M.F. Al-Kuhaili

Optical properties of gallium oxide films deposited by electron-beam evaporation

Thin films of gallium oxide were deposited by electron-beam evaporation on unheated substrates. Samples were deposited either without oxygen, or under an oxygen partial pressure of 5×10−4 mbar. The films were amorphous. Films deposited with oxygen were stoichiometric, whereas those deposited without oxygen were substoichiometric. The optical properties of the films were derived from measurements, at normal incidence, of transmittance and reflectance. Films deposited without oxygen had higher values of the refractive index and extinction coefficient. The energy gaps were 5.04 and 4.84 eV for films deposited with and without oxygen, respectively.

Characterization of thin films produced by the thermal evaporation of silver oxide

Thin films were produced by the reactive thermal evaporation of pure silver oxide (AgO) in a background of molecular oxygen. The effects of the deposition rate and oxygen partial pressure on the structural, chemical, electrical and optical properties of the films were investigated. The films were characterized using x-ray diffraction, x-ray photoelectron spectroscopy, electrical resistivity and normal-incidence transmittance and reflectance. The resulting films were found to be mainly metallic with a small oxide component that increased with the oxygen partial pressure.

Effect of preparation conditions on the optical and thermochromic properties of thin films of tungsten oxide

Abstract Thin films of tungsten oxide have been prepared by thermal evaporation. The effect of preparation conditions (heating of substrates and oxygen environment) on the optical constants (n and k) of the films has been studied. Satisfactory derivation of n and k from the measured normal incidence transmittance of the films was achieved. It was found that (a) both n and k have larger values for films deposited on heated substrates than for those deposited on unheated substrates, and at a given substrate temperature, (b) both n and k have smaller values for films deposited in the oxygen atmosphere than those deposited without an introduction of oxygen in the chamber. Thermochromic colouration of the films was carried out by annealing the films in vacuum. The annealing of the films produced significant loss in the oxygen content (measured by X-ray photoelectron spectroscopy) and modulation of the transmittance for the films deposited on unheated substrates with or without the oxygen environment and films deposited on heated substrates with the oxygen. The loss in the oxygen content and the modulation of transmittance, however, were very small for films deposited on heated substrates without the oxygen. For annealed films, satisfactory derivation of n and k was achieved for films deposited on unheated substrates, while for films deposited on heated substrates this was not possible. This study revealed that upon annealing the optical properties of the films prepared in the oxygen environment were mainly absorptance-modulated, and those of the films without the oxygen were reflectance-modulated.

Effects of preparation conditions on the optical properties of thin films of tellurium oxide

Thin films of tellurium oxide were prepared by thermal evaporation. The effects of preparation conditions and post-deposition vacuum annealing on the optical constants of the thin films were studied. Substantial changes in the optical constants, density, structure and stoichiometry were observed following changes in the preparation conditions and annealing. The majority of the films were found to be deficient in oxygen. The presence of metallic Te was detected in films deposited on heated substrates and in all the films that were annealed. All the samples showed some degree of absorption at photon energies below the band gap. One explanation for this absorption could be oxygen deficiency and the presence of metallic Te.

Investigation of the Carbon Monoxide Gas Sensing Characteristics of Tin Oxide Mixed Cerium Oxide Thin Films

Thin films of tin oxide mixed cerium oxide were grown on unheated substrates by physical vapor deposition. The films were annealed in air at 500 °C for two hours, and were characterized using X-ray photoelectron spectroscopy, atomic force microscopy and optical spectrophotometry. X-ray photoelectron spectroscopy and atomic force microscopy results reveal that the films were highly porous and porosity of our films was found to be in the range of 11.6–21.7%. The films were investigated for the detection of carbon monoxide, and were found to be highly sensitive. We found that 430 °C was the optimum operating temperature for sensing CO gas at concentrations as low as 5 ppm. Our sensors exhibited fast response and recovery times of 26 s and 30 s, respectively.

Characterization of hafnium oxide thin films prepared by electron beam evaporation

Thin films of hafnium oxide were deposited by electron beam evaporation. The effects of the substrate temperature and the oxygen partial pressure on the refractive index and carbon monoxide sensing properties of the films were studied. The films were characterized using x-ray diffraction and x-ray photoelectron spectroscopy techniques. Films deposited on unheated substrates were amorphous, whereas those deposited on heated substrates showed a mixture of amorphous and polycrystalline structure. All the films were found to be optically inhomogeneous. The inhomogeneity of the films was taken into account in the determination of their refractive indices. It was found that the porosity (as reflected by the refractive indices) of the films was the main factor that affected the sensitivity of the films in relation to their detection of carbon monoxide.

Characterization of thin films of a-SiOx (1.1<x<2.0) prepared by reactive evaporation of SiO

Thin films of a-SiOx with values of x ranging from 1.13 to 1.89 were prepared by reactive evaporation of SiO in a controlled oxygen environment. The oxygen pressure in the deposition chamber was varied so as to obtain films with different values of x. The films were studied by x-ray photoelectron spectroscopy and optical spectrophotometry. An attempt was made to analyse the Si 2p core-level spectra in terms of five chemically shifted components corresponding to basic Si bonding units Si–(Si4−nOn) with n = 0,1,...,4. The concentration of these bonding units as a function of oxygen concentration was in reasonable agreement with the random-bonding model, with the exception that the Si–(Si3O) component was almost completely suppressed for all stoichiometries. Films with x<1.65 consisted of elemental Si and oxides of silicon, while those with were almost free of Si. Films containing Si have higher refractive indices and degrees of absorption in the visible region compared with those which were free of Si. The optical properties of the films approach those of fused silica (SiO2) as the values of x increase. For the films with the largest value of x (= 1.89), the refractive index is smaller than that of fused silica. The density of these films was estimated to be smaller than that of fused silica by about 13%.

Investigation of ZnO/Al/ZnO multilayers as transparent conducting coatings

Multilayer ZnO/Al/ZnO coatings were deposited by thermal evaporation on unheated substrates. Subsequently, the coatings were annealed in air in the temperature range 300–600 °C. The structural properties of the films were investigated using x-ray diffraction and atomic force microscopy. The chemical properties were determined from x-ray photoelectron spectroscopy along with elemental depth profiling. The electrical and optical properties of the coatings were studied in order to evaluate their performance as transparent conducting coatings. The best performance was obtained with films annealed at 300 °C, for which the average visible transmittance was 75% and the resistivity was 2.9 × 10−3 Ω cm.

CO-sensing properties of undoped and doped tin oxide thin films prepared by electron beam evaporation.

Undoped thin films of tin oxide and those doped with indium oxide and nickel oxides were deposited by electron beam evaporation. The effects of the film thickness and preparation conditions (films prepared with or without the presence of oxygen environment during deposition) on the optical and carbon monoxide sensing properties of the films were studied. The films were characterized using X-ray diffraction and X-ray photoelectron spectroscopy and optical spectroscopy techniques. All the films were found to be amorphous. It was found that the sensitivity of the films to CO increased with the thickness and the porosity of the films. It was found that their selectivity to CO gas relative to CO(2) and SO(2) gases could be improved upon doping the films with indium (or nickel) oxide.

Determination of average refractive index of thin CeO2 films with large inhomogeneities

Thin films of cerium dioxide were deposited on heated (HS-films) and unheated (US-films) substrates by electron-beam evaporation. An attempt was made to determine their refractive indices from the measurement of their transmittance at normal incidence. All films were found to be optically inhomogeneous (i.e. variation of the refractive index along the depth of the film). The degree of inhomogeneity in the HS-films was far greater than that in the US-films. It is known that films of cerium oxide (HS-films) are far more inhomogeneous than the films of some other metal oxides. Satisfactory dispersion curves could be obtained for the US-films on the basis of a commonly used linear-index-variation-profile. However, in the case of HS-films satisfactory curves could not be obtained when such an index-variation-profile was used. On the other hand for the HS-films, satisfactory curves were obtained by the use of a quadratic-index-variation-profile. A simple model of a growth of film with columnar structure is proposed, for the first time, such as to justify the use of the quadratic-index-variation-profile in the case of a film with large inhomogeneity. The growth model needs to be verified physically by some other means.