Experimental evidence for a dissociation mechanism in NH3 detection with MIS field-effect devices

The gas response mechanism of ammonia detection with Pt-based metal–insulator–semiconductor (MIS) field-effect sensors was investigated. An experimental model system was designed which compares the responses of thick continuous Pt layers with controlled morphology and surface chemical composition, with the response of thin, discontinuous layers. The surface of a thick, continuous sputter-deposited Pt film is modified, either by (i) the deposition of a thin SiO2 overlayer, (ii) reactive sputter deposition of PtOx, or (iii) co-deposition of Pt with SiO2 in Ar+O2 atmospheres. We show that the ammonia response is caused by the formation of atomic hydrogen through the dissociation of NH3 at temperatures <200 °C. It is found that the modified surfaces exhibit increased ammonia selectivity compared to a pure Pt film. Results from this work indicate that the reason for the changed selectivity is the appearance of an oxidized PtOx phase or triple phase boundaries between Pt, SiO2 and the ambient gas, rather than for solely morphological reasons.