Free-band Conduction Research Articles

Electrochemical and dielectric behavior in poly(vinyl alcohol)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) blend for energy storage applications

Herein, dielectric studies of solvent-cast poly(vinyl alcohol) (PVA) with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) freestanding films are conducted in the frequency range of 100 Hz–1 MHz and temperature ranging from 303 to 453 K. An insulator to semiconducting transition is observed for PVA/PEDOT:PSS blend above 413 K. Existence of two types of conduction mechanism, hopping between nearest neighbor at low-temperature region (303–383 K) and free-band conduction at high temperatures (above 413 K) within PVA/PEDOT:PSS films under the influence of applied field, is observed and supported by electric moduli formalisms. An increase in dielectric constant and a significant reduction in $$\tan \delta$$ values are observed under high temperature and low frequency with the addition of PEDOT:PSS. Greater than 100% Coulombic efficiency and 82% specific capacitance retention at 5000 cycles are observed for 7.5 vol% PEDOT:PSS in PVA working electrode for supercapacitor. Thus, the fabricated films find potential applications in the field of flexible conventional capacitors and as working electrodes for supercapacitor.

Dielectric, conduction and interface properties of Au/Pc/p-Si Schottky barrier diode

The dielectric properties and the ability of binuclear zinc(II) phthalocyanine of clamshell type compound in passivating silicon (Si) surfaces is studied by fabricating metal–insulator–semiconductor (MIS) capacitors. The frequency and temperature dependence of the dielectric constant were discussed in the light of Koops model and hopping conduction mechanism. A detailed study of the effect of temperature on the ac conductivity of MIS structure at the temperatures between 300K and 460K was carried out. Based on the existing theories of ac conduction, it has been concluded that for low frequency region the dominant conduction mechanism in the sample is quantum mechanical tunneling at all temperatures, whereas for intermediate frequency region multihopping process is the dominant conduction mechanism. At higher frequencies, the behavior and the values of index s reveal a free band conduction mechanism. Interface properties of the fabricated MIS structure were investigated by means of conductance–voltage (GM–VG) and the combination of low frequency and high frequency capacitance–voltage (CM–VG) measurements at various fixed frequencies. The values of Dit obtained from conductance and high–low frequency capacitance measurements are 4.25×1011eV−1cm−2 and 4.90×1011eV−1cm−2, respectively. This indicates the consistency of both the methods. The observed peaks in the GM–VG characteristics indicated that the losses are predominantly due to interface states.

Synthesis, interface (Au/M2Pc2/p-Si), electrochemical and electrocatalytic properties of novel ball-type phthalocyanines

The phthalodinitrile derivative (3) was prepared by the reaction of 4,4'-(octahydro-4,7-methano-5H-inden-5-ylidene)bisphenol (1) and 4-nitrophthalonitrile (2) with dry DMF as the solvent in the presence of the base K(2)CO(3) by the method of nucleophilic substitution of an activated nitro group in an aromatic ring. The template reaction of 3 with the corresponding metal salts gave the novel bi-nuclear ball-type metallophthalocyanines, MPcs {M = Co (4), Cu (5), Zn (6)}. Newly synthesized compounds were characterized by elemental analysis, UV-vis, FT-IR (ATR), MALDI-TOF mass and (1)H-NMR spectroscopy techniques. The electronic spectra exhibit an intense π→π* transition of characteristic Q and B bands of the Pc core. The dielectric properties and interface between the spin coated films of 4-6 and a p-type silicon substrate have been studied by fabricating metal-insulator-semiconductor capacitors. The results indicated that the frequency dependence of the dielectric permittivity, ε'(ω), exhibits non-Debye type relaxation for all the temperatures investigated. The ac conductivity results indicated that the conduction mechanism can be explained by a hopping model at low temperatures (<430 K) and a free band conduction mechanism at high temperatures (≥430 K). The density of interface state calculations on these novel compounds showed that the combination of Au/4/p-Si is a promising structure with a high dielectric constant and a low interface trap density suitable for metal-oxide-semiconductor devices. The electrochemical properties of the Pc complexes were examined by cyclic voltammetry, differential voltammetry and controlled potential coulometry on platinum in non-aqueous media. The complexes showed ring-based and/or metal-based mixed-valence behaviours as a result of the remarkable interaction between the two Pc rings and/or metal centres. The mixed-valence splitting values for the complexes suggested that the mixed valence species are considerably stable. The Vulcan XC-72(VC)/Nafion(Nf)/4 modified glassy carbon electrode showed much a higher catalytic performance towards oxygen reduction than those of VC/Nf/5 and VC/Nf/6 modified ones.

Optical transparency and electrical conductivity of nonstoichiometric ultrathin InxOy films

The effect of thickness and composition on the electrical conductivity and optical transparency, mainly in the infrared, of ultrathin InxOy films was studied. InxOy films 35–470 Å thick with oxygen atomic fractions of ∼0.3 and ∼0.5 were prepared via dc magnetron sputtering. All films were polycrystalline, consisting of only the cubic bixbiyte phase of In2O3. The average grain size of the films increased from 30 to 95 nm as the film thickness increased. The weak dependence of the electrical conductivity on the frequency and the low activation energies for conduction, a few hundredths of an eV, provided an indication that free band conduction was the primary electrical conduction mechanism in the case of all ultrathin InxOy films. It was found that introducing a high degree of nonstoichiometry in the form of oxygen deficiency did not help improve the electrical conductivity, since not all vacancies contributed two free electrons for conduction and due to impurity scattering. The optical nature of these films, studied mainly by ellipsometry, was found to be dependent on the film’s composition and thickness. In the infrared, the dielectric function of all InxOy films was consistent with the Drude model, inferring that the transparency loss in this region was a result of free charge carriers. In the visible however, InxOy films under 170 Å, which had an oxygen atomic fraction of ∼0.5, were modeled by extending the Drude model to the shorter wavelengths. Films over 170 Å, with the same composition, were modeled using the Cauchy dispersion model, meaning that no absorption was measured. These results indicate that, optically, under specific compositions, ultrathin InxOy films undergo a transition from metalliclike behavior to dielectric behavior with increasing film thickness. Using a figure of merit approach, it was determined that a nonstoichiometric 230 Å thick InxOy film, with an oxygen atomic fraction of ∼0.3, had the best combination of conductivity and transparency, namely, absorption of less than 20% in the infrared, about 10% in the visible, and electrical resistance of only 230 Ω at 20 kHz. Such a film may be classified as a highly conductive transparent oxide even in the infrared.

Infrared transparency and electrical conductivity of non-stoichiometric InxOy films

Abstract In an effort to achieve both high infrared transparency and electrical conductivity, In x O y films having different oxygen atomic fractions, ranging from 0.27 to 0.6 were prepared. From AC electrical measurements it was determined that conductivity of In x O y films, having oxygen atomic fraction near 0.6, is governed by the hopping conduction mechanism via energy states located in the band gap. Conductivity of In x O y films having non-stoichiometric compositions was found to be governed by the free band conduction mechanism. The conduction activation energy was decreased from about 0.47 eV to about 0.02 eV as the deviation of the oxygen atomic fraction from the stoichiometric value of 0.6 was increased. The dielectric function of the films was determined by applying the Drude–Lorentz model to ellipsometric measurements in the infrared and visible wavelengths. In the visible range, the major source for optical transmission loss is interband absorption, which was modeled by the Lorentz model. In the infrared range, optical absorption was measured and attributed to the presence of free charge carriers according to the Drude model. Fitting the model to the optical measurements required a correction factor, which was correlated with the films polarizability. In order to determine the optimal tradeoff between optical transparency in the infrared and electrical conductivity, which were found to be affected mainly by the oxygen concentration in the films, a figure of merit parameter was established. It was found that by introducing non-stoichiometry in the form of oxygen deficiency, the electrical conductivity was improved by as much as two orders of magnitude while the infrared transparency was decreased by no more than 30% with respect to stoichiometric In 2 O 3 films.

DC CONDUCTION MECHANISMS IN AMORPHOUS THIN FILMS OF MIXED OXIDES In2O3–SnO2 SYSTEM DEPOSITED BY CO-EVAPORATION

A discussion of dc conduction mechanisms in thermally co-evaporated amorphous thin films of Al – In 2 O 3– SnO 2– Al structure is presented. Composition (in molar %), film thickness, substrate temperature, and post deposition annealing have profound effects on the electrical properties of the films. The effects of temperature on the I – V characteristics and electrical conductivity of Al – In 2 O 3– SnO 2– Al structure are also reported. The values of dielectric constants estimated by capacitance measurements suggest that high-field conduction mechanism is predominantly of Poole–Frenkel type. At low temperature and low field the electron hopping process dominates but at higher temperature the conduction takes place by transport in the extended states (free-band conduction). The transition from hopping to free band conduction is due to overlapping of localized levels and the free band. The increase in the formation of ionized donors with increase in temperature during electrical measurements indicates that electronic part of the conductivity is higher than the ionic part. The initial increase in conductivity with increase in Sn content in In 2 O 3 lattice is caused by the Sn atom substitution of In atom, giving out one extra electron. The decrease in electrical conductivity above the critical Sn content (10 mol % SnO 2) is caused by the defects formed by Sn atoms, which act as carrier traps rather than electron donors. The increase in electrical conductivity with film thickness is caused by the increase in free carriers density, which is generated by oxygen vacancy acting as two electrons donor. The increase in conductivity with substrate temperature and annealing is due either to the severe deficiency of oxygen, which deteriorates the film properties and reduces the mobility of the carriers or to the diffusion of Sn atoms from interstitial locations into the In cation sites and formation of indium species of lower valence state so that the In 3+ oxidation state may be changed to the In 2+ oxidation state.

Influence of temperature and frequency on the electrical conductivity and the dielectric properties of nickel phthalocyanine

Abstract The temperature and frequency dependence of the AC conductivity σac(ω), the dielectric constant e′(ω) and the dielectric loss e″(ω) were studied on pellet samples of nickel phthalocyanine (NiPc) with evaporated ohmic Au electrodes in the frequency range from 20 kHz to 10 MHz and within the temperature range from 303 to 600 K. The DC conductivity σdc has also been measured in the considered range of temperature. Two temperatures – induced changes in the thermal activation energy ΔE have been observed. For T ⩽ 435 K, ΔE1 = 0.322 eV; for 435 K ⩽ T ⩽ 525 K, ΔE2 = 0.497 eV; for T ⩾ 525 K, ΔE3 = 0.703 eV. These variations in the activation energy were attributed to a partial phase transformation from α- to β-NiPc phase and as a change from extrinsic to intrinsic conduction mechanism. The AC conductivity σac(ω) showed temperature independence and it has been found to vary with angular frequency as ωs with the index s ⩽ 1 suggesting a hopping conduction mechanism at low temperatures and high frequency. At higher temperatures and lower frequencies a free-band conduction mechanism was observed. Both the dielectric constant e′(ω) and the dielectric loss e″(ω) increased with temperature and decreased with frequency in the investigated ranges. Such characteristics, reveal that the tested organic NiPc exists in the form of molecular dipoles which remain frozen at low temperature, whereas at higher temperatures, when the dipoles attaining rotational freedom, the dielectric constant was found to decrease with increasing frequency and increase with increasing temperature. The increase in the dielectric loss e″(ω) with increasing temperature at low frequencies can be understood in terms of an increase in DC conductivity.

The study of electronic conduction in amorphous thin films of Al-In2O3-Al structure deposited by thermal evaporation

A discussion of electronic conduction in amorphous thin films of Al-In2O3-Al structure is presented. Particular attention is given to the question of film thickness, substrate temperature during deposition and post-deposition annealing, since these conditions are known to have a profound effect on the structure and electrical properties of the films. The effects of temperature on the V-I characteristics and effects of frequency on conductivity and capacitance of the Al-In2O3-Al structure are also reported. Activation energies for conduction processes are estimated and the results are discussed in terms of the hopping model. The conduction at higher temperature is seemingly a contact-limited, i.e. Schottky type process, so a transition from hopping to free-band conduction takes place. The capacitance decreases with the rise of frequency and the lowering of temperature. The values of dielectric constants are estimated and the results are discussed in terms of Schottky type of conduction. The increase in conductivity with the increase in temperature during measurements of electrical properties, film thickness, substrate temperature and post deposition annealing is reported and results are discussed in terms of current theory.

Effect of electrode material on AC electrical conductivity of organic zinc phthalocyanine semiconducting thin films

The AC electrical conductivity of thermally evaporated zinc phthalocyanine, ZnPc, semiconducting thin films was measured in the temperature range of 180–430K and frequency between 0.1 and 20kHz. Gold and aluminum contact electrodes were employed and both showed ohmic contacts. In both cases, the conductivity showed strong dependence on both temperature and frequency. Such dependence is related to the relevant temperature and frequency range under consideration. However, for both electrode materials, the AC conductivity σ(ω) is found to vary with angular frequency, ω, as ωs, with the index s≤1 suggesting a dominant hopping conduction process at low temperatures (<250K) and high frequency. At higher temperatures and lower frequencies free band conduction was observed in samples with Al-electrodes. The AC conductivity in the present work did not increase monotonically with temperature. For samples with Al-electrodes, σ decreased with temperature above 330K, while for samples with Au-electrodes it decreased with temperature above 430K. The former decrease in σ was attributed to oxygen exhaustion, while the latter was ascribed to α–β-phase transition.

Alternating current conduction properties of thermally evaporated α-nickel phthalocyanine thin films: Effects of oxygen doping and thermal annealing

The ac conduction properties of thermally evaporated films of α-nickel phthalocyanine (α-NiPc) were studied in situ and ex situ employing symmetric gold ohmic electrodes in the frequency range of 20−106 Hz at various temperature regimes. ac conductivity was identified to be via a hopping-type mechanism in the lower temperature region and via a free-band conduction in the high temperature region. Upon exposure of the films to dry air, the low frequency ac conductivity was found to increase by 2 orders of magnitude, which was attributed to oxygen absorption within NiPc. The doping effect was partially reversed by thermal annealing of the films under high vacuum. Measurements on the dependence of capacitance and loss tangent (tanδ) on frequency were consistent and quantitatively explained by invoking an equivalent circuit model. Oxygen doping was found to increase the low frequency capacitance of NiPc. The phenomenon was understood in terms of reduction in the value of device internal resistance induced by oxygen absorption.

Conductivity and dielectric properties of silicon nitride thin films prepared by RF magnetron sputtering using nitrogen gas

Silicon nitride is an important material in very-large-scale integration fabrication and processing. Recent work on films prepared by radio frequency magnetron sputtering using nitrogen gas have shown that the relative permittivity is typically 6.3 and that aluminium forms an ohmic contact to this material. Under direct current (DC) bias the films exhibited space-charge-limited conductivity with a bulk trap density of the order of 2×10 24 m −3. In the present work alternating current electrical measurements were made on identical samples as a function of frequency and temperature. Conductivity appeared to be by hopping at lower temperatures, giving way to a free-band conduction process with activation energy of typically 0.44 eV at higher temperatures. Over a limited range of frequency and temperature the model of Elliott was applicable, and yielded a value of 2.87×10 23 m −3 for the density of localised states, in reasonable agreement with our estimate of the trap density from DC measurements. As in the DC measurements capacitance followed a geometric relationship with relative permittivity 6.3, and showed a moderate decrease with increasing frequency and an increase with increasing temperature, tending towards a constant value at high frequencies and low temperatures. The loss tangent showed a minimum in its frequency dependence, which appeared to shift to higher frequencies with increasing temperature. The measurements are consistent with the model of Goswami and Goswami for samples having ohmic contacts, and are typical of results obtained on other insulating thin film structures.

Frequency dependence of electronic conduction parameters in evaporated thin films of cobalt phthalocyanine

A.c. dark current measurements were taken on purified cobalt phthalocyanine (CoPc) thin films in the frequency range 102−2 × 104 Hz and in the temperature range 163–433 K. Measurements revealed that the a.c. conductivity σ(ω) varied as ωs, where the index s was variable and always less than unity, generally increasing with increasing frequency and decreasing with increasing temperature. A.c. activation energy measurements at fixed frequencies showed that at temperatures below approximately 230 K the conductivity is frequency dependent with very low activation energy, while at higher temperatures a frequency independent activation energy of approximately 0.54 eV was observed. These results were interpreted in terms of hopping through a band of localized states at lower temperatures and by free band conduction at higher temperatures. Measurements of capacitance and loss tangent showed a well defined decrease with increasing frequency and an increase with increasing temperature. An interpretation is presented, based on existing theory, for the case of a thermally activated process when using ohmic contacts. A comparison of samples before and after heat treatment at 450 K showed a decrease in both conductivity and capacitance. This reduction was ascribed to desorption of oxygen, which acts as an acceptor level, as previously inferred from d.c. measurements.

Characterisation of a thin-film/thick-film strain gauge sensor on stainless steel

Thin-film strain gauges of the mixed oxide Bi 2O 3-V 2O 5 have been deposited on insulated stainless steel substrates. The thin-film gauges are insulated from the steel by application of dielectric layers printed using thick-film techniques. Various strain gauge parameters are analysed, including linearity, hysteresis, repeatability and temperature effects. Comparisons have also been made with similar gauges deposited on glass. It has been observed that the thin-film strain sensor deposited on steel performs much better than the one deposited on glass in terms of maximum applicable strain, hysteresis and linearity. However, the film on glass is much more sensitive than the one deposited on steel. The lower-sensitivity of the sensor on steel is attributed to the increased thickness of the deposited resistor and the poorer surface characteristics of the thick-film dielectric layers. The d.c. conductivity variation at high temperatures has also been studied and it has been observed that freeband conduction takes place at these temperatures.

The correlation of the electrical and optical properties of thin films of V2O5-Bi2O3

The DC conductivity variation with temperatures for V2O5-Bi2O3 thin films has been measured and the associated activation energies have been calculated. From these measurements it was concluded that the dominant conduction mechanism at low temperatures was polaron hopping with a transition to freeband conduction at higher temperatures. The refractive index was found to be composition-dependent, and a relationship was found between the conductivity and Ee, the bandwidth of the localized states. The changes in the refractive index, conductivity and Ee were all found to be consistent with a proposed increase in the concentration of oxygen vacancies. The carrier concentration and mobility were estimated as a function of composition and were found to be inversely related. Finally a tentative model for the energy band structure of the composite is proposed.

Hall effect and magnetoresistance in Au/SiO x thin films

Measurements of the temperature dependence in the range 300–520 K of the Hall coefficient and magnetoresistance of thin films of SiO containing 2 and 5 at.% Au are reported. The derived carrier density values are combined with earlier measurements of electrical conductivity to determine the temperature variation in mobility; the mobility increases with increasing temperature in the lower part of the range, as expected for a carrier hopping conduction mechanism, but decreases with increasing temperature in the higher range in accordance with the expectations of free-band conduction. Variations in the parameters with magnetic induction up to 130 mT are discussed. The free-carrier density is substantially constant with temperature, as expected for an amorphous or highly disordered material.

Some electrical properties of mixed amorphous thin films of germanium and silicon monoxide before electroforming

The co-evaporated SiO x -Ge system was studied. Thin-film MIM sandwich structures were deposited by vacuum evaporation at a pressure of ≈ 10−4 Pa and were measured at a pressure of 10−3 Pa. The conductivity at low temperature and under d.c. fields has been found to be governed by a combination of an electronic hopping process and free-band conduction. At fields greater than 2 × 106 V m−1, it is concluded that the conduction process is governed by the Poole-Frenkel effect. Comparison with earlier results on SiO x -GeO2 films showed small differences in activation energy for conduction for samples of broadly similar overall composition.

D.C. conduction in thin films of SiOx/V2O5 co-evaporated assemblies prior to electroforming

D.C. conduction in MIM sandwich structures based on SiOx/V2O5 as a dielectric has been investigated before electroforming. The electrodes make a blocking contact to the dielectric with a barrier height,φ0, dependent on the type of electrode material used. The voltage-current characteristic is studied between 165 and 413 K. Below room temperature and at low fields hopping conduction is dominant; at intermediate temperatures a transition to free band conduction is observed. At higher temperatures and fields the conduction is enhanced by Schottky barrier lowering associated with an activation energy ΔE 0.15eV. Hopping conduction has also been found to be dominant above room temperature in thin films having a high density of trapping states.