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) over bar) 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) over bar) 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. (C) 2021 The Japan Society of Applied Physics

Automated summary

Ion beam-induced polarization inversion film growth is an attractive method for fabricating various types of piezoelectric films, including thickness extensional mode and thickness shear mode structures for acoustic wave resonators. This technology shows great potential for a wide range of applications.