Electrical conduction mechanism in solution grown thin polyvinylchloride (PVC) films

The electrical transport behaviour of thin polyvinylchloride (PVC) films deposited by the solution growth technique has been investigated. The current transport in PVC films of 2500 Å thickness at temperatures below 250 K is ascribed to hopping mechanisms. The weak temperature dependence of the conductivity is attributed to interchain hopping, whereas the strong temperature dependence is attributed to the trap hopping process. The conductivity of PVC films is increased on doping with iodine. This is interpreted on the basis of the formation of charge transfer complexes in the film. The activation energy for conduction increases from 0.7 to 1.22 eV at 315 K and decreases from 2.2 to 0.8 eV at 375 K on doping PVC films with iodine (0.7 g of iodine per 100 ml PVC solution). At high (≧5 × 10⁴ V cm^-1) fields and at higher (≧350 K) temperatures, the observed conduction behaviour can be described by the Schottky emission mechanism. The height of the Schottky barrier is found to depend on the type of the metal electrode and the direction of the current. The barrier height decreases with increasing iodine concentration in the PVC films.     

Electrical properties of thin PbTe films on Si substrates

An attempt was made to reduce the carrier concentration in thin PbTe films on Si substrates by optimizing deposition conditions. A modified hot-wall method was used for reproducible growth ofp-type films with 5 × 10¹⁵ < p(77 K) < 5 × 10¹⁷ cm^-3 andn-type films with 3 × 10¹⁵ < n(77 K) < 5 × 10¹⁶ cm^-3. The IR irradiation was found to have a significant effect on the temperature variation of film resistance. The activation energy of the IR-sensitivity centers was determined to be 0.11 ± 0.005 eV at room temperature and 0.18 ± 0.005 eV between 150 and 180 K.     

Structural and electrical properties of thermally evaporated 1,4-Bis-(2-hydroxyethylamino)-9,10-anthraquinone films

Nowadays applications of organic materials as the active element in microelectronic devices is broadly investigated. In the present paper, 1,4-Bis-(2-hydroxyethylamino)-9,10-anthraquinone compound has been synthesized. The films of this compound has been deposited by thermal evaporation technique on to the glass substrates which are kept at different temperatures in vacuum of ～1.33 × 10^?3 Pa. Scanning electron microscope (SEM) shows that the films are polycrystalline in nature. Wire like crystal of width 0.204 μm is observed at higher substrate temperature (333 K). The conduction in these films is ohmic in nature. The electrical conductivity and carrier concentration of these films increases with increase in substrate temperature as well as absolute temperature of the films. The analysis of optical absorption measurements on the films indicates that the optical band gap lies in the range 1.48–1.55 eV.     

Phénomènes physiques au cours de la formation de couches minces de AgO

A method of preparation of silver oxide thin films by oxidation of silver in an oxygen plasma is described, in which the composition of the films is controlled during the oxidation by means of a quartz microbalance. The films initially contain a large excess of oxygen (28%) which is partially eliminated during the stabilisation period in vacuum, the final stoichiometric ratio O/Ag being 1.12±0.05. The desorption and diffusion of oxygen during this period have been studied by measuring the variation of the mass of the layer and its electrical resistivity. The first of these measurements allows the diffusion coefficient of oxygen in AgO to be calculated, giving 5.3x10^-10 cm² day^-1.The activation energy has been determined from the measurements of the resistivity variation with temperature between 110° and 300°K; its mean value is about 0.1 eV.AgO films are found to be relatively unstable and decompose in a few days to form Ag₂O.     

The optical properties of AgInTe2 thin films

The optical absorption in electron-beam-evaporated AgInTe₂ thin films was studied in the energy range 0.5–2 eV. AgInTe₂ was found to be a direct gap semiconductor with a room temperature gap of 1.03±0.01 eV. Another direct transition observed at 1.04±0.01 eV was ascribed to an optical transition from the crystal-field-split valence band to the conduction band minimum. A third direct allowed transition from the spin-orbit-split valence band to the conduction band was identified at 1.77±0.03 eV. An estimate of the p-d hybridization of the uppermost valence bands yields a value of about 15%.     

Superconductivity,normal state electrical and structural properties of Nb/Al2O3 cermet films

Nb/Al₂O₃ cermet films were obtained by electron-gun evaporation from two independent sources. Films containing 71 wt.% Nb (particle size about 20 Å) have an activated type of conduction mechanism due to the hopping of the charge carriers between localized states. The activation energy decreases continously from 8.8 x 10^?3 eV (300°K) to 1 x 10^?6 eV (1.9°K), which indicates that the localized states lie close to the Fermi level of the system. These states can be associated with small Nb particles (below the resolution power of the electron microscope—10 Å) in the alumina matrix. Films containing more than 85wt.%Nb (mean particle size 32 Å) have a metallic-type conduction mechanism and show superconductive properties. The transition temperature is reduced to 4.5—5.2°K compared with the Nb bulk value of 9.2°K; this is presumably due to a decrease of the electronic interaction in the surface layer of the Nb particles.     

Electrical conduction in solution-grown films of commercial grade polystyrene

The electrical conduction of solution-grown polystyrene films of thickness about 15 μm was investigated in the field and temperature ranges of about 6 × 10⁵ to 3 × 10⁷ V m^?1 and 300–400 K respectively. The results show that the Poole-Frenkel (PF) mechanism as modified by Jonscher and Ansari is the dominant mechanism in the present case. The modified PF barrier is calculated to be about 1.94 × 10^?19 J (1.21 eV).     

Electrical transport studies on heavy rare-earth tungstates

This paper reports the measurement of thermoelectric-power (S) at different temperatures (800–1100 K) and electrical conductivity (σ) as a function of electric field strength, time, ac signal frequency and temperature (650–1200 K) for pressed pellets of heavy rare-earth tungstates (HRET) with a general formula RE₂(WO₄)₃ [where RE = Tb, Dy, Ho, Er, Tm and Yb]. These tungstates are typical insulating compounds with room-temperature σ value less than 10^?10 ohm^?1 cm^?1 and become semiconductors at elevated temperature with σ values of the order of 10^?5 ohm^?1 cm^?1 at 1200 K. The S vs T^?1 and log σ vs T^?1 plots are linear, but a change in the slope of straight lines occurs at a temperature (T_B) which lies between 900–1025 K for different tungstates. These break temperatures are the same for both S and σ plots. It has been found that HRET are mixed ionic-electronic conductors. Above T_B the electronic conduction dominates over the ionic conduction, but below T_B both become comparable. The electronic conduction above T_B is intrinsic with large polaron holes as the principal charge carriers; they conduct via a band mechanism. The energy band gap lies in the range 3.2 to 4.0 eV, and the charge carrier mobility in the range 3.8×10^?2 to 2.5 cm²/V-Sec for the different tungstates. Below T_B both electronic and ionic conduction are extrinsic. The electrons conduct via a thermally activated hopping mechanism with an activation energy lying in the range 0.87 to 1.30 eV, and the holes via a diffusion process with an activation energy lying in the range 0.88 to 1.23 eV for the different tungstates.     

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.     

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 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.     

Conduction mechanism in anthracene as a function of temperature

Conduction mechanism in anthracene single crystal grown by Bridgman method was carried out. The investigations consisted of dark- and photo-current variation with respect to (i) applied electric field and (ii) temperature. The applied electric field ranged from 0·5 to 2·5 kV/cm and the temperature range was between 300 K and 450 K. Photo and dark current variations with temperature indicate, based on activation energy determination, that a band model can be applied to the conduction process. The band gap is calculated to be 1·6 eV. The band model consists of a recombination centre 0·37 eV above the valence band edge and a trap level 0·55 eV below the conduction band edge to which electrons are first thermo-optically excited and then they are thermally excited into the conduction band.     

Electrical and optical properties of TiN thin films

TiN thin films have been grown by reactive magnetron sputtering. It has been shown that an Ohmic contact to TiN thin-film can be made from indium. The TiN thin films have been shown to be n-type semiconductors with a carrier concentration of 2.88 × 10²² cm^?3 and resistivity of ρ = 0.4 Ω cm at room temperature. The activation energy for conduction in the TiN films at temperatures in the range 295 K < T < 420 K is 0.15 eV. The optical properties of the TiN thin films have been investigated. The material of the TiN thin films has been shown to be a direct gap semiconductor with a band gap E _g = 3.4 eV.     

Electrode-limited conduction in amorphous boron films I. D.C. measurements

The conduction mechanism in amorphous boron films sandwiched between plane gold electrodes has been investigated. The films, in the thickness range 3000–8000 Å, were obtained by the vacuum evaporation technique. The conduction process was examined in the temperature range 260–403 K with applied electric fields ranging from 10³ to 10⁵ V cm^-1. The temperature and field dependences of the current flowing through the insulator are understandable in terms of the thermal emission of charge carriers over the Schottky barrier of a blocking contact. The theory of this type of electrode-limited conduction, derived by Simmons, predicts a current-voltage relationship of the type $\text{J～}\text{exp}\text{(β V}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$. A good fit of the experimental J-V curves to this relationship is obtained. Furthermore, a set of parameters has been calculated: the height and the transmission coefficient of the potential barrier are respectively 0.43 eV and 10^-13, and the donor density within the insulator is equal to about 10²¹ cm^-3.     

Effect of thermal annealing on some electrical properties and optical band gap of vacuum evaporated Se65Ga30In5 thin films

Electrical properties and optical band gap of amorphous Se₆₅Ga₃₀In₅ thin films, which were thermally evaporated onto chemically cleaned glass substrates, have been studied before and after thermal annealing at temperatures above the glass transition temperature and below the crystallization temperature. The I-V characteristics, which were recorded in the temperature range (200-300 K), were obtained at different voltages and exhibit an ohmic and non-ohmic behavior at low (0-5 V) and high (5-18 V) voltages, respectively, for annealed and as-prepared films. Analysis of the experimental data in the high voltage range confirms the presence of space charge limited conduction (SCLC) for annealed and as-prepared films. The dependence of DC conductivity on temperature in the low voltage region shows two types of conduction channels: The first is in the range 270-300 K and the other at the lower temperature range (200-270 K). The conduction in the first region is due to thermally activated process, while in the other is due variable range hopping (VRH) of charge carriers in the band tails of the localized states. After annealing, the conductivity has been found to increase but the activation energy decreases. This is attributed to rupturing of Se-In weak bonds and formation of Se-Ga strong bonds. This process changes the concentration of defects in the films which in turn decreases the density of states N(E_F) as predicted by Mott's VRH model. Analysis of the absorption coefficient of annealed and as-prepared films, in the wavelength range 300-700 nm, reveals the presence of parabolic densities of states at the edges of both valence and conduction bands in the studied films. The optical band gap (E_g) was obtained through the use of Tauc's relation and is found to decrease with annealing temperature.     

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.     

Conduction processes in the Ta(-)/Ta2O5-electrolyte system

Thin tantalum oxide (Ta₂O₅) films of controlled thickness d (from 1 to 80 nm) were obtained by the anodization of tantalum in a phosphate buffer at pH 7. Experimental results for the d.c. conduction characteristics of amorphous highly imperfect Ta₂O₅ films are presented. Semiquantitative agreement is observed between the experiment and theory. For electrodes covered with very thin films, direct electron exchange between the metal and the electrolyte predominates. In the high field region, for films thicker than 30 nm, the conduction mechanism is space charge limited. The effective electron mobility is 10^-13 m² V^-1 s^-1 for ε = 13. In the low field region, the data can best be interpreted in terms of an ohmic mechanism with a resistivity of approximately 5 × 10¹⁴ Ω cm.The band gap for these films was determined from the spectral distribution of the photoconductivity and amounts to 4 eV. The flat-band potential is approximately - 1 V vs. the Ag/AgCl electrode.     

Conduction mechanism of metal-free phthalocyanine single crystals as a function of temperature

Metal-free phthalocyanine (H₂Pc) single crystals grown by vacuum sublimation were investigated for their conductivity (both in dark and light). The investigations consisted of dark- and photo-current variations with (i) applied electric field and (ii) temperature. The applied electric field ranged from 0·8 kV/cm to 6 kV/cm. The temperature range was from 300°K to around 570°K. The crystals were found to be photoconductive. Based on activation energies calculated from photoconductivity due to temperature dependence, an energy level scheme for H₂Pc single crystals is proposed. The model consists of two trapping levels within the forbidden gap — one at 0·4 eV below the conduction band edge from which electrons are thermally excited into the conduction band and the other acts as recombination centre at 0·3 eV above the valence band edge. The band gap is calculated to be 1·4 eV. Comparative study of the proposed model with that of earlier investigations on the same crystals of the H₂Pc is in good agreement, thereby indicating that H₂Pc is thermally stable even at relatively higher temperature as semiconductor.     

Al and Ni co-doped ZnO films with room temperature ferromagnetism,low resistivity and high transparence

Zn_0.91Al_0.07Ni_0.02O and Zn_0.90Al_0.05Ni_0.05O films of about 250 nm thick were deposited on glass substrates at 300 K by co-sputtering with ZnO:Al and Ni targets. The films were annealed in vacuum at 673 K for 2 h and then cooled down to room temperature under a magnetic field of 4.8 × 10⁴ A m⁻¹ applied along the film plane. After this process the films showed room temperature ferromagnetism, a resistivity of about 2 × 10⁻³ Ω cm and an average transmittance of 75% in the visible wavelength range. The films have a wurtzite structure with the c-axis orientation in the film growing direction and consist of thin columnar grains perpendicular to the substrate. A temperature dependence of the resistivity from 2 K to 300 K reveals that the carrier transport mechanism is thermally activated band conduction above 150 K and Mott's variable range hopping below 70 K.     

Temperature effects on the optoelectronic properties of AgIn5S8 thin films

Polycrystalline AgIn₅S₈ thin films are obtained by the thermal evaporation of AgIn₅S₈ crystals onto ultrasonically cleaned glass substrates under a pressure of ~ 1.3 × 10⁻³ Pa. The temperature dependence of the optical band gap and photoconductivity of these films was studied in the temperature regions of 300-450 K and 40-300 K, respectively. The heat treatment effect at annealing temperatures of 350, 450 and 550 K on the temperature dependent photoconductivity is also investigated. The absorption coefficient, which was studied in the incidence photon energy range of 1.65-2.55 eV, increased with increasing temperature. Consistently, the absorption edge shifts to lower energy values as temperature increases. The fundamental absorption edge which corresponds to a direct allowed transition energy band gap of 1.78 eV exhibited a temperature coefficient of −3.56 × 10⁻⁴ eV/K. The 0 K energy band gap is estimated as 1.89 eV. AgIn₅S₈ films are observed to be photoconductive. The highest and most stable temperature invariant photocurrent was obtained at an annealing temperature of 550 K. The photoconductivity kinetics was attributed to the structural modifications caused by annealing and due to the trapping-recombination centers' exchange.