The paper critically reviews the state of the art in the field of experimental techniques possible to be applied to the study of conductometric gas sensors based on semiconducting metal oxides. The used assessment criteria are subordinated to the proposed R&D approach, which focuses on the study, and subsequent modelling, of sensors’ performance in realistic operation conditions by means of a combination of phenomenological and spectroscopic techniques. With this viewpoint, the paper presents both the to-date achievements and shortcomings of different experimental techniques, describes – by using selected examples – how the proposed approach can be used and proposes a set of objectives for the near future.

Metal oxide-based gas sensor research: How to?

Electrochromic tungsten oxide films: Review of progress 1993–1998

Electrochromic (EC) materials are able to change their optical properties, reversibly and persistently, by the application of an electrical voltage. These materials can be integrated in multilayer devices capable of modulating the optical transmittance between widely separated extrema. We first review the recent literature on inorganic EC materials and point out that today's research is focused on tungsten oxide (colouring under charge insertion) and nickel oxide (colouring under charge extraction). The properties of thin films of these materials are then discussed in detail with foci on recent results from two comprehensive investigations in the authors' laboratory. A logical exposition is obtained by covering, in sequence, structural features, thin film deposition (by sputtering), electronic band structure, and ion diffusion. A novel conceptual model is given for structural characteristics of amorphous W oxide films, based on notions of defects in the ideal amorphous state. It is also shown that the conduction band density of states is obtainable from simple electrochemical chronopotentiometry. Ion intercalation causes the charge-compensating electrons to enter localized states, implying that the optical absorption underlying the electrochromism can be described as ensuing from transitions between occupied and empty localized conduction band states. A fully quantitative theory of such transitions is not available, but the optical absorption can be modeled more phenomenologically as due to a superposition of transitions between different charge states of the W ions (6+, 5+, and 4+). The Ni oxide films were found to have a porous structure comprised of small grains. The data are consistent with EC coloration being a surface phenomenon, most likely confined to the outer parts of the grains. Initial electrochemical cycling was found to transform hydrated Ni oxide into hydroxide and oxy-hydroxide phases on the grain surfaces. Electrochromism in thus stabilized films is consistent with reversible changes between Ni hydroxide and oxy-hydroxide, in accordance with the Bode reaction scheme. An extension of this model is put forward to account for changes of NiO to Ni2O3. It was demonstrated that electrochromism is associated solely with proton transfer. Data on chemical diffusion coefficients are interpreted for polycrystalline W oxide and Ni oxide in terms of the lattice gas model with interaction. The later part of this review is of a more technological and applications oriented character and is based on the fact that EC devices with large optical modulation can be accomplished essentially by connecting W-oxide-based and Ni-oxide-based films through a layer serving as a pure ion conductor. Specifically, we treat methods to enhance the bleached-state transmittance by mixing the Ni oxide with other oxides characterized by wide band gaps, and we also discuss pre-assembly charge insertion and extraction by facile gas treatments of the films, as well as practical device manufacturing and device testing. Here the emphasis is on novel flexible polyester-foil-based devices. The final part deals with applications with emphasis on architectural “smart” windows capable of achieving improved indoor comfort jointly with significant energy savings due to lowered demands for space cooling. Eyewear applications are touched upon as well.

/pdf/electrochromics-for-smart-windows-thin-films-of-tungsten-aw0rai8h60.pdf

Electrochromics for smart windows: thin films of tungsten oxide and nickel oxide, and devices based on these

Gas detection by means of surface plasmon resonance

This paper reviews the present knowledge on tantalum pentoxide (Ta 2 O 5 ) thin films and their applications in the field of microelectronics and integrated microtechnologies. Different methods used to produce tantalum oxide layers are described, emphazing elaboration mechanisms and key parameters for each technique. We also review recent advances in the deposition of Ta 2 O 5 in the particular field of microelectronics where high quality layers are required from the structural and electrical points of view. The physical, structural, optical, chemical and electrical properties of tantalum oxide thin films on semiconductors are then presented and essential film parameters, such as optical index, film density or dielectric permittivity, are discussed. After a reminder of the basic mechanisms that control the bulk electrical conduction in insulating films, we carefully examine the origin of leakage currents in Ta 2 O 5 and present the state-of-the-art concerning the insulating behaviour of tantalum oxide layers. Finally, applications of tantalum oxide thin films are presented in the last part of this paper. We show how Ta 2 O 5 has been employed as an antireflection coating, insulating layer, gate oxide, corrosion resistant material, and sensitive layer in a wide variety of components, circuits and sensors.

Tantalum pentoxide (Ta2O5) thin films for advanced dielectric applications

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.

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

The optical constants n and k have been determined over the wavelength range from 2000-250 nm by measurements of reflectance and transmittance at normal incidence and treating these data by the method of Denton et al. (1972). The effects of surface roughness have been taken into account. Analysis of the dependence of absorption on photon energy have shown that the experimental results may be explained by the occurrence of direct transitions from 2.42 eV to 2.82 eV, in the case of CdS, followed by combined direct and indirect transitions beyond 2.82 eV assuming the energy band to be parabolic, or equally well by assuming only direct transitions between nonparabolic bands. The results for ZnS films are similar and may be treated in the same way. It is concluded that these materials both show absorption by direct transitions just beyond the absorption edge, and that at higher energies the form of the absorption curve is probably due to the combined effects of indirect transitions together with direct transitions between nonparabolic bands.

http://iopscience.iop.org/article/10.1088/0022-3727/8/5/019/pdf

The optical constants of thin evaporated films of cadmium and zinc sulphides

Dielectric/Ag/dielectric coated energy-efficient glass windows for warm climates

X-ray photoelectron and X-ray excited Auger spectra, optical absorption and DC conductivity of vanadium phosphate glasses containing nickel oxide have been studied as a function of nickel content. The shift in the binding energy of the V 2 p 3 2 photoelectron peak with the change in nickel content provided evidence of a change in the valence states of vanadium. The Ni 2 p 3 2 core level in NiO powder is a doublet and is normally used as a fingerprint for NiO. However, a single peak, shifted to higher binding energy, was observed in the glasses. The Ni (LVV) Auger spectrum for NiO powder showed a doublet structure. In glasses, however, two well-separated peak were observed in this spectrum. The results suggest that Ni in the glass may exist in two different phases, viz. nickel oxide and nickel vanadate. Initial addition of nickel (2 wt% of NiO) increased the optical absorption and electric conductivity but large additions, (5 wt% of NiO) led to a small decrease. The conductivity had a minimum value for a concentration of about 10 wt% of NiO. An attempt has been made to account for the observed changes in optical and electrical properties in terms of the XPS and Auger results.

XPS, auger, electrical and optical studies of vanadium phosphate glasses doped with nickel oxide☆

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.

E. E. Khawaja

Papers

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

The optical constants of thin evaporated films of cadmium and zinc sulphides

Dielectric/Ag/dielectric coated energy-efficient glass windows for warm climates

XPS, auger, electrical and optical studies of vanadium phosphate glasses doped with nickel oxide☆

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