DETECTION OF COMBUSTIBLE GASES BY MEANS OF A ZNO-ON-SI SURFACE-ACOUSTIC-WAVE (SAW) DELAY-LINE

ZnO-on-Si surface acoustic wave (SAW) delay lines have been examined as detectors of some combustible gases (GO, NO, H-2), reactive gas such as O-2 and other gases (CH4, N-2, Ar). Measurements have been performed at temperatures of 20-160 degrees C, operation frequencies of 100-200 MHz and atmospheric pressure. The ZnO thin films have been prepared by reactive r.f. diode sputtering in an O-2/Ar atmosphere. The piezoelectric ZnO films deposited on the acoustic propagation path ensure both electromechanical transduction and gas detection. The relative changes of SAW phase velocity (acoustic response) upon absorption and desorption of a gas have been measured and compared with the change of electrical resistivity (electrical response) measured on the sensor in the same conditions. The effect of preliminary NO exposure on the acoustic responses for other gases is demonstrated. The time dependence of the acoustic response to NO is shown to be remarkably different from both its electrical response and from the acoustic responses to all other test gases. Therefore the shape of the time dependence can be used as a qualitative parameter for selective identification of NO. The relatively small interaction of Ar and N-2 with ZnO him makes them good carriers for other gases. Concerning the acoustic response, two possible processes of gas-molecule sorption by the ZnO film are briefly discussed.