Khalifa Aguir

Electrical properties of reactively sputtered WO3 thin films as ozone gas sensor

Abstract Electrical properties of the metallic oxide semiconductor, such as tungsten trioxide (WO3) thin films, deposited on SiO2/Si substrates by RF reactive magnetron sputtering system from a metallic tungsten target and argon–oxygen mixture gas have been investigated. This study is devoted to analyse the relationship between the electrical properties and the WO3 thin film deposition parameters (substrate temperature, oxygen partial pressure, annealing) and the sensitivity and stability of these WO3 gas sensors. The surface morphology evolution of these films has been investigated by atomic force spectroscopy (AFM). Two types of electrical measurements were performed: conductivity versus temperature and some tests under ozone at different temperatures. The activation energy evolution is correlated with the reactivity of surface sensors under oxygen partial pressure.

A novel ozone gas sensor based on one-dimensional (1D) α-Ag2WO4 nanostructures

This paper reports on a new ozone gas sensor based on α-Ag₂WO₄ nanorod-like structures. Electrical resistance measurements proved the efficiency of α-Ag₂WO₄ nanorods, which rendered good sensitivity even for a low ozone concentration (80 ppb), a fast response and a short recovery time at 300 °C, demonstrating great potential for a variety of applications.

Development of an ammonia gas sensor

Copper (I) bromide (CuBr) thin films for ammonia detection are prepared by two different techniques: (1) magnetron sputtering and (2) chemical or electrochemical oxidation of copper in presence of bromine ions. The microstructure of the films is related to these processes. The electrical properties of these films are studied and the temperature dependence of the bulk conductivity is in agreement with previous work on polycrystalline CuBr. After a preliminary strong exposure to ammonia gas, ammonia adsorption on films leads to a reversible decrease of conductivity. This effect is used to design a selective and sensitive ammonia gas sensor operating at ambient temperature. The sensitivity and the response time are related to the film preparation.

An easy method of preparing ozone gas sensors based on ZnO nanorods

One-dimensional (1D) ZnO nanorod-like structures were successfully grown via a hydrothermal method to be used as an ozone gas sensor. X-ray diffraction measurements (XRD) and field-emission scanning electron microscopy (FE-SEM) analysis revealed a preferential growth of the nanorod-like structures along the [002] direction. Electrical resistance measurements indicated a good sensitivity to different ozone concentrations (0.06–1.19 ppm) as well as long-term stability over a 6 month period at 250 °C. In addition, it was observed that the nanorods had a good sensitivity to ozone at room temperature when the sensor was exposed to UV illumination. This study provides an easy and efficient way to obtain 1-D ZnO nanorods exhibiting remarkable properties for applications as ozone gas sensing materials.

Electrical properties of a-GaAs/c-Si (p) heterojunctions

Abstract Heterojunctions have been fabricated by deposition of amorphous gallium arsenide on p-type crystalline silicon. C(V) and J(V) measurements were performed to determine electrical properties of these structures. Rectifying properties have been obtained, and capacitance-voltage behavior indicates an abrupt interface with a main discontinuity in the valence bands. The forward current involves tunneling and is successfully explained by a multi-tunneling capture-emission model. The reverse current is probably limited by the generation process.

A mobility and free carriers density fluctuations based model of adsorption–desorption noise in gas sensor

Noise spectroscopy has been proposed as a means of extracting a more selective response from metallic oxide gas sensors. In this paper, we complete our previous models of adsorption–desorption (A–D) noise by taking into account the effect of the fluctuation of the adsorbed molecules density, not only on the density of free carriers but also on their mobility. Using Wolkensteins isotherm, combined with the electroneutrality and the fluctuation of both free electron density and mobility, we derive an exact expression for the A–D noise in the case of dissociative and non-dissociative chemisorption. The model shows that the power density spectrum of the fluctuation of the sensors conductance has a cut-off frequency and a low frequency magnitude which are specifics of the adsorbed gas. The cut-off frequency is four orders of magnitude lower than the one we obtained without considering mobility fluctuations.

NiTi thin films as a gate of M.O.S. capacity sensors

Abstract In this study, a NiTi-shape memory alloy thin film is used as a gate in a M.O.S.-type structure. The shape memory effect involves a thermally induced phase transformation between a low temperature ductile phase and a high temperature high strength phase. The NiTi gate is deposited on a SiO 2 /Si substrate using RF sputtering deposition method. We have studied the evolution of C ( V ) characteristics as a function of temperature in order to provide evidence of phase transition within the NiTi layer, and also to investigate the effect of such a transition on the electrical properties of the structure. A rapid change in the C ( V , T ) curves which is a direct consequence of the modification in the NiTi structure is observed and discussed. The possibility of realizing a sensor of current to be integrated in microelectronic process and power integrated circuits is envisaged.

One-step approach for preparing ozone gas sensors based on hierarchical NiCo2O4 structures

Nanostructured semiconducting oxides have been used as resistive gas sensors of toxic and non-toxic gases, but little emphasis has been placed on ozone sensing. Here we present a new ozone gas sensor based on hierarchical NiCo2O4 cubic structures synthesized via a facile urea-assisted co-precipitation method and annealed at 450 °C, which showed a low detection level. Ozone detection was carried out through electrical measurements with an optimized performance at 200 °C, with fast response (∼32 s) and recovery (∼60 s) time with suitable concentration range (from 28 to 165 ppb) for technological applications. Furthermore, NiCO2O4 platelets are selective to ozone compared to other oxidizing and reducing gases. The low detection level can be attributed to the coexistence of 3D structures based on hexagonal platelet-like and porous flower-like shape, which were revealed by field emission scanning electron microscopy (FE-SEM). In summary, NiCo2O4 is promising for detection of sub-ppb levels of ozone gas.

Ozone Sensing Based on Palladium Decorated Carbon Nanotubes

Multiwall carbon nanotubes (MWCNTs) were easily and efficiently decorated with Pd nanoparticles through a vapor-phase impregnation-decomposition method starting from palladium acetylacetonates. The sensor device consisted on a film of sensitive material (MWCNTs-Pd) deposited by drop coating on platinum interdigitated electrodes on a SiO2 substrate. The sensor exhibited a resistance change to ozone (O3) with a response time of 60 s at different temperatures and the capability of detecting concentrations up to 20 ppb. The sensor shows the best response when exposed to O3 at 120 °C. The device shows a very reproducible sensor performance, with high repeatability, full recovery and efficient response.

Composition study of high temperature sputtered amorphous GaxAs1−x films

Abstract We present a systematic study of the affects of the sputtering parameters on the chemical composition of amorphous GaAs (a-GaAs) sputtered films. This study shows that the composition of sputtered a-GaAs films is controlled by the self-bias voltage, the temperature, and the properties of the substrate. a-GaAs films are stoichiometric when deposited at 300 ∘ C on glass substrate, at 400 ∘ C on molybdenum substrate, and at 500 ∘ C on indium tin oxide coated glass. We suggest that these observations show that the composition of the films is only governed by the As re-evaporation process at the surface of the growing film, which is mainly increased by increasing the substrate temperature. The affect of the thermal conductivity of the substrate is discussed.