Control of in-plane and out-of-plane texture in shear mode piezoelectric ZnO films by ion-beam irradiation

ZnO polycrystalline films have a strong tendency to grow their c-axis perpendicular to the film surface, even on an amorphous substrate. However, unusual (100) preferred orientations in which the c-axis lies in the substrate plane are often observed when the film is exposed to ion irradiation during its growth. To investigate the effect of ion irradiation on the (100) preferred orientation, ZnO films were fabricated using a 0-1 keV oxygen ion-beam-assisted electron-beam evaporation of zinc. The results clearly indicated that the tendency of (100) preferred orientation was enhanced with increasing ion energy and amount of ion irradiation. This demonstrated that the ion bombardment induced the (0001) preferred orientation to change into a (100) preferred orientation which corresponds to the ion channeling direction. An in-plane preferred orientation was also obtained, probably because of deviations in the incident ion-beam direction from 5° to the substrate surface normal. These in-plane textured (100) ZnO films, fabricated under the ion-beam irradiation of 0.5-1 keV, excited a shear acoustic wave without any longitudinal wave. The highest shear mode electromechanical coupling coefficient was found to be k15 = 0.16 in the film with ion-beam irradiation of 1 keV. This k15 value corresponds to 60% of that in a ZnO single crystal (k15 = 0.26)