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This work presents the development of n-type and p-type gas-sensitive materials from NiOx doped TiO2 thin films prepared by ion-assisted electron-beam evaporation. TiO2 gas-sensing layers have been deposited over a wide range of NiOx content (0-10 wt.%). The material analysis by atomic force microscopy, X-ray photoemission spectroscopy, and X-ray diffraction suggests that NiOx doping does not significantly affect surface morphology and Ni element may be a substitutional dopant of the TiO2 host material. Electrical characterization shows that NiOx content as high as 10% wt. is needed to invert the n-type conductivity of TiO2 into p-type conductivity. There are notable gas-sensing response differences between n-type and p-type NiOx doped TiO2 thin film. The responses toward all tested reducing gases tend to increase with operating temperature for the n-type TiO2 films while the response decreases with temperature for p-type TiO2 film. In addition, the p-type NiOx doping results in the significant response enhancement toward tested reducing gases such as acetone and ethanol at low operating temperature of 300 degrees C. (C) 2008 Elsevier B.V. All rights reserved.

In this paper a new C-MOS compatible fabrication process is presented for a nitrogen dioxide porous silicon (PS) sensor. Electrically insulated PS sensing layers and even free-standing PS membranes have been obtained by front side electrochemical micromachining, reducing in this way electrical leakage towards the crystalline substrate. The electrical behavior of the device in a controlled environment was measured by means of a volt–amperometric technique at constant bias. A huge variation of the current was detected at RT for NO 2 concentrations as low as 200 ppb (Δ G / G =111). The interference of humidity, ethanol, methanol, CO and ozone is also discussed.

We report about preparation and characterization of sputtered MoO3 thin film as sensing layer for gas detection. We show its capability to detect CO concentrations lower than 10 ppm in wet air, a feature that allows direct usage of this material for environmental monitoring. The research also highlights some criteria for selecting a suitable material and shows which features are important for a thin film to be a good candidate as a chemical sensor.

The structural properties of thermally evaporated Al-Fe layers were studied as a function of the aluminium layer thickness δ Al (taking values of 1,2 and 4 nm), with an iron layer of constant thickness, δ Fe = 30 nm. The quality and orientation of the material were examined by X-ray diffraction and transmission electron microscopy. Crystallites of α-Fe oriented in the (110) direction were around 7–8 nm in size for any value of δ Al except near δ Al = 1 nm where a mostly amorphous iron layer was obtained. It was also verified that a bilayer of composition δ Al = 2 nm and δ Fe = 30 nm, twice repeated, led to a two-phase metal system consisting of an amorphous component and crystalline α-Fe type material texturized along the (110) direction. The Fe 3 Al phase was also detected in the bilayer structure with δ Al = 4 nm.

Indium tin oxide polycrystalline thin films (90%In 2 O 3 + 10%SnO 2 (by weigth)) have been grown by r.f. reactive sputtering from a target of the same compound. The films exhibit columnar structure with a preferential (400) orientation with a dispersion of 8° around the normal of the film plane, as obtained by rocking curve measurements. These films exhibit an increase in resistivity when exposed to small concentrations of NO or NO 2 in air. At an operating temperature of 590K they have a sensitivity S NO,NO 2 , defined as S NO,NO 2 = ( I air − I NO,NO 2 )/ I NO,NO 2 for NO and NO 2 chemisorption, of 100 and 30 respectively for gases at room temperature and atmospheric pressure at a concentration of 1000 ppm. Four zones for oxygen ion adsorption and desorption can be distinguished in the plot of conductivity activation energy vs. temperature. The absence of long-term drift and a satisfactory reproducibility even at high NO and/or NO 2 concentrations are attributed to the excellent crystallographic properties of these films and should allow the development of detectors for nitrogen oxides at concentrations in the range 1–2000 ppm.

This polycrystalline silicon films with a crystalline grain size of 50–100 μm were grown by quasi-rheotaxy on glass substrates covered by a thin previously grown aluminium layer. The aluminium and silicon were both deposited by high vacuum electron beam evaporation. The thin silicon films exhibit only the (111) orientation for thicknesses of less than 1 μm, whereas for thicker films the (220) orientation begins to appear. The latter orientation prevails for films thicker than 10 μm. Crystalline grains as large as 10–20 μm were obtained.