G. Sberveglieri

Recent developments in semiconducting thin-film gas sensors

Abstract A critical review of thin-film semiconducting materials that are used as gas sensors is presented in this paper. These materials can be approximately divided into two main groups: the first includes sensors that detect oxygen with variations of bulk or surface conductance; the second comprises all the materials that detect oxidizing and reducing gases in air at constant oxygen partial are described. The long-term stability and the selectivity to a particular species are the main properties of a reliable gas sensor; these properties are also discussed in detail in relation to the second group of sensors. The properties of tin oxide, the most investigated material for gas sensing, are reviewed; some new ternary semiconducting compounds are also presented, together with their growth techniques. An almost novel method for growing thin films, the RGTO (rheotaxial growth and thermal oxidation) technique will be presented, since it is capable of preparing mixed oxide thin fimms with high surface area and nanosized crystallites. It is well known that the main possibility for sensor development lies in device arrays; therefore, the utilization of different materials for gas detection and the measurement techniques of signals supplied by sensor arrays will be outlined.

TiO2 thin films by a novel sol–gel processing for gas sensor applications

Novel thin films of titanium dioxide dispersed in a polymeric matrix have been prepared by a chemically modified sol–gel technique. Nanostructured films of pure TiO2 in the anatase form are obtained after annealing at 500°C. SEM, TEM and TG/DTA are used for the structure characterisation of TiO2 films. The role of the polymer in controlling the microstructure is confirmed. The first application of this technique in gas sensor field is presented in this work. Ethanol and methanol sensing properties are tested and reported. TiO2 sensors can detect very well concentration required for breath analysers.

Wo3 Sputtered Thin-Films For Nox Monitoring

We present in this paper preliminary results concerning the preparation of tungsten trioxide thin films by reactive sputtering, the characterization either of their structural properties by means of XRD measurements or of the film morphology with the AFM microscope and the electrical response of the film towards toxic and pollutant gases. WO3 thin films showed a good sensitivity towards low NOx concentrations (1–10 ppm) in the temperature interval 200–500 °C and they were also sensitive to 10 ppm NH3. These films were also selective to Nox with respect to interfering gases like CH4, CO and SO2 in the same temperature range.

UV light activation of tin oxide thin films for NO2 sensing at low temperatures

Abstract A novel approach to operate semiconductor gas sensor at low temperatures avoiding the poisoning of the surface is described. The effects of UV light illumination on the performance of SnO 2 thin film gas sensors toward an oxidising gas of increasing interest due to environmental monitoring, like NO 2 are reported. The thin films gas sensors were prepared according to the rheotaxial growth and thermal oxidation (RGTO) technique with dc sputtering in Ar atmospheres. Results has shown that there is an enhancement of the performances with UV exposure: a decrease in the response and recovery time and no poisoning effect. This is promising for the development of a sensor for NO 2 working at room temperature.

Light enhanced gas sensing properties of indium oxide and tin dioxide sensors

Abstract The effects of UV light illumination on the performance of SnO2 and In2O3 semiconductor gas sensors toward CO and NO2 are reported. The sample were prepared with DC sputtering in Ar atmospheres by the Rheotaxial Growth and Thermal Oxidation (RGTO) technique and with reactive magnetron sputtering in Ar and O2 atmosphere, respectively. Results are quite promising for the development of sensor working at room temperature.

Classical and novel techniques for the preparation of SnO2 thin-film gas sensors☆

Abstract It is well known that tin oxide is considered as one of the most important and studied materials for the fabrication of semiconductor gas sensors. Moreover, thin-film techniques seem the most suitable for the production of smart gas sensors on silicon substrates using microelectronic technologies. This paper presents a review of the different thin-film techniques for growing tin oxide gas sensors. A new deposition method with the acronym RGTO (rheotaxial growth and thermal oxidation) is also described in detail with regard to the preparation parameters. Finally, the structural and morphological characteristics of RGTO thin films and their electrical response to various gases are considered.

Investigation on the O3 sensitivity properties of WO3 thin films prepared by sol–gel, thermal evaporation and r.f. sputtering techniques

WO3 thin films have been deposited on alumina substrates provided with platinum interdigital electrodes by sol–gel (SG), r.f. sputtering (RFS), and vacuum thermal evaporation (VTE) techniques and annealed at temperatures between 500°C and 600°C for 1 to 30 h in static air. The morphology, crystalline phase and chemical composition of the films have been characterised using SEM, glancing XRD and XPS techniques. The electrical response has been measured exposing the films to O3 (10–180 ppb), NO2 (0.2–1 ppm), NOx (27 ppm NO and 1 ppm NO2) at different operating temperatures ranging between 200 and 400°C and humid air at 50% R.H. SG prepared films have shown bigger responses (S=IAir/Igas) with respect to VTE and RFS for all the investigated gases and operating temperatures. RFS prepared has resulted to be less sensitive, but faster in the response and more stable in terms of signal reproducibility. The response to O3 has been found to be at maximum at 400°C. At this temperature the response to 80 ppb of ozone has been: S=35 (SG), S=18 (VTE) and S=5 (RFS). The NO2 and NOx response reached the maximum at 200°C and becomes negligible at 400°C. Improvements on the O3 gas sensitivity and selectivity can be achieved by fixing the operating temperature of the films at 400°C.

A novel method for the preparation of NH3 sensors based on ZnO-In thin films

Abstract This paper reports the preparation of ZnO-In thin films that are capable of detecting low ammonia concentrations in air. Indium and zinc layers are subsequently deposited by thermal evaporation at 70 °C onto an alumina substrate; the metal-semiconductor phase transformation is obtained on thermal cycling of the films in a synthetic air flow at a maximum temperature of 500 °C. XRD analysis shows that these films are polycrystalline and are slightly oriented in the (002) direction. SEM micrographs show that the surface of the metal-oxide film presents a needle-shaped structure; the needle size ranges between 1 and 3 μm. ZnO-In thin films can detect ammonia concentrations in the range 1–10 ppm by operating between 200 and 500 °C and are selective towards reducing gases like CO and CH 4 ; a good response towards a few ppm NO x is also observed in the same temperature range. This sensor seems to be suitable for detecting some ppm of ammonia in air.

A new technique for growing large surface area SnO2 thin film (RGTO technique)

A new technique for growing SnO2 thin films with high surface area is described, based on tin rheotaxial growth and its thermal oxidation (RGTO). Tin thin films, when grown, present a surface characterized by spheroidal agglomerates due essentially to the surface tension of the liquid metal; these agglomerates do not seem to have any electrical continuity. By means of a thermal oxidation, both the transformation of the metal into a semiconductor and the thin film continuity are obtained, owing to the volume increase during the above-mentioned phase transformation. After this annealing cycle, SnO2 thin films are slightly oriented in the (101) direction and present an electrical resistivity equal to about 102 Omega cm. So, the surfaces of SnO2 films appear to be formed by spongeous agglomerates, which are in electrical contact with the nearest agglomerates. SnO2 thin films grown by this method show a high sensitivity (defined as the relative per cent conductance variation) to H2; in fact, sensitivity to 200 PPM H2 in synthetic air at ambient pressure is equal to 400%. It seems to be possible to prepare other metal oxide semiconducting thin films with a high surface area using this technique.

Reactively sputtered indium tin oxide polycrystalline thin films as NO and NO2 gas sensors

Abstract 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.