Investigations on the origin of mass and elastic loading in the time varying distinct response of ZnO SAW ammonia sensor

In our earlier report, we gave an account of an interesting observation of a distinct response of ZnO SAW sensor for liquor ammonia. While the sensor response consisted of a small initial decrease in differential frequency followed by large increase for moist ammonia, it showed decrease alone for dry ammonia, water vapors and other interferants. This paper attempts to investigate the fundamental cause of the observed distinct response, by exploring the contributions of various sensor mechanisms such as mass, elastic and acousto-electric loading. To carry out the investigation, four sensors having ZnO films on SAW resonators with varying structural properties were employed. ZnO thin films of same thickness but differing in crystallite size, surface roughness and stress were achieved by depositing the films using RF sputtering under different pressures (10–40 mTorr). Electrical measurements on ZnO thin film show a very small contribution of acoustoelectric effect toward sensing response for liquor ammonia. Initially the elastic loading and later mass loading are identified as the dominant mechanism. The response has been fitted theoretically to determine individual contributions toward the sensing. A mechanism has been proposed and a correlation between the properties of ZnO film and sensor response has been attempted.