E.E. Khawaja

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.

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.

X-ray photoelectron and auger spectroscopy study of copper-sodium-germanate glasses☆

Abstract The role played by copper oxide in reducing the electrical conductivity of copper-sodium-germanate glasses has been investigated by X-ray photoelectron and X-ray excited Auger spectroscopy. Evidence for the presence of copper predominantly in the Cu + oxidation state has been obtained from the “shake up” satellite structure of the Cu 2p line. The observed Cu LVV Auger line shapes are significantly different in glasses and this indicates a different bonding environment of copper in the glass matrix. The reduction of electrical conductivity with the increase of copper content is explained in terms of cluster formation and to some extent due to the bonding of copper atoms with the glass matrix.

Optical constants of zinc sulfide films determined from transmittance measurements

The optical constants of thermally evaporated thin films of ZnS have been determined by measurements of transmittance at normal incidence from two films of different thickness. A single-layer model has been successfully used for the films. The results are compared with those obtained earlier for thermally evaporated ZnS films using reflectance and transmittance measurements (where a two-layer model for a ZnS film was used). The advantage of the present method over the earlier one is the readily available measurement facilities.

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%.

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.

X-ray photoelectron spectroscopy and Fourier transform-infrared studies of transition metal phosphate glasses

X-ray photoelectron spectroscopy and Fourier transform-infrared studies were carried out on phosphate glasses containing oxides of iron, cobalt, nickel, copper and zinc. The results suggest that the glasses containing iron and zinc may have structures in which both the phosphorus and the iron (or zinc) atoms are tetrahedrally coordinated by oxygen into three-dimensional structures which resemble the polymorphic forms of silica, whereas the glasses containing cobalt, nickel and copper may consist of polymeric chains of PO4 tetrahedra bonded to adjacent tetrahedra via bridging oxygens. These polyphosphate chains are linked together by the interaction between the metal cation and the oxygens of the network former. In addition, the core level 2p shake-up satellites of the 3d-transition metal ions in these glasses were studied. The results support a suggestion that the satellites in the glass are most likely due to the electron transfer from ligand to metal 3d orbitals.

Optical properties of thin films of and mixed oxides

Optical properties of thin films prepared by thermal evaporation of , and mixed oxides , have been studied. Satisfactory derivation of the refractive and absorption indices from the measured normal incidence transmittance of the films was achieved for the films of and . However, for films of the mixed oxides this was not possible, especially for the spectral region in which these films were absorbing. Rutherford backscattering spectrometry (RBS) measurements on the films revealed that the films of and were fairly uniform but the films prepared from the mixed oxides were chemically inhomogeneous. In fact, we were able to fit the RBS data for the mixed-oxide films by assuming that the Mo content in the film decreased (while the W increased) along the direction of growth of the film (i.e. from the substrate side of the film to its surface).