Electrical properties of electrochemically prepared thin polytetrahydrofuran films II: Characterization under D.C. conditions

The electrical characteristics of electrochemically prepared polytetrahydrofuran (poly-THF) films were studied under d.c. conditions. At low fields (?10⁵ V cm^?1) a phonon-assisted hopping conduction occurs in poly-THF films 2000 Å thick. The low temperature dependence of the conductivity (T< 243 K) is related to interchain hopping, whereas the high temperature dependence of the conductivity (>320 K) is ascribed to trap hopping. At high fields (E>10⁵ V cm^?1) and high temperatures (T>320 K) Schottky conduction is observed and the Schottky barrier height depends both on the electrode metal and on the field direction. At temperatures below 320 K, Poole-Frenkel conduction occurs and the current intensity is independent of the nature of the electrodes and of the field direction. At very low temperatures (T<100 K) and high fields (E>10⁶ V cm^?1), thermally assisted tunnel effect conduction (δj∞T²) is observed. These changes in conduction with the temperature and the field are ascribed to changes in the polymer structure.     

Electrical and optical properties of r.f.-sputtered amorphous V2O5-CaO-MoO3 films

Amorphous films of the V₂O₅-CaO-MoO₃ system are fabricated by r.f.-sputtering and the d.c. conductivity and optical properties are studied. The conductivity of 1200–1400 nm thick amorphous V₂O₅-CaO-MoO₃ films with different film compositions ranges from 4.7 × 10^–4 to 1.1 S cm^–1 at 458 K. The films are n-type semiconducting. The conduction of the films is attributed to adiabatic small polaron hopping and is primarily due to hopping between V⁴⁺ and V⁵⁺ ions. The films are optically transparent in the visible range. The optical band gap energy is evaluated to be between 2.90 and 2.39 eV. The Urbach tail analysis gives the width of localized states between 0.40 and 0.58 eV. A feasibility study reveals the films to be applicable as transparent film thermistors.     

On electrical transport in CoWO4 single crystals

CoWO₄ is a p-type semiconductor with conductivity in the range 10^–8 to 10^–3 ohm^–1 cm^–1. The solid exhibits extrinsic behaviour up to 750 K and intrinsic behaviour above 750 K. In this solid conduction by hopping of small polarons seems to be dominating up to 750 K and above 750 K conduction becomes band type. The energy band gap of the solid has been found to be 2.80 eV. The values of mobility and the mean free path are estimated. The variation of dielectric constant with temperature has been attributed to changes in atomic and ionic polarization and space charge polarization of thermally generated charge carriers.     

Influence of iodine on the electrical properties of nickel phthalocyanines thin film devices

The electrical properties of pure Nickel Phthalocyanine (Ni Pc) thin films and iodine doped Ni Pc thin films with Gold and Aluminium electrode sandwiched devices have been investigated. The electrical properties and the various electrical parameters of the pure and iodine-doped films have been estimated and compared from the analysis of the current density characteristics. From our study we find that the iodine doping enhanced the electrical conductivity compared to pure Nickel Phthalocyanine and the conduction mechanism is much improved with the iodine doping. Interestingly these films after iodine doping showed remarkably increased electrical conductivity nearly ten times that of pure Phthalocyanine. This may be accounted for by the decrease in the metal-metal bond distance. At low voltages the film shows an ohmic conduction whereas at higher voltage levels the conduction is dominated by space charged limited conduction. Further the reverse conduction mechanisms have also been investigated for this sandwiched device. From the current limitations in the reverse condition a strong rectifying behaviour is evident.     

Experimental studies on substituted copper phthalocyanine Langmuir-Blodgett films

Using the Langmuir-Blodgett (LB) technique, novel copper bis(3-hydroxypropyloxy-hexaisopentyloxy)phthalocyanine molecules were deposited on quartz substrates. It is argued on the basis of pressure-area isotherms that films are deposited as monolayers. Optical absorption spectroscopy suggests that the deposited films were uniform. A.c. measurements were performed on LB films in a planar configuration at different temperatures. The power-law dependence of the conductivity on frequency is explained in terms of electronic conduction through hopping over a coulombic barrier 0.66 eV high. The Debye relaxation time is estimated to be of the order 811 µs at room temperature.     

The electronic properties of metal complexed poly(3-alkylthiophene) films

Dielectric spectroscopy has been used to extract the AC and DC electrical conductivities of P3MT complexed with the paramagnetic ion Fe²⁺, and these were compared to data for uncomplexed P3MT in the frequency range of 1 Hz-10 MHz. The polymers were nominally undoped; however, the presence of residual Fe³⁺ ions from the polymerisation reaction renders the materials p-type semiconductors. Measurements were carried out over the temperature range 123-323 K. An analysis of the frequency-dependent complex conductivity together with the DC conductivity can be used to elucidate whether free charge conduction, charge hopping or quantum tunnelling is the dominant conduction mechanism. The results were compared to the predictions of a variety of theoretical conduction models. It was found that the presence of the Fe²⁺ ions produced greater long-range order at low temperatures by complexing between the sulphur heteroatoms of four 3-methylthiophene monomer units. This provided an additional barrier hopping conduction mechanism at low temperatures (183-243 K), and the hopping occurred over a barrier height within the range 0.45-0.6 eV.     

Electrical conductivity and optical absorption in flash-evaporated CuInTe2 thin films

The flash evaporation technique produces thin films of stoichiometric CuInTe₂. From an analysis of the optical absorption spectra of the thin films, the band gap of CuInTe₂ is found to be 1.03 eV. The effect of the background absorption on the absorption spectrum is found to be negligible. The electrical conductivity of these films is limited by hopping conduction in the temperature range 77–200 K. Acceptor levels at 100 meV above the valence band maximum were observed and attributed to copper vacancies.     

Conductance and polarisability of C60 films

Thin films of C60 deposited in vacuum are studied using current-voltage (I-V) measurements and atomic force microscopy (AFM). In situ electrical measurements give an average resistivity of ca. 30 Mn cm for the as-deposited films at room temperature. The I-V dependences are found to correspond to ohmic behaviour but they have a hysteresis shape attributed to remnant polarisation due to the domain structure of the films. AFM images show a grainy surface morphology for the deposited C60. Temperature dependent measurements in the range 290-365 K provide evidence for a variable range hopping mechanism of conductance with an activation energy of 0.8-1.0 eV. With further temperature increase the C60 films restructure leading to an increase in grain size and a change of the electrical properties with I-V dependences showing Schottky barrier formation. The effect of oxygen on the conductance of the C60 films under their exposure to an ambient atmosphere is considered and discussed.     

Sn doping effects on properties of ZnO thin films deposited by RF magnetron sputtering using a powder target

In the present study, tin doped ZnO thin films (ZnO:Sn) at different contents (0–3 wt%) were deposited onto glass substrates by RF magnetron sputtering using a powder compacted target at room temperature. The effect of Sn concentration on the structural, optical and electrical properties of the ZnO:Sn thin films were investigated. The X-ray diffraction analysis shows that the pure ZnO thin film exhibits a strong intensity of the (002) peak indicating a preferential orientation along the c-axis. For Sn doped ZnO thin films, there is a change in the orientation from the (002) plane to the (101) one. The undoped ZnO thin films have transmittance 85% in the visible range and slightly increased for 0.5 wt% of Sn, while it get decreased with further increasing the Sn doping concentration. The optical band gap energy get increased with increasing the doping concentration. Moreover, the electrical conductivity and conduction mechanism are also studied by impedance spectroscopy in the frequency range of 1KHz–13 MHz at various temperatures (633–743 K). The AC conductivity in ZnO thin films increased with angular frequency. The frequency exponent S decreases with increasing temperature. Such behavior suggests that the correlated barrier hopping (CBH) model may be suitable to explain the conduction mechanism in ZnO thin films. The activation energy values calculated from angular frequency and DC conductivity are in good agreement confirming that the conduction mechanism is thermally activated by hopping between localized states.     

Frequency dependence of the conductivity of amorphous germanium films between 20 Hz and 26 GHz

Conductivity measurements on amorphous Ge films in the frequency range 20 Hz–26 GHz are described. Above 100 MHz the frequency dependence of the conductivity at room temperature satisfied the power law σ∝ω^s with s=0.45-0.7. No saturation of the conductivity was observed. When the Ge films were doped with 0.1–1 at. % Sb the conductivity was frequency independent up to 26 GHz. A.c. hopping conduction seems to be compatible with the experimental results.     

Pyroelectric and conduction properties of Z-type calix[4] acid Langmuir-Blodgett films

Non-centrosymmetric Z-type Langmuir-Blodgett (LB) films were prepared by transferring a calix[4] acid monolayer from a subphase of Millipore water (18 Momegacm(-1)) onto aluminised glass substrates. Electrical measurements were performed at room temperature on LB films with a sandwich structure comprising a 50 nm thick thermally evaporated aluminium film. A pyroelectric figure of merit of 2.23 microC m(-2)K(-1) is reported for this LB system. The low voltage value of conductivity is 1.82 x 10(-12) Sm(-1). The electrode-limited Schottky effect is responsible for the conduction mechanism at a relatively high field due to the dc bias and the barrier potential height is determined to be 1.72 eV. The ac conductance for both samples shows a typical power law dependence with a value of approximately 0.86 for the exponent.     

Electrical conductivity and optical properties of ZnO films annealed in hydrogen atmosphere after chemical vapor deposition

The d.c. electrical conductivity and optical properties of polycrystalline zinc oxide films (220–450 nm thick) annealed in hydrogen after chemical vapor deposition are investigated. A minimum film resistivity after the annealing gives 0.31 cm for the film as-deposited at a substrate temperature of 823 K. From the temperature dependence of conductivity, band conduction is confirmed for the films at temperatures above 250 K. The effect of grain-boundary scattering is due to thermionic emission of electrons over grain boundary barriers. At temperatures below 250 K, variable-range hopping transport is found to be dominant. The films are transparent in the wavelength range 400 to 1000 nm and have sharp ultraviolet absorption edges at 380 nm. The absorption edge analysis reveals the optical band gap energy for the films to be 3.18–3.23 eV. The Urbach tail analysis gives the width of localized states E_e=0.06-0.14eV.     

The substrate temperature dependent electrical properties of titanium dioxide thin films

Titanium dioxide thin films were obtained by a dc sputtering technique onto heated glass substrates. The relationship between the substrate temperature and the electrical properties of the films was investigated. Electrical resistivity measurements showed that three types of conduction channels contribute to conduction mechanism in the temperature range of 13–320 K. The temperature dependence of electrical resistivity between 150 and 320 K indicated that electrical conduction in the films was controlled by potential barriers caused by depletion of carriers at grain boundaries. The conduction mechanism of the films was shifted from grain boundary scattering dominated band conduction to the nearest neighbor hopping conduction at temperatures between 55 and 150 K. Below 55 K, the temperature dependence of electrical resistivity shows variable range hopping conduction. The correlation between the substrate temperature and resistivity behavior is discussed by analyzing the physical plausibility of the hopping parameters and material properties derived by applying different conduction models. With these analyses, various electrical parameters of the present samples such as barrier height, donor concentration, density of states at the Fermi level, acceptor concentration and compensation ratio were determined. Their values as a function of substrate temperature were compared.     

Temperature dependence of conduction mechanism of ZnO and Co-doped ZnO thin films

ZnO and Co-doped ZnO thin films have been deposited on the sapphire substrate by ultrasonic assisted chemical vapor deposition technique. The films were annealed in vacuum at 450 °C. All the films were highly c-axis orientated and contained no impurity phase. The temperature dependence of the electrical conductivity has been measured in order to identify the dominant conduction mechanism. In the higher temperature region the dominance of thermally activated band conduction was observed whereas in the low temperature region the hopping conduction was found to dominate. The hopping conduction mechanism in the lower temperature range in the film was Mott's Variable Range Hopping and not the Nearest Neighbor Hopping. The temperature region, where hopping conduction was dominant found to increase by Co doping in ZnO film. The localization length was found to be larger in the Co-doped ZnO film.     

Charge transport mechanism and photovoltaic behaviour of undoped and I2 doped tris (1,10 phenanthroline) iron (II) complex (TPFe) thin film devices

Tris (1,10 phenanthroline) iron (II) or Fe (Phen)²⁺ ₃, a metal-to-ligand charge transfer (MLCT) type complex (TPFe), was employed in the form of thin films, for the fabrication of Schottky diodes, Al/ TPFe/ITO, where ITO is indium tin oxide. The effect of iodine doping on the electrical behaviour has been emphasized. The diodes exhibit a rectification effect which improves on iodine doping. The diodes can be classified as MIS Schottky diodes with a graded dopant profile. The current-voltage (J-V ), and capacitance-voltage (C-V ) characteristics, the photoaction spectra of the devices and the absorption spectra of the complex, reveal that both doped and undoped complexes behave as a p-type organic semiconductor which form a Schottky barrier with Al and an ohmic contact with ITO. Various electrical and photovoltaic parameters were determined from the detailed analysis of J-V and C-V characteristics and these are discussed in detail. The effect of I_{_2} doping on the rectification and photovoltaic properties is also discussed.     

Electric conduction in copper gallium telluride thin films

P type copper gallium telluride (CuGaTe₂) synthesized from the elements was used as a source for the preparation of films by flash evaporation. Films of different thicknesses were prepared and their electrical conductivity was measured in the temperature range 100–300 K. While in the case of thin films the low temperature conduction could be explained by a variable range hopping process, for thicker films the conduction process could be attributed to thermally assisted tunnelling through the grain boundary barrier. The high temperature conductivity data fits well to the process of transport by thermionic emission over the grain boundaries.     

Electrical properties of NiO films obtained by high-temperature oxidation of nickel

Nickel oxide thin films are formed by high-temperature oxidation of nickel foils at 973 K, and are characterized using X-ray diffraction and scanning electron microscopy indicating the formation of a single NiO phase whose thickness grows following a parabolic law. The electrical properties of the formed films are examined by impedance spectroscopy at room temperature; and by measuring direct current (DC) and alternating current (AC) conductivities and dielectric properties at different temperatures. At room temperature, the conductivity is about 4 orders of magnitude higher than that of NiO single crystals. Below 200 K, DC conductivity displays a slight increase with increasing temperature indicating conduction by thermal activation hopping of small polarons. Above 250 K, large polaron conduction associated with holes in the 2p band of O²⁻ with activation energy of about 0.4 eV is observed. Frequency as well as temperature dependencies of the AC conductivity and dielectric constant exhibit trends usually observed in carrier dominated dielectrics.     

D.c. conduction studies on thermally evaporated neodymium oxide thin films

Aluminium-neodymium oxide-aluminium thin film capacitors have been prepared by thermal evaporation and the d.c. conduction properties of these films have been studied. The thicknesses of the films have been determined by a multiple beam interferometer. The current-voltage power-law dependence showed that the conduction in these films is space-charge limited. The linear dependence of the current density on the square root of the applied field confirmed the exponential trap distribution. The trap density has been found to be of the order of 10²⁶ m^–3. It has also been observed that the Schottky type of conduction is predominant in the high-field region and the height of the Schottky barrier has been determined. It is seen that the conduction mechanism is an activated process with the activation energy decreasing with increasing field.     

Frequency-dependent conductivity in unannealed thin films of As0.40Se0.40Te0.20

Direct current (d.c.) and alternating current (a.c.) conductivity measurements have been performed on unannealed amorphous As_0.40Se_0.40Te_0.20 thin films. The d.c. measurements were performed at temperatures between 143 K and 343 K. The d.c. behaviour of the samples indicates different hopping conduction mechanisms between 143 K-210 K and 210 K-343 K. The a.c. measurements were performed at temperatures between 143 K and 300 K and at frequencies between 110 Hz and 1 MHz. The a.c. conductivity of the films is well represented by the form Cω^s where C and s are found to be temperature dependent parameters. The data are found to fit the correlated barrier hopping model, especially at low temperatures. The comparison of a.c. data with the unified theory of the extended pair approximation shows that the results fit to the quasi universal law predicted by this model, however, qualitative calculations give unreasonable values for the decay parameter α and the exponent l.     

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

D.C. conduction in MIM sandwich structures based on SiO _x /V₂O₅ as a dielectric has been investigated before electroforming. The electrodes make a blocking contact to the dielectric with a barrier height, ₀, 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 E 0.15eV. Hopping conduction has also been found to be dominant above room temperature in thin films having a high density of trapping states.