Shear mode electromechanical coupling coefficient k15 and crystallites alignment of (11 2- 0) textured ZnO films

ZnO film, in which the crystallite c axis lies in the substrate plane [(11 2- 0) textured ZnO], is a good candidate for application in shear mode piezoelectric devices. The relationships between the degree of crystallites alignment and the shear mode electromechanical coupling coefficient k15 in (11 2- 0) textured ZnO films have been investigated. Forty pure-shear mode high overtone bulk acoustic resonators consisting of the (11 2- 0) textured ZnO film were prepared. The film was varied in crystallites alignment and film thickness. The degrees of crystallites alignment of the films were determined by x-ray pole figure analysis. The k15 values of the films were estimated from the conversion loss characteristics of the resonators. A significant correlation was observed between dispersion of the x-ray poles and the k15 values. However, the k15 values in the thinner films were clearly reduced as compared with those in the thicker films despite their similarities in crystallites alignment. In addition, unexpected second harmonic mode resonance was detected in the thinner films. We concluded that the piezoelectrically inactive layer in the initial film growth deteriorated the k15 value in thinner films.