Plasma-Assisted Epitaxy of Piezoelectric ScxAl1-xN Films on Sapphire for Use in Harsh-Environment Microwave Acoustic Sensors.

The Sc_xAl_1-xN wurtzite structure has been shown theoretically and experimentally to exhibit significantly higher piezoelectric coupling compared to pure AlN. In this work, a plasma-assisted epitaxial growth method has been used to synthesize epitaxial (0002) Sc_xAl_1-xN films on c-sapphire substrates from x = 0.07 to 0.30 by co-evaporating high-purity Sc and Al sources in the presence of a nitrogen plasma generated by an RF plasma source. Epitaxial Sc_xAl_1-xN films with highly oriented (0002) grains and in-plane registry were produced on c-sapphire substrates that were pre-exposed to the nitrogen plasma to form an oxynitride seed layer. Growth of Sc_xAl_1-xN films was carried out at 930°C under both metal-rich and N-rich conditions using precisely controlled Sc, Al, and N-plasma fluxes. Metal-rich depositions yielded non-(0002)-oriented Sc_xAl_1-xN grains and intermetallic ScAl grains. Nitrogen-rich growth with a Sc/Al flux ratio of 1/3 produced the best (0002) epitaxy as determined by x-ray diffraction analysis. Surface acoustic wave resonator (SAWR) devices were fabricated from 500-nm-thick Sc_xAl_1-xN and AlN films to extract their electromechanical coupling coefficients, k². As the Sc concentration in the films increases, the degree of (0002) epitaxy is reduced, yet the value of k² increases becasue there is more Sc in the wurzite lattice despite the decreased level of (0002) grain alignment. The use of a 10-nm-thick Si_xN_y capping layer on top of the Sc_xAl_1-xN films aids in preventing etching during SAWR device photolithography and also helps hinder film oxidation up to 800°C. [ABSTRACT FROM AUTHOR]