in mesoscopic two-dimensional electron systems

We have experimentally studied the nonequilibrium transport in low-density clean two-dimensional (2D) electron systems at mesoscopic length scales. At zero magnetic field (B), a double-peak structure in the nonlinear conductance was observed close to the Fermi energy in the localized regime. From the behavior of these peaks at nonzero B, we could associate them with the opposite spin states of the system, indicating a spontaneous spin polarization at B=0. Detailed temperature and disorder dependence of the structure shows that such a splitting is a ground-state property of low-density 2D systems.     

Electrical conduction mechanism in Fe<Subscript>70</Subscript>Pd<Subscript>30</Subscript> catalyzed multi-walled carbon nanotubes

Carbon nanotubes (CNTs) are synthesized by the catalytic decomposition of acetylene using low pressure chemical vapour deposition method (LPCVD) at 800 °C and at a chamber pressure of 10 Torr over a supported catalyst film of Fe₇₀Pd₃₀. Morphology of these CNTs is studied using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and High Resolution Transmission Electron Microscopy (HRTEM). From HRTEM image of these multi-walled carbon nanotubes (MWNTs), it is clear that these MWNTs do not possess a co-axial cylindrical structure, but are composed of imperfect and broken graphite cylinders of different sizes. The average diameter and length of the nanotubes varies between 20–70 nm and 5–60 μm respectively. Electrical transport measurements of these MWNTs are studied over a temperature range of 298–4.2 K. The results have been interpreted in terms of variable-range hopping (VRH) over the entire temperature range of 298–4.2 K. Three-dimensional variable-range hopping (VRH) is suggested for the temperature range (298–125 K), while two-dimensional VRH is observed for the temperature range (125–4.2 K).     

聚酰亚胺电导率随温度和电场强度的变化规律

介质深层充电对航天器安全运行构成了重大威胁.以聚酰亚胺为代表的此类聚合物绝缘介质的电导率受温度影响显著,又因为充电过程中局部产生强电场(10~7V/m量级),因此,其电导率模型需要综合考虑温度和强电场的影响,这对介质深层充电的仿真评估意义重大.已有的两类模型,不是低温区间不适用,就是没有充分考虑强电场的影响.基于跳跃电导理论,本文分析对比了现有电导率模型,提出了适用于较宽温度范围且合理考虑强电场增强效应的电导率新模型,并采用某型聚酰亚胺电导率测试数据做出验证.此外,为了提高新模型在强电场下的低温适用范围,尝试对强电场因子中的温度做变换,取得了满意的效果.参数敏感度分析表明新模型在电导率拟合与外推方面具有参数少、适用性强的优势.     

A critical path approach for elucidating the temperature dependence of granular hopping conduction

We revisit the classical problem of granular hopping conduction’s σ∝exp[–(T₀/T)] temperature dependence, where σ denotes conductivity, T is temperature, and T₀ is a sample-dependent constant. By using the hopping conduction formulation in conjunction with the incorporation of the random potential that has been shown to exist in insulator-conductor composites, it is demonstrated that the widely observed temperature dependence of granular hopping conduction emerges very naturally through the immediate-neighbor critical-path argument. Here, immediate-neighbor pairs are defined to be those where a line connecting two grains does not cross or by-pass other grains, and the critical-path argument denotes the derivation of sample conductance based on the geometric percolation condition that is marked by the critical conduction path in a random granular composite. Simulations based on the exact electrical network evaluation of finite-sample conductance show that the configurationaveraged results agree well with those obtained using the immediate-neighbor critical-path method. Furthermore, the results obtained using both these methods show good agreement with experimental data on hopping conduction in a sputtered metal-insulator composite Ag_x(SnO₂)_1–x, where x denotes the metal volume fraction. The present approach offers a relatively straightforward and simple explanation for the temperature behavior that has been widely observed over diverse material systems, but which has remained a puzzle in spite of the various efforts made to explain this phenomenon.     

Single-photon detection mechanism in a quantum dot transistor

We study the transport mechanisms in a quantum dot MODFET by tuning the localization induced by charge stored on the quantum dots with light. The temperature dependence of the resistivity of a macroscopic sample reveals a hopping transport when the dots contain an excess of electrons. The resistance of a mesoscopic sample however, which is capable of detecting single photons, exhibits a much weaker dependence upon temperature. This points towards source-drain tunnelling as a transport mechanism and is confirmed by a statistical analysis of the single-photon-induced conductance steps. The complexity of the conducting paths increases as the average hopping length reduces.     

Finite-size effect in shot noise in hopping conduction

We study a current shot noise in a macroscopic insulator based on a two-dimensional electron system in GaAs in a variable range hopping (VRH) regime. At low temperature and in a sufficiently depleted sample a shot noise close to a full Poissonian value is measured. This suggests an observation of a finite-size effect in shot noise in the VRH conduction and demonstrates a possibility of accurate quasiparticle charge measurements in the insulating regime.     

Dopant size dependent variable range hopping conduction in polyaniline nanorods

The present work investigates the electrical transport and dielectric relaxation of polyaniline (PAni) nanorods doped with organic camphorsulfonic acid (CSA) and inorganic hydrochloric acid (HCl) synthesized by interfacial polymerization technique. High resolution transmission electron micrographs (HRTEM) depict that initially spherical nuclei directionally grow into nanorods and CSA doped PAni produces more uniform and aligned structures. The electrical transport studies reveal that the CSA doped nanorods follow 1D Mott variable-range hopping (VRH), whereas the HCl doped nanorods exhibit 2D VRH conduction mechanism. The value of interchain charge transfer integral is found to be higher for smaller size HCl doped PAni than that for larger size CSA doped PAni. The resistivity measurements exhibit semiconducting behavior for both organic and inorganic dopants and the resistivity of the CSA doped nanorods is found to be smaller than that of the HCl doped nanorods. The dielectric relaxation studies suggest Debye type relaxation with a single relaxation peak for both the dopants and the relaxation time of the carriers of the CSA doped PAni nanorods is smaller than that of the HCl doped nanorods.     

Current voltage analysis and band diagram of Ti/TiO2 nanotubes Schottky junction

We report variable temperature resistivity measurements and mechanisms related to electrical conduction in 200 keV Ni²⁺ ion implanted ZnO thin films deposited by vapor phase transport. The dc electrical resistivity versus temperature curves show that all polycrystalline ZnO films are semiconducting in nature. In the room temperature range they exhibit band conduction and conduction due to thermionic emission of electrons from grain boundaries present in the polycrystalline films. In the low temperature range, nearest neighbor hopping (NNH) and variable range hopping (VRH) conduction are observed. The detailed conduction mechanism of these films and the effects of grain boundary (GB) barriers on the electrical conduction process are discussed. An attempt is made to correlate electrical conduction behavior and previously observed room temperature ferromagnetism of these films.     

Deconvolution of temperature dependence of conductivity,its reduced activation energy,and Hall-effect data for analysing impurity conduction in n-ZnSe

ABSTRACT

The temperature dependence of the reduced activation energy w?=?ε/k_BT of the conductivity σ has been utilised for determining the impurity conduction mechanism in doped semiconductors in many studies. Herein, the formula for deconvoluting w when plural conduction mechanisms appear is used to confirm the analysis of the data of the Hall-effect measurements on Al-doped n-ZnSe samples. The analysis is performed on the basis of an impurity-Hubbard-band model which includes ε ₂ conduction in the top Hubbard band as well as ε ₃ and Efros-Shklovskii (ES) variable-range hopping (VRH) conduction processes in the bottom Hubbard band. As the result of the analysis, transitions among the three hopping conduction mechanisms of ε ₂, ε ₃, and ES VRH are clearly shown in the temperature dependence of w as well as in that of the Hall mobility, which are hardly noticed in the temperature dependence of σ. In addition, the power-law exponent of the prefactor of ES VRH conductivity is determined through the fit to the temperature dependence of w to show that it decreases from ～ 1.5 to ～ 0 with increasing net donor concentration.     

Electrical conduction properties of Si δ-doped GaAs grown by MBE

The temperature dependent Hall effect and resistivity measurements of Si δ-doped GaAs are performed in a temperature range of 25–300 K. The temperature dependence of carrier concentration shows a characteristic minimum at about 200 K, which indicates a transition from the conduction band conduction to the impurity band conduction. The temperature dependence of the conductivity results are in agreement with terms due to conduction band conduction and localized state hopping conduction in the impurity band. It is found that the transport properties of Si δ-doped GaAs are mainly governed by the dislocation scattering mechanism at high temperatures. On the other hand, the conductivity follows the Mott variable range hopping conduction (VRH) at low temperatures in the studied structures.     

THERMALLY ACTIVATED HOPPING IN Dy1-xPrxBa2Cu3O7-δ SYSTEM

The temperature dependence of resistivity in Dy_1-xPr_xBa₂Cu₃O_7-δ system with x>0.6 was measured. The experimental results show that Pr substitution leads to the localization of mobile holes and such a localization is enhanced with increasing Pr concentration. The gradually enhancing of localization induces Anderson transitions one by one in this system, including the transition from the conduction by excitation of holes to the one by thermal activation hopping between localized states, the so called Anderson transition type-I, and the transition from nearest neighbor hopping (NNH) to variable range hopping (VRH), the Anderson transition type-II, and the Anderson transition type-lI from 3D to 2D.