Prototype structure for systematic investigations of thin-film gas sensors (original) (raw)
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Conductance analysis of (Co, Nb, Fe)-doped SnO2 thick film gas sensors
Journal of Materials Science: Materials in Electronics, 2007
Thick films prepared with undoped nanometric SnO 2 particles and with (Co, Nb, Fe)-doped SnO 2 were studied with the purpose of developing oxygen and carbon monoxide gas sensors. The ceramic powders were obtained through the Pechini method. The morphological characteristics were studied with SEM and TEM, after which, they were subjected to sensitivity tests under different atmospheres. A correlation was established between the microstructure of the material, the effects of the additives, and the electrical behavior. The response of the sensor could be explained as the result of the characteristics of the intergranular potential barriers developed at intergrains. It was determined that the SnO 2 -doped films have a greater sensitivity between 200°C and 350°C.
IOP Conference Series: Materials Science and Engineering, 2016
This research is focused on structural and electrical characterisation of tin oxide (SnO2) applied as a thick film and investigation of its properties as gas sensitive material. Micron sized SnO2 powder was milled in an agate mill for six hours to fabricate SnO2 nanopowder, which was afterwards sieved by 325 mesh sieve and characterized by XRD and SEM. This powder was used as functional part in the production of thick film tin oxide paste containing a resin vehicle with 4 wt. % nanosize glass frits acting as permanent binder. The glass frits where additionally milled for twelve hours in the agate mills to nanosized powder and sieved by a 325 mesh sieve as well. The achieved thick film paste was screen printed on alumina and fired at 850 o C peak temperature for 10 minutes in air. After the sintering process, thick film samples where characterized by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). The reflectivity was measured on the same samples by UV-VIS spectrophotometer: the band gap was determined from the slope of reflectance. After that a matrix of different interdigitated electrode structure of PdAg paste was printed and sintered using the mentioned sintering conditions. The tin oxide thick film was printed over the interdigitated electrodes as a top layer and sintered again under the same conditions. The total electrical resistance was measured as a function of the electrode spacing and temperature. A negative temperature coefficient (NTC) was identified and measured in the range from room temperature (27°C) to 180°C in a climate chamber. Finally the samples were placed into a gas reactor with NOx and CO gas and the resistance was measured in the same temperature range (27°C-200°C).
Sensors and Actuators B: Chemical, 2008
Ultrathin tin oxide films were deposited on flat hotplate templates using atomic layer deposition (ALD) techniques with SnCl 4 and H 2 O 2 as the reactants. The resistance of the SnO x ALD films across an electrode gap on the hotplate template was observed to oscillate and decrease versus the number of sequential SnCl 4 and H 2 O 2 reactions at 250 • C. The resistance also varied with exposure to O 2 and CO pressure at 300 • C and 325 • C. A wide range of SnO x ALD film thicknesses between 15.9 Å and 58.7 Å was prepared by varying the number of sequential, self-limiting SnCl 4 and H 2 O 2 reactions. The CO gas sensor response was then measured for these SnO x ALD film thicknesses at 300 • C. The CO gas sensor response increased for increasing thicknesses between 15.9 Å and 26.2 Å and decreased for increasing thicknesses between 26.2 Å and 58.7 Å. The results were interpreted in terms of the Debye length and resistance for the SnO x ALD films. The Debye length is comparable with the SnO x ALD film thickness of 26.2 Å corresponding to the maximum responsivity for CO gas sensing. For film thicknesses >26.2 Å, the responsivity decrease was explained by a larger fraction of the film with thickness greater than the Debye length that is not affected by the O 2 and CO chemisorption. For film thicknesses <26.2 Å, the responsivity decrease was attributed to the increasing resistance of the SnO x ALD film. The gas sensor response was temperature dependent and displayed its highest responsivity at temperatures between 250 • C and 325 • C. The response times of the SnO x ALD gas sensors were also faster at the higher temperatures >260 • C.
Response to oxygen and chemical properties of SnO2 thin-film gas sensors
Vacuum, 2008
The measurements of the response-in terms of the conductance changes-to oxygen adsorption of tin dioxide (SnO 2) thin-film-based gas sensors were performed. The sensing SnO 2 layers were obtained by means of the rheotaxial growth and thermal oxidation (RGTO) method. The sensor responses were measured under a dry gas flow containing oxygen in nitrogen, within the range of temperature from 25 to 540 1C. For comparison, similar studies were performed for a commercial SnO 2 thick-film (TGS 812) gas sensor. The in-depth profiles of the chemical composition of the RGTO SnO 2 layers were determined from the scanning Auger microprobe experiment. The changes in concentration ratios [O]/[Sn] and [C]/[Sn] from the near-surface region towards the grain bulk were shown.
Microstructure and morphology of tin dioxide multilayer thin film gas sensors
Sensors and Actuators B-chemical, 1997
Structural, morphological, and electrical measurements have been carried out on SnO 2 multilayer thin film grown by the rheotaxial growth and thermal oxidation method on Al 2 O 3 substrates. The analysis of X-ray and electron diffraction patterns shows that, in addition to the SnO 2 cassiterite phase, a contribution from another SnO 2 phase is present, which can be related to cassiterite by introducing microtwinning effects. The electrical measurements show that these thin films have a higher sensitivity towards CO with respect to the conventional single layer SnO 2 sensors. © 1997 Elsevier Science S.A. 0925-4005/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 9 2 5 -4 0 0 5 ( 9 7 ) 0 0 2 1 8 -9
Gas sensing properties of epitaxial SnO 2 thin films prepared by atomic layer deposition
Sensors and Actuators B-chemical, 2003
Undoped SnO 2 thin films are grown on a-Al 2 O 3 (0 1 2) (r-cut sapphire) substrates by gas phase atomic layer deposition (ALD). Two precursor pairs, SnI 4 -O 2 and SnCl 4 -H 2 O 2 , both new for ALD, are used. The films have a cassiterite structure and are [1 0 0] sapphire oriented. A good epitaxial quality and the conductivity acceptable from the standpoint of semiconductor gas sensors are achieved for ultrathin films grown from SnI 4 -O 2 at 600 8C. The sensitivity of these films to CO in air has a maximum at a thickness of about 10 nm. Response rise and decay times belonging to a several seconds interval are measured. The films are assumed to function as a single grain. #
IEEE Sensors Journal, 2013
Rutile TiO 2 (1 0 1) and cassiterite SnO 2 (1 0 1) epitaxial single and double nanolayers, the latter stacked in either sequence, are atomic layer deposited on r-cut α-Al 2 O 3 (0 11 2) substrates. Thickness of the layers is varied. Epitaxial quality of the films is characterized by X-ray diffraction (XRD), reflection high-energy electron diffraction, and transmission electron microscopy. In gas response measurements, as-grown films and the films coated with electron-beam evaporated Pt nanoclusters are exposed, at 350°C, to H 2 , CO, and CH 4 diluted in air. In response to test gas concentrations of 30 parts per million (ppm), the films with a thickness of order of 10 nm exhibit, depending on the makeup and gas, as high as two-to five-fold decrease in the resistance. It is shown that the platinum surface catalyst is effective in accelerating the response and recovery processes. The transition times of the order of a few tens of seconds are observed. The results demonstrate the feasibility of gas sensing with singlecrystal-like nanolayer films. Comparison of sensor characteristics of such quasi-2D nanostructures and the literature data relevant to individual nanowires, nanorods, and nanobelts, i.e., typical representatives of the quasi-1D structures, shows that, as to H 2 , CO, and CH 4 , both structures are worthy competitors.
Thickness Dependence of Sensitivity in Thin Film Tin Oxide Gas Sensors Deposited by Vapor Pyrolysis
International Journal of Engineering, Transactions B: Applications, 2003
Transparent SnO 2 thin films were deposited on porcelain substrates using a chemical vapor deposition technique based on the hydrolysis of SnCl4 at elevated temperatures. A reduced pressure self-contained evaporation chamber was designed for the process where the pyrolysis of SnCl4 at the presence of water vapor was carried out. Resistive gas sensors were fabricated by providing ohmic contacts on the layers obtained and the installation of a custom made micro-heater beneath the substrate. The sensitivity (S = Ra/Rg) of the fabricated sensors to acetone vapor contamination was measured at 270 ºC. S increased linearly with contamination level up to 8000 PPM, and saturation was observed at higher concentrations. The experimental relationship between S and thickness of the sensitive film was investigated in films obtained by CVD for the first time. It was shown that S was inversely related to the film thickness, and higher S values were recorded for thinner sensors. The upper limit for S was imposed by the conduction instabilities observed in the thinnest films deposited.
Undoped and Pd-doped SnO2 thin films for gas sensors
Sensors and Actuators B: Chemical, 1993
Undoped and Pd-doped SnO, thin films for gas sensors The aim of this work is the study of the structure and temperature dependence on the conductance and sensitivity of undoped and Pd-doped Snot thin films (0..5-2 pm}. Polycrystalline SnO, thin films have been synthesized by pyrolysis, cm a heated substrate (oxidized silicon (IOO}), of an aerosol produced by ultra-high frequency spraying of a volatile precursor solution. A mixture of two precursors is used to obtain Pddoped SnO,. Growth rate and microstructure of the films are particularly well controlled by the deposition temperature (460-560 "C). Conductivity measurements wem performed between 50 and 500 "C, alternately under pure air (GJ and polluted air fG) (ethanol or CO), using gold layers as electrodes. The pure SnO, sensitivity (C -Q/G, to ethanol increases when the elaboration temperature decreases, in connection with the specific area increase. P~ladium ~n~~~tjon sjgni~~ntly promotes the sensitivity. CO ~nsjt~vit~ is increased by 30 when palladium is inco~amt~ into the SnO, thin fiims and exhibits a marked Peak at low t~m~ratum. The sen~tiv~ty stability is better when the deposition temperature is higher, according with a better stability of the microstructure. A variable frequency elect&at study was carried out using complex impedance spectroscopy ( 1 mHz-20 MHz). Ethanol and CO reactions were observed and the role of grain boundaries studied.
A model for the gas sensing properties of tin oxide thin films with surface catalysts
Sensors and Actuators B-chemical, 1995
The effect of two types of surface additives (Pd and Pt) on the response of reactively sputtered thin films of SnO. gas sensors has been studied, in mixtures of zero grade air and carbon monoxide. The experimental results obtained with surface additives show an abrupt conductance increase around 500 K when carbon monoxide in the ppm range is present, a behaviour which has not been observed with plain SnO, films. A semi-empirical model that explains this behaviour is presented, that is based on the well-established theory for the conductivity of ultrathin discontinuous metal films, i.e. activated charge carrier creation and tunnelling through potential barriers. The proposed model takes into account the dependence of film conductivity both on the thickness of the noble metal deposited on it and on the working temperature. The results of the theoretical analysis are in excellent qualitative agreement with the experimental ones.