Poole-Frenkel Type Research Articles

On high-field conduction and I–V measurements in quaternary Se–Te–In–Pb nano-chalcogenide thin films

In this paper, multi-component Se79-xTe15In6Pbx (x = 0, 1, 2, 4, 6, 8 ,and 10 at.wt.%) nano-chalcogenide alloys were prepared by using melt quenching technique and their electrical properties and sub-micron structural features have been reported and discussed in detail. Sharp characteristic peaks in x-ray diffraction (XRD) spectra signify nano-range of each alloy under investigation and validated by field emission scanning electron microscopy (FESEM). The thin films of investigated nano-chalcogenide alloys were deposited on glass substrates by thermal evaporation technique. In-depth analysis of DC conductivity for studied nano-chalcogenide alloys was carried out in the temperature range 303–333 K and in the voltage range 0–150 V, to realize the conduction mechanism. I–V characteristics illustrate ohmic behavior in the voltage range 0–50 V while at high voltage non-ohmic behavior is detected. Moreover, pre-exponential factor (σo) along with activation energy (ΔE) deduced from the variation of DC conductivity with temperature, which reveals that thermally assisted tunneling of charge carriers among the localized states occurs in band tails responsible for conduction in the analyzed systems. The detailed analysis indicates that SCLC theory is not appropriate to describe the conduction process, as ln(I/V) against V plots is almost straight, although slope of these plots does not reduce linearly with temperature. The linear relation among ln(I) against V1/2 demonstrates that the conduction mechanism is either Poole–Frenkel type or Schottky emission. The obtained outcomes demonstrate that Poole–Frenkel conduction mechanism is most appropriate to describe the conduction. The deduced values of activation energy are less than 0.98 eV, which once again validated the presence of Poole–Frenkel type conduction in the examined composition. The deduced results are further corroborated by photoluminescence studies.

Electrical properties of double-wall carbon nanotubes nanocomposite hydrogels

The electrical behaviour of nanocomposite hydrogels and especially hydrogels containing carbon nanotubes is generally poorly understood. In this paper, we investigate the influence of double-wall carbon nanotubes (DWCNT) content on the electrical properties of agarose/DWCNT nanocomposite hydrogels.These nanocomposite hydrogels are potential candidates as electrode materials for transdermal drug delivery by electropermeabilization. Both alternating current (AC) and direct current (DC) measurements at different voltage amplitudes were performed, as well impedance spectroscopy (1 Hz–1 MHz). Data suggest a non-linear dependence of the conduction phenomena vs the applied electric field. From the current-voltage characteristics, the nanocomposite conduction phenomenon is narrowed to two possible mechanisms, a Schottky type or a Poole-Frenkel type. These findings are the first step towards the understanding of the conduction phenomena in such complex nanocomposite structures, comprising DWCNT, water and the 3D polymeric network. The work described in this work is of much wider interest because this kind of nanocomposites may have many other applications, while the fundamental questions about their electrical conductivity remain universal.

Electrical Conduction Properties of Sb & Se Co-Doped PbTe Thin Films

Thenanocrystalline Sb & Se co-doped PbTe thin films were prepared by using an integrated physical-chemical approach by evaporating chemically synthesized Sb& Se co-doped PbTenanopowders on glass substrates. The prepared films were analysed by XRD, SEM, EDAX and their results were discussed in detail. The DC electrical conduction mechanism in Al/SbPbTeSe/Al thin film sandwich system at different temperatures (298–423 K) was studied and was found to be Poole-Frenkel type.

Conduction mechanisms in lanthanum manganite nanofibers

Lanthanum manganites are being constantly investigated for their interesting magnetic and transport properties. In this study, polycrystalline, single phase lanthanum manganite (LaMnO3) nanofibers were synthesized using an electrospinning technique, and characterized in terms of phase, microstructure, ac and dc electrical properties. X-ray diffraction and scanning electron microscopy revealed the formation of uniform composite nanofibers (average dia.∼ 600 nm) at 100 °C and polycrystalline single phase porous LaMnO3 nanofibers (average dia.∼ 400 nm) when calcined at 600 °C. Temperature dependent impedance and current-voltage (IV) characteristics of LaMnO3 nanofibers deposited on an interdigitated electrode type device, were investigated at temperatures ranging from 298 to 378 K. The observed decrease in the dielectric constant at high frequencies may be due to decrease in the contribution from dipolar polarization. Analysis of ac conductivity indicated thermally activated type conduction mechanism in these nanofibers which could be explained by a correlated barrier hopping (CBH) model. The observed decreasing trend in activation energy with increasing frequency also supported the CBH model. Furthermore, the analysis of log (J) vs. E0.5 curves at different temperatures showed a large difference of ~5.82 between the experimental and theoretical field lowering coefficient with an internal field enhancement factor of α2 = 33.91. This result excluded the possibility of Pool-Frenkel type conduction and the observed variation might be due to the presence of some localized electric fields. Further investigation of IV characteristics revealed that the ‘space-charge-limited current (SCLC) with traps’ may be the possible conduction mechanism.

Frequency Factor of the Semiempirical Model for the Radiation-Induced Conductivity in Spacecraft Polymers

We have extended the existing Rose–Fowler–Vaisberg model of the radiation-induced conductivity (RIC) in polymers by incorporating the latest ideas borrowed from theoretical and experimental results obtained in the field of electronic transport in molecularly doped polymers. These include the Poole–Frenkel-type field-dependent mobility and the arguments of the dipolar disorder formalism based on the Gaussian disorder model. Theory validation uses a new experimental technique based on the electron-gun technology allowing combined-induced conductivity and carrier mobility measurements in a typical molecularly doped polymer. Quasi-band and hopping theories of the carrier transport are critically discussed.

Mobility of holes and polarons in polyaniline films assessed by frequency-dependent impedance and charge extraction by linearly increasing voltage

Thin films of semiconducting polyaniline hydrochloride were deposited on glass or silicon substrates. Frequency spectra of impedance measured at several voltages display regions of holes and polarons contributions and enable to evaluate their mobility 2.7×10−5cm2V−1s−1 at the electric field of 1×105Vcm−1. Good agreement of the average value 2.4×10−5cm2V−1s−1 of holes mobility was obtained by another method-charge extraction by linearly increasing voltage (CELIV). A Poole-Frenkel type of electric field dependence of mobility assessed by frequency- dependent impedance points to trapping of the charge carriers. From the value of the work function measured by the Kelvin probe and from the position of HOMO level, the hole concentration 2.9×1018cm−3 was obtained. Using this concentration together with the mobility measured by the impedance spectroscopy and the CELIV method, the conductivity 1.2×10−5Scm−1 carried by holes was estimated. Our results widen the work dealing with the mixed electron and proton conductivity of polyaniline films in aqueous solution of acids published in the literature.

Effect of graphene-oxide on the microstructure and charge carrier transport of polyaniline nanocomposites under low applied electric fields

We report a study on the preparation and characterization of polyaniline-graphene oxide (PANI-GO-X) nanocomposites focusing in the study of its structure, microstructure, and correlations with electrical transport properties under low applied electric fields. X-ray diffraction analysis showed evidence of graphene oxide (GO) blending between polyaniline fibers, and according to small angle X-ray scattering, the fractal dimensionality was quasi-one dimensional for all nanocomposites. Confocal Raman spectroscopy revealed that the addition of GO leads to a notorious decrease of the polaron population of polyaniline. A significant increase in resistivity was observed for PANI-GO-X nanocomposites with respect to pure polyaniline, in agreement with the decrease in the polaron population. The electrical transport mechanism could be explained by an Arrhenius behavior at high temperatures (T > 255 K) and a broad transition with a logarithmic dependence of the activation energy with temperature for the low temperature regime (T < 255 K). Additionally, an increase in the hopping transport dimensionality from quasi-1D to quasi-3D was observed for X = 7% GO nanocomposites. The transport properties, studied after the polarization using low applied electric fields, revealed the modification of the electrical transport at low temperatures. In addition, the electrical transport of PANI-GO-X nanocomposites followed a typical ln(σ) vs E1/2 Poole-Frenkel type dependence in the presence of low applied electric fields. At low temperatures, X = 7% GO nanocomposites showed a notorious increase in the Poole-Frenkel slope at low applied electric fields with respect to pure polyaniline. The enhancement on the electric response using low applied electric fields (E ∼ 2 V/cm) could be of great interest for the development of new organic electronic devices.

Mechanical and electrical properties of RF magnetron sputter deposited amorphous silicon-rich silicon nitride thin films

Amorphous silicon nitride thin films in a thickness range of 40 to 500nm are deposited onto (100) silicon wafers using radio frequency magnetron sputter deposition. Analysis of variance techniques are used to determine which deposition parameter has a significant impact on the film properties. The biaxial stress of the layers is found to be compressive independent of the plasma chamber pressure levels and to increase with increasing plasma power. The chemical composition of the films is silicon-rich, resulting in an index of refraction (IOR) of 2.55 independent of deposition conditions. Both IOR and X-ray photoelectron spectroscopy measurements indicate a nitrogen to silicon ratio in the range of 0.71–0.85. The etch rates for HF wet chemical etching and for CF4:O2 reactive ion etching are found to be much higher compared to direct current sputter deposited silicon nitride films with only a weak dependency on the deposition conditions. Temperature dependent leakage current measurements using Au/Cr/SiNx/Si structures between 25 and 300°C show two dominating leakage current mechanisms: ohmic conduction dominates at low applied electric field values below 0.1 MV/cm and Poole–Frenkel type conduction above 0.3MV/cm. The extracted electrical parameters such as the activation energy or the barrier height are found to be nearly unaffected by the deposition parameters.

Electrical Transport Mechanism in Au Modified Nano Porous Silicon

Electrical Transport Mechanism in Au Modified Nano Porous Silicon Nano porous silicon (PS) has drawn considerable attention as a potential material for different sensors and other device applications. A metal modified PS can be of certain scientific and practical interest in order to control its high intrinsic resistivity and current transport through it to make it an efficient electronic device material. We present here a study on the electrical transport in Au-modified nano PS layers obtained by anodic etching of p-Si with in-situ Au nano-particle incorporation from the dissolved Au salt in the etching solution. The current-voltage characteristics of the resulting Au-modified PS layers have been studied with reference to the unmodified PS layers. The results suggest that direct tunnelling is the dominating charge transport mechanism in Au-modified PS layers. With the increasing Au concentration, electrical transport mechanism deviates slowly from the hopping type in low applied field, Poole-Frenkel type in moderate applied field and Fowler-Nordheim type in high applied field range as observed in case of un-modified PS layers.

Memory switching characteristics in amorphous ZnIn2Se4 thin films

ZnIn2Se4 thin films were deposited by flash evaporation of bulk material onto glass and pyrographite substrates under vacuum. The surface aspects of films have been studied by scanning electron microscopy. The dynamic and static current (I) versus voltage (V) characteristics of brass/pyrographite/ZnIn2Se4 thin film/platinum sandwiched devices for different film thicknesses (350–650 nm) and temperatures (313–353 K) were measured. The OFF state I–V characteristics of fabricated thin film devices indicated that the conduction mechanism is of the Poole–Frenkel type. In addition, a threshold memory switching mechanism from a high resistance OFF state to a low resistance ON state was observed in these thin film structures. The I–V characteristics of the memory device were studied at different annealed and ambient temperatures. The threshold voltage was found to increase linearly with film thickness and decrease exponentially with ambient temperature. The results obtained were discussed, and a thermal model for initiating the switching process in this device system could be indicated.

Light-induced conductance switching in azobenzene based near-percolated single wall carbon nanotube/polymer composites

Photochromic molecular switches are a class of organic molecules that allow a reversible control over molecular structure, dipole moment, or conductivity with light. Incorporating these chromophores into polymer composites provides the possibility to photoswitch intrinsic properties of these materials. Here we report reversible light-induced conductance switching of near-percolated single wall carbon nanotube/polymethylmethacrylate (SWCNT/PMMA) nanocomposites containing azobenzene derivatives that do not exhibit molecular conductance switching. Stable switching amplitudes up to 28% were achieved near the percolation threshold. The results suggest a Pool–Frenkel type conduction mechanism where the chromophores are an integral part of the conduction path.

In-situ post deposition annealing of lead-free ferroelectric thin films in oxygen rich atmosphere

Nano-sized ferroelectric Ba(Zr0.15Ti0.85)O3 (BZT) thin films were deposited on Pt/Ti/SiO2/Si substrates using pulsed LASER deposition technique. X-ray diffraction results highlighted the formation of single phase perovskite structure and Atomic Force Microscopy analyses revealed the uniform distribution of the nanometer sized grains. Enhanced properties were observed by in situ annealing of deposited BZT thin films in oxygen rich environment. The observed values of saturation polarization, remanent polarization are several orders higher than those reported in literature. The possible mechanisms for enhanced properties were discussed. Annealing in oxygen rich atmosphere decreased oxygen vacancies which in turn helped in realizing high dielectric constant and better tunability. A low leakage current density was observed in 125 nm thick BZT films. The underlying transport mechanism of leakage current was found to be of Poole–Frenkel type, an interface related phenomenon. Tunability as high as 62 % was observed corresponding to in situ annealed thin films.

Improvement of Magnetic and Dielectric Properties of BaFeO<sub>3−δ</sub> Thin Films by Sn Substitution

Magnetic and dielectric properties of Ba(Fe1¹xSnx)O3¹¤ (x = 0.5­0.9) epitaxial thin films on (001) SrTiO3 single crystalline substrates have been significantly improved. All of the Ba(Fe1¹xSnx)O3¹¤ (BFSO) thin films were ferromagnetic in nature, even at room temperature, regardless of the amount of Sn-substitution. For the x = 0.7 samples, the saturation magnetization had a maximum value of 0.224 ®B/f.u. at 300K. The ferromagnetic origin of BFSO was considered to be from 180° super-exchange coupling of Fe 4+ (d 4 )-O 2¹ -Fe 4+ (d 4 ). Electrical conduction of the x = 0.9 samples could be determined to be Poole-Frenkel type. In contrast, it was found that several electrical conduction mechanisms coexist for the x = 0.7 and x = 0.8 samples. The leakage current density of the x = 0.7 samples was about 1.04 © 10 ¹4 A/cm 2 at the positive bias electric field at 400kV/cm. The dielectric constant and dielectric loss were 120 and 0.06 even at 1MHz, respectively. [doi:10.2320/matertrans.M2014402]

Field and temperature dependent electron transport properties of random network single walled and multi walled carbon nanotubes

Field and temperature dependent electron transport properties of random network single walled carbon nanotubes (SWCNTs) and multi walled carbon nanotubes (MWCNTs) were investigated and compared. The electrical characterizations of drop casted CNT samples were done by two probe measurements by varying temperatures from 80 K to 300 K in the field region 1–400 V cm−1. The charge transport mechanisms at low (<3.5 V) and high (>3.5 V) fields were analyzed from measured I–V characteristic curves at various temperatures (<300 K) with respect to applied field. At low field, the ohmic behavior was observed and at high field the charge transport appears to be Poole–Frenkel type in both types of CNTs network. Electron–electron and electron–phonon scatterings in the localized defect states dominate in SWCNTs, whereas in MWCNTs the delocalization of charge carriers as well as the scattering centers is responsible due to the presence of inner shells. Because of the different nature of chirality in random network, the SWCNTs displayed lower conduction when compared to MWCNTs. The variation in Poole–Frenkel co-efficient (β) (SWCNTs-0.193 × 10−22; MWCNTs-0.089 07 × 10−22 J V1/2 cm−1/2), activation energy (SWCNTs-90 meV; MWCNTs-60 meV for field of ) and trap energy levels (SWCNTs-109 meV; MWCNTs-37 meV) are discussed for both SWCNTs and MWCNTs.

Electro-optical Properties of Non-stoichiometric Silicon Nitride Films for Photovoltaic Applications

The optical, electrical and electro-optical properties of metal-insulator-semiconductor (MIS) devices containing Si-rich silicon nitride (SRN) films are reported. The photoluminescence and optical absorption characterization evidences that optically active centers present a defective nature. As a consequence, a Poole-Frenkel type transport mechanism is considered, which is in agreement with the electrical results. The electro-optical performance of the devices has been found to be strongly modulated by the poly-Si electrode transmittance. After taking into account this contribution, EL emission has been studied, being the obtained spectra in accordance to the PL ones, which proofs that the same luminescent centers are also being excited electrically. The electrical response of the devices has been investigated under light excitation, showing induced photocurrent and photoconductivity, which makes SRN a good candidate material for photovoltaic applications.

Temperature dependant dielectric breakdown of sputter-deposited AlN thin films using a time-zero approach

In MEMS (micro electromechanical system) devices, piezoelectric aluminum nitride (AlN) thin films are commonly used as functional material for sensing and actuating purposes. Additionally, AlN features excellent dielectric properties as well as a high chemical and thermal stability, making it also a good choice for passivation purposes for microelectronic devices. With those aspects and current trends towards minimization in mind, the dielectric reliability of thin AlN films is of utmost importance for the realization of advanced device concepts. In this study, we present results on the transversal dielectric strength of 100 nm AlN thin films deposited by dc magnetron sputtering. The dielectric strength is measured using a time-zero approach, using a fast voltage ramp to stress the film up to the point of breakdown. The measurements are performed at different device temperatures. In order to achieve statistical significance, at least 12 measurements are performed for each environment parameter set and the results are analyzed using the Weibull approach. Basically, lower breakdown fields are observed with increasing temperatures up to 300 °C with a characteristic breakdown field strength E0 following the relationship $$\sqrt {E_{0} } \propto T$$E0źT as reported in literature for similar measurements performed at silicon nitride thin films. From the intersection of this linear behavior, the Poole---Frenkel (PF) barrier height źB is determined to 0.54 eV, which is reasonable for AlN thin films. The slope of this relation is similar to values reported for silicon nitride thin films. This allows an estimation of the breakdown field at higher temperatures by extrapolation. Leakage current measurements show a dominant PF type conduction mechanism, verifying the applicability of $$\sqrt {E_{0} } \propto T$$E0źT. No breakdown occurs in negative field direction, which is attributed to the metal---insulator---semiconductor configuration of the sample and hence, the presence of a depletion layer forming in the n-doped silicon and dominating the leakage current behavior.

Effect of temperature and post-deposition annealing on Schottky barrier characterization of Bromoindium phthalocyanine/aluminum interfaces

To investigate DC electrical properties and Schottky barrier characterization between BrInPc/Al interfaces, some thin films of BrInPc in sandwich form were prepared with Al electrodes. J–V characterization showed ohmic behavior at lower voltages upto 0.3V followed by Schottky emission conduction mechanism at higher voltages. In the Schottky region two different slopes in the plot of ln (J) against V1/2 were observed and two different values of Schottky barrier height was determined for these regions. To investigate the effect of temperature on Schottky barrier behavior between BrInPc and aluminum interface, we studied the J–V characteristics of devices at the temperature range of 298–373K. By increasing the temperature, the width of Schottky depletion region decreased and the Schottky barrier height increased, and at temperatures higher than 333K the dominant conduction mechanism changed to Poole–Frenkel type. For annealed samples at 373K and 423K, the Schottky barrier height increased as the result of thermal annealing and increasing annealing temperature. The width of the Schottky depletion region decreased by annealing and increasing the annealing temperature.

The analysis of leakage current in MIS Au/SiO2/n-GaAs at room temperature

The aim of this study is to determine the reverse-bias leakage current conduction mechanisms in Au/SiO2/n-GaAs metal-insulator-semiconductor type Schottky contacts. Reverse-bias current-voltage measurements (I–V) were performed at room temperature. The using of leakage current values in SiO2 at electric fields of 1.46–3.53 MV/cm, ln(J/E) vs. \(\sqrt E \) graph showed good linearity. Rom this plot, dielectric constant of SiO2 was calculated as 3.7 and this value is perfect agreement with 3.9 which is value of SiO2 dielectric constant. This indicates, Poole-Frenkel type emission mechanism is dominant in this field region. On the other hand, electric fields between 0.06–0.73 and 0.79–1.45 MV/cm, dominant leakage current mechanisms were found as ohmic type conduction and space charge limited conduction, respectively.

Correlation between I (current)-V (voltage) characteristics and thermally stimulated depolarization current of Mn-doped BaTiO3 multilayer ceramic capacitor

I (Current)-V (voltage) characteristics of Mn-doped BaTiO3 multilayer ceramic capacitor were investigated. The increase of Mn concentration reduced the slope of current increase with increasing dc field and enhanced the dc field to the abrupt increase of current and thereby breakdown. Ohmic, Schottky, and Poole-Frenkel type conduction was successively observed with the increase of dc field in all specimens. The region of Child's law did not appear before breakdown, which suggests space charge limited (SCL) conduction with field enhanced trap barrier lowering. A peak of thermally stimulated depolarization current (TSDC) beyond 300 °C associated with trapped space charges appeared, which increased by the increase of Mn concentration. In addition, the peak intensity increased to a maximum value and then decreased again with increasing polarization dc field in TSDC measurement. Its activation energies were around 1.9 eV for all specimens, which is very close to the energy level for the ionization of Mn3+/Mn2+. These results experimentally demonstrate the role of the variable valence acceptor Mn as trapping center for injected charges, and the occurrence of field enhanced trap charge release, and therefore, the conduction mechanism can be described by the SCL model incorporating the Poole-Frenkel effect.

Size-manipulation, compaction and electrical properties of barium manganite nanorods synthesized via the CHM method

A composite-hydroxide mediated method was employed to synthesize barium manganite nanorods. Diameter, surface smoothness and uniformity of these nanorods were optimized by varying reaction temperature and reaction time. The rods with an average diameter of 200nm and length of 1–1.5μm were obtained at optimum conditions of 200°C/48h. The dielectric study of these rods reveals that they have higher value of dielectric constant at lower frequencies which was attributed to the interfacial and rotational type polarizations. Similarly, the increase in dielectric constant with temperature was attributed to the thermal activation of such polarizations. Furthermore, the analysis of ln(J) vs. E1/2 characteristics in the temperature range of 300–400K shows that possible operative conduction mechanism was of Poole–Frenkel type. The value of βexp was found to be 4.85 times greater than the expected theoretical value of field lowering coefficient with an internal field enhancement factor of α2=23.5. This high value of βexp may be due to some localized electric fields existing inside the sample.