460 Sensors and Actuators 3, IS-19 (1994) 460-463 Dependence, of sensitivity and reproducibility of CdS oxygen sensors V.A. Srn~~a and V. Golovanov Facuky of phyfics, Odessa State University, 270100, Pastera 42> Odessa (Ukmine) S. Kashulis, G. Mattogno and S. Viticoli Isttito di Chimico dei Materiali, Area di Ricerca di Roma, Monterotondo Scala (Ita&) Abstract The electrical conductance of the CdS-based sensors increased or decreased when exposed to CO or SO2 gases, depending on the operation temperature and stoichiometric composition of the tilm. A correlation between the IR spectrum and inversion of the chemisorbed complex charge state was observed. This is considered to be a favourable basis for the detection of atmospheric poilutan~ in ambient air, by combination of two sensor systems. Introduction Success in the fabri~tion of ~mi~nductor gas sen- sors is linked to sensors based on metal oxide structures characterized by improved sensitivity, stability and quick response time. The rapid desorption processes which occur during the application of semiconducting sensors at rather high temperatures, often cause a degradation of the semiconductor. Therefore, inv~t~a~ons on new structures which are sensitive to gases at lower tem- peratures are of great importance. It has been shown that the wide-gap semiconductors, like CdS, can also be used as low-temperature gas detectors [l]. On the other hand, in real circumstances, sensors have to detect the presence of one or several chemical impounds. The actual problem is lack of selective and this impedes industrial utilization of such sensors. To overcome this problem, chemically selective bulk materials and surface membranes which can respond to the desired substance must be developed. Their structure must be such that specific interaction between the sensor and the molecular products to be detected, leads to a p~si~chemical change in the detector which can be measured by a coupled physical transducer. Direct measurement of this physical change is often not possible [2]. Improving the selectivity and sensitivity of semicon- duct&g sensors gave rise to the development of sensor fusion in order to obtain non-neg~gible improvement of these fundamental parameters. Sensor arrays have been recognized to have a certain interest also in the chemical sensing frame, in conjunction with pattern recognition, that yields essential advantages for the identification and monitoring of gases in robotics [3]. This paper deals with a model that is useful for the design and evaluation of those systems based on sensors characte~ed by Iow selectivity and different moieties to CO and SO, gases. The gas sensors based upon CdS thin fihns have been studied in a low-temperature operating regime. The sensors are deposited by an electrohydrodyn- amical spray of 0.01-0.06 M solutions of CdC& and (NH&&S on a glass substrate heated at 670-750 IS in air: CdC& -t (NH,),CS f H,O - CdS 1 + 2NHJl f CO, t (1) Variation of the stoichiometric composition of the films with CdB ratios of S/l (type I sensors) and l/5 (type II sensors) was achieved by mixing the initiat solutions in different concentrations. The films were controlled by XPS and mass spectra analysis. The grain size of the investigated films obtained with SEM ob- servation was about 100 nm and was comparable with the Debye length. Evaporated indium was used for the contacts. All the contacts used were found to be ohmic and had symmetric behaviour with respect to the voltage polarity. The resulting films are highly sensitive over the entire visible part of spectrum and show steep ampere& characteristics. A quite intense absorption band at 0.58 to 0.60 mkm, the result of CO chemisorption, was 0925-4005/94/$7.000 1994 Elsevier Sequoia. All rights reserved SsDl 0925-4005(93)01038-6