High-Order Sezawa Mode Alscn/Gan/Sapphire Surface Acoustic Wave Resonators

This work reports on the first demonstration of surface acoustic wave (SAW) resonators based on epitaxial aluminum scandium nitride (AlScN) on gallium nitride (GaN)/sapphire multilayer structure operating at 3–6 GHz. Al
0.8
Sc
0.2
N epitaxial films are grown on GaN/sapphire with molecular beam epitaxy (MBE). Interdigital transducers with submicron pitch sizes are patterned with electron beam lithography (EBL). With the highly piezoelectric AlScN epi-layer, high quality (Q) factor, high-order Sezawa modes are for the first time probed in an epi-AlScN/GaN/sapphire structure, exhibiting a Bode Q factor of 571 at 4.6 GHz. On the same epitaxial stack, ferroelectric polarization switching of Al
0.8
Sc
0.2
N is demonstrated, opening monolithic integration opportunities for acoustic resonators and ferroelectric capacitors. Corroborated with COMSOL simulations, the frequency response of the epi-AlScN/GaN/sapphire SAW resonator show enhanced piezoelectric response versus the GaN/sapphire resonator. The phase velocity versus the normalized thickness of Al
0.8
Sc
0.2
N of each propagation mode is analyzed and compared with the experimental values. Furthermore, the temperature coefficient of frequency (TCF) for each mode is evaluated from the temperature-dependent frequency response, showing a stable value around -30 ppm/K up to 600 K. The thermal stability of the epi-films makes them a great candidate for high temperature applications.