DC/AC characterizations of WO 3 sensor of ethanol: mechanisms of detection A. Labidi, C. Lambert-Mauriat, C. Jacolin, K. Aguir FST St Jérôme, Université Paul Cézanne Aix-Marseille III. L2MP (CNRS UMR 6137). Marseille, France. Email-address: khalifa.aguir@l2mp.fr A. Labidi, M. Maaref IPEST. Unité de Recherche de Physique des Semiconducteurs et Capteurs (URPSC). La Marsa Tunis, Tunisia. Email-address: ahmed.labidi@l2mp.fr Abstract— In this work, both direct current (DC) and alternating current (AC) measurements were used in order to better understand the mechanisms of detection of ethanol by sensors based on polycrystalline WO 3 thin films. Firstly, DC characterizations confirmed that ethanol was oxidized by the oxygen pre-adsorbed on the WO 3 surface. Secondly, AC measurements coupled with a simple RC-model showed that this reaction occurred mainly on the surface of the grains. I. INTRODUCTION The development of chemical sensors for organic vapors is the matter of several research works in the world. Several materials are exploited for these kind of applications, such as porous silicon [1], polymer composite [2] or metal oxide semiconductors [3]. Among the metal oxide semiconductors, tungsten oxide (WO 3 ) is one of promising material for organic vapor sensor applications, particularly for ethanol vapor sensors [4]. Indeed, like others n-type metal oxide semiconductors, its electrical properties are strongly dependant on the surrounding gas [5]. In these previous works the mechanisms of detection are generally deduced from DC measurements, which give the general behavior. This work proposed to couple the DC measurements with AC spectroscopy in order to determine the regions of the device (electrodes, grains or grain boundaries) which are crucial for the detection. So, the objectives of this work are: • The understanding of the mechanisms involved in the detection of ethanol • The discrimination of the regions of the sensor device, which participate to the detection. After a brief description of experiments, DC responses under ethanol are presented and mechanisms of detection are discussed. Results obtained by AC spectroscopy are compared with a simple RC model: parts of the sensors, the more implicated in the detection, are determined. II. EXPERIMENTAL The WO 3 sensitive layer is deposited by reactive radio frequency magnetron sputtering, on Si/SiO 2 substrates. During the sputtering the Ar:O 2 ratio is 50:50 which has been found to be the best deposition condition for gas sensor applications [6]. The film thickness is around 50 nm. After WO 3 deposition, the sensor is annealed at 450°C (723 K) for 1 hour in air in order to stabilize the chemical composition and crystalline structure of the sensitive layer. Electrical measurements are performed at the working temperature T work = 300°C. The DC and AC measurements are acquired by using a pico-ammeter (HP4140B) and a frequency response analyzer (Solartron 1250) in the 0.2 Hz – 65 KHz frequency range. III. RESULTS AND DISCUSSION A. DC responses In the following, the response of sensor « S » is defined by 0 0 G G G S vap - = , (1) where 0 G and vap G are the conductance before and after introduction of ethanol vapor, respectively. Responses of sensor under ethanol mixed with dry air, in concentration range between 2 and 10 %, are presented on Fig. 1a. An increase in the conductance is observed as the vapor concentration is increased. Furthermore the coming back at the initial state is rapid with a good restoring of the baseline. This behavior could be explained by considering that the main reaction is the oxidizing of ethanol involving pre-adsorbed O - species, which are the more stable species adsorbed on the WO 3 surface at 300°C [7]. According to the works of Bârsan on SnO 2 under CO gas [8], which show that the power low exponent evolution for high vapor concentration depends on the kind of the pre-adsorbed 1384 0-7803-9056-3/05/$20.00 © 2005 IEEE.