Andreia A.S. Lopes

Influence of the deposition conditions on the gas sensitivity of zinc oxide thin films deposited by spray pyrolysis

Abstract In this work we present preliminary results on the sensitivity to methane gas of zinc oxide thin films deposited by spray pyrolysis. It was found that using highly resistive (above 10 4 Ω cm) thin films and by performing the measurements at 200°C a sensitivity better than one order of magnitude was found to detect 2000 ppm of methane. A linear dependence on the sensitivity between 100 and 2000 ppm of methane was also obtained.

Glass transition and crystallization kinetics of a barium borosilicate glass by a non- isothermal method

The glass transition and crystallization kinetics of a glass with a molar composition 60BaO-30B2O3-10SiO2 were investigated by differential scanning calorimetry (DSC) under non-isothermal conditions. DSC curves exhibited an endothermic peak associated with the glass transition and two partially overlapped exothermic peaks associated with the crystallization of the glass. The dependence of the glass transition temperature (Tg) and of the maximum crystallization temperature (Tp) on the heating rate was used to determine the activation energy associated with the glass transition (Eg), the activation energy for crystallization (Ec), and the Avrami exponent (n). X-ray diffraction (XRD) revealed that barium borate (β-BaB2O4) was the first crystalline phase to be formed followed by the formation of barium silicate (Ba5Si8O21). The variations of activation energy for crystallization and of Avrami exponent with the fraction of crystallization (χ) were also examined. When the crystallization fraction (χ) increased ...

Correlation between the microscopic and macroscopic characteristics of SnO2 thin film gas sensors

Abstract Hall effect measurements have been used to evaluate the conduction mechanism, exhibited by tin oxide thin film gas sensors deposited by spray pyrolysis. Two experiments have been carried out: (i) Hall measurements in air and (ii) in the presence of methane (first results reported), both as a function of temperature. From the measurements performed it was possible to infer the potential barrier and its dependence with the atmosphere used. The results obtained for the carrier concentration and mobility have been analysed in the light of the oxygen diffusion mechanism at grain boundaries by using the grain boundary-trapping model. In the presence of the methane gas the electrical resistivity decreases due to the lowering of the inter-grain boundary barrier height.

Silicon carbide photodiodes: Schottky and PINIP structures

Abstract This work deals with the study of the role of intra-gap density of states and films composition on the colour selection of the collection spectrum of glass/ITO/a-Si x :C 1− x :H/Al Schottky photodiodes produced in a conventional plasma-enhanced chemical vapour deposition (PECVD) system using as gas sources silane and a controlled mixtures of silane and methane. To do so, properties of the films were investigated, especially the one concerning the determination of the valence controllability of the films produced and the density of bulk states. Besides that, a PINIP device was also produced, using the a-Si x :C 1− x :layer that lead to the best Schottky diode performances.

Study of the Sensing Mechanism of SnO2 Thin-Film Gas Sensors Using Hall Effect Measurements

Hall effect measurements are one of the most powerful techniques for obtaining information about the conduction mechanism in polycrystalline semiconductor materials, which is the basis for understanding semiconductor gas sensors. In order to investigate the correlation between the microscopic characteristics and the macroscopic performances exhibited by undoped tin oxide gas sensors deposited by spray pyrolysis, Hall effect measurements were performed at different temperatures, from room temperature up to 500 K, and in the presence of two different atmospheres, air and methane. From these measurements, it was possible to infer the potential barrier and its dependence with the used atmosphere. The obtained results were analysed in terms of the oxygen mechanism at grain boundaries on the basis of the grain boundary-trapping model. In the presence of methane gas, the electrical resistivity decreases due to the lowering of the inter-grain boundary barrier height. Introduction Tin oxide (SnO2) is among all the semiconducting metal oxides the most widely used material in gas sensors fabrication. Until now, the majority of the known gas sensors are based on sintered tin oxide due to its low cost (both for the raw material and for the technological process), long-life and requirement of very simple electronics, so that little maintenance is involved [1]. These sensors exhibit a good response to several gases, both reductors and oxidants, presenting the inconvenient of a lack of selectivity in the presence of a gas mixture and a gradual performance degradation with time [2]. As these properties vary with the grain size, surface morphology and internal porosity [3], the use of this material in the form of thin film starts gaining attention. Nanostructured tin oxide presents greater densities of grain boundaries and interfaces, and its strong reactivity with gaseous species is a consequence of being small-grained. The study of the transport mechanism in polycrystalline semiconductor materials is the basis for the understanding of semiconductor gas sensors [4], and through Hall effect measurements it is possible to obtain this type of information. With the purpose of improving their operation characteristics and obtaining optimal sensors with respect to electrical conduction and microstructure characteristics, a study was performed in order to understand the correlation between the microscopic characteristics and the macroscopic performances of undoped tin oxide gas sensors [5]. Experimental details Undoped SnO2 thin films were deposited by spray pyrolysis technique. A solution of SnCl4 5H2O (Riedel-de-Haën, ref. 14550 with 98% purity) dissolved in isopropilic alcohol (Pronalab with 99.7% purity) with concentrations varying from 0.1 to 0.4 M was sprayed onto glass substrates pre-heated at temperatures between 573 and 673 K using nitrogen as carrier gas at a constant flow rate of 8l/min. The substrate-nozzle distance was kept constant at 25 cm. The film thickness (d) was Key Engineering Materials Online: 2002-10-25 ISSN: 1662-9795, Vols. 230-232, pp 357-360 doi:10.4028/www.scientific.net/KEM.230-232.357