The significance of parallel electric field on the preferred orientation and surface morphology of ZnO thin films

Abstract

Piezoelectric zinc oxide thin film, with its high electromechanical coefficient, has found applications in surface and bulk acoustic wave devices. A suitable polycrystalline zinc oxide thin-film transducer should possess the following two characteristics. First, structurally, the normals to the basal planes (0 0 2) must be predominantly oriented perpendicular to the plane of the substrate (c-axis oriented). Second, electrically, the resistivity of the film should be as high as possible. Having these requirements, polycrystalline zinc oxide can be satisfactorily used in place of ZnO single-crystal thin film in veryand ultra-high-frequency acoustic applications. Accordingly, if we could manipulate the c-axis orientation of the film, we would be able to tailor the device’s performance. We have investigated the effects of parallel electrical fields (detached electrodes) on the preferred c-axis orientation and surface morphology of polycrystalline zinc oxide thin films. Zinc thin films are oxidized at different temperatures with the application of parallel electrical fields. We have found that the application of parallel electrical fields slightly improves the c-axis orientation. The better c-axis–oriented structure is manifested by a less needle-shaped forest crystal morphology on the surface of the film.