Effects of Electric Bias on Different Sc-Doped AlN-Based Film Bulk Acoustic Resonators

Film bulk acoustic resonators (FBARs) based on aluminum nitride (AlN) and scandium-doped aluminum nitride (AlScN) exhibit tremendous application aspects in the radio frequency front-end due to achievable high-frequency characteristics, superior thermal performances and compatibility with harsh environments. Delicately controlling the resonant frequency (fs) of FBAR is essential for integrating filters or modules. In this work, we provide a practical feasibility in adjusting fs of AlN and AlScN FBAR using external direct current electric bias (EDC). When applying a negative EDC (the direction along the reversed c-axis), fs shifts to a lower frequency, whereas a positive EDC brings a higher fs. In order to extract the equivalent values of the stiffness coefficient (c33), piezoelectric coefficient (e33) and dielectric constant (εzz) of AlN and AlScN piezoelectric materials, we adopted the electromechanical equivalent Mason model. The results show that the equivalent values of c33 increase with the change of EDC from negative to positive, and, on the other hand, those of e33 and εzz decrease. Our work provides a systematic investigation on the electric field-influenced stiffening effect of AlN and AlScN piezoelectric films and opens a feasibility for frequency-tunable resonators.