Shear mode properties of c-axis parallel oriented ScxAl1−xN films grown by RF bias sputtering

Because the increase of piezoelectricity was found in Sc heavily doped AlN films, they have been well studied. Many researchers investigate the properties of the longitudinal mode bulk acoustic wave with c-axis normally oriented ScAlN films. However, there are few reports on the shear mode properties because of difficulties in growth of c-axis parallel oriented ScAlN films which can excite shear mode bulk acoustic wave. In this study, we demonstrated the film growth of c-axis parallel oriented ScAlN using RF substrate bias RF magnetron sputtering method. As a result of X-ray diffraction analyses, we obtained c-axis parallel oriented AlN, Sc_0.05Al_0.95N and Sc_0.13Al_0.87N films. Then, High overtone bulk acoustic wave resonators were fabricated with these films. They excited shear mode bulk acoustic wave, but shear mode conversion losses were large (more than 20 dB). It is necessary to obtain highly-crystallized ScAlN films.