Field-effect sensor for the selective detection of ¯uorocarbons Werner Moritz a,* , Vladimir Fillipov b , Alexey Vasiliev b , Lars Bartholoma Èus a , Alexander Terentjev b a Humboldt-University Berlin, Inst. of Phys. Chemistry, Bunsenstr. 1, 10117 Berlin, Germany b Russian Research Centre Kurchatov Institute, 123182 Moscow, Russia Received 21 July 1998; accepted 11 September 1998 Abstract A chemical semiconductor sensor for the temperature range up to 4008C was developed using silicon carbide with an epitaxial layer of SiC as the substrate. Thin layers of LaF 3 and Pt were deposited on the semiconductor/insulator structure to form a three-phase boundary with the gas under investigation. The sensor was shown to be sensitive to ¯uorine, hydrogen ¯uoride and different ¯uorocarbons. The in¯uence of the operation temperature on the sensor response signal was investigated in the range from room temperature up to 4008C. For the ¯uorocarbons CF 3 CH 2 F, CF 3 CCl 3 , CHClF 2, CF 3 CH 2 Cl and CCl 3 F a selective detection was achieved at temperatures near to 4008C. The substantial role of the ¯uoride ion conducting material LaF 3 for the sensor detection principle was proven. A mechanism including the chemisorption of the ¯uorocarbon at the Pt surface and an insertion of ¯uorine into LaF 3 was discussed. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Sensor; Selective detection; Fluorocarbons 1. Introduction Chloro¯uorocarbons have been implicated in atmospheric ozone depletion. Although they are no longer used in industrialised countries there is some production in devel- oping countries. Furthermore, large amounts of chloro¯uoro- carbons are still in use and their waste treatment has to be developed. For control of these processes and other applica- tions, where it is important to know the gas concentration, chemical sensors may be advantageous. These sensors should be selective for ¯uorine containing compounds. In addition, it may be useful to have the possibility of detection of the ¯uorohydrocarbons which are the alternative com- pounds to chloro¯uorocarbons. Sensors for the detection of ¯uorocarbons were developed using the metal±oxide-semiconductor type materials as V±Mo±Al 2 O 3 /ZnO [1] or sulphur doped SnO 2 [2]. Several ¯uorocarbons such as CF 3 CH 2 F or CCl 2 FCClF 2 have been measured at a temperature of 4008C at concentrations down to 5 ppm. It is a disadvantage of these types of sensors that there is also sensitivity to other hydrocarbons and no selec- tivity to ¯uorine containing compounds. This effect can be improved only slightly by the addition of sulphur. Moreover, it is necessary to point out that reproducible measurements with sensors based on metal±oxide semiconductors require the presence of constant oxygen concentration in investi- gated gases mixtures. This can considerably restrict the ®eld of application of these kind of sensors. Recently, we developed a ®eld-effect semiconductor sen- sor for the detection of ¯uorine and hydrogen ¯uoride in atmospheric air as well as in inert gases [3,4]. This sensor is a metal/insulator/semiconductor (MIS) device and has a structure Si/SiO 2 /Si 3 N 4 /LaF 3 /Pt. The active components of this sensor are the LaF 3 layer and the Pt gate electrode forming a three-phase boundary with the gas under inves- tigation. Already at an initial stage of sensor development it was successfully proved that such a sensor can be used for the detection of ¯uorocarbons. That is a signal was found for 1,1,1,2-tetra¯uoroethane at a temperature of 1808C [5]. Subsequent investigations showed that this temperature is too low for a stable sensor response and also for the detection of other ¯uorocarbons. Unfortunately, the silicon-based semiconductor ®eld-effect sensors can be used only up to a temperature of about 2008C because of the small band gap of Si. Therefore, we developed a high temperature gas sensor based on the large band gap semiconductor silicon carbide (SiC) using the same sensitive layers (LaF 3 and Pt). It was demonstrated that at room temperature these sensors have Journal of Fluorine Chemistry 93 (1999) 61±67 *Corresponding author. Fax: +49-3020-935-559. 0022-1139/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S0022-1139(98)00282-6