Investigation of Polarity Reversal of Bilayer AlN/ScAlN Films and Its Influence on Film Bulk Acoustic Resonators.

Polarity is one of the key properties that affect the functionality of piezoelectric materials. For typical piezoelectric materials, aluminum nitride (AlN) and scandium-doped aluminum nitride (ScAlN), the polarity of the films will affect the electromechanical performance of their acoustic devices. Here, metal organic chemical vapor deposition (MOCVD) and physical vapor deposition (PVD) are used to grow an AlN seed layer on a silicon substrate, and then use PVD to grow ScAlN on the AlN layer. The polarity of bilayer AlN/ScAlN is investigated theoretically by density functional theory based on a simplified atomic structure model, and is observed experimentally at the atomic scale with scanning transmission electron microscopy. It's found that AlN grown on silicon shows Al polarity while ScAlN shows N polarity. Furthermore, film bulk acoustic wave resonators (FBARs) are fabricated based on these films, which show that the electromechanical performance of FBARs is changed sharply when the polarity of ScAlN is opposite to AlN. After removing the AlN seed layer, the effective electromechanical coupling coefficient of the FBAR increases from 5.82% to 11.85%. This work proves the impact of the polarity reversal and provides a basis for the design of FBARs based on bilayer AlN/ScAlN at the theoretical and experimental aspects.