Scanning force microscopy and electron microscopy studies of pulsed laser deposited ZnO thin films: application to the bulk acoustic waves (BAW) devices

Abstract

New results concerning high crystalline ZnO thin films deposited on Si and sapphire substrates by laser ablation of Zn targets in oxygen reactive atmosphere are reported. Cross-section scanning electron microscopy (SEM) studies clearly evidenced a columnar structure of the layer. As a result of the preparation technique for TEM studies, the film is breaking into separate columnar groups: the ZnO columns observed are 100–500 nm thick, depending on the deposition conditions. The diffraction patterns taken on a large selected area reveal the crystalline hexagonal structure of the ZnO film with a=0.324 nm and c=0.5205 nm. The (0 0 2) diffraction spot is elongated due to the slope of the diffraction object, that is a confirmation of the fact that the columns are grown along the c-hexagonal axis. Scanning force microscopy evidenced the sharp boundaries of different domains from the uniform granular distribution on the surface. Characterization of the films was also conducted to establish their performance as piezoelectric layers in transducers for bulk acoustic wave devices in the GHz range. The insertion and conversion losses and the electromechanical coupling constant were measured using an appropriate set-up.