Microstructures and electrical conductivity of nanocrystalline ceria-based thin films

Ceria-based thin films are potential materials for use as gas-sensing layers and electrolytes in micro-solid oxide fuel cells. Since the average grain sizes of these films are on the nanocrystalline scale (< 150 nm), it is of fundamental interest whether the electrical conductivity might differ from microcrystalline ceria-based ceramics. In this study, CeO₂ and Ce_0.8Gd_0.2O_1.9−x thin films have been fabrication by spray pyrolysis and pulsed laser deposition, and the influence of the ambient average grain size on the total DC conductivity is investigated. Dense and crack-free CeO₂ and Ce_0.8Gd_0.2O_1.9−x thin films were produced that withstand annealing up to temperatures of 1100 °C. The dopant concentration and annealing temperature affect highly the grain growth kinetics of ceria-based thin films. Large concentrations of dopant exert Zener drag on grain growth and result in retarded grain growth. An increased total DC conductivity and decreased activation energy was observed when the average grain size of a CeO₂ or Ce_0.8Gd_0.2O_1.9−x thin film was decreased.     

Studying the structure and electrical conductivity of thin granulated bimetallic films

The surface resistance of bimetallic granular films prepared by means of laser-induced deposition is studied. The possibility of modeling their conductivity is demonstrated, depending on their morphological properties.     

Influence of hydrogenation on electrical conductivity of vanadium dioxide thin films

The influence of hydrogenation on electrical conductivity of vanadium dioxide thin films has been investigated. It has been shown using measurements of the electrical conductivity that the hydrogenation of vanadium dioxide thin films leads to a decrease in the temperature of the phase transition from the tetragonal phase (with “metallic” conductivity) to the semiconducting monoclinic phase. It has been found that, upon doping of vanadium dioxide with hydrogen, the electrical conductivity of the monoclinic phase can increase by several orders of magnitude. Nonetheless, the temperature dependence of the electrical conductivity of hydrogenated films exhibits a typical semiconducting behavior in the temperature range where the monoclinic phase is stable.     

Optical absorption and electrical conductivity of flash evaporated CuGaTe2 thin films

The optical absorption edge of CuGaTe₂ thin films in the energy range 1 to 2·3 eV was studied. The characteristic band gaps were found to be 1·23 eV and 1·28 eV whereas the acceptor ionization energy was 170 meV. Electrical conductivity measurements were carried out in the temperature range 300–550 K and two acceptor states with ionization energies 400 meV and 140 meV were found. The origin of acceptor states is explained based on covalent model.     

Comparative analysis of the thickness and electrical conductivity of thin chalcogenide semiconductor films

The structure and thickness of zinc and cadmium chalcogenide semiconductor films are studied by X-ray radiography. The film thickness is shown to be comparable with the half-value layer depth. The electrical conductivity of the films increases upon heating in the hydrogen atmosphere and decreases upon heating in carbon oxide. The opposite trend is observed in the ratio between the electrical conductivity and band gap of the initial and oxidized film surfaces.

     

Influence of surface roughness on the electrical conductivity of semiconducting thin films

The Green function solution of the Boltzmann transport equation has been applied in case of no magnetic field by ignoring any volume impurities. Gaussian, exponential and power law models for the surface roughness correlation function have been studied and the results have been compared with the ones obtained by other methods. It has been found that the electrical conductivity σ$\sigma$ increases with increasing correlation length l$l$ for the first two models, while for the third model σ$\sigma$ value is of the same order as the first two models. Therefore we show that the shape of the surface roughness can strongly influence the electrical properties.     

Anomalous temperature dependence of the electrical conductivity of zinc oxide thin films

The temperature dependence of the conductivity of Ga-doped ZnO thin films was examined in humid and dry atmospheres. The conductivity initially increases to the maximum (range 1) and then decreases to the minimum (range 2) and again increases (range 3) in the heating run under humid air atmosphere. The subsequent cooling reduces the conductivity to less than that for the heating run. The extent of the reduction becomes large as the cooling rate increases. The extrema character in the conductivity versus temperature curves does not disappear under nitrogen gas atmosphere provided it is humid. On the other hand, the conductivity in the ranges 1 and 2 decreases so significantly that the extrema character disappears in the driest air as possible. These behaviors are explained by assuming that the dissociatively chemisorbed states of water vapor act as electron donors and desorb at the higher temperature, and the desorption as well as the chemisorption require a time for attaining equilibrium. With this in view, a theoretical analysis is given of semiconducting films thinner than the depletion boundary layer thickness, and numerical curves fitting to the experimental results are obtained. The heat of chemisorption of nonionized water q° is 1.35 eV and the energy levels of water chemisorption donors and oxygen chemisorption acceptors are at 0.66 and 0.59 eV below the conduction band edge, respectively.     

Microstructure and electrical conductivity of YSZ thin films prepared by pulsed laser deposition

Yttria-stabilized zirconia (YSZ) is the most common solid electrolyte material used e.g. in ceramic fuel cells. Thin films of YSZ were deposited on c-cut sapphire single crystals by pulsed laser deposition using a KrF excimer laser focused on a polycrystalline 8 mol% Y₂O₃-stabilized ZrO₂ target. Depending on the substrate temperature and the oxygen background pressure during deposition, different microstructures are obtained. XRD and high-resolution SEM revealed the formation of dense amorphous films at room temperature. At 600°C preferentially (111) oriented polycrystalline films consisting of densely agglomerated nm-sized grains of the cubic phase resulted. Grain size and surface roughness could be controlled by varying the oxygen background pressure. RBS and PIXE evidenced congruent transfer only for a low number of pulses, indicating a dynamical change of the target stoichiometry during laser irradiation. The in-plane ionic conductivity of the as-deposited crystalline films was comparable to bulk YSZ. The conductivity of initially amorphous YSZ passes a maximum during the crystallization process. However, the relative changes remain small, i.e. no significant enhancement of ionic conductivity related to the formation of a nanocrystalline microstructure is found.     

Excess electrical conductivity in thin superconducting lead films above the transition temperature

Electrical conductivity measurements on thin superconducting lead films above the transition temperature are reported. It is observed that films grown at room temperature contribute to Maki-Thompson (MT) term whereas the ones grown at ∽77 K don't.     

Raman spectroscopy and low temperature electrical conductivity study of thermally evaporated CdS thin films

A systematic study has been carried out on properties of CdS thin films grown on glass substrates by thermal route and growth parameters have been optimized to achieve near stoichiometry. Raman spectroscopy of the films has been carried out to get more insight on chemical and structural information of the films. Photoluminescence study was carried out to get the knowledge of defect-states which play major role in transport mechanisms. In order to obtain electrical parameters, Hall measurement has been carried out by van der Pauw’s technique at room temperature. Also, variation of electrical resistivity of the films at temperature range 20–300 K has been studied. Finally effect of post deposition annealing on the structural, optical and electrical properties of these films has been studied.     

The influence of the structure of thin antimony films on their electrical conductivity

The eletrical conductivity of antimony films of different thicknesses was studied at different temperatures. For small thicknessd, whereL is the electron mean-free path, the film resistances varies asd ^?n , wheren=2. However, ford?L the value ofn becomes smaller than 2 which is in agreement with the theory.     

Size effect on the electrical conductivity and longitudinal gauge factor of thin metal films

The electrical and piezoresistive properties of vacuum deposited films of copper, tungsten, platinum and gold were investigated in the thickness range from 100 to 1000 . Both the conductivity and the longitudinal strain coefficient of resistance exhibit a size dependence, that was interpreted by applying the Sondheimer theory of charge transport in thin films on the assumption that the reflection of electrons by the film surfaces is partially specular. The proportion of electrons elastically scattered has been deduced from considerations based on the anomalous skin effect. The calculations are in reasonable agreement with the observations; the discrepancies can be attributed to the chemical contamination of the samples and to departures of the surface structure from the idealized flat configuration assumed in the theoretical treatment.     

Oscillations of the electrical conductivity with film thickness in very thin platinum films

Platinum films which were approximately amporphous were prepared onto glass substrates by e-gun evaporation under UHV conditions. Determination of the surface roughness showed they were smooth on atomic scale. These films fulfilled the conditions for observation of the quantum size effect (QSE) in metal films. Oscillations of the conductivity with increasing film thickness were observed during evaporation in the thickness interval from 0.5 to 2.5 nm.     

Nanostructure and conductivity of thin metal films

Amorphous silver, copper, gold, and iron films of a thickness between 6 and 350 nm are grown on polymeric substrates by vacuum evaporation. The nanostructure of the films is investigated. The dependence of the conductivity on the film thickness is obtained, and a correlation between the surface morphology and the conductivity is established.     

Kinetics of oxygen chemisorption and its effects on electrical conductivity of thin evaporated CdS films

The mechanism and kinetics of chemisorption of oxygen on thin evaporated films of CdS have been investigated through changes in electrical conductivity caused by chemisorbed oxygen. Electrical conductivity of CdS films, when exposed to oxygen atmosphere falls exponentially with time, with a characteristic time determined by several factors such as temperature, electric field applied to the sample, illumination, etc. Electric field applied in appropriate direction is shown to enhance oxygen chemisorption. Complete theoretical treatment for these changes has been provided and the agreement between theory and experiment is shown to be excellent.     

A microscopic calculation of the electrical conductivity of thin films at low adsorption coverage

We perform a microscopic calculation of the electrical resistance of thin metallic films with small concentrations of volume scatterers and of adsorbed impurities. We show that there are, in the average conductivity, two thickness dependent corrections to the bulk value, of comparable magnitudes. The first one is due to a quantum size effect, and is present even for films with pure surfaces. The second, due to the adsorbed impurities, is given in all practical cases by the dirty limit of the Fuchs-Sondheimer expression. A microscopic expression of their phenomenological diffuse reflection parameter is derived.     

Electrical conductivity of PbTe thin films

PbTe thin films were prepared by vacuum technique with different thicknesses ranging from 550 to 3000 Å. The electrical resistivity as a function of the film thickness and mobility was measured. The dependence of log (resistivity) and log (current) was studied as a function of the universal temperature. The activation energies were estimated before and after the break. The transition of conductivity from n-type to p-type is attributable to the increase in the number of migrating lead vacancies. An increase of the applied voltages on the thin films caused the shift of breaking temperature to higher temperatures. This shift is attributed to the creation of Pb vacancies which retard the break.     

Structure and electrical conductivity of ultrathin Ni-Cu films

The structure, phase composition, morphology, and electrical conductivity of Ni-Cu alloy ultrathin films having a thickness of d = 1?10 nm and a Cu concentration of 10–95 at % have been studied. All films are shown to be fcc Ni-Cu alloys; they have an island structure with an island size of 1.5–2 nm in the as-deposited films and of about 20 nm in the films annealed to 700 K. The electrical conductivity of the films depends on their thickness and morphology. For films with d ≈ 1 nm, the electrical conductivity is thermally activated with an activation energy E _a ≈ 0.086?0.095 eV. Films with d > 3 nm exhibit the metallic temperature dependence of electrical conductivity with a positive temperature coefficient of resistivity.