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

The ScxAl1-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) ScxAl1-xN films on c-sapphire substrates...

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Bibliographic Details
Published in:Journal of electronic materials 2022-01, Vol.51 (4)
Main Authors: Greenslit, Morton, da Cunha, Mauricio Pereira, Lad, Robert J.
Format: Article
Language:English
Subjects:
aluminum oxynitride
MATERIALS SCIENCE
plasma-assisted epitaxy piezoelectric film
Scandium aluminum nitride
surface acoustic wave resonator
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Summary:The ScxAl1-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) ScxAl1-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 ScxAl1-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 ScxAl1-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 ScxAl1-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 ScxAl1-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. As a result, the use of a 10-nm-thick SixNy capping layer on top of the ScxAl1-xN films aids in preventing etching during SAWR device photolithography and also helps hinder film oxidation up to 800°C.
ISSN:0361-5235
1543-186X