Plasma-Assisted Epitaxy of Piezoelectric Sc_xAl_1-xN Films on Sapphire for Use in Harsh-Environment Microwave Acoustic Sensors

Journal Article · 21 January 2022 · Journal of Electronic Materials

DOI: https://doi.org/10.1007/s11664-021-09425-2 · OSTI ID:2290245

Greenslit, Morton  ^[1]; da Cunha, Mauricio Pereira ^[2]; Lad, Robert J. ^[2]

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1. University of Maine, Orono, ME (United States); University of Maine
2. University of Maine, Orono, ME (United States)

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. As a result, 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.

• Research Organization: University of Maine, Orono, ME (United States)
• Sponsoring Organization: USDOE Office of Fossil Energy and Carbon Management (FECM)
• Grant/Contract Number: FE0031550
• OSTI ID: 2290245
• Journal Information: Journal of Electronic Materials, Journal Name: Journal of Electronic Materials Journal Issue: 4 Vol. 51; ISSN 0361-5235
• Publisher: SpringerCopyright Statement
• Country of Publication: United States
• Language: English