Interface structure of multilayer ZnO acoustic transducers

The authors have developed thin-film multilayer ZnO acoustic transducers for bulk acoustic wave generation at frequencies as high as 100 GHz. The transducers consist of vacuum-sputtered layers of ZnO with alternating crystal structure and, consequently, alternating piezoelectric constant. The conversion efficiency and maximum operating frequency depend on the sharpness of the transition between the crystal layers. The interface between the layers is examined using electron microscopy and Auger spectroscopy to determine the distance required for the c-axis of the ZnO to reorient into the crystal structure. It is shown that the transition between the layers occurs over a distance of approximately 50 angstrom. If the piezoelectric constant changes within that distance, this technique is capable of generating phonons at frequencies as high as 300 GHz. Electron diffraction studies indicate that both the 'normal' and 'tilted' layers are oriented with the c-axis parallel to the surface normal. The results of the interface studies are used in theoretical calculations of the conversion efficiency and maximum operating frequency.