A.c. conduction in evaporated MoO3/SiO amorphous thin films

The dielectric properties of vacuum-deposited MoO₃/SiO films of different compositions studied in the frequency range 10² to 10⁶ Hz at various temperatures (193 to 393 K) are reported. The properties of the film capacitor are found to be temperature and frequency dependent. The decrease in a.c. conductance with increasing concentration of SiO in MoO₃ may be attributed to the increasing number of trapping centres generated in MoO₃/SiO films during the evaporation process.     

An XPS study of amorphous MoO3/SiO films deposited by co-evaporation

X-ray photoelectron spectroscopic (XPS) core-level spectra of MoO₃/SiO thin films are presented. The effects of changes in composition, substrate temperature during deposition and annealing on the binding energy of Mo(3d) and Si(2p) core lines in mixed films are compared with those of MoO₃ and SiO. Appreciable changes in Mo(3d) peak positions and slight changes in Si(2p) peak positions are observed. The change in binding energy of the Mo(3d) doublet may be attributed to the effective incorporation of silicon ions in an MoO₃ lattice which may cause the molybdenum orbital to be a little less tightly bound. This helps in the internal electron transfer from the oxygen (2p) to the molybdenum (4d) level as a result of which the molybdenum is readily changed to lower oxidation states during heat treatment. XPS spectra show that the position of the Si(2p) core state shifts monotonically with increasing oxygen concentration from the value of 101.8 to 102.6 eV.     

The optical absorption edge in amorphous thin films of MoO3-In2O3

A study of the effects of changes in composition, film thickness, substrate deposition temperature and annealing on the optical properties of MoO₃-In₂O₃ is presented. The results are found to be compatible with the reduction in the value of optical energy gap of these materials as the molar fraction of In₂O₃ in the MoO₃ thin film increases. This decrease of optical gap may be attributed to the incorporation of In(III) ions in an MoO₃ lattice. The decrease in optical band gap with increasing thickness may be interpreted in terms of the incorporation of oxygen vacancies which are also believed to be the source of conduction electrons in the MoO₃-In₂O₃ complex. The decrease of band gap with increasing substrate temperature may be attributed to the enhanced ordering of the samples and the decrease of band gap with annealing may be attributed to a reduction in the concentration of lattice imperfections.     

The study of optical properties of In2O3 and of mixed oxides In2O3−MoO3 system deposited by coevaporation

A discussion of the optical properties of two systems of dielectric films i.e. In₂O₃ and of mixed oxides In₂O₃−MoO₃ system is presented. Film thickness, substrate temperature, annealing and composition (in molar%) have a profound effect on the structure and optical properties of these films. The decrease in optical band gap with the increase in film thickness of In₂O₃ is interpreted in terms of incorporation of oxygen vacancies in the In₂O₃ lattice. The decrease in optical band gap with the increase in substrate temperature and annealing of In₂O₃ thin films is ascribed to the release of trapped electrons by thermal energy or by the outward diffusion of the oxygen-ion vacancies, which are quite mobile even at low temperature. For the mixed oxides In₂O₃−MoO₃ system the results are found to be compatible with the reduction in the value of optical band gap of these materials as the molar fraction of MoO₃ increases in the In₂O₃ thin films and is attributed to the incorporation of Mo(VI) ions in an In₂O₃lattice that causes the indium orbital to become a little less tightly bound. The decrease in optical band gap of mixed oxides In₂O₃−MoO₃ system, with increasing film thickness is interpreted in terms of incorporation of oxygen vacancies in both In₂O₃ and MoO₃ lattice which are also believed to be the source of conduction electrons in In₂O₃–MoO₃ complex. The decrease in optical band gap with increasing substrate temperature and annealing of mixed oxides In₂O₃−MoO₃ system is due to the increasing concentration of oxygen vacancies, formation of indium and molybdenum species of lower oxidation state and indium interstitials. The blue colouration of mixed oxides In₂O₃–MoO₃ samples is due to the inter-electron transfer from oxygen 2p to molybdenum 4d level due to which Mo species of lower oxidation states are formed.     

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 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.     

Gamma-ray dosimetric properties of molybdenum phosphate glasses

In the present work, a study of the temperature dependence of the d.c. electrical conductivity and conduction activation for a series of MoO₃-P₂O₃ glass systems has been carried out. The conductivity measurements of the unirradiated glass specimens proved to be mainly dependent on both temperature and transition metal ion content in the glass matrix. The results of the present investigation have shown that the conduction mechanism would be due to the electron exchange between the low and high valency states of the MoO₃ oxide (Mo" and Mo⁶⁺). The radiation-induced conductivity of the glass system studied, produced by gamma rays, has also been measured experimentally. The d.c. electrical conductivity has proved to be dose dependent, which showed a decrease with increasing -dose. The results reflect some evidence of the-ray dosimetric potential of the glass specimens studied.     

Molybdenum ion: a structural probe in lithium–antimony–germanate glass system by means of dielectric and spectroscopic studies

In this study, we have synthesized multi-component 10Li₂O–(30 ? x)Sb₂O₃–40GeO₂–20PbO:x MoO₃ (with five values of x ranging from 1.0 to 9.0) and investigated dielectric properties over a frequency range of 10²–10⁶ Hz and in the temperature range 30–300 °C of these samples. The evaluated dielectric parameters include dielectric constant, ε′(ω); ac conductivity, σ _ac; and electric modulus, M(ω). The results were interpreted with the aid of the experimental data on optical absorption, IR, Raman, and ESR spectroscopy. The analysis of the results of spectroscopic studies have indicated that a considerable proportion of molybdenum ions reduce to Mo⁵⁺ state, form molybdenyl complexes, occupy octahedral positions, act as modifiers, and create dangling bonds in the glass network. The concentration of such molybdenyl complexes seemed to be increasing with increase in the concentration of MoO₃. The temperature dispersion of real part of dielectric constant, ε′(ω), has been analyzed using space charge polarization model. The frequency and temperature dependence of the dielectric loss parameters have exhibited relaxation character. The relaxation effects have been attributed to molybdenyl complexes and to the divalent lead ions. Electrical conductivity exhibited an increasing trend and the activation energy showed a decreasing trend with increase in the concentration of MoO₃. The increase of conductivity is attributed to (i) the increasing concentration of polaron Mo⁵⁺–Mo⁶⁺ pairs and (ii) increase in the concentration of dangling bonds in the glass network that causes the substantial decrement in jump distance for Li⁺ ions, which contribute to ionic conductivity.     

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.     

Dielectric and spectroscopic features of ZnO–ZnF2–B2O3:MoO3 glass ceramic—a possible material for plasma display panels

ZnO–ZnF₂–B₂O₃ borate glass mixed with different concentrations of MoO₃ were synthesized and subsequently crystallized. The X-ray diffraction studies revealed that the samples were embedded with crystalline phases in which molybdenum ions exist in Mo⁶⁺ and Mo⁵⁺ states. The results of spectroscopic studies (viz., optical absorption and electron spin resonance) have revealed that the there is an increasing proportion of Mo⁵⁺ ions with increase in the concentration of MoO₃ in the glass ceramic. The results of photoluminescence spectra have indicated that if the care is taken to minimize Mo⁵⁺ ion concentration, these glass ceramics are suitable for light emission in the blue, green and red regions. The analysis of the results of IR spectra have indicated that with increase in the content of MoO₃ there is an increasing degree of disorder in the glass network. The room temperature dielectric constant of these glass ceramics containing even the highest concentration of MoO₃ is always found to be in between 11.5 and 12.4 suggesting that these glass ceramics would be suitable for dielectric layer in plasma display panels (PDP). The dielectric parameters have exhibited relaxation character; the relaxation effects have been attributed to molybdenyl complexes. The observed increase in the electrical conductivity with MoO₃ content is attributed to the contribution of polaronic transfer between Mo⁵⁺ ? Mo⁶⁺ ions. Additionally, the substantial decrement in jump distance for zinc ions between the two sites in the ceramic network (because of increase in the concentration of dangling bonds) is also found to contribute to the conductivity. The value of dielectric breakdown strength for the studied materials is measured to be in the range of 10.54–12.9 kV/cm which is far greater than the required value for a material to be used as dielectric layer in PDP.     

Effect of laser power on electrical conductivity of BaTi5O11 films prepared by laser chemical vapor deposition method

BaTi₅O₁₁ films were prepared on Pt/Ti/SiO₂/Si substrates by a laser chemical vapor deposition method. The effect of laser power (P _L) on the electrical conductivity of the BaTi₅O₁₁ films was investigated. The electrical resistivity of the grains was much higher than that of the grain boundaries, which indicated that the electrical conductivity along the grain boundary was dominant. For the BaTi₅O₁₁ films, the conduction was mainly attributed to oxygen vacancies, and the electrical conductivity was strongly affected by their microstructures and the concentration of the charge carriers. With increasing the P _L, the grain size increased and the grain-boundary density decreased, which resulted in the decrease of the electrical conductivity. At the same time, the increase of T _dep led to higher concentration of charge carriers, which resulted in the increase of electrical conductivity.     

Electrical and optical properties of MoO3-V2O5 mixed oxide films

Vacuum deposited MoO₃-V₂O₅ films of different molar concentrations have been used for DC electrical conductivity studies at different temperatures. The optical absorption spectra of MoO₃-V₂O₅ films of different molar concentrations have been measured. From these measurements it is found that optical band gap and activation energy vary with molar concentration of MoO₃-V₂O₅ films.     

A study of the electrical properties of molybdenum phosphate glasses

A series of binary MoO₃-P₂O₅ and also ternary MoO₃-P₂O₅-In₂O₃ glasses were prepared and their electrical properties were investigated. The d.c. conductivity measurements show that the conduction in molybdenum phosphate glasses is by a hopping process in which the electrical charge transfers from Mo⁵⁺ to Mo⁶⁺ ion sites. The conductivity can be discussed in terms of the small polaron conduction mechanism. In contrast to vanadium phosphate glasses, the tunnelling term in the conductivity formula seems to make a significant contribution in molybdenum phosphate glasses and is associated with a hopping process in the non-adiabatic regime.     

Study on optical and electrical switching properties and phase transition mechanism of Mo6+-doped vanadium dioxide thin films

In the present work using V₂O₅ and MoO₃ powders as precursors, a novel method, the inorganic sol-gel method, was developed to synthesize Mo⁶⁺ doped vanadium dioxide (VO₂) thin films. The structure, valence state, phase transition temperature, magnitude of resistivity change and change in optical transmittance below and above the phase transition of these films are determined by XRD, XPS, four-point probe equipment and spectrophotometer. The results showed that the main chemical composition of the films was VO₂, the structure of MoO₃ in the films didn't change, and the phase transition temperature of the VO₂ was obviously lowered with increasing MoO₃ doped concentration. The magnitude of resistivity change and change in optical transmittance below and above phase transition were also decreased, of which the magnitude of resistivity change was more distinct. However, when the MoO₃ concentration was 5 wt%, the magnitude of resistivity change of doped thin films still reached more than 2 orders, and the change in optical transmittance below and above phase transition was maintained. Analysis showed that the VO₂ doped films formed local energy level, and then reduced the forbidden band gap of VO₂ as the donor defect changing its optical and electrical properties and lowering the phase transition temperature.     

MoOx thin films deposited by magnetron sputtering as an anode for aqueous micro-supercapacitors

Abstract

In order to examine the potential application of non-stoichiometric molybdenum oxide as anode materials for aqueous micro-supercapacitors, conductive MoO_x films (2  x  2.3) deposited via RF magnetron sputtering at different temperatures were systematically studied for composition, structure and electrochemical properties in an aqueous solution of Li₂SO₄. The MoO_x (x ≈ 2.3) film deposited at 150 °C exhibited a higher areal capacitance (31 mF cm^?2 measured at 5 mV s^?1), best rate capability and excellent stability at potentials below ?0.1 V versus saturated calomel electrode, compared to the films deposited at room temperature and at higher temperatures. These superior properties were attributed to the multi-valence composition and mixed-phase microstructure, i.e., the coexistence of MoO₂ nanocrystals and amorphous MoO_x (2.3 < x  3). A mechanism combining Mo(IV) oxidation/reduction on the hydrated MoO₂ grain surfaces and cation intercalation/extrusion is proposed to illustrate the pseudo-capacitive process.     

Infrared spectra of some thin amorphous films of MoO3-In2O3 deposited by vacuum evaporation

The infrared absorption spectra of vacuum-evaporated MoO₃-In₂O₃ thin films are reported for a series of films of varying compositions within the spectral range 4000 to 400 cm^–1 and interpreted in relation to the spectrum of the pure MoO₃ films. The affects of changes in composition, film thickness, substrate deposition temperature and annealing on the absorption bands are observed. In some cases a shift in band frequency is also observed. The gradual increase in ordering and decrease in valency state with increase of annealing temperature is observed in the infrared curves. Some new peaks appear which reveal the presence of a lower valency state in MoO₃. The shift in band frequency may be attributed to the incorporation of Mo(VI) ions in the indium interstitial positions.     

Electrical conductivity of polycrystalline Fe2(MoO4)3

Measurements of the electrical conductivity of polycrystalline Fe₂ (MoO₄)₃ in the temperature range 370 to 900 K and in the oxygen partial pressure region 10^?4 to 1 atm are presented. Fe₂(MoO₄)₃ is found to be a semiconductor. Differing conduction mechanisms operate, depending on the crystallographic form of Fe₂(MoO₄)₃ and the oxygen partial pressure, and their nature is discussed.     

Correlation between electron spin resonance,electrical conductivity and optical absorption edge of co-evaporated thin films of the dielectric system SiO/V2O5

Measurements of electron spin resonance, d.c. and a.c. electrical conduction and optical absorption of thin film samples of SiO/V₂O₅ prepared by a co-evaporation process at a pressure in the range 8×10⁻⁶ to 3×10⁻⁵ torr are reported. It is found that the spin density of the mixed system decreases by about one order of magnitude compared with that of an SiO film deposited under similar conditions. This reduction correlates with the d.c. electrical conductivity which also decreases as the V₂O₅ content of the complex SiO/V₂O₅ films increases. At the same time the optical energy gap also decreases. At lower temperatures (down to 198 K), a.c. conductance measurements give evidence of hopping conduction. D.c. conductance shows a transition from hopping conduction to free-band (extended state) conduction at about 263 K.     

Electrical and crystallographic properties of nanocrystalline CdSe<Subscript>0.5</Subscript>S<Subscript>0.5</Subscript> composite thin films deposited by dip method

A dip method is employed for the deposition of CdSe_0.5S_0.5 composite thin film at room temperature. Cadmium sulphate, thiourea and sodium selenosulphate were used as the basic source material. Solid solution with cubic phase was observed from X-ray diffraction studies. The specific conductivity of the film was found to be in order of 10⁻⁷ (Ωcm)⁻¹. The temperature dependence of an electrical conductivity, thermoelectrical power, carrier density and carrier mobility for CdSe_0.5S_0.5 thin films have been examined. The low temperature conductivity is governed by a variable range of conduction while grain boundary limited conduction mechanism is predominant at higher temperature.     

Phase transition behaviors and thermal conductivity measurements of nitrogen-doped Sb2Te3 thin films

Crystallization temperature of nitrogen-doped Sb₂Te₃ (ST) thin films increased with increasing nitrogen doping concentration, which indicates that the long-term stability of the metastable amorphous state can be improved by nitrogen doping. The root-mean-square (rms) roughness values of the films showed a significant decrease with nitrogen doping. Thermal conductivity of nitrogen-doped ST thin films was measured using a transient thermoreflectance (TTR) technique. It was found that the thermal conductivity decreased with increasing nitrogen doping concentration and increased with increasing annealing temperature. Nitrogen-doped ST thin films are suitable phase-change materials for low programming power consumption applications of phase-change random access memory (PCRAM).