Formation Of Conducting Channels Research Articles

Thermophoresis/diffusion as a plausible mechanism for unipolar resistive switching in metal–oxide–metal memristors

We show that the SET operation of a unipolar memristor could be explained by thermophoresis, or the Soret effect, which is the diffusion of atoms, ions or vacancies in a steep temperature gradient. This mechanism explains the observed resistance switching via conducting channel formation and dissolution reported for TiO2 and other metal-oxide-based unipolar resistance switches. Depending on the temperature profile in a device, dilute vacancies can preferentially diffuse radially inward toward higher temperatures caused by the Joule heating of an electronic current to essentially condense and form a conducting channel. The RESET operation occurs via radial diffusion of vacancies away from the channel when the temperature is elevated but the gradient is small.

Visualization of the conductive channel in a planar resistance switching device based on electrochromic materials

In this work, bipolar resistance switching behavior was realized in an Au/tungsten oxide/Au planar device, and the evolution of the conductive channel during resistance switching was successfully visualized by the in situ optical image technique based on the color-conductivity dependence of tungsten oxide. We found that there are two types of conductive channel, named parabolic channel and bar-like channel, exist in the planar device. The parabolic channel formed firstly near the cathode and then extended to but could not touch the anode. By applying opposite electric-field, the bar-like channel formed from the cathode (i.e., foregoing anode) and extended to the parabolic channel. With alternating the external electric-field polarity, the bar-like channel showed an indirect connection and nonmonotonic disconnection with the parabolic channel at the region near the foregoing anode, corresponding to the high-to-low and low-to-high resistance switching processes of the planar device, respectively. The instable RS behavior was caused by the change of bar-like channel occurring position under the high external field condition. The conductive channel formation was ascribed to the sodium ion immersion from the soda-lime glass substrate into the tungsten oxide film and then migration driven by the electric field to form sodium tungsten bronze. These results will give some insight into the resistance switching property improvement and mechanism elucidation as well as a possibility to develop electric/optical-coupled switch and data storage devices.

얇은 다공 구조 박막에서의 두께에 따른 박막 저항 변화

We have observed the change in the film resistance of thin nickel film up to 13 nm, which is deposited on a porous anodic alumina substrate, prepared by two-step anodization technique under phosphoric acid. The resulting film grows as a porous film, following the pore structure on the surface of the alumina substrate, and the value of the resistance lies above 150 kΩ within the range of thickness studied here, decreasing very slowly with the film thickness. The observed resistance value is much higher than the reported value of a uniform film at the same thickness. Since the observed value of the surface coverage with the pores is smaller than the critical value, expected from the percolation theory, the pore structure limits the formation of conduction channel across the film. In addition, by comparing to the typical model of thickness-dependent resistivity, we expect that the scattering at the pore edge further increases the film resistance.

Electrical modulus analysis on the Ni/CCTO/PVDF system near the percolation threshold

A type of Ni/CCTO/PVDF three-phase percolative composite was prepared, in which the filler content (volume fraction) of Ni and CCTO was set at 60 vol%. The dependence of permittivity, electrical modulus and ac conductivity on the concentration of Ni and CCTO fillers near the percolation threshold was investigated in detail. The permittivity of the composites dramatically increased as the Ni content approached 24 vol%. This unique physical mechanism was realized as the formation of conductive channels near the percolation threshold. Analysis on the electrical modulus showed that the conductive channels are governed by three relaxation processes induced by the fillers (Ni, CCTO) and PVDF matrix, which are the interfacial polarization derived from the interfaces between fillers (Ni, CCTO) and PVDF matrix, and the polarization of CCTO ceramic filler and PVDF matrix. The conductivity behaviour with various Ni loadings and temperature suggested that the transition from an insulating to a conducting state should be induced by charge tunnelling between Ni–Ni particles, Ni–CCTO fillers and Ni–PVDF matrix. These findings demonstrated that the tunnelling conduction in the composite can be attributed to the unique physical mechanism near the percolation threshold.

Conducting Channel Formation in Poly(3-hexylthiophene) Field Effect Transistors: Bulk to Interface

The most critical step in fabricating solution-processed organic field effect transistors (OFETs) is the solidification process of the polymer solution at the polymer/gate dielectric interface, whe...

High current-induced degradation of AlGaN ultraviolet light emitting diodes

Degradation under high current stress of AlGaN quantum well based light emitting diodes emitting at 285 and 310 nm has been studied using electroluminescence, time-resolved photoluminescence and current-voltage experimental techniques. The measurements have revealed that during aging decrease of the emission intensity is accompanied by increase of the tunneling current, increase of the nitrogen vacancy concentration and partial compensation of the p-doping. The main role in the device degradation has been ascribed to formation of tunneling conductivity channels, probably, via activation of the closed core screw dislocations with the help of nitrogen vacancies. Carrier lifetimes in the quantum wells and the p-cladding were found to be unaffected by the aging process, suggesting that the nonradiative recombination has a lesser influence on the device degradation.

Effective thermal conductivity and coefficient of linear thermal expansion of high-density polyethylene — fly ash composites

As the disposal of fly ash (FA) poses a serious problem in terms of land use and potential environmental pollution, there exists a global interest for its utilization. Utilization of fly ash as filler material in polymer composites is considered important from both economic and commercial point of view. In this communication, the effective thermal conductivity and coefficient of thermal expansion (CTE) of composites synthesized with fly ash filler embedded in high-density polyethylene (HDPE) matrix is investigated. Incorporation of fly ash in HDPE enhances both the thermal stability and the effective thermal conductivity of the composites. CTE, however, significantly decreases as the FA content increases in HDPE. Effective thermal conductivity for HDPE containing 70-volume fraction (%) fly ash becomes almost twice than that for unfilled HDPE. Results on both the effective thermal conductivity and CTE of HDPE/FA composites have been discussed in light of various theoretical models. Our analysis indicates formation of conductive channels of FA particulates in HDPE, which causes rapid enhancement in the effective thermal conductivity of the HDPE/FA composites. We also confirm the importance of the role of the interphase volume and the strength of the polymer — filler interactions to successfully predict the CTE of HDPE/FA composites.

Optical studies of degradation of AlGaN quantum well based deep ultraviolet light emitting diodes

Aging under high current stress of AlGaN quantum well based light emitting diodes with high and low Al content in the wells emitting at 270 nm and 335 nm, respectively, has been studied by scanning near field optical spectroscopy and far field electroluminescence, photoluminescence and time-resolved photoluminescence. In the high Al content devices emission band related to optical transitions in the cladding involving nitrogen vacancies has been found. Evolution of this band during aging suggests that the role of N vacancies is crucial in the aging process by aiding defect generation and formation of high conductivity channels.

Effect of different copper fillers on the electrical resistivity of conductive adhesives

The effects of different copper fillers with different morphology and particle size have been studied in terms of electrical resistivity and thermal stability on the electrically conductive adhesives. The copper fillers used in this study were prepared by wet chemical reduction, electrolytic and gas atomization method, respectively. The as cured ECAs filled with different type of Cu fillers showed significant difference in electrical resistivity. Cu filler with smaller particle size showed higher packing density and larger surface area, which would enhance formation of conductive channels and increased conductive network in the ECAs, leading to a lower electrical resistivity. In addition, thermal stability of the ECAs were investigated under high temperature exposure at 125 °C and high humidity aging at 85 °C/85% RH for 1,000 h. Results showed that ECAs with Cu fillers of relatively small particle size and rough particle surface have excellent thermal stability due to enhanced adhesion and contact area between Cu fillers and the polymer matrix. A very low resistivity at an order of magnitude of 10−4 Ω cm could be maintained for these ECAs after 1,000 h at 125 and 85 °C/85% RH.

Mesoscopic Metal−Insulator Transition at Ferroelastic Domain Walls in VO2

The novel phenomena induced by symmetry breaking at homointerfaces between ferroic variants in ferroelectric and ferroelastic materials have attracted recently much attention. Using variable temperature scanning microwave microscopy, we demonstrate the mesoscopic strain-induced metal-insulator phase transitions in the vicinity of ferroelastic domain walls in the semiconductive VO(2) that nucleated at temperatures as much as 10-12 degrees C below bulk transition, resulting in the formation of conductive channels in the material. Density functional theory is used to rationalize the process low activation energy. This behavior, linked to the strain inhomogeneity inherent in ferroelastic materials, can strongly affect interpretation of phase-transition studies in VO(2) and similar materials with symmetry-lowering transitions, and can also be used to enable new generations of electronic devices though strain engineering of conductive and semiconductive regions.

The polymer gate dielectrics and source-drain electrodes on n-type pentacene-based organic field-effect transistors

Pentacene-based organic field-effect transistors (OFETs) with different polymer gate dielectrics, such as polyvinyl alcohol (PVA), poly 4-vinyl phenol (PVP), and polystyrene (PS), are fabricated to study the influence of polymer dielectrics on the formation of the n-type conduction (electron) channel in the pentacene active layer. The output characteristics of OFETs and capacitance–voltage measurements indicate that the formation of n-type conduction channel in the active layer is hindered by the electron traps at the contact interface with PVP dielectric layers, probably due to the high dissociation constant of protons of the hydroxyl groups in PVP. The dissociated protons at PVP dielectric layer form the electron traps and restrict the formation of n-type conduction channel. In comparison, OFETs applying PVA of relatively lower dissociation constant than that of PVP as the gate dielectric present the decent n-type output characteristics. The appropriate work function of source-drain electrodes as well as a trap-free dielectric layer are essentially important to determine the performance of pentacene-based n-type OFETs. The pentacene-based OFETs applying calcium as the source-drain electrodes and PS as the dielectric layer has the electron mobility of 0.077 cm 2s −1V −1 in this study.

Current leakage and transients in ferroelectric ceramics under high humidity conditions

Lead zirconate titanate (PZT) is widely used in electromechanical devices such as ultrasonic transducers, sensors, ultrasonic motors, actuators and resonators. In harsh operating environments such as high humidity and high temperature the electrical resistance of the ceramic can decrease resulting in leakage currents. These can have a significant impact on device performance, particularly for low power applications. In this paper, the increase in leakage current is investigated and characterised under high humidity conditions. It is observed that the leakage current does not evolve smoothly over time, but is characterised by current transients. Detailed measurements of these current transients are reported, and their relationship to the conduction processes in the ceramic is investigated. A mechanism of conductive channel formation is proposed to account for the leakage current increase and current transients.

Methods of parallel integration of carbon nanotubes during the formation of functional devices for microelectronics and sensor technologies

Methods of formation of thin film structures and sensor elements based on carbon nanotubes have been developed. Methods of integration using deposition from solutions, probe micromechanics and the electrokinetic positioning of nanotubes providing compatibility with traditional microelectronic technology, are suggested. Peculiarities of the parallel integration of carbon nanotubes in the process of formation of thin films, networks, and individual conduction channels are revealed.

Conducting channel formation and annihilation in organic field-effect structures

We report measurements of displacement current to study the transient effects of conducting channel formation and annihilation at organic semiconductor/dielectric interfaces. The device structure resembled a typical organic thin-film transistor with either source or drain electrode removed. However, the channel length was very long (1–6 mm) in order to increase the transit time and enhance the displacement current. The devices consisted of a gold electrode contacting a 30 nm thick pentacene thin film, thermally deposited on SiO2 dielectric, with a heavily doped p-type Si substrate serving as a bottom electrode. Electrical measurements were performed by measuring the displacement current running through the grounded gold contact while linearly sweeping the voltage bias applied to the bottom electrode. The processes of conducting channel formation and annihilation were observed as transients in the I-V characteristics. By integrating the displacement current with respect to time, the concentrations of carriers injected into, extracted from, and trapped inside the pentacene film during the voltage sweep cycle were determined. The results are analyzed in terms of a device model that includes the effects of traps.

Direct fabrication of three-dimensional buried conductive channels in single crystal diamond with ion microbeam induced graphitization

We report on a novel method for the fabrication of three-dimensional buried graphitic micropaths in single crystal diamond with the employment of focused MeV ions. The use of implantation masks with graded thickness at the sub-micrometer scale allows the formation of conductive channels which are embedded in the insulating matrix at controllable depths. In particular, the modulation of the channels depth at their endpoints allows the surface contacting of the channel terminations with no need of further fabrication stages. In the present work we describe the sample masking, which includes the deposition of semi-spherical gold contacts on the sample surface, followed by MeV ion implantation. Because of the significant difference between the densities of pristine and amorphous or graphitized diamond, the formation of buried channels has a relevant mechanical effect on the diamond structure, causing localized surface swelling, which has been measured both with interferometric profilometry and atomic force microscopy. The electrical properties of the buried channels are then measured with a two point probe station: clear evidence is given that only the terminal points of the channels are electrically connected with the surface, while the rest of the channels extends below the surface. IV measurements are employed also to qualitatively investigate the electrical properties of the channels as a function of implantation fluence and annealing.

Bipolar space charge formation and switching effect in thin polymer films

A theoretical model of bipolar charge formation in a polymer film is proposed. High density of metastable charge pairs within a thin trapping layer results from their delayed recombination. The conditions for the highest density of charge pairs are discussed. The formation of conductive channels by charge pairs starting from the trapping layer together with the high mobility of “Coulomb holes” in the channels can switch the structure to the highly conductive state.

Energy states in GaAs delta-doped field effect transistors under hydrostatic pressure

The study of the electronic structure in GaAs-based delta-doped field effect transistors under applied hydrostatic pressure is presented. A combination of the depletion approximation and the local density Thomas-Fermi theory is used to model the potential energy profile. We present a discussion on the possible effect of the hydrostatic pressure in the formation of high conductivity channels in the system.

Formation of conducting nanochannels in diamond-like carbon films

A sharp increase of the emission current at high electric fields and a decrease of the threshold voltage after pre-breakdown conditioning of diamond-like carbon (DLC) films have been measured. This effect was observed for DLC-coated silicon tips and GaAs wedges. During electron field emission (EFE) at high electric fields the energy barriers caused by an sp3 phase between sp2 inclusions can be broken, resulting in the formation of conducting nanochannels between the semiconductor–DLC interface and the surface of the DLC film. At high current densities and the resulting local heating, the diamond-like sp3 phase transforms into a conducting graphite-like sp2 phase. As a result an electrical conducting nanostructured channel is formed in the DLC film. The diameter of the conducting nanochannel was estimated from the reduced threshold voltage after pre-breakdown conditioning to be in the range of 5–25 nm. The presence of this nanochannel in an insulating matrix leads to a local enhancement of the electric field and a reduced threshold voltage for EFE. Based on the observed features an efficient method of conducting nanochannel matrix formation in flat DLC films for improved EFE efficiency is proposed. It mainly uses a silicon tip array as an upper electrode in contact with the DLC film. The formation of nanochannels starts at the interface between the tips and the DLC film. This opens new possibilities of aligned and high-density conducting channel formation.

Influence of nanocrystalline structure of surface on boron gettering from silicon

The boron gettering from nanocrystallites of porous silicon with rate significantly greater in comparison with planar structure has been for the first time observed. As a probe of gettering process, the EPR signal intensity of P b center was used. It was established that strain effect during the surface oxidation stimulates strongly the boron gettering process from nanocrystallites bulk to SiO 2/Si interface of porous layer. Phenomenon of strong absorption of microwave radiation by porous layer after the sample was annealed in air at 400–500 °C was also detected at 37 GHz. This absorption is equivalent to high conductivity of layer ( ρ = 10 − 2 –10 − 3 Ω cm) and it is much higher (at least in 10 4 times) than of c-Si substrate. Removal of porous layer (thickness of ∼ 20 μm) from 400-μm substrate leads to where Q-factor of cavity is restored completely. The high conductivity of the porous layer comes mainly from conduction along the nanocrystalline grains, while SiO 2 layer essentially insulates their interface. One possible effect explanation is that high concentration of positive charge states formed in SiO 2 layer can lead to the creation of high density of free electrons in the SiO 2/Si interface with formation of conductive channels along each nanocrystallite. On the other hand, appreciable microwave absorption is not detected on 9.4 GHz (X-band); therefore, we cannot exclude the existence of quasi-resonant microwave absorption that is related with nanocrystallites sizes, i.e. quantum confinement and electrons tunneling between grains.

Metal nanoparticles on polymer surfaces: 4. Preparation and structure of colloidal gold films

The process of the enlargement of gold hydrosol nanoparticles adsorbed on the surfaces of glassy polymers (polystyrene and poly(2-vinylpyridine)) in mixed aqueous solution of chloroauric acid and hydroxy- lamine is studied. It is established that the character of this process depends on the intensity of metal-polymer interaction and the density of nanoparticle packing in an initial monolayer. At a high coverage of a poly(2- vinylpyridine) surface by "seeding" gold particles, their rather uniform growth is observed, whereas, at low cov- erage, the enlargement of adsorbed particles, as well as the nucleation and growth of new particles take place. At the same time, new Au nanoparticles are not formed on the polystyrene surface in the enlargement process, even at low coverages by preliminarily deposited "seeding" hydrosol particles. Adsorbed gold particles can also be enlarged after their preliminary incorporation into the polystyrene surface layer. Such an incorporation (par- tial embedding) is ensured by the annealing of a system at a temperature between "surface" ( ) and "bulk" glass transition temperatures. In this case, the value can be considerably decreased (up to room temperature) by the addition of small amounts of a homologue with a much lower molecular mass in the polystyrene matrix. Lateral conductivity of colloidal Au films formed on a poly(2-vinylpyridine) surface by the enlargement of adsorbed seeding particles is measured. According to these measurements, contacts providing the formation of conductive channels are formed in the process of nanoparticle enlargement. T g T g