Polarization control of ScAlN, ZnO and PbTiO3 piezoelectric films: application to polarization-inverted multilayer bulk acoustic wave and surface acoustic wave devices

Polarization-inverted multilayers are promising for application in bulk acoustic wave (BAW) resonators, BAW transformers, surface acoustic wave (SAW) devices and nonlinear optics crystals (NLOs). However, is difficult to obtain a polarization-inverted multilayer by a conventional polarization control technique using a buffer layer. Recently developed ion beam-induced polarization inversion film growth is attractive for multilayer fabrication. Low-energy ion beam irradiation (several hundred electron volts) during film growth enables the growth of polarization-inverted (0001)/(000 1 ¯ ) c-axis normal ZnO, AlN and ScAlN piezoelectric films. These structures excite a thickness extensional mode (longitudinal wave). In contrast, high-energy ion beam irradiation (300–3000 eV) induces c-axis parallel film growth which allows the fabrication of c-axis horizontal inversion ZnO and AlN multilayers. These structures are suitable for thickness shear mode (TSM) film bulk acoustic resonators (FBARs), TSM liquid sensors and out-of-plane NLOs. This review introduces the unusual polarization inversion film growth induced by ion beams and its applications. On the other hand, a (001)/(00 1 ¯ ) polarization-inverted layer can be obtained using ferroelectric films. This paper also provides the result of external electric field-induced polarization inversion of PbTiO3 epitaxial films.