Title:  Acoustic Resonators Based On Epitaxial AlN/AlScN Ultra Thin Piezoelectric Films

Committee: 

Dr. Ansari, Advisor       

Dr. Cressler, Chair

Dr. Ayazi

Abstract: The objective of this proposal is to introduce several advances into the ultra-thin aluminum scandium nitride (AlScN) based acoustic resonators targeted for filtering application at ku band. We approach our design considering the limitations of filter bandwidth and super high-frequency operation by marrying the advantages of highly-crystalline thin films with the piezoelectric boost from the addition of Sc to AlN alloys. This results in single-crystalline epitaxial AlScN films directly grown on Si substrate using molecular beam epitaxy (MBE). Chapter 2 presents the first demonstration of acoustic devices fabricated using epitaxial AlScN films, showing significant improvement of keff2 up to 5.3 % for LWRs with the highest operating frequency of 10 GHz. FBAR device shows the super high-frequency operation at 18.8GHz (Ku-band). Another critical performance metric of an acoustic resonator is the Qkt2 Figure of merit (FoM), with robust structures that overcome thin-film fabrication challenges. Chapter 3 illustrates the first demonstration of thin-film ferroelectric AlScN-on-silicon composite resonators, targeting high-overtone resonance modes in the sub-6GHz band with a high FoM. An ultrahigh kt2 value of 11.7% at the 3rd-order TE resonant frequency of 2.4 GHz is reported, yielding kt2×Qmax of 84, showing 2 improvement compared to the co-fabricated AlScN-only FBARs. Lastly, another objective of this proposal is to present a novel sensing scheme utilizing integrated, self-sustained, high-Q, resonant-based frequency comb sensors by employing an ultra-thin piezoelectric layer. Chapter 4 presents the first demonstrations of phononic (or mechanical) frequency combs based on non-degenerate parametric pumping using the duffing nonlinearity present in the flexural-mode ultrathin (<100 nm) piezoelectric resonators.