Hopping Transport Process Research Articles

Analysis of Low-Frequency 1/f Noise Characteristics for MoTe2 Ambipolar Field-Effect Transistors.

Low-frequency electronic noise is an important parameter used for the electronic and sensing applications of transistors. Here, we performed a systematic study on the low-frequency noise mechanism for both p-channel and n-channel MoTe2 field-effect transistors (FET) at different temperatures, finding that low-frequency noise for both p-type and n-type conduction in MoTe2 devices come from the variable range hopping (VRH) transport process where carrier number fluctuations (CNF) occur. This process results in the broad distribution of the waiting time of the carriers between successive hops, causing the noise to increase as the temperature decreases. Moreover, we found the noise magnitude for p-type MoTe2 FET hardly changed after exposure to the ambient conditions, whereas for n-FET, the magnitude increased by nearly one order. These noise characteristics may provide useful guidelines for developing high-performance electronics based on the emerging transition metal dichalcogenides.

Generation-recombination in disordered organic semiconductor: Application to the characterization of traps

The presence of traps in organic semiconductor based electronic devices affects considerably their performances and their stability. The Shockley-Read-Hall (SRH) model is generally used to extract the trap parameters from the experimental results. In this paper, we propose to adapt the SRH formalism to disordered organic semiconductors by considering a hopping transport process and Gaussian distributions for both mobile and trapped carriers. The model is used to extract multiple trap parameters from charge based Deep Level Transient Spectroscopy (Q-DLTS) spectrum. Calculation of the charge transients are given in detail. The model predicts that the activation energy of the trap should not follow an Arrhenius plot on large temperature ranges. Also, the charge transients are no longer exponential when considering Gaussian trap distributions, enlarging the Q-DLTS peaks. The model fits the Q-DLTS spectra measured on organic diodes with a limited number of trap contributions with a good agreement. It is found that an increase of the material rate of disorder reduces the extracted trap energy distances to the LUMO but has no influence on the extracted trap distribution widths. This work shows the importance of considering the specific properties of organic materials to study their properties and their trap distributions. • A trap emission model has been developed for OSCs including hopping transport and Gaussian DOS for free carriers and traps. • Model predictions: the OSC disorder, i.e., to the width of the intrinsic Gaussian DOS, strongly influences the determined trap energies. • The emission model is validated by extracting trap parameters in organic diodes from Q-DLTS measurements.

Experimental and theoretical investingation on the effect of a spacer addition in zinc phthalocyanine-based anthracene properties

phthalocyanine-based anthracene (AnPc) and phthalocyanine-based triazole anthracene (AnPc-Tr) were characterized to emphasize the effect of adding spacer existing in AnPc-Tr on electronic, optical and electrical properties of AnPc. The structures of both molecules were confirmed by infrared spectroscopy, the obtained chart was compared to the theoretical calculation chart, Electronic properties were studied using the TD-DFT approach. The obtained results are harmonious with those obtained experimentally. Experimental and theoretical UV–Visible spectra showed six maxima, corresponding to the π-π* transitions of anthracene, B band, Q band and the dimerisation phenomenon. Current-Voltage characteristics showed typical diode trend where conduction is governed by space charge limited current without distributing traps. The dynamic AC electrical transport process was studied in function of temperature. It exhibited a hopping transport process according to an equivalent electrical circuit based on a resistance (Rs) in series with a resistance (Rp) which is parallel to a Constant Phase Element (CPE).

Optical, dielectric and oxygen sensing properties of an anthracene and carbazole based π-conjugated Schiff base

Abstract An anthracene and carbazole based π-conjugated Schiff base (An2Cb) was characterized to emphasize its optical, dielectric and gas sensing properties. UV–Visible absorption spectrum shows three main maxima at 279, 340 and 420 nm with an optical gap of 2.33 eV determined according to Tauc model. Computational analysis by Time Dependent Density Functional Theory (TD-DFT) was used to obtain a UV–Visible absorption spectrum found in good agreement with the experimental chart. The 3D charge distribution was theoretically determined through B3LYP/6–311 + G* basis set. Current-Voltage characteristics of the device are typical to diode behaviour with a trap controlled space charge limited current transfer mechanism. The AC dynamic electrical transport process was studied in function of temperature. It displays a hopping transport process with a thermal activation energy of 0.88 eV. The device is assimilated to an equivalent electrical circuit based on a resistance Rs in series with a resistance Rp which is parallel to a constant phase element CPE. Gas sensing performance was tested. It shows a selective and reversible oxygen sensor at low concentration at room temperature, and an improvement in its sensitivity was observed with increasing temperature until 333 K.

Investigation on crystalline structure and dielectric relaxation behaviors of hot pressed poly(vinylidene fluoride) film

The dielectric relaxation behaviors of hot pressed poly(vinylidene fluoride) (PVDF) film have been studied using dielectric spectroscopy in the frequency domain from 20 Hz to 5 MHz at temperatures between 20 °C and 200 °C. Crystalline/amorphous interphase is suggested with methods of FTIR, XRD, and DSC. Frequency and temperature dependence of dielectric spectroscopy reveals the relaxation behavior and structural dynamics of the samples, and three types of relaxation processes are suggested, αAc relaxation process contributed by the hopping transport process near the periphery of conduction band or valence zones at Fermi energy, αc relaxation process related to the structure change of crystal lattice trapped dipoles in crystalline regions, and αa relaxation process arising from segmental dipole rearrangement of interphases in amorphous regions. Cole-Cole and Havriliak-Negami experimental equations were utilized to analyze these relaxation processes, and differences of Arrhenius parameters for αAc and αc relaxation processes obtained from Cole-Cole and Havriliak-Negami equations were discussed in detail. Activity energy of different relaxation processes obtained from Arrhenius equation and VFT equation indicates non-single thermal activation mechanism for hot pressed PVDF film.

Synthesis and investigation on optical and electrical properties of a triflato iron porphyrin: Application as an optical BPA sensor

In this study, we first describe the synthesis of the triflato(5,10,15,20-tetrakis(4-metoxyphenyl) porphyrinato) iron(III) complex with the formula [FeIII(TMPP)(SO3CF3)] (P1). This species were characterized by proton NMR and IR. Then optical and electrical properties of P1 thin films have been investigated. The triflato iron porphyrin film exhibit an absorption spectrum with a resolved electronic structure in the UV–vis range and the energy gap was determined by the Tauc method. Electrical properties of the ITO/P1/Al structure have been investigated by I–V characteristics and impedance spectroscopy measurements. The conduction is governed by space-charge-limited current (SCLC) mechanism. The impedance spectroscopy study showed a hopping transport process. Its interaction with Bisphenol A (BPA) was then investigated by UV–vis spectroscopy in a sensing window of 10−9–10−4M. The iron-porphyrin species shows a sensitive response to BPA, which induces an absorbance change to the Soret band. The interaction mechanism of P1 with BPA involved a coordination interaction.

Investigation of structural, optical and electrical properties of a new cobalt phthalocyanine thin films with potential applications in perchlorate sensor

Optical, structural and electrical properties of new cobalt phthalocyanine derivatives (Co(II)Pc-AP) thin films have been investigated. The Raman spectroscopy study shows the different vibrations bands corresponding to the metallophthalocyanine. The cobalt-phthalocyanine derivative films exhibit absorption spectra with a resolved electronic structure in the UV–vis range and the energy gap was determined by the Tauc method. Electrical properties of the ITO/Co(II)Pc-AP/Al structure have been investigated by I–V characteristics and impedance spectroscopy measurements. The conduction is governed by space-charge-limited current (SCLC) mechanism. The impedance spectroscopy study showed a hopping transport process, which is a typical behavior of disordered materials. The electrical characteristics of the devices were modeled by a single parallel resistor and capacitor network in series with a resistance.

Optical, electrical and sensing properties of β-ketoimine calix[4]arene thin films

Optical, electrical and ion sensing properties of β-ketoimine calix[4]arene thin films have been investigated. These calix[4]arene derivative films exhibit absorption spectra with a resolved electronic structure in the UV–vis and the energy gap was found to be 3.65eV. Electrical properties of ITO/β-ketoimine calix[4]arene/Al devices have been investigated by I–V characteristics and impedance spectroscopy measurements. The conduction is governed by space-charge-limited current (SCLC) mechanism. The impedance spectroscopy study showed a hopping transport process, a typical behavior of disordered materials. The device was modeled by a single parallel resistor and capacitor network in series with a resistance.The β-ketoimine calix[4]arene was used for the conception of the novel optical chemical sensor and the detection of Cu2+ ions was monitored by UV–visible spectroscopy. The linear dynamic range for the determination of Cu2+ has been 10−5–10−3.7M with a detection limit of 10−5M. The characteristics of this optode such as regeneration, repeatability, reproducibility, short-term stability, life time and ion selectivity have been discussed.

Dielectric Properties and Conductivity of Iron Oxide-Barium Titanate Composites

Sintered composites of iron oxide plus barium titanate in form of core-shell structures are investigated by impedance spectroscopy. The measurements were performed in the frequency and temperature ranges 20 Hz to 1 MHz and 150 K to 510 K, respectively. The composites exhibit strong low-frequency dispersion of electrical modulus and dielectric permittivity which is caused by different mechanisms of electron transfer at low and high temperatures. A hopping transport process with activation energy of 0.22 eV for exchange between Fe2+ and Fe3+ was tentatively assigned to reign below room temperature. It gradually changes to a conducting mechanism with activation energy of 0.45 eV above 345 K.

The Meyer‐Neldel rule in ambipolar organic thin film transistors based on F16CuPc/α6T pn heterojunction

AbstractThe temperature dependence of field‐effect mobility is investigated in ambipolar organic thin film transistors based on F16CuPc/α6T pn heterojunction. The results show that the thermally activated ambipolar mobilities obey the Meyer‐Neldel (MN) rule and activation energy (EA) steadily decreases with increasing gate voltage. At high temperatures (> 170 K), strong activated behaviour is observed, where higher EA are attributed to defects, disorder, and grain boundaries in the polycrystalline film since the transport has been described by a hopping transport process between localized trap states. Blow 170 K, much weaker activated behaviour is observed, where the charge transport is dominated by shallow traps with much lower EA. The MN energy and isokinetic temperature for both p‐channel and n‐channel are surprisingly similar to these of organic unipolar devices. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Brownian motion field dependent mobility theory of hopping transport process

A Brownian motion theory of hopping mobility has been formulated based on the one-dimensional hopping conduction model between localized states. The probability of hopping in the direction of the applied electric field and the duration of the hop between the localized states are assumed to be field dependent and thermally activated. The general form of the Brownian motion mobility model fitted well with the time of flight results measured in the low field regime and for most part of the mobility data extracted from the space charge limited conduction applied to tris-(8-oxyquinolato) aluminum (Alq3) in higher field regime. The Brownian motion model can be modified in order to account for the dependence of charge mobility in the higher electric field regime and at higher temperatures. The variation of charge mobility with applied electric field was fitted using the Brownian motion theory. The hopping time and the hopping distance were extracted from the fit and found to be about 3ps and 0.9nm, respectively for Alq3 at room temperature.

AC conductance and impedance spectroscopy of polycrystalline diamond films

Impedance measurements were performed in the frequency and temperature domain to infer the bulk and grain boundary transport of freestanding polycrystalline diamond films. Analyzed samples appear like insulators up to a threshold temperature where leaking becomes evident. Depending on deposition conditions and thickness, frequency independent conduction channels add to AC ones. The real component of conductivity, measured at different temperatures, can be reduced to a master curve according to a single hopping transport process. Therefore, also the real and imaginary component of impedance, measured at different temperatures in the 2–10 7 Hz frequency range, can be reduced to a master curve and plotted vs. a reduced angular frequency showing a circuit structure more complex than a simple Voigt element. The Nyquist plot is formed by two different lobes that have been interpreted as related to grain and grain boundary dielectric contributions. However, only a single dielectric loss peak is achieved by data elaboration. Dissimilarities among materials deposited by using different techniques are also evidenced and discussed.

Electrical transport properties of mono-dispersed single-wall carbon nanotubes formed in channels of zeolite crystal

We report the electric transport properties of mono-sized carbon nanotubes fabricated in the channels of AlPO 4-5 zeolite crystals. From X-ray data, the carbon nanotubes are shown to have a diameter of ∼5 Å. Due to the large curvature, LDA calculations show the nanotubes to have a finite density-of-states at the Fermi level for all possible nanotubes consistent with the measured diameter. However, transport measurements show the temperature dependence to be non-metallic. This could be due to electron localization caused by imperfections and impurities in the nanotubes. The observed temperature dependence of zero-field conductance, ln(σ)∼1/ T , supports the 1D hopping transport process.

Stoichiometry Effects on the Electrical Conductivity of Lithium-Manganese Spinels

The electrical conductivity of lithium-manganese spinels is analyzed in samples with different starting Li cationic fraction x. Li-rich spinels, resulting from Li-Mn substitution around the stoi-chiometric value (x= 0.333), show conductivity values higher than that observed in stoichiometric LiMn2O4 . Besides, a conductivity drop, associated with a structural phase transition at about 290 K in LiMn2O4 , progressively disappears by decreasing x, while it is absent in Li-rich samples. Stoi-chiometry effects on the concentration of charge carriers and on the available sites for the hopping transport process are evaluated, as well as the effects due to coexisting insulating phases. The role of the Jahn-Teller effect on the conductivity behaviour of stoichiometric and Li-poor spinels is also considered.

Analysis of single polaron hopping in ac conductivity of amorphous Ge20SbxSe80−x glasses

The results for the ac conductivity over temperature range (298–500 K) and frequency range (102–107 Hz) for the Ge-Sb-Se glasses with different compositions are presented. These glasses exhibit a strong temperature dependence of ac conductivity and of its frequency exponent s. The results are discussed in the light of the correlated barrier hopping model for ac conduction. All the features observed are interpreted by taking into account the contribution of both single polarons and bipolarons. It is shown that the single polarons dominate the hopping transport process in the measured temperature range. The effect of the addition of Sb is also discussed in terms of the present model. The energy levels and densities of the defect for Ge-Sb-Se glasses are deduced from the comparison between the theoretical and experimental results.

Random walk theory of a trap-controlled hopping transport process

A random walk theory of hopping motion in the presence of a periodic distribution of traps is presented. The solution of the continuous-time random walk equations is exact and valid for arbitrary intersite interactions and trap concentration. The treatment is shown to be equivalent to an exact solution of the master equation for this trapping problem. These interactions can be a general function of electric field and are not restricted to nearest neighbors. In particular, with the inclusion of trap-to-trap interactions, as well as trap-to-host interactions, an exact treatment of the change from one hopping channel to another has been obtained. The trap-modulated propagator has been derived in terms of a type of Green's function that is introduced. The results are specialized to spatial moments of the propagator, from which expressions for the drift velocity and diffusion coefficient are obtained. Numerical results for the drift velocity are presented and shown to account for the change in hopping channels in recent transport measurements in mixed molecularly doped polymers.