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 (10 1- 0) 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 (10 1- 0) 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 (10 1- 0) 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 (10 1- 0) 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 (10 1- 0) 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).