Acoustoelectric effect tudy for SAW sensors

Research has recently begun on the use of ultra thin films and nanoclusters as mechanisms for sensing of gases, liquids, etc., since the basic material parameters may change due to film morphology. As films of various materials are applied to the surface of SAW devices for sensors, the conductivity of the films may have a strong acoustoelectric effect, whether desired or not. The purpose of this paper is to reexamine the theory and predictions of the acoustoelectric effect for SAW interactions with thin conducting or semi-conducting films. The paper will summarize the theory, and predict the effects of thin film conductivity on SAW velocity and propagation loss versus frequency and substrate material. The theory predicts regions of conductivity which result in extremely high propagation loss, which also correspond to the mid-point between the open and short circuit velocities. As an example of the verification and possible usefulness of the acoustoelectric effect, recent experimental results of palladium (Pd) thin films on an YZ LiNbO₃ SAW delay line have shown large changes in propagation loss, depending on the Pd film thickness, and/or exposure to hydrogen gas. By proper design, a sensitive hydrogen leak detector SAW sensor can be designed.